The derivatives of benzo[1,2-b :4,5- b &’]dithiophene (BDT) have been broadly used in photovoltaic polymers and great success has been made in the recent years. Different strategies have been employed to improve photovoltaic properties of the BDT-based polymers, and thus efficiencies of polymer solar cell (PSC) devices based on BDT-polymers have reached over ~8%. In this work, two-dimensional (2D) conjugated structure is used to improve photovoltaic properties of BDT-based polymers and a series of new photovoltaic polymers were designed and characterized. We found that the polymers with 2D conjugated structure exhibited broader absorption bands, deeper HOMO levels, higher hole mobilities and thus better photovoltaic properties compared to their counterparts without 2D conjugated structure. For example, the power conversion efficiency (PCE) of the device based on a representative polymer with the 2D conjugated structure (name as PBDTTT-C-T) reached 7.6%, when it was used in a polymer solar cell with conventional structure; compared to the analog (named as PBDTTT-C) without the 2D conjugated structure, which shows a PCE of 6.4% in photovoltaic device, PBDTTT-C-T exhibits much better photovoltaic properties, indicating that the 2D conjugated structure can be seen as an effective strategy to optimize molecular structure of photovoltaic polymers. In this presentation, the detailed results of the 2D conjugated BDT-polymers will be presented and discussed to give an introduction to this useful strategy of molecular structure design.

Diketopyrrolo[3,4-c]pyrrole (DPP) has emerged as a promising building block of low band-gap conjugated polymers for the optoelectronic applications including both organic field effect transistors (OFETs) and polymer solar cells (PSCs). Its highly conjugated structure leads to strong π-π interaction and the electron deficiency provides the DPP with the potential use of electron-accepting unit for synthesis of low band-gap conjugated polymers. We report novel low band-gap conjugated polymers, PDTTDPP composed of dithieno[3,2-b:2&’,3&’-d]thiophene (DTT) and DPP, and PBDTDPP composed of unsubstituted benzo[1,2-b:4,5-b&’]dithiophene (BDT) and DPP as well as small molecules composed of phenylene and DPP. We demonstrates that a novel low bandgap polymer (PDTTDPP), composed of electron-rich extended heteroarene unit (DTT) and electron-deficient unit (DPP), can be used as a p-type conjugated polymer for both OFETs and OPVs. The strong intermolecular π-π stacking of PDTTDPP contributes to achieve the high hole mobility of 0.68 cm2/Vs in OFETs without post treatment. The PDTTDPP also shows a low bandgap of 1.22 eV due to strong intra-molecular charge transfer between DTT and DPP. Consequently, the high hole mobility and broad light absorption of PDTTDPP achieves a power conversion efficiency (PCE) of 6.05% with Jsc = 13.9 mA/cm2, Voc = 0.66 V, and FF = 65.7%. This work clearly demonstrates that extended heteroarene unit (DTT) is a promising building block in the molecular design of new polymers for high performance organic electronics. PBDTDPP is another interesting polymer and expected to exhibit a low-lying HOMO level to afford high Voc, because alkoxy group is removed from alkoxy-substituted BDT. Alkoxy group has strong electron-donating power, and therefore it raises the HOMO level of D-A conjugated polymer. Since the Voc of PSCs is proportional to the difference between the HOMO level of donor and the LUMO level of acceptor, PBDTDPP with low-lying HOMO level due to removal of alkoxy group exhibits higher Voc while the conjugated polymer (PBDTDPP-OR) with alkoxy group exhibits lower Voc due to its high-lying HOMO level. Consequently, the removal of the alkoxy side group from alkoxy-substituted BDT leads to achieve a promising PCE of 5.16% with a high Voc of 0.82V. These values are much higher than those of PBDTDPP-OR (2.24% and 0.61 eV). A series of simple structured small molecules based on DPP were also synthesized and their photovoltaic properties were investigated in terms of the type of electron-donating unit. By introducing a donor unit with different electron-donating power, such as thiophene (T) and phenylene (Ph), the frontier orbital energy level of small molecules can effectively be tuned. The small molecule with a weak donor unit, Ph(TDPP)2, exhibited a PCE of 4.01% with a remarkabley high Voc of 0.93 eV when it was used as a donor of active layer material in bulk heterojunction solar cells.

In this talk, we present our recent progress in development of design principles for high efficient organic solar cell polymers. We have found that the power conversion efficiencies of the donor polymers were strongly correlated to Δµge, the dipole change between excited state and the ground state of repeating monomer unit. The results reported demonstrate the utility of the calculated parameter Δµge to predict the performance of donor-acceptor copolymers in photovoltaic devices. We rationalize this result based on the large degree of polarization in the excited state which effectively lowers the Coulomb binding energy of the exciton in the excited state and leads to faster charge separation kinetics, thus facilitating the full separation of electron and hole.

Solution-processed polymer/fullerene blends are receiving extensive attention as the photoactive layer of organic photovoltaic (OPV) cells. Whilst the impressive increase in headline efficiencies highlight their potential, cell performance is known to be critically dependent on their solid-state microstructure. So far, the latter typically was ‘optimised&’ via intricate trial-and-error approaches. This unsatisfactory procedure is necessary as the relevant insight in the microstructural requirements for bulk heterojunctions is still lacking, hindering the tailoring of the donor:acceptor morphology from the outset. We present a range of photophysical, electrochemical and physicochemical data that provide evidence that formation of a crystalline phase of at least one component is necessary for driving the spatial separation of photogenerated charges. We discuss this on the example of the fullerene component and demonstrate that its crystallisation results in an increase in electron affinity, providing an energetic driving force for spatial charge separation. A functional model is proposed that is based upon charge generation in a finely intermixed polymer/fullerene phase followed by spatial separation of electrons and holes at the interface of this mixed phase with crystalline fullerene domains. This model has significant implications for the design of alternative solar cell materials as well as development of reliable processing procedures. Jamieson, F.C., Buchaca Domingo, E., MaCarthy-Ward, T., Heeney, M., Stingelin, N. & Durrant, J., Chem. Sci. 3, 3, 485 (2012)

Benzotrithiophene has recently been introduced as a new electron-rich moiety for semi-conducting polymers.[1] The incorporation of this new unit into polymers has lead to high hole mobilities of ~0.3 cm2/Vs in organic field-effect transistors and to open-circuit voltages as high as 0.8 V in organic photovoltaic devices.[2,3] We present here our recent work on random benzotrithiophene terpolymers.[4] A series of donor-acceptor type terpolymers has been synthesized with the aim to improve the charge separation and increase the polymer absorption without affecting the HOMO energy level, thus leading to higher short-circuit currents in photovoltaic devices with similar high open-circuit voltages and ultimately better overall photocurrent efficiencies. We will demonstrate how the choice of the third monomer and the monomer ratios can be used very efficiently to optimize optical absorption, charge separation and photovoltaic device performance. [1] C. B. Nielsen; J. M. Fraser; B. C. Schroeder; J. Du; A. J. P. White; W. Zhang; I. McCulloch, Organic Letters 2011, 13, 2414-2417. [2] B. C. Schroeder; C. B. Nielsen; Y. Kim; J. Smith; S. E. Watkins; K. Song; T. D. Anthopoulos; I. McCulloch, Chemistry of Materials 2011, 23, 4025-4031. [3] C. B. Nielsen; B. C. Schroeder; R. S. Ashraf; Z. Huang; A. Hadipour; B. Rand; S. E. Watkins; J. Durrant; I. McCulloch, Journal of Materials Chemistry 2011, 21, 17642-17645. [4] C. B. Nielsen; R. S. Ashraf; B. C. Schroeder; P. D'Angelo; S. E. Watkins; K. Song; T. D. Anthopoulos; I. McCulloch, Chemical Communications 2012, 48, 5832-5834.

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC₇₀BM compared to the well known low band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates stronger in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) are used to gain inside into the effect of fluorination on the field-dependence of free charge carrier generation and recombination. F-PCPDTBT is shown to exhibit a significant weaker field-dependence of free charge carrier generation compared to non-fluorinated PCPDTBT [1]. Also, fluorination increases overall generation of free charges, meaning that geminate recombination is greatly reduced. Additionally, a threefold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC₇₀BM is largely determined by the field-dependence of free carrier generation, and this field-dependence is considerably weaker compared to blends comprising the non-fluorinated polymer. For these optimized blends, a short circuit current of 14 mA/cm², an open circuit voltage of 0.74 V and a fill factor of 58% is achieved giving an energy conversion efficiency of up to 6.2%. The superior device performance and the low band-gap renders this new polymer highly promising for the construction of efficient polymer-based tandem solar cells. [1] Albrecht, S.; Schindler, W.; Kurpiers, J.; Kniepert, J.; Blakesley, J. C.; Dumsch, I.; Allard, S.; Fostiropoulos, K.; Scherf, U.; Neher, D. The Journal of Physical Chemistry Letters 2012, 640-645.

Due to the high demand for economical and eco-friendly renewable energy sources, Plextronics' new material developments target printable ink systems that allow for high efficiency solar performance. Solar cells based on Plextronics' XC-3000 low band-gap polymer showed 6.9% verified performance. Furthermore, indoor energy harvesting can be well suited to OPV technology due to potential for low cost, matching of OPV spectral response to indoor fluorescent lighting spectrum, and superior performance vs. crystalline silicon at very low light irradiance. Plextronics' inks based on p-type polymer XC-2010 can be used to achieve competitive power densities under office lighting levels (200 - 1000 LUX) to drive low power demand applications such as e-readers, motion and light sensors, and next generation displays. Development of materials for these two OPV applications will be presented along with materials properties and device data.

Loren.G. Kaake, D. Moses, G. C. Welch, G.C. Bazan and Alan J. Heeger Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106 The mechanism of charge photogeneration in organic solar cells remains controversial despite 20 years of investigation and steady advances in materials, devices, and experimental techniques. The problem is not only technological; it involves basic questions regarding the relationship between electronic structure, electron correlation and elementary photoexcitations. We have examined the ultrafast charge generation dynamics of a polymer/fullerene bulk heterojunction (BHJ) system and a molecule/fullerene BHJ system. In both cases, we find ultrafast charge transfer at times less than 50 femtoseconds. In addition, both systems exhibit a slowly rising component which peaks at 100-500 pico-seconds, and continues out to approximately 1 nano-second. The ultrafast charge transfer originates from delocalized photoexcitations immediately following photon absorption, and the slower process is attributed to exciton hopping. The majority of the charge transfer and charge generation occurs via the ultrafast process (~70%). We will summarize a first order perturbation treatment of the charge transfer exciton which yields an analytical expression describing the charge separation process.

Organic photovoltaic (OPV) devices are promising for low-cost, flexible, lightweight, large-area energy generation applications. Tremendous efforts on designing new materials, device structures, and processing techniques have been carried out in the past decade to improve the power conversion efficiency (PCE) and device lifetime. In this presentation, we will cover our recent efforts in polymer solar cell research in the following areas. 1. Tandem solar cell: We developed a series of novel mid- or low-bandgap conjugated polymers with bandgap ~1.44 eV specifically suitable for tandem structure. In the single-layer device, PBDTT-DPP based device showed a PCE over 6%. When the polymer is applied to tandem solar cells, a certified 8.62% PCE was reached. The efficiency was has been further improved by a novel low bandgap polymer with the absorption edge starting from the 900nm, which leads to certified 10.61% PCE device. This highly efficient polymer tandem solar cell represents huge promise for the commercialization of OPV technology in the near future. 2. Plasmonic Effect: We explored various approaches to use plasmonic effect to improve polymer solar cell performance. One is incorporating metal nanoparticles with different size, shape and coating to tune the plasmonic resonance wavelength, and another approach is using grating. 3. Transparent solar cell: We developed high performance all solution process highly transparent polymer solar cell with power conversion efficiency around 4% - 5%. The active materials and transparent electrode development for this new application will be presented.

In our work, functional block copolymers are utilized as compatibilizers in photoactive donor/acceptor blends, for an enhanced morphological control of the bulk heterojunction (BHJ). This methodology appears to provide a promising route for highly efficient organic photovoltaics (OPVs) that exhibit long-term stability [1]. The structural changes that are induced upon addition of P3HT-based rod-coil block copolymers in the widely used poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) BHJs are investigated herein, and these changes are correlated to device performance of the corresponding OPVs, in an effort to elucidate the structure-function relationship. The effects of two coil blocks is examined, polyisoprene (PI) and poly(4-vinyl pyridine) (P4VP); PI exhibits strong affinity with P3HT while P4VP has a preference for PCBM due to the non-covalent supramolecular interactions between the P4VP block and the PCBM moieties. For both copolymers, device performance is improved when a small amount is incorporated as additive in the P3HT:PCBM blend. 7wt% of P3HT-PI increases the power conversion efficiency (PCE) from 3.5% (for the pristine blend) to 4.5%, after annealing the active layer. Interestingly, only 4wt% of P3HT-P4VP triples PCE with respect to that of the pure P3HT:PCBM device, without the necessity for thermal treatment [2]. In order to investigate the origin of this improvement we performed an integrated morphological study of the active layers, following a preparation methodology that mimics the one followed for OPVs fabrication. The initial morphological characterization of the active layers by means of AFM shows that the incorporation of the copolymer homogenizes the blend surface, for both P3HT-PI and P3HT-P4VP. Grazing incidence X-ray diffraction (GIXD) proves that P3HT-PI enhances P3HT crystallinity in the blend, while P3HT-P4VP inhibits the aggregation of PCBM, confirming, thus, our initial hypothesis on the preferential affinity of the two coil blocks with each of the blend components. Moreover, insight in the vertical phase segregation of P3HT and PCBM upon addition of the two copolymers was gained by neutron reflectivity (NR) measurements. The scattering length density profiles indicate a better diffusivity of PCBM in the copolymer-containing blends with respect to the pristine blends, suggesting a more homogeneous mixing of P3HT and PCBM domains upon addition of the copolymer and an improved co-continuous network that allows for a more efficient exciton dissociation and charge transport. [1] a) K. Sivula et al., J. Am. Chem. Soc, 128, 13988, 2006. b) K. Sivula et al., Adv. Mat.18, 206, 2006. c) F. Richard et al., Macrom. Rapid. Commun. 29, 885, 2008. [2] C. Renaud et al., Adv. Mat., 2012, Vol 24 Issue 16 pages 2196-2201

We show that the nanostructure and internal chain configurations of poly-3(hexylthiophene) (P3HT) films can be modified through the application of nanoimprint lithography using patterned stamps with 100 nm spaced rectangular-like grooves. The film's morphological properties and the polymer backbone orientation have been investigated in-situ using grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS), respectively, over a wide temperature range[1]. The GISAXS results demonstrate the excellent fidelity of the pattern transfer into the polymer film. The GIWAXS results show the formation of face-on oriented P3HT domains and that the polymer backbones are oriented along the imprinted grooves for these face-on domains. With increasing temperature, we observed the gradual smoothing-out of the imprinted polymer nanostructure. Despite this smoothing, the imprinted-induced face-on orientation is reasonably temperature independent. In the case of nanoimprinted blends, face-on alignment is also induced, but the backbones are preferentially aligned normal to the grooves. The directed assembly of P3HT:PCBM blends atop linear grating patterns with domains of alternating high- and low-surface energy of 50 to 600 nm in width prepared by Nanoscale Oxidative Lithography of alkyl-terminated Self Assembled Monolayers on SiO2 and SiH surfaces. Tapping- and contact-mode and current-sensing Atomic Force Microscopy studies demonstrated that chemical patterns were effective at directing the 3D morphology of P3HT and Phenyl C61 Butyric Acid Methyl Ester (PCBM) blends at dimensions > 200 nm. As the dimensionality of domains approached 100 nm the chemical patterns were no longer able to direct phase segregation of the blend, evidence that a directed spinodal decompositon mechanism was responsible for the observed morphology. The work at BNL work was carried out in collaboration with Htay Hlaing , David Germack, Xinhui Lu ,Antonio Checco, Kevin Yager, and Charles T. Black. This research is supported by the U.S. Department of Energy, Basic Energy Sciences, by the Materials Sciences and Engineering Division and through the Center for Functional Nanomaterials, both of which are supported under Contract No. DE-AC02-98CH10886. This work was partially supported by Energy Laboratory Research and Development Initiative at Brookhaven National Laboratories. 1. Hlaing, H.; Lu, X., Hofmann, T.; Yager, Y.; Black, C.T.; Ocko, B.M. ACS Nano 2011, 5, 7532.

The past five years have witnessed intense research efforts leading to the development of several new conjugated polymers and small molecule electron donors and acceptors. Yet only a very few of these molecules or polymers have also been reported to have high power conversion efficiencies. While higher efficiency cells typically have low band gap conjugated molecules or polymers as active material, there is no guarantee that using low band gap polymers will result in high efficiencies. Moreover, the increased light absorption by the active material does not result in concomitant increase in efficiency. Furthermore, the efficiency of the cells drops substantially when they are fabricated over large areas. All of these issues arise from the poor control of the active layer morphology. Therefore, the challenge is to develop a general strategy to assemble the electron donors (hole conductors) and electron acceptors (electron conductors into morphologies that optimize charge separation and transport at multiple length scales. It is the critical missing link between highly efficient organic photovoltaic cells and the immense synthetic effort that has resulted in a fantastic array of new low-band gap conjugated polymers and small-molecule electron donors and acceptors. We have been developing an approach that organizes the electron-rich and electron-poor moieties into separate spherical nanoparticles and exploits their attractive interactions to organize semiconductors into segregated nanostructures.The packing of spheres to organize semiconductors has several advantages over existing methods. They include the ability to (a) independently pre-assemble the semiconductor domains with the required molecular assembly and domain size; (b) obtain stable nanoscale bicontinuous structures in a single step, through reliable self-assembly from any donor—acceptor moieties; (c) systematically alter the packing of the semiconductor through changes in the radii of the nanoparticles or interparticle interactions or both; (d) use multiple hole conductors to broaden the absorption spectrum and (e) systematically elucidate the optimal structure for an efficient PV device. In this talk, I will discuss our efforts to fabricate pi-conjugated molecules and polymers into nanoparticles, the photophysics of these particles, and the charge-transfer and transport in particle assemblies.

The miscibility of donor/acceptor mixtures can strongly affect the performance of organic photovoltaic devices comprised of such mixtures. For instance, the miscibility can strongly affect the morphological evolution thereby dictating the relationship between processing conditions and performance of devices. Furthermore, recent Energy-filtered transmission electron microscopy studies revealed that amorphous mixed phases are ubiquitous within mesostructured polythiophene/fullerene mixtures. Nevertheless, the role of mixing and miscibility within nanophases on charge transport of organic semiconductor mixtures is not fully understood. Through the combination of Flory-Huggins theory and energy-filtered transmission electron microscopy, we have estimated the miscibility limit of polythiophene/fullerene blends. We have also examined the electron mobility in amorphous blends of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester. Our studies reveal that the miscibility of the components strongly affects electron transport within amorphous blends. Immiscibility occurs at high fullerene content and promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. As a consequence, our results suggest that an optimum degree of miscibility exists to promote efficient charge transport in polythiophene/fullerene blends and organic solar cell performance.

Recent work has shown that poly(3-hexylthiophene) (P3HT) and the surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C61 (PCBM) are much more miscible than originally thought, and the evidence of this miscibility requires a return to understanding the optimal morphology and structure of organic photovoltaic (OPV) active layers. To more accurately correlate polymer:fullerene miscibility and structure to OPV performance, we have completed neutron reflectivity experiments to determine the miscibility of PCBM in a series of low bad gap polymers. In these studies, we focus on the role of the polymer structure on its ability to dissolve PCBM, in particular the impact of the presence of the 2,1,3-benzothiadiazole (BT) or 2,1,3-benzooxadiazole (BO) unit in the conjugated polymer on its miscibility with PCBM. These results provide insight into why the introduction of the BO unit into the conjugated polymer has not resulted in predicted improvement in OPV performance. We will also discuss similar results that seek to correlate the structure of the fullerene to its miscibility with P3HT and OPV performance, in order to provide a more complete picture of the importance of fullerene:conjugated polymer miscibility on the effectiveness of OPV active layers to convert sunlight into electricity.

The performance of organic bulk heterojunction solar cells is critically dependent on the solid-state structure - from the nano- to the micro-scale - of the photoactive layer. One promising route to manipulate this structure is the use of additives.[1] In our presentation we will focus on a new strategy of using commercially available, low-cost polar ester molecules to modify the supra-molecular arrangement of photovoltaic polymer:fullerene systems. We will concentrate on poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM[60]) blends as these binaries offer two advantages: (i) they display high photoluminescence quenching (>99%; analogous to the high efficiency polymers such as poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithio-phene -2,6-diyl] ], PCPDTBT) indicative of intermit mixing of essentially all the polymer in the film with PCBM, (ii) In pBTTT:PCBM[60], the intermixed phase comprises of a well-defined co-crystal, thereby facilitating structural analyses of such blend films.[2] We will demonstrate based on structural, physico-chemical and spectroscopy data that the additives successfully prevent intercalation of the fullerene between the polymer side chains - i.e. prevents formation of the pBTTT: PCBM[60] co-crystal. This promises that the fullerene content possibly can be reduced in solar cells based on such systems as crystalline PCBM[60] percolating pathways can be reached without the need to go to pBTTT:PCBM[60] weight ratios of 1:4. In addition, the microstructure can be manipulated such that intercalated regions co-exist with relatively phase pure pBTTT and PCBM domains, allowing for efficient exciton dissociation and possibly, efficient charge collection through the phase pure regions. [1] T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntner, U. Scherf, Nat Mater 2003, 2, 408. [2] a) N. C. Miller, C. E. Miller, V. Verploegen, Z. Beiley, M. Heeney, I. McCulloch, Z. Bao, M. F. Toney, M. D. McGehee, Journal of Polymer Science Part B: Polymer Physics, 2011, 49, 499; b) F. C. Jamieson, E. Buchaca-Domingo, T. McCarthy-Ward, M. Heeney, N. Stingelin, J. R. Durrant, Chem. Sci., 2012, 3, 485.

Efficiency of operation of organic photovoltaic devices that are based on the bulk hetero-junction device architecture strongly depends on the details of active layer morphology. Driven by the crystallization of regioregular poly(3-hexylthiophene) (P3HT) in a mixture with (6,6)-phenyl-C61-butyric acid methyl ester (PCBM), the optimized P3HT:PCBM blend consists of a pure crystalline phase of P3HT and a mixture of amorphous P3HT phase with PCBM. Such morphology seems to contain molecular-scale structural units that ensure efficient exciton dissociation as well as to provide with percolation pathways for electron and holetransport on the macroscopic length scale. In order to correlate structural and electronic properties of such system, we studied morphology and charge transport in regioregular P3HT by the wide-angle X-ray diffraction (WAXD) and time-of-flight (TOF) techniques, respectively. We systematically tailored the structural properties of regioregular P3HTs by the two approaches: (i) by using materials with different molecular weights and (ii) by heating these materials nearly up to their respective melting points. We also addressed the questions of morphology and charge transport in an amorphous phase independently by using a regiorandom material as a model system. As a result, we identified the temperature ranges where no significant structural changes relevant to charge carrier transport occur and where a classical Gaussian Disorder Formalism model can be applied, similar to most previous studies. More interesting, however, were the ranges of higher temperatures, where significant structural changes for each material occurred, since this enabled an unambiguous structure-property correlation. We provide a detailed description of charge carrier transport in regioregular P3HT in terms of such macroscopic characteristics as the fractions of amorphous and ordered phases and their characteristic length scales in the semi-crystalline materials, and such microscopic characteristics as polymer chain packing inside of ordered and amorphous phases.

There is a need for photovoltaic technologies with power conversion efficiencies (PCEs) exceeding 20% that can be produced with a module cost under $0.50 per Watt. We believe that making hybrid tandems with organic solar cells on top of silicon or CIGS solar cells is one of the most promising approaches to meeting this need because organic cells can generate much higher voltage than silicon or CIGS can. Organic photovoltaics have a significant processing advantage over inorganic alternatives for the top cell in a tandem because they can be easily deposited by solution processing techniques at or near room temperature without damaging the bottom cell. Furthermore, the cost of the organic layers themselves has been estimated to be less than $10/m². For hybrid tandems solar cells to be successful, it will be necessary to make organic solar cells that generate a voltage of 1.2 V and have external quantum efficiency greater than 80% without a reflective back electrode to enhance light absorption. Moreover the solar cells will need to last more than 25 years. We will show that cells with high quantum efficiency are currently limited to a voltage of 1 V because of energy transfer to the fullerene derivatives that are used as the electron acceptor in all of the best organic solar cells. We will discuss why fullerenes are the best acceptors and how it might be possible to design acceptors that work as well, but have larger band gaps. We will discuss why it is difficult to make polymer solar cells that are thick enough to absorb all of the light and still extract the charge carriers efficiently. We show examples of how charge can be collected efficiently in thick cells. Finally we will summarize the findings of our studies on the long-term reliability of polymer solar cells and lay out a plan for achieving 25 year lifetimes.

We report on fundamental studies of instability of organic solar cells under different spectra of light. Fundamental properties such as carrier mobility, midgap defect density, tail state density, interface state density are measured in-situ upon light exposure without exposing the sample to air. The solar cells sued were the standard P3HT/PCBM cells, but with varying hole transporting layers, and different structures(normal or inverted). We also report on the kinetics of degradation. We show that the increase in defect density is not linear with time, nor linear with illumination intensity. The defect density seems to be saturating, clearly showing that a reverse-photo-induced anneal process is simultaneously taking place with the degradation. The influence of various spectra is examined, and it is shown that one needs to measure the degradation under a full solar spectrum conditions, since filtering out blue photons leads to a much less degradation, even under the same short circuit current condition. We also examine the changes in fundamental properties of the material and the device for both normal and inverted cells, and when PEDOT:PSS orMoO3 is used as the hole transport layer. It is shown that significant differences in the increase in defect densities due to illumination are observed when MoO3 is used as opposed to PEDOT:PSS. Similarly, the mobility of holes in P3HT undergoes much less degradation when MoO3 is used as opposed to PEDOT:PSS. The changes in various fundamental parameters are correlated with the changes in device properties such as dark I-V, light I-V, quantum efficiency, open circuit voltage and fill factor.

Recent and continuous progress in solution processed organic solar cells has been driven by the development of novel donor polymers, enabled by an enhanced understanding of structure-property relationships in these materials. Meanwhile, the implications of these properties on stability remain largely unexplored in spite of the critical relevance to realize a viable technology. Recent investigations suggest that polycarbazoles, especially PCDTBT, have the potential to prolong the current lifetime of 1-5 years achieved with polythiophene based devices. Besides increased thermal and air stability of PCDTBT, there is also a concomitant improvement of device efficiency to 6.5%. However, the intrinsic light induced degradation of the polymer even in inert atmosphere reported by Peters et al. remains of considerable concern [1]. Here, we report a systematic investigation of the origin of the solar radiation induced failure in PCDTBT:PC70BM inverted devices employing ZnO and MoO3 as electron and hole collection layers, respectively. Although we record failure of FF and Voc independent of the device electrode and polarity in agreement with Peters et al., we show that the degradation mechanism is linked to the active layer/electrode interface, in contrast to the previous suggestion of charge trap formation in the bulk of the active layer. Upon 200 h exposure to solar illumination, device performance is reduced from 6.2% to 3.8% due to over 20% (12%) decrease of FF (Voc). Concurrently the series resistance increases ten-fold. Surprisingly, films that are aged without top electrode exhibit performance of 5.9%, similar to fresh devices upon delayed electrode deposition, proving that degradation in the bulk of the active layer can be excluded. Since similar interface failure is recorded in both conventional and inverted configurations, using either MoO3/Ag or Ca/Ag top electrode, respectively, we conclude that this interface failure is independent of the nature of the electrode. Moreover, P3HT:PC70BM devices under similar conditions exhibit stable FF and Voc, hence we can exclude degradation of the electrode materials as the origin of this failure. Further comparison of device degradation in cells prepared with a similar push-pull type polymer indicated that this light induced burn-in process strongly depends on the nature of the polymer. It is critical to note that, although the decrease of the Voc and FF follow a similar trend, the origin of their failure mechanisms probably differ. Degradation of the first parameter can be partly recovered by an annealing treatment due to de-trapping of carriers accumulated at the interface [2]. Our investigation not only contributes to the understanding of light induced failure of PCDTBT cells, but also highlights the polymer dependence of this critical degradation mechanism through comparison of the ageing with several polymer types. [1] Peters et al., Adv. Mater., 2012 [2] Kawano et al. Adv. Funct. Mater., 2009

Highly efficient, non-inverted polymer solar cells with photoactive layers based on PCDTBT:PCBM with lifetimes of about 100 hours without any encapsulation and of more than 10,000 hours are demonstrated for glass-glass sealing.. These long lifetimes were achieved by applying and careful processing of a solution processible tungsten oxide layer between the photoactive layer and the cathode metal contact. Different cathode material combinations for sealed and unsealed devices were studied by automated lifetime testing under ~1 sun intensity with in-situ IV-characterization up to 4,500 hours. The corresponding device stabilities were further correlated with information obtained from imaging methods, namely electroluminescence imaging and dark lock-in thermography. Together with external quantum efficiency measurements the degradation mechanisms could be analyzed in detail. Our results suggest that not only inverted device structures but also non-inverted architectures may be a viable approach for the commercialization of stable solar cells.

Studies of light induced degradation of organic solar cells provide important information about long term stability of devices and also about the recombination mechanisms. Measurements of the photocurrent spectral response of PCDTBT:PCBM and P3HT:PCBM show that prolonged light exposure induces deep trap states.[1] The density of induced trap states is found to be proportional to the density of recombination centers, as measured by the voltage dependence of the steady state photocurrent. These two measurements therefore identify the traps as the primary recombination centers. The energy of the optical transitions show that the traps are deep localized states between the polymer HOMO and the fullerene LUMO. Comparable trap creation is also observed with soft x-ray exposure.[2] The kinetics of trap creation and annealing have been studied. Wavelength dependent light exposure measurements show that UV light in the range 300-400 nm is primarily responsible for the defect creation. Since x-ray exposure also results in high energy excitations, the similar defect creation is not surprising. The induced states are reversible by thermal annealing to about 100C, which implies a metastable structural change with binding energy 1-1.2 eV. However, the annealing kinetics reveal three different annealing processes, even though the corresponding trap states are indistinguishable electronically. The spectral dependence of the degradation kinetics provides information about the optical filtering will be needed to control the degradation of cells in sunlight. The main effect of high energy radiation is known to be the abstraction of hydrogen from C-H bonds. Our theoretical structure calculations of possible hydrogen related defects find specific defect states that match the experimental observations, and provide values for hydrogen migration energies that are consistent with the annealing kinetics.[2] R. A. Street, A. Krakaris, and S. R. Cowan, Adv Funct. Mater, 2012, 10.1002/adfm.201200031. R. A. Street, J. E. Northrup and B. S. Krusor, Phys. Rev. B 85, 205211 (2012).

In this presentation, we intend to cover two main topics: (i) The electronic structure at the interface between a conducting oxide electrode or interlayer and an organic layer. In particular, we will discuss the mechanism by which polyamine-type polymers modify the electrode work function in plastic solar cells [1]. (ii) The organic-organic interface. We will describe our recent progress in the description of the structural configurations (“local morphology”) at the interface between the donor and acceptor components and the impact that variations in these configurations have on the nature of the interfacial charge-transfer states. [1] Y. Zhou et al., Science, 336, 327 (2012).

Saif A. Haque Centre for Plastic Electronics, Department of Chemistry, Imperial College London, UK Hybrid solar cells that comprise both organic and inorganic semiconductor materials are emerging as a promising power generation technology. This primarily stems from the attractive possibility to design devices that take advantage of the versatility and processability of organic materials and the superior electronic properties of inorganic semiconductors. The design of efficient hybrid inorganic- organic solar cells critically depends on the ability to control the charge photogeneration efficiency at the donor-acceptor heterojunction. In this talk I will report some of our recent work aimed at better understanding of the mechanisms of charge photogeneration in inorganic-organic heterojunction solar cells. In particular we will consider influence of key parameters such as interfacial energetics and nanomorphology / film structure on the charge photogeneration yields and lifetimes. These studies will be complimented with photovoltaic device fabricaiton / characterization. A key aim of this work is to develop quantitative structure-function relationships that can be used to guide the design of hybrid heterojunctions for high performance solar cells. Developing a detailed understanding of solar cell function inevitably requires control of interface architecture at the nanometre level. As such, in this talk I will also report some of our recent work addressing the development of new nanofabrication approaches for hybrid polymer - nanocrystal solar cells. Here we will focus on in-situ growth of inorganic nanoparticle networks directly in polymer films [1-4] and demonstrate that our straight forward method can be extended to the design of a range of alternative inorganic electron acceptor materials for polymer solar cells. [1] Leventis et al. Nano Letters (2010) 10, 4, 1253-1258 [2] Dowland et. al. Adv. Mater (2011), 23, 24, 2739-2744 [3] Rath et. al. Adv. Energy. Mater (2011) 1, 6, 1046-1050 [4] Reynolds et. al. Nanoscale (2012) 4, 5, 1561-1564

Organic photovoltaic cells (OPVs) are an attractive energy source for their advantages of low-cost production, flexible and large-area devices. The combination of new materials in the bulk heterojunction layer of polymer (the most common is P3HT) and PCBM has emerged as a strategy to improve the solar cell performance. Inorganic semiconductors such as ZnO nanorods (NRs) are good candidates for their higher carrier mobility and can be prepared with low cost techniques. In this work, we have modified the chemical bath deposition method applied for the synthesis of vertically-aligned NRs at low temperature. We have introduced two factors: pressure and temperature. Our results show that, for the same growth time applying the new synthesis methodology, the resulting vertically-aligned ZnO NRs are 1/5 of the original length with improved small molecule Dye adsorption and enhancement in power conversion efficiencies. The latter is attributed to the modification of the growth mechanism which results in highly homogeneous and thin NRs, and high packing density electrodes. Analyses by photoluminescence (PL) and time-resolved photoluminescence (TRPL), reveal a decrease on the surface defects of the resulting vertically-aligned ZnO Nanorods. We applied the as-prepared ZnO NRs as electrodes in OPVs, the electrode length was varied between 600 nm to 5 µm. Solar cell preparation was optimized using different conditions: organic solvents, polymer concentration and deposition of the polymer solution. A 2.0% efficiency was obtained with a 600 nm NR that is a good result compared with reported data in literature, since higher efficiencies can only be obtained with shorter NRs.

The most efficient fully organic solar cell is made of a conjugated polymer as electron donor material and fullerene derivatives as electron acceptor materials. Different advantages could be brought by the introduction of a vast library of inorganic compounds as alternative to fullerenes, such as high dielectric constant, high electron mobility and better stability, however, hybrid solar cells have not express their full potential yet. Why they are so inefficient respect to polymer/fullerene systems is still not fully understood and the issue needs to be investigated. Among inorganic compounds, metal oxides represent one of the most appealing solutions thanks to their low cost and easy synthesis. A large number of investigations have been carried out on devices based on prototypical materials such as TiO2 and Poly(3-hexylthiophene). These show poor photocurrents, though the energetics should be ideal for charge generation. In addition there is a lack of control in the three-dimensional morphology, especially in the bulk-heterojunction configuration, when simple solutions such as colloidal nanocrystals are used, maintaining the power conversion efficiency as low as 0.5%. Inspired by the successful concept of DSSCs where the metal oxide works as electron acceptor and transporting layer while providing an optimal scaffold for meeting the right compromise between high density of donor/acceptor interfaces and controlled percolation paths for good charge transport and collection, we have made hybrid solar cells by using mesoporous films of TiO2, about 1µm thick, infiltrated by P3HT. We demonstrate that by careful engineering of the TiO2/polymer interface, without using any optical and electrical active interlayer, we are able to dramatically enhance the photocurrent extracted from the nanostructured solar cell, reaching a power conversion efficiency exceeding the 1%; this represents a step change in performances for polymer-based mesoscopic hybrid solar cells. Here we investigate the device physics by combining time-resolved optical spectroscopy, XPS, and, for the first time, positron annihilation spectroscopy. The last provides a detailed mapping on the nanometer-scale of the oxide pores filling over the entire device thickness. We obtain a clear picture of the device physics, decoupling the influence of energetics, sovramolecular interactions and morphology on the device performances. Our results fully unveil the potential of a new generation of solid state devices where a new interface engineering, which goes from a molecular to a mesoscopic level, is the handle for optimizing the performances.

Hybrid composites of semiconducting polymers and n-type metal oxides are exciting systems to study from a fundamental photophysical and device point of view. Contrary to organic solar cells, hybrid architectures offer a vast library of materials for device optimization, including a variety of metal oxides, organic and inorganic absorbers, molecular, polymeric and electrolytic hole-conducting media. Beyond this, however, the versatility offered by a hybrid framework allows these new materials to be utilized in novel configurations; one example being our newly developed mesoporous metal oxide/ co-functionalized with surface adsorbed fullerene and absorber/polymer architecture. Thin-film solid-state hybrid solar cells tend to suffer from poor spectral sensitivity in the near-IR, in order to improve the overall efficiency of solid-state hybrid solar cells attention has been focused on employing spectrally-broad, light absorbing polymers such as PCPDTBT. We have reported that by functionalizing the TiO2 with a fullerene based organic electron acceptor C60-SAM we are able to enhance the electron transfer from the polymer to the oxide. Here, we extend this concept with the addition of a novel organic-inorganic absorber (IOA) as the light harvesting antenna/charge generation centre, which significantly boosts the power conversion efficiency of polymer-absorber-oxide devices. We employed ultrafast transient absorption spectroscopy and cw-photoinduced absorption spectroscopy to probe the charge generation and recombination mechanisms in metal oxide-IOA-polymer films with and without the C60-SAM functionalization. Photovoltaic devices based on this organic-inorganic absorber -polymer with the C60-SAM functionalization show a significantly increased spectral response and produce a solar power conversion efficiency of 7% under Air Mass 1.5 Global (AM1.5G) conditions delivering short-circuit photocurrents of 20 mA cm-2 and open-circuit photovoltages up to 0.9 V, the conversion efficiency is the highest reported for a polymer-oxide hybrid solar cell, as compared to less than 3% in the absence of C60-SAM. This novel absorber combined with the recently acquired knowledge about C60-SAM opens an exciting route forward for hybrid photovoltaics.

Highly efficient, solution processable, organic photovoltaics typically consist of a film ~ 100 nm thick, of a two component donor-acceptor type heterojunction structure comprised of a low bandgap conjugated polymer donor blended with a fullerene acceptor. Efficient charge extraction from these blends demands that donor and acceptor components form nanoscale phase separated percolating pathways to their respective electrodes. The formation mechanism(s), the dimensions and even the existence of the assumed phase separated bicontinuous morphology are difficult to determine however. In this talk I highlight how by using a variety of characterization methods one can begin to answer a number of fundamental polymer physics and physical chemistry questions such as the diffusive mobility of the polymer and fullerene components and the role of high boiling additives to casting solutions on the morphology of these blends. These methods include dynamic secondary ion mass spectrometry (SIMS), grazing incidence wide angle and small angle X-ray scattering (GIWAXS and GISAXS) and energy filtered transmission electron tomography (EF-TEMT) as well as near edge X-ray absorption spectroscopy (NEXAFS).

Organic photovoltaics (OPV) have been suggested as a low-cost, flexible alternative to inorganic photovoltaics. Intense research is directed towards the development of OPV with a bulk heterojunction between electron-donating conjugated polymers and electron-accepting fullerenes, e.g., poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methylester (PCBM). The greatest challenge facing the OPV development is combining high efficiency, stability and processability. While inorganic solar cells exhibit operational lifetimes in the range of 25 years, a typical operational lifetime of encapsulated OPV under full solar illumination (1 sun = 100 mW/cm2) only recently reached a few years. This achievement opens various possibilities in OPV applications but also raises new problems, e.g., the need for relevant accelerated tests of operational life-time. Heat is the conventional approach, by which acceleration factors in excess of 20 are reachable. Here we demonstrate that concentrated sunlight can be used for accelerated studies of light-induced degradation in OPV photoactive layer and describe an outdoor-indoor solar concentrator facility that allows independent control of light intensity and the sample temperature. The overall objective of this research is to formulate a protocol for accelerated stability testing of OPV materials using concentrated sunlight. Achievement of this goal requires better understanding of degradation mechanisms via correlating material- and device- degradation phenomena. Accordingly, the specific objectives are: (1) to determine the dependence of the degradation rate on the sunlight concentration and the sample temperature and (2) to compare the kinetics of degradation processes under concentrated sunlight to those observed at 1-sun conditions. Encapsulated films of P3HT and P3HT:PCBM blends were studied before and after their exposure to 1-100 suns, using UV-vis, FTIR and Raman spectroscopes. The addition of PCBM was found to significantly reduce the degradation rate of P3HT for all exposure levels. Heating was found to induce morphological changes which improve photon absorption and compete with degradation processes, but also accelerate photo-bleaching. The photon flux, i.e., the level of solar concentration, was found to be a significant factor affecting the degradation rate. Specifically, we find that the degradation rate is significantly enhanced at concentration above 50 suns.

Recently, a new class of organic semiconducting materials, conjugated polyelectrolytes, has attracted interest for a variety of optoelectronic applications. Especially, ‘cationic&’ polyfluorene (PF) derivatives have been demonstrated to function as interfacial layer in optoelectronic devices. Their cationic functionalities can effectively induce interface dipoles, resulting in reducing work function of the metal electrode, facilitating charge transport into the metal electrode and provide solubility in polar solvents, resulting in fabrication of multilayer devices. Our previous cationic PF polymers have reversed polarity to original configuration of polymer solar cells (PSCs) and affected device performance (open-circuit voltage, fill factor) in inverted PSCs. Based on these motivations, PF derivatives with various ionic groups on the side chains, which were synthesized by a palladium catalyzed Suzuki coupling reaction method, were used as cathode interfacial layer in the inverted indium tin oxide (ITO)/PF derivatives/P3HT:PCBM/PEDOT:PSS/ Ag solar cells. The electronic properties of PF derivatives for tuning work function of ITO were characterized by Ultraviolet Photoelectron Spectroscopy. Finally, we found that the devices performance was significantly influenced by the kind of ionic groups on the side chains in PF.

In this study, we demonstrate a novel interfacial anode modifier layer in organic solar cells using the oxidized Bi₂Se₃ nanocrystals through lithium intercalation approach. We found that the work function of the solution-processed Bi₂Se₃ nanocrystals on indium tin oxide (ITO) can be controlled by the use of O₂ plasma treatment. The degree of oxidation of the Bi₂Se₃ nanocrystals was further probed by X-ray photoelectron spectroscopy. Our results show that the incorporation of the oxidized Bi₂Se₃ nanocrystals between photoactive poly(3-hexylthiophene) (P3HT):phenyl-C₆₁-butyric acid methyl ester (PCBM) and the ITO electrode, largely enhance the device performance. We found that the interfacial anode modifier using oxidized Bi₂Se₃ nanocrystals can effectively reduce the carrier recombination rate. The optimized device exhibits a promising power conversion efficiency, compatible with the device using PEDOT:PSS, which is conventionally used for hole transport layers. Regarding the advantage of controllable work function and being solution processable, the Bi₂Se₃ nanocrsytals can be used as a new interfacial anode modifier for hole transport in organic solar cell or light emitting diode applications.

Bis-fullerenes have recently emerged as promising acceptors to enhance the efficiency of polythiophene (P3HT) based solar cells. Owing to their energetics, the Voc of devices is increased, resulting in >40% performance enhancement. However, only a few studies have addressed the implication of their altered material properties on stability. Although photo-oxidation of P3HT:bis-fullerene layers has been recently described, lifetime investigations on devices are scarce. Furthermore, altered molecular packing can also have considerable impact on thermal stability, though it is still unexplored. Here, we report a systematic investigation of the thermal stability of P3HT:bis-fullerene devices. We present the comparison of several bis-fullerene derivatives, namely bis-PCBM and bis-oQDMC60 and ICBA. First, we correlate the temperature induced phase segregation of the blend with the intrinsic thermal material properties of its components derived from calorimetric characterization (phase diagram and isothermal crystallization rate). Over 10% mass loss of bis-oQDMC60 and ICBA below 200°C limits their study by calorimetry. Since C60 as well as bis-PCBM are stable up to 400°C, we attribute this decomposition to the reversible addition of the sidechains. The stabilization of the blend morphology up to 150°C is ascribed mostly to the reduced tendency of bis-fullerenes to crystallize; the blend&’s glass transition temperature is only increased by 20°C. Device performance with P3HT:bisPCBM is only reduced by 10% at 85°C and 40% at 150°C over 200 h. In similar conditions P3HT:PCBM devices exhibit 40% and 90% drop in device efficiency. In agreement, clustering of fullerenes is entirely suppressed in blends with bis-fullerenes. Isothermal P3HT crystallization rate is 10-2 s-1 at 150°C both in blends of mono- and bis-PCBM. Thus, hindrance of polymer ordering by bulkier bis-fullerenes can be excluded as the origin of the morphology stabilization. Additionally, the reduction of Voc and fill factor also contributes to the failure of P3HT:bis-fullerene devices. Unchanged charge transfer states levels measured by sub-bandgap spectral response indicate no chemical alteration of the organic materials during ageing. Moreover, films that are aged without electrodes exhibit a performance similar to fresh devices upon delayed electrode deposition, proving that the failure is linked to compromised ohmic contact at the organic/metal interface. In P3HT:PCBM blends large-scale phase segregation probably clouds this secondary failure mechanism. Alternatively, the increased reactivity of bis-fullerenes might trigger this process. Since similar electrical degradation is observed in all three investigated blends, we suggest that these failure pathways are independent of the nature of the fullerene sidechains. Our detailed study contributes to the establishment of material property - stability relationships indispensable to understand and improve device lifetime.

Polymer solar cell has attracted attention as renewable energy source due to low cost and large area solution processes such as spin coating, ink jet printing, doctor blading and spray. However, to apply polymer solar cells in commercial area, roll to roll process and stability improvement are required. For this reason, the spray deposition as one of various processes has great potential having many advantages which are large area fabrication, available to roll to roll process and possible to use dilute solution. To use the spray deposition in polymer solar cells, different conditions are required unlike spin coating due to different deposition mechanism. In this study, inverted polymer solar cell was fabricated by spray deposition process. Inverted polymer solar cell can partially solve the stability problem due to stable interface layer like oxide materials and affects less to active layer during fabrication. In the inverted structure, n-type metal oxide such as titanium oxide (TiOx), zinc oxide (ZnO) and cesium carbonate (Cs2CO3) are used as electron transport layer. Among these materials, the ZnO is a good candidate because it can be applied using simple solution process by sol-gel method using zinc acetate dyhydrate (Zn(CH3COO)2 2H2O) and has high electron mobility, stability and transparent properties. A thickness of ZnO as interface layer is about 20 - 30 nm in spin coating, however, it is difficult to make thin and flat layer in spray process due to large droplet. To make thin film interface layer and optimize device efficiency, the various diluted zinc acetate dyhydrate concentration solutions were used. The characteristics of device fabricated by spray deposition were comparable to the spin coating based device.

Bulk heterojunction (BHJ) cells have an active layer that is a random dispersion of electron-accepting and electron-donating polymers, but several problems must be addressed if efficiencies are to improve. One problem is that because of the random dispersion, there are occasional dead zones where no electron-accepting polymer is within range of the electron-donating polymer, resulting in a loss of energy. Additionally, a weak link in organic polymer solar cells is the aluminum cathode, which requires evaporation under an ultra-high vacuum, making large industrial production difficult. Furthermore, aluminum is easily oxidized, leading to solar cell degradation. This research investigated the addition of polymers such as polystyrene into the active layer to improve morphology, and the incorporation of reduced graphene oxide (RGO) within these insulating polymers to enhance the conductivity. Replacing the aluminum cathode with RGO was explored, as well as creating RGO paper for use as a cathode. Exposing the hydrophobic P3HT:PCBM active layer to UV/ozone enhanced adhesion of the hydrophilic RGO suspension to the hydrophobic active layer, becoming a possible method of replacing aluminum as a cathode. To circumvent ITO corrosion at elevated room temperatures, RGO was also tested as a substitute for the anode.

Bulk Heterojunction (BHJ) polymer solar cells are an area of intense interest due to their flexibility and relatively low cost. The mixture of polythiophene derivatives (donor) and fullerenes (acceptor) is spin coated on substrate as the active layer, and are phase-separated into interconnected domains. However, due to the disordered inner structures in the active layer, donor or acceptor domains isolated from electrodes and long path conduction, the power conversion efficiency (PCE) of BHJ solar cell is low. Therefore, control of the inner structure within the active layer is required to enhance the efficiency. Our research provides the method to produce ordered self-assembly columnar structure within the active layer by introducing some common polymers into the BHJ solar cell system. Molecular dynamics simulations indicate that when nanoparticles are present in the immiscible blend, they preferentially segregate to the interfaces, allowing the interface to produce a template for the particle distribution. We first tested this concept using one photoactive polymer poly(3-hexylthiophene) (P3HT), a non active, but well characterized polymer, polystyrene, and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the charge carrier. After the blend is spun cast onto a substrate and annealed in vacuum oven for certain time, highly ordered columns (P3HT) is uniformly dispersed in PS, while PCBM nanoparticles are well confined at the interface of P3HT and PS. The surface morphology of blend thin film is measured by atomic force microscopy (AFM) and the cross section view from TEM determines the columnar structure due to the phase separation between P3HT and PS. Neutron reflectometry and top view TEM was used to demonstrate the confinement of PCBM at the interface of PS and P3HT. The different morphological structures formed via phase segregation are correlated with the performance of the PEV cells fabricated at the BNL-CFN and significant enhancement for the efficiency is observed.

4H-Cyclopenta[1,2-b:5,4-b']dithiophene (CDT) has been proven to be a perfect molecule for Organic Electronics. In donor-acceptor copolymers with benzothiadiazole (BTZ) we were able to measure ultrahigh charge carrier mobilities over 3 cm2/Vs [1]. Our group has shown that higher molecular weight (Mn > 30.000 g/mol) of the polymers strongly improve their performance in organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVCs). Currently we are working on low polydisperse (PDI < 1.8) copolymers to further increase the performance in the FET devices. The fundamental question for the good performance can be explained by the highly ordered packing of the polymers in thin films. But while the polymers with linear side chains are very good p-type polymers for FETs, the ones with branched side chains are decent ambipolar semiconductors for OPV (over 4% PCE). Keeping the backbone of the polymers constant, we can investigate how the difference in ordering influences the device performance by modifying the linear side chains to cis-trans switchable olefinic side chains. Thereby we obtained a polymer with variable electronic properties. While the all-cis polymer shows deceptively low ordering in the devices the all-trans polymer is well packed and we can measure similar charge carrier mobilities compared to CDT-BTZ polymers with linear aliphatic chains and equal molecular weights. In solution and thin films it is possible to switch the polymers from all-cis, which is far more soluble, to over 70% trans via treatment with diphenylsulfide. The decreasing polydispersity of the materials has a high influence on the materials, so it was inevitable to turn our studies also to bottom-up synthesis of defined CDT-BTZ donor-acceptor oligomers. Starting from CDT, the symmetrical final products are monodisperse, highly soluble in chlorinated solvents and easy to purify by conventional methods. Upon varying the side chains at the backbone and at the attached thiophene units we gain control over morphology and device performance of the oligomers. The materials are currently tested in OFETs and OPV cells. [1] J. Am. Chem. Soc. 2011, 133, 2605-2612

The primordial importance of short-lived delocalized states in the functioning of polymer-based bulk heterojunction solar cells has recently been recognized, both for charge dissociation of the primary polymer photoexcitation at the fullerene interface [1], and for the subsequent generation of long-range separated charge carriers [2]. The most successful polymers nowadays are however based on alternating electron-donating and electron-withdrawing units along their backbone (donor-acceptor copolymers), where the internal charge transfer character lowers the bandgap for better harvesting of the solar spectrum, but also implies some wavefunction localization of the hole and electron. In order to reconcile the notions of charge transfer character and delocalization, we have studied in detail the electronic structure of donor-acceptor copolymers (for example PCDTBT). One approach was to break down the problem by studying the intrinsic properties of the separate donor, the acceptor and the donor-acceptor dimer using (time-resolved) spectroscopy and DFT calculations [3]. We found a particularly large influence of the BT (dithienyl-benzothiadiazole) or related BBT (dithienyl-benzobisthiadiazole) acceptor group on the electronic structure of the corresponding polymers. Another approach was to study the photophysical properties in families of chemically related polymers as a function of internal charge transfer strength. Finally, we could relate photocurrent generation in pure polymer films to delocalization in the primary photoexcited species. [1] S. Cowan, N. Banerji et al., Advanced Functional Materials (2012), 22, 1116 [2] A. A. Bakulin et al., Science (2012), 335, 1340 [3] N. Banerji et al., J. Phys. Chem. C (2012), 116, 11456

Defect engineering has been of vital importance to the development of inorganic semiconductors. Here we report various chemical modifications of electrical defects in the model polymeric semiconductor, regioregular poly(3-hexylthiophene), P3HT. First, we covalently treated defect sites with either a nucleophile (e.g., lithium aluminum hydride, LAH) or an electrophile (e.g., dimethylsulfate, Me2SO4), leaving the defects of primarily opposite polarity. The work function is strongly decreased by the LAH treatment, consistent with hydride ion addition and the observed sim;400-fold decrease in p-type defect density. The exciton diffusion length (Lex) doubles to 14 nm, while the hole mobility (mu;) is improved by more than an order of magnitude to 2.4 x 10-3 cm2/Vs. By contrast, the work function is increased after the Me2SO4 treatment, consistent with methyl cation addition and, again, both Lex and mu; are increased substantially. The effective doping density can be readily tuned with these chemical treatments while notably increasing both Lex and mu;. Next, we performed successive treatments by using both nucleophilic and electrophilic, which allow us to covalently fix both positively and negatively charged defect sites in a single procedure. The effects of treating P3HT are described, for example, first with LAH to decrease the overall defect density and then with Me2SO4 to eliminate some of the remaining n-type defects. Slightly improved structural and optical differences between the starting P3HT and those after either single or dual treatments and are observed, while greatly improved electrical differences are found. All these treatments improve the performance of the photovoltaic cells and increase the stability against photodegradation. The significantly decreased series resistance and increased shunt resistance with a combined treatment suggest improved charge transport in the cell. Third, we recently introduced a new way of catalytic hydrogenation to clean the defects in P3HT. The resulting P3HT was found to lower the dark electrical conductivity by ~5 orders of magnitude while significantly enhancing the photoluminescence. Such defect-free P3HTs are expected to exhibit the superior electrical characteristics in thin film transistors, which are now under investigation. More notably, this simple hydrogenation reaction allows one to control the doping density in semiconducting polymers and even switch the doping type from its original p-type to n-type. Finally, it should be possible to employ these simple chemical treatments with any π-conjugated polymer to beneficially modify, or eliminate, some of its electronic defects, thereby providing a new method of improving the air-stability and electrical characteristics for organic (opto)electronic applications.

The active layer in a bulk-heterojunction (BHJ) organic photovoltaic device typically consists of an interpenetrating network of electron donor and electron acceptor materials. To a large extent, the underlying morphological features within this layer are controlled by the mixing and anti-mixing behavior exhibited by the two materials, which are in turn strongly influenced by processing parameters, such as the film drying rate after deposition, the extent of polymer crystallization, and any subsequent thermal or chemical treatments. Methods for directly analyzing the spatial extent of such features, and thus developing robust structure-property-processing correlations, are lacking. Transmission electron microscopy (TEM) possesses the necessary spatial resolution to make this measurement; however, the scattering contrast generated from the constituent phases is very low, rendering traditional bright-field imaging poorly suited for this task. In contrast, we and others have recently demonstrated that energy-filtered (EF)-TEM, wherein images are formed with those electrons that have lost a very specific amount of energy while traversing the specimen, can produce high-contrast, directly interpretable images of the morphological features within a BHJ film. In EF-TEM, multiple signals are available for such analysis. These include the plasmonic response (0 eV to 100 eV loss), wherein variation in electronic density produces differential contrast between the two phases, or, alternatively, the elemental core-loss signals which result from inner-shell ionization by the impinging electron beam. The latter has the advantage of being more quantitative, since the signal intensity can be related to the number of atoms contained within the thickness of the specimen at any given pixel, while the former is superior in terms of the overall electron dose required due to the far larger cross-section for plasmon scattering versus ionization. We will describe the application of EF-TEM imaging to model bi- and tri-layer specimens consisting of alternating layers of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. By employing focused ion-beam milling, thin cross-sectional specimens of these simple structures could be analyzed in order to investigate the fundamental mixing and anti-mixing behaviors within this commonly studied two-component system and its relationship with polymer crystallinity and thermal treatment.

Colloidal PbSe semiconductor nanocrystals (QDs) offer the potential for multiple carrier generation that could lead to high current densities in the next-generation of solar devices. In these devices incorporation of PbSe quantum dots in a polymer matrix without a surfactant barrier at the QD-polymer interface is important for the dissociation of excitons and subsequent collection of carriers. We have developed a laser assisted spray deposition (LAS) process form uniform coatings of surfactant-free PbSe QDs on a substrate. The LAS process for the growth of nanoparticle films offers two main advantages over the more conventional spin-coating and drop-casting methods: (1) the surfactants are burnt-out prior to deposition and thus QDs form intimate contacts between QDs and QDs and the host materials, (2) nanoparticles are sprayed uniformly over the substrate preventing local segregation of QDs. This technique involves the transient heating of aerosols containing PbSe QDs by a laser-gas interaction to burn the organic surfactants. Transmission electron micrographs (TEM) and absorption spectroscopy show, under optimum conditions, the particles remain as single crystals and maintain quantum confinement. Removal of surfactants was confirmed by optimizing the growth parameters and monitoring the degree of surfactant removal by studying the Fourier Transform Infrared (FTIR) spectrum of coating grown by LAS. LAS process is also used to deposit films of the p-type and n-type polymer composites containing P3HT and PCBM. Polymer composite and surfactant-free PbSe QDs are co-deposited by the LAS process to form a bulk heterojunction solar active layer. We have also used the LAS process to form multi-wall carbon nanotube (MWCNTs) embedded PEDOT:PSS films with enhanced conductivity. Physical properties of individual layers grown by LAS have been studied to optimize the carrier generation and charge transport. Dependence of these properties on growth parameters, physical dimensions of the layers, and concentrations of components towards the fabrication of the device structure ITO/(PEDOT:PSS/MWCNT): (PbSe-P3HT:PCBM)/Al will be presented.

The use of inorganic materials as active components within organic blends, so called hybrids, has recently attracted much attention in the field as they display several attractive properties over conventional organic blends including superior domain-size control and stability, as well as enhanced mobilities. However, these potential improvements in functionality are often hard to realise, mainly due to two conditions which limit the incorporation of the inorganic component into effective active layers: restricted nanoparticle shape and solubilising ligands. While efficient electron transfer can be traded with solubility via suitable ligand exchange, the synthesis of extended nanostructures has proved somewhat problematic. Several general techniques for incorporating inorganic nanoparticles within organic films have been pioneered, with the in-situ formation of nanoparticles showing the most promise. Our studies have focussed on the in-situ generation of metal sulfides following the thermal decomposition of soluble inorganic precursors within polymer films, and we show that the technique is appropriate for use with metals, such as bismuth, that are significantly less toxic than traditional components. Photophysical studies, for example photoluminescence and transient absorption spectroscopy, of the interfacial charge transfer mechanisms are undertaken and have been correlated with microscopy images to reveal how morphology relates to the efficiencies of photoinduced processes. Furthermore, the stability of such hybrid devices is shown to be favourable compared to standard all-organic bulk heterojunction blends. Understanding such processes is essential to the continuing development of highly efficient hybrid PV technologies.

We report on the thermal conductivities of four common semiconducting polymers in device configurations via time domain thermoreflectance: PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)), P3HT (poly(3-hexylthiophene-2,5-diyl)), PCBM ([6,6]-phenyl C61-butyric acid methyl ester), and a P3HT:PCBM blend. The thermal conductivities of these polymers span nearly an order of magnitude at room temperature. In addition, thermal annealing of P3HT:PCBM blend films leads to an increase in thermal conductivity in excess of 40 %, a difference that can be correlated to the changes in morphology that result from the anneal, i.e., polymer alignment and the formation of large crystallites. Finally, we report no change in the thermal conductivities of these devices after 30 days exposure to ambient conditions.

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high performance low-bandgap polymers is the major limiting factor for achieving high performance tandem solar cell. Here, we report the development of a high performance low bandgap polymer. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency (PCE) under standard reporting conditions (25 °C, 1000 Wm-2, IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

Rapid migration of separated charges from the bulk heterojunction (BHJ) to the appropriate electrode is critical to efficiency within organic photovoltaics (OPVs). However, methods of characterizing charge transport within complex BHJ morphologies are limited. We are exploring the use of carrier induced paramagnetic relaxation enhancement to investigate carrier transport within complex morphologies relevant to organic semiconductor BHJs. Charge carriers in OPVs typically can be paramagnetic (example radical cations in pentacene) and in these cases enhance nuclear spin relaxation. In order to probe the effect of these carriers on nuclear spin relaxation enhancement, we have doped organic semiconductors with controlled amounts of charge carrier. We have quantified spin density using EPR spectroscopy. We have measured nuclear relaxation rates in organic semiconducting materials as a function of dopant/ carrier concentration in systems relevant to organic photovoltaics such as pentacene. In the case of pentacene we find relaxation enhancement of 27% at a spin density of 0.17 mol% at 100°C. At higher doping levels (2.4 mol%), the relaxation data becomes biexponential, with a fast relaxation time of ~60-80ms and a slower relaxation time of 43-48s in the temperature range from 25-150°C. The relevant ratio of fast and slow components is approximately 1:1. These two relaxation rates are believed to result from differing morphological domains within the pentacene sample. We will also present preliminary data on relaxation enhancement within poly(3-hexyl)thiophene (P3HT), and PCBM, and blends of these two materials. Finally, as doping of organic semiconductors to improve carrier properties through trap filling becomes more important to the fabrication of organic photovoltaic devices, it is critical to understand the diffusion properties of dopants within organic semiconductors. We have performed measurements of iodine diffusion within P3HT films using attenuated total reflectance IR spectroscopy. In these measurements we calculate a diffusion constant of 9e-11 +/- 5e-11 cm2/s at RT.

Recombination of charges at the donor-acceptor interface is a key loss mechanism in organic photovoltaic devices [1,2]. We have developed a method that allows modifying the donor-acceptor interface via the insertion of an ultra-thin semiconducting polymer layer following the photo-crosslinking of the underlying polymer layer. We have fabricated P3HT(poly(3-hexylthiphene))/P(NDI2OD-T2) (poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}) devices with an ultra-thin layer of PCDTBT (poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]) polymer as an interfacial modifier. We demonstrated that the interfacial modification results in a significant reduction in charge recombination rates, which we confirm by optical transient absorption spectroscopy measurements. We report that the External Quantum Efficiency (EQE) of a modified device reaches the value of 7.6%, greatly increased from 5.7% for the unmodified bilayer. Both the Voc and Jsc increased in the modified device, giving more than 70% increase in the power conversion efficiency. Additionally, under forward bias, the measured dark currents were found to be reduced by two orders of magnitude, consistent with the observation that the injected carriers take a significantly longer time to recombine. These results suggest that the interlayer allows for effective charge disassociation in an energy-cascade fashion, with a reduced rate of charge recombination achieved by physically separating the charges across the interface. These findings open new routes for photovoltaic performance optimization by effectively tuning the recombination kinetics across the donor-acceptor interface. [1] Westenhoff, S. et al, Journal of the American Chemical Society 2008, 130 (41), 13653-13658. [2] Mihailetchi, V. D. et al, Physical Review Letters 2004, 93 (21), 216601.

With high-speed manufacturing becoming a feasible goal for photovoltaic cells, the solution processing of electrodes and electrode interlayers has become essential. It would thus be advantageous to develop a solution-based method for the synthesis of LiF, given that thermal evaporated LiF is widely used to enhance charge collection at both the metal and ITO electrodes in polymer-based solar cells. We present the use of polymeric reverse micelle reactors to synthesize LiF interlayers from solution. In addition to controlling the formation of LiF nanoparticles, the use of reverse micelles enables control over the electrode surface coverage, by successively alternating deposition and etch removal of the polymer. Characterization of the resulting interlayers reveals a work function that is tunable with LiF surface coverage of the ITO electrode. Moreover, the work function increases with surface coverage, in direct contrast to the influence of thermal evaporated LiF. This behavior is attributed to lateral depolarization between LiF particles. We present a model incorporating lateral depolarization effects to predict the work function variation with surface coverage. To validate our expectation that electrode work function can be tuned to improve charge collection efficiency, demonstrator devices are fabricated and characterized. The modeling results are supported by improvements in performance of the demonstrator devices.

Over the last decade, polymer solar cells (PSCs) have attracted a lot of attention and provided a very promising future with highest power conversion efficiency (PCE) now close to 10%. Most of the studies about improving the performance of PSCs are focused on new material, novel structures, and morphology optimization. However, PCE usually can&’t be easily improved within the same device system once the system is optimized. Here we employed an optical structure-microlens array (MLA)-to increase light absorption inside the active layer, and PCE of PSCs increased even for optimized system. Microlenses were hemispherical with a diameter of 2mu;m, and they sit on the side of glass substrate facing the incident light. Normal incident light rays are refracted at the MLA and travel a longer optical path inside the device compared to conventional one. Two of the most studied PSCs systems-Ploy(3-Hexylthiophene-2,5-diyl):(6,6)-Phenyl C61 Butyric Acid Methyl Ester (P3HT:PCBM), Poly[ [9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:(6,6)-Phenyl C71 Butyric Acid Methyl Ester (PCDTBT:PC70BM)-were investigated. In P3HT:PCBM system, MLA increased the absorption, absolute external quantum efficiency , and PCE by 5%. In PCDTBT:PC70BM system, MLA increased the absorption, absolute external quantum efficiency , and PCE by 14%. In addition, simulations incorporating optical parameters of all structure layers were performed and they supported the enhancement of absorption in the active layer with the assistance of MLA. Our results show that utilizing MLAs is an effective strategy to further increase light absorption in PSCs, in which optical losses account for ~40% of total losses. MLAs also do not pose materials processing challenges to the active-layers since they are on the other side of the transparent substrate.

Flexible large-area panels of organic photovoltaic (OPV) devices can be fabricated by modern polymer processing techniques which have cost advantages over vacuum-based methods used for inorganic solar cells. However, the efficiencies of these OPV devices are much below those of inorganic-based solar cells. Recently, research has focused on creating more efficient OPV devices by the formation of thicker bulk heterojunction (BHJ) films with the combination of donor and acceptor materials. The design of these BHJ films attempts to reconcile the need for an optical path length which is sufficient for sunlight absorption with the requirements for small domain size due to the small exciton diffusion length, e.g., 10 nm to 20 nm. It is known that the exciton diffusion length increases significantly with increasing crystallinity. These excitons must diffuse to the donor-acceptor interface to dissociate into hole and electron charges that move to generate current upon being subjected to a potential difference. Thus, the nature of the morphology and the interface between the donor-acceptor materials are essential to exciton diffusion, dissociation and also charge mobility after it dissociates. We are exploring the influence of structure and morphology on the performance of OPV devices by material characterization using solid-state nuclear magnetic resonance (NMR) methods. Of particular interest in these OPV materials is the effect of processing and in these studies we concentrate on solventless approaches using solid-state heat pressing of multi-layers of donor and acceptor films. The thickness, crystallinity and interdiffusion between the layers are controlled by the number of stacking layers, i.e. the degree of compression. ¹H spin diffusion and ¹³C NMR relaxometry are used for estimating crystallinity and domain sizes, and for characterizing molecular dynamics of these OPV devices. From ¹H spin diffusion measurements, we have observed that the size of the donor and acceptor domains decreases with decreasing layer thickness. Molecular structure and dynamics of regioregular P3HT domains in films of different layer thicknesses are being investigated by high-resolution solid-state ¹³C NMR methods.

Effective characterization of charge transport and recombination parameters within bulk heterojunction (BHJ) materials used in organic photovoltaic cells (OPV) are of vital importance for understanding how to increase the power conversion efficiency of these devices. While advantageous in many respects, vertical device structures similar to solar cells do not allow researchers to spatially probe the device electrically or optically along the direction of charge transport. We have developed a lateral BHJ device structure which overcomes these limitations by allowing the measurement of electrical and optical properties along the transport direction of charge carriers. Two terminal photocurrent measurements have confirmed that devices exhibit space charge limited transport physics under appropriate electrical biasing conditions. Two distinct space charge regions develop within the device channel, separated by a central zone where recombination is dominant and which does not contribute photocurrent. To resolve the spatial extent of these different zones in a P3HT:PCBM lateral BHJ device, electron beam lithography has been used to create structures with many voltage probes at a sub-micron resolution in the channel. Spatially resolved photocurrent measurements via local excitation of the active layer were also used to determine charge transport parameters for P3HT:PCBM and PSBTBT:PCBM lateral BHJ devices. These materials were chosen as a comparison to one another as well as due to the many reports that exist on these materials. Using the voltage probe structures, we generate a potential profile of the channel under different applied biases through potentiometric measurements. We have determined the extents of the space charge regions and the recombination mechanism throughout the device. Recombination is clearly bimolecular in the recombination zone. These voltage probe structures can also be used to evaluate mobility asymmetry. From the spatially resolved photocurrent measurements, contrast between areas of high or low local photocurrent can also determine the length of the space charge regions adjacent to each of the electrodes. Using the spatially resolved photocurrent information and the DC photocurrent measurements, the mobilities of both electrons and holes as well as the generation rate of charge carriers within the active layer can be determined using a one dimensional model which solves the current density, continuity, and Poisson&’s equations as well as simulation results based on this model. This method of measuring both carrier mobilities is one of the few methods that can resolve the mobilities of both electrons and holes even if they are approximately equal.

Atomic force microscopy (AFM) is an essential tool for characterization of surfaces of organic materials and their properties related to photovoltaic performance. In its basic function, AFM provides high-resolution visualization of surface structures and defects that are crucial for the establishment of structure-property relationship. This capability can be realized for studies of thin organic and polymer films used in solar cells and field emission transistors. Recent progress in AFM instrumentation and, particularly, developments of multi-frequency techniques, open broad capabilities for high-resolution and sensitive studies of local electric and dielectric properties of these materials and devices. AFM-based electric modes, which employ the tip-sample electrostatic force, include electric force microscopy (EFM), Kelvin force microscopy (KFM) and measurements of dielectric response (dC/dZ). KFM and dC/dZ studies can be performed in single-pass operation when the mechanical and electrostatic forces are detected simultaneously and independently at two different frequencies. In such operation the probe remains in the immediate vicinity of the sample which provides sensitive registration of surface potential and dielectric permittivity variations with a few nanometer spatial resolution [1]. The single-pass KFM and dC/dZ measurements assist in compositional mapping by revealing variations of surface potential and dielectric permittivity in heterogeneous samples of polymers, semiconductors and metals. The KFM imaging of P3HT-PCB and PEDOT-PSS blends revealed the micro-phase separation morphology of these materials and its alternations due to the preparation and annealing procedures. The changes of surface potential, which are directly detected in KFM, can be assigned to variations of the molecular dipole strength and orientation. In case of dC/dZ measurements the interpretation of the data in terms of dielectric permittivity demands knowledge of sample thickness [2]. In addition to structural and electric characterization of photovoltaics materials, the AFM techniques can be applied for studies of molecular mobility in these materials. This can be achieved by placing a charge on a sample surface and monitoring its dissipation. We will report the results of such experiments on several samples. [1] S. Magonov and J. Alexander “Single-Pass Kelvin Force Microscopy and dC/dZ Measurements in the Intermittent Contact: Applications to Polymer Materials” Beilstein Journal of Nanotechnology, 2011, 2, 15-27. [2] S. Belikov, J. Alexander, I. Yermolenko, S. Magonov “Towards quantitative local dielectric analysis of polymers” Amer. Control Conference 2012, in press.

We investigate photovoltaic performance and influence of mixed solvent systems on blend films based on poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,2&’:5&’,2&’&’:5&’&’,2&’&’&’-quarterthiophene] (P(DPP-alt-QT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM). The polymer showed high carrier mobility of 1.04 cm2/Vs and a wide light absorption up to 900 nm. Utilizing this polymer suitable for solar cells, we fabricated devices with a structure of ITO/PEDOT:PSS/polymer:PC70BM/TiO2/Al and measured their performance under 100 mW/cm2, AM 1.5G illumination. Importantly, to control the morphology, the polymer:PC70BM blend films were spin-coated from mixed solvents of CF:chlorobenzene (CB) or CF:o-dichlorobenzene (DCB). The ratio of mixed solvents varied from CF:CB (or DCB) =3:1, 4:1, to CF:CB (or DCB) = 5:1. The best power conversion efficiency (PCE) was 5.4% with an open-circuit voltage (Voc) of 0.66 V, a short-circuit current (Jsc) of 11.8 mA/cm2 and a fill factor (FF) of 0.70, which was obtained from the CF:DCB=4:1 co-solvent system. In this talk, we present how we achieved this high efficiency and also provide deep understanding on the difference on morphology and carrier transport in the blend films that were controlled by the mixed solvent system.

Morphology of nanostructured ZnO plays important roles for poly(3-hexylthiophene) (P3HT)/ZnO hybrid solar cell with an inverted structure showing the specific electrical properties and remarkable enhancement of the power conversion efficiency. In our work, ZnO nanorods and ZnO nanoparticles were synthesized and used as the electron acceptor in the devices. Dye molecules, serving as interface modifiers and photosensitizers, were adsorbed onto the surface of ZnO in order to improve interface interaction between ZnO and P3HT. The device structure of (indium tin oxide) ITO/dense ZnO nanoparticles/ nanostructured ZnO/ dye/P3HT/VO_x/Ag is prepared and characterized. Moreover, electrical properties of nanostructured ZnO in terms of energy gap, valence band edge, and so on, were investigated in order to examine charge transport properties in the devices. The results will be presented.

In this work, we demonstrated a high-performance solution-processed transparent polymer solar cell. So far, no significant progress has been made on the transparent PV device due to the lack of transparent PV materials and top electrodes. Upon the utilization of high performance low bandgap polymer, the photoactive layer absorbs more near-infrared light while being less sensitive to visible light. The transparent electrode is a fully solution-processed conducting silver nanowire-based composite film. With this combination, we have achieved 4% power-conversion efficiency for the fully solution-processed transparent solar cells on indium-tin-oxide-coated glass substrates.

In organic electronic devices, charge injection at the contacts is crucial for high electrical performance. Most of these devices require at least one electrode with a sufficient low work function (WF) to facilitate the transport of electrons in and out of various organic transporting layer. Low-WF electrodes which meet this requirement are easily available; however, they are chemically often very reactive and oxidize in ambient atmosphere. A smart way to overcome this problem is the use of molecular or polymeric dipole layers. The use of polymeric dipole layers (PDLs) to tune an electrode&’s work function can be advantageous over self assembled monolayers (SAMs) as the PDL concept can be applied to a wider range of electrode materials and provide a great alternative to chemically reactive low-WF metals. We have studied the properties of PDL-treated substrates via Atomic Force Microscopy (AFM), ambient Kelvin Probe (KP) and contact angle measurements. We have used two different materials as PDLs: polyethylenimine ethoxylated (PEIE) and branched polyethylenimine (PEI), which are both commercially available. These polymers were chosen in order to lower the work function of various substrates and to enhance electron injection in organic devices. After cleaning and plasma-treatment, solutions of the PDLs in methoxyethanol were prepared and spincoated in ambient atmosphere at room temperature onto Indium Tin Oxide (ITO)-coated glass substrates. The spin-coated films were annealed at 100°C on a hotplate in ambient air. Bare samples were studied via AFM, single-point and scanning Kelvin Probe and contact angle measurements. The Kelvin Probe method is used to investigate the effect of the PDLs on the work function. For both PDLs we see a contact potential difference (CPD) of approx. -1000 mV compared to an untreated substrate. The resulting change in the work function can be related to the intrinsic molecular dipole moments associated with the neutral amine groups contained in such polymer layer [1]. In practical use it means, that ITO can be turned from its common use as anode (WF: 4.8 eV) into a cathode (WF: 3.8 eV) for inverted or transparent organic devices. To demonstrate this, we prepared inverted solar cells based on a P3HT:PCBM bulk heterojunction layer with (PDL-treated) ITO as bottom cathode and a bilayer structure of MoO3 and Al as top anode. With a non-treated ITO cathode only poor solar cell characteristics were achieved. The characteristics of an inverted solar cell with PDL-treated ITO show comparable results to a standard solar cell with bottom anode structure. As the PDL layer is stable in ambient air, ambient-stable organic solar cells are within reach. The particular properties of PDLs can also be optimized for other applications and a polymer which increases the WF is also conceivable. Reference 1. Zhou, Y. et al. A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. Science 336, 327-332 (2012).

The efficiency of low-bandgap PCPDTBT:PCBM bulk heterojunction solar cells depends very much on the film preparation conditions, for instance on the use of cosolvents, for reasons still not entirely understood. Efficiencies of up to 5.5 % have been reported for this blend system upon addition of a cosolvent. In this contribution we present the exciton and charge carrier dynamics in pristine PCPDTBT films and in photovoltaic blends with PCBM studied by Vis-NIR broadband transient absorption spectroscopy. In particular we monitor the previously unobserved, but for low-bandgap polymers essential spectral region between 1000-2000 nm on a timescale from 150 fs up to several ms, enabling us to clearly identify the individual contributions and dynamics of singlet excitons, charge-transfer excitons as well as free and trapped polarons. This not only allows a qualitative assessment of the effect of the cosolvent, namely 1,8-octanedithiol (ODT), on the photophysics, but also the extraction of the most relevant recombination parameters that finally determine the device efficiency. Interestingly, the broadband pump probe experiments reveal that excitons are not only generated in the polymer, but also in PCBM-rich domains. Depending on the morphology controlled by the use of solvent additives, polymer excitons undergo mainly ultrafast dissociation (< 100 fs) in blends prepared without ODT or diffusion-limited dissociation in samples prepared with ODT (< 10 ps). Excitons generated in PCBM diffuse slowly to the interface in both samples and undergo dissociation on a timescale of several tens of picoseconds up to hundreds of picoseconds. The cosolvent causes a partial demixing of the material blend leading to the formation of larger domains. Furthermore, the addition of ODT reduces the fraction of interfacial charge transfer states, which rapidly recombine and thus are lost for photocurrent generation. We find that the non-geminate recombination coefficient for this material system is found to be two orders of magnitude larger than for the reference system P3HT:PCBM indicating that the competition between charge carrier extraction and recombination is shifted towards the latter in photovoltaic devices. Finally, we study whether the formation of triplet states is an additional loss channel in this material system. A comparison of the near-infrared transient absorption spectra and dynamics of free charges and triplets in photovoltaic blends reveals that triplets play only a minor role for the device performance of PCPDTBT:PCBM blends. [1] F. Etzold et al., J. Am. Chem. Soc. 2012, DOI:10.1021/ja303154g

Organic solar cells represented P3HT:PCBM type are light and flexible, so it attracts attention as next-generation small source of electrical energy. However, the conversion efficiency is still lower than Si type solar cells. Therefore, Bulk Hetero Junction type (BHJ) and Ordered Hetero Junction type (OPJ) is reported to improve the conversion efficiency. OPJ has advantages of designable the shape of PN junction and reduction recombination. However, it also have problem with effect to take particles because it is made mechanically. In this report, we propose new process to increase area of PN junction with imprint directly on the planar hetero made with spin coating. Here is the way of experiment. To increase area of PN junction, we apply directly imprint to double layer. We make two layers in order to P3HT and PCBM on the substrate. After fabricating organic-thin-layer solar cells, we press with mold for patterning with heating. After cooling, we make the area of PN junction large with releasing mold. When we make a solar cell device, we make thin layers on the ITO glass substrate in order of PEDPOT:PSS, P3HT, PCBM by spin-coating. After pressing with mold, thin layers of LiF and Al is coated by vapor deposition. We simulated behavior of deformation also. We expect that suitable thickness and so on can make large area of PN junction. The area of PN junction becomes larger with proper condition. In the past, we have observed a little improvement of conversion efficiency be assumed to be due to effect of imprint. 1) M.Zhou, et al., J. Vac. Sci. Technol. 28 (2010) C6M63 2) T. Konishi, et al., Microelectronic Engineering 83 (2006) 869.

The design of materials for organic electronic devices is driven by optimization of charge and energy transfer processes within them. Theory and simulations have substantially contributed to our understanding of these processes in amorphous organic semiconductors, in particular (extended, correlated) Gaussian disorder models and microscopic simulations. [1] In this talk, we present a stochastic network model for charge transport simulations in amorphous organic semiconductors, which generalizes the correlated Gaussian disorder model to more realistic (off-grid) morphologies, where the information on positional disorder (and other properties) are gained by microscopic simulations, i.e., the ingredients of the stochastic model are fitted to the properties of representative microscopic systems. [2] The network model is based on tools from stochastic geometry and spatial statistics. It includes an iterative dominance-competition model for positioning vertices (hopping sites) in space, distance-dependent distributions for the vertex connectivity and electronic coupling elements, and a moving-average procedure for assigning spatially correlated site energies. The field dependence of the hole mobility of the amorphous organic semiconductor, tris-(8-hydroxyquinoline)aluminum, which was calculated using the stochastic network model, showed good quantitative agreement with the prediction based on a microscopic approach. The stochastic model can be used to simulate large system sizes in realistic device geometries. [1] V. Rühle, A. Lukyanov, F. May, M. Schrader, T. Vehoff, J. Kirkpatrick, B. Baumeier, and D. Andrienko, Microscopic simulations of charge transport in disordered organic semiconductors, Journal of Chemical Theory and Computation, 2011, 7 (10), 3335-3345 [2] B. Baumeier, O. Stenzel, C. Poelking, D. Andrienko and V. Schmidt, Stochastic modeling of molecular charge transport networks, preprint (submitted)

For organic optoelectronic devices, ITO has commonly been used as the electrode due to its highly transparent and conductive properties. Specifically, recent intense research to achieve high power conversion efficiency (PCE) has led to a significant advance using ITO electrodes in organic photovoltaics (OPVs). In spite of the wide use in OPVs, high cost and brittleness of ITO are critical drawbacks when applied to flexible OPVs. Meanwhile, graphene has appeared as a promising candidate for replacing ITO due to its high transmittance, and excellent mechanical and electrical properties. However, low conductivity of graphene has limited current density and the low work function (~4.4eV) has hindered efficient charge extraction between active layer and electrode because most of the donor materials in an active layer of OPVs have an ionization potential higher than 5 eV. For these reasons, the PCE of OPVs using graphene anode has been limited until now (< 2.5%). In this work, we demonstrated flexible OPVs using graphene anodes with self-organized polymeric hole extraction layer (we term ‘GraHEL&’). The graphene films were synthesized in a thermal chemical vapor deposition (CVD) chamber using copper (Cu) foil and transferred to flexible PET substrate. The high sheet resistance of graphene was drastically decreased by p-doping graphene anodes with nitric acid (HNO3) and the surface work function of hole extraction layer (HEL) could be successfully controlled by using GraHEL. These lead to efficient charge extraction of OPVs. We eventually obtained a high PCE (> 4.0 %) in flexible OPVs. Our research demonstrates the possibility to use graphene electrode as a next generation electrode in flexible organic optoelectronics instead of ITO.

Photovoltaic technology is recognized as one of the best methods to prevent the exhaustion of fossil fuels and tackle the problem of global climate change. Organic photovoltaics (OPV) with high efficiency have attracted attention in recent years as a candidate for future photovoltaic systems. A bulk heterojunction (BHJ) device has been proposed to accomplish this objective. One of the most important methods considered to increase short-circuit current density (Jsc) in the use of long-wavelength absorbing materials, which can absorb a wider wavelength range than conventional polymers, such as poly (3-hexylthiophene). Using computational chemistry we have designed and developed a low bang gap polymer containing alternating electron-donor and electron-acceptor segments, which absorbs wavelengths up to 900 nm. Another important method to increase Jsc is to control the morphology of the active layer. Control of the morphology has long been viewed as one of the most important challenges to OPV production. The active layer was prepared by casting a mixed solution of p-type and n-type semiconductors. We used transmission electron microscopy (TEM) analysis to characterize the p/n interfacial length to determine a phase separation parameter. We have found preferable solvent systems. Herein we present our recent progress in the development of highly efficient OPV devices based on a newly developed low band gap polymer and control of the active layer morphology. Optimization of the device structure is currently under way, which will be presented as well.

Polymer-fullerene-based bulk heterojunction (BHJ) solar cells have many advantages including low-cost, low-temperature fabrication, semi-transparency, and mechanical flexibility.[1,2] However, there is a mismatch between optical absorption length and charge transport scale.[3,4] These factors lead to recombination losses, higher series resistances and lower fill factors. Attempts to optimize both the optical and electrical properties of the photoactive layer in organic solar cells (OSCs) inevitably result in a demand to develop a device architecture that can enable efficient optical absorption in films thinner than optical absorption length.[5,6,7] Here, we report the use of dual metallic nanostructures to achieve the broad light absorption enhancement, increased short-circuit circuit (Jsc) and improved fill factor (FF) simultaneously based on a new small-bandgap polymer donor of poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b&’]dithiophene-2,6-diyl]-alt-[2-(2&’-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-C-T) in BHJ cells.[8] The dual metallic nanostructure consists of 2D arrays of metallic nanograting electrode as a back reflector and the metallic nanoparticles (NPs) embedded into the active layer. Apart from the waveguide modes and diffractions, we simultaneously introduce hybridized surface plasmonic resonances (from Ag nanograting) and localized plasmonic resonances (from Au NPs) to successfully achieve a broadband absorption enhancement. The detail understanding has been described with our theoretically studies. As a consequence, we improve PCE to reach 8.79%[9] by improving both optical properties and electrical properties of OSCs through introducing dual plasmonic nanostructures which contribute to the practical application of OSCs for photovoltaics. [1] S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photon. 2009, 3, 297. [2] H.Y. Chen, J.H. Hou, S.Q. Zhang, Y.Y. Liang, G.W. Yang, Y.Yang, L. P. Yu, Y. Wu, G. Li, Nat. Photon. 2009, 3, 649. [3] D. Wöhrle and D. Meissner, Adv. Mater. 1991, 3, 129. [4] P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster and D. E. Markov, Adv. Mater. 2007, 19, 1551. [5] H. A. Atwater, A. Polman, Nat. Mater. 2010, 9, 205. [6] J. You, X. Li, F.X. Xie, W.E.I. Sha, J.H.W. Kwong, G. Li, W.C.H. Choy, and Y. Yang, Adv. Energy Mat., DOI:10.1002-aenm.201200108. [7] X.H. Li, W. E.I. Sha, W.C.H. Choy, D.D.S. Fung, and F. X. Xie, J. of Phys. Chem. C, 10, 1021, 2012. [8] L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li, J. Hou, Angew. Chem. Int. Ed. 2011, 50, 9697. [9] X. Li, W.C.H. Choy, L. Huo, F. Xie, W.E.I. Sha, B. Ding, X. Guo, Y. Li, J. Hou, J. You, and Y. Yang, Adv. Mater., DOI: 10.1002/adma.201200120.

Although various optical designs and physical mechanisms have been studied both experimentally and theoretically to improve the optical absorption of organic solar cells (OSCs) by incorporating metallic nanostructures, the effects of plasmonic nanostructures on the electrical properties of OSCs is still not fully understood. Hence, it is highly desirable to study the changes of electrical properties induced by plasmonic structures and the corresponding physics for OSCs. In this work, we develop a multiphysics model for plasmonic OSCs by solving the Maxwell&’s equations and semiconductor equations (Poisson, continuity, and drift-diffusion equations) with unified finite-difference method. Both the optical and electrical properties of OSCs incorporating a 2D metallic grating anode are investigated. For typical active polymer materials, low hole mobility, which is about one magnitude smaller than electron mobility, dominates the electrical property of OSCs. Since surface plasmon resonances excited by the metallic grating will produce concentrated near-field penetrated into the active polymer layer and decayed exponentially away from the metal-polymer interface, a significantly nonuniform and extremely high exciton generation rate is obtained near the grating. Interestingly, the reduced recombination loss and the increased open-circuit voltage can be achieved in plasmonic OSCs. The physical origin of the phenomena lies at direct hole collections to the metallic grating anode with a short transport path. In comparison with the plasmonic OSC, the hole transport in a multilayer planar OSC experiences a long transport path and time because the standard planar OSC has a high exciton generation rate at the transparent front cathode. The unveiled multiphysics is particularly helpful for designing high-performance plasmonic OSCs.

Acidic PEDOT:PSS anode buffer layer is widely used for improving efficiency in organic based solar cells. Recently, the stable P-type metal oxide, such as NiO, MoO3 and V2O5 are attempted to replace the commonly used acidic PEDOT:PSS anode buffer layer. Because PEDOT:PSS solutions are hard to coat uniformly on the anode or polymer active layer. Hereby, fabrication and characterization of various anode buffer layers are very important, because they affect efficient hole extraction and blocking of electron back flow in organic based solar cells. We have studied various MoO3 anode buffer layers formed by different fabrication methods, such as annealing of molybdenum in oxygen atmosphere, Molybdenum O2 plasma treatment. The structural properties of MoO3 layers were studies by scanning electron microscope (SEM) and x-ray diffractometer (XRD). Also, optical and electrical properties were studies by UV-Vis spectrophotometer and Hall measurement system. In particular, we found that the MoO3 phases are transformed as the annealing temperature increased and electrical properties of MoO3 are significantly changed. In addition, the photovoltaic properties of MEH-PPV/ZnO NPs hybrid solar cells with various MoO3 anode buffer layer investigated for influence of MoO3 anode buffer layer.

Incorporating zinc oxide nanoparticles as the cathode buffer layer has been a promising method to improve the performance of organic solar cells. In this work, 10% lithium doped zinc oxide nanoparticles (ZnO:Li NPs) is successfully synthesized with the virtue of lowering work function to function as a cathode buffer layer in the inverted organic solar cells with the structure of ITO/ZnO:Li NPs/P3HT:PCBM/PEDOT:PSS/Au. As a result, a higher power conversion efficiency of 3% is achieved within 0.16 cm² working area. With ultraviolet photoelectron spectroscopy (UPS), the difference of band structure between bare ZnO NPs and ZnO:Li NPs is clearly observed. Impedance measurement as a practical analysis tool is also applied to illustrate the carrier recombination behavior of organic photovoltaic devices. The ZnO:Li NPs solar cell with longer recombination life time indicates a higher probability for carrier extraction, which leads to its advanced performance.

The power conversion efficiency of organic photovoltaics (OPVs) is limited by the combined charge-extraction challenges of short (~10 nm) exciton diffusion length, bimolecular recombination, and disorder-induced low free carrier mobility. These challenges can be addressed by efficiently coupling and confining incident sunlight into an OPV with an ultrathin (< 30nm) active layer [1]. However, the realization of high-performance OPVs with such thin active layers requires an understanding of the optical properties of ultrathin organic bulk heterojunction films. To this end, we use spectroscopic ellipsometry (SE) to perform a comprehensive experimental study of the optical properties of poly(3-hexylthiophene)-[6,6]-phenyl:C61-butyric acid methyl ester (P3HT:PCBM) blends. Using SE measurements and an anisotropic five-peak Tauc-Lorentz model, we determine the complex index of refraction of P3HT:PCBM films with thicknesses from 15 nm - 100 nm. We consider optical anisotropy, and a biaxial model is used for the P3HT:PCBM active layer. Furthermore, to account for the substrate roughness, which is highly significant for such ultrathin active layers, an effective medium approximation is considered at both the air/P3HT:PCBM and P3HT:PCBM/substrate interfaces. We independently measured the thicknesses of the P3HT:PCBM films using atomic force microscopy and profilometry, and compared these results with the SE results obtained by fitting to a Cauchy model in the transparent wavelength range. Finally, we also performed Grazing Incidence Wide Angle Scattering (GIWAXS) measurements to determine the structure and order of these ultrathin films. Compared to previous measurements on relatively thick (> 80 nm) P3HT:PCBM films, our work breaks new ground into the relatively unexplored thin-film confinement regime of this important class of bulk heterojunction materials. [1] Y. Pang, D. M. N. M. Dissanayake and M. D. Eisaman, “Guided-Mode Absorption in Ultrathin Organic Photovoltaics”, 38th IEEE Photovoltaic Specialists Conference (PVSC), June 3-8, 2012.

We present silver nanowire (Ag NW) based transparent electrodes in normal and inverted organic solar cells. These devices employ either bottom ^[1] or top ^[2] electrode layers comprised of doctor bladed or spray coated Ag NWs treated at moderate temperatures. The performance of P3HT:PCBM based organic solar cells with Ag NW electrode layers was found to match very closely the performance of reference devices based on ITO bottom- or highly reflecting thermally evaporated metal silver top electrodes. Special attention is given to the optical losses of semitransparent electrodes and are investigated in detail in terms of transmission, scattering and reflection losses. Application of an external back reflector is proven to improve the light harvesting efficiency of optically thin devices. Further analysis of semitransparent devices under illumination from ITO backside as well as through the Ag NW front electrode enable the possibility to gain deep insight into the vertical microstructure related device performance. Overall, Ag NWs as bottom or top electrodes are established as a serious alternative to TCO based electrodes. Solar cell devices with Ag NWs universally applied as bottom or top electrode with comparable performance to reference devices have been realized. [1] J. Krantz, M. Richter, S. Spallek, E. Spiecker and C. J. Brabec, Adv. Func. Mater. 2011, 21 (24), 4784 [2] J. Krantz, M. Richter, S. Spallek, T. Stubhan, I. Litzov, E. Spiecker and C. J. Brabec, subm. Adv. Eng. Mat. 2012

In general, the combination of low band gap polymers and fullerene multiadduct has shown poor performance compared to poly (3-hexylthiophene) (P3HT) and fullerene multiadduct. To obtain better efficiency of polymer solar cells (PSCs), the reasons to induce poor performance should be understood. Therefore, devices using poly((4,8-diethylhexyloxyl)benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl)-alt-((5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl)(PBDTTPD), poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl](PBDTTT-C)and P3HT with phenyl C₆₁-butyric acid methyl ester(PC₆₁BM) and indene-C₆₁ multiadducts (ICMA, ICBA and ICTA)were fabricated. The photocurrent of P3HT: ICBA is higher than P3HT: PCBM while low band gap polymer, PBDTTPD and PBDTTT-C, show opposite tendency. To establish the reasons to cause the lower photocurrent in low band gap polymers/fullerene multiadduct, we measure the morphology, molecular orientation and light absorption. However, the results are not enough to explain the why low band gap show poor performance by using indene-C₆₁ multiadducts (ICMA, ICBA and ICTA) instead of PCBM. However, the G_max and P (E,T) show a considerable decrease and driving force (ΔG_CT) is also smaller than 0.1eV (ΔG_CT= E_CT - E_g) when replacing PCBM with indene-C₆₁ multiadducts. Therefore, the decreased photocurrent in PBDTTPD and PBDTTT-C BHJ-type solar cells with ICBA is attributed to the minimal driving force (ΔG_CT) to charge generation, as the external quantum efficiency (EQE) show that excitons formed from polymer absorption rarely contribute to photocurrent.

We demonstrate an inverted type organic solar cells with all nanocarbon material electrodedes under P3HT:PCBM hybrid system. We use graphene as a top electrode (silver electrode is much expensive) and single-walled carbon nanotube as a bottom electrode. The device structure is SWNTs/ZnO/P3HT:PCBM/PEDOT:PSS or graphene oxide/graphene. Therefore, both electrodes are transparent ,and enhance the light conversion efficiency and compatible with roll-to-roll printable electronics. The motivation for having an inverted type cells is because in standard organic polymer cells poly(3,4-ethyl-enedioxythiophene)poly(styrenesulfonate ) (PEDOT:PSS) coated on the ITO glass substrate damages the electrode because of the acidic property(pH~1), this is the main reason to decrease the devices performance. Thus, replacing the ITO transparent substrates with one-dimensional single-walled carbon nanotubes (SWNTs) or two-dimensional graphene is advantagous for improving the devices performance (ex: more flexible). Furthermore, changing the cell structrure into an inverted type is also a choice because the PEDOT:PSS does not contact ITO electrode and does not expose aluminium electrode which is rapid oxidation under air, thus the life span of the devices will increase. The process can be a good candidate to replace ITO and noble metal electrodes and make the transparency much better.

Organic photovoltaic (OPV) technology is a promising solution for low cost, large area and flexible light harvesting cell. Bulk heterojunction devices, one the most common type of OPV, are composed of an interpenetrated array of electron donor (such as P3HT) and acceptor (such as PC60BM) materials. The role of such blend is to dissociate efficiently excitons generated by light absorption [1]. It is well known that for domain widths much larger than the exciton diffusion length, the cell efficiency is limited by the poor rate of exciton dissociation [2]. However, for smaller domains, it has been observed that performances are also degraded [3]. Even though the physical origin of this degradation has not been clarified yet, this issue is addressed in practice by an annealing step after blend deposition [4]. Nevertheless, any model clearly identifies the fundamental origin of this degradation. In this work, an original physical interpretation of the poor efficiency for finesse interpenetrating network is proposed. An analytical model for the cell IV characteristic in dark and illuminated conditions has been derived. This model consists in extending the approach proposed by Alam et al. [5], by introducing a recombination velocity along each heterojunction interfaces, as suggested by the previous conclusions of Street et al experimental works [6]. Results show that this improved model satisfactory reproduces the cell efficiency degradation for finesse morphology. Simulations have been then compared with experiments. Samples are processed with a conventional P3HT PC60BM (ratio of 1:1, concentration of 25mg/ml, solvent chlorobenzene). A diiodooctane (DIO) additive mixed with chlorobenzene has been used to improve the initial finesse of the blend [7]. Finally, the morphology has been tuned by successive annealing steps at 100°C. The average domain size has been extracted by tapping mode AFM phase scanning measurement. EQE measurements plotted versus domain size exhibits a “bell curve”, and are in good agreement with the model predictions. These results demonstrate that the optimum EQE results from a trade off between exciton dissociation, and, as underlined in this work, charges recombination at heterojunction interface. [1] G. Dennler, M. Scharber, C. Brabec. Adv. Mater. 21, p. 1323-1338 (2009) [2] A.C. Arias et al., Macromolecules, 34(17), p.6005-6013 (2001) [3] L. Zeng, C.W. Tang, S.H. Chen, Appl. Phys. Lett. 97,053305. (2010) [4] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang. Nature Materials, 4, 11,p. 864-868 (2005) [5] B. Ray, P. Nair, M. Alam, Solar Energy Materials & Solar Cells. 95, p. 3287-3294. (2011) [6] R. Street, M. Schoenderf, A. Roy, J.H Lee Physical Review B. 81, 207301. (2010). [7] S. Lou, S. Szarko; T. Xu, T. Marks, L. Chen, J. Am. Chem. Soc. 133, p. 20661-20663. (2011)

The recent demonstration [1] of molecular miscibility/solubility between polymers and fullerenes brings many questions critical to the operation of organic devices based on these materials into focus: What is the ideal composition of a bulk heterojunction? Are there more than two domains in actually devices and is that beneficial or not? Initially, the universality and variability of such a phenomenon must be measured, but ultimately it is important to fully understand the effect of mixing at the molecular level on device performance. Here we show that fullerene solubility in semi-conducting polymers is indeed universal and varies widely between different polymers, and for the same polymer but different fullerenes [2,3]. We additionally correlate the thermodynamic miscibility in these systems to optoelectronic processes in devices. Surprisingly, we find that while some systems seem to benefit from the mixed phase, others do not. We hypothesize a complex interplay between electron trapping and percolation that is determined by the level of miscibility and the electronic nature of the polymer. Additional to the operation, molecular miscibility of the components will strongly affect the stability of the OPV morphology over time and designing a material system where the miscibility both enhances performance and cell lifetime will be critical for the future of the technology. [1] B. A. Collins et al., Journal of Physical Chemistry Letters 1, 3160 (2010). [2] B. A. Collins et al., Macromolecules 44, 9747 (2011). [3] X. He et al., Small 8, 1920 (2012).

Efficient polymer-based solar cells rely on a phase-separated bulk heterojunction (BHJ) morphology for efficient conversion of photons to free electrons.(1) The morphology of the BHJ film determines the efficiency of exciton dissociation and collection of charges. In particular, vertical phase separation (2) can critically limit charge transport towards the electrodes. We have studied the effect of the substrate on the extent of vertical phase separation on spin-cast BHJ films of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) using x-ray photoelectron spectroscopy. We have specifically considered both conventional and inverted structures, and have also studied the effect of an ultra-thin dye layer at the ZnO-Al layer which has been shown to induce an interface dipole. (3) These results are correlated with device performance data. This investigation addresses the interplay between device structure and vertical phase separation in spin-cast organic BHJ films. References 1. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 270, 1789 (1995). 2. H. Lu, B. Akgun, and T. P. Russell, Adv. Energy Mater. 1, 870 (2012). 3. J. H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, U. Scherf, A. J. Heeger, and G. C. Bazan, J. Am. Chem. Soc. 133, 8416 (2011). # This work is supported by the NSF SOLAR Grant (DMR-0934433).

Organic small molecule based solar cells show great promise in providing the necessary balance of low-cost and efficiency needed to make affordable solar energy. To make these OPV materials viable to be employed as actual devices, fundamental research of the opto-electronic processes at the intrinsic length scales is needed to systematically alter the performance for both, lifetime and efficiency. Therefore we develop complementary measuring techniques to achieve a more complete picture of the photon to exciton and charge separation processes at the nm scale. We employ true multidimensional mapping of P3HT/PCBM films by measuring in parallel hyper-spectral Tip Enhanced Raman Spectroscopy, local photo current and topography. This enables us to correlate topography, local chemistry and local photo current generation simultaneously. We observed a correlation between a local increase in photocurrent and a local increase of the Raman peak originating from benzene structure for unannealed samples whereas in case of annealed samples this correlation is less pronounced. In parallel we developed a novel method to gain insight into exciton diffusion through these OPV materials by employing optical antennae, fabricated into the bottom electrode of the PV device to optically excite organic PV material locally (near field optical spot < 15nm in diameter) while the photocurrent is measured with a conductive Atomic Force Probe. This idea has only been made feasible recently with the concept of optical antennae allows to characterized the PV materials at their critical scale below the 10 nm.

We report on the time-resolved photoluminescence in both short and long-time regimes in individual crystalline P3HT nanofibers. In contrast with structurally heterogeneous thin films and nanoparticles of P3HT, the time-resolved PL in extended P3HT nanofibers show highly reproducible short-time decay behavior, as well as pronounced long-time power-law decay associated with exciton dissociation and recombination via tunneling. We show that both of the intra-chain processes are affected by regioregularity, while changes the inter-chain recombination and charge separation processes are likely responses to solvent quality and molecular weight. One of the interesting features of the NF luminescence is the presence of an extremely long PL component, which we assign to carrier trapping created by interrupted inter-chain packing. This effect manifests as profound changes in the power law dynamics governing polaron pair recombination rates, and these findings are supported by the results of a suspension-to-solid photophysical study executed on a single fiber. Finally, we probe diffusion of bound excitons in individual NFs through polarization-resolved TRPL measurements, in which both arrival time and polarization state relative to the nanofiber axis (parallel/perpendicular) are resolved. These measurements reveal surprising new insights into intra- and inter-chain coupling in isolated P3HT nanofibers.

The bulk morphology of the active layer in a bulk heterojunction solar cell is, along with the interface and electronic structure, considered one of the most important aspects that determine device characteristics. Frequently, gracing incident scattering and diffraction is used in combination with scanning probe and advanced electron microscopy to elucidate structure/property relations. We show and discuss that soft x-ray tools (microscopy [1] and scattering [2,3]) are powerful complements that allow the determination of the domain size, the relative purity, molecular orientational ordering at interfaces, and the interfacial structure (smoothness/roughness, interdiffussion). We discuss how these parameters relate to Jsc and FF, two critical performance parameters of a device. The purity of the as-cast domains observed with scattering and microscopy will be related to the thermodynamic miscibility/solubility as measured with x-ray microscopy [1]. Of particular interest is the recently observed orientational ordering of a polymer patterned by the presence of the second, dispersed phase [3]. We show that the resulting scattering anisotropy has been observed in many polymer:polymer and polymer: fullerene based solar cell systems. Typically a face-on structure/orientation (relative to the domain interface) is observed. Soft x-ray scattering is a unique tool to assess such orientation and the role (if any) relative molecular orientation at an interface plays in the performance of devices can now be assessed. [1] B. A. Collins et al, Journal of Physical Chemistry Letters 1, 3160 (2010). [2] S. Swaraj at al., Nano Lett. 10, 2863 (2010). [3] B. A. Collins et al., Nat Mater 11, 536 (2012).

New donor-acceptor, low bandgap (Eg 0.6 - 1 eV), conjugated polymers will be presented. The unusual spectroelectrochemistry and photophysics behavior of these materials can be easily interpreted on the basis of spontaneous spin unpairing in polymers containing benzobisthiadiazole (BBT) and benzobisketopyrrolopyrrole as acceptor. The synthesis and electron spin resonance and magnetic susceptibility characterization for these systems will be presented.

The mechanism of free charge carrier generation via dissociation of photogenerated excitons in polymer:fullerene photovoltaic blends is still poorly understood, despite intensive research efforts by many groups. In particular, the role played by interfacial charge-transfer (CT) states in the generation of free charges remains largely unclear. It is often assumed that these CT-states are intermediates in the formation of free charge carriers and are themselves prone to a field-dependent splitting process in working devices. However, largely inconsistent experimental results have been presented to date, even for the same material systems. Gaining more insight into the role CT-states play is not only of fundamental interest, but also necessary to better understand the bias dependence of the photocurrent, in particular the origin of the fill factor, which largely determines the device efficiency. In this contribution we present the charge generation and recombination dynamics in state-of-the-art low-bandgap polymer:fullerene photovoltaic blends such as PCPDTBT:PCBM, Si-PCPDTBT:PCBM and PTB-type polymers blended with PCBM in comparison to the reference system regioregular P3HT:PCBM, which we investigate by broadband pump-probe transient absorption (TA) spectroscopy covering the entire dynamic range from femto- to microseconds. In particular we monitor the hitherto unobserved, but especially for low-bandgap polymers essential, near-infrared probe region between 1000-2000 nm by using a novel broadband probe and near-infrared detection scheme that we have recently developed. Our results demonstrate that by probing the entire near-infrared in addition to the visible probe region, we can identify the individual spectral contributions and dynamics of singlet and triplet excitons and charge-transfer states, as well as free and trapped polarons. This in turn not only allows us to qualitatively assess the efficiency-limiting processes in these bulk heterojunction systems, but also to extract the most relevant photophysical parameters, such as the fraction and formation rate of CT-states versus free charges, the recombination rate of interfacial CT-states and the non-geminate recombination coefficient, as well as the order of the non-geminate recombination process.

Binary solvent mixtures have been widely used in morphology control of polymer solar cell (PSC) devices and proved to be helpful in improving photovoltaic properties of several high performance polymer photovoltaic systems, such as PCDTBT[1], APFO-3[2], PTB7[3] and DPP-based polymers[4]. Especially, for PDPP3T/PC71BM based devices, only modest PCEs of ~2% were obtained when using pure CF as the casting solvent, yet the binary solvent CF/DCB or CF/DIO was used leading to the PCEs dramatically increased to ~5%. When DIO is used as the third ingredient in the binary solvent mixtures DCB/CF, a further improvement of efficiency to 6.7% is achieved, which is the highest for DPP-based device reported to date. It is thought that this significant success of using ternary solvents to process active layer of organic solar cell is mainly due to the morphology control in active layer. By utilizing Grazing-incident wide angle x-ray scattering (GIWAXS), Atomic Force microscopy (AFM) and Resonant soft x-ray scattering (R-SoXS), different structures, such as, crystallinity of polymer, surface morphology, domain spacing, domain purity and domain interface structure are compared among DCB, DCB/CF and DCB/CF/DIO mixtures used as processing solvent. The device characteristics (Jsc and FF) show significant variations when using three different kind of mixture solvent and are explained by domain size, domain purity, and interface structure in the following way. DCB leads to domains that are the largest, with a distribution of domains size, with the most impure domains and somewhat rough interfaces. This leads to high exciton dissociation efficiency, hence good Jsc, but at a cost of bi-molecular recombination and reduction in FF. The addition of CF to DCB leads to a marked improvement in domain purity and reduced domain size with sharp interfaces. This leads to good charge transport and the best FF. However, the overall domain size is too large for the most efficient charge separation, and the Jsc is not much changed relative to DCB alone. With DIO added to DCB/CF, the overall domain size is not changing much, but the rougher interfaces observed provides a shorter average distance from the average location of the photon absorption site to the donor/acceptor interface. This enhances exciton dissociation and thus Jsc. The slightly increased purity is however negated by the enhanced bi-molecular recombination at the rough interfaces with a concomitant reduction in FF. References: [1] J. K. Lee et al , J. Am. Chem. Soc. 2008, 130, 3619. [2] F. L. Zhang et al, Adv. Funct. Mater. 2006, 16, 667. [3] M. R. Hammond et al, ACS Nano 2011, 5, 8248. [4] J. C. Bijleveld et al, J. Am. Chem. Soc. 2009, 131, 16616

“Plastic” solar cells are based on interpenetrating blends of a semiconducting polymer and a fullerene and are becoming a viable technology for sustainable energy generation due to the promise of low-cost solution-based manufacturing. Both the nanoscale morphology of the two-phase film and the molecular packing within the polymer and fullerene are tremendously important to device performance. Solution deposition using high boiling point additives provides a simple and widely used fabrication option for improving the power conversion efficiencies of organic solar cells. Previous examination of the resulting device active layer directly after spin casting have shown that the use of additives induces nucleation of polymeric crystallites within two minutes of deposition as well as promotes further crystallite nucleation over a prolonged period of time. Here, we show how additives affect the morphology of furan-containing low band-gap polymers, based on PDPP2FT (see figure 1 inset), blended with PC71BM. It was recently discovered that by inserting furan moieties in the backbone of the conjugated polymers the use of relatively small solubilizing side chains is enabled because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents.1 By addition of 1-chloronaphthalene to the blend solution the device performance was improved more than 5-fold with a much higher short-circuit current density and an average power conversion efficiency of 4.7%. These polymers enabled us to tune the side chain functionality and therefore to study specifically the effect of solubility of the polymer in the additive versus the role of the high boiling point of the additive. The phase-segregation and molecular packing of the active layer was investigated using small angle X-ray scattering (SAXS) and grazing incidence X-ray diffraction (GIXD), respectively

In this talk, I will present several recent developments from my group related to organic photovoltaics: (1) New donor polymer design concepts, (2) Tuning molecular packing of small molecules and polymers and their impact on charge transport and exciton transport, (3) Air-stable solution and vacuum processable n-type dopants that can enable electron injection layers and alternative cathodes.

Organic photovoltaic devices are a promising route to lower costs via roll-to-roll manufacturing. The most promising device architecture involves a bulk heterojunction (BHJ) active layer in which nano-scale phase separation into nominally bicontinuous donor and acceptor rich regions enables both exciton dissociation and charge extraction. The performance of BHJ based devices is a strong function of the active layer processing conditions and the optimized device structure is, in general, not the equilibrium structure. Real-time techniques, such as spectroscopic ellipsometry and x-ray scattering at advanced light sources, can provide detailed insights into film thickness, composition, and microstructure. We will discuss highlights from real-time studies of P3HT:PCBM BHJ film formation where mechanisms by which small amounts of solution additives control final polymer crystallinity and nanostructure are revealed. The use of processing additives has emerged as a powerful approach for the fabrication of optimized organic photovoltaic devices. We find that additives that are both non-solvents (1,8-octanedithiol) and solvents (1-chloronapthalene) for P3HT promote improved polymer order, phase segregation, and device performance. In all cases (no additive and additive) the polymer order develops under conditions of extreme super-saturation. Both additive types (good and bad solvents) promote early polymer aggregation with respect to additive free solutions, pointing to the critical role of non-additive effects on solvent quality.

Recently, organic solar cells are very attractive for environmental energy source. Generally, active layer of the cells are bulk heterojunction structure which consists of p-type polymers and n-type fullerene derivatives. The bulk heterojunction or nano-scale phase separation structure of p-type and n-type materials in active layers form exciton dissociation interface and charge transportation path. Therefore the structure is an important factor for photovoltaic characteristics. Nevertheless formation mechanism of the bulk heterojunction structure is still unclear. And optimal process conditions are determined empirically. In order to clarify the formation mechanism of active layers, we performed a real-time observation of aggregation and crystallinity changes of bulk heterojunction structure in the drying process. In this study, a chamber was designed, which allows the control of drying rate throughout the process. Real-time Ultraviolet and Visible (UV-Vis) absorption and grazing incident wide angle X-ray diffraction (GIWAX) during the formation of bulk heterojunction layer were measured simultaneously in the same chamber. GIWAX measurement was performed by the BL19B2 beam line of SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan), equipped with a two-dimensional detector (PILATUS). Spectral changes in UV-Vis absorption were observed during the early stage of the active layer formation. From UV-Vis absorption data, P3HT microcrystals grew through the following processes; aggregation, pai stacking by the main chain plane of P3HT overlapping and twist dissolution of a P3HT main chain. After the aggregation of P3HT, the improvement of crystallinity of P3HT and PCBM were confirmed by GIWAX. The high crystallization of PCBM was observed in a slow drying condition. This technique establishes a relationship between the structure of active layer and photovoltaic properties and improves a photovoltaic performance of organic solar cells.

The morphology of high-performing BHJs is very complex. There is now good evidence that the active layer contains aggregated pure donor polymer, a fullerene/polymer solid solution where the polymer is disordered and possibly a fullerene phase. The aggregation state of the polymer affects energy levels and charge transport. We show that non-diagonal disorder in the polymer aggregates limits charge transport in most high-performance donor polymers. The role of amorphous polymer in charge transport will be analyzed as well. Because disorder affects energy levels, at the donor-acceptor interface it can affect the driving force for exciton splitting. We study a very ordered donor-acceptor interface formed by a bimolecular crystal to measure the coupling of the excited charge-transfer (CT) state to its ground state. Such coupling is directly related to the geminate recombination rate. We will show that the polymer conformation at the interface affects the energetics of the CT state as well.

The shortfall in power conversion efficiency (PCE) of many organic photovoltaics (OPVs) has been loosely ascribed to a combination of physicochemical and device complexities associated with inadequate hole transport mobility, solubility and miscibility with an appropriate acceptor, a narrow electronic band gap for efficient solar light harvesting, appropriate HOMO and LUMO energies to maximize the open circuit voltage (Voc) and electron transfer to the acceptor, and in particular the control of the multidimensional problem of bulk heterojunction (BHJ) morphology. In this talk we provide an overview of some of the recent progress towards implementing theory, modeling and simulation approaches in combination with experimental results from precision synthesis, characterization and device fabrication as a means to overcome/understand the inherent issues that limit higher PCE of particular classes of OPVs. In particular we will discuss work on controlling the donor-acceptor phase separation for promoting more efficient organic BHJ photovoltaic cells by using a poly(3-hexylthiophene)-block-polystyrene (P3HT-b-PS) diblock copolymer as a compatibilizer in a P3HT/ [6,6]-phenyl-C61-butyric acid methyl ester fullerene derivative (PCBM) blend. Quantum density functional theory calculations show that a P3HT-b-PS additive tends to promote microphase segregation, with the PCBM attracted to the PS block, and the P3HT stacking onto the P3HT block, which leads to improvements in long-range “crystallinity”. This is corroborated by grazing incidence x-ray scattering (GIXS), absorption spectroscopy, and carrier mobility studies that demonstrate an improvement in the crystallinity and orientation of P3HT, subsequently leading to enhanced hole transport. Overall there is a better balance between the electron and hole mobilities in the P3HT/PCBM active layer. Neutron reflectometry (NR) reveals a distinct scattering length density profile that shows the highest PCBM concentration in the middle layer region and a more compact and homogeneous layer, likely due to an increase in miscibility of P3HT and PCBM driven by the copolymer compatibilizer. Overall, this work demonstrates the possibility of using block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. If coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

Organic solar cells combine the advantages of low-cost and large scale production and the use of non-hazardous, environmentally friendly materials. Over the last years, the respective power conversion efficiencies improved continuously to more than 10% due to strong interdisciplinary research efforts. Improvements were often driven by the synthesis of new absorber materials that allowed for an enhanced spectral coverage of the solar spectrum or enabled high internal quantum efficiencies. In the lab, the screening and optimization of new materials and cell architectures often follow a trial and error approach though very little amounts of material are available. An alternative and material saving route to optimized organic solar cells is a comprehensive optoelectronic device simulation that reduces the experimental parameter space. This becomes in particular important for sophisticated device architectures such as tandem solar cells. However, in order to carry out meaningful simulations, a profound knowledge of the optical material constants is mandatory. If done properly, the optical model further reveals information about anisotropy within the amorphous bulk heterojunction. In this work we present a general model kit which facilitates the determination of the complex refractive index of various polymer:fullerene-blends. Therefore, the measured spectroscopic values Psi; and Δ of spin-cast films were recorded on a variable angle spectroscopic ellipsometer (VASE®, Woollam) and analyzed with the software WVASE (Woollam). In a first step, the optical constants of the fullerene derivatives PC₆₁BM and PC₇₁BM and various polymers such as poly(3-hexylthiophene), poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-bprime;prime;]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole), poly{[4,4&’-bis(2-ethylhexyl)dithieno(3,2-b;2&’,3&’-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} or poly[N-9&’-heptadecanyl-2,7-carbazole-alt-5,5-(4&’,7&’-di-2-thienyl-2&’,1&’,3&’-benzothiadiazole) (P3HT, PCPDTBT, PSBTBT, PCDTBT) are modeled independently. The absorption of the polymers is described by the superposition of various Tauc-Lorentz and Gaussians oscillators. Their center of mass energies correspond well to the characteristic features in the transmission spectra which were recorded at the very same spot. In a second step, the complex refractive index of the polymer:fullerene-blend was derived from combining the optical models of both constituents within the Bruggeman effective medium approximation (EMA). The mean squared error (MSE) was remarkably low for all fits indicating a good quality. In order to verify the results we calculated the transmissions of all polymer:fullerene blends from the optical constants. The modeled transmissions are in good agreement with the measured data showing the generality of this approach. Results from solar cell experiments are in very good agreement with the predicted material and device properties, demonstrating the generality of this approach.

We present the Harvard Clean Energy Project (CEP, http://cleanenergy.harvard.edu) which is concerned with the computational screening and design of novel organic photovoltaics. CEP has established an automated, high-throughput, in silico framework for the study of millions of potential candidate structures. The current project phase is focused on the relevant molecular properties of these candidates - for small molecule setups but also for polymer building blocks. We utilize first-principles quantum chemistry as well as ideas from cheminformatics, machine learning, and pattern recognition to characterize molecular motifs and assess their quality with respect to applications as electronic materials. In addition to finding specific structures with certain properties, it is the goal of CEP to illuminate and understand the structure-property relations in the domain of organic electronics. Such insights can open the door to a rational, systematic, and accelerated development of future high-performance materials. CEP is a large-scale investigation which utilizes the massive computational resource of IBM&’s World Community Grid (www.worldcommunitygrid.org). This cyberinfrastructure paradigm has already allowed us to characterize 8 million compounds of interest in about 100 million DFT calculations. The results are compiled and analyzed in an extensive reference database (CEPDB) and will be made available for public use shortly. Our presentation addresses the project organization and workflow, data mining, analysis, and scoring, as well as promising molecular motifs which have emerged from CEP so far. For the last point we stress our close interaction with experimentalist collaborators and the role of CEP as a community tool.

Efficient current and voltage generation in organic solar cells depends upon the efficient separation of charge pairs at the donor:acceptor heterojunction and the efficient transport of charges towards the appropriate contacts before recombination occurs. These phenomena depend on the energies and spatial extent of electronic states near the interface and the microstructure of the binary system. We have used a combination of techniques including electroluminescence spectroscopy, to probe the energy of charge transfer (CT) states relative to the energies of excited states in the donor and acceptor components and, together with steady state and time resolved device measurements, to study the relationship between the CT state and photocurrent generation. Through study of a range of polymer:fullerene combinations we show that CT state emission is positively correlated to photocurrent generation and that charge separation is switched off when the CT state energy approaches the energy of the lowest singlet excited state of the donor or acceptor. In some cases where CT emission is observed but photocurrent is poor, we show that an unfavourable microstructure inhibits charge collection. The experimental results are rationalised through theoretical calculations of the CT states and through complementary studies of blend film microstructure and charge transport. Finally, we use our results to discuss the sources of loss in heterojunction solar cells and to speculate on the limits to performance of such devices.

Organic photovoltaics (OPVs) have become an attractive technology that offer a lower cost alternative to current commercial solar conversion technologies due to their potential for low temperature, large-area, and high-throughput manufacturing. Further barriers that must be overcome prior to commercialization lie in the development of OPV materials and device architectures to result in improved efficiency and stability. To achieve this, we are developing unique tools, design rules, and new materials for both active layers and selective contacts. We begin with a high-throughput combinatorial computational method for the in silico design and evaluation of new active layer materials. This allows for the initial evaluation of estimated materials properties and helps to down-select potential materials prior to synthesis and optimization. New materials are subsequently synthesized and characterized to relate back to further improve the estimates from computation. Time-resolved microwave conductivity (TRMC) and other characterization techniques are utilized to evaluate materials while looking for the efficient charge separation, photoconductivity, and long carrier lifetimes that are correlated to high performance OPV active layer materials. This information helps to further down-select the materials that will be optimized in OPV devices. Such integration of theory and characterization help to increase the throughput and success rate for the development new materials. While the active layer materials are important for determining the ultimate performance, interfacial contact layers must be optimized both electronically and chemically for new active layer components. Such interfacial contacts are believed to improve device performance by a variety of mechanisms such as improved energy level alignment and charge carrier selectivity leading to improved charge extraction and reduced recombination. We are investigating the influence of the electronic properties of electron and hole transport layer (ETL and HTL) contacts on device performance to gain a greater understanding of the relative contributions of contact properties such as work function and band alignment. The independent control of work function and band alignment is shown to result in increased performance in both standard and inverted device architectures. Through this integrated approach we are able to actively tune the contact properties to the ever changing properties of new active layer materials. We gratefully acknowledge funding from U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory for OPV device development as well as the Center for Interface Science: Solar Energy Materials (CIS:SEM) and the Center for Energy Efficient Materials (CEEM), Energy Frontier Research Centers Funded by the U.S. Department of Energy, Office of Basic Sciences, under Award Numbers DE-SC0001084 and DE-DC0001009, respectively.

The morphological distribution of donor and acceptor withing active layer of organic solar cells significantly affects the power conversion efficiency of the devices. Morphology is affected by the fabrication conditions. Therefore, to understand how the processing conditions affect the morphology, it is important to define and extract physically meaningful morphology descriptors from morphological data. We develop a computational framework to probe 3D morphology. We develop suite of morphology descriptors based on (a) ideas from computational homology (Betti numbers); and (b) representing discretized structures as colored weighted graphs, to link process and property with the structure. Representing structures as weighted colored graphs provides the ability to leverage highly efficient graph algorithms on very large (2D and 3D) data sets. We show how physically meaningful descriptors like number of components connected to boundaries, volume fractions of components, correlation between phases, percolation length, grain size distribution, tortuosity, among others can be reformulated as graph theoretic queries, with trivially simple implementations. Our approach is generic and flexible and allows to extensively characterize both two and three phase representation of the morphology. We characterize in comprehensive and efficient way various transport properties of exciton and charges of various morphologies. We also interrogate both two and three phase morphologies.

Designing higher efficiency and more stable organic solar cells will require new materials. Computational chemistry offers the opportunity for large combinatorial sifts through potential structures without the time and expense of physical synthesis. We have developed heavily automated methods based on electronic structure calculations to probe the material parameters that control charge dynamics in these materials. In order to study charge transport in the weak coupling limit we calculate electronic transfer integrals explicitly for representative molecular assemblies generated by atomistic molecular modelling. We study the inner sphere reorganisation energies of relevant existing and hypothesised molecules, including heterocycles, non-benzenoid aromatics and non-fullerene acceptors to identify areas of possible future synthesis. We treat energetic disorder by combining a molecular dynamics approach with a tight-binding model for some classical conjugated polymers, generating densities of states which can be compared with experiment. These studies indicate that torsional freedom of the backbone is the dominant source of energetic disorder. In order to study photo-excited states and the mechanism of charge separation, Singlet and Triplet excited states are investigated with Time-Dependent (TD) Density Functional Theory (DFT) and compared to transient absorption data for newly synthesised push-pull polymers. We then apply Generalized Kohn-Sham (GKS) DFT with the ab-initio tuned BNL meta-hybrid range-separated functional to probe the nature of charge generation at the Donor-Acceptor (DA) interface in the organic solar cell and the role of the charge transfer state. The self-consistent tuning of the functional allows us to go correct for failings in standard hybrid functionals which are particularly important in charge-transferring systems such as the DA interface. We apply our results in suggesting rules for the rational choice of chemical structure in the synthesis of new materials.

An important question in organic solar cells is the mechanism of free charge carrier generation. In organic bulk heterojunction solar cells, photoinduced electron transfer between donor and acceptor molecules or polymers populated a charge transfer (CT) state at the donor-acceptor interface. The residual electrostatic binding energy between electron and hole in the CT state may be considerable and is much higher than the thermal energy. Despite the poor screening of the Coulomb potential, free charges are often created efficiently. The mechanism by which the large energy barrier for charge separation from the CT state is overcome within the short lifetime is presently not clearly understood. We investigate the influence of excess energy, i.e. the difference between photon energy and the CT-state energy, on charge separation and recombination processes in a variety of conjugated polymer:fullerene blends by employing external quantum efficiency (EQE) measurements as function of photon energy and applied bias voltage, by using photoinduced charge extraction in a linear increasing voltage (photo-CELIV) experiments, and by a kinetic Monte Carlo model. For the well-known P3HT:PCBM blend we find that excess energy liberated in free charge generation by photons with energy far above the energy of the CT state, has no significant effect on the charge generation and recombination process for both as-cast and thermally annealed P3HT:PCBM morphologies. This indicates that in P3HT:PCBM blends a large excess energy in the electron transfer step, is not a strict requirement for free carrier generation. In contrast, the electrical bias dependence of the EQE for the small band gap polymer PCPDTBT:PCBM blend shows that there is a small but significant effect of the excess energy on the charge carrier generation. This result can be interpreted as a change of the electron-hole separation distance in the CT state that can be generated at different photon energies. In the related small band gap Si-PCPDTBT:PCBM blend, this effect does not occur and hence it is not principally related to smaller energy loss from optical band gap to CT state in these small band gap systems. To rationalize the result we use a kinetic Monte Carlo model. The Monte Carlo model only contains parameters that are determined from independent measurements and predicts dissociation yields in excess of 90% for a prototypical heterojunction. In agreement with our experiments, charge separation in good heterojunctions shows little dependence on temperature or electric field. The Monte Carlo calculations demonstrate that the energetic disorder that is characteristic for organic semiconductors provides an internal energy reservoir that can be used to split the Coulombically bound electron-hole pair. The Monte Carlo simulations further suggest that it is possible to reach high dissociation yields at low energy loss, opening a route to high efficiency solar cells.

Lei Yang, Yang Shen, Leif Häggman, Erik Johansson, Anders Hagfeldt, Nick Vlachopoulos* Uppsala University, Department of Chemistry-Ångström Laboratory, Box 523, SE 751 20 Uppsala, Sweden Alan Snedden, Lars Kloo Royal Institute of Technology, Division of Inorganic Chemistry, S-10044 Stockholm, Sweden Amani Chams, Christian Perruchot, Mohamed Jouini University Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086, F-75205 Paris 13, France In recent years solid-state dye-sensitized solar cells (DSCs) have attracted a considerable interest[1]. However, the variety based on conducting polymers has not been extensively studied as compared to that containing low-molecular weight hole conductors (e.g. Spiro-OMeTAD). In this study we described a type of DSC based on PEDOT generated by in-situ photoelectrochemical polymerization in a 3-electrode cell from bis-ethylenedioxythiophene (bis-EDOT) monomer in organic solvent. The potential is maintained at a constant value vs. a suitable reference electrode[2]. The advantage of polymers generated this way as compared to pre-formed polymers or low-molecular weight hole conductors is the easy infiltration of bis-EDOT precursors into the mesoporous electrode consisting of dye-sensitized titanium dioxide. After polymerisation, the modified electrode was treated with organic solution containing LiN(CF3SO2)2 and tert-butyl pyridine for optimisation both short-circuit current and open-circuit potential. The counter-electrode is a layer of vacuum-evaporated silver directly on the polymer surface. The sensitizer is an organic charge-transfer dye (D35) based on triphenylamine conjugated to a cyanoacrylic acid anchoring group. Using this mediator-free hybrid organic/inorganic fully solid-state device, efficient photovoltaic operation has been demonstrated under simulated AM1.5 globle light conditions with good linearity of photocurrent vs. light intensity and open-circuit cell photopotential exceeding 0.6V. The performance was stable over several days without sealing. Electrochemical, current-potential, photocurrent spectroscopy, and optoelectronic (toolbox) measurements will be presented and discussed. References [1] Yang, L.; Cappel, U. B.; Unger, E. L.; Karlsson, M.; Karlsson, K. M.; Gabrielsson, E.; Sun, L. C.; Boschloo, G.; Hagfeldt, A.; Johansson, E. M. J.: Comparing spiro-OMeTAD and P3HT hole conductors in efficient solid state dye-sensitized solar cells. Physical Chemistry Chemical Physics 2012, 14, 779-789. [2] Manseki, K.; Jarernboon, W.; Youhai, Y.; Jiang, K. J.; Suzuki, K.; Masaki, N.; Kim, Y.; Xia, J. B.; Yanagida, S.: Solid-state dye-sensitized solar cells fabricated by coupling photoelectrochemically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) with silver-paint on cathode. Chemical Communications 2011, 47, 3120-3122.

For organic solar cells to successfully enter the market it is highly advantageous that attributes unique to these devices including lightweight and flexibility are exploited. However, devices under flexure may mechanically fail in a number of ways including fracture or delamination of the constituent layers. To take full advantage of the flexible nature of these materials, the mechanical behavior of organic solar cells needs to be fully characterized and optimized. To date, mechanical studies of organic solar cells have focused on the commonly used, but brittle, indium tin oxide transparent electrode and alternatives are currently being developed. The mechanical behavior of the active organic semiconductor layers on the other hand are often overlooked, but will also play a vital role in the success of flexible organic solar cells. In this study, we develop correlations between the active layer film morphology, device performance, and mechanical ductility and stiffness of the active layer. We focus on the widely studied Poly(3-hexylthiophene) (P3HT): Phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) solar cell. The morphology of the BHJ film is controlled through a combination of spin cast speed and solution concentration and the resulting films are characterized using optical spectroscopy methods. The stiffness is investigated using a buckling based metrology. This method consists of placing the film of interest on a Polydimethylsiloxane (PDMS) substrate and compressing the film stack resulting in a buckled film from which a number of mechanical characteristics can be determined including the elastic modulus. The crack onset strain is also investigated by straining the BHJ film in tension while on a PDMS substrate. We find that mechanical behavior of BHJ film is highly dependent on the film morphology. More specifically, typical processing methods commonly used to optimize the P3HT and PCBM morphology also make the film harder and more brittle. For example, we find that the crack onset strain can range from roughly 2% to over 80% depending on processing methods. The elastic modulus is also shown to have synergistic behavior, and deviates further from the rule of mixtures with increasing device efficiency. Finally, we discuss processing strategies that maximize mechanical and optoelectronic device performance.

Nanocrystalline inorganic absorbers are potentially attractive alternatives to the molecular dyes traditionally used to light sensitize mesoporous metal oxides in solid-state dye-sensitised solar cells (DSC). It is hoped that by judicious material choice and optimisation of cell design, properties such as tuneable absorption profiles and large extinction coefficients can be exploited to develop high efficiency solid state semiconductor-sensitized solar cells (SSSCs). Recently, Sb2S3-based systems have shown particular promise, with class-record energy conversion efficiency in excess of 6 % reported. However, and despite such impressive performance, detailed studies of the charge photogeneration processes that occur at the operational heart of these devices have been limited. It is expected that an improved understanding of such effects will encourage and expedite development of the design rules and structure-function relationships necessary for improved Sb2S3 device performance. With this in mind, we use time-resolved absorption spectroscopy to consider key factors influencing charge separation across metal oxide/Sb2S3/hole transport material heterojunctions and the subsequent implications for the design of efficient Sb2S3-based devices. Specifically, we find that nanocrystal size and annealing temperature can have a significant effect on the yield of long-lived charge separation and we highlight the fundamental importance of fast hole transfer.

Quantum confinement of photo-generated electrons and holes in colloidal semiconductor nanocrystals is well understood and results in a wide array of potentially important properties. Application of nanocrystals in photovoltaics requires rapid and efficient generation of separated charges, which must be transported away from the chromophore to do useful work. Interactions between excitons and surface-localized trap or ligand states are key to understanding the rate and efficiency of electron transfer in nanocrystals. We present the result of recent studies to determine, for the first time, the number and binding strength of passivating ligands on a quantum dot. We illustrate how the local environment can change the ligand coverage and determine how the number of ligands and their bond energies affects quantum dot fluorescence decay dynamics. Previous studies have determined binding strengths of ligands that strongly quench quantum dot fluorescence. Uniquely, we have been able to extract these properties from samples containing only "native" surface passivating ligands that are introduced during the quantum dot synthesis. We describe experiments that combine isothermal titration calorimetry (ITC) with time resolved photoluminescence spectroscopy (TRPL) to model surface interactions. ITC is a relatively common technique to model the heat released or absorbed upon ligand binding in biological samples. We have adapted this technique to CdSe quantum dots and trioctylphosphine oxide (TOPO) ligands and developed a multi-site binding model to extract equilibrium constants, binding enthalpies and ligand coverage from our ITC data. TRPL decays reflect both radiative exciton recombination rates and non-radiative transitions rates to extrinsic surface or ligand states; however, interpretation of fluorescence transients is not trivial and typical multi- or stretched exponential decay models yield little specific photophysical insight. We show how the thermodynamic parameters from ITC studies can be used to build a kinetic scheme to directly analyze TRPL decays, thereby illustrating how ligand type and coverage can be used to control electron transfer and recombination dynamics in nanocrystals.

The power conversion efficiencies of organic conjugated polymer-based photovoltaic technologies have been consistently increasing towards competitive values in recent years. However, short device lifetimes (~1 year) and relatively high $/Watt fabrication costs due unproven large-scale production are still significant hurdles to the successful commercialization of polymer-based organic photovoltaic devices. With regard to devices lifetimes, while the polymer semiconductor materials themselves can be inherently stable (encapsulated polymeric light-emitting diodes can exhibit device lifetimes of more than 10 years), the typical electrode materials such as poly(3,4-ethylenedioxythiophene) (PEDOT), aluminum, silver and indium-based transparent conducting oxides are major contributors to photovoltaic device degradation over time due to changing electrode performance and work-function while the devices are operated in sunlight. A particular device configuration that is receiving much attention due to the promise of significantly improved device lifetimes is the inverted bulk-heterojunction (BHJ) device. In this configuration, the metallic electrode is employed as the anode allowing more stable, high-work-function metals or metal-oxides to be employed, and unstable PEDOT layers can be replaced with more stable lower work-function materials such as zinc oxide or tin oxide. Recently we have shown that incorporation of nanostructured noble metal electrodes that support surface plasmons into an inverted BHJ device configuration can also increase light trapping in the conjugated polymer active layer. As a result, we have demonstrated absorption enhancement factors between 5 and 10 in the 570 to 750 nm wavelength range at the red of the polythiophene conjugated polymer active layer absorption band. However, to translate the enhanced absorption and improved stability of the inverted plasmonic BHJ configuration into useful photovoltaic device performance, the BHJ active layer must be deposited onto the nanostructured metal electrode and, subsequently, a transparent electrode must be applied to the active layer. This route contrasts with typical fabrication routes that apply the metal electrode layer a last step in the fabrication process. We will discuss our recent work on the application of transparent electrode layers directly to hydrophobic conjugated polymer-based BJH active layers prepared on nanostructured metal electrodes. The transparent conducting electrode layers we employ consist of solution-deposited zinc oxide nanoparticles, silver nanowires and PEDOT in both single and multilayer formats. Various solvent pre-treatments of the active layer are carried out to facilitate uniform transparent electrode coverage. The sheet resistance, transmissivity and surface structure of the solution-deposited transparent electrode layers applied to the inverted plasmonic BHJ device configuration will be described as a function of electrode processing.

To tune the HOMO and LUMO energy levels via alternating donor-acceptor monomer units, we investigated a series of CPs in which the electron withdrawing power of the acceptor group and the electron giving power of the donor group is varied, while maintaining the same conjugated chain conformation. We observed that the introduction of electron withdrawing groups lowers the LUMO level, while keeping the HOMO level almost unchanged. Conversely, inserting the electron donating groups raises the HOMO level while maintaining the LUMO level unchanged. According to these trends, designing a low band gap polymer requires strong donors and acceptors. Using first-principles calculations we investigated underlying reason. Charge localization on the electron-rich and electron-poor segments in CPs plays a key role. We identified strong correlations between the withdrawing strength of the acceptor group, the HOMO and LUMO levels, and the degree of orbital localization, which allows us to derive reliable design principles for CPs.

A systematic study on the influence of the alkyl side-chain length of C60 based fullerene derivatives in BHJ solar cells using an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) polymer (AnE-PVstat) was conducted. It is observed that the alkyl side-chain length controls the molecular interaction between polymer and fullerene resulting in strongly varying photovoltaic parameters. The most pronounced dependence on the side-chain length is found for the fill factor, spanning the range between 50 - 72%, which, dominantly controls in combination with the short circuit current, substantially defines the power conversion efficiency. The best donor/acceptor mating led to a maximum performance of 4.8%, surpassing PCBM based blends.

We studied the blending effect of differently side-chain substituted anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) copolymers (AnE-PVs) in the bulk heterojunction (BHJ). We demonstrate that this methodology is beneficial for solar cell performance compared to single polymer based donor/acceptor blends. A strong impact on photovoltaic parameters was observed. The presented system of polymer:polymer blends of an amorphous and a crystalline polymer demonstrates the importance of phase separation capability (polymer/polymer and polymer/PCBM) concerning charge generation, separation and percolation in BHJ solar cells. Hence this model system may contribute another puzzle piece in the deeper understanding of fundamental requirements in the design of highly efficient polymer/fullerene BHJ solar cells.

Morphology is a key parameter in optimizing organic solar cells based on polymer/fullerene blends. The last years have seen a tremendous progress in controlling the size of the phase segregated domains constituting the bulk-heterojunction. Among the different approaches, tempering, solvent annealing, solvent mixing and additives have been established as reliable methods to obtained the desired morphology. A more challenging structural aspect is the control of vertical morphology, while being of large relevance since carrier percolation occurs in a direction perpendicular to the films. In ideal structures a thin region rich in the hole transporting conjugated polymer should be close to the anode, while a fullerene rich one close to the cathode. Such structure may drive more efficient charge separation, minimize non-geminate recombination and improve efficiency. In this communication we present a method to prepare vertically structured polymer/fullerene solar cells from solution. Initially a pure solution of the polymer is deposited on the anode and made partially insoluble by annealing. Subsequently, a solution containing the same conjugated polymer and the fullerene derivative PCBM is spin coated on top. We have prepared solar cells with such structure for several combinations of polymer/PCBM, namely P3HT/PCBM, Si-PCPDTBT/PCBM and PTB7/PCBM. For all sets of cells we report improved efficiencies by more than 10%, which are mainly due to larger short circuit current and better fill-factor. Using a Shockley-Read-Hall based electrical model and a transfer matrix based optical model [1,2] we demonstrate that the increase in efficiency is due to 1) the polymer layer physically separating electrons and holes thus reducing recombination losses ; and 2) an increase in shunt resistance. Our results demonstrate a proof of principle easily implementable for different solution processed solar cells, with important implications for device optimization. References [1] Roderick C. I. MacKenzie, Christopher G. Shuttle, Michael L. Chabinyc, Jenny Nelson Adv. Energ. Mater. 2, 662 (2012) [2] Roderick C. I. MacKenzie, Thomas Kirchartz, George F. A. Dibb, and Jenny Nelson, J. Phys. Chem. C, 115, 9806 (2011)

Singlet exciton fission is the spin-allowed process whereby a molecule in the singlet excited state gives rise two two triplets. When combined in a heterojunction with a low band gap semiconductor such as PbSe nanocrystals, the ionization of these two triplets reduces thermalization losses in photovoltaic device, and may lead to low-cost photovoltaic devices with efficiencies above the Shockley-Queisser limit. While there has been much progress toward understanding the mechanism of singlet fission, some parameters (such as the dependence of singlet fission on film morphology or local ordering) are not well characterized. We present a mechanistic study of the rapid singlet fission in solution. The isotropic environment constitutes a model system in which to obtain mechanistic insights, and we assign an intermediate multichromophoric state to the intermediate leading to singlet fission.

Organic photovoltaics are an important subset of the solar cell industry and show promise in applications such as flexible and portable electronics, textiles, and coatings. Much research has been done to characterize the fundamental electronic properties of the solar cell active layer in order to facilitate better design and control of devices and products. In our work, the recently developed technique photo-induced charge extraction by linearly increasing voltage (photoCELIV) is used to measure a series of bulk heterojunction devices with a common polymer-fullerene pair, poly(3-hexyl thiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM61). We compare variations in film nanoscale morphology and microstructure to variations in charge carrier mobility and the rate of bimolecular recombination. These results may add insight into processing-structure-function relationships for organic photovoltaics that many researchers are pursuing.

We have characterized the morphology of mixtures of a germole-containing polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PGeBTBT), and PC71BM using Energy-Filtered Transmission Electron Microscopy (EFTEM), Resonance Soft X-ray Scattering (RSOXS) and Grazing Incidence Small Angle X-ray Scattering (GISAXS). PGeBTBT is analogous to the silole-containing polymer, PSBTBT, with a similar band gap of 1.47 eV. Sulfur elemental maps generated from EFTEM images show PGeBTBT forms a highly fibrillar structure with fibers on the order of 10 nm diameter. Complementary photovoltaic device characterization yielded high short circuit current densities of 11.2 mA/cm2 and 18.6 mA/cm2 for 65 nm and 150 nm thick films, respectively. This highly fibrillar structure of PGeBTBT/PCBM mixtures, with a probable high interconnectivity and with length scales smaller than typical poly(3-hexylthiophene)/fullerene mixtures, is conducive for efficient device performance.

Carbon nanotubes (CNTs) have large specific surface area and exceptional physical properties, such as high elastic modulus, unique longitudinal electrical and thermal conductivities. It is expected that CNTs and P3HT composite could be used in an organic photovoltaic device to yield more efficient charge transport properties and higher charge mobility in CNTs compared with fullerenes. A simple dry polymer deposition for CNT-forest is realized by photon irradiation from a 172 nm wavelength lamp. Simultaneous surface activation of CNTs, depolytmerization of P3HT, and uniform deposition and re-polymerization onto the activated CNTs are achieved by exposure of CNT samples and P3HT target to the vacuum ultraviolet lamp. P3HT is deposited onto the surface of CNTs in the CNT-forest in a N2 environment for 5 minutes. Photoluminescence spectra of the pure P3HT and the resulting coating/CNT composite show exactly the same emission peak suggesting the P3HT is indeed transferred onto the CNTs. X-ray energy dispersive spectroscopy (EDS) for a single CNT with the coating also confirms the elemental deposition of the P3HT. Molecular dynamic simulation involving a single CNT and P3HT polymer provides additional elucidation of the dynamic interaction in the coating process.

Materials like carbon nanotubes (CNT) have attracted much attention all around the world due to their exceptional electrical, mechanical, and chemical properties. CNT have been used in organic photovoltaics (OPV) as acceptor material, to enhance charge carrier transport within organic layers, and as transparent electrodes. Sometimes the J-V characteristic of OPV cells made of polymer ? fullerene - blends show an s-type curve in the third or fourth quadrant. This is mainly based on the injection/ejection properties of the electrodes. When CNT are added to these kinds of polymer polymer ? fullerene - blends the s-shape curve occurs in the first quadrant rarely. This J-V characteristic can be explained by band alignment of this blend, which enables tunnelling and the device probably works as backward-diode. We studied the device performance of different material combinations of CNT, regio regular Poly(3-Hexylthiophen-2,5-diyl) (rr-P3HT), Phenyl-C61-butric acid methyl ester (PCBM) and copper phthalocyanine (CuPc). Different device architectures were investigated and compared with each other.

Oxide contact layers are important alternatives to PEDOT:PSS in high-performance, stable bulk heterojunction (BHJ) organic solar cells. Low-temperature, solution-processed molybdenum oxide (MoOx) has recently been shown to form efficient BHJs solar cells with significantly-improved electrical stability.[1] These contact layers can have a major impact on the morphology and optoelectronic characteristics of BHJ solar cells. We have determined the impact of MoOx interlayers on the power conversion efficiency of P3HT:PCBM solar cells as a function of processing conditions. Using a combination of novel transient photocurrent measurements and morphological characteristics, we have connected the transient behavior with the vertical phase composition of the polymer:fullerene active layer determined by dynamic secondary-ion mass spectroscopy (DSIMS). Drift-diffusion simulations incorporating band tails and recombination, constrained by the experimentally-determined vertical composition profiles, reproduce the measured current-voltage responses of the cells. Our findings serve as an example of how non-idealities in solar cell behavior, manifest in a standard steady-state current-voltage curve, can be understood by a combination of active-layer composition and transient photocurrent investigations. We believe our approach can be generalized to a variety of other organic and hybrid photovoltaic systems. [1] Jasieniak, J. J.; Seifter, J.; Jo, J.; Mates, T. & Heeger, A. J.; A Solution-Processed MoOx Anode Interlayer for Use within Organic Photovoltaic Devices, Adv. Funct. Mater., 2012, 22, 2594-2605

The record efficiencies of organic tandem solar cells recently exceeded 10%. The key to further efficiency improvements will be a smart choice of complementary absorbing materials for both subcells within a properly designed device architecture. As the device absorption is ruled by thin film interferences and as the matching of currents in both subcells is of major importance in order to achieve best device performance, the thicknesses of each layer in the device have to be optimized carefully. However, experimentally optimizing numerous functional layers in a tandem device on a sample-by-sample basis is a very time and material consuming task. In this work we present a facile route to an efficient experimental screening of new materials and layer thickness optimization in solution processable single junction and tandem polymer photovoltaic devices. Therefore, we developed a method of fabricating solar cells with a graded active layer thickness. Spatially resolved mapping of the solar cell short circuit current density for different light absorbing polymers under white light allows for a quick conclusion about the optimum active layer thickness within the device. When plotting the short circuit current density versus the local active layer thickness, the influence of the thin film interference pattern within the device becomes apparent. The measured short circuit current densities are in very good accordance with optoelectronic device simulations for all layer thicknesses. For single junction devices, this technique allows for a fast material-saving functional layer thickness optimization. In tandem solar cells, this technique is an efficient tool for matching both subcell currents with only little experimental effort.

The interface between polymer-rich and fullerene-rich domains in bulk heterojunction solar cells is a critical feature of the active layer. Since photoexcited charge separation and free carrier recombination occur at these boundaries, the interface electronic and structural properties greatly influence device performance. Herein, we report on orientational patterning at the donor/acceptor interface observed with polarized soft x-ray scattering that depends on composition and correlates to device performance. Patterning is found to occur in the amorphous regions of the polymer-rich matrix next to fullerene-rich dispersions. We demonstrate that patterning is sensitive to changes in the chemical structure of the polymer using a weak donor, strong acceptor polymer with naphtho[2,1-b:3,4-b']dithiophene (NDT) and 4,7-di(thiophen-2-yl)benzothiadiazole (DTBT) alternating units as an example. For this polymer, backbone substituent atoms of either fluorine or hydrogen cause the backbone to orient face-on or edge-on/tail-on with respect to the fullerene-rich dispersions, respectively. This orientation in-turn correlates with device performance and could be a property of donor/acceptor interfaces and bulk heterojunction morphologies emerging as a critical parameter in general.

Zinc oxide (ZnO) is important to organic electronic devices for its use as an electron transport layer (ETL) and as a high-mobility component of the active layer in hybrid devices. However, the organic/inorganic interface is not optimal in terms of interfacial surface energies and energy level alignment. To address these issues, organic monolayers have been used to modify the ZnO surface properties. Organic monolayers can be used to tune the energy level alignment at an interface by introducing dipolar molecules that change the work function of the ZnO surface. In this study, we have adjusted the work function by mixing two surface modifiers with the same triethoxysilane attachment group but different dipolar terminal groups. Infrared spectroscopy showed that relative surface concentrations were the same as the relative bulk concentration in the deposition solutions. Kelvin probe measurements indicated that the work function of the ZnO surface could be tuned by over 500meV. Inverted bulk heterojunction devices were fabricated to see how the modified ZnO cathode affected device performance. The open circuit voltage (Voc) demonstrated a linear correlation with the work function with a slope of -140mV/eV while the short circuit current (Jsc) didn&’t change. All treated devices showed a double diode or S-kink in the J-V curve. Light soaking minimized the double diode while also significantly reducing the series resistance. Overall, we find that the work function of the ZnO surface can be continuously adjusted using mixed dipolar organic monolayers and that the device performance varies correspondingly. This flexibility can be exploited to engineer the ZnO-organic interface in all types of hybrid electronic systems. We acknowledge support of the NSF through grant DMR-0907409 and the Renewable Energy Materials Research Science and Engineering Center (REMRSEC).

Broadband light absorption enhancement is numerically investigated for the active light harvesting layer of an organic photovoltaic (OPV) which consists of a blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Different top and/or bottom plasmonic metallic nanostructures consisting of Ag or Al or Au, or a combination of two different metals are considered. A spectrally broadband, polarization-insensitive, and wide-angle absorption enhancement is obtained using a double plasmonic metallic nanostructure. This increase in the absorption of P3HT:PCBM is attributed to the combined excitation of localized and single-interface surface plasmon polariton modes.

Nanostructured organic photovoltaic (OPV) devices represent a promising route toward lightweight, large area solar energy conversion. While factors that affect short circuit current density in OPVs have been widely studied, those on the open circuit voltage (Voc) have not been examined to the same extent. Under ideal conditions, the maximum Voc is determined by the energy offset between the donor HOMO and acceptor LUMO levels. However, factors such as electric field distribution, recombination, and blend morphology can lower Voc. Surface photovoltage measurement is a powerful noncontact technique to characterize photoinduced charge separation at semiconductor surface and interfaces. In the context of OPV devices, surface photovoltage measurement probes the steady state charge separation of the photogenerated carriers under open circuit conditions. In this experiment, we examine the effects of thermal treatment and interfacial contact layers on the surface photovoltage signal and spectra of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction blends, and correlate the results with OPV device performance. First, we study thermal annealing of a conventional OPV device after electrode deposition (also known as post-annealing), which has been shown to drastically improve all parameters in an OPV device. The improvement has been attributed to changes in vertical composition change in the blend arising from the interaction between PCBM and Al. We compare the properties of the active layer underneath the Al electrode versus the active layer exposed to ambient using atomic force microscopy, scanning Kelvin probe microscopy, and surface photovoltage spectroscopy to probe the relationship between morphology change and Voc. Second, we find that adding interfacial contact layers above and below the bulk heterojunction increases surface photovoltage and Voc. But the surface voltage change depends on whether the electron or hole transport layer is deposited on top of the bulk heterojunction, which may explain the difference in the measured performance between conventional and inverted OPV devices. Surface photovoltage of other systems such as a low bandgap polymer blend with and without additives will also be presented.

One of the major barriers to the use of organic photovoltaics as an alternative energy source is cost. In particular, there is a growing need of materials and processes for low-cost, flexible and transparent electrodes due to the increasing price and brittleness of indium tin oxide (ITO), the material which is most widely used as a transparent electrode. Recently, inkjet-printing is being increasingly explored as an alternative to common solution processes such as spin-coating and doctor-blading because it is a localized deposition process which wastes little materials and offers the prospect of low-cost large-area manufacturing. To date, this printing process has been applied to pattern mainly an active layer of organic solar cells or an inter layer. In this presentation we report the fabrication and measurement of ITO-free inkjet-printed organic solar cells with high power conversion efficiency. Highly conductive Poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) solutions with polymer concentrations of 0.33% and 0.44% are inkjet-printed to make uniform films of ~100 nm and ~120nm thickness on glass. 5% of dimethyl sulfoxide and 0.1% fluorosurfactant are added into the solutions to increase conductivity and decrease surface tension respectively. The printed films have sheet resistance of < 100 Omega; per square with >90% transmittance at 550nm. Using the printed PEDOT:PSS films as anodes, ITO-free organic solar cells are fabricated with spin-coated Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] : [6,6]-Phenyl-C70-butyric acid methyl ester (PCDTBT:PC70BM) layer and thermally evaporated aluminium on top. The inkjet-printed solar cells exhibit a power conversion efficiency of 4% under air mass 1.5 global (AM 1.5G) irradiation of 100 mW cm-2. In comparison, solar cells made with ITO-coated glass produce an efficiency of 4.5%. The device series resistance extracted from J-V slope at open circuit is 15-17 Omega; for ITO-free solar cells and 19 Omega; for ITO-containing solar cells. The inkjet-printed transparent electrode presents a viable option for the replacement of ITO material and open new opportunities to make cost-effective photovoltaic cells. .

Generally, the electronic and morphological properties of a conjugated polymer can be designed by two constituting components: the conjugated backbone and the side chains. The backbone dictates the most important property of the conjugated polymers, the energy levels. This energy control can be modulated by the use of an electron-rich “donor” moiety and an electron deficient “acceptor” moiety, and can lead a low band gap. On other hand, side chains have an important role in improving the molecular weight and the processability of the polymer. Moreover, these side chains can adjust intermolecular interactions and enable the formation of a desired morphology. Recently, several types of low band gap polymers have been developed using the concepts briefly described to better harvest the solar spectrum . These polymers are designed to make use of internal charge transfer (ICT) from an electron-rich unit to an electron-deficient moiety within the repeating unit. The ICT intrinsic gives a strong double-bond characteristic between the repeating units, allowing a more planar configuration that facilitates the π-electrons delocalization. The work presents the synthesis and characterization of a series of conjugated copolymers D-A type, where the molecular structure is designed to have a strong and broad absorption compatible with the solar spectrum to maximize the exciton generation. Two structures where synthesized, fixing the donor segment (fluorene) and changing the acceptor moiety (benzothiadiazole and benzoselenadiazole). The insertion of selenium into the backbone produced a decrease of Eg and a broadening of the absorption spectrum that possibly will have an impact in the solar cell efficiency. Work supported by: Grupo de Polímeros Bernhard Gross, LaPPS and FAPESP.

End-functionalizing semiconducting polymers can improve their performance in solar cells [1,2]. End-functionalization changes interactions between components allowing an opportunity to better control the morphology of the bulk heterojunction (BHJ), which plays a crucial role in the efficiency of organic photovoltaics (OPVs). We have advanced this concept using an optically and electronically active end-cap molecule that also improves synthetic control. The use of an active end-cap creates an effective ternary blend BHJ with a controllable morphology. As a model system, the low band-gap polymer PDPP2FT [3], was end-capped with the chromophore perylene diimide (PDI), which has absorption from 450 to 650 nm, filling in the absorption gap of a BHJ between PC₆₁BM and the polymer. We fabricated BHJ solar cells with polymers with ratios of DPP2FT monomers to PDI end-caps ranging from 7:1 to 20:1 and found >4% efficiency with good fill factors. In order for an end-cap to perform well in a solar cell, the energy levels must be aligned between the polymer and PCBM energy levels to create a cascade effect. Despite the proximity of the HOMO of the PDI to that of PCBM, it does not act as a significant trap in the BHJ. Time-of-flight measurements will be presented that elucidate the changes to the density of states and electronic properties caused by the addition of the PDI end-caps in these BHJs. [1] B. Lim, J. Jo, S.-I. Na, J. Kim, S.-S. Kim, and D.-Y. Kim, J Mat. Chem., 2010, 20, 10919, 2010. [2] Y.-H. Chen, P.-T. Huang, K.-C. Lin, Y.-J. Huang, and C.-T. Chen, Org. Electron., 2012, 13, 283-289. [3] C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee, and J. M. J. Fréchet, JACS, 2010, 13215547-9.

Solution deposition using high boiling point additives provides a simple and widely used fabrication option for improving the power conversion efficiencies of solar cells comprised of conjugated polymer donor/fullerene acceptor blends. Previous examination of the resulting device active layer directly after spin casting have shown that the use of additives induces nucleation of polymeric crystallites within two minutes of deposition as well as promotes further crystallite nucleation over a prolonged period of time. Here, we follow how additives affect the evolution of the bulk heterojunction morphology from the solution state all the way to the solid state. It was recently discovered that by inserting furan moieties in the backbone of the conjugated polymers the use of relatively small solubilizing side chains is enabled because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents. These polymers enabled us to tune the side chain functionality and therefore to study specifically the effect of solubility of the polymer in the additive versus the role of the high boiling point of the additive. Using small angle X-ray scattering(SAXS) on the polymer and fullerene components in solution we are able to observe the effect of the additive on solution phase confirmation and aggregation properties. In order to link the solution phase behavior to the final film phase segregation and molecular packing, the active layer was investigated using SAXS and grazing incidence X-ray diffraction (GIXD) on as-deposited films.

Photovoltaic devices that employ organic semiconductor active layers typically have low power conversions (~10%) compared to traditional photovoltaic devices that employ inorganic active layers (~25%). One reason is that the absorption band width of organic semiconductors ranges from 100-200 nm, which limits the percentage of the sun&’s photons that can be harvested and converted to useful energy. Recently tandem devices incorporating multiple conjugated polymer layers, each with different band gaps, have been demonstrated with a resulting increase in power conversion efficiency due to broader overall light absorption. Here, we present a study on the development of a method to obtain a broad absorption band with multiple organic semiconductors in a single thin film ternary blend and show efficient energy transfer between them. This involved preparing a ternary solution of the following p-type organic semiconductors in chloroform: poly(9,9-dioctylfluorenyl-2,7-diyl), poly(3-hexylthiophene), and 2,3,9,10,16,17,23,24-Octakis(octyloxy)-29H,31H-phthalocyanine, and subsequent deposition via spin coating. Spectroscopic analysis of each material yielded absorption peaks at 365 nm, 530 nm, and 701 nm with widths of ~100-200 nm, respectively, and the composite thin film ternary blend yielded an absorption peak centered at 530 nm with a width of ~450 nm. Additionally, photoluminescence measurements showed significant quenching of the donor organic semiconductor fluorescence in the presence of the acceptor organic semiconductor molecules, indicating that Förster resonance energy transfer occurred with efficiencies approaching 100%. This work offers a method to enhance the light harvesting capabilities of organic semiconducting thin film active layers. Adding n-type organic molecules to this p-type ternary blend system for bulk heterojunction photovoltaics, and studying the energy transfer properties in the resulting active layer, will allow us to verify whether this approach can lead to broad band solar energy conversion.

The development of new organic semiconducting materials for organic photovoltaic applications has been the focus of considerable research in the past several years. One of the most highly investigated materials are metal phthalocyanines because of their low energy gaps, high extinction coefficients, and high hole mobilities. However, the poor solubility of metal phthalocyanines has confined them to vapor deposition methods for the solar cell device fabrication. Azatetrabenzoporphyrins (Specifically, 6-Aza-13,20,27-triphenyltetrabenzoporphyrin, 6,13-Diaza-20,27-diphenyltetrabenzoporphyrin, 6,20-Diaza-13,27-diphenyltetrabenzoporphyrin, and 6,13,27-triaza-20-phenyltetrabenzoporphyrin, in which some the nitrogen atoms at 6, 13, 20, 27 positions of the phthalocyanine ring are replaced by carbon atoms) are isoelectonic isomers of phthalocyanines. These materials have broad and strong absorption, high hole mobility, and most importantly, the possibility of modifying the structure to improve the solubility in some common solvents, making it promising candidate as solution processable donor materials for organic photovoltaic applications. In this presentation, we will introduce the synthesis and characterization of our designed Azatetrabenzoporphyrins based small molecules and polymers and their application in solution processable bulk-heterojunction solar cells. By improving the solubility we can use various acceptor materials such as PCBM rather than C60 in order to get higher open-circuit voltage while maintaining high short circuit current and fill factor to get a higher power conversion efficiencies (PCE) compared to the vapor deposited devices.

Impressive developments have been made in the field of polymer bulk-heterojunction photovoltaics (OPV) in recent years. Single junction power conversion efficiencies have reached 10%. As efficiencies have risen, the questions of lifetime and reliability of OPV have gained in importance. We recently observed a record lifetime approaching 7 years in encapsulated solar cells employing the high-efficiency polymer poly(-heptadecanyl-2,7-carbazole-alt-5,5-(-di-2-thienyl-benzothadiazole) (PCDTBT) which is double the lifetime of the well-studied polymer, P3HT. The use of fullerene acceptors other than PCBM has improved the efficiency in P3HT devices by improving the voltage; however, it is not known how these fullerenes effect device lifetime. We began the investigation into the effect of the fullerene on degradation by testing the stability of the polymer-fullerene bulk-heterojunctions (BHJ) on glass, outside of a device architecture. The samples were aged under one-sun light intensity in ambient air. We monitored degradation by measuring the decrease in the visible absorption of the BHJ over time. Five fullerenes and four polymers were tested. The degradation of the BHJ visible absorption depends on the electron affinity of the fullerene; as the electron affinity of the BHJ becomes smaller, the degradation rate increases. This can be explained by a mechanism of charge transferring from the fullerene to oxygen to form O2- which then attacks the polymer. As the electron affinity of the fullerene approaches the redox potential of oxygen, the fullerene acts as a pro-oxidant that catalyzes the degradation of the polymer. In other words, the degradation of the polymer, when combined with fullerenes that have small electron affinities, can be faster than the degradation of the polymer by itself. There is no correlation between the pholuminescence quenching rate and degradation rate; and, there is no correlation between the triplet yield and the degradation rate. To see if the degradation rate of the BHJ on glass correlates to the degradation of the device, we tested the effect of different fullerenes on the lifetime of encapsulated P3HT devices under one-sun intensity. The initial power conversion efficiency for P3HT with some of the fullerenes approaches 6%. We find that the sharp decrease in efficiency that occurs within 100 hours does depend on the choice of fullerene. Additionally, the initial decrease in efficiency for each P3HT:fullerene blend depends on the choice of anode. After the first exponential drop, all of the devices stabilize, and the degree of stabilization appears to correlate with the electron affinity of the fullerene through 1100 hours of aging.

Recent advances of hybrid solar cells have prompted for detailed studies of the processes of charge carrier generation, recombination and transfer at organic-inorganic interfaces. Among the inorganic semiconductors, mainstream III-V compounds offer excellent control over electronic properties and high flexibility in the synthesis of low-dimensional structures. Recently, hybrid systems comprising of conjugated polymers and III-V nanowires (InP and GaAs) have shown very promising photovoltaic performance. To understand the fundamental processes underlying the operation of such devices, we investigate the photophysical properties of poly (3-hexyl-thiophene) (P3HT):GaAs thin film heterojunctions using a combination of steady-state and ultrafast spectroscopy techniques including photoluminescence and transient absorption. Our measurements in samples with different thickness provide clear evidence for the formation of long-lived mobile polarons in the polymer upon charge transfer induced exciton dissociation at the heterointerface on a sub-picosecond time scale. This is consistent with the energetics of the polymer donor-GaAs acceptor system and is corroborated by first principle calculations using density functional theory (DFT). To the best of our knowledge, this is the first direct demonstration of ultrafast electron transfer in P3HT:GaAs heterointerfaces, suggesting that the efficiency of hybrid organic-inorganic photovoltaic devices may be improved by properly engineering III-V nanowires or quantum dots and by effectively controlling the polymer morphology.

Bulk heterojunction thin film polymer solar cells have drawn significant attention for potential cost-effective photovoltaic applications. The performance of these devices is inextricably linked to the three-dimensional nanoscale morphology which plays a critical role in the exciton dissociation and charge transport processes. We found that copolymer sequence strongly influences phase separation capabilities of the donor-acceptor blend in bulk heterojunction organic photovoltaic devices. Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined π-conjugated copolymers of poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS). Monomer sequence was controlled along the copolymer chain by the rate of addition of the comonomers, to achieve diblock, random and gradient copolymer chain architectures. This allowed us to study the effect of copolymer sequence and nanoscale morphology, of P3HT-P3HS copolymer/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blend via a novel low temperature annealing procedure in supercritical carbon dioxide, on device performance. With the use of energy-filtered transmission electron microscopy and conductive and photoconductive atomic force microscopy we show that the supercritical solvent protocol enabled the formation of optimum morphologies leading to an enhancement in device performance.

We demonstrate that local composition affects the rate of device aging in nanostructured polymer solar cells by comparing time-resolved electrostatic force microscopy (trEFM) and conventional device measurments on model polymer blends. Specifically, we study photovoltaics made from 1:1 blends of polyfluorene copolymers poly(9,9&’-dioctylfluorene-co-bis-(N,N&’-(4,butylphenyl))bis(N,N&’-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9&’-dioctylfluorene-co-benzothiadiazole) (F8BT). We photooxidize these films in situ using 365 nm, 405 nm, and 455 nm illumination under ambient conditions, with the wavelengths chosen to preferentially excite the different components. During photodegradation, we observe a faster loss of photocurrent generation from F8BT-rich domains, leaving the PFB-rich phases to show higher photoresponse than the F8BT-rich phases, even at wavelengths absorbed predominantly by F8BT. We attribute this effect to the more rapid degradation of PFB pathways in the F8BT-rich regions, resulting in a loss of percolation pathways for hole transport in the F8BT-rich phase. We test this hypothesis using ATR-FTIR and local hole mobility measurements taken before and after photooxidation.

Zinc oxide is a low-cost semiconductor made from earth-abundant materials; multiple routes to sol-gel processing enable the formation of thin films, making the material a promising candidate for a contact layer for next-generation thin-film photovoltaics. However previous research has observed a unique UV dependence of work function and mobility when the thin film is exposed to air. In polymer:fullerene photovoltaics, aging of the zinc oxide (ZnO) contact layer in air has been shown to effect the diode properties of the device and result in the formation of a double diode, though full initial performance can be regained through UV light soaking. This study investigates the photon energy dependence of the diode properties of PCDTBT:PC70BM bulk heterojunction (BHJ) solar cells with ZnO films formed from diethylzinc precursor. We show that the detrimental aging effect can be partially mitigated via the introduction of self-assembled monolayer dipole modifiers to modulate the work function at the interface between the ZnO and BHJ layers.

Steric interactions between neighboring repeat units in certain donor-acceptor (DA) alternating copolymers have been demonstrated to negatively impact morphologies and intermolecular electronic interactions that are necessary to obtain high performances in organic photovoltaic devices. In this light, the synthesis and properties of twelve new conjugated DA copolymers containing ethynylene linkages are investigated, with DA combinations including fluorene, benzodithiophene, and diketopyrrolopyrrole with analogues of pyromellitic diimide, thienoisoindoledione, thienopyrazine, and thienopyrroledione. Computational modeling suggests the ethynylene containing polymers can adopt virtually planar conformations, while many of the fully heterocyclic analogues are predicted to have quite twisted backbones (> 35 degrees). Despite the predicted backbone planarization that in principle might increase the effective conjugation length of these copolymers, the introduction of ethynylene linkages into these DA systems universally results in a significant blue shift in the absorbance spectra (by as much as 200 nm) as compared to their fully heterocyclic analogues. Photoconductance of the polymers are discussed, as determined by time-resolved microwave conductivity, as well as some preliminary device data. We further illustrate how these linkages can be used as a tool to bring the very low band gaps of certain DA polymers (1.2 eV) into a more optimal range (1.4 to 1.5 eV) for OPV applications.

Relative arrangement of the electronic levels in an interface between organic semiconductors is crucial for the operation of photovoltaic devices. With the addition of designed macromolecules we demonstrate that it is possible to control the relative position of the highest occupied molecular orbital and lowest unoccupied molecular orbital levels, and consequently improve the performance. The designed macromolecules consist of two end segments, each compatible with one of the interface components, and a central segment which adds functionality to the interface. The tails control the position and orientation of the functional units. If we choose the functional unit to be an electric dipole, an electrostatic field is created due to the orientation of the dipoles, which shifts the electronic levels in a controlled way. By developing a theoretical framework, based on self-consistent field theory, we study the concentration and the orientation of the central functional units. We show that the electronic levels can be shifted by as much as several tenths of an eV thus directly contributing to device performance.

Conjugated polymers have tremendous potential for use in cheap, flexible, light-weight, energy efficient opto-electronic applications, including solar cells. Despite years of work, critical fundamental aspects about their optical and electronic properties are still poorly understood. Photo absorption in pure semi-conducting polymer thin films eventually results in both free charges and bound excitons with varying branching ratios. However the identification of the nature of early excitations and charge generation is an unresolved problem. The limited exciton diffusion length is the basis of the Bulk Heterojunction (BHJ) concept and the need for nanostructure on the ~10 nm scale due to limited excition diffusion is a canonical feature of the present paradigm. Yet charge separation takes place <50-100 fs[1,2] and exciton diffusion in typical nanoscale phase-separated BHJs due to their slow hoping mechanism are incompatible and constitute a paradox.[2] There has been no direct observation of initial excitons or free electron-hole pairs, and competing views persist. In dissolved semi-conducting polymers, electronic coherences was previously observed to transport energy along a chain in ultrafast time scales,[3] however in thin films the role of electronic coherences is unclear. Here we use 2D Fourier transform spectroscopy methods to separate the spectral signatures of various processes in the photoabsorption process in a homopolymer and show that initial excitation results in an intrachain electronic coherence that can persist for more than 200 fs. As these coherences evolve, they collapse to transient population states i.e excitons, polarons and bipolarons depending on the nature of aggregation being predominantly inter chain (H-like) or intrachain (J-like). The time scale of the collapse into population states also depends on the details of the material. Preliminary analysis seems to support the picture delineated by Cowan et al. [2] References: [1] C.J. Brabec et al Chem. Phys. Lett, 2001, 340, 232-236 [2] S. R. Cowan et al. Adv. Func. Mater. 2012, 22, 1116-1128 [3] E. Collini et.al Science, 2009, 323, 369-373.

Recent increases in organic photovoltaic (OPV) efficiency have been driven largely by the development of novel active-layer materials based on what have been called push-pull or donor-acceptor copolymers. These copolymers typically have lower band gaps than the previously used homopolymers and, hence, have greater absorption of the solar spectrum. In addition, judicious choice of the donor and acceptor monomers allows both narrow bandgaps and large oxidation potentials, which can result in larger short circuit currents and open circuit voltages, respectively. Here, we describe an approach to designing new active layer polymers based on combinatorial structure generation and high-throughput electronic structure calculations. Starting with a set of donor and acceptor building blocks that differs from those used in other combinatorial studies, supplemented by a library of functional groups that can modify the building blocks, we have used custom-built software to combinatorially generate a vast number of candidate polymer materials and to compute their electronic properties automatically using density functional theory (DFT) and time-dependent DFT. The results are automatically imported into a database that can be filtered to show only materials with desired electronic properties, such as a specified band gap and HOMO level. It is central to our approach that the building blocks in our combinatorial library are chosen to be molecules that have either already been synthesized or whose synthesis seems feasible. In addition, heuristic rules prevent combinations that are likely to result in materials that would be unstable in typical polymerization conditions. Hence, among the large number of plausible candidates we produce, we expect that most of those with promising electronic properties will be readily synthesizable. Thus far, we have computed the properties of over 100,000 candidate materials, with more than 1000 candidates predicted to have desirable traits for PV applications. When generating candidates, we compute the electronic properties of hydrogen terminated oligomers with alternating donor-acceptor backbones and other, more complicated, backbone structures. A novel extrapolation method will be discussed that allows accurate extrapolation of oligomer properties to the polymer limit based on relatively small (n=1,2, and 4) oligomers. Many of the materials predicted to be promising candidates recently have been described in the literature, validating our computational and screening processes. Finally, an overview of the results contained in our combinatorial database will be presented, including comparisons of the computed results to experiment. In addition, it will be shown that the large data set reveals design rules that might not otherwise be evident and that the data suggests that some accepted rules of thumb underlying the design of donor-acceptor copolymers may not be as widely applicable as previously supposed.

The thickness dependence of the out-of-plane hole mobilities (transport direction normal to the substrate) in thin films of regioregular poly(3-hexylthiophene) (RR-P3HT) and its copolymers supported by ITO-coated glass substrates were investigated. While much is known about the structure and electrical properties of thin films of these conjugated polymers, specifically P3HT, important questions regarding the out-of-plane hole mobility remain unsolved, particularly for films in the thickness range of 100~200 nm, which is more relevant for solar cells. Using the experimental methods of Charge Extraction by Linearly Increasing Voltage (CELIV), Time of Flight (ToF) and Impedance Spectroscopy (IS), we show that the out-of-plane hole mobility in neat RR-P3HT films monotonically increased from 10^-4 cm²/Vs, for an 80 nm thick film, to a value to 10^-3 cm²/Vs, for films thicker than 700 nm. This large thickness dependence of the out-of-plane hole mobility has not been previously reported. This is largely because published ToF data for the mobilities exist only for films of thickness ~microns. While data for in-plane mobilities are numerous, the in-plane mobilities are necessarily different from the out-of-plane mobilities. Structural characterization of thin films, using Grazing Incidence X-Ray Diffraction (GIXRD) and Spectroscopic Ellipsometry (SE), reveals significant differences between the morphologies of thin films and thick films. The morphologies of the thinnest films are highly anisotropic. However in thicker films, the structure is highly anisotropic at the substrate, due to a substrate-induce-orientation of crystalline phases, but becomes isotropic at distances a few hundred nanometers from the substrate. Numerical simulations, based on the structural information, showed that the thickness dependence of the out-of-plane mobility is not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.

Zinc oxide (ZnO) is commonly used in bulk heterojunction organic photovoltaics (OPV) for charge-selective electron transfer from photoactive layers to the buried electrode in “inverted” vertically assembled devices. Here, we show that photovoltage and overall performance can be increased in devices containing ZnO from the zinc acetate sol-gel by inclusion of magnesium acetate, forming a zinc magnesium oxide (ZnMgO) electron-selective layer in the device. The band alignment is evaluated using photoemission spectroscopies; enhanced band alignment results in improved diode characteristics and enhanced device performance. We also evaluate ZnO contacts derived from a diethyl zinc precursor relative to zinc acetate-based films. The diethyl zinc precursor results in ZnO films with improved crystallinity and produces better diodes than those from the zinc acetate precursor, at temperatures less than 150 degrees C. Such modifications in the processing constraints of the ZnO contact layer result in a system that is more compatible with flexible plastic substrates for low cost roll-to-roll manufacturing.

The use of organic materials in optoelectronic devices may soon be a versatile alternative to traditional semiconductor technology. In this kind of devices the nanoscale morphology and opto-electronic properties of their building blocks -polymeric chains or complex molecules- is fundamental for their function. Scanning Probe Microscopy is thus an ideal tool for the study of this kind of materials. In the present work Dynamic Scanning Force Microscopy is used to characterise the electronic properties of MEH-PPV thin films in ambient conditions. We find surprising electrostatic properties of the MEH-PPV thin films studied. First, the polymer morphology is not correlated with the electronic surface distribution. Secondly, the small surface potential domains have an interesting time evolution, which is again uncorrelated to the morphology, since this is essentially static. This time evolution is studied using “movies” -that is the successive adquisition of images- of the sample were topography and surface potential (Kelvin Probe Microscopy) are acquired simultaneously. We find on the one hand strong fluctuations of the surface potential, and on the other a static mean corrugation of this potential. These results are tentatively interpreted as thermal fluctuation of the electrostatic potential around a fixed potential distribution. Finally, by fixing the lateral position (no scanning) and acquiring data at high speed at a specific location, relaxation experiments are performed showing a slow relaxation with a logarithmic time dependence over several decades. These results indicate that the electrostatic properties of this polymer are closer to a glass-like material than to typical insulating, semiconducting or metallic matter.

Competing processes impede the collection of photogenerated charges in conjugated polymer based photovoltaic devices: initially free charges in extended states, after becoming trapped, are subject to hopping between trap states or from trap to extended states, further localization, and recombination. Previous works aim at describing the carrier dynamics by extended Drude models, based on measurements of complex conductivities in a certain frequency range, in order to take into account contributions from partially or fully localized charges. In this work we combine the experimental methods of photocurrent transient, DC photoconductance, and impedance spectroscopy to derive a complete image of the carrier dynamics. Contributions corresponding to different degrees of localization are described by their respective oscillator strengths, dissociation energies and lifetimes. The model is completed by describing the hopping between trapped and extended bandlike states by Monte-Carlo simulations. Furthermore, morphology effects on the carrier dynamics are discussed in the context of varying distributions between differently localized charges.

Benzoporphyrins have been investigated as a candidate of organic semiconductors for solar cells due to a fairly narrow bandgap of about 2eV. However, it exhibits poor solubility in common organic solvents because of strong π-π interaction, which makes it difficult to fabricate a thin film by solution processes. Therefore, a new derivative that has high solubility in common organic solvents and high feasibility of controlling molecular orientation in a film would be suitable for the photovoltaic applications. We have paid attention to the liquid crystallinity, which often appears in the derivative chemically modified with long alkyl chains, because of high solubility attributed to long alkyl chains and the self-organization of molecules that may give high mobility in the mesophases. We have designed a new porphyrin having long alkyl chains at bay-positions of the benzoporphyrin moiety, with reference to our previous results on phthalocyanins having long alkyl chains at the same position. In this study, We synthesized 1,4,8,11,15,18,22,25-Octakis(hexyl)-29H, 31H- tetrabenzo[b,g,l,q]porphphyrin in a three steps from a commercial starting material, 3,6-Dihydroxyphthalonitrile, and characterized its optical and charge transport properties and phase transition behavior. This material exhibited liquid crystallinity at the temperature range from 149C to177Cand identified to be Columnar phase. In time-of-flight experiments, transient photocurrents of the derivative were non-dispersive and gave a clear shoulder for a transit time. We determined the hole mobility to be 0.13cm2V-1s-1 in the columnar phases, which was almost comparable to those in polycrystalline organic film effect transistor (OFET) materials, and estimated the photocarrier generation efficiency in the bulk to be 0.02 in 337nm . We compared the present results with those from the phthalocyanine derivatives and discuss how we can utilize the liquid crystallinity in solar cell materials for high device performance.

Within a systematic program for better-designed unimolecular rectifiers, we have synthesized a new molecule (IM-01), with a bis-pyrrolotetrathiafulvalene electron donor moiety, a phenyl-ethynyl-phenyl-ethynyl bridge, and an anthraquinone electron acceptor moiety. IM-01 forms a Pockels-Langmuir monolayer at the air-water interface (pressure-area isotherm), and transfers well as a Langmuir-Blodgett (LB) monolayer atop a fresh and hydrophilic Au electrode. In a separate quartz crystal microbalance experiment, IM-01 chemisorbs quantitatively to an Au substrate, thanks to its cyano termination. An LB monolayer of IM-01 between Au electrodes (top electrode deposited by the cold gold method) rectifies electrical current, with a large rectification ratio. The orientation of the molecule in the LB monolayer, relative to the bottom electrode is under study. This work was supported by NSF (CHE-0848206)

To quantify bimolecular recombination, which is a major source of photogenerated charge loss in organic solar cells, we present a new technique based on external quantum efficiency (EQE) measurement with varying-intensity white light bias. By integrating the area under the EQE spectral curve, the short circuit current (Jsc) can be obtained. We find that bimolecular recombination manifests in reduced EQE across the spectrum, particularly at the most absorbing wavelength range, as the background light intensity increases. For a device with little or no bimolecular recombination, the EQE spectrum is independent of the white light intensity, resulting in a constant integrated Jsc for all bias light intensity. In contrast, for a device with significant bimolecular recombination, the integrated Jsc decreases with increasing light bias intensity. Compared to measuring current-voltage (J-V) at different intensities, our method is more sensitive to the intensity dependence. To probe how different factors in the solar cells affect bimolecular recombination, we vary the P3HT:PCBM active layer thickness, the processing conditions, and interfacial contact layers, which improve the performance of bulk heterojunction organic solar cells when inserted between the organic active layer and the electrodes. We find that bimolecular recombination increases with active layer thickness, indicating that more light absorption in the thicker film is negated by transport inefficiency. Inverted devices that include PEDOT and MoO3 as the hole transport layer (HTL) show negligible bimolecular recombination compared to those without a HTL. Without a HTL, both device performance and EQE depend on the electrode material. In particular, while Au performs better than Al as the hole-collecting electrode, high work function alone does not produce an efficient device and bimolecular recombination is higher than with a HTL. Drift/diffusion modeling is performed at low and high intensity to compare with experimental results.

Although organic photovoltaic certified solar cell efficiencies have risen to almost 10%, our understanding of the exact mechanism by which these devices function is still in its infancy. Specifically, our knowledge of how the photogenerated excited states (excitons) are transformed into separated carriers in such high yields remains controversial. At the heart of this controversy is the role of charge-transfer (CT) states which, according to conventional photophysics, should be the dominant product of electron transfer at the low dielectric constant interface that is associated with most of the donor-acceptor systems that are studied. This presentation will examine the role that CT states play in the process of converting excitons into free charges, and will examine the possibility that they are not a state through which separated charges are generated, but a state that competes with this process. To test these ideas, results from recent studies using time-resolved microwave conductivity (TRMC) to detect the generation of the charges will be reported. TRMC provides a means of investigating the fundamental steps in systems that contain low concentrations of both donor in acceptor, and acceptor in donor. Such an approach is not easily studied using device architectures, but it can provide detailed insight into this critical charge-generating step.

The performance of organic photovoltaic cells is determined by the low field yet charge density dependent transport of typically non-ordered materials or composites. This operating regime is unique to solar cell devices and hence some of the models used to describe transport in organic materials need to be revisited. In this presentation we revisit the drift diffusion process in the context of degenerate hopping systems and show that the use of this equation, without properly accounting for the organic material, may lead to several ambiguities regarding the mobility as well as the Einstein relation rendering many modeling efforts less accurate. We show that simple, yet fundamental modification of the drift diffusion equation can resolve all relevant issues and make the modeling fully rigorous and tractable. For this purpose a semi-analytical formulation was devised by which the mobility and diffusion coefficient of charge carriers performing thermally assisted tunneling in an energetically disordered lattice can be calculated for non-dispersive transport, at low fields. To establish the mentioned results, the devised semi-analytical formulation was incorporated to calculate carrier density spatial distributions for systems at steady state under applied bias and given boundary conditions. The calculated distributions were compared with distributions obtained by Monte-Carlo simulations, run under the same conditions. The insight we report is founded on the fact that the distributions derived by the two methods bare very good agreement.

In both biological and artificial photo-conversion systems, the recombination of electron and hole is a major loss process. Biological light harvesting complexes (LHCs) prevent recombination via the use of energetic cascading structures, which lead to spatial separation of charge-carriers. In contrast, the nanoscale morphology and larger charge densities of organic photovoltaic cells (OPVs) lead to a higher rate of electron-hole encounters. This implies that OPVs should have poor quantum efficiencies if every encounter leads to recombination. However, state-of-the-art OPVs demonstrate near-unity quantum efficiencies. Here we show that the suppression of recombination is due substantially to the intrinsic spin physics of organic semiconductors. We use time resolved spectroscopy to study two model systems in which the energy of the lowest lying molecular (here polymeric) triplet exciton (T1) lies either above (for PCDTBT:PCBM), or below (for PCPDTBT:PCBM) the intermolecular charge transfer state (CT). Encounters of spin-uncorrelated electrons and holes generate intermolecular charge transfer state (CT) with both spin singlet (1CT) and spin triplet (3CT) characters. For PCPDTBT:PCBM, we observed the formation of T1 excitons following bimolecular recombination to form 3CT states between 1ns and 100ns after photoexcitation, causing effective recombination. In contrast, for PCDTBT:PCBM, the 3CT states cannot relax to either T1 states (energetically inaccessible), or the ground state (spin-forbidden). This allows sufficient time for re-dissociation of the 3CT states into free charges, thus significantly reducing recombination. Our results point to new design rules for artificial photo-conversion systems, where the use of spin will enable the suppression of electron-hole recombination.

The performance of polymer-based organic solar cells has dramatically improved during the last years. Understanding the photocurrent loss in present material systems by quantifying the efficiencies of elementary steps governing generation und extraction of carriers is of major importance for future device optimization. In this talk, generation and extraction of charge in polymer-based solar cells is quantified by performing time-delayed extraction experiments. The results are related to chain aggregation in blends of conjugated donor polymers with small molecules or polymeric acceptors. Optical spectroscopy is used to determine the degree of crystallinity and to extract information on the size and perfection of polymer aggregates in such blends.^[1] For P3HT:PCBM, the efficiency for free carrier generation in both as-prepared and annealed blends is found to be independent of electric field, meaning that the photogenerated current is entirely determined by the field-driven sweep-out of carriers in competition with non-geminate recombination.^[2] We find that layer crystallinity is not a key parameter determining the solar cell performance, while the probability to extract photogenerated holes from the blend layer is highly correlated to the degree of energetic order in the polymer crystallites. In contrast, blends of the low bandgap polymer PCPDTBT with PCBM exhibit a pronounced field-dependence of charge generation, suggesting efficient geminate recombination.^[3] We show that the efficiency of geminate and non-geminate recombination in these blends is correlated to the blend morphology, and that both decay channels are strongly reduced in blends with extensive interchain order. ^[1] S.T. Turner, P. Pingel, R. Steyrleuthner, E.J.W. Crossland, S. Ludwigs, D. Neher, “Quantitative analysis of bulk heterojunction films using linear absorption spectroscopy and solar cell performance”, Adv. Funct. Mater. 2011, 22, 4640. ^[2] J. Kniepert, M. Schubert, J.C. Blakesley, D. Neher, “Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments”, J. Phys. Chem. Lett. 2011, 2, 700. ^[3] S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J.C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf., D. Neher, “ On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: influence of solvent additives”, J. Phys. Chem. Lett. 2012, 3, 640.

We will report our recent progress on the incorporating ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) thin films and nanostructures to enhance the efficiency of organic solar cells with two distinct structures. For one, the ferroelectric polymer nanolayer was inserted at the interface between a semiconductor bulk heterojunction and the cathode, thus increasing the charge extraction and efficiency by three folds by the induced electric field. [1] For the other, increased photovoltage output was achieved by inserting a monolayer ferroelectric dipole layer between the donor and acceptor bilayer semiconductor films to increase the energy difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor and highest occupied molecular orbital (HOMO) of the donor. [2] To understand the effect of ferroelectric polarization on power conversion efficiency of organic photovoltaic devices, the effect of P(VDF-TrFE) crystallinity on its function in ferroelectric organic photovoltaic (FE-OPV) devices has been studied by several methods. Highly crystalline and amorphous P(VDF-TrFE) films have been prepared by Langmuir-Blodgett method and spin-coating from acetone solution, respectively. The polymer solar cell devices with crystalline P(VDF-TrFE) interfacial layer at cathode has larger PCE than the structures with amorphous P(VDF-TrFE) and have unique feature of switchable diode polarity and photovoltaic performance controlled by external applied voltage pulses. The obtained results confirm that the spontaneous polarization of the ferroelectric P(VDF-TrFE) layer is responsible for the enhancement of PCE in FE-OPV devices and that a highly crystalline ferroelectric polymer film is required to observe the enhancement of PCE. Amorphous P(VDF-TrFE) films act as regular dielectric layers with little poling effect on device PCE. The polarization of P(VDF-TrFE) is shown to be stable, and the photogenerated charges could be collected efficiently by the cathode rather than being compensated. [3] We will also report the synthesis of P(VDF-TrFE) nanocrystals so that they can be spin-coated on polymer semiconductor without going through the sophisticated Langmuir Blodgett deposition process. The pre-formed P(VDF-TrFE) nanocrystals with designed size and coverage on semiconductor film allows us the control the morphology and then optimize the performance of FE-OPVs. [4] 1. Yuan, Y.B., et al., Efficiency enhancement in organic solar cells with ferroelectric polymers. Nature Materials, 2011. 10(4): p. 296-302. 2. Yang, B., et al., Tuning the Energy Level Offset of Donor and Acceptor with Ferroelectric Dipole Layers for Increased Efficiency in Bilayer Organic Photovoltaic Cells. Adv. Mater., 2012. 24(11): p. 1455-1460. 3. Yuan, Y.B., et al., Understanding the effect of ferroelectric polarization on power conversion efficiency of organic photovoltaic devices. Energy and Environmental Science, 2012. In Press. 4. Xiao Z.G. et al., Synthesis and application of ferroelectric P(VDF-TrFE) nanocrystals in low bandgaporganic photovoltaic devices, Under Review

Although there is substantial interest in hybrid photovoltaics (h-PVs) based on wide band-gap oxide acceptor materials such as ZnO or TiO₂, the performance of these devices lags behind that of organic photovoltaics (OPVs) despite the apparent advantages of oxides i.e. high electron mobility, chemical and light stability, availability of nanostructures etc. To illustrate this, the best ZnO h-PV devices reported to date have power conversion efficiencies of ~0.8 % [1], around an order of magnitude lower than the state-of-the art bulk-heterojunction OPVs. Analysis of h-PV devices suggests that there are a number of factors at work, e.g. geminate recombination of charges at the interface, or poor contact between the polymer and the oxide. Work by Piris et al. [2] has also suggested that there is very little exciton dissociation at the interface in oxide-P3HT devices, thus leading to low photocurrents observed in h-PVs. It is clear that this is a key issue that must be addressed if h-PVs are to be able to compete with OPV devices, and that the use of ferroelectric (FE) materials into h-PVs may be a viable route to overcome this obstacle. Recently, papers by Yuan [3] and Nalwa [4] have investigated the incorporation of the ferroelectric (FE) polymer PVDF-TrFE into bulk-heterojunction P3HT:PCBM OPV devices in an attempt to create a large internal electric field within the solar cell, exploiting the permanent electrical polarisation of the FE material. It is postulated that this enhanced electric field would lead to a greater degree of exciton dissociation in the devices, thus increasing the photocurrent. However, in response to [3], Asadi asserts that FE polarisation cannot play a role in the observed increases in device performance as the experimental conditions are not sufficient to induce permanent polarisation in the polymer [5]. Here, we aim to address whether we do indeed see improved exciton splitting in polymer layers due to FE polarisation: layers of the FE perovskite oxide Pb(Zr_0.30Ti_0.70)O₃ are fabricated and poled using an external electric field, ensuring that a permanent polarisation is induced in the layer; the polymer is then deposited onto the PZT. Transient absorption spectroscopy (TAS) can then be used to assess any differences in exciton quenching between polarised and depolarised films. Not only will this address whether FE materials are applicable to organic-based solar cells, but a positive result may also provide a route for improving photocurrent in h-PV devices. References [1] Conings, B., et al.; Appl. Phys. Lett., 2012; 100(20), pp. 203905. [2] Piris, J., et al.; Adv. Func. Mater., 2007; 17(18), pp. 3849-3857. [3] Yuan, Y., et al.; Nat. Mater., 2011; 10(4), pp. 296-302. [4] Nalwa, K., et al.; Energy Environ. Sci., 2012. 5, pp. 7042-7049. [5] Asadi, K., et al.; Appl. Phys. Lett., 2011. 98(18), pp. 183301-183303.

Achieving a large open circuit voltage in single junction organic photovoltaics (OPVs) is critically important to improving the efficiency of tandem organic solar cells that will be required to reach power conversion efficiencies of 15%. We demonstrate polymer:fullerene solar cells with power conversion efficiencies over 7% with blends of the polymer PBDTTPD and PC60BM which achieve an open-circuit voltage (Voc) of 0.945 V and internal quantum efficiencies of 88%, and thus are an ideal candidate for the large bandgap junction in tandem solar cells. When the polymer is blended with multiadduct fullerenes, we find that the Voc is increased above 1.0 V, but the photocurrent and fill factor are greatly reduced even at low light intensities. In blends of PBDTTPD and the multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage-dependent, steady state and time resolved photoluminescence measurements indicate that energy transfer is occurring from PBDTTPD to ICBA and that back hole transfer from the fullerene to the polymer is inefficient. By analyzing the absorption and emission spectra from fullerene excitons and charge transfer states, we estimate a driving free energy of -0.14 ± 0.06 eV is required for efficient hole transfer and charge generation. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with Voc&’s above 1.0V and that non-fullerene acceptor materials with large optical gaps (>1.7 eV) may be required for devices to achieve both near unity internal quantum efficiencies and Voc&’s exceeding 1.0 V.

We report high-performance inverted polymer solar cells (I-PSCs) introducing novel functional materials, such as nonconjugated/conjugated polyelectrolytes and n- or p-doped sol-gel metal oxides, as efficient interfacial layers. Our functional layers are inserted between the photoactive layer and electrodes via solution processing and low annealing temperature below 80C, and induce significant functions in the I-PSCs as follows: (i) work function tunning of the electrodes, (ii) selective contact for charge carriers, (iii) transparent optical spacer, (iv) protection of the active layer, and (v) determination of the device polarity. As a consequence, our I-PSCs exhibit a power conversion efficiency (PCE) approaching PCE ~ 7% under AM1.5 irradiation (100 mW/cm2). In addition, we also successfully incorporate our novel functional layers into all-printed PSC modules with PCE ~ 4% fabricated using various printing techniques, such as slot-die, ink-jet, and electro-spray printing methods.

A new class of small molecules has been designed with modular features that can be used to match the optical absorption with the solar spectrum and tune molecular orbital energies so to achieve excellent power conversion efficiencies in solution deposited devices. Topological attributes can also be chosen to control bulk heterojunction phase separation and film forming behavior. Through appropriate processing conditions involving very small quantities of solvent additives, it is possible to attain power conversion efficiencies of 7 %. As shown by transmission electron microscopy, the additive content is critical for managing the crystallite size of the donor domains. Functional groups are also relevant to manage interfacial effects with charge injection layers and thereby improve charge carrier extraction. In addition to circumventing problems associated with batch to batch variations in narrow bandgap conjugated polymer counterparts, small molecule donors can also be fully characterized by single crystal X-ray diffraction. This structural insight allows understanding of lattice arrangements and the electronic coupling between structural subunits. Additionally, one can determine how molecular structure is influenced by functional groups, choice of electron rich and electron poor heterocycles in the chromophore, and pendant solubilizing side groups. These insights will be illustrated by a series of closely related structural derivatives designed to extract structure-property relationships.

Organic photovoltaic technologies have been the subject of active research and development over the past decades. Due to their ability to be processed at low temperature, over large areas, at potentially low cost, organic solar cells have experienced an accelerated development in recent years and have the potential to spawn a new generation of products with thin and flexible form factors. However, despite a steady progress in performance, many challenges and concerns about stability and cost remain before this emerging technology can unleash its full potential. In this talk, we will focus on the role of interfaces in organic solar cells. First, we will discuss the limitations of organic solar cells and why the commonly used device architectures present major challenges. In particular, we will show that the widely used indium-tin oxide (ITO) electrodes exhibit variations in the work function that impact device stability. Strategies to control and stabilize the value of the work function will be presented. Another hurdle is in the limited choice of low work function metals and their high chemical reactivity when exposed to ambient conditions. Metals such as Ca are widely used in record-high efficiency organic solar cells but require hermetic packaging. A new method to produce air-stable low work function electrodes as a substitute for Ca will be presented. This method is based on surface modification by water-soluble polymers that physisorb to the surface of various conductors and lead to large surface dipoles that shift the vacuum level (> 1 eV) reducing the injection or collection barrier for electrons. Finally, we will show that these advances in interface modification can be used to design organic solar cells with novel architectures that can overcome some of the economic hurdles of current approaches and accelerate the deployment of these technologies.

The active materials of organic bulk heterojunction solar cells based on a mixture of polymers and fullerenes are soluble in many organic solvents, allowing deposition by solution printing or coating, making organic PV (OPV) cells economically competitive. Polymer foil substrates are inexpensive, highly flexible, and are compatible with roll-to-roll processing. To reach the costs targets the preparation of working ITO-free OPV cells, of which all layers, including the electrodes, are solution processed, is a prerequisite. We are developing high volume, roll-to-roll compatible printing and coating techniques in order to produce entirely solution-processed, ITO-free OPV cells and modules. In this respect, slot die coating can be successfully used. Additionally, ink jet printing is very appealing as it constitutes a non-contact, versatile, additive and mask-less approach for deposition of thin films from solutions. We prepared organic solar cells of which most layers have been ink jet printed (IJP), without using any chlorinated solvents. A disadvantage of polymer substrates is that they do not meet by far the demanding requirements for low moisture and water permeability for OPV applications. Therefore, polymer substrates require additional barrier layer coatings, that increase the cost of solar cell manufacturing significantly. Thin metal foil substrates are particularly attractive for flexible OPV devices, because they provide an excellent barrier to water and oxygen, which are the most critical components affecting the lifetime of organic solar cells. Additionally, stainless steel foil has superior chemical resistance to most of the chemicals used during OPV device fabrication. We report on flexible large-area organic solar cell on a flexible metal foil substrate. The low sheet resistance of the steel electrode allows manufacturing of large area devices with good performance. A semitransparent top cathode was applied to complete the device stack. The individual cells with the size of 5x10 cm2 were manufactured. In order to quantify effect of the cathode transmittance, the optical simulations using the transfer-matrix formalism has been performed.

N-type conjugated polymers with high electron affinities and high ionization potentials are avidly sought after in the field of organic electronics. For example, polymer photovoltaic devices frequently comprise fullerene-based acceptor molecules, which are notoriously poor light absorbers in the visible range. Hence, there is a critical need for new n-type polymeric materials. Here we report the syntheses and characterizations of a series of aqueous-solution processable n-type conjugated polymers. The monomeric unit, based on a 1,4-bis(5-bromopyridin-2-yl)benzene skeleton, is conveniently obtained in two steps from commercially available starting materials. This monomer can polymerize under Yamamoto conditions to afford a poly(pyridine phenylene) precursor, that can undergo post-polymerization conversion into a poly(pyridinium phenylene), i.e., a water-soluble and electron-deficient polymer. A variety of co-polymers could be obtained by combining the synthesized monomer with different building blocks (e.g., thiophene- or benzothiadiazole-based units), in order to modulate the optical and electronic properties of the compounds. These materials were found to be moderately soluble in aqueous solutions, and were characterized by 1H NMR spectroscopy, gel permeation chromatography, as well as optical and electrochemical methods. In addition to advantageous solubility, the polymers were found to possess promising electronic properties, such as reversible redox behaviors, high electron affinities and useful ionization potentials. Applications involving the described polymers in fields such as electrocatalysis, chemical sensing or organic electronics are currently carried out in our laboratory.

Bulk heterojunction (BHJ) polymer solar cells (PSCs) that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based solar cells. A combination of novel polymer development, nanoscale morphology control and processing optimization has led to over 8% of power conversion efficiencies (PCEs) for BHJ PSCs with a conventional device structure. Attempts to develop PSCs with an inverted device structure as required for achieving high PECs and good stability have, however, met with limited success. Here, we report that a high PCE of 8.4% under AM1.5G irradiation was achieved for BHJ PSCs with an inverted device structure. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, leading to facilitate electron transport and suppress bimolecular recombination. These results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

Combining band-gap tunability and effective charge transport in thin-film devices, a number of recently reported π-conjugated polymers used in bulk-heterojunction (BHJ) solar cells with the fullerene acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) yield power conversion efficiencies (PCEs) approaching 10%.^[1,2] In π-functional polymers, simple molecular-structure changes, such as the incorporation of different atoms, or the addition/removal of aliphatic carbons, can greatly impact: (i) polymer solution-processability, (ii) the blend morphologies with other π-functional systems, (iii) the ability of the backbones to self-assemble, and thus, (iv) the nanoscale structural order in thin-film devices.^[1] With these variations in ordering properties, the charge dynamics can differ across analogous systems, and the solar cell efficiency parameters may change.^[3] Relying on donor-acceptor principles, low band-gap polymers of N-alkylthieno[3,4-C]pyrrole-4,6-dione (TPD) and benzo[1,2-b:4,5-bprime;]dithiophene (BDT) are some of the best-performing polymer donors in BHJ solar cells with PCBM.^[3,4] In these systems (PBDTTPD), simple changes in the side-chain configuration at the TPD acceptor induce significant variations in polymer ordering properties, yielding solar cell PCEs in the range 3-7%.^[3] In an extension of our initial work on PBDTTPD systems, we correlate polymer self-assembling properties, and performance in BHJ devices, with a number of other molecular-structure changes induced at the TPD acceptor and/or at the BDT donor. We demonstrate that these modifications impact both structural order, and charge dynamics, and the results are tied back to the BHJ device efficiency parameters. ^[1] Beaujuge, P. M.; Fréchet, J. M. J. JACS 2011, 133, 20009 ^[2] Hou, J. et al. Angew. Chem. Int. Ed. 2011, 50, 9697; You, W. et al. JACS 2011, 133, 4625 ^[3] Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet J. M. J. JACS 2010, 132, 7595 ^[4] Tao, Y. et al. JACS 2011, 133, 4250; Reynolds, J. R. et al. JACS 2011, 133, 10062

The cost potential of thin film technologies has always been regarded positive compared to traditional silicon solar cells. At Solliance we developed a cost of ownership calculation tool for future factories of both CIGS (S2S) and OPV (R2R). The calculation gives a more detailed insight in the cost buildup of the product and production process. This allows you to identify possibilities and strategic choices for design or equipment. It helps you also in identifying the sensitivity of your material costs. The cost breakdown of a R2R OPV factory will be discussed. It will be explained how a dramatic cost improvement for OPV manufacturing could be realized by sequentially replacing ITO by a metal grid, replacing a barrier for a metal foil and replacing silver electrodes for cupper or aluminum. Only these three adaptions could bring the estimated cost of OPV below $0.5/Wp.

Organic photovoltaics differs from many conventional solar conversion technologies by virtue of the fact the the core absorber materials are variable across a wide spectrum and not the optimization of a single semiconductor material. Consequently it is important to begin to sort out a broad set of possible acceptor/donor combinations to optimize performance and stability for both single junction and tandem devices. This varying set of materials of necessity demands a change in the contacts specifically the HTL and ETL materials. Optimizing these materials sets empirically may be prohibitive. The resent evolution of computational capabilities has now made it possible to predict many properties of both the organic and inorganic materials. We will report on recent work on developing DFT based tools to produce a virtual database of organic materials which can be mined for desired functionality. We will present results for the experimental realization of predicted functionality. Coupled to this is an increasing ability to design interface layers for HTLs/ETLs based on first principles theory using DFT and GW approaches to predict stable materials, doping type, work function and structure type. These two tools combined begin to develop a set of design tools for designing new optimized OPV devices. We acknowledge funding the DOE SunShot program as part of the National Center for Photovoltaics and the DOE Office of Science support for the Center for Inverse Design EFRC

Charge selective interlayers, typically with thicknesses of 10-50 nm, are increasingly required to ensure efficient and selective charge harvesting in bulk-heterojunction polymer and small molecule based organic solar cells (OPVs). Understanding and optimization of these interlayer materials becomes especially critical in extracting charge from active layers that produce high Voc, and in tandem cell configurations where recombination must be optimized between two or more sub-cells. Charge selectivity in interlayer materials is typically achieved by choosing materials whose band edge energies provide for efficient collection of only one charge carrier and minimization of surface recombination velocities involving the minority charge carrier. This talk will review our recent work in understanding the differences between electron-selective interlayer materials (e.g. ZnO, TiO2), and the role of surface/interface composition in controlling their electron-selectivity. New approaches to the formation of both TiO2 and ZnO oxide interlayers will be shown, using conventional (sol-gel) processing approaches, UV-assisted chemical vapor deposition, and plasma enhanced atomic layer deposition (PE-ALD), focusing on the surface characterization of electronically relevant defect states which appear to control charge selectivity, sub-micron heterogeneity in charge collection, and the effect of these parameters on overall OPV efficiency.

The use of charge selective interlayers in bulk heterojunction organic photovoltaics continues to be a prominent factor in controlling the overall efficiency of devices. It is well understood that thermodynamic selectivity for holes (electron blocking) is obtained using a material with a work function at or greater than the charge transport level for holes and a conduction band closer to vacuum than the transport level for electrons. However, little is understood about the influence of hole-collection rates at the interlayer/active layer interface on device performance. The impact of the work function and thermodynamic selectivity of hole collection interlayers on the open circuit voltage, photocurrent, and shunt resistance in bulk heterojunction organic photovoltaics is examined for poly(N-9‘'-heptadecanyl-2,7-carbazole-alt-5,5-(4&’,7&’-di-2-thienyl-2&’,1&’,3&’-benzothiadiazole) (PCDTBT) and [6,6]-phenyl]C71 butyric acid methyl ester (PC71BM) solar cells. Direct correlations are derived between the energy band alignments, as measured by ultraviolet and inverse photoemission spectroscopies, and the observed dark current and shunt resistances in the presence and absence of hole selective properties of the interlayer (electron blocking). Voltage-dependent external quantum efficiency measurements provide key insights into built-in voltages within the device under free carrier conditions, demonstrating contact-dependent transitions between charge sweep out and recombination dynamics. Temperature-dependent dark and light current-voltage measurements yield insight into recombination and space-charge contributions. Finally, admittance spectroscopy details the effect of interfacial/contact trap states. Clear indications of multiple mechanisms dominating recombination are found, which suggest differences in surface recombination velocities for charge collection at the hole-collecting interface.

The orbital line-up at conjugated polymer interfaces was investigated with photoemission spectroscopy. The interfaces were prepared using electrospray deposition in vacuum, i.e. are free of significant ambient contamination. This enabled the determination of the electronic structure of direct contacts enabling the comparison with small molecular interfaces, which are usually prepared in vacuum via evaporation. Two prototypical polymers, Poly(3-hexylthiophene) (P3HT) and poly[2-methoxy-5-(2&’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) were investigated in contact to Au, Ag, and Al. The results show that a linear correlation between hole injection barriers and work functions of the individual substrates exists, suggesting that the induced density of states (IDIS) model for small molecular interfaces is also valid for conjugated polymer interfaces. Charge neutrality levels (CNL) and screening factors S were determined for the two polymers. The results also confirm that the integer charge transfer model (ICT) mainly applies to interfaces prepared on spin-cast polymer films, where the presence of contamination and solvent residues results in a decoupling of the electronic systems in contact.

A relatively simple picture of free charge generation in polymer:fullerene bulk heterojunction (BHJ) blends stipulates that 1) an exciton is formed in the polymer phase, 2) the exciton diffuses to a fullerene interface, 3) it dissociates into charges that are initially bound and 4) free charges are formed. Recent experimental evidence points to a much more complex mechanism, in which charge generation is entangled with excited-state relaxation [1], where the polymer and fullerene phases are not pure [2], and where short-lived delocalized states play an important role in both transport of the neutral excitation to a fullerene interface [3], as well as in the subsequent generation of long-range separated charge carriers [4]. In order to gain more insight to the dynamics of charged and neutral excited states in thin films of PBDTTPD and photovoltaically highly efficient PBDTTPD:PCBM blends [5], we carried out detailed (polarization-sensitive) transient absorption spectroscopy in the visible and near-infrared range. In particular, varying the excitation wavelength allowed to monitor the effect of excess energy and varying its intensity allowed to understand and minimize annihilation and recombination effects caused by high laser power. [1] N. Banerji et al., J. Am. Chem. Soc. (2010), 132, 17459 [2] F. C. Jamieson et al. Chem. Sci. (2012), 3, 485 [3] S. Cowan, N. Banerji et al., Adv. Funct. Mater. (2012), 22, 1116 [4] A. A. Bakulin et al., Science (2012), 335, 1340 [5] Zou et al., J. Am. Chem. Soc. (2010), 132, 5330

We recently reported over 8% power conversion efficiency for an inverted low-bandgap polymer solar cell, which is crucial for demonstrating the viability of this technology for large-scale roll-to-roll (R2R) processing. Despite this accomplishment, we recognize that the cell geometry is not the only factor determining their compatibility for large-scale manufacturing. Another key factor is the active layer thickness required to ensure high yields in the PV modules. Active layers used for typical laboratory-scale devices are too thin to ensure high yields in the manufacturing process. These devices are also limited in their external quantum efficiency (EQE) because of reduced light absorption in the thin active layer. In order to produce solar cells with high manufacturing yields and improved light harvesting, efficient polymer solar cells with a thicker active layer must be demonstrated. However, increasing the active layer thickness of low-bandgap polymer solar cells leads to significant fill factor (FF) reduction due to the buildup of photogenerated carriers, which increases the probability of carrier recombination and hinders carrier extraction. In this work, we investigated the thickness dependence of space-charge effects in polymer solar cells employing the low-band gap polymer polydithienogermole-thienopyrrolodione (PDTG-TPD). Efficiencies over 7.4% were obtained for devices with an active layer thickness le; 200 nm, illustrating the potential of this polymer for large-scale manufacturing. Although a high average EQE was obtained for devices with active layer thicknesses > 200 nm, the cell performance cannot be maintained due to FF reduction. In comparison, P3HT:PC61BM solar cells showed almost no reduction in FF for devices with active layer thickness up to 453 nm despite having similar space-charge limited (SCL) hole mobility values compared to PDTG-TPD:PC71BM. We demonstrated that space-charge effects limit the performance of PDTG-TPD:PC71BM solar cells with thicker active layer by measuring the light-intensity dependence of the FF, and by employing a SCL photocurrent model on the devices. Finally, a first-principles calculation is presented which highlights the effect of SCL photocurrent on photocurrent loss in low-bandgap polymer solar cells.

One of the basic concerns of organic solar cells is related to the narrow absorption window of the organic conjugated polymers. Since organic conjugated polymers are dominantly molecular excitonic absorbers, their absorption spectrum is characterized by a few 100 nm width instead of the plateau absorption spectra as seen for inorganic materials. Various strategies like synthesis of A-B copolymers, where A and B are different chromophores, as well as the addition of dyes or small molecular sensitizers, have been suggested to overcome this limitation. In this work, another elegant, alternative strategy is reviewed to extend the spectral sensitivity of wide bandgap polymers in the near IR region. In detail, the concept and functionality of ternary bulk heterojunction composites, consisting of two donor polymers with one fullerene is discussed and investigated in detail. The success of this strategy results in a strong IR sensitization of wide bandgap polymers with quantum efficiencies of up to 40 % in the near IR, and allows to increase the current densities of P3HT based solar cells beyond 12 mA/cm2.

The photocurrent generation of organic bulk-heterojunction (BHJ) based fiber solar cells has been studied. The BHJ fibers were fabricated by the coaxial electro-spinning of the photoactive materials blend and supporting polymers. The structure and orientation of crystalline domain inside the fiber were characterized by GIXRD and correlated with the photocurrent generation of fiber solar cells with laterally patterned electrodes.

Crystalline nanostructures of polymeric semiconductors are attracting enormous interest as a route towards high-efficiency organic photovoltaics. Crystalline nanofibers of polythiophene have been studied extensively since their initial discovery several years ago, yet relatively little is known of the photoluminescence properties of individual nanofibers, and the relation to intra and inter-chain coupling in these structures. Using single-molecule spectroscopy techniques, we have been investigating both wavelength- and time-resolved photoluminescence approaches to probe exciton dynamics in individual isolated nanofibers. We observe multiple vibronic progressions in individual P3HT nanofibers with spectral signatures of both intrachain (J-type) and interchain (H-like) exciton coupling. In the time-domain, we find interesting effects of aggregation and charge-separation (in different time regimes) that depend on molecular weight and regio-regularity of the polymer.