DISSERTATION IMPACT OF LIFETIME VARIATIONS AND SECONDARY BARRIERS ON CdTe SOLAR-CELL PERFORMANCE

Advisor Department Head iii ABSTRACT OF DISSERTATION Impact of Lifetime Variations and Secondary Barriers on CdTe Solar-cell Performance The thin-film CdTe solar cell (generally n-CdS/p-CdTe) is one of the leading candidates for terrestrial photovoltaic applications due to its low cost and high efficiency. However, compared with single-crystal cells of comparable band gap, there remains a significant voltage difference, where the best CdTe cells are about 250 mV below the best GaAs cells when an appropriate adjustment is made for bandgap. Therefore, the fabrication of high-voltage CdTe solar cells is one of the major and critical challenges in recent years. From a device-physics point of view, variations in carrier lifetime, carrier (hole) density, and other aspects such as a back electron reflector, should be able to improve the voltage and efficiency. This dissertation systematically studies the impact of lifetime variations and secondary barriers on CdTe solar-cell performance. Numerical simulation is used to evaluate how combinations of lifetime, carrier density, interfacial recombination, and back barriers affect cell behaviors. Strategies to improve voltage and cell performance are explored. The experimentally observed characteristics with significant back-contact barrier (back-hole barrier) are explained. Current-voltage distortion which would result from a front barrier is also discussed. iv In the absence of secondary barriers, higher voltage and fill factor should be obtained, but only by a moderate amount, when the carrier lifetimes are increased from today's typical value (0.5 ns). Similarly, increased hole density (above the typical 2 × 10 14 cm-3) should lead to higher voltage, but with today's lifetimes, low collection outside the depletion region will lead to a drop in the current. Hence, both higher lifetime and higher carrier density are needed to obtain significantly higher voltage. The effect of lifetimes with secondary back barriers is also explored. The combination of a significant back-hole barrier and a typical CdTe carrier density leads to two competing mechanisms that can alter the J-V characteristics in two different ways depending on the lifetime. One is a hole limitation on current in forward bias, which reduces fill-factor and efficiency. The second is a high electron contribution to the forward diode current, which results in a reduced voltage. CdTe solar cells are particularly prone to the latter, since the combination of a wide depletion region and impedance of light-generated holes at the back contact increases electron injection at the front diode. Simulated J-V curves illustrating the two major effects are …