Influence of binary additives into the solvent for preparation of polymer and fullerene bulk heterojunction solar cells by convective deposition method

Abstract

Abstract Binary additives of 1,8-diiodooctane (DIO) and 1,3,5-trichlorobenzene (135-TCB) were utilized for preparation of poly[4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b; 4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) films for bulk heterojunction (BHJ) solar cells. The addition of binary additives of DIO 3.0 vol% + 135-TCB 2.1 vol% into PTB7-Th:PC71BM films exhibited nano-sized of PTB7-Th networks homogeneous distribution and smaller sizes than the film without additive and with DIO 3.0 vol%. The out-of-plane orientation of PTB7-Th:PC71BM films were improved by addition of binary additives and 135-TCB. A rectification ratio of the BHJ solar cells were improved by using binary additives of DIO 3.0 vol% + 135-TCB 2.1 vol%. As a consequence, fill factor (FF) and power conversion efficiency (PCE) of the device with binary additives increased up to 0.69 and 7.4% as compared with FF (0.66) and PCE (6.9%) of the device with DIO 3.0 vol%. The external quantum efficiency the device with binary additives was enhanced in the region from 550 nm to 750 nm as compared with that of the device with DIO 3.0 vol%. This result was probably caused by the improvement of PTB7-Th orientation, and phase separation between the PTB7-Th and PC71BM.