Semiconductor nanocrystals (NCs) with fewer mid gap defects or electronic traps are desired for an optoelectronic application typically in light emitting diodes LEDs, solar cells and photo detectors. The recent breakthrough in lead halides based perovskite NCs which are solution processed, and showing excellent light emitting efficiency without any surface modification suggests minimal or no role of midgap defect states1. As a result, photovoltaic solar cells based on fully inorganic CsPbI3 perovskite nanocrystals as photo absorber leads to large open circuit voltage (~1.2 V) and thus high power conversion efficiency2. While nanocrystals of these lead halide based perovskite is very promising for the design of LEDs, lasers and photovoltaic solar cells, very little work has been devoted to the basic understanding of conductive properties of nanocrystals in an ensemble system. Both DC and time resolved photocurrent measurements demonstrate that the photocurrent generation in these perovskite nanocrystal arrays is limited by the large excitonic binding energy of the material which thus limits the lifetime of the photogenerated electron hole pairs. As a result, nanotrench electrodes are employed as a strategy wherein device size is made to fit within the obtained diffusion length of the material in order to boost exciton dissociation efficiency manifested by an enhancement of the photoresponse by a factor 1000. While both DC and time resolved measurements pledge for defect tolerant nature of CsPbX3 NCs, we further investigated the electronic spectrum in absolute energy scale using photoemission measurements together with photoluminescence in one of the representative CsPb(Br0.65I0.35)3 NC arrays and revealed nearly intrinsic nature of these NCs with Fermi level slightly shifted towards conduction band. The obtained work function (≈ 4.1 eV - vacuum level vs. Fermi energy) is essential to probe and quantify Schottky barriers at the metal-NC interface.
1. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696.
2. Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M., Quantum dot–induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics. Science 2016, 354, 92-95.