Transport velocity transformation: A convenient method for performance analysis of multilayer solar cell structure

With the trend to the use of drift fields and high-low junctions in the front and base regions of solar cells, the need arises for simple modeling of structures with at least three layers in a region. A forerunner treated a one-dimensional two-layer model in closed form and showed an expansion to a third layer for a very simple case [10]. Since direct expansion of the closed-form method to additional layers is too cumbersome, a layer-by-layer modeling method was developed, based on the representation of current densities across layer interfaces by the product of the carrier density and a transport velocity. In the ease of low-level injection, space-charge quasi-neutrality, and spatially constant material parameters, including an electrostatic field, the individual layer can be treated analytically, and the basic solar cell performance parameters evaluated from three equations. The first represents the transformation of the transport velocity across the layer from the other layer boundary. The second determines the light-generated current output from the layer interface, under the influence of the transport velocities and minority-carrier density at both layer boundaries and of bulk recombination. The third equation describes the flow of these carriers across other layers. The power of the approach lies in its facility for analysis of the solar cell´s performance layer by layer, giving a clear picture of the individual layer´s influence on the cell efficiency. It thus greatly eases the task of cell-design optimization. Where the parameters are not constant within a layer, the method allows approximation by stepwise-constant parameter approach, after subdividing the layer into a suitable number of sublayers.