Publication date: 4th October 2024
Tin halide perovskites, with the general formula ABX3 [A = Formamidinium (FA), B = Sn, X = Br, I], offer a narrower bandgap (~1.3 eV) than their lead counterparts, making them suitable for the light-absorbing component in single-junction photovoltaic devices. Moreover, this attribute makes tin halide perovskites integral to all-perovskite tandems, with the best-performing cells comprising a 50-50 Sn-Pb composition in the low bandgap component. Nevertheless, record-breaking pure-Sn and Sn-Pb devices still exhibit severe losses in both open-circuit voltage (Voc) and short-circuit current (Jsc), keeping power conversion efficiencies well below theoretical limits.
Non-radiative voltage loss is directly linked to lower photoluminescence quantum yield (PLQY), and thus the prevalence of non-radiative recombination. [1] This picture is complicated in Sn-containing perovskites given that the presence of Sn4+ impurities in the precursor solution leads to both the introduction of background hole dopants (serving to increase the PLQY) and non-radiative recombination centers (serving to decrease the PLQY).[2]
We use the observation of pseudo-first order photoluminescence (PL) decay kinetics in Sn-containing perovskite films to establish a method for characterizing the hole dopant level and non-radiative recombination rate constant with combined time-correlated single photon counting (TCSPC) and photoluminescence quantum yield (PLQY) measurements. We find that untreated films exhibit large hole doping concentrations as high as p0 ~ 1 x 1019 cm-3, which can be reduced to p0 ~ 1016 cm-3 after precurosor sublimation and SnF2 treatments. While it is well known that the radiative recombination rates are increased with p0, we reveal that the non-radiative rate can also be increased. Using correlated microscopy, we find that microscale p-type regions are also centers for non-radiative recombination. We find significant PL heterogeneity even in films with moderate dopant levels, suggesting that new strategies to eliminate deleterious defects in Sn-containing perovskites should be developed. [3]