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Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C

The energetic landscape of charge transfer (CT) states at the interface of electron donating and electron accepting domains in organic optoelectronic devices is crucial for their performance. Central questions -- such as the role of static energetic disorder and vibrational effects -- are under ongoing dispute. This study provides an in-depth analysis of temperature dependent broadening of the spectroscopic absorption and emission features of CT states in devices with small molecule-fullerene blends. We confirm the validity of the electro-optical reciprocity relation between the photovoltaic external quantum efficiency ($\mathrm{EQE_{PV}}$) and electroluminescence ($\mathrm{EQE_{EL}}$), enabling us to validate the device temperature during the experiment. The validated temperature allows us to fit our experimental data with several models, and compare extracted CT state energies with the corresponding open circuit voltage limit at $0\mathrm{\,K}$. Our findings unveil that the absorption and emission characteristics are usually not symmetric, and dominated by temperature-activated broadening (vibrational) effects instead of static disorder., 11 pages, 5 figures submitted to: Physical Review Applied

Organic solar cells are approaching power conversion efficiencies of other thin‐film technologies. However, in order to become truly market competitive, the still substantial voltage losses need to be reduced. Here, the synthesis and characterization of four novel arylamine‐based push‐pull molecular donors was described, two of them exhibiting a methyl group at the para‐position of the external phenyl ring of the arylamine block. Assessing the charge‐transfer state properties and the effects of methylation on the open‐circuit voltage of the device showed that devices based on methylated versions of the molecular donors exhibited reduced voltage losses due to decreased non‐radiative recombination. Modelling suggested that methylation resulted in a tighter interaction between donor and acceptor molecules, turning into a larger oscillator strength to the charge‐transfer states, thereby ensuing reduced non‐radiative decay rates., Structural tuning improves efficiency: Modification of the molecular structure of organic molecules allows to influence the performance of organic photovoltaic devices. Addition of a methyl group in the para‐position of the external phenyl ring leads to increase in open‐circuit voltage. Quantum‐chemical calculations attribute this effect to the increase of the oscillator strength of the electronic transition between the donor and acceptor.

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control., Though single-material organic solar cells are attractive for next-generation photovoltaic technologies, designing new materials with ideal properties remains a challenge. Here, the authors report the use of orientation-dependent molecular electrostatics to realise efficient homojunction devices.

Interfaces between organic electron-donating (D) and electron-accepting (A) materials can show efficient free charge carrier generation upon illumination, enabling organic photovoltaic devices and photodetectors with photon to electron conversion yields approaching 100%. Recently, organic light-emitting diodes (OLEDs) based on charge transfer (CT) (or exciplex) emission occurring at such D-A interfaces have been shown to exhibit high electroluminescence external quantum yields (EQEEL). However, no organic D-A combination with both a high EQEEL, as well as a high free carrier generation yield has been discovered so far. Such a system would result in significantly higher operating voltages in organic solar cells, reduced dark current in organic photodetectors, and reduced driving voltages for OLEDs.

Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at similar to 800 nm with absorption coefficient of over 280 000 cm(-1). Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at similar to 790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm(-2). Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V-oc of 0.76 V.

Dynamic control of solar transmission by photovoltaic-powered electrochromic smart windows is an up-and-coming approach towards the reduction of energy consumption in buildings. Selectively-absorbing transparent organic solar cells are capable of exhibiting excellent visible-light transparency as well as respectable power conversion efficiencies. This work presents three different transparent small-molecule solar cells (TSCs) with an area of 2.52 cm2, two of which are UV-absorbing and one of which absorbs in the NIR, each in combination with an organic electrochromic device (EC). The NIR-absorbing TSC uses a BDP-OMe:C60 combination, and has a power conversion efficiency of 4% with an average visible light transmission (AVT) of 40%. The two UV-harvesting systems, α-6T/B4PYMPM and NPB:B4PYMPM, each have an AVT of 50–65%, as well as high open-circuit voltages of 1.5–2.0 V. We demonstrate that these photogenerated voltages are sufficient to power a fluoran dye-based organic EC, which has excellent optical properties: its color ranges from highly transparent to deep black, and its on/off contrast ratio is higher than 80% in the range between 390 nm and 640 nm. Self-powered smart windows made by the combination of TSCs and ECs provide a way to significantly reduce energy consumption from air conditioning and lighting, towards zero-energy buildings.

Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture., Strong light-matter coupling can tune exciton properties but its effect in photovoltaics remains unexplored. Here Nikolis et al. show that the photon energy loss from optical gap to open-circuit voltage can be reduced to unprecedented values by embedding organic solar cells in optical microcavities.

Achieving long-term stability in organic solar cells is a remaining bottleneck for the commercialization of this otherwise highly appealing technology. In this work, we study the performance and stability differences in standard and inverted DBP/C-70 based organic solar cells. Differences in the charge-transfer state properties of inverted and standard configuration DBP/C-70 solar cells are revealed by sensitive external quantum efficiency measurements, leading to differences in the open-circuit voltages of the devices. The degradation of standard and inverted solar cell configurations at ISOS aging test conditions (ISOS-D-3 and ISOS-T-3) was investigated and compared. The results indicate that the performance drop in the small molecule bilayer solar cells is less related to changes at the D-A interface, suggesting also a pronounced morphological stability, and instead, in the case of inverted cells, dominated by degradation at the electron transport layer (ETL) bathocuproine (BCP). Photoluminescence measurements, electron-only-device characteristics, and stability measurements show improved exciton blocking, electron transport properties and a higher stability for BCP/Ag ETL stacks, giving rise to inverted devices with enhanced performance and device stability. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/under REA Grant Agreement No. 607232, THINFACE. J.B., D.S. and K.V. acknowledge the German Federal Ministry for Education and Research (BMBF) for funding the project through the InnoProfile project "Organische p-i-n Bauelemente 2.2".

Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11). This work was supported by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile project 'Organische p-i-n Bauelemente 2.2' (03IPT602X) and by the German Research Foundation (DFG) project Photogen (VA 1035/5-1). X.J. and Y.L. acknowledge support from the China Scholarship Council (nos. 201706140127 and 201506920047, respectively). The authors also acknowledge the DFG for supporting K.T. (project 382633022 'RECOLPER'), F.P., S.Ro. and D.N. (SFB 951 'HIOS') and A.F. (RE 3198/6-1 'EFOD').

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 with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at similar to 6 mol % in C-60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well. The work was funded by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile Projekt "Organische p-i-n Bauelemente 2.2" (03IPT602X). This work was supported by the Deutsche Forschungsgemeinschaft (project OR 349/1-1), by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 (CAOP MUM), by the German Ministry of Science and Education (BMBF), project UNVEiL, and through a state sponsored scholarship for graduate students to Y.F. from the China Scholarship Council. M.N. acknowledges funding from "Svenska Tekniska Vetenskapsakademien i Finland". Grants for computing time from the Center for Information Services and High Performance Computing (ZIH) are gratefully acknowledged.

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

Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single-junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to (1.45-1.65) eV, that is, (0.2-0.3) eV higher than for technologies with minimized non-radiative voltage losses.

High photon energy losses limit the open-circuit voltage (V-OC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the V-OC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (alpha-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the V-OC of an alpha-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D-A interface. By accurately measuring the optical gap (E-opt) and the energy of the charge-transfer state (E-CT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. E-opt - qV(OC) losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the V-OC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices.

In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor–acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor–acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor–acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%–10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules, over distances of up to ≈4 nm. Hence, these results question the relevance of “pristine” phases and whether a continuous interpenetrating donor–acceptor network is the ideal morphology for charge transport. Funding agencies: German Federal Ministry for Education and Research (BMBF) through the InnoProfille project "Organische p-i-n Bauelemente 2.2"; Research Council of Lithuania [MIP-85/2015]; Science Council of Sweden; Knut and Alice Wallenberg foundation; Wallenberg Scholar

We investigate the photodegradation in inert atmosphere of (poly 3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester) (P3HT:PCBM) heterojunction solar cells under continuous illumination using advanced electrical characterization and a device modeling tool. Our results indicate that different failure mechanisms contribute to the performance loss. The first 250 h of illumination induced p-type doping and recombination related to traps in the blend which mainly decreases the short-circuit current and the efficiency of the cells. Device modeling and simulation allowed us to prove that increased p-type doping of the blend provoke the loss in the short-circuit current and the quantum efficiency by simultaneous reduction of charge carrier mobility and the electric field together with a shrink of the space charge region. Transmission electron microscopy (TEM) measurements reveal a change in the blend morphology upon long illumination times manifested by phase segregation in the blend. The reduction in the open-circuit voltage is reported to be related to a slight reduction of the charge transfer energy (CT) upon 700 h of illumination aging. The final failure mechanism was a rapid drop in the fill factor which occurs upon 1000 h of illumination and manifested by the appearance of an S-shape J–V characteristic. This failure mechanism is linked to the reduction of charge extraction caused by a reduced surface recombination velocity at the contacts.

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

P3HT:PCBM blends applied as active layers for bulk heterojunction organic solar cells generally show unstable morphologies upon prolonged thermal annealing, severely limiting the lifetime of the devices. As such, the thermodynamic instability of the blend is a limiting factor in the overall performance of organic photovoltaics, and a strong disadvantage in the fierce competition with other photovoltaic technologies. This paper shows whether different blend preparation conditions and intrinsic structural changes in the side chains of poly(3-alkylthiophene) (P3AT) derivatives can influence the thermal stability of the resulting solar cells. A combination of Bright Field Transmission Electron Microscopy (BFTEM) and the analysis of Selected Area Electron Diffraction (SAED) patterns revealed that the investigated preparation conditions do not really affect the thermal stability, whereas the introduction of a small ratio (10%) of specific functional moieties in the side chains of random P3AT copolymers does improve the thermal stability significantly. It was demonstrated that demixing of the blend components upon prolonged thermal annealing is strongly delayed in the functionalized P3AT:PCBM blends. The enhanced thermal stability was confirmed by in-situ monitoring of the short circuit current of organic solar cells based on the respective active layers. The introduction of functionalized side chains hence represents an attractive approach to increase the operational stability of organic photovoltaics based on the bulk heterojunction concept.

To improve the thermal stability of polymer:fullerene bulk heterojunction solar cells, a new polymer, poly[2-methoxy-5-(2'-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV), has been designed and synthesized, which showed an increased glass transition temperature (Tg) of 111 °C. The thermal characteristics and phase behavior of MPE-PPV:[6,6]-phenyl C61-butyric acid methyl ester ([60]PCBM) blends were investigated by means of modulated temperature differential scanning calorimetry and rapid heating–cooling calorimetry. The thermal stability of MPE-PPV:[60]PCBM solar cells was compared with devices based on the reference MDMO-PPV material with a Tg of 45 °C. Monitoring of the photocurrent–voltage characteristics at elevated temperatures revealed that the use of high-Tg MPE-PPV resulted in a substantial improvement of the thermal stability of the solar cells. Furthermore, a systematic transmission electron microscope study of the active polymer:fullerene layer at elevated temperatures likewise demonstrated a more stable morphology for the MPE-PPV:[60]PCBM blend. Both observations indicate that the use of high-Tg MPE-PPV as donor material leads to a reduced free movement of the fullerene molecules within the active layer of the photovoltaic device. Finally, optimization of the PPV:fullerene solar cells revealed that for both types of devices the use of [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM) resulted in a substantial increase of current density and power conversion efficiency, up to 3.0% for MDMO-PPV:[70]PCBM and 2.3% for MPE-PPV:[70]PCBM.

Lately, research efforts in photovoltaics towards hybrid solar cells based on nanostructured metal oxides and conjugated polymers have been intensifying. However, very limited effort has been spent so far to investigate their recombination kinetics in comparison with their fully organic counterpart. In this work, impedance spectroscopy under different illumination intensities is used to probe the recombination kinetics of hybrid solar cells based on ZnO nanorod arrays and poly(3-hexylthiophene). A recombination-based model developed for fully organic solar cells is effectively applied in our hybrid solar cells, demonstrating their similarity in device physics and establishing the nanorod array/polymer compound as true bulk heterojunction.

Electrical transport properties of poly (3-hexylthiophene) (P3HT) (6,6)-phenyl C61-butyric acid methyl ester (PCBM) solar cells, with and without encapsulation, have been investigated and analyzed using admittance spectroscopy and capacitance voltage measurements at different temperatures. The admittance spectroscopy clearly reveals two defect states with activation energies of 53 and 100 meV, and a concentration ten times higher in the unencapsulated sample. These defects seem to have a strong effect on the charge transport and the solar cell performance when they are present with a high concentration, since they lead to a decrease of the mobility and also the short-circuit current and the efficiency. The origin of these defects has been assigned to reaction of the blend with O2 which is also known to induce p-type doping in pure P3HT. In an attempt to understand the effect of these defects on the organic solar cell performance, modeling and simulation were carried out using the effective medium layer model and gave good agreement with the measurements results.

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C60 via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C60 excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices., The commercialisation of organic photovoltaic technology calls for research on material degradation mechanisms. Ramirez et al. show that triplet excitons produced by back charge transfer can significantly impact the photo-stability of fullerene-based devices even in the absence of water and oxygen.

The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses. This work was supported by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile project "Organische p-i-n Bauelemente 2.2" and the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska Curie Grant agreement No. 722651 (SEPOMO). F.P. and D.N. acknowledge funding by the German Research Foundation (DFG) via the SFB 951 "HIOS". The authors acknowledge Prof. K. Leo and V. C. Nikolis for fruitful discussions. The authors thank Dr. B. Beyer for supplying ZnF4Pc and CuF4Pc as well as Dr. M. Lau for the synthesis of F4CuPc. Additionally, the authors thank Prof. Bauerle from University of Ulm for the supply of DH4T, DH6T, and several DCV2-nT-R. Furthermore, the authors acknowledge Dr. F. Holzmueller, M. Saalfrank, and Dr. R. Meerheim for providing OSC devices for this study. Computational resources were provided by the Consortium des Equipements de Calcul Intensif (CECI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Federation Wallonie-Bruxelles, infrastructure funded by the Walloon Region under Grant Agreement No. 1117545. D.B. is a FNRS Research Director.