Cu2ZnSnS4 (CZTS) shows great potential for cheap, efficient photovoltaic devices. However, one problem during synthesis of CZTS films is the loss of Sn as a result of decomposition and evaporation of SnS. This paper uses kinetic models to show that the mechanism of the decomposition reaction probably occurs in at least two stages; first, a loss of sulfur which causes dissociation of the structure into binary sulfides, and only then the evaporation of SnS. Knowledge of the reaction mechanism helps to identify the driving force for decomposition as arising from the relative instability of Sn(IV) in CZTS against reduction; this theory is backed up by thermodynamic data. The volatility of SnS further exaggerates the decomposition by rendering it irreversible. This insight, alongside experimental data, allows prediction of the annealing conditions required to stabilize CZTS surfaces. A fundamental incompatibility of CZTS with high-temperature, vacuum-based processing is exposed, distinguishing it from related indium-containing compounds. This offers an explanation as to why the most efficient CZTS devices to-date all arise from “two-stage” fabrication processes involving low temperature deposition followed by annealing at high pressure, and provides key information for designing successful annealing strategies.

Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell material Cu(2)ZnSn(S,Se)(4) (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS(2) and secondary phases at the CZTS|Mo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.

Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuIn x Ga 1Àx Se 2 , are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results. Broader contextSolar energy is one of the most attractive renewable energy sources as it is abundant, wide spread and basically free. The price for capturing this energy is now starting to get compatible with other energy sources which raises the concern of the intermittent nature of solar irradiation. The production of solar generated hydrogen is both a possible alternative for dealing with this intermittency, as well as for providing a potent fuel and an important feedstock for the industry. Much research has been done in the development of materials usable as photo-electrochemical cells for this purpose. An inherent problem in the design of these devices is the mismatch between the solar spectrum and the thermodynamic and kinetic requirements for the solar water splitting reaction. The standard proposed solution to this problem is to construct tandem devices. Here we instead explore the idea of connecting efficient photo-absorbers in series to obtain the required photo-potential and discuss the similarities between PEC and PV-electrolysis cells. To demonstrate the potential of this approach we manufacture a monolithic device based on series interconnected CIGS cells that reach 10% solar to hydrogen efficiency.

A theoretical analysis of different device concepts for solar hydrogen production, demonstrating the close similarities between photoelectrochemical cells and PV-electrolyzers.

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Photoelectron spectroscopy, optical characterization, and density functional calculations of ZnO1-xSx reveal that the valence-band (VB) offset E(v)(x) increases strongly for small S content, whereas the conduction-band edge E(c)(x) increases only weakly. This is explained as the formation of local ZnS-like bonds in the ZnO host, which mainly affects the VB edge and thereby narrows the energy gap: E(g)(x=0.28) approximately E(g)(ZnO)-0.6 eV. The low-energy absorption tail is a direct Gamma(v)-->Gamma(c) transition from ZnS-like VB. The VB bowing can be utilized to enhance p-type N(O) doping with lower formation energy DeltaH(f) and shallower acceptor state in the ZnO-like alloys.

This contribution concerns the effect of the Ag content in wide‐gap AgwCu1‐wIn1‐xGaxSe2 (ACIGS) absorber films and its impact on solar cell performance. First‐principles calculations are conducted, predicting trends in absorber band gap energy (Eg) and band structure across the entire compositional range (w and x). It is revealed that a detrimental negative conduction band offset (CBO) with a CdS buffer can be avoided for all possible absorber band gap values (Eg = 1.0–1.8 eV) by adjusting the Ag alloying level. This opens a new path to reduce interface recombination in wide‐gap chalcopyrite solar cells. Indeed, corresponding samples show a clear increase in open‐circuit voltage (VOC) if a positive CBO is created by sufficient Ag addition. A further extension of the beneficial compositional range (positive CBO at buffer/ACIGS interface) is possible when exchanging CdS with Zn1‐ySnyOz, because of its lower electron affinity (χ). Nevertheless, the experimental results strongly suggest that at present, residual interface recombination still limits the performance of solar cells with optimized CBO, which show an efficiency of up to 15.1% for an absorber band gap of Eg = 1.45 eV.

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