Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se2 solar cells

Abstract

The carriers’ behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se2 thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely VSe, InCu and (InCu+VCu), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of EC-0.08, EC-0.20 and EC-0.25 eV, respectively. The InCu and (InCu+VCu) deep level defects are non-radiative recombination centers at room temperature. The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se2 layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (InCu+VCu) and Vse defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se2 conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.

Alternative alkali fluoride post‐deposition treatment under elemental sulfur atmosphere for high‐efficiency Cu(In,Ga)Se2‐based solar cells

“…The same four peaks were found in all the fitted spectra, and they are shown in Figure 5. The P3 peak is attributed to the band‐to‐band transition of the CIGSe film 25,26 . The P3 peak energy at 1.145 ± 0.005 eV for all the samples is slightly lower than the bandgap energy of 1.155 eV measured by the EQE curve (see Figure 1).…”

Study of Ammonium Sulfide Surface Treatment for Ultrathin Cu(In,Ga)Se2 with Different Cu/(Ga + In) Ratios

“…We propose that this defect could be related either to the neutral complex defect (2 V Cu + In Cu ) or the acceptor‐type defect O Se , as the difference in the position of the PL peaks matches the reported energies for these defects. [ 23–26 ] However, Ishizuka et al show that the behavior of the acceptor O Se defect changes with sodium incorporation. [ 26 ] Thus, this defect cannot be the defect that we observed at low energy, because our samples have all been supplied with the same amount of sodium.…”

Luminescence properties of pulsed laser deposited CuInxGa1−xSe2 films

“…An assignement of the deep levels participating in the emissive peaks 1 and 2 on specific CIGS point defects is challenging due to the spatial and energetic disorder that broadens deep levels to a manifold of states as evidenced by the associated broad luminescence peaks. Defects with the lower formation energy in CIGS are I Cu (interstitials) and V Cu (vacancies) acting as individual donor and acceptors, respectively or correlated In Cu +V Cu defect complexes [13,14]. The latter complex induces a donor-to-acceptor emission at roughly ∼200 meV below the gap [14] that based on the energetics may be associated with peak 1.…”

“…Defects with the lower formation energy in CIGS are I Cu (interstitials) and V Cu (vacancies) acting as individual donor and acceptors, respectively or correlated In Cu +V Cu defect complexes [13,14]. The latter complex induces a donor-to-acceptor emission at roughly ∼200 meV below the gap [14] that based on the energetics may be associated with peak 1. According to our previous work [7], the films are slightly In-rich and Ga-deficient so In interstitials and/or Ga vacancies are also candidate deep levels involved on the emissive peaks 1 and 2.…”

“…Band-edge transitions are generally expected to exhibit larger recombination rates compared to sub-gap transitions due to higher radiative rates and the presence of non- radiative trapping channels that feed the lower energy peaks and effectively quench the higher energy emission lifetime. Furthermore increase of the temperature typically benefits non-radiative quenching over radiative recombination [14,16]. Interpretation of trend (iii) requires more elaborate thinking.…”