Search

Alkali metal doping and grain boundaries (GB) have been at the center of attention within the Cu(In,Ga)(S,Se)2 photovoltaics community for years. This study provides the first experimental evidence that the GB of sodium-doped CuInSe2 thin films may undertake reversible oxidation even at room temperature, whereas undoped films may not. The findings are corroborated by cathodoluminescence imaging, secondary ion mass spectrometry, and Kelvin probe force microscopy on air-exposed films subsequently subject to vacuum. A thermochemical assessment identifies the likely solid–gas equilibria involved. These reactions open new research questions with respect to the beneficial role played by alkali metal dopants in chalcopyrite solar cells and may steer the community toward new breakthroughs.

The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells.

Heavy alkali‐metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low‐cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO3) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin‐coating CsCO3 solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open‐circuit voltage. X‐ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High‐resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low‐cost route for improving the performance of CIGS solar cells by surface modification using a solution process. [ABSTRACT FROM AUTHOR]

Alkali metal doping and grain boundaries (GB) have been at the center of attention within the Cu(In,Ga)(S,Se)2 photovoltaics community for years. This study provides the first experimental evidence that the GB of sodium‐doped CuInSe2 thin films may undertake reversible oxidation even at room temperature, whereas undoped films may not. The findings are corroborated by cathodoluminescence imaging, secondary ion mass spectrometry, and Kelvin probe force microscopy on air‐exposed films subsequently subject to vacuum. A thermochemical assessment identifies the likely solid–gas equilibria involved. These reactions open new research questions with respect to the beneficial role played by alkali metal dopants in chalcopyrite solar cells and may steer the community toward new breakthroughs. [ABSTRACT FROM AUTHOR]

The effect of selenium‐free annealing on cesium fluoride (CsF)‐treated Cu(In,Ga)Se2 (CIGS) thin films is investigated and their solar cell performance is evaluated. Annealing of CsF‐treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained, because of an increased buffer/CIGS interface recombination as a consequence of re‐evaporation of the alkali‐containing layer from the surface region upon annealing. On the other hand, CsF‐treated CIGS thin films without any annealing after the post‐deposition treatment (PDT) preserve the alkali‐containing layer at the surface, thereby leading to an improved buffer/CIGS interface. Consequently, the open‐circuit voltage (VOC) and fill factor improve significantly in these devices. The Urbach energies of both annealed and nonannealed solar cells are calculated from external quantum efficiency measurement to understand their impact on VOC and VOC deficit. No correlation between the VOC deficit and the Urbach energies is observed. [ABSTRACT FROM AUTHOR]

Cost-effective, active, and durable electrodes are required for the development of the efficient alkaline water electrolysis towards green hydrogen production. Here, we report successful fabrication of such electrodes by means of robust radio frequency magnetron sputtering that produces uniform, crystalline, and phase pure catalytic iridium thin films over nickel foam current collectors. The effect of working pressure of argon gas during sputtering on the physicochemical and electrochemical properties of the as-fabricated electrodes has been investigated. Specifically, it has been established that the employment of working pressure of 3 Pa afforded the electrodes with the lowest mass loading of precious iridium metal (≈0.32%), but exhibiting the best alkaline water electrolysis performance. The electrodes demonstrated favourable overpotentials of 14 mV and 318 mV at current densities of ±10 mA/cm2 toward hydrogen (HER) and oxygen (OER) evolution reactions, respectively. Furthermore, as a cathode and an anode for overall water splitting (OWS) in alkaline electrolyte, the electrode exhibited the cell voltage of 1.57 V at 10 mA/cm2 and stably performed electrolysis at different applied current densities. The electrochemical performance of the electrodes has been corroborated by work function measurements using Kelvin probe force microscopy. • Phase pure Ir thin films on nickel foam were fabricated by RF magnetron sputtering. • The effect of the working gas pressure on HER and OER performance was observed. • Bifunctional NiF@Ir-3 electrode can stably sustain OWS at several current densities. • Work function measurements prove to be a useful tool to validate electrochemical results. [ABSTRACT FROM AUTHOR]

2D materials are considered the future of electronics and photonics, stimulated by their remarkable performance. Among the 2D materials family, β‐In2Se3 shows good mobility, excellent photoresponsivity, and exotic ferroelectricity, making it suitable for a wide variety of applications. To date, most reported devices from 2D materials in general, and β‐In2Se3 in specific, rely on cumbersome fabrication methods using mechanical exfoliation and transfer of layers onto other substrates. However, for a successful adoption of 2D materials in industry, reliable and reproducible large‐area growth of 2D materials is required. Here, the wafer‐scale epitaxial growth of 2D β‐In2Se3 on c‐sapphire using molecular beam epitaxy is demonstrated. Excellent materials quality of thick (90 nm) and very thin films, down to two quintuple layers (2 nm) is confirmed. Furthermore, the fabrication of hundreds of photodetector devices on a 2 in. wafer, using five quintuple layers of β‐In2Se3, is demonstrated. They are sensitive to near‐infrared light up to 898 nm wavelength and show a response time of ≈7 ms, which is faster than any result previously reported for β‐In2Se3 photodetectors. The devices are produced using photolithography and other standard semiconductor processing methods, which allows easy integration into the current Si technology. [ABSTRACT FROM AUTHOR]

Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries. [ABSTRACT FROM AUTHOR]