Effects of defects in semiconductors on reproducibility and performance of thin-film photovoltaic solar cells

Thin-film solar cells based on GaAs/AlxGa(1-x)As showing open circuit voltages of ( 1100-1170) mV and fill factors of ( 0.80-0.87) have been exposed to external electrical stresses, and current-voltage characteristics were monitored in order to study the effects of defects present in the device structure. It has been found that peculiar kinks, sudden jumps and various deformations occur in current-voltage curves which could be caused when the Fermi level moves position at the device interface. These changes occur when there is a group of defect levels and the Fermi level is forced to move across these levels. The reproducibility and performance of solar cells heavily depend on the properties of these defect structures, and external forces such as electrical stresses, illumination and temperature variations cause changes in the measured current-voltage characteristics. The observed variations during this work together with examples from the literature are presented and discussed in this paper. It is concluded that the control of these defects present in device structures is crucial in developing stable, durable and high-efficiency thin-film solar cells.