Evolution Pathway of CIGSe Nanocrystals for Solar Cell Applications

CuInxGa1-xSe2 nanocrystals synthesized via the hot injection route have been used to make thin film solar cells with high power conversion efficiency. Thus, CuInxGa1-xSe2 nanocrystals have the potential to provide a low cost and high efficiency solution to harvest solar energy Stoichiometry control of these nanocrystals offers the possibility of tuning the band gap of this material. It is important to understand how the composition of quaternary CuInxGa1-xSe2 nanocrystals evolves to control the stoichiometry of this compound. We report a systematic study of the growth and evolution pathways of quaternary CuIn0.5Ga0.5Se2 nanocrystals in a hot coordination solvent. The reaction starts by the formation of a mixture of binary and ternary nanocrystals, which transforms subsequently to CuIn0.5Ga0.5Se2 nanocrystals. These binary and ternary compounds dissolve in the course of the reaction, so as to provide the molecular precursor for monophasic CuIn0.5Ga0.5Se2 nanocrystals to form. Here, we study the growth sequence of these spherical, monophasic CuIn0.5Ga0.5Se2 nanocrystals as a function of time. Control experiments indicated that the phase changes of CuIn0.5Ga0.5Se2 nanocrystals are temperature- and time-dependent. The change in the stoichiometry of CuIn0.5Ga0.5Se2 during growth was estimated using Vegard's law.