Electronic Properties of Modified CuGaSe 2 Solar Cells
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Electronic Properties of Modified CuGaSe2 Solar Cells Jehad A. AbuShama1, Steve W. Johnston, David L. Young, and Rommel Noufi, 1617 Cole Blvd., National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. 1 Department of Physics, Colorado School of Mines, Golden, CO 80401, U.S.A. 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 Ec − 0.06 to Ec − 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. INTRODUCTION CuInxGa1-xSe2 (CIGS) thin film solar cells are important candidates for photovoltaic applications. They are among the most commercially feasible solar cells. Recently, new record total-area efficiencies of 15%, 19.5% and 10.2% for CdS/CuIn1-xGaxSe2 solar cells have been achieved for x~0 (CuInSe2 or CIS), x~0.3 (CuInGaSe2 or CIGS), and x=1 (CuGaSe2 or CGS), respectively [1-3]. In this work, we focus on the electronic properties of the 10.2% [2] and the 9.53% [4] CGS cells. CGS has a bandgap of 1.68 eV, which makes it a possible candidate as an absorber layer in wide bandgap thin film solar cells and in thin film tandem devices. As CGS cells have higher open-circuit voltage, and lower currents, they will result in better module performance. However, it is a challenging task to increase the open-circuit voltage in CGS devices. This is because CGS has relatively poor transport properties [5].
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EXPERIMENTAL We deposited CGS thin films on Mo-coated soda lime glass substrates
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