Bulk-heterojunction polymer solar cells (BHJ-PSCs) have attracted considerable attention because of their unique advantages of lightweight, low cost, mechanical flexibility and suitable for large-area fabrication [1–3]. In the last decades, much attention has been paid to the donor and acceptor system P3HT:PCBM, However, because of the relatively large bandgap of P3HT (∼1.9 eV) and the relatively small energy difference between the lowest unoccupied molecular orbital (LUMO) of PCBM and the highest occupied molecular orbital (HOMO) of P3HT, the photovoltaic performance of the PSCs based on P3HT:PCBM is still significantly lower than the inorganic solar cells. Recently more work has been done on the novel donor materials which have a reduced energy gap with an ability of harvesting more of the sun’s spectral emission and a high charge carriers mobility for charge transport. One of the most promising new donor polymer is poly [N-9"-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3' -benzothiadiazole)] (PCDTBT) with a HOMO level of 5.5eV which is 0.4 eV down-shifted than that of P3HT. When PCDTBT is blended with the fullerene acceptor PC70BM, it showed excellent photovoltaic performance with a power conversion efficiency of ∼ 6%. [6]
Improved photovoltaic performance of inverted polymer solar cells through a sol-gel processed Al-doped ZnO electron extraction layer