Open-circuit and doping transients of Cu(In,Ga)Se2 solar cells with varying Ga content

Abstract

Solar cells based on Cu(In1−x, Gax)Se2 typically show time transient behavior of the open-circuit voltage Voc under illumination. In this work, we study both the red-light Voc(t) transient and the red-light capacitance transient at different temperatures of samples with different x. From the capacitance transient, we calculate a transient behavior of the Cu(In1−x,Gax)Se2 doping density NA,a(t). Then, using established models on the NA,a dependence of the dominant recombination mechanisms, we derive from Voc(t) that Cu(In1−x,Gax)Se2 samples with x = 0, 0.3 are dominated by bulk recombination and a sample with x = 1 is dominated by interface recombination—in agreement with the expectation. Further, the transients of NA,a(t) can be used to recalculate Voc(t) transients which are then compared with the measured Voc(t) transients. From the excellent agreement, we conclude that under red-light illumination Voc(t) indeed is dominated by NA,a(t) and other transient effects are of secondary importance. We further conclude that the sample with x = 1 can be described by an absorber/buffer/window energy band diagram with fully depleted buffer layer which here is CdS.