Impact of the selenisation temperature on the structural and optical properties of CZTSe absorbers

Abstract

We present structural and optical spectroscopy studies of thin films of Cu2ZnSnSe4 (CZTSe) with strong copper deficiency deposited on Mo/Glass substrates and selenised at 450, 500 or 550 °C. Solar cells fabricated from these films demonstrated efficiencies up to 7.4% for selenisation at 500 °C. Structural analysis based on X-ray diffraction and Raman spectroscopy revealed the presence of SnSe2 in the film selenised at 450 °C but not detected in the films selenised at higher temperatures. A progressive decrease of the Sn and Se content was observed as the selenisation temperature increased. Photoluminescence excitation was used to determine the bandgaps at 4.2 K. Detailed measurements of the temperature and excitation intensity dependencies of the photoluminescence spectra allow the recombination mechanisms of the observed emission bands to be identified as band-to-impurity and band-to-band transitions, and their evolution with selenisation temperature changes to be analysed. The strongest band-to-band transition is recorded in the PL spectra of the film selenised at 500 °C and can be observed from 6 K to room temperature. The compositional and structural changes in the films and their influence on the optoelectronic properties of CZTSe and solar cells are discussed

A luminescence study of Cu2ZnSnSe4/Mo/glass films and solar cells with near stoichiometric copper content

“…As in the film the P2 band quenches by 70 K. At temperatures above 200 K the solar cell spectra show a broad band at 0.9 eV. Such bands, observed earlier in CZTS [28] and CZTSe [20,23,24], were attributed to band-to-band (BB) transitions, involving the recombination of free electrons from the conduction band with free holes from the valence band. The P3 band in the PL spectra of the solar cell gradually quenches with raising temperature however it retains a relatively high intensity up to temperatures of 300 K. High concentrations of charged defects generate tails in the density of states (DOS) of the conduction and valence bands of semiconductors.…”

“…The rate of this shift (j-shift) is 14 meV per decade of laser power change for the CZTSe/Mo film and decreases to 13 meV/decade for the cell spectra as shown in Table 1. The characteristic asymmetric shape of the P1 band, the invariance of this shape on the excitation intensity and the significant j-shifts suggest that the radiative recombination mechanism of this band is associated with band tails generated by spatial potential fluctuations due to high concentrations of charged defects [18,20,[23][24][25][26][27]. The P3 band in the PL spectra of the cell also increases its intensity at higher excitation.…”

“…Thin films of CZTSe used in high performance solar cells are generally considered to have high concentrations of both donors and acceptors [3,18,20,23,31,32]. A semiconductor is highly doped if the mean distance between defects is smaller than their Bohr radii [30].…”

CZTSe solar cells developed on polymer substrates: Effects of low‐temperature processing

“…The hypothesis of SnSe 2 formation due to a low supply of thermal energy during annealing is supported by observations performed in other works. Márquez et al observed a strong formation of SnSe 2 when processing CZTSe at 450°C that disappeared for samples processed at 500°C . The duration of the annealing was the same at both temperatures so the energy supplied to the sample was directly related to the annealing temperature.…”

“…47,[69][70][71] when processing CZTSe at 450°C that disappeared for samples processed at 500°C. 75 The duration of the annealing was the same at both temperatures so the energy supplied to the sample was directly related to the annealing temperature. What is more, Temgoua et al fabricated CZTSSe devices at 600°C for different annealing times and observed the formation of SnSe 2 from 1-to 30-minute annealings that vanished for longer annealing times.…”