Earth‐abundant Cu2BaSnS4 (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 104 cm−1, and a p‐type conductivity, suitable as a top‐cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open‐circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine‐doped SnO2 (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum‐doped ZnO (front contact).

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV

S olar energy sources, such as hydrogen fuel produced by photoelectrochemical (PEC) water splitting, offer the promise of essentially limitless clean energy for powering our society. Achieving high efficiency for solar-to-hydrogen conversion is essential for reducing the cost of these technologies. The tandem device concept of stacking a wide-

We report the application of thin film nanocrystalline (NC) FeS 2 as the copper-free back contact for CdTe solar cells. The FeS 2 -NC layer is prepared from solution directly on the CdTe surface using drop-casting coupled with a hydrazine treatment at ambient temperature and pressure, and requires no thermal treatment. Copper-free solar cells based on the CdS/CdTe/FeS 2 -NC/Au architecture exhibit device efficiencies >90% that of a standard Cu/Au back contact devices. The FeS 2 -NC back contact solar cells show good thermal stability under initial tests. Devices prepared with untreated FeS 2 -NC back contacts display a strong "S-kink" behavior which we correlate with a high hole-transport barrier arising from inter-NC organic surfactant molecules.

Device grade polycrystalline CdTe films, grown by rf-sputtering (yielding up to 13% efficient devices) and closespaced sublimation (CSS) (yielding up to 16% efficient devices), have been analyzed using scanning microwave impedance microscopy (sMIM). In order to study the effect of Cl on the electronic properties of grain boundaries in CdTe, sMIM based capacitance measurements were performed on the surfaces of asdeposited and CdCl 2 annealed CdTe films. The measurements were performed with an sMIM instrument operating at ∼3 GHz probe signal frequency in conjunction with a combination of ac and dc voltage biases applied to the tip. It was found that CdCl 2 annealed CdTe samples deposited using both CSS and sputtering exhibit carrier depletion along the grain boundaries as compared to the grain bulk. The grain boundary carrier depletion was not observed in samples that have not been CdCl 2 annealed and hence have no incorporated Cl.