Influence of the polymer processing conditions on the performance of bulk heterojunction solar cells

We performed a systematic study of the effect of processing conditions on the performance of P3HT:PCBM solar cells. We have investigated the influence of the source material, solution preparation (stirring vs. sonication), additives (such as 1,8-octanedithiol), pre-formation of P3HT nanowires, and annealing on the device performance and/or morphology and phase separation in the active layer. Furthermore, the influence of spin-coating PCBM on top of P3HT and PCBM on top of the P3HT: PCBM layer has been investigated. We found that all of these factors affect the performance of the solar cells, although there are several alternative methods which can result in similar improvements of the performance. We found that the improvement trends for various procedures (additives, PCBM top layer, P3HT nanowires, etc.) are similar and also strongly dependent on the different source materials. The main factor determining in the obtainable efficiency is the solution preparation and the source materials. Obtained results and the implications on further improvement of solar cell performance are discussed in detail.