The recent focus on tandem cell structures has generated a renewed interest in mixed-composition perovskite materials with bandgaps in the range of 1.7–1.9 eV. These wide bandgap absorbers are typically obtained by increasing the fraction of bromide ions present in the X-site of the ABX3 perovskite lattice. Indeed, it is already well established that the perovskite bandgap can be reliably tuned over a wide range by adjusting the Br/I ratio.[1] These early studies also revealed, however, that certain compositions are more stable than others, and, in addition, the homogeneous distribution of I- and Br- ions in the perovskite lattice tended to spontaneously and reversibly de-mix under strong light, an effect that is generally referred to as “halide segregation”.[2]

While it is often suggested that halide segregation presents an intrinsic limit for the performance of wide bandgap perovskite solar cells, recent work by Brinkmann et al., featuring monolayer-based n-i-p devices would contradict this assessment, as they were able to obtain an open circuit voltage of 1.35 eV from a perovskite absorber with a 1.85 eV bandgap and 50% bromide fraction.[3]  In light of these new results, it is useful to re-evaluate the influence of the halide ratio on device performance and operational stability.

In this work, two triple-cation mixed halide lead perovskite absorbers are compared, one with high bromide content (Br/I ratio 1:2, bandgap 1.72 eV, 16.1% power conversion efficiency) and one with low bromide content (Br/I ratio 1:11, bandgap 1.57 eV, 19.1% power conversion efficiency).[4] Both materials demonstrated good stability while operating under simulated sunlight at the maximum power point for 100 h. After 100 h operation, however, the measured device efficiency fell temporarily due to a transient loss of output current before returning to the nominal level after a long recovery period in the dark. These transient losses were more apparent in the wide bandgap device under strong light and were likely caused by light-induced halide segregation. After recovery, however, both devices retained good performance under a wide range of light intensities. Overall, we conclude that the monolayer-based p-i-n device structure with wide bandgap perovskite absorber layers shows great promise for long-term deployment in both solar and ambient-light harvesting applications from the standpoint of both efficiency and operational stability.