Metastable shunt paths in a-Si solar cells

The transitory and erratic nature of shunt currents, whether caused by light-soaking or electrical biasing, in amorphous Si (a-Si) single- and triple-junction solar cells has been a real puzzle and made the study of these cells difficult. The authors present a careful study of the time/voltage dependence of these current transients in several different a-Si solar cell structures and find they reveal more about the basic shunt mechanism. In single-junction cells, they see stepwise current changes that increase in size and number with reverse bias and can be removed with forward bias. This stepwise, on and off switching suggests a discrete shunt path conduction mechanism. The kinetics of these metastable shunt paths show that both the "on-state" and "off-state" possess memory. Cells without (Al)ZnO show no metastable switching. The authors associate the stepwise features with the textured substrate and the switching metastability with contact to (Al)ZnO. Switching in triple-junction cells occurs with hundreds of oscillations at each step.