Highly Stable Polymer Solar Cells Based on Poly(dithienobenzodithiophene-co-thienothiophene)

초록

It is important to develop new denor (D)acceptor (A) type low band gap polymers for highly stable polymer solar cells (PSCs). Here, we describe the synthesis and photovoltaic properties of two D-A type low band gap polymers. The polymers consist of dithienobentodfthiophene (DTBDT) moieties with expanded conjugation side: groups as donors and 2-ethyl-1-(thieno[3,4-b]thiophen-2yl)hwtan-1-one (TTEH) or 6-octyl-5H-thieno[3',4':4,5]thieno[2,3-c]pyrrole-5,7(6H)-dione (DTPD) as acceptors to give pDTBDT-TTEH and pDTBDT-DTPD polymers, respectively. The pDTBDT-TTEH is quite flat, resulting in a highly crystalline film. In contrast, the pDTBDT-DTPD is highly twisted to yield an amorphous film. Photovoltaic devices based on pDTBDT-TTEH and pDTBDT-DTPD exhibited power conversion efficiencies (PCEs) of 6.74% and 4.44%, respectively. The PCE difference results mainly from morphological differences between the two polymer:PC71BM blend films; the pDTBDT-TTEH polymer formed a nanoscopically networked domains in the blend state, while the pDTBDT-DTPD polymer film contained aggregated domains with large phase separation between the poly-mer and PC71BM molecules. Importantly, we observed that pDTBDT-TTEH-based devices showed excellent stability-in air, retaining 95% of the initial PCE after storage for over 1000 h without encapsulation. The high stability of the pDTBDT-TTEH-based device was originated mainly by the crystalline nature of the pDTBDT-TTEH:PC71BM film. This work suggests that designing highly conjugated planar backboned polymers is crucial to improve not only the photovoltaic performance but also the stability of PSCs.

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