Electronic Properties of Modified CuGaSe₂ Solar Cells

Abstract

We report on the electronic properties of the state-of the art surface-modified CuGaSe₂ solar cells. We compare between the 10.2%-efficient surface-modified and the 9.53%-efficient unmodified CuGaSe₂ solar cells. We examined our cells using deep level transient spectroscopy (DLTS) and drive level capacitance profiling (DLCP). The DLTS data for the modified CGS exhibits minority traps with activation energies ranging from E_c — 0.3 to E_c — 0.6 eV, whereas that for the pure CGS exhibits minority traps with activation energies ranging from E_c — 0.06 to E_c — 0.1 eV (where E_c is the energy of electrons at the conduction band minimum). While varying the filling pulse duration, we observed the gradual increase in the amplitude of the DLTS signal for these states until it apparently saturates at a pulse duration of ˜1s for the 10.2% cell, and 0.05 s for the 9.53% cell. Increasing the duration of the filling pulse also results in broadening of the DLTS signals and shifting of the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. Using a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states. The DLCP data shows that the 10.2% cell has a lower carrier concentration, a more uniform defect density profile, a larger depletion width, and a higher drift collection length for photo-generated carriers as compared to our 9.53% CuGaSe₂ cell. We also recorded the transient capacitance versus time and found that the 10.2% has responded differently compared with the 9.53% one. The transient capacitance decay curves for these two cells are different.