In Situ Ellipsometry Measurements on the Halide Phase Segregation of Mixed Halide Lead Perovskites**

Abstract

Methylammonium lead iodide bromides MAPb(BrxI1‐x)3 are a class of mixed halide lead perovskites, materials that offer high‐power conversion efficiencies and bandgap tunability. For these reasons, they are a promising absorber material for future solar cells, although their use is still limited due to several factors. The reversible phase segregation under even low light intensities is one of them, lowering the effective bandgap due to local segregation into iodide‐rich and bromide‐rich phases. While several studies have been done to illuminate the mechanism and suppression of phase segregation, challenges remain to understand its kinetics. We obtained dynamic ellipsometric measurements from x=0.5 mixed halide lead perovskite thin films protected by a polystyrene layer under green laser light with a power density of ∼11 W/cm2. Time constants between 1.7(±0.7)×10−3 s−1 for the segregation and 1.5(±0.6)×10−4 s−1 for recovery were calculated. The phase segregation rate constants are surprisingly two orders of magnitude slower than and the recovery rate is consistent with those measured using photoluminescence methods under similar conditions. These results confirm a concern in the literature about the complexity in the phase separation kinetics measured from photoluminescence. We expect ellipsometry to serve as a complementary technique to other spectroscopies in studying mixed‐halide lead perovskites phase segregation in the future.

Monitoring Phase Separation and Dark Recovery in Mixed Halide Perovskite Clusters and Single Crystals Using In Situ Spectromicroscopy

“…In our previous work, a fast initial phase shift in millisecond time scale was observed for MAPb(I 1– x Br x ) 3 thin films that also exist as aggregates in contrast to the peak shifts <100 s in CsPbBr 1.2 I 1.8 clusters and single NCs. , We do not see the PL from the Br-rich domain which is consistent with literature reports done under relatively low excitation power. , This suggests that a large part of photogenerated charge carriers is funneled to the lower-band-gapped I-rich regions to undergo radiative recombination. Besides, the binding energy associated with ionization of the longer and weaker Pb–I bond ,− is low compared to that in the Pb–Br bond. , The reason for the initial drop in PL intensity could be nonradiative Auger recombination centers creating trap states that dissipate excitons before emitting photons, , consistent with our previously reported trap formation dynamics on single-crystal perovskite nanorods …”

“…In our previous work, a fast initial phase shift in millisecond time scale was observed for MAPb(I 1−x Br x ) 3 thin films that also exist as aggregates in contrast to the peak shifts <100 s in CsPbBr 1.2 I 1.8 clusters and single NCs. 13,28 We do not see the PL from the Br-rich domain which is consistent with literature reports done under relatively low excitation power. 42,43 This suggests that a large part of photogenerated charge carriers is funneled to the lower-band-gapped I-rich regions to undergo radiative recombination.…”

“…Consequently, this ion migration leads to the display of contrasting bandgap energies compared to that of the original mixture. It is believed that the initiation of halide migration occurs by the accumulation of photoexcited charge carriers on grain boundaries providing a driving energy or an internal bias allowing halide anion mobility, without which the material is entropically stable and phases can be remixed in the dark . The charge carriers in the demixed phases have different energies defined by the local bandgap and prefer to funnel to the phase with a lower bandgap .…”

Temperature‐Dependent Reversal of Phase Segregation in Mixed‐Halide Perovskites

“…Under above-bandgap illumination [27] or charge-carrier injection, [30][31][32] these materials undergo a demixing process, resulting in the formation of localized regions of iodide-rich and bromide-rich phases. [33][34][35][36] Removal of the external stimulus results in recovery from the segregation. [27,[37][38][39] Although this reversible phase separation only affects a small minority of the perovskite volume, [34,40] the resultant spatially inhomogeneous bandgap seriously undermines the suitability of mixed-halide perovskites for tandem cell applications, not only by limiting the bandgap tunability necessary for such devices [41] but also through adversely affecting charge-carrier extraction [41][42][43] and recombination, [44] and causing voltage losses.…”

“…[33][34][35][36] Removal of the external stimulus results in recovery from the segregation. [27,[37][38][39] Although this reversible phase separation only affects a small minority of the perovskite volume, [34,40] the resultant spatially inhomogeneous bandgap seriously undermines the suitability of mixed-halide perovskites for tandem cell applications, not only by limiting the bandgap tunability necessary for such devices [41] but also through adversely affecting charge-carrier extraction [41][42][43] and recombination, [44] and causing voltage losses. [40,45] Consequently, considerable research attention has been devoted to understanding this phenomenon in order to prevent it, as laid out in several recent review articles.…”