COMMUNICATION 541 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Adv. Energy Mater. 2012, 2, 541-545Despite recent achievements in organic photovoltaic (OPV) technologies, the solar cells based on them still do not make a large contribution to the energy supply owing to their low energy conversion effi ciency and lack of long-term stability. Prepared from organic semiconductors such as molecular or polymeric fi lms, they offer promising advantages for low-cost solar energy applications. [ 1 , 2 ] Intensive research in recent years on bulk heterojunction (BHJ) polymer-fullerene solar cells led to fabrication of devices with power conversion effi ciencies (PCEs) between 4% and 5% for poly(3-hexylthiophene) and phenyl C61 butyric acid methyl ester (P3HT:PCBM) [ 3 , 4 ] and up to 8% for the new generation of low-bandgap polymers. [5][6][7] The growing importance of these devices is related to the possibility of preparing them by wet processes on lightweight fl exible substrates, making them potentially suitable for large-area low-cost fabrication. [ 8 , 9 ] Different ways to improve the device performances (mainly their effi ciencies and stability) have been explored: i) changing the photoactive donor-acceptor system [ 10 ] or ii) proposing new device architectures. The conventional BHJ polymeric solar cell (PSC) with an active layer sandwiched between a low-work-function aluminum cathode and a hole-conduction poly(3,4-ethylenedioxylenethiophene): poly(styrenesulfonicacid) (PEDOT:PSS) layer on top of an indium tin oxide (ITO) substrate is the most widely used and investigated confi guration. Under prolonged exposure to illumination these PSCs suffer from rapidly reduced performances. A very effective way to circumvent this problem was proposed with the so-called inverted device. [ 9 , 11 , 12 ] By taking advantage of the ability to reverse the polarity of charge collection in an inverted confi guration, an airstable high-work-function metal such as Ag is substituted for the air-sensitive Al as the anodic electrode for hole collection, while metal oxides such as TiO x and ZnO act as the electroactive interface at the ITO interface. Thus for high-performance PSCs, an important key parameter appears to be the appropriate selection of this electron-transporting layer (ETL), the purpose of which is also to provide hole-blocking capability and a low-resistance pathway for effi cient electron extraction. A literature survey shows that the integration of ZnO layers as an ETL leads to an increased effi ciency and stability of inverted PSCs. [ 13 ] Furthermore, different nanostructures, such as ZnO nanorods [14][15][16][17] or ZnO nanoridges, [ 18 ] have been used to improve the electron transport in BHJ solar cells. Inspired by this fi rst work, very recently PCEs of more than 4% (but for an active surface area of 0.06 cm 2 ) on glass substrates with an electrodeposited ZnO interlayer have been reported. [ 19 , 20 ] Nevertheless, even with such impressive PCEs, it is still challenging to develop polyme...