Individual losses in thin-film CdTe solar cells

Thin-film polycrystalline CdTe solar cells have been analyzed using current-voltage, reflection, quantum efficiency, and capacitance measurements. The objective is to quantify the individual current and voltage losses in cells from different sources. Compared to an optimum photocurrent density of 30.5 mA/cm², they typically lose 2 mA/cm² to reflection, 2-3 to uncollected CdTe carriers, and 2-6 to window-layer absorption. Voltage loss at maximum power is of the order of 200 mV because of the polycrystallinity, 100 mV due to light-dark differences in forward current, and 50 mV resulting from series resistance. Individual voltage loss values vary considerably among samples. The capacitance measurement implies that a significant fraction of the CdTe is a highly compensated i-layer and that the extraneous bandgap state density is above 10¹¹ eV^-1 cm^-2 under operating conditions