Poly [4,4-bis(2-ethylhexyl)cyclopenta [2, 1-b;3,4-b′] dithiophene-2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl], a New Low Band Gap Polymer in Polymer Solar Cells

A low band gap polymer named as poly [4,4-bis(2-ethythexyl)cyclopenta[2, 1-b;3,4-b']dithiophene-2,6-di ylalt-2,1,3-benzoselenadiazole-4,7-diyl- PCPDTBSe, was designed and synthesized. The band gap of PCPDTBSe is 1.35 eV, which is 0.1 eV lower than that of poly [4,4-bis(2-ethy 1 hexyl)cyc lopenta[2,1 -b;3,4-b']dithiophene2.6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl], PCPDTBT. In the visible range, the PCPDTBSe film exhibits weaker absorption than that of PCPDTBT, making PCPDTBSe a promising candidate as a low band gap material for tandem cells. Polymer solar cell (PSQ devices using PCPDTBSe as the electron donor material and 6,6-phenyl-C-71-butyric acid methyl ester (PC70BM) as the electron acceptor material were fabricated and characterized to investigate the photovoltaic properties of PCPDTBSe. The external quantum efficiency spectrum indicates that the response range reaches 1000 nm. Under AM 1.5 G, 100 mW/cm(2) illumination, a PCE of 0.89% was recorded, with a short circuit current of 5.0 mA/cm(2), an open circuit voltage of 0.52 V, and a fill factor of 34.3%. Based on the results from the absorption spectrum and the carrier mobility of the PCPDTBSe:PC70BM blend film, we found that the photovoltaic performance of the PCPDTBSe-based PSC device is limited by the weak absorbance and an imbalance in hole and electron transport of the active layer. In conclusion, based on preliminary results, PCPDTBSe is a promising low band gap material for applications in PSCs, especially for PSCs with tandem structures.