© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 836 FULL PAPER wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Adv. Energy Mater. 2011, 1, 836–843 1. Introduction Organic photovoltaics (OPV) has been suggested as a low-cost, lightweight, flexible alternative to inorganic photovoltaics. In particular, intense research is directed towards the development of OPV with a bulk heterojunction (BHJ) between donor-type conjugated polymers and acceptor-type fullerenes (e.g., poly(3- hexylthiophene) (P3HT)) and fullerene derivative, phenyl-C61- butyric acid methylester (PCBM)). [1,2] The most challenging problem in OPV technology to be solved yet is the development of devices that unite high effi- ciency, stability and processability. While inorganic solar cells exhibit operational lifetimes in the range of 25 years, a typical operational lifetime of encapsulated OPV under full solar illumination (1 sun = 100 mW/cm 2 ) has for a long period of time been in the range of only days or weeks. [3] Recently BHJ cells with operational life- time of 1 year and longer have been devel- oped. [4–7] This achievement opens various possibilities in OPV applications but also raises new problems, e.g. the need for rel- evant accelerated tests of operational life- time. Degradation mechanisms in OPV are complex and include a variety of processes: photo-bleaching of the photoactive layer and trap generation, [8,9] degradation of the hole conducting PEDOT:PSS layer, [10] ion migration from the electrodes and mor- phological changes of the device, [8] etc. These processes share the same catalysts (exposure of the device to light, heat, water and oxygen) and therefore are almost inseparable. This compli- cates the task of revealing the responsible process of a specific degradation phenomenon. Recently we suggested using concentrated sunlight for accelerated study of the degradation of OPV materials [11] and devices. [12] The degradation experiments with single layers of various polymers demonstrated an acceleration of the polymer photo-bleaching, conserving the chemistry observed at 1 sun even at highest illumination intensities. In particular, accelera- tion factors exceeding 100 were obtained for P3HT illumination by 200 suns (a level not obtainable by conventional heat-assisted acceleration [13] ). However, the performance of similar experi- ments with OPV cells can be more problematic and less predict- able due to the increased complexity of the system. Increased light intensity can not only accelerate the degradation processes occurring at 1 sun operation but also stimulate some “hidden” degradation mechanism. Here, we report an example of this situation by the study of BHJ OPV of inverted device architecture under concentrated light of ∼4 suns. This concentration level is not enough for suf- ficient acceleration of the photobleaching or trap-generation in the photoactive layer but it considerably stimulates such “hidden” degradation mechanism as generation of shunts in ZnO hole blocking layer. Inverted geometry OPV cells have developed to exclude low work function metal electrode that requires evaporation of metal onto the spin-coated organic layers. For large scale OPV processing such as roll-to-roll printing, metal vacuum evapo- ration is not suitable. Using high work function Ag electrode A. Manor, Dr. E. A. Katz Dept. of Solar Energy and Environmental Physics J. Blaustein Institutes for Desert Research Ben-Gurion University of the Negev Sede Boker Campus 84990, Israel E-mail: keugene@bgu.ac.il Dr. E. A. Katz The Ilse Katz Institute for Nanoscale Science and Technology Ben-Gurion University of the Negev Beersheva 84105, Israel T. Tromholt, Dr. F. C. Krebs Risø National Laboratory for Sustainable Energy Technical University of Denmark Frederiksborgvej 399, DK-4000 Roskilde, Denmark DOI: 10.1002/aenm.201100227 Assaf Manor, Eugene A. Katz,* Thomas Tromholt, and Frederik C. Krebs Electrical and Photo-Induced Degradation of ZnO Layers in Organic Photovoltaics We present the case of degradation of organic solar cells by sunlight concen- trated to a moderate level ( ∼4 suns). This concentration level is not enough for sufficient acceleration of the photobleaching or trap-generation in the photoactive layer and therefore such short treatment (100 minutes) does not affect the short-circuit current of the device. However, a significant degrada- tion of V OC and FF has been recorded by measurements of the cell current- voltage curves with a variation of light intensity, for the devices before and after the treatment. The same degradation was found to occur after short application of forward voltage biases in the dark. This kind of degradation is found to be repairable, and could even be prevented by simple electrical treat- ment (short pulses of the reverse bias). Moreover, even the fresh cells can be improved by the same process. Generation and degeneration of shunts in ZnO hole-blocking layer as underlying physical mechanisms for the cell degradation and restoration, respectively, can explain the results.