Bulk Heterojunction Solar Cells with Ternary Mixed PTB7:PCDTBT:PC₇₁BM Active Layers

We report on bulk-heterojunction solar cells fabricated based on ternary mixed solutions of two donors of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b’] dithiophene-2, 6-diyl] [3-fluoro-2- [(2-ethylhexyl) carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT), and an accepter of [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The solar cells had a glass/ITO/NiO/PTB7:PCDTBT:PC71BM/LiF/Al structure. Solar cells containing a 1-2% PCDTBT weight fraction showed a noticeable improvement in short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE). Solar cells with a 2% PCDTBT weight fraction exhibited an open circuit voltage (Voc) of 0.77 V, Jsc of 13 mA/cm2, FF of 0.42, and PCE of 4.23%. Possible mechanisms for the solar cell performance improvement by the introduction of the small amount of PCDTBT in the PTB7:PC71BM active layer was discussed based on the active layer morphology changes and carrier transport mechanisms.