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.

Impact of air‐light exposure on the electrical properties of Cu(In,Ga)Se2 solar cells

“…The increase in E a due to HLS of the non‐ALE cell (from 1.24 to 1.28 eV) may be due to changes in the band bending in this graded‐bandgap device and requires further investigations. Inspecting the metastability behavior, the ALE cell now after HLS shows an ascending V OC ( t ) transient in Figure B (in contrast to before HLS), which is a clear sign for bulk recombination . Thirdly, Figure C shows that the additional admittance structure in the surface plot of the −ω (dC/dω) signal from Figure B is no longer visible.…”

“…To this end, we measured the transient V OC ( t ) and the relation V OC ( T ) where t is the time, T the temperature, and V OC the open circuit voltage. Both experiments are apt to discriminate between bulk and interface recombination as the limiting recombination process for the device at open circuit voltage . With the ideal diode equation J ( V ) and the saturation current density J 0 , an expression for V OC is given by Equation .…”

“…Further information on the dominant recombination region can be obtained by measuring the transient V OC ( t ) signal under red light illumination. The red light effect of CIGSe solar cells is due to an increased acceptor concentration in the absorber, ie, an increased N A,a . For an interface recombination limited heterojunction assuming type inversion at the interface, the reference current without tunneling is given by Equation .…”

“…Combining Equations and , we expect a decreasing V OC with time—for a device that is limited by interface recombination—due to an increasing N A,a with time. Instead, for bulk recombination (in the space charge region or in the quasi‐neutral region), J 00 will decrease with increasing N A,a . Thus, the V OC ( t ) measurement is appropriate to discriminate between interface and bulk recombination.…”

Large metastability in Cu (In,Ga)Se2 devices: The importance of buffer properties

“…Each light soak is maintained for 20 minutes. Choice of light soak time is somewhat arbitrary, as metastable changes decrease in rate with time, but do not saturate . Twenty minutes is chosen for this study because it allows measurement of several samples per day, while still spanning a time period over which metastable changes are clearly observable.…”