Vertically phase-separation based on amination-functionalized fullerene derivatives in inverted polymer solar cells

Abstract

Vertical stratification of the polymer solar cells in inverted device architecture is an efficient method to obtain high photovoltaic conversion characteristics. Herein, we establish a connection between the vertical stratification and interfacial engineering, which can simplify the device fabrication process and achieve preferable device performance. Then a straight forward synthetic amination-functionalized fullerene derivative PCBDCU has been synthesized and utilized as an additive in the inverted polymer solar cells. When doped with 10 wt% PCBDCU (relative to PC61BM), the average power conversion efficiency of ITO/P3HT:PC61BM: PCBDCU/MoO3/Ag achieves 3.06%, which is obviously better than that of the reference device with the structure of ITO/P3HT: PC61BM/MoO3/Ag (1.21%). The reason for the eminent enhancement of PCE can attribute to that the spontaneous activity of PCBDCU in donor-acceptor bulk heterojunction blends formed a beneficial vertical phase separation. A small quantity of PCBDCU moved to buried ITO surface and kept an intense interaction with ITO, while the electron donor was rich on air interface that is close to anode. The XPS analysis proved that PCBDCU might migrate to ITO and confirmed a strong interaction between ITO and PCBDCU. We speculate that the strong interaction probably may come from the formation of hydrogen bonds between N(H)CO moiety from PCBDCU and oxygen atoms from ITO, leading to lower work function of ITO. Similar phenomena were also found in PTB7-Th: PC71BM system with the device architecture of ITO/photoactive layer/MoO3/Ag. With 10% doping of PCBDCU, the device exhibits PCE of 6.37%, which is double of PCE of reference device (3.20%). The above results imply that vertical stratification caused by the self-organization of PCBDCU is a universal phenomenon, which provides a preferable procedure toward low cost inverted polymer solar cells without cathode interface layer.