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In recent years, the power conversion efficiency of perovskite solar cells has increased substantially, reaching today's values of above 25%. This has also placed an increased focus on device stability, which still limits commercial use. Electron transport layers (ETL) are critical for both factors in perovskite solar cells and have received much attention since the research focus boomed for this technology. In this chapter, requirements on ideal ETL are first provided, including electronic and photonic properties, energy band alignment, morphology, defect states, and chemical stability. Different metal oxide and organic electron transport materials used for perovskite solar cells are reviewed, focusing on optimizing ETL using various doping strategies to improve the cells' efficiency. Finally, the progress made on electron transport materials used in mesoscopic perovskite solar cells, as well as normal (n–i–p) and inverted (p–i–n) planar perovskite solar cells is briefly discussed, highlighting device architecture considerations. This chapter reviews the ETL used in perovskite solar cells, emphasizing the considerations needed when embedding such layers inside perovskite solar cells.

The introduction of nonfullerene acceptors (NFAs) has pushed the power conversion efficiency and organic photovoltaics (OPV) device stability to new standards. In this aspect, removal of trace impurities from one purification stage to the next is frequently stressed throughout the synthesis of photoactive OPV materials and NFAs to obtain the highest-purity material. However, detailed studies of the effect of purification on device performance are less reported. Herein, the role of NFA trace impurities on the optoelectronic characteristics and lifetime of resulting OPV devices is studied. The optimization of PBDB-T:ITIC-X devices, with various ITIC purity levels (X), has been thoroughly studied via a combination of photophysical, chemical, morphological, electrical, and optical characterization techniques, to shine light on the role of these impurities on device performance and lifetime. The findings suggest that, even in materials with larger concentrations of trace impurities, careful tuning can produce high efficiencies. Interestingly, the less-pure materials lead to longer device lifetimes along with an enhancement in accumulative power generation by a factor 3, compared to the purest ITIC-based devices. This demonstrates that selecting a material with the highest purity may not always be the best option for NFA OPV and that any positive effects of NFA purification must be carefully considered in light of both the device efficiency and stability.

The performance of organic solar cells has improved significantly in recent years due to the use of non-fullerene acceptors (NFA). While processing additives are typically added to the active layer blends to enhance device performance in NFA organic solar cells, their impact on device degradation remains unclear. In this work we have compared the performance, in pristine and degraded state, between air-processed slot-die coated NFA ITO-free organic solar cells with and without the processing additive DIO, using a structure of PET/Ag/ZnO/PBDB-T:ITIC/FHC PEDOT:PSS. We observed an improvement in the power conversion efficiency of the devices when adding DIO, from 4.03% up to 4.97%. The evolution of the performance for both devices under ISOS-L1 life testing protocol reveals that the drop in efficiency is mainly due to a decay of JSC for both cells. In the short time scale the efficiency of non-DIO cells decays faster than the DIO cells, whereas in the long time scale the efficiency of non-DIO cells tends to stabilize sooner. Carrier mobilities estimated from impedance measurements decrease with time at similar rate for both degraded samples. Besides, DIO devices present a steep increase of the series resistance with time causing a decrease of the FF and thus of the efficiency. Moreover, in both degraded devices, the open-circuit voltage saturates with increasing illumination intensity. Numerical simulations reveal that a reduced anode work function of 5 eV is needed to fit experimental data.

Thin-film organic and perovskite solar cells have attracted the interest of research and industry. The main reason behind this is the low price, high tunability, and great potential of these technologies. Lightweight, color tunability, translucence, flexibility, and ease and low cost of processing are a few of the excellent characteristics of these technologies. However, they have a high sensitivity to light, oxygen, temperature, and humidity, which are common environmental conditions. Upon exposure to these environmental stresses, the chemical structure and morphological arrangement undergo complex changes that result in dramatic reduction of device functionalities over time. This chapter collects the approaches currently reported in the field of additive-assisted stabilization, a method in which solar cells’ active layers are blended with additives that promote their stability (e.g., hydrogen donors, radical scavengers, hydroperoxide decomposers, quenchers, UV absorbers).

The power conversion efficiency of thin film solar cells based on organic and perovskite materials has improved dramatically in recent years, currently reaching above 18% for organic photovoltaics and above 25% for perovskite solar cells. Combined with their appealing properties, such as mechanical flexibility, light weight, semi-transparency, and low-cost large-scale roll-to-roll (R2R) processing compatibility, the high conversion efficiencies have placed organic and perovskite solar cells at the center of attention in terms of promising PV technologies. In this context, 2D materials are, due to their unique properties such as high charge carrier mobility, high optical transparency and especially tunable electronic structure, ideal contact layer materials in thin film solar cell devices. They can be applied as electrodes, hole (HTL) and electron (ETL) transport layers, and additives in active layers. This review paper focuses on the integration of 2D graphene and its derivatives, and 2D materials beyond graphene, i.e. transitional metal dichalcogenides (TMD), MXene, black phosphorous (BP) and boron nitrides in organic and perovskite solar cells, and discusses the positive influence these material systems have shown on both the fundamental photophysical processes, as well as on device stability and lifetime. Furthermore, this review addresses the future potential of 2D materials for the development of lightweight, eco-friendly, high performance and cost-effective flexible solar cells, provided by the high mechanical flexibility, high environmental stability, low electrical resistivity, and low environmental impact of these 2D material systems.

Photochemical and mechanical stability are critical in the production and application of organic solar cells. While these factors can individually be improved using different additives, there is no example of studies on the combined effects of such additive-assisted stabilization. In this study, the properties of PTB7:[70]PCBM organic solar cells are studied upon implementation of two additives: the carotenoid astaxanthin (AX) for photochemical stability and the silicone polydimethylsiloxane (PDMS) for improved mechanical properties. A newly designed additive, AXcPDMS, based on astaxanthin covalently bonded to PDMS was also examined. Lifetime tests, produced in ISOS-L-2 conditions, reveal an improvement in the accumulated power generation (APG) of 10% with pure AX, of 90% when AX is paired with PDMS, and of 140% when AXcPDMS is added in the active layer blend, as compared to the control devices. Singlet oxygen phosphorescence measurements are utilized to study the ability of AX and AXcPDMS to quench singlet oxygen and its precursors in the films. The data are consistent with the strong stabilization effect of the carotenoids. While AX and AXcPDMS are both efficient photochemical stabilizers, the improvement in device stability observed in the presence of AXcPDMS is likely due to a more favorable localization of the stabilizer within the blend. The mechanical properties of the active layers were investigated by tensile testing and cohesive fracture measurements, showing a joint improvement of the photooxidative stability and the mechanical properties, thus yielding organic solar cell devices that are promising for flexible photovoltaic applications.

Degradation of perovskite solar cells (PSCs) is often found to be partially or fully reversible when the cells are allowed to recover in the dark. Unlike the dynamics of degradation, knowledge about the dynamics of PSC cell recovery is very limited. Here, we demonstrate that the PSC recovery strongly depends on the electrical bias conditions during the light-induced degradation and that it can be manipulated by applying an external electrical bias during the recovery phase. Investigation of the recovery dynamics allows us to analyze the degradation mechanisms in detail. More specifically, we aged a mixed-cation mixed-halide PSC with a n-i-p structure under illumination in open-circuit (OC) or short-circuit (SC) conditions, and periodically measured their characteristics during the recovery. PSCs aged in SC degrade faster and fully recover after the light is switched off, while the performance of the cells aged in OC does not recover but instead further decreases after the light is switched off (“drop-in-dark” effect). With the use of transient photoluminescence, secondary ion mass spectrometry, and drift-diffusion-based simulations, we hypothesize that extrinsic ion migration causes the drop-in-dark effect, by forming an electron extraction barrier at the metal oxide electron transport layer. The applied bias alleviates this effect. Our results are relevant for gaining a deeper understanding of the multiple degradation mechanisms present in perovskite solar cells, and for finding a practical way to assist their recovery.

The development of nonfullerene acceptors (NFAs) has led to dramatic improvements in the device efficiencies of organic photovoltaic (OPV) cells. To date it is, however, still unclear how those laboratory-scale efficiencies transfer to commercial modules, and how stable these devices will be when processed via industrially compatible methods. Herein, the degradation behavior of lab-scale and scalable OPV devices using similar nonfullerene-based active layers is assessed. It is demonstrated that the scalable NFA OPV exhibits completely reversible degradation when assessed in ISOS-O-1 outdoor conditions, which is in contrast to the laboratory-scale devices assessed via the indoor ISOS-L-2 protocol. Results from transient photovoltage (TPV) indicate the presence of charge trap formation, and a number of potential mechanisms are proposed for the selective occurrence of this in laboratory-scale devices tested in ISOS-L laboratory conditions—ultimately concluding that it has its origins in the different device architectures used. The study points at the risk of assessing active layer stability from laboratory-scale devices and degradation studies alone and highlights the importance of using a diverse range of testing conditions and ISOS protocols for such assessment.

Degradation of perovskite solar cells is shown to be partly reversible in the dark, which significantly affects the lifetime of the cell under operational conditions. In this contribution, complex interplay between reversible and irreversible degradation is discussed on the basis of indoor illumination-recovery experiments and outdoor exposure to natural diurnal cycle. We demonstrate that both photodegradation and subsequent recovery processes are strongly affected by the electrical bias condition during the aging. Light-induced ion migration and metastable traps formation with different dominance under different biases and at different degradation stages might explain observed features.

In this work, we have studied degradation mechanisms of nonfullerene-based organic solar cells with PET/Ag/ZnO/PBDTB-T:ITIC/PEDOT:PSS/CPP PEDOT:PSS device structure. We compare pristine and degraded samples that were subjected to outdoor degradation following the standard ISOS-O2 protocol. The ideality factors for different incident wavelengths obtained from open-circuit voltage vs irradiation level and current density–voltage (J–V) measurements at different temperatures indicate that for aged samples recombination is governed by the Shockley–Read–Hall mechanism occurring in a region near the anode. Samples were also characterized using impedance spectroscopy (IS) and fitted to an electrical model. Impedance parameters were used to obtain mobility, indicating a clear degradation of the active layer blend for aged samples. The change in the chemical capacitance also reveals a worsening in carrier extraction. Finally, two-dimensional (2D) numerical simulations and fits to experimental J–V curves confirm the existence of a layer near the anode contact with poorer mobility and a decrease in the anode work function (WF) for the degraded samples. This work was supported by Comunidad de Madrid under the SINFOTON2-CM Research Program (S2018/NMT-4326-SINFOTON2-CM) and the Spanish Ministry of Economy, the Agencia Estatal de Investigación, and European Union's FEDER under the TEC2016−77242-C1/C2/C3 AEI/FEDER, UE Projects. The work of E.L.-F. was supported by the Ministerio de Educación y Formación Profesional for his Doctoral Grant through the FPU Research Fellowship under Grant FPU17/00612. M.M., J.L., E.D., and V.T. acknowledge that part of this work was developed within the RollFlex project, part-financed by Interreg Deutschland-Danmark with means from the European Regional Development Fund and the Southern Denmark Growth Forum. M.M. and V.T. acknowledge the support from the Villum Foundation for Project CompliantPV (Grant No. 13365). Finally, all authors acknowledge the support from the EU Framework Program Horizon 2020 for MNPS COST ACTION MP1307 StableNextSol. Publicado

Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.

Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis., This article is based upon work from COST Action StableNextSol MP1307 supported by COST (European Cooperation in Science and Technology). M.V.K., E.A.K., V.B. and A.O. thank the financial support of the United States – Israel Binational Science Foundation (grant no. 2015757). E.A.K., A.A. and I.V.-F. acknowledge partial support from the SNaPSHoTs project in the framework of the German-Israeli bilateral R&D cooperation in the field of applied nanotechnology. M.S.L. thanks the financial support of National Science Foundation (ECCS, award #1610833). S.C., M.Manceau and M.Matheron thank the financial support of European Union’s Horizon 2020 research and innovation programme under grant agreement no 763989 (APOLO project). F.D.R. and T.M.W. would like to acknowledge the support from the Engineering and Physical Sciences Research Council (EPSRC) through the SPECIFIC Innovation and Knowledge Centre (EP/N020863/1) and express their gratitude to the Welsh Government for their support of the Ser Solar programme. P.A.T. acknowledges financial support from the Russian Science Foundation (project No. 19-73-30020). J.K. acknowledges the support by the Solar Photovoltaic Academic Research Consortium II (SPARC II) project, gratefully funded by WEFO. M.K.N. acknowledges financial support from Innosuisse project 25590.1 PFNM-NM, Solaronix, Aubonne, Switzerland. C.-Q.M. would like to acknowledge The Bureau of International Cooperation of Chinese Academy of Sciences for the support of ISOS11 and the Ministry of Science and Technology of China for the financial support (no. 2016YFA0200700). N.G.P. acknowledges financial support from the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF-2012M3A6A7054861 and NRF-2014M3A6A7060583 (Global Frontier R&D Program on Center for Multiscale Energy System). CSIRO’s contribution to this work was conducted with funding support from the Australian Renewable Energy Agency (ARENA) through its Advancing Renewables Program. A.F.N gratefully acknowledges support from FAPESP (Grant 2017/11986-5) and Shell and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. Y.-L.L. and Q.B. acknowledge support from the National Science Foundation Division of Civil, Mechanical and Manufacturing Innovation under award no. 1824674. S.D.S. acknowledges the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement no. 756962), and the Royal Society and Tata Group (UF150033). The work at the National Renewable Energy Laboratory was supported by the US Department of Energy (DOE) under contract DE-AC36-08GO28308 with Alliance for Sustainable Energy LLC, the manager and operator of the National Renewable Energy Laboratory. The authors (J.J.B, J.M.L., M.O.R, K.Z.) acknowledge support from the ‘De-risking halide perovskite solar cells’ program of the National Center for Photovoltaics, funded by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technology Office. The views expressed in the article do not necessarily represent the views of the DOE or the US Government. H.J.S. acknowledges the support of EPSRC UK, Engineering and Physical Sciences Research Council. V.T. and M.Madsen acknowledge ‘Villum Foundation’ for funding of the project CompliantPV, under project no. 13365. M.Madsen acknowledges Danmarks Frie Forskningsfond, DFF FTP for funding of the project React-PV, no. 8022-00389B. M.G. and S.M.Z. thank the King Abdulaziz City for Science and technology (KACST) for financial support. S.V. acknowledges TKI-UE/Ministry of Economic Affairs for financial support of the TKI-UE toeslag project POP-ART (no. 1621103). RC thanks the grants for Development of New Faculty Staff, Ratchadaphiseksomphot Endowment Fund. A.D.C. gratefully acknowledges funding from the European Union’s Horizon 2020 Research and Innovation Program (grant agreement no. 785219-GrapheneCore2 and no. 764047-ESPResSo). M.L.C. and H.X. acknowledges the support from Spanish MINECO for the grant GraPErOs (ENE2016-79282-C5-2-R), the OrgEnergy Excellence Network CTQ2016-81911- REDT, the Agència de Gestiód’Ajuts Universitaris i de Recerca (AGAUR) for the support to the consolidated Catalonia research group 2017 SGR 329 and the Xarxa de Referència en Materials Avançats per a l’Energia (Xarmae). ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant no. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya.

Recent efficiency records of organic photovoltaics (OPV) highlight stability as a limiting weakness. Incorporation of stabilizers is a desirable approach for inhibiting degradation-it is inexpensive and readily up-scalable. However, to date, such additives have had limited success. We show that β-carotene (BC), an inexpensive and green, naturally occurring antioxidant, dramatically improves OPV stability. When compared to nonstabilized reference devices, the accumulated power generation of PTB7:[70]PCBM devices in the presence of BC increases by an impressive factor of 6, due to stabilization of both the burn-in and the lifetime, and by a factor of 21 for P3HT:[60]PCBM devices, owing to a longer lifetime. Using electron spin resonance and time-resolved near-IR emission spectroscopies, we probed radical and singlet oxygen concentrations. We demonstrate that singlet oxygen sensitized by [70]PCBM causes the "burn-in" of PTB7:[70]PCBM devices and that BC effectively mitigates it. Our results provide an effective solution to the problem that currently limits widespread use of OPV.

Photo-initiated, oxygen-mediated degradation of the molecules in the active layer of organic photovoltaic, OPV, devices currently limits advances in the development of solar cells. To address this problem systematically and at a molecular level, it is informative to quantify the kinetics of the pertinent processes, both in solution phase and in solid films. To this end, we examined the oxygendependent photophysics and photochemistry of selected functionalized fullerenes, thiophenederivatives, and a subphthalocyanine commonly used in OPV devices. We find that the photosensitized production of singlet molecular oxygen, O2(a1 Δg), by these molecules is a key step in the degradation process. We demonstrate that the addition of either β-carotene or astaxanthin as antioxidants can inhibit degradation by a combination of three processes: (a) deactivation of O2(a1 Δg)to the oxygen ground state, O2(X3 Σg−), (b) quenching of the O2(a1 Δg) precursor, and (c)sacrificial reactions of the carotenoid with free radicals formed in the photo-initiated reactions. For OPV systems in which reaction with O2(a1Δg) contributes to the degradation, the first two of these processes are desired and should have appreciable impact in prolonging the longevity of OPV devicesbecause they do not result in a chemical change of the system.

High work function metal oxides such as molybdenum oxide (MoOx) have demonstrated good hole contact properties in organic photovoltaic (OPV) devices in the past years and have replaced the otherwise conventionally used PEDOT:PSS. In this work, we introduce new crystalline MoOx layers that outperform the otherwise commonly used thermally grown MoOx layers in OPV devices. These hole contact layers are developed from superoxidized MoO3.2 films grown by reactive sputtering, followed by postannealing in high vacuum to induce crystallization of the otherwise amorphous MoOx films. Standard configuration organic solar cell devices based on DBP as electron donor and C70 as electron acceptor were developed on top of the sputtered MoOx, and a large increase in power conversion efficiency as a function of the MoOx annealing temperature was observed, which is in clear contrast to devices grown on thermally deposited MoOx. The crystallization of the sputtered MoOx at elevated temperatures is shown to lead to high work function films with improved conductivity, resulting in the appealing device properties. Importantly, long-term stability investigations revealed that devices based on these crystalline MoOx films exhibit superior stability as compared to devices based on thermally grown MoOx. These characteristics show that crystalline MoOx prepared by postannealing sputter deposited films forms a superior hole contact layer material for future air-stable organic optoelectronic devices.

The operational stability of perovskite solar cells (PSCs) remains a limiting factor in their commercial implementation. We studied the long-term outdoor stability of ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-OMeTAD/Au cells, as well as the dynamics of their degradation, under simulated sunlight indoors and their recovery in the dark. The extent of overall degradation was found to depend on processes occurring both under illumination and in the dark, i.e., during the daytime and nighttime, with the dynamics varying with cell aging. Full recovery of efficiency in the dark was observed for cells at early degradation stages. Further cell degradation resulted in recovery times much longer than one night, appearing as irreversible degradation under real operational conditions. At later degradation stages, very different dynamics were observed: short-circuit current density and fill factor exhibited a pronounced drop upon light turn-off but strong improvement under subsequent illumination. The interplay of reversible and irreversible degradation processes with different recovery dynamics was demonstrated to result in changes in the cell's diurnal PCE dependence during its operational lifespan under real sunlight conditions.

The development of hybrid organic–inorganic halide perovskite solar cells (PSCs) that combine high performance and operational stability is vital for implementing this technology. Recently, reversible improvement and degradation of PSC efficiency have been reported under illumination–darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variations is challenging. We report the outdoor stability measurements of two types of devices showing either reversible photo-degradation or reversible efficiency improvement under sunlight. Instead of the initial (or stabilized) efficiency and T80 as the figures of merit for the performance and stability of such devices, we propose using the value of the energy output generated during the first day of exposure and the time needed to reach its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of process prevailing in a given perovskite cell., This work was supported by the European Commission’s StableNextSol COST Action MP1307.

Under general working conditions, organic solar cells are exposed to a multitude of degradative stresses, including light, oxygen, temperature and humidity. We selectively exposed the layer stacks of three different material systems, P3HT, PCDTBT and AnE-PVcc in bulk heterojunction with fullerene, to different combinations of stresses, starting from one single stress and then iteratively adding a new stress to the combination. This allowed us to determine how harshly different polymers get affected by individual stress factors. P3HT based devices show an overall higher stability compared to PCDTBT or AnE-PVcc. All of the devices are affected the most by combined influences of light, oxygen, humidity and temperature, and they cease functioning within a couple of hours of exposure. A thermodynamically driven formation of a thin PCBM layer at the anode is observed with the prolonged storage, which causes an increase in the current densities for the large reverse voltages. We explain this phenomenon by tunneling from the fullerene HOMO to the LUMO, as a result of a large gradient in the electric potential over this layer. The increase of open circuit voltage of the pre-annealed P3HT devices upon UV photolysis was investigated, and attributed to the UV irradiation facilitated destruction of the face-on P3HT layer grown on the surface of the active layer. The effect of the sequence in which the annealing step is performed with respect to evaporation and degradation on the performance upon degradation is discussed.

We used spectroscopic ellipsometry to simultaneously reveal the distribution of the fullerene component, as well as the formation of higher ordered polymer domains, within the active layer of organ...

This study correlates the device performance of organic solar cells and the electronic charge transport within polymer/fullerene films, directly to the optical order of the polymer. The optical order was measured by spectroscopic ellipsometry and evaluated by our previously derived model. We were able to determine the in-depth distribution of higher and lower ordered poly(3-hexylthiophene) (P3HT) domains within an [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) matrix. The over the film thickness integrated volume fraction of highly ordered P3HT domains could be directly correlated to the corresponding solar cell device performance. We are able to describe various thermally annealing conditions between room-temperature and 200 °C.

Stability of organic solar cells (OPV) remains a big problem on the way to their commercialization. Different approaches are being investigated: development of intrinsically more photochemically stable materials, optimization of encapsulation, and implementation of getter and UV blocking layers. In this study, we investigate stabilization of OPV devices using hydroperoxide decomposers as stabilizing additives. A set of five commercially available additives of organophosphorus, organosulfur, Ni chelate, and blocked thiol type are compared, ternary blended into the active layer, under exposure to aging under ISOS-3 degradation conditions. Improvements in long-term performance of OPV devices were observed upon stabilization with Advapak NEO-1120, lifetime was prolonged by a factor of 1.7, and accumulated power generation increased by a factor of 1.4. The stabilizing mechanisms are discussed using spectroscopic and microscopic measurements.

We apply spectral ellipsometry (SE) to analyze the in-depth morphology of a polymer/fullerene film. Observed fullerene concentration gradients within the film are investigated with respect to temporal changes. The changes within the fractional composition of the polymer/fullerene film are resolved and can be correlated to the formation of fullerene segregations near the surface.

The changes in the blend morphology of the thin films accelerate when thermally annealed at increased temperatures. Tapping-mode atomic force microscopy (AFM) measurements provide deeper insight into the nano- and micro-meter scale of the phase separation observable on the surface of the film. Furthermore, to prove the coarsening of phase separation on various length scales, optical microscopy, UV–Vis, photoluminescence and measurements were conducted. Of special interest is tracking down the formation of fullerene aggregates and correlation of their growth in time. We demonstrate ways to detect characteristic stages of phase separation directly using simple optical measurements.

The influence of the polymer/fullerene blend ratio on the morphological properties of organic solar cells is investigated. Spectroscopic ellipsometry is applied as a tool for analyzing fullerene domains and the influence on polymer crystallinity within the film. Furthermore, the measurements are correlated with a percolation limit of as-cast films at around 40 wt% fullerene content.

We report on the improvement of long-term stability of organic solar cells (OPV) using hindered phenol based antioxidants as stabilizing additives. A set of seven commercially available hindered phenols are investigated for use in bulk-heterojunction OPV. Polymer:fullerene films based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are characterized with respect to the initial power conversion efficiency and the long-term stability improvement under illumination in ambient conditions. FTIR spectroscopy is used to trace chemical degradation over time. OPV performance is recorded under ISOS-3 conditions, and an improved long-term performance of OPV devices, manifested in increased accumulated power generation (APG), is found for octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. Using this additive, APG is increased by a factor of 3 compared to the reference. Observed differences in the stabilization of tested additives are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological changes, and chemical structure.

UV-blocking layers can increase the long term stability of organic solar cell devices; however, they limit the amount of light that can be utilized for energy conversion. We present photon recycling and down-conversion via a luminescent layer across a UV-blocking TiO2 layer. Our results show that the use of an additional UV-blocking layer does not necessarily reduce the overall efficiency of organic solar cells, since the loss in photocurrent due to the UV-absorption loss can be partially compensated using high energy photon down-conversion via luminescence layers.