2, Universität Heidelberg, Heidelberg, , Germany
Preventing hysteresis and enhancing stability remain key challenges that could be resolved with the aid of judicious device design. We report numerical study of a solar cell model system that is based on a mixed electron-ion conducting perovskite active layer having various device configurations. In the full picture we allow for both mobile ions and the polarizability due to the easy-rotational methylammonium (MA). We then compare with cells where the MA rotation is frozen and/or the ions are non-existing. Several insights, resulting from these detailed simulations, will be presented.
For example: Even when there is no indication of hysteresis and the device’s characteristics can be modelled using ionic free model, the actual electron and hole distributions may be vastly different to the predictions by ionic free model. The low effective DOS promotes higher Voc but makes it more difficult to overcome energy level mismatch. These are related to the fact that the ionic motion is not only causing the hysteresis, it also allows for large deviations between electron and hole densities. Also, when a large energy mismatch exists between the BL and the perovskite the charge density distribution self-adapt to create an effective dipole at the interface. Such self-induced dipole can compensate for 0.4eV mismatch and thus prevent any loss in Voc. In this context formamidinium is preferred to MA.
We also found that while the use of doped BLs is effective to reduce serial resistance and potential S shapes it also results in a relatively pronounced ionic motion. We note that even in hysteresis-free cells the ions still redistribute as a function of bias. We show that by keeping a certain level of resistivity, as in undoped BL, the ionic motion is significantly reduced. We expect this to have significant impact on device long term stability.