Effect of Cs+ and K+ incorporation on the charge carrier lifetime, device performance and stability in perovskite solar cells

Abstract Incorporation of a third monovalent cation has been shown to improve the perovskite solar cell performance and stability. In this work, an in-depth study has been carried out to understand the effect of Cs+ and K+ cations on charge carrier dynamics in methylammonium-Formamidinium based solar cell. Different spectroscopic techniques together with electrical studies have been employed to determine the role of these cations on the charge carrier traps, charge transport, charge carrier extraction, recombination, etc. Incorporation of Cs+ leads to superior crystallinity and larger grains resulting in improved passivation, reduction of non-radiative losses and improved charge transport within the perovskite layer which resulted in improved cell performance than that with K+ incorporated device. The PL quenching at short-circuit, high recombination resistance deduced from the impedance spectra, lowest Urbach energy, and high electroluminescence of the Cs+ incorporated perovskite concur with the large and colunmnar grain growth with Cs+ incorporation. With Cs+, the device efficiency reach 19.8% with high fill factor of 80.28%, whereas K+ incorporated devices demonstrate only 16.16% efficiency with a fill factor of 71.76%. Although K+ substitution leads to better stability, MAFA:Cs+ devices are superior in terms of cell performance.