Revisiting radiative deep-level transitions in CuGaSe2 by photoluminescence

Abstract

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10 K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined from the temperature quenching depend on the excitation level and show unexpected values of 600 meV and higher. The thermal activation of non-radiative processes can explain the distortion of the ionization energies. Both the deep levels play a major role as radiative and non-radiative recombination centers for electrons and can be detrimental for photovoltaic applications.

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.…”

Deep level emission in polycrystalline CuGaSe2 thin-films observed by micro-photoluminescence

“…Recently similar deep level emission, DDA2 has been reported by Spindler et al around 1.1 eV in PL spectrum of a epitaxially grown CuGaSe 2 thin film. 20) Optical transition was explained with a shallow acceptor level around 100 meV above E V and deeper donor-level <530 meV below E C . As location of the deep level associated with this DL emission is close to the mid-gap position in CuGaSe 2 in this study, it might work as an active recombination center, i.e., nonradiative recombination of minority carrier electrons trapped at the level, E C − 630 meV may be possible with holes at E V .…”

Chalcopyrite compound semiconductors for thin film solar cells

“…It was only recently understood that this difference is not in the first place due to a difference in interface properties, but to bulk properties of Cu(In,Ga)Se 2 : in pure CuInSe 2 the quasiFermi level splitting, which is a measure of the opencircuit voltage an absorber could achieve, is higher in Cu-rich material than in Cu-poor [70,71], whereas in Cu(In,Ga)Se 2 (with Ga) the quasi-Fermi level splitting is lower in Cu-rich material than in Cu-poor, already in the absorber without any interface [72]. It has been proposed that the difference is due to a deep Ga Cu antisite defect [73,74], which forms a less detrimental pair with copper vacancies in Cu-poor material.…”