Tin-lead (Sn–Pb) halide perovskites hold promise as narrow-bandgap semiconductors in future solar cells. Currently, non-radiative recombination induced open-circuit voltage losses limit their full potential. Additives are commonly used to increase the performance of metal halide perovskite solar cells.

We investigated the effect of glycine hydrochloride as additive during solution processing of Sn–Pb perovskites.^[1] By combining photovoltaic performance and stability, with determining the quasi-Fermi level splitting (QFLS), time-resolved microwave conductivity, and morphological and elemental analysis a comprehensive insight is obtained. Glycine hydrochloride retards the oxidation of Sn²⁺ in the precursor solution improves the grain size distribution and crystallization of the perovskite causing a smoother and more compact layer, reducing non-radiative QFLS on perovskite layers without and with charge transport layers it is found that glycine hydrochloride primarily improves the bulk of the perovskite layer but does not contribute significantly to passivation of the interfaces of the perovskite with either the hole or electron transport layer.

QFLS measurements were also used to study the interfacial non-radiative recombination losses of Sn–Pb perovskite solar cells and the passivation strategies.^[2] The intrinsic losses in the perovskite semiconductor and at its interfaces with PEDOT:PSS and C₆₀ charge transport layers contribute significantly to the overall voltage deficit. Surface passivation with alkane-diammonium iodides or cadmium iodide mitigates the non-radiative recombination induced by the C₆₀ electron transport layer by eliminating direct contact with the perovskite semiconductor. While each of these passivation strategies are beneficial, shortcomings remain in implementing them in actual devices because effective passivation of the perovskite can limit the efficient extraction of charges.

These were largely overcome by co-depositing a mixture of alkane-diammonium diiodides and amphiphiles at the interface. This bi-molecular strategy, eliminates C₆₀-induced losses and enhances the open-circuit voltage of Sn-Pb perovskite solar cells to 90% of the detailed balance limit. Interestingly, a similar approach also reduces non-radiative recombination at the interface between the perovskite and PEDOT:PSS layer and enhances the open-circuit voltage to 0.91 V, or 93% of the detailed-balance limit. The double-sided passivation strategy enables narrow-bandgap single-junction solar cells with an efficiency of nearly 23%.

Incorporating bromide into metal-iodide perovskites is a commonly used approach for widening the bandgap of lead-halide perovskites. We now explored  mixing of iodide and bromide in Sn–Pb perovskites to create a mixed-metal mixed-halide perovskite composition, achieving the optimal bandgap of 1.34 eV for single-junction solar cells.^[3] Supported by in-situ absorption measurements, it is found that the delay time between starting the spin-coating of the perovskite precursor and depositing the antisolvent is key in controlling the film morphology. The optimized Sn–Pb–I–Br perovskite did not show signs of light-induced halide segregation during prolonged illumination. Applying passivation to reduce non-radiative recombination at the perovskite - electron transport layer interface and optimizing the device configuration results in a power conversion efficiency of 19.0%. This is among the highest for perovskites in the 1.3 − 1.4 eV bandgap range reported to date.