Optical properties of the chalcopyrite semiconductors CuInSe₂, CuInS₂ and CuGaSe₂

• Main Author: Luckert, Franziska
• Published: University of Strathclyde 2012
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.668853

CuInSe₂, CuInS₂ and CuGaSe₂ are I-III-VI₂ compound semiconductors with a chalcopyrite structure. These ternary compounds exhibit favourable properties, such as direct band gaps and high absorption coefficients, for application as absorber layers in thin-film solar cells. Recently Cu(In,Ga)Se₂ based photovoltaic devices have demonstrated conversion efficiencies of 20.3 % which is the highest amongst polycrystalline thin-film solar cell technologies. This thesis describes a study of excitonic recombination processes in high quality CuInSe₂, CuInS₂ and CuGaSe₂ single crystals using photoluminescence (PL) spectroscopy as a function of excitation power, temperature and applied magnetic field. Excitation power dependent measurements confirm the identification of the free excitons in the PL spectra of the three chalcopyrite semiconductor compounds. Additional sharp lines in the PL spectra appear to be due to the radiative recombination of excitons bound to shallow hydrogenic defects. PL lines due to excitons bound to more complex defects with a low concentration of defects are also found in CuInSe₂ and CuInS₂. Analysis of the temperature dependent PL spectra lead to activation energies of the free and bound excitons in CuInSe₂, CuInS₂ and CuGaSe₂. In addition, phonon energies have been obtained from the temperature dependence of the free exciton spectral positions and of the full width at half maximum. PL spectra measured in applied magnetic fields allow estimation of the diamagnetic shift rates for CuInSe₂, CuInS₂ and CuGaSe₂. A first-order perturbation model leads to values for the excitonic reduced masses and the effective hole masses can be estimated. For CuInSe₂ a theoretically predicted anisotropy of the effective hole masses is demonstrated. The study of the excitonic states in CuInSe₂, CuInS₂ and CuGaSe₂ provides a deeper understanding of the electronic material properties which can facilitate further improvements in solar cell efficiencies.