Symposium EM4 : Structure-Property Relationships of Organic Semiconductors

One of the most promising technologies to address the energy generation from renewable sources is the organic and printed photovoltaics (OPVs). OPVs have the potential to provide energy harvesting capabilities to numerous consumer products, such as automotive and transport, buildings, wearables, portable devices, and consumer applications. Their outstanding advantages include lightweight, thin film form factor, conformability to complex surfaces in combination to high aesthetics and tunable optical performance. Furthermore, they can be combined to numerous other electronic devices, such as sensors, to provide energy efficiency and independence to the electric grid.
This potential of OPVs will be unleashed only after their large-area fabrication by cost-effective Roll-to-Roll (R2R) manufacturing processes, that will enable their commercialization and market implementation. Nevertheless, one of the major challenges that need to be overcome, is the capability to manufacture high quality OPV nanolayers and devices with reproducible and tailored properties (optical, electrical, structural), homogeneity in thickness and structure, as well as high performance over large areas, by R2R production processes.
Robust optical metrology techniques have an enormous potential to be used as quality control tools for R2R manufacturing processes. This is attributed to their flexibility for adaptation to pilot and production equipment, to their high speed (capability to measure optical spectra in ms) and to the capability to use sophisticated modelling approaches to determine the optical properties, composition and thickness of the material under study.
In this work, we describe the novel methodology for the in-line optical metrology of R2R printed nanomaterials (e.g. transparent electrodes, polymer-based organic semiconductors, barrier nanolayers) onto flexible substrates for manufacturing of OPVs. Sophisticated modelling procedures and methodologies has been developed to investigate the optical properties, homogeneity, thickness, quality and surface-property relationships of bulk-heterojunction photoactive layers for printed OPVs that consist of electron donors (e.g. polythiophene and carbazole based polymers) and acceptors (e.g. fullerene derivatives, indene C60 and C70 bis adduct, etc.).
Finally, the in-line optical metrology has been implemented for the optimization of the flexible OPV manufacturing and the achievement of high efficiency printed OPV devices. This innovative methodology establishes the importance and applicability of optical monitoring tools to be used as necessery components for R2R pilot lines for the cost-effective manufacturing of novel organic and inorganic nanolayers for several Organic and Printed Electronics applications.

Polymer acceptors are promising fullerene alternatives because their spectral absorption can extend into the visible and near-infrared regions of the solar spectrum. Their synthetic modularity provides leverage for tunable molecular frontier orbitals (HOMO and LUMO) and optical bandgaps that can be complementary to those of their polymer donor counterpart in bulk heterojunction (BHJ) solar cells. Spectral complementarity helps achieve higher device photocurrents and, in turn, high BHJ solar cell efficiencies. Because polymers show better morphological and mechanical resilience under stress compared with molecular donors and acceptors, “all-polymer” BHJ solar cells benefit from longer-term stabilities compared with that of fullerene-based BHJ devices (e.g. PC₆₁BM, or its C₇₁ analogue). At this time, most efficient polymer acceptors are based on perylenediimide (PDI) or naphthalenediimide (NDI) motifs, and a few recent reports have shown that PDI/NDI-based polymers blended with selected polymer donors can achieve power conversion efficiencies (PCE) >6%. Several other acceptor motifs are currently being studied across the community, such as diketopyrrolopyrrole, benzothiadiazole, isoindigo, B←N bridged bipyridyl, and various nitrile (-CN) derivatizations; however, only a few polymer systems can achieve PCEs comparable to those of PDI/NDI-based polymer acceptors and identifying material systems that can gradually improve device performance beyond currently reported PCEs remains a critically important direction. Our recent studies geared to the design, examinations, and synthetic derivatizations of PDI/NDI-free polymer acceptors for all-polymer BHJ solar cells indicate that adequately-substituted “all-thiophene” polymer systems can also act as efficient polymer acceptors, used as alternatives for fullerenes with polymer donors. Using several sets of branched alky-substituted polymer acceptors, we show that several analogues can achieve PCEs of 2-5% in BHJ solar cells with PCE10 (and analogues) as the polymer donor. In the optimized BHJ devices, the polymer donor (E_opt ~1.5-1.6 eV) and acceptor (E_opt ~1.7-2.0 eV) counterparts absorb in complementary regions of the UV-vis spectrum (spanning 300-800 nm), yielding high short-circuit current densities (J_SC) (6-10 mA/cm²) and some of the best open-circuit voltage (V_OC) figures (up to ca. 1.1 V) reported to date for BHJ solar cells. Our morphological characterizations indicate that the polymer acceptors tend to aggregate, inducing the phase separation network between the polymer donor and acceptor. The morphology changes translate into variations in the carrier mobility patterns across the BHJ thin films (measured via SCLC model); and we show that the balance between hole (µ_h) and electron (µ_e) mobilities correlate with the BHJ solar cell efficiencies.

Environmental stability is one of the primary issues limiting widespread applicability of organic semiconductors in flexible applications. Predicting lifetime of these materials is very challenging as these technologies have not been in use for long enough and, therefore, there is no real data to validate predictive models. Additionally, for organic photovoltaics, the technology changes constantly which can alter the main degradation modes and therefore affect any predictions based on accelerated stability tests.

At the National Physical Laboratory we have developed a unique stability testing rig which can control the atmospheric H₂O and O₂ content within a sample chamber to the part-per-million level, as well as illumination intensity, temperature and electronic stress, in an arrangement that allows the detailed study of the degradation of organic semiconductor materials and devices under highly controlled and reproducible conditions. The test rig can be coupled to different characterisation instrumentation for detailed in-situ studies.

Here we present a full investigation of the efficiency degradation of printed, semi-transparent, organic photovoltaic devices (OPV) on flexible substrates in various oxygen-rich and humid atmospheric conditions. Periodic measurements of the current-voltage characteristics of many devices allows the effect of O₂ an H₂O to be isolated from degradation due to extended exposure to 1 sun irradiation. We demonstrate an approach of using a whole-device computational model to fit the time-varying device parameters throughout the test. We show how a combination of the device model analysis with complementary in-situ characterisation measurements, such as external quantum efficiency and photocurrent mapping (LBIC), to investigate the simultaneous changes in different layers within the solar cell allows to elucidate the precise mechanisms of environmental degradation in these OPV devices.

Optimizing the energy levels at the donor-acceptor interface of organic solar cells has driven their efficiencies to above 10%. However, further improvements towards efficiencies of typical for the best inorganic solar cells are difficult because of empirical limits for the open-circuit voltage of polymer fullerene solar cells which hardly ever exceeds 1V due to voltage loss >0.6 eV (Figure 1). Here we show that this limit is overcome with a non-fullerene acceptor where a rhodanine flanked non fullerene small molecule acceptor FBR serves as acceptor with and a low band gap polymer PffBT4T-2DT as donor material. The combination of FBR and PffBT4T-2DT allows both a high open-circuit voltage of 1.12V and extremely low voltage loss ~ 0.48 V whilst keeping the external quantum efficiencies >55 % which could not been achieved with fullerene solar cells so far. These unconventional features result in photovoltaic performances as high as 7.8%. This study will inspire organic-photovoltaic community to focus on non-fullerene acceptors for further improvement of power conversion efficiencies without any complex multi-layer or -component device architectures.



Figure 1. Comparison of the maximum external quantum efficiency and the voltage loss between E_g/q and the open circuit voltage V_oc for different types of organic solar cells. The grey line indicates the empirical limit for the maximum EQE possible for a given voltage loss given by Li et al.(REF) The blue line is determined using the result in the current work. Traditionally, there are hardly any cases of EQE_max > 0.5 with a voltage loss < 0.6 V. Novel results presented here as well as the results from Kawashima et al. show that it is possible to overcome this barrier.
[1] W. Li, K. H. Hendriks, A. Furlan, M.M. Wienk, R. A. J. Janssen “High Quantum Efficiencies in Polymer Solar Cells at Energy Losses below 0.6 eV”, JACS, Vol. 137, No. 6, (2015), pp 2231-34.

Recently, organic bulk heterojunction solar cells that employ non-fullerene acceptors such as n-type small molecules and polymers instead of the ubiquitously used fullerene derivatives have shown significant increases in efficiency, now approaching or even exceeding those previously achieved with fullerenes. While the photophysics of polymer and small molecule donor – fullerene acceptor bulk heterojunctions have been studied intensively, which in turn has significantly improved our understanding of the efficiency-limiting processes in these systems, the precise interplay between structure, morphology, photophysics, and efficiency of non-fullerene acceptor devices remains to be better understood to allow for a more guided material design.
In this contribution I will present our recent spectroscopic studies on different small molecule and polymer donor:acceptor blends. Specifically, I will discuss the photophysics of small molecule and polymer non-fullerene acceptors in comparison to blends that use fullerene derivatives as acceptors. Using advanced Vis-NIR broadband ps-µs transient absorption spectroscopy, transient IR vibrational, and ps-ns time-resolved photoluminescence spectroscopy in combination with sophisticated data analyses techniques such as multivariate curve resolution (MCR) analysis we are able to deconvolute the spectral components of excitons and charges and to monitor their individual dynamics in OPV blends. Furthermore, using photothermal deflection spectroscopy in combination with electroluminescence experiments we evaluate the impact of the CT-state energy on the open-circuit voltage in non-fullerene blends. In addition, time-delayed collection field experiments reveal the field-dependence of charge generation and its dependence on structure and blend morphology. Combining the results of the aforementioned techniques we are able to track the charge generation and recombination processes across a wide dynamic range and can develop a precise picture of the efficiency-limiting processes in devices that use novel non-fullerene acceptors.

Organic photovoltaics (OPV) is a promising candidate technology for the low-cost fabrication of modules to harvest solar energy. Although OPV technology has significantly matured over the past few years, there remain significant challenges in addressing the gap between lab-scale devices and real manufacturing. Structure-property-performance relationships for OPV devices are still underdeveloped, and relationships based on one system are not necessarily transferrable to new, higher-performance systems. I will highlight our device and morphology studies of an organic photovoltaic (OPV) system deposited by a high volume manufacturing technique, blade-coating, that achieves greater than 9.5 % power conversion efficiency (PCE). The average crystal domain orientation and characteristic phase separation length distribution are markedly different when deposited by blade-coating rather than spin-coating. This result allows us to determine which aspects of morphology are not relevant to the PCE of this system. Whether the crystallites are "face on" or "edge on" does not appear to impact the PCE of system, nor does the length scale or "hierarchical" nature of the phase length scale. Persistent morphological qualities that may be associated with high PCE in this system are relatively pure phases and relatively strong diffraction. We posit that OPV systems in which the PCE is less sensitive to morphology may also be less sensitive to film thickness, enabling some to maintain high PCE in active layers thicker than greater than ≈200 nm. We confirm that blade-coating is a suitable prototyping technique for R2R coating by demonstrating nominally identical morphologies for both piece blade-coating and continuous-web, slot-die coating.

Organic photovoltaics is one of the most promising technologies for long-term green energy supply. The widely used fullerene-based acceptor, phenyl-C₆₁-butyric acid methyl ester (PCBM), displays a limited open circuit voltage and unsatisfactory long-term stability in combination with many of the state-of-the-art organic donors due to microstructural incompatibilities and subsequent demixing. Rational design rules for novel fullerenes with enhanced compositional miscibility are employed to develop novel fullerenes overcoming this limitation. We demonstrate that binary organic solar cells based on PTB7-Th:fullerene and pDPP5T-2:fullerene composites with decent photovoltaic performance and extraordinary high thermal stability. The solar cells based on these novel fullerene acceptors maintain almost 100% initial performance after baking at 140 °C for 24 hours under inert atmosphere while PCBM based devices are with only 19% or 31% left under the same examined condition.

We further demonstrate that the low miscibility between PCBM and pDPPT5-2 or PTB7-Th is the fundamental origin of the low thermal stability. On the contrary, two novel fullerenes, PyF5 (amorphous) and FAP1 (crystalline), with a significantly higher chemical compatibility are introduced to overcome these limitations. PyF5 and FAP1 exhibit more optimized energy levels and a higher V_OC than PCBM does, allowing for keeping the time-zero performance losses at a minimum. Most importantly, the better chemical miscibility with PTB7-Th and pDPP5T-2 allows overcoming short-time J_SC burn-in losses and furthermore results in OPV devices with superior long-term stability. The combination of promising optoelectronic properties and thermal stability underlines the necessity for novel acceptor design rules balancing performance and stability. The benefit of chemical miscibility as a novel design principle for improved stability is expected to pave alternative guidelines towards designing and developing novel acceptors for efficient and thermally stable OPV devices.

Here we report a comparison of the morphology as well as the charge carrier dynamics for organic bulk heterojunction solar cells employing the high crystalline conjugated polymer named PffBT4T- 2OD in blends with two electron acceptors: fullerene derivative, PC₇₁BM and rhodanine- based non- fullerene small molecule. We employ femtosecond transient absorption spectroscopy and atomic force microscopy and GIWAXS to show a correlation between morphology and the kinetics of polaron generation from excitons in donor or acceptor, and relate these observations to device efficiency. Charge generation and charge carrier dynamic are found to be strongly affected by blend films morphology with both acceptors. We is found that rhodanine acceptor matches well with PffBT4T-2OD and non-fullerene OSCs with an impressive V_OC of 1.11 V, and a high power conversion efficiency of 7.0% is achieved. The relatively lower device performance for the rhodanine acceptor (compared to 10.3 % for the equivalent PC₇₁BM device) is shown to result from a lower yield of dissociated polarons, increasing geminate recombination losses.

Organic photovoltaics have surpassed 11% power conversion efficiency through the careful design of low bandgap polymer donors to be used with fullerene acceptors. Despite this impressive progress, most of these high performance devices exhibit poor stability. Additionally, P3HT remains the only cost-effective donor polymer and the limitations of using fullerenes in photovoltaic devices continue to persist. We have recently shown that the efficiencies of P3HT devices can reach as high as 6.4% by replacing PCBM with a non-fullerene small molecule acceptor.¹ The use of these acceptors in multi-component heterojunctions can further increase the efficiencies achieved by the P3HT devices.
We have developed a novel approach towards high efficiency organic photovoltaic devices; where P3HT is combined with two non-fullerene acceptor molecules in a ternary blend. Ternary blends that make use of two polymer materials have been explored in the past, however the lack of entropic driving force and the potential for intermolecular interactions between polymer chains renders this approach problematic.² The introduction of a newly synthesized small molecule acceptor, IDFBR, into the P3HT binary devices leads to an increase in efficiency from 6.4% to 7.7% and importantly a vast improvement in device stability. Tuning of the second acceptor’s LUMO allows a higher V_OC to be achieved, and through careful selection of the acceptors and their ratios a favourable blend morphology can be achieved, thereby reducing recombination in the blends. This is the highest efficiency value reported for P3HT based solar cells, to date.

1. S. Holliday et al. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor, Nat. Commun., 2016, 7, 11585
2. C. B. Nielsen et al. Non-fullerene electron acceptors for use in organic solar cells, Acc. Chem. Res., 2015, 48, 2803-2812

Water-based polymer nanoparticle dispersions (solar paint) offer the prospect of simultaneously controlling the nanoscale architecture of the active layer whilst eliminating the need for hazardous organic solvents during device fabrication. However, the behaviour of these nanoparticulate devices is complex and thus understanding their structure-function relationships requires characterisation techniques that can probe chemical structure on the nanoscale. In this paper we review our progress in understanding the structure-function relationships of organic electronic nanoparticulate thin films. In particular, we discuss how scanning transmission X-ray microscopy is an invaluable tool for characterising these materials. Finally, we explore the future prospects and economic for large scale manufacture of solar cells based on printing.

This presentation will describe the synthesis and characterization of conjugated aromatic acid and silane based surface ligands that can significantly change the optoelectronic properties of silicon and lead sulfide quantum dots, and the work-function of common oxide electrodes. For example, we have used mild Heck chemistry to design and prepare electron rich or poor aromatic acids that, when attached to 2-5nm diameter SiQD surfaces using vinylsilyl linkages, can lower the resultant materials bandgap and potentially increase the charge transport properties. Furthermore we have prepared acid ligands with extreme dipole moments that have been used for tuning the work-function of ZnO, ITO, GaInP2, and NiOx oxide electrodes by 2eV, a magnitude of which has not been reported to date. Lastly the mild synthetic procedure allows us to prepare novel di- and tri-aromatic acids with a variety of functional groups used for preparing metal organic frameworks (MOF) with unique properties for catalysis and gas separations.

Solution processable conjugated polymers and small molecules based on alternating electron donating (D) and electron accepting (A) building blocks have lead to state-of-the-art solar cell materials governing power conversion efficiencies up to 10% and organic Field-Effect Transistors OFETs with mobilities in the order of 10 cm²/Vs. Unfortunately, the coupling of D-A building blocks does not always proceed as planned, which can result in the generation of ‘defects’ in the alternating DA-polymer chain such as homo-couplings (i.e. alternation of sequence of multiple instead of single D or A building blocks). Previous studies reported a reduced performance in polymer solar cells and, very recently, in small molecule solar cells. A general consensus on the impact of homo-couplings on device performance is needed as well as a deeper understanding on the root of the device deterioration. To eliminate the combined effect of molecular weight and homo-couplings in polymer solar cells, a systematic study on a small molecule system (p-DTS(FBBTh₂)₂) is presented. The impact of homo-couplings on the nanomorphology and electro-optical properties are investigated and it is demonstrated that very low quantities of homo-couplings (<5%) already lead to sub optimal device performance. These results highlight the importance of material purity in conjugated polymer and small molecule opto-electronic devices as homo-coupled products could be one of the major reasons for batch-to-batch variation.

The most performing conjugated polymers for Organic Photovoltaics (OPVs) are usually synthesized via Pd-catalyzed cross-coupling reactions of organometallic reagents, and the Stille coupling polymerization is frequently the protocol of choice.[1] On the other hand, the highly toxic organo-tin compounds and the stoichiometric amount of metal-containing wastes produced by this reaction do not comply with industrial scalability. In this context, Direct (Hetero)Arylation Polymerization (DHAP) via C-H bond activation represents a simpler and greener synthetic tool compared to conventional cross-coupling reactions, avoiding the use of toxic organometallic reagents which can strongly limit large scale processes of industrial interest.[2]^,[3]
The communication will discuss the synthesis of one of the most promising polymers for plastic solar cells, (poly[(benzo[1,2-b:4,5-b’]dithiophene)-alt-(4H-thieno[3,4-c]pyrrole-4,6(5 H)-dione) PBDTTPD, via Pd(PPh₃)₄-catalyzed Direct (Hetero)Arylation Polymerization (DHAP). Molecular weight distribution, thermal stability and embedded metal impurities of the polymer obtained with this protocol favourably compare with those of the same material prepared by the Stille polycondensation. In addition, the polymer synthesized via DHAP is demonstrated to outperform the Stille reference material in photovoltaic devices under the same processing conditions. Moreover, we report some preliminary results on the synthesis of double-acceptor / donor ternary random copolymers via the same protocol
Our results undoubtedly confirm the potential of DHAP as a straightforward and scalable synthetic approach to semiconducting polymers for plastic solar cells.

References
[1] Marzano, G.; Ciasca, C.V.; Babudri, F.; Bianchi, G.; Pellegrino, A.; Po, R.; Farinola, G.M. Eur. J. Org. Chem 2014, 30, 6583-6614.
[2] Mercier, G.L. ; Leclerc, M. Acc. Chem. Res. 2013, 46, 1597−1605.
[3] Marzano, G.; Kotowski, D.; Babudri, F.; Musio, R.; Pellegrino, A.; Luzzati, S.; Po, R.; Farinola, G.M. Macromolecules 2015, 48, 7039-7048.

The exploitable electronic and photophysical properties of organic materials rely both on the inherent properties of the particular chromophore, and on the specific nature of intermolecular interactions in aggregates, thin films, or crystals. We have developed simple models to estimate crystal packing in acenes, and have successfully applied these models to linear hetero-acenes such as anthradithiophenes. We have recently demonstrated the impact of isomer purity and rotational disorder in anthradithiophene crystals on the charge transport properties. In the interest of exploring our order-tuning approach with more unusual or extended chromophores, we have prepared and studied a wide array of new aromatic cores aiming to better understand the impact of chromophore configuration on charge transport. Further, I will describe how these studies have allowed us to begin developing relationships between molecular structure and critical parameters such as thermal and photo stability, solubility, and thermal conductivity.

The incorporation of heavy main group elements into conjugated polymer backbones has been an area of growing interest over recent years. Here we establish structure – property relationships for conjugated polymers containing a variety of heteroatoms. In particular the role of the bridging atoms will be reported for various classes of dithienometallole and diselenometallole containing materials. The properties of recently reported dithienoarsole polymer will be further explored in a variety of device applications.

We present a series of poly(3-hexylthiophene)-block-poly(ethylene glycol) (P3HT-b-PEG) with constant P3HT block length block and variable PEG block length. Alkyne functionalized P3HT with high absolute molecular weight of 11.4 kg/mol is combined with azide-functionalized PEGs via Copper Catalysed Alkyne Azide Cycloaddition (CuAAC click). The resulting P3HT-b-PEG block copolymers have PEG weight fractions between 15 and 64 wt%. In bulk materials, the crystallinity in the conjugated block is similar to pure P3HT, while the crystallinity of PEG is influenced with decreasing PEG block length. On addition of MeOH as a non-solvent for the P3HT block, these block copolymers are able to form stable micellar aggregates if the PEG fraction is >31 wt%. The preparation of P3HT-b-PEG micelles is done in a controlled way by using a syringe pump. Subsequently, the micelles can be transferred from THF/MeOH solution to water by dialysis. The structures of the micelles in aqueous solution were studied using cryo-TEM and DLS. The block copolymer micelles are found to have a spherical or cylindrical shape with diameters of around 25 nm and lengths between 40 nm up to some hundred nm. It is found that short PEG blocks lead to bigger block copolymer micelles. By spin-coating these micelles on mica substrates we could measure the size and shape of the dried micelles by AFM. The observed sizes and structures of the micelles in AFM confirm the results from cryo-TEM. Thus, a correlation of composition on solution structures and its consequences on the crystallization of both blocks are given. These findings help to understand the structure formation of amphiphilic block copolymers with one conjugated block. This systematic study on micellar structures of amphiphilic semiconductor block copolymers allows further investigation in different fields such as self-assembled hybrid materials, infiltration of biomembranes and their optical imaging.

Yulun Wang, Qiaogan Liao, Han Guo, and Xugang Guo*

Department of Materials Science and Engineering, South University of Science and Technology of China (SUSTC), No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China
Email: [email protected] (Y.W.)


Oligothiophenes (nTs) have been widely employed for constructing high-performance polymeric semiconductors for applications in organic-thin film transistors (OTFTs) and polymer solar cells (PSCs) due to their appropriate electrical properties and self-assembly characteristics. However, as the conjugation length of nTs extends, the corresponding polymers typically result in elevated HOMOs. When bithiophene (2T) is incorporated into polymers, the materials typically suffer from limited solubility (low molecular weight) and/or low degree of backbone conjugation. Hence, among various nTs, terthiophene (3T) and tetrathiophene (4T) are the most promising ones for enabling high-performance polymer semiconductors and bithiophene (2T)-based semiconductors typically show lower performance versus 3T and 4T-based counterparts.

3,3’-dialkyl-2,2’-bithiophene (BTR) is highly avoided in materials design since the accompanying steric hindrance leads to a twisted polymer backbone and amorphous film crystallinity. Replacing the alkyl chain with a less steric demanding alkynyl should lead to reduced steric hindrance due to the elimination of two methylene H atoms on the alkynyl substituents, and at the meantime maintain good materials solubility. Density functional theory (DFT) computation reveals a highly planar conformation between the two thiophenes with a low-lying HOMO in 3,3’-dialkynyl-2,2’-bithiophene (BTRy). Herein, we report the design and synthesis of a novel electron donor unit 3,3’-dialkynyl-2,2’-bithiophene (BTRy), and its incorporation into polymer backbones leads to the BTRy-based polymer semiconductors with a high degree of planar backbone, good solubility, substantial conjugation, and narrow bandgap (~1.6 eV) with a low-lying HOMO of -5.4 eV, which is greatly lower than that of 3,3’-dialkoxy-2,2’-bithiophene (BTOR)-based polymer analogue. When integrated into OTFTs, the 3,3’-dialkynyl-2,2’-bithiophene-based polymers show a preliminary charge carrier mobility on the order of 10^-1 cm²/Vs. When incorporated intosolar cells, the PSCs exhibit a PCE great than 6% with a substantial open-circuit voltage of 0.9 V. Via a series of complementary thin film and device characterization techniques, the materials structure-property-device performance correlation is established. The results indicate that the alkynyl-functionalized head-to-head linkage containing bithiophene is a promising building block for organic electronics.

Donor-acceptor block copolymers are promising candidates for photovoltaics due to their ability to microphase separate at length scales commensurate with exciton diffusion lengths. Conjugated block copolymers can also serve as model systems to study the relationship between molecular structure, microstructure, and optoelectronic properties of conjugated polymers. The most widely used synthetic route for conjugated block copolymers is the polymerization of the donor block by Grignard metathesis (GRIM), followed by chain extension through a Stille or Suzuki-Miyaura polycondensation to grow the acceptor block. Due to the step-growth mechanism of the second reaction, this approach leads to a mixture of products including homopolymers, diblock copolymers, and muliblock copolymers and large dispersities. Thus, chain-growth polymerizations that lead to well-defined architectures and low dispersities may be crucial to realize the full potential of conjugated block copolymers. We have demonstrated the synthesis of fully conjugated donor-acceptor block copolymers through chain-growth polymerizations such as GRIM associated with a new Ni catalyst for both electron donating and accepting monomers. Chain extensions were tracked by NMR as a function of the reaction time, and formation of block copolymers was confirmed by GPC using a UV-vis detector. These well-defined donor-acceptor conjugated block copolymers are expected to enable finer control of the microstructure, lead to enhanced power conversion efficiencies, and serve as good model systems for studies of the microstructure and photophysics of donor-acceptor systems.

This contribution focuses on the characterization of electronic main-chain defects unintentionally formed during the synthesis of conjugated polymers under non-optimum conditions and their consequences in electronic devices. While a plethora of new building blocks and suitable candidates of high performance conjugated polymers have been presented recently, much less attention has been devoted to batch-to-batch reproducibility, device reproducibility and comparability of literature results. I will show that main chain electronic defects unintentionally formed through homocoupling reactions are prevalent and significantly impact the performance of electronic devices (e.g. solar cells).^[1-6] Importantly, such defect structures can form independent of the polycondensation method used, and thus need to be analyzed from case to case. Several examples of high-performance conjugated polymers made by C-H activation polycondensation or Suzuki polycondensation will be presented in which main chain defects are analyzed on a quantitative basis. It is suggested that main chain defects must be considered in addition to molecular weight and dispersity to explore the full potential of a polymeric semiconductor, and to enhance comparability of results.^[6]

[1] Lombeck, F.; Komber, H.; Gorelsky, S.; Sommer, M. ACS MacroLett 2014, 3, 819.
[2] Luzio, A.; Fazzi, D.; Nübling, F.; Matsidik, R.; Straub, A.; Komber, H.; Giussani, E.; Watkins, S.; Barbatti, M.; Thiel, W.; Gann, E.; Thomsen, L.; McNeill, C. R.; Caironi, M.; Sommer, M. Chem. Mater. 2014, 26, 6233-6240
[3] Lombeck, F., Matsidik, R., Komber, H., Sommer, M., Macromol. Rapid Commun. 2015, 36, 231
[4] Matsidik, R., Komber, H., Luzio, A., Caironi, M., Sommer, M, J. Am. Chem. Soc. 2015, 137, 6705-6711
[5] S. Broll, F. Nübling, A. Luzio, H. Komber, M. Caironi, M. Sommer, Macromolecules 2015, 48, 7481–7488
[6] F. Lombeck H. Komber, D. Fazzi, D. Nava, J. Kuhlmann, D. Stegerer, K. Strassel, J. Brandt, A. de Zerio Mendaza, C. Müller, W. Thiel, M. Caironi, R. H. Friend, M. Sommer: On the role of prevalent carbazole homocoupling defects on the photovoltaic performance of PCDTBT:PC₇₁BM solar cells, Adv. Energy Mater., 2016, article asap, DOI: 10.1002/aenm.201601232

Organic photovoltaic (OPV) devices have attracted recent attention because of their flexibility, transparency, cost-effectiveness etc. The active layers, which convert light energy to electrical energy, consist of organic donor and acceptor materials. For the commercialization of OPV, it is important to improve both the power conversion efficiency and stability of the materials.
We have focused on the photostability of the materials used for active layers. Previously McGehee and co-workers reported that the photostability of donor materials mixed with acceptor materials depends on the energy levels of the acceptor materials [1]. On the other hand, the relationship between nanostructures of bulk-heterojunction (BHJ) films and their photostability has been unclear, although that for organic semiconductors without mixing with other materials was studied and it was reported that poly(3-hexylthophene) with higher crystallinity shows better stability [2,3]. In this study, we performed photooxidation studies on DTS(FBTTh₂)₂:fullerene BHJ films, one of the promising small-molecule materials for OPV cells [4]. Their nanostructures were examined by two-dimensional (2D) grazing-incidence wide-angle X-ray scattering (GIWAXS), absorption spectroscopy, and atomic force microscopy (AFM). The results of 2D-GIWAXS and absorption spectroscopic studies indicated that increasing the crystallinity of DTS(FBTTh₂)₂:fullerene BHJ films improved their photostability. The AFM results suggest that the roughness of the original DTS(FBTTh₂)₂:fullerene films was also related to their photooxidation rates. These results showed that controlling the nanostructures of the thin films is effective for improving the photostability of OPV cells. Reference:[1] E. T. Hoke, I. T. Sachs-Quintana, M. T. Lloyd, I. Kauvar, W. R. Mateker, A. M. Nardes, C. H. Peters, N. Kopidakis, M. D. McGehee, Adv. Energy Mater. 2012, 2, 1351–1357. [2] A. Dupuis, P. Wong-Wah-Chung, A. Rivaton, J.-L. Gardette, Polym. Degrad. Stab. 2012, 97, 366–374. [3] M. V. Madsen, T. Tromholt, A. Böttiger, J. W. Andreasen, K. Norrman, F. C. Krebs, Polym. Degrad. Stab. 2012, 97, 2412–2417. [4] T. S. van der Poll, J. A. Love, T.-Q. Nguyen, G. C. Bazan, Adv. Mater. 2012, 24, 3646–3649.

The alignment of the electronic bands across heterointerfaces governs the electric response of inroganic and organic heterodevices as e. g. light emitting diodes, solar cells, and field effect transistors to large extents. For this reason the contact properties of semiconductor devices prepared by in-situ step-by-step deposition experiments have been intensively studied by photoelectron spectroscopy. As for inorganic semiconductors it has been found for very many heterocontacts of organic semiconductors that they tend to show a strong interfacial dipole and a shift oft he Fermi level in contact formation. Despite these numerous studies there is still no widely accepted single mechanism which is condisered tob e the reasons of observed band energy alignments.
We will present s summary of our surface science investigations of different organic semiconductors deposited onto inorganic (oxides and layered semiconductors) as well organic semiconducting substrates. The deposited organic semiconductors have been purified in a number of vacuum sublimation steps and are considered to be pure on a standard level. Despite this fact we observe a Fermi level position in most organic films which is deviating from its midgap position and is mostly independent on the substrate work function.
Most heterointerfaces studied in our experiments show two principle effects: 1. There is an alignment of Fermi level position as for inorganic semiconductors indicating band bending which saturates for most systems already for layer thicknesses of 100 nm or below. 2. The band offsets are strongly influenced by additional double layer potential drops (interface dipoles), which tend to decrease the original given work function difference. For this reason we assume a charge transfer across the very interface (integer charge transfer interface dipole) but cannot exclude additional effects induced by other interface dipole producing processes.
Our results indicate that remaining charge carrier traps in the organic layers strongly influence the contact formation also in heterointerfaces with organic semiconductors. The density of states distribution and origin of these traps or defects (rechargable electronic states) need to be studied in more detail and experimental procedures to obtain ideal trap free materials must be further investigated.

Organic solids promise low-cost large-scale printing of optoelectronic devices on flexible substrates. Additionally, organic photovoltaics offer the potential to overcome the Shockley-Queisser limit for photovoltaic devices. It sets the upper boundary for conversion efficiencies in conventional photovoltaic devices by assuming that the energy difference between the exciting photon and the band gap is lost by dissipation. However, carrier multiplication schemes such as singlet exciton fission in organic semiconductors may help to overcome these limitations. Here, one dipole-allowed singlet exciton is converted into two triplet states. These are parity-forbidden for individual molecules, however, once excited they may contribute to charge extraction and hence increase the quantum efficiency, eventually above unity. At least two participating molecules are necessary due to Pauli’s exclusion principle. This mechanism requires effective inter-molecular coupling, which is rarely found in van-der-Waals-bound solids. The prevailing packing motif in the crystalline phase of p-conjugated molecules is the face-to-edge herringbone structure.
Here, we experimentally show that singlet-exciton fission is greatly enhanced for the slip-stacked molecular arrangement compared to the prevailing Herringbone motif, verifying a universal correlation between the electronic properties and molecular arrangement in organic semiconductors. In particular, we monitor the singlet-exciton-fission dynamics with crystal axis resolution in time-resolved pump-probe spectroscopy by utilizing the specific epitaxial growth relation of perfluoropentacene (C₂₂F₁₄, PFP) thin films on KCl(100) and NaF(100) substrates. These allow us to optically address all three crystal axes in a single crystal independently by linearly polarized light. Unlike its hydrogenated counterpart pentacene, PFP exhibits drastic differences in the stacking pattern for the three crystalline axes which are already observed in the linear optical responses. A Davydov-like splitting of the exciton is found for the b- and c- axes, while only residual absorption is found along the a-axis around 1.7-1.8 eV. The spectral features observed in pump-probe experiments are highly anisotropic caused by a strong uniaxial delocalization of the excitations which, in turn, leads to weakened molecular selection rules. The pronounced slip stacking of the molecules along the b-axis enhances the intermolecular coupling, accompanied by delocalization of singlet excitons and direct coupling to the correlated triplet pair. These findings significantly enhance the fundamental understanding of the relation between molecular arrangements and electronic responses laying the foundation for future high-performance organic electro-optical devices.

In this work, the role of thermal annealing on the component processes responsible for photoconversion is elucidated for a promising set of donor-acceptor-acceptor (D-A-A) electron donating materials. Here, D-A-A donors that combine, thieno[2,3-b]indole-2,1,3-benzothiadiazole-dicyanovinylene, are found to show broadband absorption with high extinction coefficients. A bulk heterojunction (BHJ) device that uses C₇₀ as an electron acceptor shows a champion power conversion efficiency (η_P) of 5.2% under AM1.5G (100 mW cm^-2) solar simulated illumination. This efficiency is limited by a low fill factor (FF). In order to better optimize the BHJ, a separate study of planar heterojunction devices was carried out, with particular attention paid to the behavior of the donor upon annealing. For planar heterojunction devices, thermal annealing of the D-A-A donor is found to increase the short-circuit current density, FF and η_P. Annealing leads to enhanced donor layer crystallinity, reduced light absorption efficiency, and a significant increase in the charge collection efficiency and photocurrent under forward bias. The significant increase in charge collection efficiency observed with annealing holds promise for further optimization of crystallinity in BHJ devices and corresponding increases in efficiency.

During solution deposition a supersaturated state forms before the film transforms to a solid. Molecular aggregates are observed by optical reflection spectroscopy in the supersaturated state, corresponding to subcritical nuclei in the crystallization process. This observation shows that a two-step nucleation occurs during organic semiconductor crystallization. Results for TIPS-pentacene and diF-TES-ADT with both polar and non-polar solvents will be reported. In addition, blending polar solvents with different vapor pressures leads to well aligned, extremely thin films (<30 nm) with high carrier mobility.
The strain-stabilization of the high-mobility polymorph known as Form II has also been studied in detail. When crystalline thin films deposited at high temperature are cooled to ambient temperature they become strained. Below a critical thickness, cracking is not observed and the films are constrained to the lattice constants corresponding to the temperature at which they were deposited. To study the strain effect on the mobility, aligned thin films are obtained in a temperature range from 25 centrigrade to 135 centigrade. We have studied organic field-effect transistors using these films as active layers and have observed the mobility greatly improved when the film is more strained. A fourfold improvement in the mobility was obtained if we compared the mobility of film deposited at 135 centigrade to film deposited at 25 centigrade.

Covalent organic frameworks (COFs) are self-assembled 2D or 3D crystals based on covalent bonds between building blocks of organic molecules. Predictable formation of ordered porous networks, in combination with a large catalogue of possible building units, allows tailoring the optoelectronic properties and creating novel functional materials for a variety of applications. Of great interest is a new class of fully conjugated COFs, where the π-electron cloud is fully delocalised in a 2D plane. However, very little is known about the photophysical properties of these systems. Here we present a detailed photophysical study on a series of COFs structures, using ultrafast transient absorption spectroscopy. Excitation with pulses shorter than 20 fs allows detecting the characteristic vibrational modes of the framework, in addition to the ultrafast electronic processes. Our results highlight how the conjugation of organic molecules in a large periodic structure leads to novel phenomena, such as the delocalisation of wavefunctions within the extended 2D structures and excited state lifetimes of hundreds of ns, as opposed to a few ns for typical conjugated polymers.

Fullerenes are an intriguing class of organic semiconductors that feature a high electron affinity and exceptional charge transport properties, desirable for a number of opto-electronic applications. Typically, a trade-off has to be made between the higher electron mobility of pristine fullerenes and the superior solubility of substituted (exohedral) fullerenes, which however require additional synthesis steps that result in a higher energy footprint and materials cost. We therefore revisit the use of pristine fullerenes and demonstrate that the increase in configurational entropy associated with co-dissolving C₆₀ and C₇₀ leads to a significantly enhanced solubility in a variety of organic solvents. We utilize this ‘entropic dissolution’ to prepare state-of-the-art opto-electronic devices with pristine fullerene mixtures: field-effect transistors with an electron mobility of 1 cm² V^-1 s^-1 and polymer solar cells with a highly reproducible power-conversion efficiency of 6 % as well as a thermally stable active layer

Photoactive materials with enhanced visible light absorptivity, charge transport capabilities, and extended excited state lifetimes that render efficient exciton electron - hole separation are necessary for high performance solar energy conversion materials. Promising photoactive materials include acceptor-donor-acceptor (ADA) and/or acceptor-donor (A-D) organic semiconductor molecules because they increase electronic delocalization on the molecule and generate a strong intra-molecular charge transfer state. Excited-state charge transfer increases red light absorptivity, electron donating ability, and the lifetime of the singlet state enabling longer exciton diffusion lengths. In this study, porphyrin donor molecules (either 5,15-bis(4-carbomethoxyphenyl)porphyrin or 5,15-bis(2,6-dioctoxyphenyl) porphyrin) were coupled to asymmetrically engineered, relatively electron deficient acceptor 2-(4-octoxyphenyl)-7-(4-ethynylphenyl) thiazolo[5,4-d]thiazole (asymmetric TTz). Thiazolo[5,4-d]thiazole is a rigid, planar, fused heterocyclic molecule that forms strong π-π stacking interactions, is characterized by strong oxidative and thermal stability, and has high charge carrier capability. Asymmetric TTz was prepared in a mixed pot condensation reaction with rubeanic acid, trimethylsilylaldehyde and 4-octoxybenzaldehyde. The excited state dynamics of these ADA and AD molecules have been studied using UV-Vis spectroscopy, steady state emission, time-resolved and delayed photoluminescence. These materials show promise for use in molecular photovoltaic applications with unique properties that will influence exciton generation/diffusion, charge separation and mobility.

Fully conjugated block copolymers, consisting of an electron donor and an electron acceptor block, can serve as the active layer in organic photovoltaic devices. Incorporating the donor-acceptor interface within the chemical structure enables model studies of energy and charge transfer. We synthesized a series of block copolymers consisting of a P3HT electron donor and four different push-pull polymer electron acceptors, either poly-(4-(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole)-2,6-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2"-diyl) (PDPT6BT), poly-((9-(9-heptadecanyl)-9H-carbazole)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2"-diyl) (PCT6BT), poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2"-diyl) (PFT6BT), or poly-((2,5-dihexylphenylene)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2"-diyl) (PPT6BT). By altering only the electron rich unit of the acceptor, we adjust the energy difference between the HOMO of the donor and acceptor by tenths of an eV, leading to offsets between the HOMO levels of 0.1 to 0.6 eV. Dynamic light scattering confirms that the synthesized block copolymers can be fully dispersed in dilute solutions, enabling studies of photoluminescence quenching within individual chains. The absorption and emission spectra of the block copolymers can be deconvoluted to extract the contributions from each of the blocks. Using this, we quantify the yield of intramolecular charge transfer states. Taking the data from all block copolymers together, we find a critical driving force required to generate charge transfer states that depends on the dielectric constant of the solvent.

Small molecule organic photovoltaics (SM-OPV) have garnered great interest because the small molecules are monodisperse, easy to synthesize and purify, and can aggregate into ordered domains upon thermal or solvent annealing, leading to high charge mobility.
In this work we introduce a more accurate assignment of the excited states of a promising squaraine (SQ) targeted for OPV application. From this assignment we conclude that a mixed population of aggregates and monomers exists in spin-cast thin film samples. In as cast SQ:PCBM blend films, SQ are predominantly in the monomer form, indicating a rather amorphous morphology. Since crystallinity is critically important because associated high mobility and exciton diffusion rates lead to higher PCE, we reduce the amorphous regions through thermal annealing. Our analysis of annealed films demonstrates a delicate trade-off between increased crystallinity and larger domain sizes; crystallinity improves, but often at the expense of larger crystal size, as supported by XRD and TEM study. These large domains in the blend films significantly reduce the interfacial area between SQ and PCBM, leading to a drastic deterioration of solar cell performance.
In conclusion we highlight the importance of chemical compatibility when designing small molecules for use in high efficiency bulk heterojunction devices. This work has a substantial impact in that we have connected theoretically validated spectroscopic assignment with a full study of morphology and domain size control as they affect small molecule BHJ active layers.

In this work, the morphology of semiconducting polymer/insulating polymer blends from inkjet printing and their field effect transistor (FET) properties have been investigated. We attempted to study the effect of molecular weight of insulating polymer on the nano-scale morphology and function of the blends. The morphologies of all the samples from inkjet printing are characterized by small angle neutron scattering (SANS), atomic force microscopy (AFM), and grazing incidence x-ray diffraction (GIXD). The SANS results shows that the domain size of the blends increases with an increase of the molecular weight of insulating polymer, while the domain purity reaches the maximum with proper molecular weight of insulating polymer. AFM images show that the connectivity of semiconducting polymer domains is disrupted with addition of polystyrene (PS) with low molecular weight , while well interconnected domains are observed with addition of PS with high molecular weight. GIXD results indicate that the π-π stacking distance of semiconducting polymer can be shortened with addition of PS and decreases with an increase of PS molecular weight. This work demonstrates that the domain purity, connectivity of semiconducting polymer domains, molecular packing are crucial for the charge transport.

Organic solar cell is one of the important growing technologies, and various approaches have been made to improve power conversion efficiency (PCE), including design of novel conjugated polymers and molecules. Introducing electron-donor/acceptor units to conjugated copolymers is an effective way to cover the broad sunlight spectrum from visible to near-infrared. In this work, new electron accepting molecules named as thienoisoindigo (TIDG) and benzothienoisoindigo (BTIDG) were synthesized and coupled with donor moieties. TIDG has a high planarity because of sulfur-oxygen interaction, affording the absorption spectra expanded up to 1400 nm. The 1.4 % PCE was achieved for the fluorene-TIDG based copolymer, despite its edge-on orientation unsuitable for OPV. On the other hand, BTIDG copolymers bearing optimal number of alkylthiophene spacer and benzobisthiazole (BBTz) donor unit provided a notable face-on orientation. In addition, unsymmetric BTIDG molecule inherently allows for improvement of solubility. As a consequence, we achieved 4.2 % PCE for the BTIDG-BBTz copolymer blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). We will discuss the structure-property relationship which could contribute to guiding the design of polymer –fullerene solar cell towards high efficiency.

In this work, we prepared four star-shaped conjugated small molecules, the triphenylamine dithiophene (TBT) derivatives, namely TBT-H, TBT-Br, TBT-OH, and TBT-N₃ presenting hydride, bromide, hydroxyl, and azide terminal functional groups, respectively. These TBT derivatives were used as additives in the active layers of organic photovoltaics to investigate the effect of intermolecular interactions (TBT-H, TBT-OH) or crosslinking (TBT-N₃, TBT-Br) on the long-term thermal stability of the devices. From analyses of blend film morphologies, and optoelectronic and device performance, we observed significant enhancements in thermal stability during accelerated heating tests at 150 °C for the devices incorporated with the additives TBT-N₃ and TBT-Br. These two additives functioned as crosslinkers, and constructed local borders that effectively impeded heat-promoted fullerene aggregation, thereby leading to highly stable morphologies. When compared with corresponding normal devices, the TBT-N₃–derived devices based on poly(3-hexylthiophene) exhibited greater stability, with the power conversion efficiency (PCE) remaining as high as 2.5% after 144 h at 150 °C. Because of this enhancement, a device based on an amorphous low-bandgap polymer, namely poly(thieno[3,4-b]thiophene-alt-benzodithiophene), with the addition of TBT-N₃ was fabricated. We observed a significant improvement in device stability, retaining approximately 60% (from 5.0 to 3.3%) of its initial PCE under accelerated heating (150 °C). In contrast, the PCE of the corresponding normal device decayed to 0.01% of its initial value.

A novel conjugated copolymer, pDPP-(TV)₂B-2DO, including donor (D) and acceptor (A) blocks of (TV)₂B-2DO, 1,4-bis((E)-2-(thiophen-2-yl)vinyl)benzene with dodecyloxy moieties and diketopyrrolopyrrole (DPP) derivatives, respectively, was synthesized by Stille-coupling polymerization. The 60 kDa pDPP-(TV)₂B-2DO showed good solubility in organic solvents and strong molecular interaction during film procedures. Semiconducting thin films were spun-cast on polymer-treated SiO₂ from chloroform and annealed at temperatures ranging from 150 to 300 °C for 1 h. The electrical properties of these films in organic field-effect transistors (OFETs) were evaluated and correlated with π-conjugated ordering of the pDPP-(TV)₂B-2DO, before and after annealing. Featureless aggregates of the face-on copolymer by the bulky B-2DO in the D block were dominant in the as-spun film and transferred to ordered rods or fibrils containing highly ordered edge-on polymers on the substrates after annealing. In terms of π-conjugated ordering, the as-spun and 250 °C-annealed pDPP-(TV)₂B-2DO films contained highly-consistent chains with face-on and edge-on structures, respectively. The resulting OFETs yielded similar field-effect mobilities of 0.07 and 0.08 cm² V^-1 s^-1, respectively, as well as on-off current ratios greater than 10⁶ and a threshold voltage near 0 V. Therefore, the π-overlap of edge-on semiconducting polymers on gate dielectrics was not necessary to enhance the electrical performance of OFETs, but the high consistency in chain ordering was very important.

In this work, we developed a series of pyrimidine/acridine-based blue thermally activated delayed fluorescence (TADF) emitters[1], and investigated the effect of the position of nitrogen atoms toward the electroluminescent efficiency in organic light-emitting devices. We used 4-phenylacridine as a donor unit, and tri-substituted pyrimidine as an acceptor unit. As a result, when 4-phenylacridine was introduced into 2-position of pyrimidine unit to give Ac-46DPPM, the photoluminescent quantum yield (PLQY) of the Ac-46DPPM-doped bis[2-(diphenylphosphino) phenyl]ether oxide (DPEPO) film was observed to be 61% with an emission peak at 454 nm. Whereas, when 4-phenylacridine was introduced into 4-position of pyrimidine unit to give Ac-26DPPM, the PLQY of the Ac-26DPPM-doped DPEPO was observed to be 81% with an emission peak at 475 nm. Although the differences in chemical structure are very small, however, Ac-26DPPM-based OLED showed 1.6 times higher EQE of 18.6% with much smaller effieicncy roll-off than that with Ac-46DPPM (EQE 11.8%). We would like to discuss the structure-property relationships among these pyrimidine/acridine conjugates in detail.
[1] R. Komatsu, H. Sasabe, Y. Seino, K Nakao, J. Kido, J. Mater. Chem. C 2016, 4, 2274–2278.

It is considered that carborane (B₁₀C₂H₁₂) block in the main chain of polymer gives an intriguing property. For instance, this polymer is known to show fluorescence in both of liquid phase and solid-phase. It is thought that there are some relationships to the modulations of π conjugate characteristics of the main chain and the occurrence of AIE (Aggregate Induced Emission) as a function of carborane at this time. In the present research simple models of π-conjugated polymer, poly(p-phenylene ethynylene) (PPE), containing carborane block have been designed and theoretically studied with respect to the control of their electronic states.
Crystal orbital (CO) method has been employed for analyses of the energetically optimized structures, band structures, CO phases, and miscellaneous electronic properties deduced from the band structures using both Gaussian 09 and Crystal 09 program packages. In parallel, the structures and the electronic states of o- and p-carborane molecules have also been examined for reference, which has clarified that these carborane molecules have σ-conjugated bonding character, σ aromaticity, and the tendency easily accommodating an excessive electron.
The electronic states of the designed polymers differ depending on the relative position (o- and p-) of the carborane block in the main chain. It has been shown that o-carborane (benzocarborane) PPE has similar degree of π-conjugation with smaller band-gap value than that of PPE itself. The HO and LU bands of p-carborane PPE are also of π characteristics, but the band widths are rather narrow and the π conjugation throughout the polymer is shown to be segregated.
In conclusion, it has been revealed that characteristics of the carborane molecule such as considerably large electron affinity, molecular orbital features, and so on influence the electronic properties of the whole polymer acting as the effective element-blocks.
This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No.2401)” (JSPS KAKENHI Grant Number JP24102014).

In this work, we designed and synthesized a novel isonicotinonitrile/phenoxazine-based green thermally activated delayed fluorescence (TADF) emitter named PXZINN for high-performance TADF organic light-emitting devices (OLEDs). The ionization potential of PXZINN was observed at 5.6 eV, whereas, the electron affinity was estimated to be 3.1 eV. The energy difference between singlet and triplet excited states was observed to be 0.06 eV. The photoluminescent quantum yield (PLQY) of the 10 wt% PXZINN-doped 4,4'-bis(carbazol-9-yl)biphenyl (CBP) film was observed to be 79% with an emission peak at 522 nm. An OLED with a structure of [ITO/triphenylamine-containing polymer: 4-isopropyl-4′-methyldiphenyl-iodonium tetrakis(pentafluorophenyl)borate (PPBI)/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC)/PXZINN-doped CBP/3,3'’,5,5'-tetra(3-pyridyl)-1,1’;3’,1''-terphenyl (B3PyPB)/LiF/Al] were fabricated. The device exhibited operating voltages of 2.84, and 3.39 V at luminances of 1 and 100 cd m–2, respectively. At 100 cd m–2, this device gave a power efficiency of 67.4 lm W–1, a current efficiency of 72.7 cd A–1, an external quantum efficiency of 22%.

Conjugated polymers are promising materials that can serve as the active layer in a variety of electronic devices, such as solar cells and thin-film transistors. However, fundamental properties, including the glass transition temperature (T_g) and the entanglement molecular weight (M_e), are still in dispute for most conjugated polymers. These parameters play a central role in the microstructures, such as intercrystalline connectivity and tie chains, which are believed to ultimately influence bulk electrical charge transport. Firstly, we have studied a wide range of molecular weights of both regiorandom (RRa) and regioregular (RRe) Poly(3-hexylthiophene-2,5-diyl) (P3HT) using oscillatory and steady shear rheology. Coupled with the absolute Mw from static light scattering and the molecular weight distribution from GPC, M_e was extracted by fitting the linear viscoelastic data of multiple molecular weight samples using the classic tube reptation model. Furthermore, by using low-temperature oscillatory shear, two glass transitions (T_gs) for both RRe and RRa P3HT were clearly identified. The higher T_g, corresponds to segmental motion, follows the Flory-Fox equation well for various MWs with 1 C < T_g < 14 C, and yields a high MW T_g∞ ~ 20 C. RRe P3HT has a significantly larger M_e than RRa P3HT, which might originate from the difference in side chain packing evidenced by their different lower T_g values. This difference in chain packings was also investigated in the context of dilute solution by both the Ubbelohde viscometry and dynamic light scattering, in order to further understand entanglements in these semiflexible polymers. In addition, this linear viscoelasticity study was also expanded to other high performance conjugated polymers, such as poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2”-diyl) (PFTBT) , poly{[ N , N 9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt -5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PBDTT-FTTE), to determine their molecular weight dependences of T_g, M_e and even nematic to isotropic transition temperatures.

Alkyl-substituted polyhedral oligomeric silsesquioxane (POSS) cage was combined with 3,4-ethylenedioxythiophene under the same roof. The corresponding monomer called EDOT-POSS was used to get soluble poly(3,4-ethylenedioxythiophene) (PEDOT-POSS). Both chemically and electrochemically obtained polymer was soluble in common organic solvents like dichloromethane, chloroform, tetrahydrofuran, etc. The PEDOT-POSS has somewhat higher band gap (1.71 eV at 618 nm) than its parent PEDOT and as expected the PEDOT-POSS exhibited higher optical contrast (69%) and coloration efficiency (582 cm²/C), lower switching time (0.9 s), higher electrochemical stability (93% of its electroactivity retains after 5000 cycles) when compared to PEDOT. A number of advantages of the PEDOT-POSS over PEDOT can make it a promising material in the areas of electro-optical applications.

Organic solar cells based on blend system of electron donor and electron acceptor for photoactive layer are receiving extensive academic and commercial interests due to their unique advantages such as solution processablity and flexibility on the plastic substrates. For the organic solar cells to become a viable energy source in near future, light absorption and charge transport properties should be improved. Although long alkyl side chains have been introduced are in general use for solubilizing group in organic semiconductors, they are known to be electrically insulating and are detrimental to the charge injection and transport. In this study, we designed an unsymmetrical structure fFor solution processable high efficiencynovel small molecule without long insulating alkyl side chain. , they should have unsymmetrical structure to have both solubility and charge transporting properties. Triphenylamine (TPA) unit as the bulky solubilizing unit are 3D structures and also has good transporting properties of the hole carrier. Further improvement in transport pathway for charge carriers, the large π-π overlap within self-organized stacks of disk-shaped unit was chosen to be the stacking unit. In this presentation, we report synthesis of unsymmetrical small molecule which consists of donor and acceptor through π-conjugated spacer to check the relationship between alkyl-free unsymmetric structure and properties by measuring thermal analysis, UV-Vis absorption, cyclic voltammetry, x-ray diffraction measurement. Finally, organic solar cells were fabricated using new small molecule:PCBM systems and their performance was investigated.

We have fabricated optical devices using single crystals of thiophene/phenylene co-oligomers (TPCOs) combined with one-dimensional (1D) diffraction gratings. TPCOs are oligomer materials that show good carrier mobility and fluorescence efficiency. We chose BP3T from among TPCOs and its single crystals were grown in a vapor phase. We spin-coated a photoresist polymer (SU-8) film on SiO₂/Si substrates or BP3T crystals laminated on the substrates and formed 1D diffraction gratings by interference exposure of the SU-8 films using a Lloyd mirror set up. Thus, we formed two types of optical devices. One had the BP3T crystal placed the 1D diffraction grating fabricated preliminary, and the other had the grating fabricated on the crystal.
We excited the former devices by an ultraviolet light from a mercury lamp. The devices indicated spectrally-narrowed emission peaks along the direction parallel to the crystal surfaces. On the basis of the grating period and peak locations, we evaluated effective refractive indices and diffraction orders. Using them, we designed and fabricated the 1D diffraction gratings that produced the narrowed emission lines located at the wavelength where the BP3T indicated emission maxima (616 nm).
For the latter devices, we used ~100-nm-thick SU-8 layers. The thickness (~100 nm) was smaller than that for the former devices (~300 nm). In the case of the latter devices, we expected that the 1D gratings were closely contact with the BP3T crystals because the gratings were directly fabricated on the crystals. Detailed results will be presented on the day.

Organic solar cells (OSCs) have been recently adopted inverted-type structure for high efficiency and long term stability. A variety of cathode buffer layers have been used to decrease the work function of ITO or electron transport layer (ETL) for efficient charge extraction. It has been reported that high efficient OSCs can be fabricated by applying polyelectrolyte and metal oxide bilayer as ETL. However, bilayer ETL need addtional annealing process that increase the fabrication complexity. In this work, we adopted a composite ETL by blending a ZnO nanopaticle with a polyelectrolyte solution to reduce addtional process. We show that both efficiency and stability of OSCs were enhanced by adopting the composite ETL. The Power conversion efficiency (PCE) of polyelectrolyte blended ZnO device were 8.43% and decreased 29% after degradation while that of ZnO device were 7.93% and decreased 35%. This result can be attributed to the low trap density and apropriate energy level of modified ZnO. For further understanding, ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), absorbance, impedance spectroscopy (IS) measurements were supported. In summary, we analyzed the effect of polyelectrolyte blended ZnO on device and confirmed that device with modified ZnO showed better performance and lifetime compare to that of device with ZnO.

The nanoscale organization and corresponding electronic properties of a photoactive blend layers of regioregular poly (3-hexylthiophene) (RR-P3HT) and fullerene derivative [6,6]-phenyl-C71 butyric acid methyl ester (PCBM) on Au(111) substrate has revealed by scanning tunneling microscopy and spectroscopy (STM/STS). Subsequent to annealing treatment, STM topography and images are observed as a combination of phase-separated polymer-rich, fullerene-rich, and mixed polymer-fullerene domains. This technique permits to explore simultaneously the quantitative linkage between the nanoscale morphologies and corresponding local electronic properties. We determine the HOMO and LUMO-edges at the individual domains and interfacial band alignments of the donor (RR-P3HT)-acceptor (PCBM) interface. We could observe a noteworthy deeper HOMO energy of RR-P3HT by 0.35 eV in mixed-region associated primarily with the degree of disorder-induced band gap widening of the polymer and polymer:fullerene intermolecular interactions. And LUMO of the PCBM in the mixed region raised by 0.15 eV. These energetic difference in the mixed phase is likely to be responsible for the reduced recombination in bulk heterojunction (BHJ). So, this characterization provides nanoscale insight to the annealing-induced morphological organization and corresponding local electronics properties account for an impressive increase of the charge generation, transport and corresponding device performance of the polymer solar cells.

Thermoelectric plastics, i.e. malleable polymer-based thermoelectric materials that can be readily shaped into a wide range of thin-film to bulk architectures, offer an interesting alternative since they promise more cost-effective processing from melt or solution. In particular conjugated polymers are of interest, as they are based on naturally-abundant materials, easily tuned chemical structure, mechanical robustness, low-weight and typical low thermal conductivities.

To optimise the thermoelectric properties of semiconducting polymers the charge carrier density must be increased, which is commonly done through the addition of a molecular dopant, either during or after processing. Typical post-processing doping treatments either involve immersion of thin polymer films in dopant solutions (solution doping) or introduction of the dopant via sublimation (vapour doping).

However, unlike most organic electronic applications, thermoelectric devices require thick layers to maintain a certain temperature gradient. As a result, doping of such bulk architectures is difficult because it is limited by the diffusion rate of the dopant. The dopant can be introduced by co-processing but requires elevated processing temperatures to minimize complex formation that otherwise renders the polymer intractable.

Here, we will present alternative doping methods that allow to control the distribution of the dopant, either through microstructure design of the semiconductor polymer or the molecular design of the dopant.

Molecular alignment is usually effective to increase the electrical conductivity in organic semiconductors. One of the practical techniques to improve the molecular alignment is stretching in the case of polymeric materials. In order to improve the thermoelectric properties the stretching technique have been used historically. For example, the stretching of camphorsulfonic acid-doped polyaniline films at an elevated temperature did increase the electrical conductivity, which resulted in high thermoelectric figure-of-merit, ZT=0.04.¹⁾ The stretching technique was also applied to the precursor polymers of poly(p-phenylenevinylene) (PPV) derivatives. The PPV derivatives produced by decomposition reaction of the stretched precursor polymers showed higher electrical conductivity than the pristine ones, resulting in organic thermoelectric films with the ZT value as high as 0.1 after doped with iodine.²⁾ In these cases the stretching offered the better molecular alignment, which caused the high electrical conductivity and high ZT value. Recently much attention is being denoted to the organic films with high electrical conductivity easily prepared from commercially available poly(3,4-ethylenedioxy-thiophene) poly(styrenesulfonate) (PEDOT-PSS) dispersions. In the case of thin films a spin-coating technique is known to be effective for molecular alignment. In the case of thick films, however, the PEDOT-PSS films are hard and stiff, and difficult to apply the stretching technique. Thus, we used the composites of PEDOT-PSS with xylitol.³⁾ The composite films are thick and soft. We applied the stretching technique and solvent treatment to the composites films. The electrical conductivity of the composite film (278 S/cm) was improved to 677 S/cm by DMSO addition and water rinse, and further improved to 1155 S/cm by additional application of the stretching technique. Thus, the combination of both techniques was proved to offer the molecular alignment of PEDOT in the composites, resulting in improvement of the electrical conductivity and the thermoelectric performance.
References: 1) H. Yan, et al., Macromol. Mater. Eng., 2001, 286, 139. 2) Y. Hiroshige, et al., Synth. Met., 2007, 157, 467. 3) S. Ichikawa, et al., Polym. J., 2015, 47, 522.

The introduction of p- and n-doped layers into the device architecture of organic opto-electronic devices can greatly improve their performance. Charge carrier extraction or injection from or into the organic photo-active layer is enhanced by doping, resulting in a reduction of ohmic losses. Hole and electron conductive layers can be realized by molecular doping of organic host materials moving the Fermi level to the appropriate position, enabling electron or hole selectivity.
In this work, we use thermovoltage (Seebeck effect) and temperature-dependent conductivity measurements to determine the dominating type of charge carriers introduced by the dopant and to gain insight into the position of the transport level with respect to the Fermi level. The investigation of fullerene dopants with a high degree of fluorination in various amorphous host materials allows us to tune the energy level offset between host and dopant and to study their influence on Fermi level position and overall doping efficiency systematically. We find that even low HOMO materials can be doped efficiently by highly fluorinated fullerenes. Moreover, we observe a clear influence of the energy level offset between matrix and host on the thermoelectric properties and the conductivity. This work provides guidelines for future dopant-matrix combination with a maximum doping efficiency.

Conjugated, semi-crystalline polymers are widely used as semiconductors in the field of organic electronics. In order to achieve high performing devices, precise control of polymer morphology is of great importance. As most preparation techniques lead to fast film formation, the morphology of the polymer films has to be tuned in an additional step, e.g. by using thermal or solvent vapor annealing.^[1,2] In this contribution we present strategies to control the morphology of the n-type semiconducting polymer poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} P(NDI2OD-T2) in thin films. Solvent vapor annealing is applied to induce a reorientation of aggregated polymer chains in the swollen film. This treatment results in the formation of spherulitic structures with a tuneable size of over two orders of magnitude up to several hundred micrometers. Additional thermal treatment of the polymer films enables control over the crystallinity and leads to the formation of two different polymorphs.^[2] We monitor such morphological changes by in-situ absorption spectroscopy measurements.
Furthermore we studied the electrochemical behavior via cyclic voltammetry which gives valuable insight into the doping behaviour of such well-crystallized films and is interesting for applications in electrochemical devices and thermoelectrics.^[3]

References

[1] F.S.U. Fischer, D. Trefz, J. Back, N. Kayunkid, B. Tornow, S. Albrecht, K.G. Yager, G. Singh, A. Karim, D. Neher, M. Brinkmann, S. Ludwigs, Adv. Mater. 2015, 27, 1223.
[2] K. Tremel, F.S.U. Fischer, N. Kayunkid, R. Di Pietro, R. Tkachov, A. Kiriy, D. Neher, S. Ludwigs, M. Brinkmann, Adv. Energy Mater. 2014, 4, 1301659.
[3] D. Trefz, A. Ruff, R. Tkachov, M. Wieland, M. Goll, A. Kiriy, S. Ludwigs, J. Phys. Chem. C 2015, 119, 22760.

Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work we first focus on how processing parameters of doped conjugated polymers determine morphological and electronic properties and therewith device-relevant parameters such as the conductivity. We study the p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) spin cast from two different solvents by means of Infrared and UV-vis spectroscopy as well as electron diffraction performed in a transmission electron microscope and electrical measurements. We demonstrate that structural order and electronic properties in doped films can be predefined by the interaction of the solvent with charge-transfer complexes already in solution.¹ However, not only the process of film formation is dynamic: The usually unwanted but still prevalent effect of dopants moving in organic semiconductors is known for various dopant molecules and – if not controlled or suppressed – can cause device degradation. We aim at controlling this dynamic process of charged dopant molecules drifting in thin films under an applied electric field. We study the drift and diffusion behavior and compare different dopants within organic host materials. This effect can be furthermore utilized to create highly doped and almost undoped regions within one thin film and therefore to spatially alter the conductivity. To demonstrate the applicability, we show first data of a proof-of-principle memristor device that is based on a spatially controlled dopant distribution.

References

¹Müller et al., Chemistry of Materials, Article ASAP, DOI: 10.1021/acs.chemmater.6b01629

Conducting polymers (CPs) have recently gained the attention of the scientific community due to their prospective use in thermoelectric applications [1,2]. Particularly, it has been proven that an important parameter for tuning the thermoelectric properties and the charge transport behavior of the CP is the shape of the DOS in the band edge, where a steeper band edge would be translated in a semi-metallic behavior for the system, with higher thermoelectric efficiencies. However this shape can also be affected by the oxidation levels of the systems [3,4]. In the present study we managed to elucidate the correlation between material structure, electronic structure and electronic/ thermoelectric properties, through proper material design and keeping constant the carrier concentration of the CPs. Additionally, for these systems a metal-to-insulator transition was observed in low temperatures near 30K, which was diminished with increasing thin film crystallinity. Our results underline that CPs with higher crystallinity will result in a 2-D charge transport behavior and a steeper band edge, which are translated into higher carrier mobility (12.4x10^-4 cm²/Vs) and higher Seebeck coefficient (44 μV/K). Finally, an experimental relationship between carrier mobility and Seebeck coefficient was extracted highlighting the importance of CPs with high carrier mobility for thermoelectric applications.

References :

[1] Bubnova, O. et al. Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene). Nat. Mater. 10, 429–433 (2011).

[2] Kim, G.-H., Shao, L., Zhang, K. & Pipe, K. P. Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. Nat. Mater. 12, 719–723 (2013).

[3] Bubnova, O. et al. Semi-metallic polymers. Nat. Mater. 13, 190–194 (2013).

[4] Bubnova, O. & Crispin, X. Towards polymer-based organic thermoelectric generators. Energy Environ. Sci. 5, 9345 (2012).

ACKNOWLEDGEMENTS

The authors acknowledge financial support from Arkema and the Région Aquitaine as well as from the Industrial Chair (Arkema/ANR) within the grant agreement no. AC-2013-365. I.P. is grateful to the Région Aquitaine for financial support (Ph.D. grant #20111101004). The ESRF and NWO are acknowledged for allocating beam time at the Dutch-Belgian beam line (DUBBLE) for the GIWAXS experiments. This work was performed within the framework of the Labex AMADEUS ANR-10-LABEX-0042-AMADEUS with the help of the French state Initiative d’Excellence IdEx ANR-10-IDEX-003-02.

Enhancing the electrical conductivity of semiconducting polymers has been of great interest for many applications in organic electronics including electrode layers and thermoelectric devices. One method to increase the electrical conductivity of polymers is through chemical doping with dopant molecules that increase charge carrier density through charge transfer between the polymer chain and the dopants. Finding an efficient doping method is important as doping methods in thin films affect the film morphology, which is closely related to charge transfer. Previous work with PBTTT and F4TCNQ has shown that films doped through the vapor doping method exhibit higher conductivities compared to those doped through solution doping method. The higher doping efficiency in vapor doping is attributed to the morphology setting prior to the dopant addition and the higher amount of dopants that can be added to the film.

In this work, we used the vapor-doping method to examine the electrical conductivity of the widely studied polymer, P3HT. Previously, doping methods such as solution and sequential doping has been explored for this system and show two phases (doped and undoped) in the resulting films. We used grazing incidence wide-angle scattering (GIWAXS) to show that upon vapor doping, a second crystalline phase appears similar to other methods. GIWAXS suggests formation of a larger number of heavily doped regions from the intensity of π stacking peaks, but the maximum measured electrical conductivity was similar. In order to better understand the impact of doping on morphology and electrical properties, we used resonant soft x-ray scattering (RSOXS) on doped films to probe amorphous and crystalline regions of the films over larger length scales. RSOXS showed that vapor doping has little effect on the connectivity between the crystalline regions explaining the lack of a significant increase in conductivity in the P3HT films compared to the PBTTT films. The effects of molecular weight and the side chain chemistry will be discussed. This work allows us to better understand morphological influences on chemical doping methods, which can open up new possibilities of exploring efficient doping methods.

In conducting polymers, charge transport properties have been limited by structural and energetic disorder created by dopants upon doping. We have recently demonstrated an efficient solid state doping method for conjugated polymers to achieve a coherent charge transport in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) with one of the highest Hall mobilities for conducting polymer [1]. This enabled observation of a nearly-ideal Hall effect in addition to a low-dimensional quantum mechanical phenomenon that is in agreement with free-electron like magnetic susceptibility. The coherent charge transport in the doped PBTTT can be attributed to a preserved lamellar microstructural order upon doping; the dopant molecules were found to intercalate in the sidechain regions and do not disturb the π-stacking of the polymer backbones. Comparison with other polymers with disordered microstructure further reveals a strong correlation between the degree of structural order and resulting charge transport properties.

The different charge transport regimes accessible in the conducting polymer also presumably affect their thermoelectric properties. Especially, Seebeck coefficient is a powerful tool which could reveal the degree of energetic disorder in charge transport [2]. The Seebeck coefficient for the doped PBTTT film was measured over a wide conductivity range realised with the solid state doping method to strengthen the evidence of the observed metallic transport. The demonstrated solid state doping method can also be extended to uncover charge transport properties of other thiophene-based and donor-acceptor polymers.

[1] K. Kang, S. Watanabe, K. Broch, A. Sepe, A. Brown, I. Nasrallah, M. Nikolka, Z. Fei, M. Heeney, D. Matsumoto, K. Marumoto, H. Tanaka, S.-I. Kuroda, H. Sirringhaus, Nat Mater (2016).

[2] D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A.J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, H. Sirringhaus, Nature 515 (2014) 384–388.

Organic semiconductors (OSCs) have emerged as valuable alternative to their inorganic counterparts owing to the ease of tuning the optical gap through their chemical variability and high potential for low-cost, large-area processing on flexible substrates. In contrast to inorganics, however, the potential of doping OSCs for enabling new functionality and improving opto-electronic device performance has only recently been established. There, p-doping OSCs is done by admixing strong molecular acceptors like tetrafluoro-tetracyanoquinodimethane (F4TCNQ). Compared to inorganic semiconductor doping, however, the doping efficiency remains conspicuously low for OSCs, which highlights the fact that, still, the underlying fundamental mechanisms are less well understood.

Here, the broad range of phenomena observed upon molecularly doping conjugated polymers (CPs) and molecules (COMs) is discussed, from which, finally, two different competing scenarios emerge [1]: (i) the formation of both OSC and dopant ions through integer-charge transfer, i.e., an ion pair (IPA), and (ii), the emergence of OSC/dopant ground-state charge transfer complexes (CPXs) through supramolecular hybridization of their respective frontier molecular orbitals leading to unoccupied states in the fundamental gap. On the basis of the characteristic spectral signatures of the individual species observed in various spectroscopic techniques, experimental evidence for the exclusive occurrence of always one scenario is presented for a number of prototypical doped CPs/COMs.

In particular, the F4TCNQ doping of poly(3-hexylthiophene) (P3HT) is juxtaposed with that of quaterthiophene (4T) where we find, for both systems, an increase in thin-film conductivity by several orders of magnitude. The underlying doping mechanisms at work are, however, fundamentally different [2]: IPA formation occurs for the polythiophene while CPX formation is found for the 4T oligomer, as deduced from characteristic shifts of the F4TCNQ cyano-vibrational bands in infrared absorption spectroscopy, which allow quantifying the degree of partial charge transfer to only 0.2 for the latter. This finding is particularly surprising as, in both cases, grazing-incidence X-ray diffraction experiments reveal phase separation between the OSC and a mixed crystalline OSC/F4TCNQ film-portion of similar microstructure, in which enhanced mutual frontier-molecular orbital overlap would be expected to equally favor CPX formation.

For both cases (i) and (ii), the doping-induced modification of the OSC density of states (DOS) is discussed in general for both p- and n-doping and its Fermi-Dirac occupation is modelled by employing extensive numerical simulations. Therefrom finally emerges that engineering the DOS of doped OSCs, the occupation of which ultimately determines the doping efficiency, represents a key challenge in dopant design.

[1] I. Salzmann et al., Acc. Chem. Res. 49, 370 (2016)
[2] H. Méndez et al., Nature Commun. 6, 8560 (2015)

It is well known that conjugated polymers like P3HT form nanocrystals in poor solvent solutions and that these same polymers “crash out” of solutions containing p-type dopants. We examine the relationship between polymer doping and nanostructure formation using a combination of small angle neutron scattering (SANS) and inelastic neutron scattering (INS). We use SANS to monitor the formation of P3HT nanocrystals in solution and examine how the presence of the p-type dopant (F4TCNQ) changes the crystal size, structure, and formation kinetics of P3HT nanocrystals. Next we use INS to determine P3HT configurations both inside and outside of crystalline domains for both doped and undoped polymers. These results demonstrate that only a small percentage of the volume is occupied by the ideal structure determined by x-ray crystallography and that the side chains are much more disordered than presented in literature. In addition, we fit the P3HT/F4TCNQ configuration using DFT modeling. This microscopic information is related to macroscopic processing. Both the solubility of the dopant in solvent and miscibility of a dopant in the polymer are used to demonstrate selective placement of dopants in amorphous and crystalline phases. This processing control has significant effects of the conductivity and mobility of charges as a function of doping density. Strategies to synthetically control dopant miscibility and solubility will be explored.

Many organic and hybrid thin film electronic devices (e.g. memory diodes, solar cells, light emitting diodes, transistors, capacitors) typically contain a functional layer based on a blend comprising multiple polymeric or small-molecular species whose properties cooperatively give rise to a specific function. Depending on the desired functionality, phase separation during film processing is either encouraged or suppressed. As usually at least one blend component is polymeric, mutual segmental repulsion readily overcomes the entropic driving force to form stable mixtures. For this reason, it is often observed that during solution-casting droplet-like demixed structures emerge due to liquid-liquid demixing, which may or may not be desirable.
This presentation will focus on the role of liquid-liquid demixing in thin film electronics by demonstrating how local phase composition and characteristic morphological features affect (opto-)electronic performance. Calculations based on mixing thermodynamics as well as results from dynamic phase field simulations will be placed in direct connection to electronic processes such as charge injection, recombination and trapping. As the community heavily relies on controlling morphology development during solution-processing, we would in the second part of the presentation like to share novel insight concerning the influence of solvent evaporation on spinodal decomposition. We quantitatively show how both the early and late stages of demixing are affected by the continuous drift in composition imparted by steady evaporation and why significant deviation from the “classical” laws governing structure formation and domain growth is observed.

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to- roll production and thermomechanical reliability of organic electronic devices. These material properties are a complex function of many factors—including molecular structure, composition, and processing conditions—making them difficult to design from chemical intuition and empiricism alone. Here, we apply both classical atomistic and coarse-grained molecular dynamics simulations to predict the solid-state morphology, thermal, and tensile behavior of a representative set conjugated polymers and composites (including pure P3HT, several low bandgap polymers, and composites with fullerene derivatives). First, we validate the approach through comparison to experiment for important thermomechanical properties including density, tensile modulus, and glass transition temperature. Next, we perform detailed analysis to uncover the underlying structural mechanisms for strain accommodation in this class of materials including the role of solid-state packing, chain conformations, and entanglement on bulk mechanical properties. Our results suggest that classical molecular dynamics simulations can be used for in Silico screening of candidate materials to gain a fundamental understanding that links molecular structure with mesoscale morphology and macroscopic properties to guide the experimental design and processing of new material systems with co-optimized optoelectronic and mechanical properties for applications in stretchable, and ultra-flexible electronics.

The dynamics of the active layer of organic photovoltaic (OPV) devices are known to influence their performance and lifetime. Poly(3-hexylthiophene-2,5-diyl) (P3HT) is a widely studied conjugated polymer, which exhibits a glass transition around room temperature, and consequently, is sensitive to temperature variations. We study the dynamics of P3HT and of blends of P3HT and Phenyl-C61-butyric acid methyl ester (PCBM) as a function of temperature, by comparing their quasi-elastic neutron scattering (QENS) with molecular dynamics simulations (MD). We use the deuteration technique that allows us to access not only the dynamics of the polymer but also the dynamics of PCBM within the blends.¹ We find a good agreement between the simulated and experimental data within the explored time window for the polymer dynamics, demonstrating that the force fields used in MD simulations are appropriate and that the QENS technique can be used as a validation of such force fields.² Using MD allows us (i) to identify and to assign contributions to the QENS signal from different parts of P3HT and PCBM, (ii) to study separately different phases of the active layers e.g. neat P3HT and PCBM phases and amorphous mixtures of P3HT and PCBM and (iii) to determine the activation energies of the different motions.

¹. Anne A. Y. Guilbert, Mohamed Zbiri, Maud V. C. Jenart, Christian B. Nielsen, and Jenny Nelson, “New Insights into the Molecular Dynamics of P3HT:PCBM Bulk Heterojunction: A Time-of-Flight Quasi-Elastic Neutron Scattering Study”, DOI: 10.1021/acs.jpclett.6b00537.
². Anne A. Y. Guilbert, Antonio Urbina, Jose Abad, Carlos Díaz-Paniagua, Francisco Batallán, Tilo Seydel, Mohamed Zbiri, Victoria García-Sakai, and Jenny Nelson, “Temperature-Dependent Dynamics of Polyalkylthiophene Conjugated Polymers: A Combined Neutron Scattering and Simulation Study”, Chem. Mater., 2015, 27 (22), pp 7652–7661.

A comprehensive understanding of the connections among chemistry, structure, and dynamics of polymers is needed to improve performance and efficiency in numerous polymer-based applications. However, these relationships can be difficult to probe, especially under in situ or operando conditions, and proper simulations are needed to complement and unravel experimental results. We show an example of how first-principles calculations of X-ray absorption spectroscopy can be used to understand the fundamentals of electronic structure in conjugated polymers, and help elucidate structural parameters such as backbone tilt or polymer chain axis orientation. The important effects of various molecular parameters such as polymer chain length, side chain atoms, and backbone orientation on simulated spectra is demonstrated for model conjugated polymers. In addition to core-level spectroscopy, which is sensitive to chemical moieties and electronic structure, soft X-rays provide the ability to probe spatial information, dynamics and chemical kinetics through transmission microscopy, resonant scattering and photon correlation spectroscopy, and this suite of soft X-ray methods is naturally complementary. We will also discuss current developments and outlook for combining theory and soft X-ray experiments to investigate functional polymers under in situ and operando conditions, for example, in the presence of liquid or gas flow, or during electrochemical reactions. Advancements in X-ray methodologies and complementary theory will drive capabilities to probe chemistry and morphology of polymers and soft matter in a time-resolved manner.

We study the role of disorder, electronic polarization, and electron delocalization on the nature of charge-transfer states in model donor-acceptor systems. We use a combination of electronic-structure calculations, molecular mechanics and molecular dynamics simulations complemented by ensemble and time average approaches to separate the static and dynamic disorder components. The implications of the charge-transfer states, interface morphology, density of states and charge carrier concentration on charge transport and charge recombination dynamics is investigated as well.

Understanding excitation properties and charge transport phenomena of curved π -conjugated materials is critical for a rational utilization of buckybowls as electrically active materials in solid state devices. In contrast to planar polyarene, materials systems based on the smallest bowl-shaped fullerene fragment, corannulene (C20H10), offer a unique possibility for building up scaffolds with a tunable spectrum of structural and electronic properties.[1] When assembled in the solid state, these molecular fragments provide an array of materials supported in varying complex environments, which are shown to be exploited as active components in optoelectronic applications (OLEDs, OPVs, OFETs),[2,3] aggregated as monolayers on metallic surfaces for work-function (WF) engineering and self-assembly studies, [4,5] or, as single molecules in junctions for transport processes.[6] Moreover, most functionalized derivatives show columnar arrangement along a particular stacking axis where the great bowl overlap and the enhanced π-π interactions have the potential to facilitate interstack and charge-transfer excitations in the crystal. [2]
In this study many body perturbation theory (MBPT) within the GW approximation and Bethe Salpeter equation (BSE) approach [7] are applied to investigation and predictions of charge transfer excitations across a series of experimentally established C20H10 based material systems, quantitatively addressing IP, EA, optical absorption spectrum and the nature of the excitonic transitions. The effects of a stepwise build-up of size, curvature, and strain at the C20H10 core on the resulting 1D columnar stacks and excitonic properties are specifically addressed, with a special emphasis on design aspects of functional materials relevant in the experimental domain. Theoretical predictions together with experimental findings show the ability to predict the molecular substitution/packing motifs most suitable for device purposes, illustrating the possibility to integrate corannulene electronic functions in molecular devices.
[1] Zoppi, L.; Martin-Samos, L.; Baldridge, K. K., Acc. Chem. Res. 2014, 47, 3310-3320.
[2] Zoppi, L.; Martin-Samos, L.; Baldridge, K. K., J. Am. Chem. Soc. 2011, 133, 14002-14009
[3] R.-Q.; Zhou, Y.-N.; Yan, X.-Y.; Shi, K.; Zheng, Y.-Q.; Luo, M.; Wang, X.-C.; Pei, J.; Xia, H.; Zoppi, L.; Baldridge, K. K.; Siegeld, J. S.; Cao, X.-Y., Chem. Commun. 2015, 51, 1681-1684
[4] Bauert, T.; Zoppi, L.; Koller, G.; Garcia, A.; Baldridge, K. K.; Ernst, K.-H., J. Phys. Chem. Lett. 2011, 2, 2805-2809;
[5] Bauert, T.; Zoppi, L.; Koller, G.; Siegel, J. S.; Baldridge, K. K.; Ernst, K.-H., J. Am. Chem. Soc. 2013, 135, 12857–12860
[6] Zoppi, L.; Ferretti, A.; Baldridge, K. K., J. Chem. Theory Comput. 2015, 11, 4900-4910
[7] Onida, G.; Reining, L.; Rubio, A., Reviews of Modern Physics 2002, 74, 601-659.

Excitons and polarons are the major players in governing energy and charge transfer phenomena in soft materials [1]. They are the primary outcome of light-matter interactions.
Our understanding about their formation and relaxation mechanisms has significantly improved, however several features are still obscure. For instance, there is not a clear comprehension yet about the high-energy ultrafast exciton relaxations in complex molecular systems, leading to polaron formation / recombination phenomena.
In this contribution, via time dependent density function theory (TD-DFT) calculations, surface hopping techniques [2,3] and experimental spectroscopic investigations, we get mechanistic insights into three processes: 1) ultrafast exciton relaxations and charge transfer dynamics [2,3], 2) singlet-triplet excitons dynamics and spin-orbit coupling mechanisms [4], 3) polarons generation and spectroscopic signatures [5].
A survey of the above processes over different classes of organic materials is presented, spanning from thiophene-based molecules vs. aggregates, homo- vs. low band gap co-polymers (e.g. P3HT vs. C- and Si-PCPDTBT, P(NDI2OD-T2) vs. BBL), and donor/acceptor interfaces.
Fundamental mechanisms are tackled and critically discussed, proposing molecular design rules to effectively improve the light-to-current conversion efficiency and the charge transport [6] in functional organic materials.

[1] Savoie B. M. et al., Acc. Chem. Res., 2014, 47, 3385.
[2] Fazzi D. et al., PCCP, 2015, 17, 7787.
[3] Fazzi D. et al., J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.5b13210.
[4] Akimov A. V., et al., Chem. Rev., 2013, 113, 4496.
[5] Yin J., et al., J. Phys. Chem. C., 2016, 120, 1994.
[6] D. Fazzi, M. Caironi, PCCP, 2015, 17, 8573.

Semiconducting polymers are soft materials with many conformational degrees of freedom. The interaction between electronic and nuclear degrees of freedom leads to the localization of charge carriers. An understanding of the mechanisms that cause the localization of charges is necessary for the development of charge transport models that establish a clear link between the polymer structure and electronic properties. We use a coarse-grained approach based on the tight-binding approximation to model the electronic degrees of freedom of poly(3-hyxylthiophene), with an effective account of nuclear degrees of freedom. Dihedral rotations and vibrational ring distortions, known to disrupt extended electronic states, are particularly important. The initial stage of our model successfully captures the effects of dihedral rotations on the valence and conduction states of the chains. The coupling between dihedral rotations and carriers is characterized using density functional theory (DFT) calculations of the one-dimensional band structures for chains with imposed periodic variations in dihedral angles. The model predicts the formation of localized electronic states due to thermal dihedral fluctuations—a disorder induced localization. We extend the model to include the effects of vibrational ring distortions. The vibrational modes and their coupling constants to carriers are computed using DFT. Combined with the variational method, the extended model predicts the formation of a self-localized carrier—a polaron stabilized by the reorganization of rings in the absence of disorder. We also present a third mechanism of localization which relies on the dielectric stabilization of a localized charge carrier on a given chain surrounded by other chains, such as in the crystal or amorphous melt. This polaron forms as the dielectric medium distorts to favor the localized carrier. Finally, we infer the relative importance of these different localization mechanisms by comparing the localization lengths and binding energies.

Charge transport mobility in organic semiconductors is limited by energetic and configurational disorder. The energetic disorder arises partly from intrinsic effects such as variations in the strength of intra and intermolecular coupling, and partly from extrinsic effects such as chemical defects. Using fullerene films as a model system we study the effects of intrinsic and extrinsic phenomena on the electronic structure, density of states and electron transport in fullerene assemblies generated by coarse grained molecular dynamics [1]. We do this using a tight binding model of the electronic structure of the assembly, in which the self-trapping of the charge through the dielectric response of the surrounding medium (i.e. polaron formation) is included self-consistently. We calculate the polaron binding energy from calculations of the static and optical dielectric function of the medium. By comparing the results to experimental data we can evaluate the relative effect of extrinsic defects and intrinsic disorder on charge transport in fresh and aged films. We show how the tight binding approach can be applied to other problems such as the electronic structure in disordered macromolecular systems, and to the composition dependent density of states of multicomponent molecular systems.

[1] F. Steiner et al, Materials Horizons, 2014, 2, 113-119.

Organic electronic materials are highly spatially disordered. Resultant fluctuations in wavefunction overlap leads to band tailing in the electronic density of states. In turn this limits the charge carrier mobility. Methods to understand these relationships will enable the design of higher performance organic semiconductors.

We will discuss the multi-scale simulation methods required to solve the electronic density of states of a conjugated polymer, focusing on amorphous P3HT[1]. Here we undertook atomistic molecular dynamics, calculated transfer integrals from frozen snapshots with the molecular orbital overlap method, and then solved a tight binding model, to have an entirely ab-initio prediction of the Urbach tail of charges. Our chief results were: inter-monomer torsional disorder dominates intra-chain disorder; and that the Urbach tail was composed of extremal configurations, and so required very large calculations to converge on a value.

Our latest work calculates the electronic density of states for conjugated polymer chains, with the off-diagonal disorder simulated by statistical mechanics based on an ab-initio torsional potential[1,2]. The method uses the linear-scaling Sturm sequences to solve the tight binding Hamiltonian and construct the densities of states. This enables an extremely high signal to noise ratio, for minimal computational effort. This enables us to make quantitative predictions on how varying the backbone of a conjugated polymer can directly influence the resulting charge transport characteristics of both holes and electrons, and so indicate routes in chemical synthesis to materials of superior performance.

Finally we will discuss how organic materials can offer unique performance benefits over crystalline, with the use of Wavefunction Engineering to engender unique excited state properties.

[1] Parameter free calculation of the subgap density of states in poly (3-hexylthiophene), JM Frost et al, Faraday discussions 174, 255-266
[2] https://github.com/jarvist/Teclo

We report on device optimization of bistetracene thin film transistors using solvent vapor annealing. The disparity between the excellent mobility of a single crystal (6 cm²/Vs) and an as-cast thin film (1x10^-4 cm²/Vs) is explored with a detailed morphology analysis. Charge transport is known to be very sensitive to slight variations in molecular packing and SVA provides a powerful handle to adjust the thin film morphology of this 2D polycyclic aromatic hydrocarbon to induce and characterize different polymorphs. To understand the morphological changes, in-situ and static GIXD were employed to map changes in thin film crystal packing that rationalize our observations in charge transport. Charge transport anisotropy and variable temperature mobility measurements were also conducted to understand the effects that the crystal structure variations have on the transport of charge.

Organic-based devices are currently attracting great attention for applications where low-cost, large area coverage and flexibility are required. The best performing electronic devices to date are those comprising single crystals of organic semiconductors, although they are neither suitable for large-area applications nor compatible with fast high-throughput industrial-scale fabrication. Thus, engineering processing techniques that could give rise to highly crystalline and homogenous semiconducting films potentially resulting in reproducibly high mobility devices is a current challenge. We report here the bar-assisted solution shearing (BAMS) of organic semiconductor blends based on small semiconducting molecules and the insulating polymer polystyrene (PS).[1-3] This technique results in highly crystalline thin films that showed ideal OFET characteristics. Further, we investigated the influence of the deposition parameters and solution formulation on the thin film morphology and polymorphism, which in turn, has a crucial impact on the device performance.

[1] F. G. del Pozo, S. Fabiano, R. Pfattner, S. Georgakopoulos, S. Galindo, X. Liu, S. Braun,M. Fahlman, J. Veciana, C. Rovira, X. Crispin, M. Berggren, M. Mas-Torrent, Adv. Funct. Mater. 2016, 26, 2379.
[2] S. Georgakopoulos, F. G. del Pozo, M. Mas-Torrent, J. Mater. Chem. C 2015, 3, 12199.
[3] I. Temiño, F. G. del Pozo, M. R. Ajayakumar, S. Galindo, J. Puigdollers, M. Mas-Torrent, Adv. Mater. Technol. 2016, DOI 10.1002/admt.201600090.

Over the course of the last decades, the field of semiconducting polymers has experienced huge developments, both in the fields of physics and materials chemistry. For a long time, chemists primarily focused on developing new and ever more exotic conjugated materials. Even though novel organic semiconductors became more challenging to synthesize, physical properties like charge carrier mobilities started to stagnate. The focus therefore shifted from new material design to gradually improving the physical properties of organic semiconductors by modest chemical alterations. The most striking example of this development is increased interest in the alkyl side chains, which are no longer regarded primarily as solubility providing groups, but as attractive functional groups that can have a profound impact on the physical properties of organic semiconductors.

Early studies on conjugated polymers came to the conclusion that an edge-on orientation of the polymer chains on the substrate was beneficial for efficient charge transport. This assumption however does not hold-up anymore with the emergence of high performing “push-pull” polymers, which show exceptionally high charge carrier mobilities in OFET devices without necessarily adopting a highly ordered edge-on orientation. In contrast, those materials show a strong preference for face-on orientations and relatively poor long range order. In this work, we will outline how modest structural changes to the alkyl side chains of a high-performing diketopyrrolopyrrole based polymers can lead to dramatic morphological changes and consequently affects the charge carrier mobility. These findings help to establish new design, respectively processing rules, and show that careful side chain design is absolutely essential in maximising the device performance of conjugated polymers.

Organic electronics have attracted considerable interest over the last decades promising an alternative to conventional, inorganic electronics platforms. To fully exploit the touted potential of this plastic electronics platform, however, other prerequisites need now to be fulfilled: for example, good mechanical stability, ease of processing and device reliability. A possible method to overcome these issues is the employment of insulating:semiconducting polymers blends, which have been demonstrated to display favourable rheological and mechanical properties without negatively affecting the electric performance.^1–4
In this work we will firstly show how the processing conditions and crystallization processes of the various components in such blends plays a crucial role. In order to do so, we use a combination of in-situ techniques including grazing incidence wide-angle X-ray scattering (GIWAXS), optical microscopy and UV-vis spectroscopy to analyse the crystallization of these systems.
We then present a range of organic field-effect transistors (OFETs) comprising an electrically insulating material, high-density polyethylene (HDPE), blended with different organic semiconductors. We discuss the effects of the insulating polymer on the electric characteristics of the devices and how the reliability is enhanced. We will focus on bias-stress behaviour, showing how the insulating polymer has a stabilizing effect on the transistor operation.
Finally, we will show how this approach can be extended in order to produced self-standing, fully printed, top-gate top-contact transistors as a possible route towards stable fully printed electronics.


1. Scaccabarozzi, A. D., & Stingelin, N. Semiconducting:insulating polymer blends for optoelectronic applications—a review of recent advances. Journal of Materials Chemistry A, 2, (28), 10818 (2014)
2. Müller, C. et al. Tough, Semiconducting Polyethylene-poly(3-hexylthiophene) Diblock Copolymers. Advanced Functional Materials 17, 2674–2679 (2007).
3. T. A. Ferenczi, C. Müller, D. D. C. Bradley, P. Smith, J. Nelson, and N. Stingelin, “Organic semiconductor:insulator polymer ternary blends for photovoltaics.,” Advanced materials (Deerfield Beach, Fla.), 23, (35), 4093-7, (2011)
4. Wolfer, P., Müller, C., Smith, P., Baklar, M.A. & Stingelin-Stutzmann, N., “α-quaterthiophene-polyethylene blends: phase behaviour and electronic properties” Synth. Met. 157, 827 (2007)

Bioelectronic sensing and stimulation calls for the seamless bridging of biological systems (rich in ionic and biomolecular signaling) with the realm of microelectronics in order to monitor health, diagnose or treat disease. Conjugated polymers offer unique characteristics which can meet these needs: mixed ionic/electronic transport can allow for efficient signal transduction, and softer mechanical properties and synthetic functionalization can impart biocompatibility and reduced inflammatory response. Organic electrochemical transistors (OECTs) are one class of devices that utilize organic mixed conductors as the transistor channel, and have shown considerable promise as amplifying transducers due to their stability in aqueous conditions and high transconductance. These devices can be fabricated in flexible, conformable form factors for electrophysiological and biomolecular sensing applications. The majority of high performance bioelectronic devices are based on conducting polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS. Combining structure-transport studies on such prototypical materials with device characterization, we are able to construct design rules for new materials. Film microstructure and morphology critically affect both electronic transport and the capacity for charge storage/ion penetration (and thus OECT performance), and can be tailored through processing, formulation or molecular design. For example, introducing glycolated side chains to carefully selected semiconducting polymer backbones has enabled a new class of OECT materials that feature high volumetric capacitance, transconductance >10 mS (device dimensions ca. 10um), and steep subthreshold switching characteristics. A sub-set of these materials outperform PEDOT:PSS and show significant promise for low power in vitro and in vivo biosensing applications.

Polymer semiconductors with steadily improved electronic properties are being synthesized, achieving charge mobility in excess of 5 cm²/Vs for electrons and holes. Such performances are sufficient for a large range of applications of printed, light-weight and mechanically robust circuits, in diverse fields such as wearable electronics, smart packaging, and bio-electronics. Yet, charge transport properties in high mobility donor-acceptor polymer films is still under debate. In this presentation I will show the useful information that can be gathered on the nexus between film microstructure and electronic properties, and on the nature of charge carriers thanks to charge modulation spectroscopy (CMS) and microscopy (CMM), techniques which can selectively probe and map carriers at the buried semiconductor-dielectric interface in a working field-effect transistor (FET). In particular, I will show that CMS, combined with electrical measurements, allows to reveal a neat two-dimensional charge transport regime in molecularly ordered, nanometers thin, Langmuir-Schäfer monolayers of an electron transporting polymer, where charge transport and electro-optical properties do not depend on the number of layers deposited one on top of the other. As an effect of the formed microstructure, the channel results confined and does not extend beyond a single polymer strand because of an inter-layer hopping penalty. As another example, I will show how local charge-modulation spectra recorded with a focused probe in different points of a polymer FET channel allow to distinguish genuine charge-induced spectral features from field-dependent spurious effects related to non-optimal charge injection, thus allowing a proper spectral assignment and a correct assessment of the nature of carriers. This allows to safely map the charge distribution of carriers in polymer FETs under different bias conditions, from unipolar to ambipolar accumulation regime. CMM also enables the investigation of the specific spatial distribution of transition dipole moments associated with holes and electrons accumulation regime over the same film microstructure.

With hole and electron mobilities of 1-5 cm² V^-1s^-1, polymer semiconductors have become viable candidates for photovoltaics and all-organic flexible displays. Recent advances in charge transport have been materials driven: the development of donor-acceptor (D-A) copolymers has shown that high mobilities can be achieved in relatively disordered morphologies compared to the highly crystalline benchmark materials. The exceptional performance and low energetic disorder of such D-A polymers has been linked to planar backbones that facilitate delocalisation along the polymer chain[1] and the availability of tie chains that connect crystallites over amorphous regions[2]. There is first experimental evidence for coherent charge-transport at high carrier densities (Hall effect, band-like temperature dependence)[3,4] and transistors with aligned polymer films indicate that charge transport is limited by hopping events between crystallites while transport along the polymer chain takes places in highly delocalised states[5]. However, it remains elusive under which conditions such a transition between hopping and band transport occurs.

In this study, we probe charge transport at the microscopic level via electron spin resonance on field induced charges in a transistor structure (FI-ESR). From the narrowing of resonance lines due to the motion of charges[6], we determine trap residence times of polarons down to 10 K where electric measurements are no longer possible. The comparison with field effect mobilites from the same device allows us to estimate the average distance between trapping events. The large lengthscales, e.g., 16 nm for indacenodithiophene-co-benzothiadiazole (IDT-BT) at 250K, support a multiple trap and release (MTR) model with band transport between trap states as opposed to hopping between traps in a disordered density of states (DOS). We compare the resulting values for IDT-BT, which shows an exeptionally low degree of energetic disorder and ideal transistor characteristics [1], with polymers that exhibit stronger backbone torsion and varying degrees of crystallinity.
In addition, the magnetic susceptibility of IDT-BT shows a Pauli temperature dependence for a significant fraction of the field induces charges. This microscopic evidence for a band-structure[7] allows us to calculate the DOS at the Fermi level and monitor the ratio between trapped and delocalised charges as a function of the carrier concentration (gate voltage).
In summary, we present a microscopic study of the transition between hopping and band transport via FI-ESR by comparing IDT-BT to other high performing IDT and diketopyrrolopyrrole (DPP) based polymers.

[1] D. Venkateshvaran et al. Nature 515 (2014).
[2] R. Noriega et al. Nat. Mater. 12 (2013).
[3] K. Kang et al. Nat. Mater. (2016).
[4] S.P. Senanayak et al. Phys. Rev. B 91 (2015).
[5] S. Schott et al. Adv. Mater. 27 (2015) 7356.
[6] H. Matsui et al. Phys. Rev. Lett. 100 (2008).
[7] H. Tanaka et al. Adv. Mater. 26 (2013).

Organic thin-film transistors (OTFTs) are emerging as a promising technology for integration of various functionalities on large scale, flexible substrates. The application of a gate voltage leads to the modulation of the charge carrier density and thereby the transistor current. Here we demonstrate a new method to characterize the charge carrier density in the transistor channel based on ultrafast optical measurements. We use the fact that photoluminescence (PL) lifetime is affected by exciton-charge carrier quenching to detect the carrier density under bias conditions. Using a beam scanning approach, we are able to also spatially resolve the varying carrier density along the transistor channel. By comparison with a drift-diffusion model, we establish a correlation between the charge carrier density and the observed PL lifetime.

PBWJ, TL, and YL have contributed equally to this work

The space-charge-limited current (SCLC) measurement is a commonly used method for determining charge-carrier mobilities in organic, inorganic and hybrid semiconducting thin films. SCLC measurements are popular and convenient since they are simple to perform, but the obtained data is prone to misinterpretation. It is common to fit analytical equations to the obtained current density-voltage (J-V) curves from single-carrier devices, such as the Mott-Gurney (MG) square law to extract the charge-carrier mobility (Mott & Gurney, 1940) and, recently also, Ohm’s law to extract the conductivity, and hence the charge-carrier density when used in combination with the MG law (Shi et al., 2015). Whereas the MG law does not allow for factors such as traps, internal and external resistances, built-in voltages, effects caused by injection barriers, or doping, something which will heavily influence the value for estimated mobility when not accounted for, Ohm’s law will only apply when the device is in a high doping limit. We show that using these two equations in combination for the characterisation of lowly doped materials will yield results which are wrong by up to many orders of magnitude. Furthermore, we question the validity of the MG law, even for the ideal case of perfect injection into an intrinsic semiconductor, and evaluate under which “real-life” conditions the MG can yield reasonable results for charge carrier mobility. We finally show that it is possible to extract correct mobilities and trap characteristics from single-carrier devices using a drift-diffusion solver approach, and by accounting for traps with the model, the extracted charge-carrier mobility is of an order of magnitude higher than when analytical equations for trap-free systems are used. We apply our approach to a number of popular semiconductors used in printed electronics, such as Spiro-OMeTAD (and derivatives), polyfluorene, fullerene derivatives and lead halide perovskites.

The performance of most organic electronic devices has not yet reached the level required for incorporation in consumer technologies. Impurities and crystalline defects diminish the electrical performance by giving rise to trap states. We show that the microstrain induced in the semiconductor film as a result of the mismatch in the thermal expansions of the consecutive layers present in organic field-effect transistors (OFETs) introduces traps, lowers charge carrier mobility and can even induce a crossover from a band-like to an activated charge transport. We evaluate the temperature effects on OFETs consisting of different small molecule organic semiconductors in combination with “vacuum-gap”, polymer or SiO₂ dielectrics. We tune film texture and structure via processing to access ratios between the coefficients of linear thermal expansions (CTE) of the organic semiconductor and the dielectric layer varying between 1 and ~ 200 ppm/K. We observe band-like transport only in devices of low intrinsic defect density and characterised by very small CTE mismatch. We find a general relation between grain size, thermal expansion mismatch and trap densities at the semiconductor/dielectric interface. By combining our data with results published on other systems, we discover an universal scaling between the activation energy of the devices and the interfacial thermal expansion mismatch coefficient, in which a band-like transport is observed for similar coefficients of thermal expansion, and activated transport otherwise, with the magnitude of the activation energy being proportional with the relative thermal expansion coefficients. Our results provide evidence that a high quality semiconductor layer is necessary, but not sufficient, in obtaining high performance, and therefore underline the importance of device design in achieving the fundamental performance limits of a given semiconductor.

Acenedithiophenes, represented by benzodithiophenes (BDT), naphthodithiophenes (NDT), and anthradithiophenes (ADT), have been an important class of pi-frameworks for developing new small-molecule semiconductors and semiconducting polymers [1]. Thanks to their extended pi-system, they have generally high-lying HOMO suitable to p-type organic semiconductors and building blocks for semiconducting polymers. In fact, the small-molecule semiconductors based on ADT [2] and semiconducting polymers based on BDT [3] have played important role in the solution-processed OFET and OPVs, respectively. On the other hand, one of the structural characteristics of acenedithiophenes is to possess two terminal thiophene rings, which provide chemically reactive sites for further modification. By such chemical modifications, it is possible to incorporate acenedithiophene framework into quinoidal conjugation system terminated with ketone, dicyanomethylene, or other groups. In this contribution, we present the synthesis and characterization of a series of acenedithiophene quinoidals, and their potential applications to n-type organic semiconductors [4,5].

[1] Nakano et al. J. Org. Chem. 2012, 77, 8099–8111.
[2] Anthony et al. J. Am. Chem. Soc. 2005, 127, 4986–4987.
[3] Yu et al. Adv. Mater. 2014, 26, 4413–4430.
[4] Suzuki et al. J. Am. Chem. Soc. 2010, 132, 10453–10466.
[5] Mori et al. Org. Lett. 2014, 16, 1334–1337.

Incorporation of nitrogen into extended aromatic frameworks has been investigated extensively, often by placing pyridine, pyrazine and pyrrole moieties into the framework.¹ N-Fused heteroaromatic compounds such as indolizine present interesting new possibilities for incorporating nitrogen into large aromatic systems. Indolizine is a bicyclic N-heterocycle, isoelectronic with naphthalene, where the nitrogen is placed at a bridge-head position of the molecule, thus fully incorporating the nitrogen lone pair into the π-electron system.² Indolizine is a non-alternate hydrocarbon, demonstrating some structural and chemical similarity to non-traditional aromatic molecules such as azulene, which tend to have dipole moments and unusual molecular orbitals.³ The frontier molecular orbitals of indolizine containing molecules have expanded structures compared to molecules with similar geometries, leading to interesting ring currents, bright colors and strong fluorescence, demonstrated by recently published molecules.⁴
We have investigated several N-fused heteroaromatic compunds incorporating indolizine units to date. Benzo[2,3-b:5,6-b’]diindolizine has a zig-zag shape like dibenz[a,h]anthracene, but has optoelectronic properties very similar to pentacene, including singlet fission activity in aggregate. Crystal engineering by utilization of steric solubilizing groups allowed us to target 2-D π-stacking motifs in order to achieve derivatives suitable for field effect transistors, with maximum mobility measurements of 0.1 cm²/Vs. We incorporated the indolizine unit in other molecules by way of end capping an acene with aceindolizylene; an acenaphthene-like tricyclic unit where the nitrogen atom is the tertiary ring fusion site in the center of the unit. This aceindolizylene was placed at the long axis terminus of an acene such as anthracene, decreasing the HOMO-LUMO gap versus the base acene and leading to substantial bathochromic shifts in UV-vis spectra of 200 nm or more. Despite the shifts in absorption spectra, optoelectronic properties often associated with acenes were maintained, e.g. vibronic structure in the UV-vis and fluorescence spectra. Anthracene with two of these groups, one aceindolizylene on both sides of the long axis, has visible absorption up to 700 nm and emission over 700 nm.
1. Bunz, U.H.F.; Engelhart, J.U.; Lindner, B.D.; Schaffroth, M., Angew. Chem. Int. Ed., 2013, 52, 3810.
2. Uchida, T.; Matsumoto, K., Synthesis, 1976, 4, 209.
3. Michl, J.; Thulstrup, E.W., Tetrahedron, 1976, 32, 205.
4. a. Kim, E.; Lee, Y.; Lee, S.; Park, S.B., Acc. Chem. Res., 2015, 48, 538. b. Huckaba, A.J.; Giordano, F.; McNamara, L.E.; Dreux, K.M.; Hammer, N.I.; Tschumper, G.S.; Zakeeruddin, S.M.; Grätzel, M.; Nazeeruddin, M.K.; Delcamp, J.H., Adv. Energy. Mater., 2015, 5 (7), 1401629.

Amorphous thin films of small molecule organic compounds are commonly used as active layers in organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). While the long-range morphological homogeneity of solution-cast or liquid-cooled glasses is desirable, the lack of useful structure in the form of molecular orientation diminishes device performance through poor light out-coupling. We used physical vapor deposition to prepare nanometer-scale films of three compounds commonly used in OLEDs (TPD, NPB and DSA-Ph) and used spectroscopic ellipsometry to measure their optical and material properties. We found that this processing route results in dichroism when the substrate temperature during deposition (T_Substrate) is below the glass transition temperature (T_g), and that dichroism is primarily controlled by T_Substrate/T_g. T_Substrate was varied using a high-throughput technique from T_g to 0.6 T_g for all three systems. The average molecular orientation is calculated using the dichroism of the molecular long axis and varies from slightly aligned with the substrate normal (S_z = +0.2) to primarily lying in the plane of the substrate (S_z = -0.4). In addition, these materials have elevated densities and thermal stabilities which are unobtainable by other means. We explain both the tunable molecular orientation and favorable physical properties of the films as resulting from competition between the thermodynamic driving force to equilibrium and the kinetic limitation of mobility at the free surface during deposition. The controllability of the molecular orientation with the ratio T_Substrate/T_g provides a new framework for material design by guiding both chemical synthesis of new molecules and device construction.

Rubrene is a benchmark high performance small-molecule organic semiconductor. Single-crystal transistors grown via physical vapor transport exhibit mobilities as large as 20 cm²/Vs. Rubrene has also shown to exhibit extraordinary reversible wrinkling in OFETs which makes this material an excellent candidate for flexible device applications. The problem with Rubrene is that solution-cast thin film transistors do not exhibit any carrier transport at all- likely due to the fact that Rubrene oxidizes within a matter of minutes in chlorinated solvents and it forms disordered films. So why does Rubrene oxidize so readily in solution and how one can synthesize derivatives that are chemically stable to enable functional thin film devices? In this talk I will discuss our recent efforts in single-crystal Rubrene devices. Discussion on new Rubrene derivatives and their enhanced chemical stability will also be presented.

Solution deposited single-crystal organic semiconductor thin films have great potential in large-area manufacturing of flexible electronics such as solar cells and displays. However, the control over morphology has been a long-existing key challenge towards high performance devices, especially when high-speed processing is required. The hollow pen writing method is used to deposit 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) organic semiconductor thin film from solution. Millimeter-scale grains and average carrier mobility of 3.0 cm²/V.s were achieved under very high writing speed (25 mm/s). In-situ synchrotron X-ray scattering experiments show that very large grain size at high speed is related to the formation of transient liquid-crystalline and crystalline phases during the writing process. We were able to stabilize the transient crystalline phase and compare its properties with the stable phases by control of substrate temperature and film thickness. With further patterned substrate by hydrophobic self-assembly monolayer, the crystal morphology can be controlled by engineering the nucleation in order to fabricate single-crystal domains. The results suggest a promising step to large-area manufacturing flexible electronics that utilize high-performance organic single crystal field effect transistors.

Discrete oligomers of benzodithiophene have proven difficult to solubilize and subsequently investigate as potential candidates for solution processable organic semiconductors. Herein, we have synthesized a series of soluble, partially fluorinated, benzodithiophene oligomers containing a centrally located tri(alkyl)silyl ethynyl group. The degree of rotational freedom about the bond connecting the benzodithiophenes leads to a varying and seemingly random amount of disorder in the crystal structure, with adjacent benzodithiophenes taking either a syn or anti relationship to one another. It is likely that this disorder impedes device performance and could also cause the polymorphism observed in solution cast thin-films of these molecules. By replacing hydrogens with flourines at the benzodithiophene junctions, the presence of the syn isomer in the crystal structure can be eliminated. However, the inclusion of the fluorine atoms also has an unpredictable impact on both the crystal packing and the solubility of the molecule. Through modulation of the tri(alkyl)silyl group and placement of flourines on the external benzodithiophenes the crystal packing can be attempted to be engineered to one that is suitable for the active layer of solution processed OFETs.

As subtle changes in the crystalline packing motif of molecular semiconductors can influence charge transport, it is imperative to obtain a thorough understanding of their polymorphic landscapes and how differences in polymorphs impact charge transport. While many groups have focused on single-crystal polymorphic studies, we have instead chosen to focus on polymorphic transformations in more technologically relevant but polycrystalline thin-film counterparts. Starting with a series of core-chlorinated naphthalene tetracarboxylic diimides (NTCDI) that adopt two distinct polymorphs in single crystals, we demonstrate reversible accessibility of these polymorphs in thin films with post-deposition annealing and reveal how their access is predominantly driven by subtle differences in the alkyl substitution at the imide functionalities of the conjugated core. NTCDI-CH₂C₃F₇, for example, adopts its β-phase in thermally evaporated thin films; solvent-vapor annealing induces a reversible transformation to its α-phase [1]. Extending its fluoro-alkyl substituents by one methylene unit, on the other hand, hinders polymorphic transformation with post-deposition annealing; thin films of this derivative retains its β-phase polymorph with all the post-deposition annealing conditions we have explored. Conversely, the β-phase of the derivative possessing hydrogenated, as opposed to fluorinated alkyl substituents is considerably less stable; transformation from its β-phase to the α-phase is reversible and readily accessed with a myriad of post-deposition processing conditions. NTCDI derivatives adopt layered structures in both their α- and β-phases; the ease with which these polymorphs are reversibly accessed with solvent-vapor annealing is anti-correlated with the extent of short intermolecular close contacts between the alkyl substituents in adjacent layers.

Interestingly, tuning molecule-substrate interactions by altering substrate surface energy influences the nucleation density, and thus the resulting grain size, of the incoming polymorph without affecting the activation energy of transformation. This tuning knob has thus enabled us to vary the grain-boundary density within the active channels of thin-film transistors. Finally, tuning molecule-solvent interactions through judicious selection of the annealing solvent or altering the solvent-vapor concentration alters the transformation rate along the pi-stacking direction, effectively providing us access to crystal habits ranging from needles to plates of the incoming polymorph.

Altering intermolecular, molecule-substrate and molecule-solvent interactions has thus enabled unique and independent access to the morphological evolution of NTCDI, allowing us to traverse the complex phase space of these derivatives in their thin-film format.

[1] G.E. Purdum, N. Yao, A. Woll, T. Gessner, R.T. Weitz, Y.-L. Loo. Adv. Funct. Mater. 2016. 26, 2357-2364.

Disordered semiconductors often exhibit low charge transport and, thus, poor device’s performance. One of the key roles to suppress disorder, especially the structural type of disorder, is the control of morphology of π-conjugated materials. Morphology-tuning techniques such as thermal and solvent treatments as well as slow growth have been investigated. Aforementioned techniques produced polymeric thin films with improved crystallinity, better connectivity in-between polymer chains and, thus, carrier mobility. A method to reduce the structural disorder in π-conjugated matrix by static electric field is here presented. Immediately after their spin-coating on an indium-doped tin oxide (ITO) substrate, regioregular (RR) and regiorandom (RRa) poly(3-hexylthiophene) (P3HT) thin films were exposed to the electrostatic field (E-field) of Van de Graaff generator until they were dry. For comparisons, untreated (control) thin films were fabricated at the same time under the same conditions. After that, active layer was sandwiched between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and thermally evaporated MoO_X and aluminum to batin hole-only devices [structure ITO/ PEDOT:PSS/ active layer/ MoO_X/ Al]. Measuring the current-voltage (I-V) characteristics in the dark provide us with carrier mobility from fitting the space–charge limited current (SCLC) regime. We also obtain the activation energy (△) and structural disorder (σ) parameters from the Arrhenius dependence of mobility on temperature at different temperatures. △ (σ) values of untreated RR- and RRa-P3HT thin films were ~182.3 (65.3) and ~193.5 (67)meV, respectively. E-field treatment significantly suppresses the △ (σ) values to ~99.1 (47.9) and ~106.8 (50) meV for RR- and RRa-P3HT devices, respectively. To check its universality to reduce the structural disorder in semiconducting materials, small molecules such as SMPV1 were also treated with E-field. In SMPV1 hole-only devices, the activation energy decreases from 9.6meV for the neat case to 2.9meV for films processed in static-field. Extracting the structural disorder parameters from impedance spectroscopy measurments also validates our SCLC results. Intestinally, our results are in good agreement with the literature^{1, 2}. It is worth mentioning that static field yields RR- and RRa-P3HT and SMPV1 hole-only devices with enhanced mobility values relative to untreated devices.


1- [Noriega, NATURE MATERIALS, VOL 12, NOVEMBER 2013]
2- [Carr, Applied Physics Letters, VOL 100, 21, 2012]

Large-area OLED up to 320 mm × 320 mm in size are commercially available (LG Display) but are still suffering from the requirement for gridlines in combination with stacking to achieve high luminance uniformity. Especially when OLED sizes are growing further, gridlines limit the active area and are not aesthetical. For larger sizes, even broader gridlines will be required.
We have investigated OLED backside contacting as a superior alternative to gridlines. In this method, the low-conductivity anode (e.g. ITO) of a conventional bottom-emitting OLED is supported by a high-conductivity auxiliary electrode layer on top of the thin-film encapsulation (TFE). This auxiliary electrode is insulated from the Al cathode by the TFE and connected to the anode by etched and metallized via holes spread over the active OLED area. Electrical simulations of large-area OLED have predicted that this method allows comparable luminance uniformity while sacrificing significantly less active area compared to the gridline approach. We will not only present the fabrication method results of backside-contacted OLED, but also show that virtually invisible via holes could be realized.
The method for fabricating backside contacts is comprised of five steps: (1) TFE of the OLED with 115 nm AlO_x deposited by ALD, (2) Patterning of the OLED surface with lithography (resist mask defining via hole positions), (3) Via hole formation by a plasma etching process (low-energy BCl₃/Ar ICP-RIE) through TFE, Al cathode and organic layers, (4) Sidewall insulation by oxidation of the aluminum cathode and organic residues removal using oxygen plasma. This process prevents short circuit of anode and cathode and exposes the ITO anode in the via holes, (5) Contacting of the anode with a high-conductivity auxiliary electrode by thermal evaporation of 600 nm aluminum. The etching process needs to be stable and reproducible as etching rates of ITO are similar to those of e.g. aluminum (cathode material). Details and characterization results of each step will be shown with particular focus on the ICP-RIE etching process and the sidewall insulation.
Backside-contacted OLED processed by organic vapor phase deposition (OVPD) show negligible optical defect densities (limited by the simple single-layer TFE) and high luminance uniformity. Scanning electron microscopy (SEM) pictures and electrical breakthrough measurements (>2 V reverse bias) confirm efficient sidewall insulation.
The etching process creates a pyramidal ITO surface which was characterized by SEM. Due to the textured interface between the ITO and the auxiliary anode, trapped ITO modes – otherwise lost – are coupled out of the OLED at the via hole positions. With optimized etch parameters, the ITO surface may be structured in such a way that virtually invisible via holes can be realized.

Semiconducting materials are an integral part of everyday electronic circuits. Conventional electronics is based on silicon as semiconductor and therefore can be fabricated only on rigid substrates such as glass. Organic semiconductors on the contrary enable plastic-based flexible circuits and displays. For efficient organic electronic circuits, both electron as well as hole conducting semiconductors are required. While p-type materials with high charge carrier mobility and good ambient stability are readily available, the development of such electron conducting materials still lags behind.
We have developed a process for fabrication of large highly crystalline layers of a novel n-channel semiconductor that shows a maximum charge carrier mobility of 4.3 cm2/Vs. To understand the charge transport in this system we have performed a detailed temperature dependent transport study. We find that the more charges are confined to the semiconductor / insulator interface (this can be obtained either by decreasing semiconductor film thickness or via an electric field) the more the mobility drops with increasing temperature above 260K. Our observations are consistent with lattice vibrations or lattice imperfections at the semiconductor/insulator interface dominating the charge transport.

We have designed and synthesized a selenophene and fluorinated benzothiadiazole-based conjugated polymer, PDFDSe, as channel material for OTFT devices. PDFDSe TFTs showed the high charge carrier mobility of 2.72 cm²V^-1s^-1 in TG/BC OTFTs using PMMA dielectrics. This high carrier mobility can be ascribed to (i) the planar molecular backbone facilitating efficient π-π stacking of polymer chains, (ii) highly polarizable selenophenes, and (iii) fluorinated benzothiadiazole units enforcing interchain interactions. These featured were confirmed by UV-visible absorption spectroscopy, grazing-incidence X-ray patterns, and cyclic voltammetry, and surface morphology of PDFDSe polymer films.

Acquiring chemical orthogonality for solution-processed polymer semiconductor films against various organic solvents plays a critical role in realizing fabrication of electronic devices through low-cost solution processes. This is because stacking of multiple functional materials in electronic and optoelectronic devices would be hardly achieved through solution-processes, unless chemical orthogonality between the layers is guaranteed. To address this issue, we have recently devised a universal conversion method that forms heterogeneous interpenetrated network (HIPN) between polymer semiconductor and organosilane gel. The conversion method not only allowed one to apply conventional photolithography to form sub-micron patterns of polymer over a large area, but also led the resulting performance of the electronic devices based on the films in HIPN to be comparable to those of pristine polymer semiconductor films. This was, in fact, a surprising result, since the macroscopic ordering of the polymers confirmed from X-ray analysis was inferior for the polymer in HIPN compared to that of the pristine film.
In this presentation, we investigate the electrical transport through poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPP-DTT) polymer semiconductor that forms HIPN with organosilane gel in comparsion to that through pristine DPP-DTT film using a thin-film transistor (TFT) device testbed. Specifically, the electrical transport behavior of DPP-DTT near the semiconductor/gate dielectric interface was analyzed at different gate voltages (-40−-100 V) at different temperatures (300-200 K). The temperature-dependent transport study revealed that activation energy for electrical transport on the DPP-DTT in organosilane interpenetrated network (127 meV) is smaller compared to that in pristine DPP-DTT (241 meV), which led higher carrier mobility from the network. Despite the poor macroscopic ordering of the film in HIPN matrix, the short range ordering of the macromolecules, evidenced by the enhanced absorbance peak associated with pi-pi interaction, turned out to be stronger. Using the 3 dimensional charge transport model1,2 to describe the electrical properties of the film, our results suggest that the confinement of the polymer not only makes the material orthogonal to subsequent chemical processes but also helps forming efficient conduction pathway.

Semiconductor monolayers have great value in understanding the carrier transport behavior of field-effect transistors (FETs), as charge accumulation usually happens at only a few nanometers from the semiconductor/dielectric interfaces. Here we develop a new method to deposit 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) ultra-thin films. Our dual solution shearing (DSS) method can produce highly crystallized C₁₀-DNTT with monolayer thickness (4 nm). This monolayer device shows comparable mobility with the thicker film up to 10 cm²V^-1s^-1. This unipolar C₁₀-DNTT monolayer, can be converted to ambipolar by thermal evaporating a layer of Copper(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine (F₁₆CuPc) on it. The bilayer organic thin-film transistor(OTFT) devices show remarkable enhancement in both hole and electron mobilities. The highest mobility of hole and electron can reach 30 cm²V^-1s^-1 and 0.06 cm²V^-1s^-1 respectively, three times higher than that of pristine monolayer C₁₀-DNTT and thermal evaporated F₁₆CuPc. Since both p- and n- channels are extremely closed to the dielectric, the electrons and holes accumulation at the interface of bilayer heterojunction is more efficient and the localized trap states can be easily filled under the applied gate field. Through the AFM, XRD, TEM and SAED measurements, we confirmed that the flat and smooth monolayer C₁₀-DNTT surface can act as a growth template for the F₁₆CuPc and contributes to their high crystallinity, which favor electron transportation and interfacial charge-transfer between two n-type and p-type semiconductors.

The performance enhancement of organic optoelectronic integrated device (OID) was realized by using a wide-bandgap thermally activated delayed fluorescence (TADF) material namely, 3,4,5,6-tetrakis(carbazol-9-yl)-1,2-dicyanobenzene (4CzPN) as a dopant and a triplet-triplet annihilation (TTA) featured material of (E)-2-[4-(t-butyl)phenyl]-6-{2-[6-(diphenylamino)naphthalene-2-yl] vinyl}-1H-benzo [de]isoquinoline-1,3(2H)-dione (NA-TNA) as a host in a doping system to be an active layer. The result showed that the OID with a doping system had a maximum luminance of 7000 cd/m², which was 2 folds higher than that of the non-doped OID. And the 4CzPN doped OID had a low turn-on voltage of 2.7 V as OLED. Meanwhile, the 4CzPN doped OID exhibited a high detectivity of 0.6×10¹² Jones at a bias of -1 V under the UV-365 nm illumination, which was 14.6 folds higher than that of the pristine NA-TNA based device. Based on the analysis of the energy transfer model of devices, it was found that the improved electroluminescent performance of OID was attributed to the efficient energy transfer from NA-TNA to 4CzPN. And by analyzing the dark current behavior of device, the high ultraviolet detective property of OID was mainly caused by the wide band gap of the active layer when the dopant 4CzPN was added into the active layer, because the wide band gap of the active layer was contributed to reducing the dark current of OID.

The precise alignment of molecular structure and energy levels at donor/acceptor (D/A) interfaces are intrinsically tied to the resulting overlap of frontier orbitals. This relationship enables energetic control of hybrid charge transfer (CT) states and thereby electrical doping via CT complex formation [1] or charge separation in solar cells.[2] Here, perylene derivatives are used due to their high charge carrier mobilities, strong light absorption, stability and chemical tunability. Diindenoperylene (DIP) as donor is combined with a dicyanoperylene-3,4:9,10-bis(dicarboxyimide) derivative (PDIR-CN₂) as acceptor. Their structural properties, optical, and (photo-)electrical properties in planar and planar-mixed D/A heterojunctions are investigated.
A new polymorph in the blended films was found by X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD). In particular, this mixed crystal features a molecular realignment towards a face-to-face stacking of both molecules, which was furthermore corroborated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. The energy levels of the frontier orbitals result in a type II interface with staggered gap, as confirmed by photoelectron spectroscopy (UPS).
Several new CT transitions were observed and quantified by means of optical absorption, photo- and electroluminescence spectroscopy. Contrary to typical photovoltaic systems, a broad absorption band is present here, which diminishes the performance of photovoltaic cells. Fabricated photovoltaic devices show a rather interesting behavior. While the open circuit voltage is equivalent for planar and planar-mixed heterojunctions, the photocurrent is reduced in the case of blended films.
A combined experimental and theoretical approach identifies the CT complex in both ground and excited states. The ground state interaction between the two D/A molecules and the effects of hybrid CT transitions on charge carrier transport and optoelectronic properties are quantified. CT energy levels as well as the structural reorganization of the D/A molecules are correlated to the energy levels of the ground state. A critical discussion shows that in this material system, the improved absorption due to CT transitions is overcompensated by the limited transport and low energy offset in blended films, which explains the overall low efficiency of the photovoltaic cells.


References

[1] Salzmann, I., Heimel, G., Oehzelt, M., Winkler, S. & Koch, N. Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules. Acc. Chem. Res. 49, 370–378 (2016).
[2] Vandewal, K. et al. Efficient charge generation by relaxed charge-transfer states at organic interfaces. Nat. Mater. 13, 63–68 (2014).

Certain oligomers can self-assemble into crystalline nanofibers that can be used as the active, organic material in e.g. field-effect transistors [1] and light-emitting [2] and light-sensing [3] devices. The strong potential in the field of photodetectors is due to the combination of a high photogeneration yield over a large spectral range with low cost and low-temperature processing. However, in such devices the dissociation of optically generated excitons competes with several other processes such as radiative decay and exciton quenching. This can be influenced e.g. by an external electric field or by the use of a suitable donor-acceptor material blend. However, even in pristine semiconductors in the absence of an applied electric field, charge splitting can occur. It has been proposed that this can occur either through autoionization, in which excess photon energy results in charge separation or through a process that involves the exciton diffusion to sites with a low energy barrier towards charge separation. In this work, we have investigated charge splitting in pristine organic, crystalline semiconductor nanofiber upon illumination with a focused laser beam using electrostatic force microscopy. We demonstrate that the photogenerated excitons split and result in a localized build-up of negative charge at the illumination position and positive charge at the distant nanofiber end as a consequence of the difference in effective charge carrier mobilities between electrons and holes. At higher laser powers, the saturation behavior of the localized charge density is observed as a result of increased recombination. We employ drift-diffusion based modelling to interpret the results and from this determine carrier transport and recombination properties.[4]
[1] L. Tavares, J. Kjelstrup-Hansen, K. Thilsing-Hansen, and H.-G. Rubahn, 2011, Nanoscale Reasearch Letters, 6, 319.
[2] L. Tavares, J. Kjelstrup-Hansen. and H.-G. Rubahn, 2012, Nanotechnology, 23, 425231.
[3] X. Liu, L. Tavares, A. Osadnik, J. L. Lausen, J. Kongsted, A. Lützen, H.-G. Rubahn, and J. Kjelstrup-Hansen, 2014, Organic Electronics, 15, 1273.
[4] L. Tavares, Y. Liu, D. Behn, A. Mews, J. Kjelstrup-Hansen, Laser-Induced Charge Separation in Organic Nanofibers. In preparation.

In recent years, organic semiconductor materials have made rapid advances in the field of optoelectronics. Devices such as organic light emitting diodes and organic photovoltaic cells are already available on the market. Despite of its commercial success, the fundamental understanding of this material class and the underlying mechanisms such as exciton generation and charge separation remain disputed. Microscopic theories with sufficient predictive power are only recently emerging, while effects as the influence of disorder still need to be attributed for experimentally. In this work, we present a comprehensive study of the optical properties of the model organic semiconductor perylene. Experimental and theoretic methods are used to investigate the correlation of molecular packing motifes and optical properties. We discuss the delocalization of excitons in relation to crystalline orientation and identify pronounced reflection signatures. A detailed theoretical analysis reveals that model calculations need to be performed beyond the single particle picture, with the Tamm-Dancoff approximation losing validity. Our findings on the model system of perylene are to be understood as the ground work for the understanding of more complex molecular solids and for tailored functionalization of future materials.

Excitonic spin plays a crucial role in the design of materials for organic light emitting diodes (OLEDs). The random spin statistics of recombining charge sets an upper limit of 25% on the fraction of singlet (spin-0) excitons formed by electrical excitation. Without efficient emission from triplet (spin-1) excitons, the same limit applies to the internal quantum efficiency (IQE) of fluorescent OLEDs. Phosphorescent OLEDs, which utilise the heavy atom effect in platinum or iridium compounds to render triplets emissive, and thermally activated delayed fluorescence (TADF) OLEDs based on thermally-assisted triplet-to-singlet up-conversion in systems with low exchange energies are currently the most promising routes to enable emission from triplet states. Both approaches assume that triplets are the lowest-energy excitonic species.

Here we show that the effective exchange energy in a family of linear copper and gold carbene metal amide compounds can be tuned via rotation about the metal-amide bond from positive to negative values, even in the solid state. Through molecular rotation, the energetic ordering of spin-states can thus be inverted, enabling triplet-to-singlet down-conversion. This unusual spin-state ordering allows rapid conversion of triplets into emissive singlets. Employing these materials as the emissive dopants in solution-processed OLEDs leads to extremely efficient devices with near 100% IQE (external quantum efficiencies >27%). These devices also exhibit current efficiency, power efficiency and brightness comparable to or exceeding those of state-of-the-art vacuum-deposited OLEDs and quantum dot LEDs.

We describe the experimental and theoretical evidence for the rotationally-accessed spin-state inversion mechanism. We show how the resulting emission depends strongly on the interplay between rotational energetics, temperature, oscillator strength and the nanomorphology of the emissive layer. The latter plays a crucial role – without sufficient free volume in the surrounding matrix, molecular rotation is arrested and emission is restricted to less-effective phosphorescence. Tuning the emissive layer nanostructure is therefore vital for achieving rapid intersystem crossing and efficient emission, with the rate of triplet-to-singlet conversion directly linked to material plasticity.

Controlling the orientation of luminescent organic molecules can improve the optical outcoupling of organic light emitting devices (OLEDs). The crystallinity and orientation of molecules is dependent on the deposition conditions and interactions between them. Here we employ a thin ordered molecular template layer that controls the orientation of a subsequently-deposited light emitting layer comprising platinum-based phosphorescent molecules. We also investigated the relationship between molecular orientation and crystalline order and found the correlation between crystallinity and alignment of molecules. We achieve an increase of nearly 60% in horizontally aligned phosphorescent molecules relative to the substrate plane via the use of template layers when compared with molecules deposited directly onto the substrate surface. This preferential orientation leads to an approximately 40% increase of the emission intensity normal to the substrate plane. The method of employing a template layer has the potential to achieve high outcoupling efficiency in OLEDs.

All-polymer solar cells (all-PSCs), which consist of polymer donors and polymer acceptors, have been investigated to replace conventional polymer:fullerene PSCs in recent years since they have far superior mechanical strength and flexibility. To increase the power conversion efficiency (PCE) of all-PSCs, there are critical challenging issues, including enhancing electron mobility to reach the levels of fullerene derivatives, controlling the molecular orientation of n-type polymers, and the control of phase separation with intermixed network of nano-crystalline domains. Among them, molecular orientation, with respect to donor/acceptor interface and electrodes, plays a critical role in determining the performance of all-PSCs, but is often difficult to rationally control. Here, an effective approach for tuning the molecular crystallinity and orientation of naphthalenediimide-bithiophene-based n-type polymers (P(NDI2HD-T2)) by controlling their number average molecular weights (M_n) is reported. A series of P(NDI2HD-T2) polymers with different M_n of 13.6 (P_L), 22.9 (P_M), and 49.9 kg mol⁻¹ (P_H) are prepared by changing the amount of end-capping agent (2-bromothiophene) during polymerization. Increasing the M_n values of P(NDI2HD-T2) polymers leads to a remarkable shift of dominant lamellar crystallite textures from edge-on (P_L) to face-on (P_H) as well as more than a twofold increase in the crystallinity. For example, the portion of face-on oriented crystallites is dramatically increased from 21.5% and 46.1%, to 78.6% for P_L, P_M, and P_H polymers. These different packing structures in terms of the molecular orientation greatly affect the charge dissociation efficiency at the donor/acceptor interface and thus the short-circuit current density of the all-PSCs. All-PSCs with PTB7-Th as electron donor and P_H as electron acceptor show the highest PCE of 6.14%, outperforming those with P_M (5.08%) and P_L (4.29%).

Organic EL materials often show spontaneous orientation polarization in evaporated films with giant surface potential (GSP)[1]. This polarization induces positive and negative fixed charges on both ends of the polarized layer, leading to interface charges at organic/organic and organic/electrode interfaces in organic light-emitting devices. These interface charges often attract and repel carriers, and significantly affects the device performance. In this study, the impact of orientation polarization was investigated from the viewpoints of carrier behavior and electronic structures.
Displacement current measurement (DCM) was performed for ITO/a-NPD/Alq₃/Al and ITO/a-NPD/Al(7-prq)₃/Ca devices. Al(7-prq)₃ is a derivative of Alq₃; the position 7 is replaced by propyl group. These two molecules show opposite polarity of polarization; the cathode side of Alq₃ has positive interface charge, but negative for Al(7-prq)₃. DCM results suggested that positive interface charge near the cathode can support electron injection, while negative charge increases the resistance [2]. This trend was also supported by photoemission experiments as below.
By using low energy high-sensitivity ultraviolet photoemission spectroscopy (LE-UPS) and photoelectron yield spectroscopy (PYS), we have found unusual photoemission from films of Alq₃[3], BCP, TPBi and 4CzIPN with GSP. Even if the photon energy is smaller than their ionization energy, photoelectrons are clearly observed. This can be ascribed to photoemission from negative carriers (anion radicals) which are captured by fixed positive charges at the surface region due to the orientation polarization. Thus the ionization energy of anion corresponds to the electron affinity of the neutral. The estimated values of the electron affinity in this study tend to be about 1eV larger than that by inverse photoemission spectroscopy (IPES). This energy difference can be ascribed to the attractive interaction between the anion and the positive polarization charge at the film surface. This energetic stabilization suggests that the orientation polarization in OLED can assist the carrier injection, leading to the significant reduction of injection barrier. So, the control and enhancement of the polarization order is significant to improve the device performance.

References
(1) Y. Noguchi, Y. Miyazaki, Y. Tanaka, N. Sato, Y. Nakayama, T. D. Schmidt, W. Bruetting, and H. Ishii, J. of Appl. Phys. 111, 114508(2012).
(2) Y. Noguchi, H. Lim, T. Isoshima, E. Ito, M. Hara, W. W. Chin, J. W. Han, H. Kinjo, Y. Ozawa, Y. Nakayama, and H. Ishii, Appl. Phys. Lett. 2013, 102, 203306.
(3)H. Kinjo, H. Lim, T. Sato, Y. Noguchi, Y. Nakayama and H. Ishii
Appl. Phys. Exp., 9, 021601-1(2016).

We systematically designed and synthesized 2D-conjugated n-type polymers based on naphthalenediimide and focused on the effect of conjugated side chain on the polymer properties. The phenyl rings having different alkoxy chain were directly introduced on naphthalenediimide to synthesize three different 2D-conjugated polymers. The optical and electrochemical properties of newly synthesized 2D-conjuated polymers were investigated and the structural change of the polymers in terms of the length of conjugated side chain was deeply explored using molecular simulation. In addition, the photovoltaic performance as polymer acceptor was examined by fabricating all-PSCs with polymer donor, PTB7-Th. The 2D-conjuated polymer having buthyloctyl chain as solubilizing group (PBO) showed optimal device performance which was determined by two major factor in this system, solubility and steric hindrance, and finally the highest power conversion efficiency (PCE) of 4.31 % was achieved using PBO among three different polymers.

Organic semiconductors are promising photovoltaic materials that can efficiently separate charge despite large exciton binding energies. To design improved organic solar cell materials, the mechanisms that enable efficient charge separation in organic semiconductors must be better understood. Intriguingly, Rumbles has proposed that in good organic photovoltaic systems, charge recombination is slow because the exciton and charge separated states are set up for optimal Marcus electron transfer, while the exciton to charge transfer state electron transfer is in the Marcus inverted region [1-2]. Scanned probe microscopy measures topography, surface potential and tip-sample capacitance at nanometer resolution, allowing researchers to explore spatial variations in charge generation and transport. In particular, the tip-sample photocapacitance charging rate, measured using time-resolved electric force microscopy (tr-EFM), has been shown to be proportional to external quantum efficiency in benchmark photovoltaic systems [3–5]. The time resolution of the tr-EFM measurement, however, is fundamentally limited by detector and demodulation bandwidth. The well-understood physics of cantilever dynamics can be exploited to arrange an indirect measurement that evades detector and demodulation bandwidth limitations [6–8]. We use the cantilever as a gated integrator to measure photocapacitance indirectly, measuring the change in cantilever phase when the cantilever's dynamics are altered by precisely timed tip voltage and sample-illumination pulses. We demonstrate that this indirect, "phasekick" technique gives results consistent with the direct tr-EFM experiment for time constants from 40 µs to 10 ms. The technique's ability to measure sub-cycle, nanosecond dynamics is verified by measuring the 10s of nanosecond probe wiring time constant in 100 milliseconds of total acquisition time. With access to nanosecond charging dynamics, we can test the Marcus inverted region hypothesis for high performance organic donor/acceptor blends prepared on standard electrodes.
[1] Coffey, D. C. et al. J. Phys. Chem. C, 116, 8916–8923 (2012).
[2] Rumbles, G. Excited State Processes in Electronic and Bio Nanomaterials. Santa Fe, New Mexico (2014). Conference Presentation.
[3] Coffey, D. C., and Ginger, D. S. Nat. Mater. 5, 735–740 (2006).
[4] Giridharagopal, R. et al. Nano Lett. 12, 893–898 (2012).
[5] Karatay, D. U. Rev. Sci. Instrum. 87, 053702 (2016).
[6] Moore, E. W. et al. Proc. Natl. Acad. Sci. U.S.A. 106, 22251–22256 (2009).
[7] Inami, E. and Sugimoto, Y. Phys. Rev. Lett. 114, 247603 (2015).
[8] Murawski, J. et al. J. Appl. Phys. 118, 154302 (2015).

The structural and morphological properties of the buried interfaces, in organic photovoltaic (OPV) devices, significantly affect the device performance. For example, in OPV devices, the fill factor (FF) and series resistance (R_s) are largely dependent on the homogeneity of donor-acceptor bulk heterojunction (BHJ) layer, intermixing of BHJ and transport layers at the interfaces. An understanding of the correlation between the structural properties of various interfaces involved in OPV devices and their electrical characteristics is essential. A combination of structural and electrical characterization tools are required to understand this correlation.
In this report, we present an investigation of the buried interfaces in OPV device with inverted architecture, ITO/ZnO/PTB7-Th:PC₇₁BM (BHJ)/MoO_x/Ag, fabricated in ambient condition. We have used electron transport layer (ZnO) processed by two different methods, sol-gel based (high temperature) and diethylzinc (DEZ) based (low temperature) processes. The sol-gel ZnO and DEZ ZnO based OPV devices have exhibited the power conversion efficiency (PCE) ~ 5.8 % and ~ 6.3 %, respectively.
ITO/ZnO, ITO/ZnO/BHJ, ITO/ZnO/BHJ/MoO_x structures were fabricated in ambient condition for performing the X-ray reflectivity (XRR) measurements at each interface similar to the actual devices. Scattering length density (SLD) has been derived for each layer by the fitting of XRR data using Parratt’s recursive method. We observed a signature of additional intermixing zones at the interfaces of ZnO/BHJ and BHJ/MoO_x, which may be originated due to the surface roughness and/or molecular interdiffusion. Our results show smoothening of interfacial junctions probably due to insertion of PC71BM molecules into roughness, arises at ZnO/BHJ interface. Such change at this interface is expected to improve charge transport. However, SLD value of interface at the junction of BHJ/MoO_x is found to be close to MoO_x layer. It can be attributed to the planarization of active layer (BHJ) roughness through the intrusion of thermally evaporated MoO_x molecules.
We have also characterized the individual layers of OPV devices for morphology and current-voltage (I-V) spectroscopy in the dark using conducting mode Atomic Force Microscopy (c-AFM). The results of AFM are consistent with the findings of XRR measurements. In this presentation, a comparative study of structural and electrical properties of the buried interfaces involved in sol-gel ZnO and DEZ ZnO based inverted OPV devices will be discussed.

The control of the bulk heterojunction (BHJ) morphology in polymer/polymer blends remains a critical hurdle for optimizing all-polymer solar cells (all-PSCs). The relationship between donor/acceptor phase separation, domain size, and the resulting photovoltaic characteristics of poly{2,5-di(2-thienyl)thiophene-alt-6,7-difluoro-2,3-bis(3,4-bi(octyloxy)phenyl)quinoxaline)} and poly{N,N’-bis(2-octyldodecyl-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5’-(2,2’-bithiophene)}-based all-PSCs was investigated. We varied the film-processing solvents (chloroform, chlorobenzene, o-dichlorobenzene and p-xylene), thereby manipulating the phase-separation of all-polymer blends with the domain size in the range of 30-300 nm. The different volatility and solubility of the solvents strongly influenced the aggregation of the polymers and the BHJ morphology of polymer blends. Domain sizes of all-polymer blends were closely correlated with the short-circuit current density (JSC) of the devices, while the open-circuit voltage (0.80 V) and fill factor (0.60) were unaffected. All-PSCs with the smallest domain size of ~ 30 nm in the active layer (using chloroform), which is commensurate with the domain size of highly-efficient polymer/fullerene solar cells, had the highest JSC and power conversion efficiency of 5.11% due to large interfacial areas and efficient exciton separation. Our results suggest that the BHJ morphology was not fully optimized for most of the previous high-performance all-PSC systems, and their photovoltaic performance can be further improved by fine-engineering the film morphology, i.e., domain size, domain purity and polymer packing structure.

Since most organic light emitting diodes (OLEDs) have air-sensitive layers and interfaces near the top cathode, encapsulation of device is usually required [1]. In contrast, inverted OLEDs with a bottom cathode is expected to have more air-stability because they use inert oxide materials such as indium tin oxide (ITO) for the cathode, instead of reactive metals such as aluminum [2]. Furthermore, in the fabrication process of inverted OLEDs, electron injection layer (EIL) can be formed before depositing organic layers, which makes more materials and processes applicable to EIL [3].
One major problem of inverted OLED is high driving voltage, which is associated with the large injection barrier between the cathode and organic layer. Since Zhou et al. proposed polyethyleneimine (PEI) for a universal surface modifier that reduces work function of several electrodes by about 1 eV [4], efficient inverted OLEDs utilizing PEI as EIL has been reported [3].
The improvement can be attributed to a better energy alignment between the cathode and organic layer, which is explained in terms of intrinsic molecular dipole moments and charge-transfer character of amine groups contained in PEI [4]. The discussion is so far based on the electronic structure of the PEI/electrode interface and does not consider the influence of the contact between the organic layer and either electrode or PEI. The effect of PEI on the vacuum level shift at these contact and orientation polarization, which organic materials used in OLEDs often have, is still unclear.
In this study, we fabricated Bebq₂/PEI/ITO model interfaces and measured electronic structure of each interface and bulk of Bebq₂ by using high-sensitivity ultraviolet photoemission spectroscopy (HS-UPS). The result indicates that PEI reduces the work function of ITO by 1.4 eV and small vacuum level shift at the Bebq₂/ITO and Bebq₂/PEI, and reduced injection barrier from 1.8 eV to 0.6 eV. In addition, our high-sensitivity measurement revealed that a state in Bebq₂ layer emerges at 1.4 eV above the Fermi level. The observed shallow state can be related to the good electron injection nature of the interface. The details of HS-UPS spectra as well as photoelectron yield spectra will be also presented in relation to weak structural order in the film leading to orientation polarization.
[1] P. E. Burrows et al., Display 22, 65 (2001)
[2] H. Fukagawa and T. Shimizu, NHK STRL R&D 145, 48 (2014, in Japanese)
[3] H. Fukagawa et al., Appl. Phys. Express 7, 082104 (2014)
[4] Y. Zhou et al., Science 336, 327 (2012)

Solution shearing has been demonstrated as a promising approach to induce highly aligned crystalline structures. However, there are only a few studies focused on the alignment of semiconducting polymers due to their chain entanglement effects, which can severely hinder the alignment and π- π stacking of polymer chains. Herein, a synergistic combination of ultraviolet-irradiation and solution aging technique has been proposed to facilitate the self-assembly of P3HT into long nanofibrillar structures prior the fast solution shearing process. It was found to be more straightforward to control nucleation and crystal growth directly in the solution state. The degree of polymer aggregation can be easily modulated via the sequential application of both solution treatments. The surface morphologies of deposited films reveal that the degree of chain alignment was greatly improved with increased polymer assembly, which can be controlled by the solution aging time. The development of birefringence and increased dichroic ratio determined from polarized optical microscopy and UV-Vis, respectively, provide further support for the macroscopic ordering and polymer chain alignment. According to image analysis, which provides insight into the orientation distribution of polymer chains, the P3HT nanofibers are shown to align in the solution shearing direction, leading to significant charge transport anisotropy. The correlations between highly oriented nanofibrillar structures and their two dimensional charge transport properties were systematically investigated by shearing the pretreated solution parallel and perpendicular to the active channel. Spin coated thin films, which exhibit randomly oriented nanofibrillar structures will also be discussed. This facile solution coating method suggested an effective approach to investigate the charge transport behavior within long-range and highly oriented nanofibrillar structures, which can be obtained by directly controlling their intrinsic solution properties without the need for extrinsic techniques such as grooved substrates or blade patterning.

Halogenation is an effective and facile way to tune the energy levels and phase behaviors of organic semiconducting materials. However, the fluorinated organic semiconductors are far more used in the fabrication of polymer solar cells (PSCs) than the chlorinated materials. Herein, in order to enrich the diversity of chlorination chemistry, we designed and synthesized a series of chlorinated polymers having donor-acceptor (D-A) structures, in which the benzo[1,2-b:4,5-b]dithiophene-chlorinated benzothiadiazole and the asymmetric alkyl thiophene are conjugated through the thiophene π-bridge. It was found that these chlorinated polymers showed reduced molecular planarity, and lower highest occupied molecular orbital (HOMO) energy level. Combined with the highest photovoltaic conversion efficiency of 8.17% with about 250 nm active layer, it can be concluded that the chlorinated polymers behave as well or better than their fluorinated counterparts. Although the introduction of one large chlorine atom increases the torsional angle of the polymer backbone, the chlorinated polymers still exhibit high crystallinity, good mobility, and favorable backbone orientation in the blend films, all of which contribute to the respectable device performances from the thick-film devices.

Solution processable conjugated polymers are being intensively studied as potential materials for flexible electronics applications. Recent studies have shown that high degrees of polymer backbone alignment in solution processed thin films sensitively influence the charge transport properties. Although multiple methods have been demonstrated to induce the alignment of conjugated polymers, it remains a challenge to control thin film orientational order during the solution coating/printing process for the lack of mechanistic understanding of the multiscale assembly process during coating/printing. In fact, the literature often reports conflicting results that for similar systems, the orientational order of the coated thin films can vary from almost isotropic to highly aligned. Herein we show that it is important to consider the interfacial morphology separately from the bulk morphology, which is also directly relevant to charge transport, which occurs predominantly in the interfacial region. We observed distinct interfacial alignment compared to the bulk film in solution coated diketopyrrolopyrrole based conjugated polymer thin films. Grazing incident X-ray diffraction results indicated that the preferred molecular stacking orientation near the film surface was orthogonal to that in the bulk film with a higher molecular stacking anisotropy at the film surface. Meso-scale morphology at the top air-film interface revealed the presence of nanofibers aligning parallel to the coating direction. On the contrary, the buried film-substrate interface contains significantly smaller nanofibers which less degree of alignment. As a result, higher mobility anisotropy was observed at the top surface of the film with aligned crystalline nanofibers. We further propose the mechanism of interfacial alignment by considering the anisotropic mass and momentum transport processes during meniscus-guided unidirectional coating/printing.

We present the first crystal structure of a photoconducting ionic porphyrin supermolecular assembly, fabricated from tetra(N-methyl-4-pyridyl)porphyrin (TMPyP) and tetra(4-sulfonatophenyl)porphyrin (TSPP) in a 1:1 stoichiometric ratio. Rod like crystals large enough for single crystal diffraction studies were grown by utilizing a nucleation and growth model described in our previous work.^[i] {Adinehnia, 2014, Predicting the Size Distribution in Crystallization of TSPP:TMPyP Binary Porphyrin Nanostructures in a Batch Desupersaturation Experiment}This model provides practical guidelines for controlling growth of nano to micro-sized organic structures in general. The unit cell of the TMPyP:TSPP crystals is monoclinic P21/c and the cell constants are a = 8.3049(11) Å, b = 16.413(2) Å, c = 29.185(3) Å, = 92.477(9)°. These crystals have smooth well defined facets and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions that are stacked face to face. Additional microscopic and spectroscopic characterization of the TMPyP:TSPP solid was performed using XRD, AFM, SEM, TEM and DRS UV-vis. For the first time crystal morphology (habit) of an ionic porphyrin solid is predicted by using the crystal structure data and applying the Bravais–Friedel, Donnay–Harker model and Attachment energy (AE) model. The predicted habit is in good agreement with the experimental structural morphology observed in the AFM, SEM and TEM images of the TMPyP:TSPP crystalline solid. The TMPyP:TSPP crystals are non-conducting in the dark but become photoconducting with illumination; their a photoconductivity action spectrum closely parallels the UV-vis absorption spectrum. The photoconductivity is linearly proportional to the light intensity of the laser up to 40 mW throughout the visible. The photoconductive response is significantly faster with excitation in the Q-band than with excitation in the Soret band. Field effect experiments show that the TMPyP:TSPP crystals are n-type semiconductors. In order to understand/explain the photoconduction of the crystalline porphyrin assemblies, DFT calculations were performed to determine their electronic band structure and density of states. Based on the theoretical results we propose a model in which two types of photoconductivity occur: (1) a direct band conduction which occurs at all excitation wavelengths and (2) a hopping conductivity which is facilitated by the formation of metastable photoinduced defects that are primarily formed at higher energy excitations. The TMPyP:TSPP crystalline system is a useful supermolecular model structure for quantitative studies that combine the elements of molecular organization and morphology along with theory and correlate them with electronic and optical electronic properties for engineering highly-organized functional materials from organic p-conjugated molecules.

A current challenge in polymer-based bulk heterojunction (BHJ) solar cells is to push the efficiency to a level where they can become economically viable while keeping the attractive simplicity in materials and processing. Optimization of the electron donor active layer plays a crucial role in improving the efficiency of the solar cells. One of the parameters that must be optimized is the light absorption of the donor polymer across the solar spectrum with a high absorption coefficient. Broadening the spectral absorption is one of the key factors usually targeted via the perfectly alternating donor/acceptor approach, where electron-rich donor and electron-deficient acceptor units are polymerized in an alternating fashion along the polymer backbone, resulting a narrow polymer band gap (E_g). However, the primary weakness of the perfectly alternating donor/acceptor strategy is that the polymer absorption profile is frequently observed to red-shift as opposed to broadening in the visible and near-infrared regions. The resulting decrease in the polymer absorption in the visible region of the solar spectrum can hinder the desired increase in the short circuit current density (J_sc) and ultimately the efficiency.
Random conjugated polymers have been developed in order to overcome the limitations of the perfectly alternating donor/acceptor approach where multiple chromophores can be used in a random fashion. Recent studies have shown that randomized incorporation of the donor and acceptor units in the polymer backbone generates broadened absorption in the visible and near-IR region that significantly increases the J_sc value of the solar cell, and the corresponding efficiency. In addition to increased absorption, the random approach is an attractive tool for tuning the open circuit voltage (V_oc) values by simply varying the donor/acceptor composition on the polymer backbone, leading desirable solar cells with tunable properties.
In this study, we developed a family of random copolymers with varying optical and electronic properties. The goal is to expand the conjugated polymer library for ternary blend BHJ solar cells by designing synergistic components that are eligible for organic alloy formation. Toward this end, we synthesized random donor-acceptor copolymers by incorporating two to three acceptor monomers into the polymer backbone based on carbazole unit as a donor and DTBT, DPP and TPTI units as acceptors. These polymers have been fully characterized including optical, electrochemical and photophysical properties and tested on BHJ solar cells.

Organic semiconductors offer a wide range of possible applications, from thin-film transistors to sensors and solar cells. In all these applications, the desired physical properties (optical and electronic) of a given organic semiconductor are critically linked to its final structure. On another front, 2D materials, such as graphene, molybdenum disulfide (MoS₂) and others, enable a myriad of optoelectronic applications. In many cases, the functionalization of a 2D material surface, which modulates a physical property of interest, is required in order to improve device performance (e.g., doping level control; work function tuning). The present work brings these two fronts together by demonstrating a highly ordered retinoic acid (RA) monolayer on 2D materials (graphite and MoS₂). Scanning probe microscopy (SPM) techniques along with optical spectroscopy (photoluminescence and Raman) and theoretical calculations were employed to study the physical properties of these systems. RA is a semiconducting organic dye molecule, which is optically active in the visible region. In the RA/graphene system, the monolayer emission efficiency was observed to remain constant even at low temperatures, indicating the organized molecular arrangement, which was also confirmed by atomic force microscopy. Additionally, electrical force microscopy and scanning Kelvin probe microscopy photo-assisted experiments are consistent with graphene doping and reveal photo-charge density generation and surface potential change on the RA/graphite system via monolayer excitation. Raman spectroscopy measurements and ab initio calculations indicated a graphene p-type doping of the order of 10¹³cm^-2 corroborating with SPM experiments. The results from the RA/MoS₂ system indicate that RA molecules are further organized on three preferential directions. Thus, as a signature of structure-property relationship, the RA-monolayer/supporting-substrate interaction influences its radiative charge recombination: the RA/MoS₂ emission efficiency is larger than the RA/graphite, suggesting the presence of significant quenching mechanisms of the luminescence on the last system. In summary, the realization of highly organized structures atop 2D materials may have an important impact on both design and operation of π-conjugated nanomaterial-based hybrid systems.

We have investigated the optical and electrical properties of poly[2,5-bis(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (BDMO-PPV) and poly[(9,9-dihexylfluoren-2,7-diyl)-co-(9-ethylcarbazol-2,7-diyl)] (PDHF-ECZ) blends on top PMMA by photoluminescence (PL) and optically assisted electrostatic force microscopy (EFM). We observed a giant shift in the PDHF-ECZ surface potential (contact potential difference) of about 2eV when a 371nm laser is focused on top of the EFM tip and the EFM phase signal versus bias is monitored. The dependence on laser power was measured and we have observed two distinct regimes: one, for low power, where there is a shift in surface potential but no significant change in the capacitance of the system; and a high power regime, where its surface potential tends to recover the no-illumination situation and the second derivative of the system capacitance increases. We have associated these results with photo-oxidation of the sample, which lowers its surface potential and changes its capacitance (due to changes in its permittivity). Such hypothesis is corroborated by PL measurements. We have also observed that, for PDHF-ECZ, the photo-assisted EFM phase signal is not a symmetric parabola as a function of applied bias, but rather there is a preferred bias where the illumination effect is minimal. We have associated such preferred direction with an electric dipole of the polymer on the PMMA surface. When the same experiments are carried out on a blend PDHF-ECZ: BDMO-PPV (1:11 and 1:25), we observed no significant illumination effect, no giant shift in surface potential and no capacitance change. These results are associated with induced charges transferred to BDMO-PPV and, hence, there are no net surface charges which could lead to a change in surface potential. In summary, the present work illustrates the possibility of observing change transfer using optically-assisted EFM measurements.

Side-chain substitution may control by large extend the conjugated polymer structure and thus its electronic and optical properties. Tuning polymer order or aggregation was obtained by several approaches - either by blending semicrystalline with amorphous components or via variation of aggregation obtained from solution. It is shown that structure indeed controls or dictats opto-electronic properties. For example photovoltaic devices can be either limited by charge generation or by charge recombination, depending on the state of order and aggregation.

Microphase separated block copolymers provide an opportunity to control the mesoscale structure in the active layer of organic electronic devices. Donor-acceptor block copolymers have been shown to self-assemble into lamellar morphologies in the active layer, leading to power conversion efficiencies in photovoltaic devices as high as 3% with remarkable open-circuit voltages above 1.2 V. These materials can also serve as testbeds to address fundamental questions regarding charge photogeneration. For example, we have explored the role of crystal texturing in microphase-separated active layers on the performance of solar cells. We have also utilized conjugated block copolymers as model donor-acceptor complexes, allowing us to explore the role of energetics and dielectric constant on the formation of intramolecular charge transfer states using steady state and ultrafast spectroscopy. Furthermore, coupling a strongly crystallizable block to a weakly crystalline block perturbs crystallization within films; in particular, order within the weakly crystalline phase is enhanced. As a consequence, electron mobilities higher than either of the homopolymers are extracted using bottom gate, bottom contact transistors with aluminum source and drain contacts and block copolymer active layers. Thus, our results suggest that block copolymer architectures can be used to enhance charge mobilities in polymer semiconductors.

'Post-polymerization functionalization is a versatile technique to decorate polymers with functionalities which are incompatible with the conditions of polymerisation. Here we report the direct post-polymerisation functionalization of conjugated polymer backbones with functionalised thiols, in high yield. A series of co-polymers containing either the fluorinated benzothiadiazole or benzotriazole units, are utilised. We demonstrate the potential of this approach with two examples:

1) Enhancing thermal stability of devices

The poor thermal stability of organic solar cells constitutes major barrier to achieving commercial viability. Cross-linking the blend components in the device has been a popular approach to solve this issue. We report the novel incorporation of moisture sensitive trimethoxysilane groups onto the backbone of a conducting polymer (PCPDTFBT). The resulting polymer undergoes cross-linking without the need for additives, external stimuli or exposure to air. Optimised solar cells with the modified material gave an impressive 22% increase in the power conversion efficiency (PCE) after prolonged heating (336 h at 120 °C) compared a 64% decrease in PCE for the non-modified analogue.

2) Enhancing Polymer Absorption

Conjugated polymers have limited absorption ranges, and a proportion of sunlight that reaches a solar cell is not therefore utilised by the polymer. In this example we report the incorporation of a thiol -functionalised fluorescent dye (coumarin) onto a conjugated polymer backbone. The dye absorbs strongly at wavelengths where the polymer is weakly absorbing, but emits where the polymer absorbs strongly. The incorporation of the dye resulted in an overall increase in absorptivity of the polymer and a reduction in the lifetime of the fluorescence, suggesting occurrence of a FRET mechanism. When integrated into solar cells, the resulting device had a 9% increase in short circuit current and an increase in the external quantum efficiency at wavelengths where the dyes absorbs in comparison to polymers without the dye attached.

The performance of organic solar cells and photodetectors are intricately tied to the phase morphology of the donor and acceptor components that make up the typical bulk heterojunction structure. The processes that make these devices work (charge generation, charge transport and extraction, and charge recombination) are all dependent on the details of the morphology from the molecular scale to the mesoscale. To produce efficient devices, charge carriers must be quickly extracted from the active layer, and this property is typically characterized by determining the mobility of the charge carriers. While a great deal of experimental and theoretical work has been focused on relating the morphological structure on molecular length scales to the charge carrier mobility, understanding of how the nano- to meso-scale morphological features affect the mobility is still lacking. Determining the three-dimensional connectivity of the donor- and acceptor-rich domains and understanding how this structure impacts charge transport is of great interest.

One particularly useful metric for quantifying the quality of three-dimensional connectivity is the tortuosity of the charge transport pathways. Morphologies with good domain connectivity and direct charge transport pathways have a low average tortuosity and should demonstrate a higher charge carrier mobility than a morphology with a high average tortuosity. The average tortuosity of a particular morphology will depend critically on the film processing conditions, so fabrication procedures should be optimized to reduce tortuosity. However, determining the tortuosity via tomographic techniques is very challenging and time-consuming. Here, we perform simple phase separation simulations and show how the interaction energy between components can affect the average tortuosity of the resulting morphologies. Using these model bulk heterojunction morphologies and kinetic Monte Carlo simulations, we then show the fundamental effect that tortuosity has on the electric-field and temperature dependence of the charge carrier mobility. We further propose that measurements of the field-dependence of the mobility can be used to determine the quality of the charge transport pathways and, as a result, can be used as a tool to help optimize the morphology of bulk heterojunction blends in cases where full three-dimensional morphology characterization is too difficult.

Solution-processable small molecule (SM) donors are promising alternatives to polymer donors in bulk-heterojunction (BHJ) solar cells with fullerene acceptors.[1] While BHJ device efficiencies have quickly improved from ca. 4%[2] to >9%[3] in recent years, further efficiency improvements with SM donors are bound to forging a more precise understanding of how molecular structure and functional substitutions impact the development of molecular packing effects, BHJ morphologies and carrier transport in thin films. Our recent studies with various sets of analogous SM donor systems show that: (i) the pattern of solubilizing alkyl- and ring-substituents appended to the π-extended backbone governs the morphologies achievable with SM donors;[4] (ii) the role of processing additives and solvent-vapor-annealing steps is not independent of the molecular structure of the SM used in the donor/acceptor blend;[5] and (iii) various donor and acceptor unit sequences along the π-extended backbone induce distinct molecular packing effects and charge transport patterns in thin films,[6] which in turn impact material and BHJ solar cell efficiencies. By successive molecular structure derivatizations our studies show that SM donors can perform increasingly well, achieving BHJ device efficiencies ranging from ca. 1% to >8% upon concurrent optimizations of processing parameters and device configuration.[7] Our parallel effort on SM acceptors that can serve as fullerene alternatives in efficient BHJ solar cells with polymer donors (4-10%) indicates that adequate functional end-group substitutions can effectively promote the electron-transport character of the π-extended backbone,[8] while the exact role of the π-bridge remains a matter of examinations.[9] Interestingly, efficient SM acceptors tend to form relatively disordered morphologies,[8] which contrast with the textured morphologies obtained when some of the best-performing SM donors are blended with fullerenes. Understanding the origin of those differences, the relative selection rules for the polymer donor counterpart, and the interplay between electron donor and acceptor components in the BHJ, remains an important step to take in furthering the efficiency of SM acceptors for BHJ solar cells.

[1] T.-Q. Nguyen et al. Chem. Mater. 2011, 23, 470-482; X. Zhan et al. Chem. Soc. Rev. 2012, 41, 4245-4272; G. C. Bazan et al. Acc. Chem. Res. 2014, 47, 257-270; [2] T.-Q. Nguyen et al. Adv. Funct. Mater. 2009, 19, 3063; [3] D.J. Jones et al. Nat. Commun. 2015, 6, 6013; Y. Chen et al., JACS 2015, 137, 3886; [4] P. M. Beaujuge et al. Chem. Mater. 2016, 28 (7), 2058–2066; [5] P. M. Beaujuge et al. 2016, Submitted; [6] P. M. Beaujuge et al. 2016, Submitted; [7] P. M. Beaujuge 2016, Submitted; [8] P. M. Beaujuge et al. Chem. Mater. 2016, 28 (7), 2200–2208; [9] X. Zhan et al. Adv. Energy Mater. 2015, 5, 1501063.

Conjugated polymers have received a lot of attention since they combine the best features of metals or semiconductors with those of synthetic polymers. For instance, solar cells based on poly(2,7-carbazole) and poly(thienopyrroledione) derivatives have revealed power conversion efficiencies up to 8-9 %. This class of materials could lead to printable and flexible photovoltaic devices as well as other plastic electronic devices. Along these lines, we will describe novel synthetic methodologies for a simple and “green” preparation of such well-defined conjugated polymers. These new synthetic methods are based on direct (herero)arylation reactions that allow the formation of carbon-carbon bonds between heteroarenes and aryl halides. They do not require organometallic intermediates thereby significantly reducing synthetic steps, metallic by-products, and cost.

The morphology of organic semiconductor films, which often includes ordered, disordered and mixed domains, determines the overall device performance. However, due to the “soft” features of the materials, the morphology is prone to kinetically and/or thermodynamic changes with time, temperature and treatments. For example, provided the appropriate driving force, molecules can diffuse through the active layer. In this presentation, we will discuss this diffusion, possible driving forces, outcome and applications. We will show that diffusion of selected additives from the active layer to the interfaces can be used to spontaneously generate interlayers that significantly enhance the device performance and stability. On the other hand, molecules can also diffuse from the gas phase into the pre-formed films. Because the diffusion depends on the morphology of the film, it can be used to map the film morphology. We demonstrate this approach on a variety of solar cell films composed of polymer-fullerene and small molecule-fullerene blends. The apparent differences between the diffusion through a small molecule blend and through a polymer-based blend and their implications on issues such as stability and doping will be discussed.

Fullerene Intercalation has been extensively studied; especially with poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT). The perceived benefits in exciton generation from intimate contact of the P-type (polymer) and N-type (fullerene) domains are far outweighed by the loss in free carriers due to recombination; leading to lower than expected PCEs. To better understand this phenomena, we characterized the pBTTT-Dimer : Fullerene system. This all-small-molecule intercalation, first observed by Zhang & Briseno et al., shows similar morphologic behavior to the parent polymer system but with marked differences in thermal and electronic properties. While these oligomers still suffer from similar performance limitations to the polymer analogues, these well-defined, monodisperse dimers of pBTTT offer unique insight into the thin film morphology and characterization of an intercalated system. We combine TEM, GIXD, NEXAFS, TA and DSC to comprehensively rationalize the thin film morphology and device performance through the full range of blend fractions.

Control over the bulk heterojunction (BHJ) morphology is a key issue in the development of stable and highly efficient organic photovoltaics (OPV). While most approaches to control BHJ morphology involve the use of annealing or solvent additives, these approaches are very limited. In contrast, molecular engineering approaches have the potential to offer unprecedented control over BHJ morphology by controlling the covalent connectivity and self-assembly of the BHJ components. Herein, we describe a novel “flexible linker” ¹ strategy to control the BHJ morphology in OPV. Efficient diketopyrrolopyrrole based donor small molecules (DPP(TBFu)₂) were chosen as a model system to explore the effect of the aliphatic linker connectivity on the self-assembly, crystallinity, and charge transport in neat films and BHJ devices. Different linking positions have been tested with a pair of dimers and a polymer which all retain the original DPP conjugated core. The flexibly-linked polymer was found to be an amorphous and electronically inactive nucleation promoter while dimers exhibited varying degrees of crystallinity and hole mobility similar to DPP(TBFu)₂. Differential scanning calorimetry and X-ray diffraction suggest that these new flexibly linked dimers promote alternative polymorphic crystal structures, opening new ways to engineer the crystal structure. In addition, we show that the flexible linker strategy can lead to significant improvement of the BHJ stability under thermal stress, by controlling the crystallization of the donor phase.²
1. Gasperini, A., Bivaud, S. & Sivula, K. Controlling conjugated polymer morphology and charge carrier transport with a flexible-linker approach. Chem. Sci. 5, 4922–4927 (2014).
2. Gasperini, A., Jeanbourquin, X. A., Rahmanudin, A., Yu, X. & Sivula, K. Enhancing the Thermal Stability of Solution-Processed Small-Molecule Semiconductor Thin Films Using a Flexible Linker Approach. Adv. Mater. 27, 5541–5546 (2015).

Here we will take a close look at charge generation in organic bulk heterojunction solar cells. Focus will be on discussing the key mechanisms during charge generation in polymer:fullerene blends and how they can be linked to specific structural features of these systems, in an attempt to resolving the long-standing question of how free charges are generated in such donor:acceptor blends that are used in organic solar cells. For this we link the picture of the generally complex phase morphology of polymer:fullerene where intermixed and neat phases of the donor and acceptor material co-exist, with Stark effect spectroscopy data (performed in Natalie Banerji’s laboratory at the University of Fribourg, Switzerland) obtained in the absence and presence of externally applied fields. Reconciling opposing views found in literature, we will demonstrate that the fate of photogenerated electron-hole pairs – whether they will dissociate to free charges or geminately recombine – is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower.

A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the open-circuit voltage by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton state called electron back transfer, which is detrimental to device performance [1]. In several polymer:fullerene blends, increased fullerene domain size has been correlated with a reduced triplet exciton yield due to increased charge delocalization. To test this, we study triplet formation in the various high performing bulk-heterojunction systems and determine the impact of the morphology-optimizing additive 1,8-diiodoctane. Using photoluminescence and spin-sensitive photoluminescence detected magnetic resonance measurements, we identify triplet excitons formation pathways via intersystem crossing and electron back transfer. The latter process is found to be significant only at low temperatures and does not represent a dominant loss pathway at room temperature. Using electrically detected magnetic resonance, the presence of triplet excitons was confirmed even at room temperature, highlighting that triplet excitons form via intersystem crossing during solar cell operation [2].
References
[2] M. Liedtke, et al., J. Am. Chem. Soc. 133, 9088 (2011).
[2] H. Krauss, et al., Sci. Reps. accepted (2016).

Spatially-resolved photoluminescence (PL) degradation of a polyfluorene blend commonly used in organic light emitting diodes, F8BT/PFO, was characterized using near-field scanning optical microscopy (NSOM). The emission from both the PFO-rich phase and the F8BT-rich phase was dominated by green fluorescence due to efficient energy transfer from PFO to F8BT. In the initial NSOM scans the emission was dominated by the PFO-rich phase. This behavior changed at longer operation times, where the emission was dominated by the F8BT-rich phase. Using macroscopic investigations we note that while the degradation of F8BT was driven by photo-bleaching and the absorption remained almost unchanged, both faster absorption and photo-bleaching processes dominated the degradation of PFO. Hence the efficiency of the energy transfer from the PFO to the F8BT is hampered, resulting in the contrast change observed by NSOM mapping. This implies that energy transfer does not protect the PFO from degradation and does not improve its resistance to oxidation. High resolution PL mapping therefore allowed visualizing the PL time dependence of phase compositions that do not exist macroscopically in equilibrium, and its correlation with PL degradation characterization of macroscopic blend compositions.

Understanding charge generation and energy transfer processes in conjugated polymers is critical in the development of high-efficiency organic solar cell (OPV) devices. Recent spectroscopic studies have shown the link between multi-phase (amorphous and semi-crystalline) morphologies and increased device performance and charge generation in polymer:fullerene blends [1], as well as in neat polymer films [2].

We present results from ultrafast (100 femtosecond resolution) transient absorption spectroscopy measurements of a series of neat poly(9,9-dioctylfluorene) (PFO) thin films. We systematically alter 2 variables: 1) the relative fractions of the amorphous, glassy-phase and the semi-crystalline, ordered beta-phase [3]; and 2) the pump wavelength in order to selectively excite each phase. This allows us to investigate the effect of varying both the film morphology and the location of the initial excited states on polaron and triplet generation processes. The effect of pump fluence on triplet and polaron generation rate and yield is also examined.

The key results of our study are the observations that ultrafast (sub-picosecond) charge generation is significantly increased in multi-phase PFO films containing beta-phase chains, and that charge yield is higher within these films when selectively exciting the beta-phase chains. We also observe, for the first time, ‘uphill’ energy transfer of excited state species from the lower energy beta-phase to the higher energy glassy-phase.
Our results highlight the importance of mixed-phase morphologies with energy offsets between phases for ultrafast generation of charges within neat polymer films, analogous to the donor:acceptor blends used in state-of-the-art OPVs. Knowledge of this structure-property relationship could be exploited in future OPV material and device design, through controlled manipulation of material crystallinity and smart patterning of active layers. Such design could increase early charge generation within both the donor and acceptor components of OPVs in addition to at the interfaces between components, potentially leading to higher efficiency devices.

1. J. K. Gallaher et al., Energy Environ. Sci. 8, 2713 (2015).
2. F. Paquin et al., J. Mater. Chem. C 3, 10715 (2015).
3. M. Grell et al., Macromolecules 32, 5810 (1999).

Recently, an elegant alternative approach have been introduced to overcome the photocurrent and the performance limitation of polymer:fullerene solar cells in a simple single-junction structure by implementing a sensitizer with complementary absorption profile into the host matrix.[1] The power conversion efficiencies over 10% have been reported for the ternary organic solar cells, mainly based on the enhanced short circuit current density (Jsc).[2] However, we have recently demonstrated the multi-functionality of the ternary blend concept by which the sensitizer addition adjusts the recombination mechanism and transport of the host matrix and consequently increases the fill factor (FF) in addition to the Jsc.[3] Most interestingly, the dependence of open circuit voltage (Voc) on the sensitizer concentration has been reported by several groups. Although different hypotheses are reported in literature to explain the origin of Voc increase, for example by using alloy model, [4] 2-charge transfer (2-CT) state/ or parallel like model,[5] or local morphological change at the interface,[6] no clear proof and distinguishable approach has been presented so far.
In this work, we investigate the influence of the sensitizer content on the Voc for different ternary blend systems, such as polymer1:polymer2:fullerene and polymer:fullerene1:fullerene2. The Voc shows a continuous gradual increase by increasing the sensitizer content (polymer2 / or Fullerene2), representing either a linear or sub-linear trend in each specific case. To illuminate on the origin of the Voc, we employ different optoelectronic spectroscopy techniques, such as electroluminescence (EL), photoluminescence (PL), and fourier transform photocurrent spectroscopy (FTPS). Furthermore, we perform Monte Carlo simulation to correlate the experimental results to the proposed morphological models. Lattice kinetic Monte Carlo (kMC) computational code, previously applied to the modeling of binary polymer:fullerrene solar cells, is extended to treat ternary blends. The unprecedented work allows to correlate nano-morphological features with measured Voc, a crucial capability for the design of optimized, high performance ternary blends.
We will discuss the achieved results in details for various prototype organic ternary systems.

[1] a) T. Ameri et al, Adv. Mater. 2013, 25, 4245; b) Q. An et al, Energy Environ. Sci. 2016,9, 281.
[2] J. Zhang et al, J. Am. Chem. Soc. 2015, 137, 8176.
[3] N. Gasparini, X. Jiao, T. Heumueller, D. Baran, G. J. Matt, S. Fladischer, E. Spiecker, H. Ade, C. J. Brabec, and T. Ameri,“Counterintuitive design rules yield ternary OPVs with reduced recombination”, Submitted to Nature Energy, 2016.
[4] a) Robert A. Street et al., J. Am. Chem. Soc. 2013, 135, 986; b) Petr P. Khlyabich et al., Adv. Funct. Mater. 2015, 25, 5557.
[5] Sandra Kouijzer et al., J. Photon. Energy. 2015, 5, 057203.
[6] Sonya A. Mollinger, et al. Adv. Energy Mater. 2015, 5, 1501335.

Conjugated polyelectrolytes have a conjugated backbone with ionic side chains. They unite the optoelectronic properties of conjugated polymers with water-solubility, and they assemble with charged biomolecules. This was exploited for DNA detection using a cationic polythiophe (CPT) [1]. Interestingly, the photophysical properties of the polymer in assemblies with short single DNA strands are strongly dependent on the sequence of the DNA. Very different absorption spectra are thus obtained when CPT is mixed with different DNA strands. We have investigated the conformation of CPT in the assemblies using analysis of the absorption spectra as well as resonance Raman spectroscopy, and related this to the excited-state properties determined by ultrafast spectroscopy. While we find that the polymer in PBS buffer solution has a tendency to form aggregates, especially at low temperature, the polymer chains are isolated from each other in the DNA assemblies, but differ in the extent of backbone planarity according to the DNA sequence.

[1] I. Charlebois, C. Gravel, N. Arrad, M. Boissinot, M. G. Bergeron, and M. Leclerc, "Impact of DNA sequence and oligonucleotide length on a polythiophene-based fluorescent DNA biosensor". Macromolecular Bioscience, Vol. 13, (2013), pp 717-722.

We report measurements of the homogeneous excitation linewidth of regioregular poly(3-hexylthiophene), a model semicrystalline polymeric semiconductor, by means of two-dimensional coherent photoluminescence excitation spectroscopy. At a temperature of 4 K, we find a linewidth that depends on microstructure but is always >140meV full-width-at-half-maximum, which is a significant fraction of the total linewidth. We discuss how such broad excitation spectrum is dominated by fluctuations in off-diagonal coupling between excitations and their bath, and how mesoscopic polarisation plays a role in driving these dynamics. The relationship between the underlying optical dephasing dynamics and solid-state microstructure is a general and fundamental illustration of structure/property relationships in macromolecular semiconductor materials.

Recent experimental results suggest that the spatial extent of charges in organic semiconductors significantly influences the carrier mobility and separation efficiency at the heterojunction interface of organic solar cells[1,2]. A deeper especially experimental insight in the localization of charges would therefore significantly improve the understanding of organic solar cells, transistors and light emitting diodes.
Here we investigate the delocalization of holes in the semicrystalline conjugated polymer PBTTT (Poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene]) by directly measuring the hyperfine coupling (HFC) between photogenerated polarons and bound nuclear spins using electron nuclear double resonance (ENDOR) spectroscopy. An extrapolation of the corresponding oligomer spectra reveals that charges tend to delocalize over 4.0-4.8 nm with delocalization strongly dependent on molecular order and crystallinity of the PBTTT polymer thin films. Density functional theory (DFT) calculations of hyperfine couplings confirm that long range corrected functionals properly describe the change in coupling strength with increasing oligomer size as confirmed by corresponding ENDOR spectra. Our discussion presents general guidelines illustrating the different pitfalls and opportunities when deducing polaron localization lengths from hyperfine coupling spectra of conjugated polymers.


[1] Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Nature 2014, 515, 384
[2] Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. Science 2012, 335, 1340

Compared to conventional photovoltaic devices, organic solar cells (OSC) suffer from large photo-voltage losses. Although various empirical rules exist to estimate the magnitude of these losses, their origin remains unclear. Here, we investigate energy losses due to thermalization of photo-generated charge carriers. By a combination of time-resolved experiments and numerical techniques we show that as the photo-generated carriers are transported to the electrodes a substantial amount of their energy (200-600 meV per carrier pair) is lost by gradual thermalization in the disorder broadened density of states (DOS). We demonstrate that thermalization in OSC is a relatively slow process which can extend up to a hundred ns or microsecond timescale. This enables the photo-generated charge carriers to be extracted before reaching thermal equilibrium – not all excess energy is lost.
We identify thermalization in OSC as a two-step process: on an ultrafast (ps-ns) time scale a large fraction of excess energy (50-200 meV) is wasted by fast diffusive motion. Following this step, an additional loss of the order of 25-100 meV occurs while charge carriers drift to the electrodes. The magnitude of both of these losses may possibly be reduced by rational material design and morphology control.
Since thermalization occurs downhill in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by experiments. We identify the time and distance scales relevant for charge extraction and show that experimental techniques, which probe the mobility of (almost) thermalized charge carriers are not meaningful to OSC operation. Therefore, for the correct description of OSC operation, non-equilibrium effects related to the gradual thermalization of the photo-generated carrier populations must be taken into consideration.

Narrow bandgap conjugated polymers have garnered recent interest based on their performance in optoelectronic devices. Specifically, PCPDTBT has been studied as a donor material in organic solar cells and regioregular PCDTPT has been studied in organic field effect transistors. However, the secondary structure of donor-acceptor conjugated polymers remains poorly understood. Optical properties, more specifically chiroptical properties, can provide useful structural information. To study the chiroptical properties of these two polymers, the racemic 2-ethylhexyl side chain was substituted with a chiral 2-(S)-ethylhexyl side chain to produce PCPDTBT* and PCDTPT*. The only structural difference between the PCPDTBT and PCDTPT is the replacement of the –CH group with a pyridal nitrogen on the benzothiadiazole ring. Assuming the asymmetry is translated from the side chains to the optically active backbone, these two polymers can be studied using circular dichroism (CD) spectroscopy.

Optical absorption and CD spectroscopies reveal well-dissolved polymer chains in good solvents and chiral aggregates in poor solvent systems in the case of both polymer structures. However, chiral ordering is found to translate to the solid state microstructure of PCPDTBT* but not PCDTPT*. A possible explanation for the differing chiroptical properties of PCPDTBT* and PCDTPT* is rooted in differences in the rotational barrier of their monomer units. DFT calculations reveal a higher rotational barrier for the CDT-PT monomer compared to the CDT-BT monomer. A higher rotational barrier hinders the twisting of the PCDTPT* chains compared to PCPDTBT* necessary to form a helical structure as observed by CD spectroscopy. Thus, the preference of PCDTPT* to form a planar structure likely contributes to its OFET performance.

Organic semiconductors in amorphous films typically exhibit exciton diffusion lengths (L_D) that are much smaller than the optical absorption length. This trade-off limits the practical thickness of planar heterojunction organic photovoltaic cells (OPVs). Crystallization and molecular ordering can be used to manipulate the spatial arrangement of molecules, enhancing the electrostatic coupling between nearby transition dipoles to increase the rate of excitonic energy transfer, and L_D. Here, the electron donor boron subphthalocyanine chloride (SubPc) is templated by 5,5’-bis(4-biphenylyl)-2,2’-bithiophene (BP2T), resulting in an over 60% increase in L_D, as measured in a planar heterojunction OPV by a photovoltage-based technique. This improvement is attributed to an increase in the exciton diffusivity, as the exciton lifetime is unaffected by the templating. Calculations based on the SubPc crystal unit cell show a reduction in the intermolecular spacing and an enhancement in molecular coupling due to spatial orientation upon crystallization. A narrowing of the excitonic density of states could also contribute, increasing the rate of energy transfer and mean squared displacement. These effects and their implications will be discussed. Devices constructed using templated films show ~20% improvement in responsivity, primarily arising from enhanced harvesting of SubPc excitons. These results generalize the use of molecular templating to exciton transport among non-planar phthalocyanines, and demonstrate the feasibility of this method to realize more efficient exciton harvesting in planar heterostructures.

The photophysics of isolated chains and a bulk film of conjugated polymers such as poly(3-hexylthiophene) (P3HT) differ substantially [1]. Most prominently, the fluorescence quantum yield drops by at least one order of magnitude upon the transition from dissolved chains to the bulk film. The mechanisms behind these differences, however, are still not fully understood.
In this contribution we show that we can clarify the question of fluorescence quenching in conjugated polymers by using time-resolved fluorescence microscopy techniques which can be applied to single polymer chains as well as to isolated aggregates consisting of several polymer chains. By investigating the photon statistics in fluorescence from a single chain, we show that the fluorescence quenching mechanism in isolated chains of P3HT can be attributed to the efficient formation of long-lived triplet states, which are able to quench the fluorescence of the entire chain by singlet-triplet annihilation [2].
By applying solvent vapor annealing, we are able to grow aggregates of a controlled number of conjugated polymer chains [3]. Single-aggregate fluorescence correlation spectroscopy identifies additional “dark” states, which are more rapidly formed compared to the triplet state and effectively quench the triplets. We attribute these “dark” states, which only evolve in the interchain mesoscopic size regime of P3HT, to charge transfer states, which play the main role in the strong quenching of fluorescence at the transition from isolated molecules to the bulk film.


[1] A. Thiessen, J. Vogelsang, T. Adachi, F. Steiner, D. Vanden Bout, and J. M. Lupton, "Unraveling the chromophoric disorder of poly(3-hexylthiophene)", Proc. Natl. Acad. Sci 110, E3550 (2013)
[2] F. Steiner, J. Vogelsang, and J. M. Lupton, "Singlet-triplet annihilation limits exciton yield in poly(3-hexylthiophene)", Phys. Rev. Lett. 112, 137402 (2014)
[3] T. Stangl, P. Wilhelm, K. Remmerssen, S. Höger, J. Vogelsang, and J. M. Lupton, "Mesoscopic quantum emitters from deterministic aggregates of conjugated polymers", PNAS 112, E5560 (2015)

Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to mis-interpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells.

We have characterized density of trap states (trap DOS) in organic thin film transistors (OTFTs) by using DLTS with extremely longer filling times up to 1000 s. Bottom gate top contact OTFTs are fabricated using alumina gate dielectric deposited by ALD on thienothiophene derivatives (C10-DNTT) as the organic semiconductor. Cr and Au are evaporated as the gate and source/drain electrodes, respectively. DLTS measurements are performed by applying the gate bias pulse for OTFTs with the source electrode shorted to the drain electrode. This MOS capacitor is biased into accumulation to fill traps with holes. Subsequently, it is biased into depletion to emit holes from filled traps. This causes current transients which are processed into DLTS signals in the unit of coulomb using a bipolar rectangular weighting function [1]. Filling times in the accumulation are varied from 1 ms to 1000 s. Since the long filling times are used, DLTS measurements are performed in the isothermal condition at 300 K. The broad isothermal DLTS spectra are observed, indicating continuously energy distribution of trap states. It is found that the longer filling times are needed to saturate DLTS signals for traps with longer emission time constants, i.e., deeper traps. This might be ascribed to fewer holes available to fill traps in OTFTs. This result will be discussed in connection with the effect of bias stress on the TFT characteristics. The authors acknowledge Nippon Kayaku Co., Ltd for supplying the DNTT materials. This work was financially supported by NEDO.
[1] K. Nakamura et al., Materials Research Society Fall meeting, M5.80, 2013.

Harvesting solar energy presents a challenge in using eco-friendly, earth abundant and inexpensive materials.The challenges imposed and the quest for more affordable and efficient clean energy production to find a cheaper alternative led to the development of polymer-based solar cells which are easily processed at low temperatures. In organic donor-acceptor systems, ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) are key determinants of the overall performance of photovoltaic devices. However, the relatively low power conversion efficiency of the polymers poses as an obstacle and hinders its applicability for industrial widespread utilization. The time-resolved laser spectroscopic measurements and high-resolution electron microscopy revealed the fundamental information requisite to fabricate and optimize organic solar cell devices. In real time, the CT and CS at the interface between three fullerene acceptors (FAs) (PC₇₁BM, PC₆₁BM, and IC₆₀BA) and the PTB7-Th donor polymer are monitored. Femtosecond transient absorption (fs-TA) data demonstrated that photo-induced electron transfer from the PTB7-Th polymer to each FA occurs on the sub-picosecond time scale, leading to the formation of long-lived radical ions. The power conversion efficiency improved from 2% in IC₆₀BA-based solar cells to > 9% in PC₇₁BM-based devices, further supporting the time-resolved results. The insights reported in the current work provide a clear understanding of the key variables involved at the device interface, paving the way for the exploitation of efficient CS and subsequently improving the photoconversion efficiency.

The use of organic materials in electronic devices is progressing rapidly with a growing number of applications (organic light emitting diodes (OLEDs), organic photovoltaic devices, co-block polymer films for advanced lithography, to name but a few). Devices for organic electronics typically consist of several thin layers of both organic and inorganic materials. Two of the major issues that need to be addressed in the characterization of organic devices are how to characterize the effects of ageing (environmental or electrical) and how to image the fine scale ordering that may take place during film processing. Until recently characterizing such organic layer systems without damaging them was extremely difficult by surface analysis techniques such as time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) due to the irradiation damage induced by conventional sputter sources. The recent introduction of argon gas cluster ion beam (GCIB) sources allows organic layers to be profiled with little or no damage accumulation during sputter depth profiling [1]. However, commercial argon cluster ion beam sources can deliver a wide range of projectile sizes (in the range 500 to 5000 atoms per cluster) at impact energies ranging from 2 to 20 kV. There is therefore a need to understand the effects of cluster size and impact energy as a function of material in order to find the optimum sputter and analysis conditions.
Here we will show how a correlative approach using TOF-SIMS, XPS and AFM combined with argon cluster sputtering can give a more complete understanding of the effects of ageing or process conditions. Several model systems will be used to illustrate the potential of argon cluster sputtering. Firstly we studied a range of technologically relevant organic materials to investigate the effect of cluster size and energy. Correlating TOF-SIMS, XPS and AFM results enabled the molecular information, chemical damage and roughness to be evaluated for different cluster sizes and energies and the results compared to the recently published universal equation for argon cluster sputtering [2]. Using the optimized conditions found in this study we then show how TOF-SIMS depth profiling with Argon clusters can detect the effects of environmental ageing on organic layers such as those found in OLED devices. Then co-block polymer layers of PS and PMMA organized as spheres, cylinders or lamella were used to demonstrate the ability to resolve domains as small as 20 nm by argon cluster depth profiling. It was possible to discriminate between different organizations, measure the period and evaluate the degree of organization.
This work was carried out on the Platform for Nanocharacterization (PFNC) at the CEA Grenoble.
[1] Ninomiya, S ; Ichiki, K; Yamada, H ; Nakata, Y ; Seki, T ; Aoki, T ; Matsuo, J Rapid Comm. Mass Spec. 23 20 (2009) 3264.
[2] Seah, MP ; Havelund, R ; Gilmore, IS J. Phys. Chem. C 118 24 (2014) 12862

Controlling the alignment of the emitting molecules used as dopants in organic light emitting diodes is an effective strategy to improve the outcoupling efficiency of OLED devices. Even though there are numerous studies describing how different emitters align in emissive layer of the Organic light emitting diodes (OLED) there are very few reports explaining the structure-property relationship of these emitters in controlling the alignment. Apparently, heteroleptic Pt complexes provide us with a simpler system to study the emitter orientation because they possess square planner geometry containing only one chromophoric ligand which makes is easier to understand the transition dipole moment orientation of the emitter material. The chromophoric ligand used in this study is an analog of Dibenzoquinoline (DBQ) and the ancillary ligands used were β-diketonate type ligands. Angle dependent photoluminescent analysis was used to measure the orientation of transition dipoles of these emitters in a host matrix. Modifications in the ligands around the Pt center allow a change in emitter orientation and provide with a valuable handle to understand and control the orientation associated outcoupling efficiency in OLED devices.

This study presents the effects of the polymer molecular weight (M_w) of PBDTTT-C and processing solvents on the PBDTTT-C:PC₇₁BM nanoscale morphology and device performances. In particular, this work demonstrates that different M_w values of the PBDTTT-C are optimized by different processing solvents, leading to various morphological characteristics, as thoroughly examined by atomic force microscopy, transmission electron microscopy and X-ray diffraction. When PBDTTT-C with a relatively low molecular weight is used, an optimal blend texture that supports efficient charge transport to ideally connected crystalline structures and significantly enhanced power conversion efficiency (PCE, over 5.5%) can be obtained through a treatment with chloroform as a processing solvent. These findings highlight the important role of the molecular weight and processing solvents with regard to device performance capabilities and may help in the further development of novel, efficient donor polymers with lower bandgaps for photovoltaic applications.

Youngrok Kim^a, Daekyoung Yoo^a, Jingon Jang^a, Younggul Song^a,
Hyunhak Jeong^a, Kyungjune Cho^a, Wang-Taek Hwang^a_, Woocheol Lee^a,
Tae-Wook Kim^b, and Takhee Lee<sup>a,*

a</sup>Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea
^bSoft Innovative Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Joellabuk-do 55324, Korea
E-mail address (corresponding author): [email protected]

Organic resistive memory devices have been developed as a promising future data storage media to substitute silicon-based memory devices in flexible electronics application. Among them, the mixture of polyimide (PI) and 6-phenyl-C61 butyric acid methyl ester (PCBM) (denoted as PI:PCBM) has been employed to various organic resistive memory applications [1]. However, until now there were few reliability studies of PI:PCBM memory devices under thermal stress. In this presentation, I will report the result of thorough investigation of the reliability issues against thermal stresses about nonvolatile organic resistive memory devices using PI:PCBM [2]. We confirmed the thermal robustness of PI:PCBM organic resistive memory devices through verification of their electrical and structural robustness upon increasing the temperature from room temperature to 470 K. PI:PCBM organic memory devices exhibited stable resistive switching performance such as similar current levels, steady ON/OFF ratio, and unchanged conductive features. From the retention more than 10⁴ seconds, endurance cycles more than 200 switching cycles, and thermal stress tests over 1 hour, stable resistive switching of the PI:PCBM memory devices was maintained at high temperature. Moreover, we verified the structural robustness of our PI:PCBM organic memory under thermal stress by cross-sectional transmission electron microscopy, energy dispersive X-ray spectroscopy, and atomic force microscopy analysis of the active layer after a retention test at 470 K for 10⁴ seconds. The structural robustness of organic layer implies the nanocomposite structures of PI:PCBM are maintained under thermal stress. Form this result, we inferred that the depth of trap sites which is about 2 eV is maintained under thermal stress [3]. This study demonstrated the electrical stability and structural robustness of the matrix polymer under thermal stress. This thermal robustness of the PI:PCBM nanocomposite system promises a stable functionality in high temperature environments.
[1] B. Cho, T.-W. Kim, S. Song, Y. Ji, M. Jo, H. Hwang, G.-Y. Jung, T. Lee, Adv. Mater. 22, 1228 (2010).
[2] Y. Kim, D. Yoo, J. Jang, Y. Song, H. Jeong, K. Cho, W.-T. Hwang, W. Lee, T.-W. Kim, T. Lee, Org. Electron. 33, 48 (2016).
[3] Y. Song, H. Jeong, J. Jang, T.-Y. Kim, D. Yoo, Y. Kim, H. Jeong, T. Lee, ACS Nano, 9, 7697 (2015).

In this study, we propose an ideal interfacial structure of organic solar cells emerged through the investigation of planar heterojunction device. To realize high performance in organic solar cells, donor/acceptor (D/A) interface must enable efficient charge separation and suppressed charge recombination at the same time. Cascade interlayer between the donor and the acceptor domains has been reported as one of the promising candidates; however, understanding of the role of the cascade layer is far from complete. It is essential to know how the cascade layers work in photoelectron conversion processes and how the intermixing in the cascade layer affects device performance.

We investigated tri-layered planar heterojunction structure of PCBM//cascade layer (regiorandom-P3HT or PTB7)//P3HT fabricated by contact film transfer method. We found three fundamental insights. (1) The energy gap of the cascade layer must be narrower than bulk P3HT to allow the exciton diffusion, otherwise the exciton in P3HT cannot contribute to the photocurrent. (2) Open circuit voltage (V_OC) was enhanced by the cascade layer mainly because increased energy of charge transfer state (E_CT). E_CT increased owing to deep ionization potential of the cascade layer and long separation distance between electron and hole. (3) Intermixing of the cascade layer with PCBM acceptor lost the enhancement of V_OC, because it opened the recombination pathway through low energy CT state. These results indicate that the cascade layer for both charges and excitons without intermixing can be an ideal interfacial structure.

1,4,5,8-Naphthalenediimide (NDI) is a well-known prototypical p-skeleton with its strong electron deficient nature (E_LUMO ~ -3.8 eV). Recently, NDI derivatives modified with π-conjugated substituents, often referred as core-extended NDIs, have been developed and used as dyes, pigments, sensors, aggregates, and organic semiconductors.
As a novel core-extended NDI, we have recently developed naphtho[2,3-b:6,7-b’]dithiophene diimide (NDTI) which has two anti-fused thiophene rings (J. Am. Chem. Soc., 2013, 134, 11445-11448.). The fused thiophene rings gave the NDI skeleton lower LUMO energy level (E_LUMO ~ -4.0 eV) and “chemical flexibility” via chemical modifications at the thiophene α-positions. In this presentation, we report the synthesis of naphtho[2,3-b]thiophene diimide (NTI), which has one-fused thiophene ring. NTI also showed lower LUMO energy level (E_LUMO ~ -3.9 eV) than that of NDI, and further π-extension was achieved by modification of the α-position of fused thiophene. By using the general coupling chemistry, the NTI unit was readily integrated into NTI-ended π-conjugate systems.
Using NDTI and NTI units, we developed electron deficient π-conjugate molecules. They showed narrow HOMO-LUMO gaps and low LUMO energy levels (E_LUMO = -3.9 ~ -4.3 eV), which are suitable as an n-type organic semiconducting material. These thiophene-fused naphthalenediimide-based molecules worked as air-stable n-type OFET materials (μ_electron ~ -0.43 cm^-2V^-1s^-1_, spin-coat or vacuum deposition) and electron accepting materials for organic photovoltaic cells (PCE ~ 2.5 %, inverted cell with PTB-7-Th as an electron donor material). We also characterized their molecular and packing structure in the single-crystal as well as electronic structure.

In recent years, carrier doping in organic semiconductors has been intensely studied and developed. For instance, in organic thin-film devices, contact doping is one of the primary techniques for making ohmic-like contacts in order to increase the current and to enhance device performances. Since the great importance of the carrier doping in organic materials is well recognized, the evaluation method of charge carrier concentration in these films should be established. In this study, we attempted direct current (DC) Hall-effect measurement with the van der Pauw geometry for highly-doped organic films with the aid of a micro-scale four-finger electrode structure and site-selective inkjet-deposition of organic films.
Poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) was selected as a measurement object because of its potential toward various applications in future printed electronics. A commercially-available PEDOT/PSS ink of high conductivity grade was employed. Sample structures for the Hall-effect measurement were prepared by inkjet deposition of the PEDOT/PSS ink in the central part of the microscale electrode gap pattern for fabricating the van der Pauw configuration. Then, Hall voltages in the PEDOT/PSS films were evaluated using the dc magnetic field. The polarity of the applied magnetic field was changed by the 180° mechanical rotation of the permanent magnet under the constant flow of the dc current. The time variation of the Hall voltage against the polarity change in the magnetic field was monitored continuously. All the measurements were carried out at room temperature under ambient conditions.
The Hall voltage measured for the inkjet-deposited PEDOT/PSS films containing ethylene glycol (EG) showed rectangular changes according to the reversal of external magnetic field. In addition, the polarity of these rectangular potential changes against the magnetic field switching was also inverted when the polarity of the dc current was altered. These phenomena support the clear detection of Hall voltage. Carrier concentration and Hall mobility of the PEDOT/PSS film treated with EG were estimated to be in the order of 10²¹ cm^-3 and 0.1 cm²/(Vs), respectively. The Hall voltage displayed an n-type behavior, while the thermoelectric current measurement indicated p-type. Although non-uniformities in the sample structure should be noted, this sign anomaly of the Hall voltage, which often appeared in such low-mobility materials as amorphous silicon and chalcogenide glasses, may be due to hopping transport in the conducting polymer. Anyhow, this work suggests that the combination of well-designed microelectrode patterns and inkjet deposition may contribute to the high-sensitivity Hall voltage detection for organic conducting films.

The solvation behavior of C₆₀ fullerene has received much attention since C₆₀ was found to exhibit exceptionally promising properties for both biological and electronic applications. This heightened interest has led to the current understanding of C₆₀ in solvents in which the fullerene molecule, balanced in size between the colloidal regime and that of the surrounding solvent molecules, forms complex and potentially tunable nanostructures as a result of solvent interactions. Herein, we use all-atom molecular dynamics (MD) simulations to gain an insight into the little-understood structural and dynamic properties of the self-assembly of solid C₆₀-solvent complexes. Our results revealed a strong positive correlation between the regularity of solvent molecule orientation and experimentally obtained solubility limits. In addition, we were able to explain the formation of metastable solid C₆₀ complexes that form in a variety of aromatic solvents, clarifying a direct relationship between solvent geometry and the formation of solvates as suggested by experimental studies. Furthermore, we investigated the crystalline state of C₆₀ solids which revealed solvent-dependent structural transitions. These small changes in crystal structures are known to have drastic effects in the formation of various nanostructures from C₆₀ solvates; however, the transitions from one structure to another have yet to be directly observed. These insights provided by MD simulations into the formation of C₆₀ solvents may allow highly predictive tuning of fullerene-based materials on a nanostructural and electronic level to fully realize the potential of this unique molecule.

Extensive efforts have been focused on the study of wide-band gap (WBG) polymers due to their important application in multiple junction and ternary blend organic solar cells. Herein, three WBG copolymers named PBDT(X)-T1 (X = O, S, Se) were synthesized based on benzodithiophene (BDT) donor unit and 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (T1) acceptor unit. Different aromatic heterocycle groups (furan, thiophene and selenophene) were introduced to modify BDT unit to investigate the influence of conjugated side chains on photovoltaic properties of conjugated polymers. Photophysical properties, electrochemistry, charge transport and crystalline properties of the polymers were studied to discuss the role of chalcogen atoms on the performance of conjugated polymers. Solar cells based on these three WBG copolymers were fabricated. Among them, the PBDT(Se)-T1-based solar cell show the best photovoltaic performance with a highest power conversion efficiency (PCE) of 8.52%, a open-circuit voltage (V_oc) of 0.91 V, and a high fill factor (FF) of 72%. In comparison with PBDT(Se)-T1-based device results, PBDT(S)-T1-based solar cell show a slightly lower PCE (7.48%) and a FF (68%), which is still much higher than the performance (PCE = 4%, FF = 49%) of PBDT(O)-T1-based cells. The results indicate the important role of chalcogen atoms in conjugated side chains and high photovoltaic performance can be realized through side chain engineering of BDT-based WBG conjugated polymers.

In this work, we present organic infrared photo-detectors based on the absorption of charge-transfer states between molecular electron donors and C₆₀ as acceptor material. We reach external quantum efficiencies above 1% at 1064nm for ZnPc:C₆₀ blend layers, well below (200 nm) the optical gap of ZnPc. We measure transient photo-current responses at wavelengths of 355nm, 532nm, and 1064nm, exciting dominantly C₆₀, ZnPc or the ZnPc-C₆₀ CT state respectively, and obtain rise and fall times of a few nanoseconds. We find that the response upon CT excitation is at least as fast as upon ZnPc or C₆₀ excitation. The transients are described with time dependent simulations of electrons and holes and reconstruct the photo-current signal, including capacitance and series resistance effects, and identify the hole mobility of the donor material as the limiting factor for the high frequency response.

To target the charge and energy transport issue at multi-interfaces in organic light emitting diodes (OLEDs) and traditional organic alternating driven electroluminescent (AC-OEL) devices with high-k dielectric layer, recently we demonstrated that a novel AC-OEL device with “gate”, a n-type semiconductor with wide band gap, exhibited an outstanding management of the carrier injection and extraction at the interface of hole generation layer (HGL) and the cathode. Under intense high-frequency (over 40,000Hz) electric field, charge generation mechanisms in the HGL are carried out and confirmed by multiple experiments. Meanwhile, we investigate the role of gate layer on carriers management in the forward and reverse bias of AC cycles via band alignment analysis of hole-only devices due to the yield of n-type semiconductor. A high performance of 12300 cd/m^2 and over 70 lm/W is achieved in ZnO gated AC-OEL devices and the flexible devices are also demonstrated.

Molecular packing in organic single crystals greatly influences their charge transport properties but can hardly be predicted and designed because of the complex intermolecular interactions. In this work, we have realized systematic fine-tuning of the single-crystal molecular packing of five benzodifurandione-based oligo(p-phenylenevinylene) (BDOPV)-based small molecules through incorporation of electronegative fluorine atoms on the BDOPV backbone. While these molecules all exhibit similar column stacking configurations in their single crystals, the intermolecular displacements and distances can be substantially modified by tuning of the amounts and/or the positions of the substituent fluorine atoms. Density functional theory calculations showed that the subtle differences in charge distribution or electrostatic potential induced by different fluorine substitutions play an important role in regulating the molecular packing of the BDOPV compounds. Consequently, the electronic couplings for electron transfer can vary from 71 meV in a slipped stack to 201 meV in a nearly cofacial antiparallel stack, leading to an increase in the electron mobility of the BDOPV derivatives from 2.6 to 12.6 cm² V^–1 s^–1. The electron mobility of the five molecules did not show a good correlation with the LUMO levels, indicating that the distinct difference in charge transport properties is a result of the molecular packing. Our work not only provides a series of high-electron-mobility organic semiconductors but also demonstrates that fluorination is an effective approach for fine-tuning of single-crystal packing modes beyond simply lowering the molecular energy levels.

Despite the tremendous interest in conjugated polymers for organic electronics, relatively little attention is being paid to the underlying physical chemistry that governs structure formation in this intriguing class of materials. Here, we investigate isothermal crystallization of poly(3-(2′-ethyl)hexylthiophene) (P3EHT) with differential scanning calorimetry (DSC) and establish a clear view of the nucleation and crystal growth behavior through Avrami analysis. We find that classical understanding, which has been developed for commodity polymers, can be readily applied to P3EHT. Isothermal crystallization of thin films of P3EHT supports this insight with matching microstructure as predicted from the kinetic parameters obtained from DSC. Thus, we are able to provide a clear reference point to guide understanding of microstructure development in conjugated polymers.

Halogenation is an effective and facile way to tune the energy levels and phase behaviors of organic semiconducting materials. However, the fluorinated organic semiconductors are far more used in the fabrication of polymer solar cells (PSCs) than the chlorinated materials. Herein, in order to enrich the diversity of chlorination chemistry, we designed and synthesized a series of chlorinated polymers having donor-acceptor (D-A) structures, in which the benzo[1,2-b:4,5-b]dithiophene-chlorinated benzothiadiazole and the asymmetric alkyl thiophene are conjugated through the thiophene π-bridge. It was found that these chlorinated polymers showed reduced molecular planarity, and lower highest occupied molecular orbital (HOMO) energy level. Combined with the highest photovoltaic conversion efficiency of 6.27%, it can be concluded that the chlorinated polymers behave as well or better than their fluorinated counterparts. Although the introduction of one large chlorine atom increases the torsional angle of the polymer backbone, the chlorinated polymers still exhibit high crystallinity, good mobility, and favorable backbone orientation in the blend films. The results indicate that the introduction of chlorinated counterparts is a feasible way to achieve highly efficient polymer solar cells.

Organic dyes have been incorporated into the design of pi-conjugated materials as low-cost organic building blocks that offer strong absorptive properties, ideal for light harvesting technologies. Utilizing these organic dyes as the central unit in small molecule frameworks allows for the material properties to often be dictated by the nature of the organic dye; however, we can fine-tune material properties by varying the choice of terminal unit or the pi-bridging unit that links the core and end-cap together. Using this template, we have synthesized organic dye-based pi-conjugated small molecules that were designed to be accessible from an optimized direct heteroarylation protocol. The versatility of this approach has facilitated the synthesis of a series of new pi-conjugated small molecules with applications in solution-processable organic electronics. Materials design, the optimization of silica-supported direct heteroarylation and optoelectronic characterization will be discussed in relevance to structure-property relationships and their influence on electrical mobilities and photovoltaic device performance.

Perylene dimide (PDI) based compounds are an important class of opto-electronic materials that have found utility as active components in a wide range of electronic devices. Most recently, PDI based materials have been shown to be effective at replacing fullerenes in bulk-heterojunction organic solar cells. In the pursuit of new materials to replace fullerenes, our research group has recently developed an optimized synthetic pathway towards N-annulated PDI structures. N-annulation at the bay position of the PDI chromophore allows for installation of functional groups that can fine tune solubility, self-assembly, optical and electronic properties. This presentation will detail our synthetic methodology, introduce several new monomeric, dimeric and trimeric PDI materials, cover structure-property relationships including single crystal X-ray analysis of twisted PDI molecules, and highlight selected photovoltaic performance that rival fullerene based organic solar cells.

Solvent annealing is a ubiquitous protocol used during the fabrication of organic semiconductor devices. It is especially used to control morphology in polymer blend and small molecule based thin films. Current methodologies utilizing this technique are mostly trial-and-error based, with time- and effort- spent on identifying the optimal annealing conditions. However, this approach is resource intensive and is usually limited to exploring a narrow range of conditions. A computational model that predicts morphology evolution under solvent rich atmospheres could enable virtual exploration to identify promising annealing conditions. In this work, we utilize a phase field model to simulate the effect of solvent annealing on a polymer thin film. We integrate the model with parallel workflows to perform high throughput computing to identify the effect of various processing parameters on morphology. We particularly link the processing conditions to morphology and to performance measures (specifically charge transport) via morphology descriptors. Morphologies are quantified using a graph based method to interrogate the charge transport characteristics. We construct phase diagrams quantatively illustrating how annealing conditions affect morphology traits. We anticipate that this framework will greatly enable experimental colleagues to quickly and efficiently identify optimal aneealing conditions for efficient device design.

Recently, non-fullerene acceptors as replacement of fullerene which have been commonly used in bulk heterojunction (BHJ) solar cells have been vigorously investigated because of their advantages of easily tunable energy levels, broadening absorption spectra, good thermal/photochemical stability, controlling molecular structures and low production costs.
Among the reported non-fullerene accepting small molecules, perylenediimide (PDI) derivatives have been investigated most extensively due to proper properties, however, the structural properties of PDI were strong intermolecular π-π stacking induced by a highly planar conformation.
In this presentation, we have newly synthesized M- and V-shaped PDI-based small molecules (i.e., CP-M and CP-V) including a unique sp³ σ-bridge (i.e., 1,1-diphenylcyclohexane), which were rarely used to connect PDIs to overcome the drawback of the strong self-association of PDIs. In addition to the dependence of the binding geometry of the PDIs into the cyclohexane core, the intrinsic and photovoltaic properties were investigated in detail. In the case of the V-shaped n-type material rather than the M-shaped molecule, the best PSC performance with PPDT2FBT was revealed to give a PCE of 5.28% with a J_sc of 10.04 mA/cm², a V_oc of 0.87 V, and FF of 60.16.

We discuss the relationship between the structure, property and function of a novel small molecule donor, hexyl-TBBT, which absorbs near-infrared light (700-900nm). Despite favorable optical and electronic properties, solution processed films of hexyl-TBBT:fullerene surprisingly exhibit negligible solar cell efficiency. X-ray scattering experiments indicate that hexyl-TBBT readily forms large crystallites during solution deposition, due to its planarity and small size. Consisting of many grain boundaries and discontinuous patches, the resulting film morphology has poor transport properties. However, thermally evaporating this small molecule donor with C60 into bilayer or gradient-layer films creates solar cells with increased photovoltaic efficiency. We investigate how these different deposition methods impact the blend film morphologies to produce these drastic differences in transport properties.
As a semi-transparent material, near-infrared donors like hexyl-TBBT extend organic solar cell applications into unique niches such as tandem architectures and solar windows. Tandem solar cells provide an opportunity to increase absorption and photovoltaic conversion by extending the effective operating wavelength range of existing solar cell donors like P3HT and PTB7. Such transparent cells can also be incorporated into windows in commercial and agricultural structures, such as high-rise buildings and greenhouses.

We introduce a method that significantly improves the time resolution of scanning Kelvin probe microscopy, enabling the rapid acquisition of photocapacitance transients in solar-cell films. This work is motivated by Ginger et al.’s finding that the tip-sample photocapacitance charging rate measured in a time-resolved electrostatic force microscope experiment is proportional to the external quantum efficiency in benchmark organic photovoltaic systems [1]. In our new method a light pulse generates free carriers in the sample while a nearby charged microcantilever is used as a voltage-gated mechanical integrator to encode the time evolution of the subsequent carrier recombination as a change in the cantilever’s phase of oscillation. We demonstrate the method by using it to reveal a biexponential photocapacitance buildup in a polymer-blend solar-cell film, PFB:F8BT on ITO, with the fast component having a risetime of 40 microseconds at high light intensity. We demonstrate the superior signal-to-noise and time resolution of the new method by using it to record the 10’s of nanoseconds probe-wiring time constant of our apparatus in ~100 ms of total acquisition time.
Rumbles has proposed a tantalizing design rule for molecular solar-cell materials: in good molecular photovoltaic materials, the exciton to charge-separated (CS) state transition is set up for optimal Marcus electron transfer while the exciton to charge-transfer state transition operates in the Marcus inverted region and is therefore slow [2]. Only a few experiments have shown clear evidence of Marcus electron transfer taking place in a molecular photovoltaic material, however [3]. These measurements largely used time-dependent microwave conductivity which measures charge recombination with nanosecond time resolution but has no spatial resolution and cannot study films with metal contacts present. By enabling the study of charge generation with high spatial resolution and 10’s of nanosecond temporal resolution, our new scanned-probe photocapacitance measurement opens up exciting new avenues for testing the Rumbles’ hypothesis in device-relevant samples.

[1] Coffey, D. C. & Ginger, D. S Nat. Mater., 2006, 5:735; (b) Giridharagopal, R.; et al. & Ginger, D. S Nano Lett., 2012, 12:893; (c) Karatay, D.U; et al. & Ginger, D.S. Rev. Sci. Instrum., 2016, 87:053702.

[2] (a) Coffey, D. C.; et al. & Rumbles, G. J. Phys. Chem. C, 2012, 116:8916; (b) Rumbles, G. Charge-generating mechanism in organic solar cells. Excited State Processes in Electronic and Bio Nanomaterials (ESP-2014); Santa Fe, New Mexico; June 9 – 12, 2014.

[3] (a) Rand, B. P.; Burk, D. P. & Forrest, S. R. Phys. Rev. B, 2007, 75:115327; (b) Ward, A. J.; et al. & Samuel, I. D. W. Adv. Mater., 2015, 27:2496; (c) Ihly, R.; et al.; Rumbles G. & Blackburn, J. L. Nature Chem, 2016, 8:603.

[4] Dimitrov, S. D. & Durrant, J. R. Chem. Mater., 2014, 26:616.

Conjugated polymers are important components in optoelectronic devices such as light-emitting diodes, field-effect transistors, and photovoltaic cells.

A comprehensive understanding of exciton dynamics in conjugated polymers is challenging, given the effects of electron-electron interactions , electron-nuclear coupling, and disorder on a wide range length scales—from molecular level up to the device scale—has on electronic and optical properties of polymers.

Here we present a new phenomenological model for simulating the dynamics of excitons in long-chain organic conjugated molecules. In our model, the polymer is described as a time-dependent array of ring-ring torsion angles. These angles define the conjugation environment that determines the position, size, and energy of the excitons. Exciton dynamics arise in direct response to the evolution of this torsional landscape along its excited state potential energy surface, which includes exciton-induced forces (such as those that lead to self-trapping). The framework for generating an accurate description of these heterogeneous excited state forces was developed based on the analysis of mixed QM/MM simulations known as QCFF/PI with a semi-empirical Pariser-Parr-Pople(PPP) Hamiltonian to describe the pi-electron system to accurately reproduce the local structure and dynamics of an atomistic long-chain polymer (e.g. polythiophene)

We show that this model can reproduce transient pump-probe experiments; we remark on the importance on the excited state force field when describing these systems. Then we go on to present molecular-level physical insights into exciton dynamics in these polymer materials, which have been previously speculative, to help better engineer organic solar cell devices.

Recently, non-fullerene acceptors have been promising alternative of fullerene derivatives in organic solar cells (OSCs) with bulk heterojunction (BHJ) structures. Although fullerene acceptors exhibited high efficiencies over 11% of PCE, they possess weak absorption and limited tuning ability of electronic energy levels. On the other hand, absorption bands and energy levels of n-type small molecules can be tuned relatively easy under modification of structure. In addition, non-fullerene small molecule acceptors can be synthesized from low-cost building blocks and easily purified.
In this presentation, we synthesized non-fullerene small molecules with strong electron deficient cyano groups. Difference of the number of nitrile functional group and extended π-conjugation length of donor block affected optical and electrochemical properties of small molecules. By tuning absorption region and lowest unoccupied molecular orbital (LUMO) energy level, we investigated unique characteristics of small molecules in bulk hetrojunction polymer solar cells with PTB7-Th as a donor.

As a key part of future electronics, organic field-effect transistors (OFETs) have been attracted great attention due to their advantages such as low cost, light-weight, mechanical flexibility, and solution processability. Charge carrier mobility in OFETs is directly proportional to semiconductor conductivity, and is thus directly related to the performance of the device. In order to improve charge carrier mobility, it is important to enhance charge transport of organic semiconducting materials. One of the most successful strategies to design molecular structure for effective charge transport is incorporation of π-extended conjugated structure due to their high planarity and rigidity. Introduction of such structures into the conjugated polymer backbones enhances intrachain transport by enlarging conjugation length as well as interchain transport due to shortening the π-π stacking distance. Among the various π-extended molecules, quinoidal structures which connected with double bond linkage between aromatic rings are rarely researched in the fields related to conjugated polymers. Quinoidal structures have several advantages as semiconducting materials for organic electronics such as high planarity, reducing band-gap and amphoteric redox behavior. Until now, quinoidal conjugated molecules are usually focused on small molecules because of restricted end groups for polymerizing them; for example, dicyanomethylene (DCM) group.
Here, we have exploited novel conjugated polymers containing quinoidal building block by substituting isatin end groups into quinoid structure with high solubility through alkyl side chain at N-position. This building block contributed greatly to the achievement of high charge carrier mobility, as well as successful ambipolar behavior. Quinoidal conjugated polymers were synthesized via Stille polymerization, and top-gate/bottom-contact (TG/BC) OFET devices were fabricated to measure their electrical properties. A low band-gap and ambipolar field-effect characteristics were observed for these polymers. By optimization of annealing temperatures of the polymers in thin film, we obtained conspicuously enhancement of charge carrier mobility. To explain the correlation between molecular orientation/crystallinity and OFETs performance, we investigated thin film microstructure of these conjugated polymers, based on grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM).

We investigate the relationship between sheet conductivity on rubrene single crystals as a function of crystal thickness. We compared pristine and surface-doped single crystal by putting graphite electrodes. In case of pristine crystals, the sheet conductivities decreased exponentially as function of thickness increase until ~ 1 μm, and saturated regardless of their thickness change. Additionally we show, that doping the single crystals with fluoroalkyl trichloro-silane (FTS) enhanced the sheet conductivities to the same value regardless of their thicknesses. Investigation of the surface morphology using atomic force microscopy (AFM) and optical profilometer revealed that thicker crystals have a higher number of molecular steps, increasing the overall surface roughness compared to thin crystals. Higher charge trap density on the surface of thick crystals was observed from the subthreshold slope of the transfer curve in single-crystal transistors. We found that increased molecular steps on the crystalline surface correlates with higher trap density hindering the charge transport. Conversely, the FTS doping gives an external dipole moment on the crystal surface and activates a highly conductive channel. With this fundamental research about the thickness dependence of sheet conductivity and the effect of FTS doping, we expect that the mechanism of the charge transportation on organic semiconductor can be understood better.

Bulk heterojunction (BHJ) organic solar cells are an emerging technology that has the potential to provide a low cost photovoltaic devices. It is well known that the nanomorphology of the polymer:fullerene BHJ is a critical factor which affects the solar cell performance. The addition of processing additives such as 1,8-diiodooctane (DIO) is widely used approach to increase power conversion efficiencies for many organic solar cells[ 1-2 ]. Solid-state solar NMR spectroscopy offers several techniques for the investigation of the morphological, structural, and dynamics properties of BHJ organic solar cells.
We have investigated the effects additives(1,8-diiodooctane(DIO), 1,8-Octanedithiol(ODT) and 1-Chloronaphthalene(CN) and their mixtures) of P3HT/PCBM BHJ films on the local crystallinity andmorphology by using 13 C and 1 H solid-state NMR spectroscopy. We also investigated the drying time dependence and annealing effect of BHJ thin film on their local crystallinities and morphologies.Mixed solution of P3HT/PCBM (1:1(w/w)) was prepared by using chlorobenzene at a concentration of 1 wt%. for 50 hrs in a glove box under argon atmosphere. DIO, ODT, CN or their mixtures was then dropped into the solution and then stirred for 1 hr. The solution was filtered using 0.45μm filter before making films to remove undissolved materials. P3HT/PCBM BHJ films were prepared by dropping the solution in a glass plate and dried in a glove box under argon atmosphere for 40 hrs and then put in the vacuum. Dried film were removed from the glass plate and sealed into 4 mm zirconia NMR rotor. Solid-state NMR spectra ( 13 C-CP/MAS- NMR, 1 H-MAS NMR, and their heteronuclear correlation-NMR) and T 1 were measured byusing Bruker AVANCE300 spectrometer. The detailed results will be presented.

Recently, the organic light-emitting diodes (OLEDs) have attracted considerable attentions because of their unique properties of light weight and high mechanical flexibility in display and lighting applications. ^[1] To realize these next-generation electronics, high performance flexible and transparent electrodes will be required to replace conventional transparent electrodes based on brittle and expensive indium tin oxide (ITO). In this respect, the need for the development of alternative transparent electrodes to replace ITO rapidly arises. In particular, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)^[2], a conductive polymer, is regarded as a promising alternative transparent electrode due to is high conductivity, high transmittance, and excellent mechanical flexibility. Here, we demonstrate on a highly conductive PEDOT:PSS thin films doped with 1,3-dimethyl-2-imidazolidinone (DMI). DMI significantly increases the conductivity of PEDOT:PSS films up to 812.1 S/cm. The conductivity of the PEDOT:PSS films mixed with DMI are further improved by the solvent post-treatment. The obtained highest conductivity of post-treated PEDOT:PSS with DMI films is 986.2 S/cm. Furthermore, the solvent post-treated PEDOT:PSS films are successfully adopted as transparent electrodes for white OLEDs. The maximum external quantum efficiency (EQE) of OLEDs with solvent post-treated PEDOT:PSS films is 16.2 %, which is higher than that of reference OLEDs based on PEDOT:PSS doped with EG (13.5 %). The results indicate that the optimized PEDOT:PSS films with the new solvent of DMI.
[1] M. C. Gather, A. Köhnen, K. Meerholz, Adv. Mater. 2011, 23, 233.
[2] Y. H. Kim, J. Lee, S. Hofmann, M. C. Gather, L. Müller-Meskamp, K. Leo, Adv. Funct. Mater. 2013, 23, 3763.

Organic materials have been drawing a lot of attention over the past few decades with recent commercial applications such as organic photovoltaics, OLEDs, and flexible organic displays. One of the key components of creating organic materials suitable for electronic devices is a fundamental understanding of excitonic behaviors such as superrradiance and superfluorescence.^1,2
To this end, we attempted to study the exciton delocalization and superradiant behaviors of small organic molecules. The organic alloys of metal free and metal based octabutoxy phthalocyanine (MOBPC_xOBPC_1-x) systems were prepared with different ratio from x =0.001 to 0.5 in concentration. Crystalline thin films of these materials were deposited using an in-house developed pen writing technique³ that results in macroscopic long-range order even at the ratio of x = 0.5, which is unique and important for spectroscopic studies.⁴ Our temperature dependent polarization resolved photoluminescence (PL) experiment revealed the existence of a delocalized bandgap exciton⁵ that extends over approximately 10 molecules in the crystal. The formation of this delocalized exciton is inhibited for x > 0.09.⁶
However, due to the quenching of luminescence in metal OBPCs, the ratio of alloys couldn’t exceed x = 0.5. Thus, another strong luminescence compound with extended π-conjugated rings, octabutoxy naphthalocyanine (H₂OBNC), was used to make alloyed systems with H₂OBPC. Crystalline thin films of mixtures H₂OBPC_1-xH₂OBNC_x with ratio ranging from x = 0.1 to x = 0.9 was made to further investigate the coherent emission over the entire range of mixing ratios. We demonstrated that the band gap exciton energy in thin films is tunable over a 30nm range as a function of mixing ratio.
Furthermore, we also investigate the superradiant behaviors in various bio-inspired organic conjugated molecules such as quinacridone and HB194 merocyanine. Our time and polarization resolved PL of HB194 demonstrated the enhancement of radiative decay as the temperature decreases and the strong temperature dependence of band gap emission.
¹Lim S. et. al., Phys. Rev. Lett., 2004, 92 (10), 107402.
²Meinardi F. et. al, Phys. Rev. Lett., 2003, 91 (24), 247401.
³Headrick R.L. et al., Applied Physics Letters, 2008, 92, 063302.
⁴Pan. Z., et. al, Nat. Comm., 2015, 6.
⁵Rawat R. et. al, J. Phys. Chem. Lett., 2015, 6 (10), pp 1834–1840.
⁶Manning L. W. et. al., J. Phys. Chem C, 2016, 120, 11966−11976.

In organic light emitting devices (OLEDs), the excited states are formed in a 25% singlet and 75% triplet state ratio. However, in fluorescent devices, only the singlet states can decay radiatively. This limits the internal efficiency of fluorescent OLEDs drastically. The limitation can be overcome by the concept of thermally activated delayed fluorescence (TADF). In TADF molecules, the non-radiative triplet excited states are converted into radiative singlets via reverse inter-system crossing (RISC) by thermal energy. For this process, the energy splitting between the lowest excited singlet and triplet states (ST-splitting, ΔE_ST) has to be sufficiently low. A small ST-splitting can be achieved by spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) on different functionalized componends of the molecule, creating intra-molecular charge transfer (CT) states. With this concept, all excitons that are formed on the emitter can theoretically decay radiatively, leading to an enhanced internal quantum yield of TADF devices in comparison to fluorescent OLEDs and thus also to a higher external quantum efficiency (EQE).

As demonstrated before by Uoyama et al. [1], highly efficient TADF emitters can be built by combining a functionalized phenyl-based acceptor unit with various carbazole donor groups. In this work, a new homologous series of TADF emitters is presented, where the phenyl-based acceptor moieties are optimized towards higher lying CT energies. Thereby the emission of the molecules can be pushed towards shorter wavelengths (i.e. higher energies). Newly designed sky-blue TADF emitters with emission wavelengths below 500 nm, which qualify for white light generation owning to their rather symmetric emission bands with strong blue contribution, are reaching photoluminescence quantum yields of 68 % (in toluene) and EQEs up to 18 % in OLEDs.

Combining the blue TADF emission with a red phosphorescent emission, hybrid two-color warm white OLEDs are presented. Due to the broad emission spectra of the TADF emitters, a deep blue or a further yellow emitter is no longer needed to reach high color rendering indices (CRIs) over 80 and warm white color correlation temperatures (CCTs) about 3000 K. The concept of two-color white OLEDs enables a simpler device structure and an easy tunability of the spectral parts of the emission to match the white point the CIE1931 color space.
Additionally, device lifetime studies for different matrix-emitter systems and the dependence of the degradation processes on the doping concentration and the choice of charge transport materials is shown.

[1] H. Uoyama et al. “Highly efficient organic light-emitting diodes from delayed fluorescence“. Nature 492, 234–238 (2012)

All-polymer solar cells employ an electron deficient conjugated polymer as an acceptor in lieu of the ubiquitous fullerene acceptor. All-polymer solar cells have seen a steady rise in efficiency over the past few years going from about 2% to 8.3% with the development of new donor and acceptor polymers. All-polymer solar cells have certain advantages over their fullerene counter parts in terms of tunability, morphological stability and possible cost benefits. It is due to these reasons that the study of various aspects like morphology, physics and chemistry of all-polymer systems is garnering more attention amongst the research community.
With an aim to further the development of polymer acceptors, we have developed and studied three new candidates P(NDITPhT), P(NDITF2T) and P(NDITF4T). These polymers are analogous to the well-studied P(NDI2OD-T2), with P(NDITPhT) possessing a phenyl ring between the two thiophene units, and P(NDITF2T) and P(NDITF4T) possessing fluorinated phenyl rings with increasing degrees of fluorination. When paired with the donor polymer PTB7-Th, we find that the overall power conversion efficiency (PCE) increasing with increasing degree of fluorination despite a systematic decrease in open-circuit voltage. With further optimisation of molecular weight, an efficiency of up to 5.4% is achieved with P(NDITF4T) as the acceptor, higher than that achieved with the benchmark P(NDI2OD-T2) (4.5%). The effect of fluorination of the morphology and photophysics of these blends is also discussed.

Polymer acceptors are promising replacements of fullerenes in all-polymer donor:acceptor bulk-heterojunction (BHJ) solar cells due to their high absorption coefficient across the visible spectral region, the ability to achieve complementary absorption profiles in two-component donor:acceptor blends, which leads to a wider spectral coverage, and the potentially better photostability of polymers compared to fullerene acceptors.

Here we present the photophysics of a new class of polymer acceptors incorporating thieno[3,4-c]pyrrole-4,6-dione (TPD) and 3,4-difluorothiophene ([2F]T) motifs with branched alkyl-chains. These PTPD[2F]T polymers blended with the commercial low-bandgap polymer donor PCE10 reach device efficiencies of up to 4.4%. However, this is much below the maximum theoretical performance for such an all-polymer solar cell system for reasons not yet understood.

The intention of this study is thus to unveil the reasons behind the moderate device efficiency compared to PCE10:fullerene blends. Here we use sub-picosecond to millisecond pump-probe transient absorption (TA) spectroscopy to track the dynamics of excitons and charges in the donor and acceptor, which allows us to follow the charge carrier generation and recombination processes. Additionally, time-resolved photoluminescence (TRPL) spectroscopy is used to monitor the decay of emissive species. The combination of these two techniques allows us to disentangle energy and charge transfer processes between the donor and acceptor polymers. Finally, time-delayed collection field (TDCF) experiments are used to get a better understanding of the processes that limit the fill factor of these all-polymer solar cells.

Nanocrystals synthesized from the non-covalent self-assembly of molecular precursor exhibit unique electronic and optical properties stemming from the molecular building blocks.^[1,2] More importantly, the ability to control size and shape provides can enhance properties due to size- and shape-dependent effects and collective behavior from self-assembled vicinity building blocks.^[3,4] Here we report the synthesis of hierarchical structured nanocrystals through an interfacial self-assembly driven micro-emulsion (µ-emulsion) process. An optically active macrocyclic building block Sn (IV) meso-tetraphenylporphine dichloride (SnTPP) is used to initiate non-covalent self-assembly confined within µ-emulsion droplets. In-situ studies of dynamic light scattering, UV-vis spectroscopy, and electron microscopy, as well as optical imaging of reaction processes suggest an evaporation-induced nucleation and growth self-assembly mechanism. The resulted nanocrystals exhibit uniform shapes and sizes from ten to hundred nanometers. Due to the spatial ordering of SnTPP, the hierarchical nanocrystals exhibit collective optical properties resulted from coupling of molecular SnTPP and photocatalytic activities in reduction of platinum nanoparticles and networks and in photo degradation of methyl orange (MO) pollutants.
1. Bai, F.; Sun, Z.; Wu, H.; Haddad, R. E.; Coker, E. N.; Huang, J. Y.; Rodriguez, M. A.; Fan, H.* Nano Lett. 2011, 11, 5196-5200
2. Bai, F.; Sun, Z.; Wu, H.; Haddad, R. E.; Xiao, X.; Fan, H.* Nano Lett. 2011, 11, 3759-3762
3. Zhong, Y.; Wang, Z.; Zhang, R.; Bai, F.*; Wu, H.; Haddad, R.; Fan, H.* ACS Nano 2014, 8, 827-833
4. Zhong, Y.; Wang J.; Zhang, R. Wei W.; Wang H.; Lv X.; Bai F.*; Wu, H.; Haddad, R.; Fan, H.* Nano Letters 2014, 14, 7175

Organic small molecule acceptors are significantly advancing alternatives to the ubiquitously used fullerene (PC61/71BM) acceptors in bulk heterojunction (BHJ) organic solar cells. Besides providing cost effective and scalable processing, they often outcompete fullerenes by better photostability, significantly stronger absorbance in the visible spectral region, and enhanced modifiability of their electronic properties. While fullerene-based photovoltaic devices have been extensively studied in the past two decades and have now reached efficiencies exceeding 11-12%, the photophysics, specifically the efficiency-limiting processes, in non-fullerene blends are not yet well understood and the efficiencies still lack behind those of their fullerene-based counterparts.
Here, we investigate the photophysical processes such as exciton dissociation, charge carrier generation and recombination in blends of a commercial low-bandgap donor polymer, namely PCE10 (benzo[c][1,2,5]thiadiazol-4-ylmethylene), with novel malononitrile (BM)-terminated small molecule acceptors, for which power conversion efficiencies as high as 8.1% have been achieved, by means of ultrafast transient absorption spectroscopy, employing a broadband detection scheme with spectral coverage across 500-1600nm and a dynamic range from sub-picoseconds to hundreds of microseconds. In addition, sensitive external quantum efficiency measurements were performed to extract the energy of interfacial charge transfer states. Finally, time-resolved photoluminescence spectroscopy has been conducted to study energy transfer processes in these polymer:non-fullerene acceptor blends.

The creation of bulk heterojunction architecture enabled a substantial increase on device performance of organic solar cells, and this device structure has become the most widely used at present(Yu, Gao et al. 1995). Although, this intimate mixing structure large increase the exiton dissociation, the recombination of separated polarons also substantially increased due to large interface area. This recombination also known as the non-geminate recombination becomes the main loss process in bulk heterojunction organic photovoltaics device (Koster, Mihailetchi et al. 2005, Koster, Mihailetchi et al. 2006). Transient photovoltage (TPV) is an efficient way to study this recombination dynamics within the device. Instead of achieving higher device performance, stability is another most important issue and has been intensively researched, however, the understanding the effects of degradation of organic semiconductor on recombination is unclear. In this study, TPV measurement was applied on the PCDTBT devices with different levels of degraded PCBM, the Voc loss of PCBM degraded device was found mainly due the energetic change which is caused by introduced amount of trap states in device, moreover, the mobility was also found decreased in degraded device. The non-geminate recombination loss was quantified, and was successfully made the Voc reconstruction at the range of light intensity measured.

Reference:
Koster, L. J. A., et al. (2006). "Bimolecular recombination in polymer/fullerene bulk heterojunction solar cells." Applied Physics Letters 88(5): 052104.

Koster, L. J. A., et al. (2005). "Light intensity dependence of open-circuit voltage of polymer:fullerene solar cells." Applied Physics Letters 86(12): 123509.

Yu, G., et al. (1995). "Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions." Science 270(5243): 1789-1791.

Control over the crystallinity and orientation of conjugated polymers is highly desirable since it directly affects the electrical charge transport and optical absorption. Many applications could directly benefit from an increased control over these properties.
Here we present a quick and simple method to deposit spherulites, polycrystalline regions extending radially from a nucleation site, directly from solution. Poly(3-hexylthiophene) (P3HT) is deposited from a chlorobenzene solution containing the additional crystallizable solvent 1,3,5-trichlorobenzene (TCB). During spin- or blade-coating, macroscopic crystals of TCB form as the solvent evaporates. These serve as templates for the epitaxial crystallization of P3HT. After the sublimation of TCB, oriented domains of pure P3HT remain.
We demonstrate different methods that allow to control the size, location and number of P3HT spherulites, as well as combine them with completely isotropic regions of polymer on the same sample.
We establish the potential of this method by preparing several types of P3HT:PCBM bilayer devices of increasing complexity. In these devices, both the absorption and the external quantum efficiency depend on the angle of polarization of the incident light.
Importantly, this method is not limited to P3HT and TCB only. Instead, it works with a variety of conjugated polymers (e.g. PCPDTBT, PFO, PFBT) and crystallizable solvents (e.g. naphthalene).

Organic semiconductors have generated a lot of interest for low cost and thin organic electronic devices. One of the most studied organic semiconductors for photovoltaic (OPV) applications has been poly(3-hexylthiophene) (P3HT), which possesses high optical absorbance. However P3HT, and other semiconducting polymers, has shown low out-of-plane hole mobilities (10^-4-10^-3 cm²/V.s) compared to in-plane (field-effect transistors) charge transport and mobility.^1,2 A higher out-of-plane mobility is desirable for devices with a diode configuration (OPVs and OLEDs).

Here we report patterned P3HT films with out-of-plane mobilities in excess of 10 cm²/V.s, which is ≈3-4 orders of magnitude higher than typical mobilities in the diode configuration.³ The films were characterized by synchrotron grazing incidence X-ray diffraction (GIXD), by macroscopic mobility measurements, and by conductive atomic force microscopy (c-AFM).

The exceptional long-range charge transport is explained by vertical chain alignment and chain-on crystallite orientation, which results in fast charge transport through the chain backbones of P3HT. The data is well fitted with a model of charge transport in an undoped semiconductor.

A relationship between mobility and the degree of chain alignment will be shown, which is explained in terms of nanoscale fluid flow during formation of the micro- and nano-patterns. Finally, application of this method to other types of semi-conducting polymers will be discussed. These results may open a new route for easily processed semiconducting polymer films with high mobility for diodes and OPVs.

References:
[1] Enhanced hole mobility in regioregular polythiophene infiltrated in straight nanopores, K. Coakley et al., Adv. Funct. Mater. 15, 1927–1932, 2005.
[2] Nanostructured organic-inorganic hybrid solar cells, M. D. McGehee, MRS Bulletin 34, 95–100, 2009.
[3] Ultra-high mobility in an organic semiconductor by vertical chain alignment, V. Skrypnychuk, G-J Wetzelaer, I. Gordiichuk, S. Mannsfeld, A. Herrmann, M. F. Toney and D. R. Barbero, Advanced Materials, 28(12), 2359-2366, 2016.

There is increasing interest in understanding how the molecular order in thin films of semiconducting polymers influences charge transport. It is well known that charges can easily move along the backbones of conjugated polymers and in the pi-pi stacking direction. However, charge transport between adjacent polymer crystallites is not as well understood. Direct imaging of the local order and lattice planes within crystalline regions of organic thin films is possible using high resolution transmission electron microscopy (HR-TEM) and may elucidate potential pathways for charges to move across grain boundaries in a semicrystalline film. However, this technique is often limited by large area analysis capabilities leading it to mostly be used as qualitative tool for understanding film morphology on a length much smaller (nanometers) than that over which charge transport is occurring (microns to millimeters).

To solve this problem, a rigorous, quantitative, and automated framework has now been developed to quickly analyze HR-TEM images of polymer thin films. GRATE (GRaphic Analysis of Transmission Electron microscopy images) converts HR-TEM images covering areas of 10s of microns into line-drawings of the lattice fringes and quickly calculates lattice spacings, crystallite dimensions, average chain length, and overlap angle of adjacent crystallites. Here, the program is tested by examining the molecular organization of thin films of PBDTTPD, a successful donor polymer within BHJ solar cells. GRATE is used to study the morphological differences that arise when the length and branching of the alkyl side-chains on the same polymer backbone are varied. Using GRATE, it is possible to look for patterns in the microstructural organization which can be related back to the specific physical properties of the polymer and achieve a great understanding of how different morphologies form and what is needed for effective charge transport.

Despite of the rapidly increasing demand for wearable and bio-implantable devices, the lack of stretchable electronic materials, especially semiconducting materials has been limiting factors for developing high performance stretchable electronics. The existing approaches typically find high stretchability with trade-off for good charge-transport mobility. Here, we present a concept based on nanoconfinment effect of polymers to improve their ductility, for developing stretchable semiconductors. As a result, the fabricated semiconducting layer can be stretched to 100% strain with little drop in the mobility, giving a record high mobility of 1.32 cm²/Vs at 100% strain, which is 2 orders of magnitude higher than that from the neat semiconducting counterpart. With the universal applicability of this methodology, it enabled 4 different conjugated polymers with mobility above 1 cm²/Vs at 100% strain. Since this methodology is readily applicable to any high-performance semiconducting polymers to impart a significantly stretchability maintains their high charge carrier mobility, this concept presents highly versatile approach for realizing stretchable semiconductors as a critical component for stretchable electronics.

Knowledge of the mechanical properties of organic semiconductors is critical for the long-term stability of flexible devices fabricated using roll-to-roll printing techniques. Moreover, increasing the elastic range and resistance to fracture of these materials will enable stretchable and ultra-flexible devices for portable, wearable, implantable, and disposable applications. Despite the importance of the mechanical properties of low-bandgap (i.e., donor-acceptor) conjugated polymers, there are no design rules available that have the goal of maximizing the charge transport properties along with the mechanical deformability. This talk describes my group’s experimental and theoretical approaches to understanding the molecular and microstructural determinants of the mechanical properties of these materials. Our approach began with the measurement of a large library containing over 50 donor-acceptor conjugated polymers. Among the design rules that emerged from this study is the importance of (1) branched side chains and (2) isolated rings (e.g., bithiophene) as opposed to fused rings (e.g., thienothiophene) along the backbone. To obtain a deeper understanding of these properties, we are developing a coarse-grained molecular dynamics approach to determine the glass transition temperature, elastic moduli, Poisson ratio, and toughness of a representative subset of these materials. Among our findings are that the density of entanglements, which critically determines the fracture energy and toughness, is highly dependent on the conformation of the chains in solution prior to deposition. Finally, we describe the first study of the mechanical properties of solution-processed small-molecule semiconductors. These materials exhibit a surprisingly high ductility in pure form, with some films able to withstand ≥10% tensile strain without fracture.

In my talk I will present some of our current examples on the manipulation of semiconducting polymer thin films via polymer synthesis and controlled crystallization techniques for organic electronic devices.
The influence of atom modifications in (poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b:3,4-b’)dithiophene]-2,6-diyl-alt-(2,1,3-benzo-thiadiazol)-4,7-diyl}) (PCPDTBT)^[1] and its Si- and F-derivatives as well as differences in regioregularity in the case of poly{[N,N’-bis(2-octyldodecyl)-1,4,5,8-naphthalene-dicarboximide-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} (PNDI2OD-T2)^[2] on the crystallization behavior will be discussed. Solution crystallization and solvent vapor annealing are presented as methods to tune and guide crystallization.^[3] Absorption spectroscopy proves to be an excellent tool to monitor aggregation and crystallization in solution and films in this context.
Direct correlations between the molecular and mesoscopic structure with optical, electrochemical and opto-electronic properties will be established.

[1] F.S.U. Fischer et al. Adv. Mater. 27, 2015, 1223.
[2] K. Tremel et al. Adv. Energy Mater. 4, 1301659 (2014).
[3] E. Crossland et al. Adv. Mater. 24, 2012, 839.

Solution processable semiconducting polymers with excellent film forming capacity and mechanical flexibility are emerging as alternatives to conventional inorganic semiconductors. However, the required optimization of the charge transport within conjugated polymers can be obtained only through a full control of the molecular assembly at the different length scales.
Here, we present a general strategy, based on Langmuir-Schäfer deposition, to control the polymer organization to form uniaxially aligned architectures at the water-air interface prior to their transfer onto a flat substrate. In particular, we demonstrate that the hole mobility of isoindigo-based conjugated polymer, [1] in our films processed in air is comparable with the values previously reported for films prepared under N₂ atmosphere. Interestingly, the transport is anisotropic: XRD and high resolution AFM show that the mobility is higher in the direction perpendicular to the polymer backbone.
This method, based on the (macro)molecular pre-assembly at the water-air interface, can be applied to grow layer-by-layer structures of polymers possessing different electronic properties.

References
[1] Lei T., Dou J-H, Pei J., Adv.Mater., 24, 6457-646 (2012).

Precise integration of π-conjugated units is a key issue to achieve molecular (nano) electronics devices based on organic semiconductor materials. Molecular electronics requires versatile and facile approach to low dimensional (1D or 2D) integration of π-conjugated units, which is hardly covered by the conventional studies on organic electronics. Therefore, there should develop a novel methodology for manipulating wide variety of molecules in low-dimension and high-density, which brings about precise integration of various π-conjugated molecules at the nanometer-sized space. In this study, we focused on Langmuir-Blodgett (LB) technique and propose a versatile and facile low-dimensional integration approach of π-conjugated units. The LB technique completes at room-temperature and ambient pressure conditions and provides highly ordered molecular monolayers, which was revealed by our previous studies. Namely, acrylamide-based amphiphilic polymers with long alkyl chains such as poly(N-dodecylacrylamide) (pDDA) forms highly stable polymer monolayers at air-water interface. Herein, acrylamide-based polymers with several π-conjugated units were designed to demonstrate versatile approach to low-dimensional integration: pCzAA with carbazole and p(PhAA-mHBT) with bithiophene. For these homopolymers pCzAA and p(PhAA-mHBT), only a small amount (less than 20 mol%) of pDDA enhances monolayer stability. These high mixing ratio of polymers with π-conjugated units (over 80 mol%) exceeds that of previous studies based on copolymerization (30 mol%) and also the percolation threshold in two-dimensional plane (45 mol%), which assures charge transport pathway in the in-plane direction. The small amount of pDDA into pCzAA or p(PhAA-mHBT) also enabled successive transfer of the blend monolayers on hydrophobic substrates, indicating the possibility of continuous tuning of dimensionality between 2D and 3D by the multilayer stacking process. Furthermore, x-ray reflectivity (XRR) measurements show that the pDDA:pCzAA films had a highly ordered layered structure with uniform monolayer thickness of 1.5-1.7 nm depending on the mixing ratio. Therefore, we conclude that low-dimensional and high-density integration of π-conjugated units can be realized by LB technique with acrylamide-based polymers with π-conjugated units.

Semiconducting polymers present a low-cost and flexible alternative to inorganic semiconductors for applications ranging from environmental and biomedical sensors to electrochromic windows and photoelectrochemical devices. However, technological implementation so far has been hindered by a lack of control over polymer microstructure and, as a result, over the flow of electronic and ionic charge carriers. Improving the understanding of correlations between preparation conditions, structural properties, and functionality is therefore crucial to promote further advancement of next-generation polymeric electronics. In the present work, the effects of varying morphology and molecular order in thin films of poly-(3-hexylthiophene) (P3HT) on charge transport and interfacial charge transfer in contact with a liquid redox electrolyte were investigated.
Preparation of P3HT films by spin casting or by electrochemical deposition under potentiostatic or pulsed potential control yielded polymer structures with varied degree of crystallinity and electronic structure, as observed by (spectro)electrochemical measurements, wide angle X-Ray scattering (WAXS), and photoelectron spectroscopies. Electrochemical impedance spectroscopy (EIS) at varied bias potentials revealed a comprehensive picture of mass transport and interfacial charge transfer in P3HT/solution junctions at different doping levels of the polymer. Scanning electrochemical microscopy (SECM) was utilized to assess how the variation in preparation conditions affected both the morphology of the liquid-contacted polymer surface as well as the spatial distribution of electrochemical activity across the surface. The rate constant of hole transfer from the polymer to standard redox probes in solution was measured at selected sites to gain a more detailed view of correlations between surface morphology and interfacial charge transfer behavior.
Important implications regarding the optimization of conductive polymer deposition for electrochemical and photoelectrochemical applications will be discussed.

Controlling crystallinity and molecular packing at nano- and macroscopic length scales in conjugated polymer thin films is vital for improving the performance of polymer based electronic devices. I will present about how the inherent amphiphilicity of rigid donor-acceptor copolymers used in high performance polymer electronics can be leveraged to allow the formation of highly ordered lyotropic mesophases. By increasing the length and branching of solubilizing chains, amphiphilicity can be increased, and lyotropic liquid crystalline mesophases are observed in selective solvents. These lyotropic mesophases consist of chain extended polymers exhibiting close, ordered π-stacking. This is evidenced by birefringent solutions, and red-shifted absorbance spectra exhibiting pronounced excitonic coupling. Crystallinity developed in solution can be transferred to the solid state, and thin films of donor-acceptor copolymers cast from lyotropic solutions exhibit improved crystalline order in both the alkyl and π-stacking directions. Due to this improved crystallinity, transistors with active layers cast from lyotropic solutions exhibit a significant improvement in carrier mobility to those cast from isotropic solution, reaching a maximum value of 0.61 cm²V^-1s^-1. This approach of rational side chain design bridges the gap from solution structure to solid state structure, and is a promising and general approach to allow the expression of lyotropic mesophases in rigid conjugated polymers.

For the solution processing of organic photovoltaics on an industrial scale, the exclusion of halogenated solvents is a necessity. Inkjet printing is a mask-less and non-contact deposition technique that is potentially suited for rapid prototyping and/or manufacturing of large-area thin-film polymer organic semiconductor devices. Nevertheless two challenges need to be overcome before flat films with a suitable macromorphology are available with inkjet printing. On the one side is the limited solubility of most semiconducting polymer/fullerene blends in non-halogenated solvents results in ink formulations with low viscosities which poses limitations to the use of roll-to-roll compatible deposition processes. On the other side the fluid migration over the substrate and evaporation-induced solvent flow within the fluid puddle, can severely distort film shape and profile, e.g., through the "coffee-stain" effect. Here we report the application of an amorphous absorber polymer based on fluorene with different content of dialkyl substituted diphenyl-benzopyrazine or diphenyl-thienopyrazine and triphenylamine units (PFDTBTP). This polymer was synthesized with high molecular weights (M_n ~ 10⁵ g/mol) over the environmentally friendly Suzuki polymerization with very good solubility in non-chlorinated aromatic solvents. The blend with PC₇₀BM leads to moderate efficiencies in inverted OPV cells of about 6 % performance. In this case the deposition of the photoactive layer was done by spin coating from dichlorobenzene. Based on these results the blend layers were prepared with inkjet printing and a special emphasis was placed on finding a chlorine-free solvent system which could be used to produce devices with high performance. Solar cells with inkjet printed photoactive layers that were deposited from chloro-/trichlorobenzene had efficiencies of 3.5% which compares to 2.7% for layers that were printed from anisole/tetralin in a normal device structure. Printed blend films were analyzed AFM and EF-TEM to better understand the device efficiency. A coarser phase separation was found for films that were printed from anisole/tetralin which could be related to the partial solubility of PFDTBTP and PC61BM in anisole. In general, chlorine-free solvent systems can be used to inkjet printed PFDTBTP:PC61BM without greatly reducing the device efficiency. PFDTBTP and other polymers with similar structures are good candidates for industrial scale processing of organic solar cells because they can be printed and hazardous chlorinated solvents are not needed to achieve acceptable performance.