Inverted Polymer Solar Cells with Reduced Interface Recombination

Conjugated polymer solar cells are attractive as a source of renewable energy due to their compatibility with low cost rollto- roll printing processes. [ 1 ] The most effi cient of these polymer cells utilize a bulk heterojunction (BHJ) morphology, most often fabricated by blending a light-absorbing, electron-donating, and hole-transporting polymer with an electron-accepting fullerene derivative to form an active layer, which separates and transports charge carriers to the electrodes. While conventional BHJ cells are fabricated with a top cathode which requires a low work function metal for effi cient electron extraction, [ 2-5 ] this device architecture is not viable for large area processing due to the poor stability of these cathode materials. To circumvent this problem, an inverted cell geometry is a more practical architecture for existing roll-to-roll processes with a bottom cathode prepared using a modifi ed transparent electrode. [ 6 ] To fabricate inverted polymer solar cells, one widely used approach is to deposit a thin fi lm of zinc oxide (ZnO) nanoparticles as an electron extraction layer on top of a transparent indium tin oxide (ITO) coated substrate. Due to the simple synthesis routine [ 7 , 8 ] and the capability of low-temperature fi lm processing, ZnO colloidal nanoparticles (NPs) is the most commonly used electron extraction material for inverted polymer cells. [ 9-12 ] However, the reported inverted cells generally have a lower power conversion effi ciency (PCE) than the conventional cells with a top electron extraction layer. It is known that up to 30% of the atomic bonds in ZnO NPs are dangling bonds [ 13 ] and these defects give rise to a high density of recombination centers resulting in low power conversion effi ciencies in these inverted cells. [ 14 ] These defect states are sensitive to light. It has been documented that a brief exposure of the device to ultraviolet (UV) light helps partially fi lling the defect states in ZnO NPs and results in enhanced device performance. [ 15 ] Typically, this light soaking is done by exposing the device to a solar simulator and the effect lasts for a period of several days with the device characteristics ultimately degrading over time. In addition to colloidal ZnO NPs, several groups reported using sol- gel ZnO as an alternative for the electron extraction layer. [ 16-19 ] Sol-gel ZnO fi lms are formed by annealing zinc acetate fi lms at high temperatures. Using this approach, we recently demonstrated an inverted cell with a PCE exceeding 8%. [ 15 ] However, sol-gel derived ZnO requires an annealing temperature above 200 ° C. Therefore, colloidal ZnO NPs is a choice of materials for electron extraction in polymer solar cells due to its low temperature ( < 100 ° C) processing capability. In this communication, we fi rst show that light soaking cannot completely prevent photocurrent loss. Based on our photoluminescence and recombination study, we demonstrate a UV-ozone (UVO) treatment of ZnO NP fi lms immediately after processing can effectively passivate the defect states leading to longer carrier lifetimes in PV devices compared to devices treated only by light soaking. The resulting polymer PV devices with this treatment give enhanced short-circuit currents ( J sc ). Employing one specifi c high performance donor-acceptor (DA) copolymer, cells with PCEs exceeding 8% are demonstrated. © 2012 WILEY-VCH Verlag GmbH & Co.