Organometal halide perovskites are a class of photovoltaic (PV) materials which have fascinated PV community for demonstrating outstandingly high power conversion efficiencies (PCE) comparable to established PV technologies. Despite their high PCEs, perovskite solar cells (PSCs) are yet not a marketable product. This is due to their low stability towards the key stress factors of solar modules in outdoor installations like moisture, oxygen, and light exposure; especially the ultraviolet (UV) light. Although there is consensus on the general problem of low stability, yet the existing studies on photostability of PSCs are not conclusive. Neither the origin of UV degradation is fully understood, nor the harmful UV spectral range and intensities are well-known.

In this work, we systematically study the impact of different UV spectra, in order to shade light on the inconclusive literature and define the harmful UV spectrum. For this purpose, we consider two UV spectra: (i) 310-316 nm (ii) 360-380 nm and stress various device architectures based on spin coated methylammonium lead iodide (CH₃NH₃PbI₃) as absorber layer with various electron transport layers (ETLs). We show that only the deep UV wavelengths (310-316 nm) are responsible for degradation in PSCs and this finding is consistent for all four ETLs (i.e. (i) SnO₂ (ii) compact-TiO₂ (iii) electron-beam TiO₂ and (iv) nanoparticles-TiO₂) used in this study. This instability is triggered as perovskite absorber material undergoes decomposition after absorbing UV photons, resulting in diminished photocurrent and the low stabilized PCEs. This observation is independent of the intensity of UV radiation as low intensities (10%) showed same trend in the degradation behaviour. Thus, we assume that it occurs independent of the magnitude of UV light intensity.

Different remedies are proposed to prevent UV triggered degradation of PSCs. One strategy is to cut-off the UV wavelengths which are primarily responsible for degradation. However, this comes at a cost of fractional loss in photocurrent. The other scheme is to use luminescent downshifting (LDS) layers to regain about half of the lost photocurrent as well in addition of blocking the harmful UV content. Potential loss in short-circuit current density (J_SC) by eliminating harmful UV photons and amount of recovered J_SC in result of downshifting is also presented in this work.

Synoptically, the deep UV photons degrade the PSCs irrespective of whichever ETL and light intensity we use.