Metastable shunt paths in a-Si solar cells

We have studied the time/voltage dependence of current transients in several different a-Si solar cell structures and have found they reveal more about the basic shunt mechanism. In single-junction cells, we 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 the Al doped ZnO layer (Al)ZnO between the cell and the back contact metal show no metastable switching. We 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.