Synthesis and characterization of Cu-Chalcogenide nanocrystals for potential applications in low-cost solar cells
Date of Issue2013
School of Materials Science and Engineering
One of the most popular Cu-chalcogenide materials used for the fabrication of thin film solar cells is CuInxGa1-xSe2. The device performance of cells made using Cu(In,Ga)(S,Se)2 nanocrystal (NC) inks synthesized via hot injection method have yielded efficiency up to 12 % recently. Stoichiometry control of these NCs offers the possibility of tuning the band gap of this material. We report a systematic study of the growth and evolution pathways of quaternary CuIn0.5Ga0.5Se2 NCs in a hot coordination solvent synthesis. The reaction starts with the formation of a mixture of binary and ternary NCs, which transforms subsequently to CuIn0.5Ga0.5Se2 NCs. These binary and ternary compounds dissolve in the course of the reaction, so as to provide the molecular precursor for monophasic CuIn0.5Ga0.5Se2 NCs to form. The growth and evolution of these spherical, monophasic CuIn0.5Ga0.5Se2 NCs was studied as a function of time. Control experiments indicated that the phase changes of CuIn0.5Ga0.5Se2 NCs are temperature and time-dependent. The change in the stoichiometry of CuIn0.5Ga0.5Se2 during growth can be estimated using Vegard’s law. Detailed crystallographic information of these NCs is scarcely documented. Therefore, in this study, a thorough investigation of the composition and crystal structures of CuInxGa1-xSe2 NCs over the entire composition range (0 ≤ x ≤ 1) synthesized from the hot injection method was also conducted. Raman spectroscopy of the NCs complemented the information that were derived from XRD and EPMA for the first time. The EPMA result indicates the good controllability of Ga/(In+Ga) ratio in the NCs using this synthesis method. Raman spectroscopy was found to be a useful technique to differentiate the crystal structures in synthesized NCs. Dominant Raman modes were observed for CuInSe2 NCs at 175 cm-1 with a shoulder at 182 cm-1, assigned to the A1 mode of chalcopyrite and S mode of sphalerite with disordered cations atom, respectively. These peaks shift to higher wave number from CuInSe2 to CuGaSe2 NCs. This study also revealed that CuGaSe2 NCs are purely chalcopyrite. The lattice parameters determined from XRD was found to deviate from that calculated parameters using the Vegard’s law for all compositions. Hence, it can be deduced that the lattice is distorted and this likely to result in micro-strain in the crystal. The band gap of CuInxGa1-xSe2 NCs was measured optically (UV-Vis spectroscopy) and electrochemically (cyclic voltammetry). The optical and electrochemical band gap of CuInxGa1-xSe2 NCs increases as the Ga content increases. The energy band gap deviates from the theoretical values, which could be related to the contribution from the disordering of cations and strain. These results help to tailor the opto-electrical properties of semiconductors which inherently depend on the crystalline quality, strain and composition. Other than developing CuInxGa1-xSe2 NCs for low-cost solution-processed solar cells, searching for alternative Cu-chalcogenide NCs that made use of elements that are abundant and low cost is another strategy for future energy conversion. Therefore, the other focus for this work is the controlled synthesis and characterization of an alternative solar absorber material to conventional quaternary CuInxGa1-xSe2 - ternary chalcogenide Cu2SnSe3 NCs. We have successfully synthesized highly crystalline Cu2SnSe3 NCs with a narrow size distribution using the hot injection method and hexadecylamine (HDA) was used as the capping ligand for the first time in this material system. Monoclinic Cu2SnSe3 NCs with an optical energy band gap of 1.3 eV were synthesized. Since the opto-electrical properties of the NCs are highly dependent on the composition and crystal structure, X-ray diffraction, selected area electron diffraction, convergent beam electron diffraction and Raman spectroscopy were used to determine their crystal structure. The chemical composition and valence states of the NCs were obtained using electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). These NCs are shown to be photosensitive in the range of wavelengths corresponding to the solar spectrum which makes them highly promising as alternative photon absorber materials for solar cell applications.