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

Abstract

Cu 2 ZnSnSe 4 (CZTSe) is one of the leading candidates for the absorber layer in sustainable solar cells. Thin films of CZTSe with a near stoichiometric [Cu]/[Zn+Sn] were used to produce solar cells with conversion efficiency η = 6.4% by a standard solar cell processing including KCN etching and the deposition of CdS and ZnO. Both CZTSe films and solar cells were examined using photoluminescence (PL) to analyse the nature of radiative recombination and photoluminescence excitation (PLE) at 4.2 K to determine the bandgap (E g). Low temperature PL spectra of the films reveal an intense band P1 at 0.81 eV and a low intensity band P2 at 0.93 eV. Their temperature and excitation intensity dependencies suggest that they both involve recombinations of free electrons with holes localised at acceptors with the energy level influenced by potential fluctuations in the valence band. We associate P1 and P2 with different fractions of CZTSe: with a lower and higher degree of order of Cu and Zn on the cation sub-lattice, respectively. Device processing reduced the intensity of P1 by 2.5 whereas the intensity of P2 increased by a 1.5. We assign this to a low temperature annealing due to CdS and ZnO deposition which increased the fraction of CZTSe with high degree of Cu/Zn order and decreased the fraction with low degree of Cu/Zn order. Device processing increased E g , blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can also be related to the annealing and/or KCN etching and the chemical effect of Cd, due to CdS replacing copper at the CdS-CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV (quenching with E a = 200 meV) which we attributed to defects in the CdS layer.

Mapping the Energetics of Defect States in Cu2ZnSnS4 films and the Impact of Sb Doping

“…The dependence of the primary PL peak intensity with excitation laser power is shown in Figure d, which is fitted to the power law where I is the PL peak intensity, A is a proportionality constant, P is excitation power, and the exponent k is a parameter associated with the recombination mechanism . Values of k obtained for ND and Na:Sb doped were below 1, indicative of localized defect mediated transitions. − On the other hand, Sb-doped films exhibit k = 1.2, which strongly suggests a nonlocalized defect as reported in other studies involving chalcopyrites and kesterites. − This shift from localized to nonlocalized defect mediated transitions is an indication that the positions of the states involved in the PL transition are shifted with respect to the band edges depending on the Sb content.…”

The electrical and optical properties of kesterites

“…These fluctuations usually determine the asymmetric shape of PL bands, where the shape of the low energy side of the PL spectra is determined by the density of states function. In kesterite samples, γ values in the range γ≈25-80 meV were found [2,3,5,29,[31][32][33][34][35]. Typical low-temperature measurements for kesterites show a broad peak with increasing width and assymmetry for higher sulfur concentrations as shown in figure 2(a).…”

Microwave-assisted synthesis of organic–inorganic hybrid porous g-C3N4/CdS–diethylenetriamine S-scheme heterojunctions with enhanced visible light hydrogen production

“…is non-polluting, renewable, and rich in resources due to its combustion: it uses water as a raw material, emits a lot of heat, and can also be recycled [1][2][3][4][5][6][7]. There are many ways to produce hydrogen, such as fossil fuel hydrogen production, pyrolysis hydrogen production, and hydrogen production by electrolysis of water [8]. The use of sunlight to decompose water under the action of a catalyst to produce hydrogen is considered to be the current solution to environmental and energy crises.…”