The effects of SQ additive on charge carrier transport and recombination in PCDTBT:PC71BM based ternary organic solar cells

Abstract

The ternary solar cell has been proved as an elegant strategy to realize efficient bulk heterojunction (BHJ) organic solar cells (OSCs) by harvesting a larger range of photons than binary devices. In order to design such an efficient system, the charge carrier transport and recombination in ternary OPVs must be considered. We fabricated various ternary devices by adding squaraine derivatives (SQ-1, SQ-2, SQ-3, SQ-4 and SQ-5) with complementary absorption spectrum in near infrared region as the additional donor material into Poly [[9-(1-octylnonyl)-9H-carbazole-2, 7-diyl]-2, 5-thiophenediyl-2, 1, 3-benzothiadiazole-4, 7-diyl-2, 5-thiophenediyl] (PCDTBT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host binary blend film in this paper. Through the charge carrier mobility and transient photovoltage (TPV) measurements, it is found that the incorporation of 10% SQ-5 with shallower Highest Occupied Molecular Orbital (HOMO) level hinders holes in transferring from SQ to PCDTBT, which leads to unbalanced charge carrier transport, significant charge recombination, and decrease of performance. On the contrary, the performance of ternary devices incorporated with 10% SQ-3 with deeper HOMO energy level is improved by 21% compared with binary devices, which can be attributed to increased photon harvesting and changeless charge carrier characteristics.