Abstract
Cesium lead iodide (CsPbI3) perovskite nanocrystals (NCs) suffer from a known transformation at room temperature from their red-emitting (black) to non-emitting (yellow) phase, induced by the tilting of PbI6 octahedra. While the reported attempts to stabilize CsPbI3 NCs mainly involve Pb2+-site doping as well as compositional and/or NC surface engineering, the black phase stability in relation only to the variation of the reaction temperature of CsPbI3 NCs is surprisingly overlooked. We report a holistic study of the phase stability of CsPbI3 NCs, encompassing dispersions, films, and even devices by tuning the hot-injection temperature between 120-170 °C. Our findings suggest that the transition from the black to the yellow phase occurs after over a month for NCs synthesized at 150 °C (150@NCs). Structural refinement studies attribute the enhanced stability of 150@NCs to their observed lowest octahedral distortion. The 150@NCs also lead to stable unencapsulated solar cells with unchanged performance upon 26 days of shelf storage in dry air. Our study underlines the importance of scrutinizing synthesis parameters for designing stable perovskite NCs towards long-lasting optoelectronic devices.
Highlights
In the plethora of possible halide perovskite compositions, cesium lead iodide perovskite (CsPbI3) remains a key material for single-junction solar cells, since its red-emitting black phase has an optimal bandgap (1.6–1.8 eV)[1] for matching the solar spectrum
We report a holistic study of the phase stability of CsPbI3 NCs, encompassing dispersions, films, and even devices based on the targeted perovskite nanomaterials
We synthesized CsPbI3 NCs by following a reported hot-injection synthetic route with minor adaptations,[8] in which the injection temperature was tailored in the range of 120–170 °C
Summary
The exposure of CsPbI3 NCs to moisture, light, or heat dramatically (i) reduces their light absorption and photoluminescence (PL) quantum yield (PLQY), (ii) enables phase transitions, and (iii) affects morphological transformations.[6]. One exception is provided by Dutta et al, who reported the tuning of the hot-injection temperature at higher values than the commonly adopted ones (160–180 °C).[14] In that work, the synthesis of CsPbI3 NCs at a high reaction temperature (260 °C) enabled phase-stable CsPbI3 NCs up to 30 days under ambient conditions via strong binding of alkylammonium ligands to the NC surface. No study has outlined if long-term phase stability of CsPbI3 NCs can be achieved by tuning the hot-injection temperature below the typically reported values (160–180 °C) and without the need of demanding ligand engineering. The transition from the black (γ-orthorhombic) to the yellow (δ-orthorhombic) phase of CsPbI3 NCs occurs only after 33 days of storage under ambient conditions (RH = 40%, T = 25 °C), when the hot-injection temperature is set to 150 °C. The corresponding unencapsulated PSCs, employing the NCs formed at 150 °C, fully retain their initial performance upon 26 days of shelf storage in dry air
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