Defect Mediated Photoluminescence and Solar Cell from Colloidal I-III-VI Semiconductor Nanocrystals

Abstract

Semiconductor nanocrystals (NCs) investigated in this thesis are mainly AgInS2 and CuInSe2 (along with their derivatives), which are constituted by group-I (Cu, Ag), group-III (In), and group-VI (S, Se) elements of the periodic table. These semiconductors exhibit suitable band gap in the visible to near infrared region and small effective mass of charge carriers, which are desired for an optoelectronic application. Therefore, we prepared colloidal nanocrystals of these semiconductors, studied their photophysical properties, and then fabricated solar cells. AgInS2-based NCs exhibit high photoluminescence (PL) efficiency, along with sub-microsecond long PL lifetimes. We established that the PL is originated involving both delocalized band edges and localized point defects. Such PL properties are desired in a material used for solar cell. But the band gap of these NCs is somewhat higher for absorbing near-infrared region of solar light. Consequently, we prepared AgInS2-Ag2S heterodimer NCs, where Ag2S component can absorb near-infrared light, and the interface between AgInS2 and Ag2S has the possibility to provide a nanoscale pseudo p-n junction required for efficient exciton dissociation. A detailed mechanistic study suggests that the charge separation across the interface is perhaps driven by defects, leading to an enhancement in solar cell performance. But still the solar cell efficiency was low (1.3 %), because of the improper alignment of conduction band minimum of Ag2S part with that of TiO2 anode material used in the quantum dot sensitized solar cell. Finally, colloidal CuInSe2-based NCs could combine both broad absorption of solar light, and desired band alignment with TiO2, leading to solar cell efficiency 3.6%. For all these works, surface chemistry of NCs was optimized to facilitate the charge transport across the NC films. This work shows the potential of colloidal AgInS2 and CuInSe2 based NCs as environmentally benign and solution-processed optoelectronic material, without the use of toxic Cd and Pb.