Electronic defects in Cu(In,Ga)Se2 : Towards a comprehensive model

Abstract

The electronic defects in any semiconductor play a decisive role for the usability of this material in an optoelectronic device. Electronic defects determine the doping level as well as the recombination centers of a solar cell absorber. Cu(In, Ga)Se 2 is used in thin-film solar cells with high and stable efficiencies. The electronic defects in this class of materials have been studied experimentally by photoluminescence, admittance, and photocurrent spectroscopies for many decades now. The literature results are summarized and compared to new results by photoluminescence of deep defects. These observations are related to other experimental methods that investigate the physicochemical structure of defects. To finally assign the electronic defect signatures to actual physicochemical defects, a comparison with theoretical predictions is necessary. In recent years the accuracy of these calculations has greatly improved by the use of hybrid functionals. A comprehensive model of the electronic defects in Cu(In, Ga)Se 2 is proposed based on experiments and theory. The consequences for solar cell efficiency are discussed.

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

“…Besides, the Cu In and Cu i have clear PL signatures (cf. Spindler et al 4 ) and do not correlate with the 200 ± 20 meV defect.…”

Wide‐gap (Ag,Cu)(In,Ga)Se2 solar cells with different buffer materials—A path to a better heterojunction

“…Pohl et al calculated that In Cu in CuGaSe 2 is a much shallower donor than Ga In , suggesting that only Ga In can act as a detrimental recombination center in wide‐gap absorbers . In fact, the presence of intrinsic Ga Cu donor defect was experimentally confirmed in CuGaSe 2 by low‐temperature photoluminescence studies . On the other hand, Hanna et al found that the density of a deep acceptor in the absorber bulk is the lowest for samples with GGI ≈ 0.26, which also exhibit the lowest V OC losses, while its density increases monotonously for higher GGI values, resulting in V OC saturation .…”

“…12 In fact, the presence of intrinsic Ga Cu donor defect was experimentally confirmed in CuGaSe 2 by low-temperature photoluminescence studies. 13,14 On the other hand, Hanna et al found that the density of a deep acceptor in the absorber bulk is the lowest for samples with GGI ≈ 0.26, which also exhibit the lowest V OC losses, while its density increases monotonously for higher GGI values, resulting in V OC saturation. 15,16 An alternative explanation for the observed V OC vs E g trend is an increased grain boundary recombination due to Cu-enrichment in grain boundaries (ie, no hole repulsion) for Ga-rich compositions.…”

Challenge in Cu-rich CuInSe2 thin film solar cells: Defect caused by etching

“…Several approaches have been developed to combine Cu-rich absorbers with a Cu-poor surface by implementing different surface treatments [11,16,[19][20][21] that were reported to increase the V OC of Cu-rich cells through the reduction of interface recombination. Recently, it was found that the V OC gap between Cu-rich and Cu-poor cells is related to the bulk recombination and not only to the interface: the quasi-Fermi-level splitting (qFLs), that represents the highest V OC an absorber can achieve, is lower for Cu-rich Cu(In, Ga)Se 2 than for Cu-poor Cu(In, Ga)Se 2 , even before the interface with the buffer is formed [22,23]. The same study demonstrated a considerably larger difference between qFLs and V OC in Cu-rich solar cells, indicating that interface recombination poses an additional loss mechanism in Cu-rich Cu(In, Ga)Se 2 .…”