Polyol-mediated synthesis of Cu2ZnSn(S,Se)4 kesterite nanoparticles and their use in thin-film solar cells

Cu₂ZnSnS₄ kesterite nanoparticles (CZTS) with a particle diameter of 10–20 nm are prepared by a polyol-mediated synthesis with diethylene glycol as the liquid phase. The polyol – a high-boiling multidentate alcohol − allows controlling the particle size and agglomeration as well as preparing readily crystalline nanoparticles. The as-prepared kesterite nanoparticles exhibit an overall composition of Cu_1.56Zn_1.29Sn_1.16S_4.59 and a band gap of 1.37 eV. As a first test, thin-film solar cells are manufactured after layer deposition of the as-prepared CZTS nanoparticles and conversion to Cu₂ZnSn(S,Se)₄ (CZTSSe) via gas-phase selenization. The volume increase of about 15% due to the CZTS-to-CZTSSe conversion supports the formation of a dense layer, reduces the interparticulate surfaces and leads to a reduction of the band gap to 1.14 eV. The chemical composition of the as-prepared CZTS nanoparticles and of the deposited CZTSSe thin film prior and after selenization are studied in detail by energy-dispersive X-ray spectroscopy, Raman spectroscopy and X-ray fluorescence analysis. All these methods confirm the intended copper-poor and zinc-/tin-rich CZTS/CZTSSe composition. The resulting thin-film solar cells show an open-circuit voltage of 247.3 mV, a short-circuit current density of 21.3 mA/cm², a fill factor of 41.1% and a power-conversion efficiency of 2.2%.     

Effects of Cu2−xS phase removal on surface potential of Cu2ZnSnS4 thin-films grown by electroplating

Cu₂ZnSnS₄ (CZTS) has an optical band gap of 1.4–1.5 eV, which is similar to that of Cu(In,Ga)Se₂ (CIGS), and a high absorption coefficient (>10⁴ cm⁻¹) in the visible light region. In previous reports, CIGS thin-film solar cells have been shown to improve the performance of the device since the secondary phase is removed by Potassium cyanide (KCN) etching treatment. Therefore, in this study we applied a KCN etching treatment on CZTS and measured the effects. We confirmed the removal of Cu_2−xS via Kelvin probe force microscopy (KPFM) and Raman scattering spectroscopy. The effects of the experiment indicate that we can define with precision the location of the secondary phases, and therefore the control of the secondary phases will be easier and more efficient. Such capabilities could improve the solar cell performance of CZTS thin-films.     

Properties of Cu2ZnSn(SxSe1−x)4 thin films prepared by one-step sulfo-selenization of alloyed metal precursors

The pentenary system, Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe), is a promising alternative for thin film solar cells. In this study, CZTSSe thin films were prepared using a two-stage process involving the thermal diffusion of sulfur (S) and selenium (Se) vapors into sputtered metallic precursors at approximately 450 °C. The effects of the sulfur content on the composition, structure, optical and electrical characteristics of the CZTSSe thin films were investigated. The films showed a kesterite structure with a predominant (112) orientation. X-ray diffraction and Raman spectroscopy confirmed the formation of a single phase CZTSSe compound. The band gap was dependent on the sulfur content and was calculated to be 1.25 eV, 1.33 eV and 1.40 eV for CZTSSe films with a S/(S + Se) ratio of 0.3, 0.5 and 0.7, respectively. All films exhibited p-type semiconductor properties.     

Photoluminescence study of disordering in the cation sublattice of Cu2ZnSnS4

In this study we investigated the influence of the degree of disordering in the cation sublattice on low temperature photoluminescence (PL) properties of Cu₂ZnSnS₄ (CZTS) polycrystals. The degree of disordering was changed by using different cooling rates after post-annealing at elevated temperatures. The results suggest that in the case of higher degree of cation sublattice disorder radiative recombination involving defect clusters dominates at T = 10 K. These defect clusters induce local band gap energy decrease in CZTS. The concentration of defect clusters can be reduced by giving more time for establishing ordering in the crystal lattice. As a result, radiative recombination mechanism changes and band-to-impurity recombination involving deep acceptor defect with ionization energy of about 200 meV starts to dominate in the low temperature PL spectra of CZTS polycrystals.