ИСТИНА

The star-shaped oligothiophenes (SSO) are promising donor materials for bulk heterojunction organic photovoltaics. The efficiency higher than 5% was achieved with SSO using the normal structure of organic solar cell with bottom transparent high work-function electrode and top low work-function electrode . However, the inverted structure usually provides higher air stability of organic solar cells . Moreover, when studying new materials for bulk heterojunction solar cells both the structures should be compared because the vertical phase segregation of donor and acceptor phases may occur in the active layer. If the donor is on top of the active layer and the acceptor at the bottom, the inverted structure will be more preferable for efficient collection of photogenerated charges. In this work, we study three different SSO as donor components for bulk heterojunction organic solar cells and compare their photovoltaic performance in the inverted and normal structures. Figure 1 shows the cell structures, general formulas of studied SSO, and I/V curves for one of the studied SSO. For the inverted cell, the short-circuit current is a bit higher than for the normal one. However, the open-circuit voltage and fill factor are slightly lower in the former probably because of leakage currents as implied from the dark characteristics. The energy conversion efficiency for both architectures is about 4%. The roles of vertical donor-acceptor phase segregation, electrode buffer layers and shunts in difference of photovoltaic performance for the inverted and normal structures with the different SSO are discussed. The work on synthesis of SSO was supported by Russian Science Foundation (grant 14-13-01380) and photovoltaics study was supported by RFBR (project № 14-02-31823) and European Comission’s StableNextSol COST Action MP1307.