Voltage losses due to the perimeter and dark area in micro-concentrator solar cells

For concentrator photovoltaic modules, a promising approach is the miniaturization of the solar cell and optics. In solar cells, the perimeter is a source for the recombination of minority charge carriers, which leads to a voltage loss. With decreasing cell size, the ratio between perimeter and solar cell area increases and therefore the perimeter recombination becomes more relevant. In this work, we quantify this loss using the open-circuit voltages derived from I-V measurements of triple-junction concentrator solar cells of different sizes under varying irradiances. For the simulations, a one-dimensional diode model with two parallel interconnected diodes is applied with ideality factors of n1 = 3 and n2 = 6 for the triple-junction solar cell. As we compare solar cell designs that differ in size and fraction of the metallized area, we must take into account that the parallel resistance and the dark saturation currents differ. In the model, this discrepancy is conveyed by a dependency on the perimeter-to-area ratio and incorporating the ratio between the active and total areas into the model. Therefore, the areas need to be determined. For small solar cells, the uncertainty of the measurement is high. We account for this uncertainty by investigating areas derived from the photolithography masks and measurements. The model is applied to investigate the influence of the voltage loss on solar cells smaller than 3 mm in diameter. We show that, despite the simplification of the ideality factors, the model agrees well with the measurements. It is shown that for high concentrations (C > 500), the voltage loss is below 2% even for small cells with P/A up to 120 cm−1. We show that the dark area voltage loss for this cell and concentration is of the same order of magnitude as the loss due to the perimeter.