The Effect of Dominant Junction on the Open Circuit Voltage of Amorphous Silicon Alloy Solar Cells.

Abstract

We have theoretically studied the role of the doped contacts on the open circuit voltage (V_oc) of amorphous silicon, pin solar cells by numerical solution of coupled Poisson and current continuity equations for the bulk and the doped regions. The built-in potential (V_bi) in the cell was found to be split asymmetrically between the p⁺/intrinsic and i/n⁺ regions. The first one forms a p⁺/n junction (dominant junction) since the undoped intrinsic is slightly n-type; the latter forms a n/n⁺ or low/high junction.

We found that V_oc is determined by the built-in potential at the dominant junction (and hence by the conductivity activation energy of the p⁺ layer) and is fairly insensitive to the activation energy in the n+ region. If one dopes the intrinsic with phosphorus, Voc is even less dependent on the n+/i junction potential but becomes very sensitive to the activation energy in the p⁺ layer. However, when the i-layer becomes p-type by boron doping, the dominant junction is created at the n⁺/i interface and Voc then becomes insensitive to the quality of the p⁺ layer and is determined by the n⁺ contact potential. The results are interpreted by considering the modification of the electric field within the bulk by the junction potentials and the recombination mechanisms at the two interfaces.