Do Built-in Fields Improve Solar Cell Performance?

Intrinsic fields can be built into solar cells by varying the doping level and/or the cell bandgap with potential benefits long recognised. A counteracting effect is that varying the doping or bandgap from the optimum for a particular material system will result in poorer material quality. New solutions are described to the standard semiconductor transport and recombination equations that allow such effects to be incorporated and the impact of these fields on device current and voltage to be assessed. Clear boundaries are found between when built-infields are beneficial or deleterious. For the case of doping gradients, built-infields decrease both cell current and voltage if carrier lifetime decrease more quickly than as tire inverse square of doping; decrease current but can increase voltage if the inverse variation is between linear and square; can improve both if less than linear but only significantly if lifetime varies less than the inverse square root of doping. In the case of a graded bandgap, an optimal field exists as a trade-off between decreased absorption and increased carrier collection. Copyright (c) 2008 John Wiley & Sons, Ltd.