Influence of the copper content on the optical properties of CZTSe thin films

Abstract

We present an optical spectroscopy study of Cu 2 ZnSnSe 4 (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap E g , determined using photoluminescence excitation (PLE) at 4.2 K, from 0.99 eV to 1.03 eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83 eV to 0.95 eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83 eV and a lower intensity one at 0.93 eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean 2 depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens E g due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the E g widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93 eV.

Direct evidence of causality between chemical purity and band-edge potential fluctuations in nanoparticle ink-based Cu2ZnSn(S,Se)4 solar cells

“…Conversely, LP devices have a higher J SC value (26.1 mA/cm 2 ) in comparison to that seen in HP devices (23.7 mA/cm 2 ). Similar observations were made by Yakushev et al for Cu 2 ZnSnSe 4 (CZTSe) devices with varying Cu/(Zn+Sn) and Zn/Sn ratios and were attributed to the degree of Cu-Zn order/disorder in the crystal lattice (Yakushev et al, 2017). EQE plots for LP and HP devices are shown in Fig.…”

“…revealed comparable temperature trends in PL spectra of CZTSe thin lms with varying Cu content (Yakushev et al, 2017).…”

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.…”

Spectroscopic ellipsometry study of Cu2ZnSnS4 bulk poly-crystals

“…However, this tendency is opposite to those found in Cu2ZnSnSe4, where the band gap increases with copper deficiency. 38 On the other hand, the Cu-Zn lattice disorder in CZTS can induce a change in the band gap of about 0.2 eV. 39 According to this concept, an increase of the band gap is related to an increase of degree of ordering in the samples.…”