Due to the presence of a “built-in” electric field F(x) within the a-Si:H-based pin solar cell device, the bands are tilted, for EC according to eF(x) = dEC(x)dx; however, these built-in fields cannot be measured directly.
The built-in fields across the i – layer stems from the space charges created between the n and p layers on both sides. This is caused by the connection of the Fermi energies at both sides (as connected to the respective electrodes). With electrons donated from the n-layer to the p-layer a built-in potential Vbi can be read. With light, electrons and holes are generated that will drift in the built-in field toward the respective electrodes.
Several computer programs were developed for solar cell calculations with a – Si:H and nc-Si:H (Burgelman et al. 2004; Zhu et al. 1999); in amorphous semiconductors, band tail states may determine some opto-electronic properties (for a review see Schropp and Zeman 1998).