Investigation Of Bulk-heterojunction Photovoltaics Based On Small Molecules

Organic photovoltaic solar cells have received great attention from both academic and industrial sectors taking advantage of their ability of solution processing and and low cost production. In recent years, devices based small molecule donor materials have got great progress as their power conversion efficiency get closer to those based on polymer solar cells. But both polymer and small molecule solar cells are still inferior to their inorganic counterparts in efficiency and stability. In order to advance large-scale commercialization, their performance must be improved. In this dissertation, we worked on photovoltaic devices based on small molecule donors via various ways of processing for the purpose improving the performance of small molecule solar cells.First, the performance of small molecule solar cells(SMSCs) based on IDT core small molecules SM1 and SM2 are investigated. Due to the good self crystallinity, we found that devices based on SM1 and SM2 get better performance without thermal annealing than those annealed samples. When blended with PC61 BM, SM2 based devices show better photovoltaic performance than that of SM1 based devices. But when we use PC71 BM instead of PC61 BM as acceptor, devices based on SM1 are dramatically improved, and devices based on SM2 act the other way. The morphology of blended thin films were characterized. Films of SM1:PC71BM were quite smooth and uniform. The smaller roughness of the SM1 based blend films with PC71 BM indicated that SM1 has good miscibility with PC71 BM. In comparison, the films blended with SM2 exhibit a larger roughness, which indicates an unfavorable film morphology that is a major disadvantage for charge separation from the donor to the acceptor. Through the observation of EQE curves, we found that PC71 BM would change the absorption of SM2 when they are blended.In the second part, we studied the performance of two DPP core small molecules DPP(TNa)2, DPP(TFNa)2 based photovoltaic cells. We found that these two small molecule are very sensitive to thermal annealing, related devices got dramatic improved performance after thermal annealing. The films based on these two small molecules were characterize by AFM. We found there existed many rod-like fibers on the surface of films based on DPP(TNa)2. And we also found that these two molecules did not act like the two IDT core molecule. When blended with PC71 BM, they all showed reasonably better performance.In this part, we systematically investigated on inverted small molecule solar cells based a DPP core small molecule, DPP(TBFu)2. Power conversion efficiency of 4.46% inverted small molecule solar cells was achieved. We had also studied the effect of DIO in ISMCs. When DIO was added, FF can be improved, but JSC decreased. When the ratio of PC61 BM increased, the effect on the JSC become weak. Through the characterization of GXID and Rso XS, we found that PC61 BM in the blend was beneficial to the crystallization of DPP(TBFu)2, and DIO could obviously influence the aggregation of PC61 BM in the blend films.