Traditional Quantum Dots to Defect-Tolerant Cesium Lead Halide Perovskite Nanocrystals for Optoelectronics

Abstract

Colloidal semiconductor nanocrystals, also known as quantum dots, exhibit promising optoelectronic properties because of their quantum size effect and solution processibility.One of the major challenges to bring these materialsinto real life application isthehuge number of under-coordinated atoms/ions present on thesurfaces with high surface to volume ratio.These under-coordinated defects energetically lie deep within the bandgap for traditional quantum dots like CdSe and InP. These deep defect states trap charge carriers, drastically decreasing the efficiencies of optoelectronic processes. Owing to the small size of nanocrystals, a high density of surface defects is inevitable in nanocrystals. However, one can get rid of these traps by pushing energy levelsof defects near to (shallow region) or inside the conduction band and/or valence band. These shallow defects do not trap the charge carriers. Semiconductors that exhibit efficient optoelectronic properties, in spite of having defects (typically shallow ones) are termed as defect-tolerant materials. In this thesis, we show that colloidal cesium lead halide (CsPbX3) perovskitesnanocrystals are such defect-tolerant material, showing high efficiencies of photoluminescence (~90%), solar cell ( >10%), and other optoelectronic parameters. Interestingly both photoluminescence and electroluminescence of CsPbBr3nanocrystals are very narrow (FWHM< 100 meV), which are useful for high color-purity displays. Suppression of photoluminescence blinking, along with negligible detrimental effect of size-distribution on both self-absorption and FRET is observed. Interestingly, surfaces of the nanocrystals stabilize theblack (α-) phase of CsPbI3at room temperature,unlike their bulk counterpart.This black phase has the desired bandgap for solar cell and visible-light photodetectors. After developing the colloidal nanocrystals, we developed a novel surface chemistry to improve the charge transportacross nanocrystal films.Efficiencies of these nanocrystalfilms were tested with the help of collaborators for solar cell (10.77% efficiency), photodetectors (detectivity = 1.8 X 1012 Jones) and light emitting diodes (~5 Cd/A). The key advantage of these all-inorganic CsPbX3nanocrystal based devices is higher thermal stability compared to devices made of volatile organic-inorganic hybrid perovskites.