Abstract
$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ is an emerging material for solar cells, reaching already efficiencies above 15%. For any solar cell material, it is essential to understand the recombination processes and doping behavior of its defects. Therefore, in this work, we present a detailed photoluminescence and admittance investigation of polycrystalline $\mathrm{CuIn}{\mathrm{S}}_{2}$ thin films to study the shallow intrinsic defects. We find evidence for two acceptors, 105 and 145 meV from the valence band, their predominance depending on the Cu excess, plus one donor, 30 meV from the conduction band. The high crystalline and electronic quality of our samples allows the detection of low intensity luminescence peaks. Based on these emissions we can analyze the phonon coupling of the observed defects and show that an emission previously attributed to a second donor is in fact a phonon replica of the first donor-acceptor transition.
Highlights
Thin film solar cells present the electricity source with the lowest carbon footprint [1]
A step forward was recently made for the sulphide chalcopyrite Cu(In, Ga)S2, achieving a record efficiency of 15.5% [12] and open circuit voltage Voc up to 973 mV [13,14]
The general agreement is that the near band gap luminescence is characterized by the presence of two shallow acceptors and two shallow donors, even if there is no agreement on their activation energies [17,18,19,20]
Summary
Thin film solar cells present the electricity source with the lowest carbon footprint [1]. The general agreement is that the near band gap luminescence is characterized by the presence of two shallow acceptors and two shallow donors, even if there is no agreement on their activation energies [17,18,19,20] Another common feature reported in low temperature PL spectra is the presence of a broad deep band in the range of 1.2–1.3 eV, with an intensity comparable to the near band edge transitions [20,21]. The high quality of the absorbers investigated leads to a very low intensity of the deep defect luminescence in the range of 1.2–1.3 eV This feature allows one to doubtlessly attribute the defect related emission at lower energies than the two DA transitions to phonon replicas of those transitions, in contradiction to the models reported in literature. We demonstrate the remarkable similarity between the sulphide material and the better investigated selenide material [23]
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