Controlling competing photochemical reactions stabilizes perovskite solar cells

Abstract

Metal halide perovskites have become a popular material system for fabricating photovoltaics and various optoelectronic devices. However, long-term reliability must be assured. Instabilities are manifested as light-induced ion migration and segregation, which can lead to material degradation. Discordant reports have shown a beneficial role of ion migration under illumination, leading to defect healing. By combining ab initio simulations with photoluminescence measurements under controlled conditions, we demonstrate that photo-instabilities are related to light-induced formation and annihilation of defects acting as carrier trap states. We show that these phenomena coexist and compete. In particular, long-living carrier traps related to halide defects trigger photoinduced material transformations, driving both processes. Defect formation can be controlled by blocking under-coordinated surface sites, which act as a defect reservoir. By use of a passivation strategy we are thus able to stabilize the perovskite layer, leading to improved optoelectronic material quality and enhanced photostability in solar cells.

Photobrightening in Lead Halide Perovskites: Observations, Mechanisms, and Future Potential

“…Motti et al proposed that the long‐lived carrier traps associated with halide defects govern the photobrightening and photodarkening processes (even in the absence of oxygen) by inducing photochemical reactions of mobile ionic species at those sites. At low charge carrier excitation densities (longer excitation wavelengths), low repetition rates (1000 pulses s −1 ), and low temperatures (≈200 K), they propose that photobrightening can take place in MAPbI 3 through electron trapping at iodide interstitial defects and subsequent Frenkel pair trap annihilation (Figure b) . This is also consistent with the recent mechanism proposed by Mosconi et al in which light illumination leads to the annihilation of Frenkel pairs .…”

“…These include long‐chain 2D molecules that incorporate into the perovskite surface structure and sulfur, nitrogen or phosphorus‐containing molecules that interact with surface defects . Use of these molecules to bind to undercoordinated lead may impede the formation of I 2 at these sites and associated photodarkening processes . One key advantage of the light treatment is that charge carriers can be generated in selected regions of the film (dictated, e.g., by the excitation wavelength) and can instigate photochemical reactions in those specific regions.…”

Sensitive Direct Converting X‐Ray Detectors Utilizing Crystalline CsPbBr3 Perovskite Films Fabricated via Scalable Melt Processing

“…Contrary, for negative Ga potentials the dark current is instable and is continuously increasing with time. The detailed mechanism for such slow in time instable dark current is controversial discussed in the literature . But it is commonly agreed that the dark current is of dominant ionic nature and the current instabilities are caused by ion migration of the mobile interstitial halogen (i.e., Br − ) and the associated positively charged vacancy.…”

Reducing Detrimental Defects for High‐Performance Metal Halide Perovskite Solar Cells

“…Similarly, I 0 can also serve as carrier recombination centers . Pb 0 and I 0 were identified to be generated during long‐term operational stability under light, electric field, and thermal stress . More recently, Yang et al .…”

“…[97,98] In addition to charged defects,previous XPS studies have shown the existence of neutral species,s uch as unsaturated Pb 0 formed on the surface of perovskite induced by the oxidation of I À into I 2 (Figure 1b). [5,99] Pb 0 is associated with at ype of defect leading to deep trap state in perovskites. [100] Similarly,I 0 can also serve as carrier recombination centers.…”