Abstract
A detailed analysis of the radiative recombination processes in Ag(InGa)Se2 thin films grown by a three-stage method was carried out by photoluminescence. The temperature and excitation dependence of the photoluminescence spectra was used to identify the recombination types and determine the ionization energy of the defects in the films. Significant differences were observed between the spectra of the Ag-rich and Ag-poor samples. The Ag-rich films were dominated by two emission peaks of donor acceptor pairs (DAPs). The DAP at lower energy level is attributed to recombination of donor level 13.8 meV (Agi) with acceptor level 70.3 meV (AgIn), while the one at high energy level is assigned to recombination of donor level 18.5 meV (Agi) with acceptor level 108.9 (AgSe). When Ag/III atomic ratio was near 2.00, a phonon related-structure began to appear, which is attributed to the phonon replica of the high energy level DAP. In the case of Ag-poor AIGS samples, the dominant broad asymmetric peaks of AIGS films with different Ag/III atomic ratios were related to potential fluctuation at low temperature, and the compensation level decreased with increasing Ag/III atomic ratio. The emission line was assigned to recombination of donor level 12.7 meV (Agi) with acceptor level 175 meV (AgGa2). When the excitation power and temperature were increased, new free-bound and DAP emission lines began to appear. The free-bound was assigned to the transition from the conduction band to an acceptor level of 80 meV (AgIn). The DAP was assigned to recombination of donor level 20 meV (VSe) with acceptor level 145 meV (AgGa).
Highlights
Ag(InxGa1−x)Se2 (0 ≤ x ≤ 1) (AIGS) crystals have been recognized as promising candidates for photovoltaic applications.[1,2,3] The bandgap of AIGS can be adjusted from 1.24 (AgInSe2) to 1.78 eV (AgGaSe2), which can be used as the wide bandgap material.[4]
Significant differences were observed between the spectra of the Ag-rich and Ag-poor samples
The donor acceptor pairs (DAPs) at lower energy level is attributed to recombination of donor level 13.8 meV (Agi) with acceptor level 70.3 meV (AgIn), while the one at high energy level is assigned to recombination of donor level 18.5 meV (Agi) with acceptor level 108.9 (AgSe)
Summary
Ag(InxGa1−x)Se2 (0 ≤ x ≤ 1) (AIGS) crystals have been recognized as promising candidates for photovoltaic applications.[1,2,3] The bandgap of AIGS can be adjusted from 1.24 (AgInSe2) to 1.78 eV (AgGaSe2), which can be used as the wide bandgap material.[4] There have been several reports on the application of AIGS thin films to solar cells, and as reported in our former papers, a highest efficiency of 10.7% has been achieved.[5] the fill factor of AIGS thin film-based solar cells is still very low because of strong recombination, which hinders further increase in performance. It is obvious that a high level of defects in the semiconductor will cause recombination to take place more . To further increase the solar cell performance, it is necessary to study the optical properties of AIGS films. Understanding of the type and energy level of defects in AIGS films is essential for crystal quality control
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