Universal Features of Electron Dynamics in Solar Cells with TiO₂ Contact: From Dye Solar Cells to Perovskite Solar Cells

The electron dynamics of solar cells with mesoporous TiO₂ contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current–voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 μs, 1 order of magnitude longer, is required to reproduce the current–voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO₂ contact with electrons

Universal Features of Electron Dynamics in Solar Cells with TiO₂ Contact: From Dye Solar Cells to Perovskite Solar Cells

The electron dynamics of solar cells with mesoporous TiO₂ contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current–voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 μs, 1 order of magnitude longer, is required to reproduce the current–voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO₂ contact with electrons

Origin and Whereabouts of Recombination in Perovskite Solar Cells

The success of metal halide perovskite solar cells stems from high absorption combined with a low recombination rate. Despite the fact these properties are inherent to the perovskite material, the choice of selective contacts is critical to achieve high voltages according to experimental evidence. In this work, the impedance and the open-circuit photopotential are measured for two excitation wavelengths (blue and red light), in two illumination directions (back and front), and at different temperatures. The open-circuit recombination characteristics of two different perovskite compositions, i.e., pure MAPbI₃ and mixed ion-based (FAPbI₃)_0.85(MABr₃)_0.15, and with two different hole selective layers (Spiro-OMeTAD and P3HT) have been studied. Our results indicate that, for the studied devices, the recombination process that determines the open-circuit potential is governed by the bulk of the perovskite layer via a trap-limited mechanism, but surface-mediated recombination cannot be ruled out for P3HT contact or degraded devices. Further, we propose a model that provides a general interpretation of the nature of recombination in perovskite solar cells