Abstract
The approach via ternary blends prompts the increase of absorbed photon density and resultant photocurrent enhancement in organic solar cells (OSCs). In contrast to actively reported high efficiency ternary OSCs, little is known about charge recombination properties and carrier loss mechanisms in these emerging devices. Here, through introducing a small molecule donor BTR as a guest component to the PCE-10:PC71BM binary system, we show that photocarrier losses via recombination are mitigated with respect the binary OSCs, owing to a reduced bimolecular recombination. The gain of the fill factor in ternary devices are reconciled by the change in equilibrium between charge exaction and recombination in the presence of BTR toward the former process. With these modifications, the power conversion efficiency in ternary solar cells receives a boost from 8.8 (PCE-10:PC71BM) to 10.88%. We further found that the voltage losses in the ternary cell are slightly suppressed, related to the rising charge transfer-state energy. These benefits brought by the third guest donor are important for attaining improvements on key photophysical processes governing the photovoltaic efficiencies in organic ternary solar cells.
Highlights
The recent efforts on organic bulk heterojunction (BHJ-organic solar cells (OSCs)) solar cells have pushed forward this photovoltaic technology toward a meaningful solution for generating the electricity at lower expenses
We show that the low recombination rate in the benzodithiophene terthiophene rhodanine (BTR):PC71BM binary OSC provides an opportunity for mediating recombination losses in the power conversion efficiencies (PCE)-10:BTR:PC71BM ternary devices
Hereinafter, we focus on the analysis based on the optimal blend ratio at 0.8:0.2 (w/w) for PCE-10:BTR that yields the best PCE of 10.88%. (Chen et al, 2017) With non-optimal blend ratios, more significant geminate losses may be present, which complicates our analyses on bimolecular recombination properties
Summary
The recent efforts on organic bulk heterojunction (BHJ-OSCs) solar cells have pushed forward this photovoltaic technology toward a meaningful solution for generating the electricity at lower expenses. Based on an optimal blend ratio (0.2:0.8 w/w for BTR:PCE-10), the enlargements of Jsc and FF in the ternary cell are found associated with a stronger reduction factor with regard to the polymer binary device. The suppressed charge recombination in ternary devices leads to more efficient carrier extraction, which explains the simultaneously enhanced photocurrent and fill factor in the presence of BTR donor.
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