•Ambient-exposed CZTSSe thin films possess an overlayer.•KCN etching removes the overlayer completely from CZTSSe absorbers.•After NH4OH treatment overlayer residues still remain on CZTSSe absorbers.•KCN-etched CZTSSe devices demonstrate better device characteristics.•KCN etching equalizes surface potential of grain surfaces and grain boundaries.Despite the many similarities between Cu2ZnSn(S,Se)4 (CZTSSe) and Cu(In,Ga)(S,Se)2 materials and device architecture, open questions remain about the optimal surface preparation steps for CZTSSe absorbers, including whether differences exist for absorber layers deposited by different methods. In this work, we investigate KCN etching and NH4OH treatment as surface preparation methods for the absorber/CdS interface for two-stage processed CZTSSe and co-evaporated CZTSe absorber layers. Ambient-exposed, thus oxidized and contaminated, thin film absorbers are utilized to examine the effectiveness of these surface preparation methods and to elucidate their effects on device performance. Topography and surface potential images simultaneously obtained by Kelvin probe force microscopy (KPFM) show the existence of an overlayer on the ambient-exposed absorbers. Moreover, KPFM results also demonstrate that although NH4OH treatment removes much of the overlayer from the CZTSSe surface, KCN etching removes the overlayer completely. In addition, differences in the deposited CdS layer and depletion region width result depending on the surface preparation method, with the NH4OH-treated solar cells having narrower depletion region. This is reflected in device results in which KCN-etched solar cells outperform their NH4OH-treated counterparts due to increases in external quantum efficiency at long wavelengths and in open circuit voltage. KPFM measurements also demonstrate that grain boundaries (GBs) in the KCN-etched two-stage processed CZTSSe thin films are either negatively charged or neutral. On the other hand, KCN etching makes the surface almost equipotential in the co-evaporated CZTSe thin film by causing disappearance of positively charged GBs that existed before etching.

CuInSe2 (CISe) is a prototype material for the I–III–VI chalcopyrites such as Cu(In,Ga)(S,Se)2 used as absorber layers in thin film photovoltaic cells. Carefully-controlled pulsed-laser annealing (PLA) is a unique annealing process that has been demonstrated to improve the device performance of chalcopyrite solar cells. Here, we investigate the changes in defect populations after PLA of electrochemically-deposited CISe thin films previously furnace annealed in selenium vapor. The films were irradiated in the sub-melting regime at fluences inducing temperatures up to 840 ± 100 K. Deep-level transient spectroscopy on Schottky diodes reveals that the activation energy of the dominant majority carrier trap changes non-monotonically from 215 ± 10 meV for the reference sample, to 330 ± 10 meV for samples irradiated at 20 and 30 mJ/cm2, and then back to 215 ± 10 meV for samples irradiated at 40 mJ/cm2. A hypothesis involving competing processes of diffusion of Cu and laser-induced generation of In vacancies may explain this behavior.Highlights► Pulsed laser annealing (PLA) effects studied on CuInSe2 films ► PLA improves crystalline order parameter. ► PLA induces changes in majority carrier defect levels.

A large amount of current research in thin film photovoltaics based on inorganic semiconductors aims to deposit and process these active layers using inexpensive and low temperature processing methods resulting in high densities of electronic states in the bandgap associated with structural and point defects. We define semiconductors in which the Fermi level is pinned throughout the bulk as being at the defective limit and explore the consequences for heterojunctions designed to separate photocarriers. Using this novel concept of the defective limit, heterojunction band offsets can be predicted for less-common materials such as the earth-abundant materials also under intense investigation. Because of the Fermi level pinning in the bulk, only Type II heterojunctions will separate charges but the high defect concentrations will also result in small minority carrier diffusion lengths. These limitations require the use of nanostructured device architectures for cells of such materials with appreciable power conversion efficiency.

We investigate the synthesis of kesterite Cu2ZnSnS4 (CZTS) polycrystalline thin films using cosputtering from binary sulfide targets followed by annealing in sulfur vapor at 500°C to 650°C. The films are the kesterite CZTS phase as indicated by x-ray diffraction, Raman scattering, and optical absorption measurements. The films exhibit (112) fiber texture and preferred low-angle and Σ3 grain boundary populations which have been demonstrated to reduce recombination in Cu(In,Ga)Se2 and CdTe films. The grain growth kinetics are investigated as functions of temperature and the addition of Na. Significantly, lateral grain sizes above 1 μm are demonstrated for samples grown on Na-free glass, demonstrating the feasibility for CZTS growth on substrates other than soda lime glass.

We report on the heteroepitaxial growth of thin films of rocksalt GdN on c-plane (0 0 0 1) wurtzite GaN by molecular beam epitaxy (MBE) using either an N2 plasma or NH3 as the nitrogen source. In both cases, epitaxial films with fully oriented GdN (1 1 1)∥GaN (0 0 0 1) were deposited as demonstrated by θ–2θ X-ray diffraction. φ scans of GdN peaks demonstrate 6-fold symmetry along the growth axis implying the presence of two 3-fold-symmetric GdN (1 1 1) crystal variants in-plane. Electrical transport and magnetometry measurements on films grown using N2 plasma show that these GdN films are ferromagnetic below TC=70 K and degenerately doped or metallic from 10 to 300 K with magnetotransport signatures associated with TC.