Symposium EP02 : Excitonic Materials—Physics, Characterization and Devices

Through almost 30 years’ research and development, starting from fluorescence molecules, organic light emitting diodes (OLEDs) finally realized the ultimate electroluminescence (EL) efficiency, i.e., nearly 100% electron to photon conversion by using a novel conceptual pure aromatic emitter of thermally activated delayed fluorescence (TADF)¹. TADF enabled harvesting all electrically generated singlet and triplet excitons by engineering reverse intersystem crossing. In fact, the diversity of molecular design allowed a wide variety of new compounds as TADF emitters. Successively, as the post OLEDs, the realization of organic semiconductor laser diodes (OSLDs) has been anticipated for a long time. For realizing current driven lasing, however, we have to overcome some critical issues such as high current injection associated with joule heating, and exciton dissociation induced by annihilation and quenching processes. In this talk, we will review the development of OLEDs’ emitters along with the clarification of the exciton dissociation processes peculiar to electrical excitation. In particular, we highlight our recent success in realizing quasi-CW lasing by engineering both singlet and triplet excited states².

Refs. [1] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature, 492, 234-238 (2012). [2] A. S. D. Sandanayaka, T. Matsushima, F. Bencheikh, K. Yoshida, M. Inoue, T. Fujihara, K. Goushi, J.-C. Ribierre, C. Adachi, Sci. Adv., 3, 4, e1602570 (2017).

Mixed host emissive layers have been widely employed to achieve high efficiency phosphorescent organic light-emitting devices (OLEDs), and are particularly promising for reducing efficiency roll-off and improving lifetime. These co-host devices are known to yield broad recombination zones, and hence reduce exciton density. However, the relationship between co-host properties and degradation is not well understood, and there are no established co-host design principles to optimize lifetime. We apply in situ measurements of photoluminescence (PL) efficiency during device operation to understand how properties of mixed hosts influence PL and electroluminescence (EL) degradation. The wide recombination zone in these devices is found to directly mitigate PL degradation while not significantly decreasing other loss mechanisms, such as exciton formation efficiency. Notably, the kinetics of degradation in PL and exciton formation are found to differ, suggesting distinct degradation mechanisms. An important implication of this finding is that a single mechanism cannot be assumed when attempting to model OLED degradation, and that losses to exciton formation efficiency may originate outside the emissive layer. These results yield deeper insight into degradation mechanisms in co-host OLEDs, and may suggest different roles for excitons and polarons in overall device degradation.

For the first time multiple exciton generation (MEG) is observed in all-inorganic perovskite nanocrystals (IP-NCs): we demonstrate this effect in a colloidal dispersion of CsPbI₃ NCs with a bandgap energy of 1.78 eV.[1] Due to the recent demonstration of a stable solar cell based on CsPbI₃ NCs, [2] this material has suddenly changed its status from being a scientific curiosity to a highly-promising new alternative for perovskite-based applications. MEG is of interest because it can significantly enhance the efficiency of energy conversion processes in photo-detectors and solar cells. For solar cells in particular, an overall increased efficiency of 44% can be expected for devices that make use of MEG. [3] Previous investigations conducted larger bandgap Br-containing IP-NCs failed to reveal MEG, showing no specific increase in carrier generation rate when exciting with a photon energy (more than) twice the bandgap. Here, we explicitly demonstrate the MEG effect in CsPbI₃ NCs by making use of ultrafast transient absorption spectroscopy. By comparing the photo-induced transients at different pump photon energies, typically below and above the threshold energy for CM, we observed the fingerprint of CM, in the form of a fast transient induced by Auger recombination. We confirm this observation by quantifying the fast decay as being induced by Auger interaction between multiple carriers co-localized in one IP-NC.

[1] C. de Weerd & L. Gomez et al., 2018, submitted
[2] Swarnkar et al. Science 2016
[3] A.J. Nozik, Nano Letters 2010

We demonstrate a facile route to obtain strong and broad-band circular polarization of electro-luminescence in single-layer polymer OLEDs. As light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that the polymer self-assembles into a multi-domain cholesteric film, simply requiring thermal annealing. We achieve high levels of circular polarization of electro-luminescence (up to 40% excess of right-handed polarization), which are between the highest reported for polymer OLEDs. We model the surprisingly strong and broad-band polarization by taking into account circular selective scattering of electro-luminescence in the emitting cholesteric layer. These results indicate an easily accessible route to develop full circular polarization in OLEDs. More generally, our work demonstrates that chiral supramolecular self-assembly can be used to combine photonic and semiconducting properties for advanced control of light in organic optoelectronic devices.

High efficiency solid state lighting devices have the potential to significantly reduce lighting energy usage while also offering good color rendering and longer lifetimes than conventional lighting sources. While organic light emitting diodes are promising candidates for this application, their operational lifetime is limited by the blue phosphorescent chromophore. We demonstrate stacked white phosphorescent light emitting devices (SWOLEDs) with lifetimes (as determined from the time it takes to lose 30% of the initial luminance of 1000 cd/m²) of up to 80,000 hours. The correlated color temperature of the devices ranges between 2780-3300 K with color rendering index as high as 89. The three emitter devices (red, green, and blue) contain up to five stacked elements, and employ red emitting blocking layers, stable charge generation layers, graded doping, and hot excited state management to achieve long lifetime. The materials and layer structures used and design principles for SWOLEDs are discussed.

In a molecular photovoltaic device, charge separation and energy conversion result from the evolution of a photogenerated exciton into a charge separated state, in competition with recombination to ground. The efficiency of charge separation is a function of the molecular packing and energy level alignment near the interface, and of disorder in these properties. Understanding the effect of structure, energetics and disorder on the competition between charge separation and recombination help to identify the factors controlling device photovoltage and ultimately conversion efficiency. Here, we address the factors controlling photovoltage in molecular donor: acceptor solar cells using a combination of electrical and spectroscopic measurements and numerical models. We explore the limits to Voc using a model of non-radiative recombination, building on prior work [1], and demonstrate how choice of materials and control of processing may influence voltage losses. We use these results to consider the relative importance of interface and bulk regions of the materials and the ultimate limitations placed on solar to electric conversion by the molecular nature of the materials. In a second application example we address the role of chemical structure, molecular organisation and environment on the efficiency of charge separation, and subsequent photocatalytic activity, in conjugated polymer photocatalysts. We consider the relevance of models of charge separation in organic solar cells to photocatalysis.
[1] J. Benduhn et al. Nat. Energy (2017) DOI: 10.1038/nenergy.2017.53

Recently in the organic photovoltaics community, there has been increased attention to the role of local morphology on the energetics and behavior of organic donor/acceptor charge transfer (CT) states. Different molecular orientations, local dielectric environments, and degrees of exciton delocalization at the donor/acceptor interface have been shown to significantly impact the CT state energy. This morphological sensitivity of the CT state energy is of particular importance in singlet fission and upconversion solar cells, where a specific alignment of the CT state with respect to the triplet state of the singlet fission or upconversion material is a necessary condition to the function of each device. In a singlet fission solar cell, the CT state must be lower in energy than the triplet state of the fission material in order to take advantage of the multiple exciton generation process.

Pentacene/C₆₀ is one of the most widely studied singlet fission donor/acceptor systems, and in the literature, it is usually assumed that its donor/acceptor interface is triplet dissociating. In our study, we use sensitive external quantum efficiency and photothermal deflection spectroscopy methods to investigate the CT state spectrum of pentacene/C₆₀ planar and bulk heterojunction solar cells under different blend morphologies and find that the interface is triplet dissociating for most, but not all, of the device configurations. We find that the CT state energy is raised in pentacene:C₆₀ blends with low pentacene content, and if raised sufficiently, the interface presents an energetic barrier for pentacene triplet dissociation. We also investigate the impact of a poly(3-hexylthiophene) (P3HT) underlayer on the CT state spectra, and find that the CT state energy remains lower than that of the pentacene triplet for all of the studied blends. P3HT is often used as a triplet blocking layer in pentacene/C₆₀ solar cells, but based on these results, the underlayer may also have the added benefit of promoting a blend morphology that induces a low energy CT state that is always able to dissociate pentacene triplets. Our spectroscopic studies are complemented by atomic force microscopy imaging, which is used to correlate the patterns in the CT state energy spectrum to the nanoscale morphology of different pentacene/C₆₀ films. The results of our study demonstrate the morphological sensitivity of donor/acceptor CT states and highlight the need for careful CT state characterization in singlet fission solar cells.

Photon up-conversion (UC) enables the generation of high energy light from a lower energy excitation, thus it is a process of great interest for solar energy applications. UC allows for large anti-Stokes emission, recovering a fraction of photons not absorbed by devices and converting them to an energy range that is adequate for absorption, thus increasing solar cell efficiency. Among all up-conversion processes, sensitized triplet-triplet annihilation based UC (sTTA-UC) is the most promising for real-world applications, thanks to its high efficiency (~30%) that can be reached at excitation intensities comparable to the solar irradiance. In sTTA-UC, high energy light is generated after the annihilation of metastable triplet excitons on emitter molecules that generates a high-energy emissive singlet. These triplets are optically dark and thus they are populated via Dexter energy transfer from a sensitizer moiety employed as light harvester. Conventional sTTA-UC systems use metalated porphyrins as sensitizers and aromatic polyacenes as emitters. However, solid state systems for sTTA-UC are generally characterized by low diffusivity of the dyes, which strongly limits the UC yield at low powers, and have to face serious problems due to the aggregation of the optically active molecules at the high concentrations employed.

Here we introduce a novel approach towards the development of solid-state sTTA-UC by using optically active porous aromatic frameworks nanoparticels (PAFs), organic structures composed of sTTA-UC emitters covalently connected by optically inert cross-linking units. The high density of emitters in the PAF structure allows excitons to migrate and annihilate even in disordered networks. We synthesized a series of PAF by linking 9,10-diphenylanthracene (DPA) molecules with tetraphenylmethane units in different proportions. By using Pt(II)-octaethylporphyrin, we achieved green to blue UC, demonstrating for the first time sTTA-UC with a 10% quantum yield in porous, organic and ultra-stable covalent structures. Remarkably, we were able to achieve the maximum UC efficiency with an irradiance as low as 10 suns, highlighting the potential of these advanced materials for future applications in photonics.

The ability to regulate energy transfer pathways through materials is an important goal of nanotechnology, as a greater degree of control is crucial for developing sensing, solar energy, and bioimaging applications. Such control necessitates a toolbox of actuation methods that can direct energy transfer based on user input. Here we propose a novel molecular exciton gate, analogous to a traditional transistor, for controlling exciton migration in chromophoric systems. The gate may be activated with an input of light or an input flow of excitons. Unlike previous gates and switches that control exciton transfer, our proposal does not require isomerization or molecular rearrangement, instead relying on excitation migration via the second singlet (S2) state of the gate molecule--hence the system is named an "S2 exciton gate." After presenting a set of system properties required for proper function of the S2 exciton gate, we show how one would overcome the two possible challenges: short-lived excited states and suppression of false positives. Precision and error rates are studied computationally in a model system with respect to excited-state decay rates and variations in molecular orientation. Finally, we demonstrate that the S2 exciton gate gate can be used to produce binary logical AND, OR, and NOT operations, providing a universal excitonic computation platform with a range of potential applications, including e.g. in signal processing for microscopy.

Exciton-photon polaritons that emerge in the strong light-matter coupling regime have been studied extensively in optical microcavities using a variety of organic and inorganic semiconductors. Being comprised of charge neutral excitons and photons, the resulting polaritons also carry no charge and therefore their motion cannot be manipulated directly with applied electric fields. The first half of this talk will focus on strong coupling between light and charge-carrying polaron optical excitations in an organic semiconductor at room temperature. We show that a radical cation transition of hole-doped TAPC can be strongly coupled to the optical field in a planar microcavity to yield polaron polariton states with a vacuum Rabi splitting >0.3 eV. The resulting polaron polaritons are unique to organic semiconductors and may lead to increased Coulombic polariton-polariton interaction that reduces the threshold for phenomena such as parametric amplification and Bose-Einstein condensation as well as providing a pathway to exploit charged polaritons in practical optoelectronic devices.

The second half of the talk will focus on bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment. These states have a long history that dates back to the early work in conducting polymers; however, they are considered unlikely to form in organic semiconductor devices such as LEDs, photovoltaics, and transistors because of the strong on-site Coulomb repulsion energy penalty, known as the Hubbard energy. Here, we use charge modulation spectroscopy to directly reveal a bipolaron sheet density >10¹⁰ cm^-2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor TPD. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic semiconductor devices than previously thought.

We will present recent nonlinear optical experiments on organic microcavities in the strong-light matter coupling regime. In this regime, new quasiparticles called polaritons are typically introduced to describe the hybrid exciton-photon state. When this occurs, the photon inherits the intrinsic nonlinearity of the exciton and furthermore, the nonlinearity is resonantly enhanced. We will use this fact to demonstrate room-temperature superfluidity of exciton-polaritons. In this regime, a laser is used to create a superfluid flow of light, which can flow past defects without scattering. Using interferometric imaging, we can clearly observe the vanishing of turbulance and vortex formation in the superfluid regime. This confirms a 20 year old prediction on the superfluid flow of light by Chiao et al. Then, we will show that infrared organic polaritons can be used to generate efficient and tunable third-harmonic generation throughout the visible spectrum and we will give some perspective on further excitonic effects that could potentially be harnessed using polaritons.

Photocurrent generation in organic photovoltaic (OPV) devices is driven by the creation and subsequent separation of excitons into free electrons and free holes. Materials with low exciton binding energies are able to efficiently separate into free charge carriers and limit recombination pathways. The binding energy of an exciton is dependent on a number of factors, such as that described by the Wannier-Mott construct, which states that the exciton binding energy is proportional to the inverse square of the dielectric constant. Additionally, according to the Clausius-Mossotti relation, the dielectric constant of a material can be related to its density, with increased density providing a higher dielectric constant. As a result, it is expected that increasing the dielectric constant of a material will decrease exciton binding energy, reducing the energy losses for exciton separation.

At present, there has been limited development on increasing the dielectric constant of organic semiconductors. However recent work has shown that the inclusion of triethylene glycol monomethyl ether chains can dramatically increase the dielectric constant at low frequency, while maintaining both the optical and electronic properties of their alkylated derivatives. Additionally, it has been shown that the inclusion of these glycolated units has the ability to increase the molecular packing density of the film, enhancing the high (optical) frequency dielectric constant, and thus, homojunction device performance.

Based on these considerations, this presentation will describe a library of novel glycolated materials that have been engineered with the aim of tuning the dielectric constant to decrease exciton binding energy. Additional considerations such as broad absorption within the visible light spectrum, solution processability, and engineered energy levels to optimise the open circuit voltage, to maximise charge generation and extraction will be discussed. Finally, the applicability of dielectric constant manipulation will be reported in the context of homojunction OPV device design and fabrication.

Adv. Energy Mater. 2012, 2, 1246–1253
Chem. Commun., 2015, 51, 14115-14118
J. Mater. Chem. C, 2017, 5, 3736-3747

Very recently, we have developed a nano-lens array (NLA, lens size: <1 um) fabrication technology using a one-step vacuum deposition.¹ The sequence of the dry NLA process was as follows. First, N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB, purity : >99.9%) powders were put into a vaporize. Secondly, they were heated to generate organic vapors. Thirdly, the vapors were transferred by means of nitrogen gas and deposited on the samples. In contrast to the two-step fabrication of a micro-lens array (MLA, lens size: several hundred micrometers) which is generally used to improve the efficiency of organic light emitting diodes (OLEDs), our prosess is spontaneous and obviates the need of masking, modling and curing, hence it can be easily applicable to the industry. Additionally, benefited from their small sizes of sub-um, NLAs do not distort the image of the active matrix OLED (AMOLED) displays with a pixel size of same or less than 50 um. The other advantages of NLA technology were size controllable between ~50 nm and ~500 nm, and easily formable on both a rigid and a flexible substrates. The NLA integration in top-emitting OLEDs (TOLEDs) by forming the NLA on a indium zinc oxide (IZO) significantly enhanced a light extraction efficiency and color stability.^1,2
Currently, flexible AMOLED market is experincing rapid growth. Moreover, TOLEDs better fit into AMOLEDs, as TOLEDs are structurally unaffected by the number of thin film transistors integrated on a substrate. Therefore, extensive studies on the efficiency and color stability of flexible TOLEDs have become a greater importance. In this talk, the device characteristics of flexible TOLEDs using a polyethylene-naphthalate film (Teonex, DuPont Teijin Films) as a substrate, without and with NLAs, are presented. Prior to the flexible TOLEDs fabrication, rigid TOLEDs using a glass substrate were prepared to investigate the feasibility of a thin film encapsulation layer (Al₂O₃, 50nm) deposited on the OLED layer. The difference between flexible and rigid TOLEDs is a substrate material. The Al₂O₃ layer was formed using atomic layer deposition equipment (LUCIDA D100, NCD). Trimethylaluminium and ozone were used as precursors. The Al₂O₃ thickness was measured using ellipsometry (Nanospec 9100, Nanometricslpitas). Lighting tests using rigid TOLEDs indicate that air exposure does not damage the Al₂O₃ encapsulated OLED device. Furthermore, NLAs with various sizes are employed in the OLEDs, and the effects of the introduction of NLAs on the OLEDs will be discussed.

References
1. Y. -S. Park, K. -H. Han, J. Kim, D. -H. Cho, J. Lee, Y. Han, J. T. Lim, N. S. Cho, B. Yu, J. -I. Lee and J. -J. Kim, Nanoscale, 9, 230 (2017)
2. K. -H. Han, Y. -S. Park, D. -H. Cho, Y. Han, J. Lee, B. Yu, N. S. Cho, J. -I. Lee and J. -J. Kim, Nanoscale, submitted (2017)

Deducting the exact molecular alignment of heavy metal phosphors, such as Ir-complexes, performing as emissive species in organic light emitting diodes has been in the focus of research within the last few years. Although substantial progress has been made using angular dependent spectroscopy^[1], exciton lifetime measurements^[2] and several computational approaches^[3,4], the exact morphology is still not unveiled.

In order to pinpoint the dye alignment, we introduce a novel measurement technique, identifying the preferential orientation of the permanent dipole moment, which is present in many common emissive molecules. Therein the experimental procedure is based on impedance spectroscopy, measuring interfacial polarization induced by alignment of electrical dipoles. This technique has previously been used to identify the preferential alignment of Tris(8-hydroxyquinolinato)aluminium, which has no indication for optical anisotropy. ^[5]

The application of this new measurement technique reveals a huge interfacial polarization of the emissive guest host system for heteroleptic dyes, while its homoleptic counterparts show no evidence for this effect. For heteroleptic Ir-complexes the resultant polarization depends on the guest-host composition, whereupon the dependence is non-linear indicating concentration dependent processes like aggregation.

Combining both, optical and electrical experiments, allows for a more detailed insight into the morphological behavior of heteroleptic dye molecules. It is important to note that due to the amorphous nature of the film not only specific molecular alignments, but a widespread distribution has to be taken into account. The results reveal a very distinct alignment of the molecular C2 symmetry axis at low guest concentrations, wherupon the values are in agreement with common predictions for molecular orientation.

While the presented film characterization technique is of huge importance for the in depth understanding of Ir-dye molecules, it can also be applied to further polar organic systems. Thus, enabling a new characterization technique for thin films in organic electronics.

[1] M. Flämmich, J. Frischeisen, D. S. Setz, D. Michaelis, B. C. Krummacher, T. D. Schmidt, W. Brütting, N. Danz, Organic Electronics 2011, 12, 1663.
[2] R. Mac Ciarnain, D. Michaelis, T. Wehlus, A. F. Rausch, S. Wehrmeister, T. D. Schmidt, W. Brütting, N. Danz, A. Bräuer, A. Tünnermann, Scientific reports 2017, 7, 1826.
[3] C. Tonnelé, M. Stroet, B. Caron, A. J. Clulow, R. C. R. Nagiri, A. K. Malde, P. L. Burn, I. R. Gentle, A. E. Mark, B. J. Powell, Angewandte Chemie (International ed. in English) 2017, 56, 8402.
[4] C.-K. Moon, K.-H. Kim, J.-J. Kim, Nature communications 2017, 8, 791.
[5] L. Jäger, T. D. Schmidt, W. Brütting, AIP Advances 2016, 6, 95220.

For transparent OLED (TOLED), additional technological challenges beyond those of conventional bottom emitting OLEDs (BOLEDs) have to be met. Unlike in BOLEDs, TOLEDs cannot benefit from scattering layers for improved light extraction. If a scattering layer is employed in a TOLED, a frosted look unacceptable for window-integrated lights or head-mounted displays would result. Thus, in TOLEDs, a large amount of light is trapped in wave-guided modes, and their luminous efficacy is significantly lower than that of conventional BOLEDs. Additionally, an ITO top electrode is technologically challenging to fabricate without damaging the underlying organic layers. Other approaches to fabricate a transparent top-electrode using thin metal layers (Ag or Au) promise simplified manufacturing but suffer from strong reflections sacrificing light extraction efficiency and optical transmittance. To increase light extraction through such a metal top electrode, several groups introduced transparent capping layers. Those layers serve as anti-reflective coatings and do not only increase luminous efficacy but also transmittance.
In this work, we use the transfer matrix method (TMM) to optimize TPBi capping layers with respect to light extraction and transmittance in TOLEDs. Our devices are prepared on pre-structured ITO-on-glass substrates using organic vapor phase deposition (OVPD) in an AIXTRON Gen1 OVPD tool. The TOLEDs comprise three organic semiconductors (CBP, Ir(ppy)₃ and TPBi) forming an efficient simplified phosphorescent OLED stack. A transparent cathode of 2 nm Cs₂CO₃, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The refractive indices of all materials in the TOLEDs (glass, ITO, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance measurements. With these spectrally resolved data, we calculate the transmittance of TOLEDs with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm – 1100 nm) by a series of experiments. By chosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated TOLEDs with an optical transmittance which was strongly enhanced from 47 % (reference without capping layer) to 65 %, measured at 555 nm. Simultaneously, the bottom-to-top ratio of luminance is tuned by the capping layer in accordance with our TMM simulations. That ratio was changed from 3.6 (reference device without capping layer) to 2.4 for the transmission-optimized device. A reference BOLED with the same organic stack but with an opaque 2 nm Cs₂CO₃/120 nm Al cathode reaches a luminous efficacy of 24 lm/W (at 1000 cd/m²). However, our transmission-optimized TOLED has a value of 6 lm/W (1000 cd/m² total luminance). The reduced efficacy is probably owed to an inferior activation of the Cs₂CO₃ contact dopant by the 2 nm Al film compared to the flash-evaporated 120 nm Al layer of the BOLED.

A general strategy for measuring the energy transfer efficiencies in multi-fluorophore assemblies is demonstrated. The present method is based on spectral shaping of the excitation light with molecular solutions representing donor and acceptor fluorophores, which causes a suppressed excitation of the respective donor and acceptor molecules in the sample. The changes in the acceptor emission resulting from spectral shaping of the excitation light are then used to determine the energy transfer efficiencies (E_x) associated with all participating donor-acceptor pairs. Here, the technique is demonstrated through energy transfer (ET) measurements in a 4-fluorophore construct featuring a DNA supported assembly of three donor/donor-relay (Cy3, Cy3.5, and Cy5) and one acceptor (Cy5.5) molecules. The resulting E_x were validated using the standard photoluminescence (PL) quenching approach as well as measurements of partial 2- and 3-dye assemblies. The present work highlights general benefits of the spectrally-shaped excitation approach to measuring donor-acceptor energetics, including the ability to resolve the spectral cross talk between multiple fluorophores and to exclude charge transfer contributions into donor PL quenching

The upconversion system involving CdSe nanocrystals (NC) light absorbers and organic emitter molecules has shown advances in efficiency and holds promise for robust applications where ultraviolet light is generated from the visible. Further advances in efficiencies will rely heavily on a thorough understanding of the underlying energy transfer systems. This work explores the charge, and triplet energy transfer from five differently sized nanocrystals, ranging from 2.2 to 4.2 nm in diameter, corresponding to decreasing band gap. First, by using dynamic and static quenching of benzoquinone, a known probe of photo-induced electron transfer, as well as transient absorption measurements of the triplet energy transfer (TET) to bound 9-anthracene carboxylic acid (9-ACA) ligands, we can directly calculate the rate of charge and energy transfer from CdSe NCs. We can compare the transfer mechanism of triplets on NCs to the two electron Dexter energy transfer rates. Second, by synthesizing a range of NC sizes, we can vary the driving force for electron and energy transfer in the CdSe-9-ACA system. This allows us to determine the mechanism, as either Marcus-Hush, which treats the NC as a sphere and the solvent as a dielectric, or Marcus-Jortner, which considers the coupling between the NC and the vibrational mode of the solvent. Determining the mechanisms can guide us to design a better system with improved energy transfer for engineering photon upconversion systems.

Conventional solar cells are limited to a theoretical maximum power conversion efficiency of 34%, commonly known as the Shockley-Queisser limit. Singlet exciton fission, a process by which one high-energy singlet exciton is converted into two triplet excitons of half the energy, is a promising way of overcoming this limit. This process has been observed in a range of organic semiconductors where pentacene is the most studied for this prospect. Triplet excitons cannot decay to radiatively to the ground state, so that trap states are presumably the main decay channel. To understand behavior of tripled excitons we investigate the charge-carrier-trap distribution in pentacene-based devices using deep-level transient spectroscopy and admittance spectroscopy. We find that Schottky diodes made from pristine Pentacene have a trap state with a density of about 10¹⁷ cm^-3 and an activation energy of 0.5 eV. By systematically adding impurities into the pristine pentacene layer we show that the variety and the density of charge-carrier traps can be altered, affecting the triplet lifetime. We then investigate the performance of singlet fission solar cells with various trap state densities.

Hybridisation of quantum emitters and plasmonic nano-structures has attracted much attention over the last years, due to their interest in the design of plasmon-based nano-lasers [1,2] or to achieve long-range qubit entanglement [3,4]. Recent theoretical studies [5,6] suggest a plasmonic super-radiant mechanism to increase the rate of emitters, similar to Dicke super-radiance [7].
In this talk, we will report a review of our work in these domains and explain the salient features of the involved effects.
As such, i) we provide experimental evidence of plasmonic super-radiance of organic emitters close to a metal nanosphere at room temperature. This observation of plasmonic super-radiance at room temperature opens questions about the robustness of these collective states against decoherence mechanisms which are of major interest for potential applications.
Ii) We propose a new type of nanodevice, capable of both path-selectivity and anisotropic lasing that is based on loss-compensation and amplification by a localized plasmon polariton [8]. The nano-device is a Y-shaped plasmonic nanostructure embedded in an anisotropic host medium with gain. The anisotropy leads to the path selectivity, an effect which is more pronounced once gain is included. The path-selectivity may be coupled with activation of a rotation of the anisotropic host medium for inducing a light-guiding switching functionality.
Finally, iii) we demonstrate both experimentally and theoretically how to manipulate strong coupling between the Bragg-plasmon mode supported by an organo-metallic array and molecular excitons in the form of J-aggregates dispersed on the hybrid structure [9]. We observe experimentally the transition from a conventional strong coupling regime exhibiting the usual upper and lower polaritonic branches to a more complex regime, where a third nondispersive mode is seen, as the concentration of J-aggregates is increased. The numerical simulations confirm the presence of the third resonance. We attribute its physical nature to collective molecule-molecule interactions leading to a collective electromagnetic response. It is shown that at the energy of the collective mode molecules oscillate completely out of phase with the incident radiation acting as an effective thin metal layer.
References
[1] J.G. Bohnet et al. Nature 484, 78–81 (2012).
[2] M.A. Noginov et al., Nature 460, 1110–2 (2009).
[3] R. Kolesov et al., Nature Physics 5, 470–474 (2009).
[4] A. Gonzalez-Tudela et al., Phys. Rev. Lett.106, 020501 (2011).
[5] V.N. Pustovitet al., Phys. Rev. Lett.102, 077401 (2009).
[6] D. Martín-Cano et al., Nano Letters 10, 3129–3134(2010).
[7] R.H. Dicke. Phys. Rev.93, 99-110 (1954).
[8] A. Yamada, D. Neuhauser and R. A. L. Vallée. Nanoscale 8, 18476–18482 (2016).
[9] P. Fauché, C. Gebhardt, M. Sukharev, M., R. A. L. Vallée (2017). Scientific Reports 7, 4107 (2017).

Semitransparent organic solar cells (ST-OSCs) show great promising applications in building integrated photovoltaics, solar-powered automotive and wearable electronics. However, the development of ST-OSCs is significantly lagging behind opaque OSCs, probably due to the lack of suitable photoactive materials. In this work, four different squaraines, IDPSQ, SQ-BP, D-BDT-SQ and AzUSQ are successfully used as donors in ST-OSCs (Figure 1), whose colors can be tuned form purple to blue to green to dark green, respectively. When using 10 nm Ag as the transparent electrode, the ST-OSCs fabricated with IDPSQ: PC₇₁BM, SQ-BP: PC₇₁BM, D-BDT-SQ: PC₇₁BM, and AzUSQ: PC₇₁BM yield power conversion efficiencies (PCEs) of 2.96%, 4.36%, 4.57%, and 1.63%, respectively, whose colors are purple, green, reddish brown, and light brown, respectively. Remarkably, compared with their traditional opaque OSCs (PCEs of 3.95%, 5.45%, 5.84%, and 1.91%, respectively), all of the decreases in the PCEs of ST-OSCs are less than 25%, which is very rare for ST-OSCs. Moreover, all of the ST-OSCs exhibit good average visible transmittance (AVT) of 25-30%, favorable CIE color coordinate closely to the white color point, and high color rendering index (CRI) of 72-97%. Finally, by changing the thickness of the electrode (Ag), the relationships between the PCEs and AVTs are investigated in detail, and an impressive PCE of 4.91% with an AVT of 25.1% and a CRI of 97% can be obtained in the D-BDT-SQ: PC₇₁BM-based ST-OSCs, which is the highest value for small molecules-based ST-OSCs. This remarkable result first demonstrates squaraines should be very promising candidates for promoting the development of ST-OSCs. These colorful squaraines also offer a room for the development of tandem solar cells and solar sharing for agriculture.

Abstract
Thermally activated delayed fluorescence (TADF) so called third-generation organic light emitting diodes (OLEDs) materials are now being actively studied to replace current phosphorescent and fluorescence materials. Such TADF can achieve 100% internal quantum efficiency by harvesting electrical excited triplet excitons through spin up-conversion from triplet (T₁) to singlet (S₁) state [1]. To incresase the efficiency of TADF OLEDs, high T₁ host materials are desired to suppress triplet exciton quenching from dopant to host material. In addition, bipolar type host materials are desired for good charge balance and suppress the exciton polaron annihilation in the dopant. Furthermore, bipolar hosts could result in good device stability and better device performances such as low driving voltage, high efficiency, and low efficiency roll-off characteristic. To date, phosphine oxide series have been widely used due to their high T₁ characteristics, but their bipolar characteristics and chemical stability are known to be poor [2].
In this work, we report newly synthesized two bipolar host materials, KHU-TBH 1 and KHU-TBH 2. For the bipolar characteristic, our hosts have carbazole as a hole-transport type moiety and carboline and pyridine as electron-transport type moieties. These host molecules were designed to have high T₁ values by reducing the conjugation length with introducing steric hindrance between each moiety. The measured T₁ values of KHU-TBH 1 and KHU-TBH 2 were 2.98 eV and 2.97 eV, respectively. It indicates that our bipolar hosts have suitable high T₁ values for blue TADF OLEDs. We evaluated our two host materials with well-known hole-transport type host, 1,3-bis(N-carbazolyl)benzene (mCP). For a blue TADF dopant, DMAC-DPS was employed for the device fabrication [2]. As expected, the current density-voltage-luminance characteristics of KHU-TBH 1 and KHU-TBH 2 were better than mCP due to thier bipolar characteristics. In addition, the device efficiencies of two hosts were higher than that of mCP. The maximum EQEs of KHU-TBH 1, KHU-TBH 2, and mCP were 22.9%, 18.8%, and 17.5% and the reduced efficiencies from the maximum to 1,000 cd/m² were 0.16, 0.22, and 0.38, respectively.

Acknowledgement
This work was supported by Grant No. NRF-2016R1A6A3A11930666 and the Human Resources Development program (no. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

References
1. K. Sato, K. Shizu, K. Yoshimura, A. Kawada, H. Miyazaki and C. Adachi, Phys. Rev. Lett., 2013, 110, 247401.
2. Q. S. Zhang, B. Li, S. P. Huang, H. Nomura, H. Tanaka and C. Adachi, Nat. Photonics, 2014, 8, 326–332.

Singlet fission is a process in molecules where multiple excitons are generated per photon absorbed. In certain molecules like diphenylhexatriene (DPH), a photon creates an excited state, which can then undergo singlet fission to create more than one electron-hole pair. Bidentate diphenylhexatriene ligands can bind to two inorganic semiconductor nanocrystals or quantum dots at each end of the ligand. This provides a percolating pathway for electrons, holes and excitons, and potentially creates the right morphology for singlet fission. Here, we synthesized DPH with the carboxylic acid, amino, pyridine and imidazole functional groups attached in a bidentate geometry to bind to the PbS nanocrystals. This work will examine the thin film morphology required for efficient singlet fission, charge and triplet exciton transport for inexpensive, solution processed, next-generation solar cells.

Controlling ultrafast charge carrier dynamical processes at donor−acceptor interfaces remains a major challenge for physical chemistry and solar cell communities. The process is complicated by the involvement of other complex dynamical processes, including hydrogen bond formation, energy transfer, and solvation dynamics occurring on similar time scales. In this study, we explore the remarkable effect of hydrogen-bond formation and dynamics on the interfacial charge transfer between a negatively charged electron donating anionic porphyrin and a positively charged electron accepting π-conjugated polymer, as a model system in solvents with different polarities and capabilities for hydrogen bonding using femtosecond transient absorption spectroscopy. Unlike the conventional understanding of the key role of hydrogen bonding in promoting the charge-transfer process, our steady state and time-resolved results reveal that the intervening hydrogen-bonding environment and, consequently, the probable longer spacing between the donor and acceptor molecules significantly hinders the charge-transfer process between the donor and acceptor units. These recent results show that site-specific hydrogen bonding and geometric considerations between donor and acceptor can be exploited to control both the charge-transfer dynamics and its efficiency not only at donor−acceptor interfaces but also in complex biological systems.

The superior optical extinction characteristics of noble metal nanoparticles have long been considered for enhancing the solar energy absorption in light-harvesting devices. The energy captured through a plasmon resonance mechanism can potentially be transferred to a surrounding semiconductor matrix in form of excitons or charge carriers offering a promising light-sensitization strategy. Of a particular interest is the plasmon near-field energy conversion, which is predicted to yield substantial gains in the photocarrier generation. Such short-range interaction, however, is often inhibited by processes of backward electron and energy transfer, which obscure its net benefit. Here, we employ the sample-transmitted excitation photoluminescence (STEP) spectroscopy to determine the quantum efficiency for the plasmon induced energy transfer (ET) in assemblies of Au nanoparticles and CdSe nanocrystals. The present technique distinguishes the Au-to-CdSe ET contribution from metal-induced quenching processes thus enabling accurate estimates of the photon-to-exciton conversion efficiency. We show that in the case of 9.1-nm Au nanoparticles, only 1-2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0-nm Au, the photon-to-exciton conversion efficiency increases to 29.5%. The results of present measurements were used to develop an empirical model for estimating the maximum gain in plasmon-induced carriers versus the mass-fraction of Au in a film.

Since the past few years, porphyrin derivatives have attracted scientists' attention to be involved in the photon-to-charge conversion process in organic solar cells (OSCs). Here, we adopt a supramolecular assembly concept to produce a brand new derivate while preparaing donor-acceptor bulk heterojunction composite for the photoactive material of OSCs. The complexation of nitrogen lone pairs in the bidentate ligands to the axial orbitals of both zinc atoms in zinc-metalated porphyrin dimers (KC2s) forming a new porphyrin derivative, KC2-duplex. According to the UV-vis absorption spectrum, there is a red-shift of Q-band after assembly, indicating an improvement of intermolecular interaction. With this new mixture composed of KC2-duplex and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the photoactive material, the fabricated devices demonstrate a 38.7% enhancement of short circuit current density (J_sc), which is attributed to the improved hole mobility and exciton dissociation probability.

Through careful selection of materials and rigorous engineering of device interfaces, organic photovoltaic devices have broken the 10% power conversion efficiency mark. However, these devices are still operating far below their thermodynamic potential and enhanced understanding of the excitonic processes and free carrier dynamics can bridge that gap. Amongst various loss factors, foremost is a substantial offset between the optical gap of the donor-acceptor materials and the open-circuit voltage of the device. This offset is a direct representation of the energy losses during carrier generation and recombination processes, and are linked to the charge transfer (CT) states of the device. Thus understanding its density of states (DOS) can give us valuable insights to free carrier generation and recombination mechanisms. In this work, we investigate the CT state DOS by applying various external stresses to the normal operating condition of the solar cell. We measure the effect of electric field, excess free carriers and dynamic disorder on the CT states; through voltage bias, background light intensity and temperature controlled external quantum efficiency (EQE) measurements. Analyzing the behavior of the CT states at these stressed conditions we intend to understand the nature of the CT state DOS and its role towards successful free carrier generation and non-radiative recombination processes.

In our voltage bias measurements, we apply an external DC voltage across the solar cell and measure the EQE. Changing the bias voltage, we can operate the solar cell anywhere between the open circuit and short circuit condition, which forces the device to go through different degrees of free carrier recombination. The EQE in Frenkel exciton absorbing region is enhanced at reverse bias and degraded at forward bias, as expected due to exciton polaron annihilation. However, the CT region is less affected by the applied bias voltage compared to the Frenkel region. This is possibly due to a shift in CT energy with increasing bias. In addition, the CT energy shifts are less visible in bulk heterojunction devices compared to planar heterojunctions, possibly due to relatively lower junction electric fields. To probe the role of dynamic disorder on the findings of the voltage bias, we performed temperature controlled EQE measurements. Data showed well-resolved red shifts in CT energy with decreasing temperature, consistent with the literature. But the relative change in EQE vs. temperature reveals that the CT region is also less affected by the decreasing temperature when compared to Frenkel region, similar to the forward voltage bias case. Ongoing EQE measurements under background light bias to selectively flood the CT region with excess excitons and measure how that affects the quantum efficiency, coupled with the electric field and temperature stresses if possible.

Organic semiconductor materials are of interest for optoelectronic applications due to their low cost, solution processability, and tunable properties. Recently, organic pigments derived from natural products have attracted attention, exhibiting extraordinary environmental stability combined with high (photo)conductivity, in spite of their molecular structures not having a fully conjugated core. A subset of such pigments, fungi-derived pigments, represents a naturally sourced, sustainable class of materials that are completely unexplored as organic semiconductors. We explored optical and electronic properties of several fungi-derived pigments, an example of which is a blue-green pigment xylindein, which is secreted by the wood-staining fungi Chlorociboria (C.) aeruginosa and C. aeruginascens. Xylindein exhibits an extraordinary long-term thermal and photostability in solution and in films, a hole mobility of >0.2 cm²/(Vs) in amorphous films, and a photoresponse throughout the ultraviolet and visible wavelength range. In order to understand these properties, we carried out a detailed study of exciton and charge carrier dynamics in xylindein and selected other pigments (such as red and yellow pigments derived from Scytalidium cuboideum and Scytalidium ganodermophthorum, respectively), which will be presented.

A particularly important aspect of the pigment photophysics is related to their ability to form both intermolecular and intramolecular hydrogen bonds. The relative contribution of these interactions to the optical properties depends on the environment, which was established using measurements of optical absorption and time-resolved photoluminescence (PL) of pigments in various solvents and pH buffers, depending on the pigment concentration. In thin films, optical properties are determined by an interplay of p-p stacking and hydrogen bonding resulting in an aggregate formation. The aggregate properties were quantified using temperature and polarization dependent optical absorption and PL spectra and Spano’s theoretical analysis of molecular aggregate spectral characteristics. The optical properties were then related to the molecular packing and film morphology, obtained from XRD and SEM.

Pristine xylindein when deposited from solution forms porous amorphous films; towards improving film morphology, blends of xylindein with several polymers were explored. The (opto)electronic properties of xylindein and xylindein dispersed in polymer matrices were systematically studied to determine electron and hole mobilities, depending on the blend content and film structure and morphology. Measurements of photocurrents in pristine films and in donor-acceptor blends in which a polymer serves as a donor and xylindein as acceptor were also carried out. (Opto)electronic properties of solution-grown crystals formed by a red pigment derived from Scytalidium cuboideum will also be presented.

In organic photovoltaics, fluorination of electron donor molecules or polymers at appropriate positions can lead to an improvement in photovoltaic (PV) device efficiency. Fluorination can change the energetics at the donor-acceptor interface and the morphology of the organic semiconductor blend, which can result in changes in charge separation efficiency, reduced charge-carrier recombination, and increased PV performance. Using ultraviolet photoemission spectroscopy (UPS) measurements of interfacial energetics and external quantum efficiency (EQE) measurements of charge-transfer (CT) states, we have investigated the effect of halogenation on the energetics of model anthradithiophene (ADT) compounds. We investigated bilayer ADT/fullerene C₆₀ interfaces and ADT:C₆₀ blends to emulate interfaces between pure donor-acceptor phases and mixed phases found in typical bulk-heterojunction photovoltaics. Non-halogenated ADT shows a lower energy CT state in ADT/C₆₀ bilayers and higher energy CT states in ADT:C₆₀ blends, while halogenated ADT shows the opposite trend. Surprisingly, in blend systems of halogenated ADTs with C₆₀, the CT states display lower energies as compared to the bilayer system. In bulk-heterojunction photovoltaics, the lower energy CT states in the mixed phase for the halogenated derivatives will likely decrease the probability of charge separation and increase charge-carrier recombination. These CT energies combined with UPS measurements of the interfacial energy landscapes suggest that the beneficial effects often observed upon fluorination are not likely due to the affect of fluorination on energetics at the D-A interfaces.

Organic semiconductors have attracted attention due to their low cost, enhanced processability, and tunable properties. Charge carrier mobilities exceeding 10 cm²/Vs in organic thin-film transistors and power conversion efficiencies over 10% in organic solar cells have been demonstrated in several systems. Another area that has seen dramatic progress is in organic polaritonic devices relying on strong exciton-photon coupling in organic microcavities or on plasmonic nanostructures. The hybridized light-matter states formed in these devices have opened up a host of new and exciting phenomena to explore, such as low-threshold polariton lasing and polariton Bose-Einstein condensation. While these exotic phenomena are being investigated, there are still unanswered questions pertaining to the basic physics of organic polaritons, particularly in crystalline media. One such question involves the dependence of the exciton-photon coupling—enabled by the organic crystal placement in a cavity (or on a plasmonic surface)—on the intermolecular coupling in crystal, which determines exciton physics for “bare” (cavity-free) crystals. We present a systematic study of intermolecular interactions and their effect on photophysics and coupling to cavity/plasmon modes in high-performance functionalized anthradithiophene (ADT) derivatives, depending on the molecular packing.

The fluorinated ADT derivatives (diF R-ADT) with varying side groups R, which we chose as model systems, exhibit identical optical properties in solution, but considerably different properties in the solid state owing to the side groups R controlling the molecular packing and, thus, exciton dynamics. In order to quantify intermolecular interactions, we explored in detail temperature- and polarization-dependent optical absorption and photoluminescence properties of single crystals of several diF R-ADT derivatives, which were analyzed using Spano’s model of molecular aggregate spectra. Strongly anisotropic characteristics and temperature-dependent exciton dynamics were observed, which depended on the molecular packing. The polarization dependence was well described in the framework of switching between the J-aggregate- and H-aggregate-dominated behavior depending on the polarization of light with respect to the crystal axes. The prominence of such behavior could be controlled with a choice of the side group R.

The changes in the exciton dynamics that occurred when diF R-ADT crystals were placed in optical cavities and on plasmonic nanostructured substrates were then studied in detail. Coupling between the cavity (plasmon) modes and excitons was observed, depending on the R group, polarization, and cavity detuning. The properties of light-matter hybrid states were then correlated with the exciton dynamics in “bare” crystals and J/H-aggregate switching behavior in particular.

Flexible plastic substrates have received attention as components in next-generation optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic solar cells because of lightweight, inexpensive, and enable to roll-to-roll mass production. To improve the performance of devices based on flexible substrates, nano-structuring technology, which applies haze in substrate has become indispensable, because planar structure causes unwanted surface reflection and total internal reflection. Recently, there have been tremendous efforts to fabricate nanostructures on substrates such as low index-grid structure, refractive index modulation layer, micro-lens array attaching, and surface modulation including nanoimprinting, embedding scattering particles in film. However, the low index-grid structure requires high temperature process or acid treatment that can damage the polymer substrates. Also, micro-lens array attaching and the surface modulation techniques have complex fabrication process and reduce the total transmittance due to Fresnel reflection and light absorption. Therefore, the hazy substrate which is fabricated at low temperature and simple process is highly required.
In this work, we report the high haze film that manufacture scattering centers in which air is trapped in flexible film by coating a planarization layer on a micro-patterned substrate. Micro-patterns are formed on PET using AgCl nanorods as an etching mask and oxygen plasma treatment. Also, we can control the haze by changing the aspect ratio of micro-patterns to trap large air sites with plasma treatment time. After planarization treatment with colorless hybrid UV-curable polymer, Ormoclear, the average roughness of surface reduced 181 nm to 0.72 nm. The average haze from 0.9 % to 87.92 % has been achieved with the extremely flat surface. Also, the average transmittance has been enhanced from 89.84 % to 93.34 %. The finite-difference time-domain (FDTD) simulations was conducted to demonstrate that the air-trapped sites in the film can effectively extract light with wide angle distribution by inducing haze.

The samples of Er-doped Si-rich-HfO2 films were grown by means of radio-frequency magnetron sputtering method. The samples were divided into three groups: the first group is left as deposited, the second and third groups were treated with thermal annealing at 950^oC and 1100^oC, respectively, in a nitrogen flow environment. To study the effect of thermal annealing temperatures on the optical and structural properties of the films, the X ray diffraction (XRD), scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS) and photoluminescence (PL) have been used.

Photoluminescence spectroscopy (PL), X-Ray Diffraction (XRD), Enegy dispersive spectroscopy (EDS) and Scanning Electron Microscopy (SEM) were used for the comparative characterization of ZnO and ZnO Er nanocrystals (NCs) obtained by spray pyrolysis method with different Er-doping (0, 1, 2, 3, 4 and 5at%). High temperature annealing in the range of 400-900 ^oC for 2 hours in nitrogen flow has been applied after the spray pyrolysis process. SEM study has confirmed the nanocrystal (NC) structure of obtained ZnO and ZnO Er films. X-Ray diffraction has detected the wurtzite crystal structure in ZnO and ZnO Er NCs.
Presented results have shown that the crystal quality of ZnO:Er NC films can be improved at a small level of Er-doping (≤ 3at%) together with intensity enlarging the near band edge (NBE) emission and PL bands related to the inner-shell optical transitions in Er ions. In contrary, at high level of Er-doping (≥3at%) the ZnO:Er crystallinity falling down, the PL intensities of NBE and Er-related PL bands decrease, but the intensity of green PL band related to native defects enlarges. The impact of temperatures at annealing and the mechanisms of mentioned emission transformations are discussed as well.

Ultrasound spray pyrolysis system was used to obtain the Al doped ZnO nanocrystals. Aqueous solutions of zinc acetate (C₄H₆O₄Zn-xH₂O) was used as Zn precursors, and aluminum nitrate (Al(NO₃)₃) was used as Al precursors. All the used solutions were 0.2M. The Al/Zn atomic ratio in the solution was 0%, 1%, 3% and 5%. ZnO nanocrystals (NCs) deposited by ultrasonic spray pyrolysis at 450°C on the Si substrates have been investigated.Scanning electronic microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL) methods have been used for the comparative investigation of ZnO and ZnO:AL NC films. All NCs are characterized by wurtzite crystal lattice, but the size of NCs change in dependence on Al contents. The SEM image of undoped ZnO NCs represents hexagonal grains with the size of 200 nm. The NC size decreases dramatically with rising the Al content in the films. PL spectra of all studied samples were studied at room temperature, as well as in the temperature range 10-300K. PL spectra were complex and onclude the near band edge (NBE) and defect-related emission bands. The Impact of Al doping on the emission and structural parameters has been discussed.

We present a method to measure the exciton diffusion length (L_D) of optically dark triplet states in organic semiconductor thin films. In order to probe these states, we optically inject triplets via energy transfer from an adjacent phosphorescent thin film. Injected triplet excitons migrate through the full thickness of the material of interest before undergoing energy transfer to a luminescent phosphorescent sensitizer. By measuring photoluminescence from the sensitizer as a function of active layer thickness and sensitizer layer concentration, we are able to extract both L_D and the transfer efficiency to the sensitizer. This is in contrast to much previous work that assumes a unity quenching efficiency in measurements of L_D. Here, we demonstrate this method on the archetypical organic light-emitting device (OLED) hole transport material N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) (NPD). An injection layer of tris[2-phenylpyridinato-C²,N] iridium(III) (Ir(ppy)₃) is used with a sensitizing layer platinum octaethylporphyrin (PtOEP) diluted into tris(4-carbazoyl-9-ylphenyl)amine (TCTA). In extracting a value of L_D = (30±5) nm for NPD, we find a direct correlation between sensitizer concentration and the transfer efficiency. Consequently, it is essential to determine the transfer efficiency to the sensitizer in order to extract the correct value of L_D. We expect this technique to be widely applicable with appropriate choice of injection and sensitizer materials.

The most widely used electro-optic devices today employ inorganic semiconductors because of their high dielectric constants, but organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs) are beginning to claim market share because organic materials are cheap, manufacturable on a large scale, non-toxic, and plentiful. The fundamental chemical challenge with organic optical materials, however, arises from their low dielectric constants and localized excitons. Excitations in these materials couple to vibrational modes which substantially hinders migration of the exciton to a heterojunction, thereby fundamentally limiting device efficiency. By exploiting long-range transport in dark states of organic H-Aggregates, the limitations of exciton diffusion length can be circumvented altogether---an approach which is fundamentally different from current state-of-the-art materials for organic photovoltaics. Today, bulk heterojunction OPVs create topologies that eliminate the need for long diffusion lengths, but the blended nature of the donor and acceptor layers significantly increases the probability of charge carrier recombination and trapped excitons. My approach couples individual molecular dipoles to allow coherent energy transfer through a dark state that is delocalized across many molecular units. Phthalocyanines, a close cousin to the biologically relevant solar-harvesting porphyrins, are being synthesized and utilized as a model system. Energies and couplings between electronic states will be captured in real time while imaging both coherent and incoherent transport mechanisms utilizing ultrafast Gradient Assisted Photon Echo Spectroscopy (GRAPES). Further utilizing nanosecond transient absorption in conjunction with GRAPES will provide spectral information that spans twelve orders of magnitude in time and can inform how the dark states in H-Aggregates participate in exciton relaxation and diffusion.

The molecular orientation is an important factor affecting electrical and optical properties of the organic semiconducting layers. In organic light emitting diodes (OLEDs), the outcoupling efficiency of light significantly relies on the emitting dipole orientation (EDO). The horizontally aligned dipole moment embedded in a 2-dimensional microcavity structure contributes to larger far-field emission than the vertically aligned dipole moment. Therefore, employing the horizontally oriented emitter is one of the effective methods to enhancing the outcoupling efficiency of OLEDs. Ir complexes are excellent phosphorescent dyes that have been used in OLEDs for decades because they have high photoluminescence quantum yield and allow electroluminescence from triplet excited states. However, it is only recent years to attract attention on the EDO of Ir complexes as doped in the emissive layers because their iridium-centered octahedral structures and the amorphous surrounding nature in the emissive layers are far from having strong molecular alignments.^[1-5] Recently, many Ir complexes have been known to have horizontal EDOs and expected to increase external quantum efficiencies (EQEs) of OLEDs.
Here, we reveal the origin of the molecular alignment of Ir complexes by simulation of vacuum deposition using molecular dynamics along with quantum chemical characterization. The molecular alignment of the dye varies largely depending on the type of the host materials. Close observation of the molecules on the film surface by the vacuum deposition simulation indicates that the interactions between the phosphor and nearest host molecules on the surface, minimizing the non-bonded van der Waals and electrostatic interaction energies determines the molecular alignment during the vacuum deposition. Parallel alignment of the main cyclometalating ligands in the molecular complex due to host interactions rather than the ancillary ligand orienting to vacuum leads to the horizontal emitting dipole orientation.

Organic semiconductors are attractive materials for displays, solar cells, lighting and lasers because of their combination of desirable optoelectronic properties with simple processing and fabrication. An increasing trend is to be able to manipulate these properties through not only through molecular design but also through processing and the use of wavelength scale cavities. In this talk two examples of modifying exciton behaviour will be presented.
The first is engineering exciton diffusion length to enhance device efficiency in organic photovoltaics. The strong exciton binding energy means it is desirable to increase exciton diffusion length so that excitons can reach a heterojunction and charge separation can occur. We explore the effect of a range of processing methods on exciton diffusion and find that for the widely studied small molecules DR3TBDT and SMPV1, solvent vapour annealing leads to a doubling of the exciton diffusion length. It also leads to larger acceptor domains which would normally reduce charge separation. However, the increased exciton diffusion length means that exciton harvesting is still efficient even in larger domains. The larger domains lead to improved charge extraction and higher device efficiency.
The second aspect of controlling the properties of excitons is in cavities in which strong light-matter coupling can occur. Here we will present low threshold polariton lasing.

Organic solar cells based on vapor-deposited small molecule materials are entering the commercial stage due to their advantages such as combined high efficiency and long lifetime and the possibility to easily realize multijunctions. However, better near infrared absorber materials are needed to cover larger parts of the solar spectrum. In this talk, we will discuss recent developments on two donor materials classes, boron dipyrromethenes (BODIPYS) and merocyanines with a quinoid structure for vacuum-deposited organic solar cells. The materials show high absorption coefficients up to wavelengths of 1100 nm. BODIPYS allow to realize solar cells with – for their wavelength range – comparatively high efficiencies above 6%. The quinoid merocyanines have so far achieved only low efficiencies, but show very unusual photophysical properties, such as an extremely large blue shift of the main absorption peaks of the thin film as compared to the solution, indicating H-aggregation with very large coupling. Morphology investigations with scanning electron microscopy and electron diffraction indicate crystalline growth in nanowires.

Using diketopyrrolopyrrole (DPP) polymers with different chemical structure and molecular weights, the device performance of polymer:fullerene solar cells was systematically optimized by considering device polarity, morphology, and light absorption. More soluble derivatives show a 10-25% enhanced PCE in inverted device configurations, while the device performance is independent of device polarity for less soluble DPP polymers. Optimization of the nature of the cosolvent to narrow the fibril width of polymers in the blends towards the exciton diffusion length enhanced charge generation. Additionally, the use of a retroreflective foil increased absorption of light. Combined, the effects afforded a PCE of 9.6% for small band gap polymers.

Using a combination of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, diluted in near azeotropic water/n-propanol dispersions as hole transport layer, and metal (Zn, Sn) oxide nanoparticles, dispersed in alcohols as electron transport layer, novel, versatile charge recombination layers have been developed for solution-processed multi-junction solar cells in n-i-p and p-i-n architectures. These have been incorporated in multi-junction solar cells, employing a range of different polymer-fullerene photoactive layers, without the need of adjusting the formulations or deposition conditions. The approach permitted realizing complex devices in good yields, providing PCE up to 10%.

Further a new protocol, involving optical modeling, has been developed to correctly measure the EQE of triple-junction organic solar cells. Apart from correcting for the build-up electric field, the effect of light intensity is considered with the help of representative single-junction cells. The short-circuit current density determined from integration of the EQE with the AM1.5G solar spectrum differs by up to 10% between corrected and un-corrected protocols. The results were validated by comparing the EQE experimentally measured to the EQE calculated via optical-electronic modeling, obtaining an excellent agreement.

The bulk heterojunction (BHJ) photoactive layer design remains attractive for achieving cost effective and efficient organic photovoltaic (OPV) cells. Performance of such OPV devices critically depends on whether the nanoscale morphological structure of the BHJ is conducive to exciton transport to donor-acceptor interfaces and charge transport within each material phase. We are exploring a supramolecular approach based on the hydrogen-bond (HB) directed self-assembly of molecular donors to exercise control over donor-acceptor thin film blend morphology. Our initial work explored simple quaterthiophene donors functionalized with a phthalhydrazide HB unit to promote self-organization. Scanning tunneling microscopy (STM) was used to directly observe the trimer assembly of such HB-functionalized donor molecules. In vacuum-deposited thin film blends (with C₆₀ and C₇₀) the installed H-bonding interactions operate synergistically with π-stacking and have a strong and favorable impact on the absorption properties, molecular donor surface orientation, and phase separation. OPV devices for the H-bond capable donors show improved charge transport characteristics, external quantum efficiencies, and a 2-3 fold increase in power conversion efficiency over devices based on comparator HB-disabled donor molecules with the same donor chromophore design. We further explored the modularity of the supramolecular design with respect to donor chromophore structure and H-bond assembly motif to extend light absorption to longer wavelengths and realize other supramolecular assembly topologies (tetramers and hexamers). OPV cells based on such lower-gap oligomers indeed still possess more than two-fold increase in efficiency than those with HB-disabled comparator molecules, although there remain challenges in controlling the molecular orientation and stacking in these solution-processed films.

After enhancing the power conversion efficiencies of organic solar cells beyond 10%, their long term stability has become the most urgent challenge in order to eventually integrate organic solar cells into end-user products. Solar devices may have to endure harsh conditions already during the fabrication of tiles or façade elements, typically requiring lamination temperatures up to 120°C, critical for the initial performance of organic solar modules.
In this work, we demonstrate polymer:fullerene bulk-heterojunctions with significantly enhanced thermal stability at 120°C and beyond, by incorporating a novel crosslinkable bisazide that can lock the bulk-heterojunction morphology. The bisazide molecule is easy to synthesize and offers large-scale accessibility.
The solar cells clearly outperform the thermal stability of reference devices without the crosslinking bisazides.
We investigated bulk-heterojunctions comprising a variety of light-harvesting copolymers, combined with the industrially relevant fullerene acceptor PC₆₁BM. Upon thermal annealing, the reference blends without the crosslinking bisazides exhibit only moderate thermal stability and loose more than 70% of their initial performance, mainly originating from crystallization and aggregation of the fullerene. In contrast, polymer:fullerene blends comprising 7wt.% bisazide crosslinkers show effectively no degradation but retain their initial performance: Even after 200 hours of continuous thermal annealing at 120°C, the respective solar cells still exhibit over 90% of their initial performance.

Organic semiconductors are in general known to have lower mobility compared to their inorganic counterparts. As such, the bimolecular recombination rate of holes and electrons, usually referred to as Langevin recombination, is typically an important loss mechanism. Here, we elucidate the photophysics of BHJ solar cells based on a non-fullerene acceptor (IDTBR) in combination with various polymers showing an unprecedented low bimolecular recombination rate despite the unbalanced charge carrier mobility. The high FF observed (above 65%) is attributed to the non-Langevin behaviour with a beta/betaL ratio of 1x10-4. We calculated high charge carrier lifetimes without parasitic recombination in P3HT:IDTBR solar cells, leading to an almost perfect bimolecular recombination. Most interestingly, light intensity mobility measurements reflect a strongly non-thermalized carrier transport, indicating the origin of this unusual slow recombination kinetics. As a consequence of this peculiar recombination kinetics, devices between 80 nm and 450 nm and found to show a thickness- independent PCE.
The second part investigates the radiative and non-radiative voltage losses related with NFAs. Voc values of up to 1.15 are found for semiconductor composites with a bandgap below 1.75, and record Voc values of up to 1.25 are observed for semiconductors with slightly larger bandgaps. Non-radiative Voc losses as low as 0.2 V in combination with low radiative losses due to the absence of CT states open a technical venue to construct OPV devices with efficiencies beyond 15 %.

The power conversion efficiency (PCE) of single junction organic solar cells has increased significantly during the last decade and now approaching the threshold considered necessary for commercialisation. During this period, the structural diversity of semiconducting donor polymers for solar cells has increased dramatically, enabling accelerated development of bulk heterojunction (BHJ) organic solar cells based on polymer donor materials and molecular fullerene derivatives. However both the fullerenes, and the low bandgap polymers typically suffer from low absorption coefficients due to weak oscillator strength. Our approach is to use P3HT as a p-type hole acceptor, and design highly absorbing, low-bandgap n-type small molecules to replace fullerenes. These fullerene acceptors not only have weak absorption, but also poor tunability of absorption over the longer wavelengths of the solar spectrum; morphological instability in thin film blends over time; high synthetic costs and limited scope for synthetic control over electronic and structural properties. For these reasons, we have developed new, synthetically simple electron acceptor materials, based on rhodanine end groups, which have much larger absorption coefficients than fullerenes, coupled with high lying LUMO energy levels, to maximize cell voltages. In BHJ devices with P3HT donor polymer, the rhodanine molecules were demonstrated to outperform fullerenes. The highest performing P3HT devices have power conversion efficiencies approaching 8%, based on a ternary blend of two rhodanine acceptors. We also demonstrate performances of over 12% with one non-fullerene acceptors in combination with lower bandgap polymers, deposited from non-chlorinated solvents.

In this presentation, we introduce innovative approaches for the stabilization of ideally packaged (encapsulated) and completely unpackaged organic solar cells. While ideally packaged devices allow to study the intrinsic stability limitations of the device, solar cells fully exposed to the environment allow to investigate methods for preventing photo-oxidation of photoactive materials, thus alleviating the need for costly barrier materials.
We first compare the lifetime of continuously light soaked, ideally packaged polymer:fullerene (P3HT:PC60BM) and polymer:non-fullerene (P3HT:IDTBR) solar cells in the course of 2000 h to conclude that replacing PCBM with IDTBR fully inhibits short-circuit current losses. In fact, even the open circuit voltage and fill factor are only minimally affected, leading to photovoltaic devices with no burn-in (early, exponential loss in device performance), as opposed to P3HT:PCBM that shows a very pronounced burn-in. We elucidate the device physics of pristine and degraded devices for both material systems and identify clear differences in charge trapping and carrier lifetime behavior. Importantly, the stabilization extends to polymers with very different molecular weights.[1]
We then present a pronounced photo-stabilization effect in air in a wide range of prominent semiconducting polymers in the presence of the antioxidant nickel(II) dibutyldithiocarbamate, Ni(dtc)₂.[2] We show that Ni(dtc)₂ acts as a broadband stabilizer that inhibits both the formation of reactive radicals and singlet oxygen. Ultrafast pump–probe spectroscopy reveals quenching of triplet excited states as the central mechanism of singlet-oxygen induced photo-oxidation. When introduced into the active layer of organic photovoltaic devices, Ni(dtc)₂ retards the short circuit current loss in air without affecting the sensitive morphology of bulk heterojunctions and without major sacrifices in semiconductor properties. We conclude that antioxidants based on nickel complexes render organic semiconductors less susceptible to oxygen and represent a cost-effective route toward organic electronic appliances with extended longevity.

References:
[1] Nicola Gasparini, Michael Salvador, Sebastian Strohm, Thomas Heumueller, Ievgen Levchuk, Andrew Wadsworth, James H Bannock, John C de Mello, Hans-Joachim Egelhaaf, Derya Baran, Iain McCulloch, Christoph J Brabec, Adv. Energy Mater., 7, 1700770 (2017).

[2] Michael Salvador, Nicola Gasparini, José Darío Perea, Sri Harish Paleti, Andreas Distler, Liana N Inasaridze, Pavel Troshin, Larry Lüer, Hans-Joachim Egelhaaf, Christoph J Brabec, Energy Environ. Sci., 10, 2005-2016 (2017).

Non-fullerene acceptors (NFA’s) have been receiving increasing attention for application in polymer-based bulk-heterojunction organic solar cells, as they have demonstrated improved photovoltaic performances over more conventional polymer-fullerene blends. Here, polymer solar cells based on statistically substituted anthracene-containing poly(p-phenyleneethynylene)-alt-poly(p-phenylenevinylene)s (PPE–PPVs) copolymer (AnE-PVstat) were investigated in combination with various electron accepting materials. In contrast to blends with PCBM, strong photoluminescence quenching of specifically the polymer indicates fine-scaled intermixing of materials. This was accompanied by a very weak photovoltaic function. By application of Time-Delayed Collection-Field (TDCF) measurements, it could be seen that charge generation and extraction was strongly limiting performance in these blends.

Light intensity dependent measurements of the open-circuit voltage of a photovoltaic cell, the so called Suns-Voc method, are often used to identify the free carrier recombination mechanism of electrons and holes. Additionally varying the temperature during such a study by performing the measurement in a cryostat allows to access a larger data space and by that a deeper understanding of the processes at play [Tvingstedt et al., Adv. Energy Mater. 2016, 1502230].

Here, we present a modified Suns-Voc method that makes use of a temperature variation which is simply induced by the absorption of the light [Ullbrich et al., submitted]. In this case, the open-circuit voltage continually decreases with respect to the isothermal case as a consequence of the absorption induced temperature rise. An exact mathematical treatment of the problem reveals that the open-circuit voltage can even decrease although the incident light intensity increases. The entire characterization can be done at ambient conditions using a strong light source which sufficiently heats up the sample without the need for an external heating.

We test our model on different photovoltaic cells based on small molecules, polymers, perovskite and silicon. A very good agreement between fit and experimental data proves the general validity and allows us to extract important information. First, the energy gap over which recombination between electron and holes takes place are determined as it is a direct fit parameter. Second, we can demonstrate that the ideality factor can be assumed to remain constant (close to 1 in case of organic semiconductor devices) allover the measured light intensity range and third, the fitted thermal resistance yields the temperature rise of the sample.

A general aspect of our finding is that the often found voltage turnover in Suns-Voc measurements do not necessarily arise from recombination at the contacts, as often believed, but can simply stem from thermal effects.
The turn-around effect can basically be understood by a strong temperature-dependent broadening of the Fermi-Dirac distribution, representing an increasing number of charge carriers even for back-shifting quasi-Fermi levels.
From a mathematical point of view, it is very similar to the electrothermal feedback caused by Joule-self-heating in semiconductor devices with increasing electrical conductivity towards higher temperatures [Fischer et al., Phys. Rev. Lett. 2013, 110, 126621].

Finally, we will shortly discuss that this effect cannot only be found in photovoltaic cells but also all kind of devices which produce an open-circuit voltage under light illumination, e.g. organic light-emitting diodes.

Fluorescent materials that efficiently convert triplet excitons into singlets through reverse intersystem crossing (RISC) rival the efficiencies of current state-of-the-art organic light-emitting diodes (OLEDs) without requiring expensive heavy metals such as platinum and iridium. The efficiency with which triplet excitons are upconverted into singlet excitons, a phenomenon known as thermally activated delayed fluorescence (TADF), is dictated by the rate of RISC, a material-dependent property that is not directly measureable. In this work, an analytical model is developed to determine RISC, as well as several other important photophysical parameters such as exciton diffusion and the rate of intersystem crossing (ISC), all from simple time-resolved photoluminescence measurements. This new analytical model has been used to investigate 5 different TADF materials in order to elucidate structure-property relationships and understand how RISC can be modulated. The bromination of TADF materials results in a dramatic increase of k_ISC, k_RISC, spin cycling, and exciton diffusion length due to the spin-orbit coupling of the heavy bromine atoms. This general methodology can be used to better understand how molecular structure affects solid state quantum yield and spin dynamics, enabling the rational design of molecules with desirable spin characteristics.

Electrically injected charge carriers in organic light-emitting devices (OLEDs) undergo recombination events to form singlet and triplet states in a 1:3 ratio, representing a fundamental hurdle for achieving high quantum efficiency in devices. Dopants based on thermally activated delayed fluorescence (TADF) traditionally rely on a small singlet-triplet exchange energy to activate luminescence pathways for weakly emissive triplet states. We use transient electron spin resonance (TrESR) and density functional theory (DFT) to show that when the photoexcitation is at the lowest singlet excitation energy, the spin-orbit coupling (SOC) mechanism drives spin conversion for the benchmark 4CzIPN and 2CzPN TADF molecules, even in the absence of heavy metals. Furthermore, the polarization of the observed triplet ESR signals support the proposal for second-order, vibronically-mediated SOC. Departure from a ‘static’ picture for the excited states involved in TADF removes the problem of balancing the promotion of fluorescence and the rate of reverse intersystem crossing; these are contradictory molecular properties under the adiabatic approximation. Critically, torsional freedom between the electron acceptor and donor components open additional ISC pathways between S₁ and T₁, and should be considered as a possible design rule for new TADF molecules.

Exciton diffusion in organic semiconductors is often probed using measurements of photoluminescence (PL), a method only applicable to luminescent materials. A more general approach to extract the exciton diffusion length (L_D) is from the photocurrent current spectrum of a bilayer organic photovoltaic cell (OPV). This method, however, is limited by often unknown recombination losses that can occur during charge collection. Here, we demonstrate a device-based measurement to extract the intrinsic L_D without the need to assume a value for the charge collection efficiency (η_CC). Since η_CC is identical for carriers originating from both the donor and acceptor, a ratio of the donor and acceptor internal quantum efficiencies (η_IQE) cancels this unknown quantity. In this work, a fluorescent material boron subphthalocyanine chloride (SubPc) is used as a test of this method. We measure both donor to acceptor η_IQE ratio and PL emission of SubPc-C₆₀ bilayer OPVs as a function of SubPc thickness. The L_D extracted from the thickness dependence of the η_IQE ratio agrees well with that extracted from device PL. The extracted L_D is also in agreement with thickness dependent PL quenching measurement taken on glass substrates, further confirming the viability of this technique. We expect this simple, device-based approach for the extraction of L_D will have wide applicability to any excitonic material capable of integration into a simple, bilayer photovoltaic device.

Excited charge transfer complexes (Exciplex) formed between donor and acceptor materials are frequently encountered in organic photonic devices such as in organic light emitting diodes and organic photovoltaics. Formation of exciplexes can be easily identified by the observation of the red shifted emission from those of the component molecules. Generally the PL efficiency of the exciplexes is low so that OLEDs are designed not to form exciplexes at the organic/organic junctions. Formation of exciplexes at the D/A junction is also to be avoided in OPVs since it reduces the dissociation probability of geminate electron-hole pairs formed at the interface. In this presentation we will firstly discuss on the nature of exciplex including the electronic structure, emission processes and diffusion. Further discussion will be given to the application of exciplex forming systems as the triplet harvesting fluorescent molecular system and as the co-host for phosphorescent and fluorescent dopants for ultimate efficiency in OLEDs.

Organic light emitting diodes (OLEDs) are being used in an increasing number of display and lighting applications due to their high efficiencies and vibrant colors. The high efficiency of these devices is enabled by the use of iridium(III) complexes as phosphorescent dopant emitters. One feature that favors these Ir phosphors as emissive dopants is their fast radiative rate constants (k_r > 10⁵ s^-1) which help boost their luminescent efficiencies. We have investigated luminescent copper(I) complexes for a number of years as alternative phosphorescent dopants. These Cu(I) complexes typically have radiative rates that are an order of magnitude slower than values found for the Ir(III) complexes. The disparity in radiative rates is attributed to the relatively weak spin-orbit coupling induced by the light Cu atom. We have recently prepared a series of Cu(I) carbene complexes that have luminescent efficiencies and radiative rates comparable to those of the heavy Ir(III) complexes. We can tune the emission energies of these Cu(I) complexes from blue to red by varying the electron affinity of the carbene ligand, yet still retain the fast radiative rate constants. In this presentation, we will discuss the photophysical properties of these new phosphors and propose a mechanism for their unusually rapid radiative behavior.

Recent portable electronic devices such as flagship smartphones and wearable VR/AR have adopted organic light-emitting diodes (OLEDs) as a display panel due to their lightness, high color purity, contrast, high-speed, and applicability to versatile designs. Those emerging devices often need even greater efficiency than conventional devices because the capacity of their battery is often severely limited. In this regard, unlocking the full optical potential of OLEDs is strongly desired for long operational hours and, not to mention, for performance improvements.
It is well known that most of the generated photons in an OLED are trapped in the substrate and organic layer as a waveguide mode or dissipated in the metal surface as a surface plasmon polariton (SPP) mode. The portion of energy loss induced by SPP can be as high as 30% in conventional OLEDs, and its extraction or recovery has been considered as the most challenging among the various loss mechanisms. For this reason, many researchers have focused on extracting waveguide or SPP mode using nano- or micro- structures; however, applying internal structure in an OLED easily causes electrical shorts and makes it subject to rather expensive fabrication cost. Increasing the distance between the metal and emitter is another way to reduce SPP loss, however, this approach increases the portion of waveguided modes and weakens Purcell effect in cavity–resonance structures, tending to render its efficiency to drop. To place low-index materials between the emitter and metal electrode is an alternative way to reduce SPP. By introducing the low-index medium adjacent to metal, the portion of dissipated power from SPP loss is suppressed and shifted into a smaller in-plane wavevector region, contributing to the improvement in outcoupling efficiency. From this perspective, developing a low-refractive-index material with proper electrical properties (alignment of energy levels and carrier mobility) for the buffer layer is becoming an important agenda.
In this study, we propose a very low-refractive index layer based on a conducting polymer with perfluorinated ionomer, which can effectively mitigate SPP mode generated along the dielectric/ metal interfaces. In addition, by adopting an anisotropic material for transport layer having a lower extra-ordinary refractive (n_e) index, the SPP loss associated with semitransparent metal layers is also suppressed effectively. Based on this combinatory method, we demonstrate a highly efficient top-emitting OLED by suppressing SPP loss from metal electrodes. The theoretical predictions are verified in an experiment that compares OLEDs with and without the proposed low-index buffer layer and anisotropic transport layer.

Extracting the “internally waveguided” light from OLEDs, which together with losses to surface plasmons at the metal cathode typically account for > 50% of the light generated in the emission zone, has proven to be a particularly challenging problem. To address this problem, we describe devices fabricated on nano-patterned plastic substrates that disrupt the internal waveguiding and surface plasmon modes. We describe thermally evaporated small molecule phosphorescent OLEDs (PhOLEDs) fabricated on corrugated polycarbonate (PC) and polyethylene terephthalate substrates nanopatterned in a roll-to-roll process. We compare the devices fabricated on glass/ITO to those on plastic/PEDOT:PSS. Depending on the height and pitch of the pattern, up to a 2.5 fold increase in the outcoupling factor is observed relative to the flat substrate, resulting in green and blue PhOLEDs with an external quantum efficiency that reaches 50% and 32%, respectively. Issues related to the fidelity of the conformal deposition of the various layers on the patterned plastic, imaged by AFM and focused ion beam (FIB), are also discussed.

Top-emission organic light-emitting diodes (TE-OLEDs) are popular in high-resolution displays or in aperture-controlled transparent displays as they can maximize the areal utilization of emitting devices. Semi-transparent metal layers are generally used for a top electrode in TE-OLEDs because they can be easily formed via thermal evaporation and can control a micro-cavity resonance effect, thereby achieving wide color gamut. Nevertheless, metal based top electrodes which show low transmittance and high reflectance, have serious limitations such as photon energy loss due to strong surface plasmon polariton (SPP) modes, and viewing angle color distortion.

To overcome these problems, we here report a non-metal based transparent conductive film as a top electrode, which consists of the transparent conductive electrode and the transparent UV curable polymer layer. The proposed conductive film can be uniformly transferred by a novel vacuum lamination process onto an OLED stack. The angular distribution of the electroluminescence (EL) intensity of the laminated devices shows that of nearly ideal Lambertian, which is enabled by high transmittance and spectrally neutral character of the proposed film over the whole visible range.

Furthermore, we successfully demonstrate outcoupling structures at the surface of the proposed film for enhancing the light extraction using an imprinting method. Outcoupling structures and high refractive index of the transparent polymer layer can significantly increase the external quantum efficiency (η_EQE) of TE-OLEDs as shown in our simulation results. These results indicate that the waveguided modes in the organic and top electrode layers can be extracted to the substrate-confined modes due to high refractive index of the transparent polymer layer, and the confined light in the substrate (the transparent polymer layer) can be greatly extracted through the out-coupling structures. Consistent with the simulation results, laminated devices with concave hexagonal structures (η_{EQE. w/ structures} = 38.6%) shows 1.7 times higher η_EQE than those with planar structures (η_{EQE. w/o structures} = 23.0%). In addition, while we formed out-coupling structures in the film, laminated devices are shown to exhibit little optical blurring due to thin film thickness (~ 60 µm).

Gold nanoparticles located at a metal oxide/hole conductor interface generate photocurrents upon visible light illumination. We demonstrate that the quantum efficiency of this process depends on the nanoparticle size. Gold nanoparticles (5 nm) show a maximum absorbed photon-to-electron conversion efficiency (APCE) of 13.3%. For increasing particle sizes, the average APCE decreases to 3.3% for the largest particles (40 nm) investigated. Three possible causes for this efficiency change are discussed.

Atomically thin transition metal dichalcogenides (TMDCs) have intriguing nanoscale properties like high charge mobility and photosensitivity, layer-thickness-dependent bandgap, mechanical flexibility, all appealing for the development of next generation optoelectronic, catalytic and sensory devices. Their atomically thin thickness however renders TMDCs poor absorptivity. Here we combine bilayer MoS₂ with core-only CdSe QDs and core/shell CdSe/ZnS QDs to obtain hybrids with increased light harvesting and exhibiting interfacial charge transfer (CT) and nonradiative energy transfer (NET), respectively. Field-effect transistors (FETs) based on these hybrids and their responses to varying laser power and applied gate voltage were investigated with scanning photocurrent microscopy (SPCM) in view of their potential utilization in light harvesting and photodetector applications. We found CdSe-MoS₂ hybrids to exhibit encouraging properties for photodetectors under low light exposure while CdSe/ZnS-MoS₂ hybrids showed an enhanced charge carrier generation with increased light exposure, making them suitable for photovoltaics. While distinguishing optically between CT and NET in QD-TMDCs is non-trivial, we found they can be differentiated by SPCM as these two processes exhibit distinctive light-intensity dependencies: CT causes a photo-gating effect, decreasing the photocurrent response with increasing light power while NET increases the photocurrent response with increasing light power, opposite to the CT case.

Singlet Fission sensitized solar cells are a possible solution to overcome the Shockley Queisser efficiency limit for single junction solar cells. Combining singlet fission materials with silicon is promising since silicon is the dominating material for solar cells on the market.
In one implementation, a singlet fission layer made from tetracene absorbs high-energy photons (> 2.5 eV) whose energy is split into two triplet excitons with half the energy (1.25eV) via singlet fission. Combining the tetracene sensitizer material with silicon (bandgap ~1.1eV) can increase the theoretical maximum solar cell efficiency to around 44%
However, transferring triplet excitons from tetracene into silicon stays elusive. It has been shown that it is possible to transfer triplet excitons from tetracene to PbS quantum dots. Once in the quantum dot, the energy can be transferred by Förster resonant energy transfer (FRET). Here we calculate the FRET efficiency from PbS quantum dots into silicon, using realistic material parameters. We study the influence of the device geometry, donor-acceptor distance, and quantum dot bandgap on the energy transfer efficiency. We find that a short spacing of < 1 nm between the quantum dots and silicon is necessary to obtain a transfer efficiency of >80%. The small absorption coefficient of silicon leads to poor efficiencies for larger distances. Our work lays the foundation to enable singlet fission solar cells with an intermediate QD layer that facilitates energy transfer.

The highest efficiency reported for infrared-to-visible photon upconversion at sub-solar excitation densities¹ used PbS QDs (quantum dot) light absorbers with CdS shells. Despite these encouraging results, the mechanism of passivation is not very well understood. In this presentation, we demonstrate the importance of synthesis and shell composition in this hybrid photon upconversion system. Using oleylamine to install different shells of ZnS, SnS, CdS, InS, NiS, and BiS on PbS QDs, we find only the ZnS and CdS shells enhance the upconversion quantum yield (QY) to 0.28% and 0.13% for oleic acid capped ZnS-PbS and CdS-PbS core-shell QDs respectively. This is comparable to the previously reported upconversion QY of PbS-CdS core-shell QDs (0.20%)¹. Interestingly, here, the enhancement in photon upconversion is accompanied by a decrease in the photoluminescence QY at much thinner shell thickness (less than 0.5 Å) compared to the 2.3 Å CdS shell thickness reported earlier. Time correlated single photon counting (TCPSC) measurements show that the primary reason for the enhancement of the upconversion QY is due to the decrease in non-radiative transition rates. The very thin nature of the shell allows 970 nm near infrared photons to be efficiently converted to visible photons. This is a 120 nm red shift compared to our previous reported. We believe our work provides a new insight on the surface chemistry modification of the QDs that can be applied in solar cells, bio imaging, cancer therapy, and sensors.

Organic solar cell (OSC) has been an active research field over the past decades due to the intrinsic advantages, Small molecule vacuum deposition is regarded as a promising fabrication method for the realization of multi-junction tandem solar cells (TSCs). One of the challenging topics is the development of novel photoactive electron donor materials that absorb in near infrared (NIR) region (λ > 780 nm). This kind of molecules is extremely rare compared to those whose absorption bands are in the visible light region (400<λ<780 nm), but they can make significant contribution in TSCs by completing the absorption. Taking advantage of the flexible chemical modification and tunable photophysical properties of boron dipyrromethene (BODIPY), we have been investigating high performance OSCs based on NIR BODIPY derivatives as electron donors.
Three aza-BODIPYs are synthesized via a modified route using phthalonitrile and organolithium reagents. Heterocyclic aromatic substituents (N-methylpyrrole, N-methylindole and 2-trimethylsilyl thiophene) with different electronic properties are introduced to tune the absorption positions. Moreover, the synthetic methods of aza-BODIPY derivatives with BF(CN) or B(CN)₂ moiety are demonstrated, and three aza-BODIPY derivatives with BF(CN) moiety are prepared. The absorption bands of all the dyes are broadened and red-shifted from solution to solid state, and the maxima are all over 830 nm in thin film.
These aza-BODIPYs are used as electron donors in vacuum-processed bulk heterojunction (BHJ) OSCs with C₆₀ as the acceptor. The optimal device presents a short-circuit current (j_sc) of 9.0 mA cm^-2, a moderate open-circuit voltage (V_oc) of 0.61 V and a fill factor (FF) of 53%, giving a PCE of 3.0%. It is noteworthy that its external quantum efficiency (EQE) spectra covers from 650 to 950 nm, peaking around 850 nm, and the PCE value is reasonable for such long wavelength absorbing donor material.
A BODIPY with intense and long wavelength absorption can be achieved by an extension of the π-system and the introduction of an electron withdrawing group on the meso-C. Furan-fused BODIPYs with a CF₃ group on the meso-C are synthesized, and their planar molecular structures are demonstrated by the single crystal X-ray diffraction measurements. Their intense sharp absorption bands in solution are greatly broadened in solid state, covering a wide region from 500 to 950 nm.
The best device gives a PCE of 6.1% with a high j_sc of 13.3 mA cm^-2. Moreover, its EQE spectrum spans a wide region from 550 to 900 nm, making it a suitable NIR donor for a TSC in cooperation with a “green” absorber. The serial TSC has complementary absorption peaking at 550 and 800 nm. Due to the matching photocurrent generated by the both sub-cells, a j_sc of 9.9 mA cm^-2 is achieved. With a high V_oc of 1.70 V and a reasonable FF of 59%, a high PCE of 10% is obtained, which is an excellent value for vacuum-deposited TSCs.

Since the discovery of the first laser in 1960, wide varieties of applications have been found, including fundamental usage such as scientific optical excitation and photolithography, industrial laser cutting, drilling, military applications, medical imaging and surgery. The key advantages of using organic semiconductors are low cost, light weight and high flexibility.¹ A solid-state optically pumped organic laser was first demonstrated about two decades ago² while an electrically pumped organic laser (i.e. using direct electrical pump) is still to be realised.
The key challenges for electrically pumped organic lasers include finding suitable materials with high photoluminescence and low losses like self-absorption as well as designing appropriate device architectures to achieve high current density and high brightness as well as low gain losses associated with polarons and electrodes.^3,4 In this work, we report our investigation of an organic fluorescent chromophore for its potential as an organic laser dye. New synthetic route to the chromophore will be reported. Its low amplified spontaneous emission (ASE) threshold of 21 mJ/cm² in solution, comparable to a common commercial laser dye, Rhodamine 6G, will be discussed. Notably, the material had the lowest solid-state ASE threshold (2.4 μJ/cm²) for similar classes of organic laser dyes prepared from solution. In this report, organic light-emitting diode performance based on the dye with a maximum external quantum efficiency of 1.9% will also be presented to show its potential toward the realisation of organic laser diodes.
References:
1. I. D. W. Samuel, Laser Physics: Fantastic Plastic. Nature 2004, 429, 709-711.
2. F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, Semiconducting Polymers: A New Class of Solid-State Laser Materials. Science 1996, 273, 1833-1836.
3. I. D. W. Samuel, G. Turnbull, Organic Semiconductor Lasers. Chemical Reviews 2007, 107, 1272-1295.
4. K. Hayashi, H. Nakanotani, M. Inoue, K. Yoshida, O. Mikhnenko, T.-Q. Nguyen, C. Adachi, Suppression of Roll-Off Characteristics of Organic Light-Emitting Diodes by Narrowing Current Injection/Transport Area to 50 nm. Applied Physics Letters 2015, 106, 093301(1)-093301(5).

Resonance Raman (RR) spectroscopy is an effective tool for probing the extent of exciton-phonon coupling in colloidal semiconductor nanocrystals (NCs); however, there are conflicting reports in the literature of whether excitons also couple to covalently bound surface ligand vibrational modes.^1-3 Recent theoretical and computational studies have posited that such couplings can occur through resonance with ligand-nanoparticle charge-transfer states, particularly in the case of asymmetric vibrational modes of ligands with states that lie within the semiconductor bandgap.^4-5 Here we show that such predictions can be compared with experiment. We utilize a simple approach to prepare a set of 2-5 nm colloidal CdSe NCs capped with thiophenolate (PhS), which is a widely employed surface-enhanced Raman tag. The use of PhS also mitigates the strong background photoluminescence which generally impedes RR measurements of semiconductor nanocrystals. This system is further contrasted with obtained Raman spectra of an established studied set of molecular analogues (PhS, Cd(SPh)₂, Cd₄(SPh)₁₀(MeN₄), Se₄Cd₁₀(SPh)₁₆(Me₄N₄), Se₄Cd₁₀(SPh)₁₀). We will also present our recent efforts to expand this methodology to investigate sub 2 nm CdSe NCs. In this size regime, a secondary broad emissive feature appears, which we attribute to the formation of a radiative self-trapped exciton that is strongly coupled to the longitudinal optical phonon mode of CdSe.


1. Grenland, J. J.; Maddux, C. J. A.; Kelley, D. F.; Kelley, A. M., Charge Trapping versus Exciton Delocalization in CdSe Quantum Dots. The Journal of Physical Chemistry Letters 2017, 8, 5113-5118.
2. Wang, Y.; Zhang, J.; Jia, H.; Li, M.; Zeng, J.; Yang, B.; Zhao, B.; Xu, W.; Lombardi, J. R., Mercaptopyridine Surface-Functionalized CdTe Quantum Dots with Enhanced Raman Scattering Properties. The Journal of Physical Chemistry C 2008, 112, 996-1000.
3. Lifshitz, E., Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots. The Journal of Physical Chemistry Letters 2015, 6, 4336-4347.
4. Lombardi, J. R.; Birke, R. L., Theory of Surface-Enhanced Raman Scattering in Semiconductors. The Journal of Physical Chemistry C 2014, 118, 11120-11130.
5. Swenson, N. K.; Ratner, M. A.; Weiss, E. A., Computational Study of the Resonance Enhancement of Raman Signals of Ligands Adsorbed to CdSe Clusters through Photoexcitation of the Cluster. The Journal of Physical Chemistry C 2016, 120, 20954-20960.

Pristine, defect-free, suspended carbon nanotubes (CNTs) are an ideal system to study photocurrent generation in the limit of strong Coulomb interactions. Photon-to-electron conversion efficiencies exceeding 100% have already been shown in quantum dot solar cells, and carrier multiplication effects have been reported in CNT photodiodes. However, measurements of CNT photodiodes have so far yielded disappointing internal quantum efficiencies (< 70%). Our experiments utilize fully-suspended dual-gated carbon nanotubes with axial electric fields up to ~ 15 volts per micron. Building on our past results (Aspitarte, Nano Letters 2016), we use stronger electric fields to increase the exciton dissociation rate and unlock the possibility of harnessing carrier multiplication. We present our recent progress toward demonstrating internal quantum efficiencies greater than 100%.

The wavelength-selectivity in narrowband photodetectors is an important parameter in color photography, medical imaging, intelligence surveillance and in energy harvesting. Conventional photodetectors have a broadband response beyond the wavelength of interest which hinders their application in visible-opaque, infra-red selectivity or vice-versa. Moreover, it is highly challenging to obtain photodetectors with a full-width at half maximum (FWHM) of less than 100 nm. Here, we show that we can overcome these limitations by tailoring the supramolecular assemblies using organic dyes for photodetection. Strong molecular coupling enhances the absorption coefficient and improves exciton diffusion in these photodetectors. Self-assembly of these organic dyes in solution results in a narrowband J-aggregate with FWHM of around 15 nm. Three individual photodetectors with different wavelength-selectivity at 580, 780 and 1000 nm are demonstrated in this study. Using ink-jet printing, homogeneous thin layers of compact TiO₂ (20 nm), mesoporous TiO₂ (50 nm) and the active layer are printed and the device was completed by other deposition processes. All these photodetectors show an external quantum efficiency of around 20 % and internal quantum efficiency of more than 90%. A high transparency of around 80% in the visible region for the NIR dyes is achieved in these photodetectors without any anti-reflective coatings.

Recently, there have been several efforts in creating Van der Waals heterostructures of 2D materials to investigate many body interactions of their excitons and trions, which often give rise to unusual functionalities at the interface. However, there are certain exclusive functionalities like presence of soft phonon modes, which is seldom observed in 2D materials due to stringent symmetry requirements. SrTiO₃ (STO) is a transition metal oxide well known for the presence of a soft phonon mode below 105 K. This phonon mode is also polar in nature, thus enabling columbic interactions with quasiparticles around it. In this work, we have been able to create a heterostructure of MoS₂ and SrTiO₃ (STO) by direct CVD growth, and we observe from photoluminescence measurements that as the soft phonons evolve in STO, the trion peak exhibits a significant red shift as compared to emission from MoS₂ grown on various other substrates without soft phonons. From femtosecond pump probe spectroscopy we calculate the lifetimes of the trion in MoS₂ (10 ps), which is in the same time scales as the frequency of polar soft phonon modes in STO. This enables the polar phonons and the trion to selectively couple with the charged trions in MoS₂ to form a quasi-stationary bound state, which we call the ‘Polaronic Trion’. We demonstrate tunability of the light emission from the Polaronic Trion by almost 90 meV by simply varying electric field (which suppresses the phonons), temperature or even crystal orientation. Such an unprecedented selective tunability of the trion could probably guide actual applications for the much investigated quasiparticles in 2D materials.

After 60 years of research, the power conversion efficiency of silicon solar cells is slowly approaching the Auger-recombination-constrained Shockley−Queisser limit close to 30%. Conventional silicon solar cells lose a major part of incident sunlight energy via thermalization of excited charge carriers. Using singlet exciton fission, a process in which converts one high-energy photon into two charge carriers with half the energy, is a promising way to reduce such thermalization losses. Combining highly-optimized silicon solar cells with singlet fission has the possibility to further increase the power conversion efficiency of conventional silicon solar cells while simultaneously reducing the cost per kWh. Here we study the implementation of a singlet fission material on top of conventional silicon solar cell. We investigate the charge transfer behavior of triplet excitons from the singlet fission layer to the silicon solar cell by introducing intermediate molecular acceptors at the hybrid interface of silicon and singlet fission materials. We find that the energetics are unfavorable for charge transfer directly from tetracene, even with a C60 interlayer. We further investigate means to optimize the energy level alignment between the singlet fission layer, the intermediate acceptor and silicon.

We obtained efficient sensitized triplet-triplet annihilation based photon up-conversion (sTTA-UC) in a solid-state system consisting in a low viscosity transparent host matrix doped with molecular sensitizer dyes and loaded with annihilating/emitting MOF nanocrystals with size of tenths of nanometers. The maximum up-conversion yield of 5% has been achieved well below the solar irradiance, thanks to the peculiar properties of the nanocrystals embedded. Each one consists in a framework of identical and non-interacting optical active emitters that preserve the ground state electronic properties of the isolated molecules allowing for both the fast diffusion and the annihilation of the sensitized triplet excitons that generate higher energy fluorescent singlets. Specifically, each MOF nanocrystal can be considered as a giant supramolecular energy acceptor that works as triplet excitons collector. Thanks to its size, larger than that one of its molecular components, the collisional probability with the excited light harvesters is strongly enhanced, resulting in an effective sensitization of the nanocrystal triplets even with an ultra-low loading of the polymer matrix. These findings can be crucial for the development up-converting nanocomposite materials loaded with low concentrations of optically active species, with a remarkable simplification of the synthetic protocols without detrimental consequences for the material structural and optical properties.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are very promising candidates for future optoelectronics due to their enhanced light-matter interactions and stability at atomic thicknesses. However, as-fabricated 2D TMDC devices suffer from low induced current, severely limiting their application. Several post-processing treatments have been proposed to address this problem and increase 2D TMDC quality and device performance. We report here a 50x increase in the photocurrent response of photodetector MoS₂ devices using a non-destructive 1,2-dichloroethane (DCE) treatment. This treatment significantly improves 2D MoS₂ optoelectronic device performance without using contact-corroding acids. In addition, the treatment works well on as-grown CVD MoS₂ and does not require any secondary transfers to be effective. These features are here experimentally verified for the first time, to the best of our knowledge, enabling a straightforward process for 50x enhancement in photocurrent generation of large-scale MoS₂ optoelectronic devices post-fabrication.
Trilayer MoS₂ was grown using a thermal vapor sulfurization (TVS) process developed in previous works. The growth yields centimeter-scale MoS₂ with high crystallinity as measured by Raman spectroscopy and photoluminescence. Back-gated field effect transistor (FET) devices with 2 um channel length and 7 um channel width were fabricated using e-beam lithography, and Ti/Au source and drain contacts were deposited using e-beam evaporation. The fabricated devices were soaked in DCE at 60^° C for 20 hours. Spectral photocurrent was measured with 6V source-drain bias under illumination from a supercontinuum light source and a laser line tunable filter. Photocurrent was measured before and immediately after the DCE treatment, revealing a 50x photocurrent increase at the 420nm C-peak (from 0.5 nA before treatment to 25 nA immediately after treatment) and a 30x enhancement throughout the rest of the visible spectrum. However, this enhancement degraded over time when exposed to ambient air and subsequent measurements in a regular time interval show a gradual decrease in the peak height until it settles at ~30x enhancement (15 nA) for the C-peak; encapsulation methods are currently under investigation to mitigate this degradation.
The enhanced photocurrent is caused by a combination of reduced contact resistance between the metal contacts and the semiconductor, improved n-dopant concentration in the MoS₂ layer, and the passivation of dangling bonds to reduce Shockley-Reed-Hall (SRH) recombination. The interplay of these three components will inform the implementation of this treatment in optoelectronic device applications and as such, it is expected to greatly enhance the performance of 2D TMDC detectors, emitters, and photovoltaics and accelerate their development into practical technologies.

Organic vapor phase deposition (OVPD) technology, providing fast, particle-free, uniform deposition on large substrates, is a potential candidate for the production of large-area displays. Nevertheless, classical vacuum thermal evaporation (VTE) materials for electron injection layers (EIL) such as LiF or Cs₂CO₃ cannot be used in pure OVPD processes because of their high sublimation temperatures. This limits the choices for combinations of efficient EIL and transport layers (ETL) in OLED fabrication by OVPD. Kathirgamanathan et al. [1] have reported on a novel ETL material – zirconium tetrakis(8-hydroxyquinolinolate) (Zrq₄) – combined with lithium Schiff-base cluster complexes as EIL, which show comparable performance to LiF and 8-hydroxyquinolinolato lithium (Liq). Due to the oligomeric nature (clusters) of lithium complexes, such molecules can be evaporated at a relatively low temperature of 200 – 400 ^oC.
In this work, we examine lithium 2-((o-tolylimino)methyl)phenolate (EI-111-2Me) and Zrq₄, as EIL and ETL, respectively, in green PHOLED processed by an AIXTRON Gen1 OVPD tool. For comparison, reference devices having TPBi as ETL and Cs₂CO₃ as EIL (deposited by VTE) are fabricated. All devices are prepared on pre-structured ITO-on-glass substrates with a 2 nm layer of MoO_x on the ITO surface to enhance hole injection. The OLED stack consists of a graded Ir(ppy)₃/CBP organic layer serving as combined transport and emission layer. After depositing ETL and EIL, an opaque Al (120 nm) cathode is formed by VTE. To investigate the impact of EI-111-2Me and Cs₂CO₃ on electron injection, unipolar devices consisting of 150 nm undoped Zrq₄ in combination with the respective EIL are produced. Electron-only devices of Zrq₄ show a significant improvement in electron injection with reduced turn-on voltages from 11.1 V down to 4.4 V or to 4.3 V after introducing EI-111-2Me (1 nm) and Cs₂CO₃ (2 nm) as EIL, respectively.
Reference ETL and EIL composed of TPBi and Cs₂CO₃ are successfully replaced by Zrq₄ and EI-111-2Me in the PHOLED stack. However, the HOMO level of Zrq₄ is not sufficient for an effective confinement of holes in the emissive layer, therefore an additional 5 nm hole-blocking layer of undoped TPBi is added. After implementation of this hole-blocking layer, the luminous efficacy strongly increases from 7.7 to 21.9 lm/W and the EQE from 2.1 to 11.0 % in the investigated PHOLED containing Zrq₄ and EI-111-2Me. Further enhancement of emission properties to 26.3 lm/W and 11.7 % is observed for OLED with Zrq₄ and Cs₂CO₃ (all values at 1000 cd/m²). Obtained values are comparable to those of the reference PHOLED with TPBi and Cs₂CO₃ (24.4 lm/W and 11.2 %), whereas combining TPBi and EI-111-2Me yielded inferior results.

1 Kathirgamanathan P. et al., J. Mater. Chem., 2012, 22, 6104-6116

The research leading to these results has received funding from the European Union‘s Horizon 2020 Research and Innovation Programme under grant agreement No 674990 (EXCILIGHT).

Aggregation of organic semiconductors positively impacts the device performance of polymer-based bulk heterojunction solar cells. Aggregation and specifically intermolecular electronic coupling leads to extended absorption spectra by formation of new absorption bands. Furthermore, it has been demonstrated that more ordered domains of organic semiconductors improve the free charge carrier generation and also reduces the recombination of free charges located in the ordered phases due to energy relaxation. Finally, aggregation enhances charge carrier transport and lifetimes, resulting in an improved charge extraction under operating conditions.
The advantage of controlling the aggregation in solution is a higher reproducibility of bulk heterojunction domain sizes – largely independent of local drying conditions which are varying for different casting techniques during film formation. Furthermore, pre-aggregation in solution makes post-processing techniques mostly evitable and, hence, simplifies the production process of the solar cell devices.
In this study, progress in controlling polymer aggregation in solution by introduction of anti-solvent additives is reported. The impacts of polymer aggregation on parameters determining photovoltaic performance were investigated. Various spectroscopic and structure revealing methods were applied in order to further elucidate correlations between device properties and conformational state of materials and interfaces.

We develop a theory for the intra-intermolecular exciton intermixing and polarization dynamics in periodic 1D chains of planar organic molecules with two isolated low-lying Frenkel exciton states [1,2], typical of transition metal phthalocyanines [3]. We formulate the Hamiltonian and use the exact Bogoliubov diagonalization procedure to derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer (CT) exciton state [1]. By comparing our theoretical spectrum with available experimental absorption spectra of crystalline copper phthalocyanine (CuPc) thin films, we obtain the parameters of the Frenkel-CT exciton intermixing for this organic semiconductor material. The two Frenkel exciton states here are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the CT exciton, showing the coupling constant 0.17 eV in agreement with earlier electron transport experiments [4]. The third-order nonlinear polarization response function we derive [2] shows dynamical reorientation of the exciton transition dipole polarization (initially excited in the molecular plane) towards the axis of the 1D molecular chain. Such a dynamical reorientation pinpoints the direction of the charge separation process, whereby an originally excited intramolecular (Frenkel) electron−hole configuration gives one of its charges over to a neighboring molecule of the 1D molecular chain, to form a stationary eigen state of the 1D periodic molecular crystal lattice. Our results can be used for the proper interpretation of the physical properties of crystalline transition metal phthalocyanines – next generation organic semiconductors for advanced optoelectronics.

Acknowledgements: DOE-DE-SC0007117 (I.B.), NSF-ECCS-1306871 (A.P.)

References:
[1] I.V.Bondarev, A.Popescu, R.A.Younts, B.Hoffman, T.McAfee, D.B.Dougherty, K.Gundogdu, and H.W.Ade, Appl. Phys. Lett. 109, 213302 (2016).
[2] A.Popescu, R.A.Younts, B.Hoffman, T.McAfee, D.B.Dougherty, H.W. Ade, K.Gundogdu, and I.V.Bondarev, Nano Lett. 17, 6056 (2017).
[3] L.Edwards and M.Gouterman, J. Mol. Spectr. 33, 292 (1970).
[4] I.G.Hill, A.Kahn, Z.G.Soos, and R.A.Pascal, Jr., Chem. Phys. Lett. 327, 181 (2000).

Cyanine dyes are a class of organic chromophores well known for their strong light absorption and emission in the visible and near-infrared range. In the past, cyanine dyes have been used as light sensitizer for silver halide photography, in recording media technology or as contrast agent for biological applications. They are currently being investigated in the form of amorphous thin films as active layers for photovoltaic solar cells or light-emitting devices. The optical properties of cyanine molecules are highly anisotropic. They depend on the orientation of the transition dipole moment and the dipole-dipole interactions with neighboring molecules. However, in amorphous films the macroscopic optical properties become isotropic due to the random orientation of individual chromophores. Therefore, controlling the molecular orientation in cyanine thin films by fabricating single crystals could be an important strategy to tune the opto-electronic properties of cyanine-based devices.

In this work we fabricate single crystals of the dye 1,1’-diethyl-3,3,3’,3’-tetramethylcarbocyanine perchlorate directly on substrates through a solution-based process. The single crystals show a platelet morphology making them potentially interesting for use in thin film devices. Using X-ray crystallography, we show that the structure is made of strongly interacting molecular layers, each displaying a different herringbone-type arrangement. This leads to the splitting and polarization of the optical absorption bands through excitonic coupling of the transition dipole moments of the chromophores. Additionally we also measure the photoluminescence of single crystals which can also be related to the crystal structure. We show that compared to amorphous film, the crystals exhibit an additional intense emission peak. The experimental results are compared to theoretical calculations and a good correlation is found. In addition, the theoretical approach allows pinpointing specific molecular exciton contribution to the different spectroscopic bands.

With this work we demonstrate a simple method to fabricate thin film crystals and provide new insights in the photo-physics of cyanine dyes which opens the way for applications in optoelectronic devices.

Localized surface plasmon resonances have long been utilized for enhancing spectroscopic signals of molecules, semiconductor quantum dots and thin films. To date, the focus has been on understanding the enhancement effect on the rate of photon absorption and emission (and other non-radiative processes), where the total enhancement is determined by the product of the two rates. In this work, we utilize localized surface plasmon resonances to enhance the rate of photon absorption and exciton generation at a excitation frequency much higher than that of the emission and at a precisely defined distance away from the emitter. We first study the distance dependence of emission enhancement as a function of distance at room temperature. The enhancement factor is then investigated as a function of temperature (10 – 300 K). This approach has allowed us to obtain new insight into the exciton dynamics (generation, diffusion, capture and recombination) in different semiconductor heterostructures. We believe that these novel observations will have far reaching implications and will be shared in this presentation.

Organic infrared (IR) photodetectors are especially promising due to their strong absorption in the near-infrared (IR) regions, color selectivity, and compatibility with low-cost roll-to-roll processing. In this work, Tin(IV) 2,3-naphthalocyanine dichloride (SnNcCl₂) was used as the IR absorbing small molecule donor for fabricating IR photodetectors as well as IR sensitive organic light-emitting diode (IR-OLED). The SnNcCl₂ devices show strong IR response beyond 1100 nm which commercially available Si-based devices cannot offer. SnNcCl₂ as a donor needs to have an acceptor with a proper energy band alignment to efficiently dissociate photo-generated excitons because of the nature of the excitonic material with strong binding energy. C₆₀ and Phenyl-C61-butyric acid methyl ester (PC₆₀BM) were used as the acceptors with different lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels. Using a C₆₀ acceptor, the SnNcCl₂ IR photodetectors showed very high dark currents (~ 3 × 10^-5 A/cm² at -1 V). It is because the HOMO level of a SnNcCl₂ is too close to the LUMO level of C₆₀ acceptor, thus resulting in the charge generation effect between the HOMO level of a SnNcCl₂ donor and the LUMO level of a C₆₀ acceptor. Using a PC₆₀BM acceptor with high-lying LUMO level, in contrast with C₆₀, the dark current of the SnNcCl₂ photodetectors was significantly reduced to ~ 4 × 10^-8 A/cm² at -1 V. It means that the high-lying LUMO level of the PC₆₀BM acceptor sufficiently suppresses the charge generation effect. The acceptors with different LUMO levels also significantly affected the IR sensitive OLED with the SnNcCl₂ IR absorbing donor. Upon IR irradiation, both devices with PCBM and C₆₀ acceptors turned on at ~4.5 V. In dark, however, while the PC₆₀BM device showed higher turn-on voltage (10 V), the C₆₀ device has no significant different turn-on voltage with IR irradiation. Upon IR irradiation, both devices with PC₆₀BM and C₆₀ acceptors turned on at ~4.5 V. In dark, however, while the PC₆₀BM device showed high turn-on voltage (10 V), the C₆₀ device showed low turn-on voltage (5V), which is similar to that under IR irradiation condition. It means that the SnNcCl₂/C₆₀ layer as the charge generation layer can efficiently supply the hole current to light-emitting layer even without IR irradiation. A systematic study of these devices and the underlying device physics will be presented.

Monolayer transition metal dichalcogenides (TMDs) are attractive building blocks for future optoelectronic and valleytronic applications due to their direct band gap and preferential valley filling in response to circularly polarized light. Despite this promise, few processes exist to synthesize single crystals of primarily monolayers over a large area. Chemical vapor deposition (CVD) using solid precursors is an attractive option to produce single crystals of TMDs at the research scale; however, selenide-based TMDs (i.e., MoSe₂ and WSe₂) are particularly difficult to synthesize. To promote lateral growth, organic molecules such as perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) are frequently deposited onto the substrate prior to growth. The role of this organic semiconductor during synthesis and its subsequent effect on TMD properties is unclear. Its use also precludes the development of robust chemistry-structure relationships. In this work, we synthesize monolayer MoSe₂ on SiO₂ with and without PTAS and find significant differences in their growth behavior. We use a variety of post growth techniques such as PL, Raman, AFM, and XPS to understand the variations between the synthesized monolayers. Our results suggest that PTAS acts as a reservoir to increase the local concentration of Mo and Se precursors, thereby promoting MoSe₂ growth. We explain the diminished PL in the PTAS-assisted MoSe₂ as a consequence of the shorter nucleation time, which results in exposure to the deleterious effects of high temperature. This work is the first to address the morphological and optoelectronic differences in monolayer TMDs synthesized with and without seeding promoters. These findings serve as important steps towards the scalable growth of monolayer TMDs.

Colloidal quantum dots (CQDs) are attractive materials for lasers, displays and light-emitting applications due to their narrow and brighter spectral emission bandwidth, size-tunable bandgap and high-photoluminescence quantum yield (PLQY). However, these CQDs undergo inevitable degradation of their unique optical properties overt time due to their sensitive surface chemistry. To overcome these limitations, several approaches have recently been used such as over coating with an inorganic semiconductor shell of a wider band gap (core-shell hetrostructures), surface functionalization with new ligands or polymer coating and composites etc. In particular, core-shell heterostructured quantum dots with thick shell (so called “giant” quantum dots (g-QDs)) has shown higher PLQYs and improved photochemical stability than traditional thin-shelled or core only CQDs. The outstanding properties of g-QDs essentially depend on both the structure (defects, surface chemistry etc.) and the properties of interfacial layer (sharp or smooth core/shell interface). These structural and interfaces properties of g-QDs are strongly influenced and can be tailored by the synthetic parameters. Here, we will present our recent results on understanding the behavior of interfacial layer and structural defects on the photophysical properties of g-QDs.

Organic light-emitting diodes (OLEDs) have progressively attracted generous attention because of their versatile applications in solid-state lighting and displays. High-efficiency is crucial for OLEDs being energy-saving and to have a long lifetime. Numerous approaches have been attempted to attain high-efficiency OLEDs through the synthesis of novel organic materials, the design of light extraction structures, and the design of efficiency-effective device architectures. The organic materials used in optoelectronic devices have inherently low dielectric constant.
In this work, we demonstrate a comprehensive model to quantitatively investigate the role of dielectric constant of the electron transporting materials on the electric field distribution, charge drift, and exciton recombination probability in the emissive zone (EML) and electron transporting layer (ETL) of the organic light-emitting diode via commercialized electrical simulation package SETFOS. The simulation outcomes show the electric field distribution in the EML and ETL is heavily influenced by the dielectric constant of the ETL’s material. The electric field in the ETL dramatically decreases from 2.1 to 1.46 MVcm^-1, a decrement of 30%, as its dielectric increases from 1 to 5 respectively, whilst electric field across the EML increases from 0.78 to 2.21 MVcm^-1, that jeopardize the device reliability at elevated applied voltages. The organic layer with low dielectric constant material reduces the electron mobilities in the EML region, as the consequence electron and holes cannot overcome to their binding energy. It exceeds the internal thermal energy of the device at room temperature and finally leads to the deleterious effects on OLEDs performance.
Moreover, the recombination rate in EML increases from 11.12 to 28.21 cm^-2s^-1 as dielectric constant increases from 1 to 5 respectively, while in the ETL it decreases from 3.08x10^-2 to 4.01x10^-5 cm^-2s^-1. The simulation results show that the ETL with high dielectric constant material not only increases the number of electrons but also reduces the number of holes entering into the EML that leads the charge-balance into the emissive-recombination zone. In short, the ETL with a high dielectric constant enables low-operation voltage, balance-charge-distribution, and enhanced light-emissive exciton, hence better device performance.

Flexible organic light-emitting diodes (OLEDs) have potential application in next-generation display technology such as smart windows, mobile devices, virtual reality and wearable electronics. Typically, flexible OLEDs are based on flexible plastic substrates. However, these plastic substrates cause Fresnel reflection (10.4 %) and internal reflection (89%), so generated light from an emissive layer remains inside the OLED and substrate. To enhance the efficiency of the OLEDs without haze, it is known that subwavelength structures (< 250 nm, SWS) are essential. To obtain SWS on the plastic film, nanoimprint lithography (NIL) has been proposed due to the thermal and chemical weakness of polymer. However, the NIL requires master mold and complex surface treatment of the mold and substrate. And the master mold may deform after repeated use. Such problems could be solved by exposing chlorine (Cl₂) plasma treatment onto a thin Ag-coated plastic substrate. The low ionization energy of Ag can react with Cl₂ easily, producing subwavelength-scale AgCl nanostructure quickly. The SWS AgCl nanostructure can be directly formed on the surface of the flexible plastic film without surface treatment and additional etching mask.
In this work, we design the optimal SWS structure for highly transparent and haze-free plastic film, and demonstrate the AgCl SWS on the plastic substrate by a sequential process of Ag deposition and Cl₂ plasma treatment. To design the optimal SWS structure, rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) simulation were systematically calculated with respect to the various geometry of the nanostructure such as period, height, and gap. From the scanning electron microscopy (SEM) and UV-Vis spectrophotometer, we determined the optimal AgCl nanostructure. And X-ray diffraction and X-ray photoelectron spectroscopy were examined to understand forming mechanism of AgCl SWS. The Ag film reacts with Cl radicals in Cl₂ plasma to form AgCl. AgCl has larger molar volume (25.7 cm³ mol^-1) than that of Ag (10.2 cm³ mol^-1), so surface morphology drastically changed from a two-dimensional (2D) flat Ag surface to three-dimensional (3D) SWS to release volume expansion during the plasma treatment. Consequently, the optimized AgCl nanostructure drastically enhanced the current efficiency of the OLEDs up to 10.8% without modifying a shape of angular emission patterns.

Two-dimensinal van der Waals perovksites (RNH₃)₂PbX₄ are self-assembled by alternating layers of inorganic lead halide and organic ligands. Due to reduced screening from organic layers, the exciton confined in inorganic lead halide layers show binding energy of several hundred meV. The highly stable excitons, along with direct bandgap nature, render (RNH₃)₂PbX₄ an ideal candidate for exciton or polariton studies. While most of these materials/devices were synthesized/fabricated from solution methods, vapor-based synthesis are still needed to minimize impurity and defects. In this work, the vapor-phase growth of single crystalline (C₄H₉NH₃)₂PbI₄ flakes with high optical quality is reported. Individual single crystalline domains with lateral size about 5-10 µm and thickness 15-100 nm were deposited on Si, SiO₂/Si or muscovite mica, showing well-defined rectangular shape. Epitaxial relation were observed between perovskite flakes and mica/Si substrates. Due to the substrate effect therein, the structural phase transition at around 240 K were hindered and room-temperature phase were stabilized till liquid nitrogen temperature. Room temperature photoluminescence (PL) showed full width at half maximum (FWHM) of 70 meV and decay lifetime of several nanoseconds, indicating comparably high quality with mechanically exfoliated counterparts. Based on temperature dependent PL intensity, exciton binding energy 279 ± 46 meV and electron–phonon coupling (Fröhlich) strength around 20 meV are revealed. This vacuum-based method showed better controllability over thickness and structure, and may provide a solution for integrating layered perovskites into optoelectronic devices and systems.

Undoped and tin doped cadmium oxide (CdO) based thin films are prepared on corning glass substrate by sol-gel spin coating technique. In the present work the charge nutrality level (CNL) in highly conducting CdO thin films is demonstarted by the observed variation in the band gap upon annealing and doping. The increase in crystallite size with tin doping is a signature of decrease of CdO stoichiometry by substitutional replacement of Cd with Sn. Each Cd²⁺ ions are substituted by Sn²⁺ ions with reduction of Sn⁴⁺ via creating oxygen vacancies in the lattice which also enhnaces the carrier concentration in the tin doped thin film. The band gap enhancement can not be explained by Burstein-Moss Shift (BMS) only but can be explained in the frame work of CNL. The level of local CNL resides at the branch point of virtual gap states (ViGS) generation of which is the consequence of tin doping in CdO lattice. Further investigations using soft x-ray absorption spectroscopy (SXAS) at Oxygen k and Cadmium M4,5 edge show the reduction of Sn⁴⁺ to Sn²⁺. In CdO the interaction between Cd 4d and O 2p determines the type of band gap of CdO. The indirect band gap due to displaced valence band minima from Brillouin zone centre can be speculated to be the direct consequence of the hybridization of Cd 4d with O 2p states combined with octahedral point symmetry. The analysis of the spectral features has revealed an evidence of p-d interaction between O 2p and Cd 4d orbitals that pushes the valence band minima at higher energies which is symmetry forbidden at Γ causing a positive valance band dispersion away from the zone centre in the Γ ~ L, K direction. Thus, origin of the CNL is attributed to the high density of the Oxygen vacancies as confirmed by the change in the local electronic structure and p-d hybridization of orbitals.By further irradiating the thin films with 84 MeV Si⁶⁺ and 120 MeV Ag⁹⁺ ions with fluence 5e12 ions cm^-2 we found an unusual band gap enhancement via generation of oxygen vacancies due to huge electronic energy deposition inside the lattice by Ag and Si ions.The electrons from oxygen vaccancy would fill the ViGs more above the conduction band mininma and as a result the Fermi level tends to be more towards the CNL and a band gap enhancement is observed which has been substantiated by developing an schematic block diagram.

Singlet fission is an exciton splitting process that occurs in selected molecular systems that can boost solar energy conversion efficiencies by generating two electron-hole pairs from one high-energy photon. Molecular materials are fundamentally capable of supporting quantitative (i.e., lossless) carrier multiplication, and significant efforts have been made to better understand the process so as to ensure that quantitative yields, a prerequisite for the practical implementation of singlet fission, are actually achieved. A critical intermediate, known colloquially as the triplet pair, enables the process and precedes the formation of the two independent triplet excitations required for carrier multiplication. While a detailed understanding of the first step of singlet fission, i.e., triplet pair formation, has emerged, there are a number of details regarding the dissociation of the triplet pair into independent triplet excitations that remain to be clarified.

In this talk, I show how side chains can be used to effectively balance intermolecular coupling in amorphous solids of several pentacene derivatives and achieve overall highly-efficient singlet fission. I show how side chain sterics sensitively govern local packing in these amorphous solids and how this is in turn governs both triplet pair formation and decay rates. While triplet pairs form in quantitative yields in all cases, it is found that compact side chains promote stronger couplings that cause triplet pairs to effectively couple to the ground state, resulting in detrimental losses. In contrast, bulkier side chains cause triplet pairs to appear more like two independent and long-lived triplet excitations, and thus promote quantitative yields. Our results clarify many outstanding aspects of the triplet pair especially relevant to losses—we find that the triplet pair is non-emissive, that it is not bound, and that the constituent triplets cannot be considered in an independent nature. This work represents an important step toward better understanding intermediates in singlet fission and how molecular packing and couplings govern overall triplet yields.

In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and the mixed donor-acceptor phase are generally regarded as detrimental. Here, we show experimental evidence that a continuous donor network is not strictly necessary – hole motion between isolated donor sites can occur efficiently by long-range tunneling.

Using Time-Resolved Electric-Field-Induced Second Harmonic generation (TREFISH) combined with photocurrent measurements we have measured the motion of photo-generated charges from first hopping events (with sub-picosecond time resolution) to full extraction in complete solar cell devices based on α-sexithiophene (α-6T) dispersed in a buckminsterfullerene (C₆₀) matrix. Using vacuum deposition, we carefully vary the molar fraction of α-6T in C₆₀ from homogeneously diluted (<10% molar), to a point where α-6T begins to form isolated aggregates (>10-25% molar) or is strongly aggregated (50% molar). We thus vary the distance between the donor sites in a controlled manner. We quantitatively show that even highly diluted α-6T sites (5.7-10% molar) in a C₆₀ matrix enable hole transport, which occurs between isolated donor sites by hole tunneling through several C₆₀ molecules (tunneling distance ≈ 4 nm). Furthermore, at such low donor amounts (<10% molar) electron transport in the buckminsterfullerene phase remains unperturbed, thus facilitating ambipolar transport.

These results question the relevance of ‘pristine phases’ and whether a continuous interpenetrating donor-acceptor network is the ideal morphology for charge transport. The limits of this charge transport mechanism are yet to be explored.

This talk will describe why structural dynamics are important and provide ideas on how to control structural dynamics in nanocrystals (NCs). I will begin by describing the type of structural dynamics that occurs in NCs and the methods that we can use to measure structural dynamics, such as inelastic neutron scattering and inelastic x-ray scattering, or simulate it, such as ab initio molecular dynamics. Then I will describe the theory of how electrons and vibrations interact, and the challenges we face in calculating non-radiative electronic transitions rates. Finally, I will explain why we should care about structure dynamics, specifically the impact the type of dynamics has on optical properties such as carrier cooling, homogenous linewidths, thermal broadening, and electronic properties such as mobility and Shockley Read Hall non-radiative recombination.
In my talk, I will rely on the example of NCs, where structural dynamics can explain for example the large thermal broadening and fast carrier cooling rates experimentally observed in Pb-chalcogenide NCs. Furthermore, changing the structural dynamics (e.g., by changing the surface of NCs) can reduce the thermal broadening and slow carrier cooling due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. However, it is important to remember that these phenomena are relevant not only for NC, but also for bulk semiconductors and small molecules.

Semiconductor nanocrystals have risen as interesting excitonic materials in photovoltaics and light-emitting diodes. Depending on the device architecture, the quantum dots may have to perform optically and electronically (e.g. absorption and charge transport in a solar cell, charge transport and emission in an LED), or only optically (e.g. lumophores for luminescent concentration). Because of their small size, nanocrystals have the potential to be virtually defect-free in their interior. The surface of the crystal however is a source of defects, which become more critical the smaller the nanocrystal. Indeed, much work focuses on passivating nanocrystal surfaces to get rid of such defects. While the effect of these defects is often observed in device performance, the direct characterization of these defects remains a challenge, which affects the ability to correlate systematically processing to structure and properties.
I will first show how an ultra-sensitive absorption spectroscopy (photothermal deflection spectroscopy-PDS) can be used on model materials to correlate surface stoichiometry and passivation to deep defects and Urbach tails. We will use a series of well-defined PbS nanoparticles to directly observe emergence of deep states due to degradation and the slope of the Urbach tail. IN order to do so, we are able to measure absorption over an unprecedented 5 orders of magnitude in these materials. We will also show that states not predicted by theory are observed in the smallest particles. We will also relate particle size to the slope of the Urbach tail.
In the second part of the talk I will show how combining PDS with emission spectroscopy allows to measure the quantum yield (QY) of very luminescent particles (CdS/CdSe core/shell quantum dots) used as lumophores in concentrators with unprecedented accuracy. In order to obtain the high concentration factors needed for the practical application of these devices, the lumophores’ QY must approach 100%. We are able to measure QYs higher than 99% with 0.5% or better accuracy. This accuracy is needed in designing materials for luminescent concentrators as any non-radiative loss decreases the concentration factor.

Quantum-confined semiconductor nanocrystals offer tunable energy gaps, strong photoluminescence, and, in some cases, optical gain and lasing [1]. We report ultrafast optical pump, X-ray diffraction probe experiments performed at Argonne National Lab’s Advanced Photon Source with CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveyed transient lattice disordering (or melting). For smaller NCs, melting was observed upon absorption of as few as ∼15 electron–hole pair excitations per NC on average (0.89 excitations/nm³ for a 1.5 nm radius) with a similar electron-hole pair density inducing disordering for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting.
[1] Klimov et al. Science, 290, 314 (2000).
[2] Kirschner et al. Nano Lett. 17, 5314 (2017).

Solution-processed colloidal semiconductor nanocrystals are promising building blocks for next generation electronics and optoelectronics. In order to improve carrier transport wherein long native insulating ligands act as huge potential barrier, they are replaced by shorter ligands to enhance inter-nanocrystal coupling. However, in these devices carriers transport at band edge or mid gap states, leading to a significant reduction of the modulation ratio in photoconductivity (photocurrent/dark-current ) and transistors (with gate bias/ without gate bias ). Here we show a peak photocurrent density of more than in arrays of PbSe nanocrystals capped with native oleic acid ligands in a high-speed photoconductor configuration with a sub- response time. The ratio between peak photocurrent and dark current is more than ten orders of magnitude. The effective mobility is in the order of , more than eight orders of magnitude higher than previous results in PbSe, CdSe, and CdTe nanocrystals capped with native ligands. We attribute the ultra-high photocurrent density to multiple photo-generated excitons undergoing Auger recombination, followed by resonant, rapid tunneling at double bandgap excited states. This mechanism is demonstrated for a wide range of PbSe nanocrystals sizes (diameter from to ) and device dimensions. Our work demonstrate that native ligand capped nanocrystals ink are promising for optoelectronics applications wherein multiple carriers can be photoexcited.

Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2−4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets.

Two-dimensional transition metal dichalcogenides (TMDs) have garnered significant recent interest due to their efficient visible light absorption, emission, and photocatalytic activity. While the electronic structure and excitonic transitions governing their optoelectronic properties are becoming well-understood, less is known about the photophysics and carrier dynamics following light excitation. This work monitors relaxation dynamics of single-layer chemical vapor deposited MoS₂ and few-layer liquid exfoliated samples with femtosecond to nanosecond transient absorption spectroscopy featuring tunable pumping and broadband visible probing. Femto to picosecond dynamics exhibit a simultaneous bleaching of excitonic states as well as a red-shifted absorption spectrum attributed to both bandgap renormalization and dielectric effects. Long-lived, nanosecond scale features exceeding reported PL lifetimes and spectral differences at ca. 100-fold variation in nanosheet length could be critically important in understanding edge and carrier trap sites for device optimization. Studying the transient absorption behavior under redox-active conditions provides insight into dynamic charge transfer and chemical reactions catalyzed by both substrate-supported and liquid-phase 2D TMDs. Overall, these results provide an important step forward for understanding and implementing these emerging materials into photocatalytic and optoelectronic applications.

I will update on the use of reduced-dimensional (2.5 and 2D) halide perovskites. I will discuss strategies to make the active layer efficiently luminescent at low excitation thresholds. I will then review advances in electroluminescent device engineering and performance.

Solution-processed materials have significant promise for thin film electronics ranging from solar cells to LEDs to transistors. Hybrid organic metal halides, such as CH₃NH₃PbI₃, have garnered significant attention because they are earth-abundant, solution processable materials that can be used to form solar cells with high power conversion efficiency. We will present our work on elucidation of the optoelectronic properties of organic metal halide semiconductors with layered structures formed by replacement of halide ions by the pseudohalide thiocyanate (SCN^-) and by the introduction of a mixed organic cations to form Ruddlesden-Popper structures. Using time-resolved microwave conductivity (TRMC) experiments, the carrier mobility in-plane in thiocyanate compounds was found to be relatively high and comparable to that of polycrystalline methylammonium lead iodide (MAPbI₃) along with similar carrier lifetimes. In contrast, the layered Ruddlesden-Popper compounds, (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 (n = 1, 2, 3, 4), do not show lifetimes comparable to CH₃NH₃PbI₃. Instead the observed behavior by TRMC is a complex function of the “perovskite” thickness (n) and the morphology of the thin films. We will present structural studies of polycrystalline thin films of the Ruddlesden-Popper compounds using X-ray scattering that help to address this difference in behavior. Measurements of the band tail of these materials using solar cells also reveal key insights into the electronic structure of spin coated films.

Understanding the nature and energy distribution of optical resonances is of central importance in low-dimensional materials and its knowledge is critical for designing efficient optoelectronic devices. Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where A, A’ are cations, M is a metal, X is a halide, and their optical and electronic properties can be tuned by varying the perovskite layer thickness (n value). They have recently emerged as efficient semiconductors for light emission and photovoltaics, with technologically relevant stability [1-3]. However, fundamental questions concerning the nature of optical resonances (excitons or free-carriers), their scaling with quantum well thickness, and the physics behind the exciton properties, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5 (equivalent quantum well thickness from 0.64 to 3.14 nm) [4]. Comprehensive modelling of exciton states in 2D perovskites enable the understanding of dielectric confinement effects which prevail over quantum confinement in thin 2D perovskites. In addition, from these results we produce a general scaling behaviour for the binding energy of Wannier-Mott exciton states in Ruddlesden-Popper perovskites, which predict the exciton binding energy for any given thickness and organic spacer group. Our work demonstrates the dominant role of Coulomb interactions in 2D solution-processed quantum wells and presents unique opportunities for next-generation optoelectronic and photonic devices.

[1] Tsai et al., Nature (2016), 536, 312-316.
[2] M. Yuan et al., Nat. Nanotechnol. (2016), 11, 872-877.
[3] Blancon et al., Science (2017), 355, 1288-1292.
[4] Blancon et al., arXiv:1710.07653.

Fully inorganic cesium lead halide (CsPbX₃, X = I, Br, or Cl) perovskite nanocrystals (PNCs) are a novel optoelectronic material with outstanding optical properties. In order to be successfully employed in solid-state optoelectronic devices, the process of carrier propagation in PNC solids must be understood. In the present work we fabricated controlled 2D assemblies of PNCs, allowing for the direct measurement of exciton diffusion by steady state and time-resolved photoluminescence (PL) microscopy. The results of each technique demonstrate a long-range FRET-mediated diffusion process characterized by a 200 nm exciton diffusion length and diffusivity as high as 0.5 cm²/s. These findings are comparable or greater than those measured in bulk inorganic perovskite and hybrid polycrystalline perovskites, exceeding reported values for chalcogen-based QDs solids by orders of magnitude.

Hybrid perovskites are one of the most promising materials for low-cost and high-efficiency photovoltaic technologies. However, the understanding of their physical properties has lagged behind the impressive improvement of power-conversion efficiency (>20%). In my presentation I will report about the optical properties of FAPbI3 perovskite films based on different precursor systems. Exciton binding energies of 8.1 meV for the high temperature phase and 18 meV for the low temperature phase are extracted. At room temperature, we found the lifetimes to vary from tens to hundreds nanoseconds. FAPbI3 film made from the new precursor, exhibits the longest lifetime of 439 ns, suggesting a lower number of defects and lower non-radiative recombination losses compared with FAPbI3 obtained with the other two precursors set. From the low temperature data we estimated the exciton-optical phonon-coupling coefficient to be about 35 meV, and the optical phonon energy 18 meV. This latter value is assumed to be the effective average, since the FAPbI3 lattice possesses more than one optical phonon.

This presentation will focus on understanding the fundamental steps of photoinduced electron transfer in thin films conjugated polymers doped with low concentrations of molecular electron acceptors. The goal is to understand how these systems produce very high yields of long-lived, separated charges when exciting the acceptor directly, or indirectly through the conjugated polymer, which can serve as both a light harvesting antenna system and electron donor. An unusual feature of these systems is the large discrepancy in timescales of the initial photoinduced electron transfer step, which is sub-picosecond, and the recombination process, which can take as long as milliseconds.
By controlling the solid-state microstructure of the conjugated polymer through the degree of regioregularity, or by changing sample temperature we demonstrate how the yield of long-lived carriers, detected by transient microwave and optical absorption spectroscopies, is controlled by the delocalization effect afforded by regions of order within the polymer film. The influence that this control exerts on the carrier recombination process through the formation of a long-lived, emissive exciplex state will also be discussed.

Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S1) into two spin-triplet excitons (T1), could provide a means to overcome the Shockley-Queisser limit in photovoltaics [1,2]. Recent results have shown that the process is strongly governed by the coupling of electronic and vibrational degrees of freedom [3-5] and strongly coupled triplet pair states [6,7]. This gives rise to a host of interesting phenomena such as multi-molecular conical intersections at which singlets convert to entangled triplet pair states and the formation of exchange-coupled spin-2 quintet states. In this talk we will present recent results from our investigations into the coupled triplet pair in these systems using a combination of ultrafast spectroscopy and high magnetic fields. These results help to elucidate the nature of the wavefunction of the coupled triplet pair state and how it can be engineered via molecular design.

Smith, M. & Michl, J. Singlet fission. Chem. Rev. 110, 6891–6936, (2010).
Rao, A. & Friend, R. H. Harnessing singlet exciton fission to break the Shockley–Queisser limit. Nat. Rev. Mater. 2, 17063, (2017).
Bakulin, A. A. et al. Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy. Nat. Chem. 8, 16–23, (2016).
Musser, A. J. et al. Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission. Nat. Phys. 13, 182–188, (2015).
Stern, H. L. et al. Vibronically coherent ultrafast triplet-pair formation and subsequent thermally activated dissociation control efficient endothermic singlet fission. Nat. Chem. http://dx.doi.org/10.1038/ nchem.2856, (2017).
Weiss, L. R. et al. Strongly exchange-coupled triplet pairs in an organic semiconductor. Nat. Phys. 13, 176–181, (2016).
Tayebjee, M. J. Y. et al. Quintet multiexciton dynamics in singlet fission. Nat. Phys. 13, 182–188 (2016).

The electronic structure and exciton dynamics of organic molecules can change dramatically upon aggregation. The exciton dynamics of molecules in solution and in thin films of aggregates can be measured using transient absorption spectroscopy, but the exciton dynamics of intermediate aggregation states during thin film formation are typically unknown since measurements cannot be performed quickly enough to collect accurate transient absorption spectra of these species. By increasing the speed of data collection, the exciton dynamics of evolving material systems can be measured. A novel implementation of transient absorption spectroscopy is introduced that can measure transients with up to a 45 ps pump-probe time delay in one shot. The exciton dynamics measured during the solution deposition of a thin film is validated by comparing the initial exciton dynamics of the solution and the final exciton dynamics of the dry film to traditional transient absorption measurements. The exciton dynamics of intermediate aggregation states will be presented for the first time. The information gained using this technique can be used to modify environmental parameters during the film formation process to kinetically trap aggregates with exciton dynamics tailored for a particular application.

Singlet Fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improving organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit SF, most of which are acene derivatives. Recently, there has been a growing interest in the rylene family of polycyclic aromatic hydrocarbons (PAHs) as potential SF materials. We use many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation (BSE) to investigate the possibility of SF in crystalline quaterrylene. A new method is presented for determining the percent charge transfer character (%CT) of an exciton wave-function from double-Bader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (E_S-2E_T) and %CT, crystalline quaterrylene is a promising candidate for SF. Furthermore, quaterrylene is attractive for OPV applications thanks to its high stability, narrow optical gap, and lowest lying triplet excitation energy in the optimal range to maximize solar conversion efficiency.

Singlet fission in organic semiconductors is a promising and potentially inexpensive approach to surpass the Shockley-Queisser limit of single-junction solar cells. By converting one high-energy excitation into two lower-energy excitons, singlet fission can efficiently down-convert high-energy light of the solar spectrum. The coupling between molecules has proven to be critical for efficient singlet fission, but exactly how the intermolecular distances and stacking will influence the rate and mechanism of singlet fission is not clear.
Here we use hydrostatic pressure as a simple and clean tool to tune the intermolecular distance and stacking in thin films of tetracene, a highly efficient singlet fission material and rubrene, which has a relatively low singlet fission efficiency. In the case of tetracene, a slower singlet decay rate is observed at higher pressure and may be attributed to a higher energy barrier between the first excited singlet state and the multi-exciton state. In rubrene, the rate of singlet fission is increased at higher pressure, which is accompanied by a significant drop in photoluminescence intensity, indicating a higher singlet fission efficiency.
Our study shows that increasing intermolecular coupling may have different effects for different singlet fission materials and provides insights for designing new efficient singlet fission materials.

One way to overcome the theoretical limit for the efficiency of single junction photovoltaics (PV) is to reduce the quantum defect via multiple exciton generation. Singlet exciton fission in tetracene and other polyacenes has been proven to be a highly efficient pathway to generate two triplet excitons from one singlet exciton. If those triplet excitons could be transferred to silicon, the efficiency of conventional Si solar cells could be dramatically improved. Triplet excitons in tetracene have an energy of 1.25 eV which is slightly higher than the bandgap of Si (1.1 eV), a requirement for triplet exciton energy transfer.
To enable transfer of triplet excitons, we functionalize a Si (111) surface with different benzene derivatives to control the alignment of the tetracene on the Si surface, keep the Si surface oxide free and allow for a close distance of ~1 nm between tetracene and Si. The energy of triplet excitons can transfer only via Dexter energy transfer, which requires the wavefunction of initial and final states to overlap, so aligning the wavefunction of Si and tetracene will allow to improve the energy transfer rate dramatically.
We study the energy transfer from the dependence of the decay rate on tetracene thickness. Tetracene growth in islands on the silicon surface, where each island has a different thickness. We correlate the thickness of the tetracene islands with the lifetime of the triplet excitons for each island individually. We find that the delayed photoluminescence (PL) decay lifetimes of the thicker islands is longer. This means that the triplet excitons that experience the interface during their lifetime are quenched, consistent with energy transfer into silicon.

Solution-processed thin film solar cells based on colloidal quantum dot (CQD) and organic bulk heterojunction (BHJ) absorbers are compatible with low-cost, large-area manufacturing processes amenable to efficient, lightweight modules. In recent years, both CQD and organic photovoltaic devices have seen significant efficiency improvements, with single-junction devices reaching power conversion efficiencies (PCE) of over 12%. Yet, the progress in both fields has been seen primarily in laboratory scale champion devices prepared almost exclusively using spin-coating and employ halogenated additives (organics) or tedious ligand exchange schemes (CQDs). Surface traps in CQDs have also, indirectly, impeded scalable and industry-compatible fabrication of these solar cells as some of the CQD preparation steps need to be performed in low humidity environments.
CQDs benefit from a size-tunable bandgap that allows absorption of photons with a relatively wide range of wavelengths, but a weaker absorption in the red and near-infrared (NIR) regions of the solar spectrum. Meanwhile, organic absorbers can have a narrower and more tunable spectral absorption than that of CQDs, making organic absorbers especially attractive for tandem solar cells, including with CQD absorbers. The opportunity exists to increase beyond the state-of-the-art PCE by tapping into the complementary spectral range of quantum-dot and organic absorbers and combining the two material types into hybrid tandem solar cells.
The first part of this talk will discuss our recent efforts aimed at fabricating highly efficient organic and CQD solar cells using large-area upscalable coating techniques. This effort has thus far yielded CQD solar cells with PCE > 10% and organic solar cells with PCE>11% using active layers coated in ambient air at high speed (>10 m/min). The second part of this talk will discuss the challenges and recent progress in the monolithic hybrid tandem integration of CQD and organic solar cells and the pathway toward achieving PCE>12%.

2D crystals can, in effect, be thought of as huge J-aggregates, focusing electronic oscillator strength in a small set of dominant excitonic transitions. The radiative lifetime of excitons, created within the light cone of the dispersion relation, can be exceedingly short, of order 100 fs. With such short lifetimes it is possible to perform a range of unusual experiments which are hard to conceive with conventional materials. For example, excitation energy can be harvested from plasmon excitations of metallic nanostructures. Whereas metal nanoparticles are often used to quench the fluorescence of dye molecules by FRET, the opposite process is possible with 2D crystals. We excite the plasmon of a single nanoparticle electrically and funnel the spectrally broad excitation energy into the narrow excitonic band of the 2D crystal. In this way, EL can be tuned by the monolayer and is both down- and up-converted to the exciton resonance. Photon correlation spectroscopy demonstrates that emission occurs from nanoscale volumes and reveals signatures of single-photon generation on demand, at room temperature [1].

A second demonstration of the unusual excitonic properties of these materials is given by their pronounced non-linear optical characteristics. With careful tuning of the excitonic resonances, by temperature and dielectric environment, quantum interference phenomena between transitions of bright and dark excitonic states become apparent. This interference is manifested in resonantly enhanced second-harmonic generation, and is analogous to electromagnetically induced transparency, EIT, providing a pathway to ultrafast optical switching and inversion-less lasing.

[1] Puchert, Lupton et al., Nature Nano. 12, 637 (2017).

The strong absorption and visible spectrum energy bandgaps for the transition metal dichalcogenides (TMDCs) of molybdenum and tungsten render them as attractive candidates for photovoltaics (PV) and optoelectronics. Further, the atomically thin nature is favorable for efficient separation and collection of photo-excited charge carriers. Therefore, if the three major optoelectronic criteria, i.e., i) sunlight absorption, ii) carrier collection and iii) operating voltage can be addressed for TMDC materials, they may be candidates for high efficiency photovoltaics. We have recently demonstrated near-unity broad-band absorption of above band-gap photons for < 15 nm TMDC layers.¹ This has been further extended to concurrently achieve high external quantum efficiency in < 10 nm thick active layer photovoltaic devices in a p-n junction of WSe₂/MoS₂ with graphene contacts², thereby addressing the first two criteria. However, achievement of high open-circuit voltage (V_oc) remains an outstanding challenge towards achieving high photovoltaic efficiency.
Here, we experimentally demonstrate high open-circuit voltages (V_oc) in TMDC absorbers based photovoltaic devices. We achieve this by tailoring the conduction and valence band-alignments between a single TMDC absorber layer and carrier-selective contact layers for electron collection (titanium oxide) and hole collection (nickel oxide), respectively. We further verify the band alignments using X-ray photoelectron spectroscopy to demonstrate the asymmetric and selective nature of the metal oxide carrier-selective contacts. We observe open circuit voltage values exceeding 700 mV under AM 1.5 illumination at 1 Sun, a record for TMDC-based photovoltaic devices. In contrast, the current maximum V_oc in similar class of devices is limited to 300-400 mV. We will also present detailed electrical characterization of these devices, combined with coupled optoelectronic simulations combining device physics modeling with full wave electromagnetic simulations of optical absorption. Devices form using both sulfides and selenides of molybdenum and tungsten will be presented, in a comparative analysis of performance, material quality, doping levels and band alignments. Our results presented here will serve as guiding design principles towards overcoming the final major hurdle in achieving high power conversion efficiency in devices with ultrathin van der Waals absorber layers.³
References:
1. Jariwala, D.; Davoyan, A. R.; Tagliabue, G.; Sherrott, M. C.; Wong, J.; Atwater, H. A. Nano Lett. 2016, 16, (9), 5482-5487.
2. Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. ACS Nano 2017, 11, 7230–7240.
3. Jariwala, D.; Wong, J.; Davoyan, A. R.; Atwater, H. A. ACS Photonics 2017, doi: 10.1021/acsphotonics.7b01103.

Third-generation photovoltaics seek to improve solar technology, potentially beyond the Shockley-Queisser limit. One proposed method to achieve this goal, is to generate multiple excitons per absorbed photon. Multiple exciton generation in quantum dot photovoltaics, and singlet fission in organic photovoltaics have been able to produce photovoltaic external quantum efficiencies above one hundred percent. In addition, these processes have the potential to improve solar cell efficiency by generating additional photocurrent from these multi-exciton processes. This work seeks to create a hybrid nanoparticle-ligand system as the active absorbing layer in a p-n heterojunction solar cell. This active layer will utilize strongly absorbing PbS nanocrystals, with bidentate derivatives of diphenylhexatriene (DPH) functionalized with carboxylic acid linkers as ligands capable of replacing the native ligands on the nanocrystal. DPH in the solid state has shown singlet fission quantum yields of 90% and triplet lifetimes greater than 50 microseconds. By designing PbS nanocrystals with conduction bands below the triplet energy of the DPH analogs, triplet excitons generated in the DPH ligands can transport into the PbS nanocrystals and then undergo charge separation and transport to their respective electrodes. The purpose of this work is to observe singlet fission at the nanocrystal-ligand interface and use the triplets generated to produce additional photocurrent in a photovoltaic device.

We report on the synthesis and spectroscopy of energy-gradient nanostructures that support the formation of two-dimensional excitons in the shell domain. The developed geometry places a wide-gap semiconductor (CdS) at the core of the composite nanoparticle in order to funnel the photoinduced energy into the low-gap CdSe surface layer. As a result, the quantum confinement is achieved in nanoparticles which total size exceeds the exciton Bohr radius. The formation of excitons in the CdSe shell layer was manifested through a size-tunable emission and the characteristic step-like absorption profile. Transient absorption measurements further elucidate the dynamics of the photoinduced energy relaxation in CdS/CdSe nanoshells providing evidence that excitations of the bulk-like core domain result in a rapid, ~ 2-ps recovery of the CdS bleach attributed to electron cooling. The subsequent decay of excitons in the shell proceeds at a lower rate with a markedly multi-exponential temporal character. The charge transport characteristics of nanoshell assemblies were evaluated through a side-by-side comparison with CdSe quantum dot solids. According to photocurrent measurements, nanoshell solids showed an enhanced photoconductivity relative to similarly processed films of spherical CdSe nanocrystals. We expect that the developed nanoshell architecture could potentially be extended to a broader range of semiconductors (e.g. CdS/PbS, ZnS/CdS) facilitating the development of quantum confined solids offering improved charge transport characteristics.

Tailoring semiconductor heterostructures for specific functionalities has led to varied opto-electronic devices including solar cells, photodetectors, light-emitting diodes and lasers. An emerging class of heterostructures involves monolayer semiconductors such as many of the transition metal dichalcogenides (TMDs) which can be combined to form van der Waals heterostructures (vdWHs). VdWHs offer novel functionalities making them promising hosts for future devices. One unique new heterostructure property is an interlayer exciton (ILE), a spatially indirect, bound electron-hole pair with the electron in one TMD layer and the hole in the other. Here, using state-of-the-art preparation techniques, we are able to resolve emission from the ILE in a MoSe₂/WSe₂ heterostructure into two distinct peaks separated by 24 meV at zero field. Furthermore, we demonstrate that, when excited with circularly polarized light, the two emission peaks have opposite circular polarizations. Ab initio calculations provide an explanation of this unique and potentially useful property and indicate that it is a result of the indirect character of both electronic transitions. These peaks are double indirect excitons. i.e. indirect in both real and reciprocal space, split by relativistic effects.
This work was supported by core programs at NRL and the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research #AOARD 14IOA018-134141. This work was also supported in part by a grant of computer time from the DoD High Performance Computing Modernization Program at the U.S. Army Research Laboratory Supercomputing Resource Center.

Van der Waals heterostructures of two-dimensional (2D) materials are a promising platform for understanding the nanoscale energy transport of room temperature excitons. Since the orientation of exciton dipole in type-II heterostructures of 2D transition metal dichalcogenides (TMDCs) is fixed due to electron and hole existence in different monolayers [1], it offers a controlled platform to study excitonic transport.
In this work, we experimentally demonstrate a spatial, temporal, and spectral visualization of exciton transport in monolayer molybdenum disulfide (MoS₂), tungsten diselenide (WSe₂) and a MoS₂/WSe₂ heterostructure. The sample was made by stacking two mechanically exfoliated MoS₂ and WSe₂ monolayers using a dry transfer technique followed by high temperature annealing [2,3]. The photoluminescence (PL) measurement of the heterostructure revealed a dominant spectral peak at 1.59 eV that was red shifted from the PL emission peak of monolayer MoS₂ (1.90 eV) and WSe₂ (1.66 eV) confirming the existence of the interlayer exciton in the heterostructure. By performing diffusion experiments using an avalanche photodiode (APD), we were able to estimate the lifetime, diffusivity and diffusion length of each excitons [4]. At room temperature, interlayer excitons in the MoS₂/WSe₂ heterostructure showed a longer lifetime (0.955 ns) compared to the intralayer excitons in MoS₂ monolayer (0.236 ns) and WSe₂ monolayer (0.705 ns). Furthermore, the diffusivity and diffusion length of interlayer excitons (2.25 cm²/s, 654 nm) was found to be higher than the intralayer excitons in the MoS₂ monolayer (0.44 cm²/s, 56 nm) and WSe₂ monolayer (1.4 cm²/s, 444 nm) reconfirming the electronic coupling in the heterostructure.
In conclusion, our results quantify and confirm that interlayer excitons live longer and diffuse faster and further than intralayer excitons. Controlling the transport of such intralayer excitons can potentially enable a platform to fabricate room temperature, energy efficient, exciton-based information processing devices.
1. P. Rivera et al. Observation of long-lived interlayer excitons in monolayer MoSe₂-WSe₂ heterostructures. Nat. Commum. 6, 6242(2015).
2. Andres Castellanos-Gomez et al. Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Mater. 1 011002(2014).
3. Zhixing Lu et al. Universal Transfer and Stacking of Chemical Vaper Deposition Grown Two-Dimensional Atomic Layers with Water-Soluble Polymer Mediator. ACS Nano. 10, 5237-5242(2016)
4. Gleb M. Akselrod et al. Visualization of exciton transport in ordered and disordered molecular solids. Nat. Commum. 5, 3646(2014)

Organic semiconductors are promising optoelectronic materials with strong light absorption and excitonic effects. For these systems, the interplay between inter- and intra-molecular interactions results in a complex behavior of excitons that is not yet well-controlled in devices. Here, we utilize first-principles theory to understand the role of inter-molecular orientation on the nature and energy of excitons within ordered organic materials. For a series of acene derivatives in the bulk crystalline phase, highly accurate many-body perturbation theory calculations predict significant exciton binding energies and charge-transfer character, which can be controlled through solid-state morphology or change of conjugation length. Additionally, we investigate the role of inter-molecular vibrational interactions on the optoelectronic properties of PTCDI-based molecular wires. Using a density functional theory analysis, we demonstrate that strong inter-molecular interactions lead to distinct vibrational, electronic, and optical properties. Overall, this work suggests a new strategy for design of new electronic and organic optoelectronic materials.

The migration of Frenkel excitons, tightly-bound electron-hole pairs, in photosynthesis and in organic semiconducting films is critical to the efficiency of natural and artificial light harvesting. While these materials exhibit a high degree of structural heterogeneity on the nanoscale, traditional measurements of exciton migration lengths are performed on bulk samples. Since both the characteristic length scales of structural heterogeneity and the reported bulk diffusion lengths are smaller than the optical diffraction limit, we adapt far-field super-resolution fluorescence imaging to uncover the correlations between the structural and energetic landscapes that the excitons explore. By combining the ultrafast super-resolved measurements with exciton hopping simulations we furthermore specify the nature (in addition to the extent) of exciton migration as a function of the intrinsic and ensemble chromophore energy scales that determine a spatio-energetic landscape for migration.

In an organic solar cell (OPV), the key photophysical process is free charge carrier generation. Ultrafast long-range charge separation have been proposed¹, there is however no universal consensus about the role of the charge-neutral electron-hole pairs (excitons) and charge transfer (CT) state in this process².
Here, we aim at getting more insight on the timescale and process leading to the formation of the charge carriers from thermalized excited states, by using ultrafast transient absorption spectroscopy with 20fs time resolution based on one efficient ternary D₁-D₂-A OPV system. The use of extremely short pulses enables us to measure the characteristic vibrational modes in the excited states of the system. The analysis of these modes allow to study how they evolved during charge generation, in addition to unraveling the exciton photophysics in this process.
The vibrational modes created by the pump are clearly visible from the TA data (fig.1, a). The electronic response shows that the ternary blend have significant oscillation signal instantly while binary blend short of distinct signal. Time-resolved photoluminescence data (fig.1, b,c) showing that exciton in the ternary blend dissociate almost totally in the initial 10ps, while the binary blend still emitted photons after 1ns. In combination with other time –resolved and steady state optical measurements, we can identify that compared to binary blend, the ternary blend show less charge transfer state and more charge carriers come from exciton directly. This shows that thermalized excited states is not hindering charge generation as the vibronic coupling accelerates ultrafast charge separation dynamics.

References:
1. Gelinas S et al, Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes. Science 2014, 343(6170): 512-515.

2. Lienau C et al, Coherent ultrafast charge transfer in an organic photovoltaic blend. Science 2014, 344(6187): 1001-1005.

The morphology of organic semiconductors influences exciton and charge transport, which are main factors in the performance of organic electronic devices. J-aggregates which are crystalline in nature and show coherent exciton migration experience strong exciton recombination at the grain boundaries. However, improving the crystallinity by tuning their morphologies is limited. Here, we demonstrate that surface-guided molecular assembly helps in realizing different morphologies like monolayer and lamellar crystals with improved crystallinity, which suppresses the non-radiative decay path and improves exciton diffusion.
In this study, we have deposited J-aggregates of a cyanine dye onto a PAMAM (polyamidoamine) dendrimer functionalized surface using a solution-immersion route. The presence of J-aggregates in the solution (water) limits to only island-type growth following the ‘Volmer-Weber’ model as reported so far. The platelets obtained from this growth show a lot of small coherent domains isolated by grain boundaries.
Tuning the size of the J-aggregates in a binary solvent mixture induces strong substrate-molecules interaction following a ‘Stranski-Krastanov’ growth model. The J-aggregates selectively nucleate on the PAMAM surface to form a complete monolayer. Nevertheless, with prolonged exposure, the molecule-molecule interaction dominates the substrate-molecule interaction and quasi-epitaxial vertical crystals were obtained. Furthermore, increasing the dye concentration affects the crystal growth, resulting in lamellar shapes with improved crystallinity. Our finding suggests that having J-aggregates in critical nucleus size is the most important criteria to tune the aggregate morphology on the surfaces.
Optical properties such as absorbance and photoluminescence were characterized in this study. Both platelets and lamellar crystals show similar absorbance at room temperature. Steady-state photoluminescence shows higher fluorescence quantum yield for the lamellar films than platelets. Temperature-dependent photoluminescence studies from 6 K to room temperature also show a similar trend. Using time-resolved photoluminescence spectroscopy, different non-radiative decay pathways were identified. The influence of exciton diffusion on the interplay of energetic disorder and non-radiative decay will be presented. The results suggest that the difference in domain size and order give rise to significantly enhanced radiative decay from lamellar films as compared to platelets or films deposited by spin-coating.