Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I_0.5Br_0.5)₃ Perovskite Nanocrystals

Abstract

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. Here, we quantitatively compare experimental, CsPb(I 0.5 Br 0.5 ) 3 nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size-and excitation intensity-dependent photosegregation data agree with model predicted size-and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard's law expressions of composition-dependent band gaps.

Thermodynamic Band Gap Model for Photoinduced Phase Segregation in Mixed-Halide Perovskites

“…To simplify subsequent thermodynamic analyses, eq is alternatively expressed as ( 1 − x init ) normalA normalP normalb normalI 3 + x init normalA normalP normalb Br 3 ↔ 1 2 normalA normalP normalb false( normalI 1 − x Br x false) 3 + 1 2 normalA normalP normalb false( normalI 1 − 2 x init + x Br 2 x init − x false) 3 where the mixed-halide alloy, APb false( normalI 1 − x init Br x init false) 3 , has been written in terms of its starting pure halide precursors. Of note in either eqs or are equal quantities of I-enriched and Br-enriched products. ,,, As will be described next, other chemical descriptions of photosegregation exist.…”

Photolysis of Mixed Halide Perovskite Nanocrystals

“…Explicitly, the rate of PL blueshifts increases with increasing excitation energy and intensity. This alludes to a link between the NC photolysis rates and charge carrier density as has been observed in reversible halide segregation for mixed halide thin films. ,,, …”

“…This alludes to a link between the NC photolysis rates and charge carrier density as has been observed in reversible halide segregation for mixed halide thin films. 5,10,14,20 Here, we have tracked the photolysis of both CsPb(I 1−x Br x ) 3 and CsPb(Cl 1−x Br x ) 3 alloys via mass spectrometry, electron microscopy, and temperature-dependent emission. The data show that both mixtures behave similarly during this process.…”

“…Colloidal CsPbX 3 (X = I – , Br – , Cl – ) perovskite nanocrystals (NCs) with edge lengths ( l ) of ∼10 nm have outstanding emissive properties (e.g., high photoluminescence (PL) quantum yields, narrow PL line widths, and fast radiative lifetimes). , Size-tuning and/or halide alloying , can be exploited to engineer their emissive properties from ∼400 nm ( x = 0) to ∼515 nm ( x = 1.0) for CsPb­(Cl 1– x Br x ) 3 alloys and ∼515 nm ( x = 1.0) to ∼780 nm ( x = 0) for CsPb­(I 1– x Br x ) 3 alloys. As a direct result of bright, readily tunable emission, mixed halide perovskite NCs have strong potential to be implemented in next-generation lighting and color conversion technologies (e.g., backlit quantum dot displays) as well as a broader class of energy applications (e.g., multijunction solar cells, scintillators, single photon sources). , Unfortunately, mixed halide perovskite NCs undergo a photochemical degradation (i.e., photolysis) during continuous wave (CW) ultraviolet–visible irradiation that considerably alters their optical and structural properties, quashing appeal for practical use in devices. ,− …”

“…Ligand sublimation may have implication in changes to particle sizes and morphology as will be discussed later. Importantly, a wide range of surface passivation strategies now exist for CsPbX 3 NCs (e.g., branched mondentate ligands, bidenate ligands, zwitterions, or inorganic shells) , While the focus here is on oleic acid/oleylamine, utilizing different surface passivation may be a suitable route toward mitigating the photochemical degradation of l ≈ 10 nm CsPbX 3 NCs.…”