An outstanding property of lead-halide perovskites is the incredibly low band-gap to open circuit voltage loss, which is optimized devices is close to the thermodynamic limit (1.6 eV gap; 1.3 V voltage, loss ~0.3 V). The high open circuit voltage is extremely attractive for both photovoltaics and water splitting, since it requires no more than two series connected cells to operate water electrolysis. These observations suggest an apparently low density of traps in MAPbI3, contrary to the expectedly high defect density of a low-temperature, solution-processed material, suggesting that metal halide perovskites are inherently defect tolerant due to dominant defects introducing only shallow traps in the material band-gap.[1] A mechanism for the protection of charge carriers implying large polarons has also been invoked to explain the long carrier lifetimes and diffusion lengths favoring efficient carrier collection at selective contacts, leading to power conversion efficiencies competing with established thin film photovoltaics. [2]

We present on overview of first-principles computational analyses devoted to understanding the outstanding optoelectronic properties of lead-halide perovskites. We show that despite the fairly high defect density due to lead and MA-related defects, only less abundant iodine defects introduce deep electron and hole traps in MAPbI3.[3, 4] The peculiar iodine redox chemistry leads, however, to kinetic deactivation of filled electron traps, leaving short-lived hole traps as potentially harmful defects. Hole traps can be eventually converted to electron traps under mild oxidizing conditions, clarifying the defect tolerance. A polaronic mechanism, triggered by a photoinduced structural deformation, is presented which is also responsible for the reduced electron/hole recombination observed in lead-halide perovskites. [2,5]

The two ingredients, intimately related to the constituting lead-halide chemistry, represent the key to the success of perovskite-based PV and can represent the basis for development of new materials with similar target characteristics, possibly avoiding the environmental risks posed by lead.

References

1) W. J. Yin et al. Appl. Phys. Lett. 2014, 104, 063903.

2) K. Miyata et al. Sci. Adv. 2017, 3, e1701217.

3) D. Meggiolaro et al. ACS Energy Lett., 2017, 2, 2794.

4) E. Mosconi et al. Energy Environ. Sci. 2016, 9, 3180.

5) F. Ambrosio et al. Energy Environ. Sci., 2018, DOI: 10.1039/C7EE01981E.