Enhanced Conversion Efficiency Enabled by Species Migration in Direct Solar Energy Storage

Solar energy can be stored via either an indirect route in which electricity is involved as an intermediate step, or a direct route that utilizes photogenerated charge carriers for direct solar energy conversion. In this study, we investigate the fundamental difference between the direct and indirect routes in solar energy conversion using a new photoelectrochemical energy storage cell (PESC) as a model device. This PESC centers on a liquid junction that utilizes CH₃NH₃PbI₃ perovskite to drive photoelectrochemical reactions of Benzoquinone (BQ) and Ferrocene (Fc) redox species. The experimental studies show that the equilibrium redox potentials are 0.1 V and −0.78 V (vs Ag/AgNO₃) for Fc⁺/Fc and BQ/BQ^.−, respectively, which would produce a theoretical open-circuit voltage of 0.88 V for the storage device. The physics-based computational analysis shows a relatively flat reaction rate distribution in the electrode for the indirect route; however, in the direct route the photoelectrochemical reaction rate is critically affected by electron concentration due to strong light absorption of the perovskite material, which has been shown to vary by at least 10-fold in the transverse direction across the photoelectrode. The drastic variation of reaction rate in the photoelectrode creates an electric field that is 7.5 times stronger than the bulk electrolyte, which causes the photo-converted reaction product (i. e., BQ^.−) to drift away from the photoelectrode thereby creating a constant reaction driving force. As a result, it has been shown that the intrinsic solar to chemical conversion (ISTC) efficiency improves by ∼40 % for the direct route compared to the indirect route at 0.05 mA/cm².