Search

While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading band gaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub-cell. Using the resulting design rules, three systems are selected, namely PBDB-T-2F:IO-4Cl, PBDB-T-2F:Y6 and PTB7-Th:COTIC-4F, and their potential as sub-cells in this geometry is experimentally investigated. With the aid of a custom built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub-cell. Finally, a working proof of concept monolithic device consisting of two sub-cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept. This article is protected by copyright. All rights reserved., The Spanish "Ministerio de Ciencia e Innovación (MICINN)" is gratefully acknowledged for its supportthrough grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo OchoaCentre of Excellence program and the AEI/FEDER(UE) grants PGC2018-095411-B-I00(RAINBOW),TED2021-131911B-I00and PID2021-128924OB-I00(ISOSCELLES). The authors also thank the Cata-lan agency AGAUR for grant2021-SGR-00444. MCV acknowledges a FPI fellowship (PRE2019-089855)from MICINN co-financed by the European Social Fund and MGR acknowledges the scholarship FPU16/02631 from the Spanish "Ministerio de Educación". MCV and MGR also thank the PhD programme in Materi-als Science from Universitat Autònoma de Barcelona in which both were enrolled. KV and QL acknowl-edge funding by the European Research Council (ERC, grant agreement864625)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

3D hybrid perovskites (APbX3) have made a significant impact on the field of optoelectronic materials due to their excellent performance combined with facile solution deposition and up-scalable device fabrication. Nonetheless, these materials suffer from environmental instability. To increase material stability, the organic cation (A) is substituted by the non-volatile cesium cation. However, the desired photoactive cesium lead(II) iodide black phase is metastable in ambient conditions and spontaneously converts into the photo-inactive yellow δ-phase. In this work, the black phase is stabilized by the formation of a quasi-2D perovskite containing a benzothieno[3,2-b]benzothiophene (BTBT) large organic ammonium cation. Thermal analysis shows that degradation of the butylammonium (BA)-based quasi-2D perovskite (BA)2CsPb2I7 sets in at ≈130 °C, while (BTBT)2CsPb2I7 is phase-stable until ≈230 °C. Additionally, the (BTBT)2CsPb2I7 film does not show any sign of degradation after exposure to 77% Relative Humidity in the dark for 152 days, while (BA)2CsPb2I7 degrades in a single day. Photoconductor-type detectors based on (BTBT)2CsPb2I7 demonstrate an increased external quantum efficiency and a similar specific detectivity compared to the BA-based reference detectors. The results demonstrate the utility of employing a BTBT cation within the organic layer of quasi-2D perovskites to significantly enhance the stability while maintaining the optoelectronic performance.

Spectroscopic photodetection plays a key role in many emerging applications such as context-aware optical sensing, wearable biometric monitoring, and biomedical imaging. Photodetectors based on organic semiconductors open many new possibilities in this field. However, ease of processing, tailorable optoelectronic properties, and sensitivity for faint light are still significant challenges. Here, the authors report a novel concept for a tunable spectral detector by combining an innovative transmission cavity structure with organic absorbers to yield narrowband organic photodetection in the wavelength range of 400-1100 nm, fabricated in a full-vacuum process. Benefiting from this strategy, one of the best performed narrowband organic photodetectors is achieved with a finely wavelength-selective photoresponse (full-width-at-half-maximum of approximate to 40 nm), ultrahigh specific detectivity above 10(14) Jones, the maximum response speed of 555 kHz, and a large dynamic range up to 168 dB. Particularly, an array of transmission cavity organic photodetectors is monolithically integrated on a small substrate to showcase a miniaturized spectrometer application, and a true proof-of-concept transmission spectrum measurement is successfully demonstrated. The excellent performance, the simple device fabrication as well as the possibility of high integration of this new concept challenge state-of-the-art low-noise silicon photodetectors and will mature the spectroscopic photodetection into technological realities. China Scholarship CouncilChina Scholarship Council [201706070125, 201706890003, 201706140127]; DFGGerman Research Foundation (DFG)European Commission [VA 1035/5-1]; Sachsische Aufbaubank [100325708]

Two novel aza-BODIPY derivatives with the fluorine atoms in the BF2 moiety replaced by cyano groups are synthesized. The introduction of cyano groups changes the phenyl substituents on the 3, 5 positions from parallel to antiparallel. The HOMO/LUMO energy levels are stabilized gradually when increasing the number of cyano groups and the photophysical properties show corresponding shifts. With high thermal stability, the derivatives can be purified by sublimation and to prepare vacuum deposited thin films. Our research extends the family of aza-BODIPY with cyano substituted derivatives.

Organic materials for near-infrared (NIR) photodetection are in the focus for developing organic optical-sensing devices. The choice of materials for bulk-type organic photodetectors is limited due to effects like high nonradiative recombination rates for low-gap materials. Here, an organic Schottky barrier photodetector with an integrated plasmonic nanohole electrode is proposed, enabling structure-dependent, sub-bandgap photodetection in the NIR. Photons are detected via internal photoemission (IPE) process over a metal/organic semiconductor Schottky barrier. The efficiency of IPE is improved by exciting localized surface plasmon resonances, which are further enhanced by coupling to an out-of-plane Fabry–Perot cavity within the metal/organic/metal device configuration. The device allows large on/off ratio (>1000) and the selective control of individual pixels by modulating the Schottky barrier height. The concept opens up new design and application possibilities for organic NIR photodetectors.

Efficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single-component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field-assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor-acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open-circuit voltage is maximized. V.C.N. and Y.D. contributed equally to this work. The authors would like to thank Prof. Dr. Dieter Neher for providing measurement time on the electroluminescence setup of his lab at University of Potsdam. They would also like to acknowledge the Optoelectronics group in the University of Cambridge for sharing the global analysis codes. This work was supported by the German Federal Ministry of Education and Research (BMBF) through the InnoProfile project "Organische p-i-n Bauelemente2.2" (FKZ 03IPT602X). A.A.B. is a Royal Society university research fellow. Open access funding enabled and organized by Projekt DEAL. Nikolis, VC (corresponding author), Tech Univ Dresden, Dresden Integrated Ctr Appl Phys & Photon Mat IAP, Nothnitzer Str 61, D-01187 Dresden, Germany ; Tech Univ Dresden, Inst Appl Phys, Nothnitzer Str 61, D-01187 Dresden, Germany. Bakulin, AA (corresponding author), Imperial Coll London, Mol Sci Res Hub, London W12 0BE, England. Vandewal, K (corresponding author), Hasselt Univ, Inst Mat Res IMO IMOMEC, Wetenschapspk 1, B-3590 Diepenbeek, Belgium. vasileios_christos.nikolis1@tu-dresden.de; a.bakulin@imperial.ac.uk; koen.vandewal@uhasselt.be

Organic solar cells are a promising technology for a large area conversion of sunlight into electricity. In particular for solar cells based on oligomers (small molecules), efficient donor materials absorbing wavelengths larger than 780 nm are still rare. Here, we investigate three aza-BODIPY dyes absorbing in the infrared. The addition of side groups leads to a red shift of the optical gap from 802 to 818 nm. In optimized devices using these donors in a bulk heterojunction with C-60, we observe a higher charge carrier mobility and a higher power conversion efficiency for the molecules without a methyl or methoxy side group lowering the molecular reorganization energy. Surprisingly, the donor-acceptor blend with the lowest energy loss during the electron transfer to the C-60 yields the highest short circuit current. With increasing size of the attached side chain, the devices exhibit a larger trap density, measured by impedance spectroscopy. Based on the investigation of different blend ratios, we conclude that these traps are mainly present in the donor phase. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The authors thank the German BMBF for funding within the scope of the projects InnoProfile 2.2 (03IPT602X) and MEDOS (03EK3503A) as well as the European Commission within the scope of the Career Integration Grant (FP7, MSCA, 630864). I.D.W.S. and M.T.S. acknowledge support from the European Research Council (grant number 321305) and from EPSRC (grant number EP/L017008/1). I.D.W.S. also acknowledges a Royal Society Wolfson Research Merit Award. K.L. is a fellow of the Canadian Institute for Advanced Research (CIFAR). The research data supporting this publication can be accessed at https://doi.org/10.17630/af263bfb-620c-40a8-9929-86658e5187d3 A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results. Postprint Postprint

Acknowledge support from the Office of Naval Research (N00014-14-1-0142), KAUST Center for Advanced Molecular Photovoltaics at Stanford and the Stanford Global Climate and Energy Program, NSF DMR-1303742 and the National Natural Science Foundation of China (Projects 21174004 and 21222403). Soft X-ray characterization and analysis by NCSU supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under Contract DE-FG02-98ER45737. Soft X-ray data was acquired at beamlines 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Professor Michael D. McGehee, Dr. George F. Burkhard and Dr. Eric T. Hoke for their help in discussion of the recombination mechanism.

The field dependence and yield of free charge carrier generation in polymer:fullerene blends with varying energetic offsets is not affected when the excitation energy is varied from above band-gap to direct CT state excitation. Instead, the ability of the CT state to split is dictated by the energetic offset between the relaxed CT state and the charge separated (CS) state.

The effects of sterics and molecular orientation on the open-circuit voltage and absorbance properties of charge-transfer states are explored in model bilayer organic photovoltaics. It is shown that the open-circuit voltage correlates linearly with the charge-transfer state energy and is not significantly influenced by electronic coupling.

An N-Type organic thin-film transistor (OTFT) based on doped 6,13-Bis(triisopropylsilylethynyl)pentacene is presented. A transition from p-type to n-type occurrs with increasing doping concentrations, and the highest performing n-channel OTFTs are obtained with 50 mol% dopant. X-ray diffraction, scanning Auger microscopy, and secondary ionization mass spectrometry are used to characterize the morphology of the blends. The high performance of the obtained transistors is attributed to the highly crystalline and aligned nature of the doped thin films.

For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo-stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light-induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1209–1214

Polymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC61BM. These devices achieve open-circuit voltages (Voc) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. Voc’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with 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 occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for efficient hole transfer. 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 values above 1.0 V and that non-fullerene acceptor materials with large optical gaps (>1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of Voc exceeding 1.0 V.

Semi-transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA-1 modified ITO coated glass substrate as the ohmic electron-collecting cathode and PEDOT:PSS PH1000 as the hole-collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition ( ∼ 90%) and high transmittance ( ∼ 50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency ( PCE ) compared to a single ST solar cell can be constructed by connecting the stacked two ST subcells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking fi ve separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell. Refereed/Peer-reviewed

In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (ED*) and the energy of the charge transfer state (ECT) will optimize the open-circuit voltage (Voc). In this work, this energy loss ED*-ECT is measured directly via Fourier-transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI-1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI-1 are similar (1.4 eV–1.5 eV), the optimized photovoltaic devices show large differences in Voc and internal quantum efficiency (IQE). For P3TI:PC71BM blends a ED*-ECT of ∼ 0.1 eV, a Voc of 0.7 V and an IQE of 87% are found. For PTI-1:PC61BM blends an absence of sub-gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher Voc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI-1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between ECT and qVoc due to radiative and non-radiative recombination are quantified for both material systems, indicating that for the PTI-1:PC61BM material system, Voc can only be increased by decreasing the non-radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (

The voltage loss, determined by the difference between the optical gap (E-g) and the open-circuit voltage (V-OC), is one of the most important parameters determining the performance of organic solar cells (OSCs). However, the variety of different methods used to determine E-g makes it hard to fairly compare voltages losses among different material systems. In this paper, the authors discuss and compare various E-g determination methods and show how they affect the detailed calculation of voltage losses, as well as predictions of the maximum achievable power conversion efficiency. The aim of this paper is to make it possible for the OSC community to compare voltage losses in a consistent and reasonable way. It is found that the voltage losses for strongly absorbed photons in state-of-the-art OSCs are not much less than 0.6 V, which still must be decreased to further enhance efficiency. Funding Agencies|Swedish Research Council VR [2017-00744]; Swedish Energy Agency Energimyndigheten [2016-010174]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; DFG [KI-1571/2-1]; China Scholarship Council

Efficient polymer solar cells typically contain the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which promotes dissociation of excited states and enhances charge transpo ...

The morphological, bipolar charge-carrier transport, and photovoltaic characteristics of poly(3-alkylthiophene) (P3AT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side-chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3-butylthiophene) (P3BT, m = 4), poly(3-pentylthiophene) (P3PT, m = 5), and poly(3-hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo-current yield (exceeding 10 mA cm−2) irrespective of side-chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole-only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field-effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.

Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]-phenyl C61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground-state charge-transfer complexes in the studied polymer–fullerene blends. The effective band gap (Eg) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open-circuit voltage (Voc) scales linearly with Eg. The redshift of the CT band upon thermal annealing of regioregular poly(3-hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open-circuit voltage of high-temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of Eg, proportional with the observed changes in Voc for different PPV:PCBM ratios. As Eg corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.

Evidence is presented for the formation of a weak ground-state charge-transfer complex in the blend films of poly[9,9-dioctylfluorene-co-N-(4-methoxyphenyl)diphenylamine] polymer (TFMO) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), using photothermal deflection spectroscopy (PDS) and photoluminescence (PL) spectroscopy. Comparison of this polymer blend with other polyfluorene polymer/PCBM blends shows that the appearance of this ground-state charge-transfer complex is correlated to the ionization potential of the polymer, but not to the optical gap of the polymer or the surface morphology of the blend film. Moreover, the polymer/PCBM blend films in which this charge-transfer complex is observed also exhibit efficient photocurrent generation in photovoltaic devices, suggesting that the charge-transfer complex may be involved in charge separation. Possible mechanisms for this charge-transfer state formation are discussed as well as the significance of this finding to the understanding and optimization of polymer blend solar cells.

K.R.G. and G.O.N.N. contributed equally to this work. K.R.G. and A.A. acknowledge SABIC for a postdoctoral fellowship. G.O.N.N., K.R.G., M.D.M., and A.A. acknowledge the KAUST GCR for a GRP-CF award. M.D.M. and S.S. acknowledge support from the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580. J.J.C and M.E.T. acknowledge support from the National Science Foundation Award No. CBET 1511757. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

One of the factors limiting the performance of organic solar cells (OSCs) is their large energy losses (E loss) in the conversion from photons to electrons, typically believed to be around 0.6 eV and often higher than those of inorganic solar cells. In this work, a novel low band gap polymer PIDTT-TID with a optical gap of 1.49 eV is synthesized and used as the donor combined with PC71BM in solar cells. These solar cells attain a good power conversion efficiency of 6.7% with a high open-circuit voltage of 1.0 V, leading to the E loss as low as 0.49 eV. A systematic study indicates that the driving force in this donor and acceptor system is sufficient for charge generation with the low E loss. This work pushes the minimal E loss of OSCs down to 0.49 eV, approaching the values of some inorganic and hybrid solar cells. It indicates the potential for further enhancement of the performance of OSCs by improving their V oc since the E loss can be minimized. Refereed/Peer-reviewed

The nanoscale morphology of the bulk heterojunction absorber layer in an organic solar cell (OSC) is of key importance for its efficiency. The morphology of high performance vacuum-processed, small molecule OSCs based on oligothiophene derivatives (DCV5T-Me) blended with C60 on various length scales is studied. The analytical electron microscopic techniques such as scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, highly sensitive external quantum efficiency measurements, and meso and nanoscale simulations are employed. Unique insights into the relation between processing, morphology, and efficiency of the final devices are obtained. It is shown that the connectivity of the oligothiophene-C60 network is independent of the material domain size. The decisive quantity controlling the internal quantum efficiency is the energetic disorder induced by material mixing, strongly limiting charge and exciton transport in the OSCs.

Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open-circuit voltage (Voc) with increasing amounts of a second donor or an acceptor. The Voc is strongly correlated with the energy of the charge transfer state in the blend, but this value depends on both local and mesoscopic orders. In this work, the behavior of Voc in the presence of a wide range of interfacial electronic states is investigated. The key charge transfer state interfaces responsible for Voc in several model systems with varying morphology are identified. Systems consisting of one donor with two fullerene molecules and of one acceptor with a donor polymer of varying regio-regularity are used. The effects from the changing energetic disorder in the material and from the variation due to a law of simple mixtures are quantified. It has been found that populating the higher-energy charge transfer states is not responsible for the observed change in Voc upon the addition of a third component. Aggregating polymers and miscible fullerenes are compared, and it has been concluded that in both cases charge delocalization, aggregation, and local polarization effects shift the lowest-energy charge transfer state distribution.

The photoinduced open-circuit voltage (Voc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn-in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the Voc losses observed during burn-in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the Voc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the Voc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against Voc losses during burn-in.

Organic solar cells lag behind their inorganic counterparts in efficiency due largely to low open-circuit voltages (Voc). In this work, a comprehensive framework for understanding and improving the open-circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is first shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of Voc losses to a combination of interfacial energetic disorder, non-negligible CT state binding energies, large degrees of mixing, and sub-ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature-dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63–104 meV of disorder is found in five systems, contributing 75–210 mV of Voc loss. This work provides an intuitive explanation for why qVoc is almost always 500–700 meV below the energy of the CT state and shows how the voltage can be improved.

DOI: 10.1002/admi.201400135 conventional oven for a period of 0 to 24 h, prior to delamination. The measured adhesion energy, G c (J m − 2 ), is shown as a function of annealing time in Figure 2 a. For any particular annealing temperature, the adhesion energy increased with annealing time. For example at 130 °C, G c increased from 1.3/ for no annealing to 2.76 J m – 2 and 2.81 J m – 2 for 30 min and 24 h annealing, respectively. The largest increase in G c generally happened within the fi rst 2 h of annealing. Conversely, at a constant annealing time, the G c increased with annealing temperature. The impact of such an adhesion increase has large implications on the reliability of these inverted OPV devices. After delamination, every adhesion specimen is split into two fractured halves: one includes the Ag electrode, referred to as the “Ag side” and the other includes the ZnO, referred to as the “ZnO side”, as shown on Figure 1 . Note that it does not refer to the interface between P3HT:PCBM and the ZnO layer or the interface between PEDOT:PSS and the Ag electrode. In order to understand the interfacial reinforcement mechanism responsible for the increase in G c , these delaminated surfaces were characterized using NEXAFS, UV-VIS absorption and X-ray photo spectroscopy (XPS). NEXAFS in Total Electron Yield (TEY) mode was used to quantify the surface composition at the top few nanometers of the delaminated surfaces (see experimental details). The carbon K-edge NEXAFS TEY spectra from pristine P3HT, PCBM and PEDOT:PSS are shown in Figure 3 a. The NEXAFS intensity is dominated by resonances arising from transitions from the 1s core level to unfi lled molecular orbitals of π* and σ* character, which are specifi c to the bonding within different functional groups. [ 11 ] We observed the characteristic π* and σ* resonances for the pure compounds consist with previous literature. [ 12 ]

There is a need to find electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open-circuit voltage (VOC), do not absorb strongly and are expensive. Here, a phenylimide-based acceptor molecule, 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (HPI-BT), that can be used to make solar cells with VOC values up to 1.11 V and power conversion efficiencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum efficiency of 56%, compared to 75–90% for polymer-fullerene devices, results from less efficient separation of geminate charge pairs. While favorable energetic offsets in the polymer-fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (