Off-stoichiometry in I-III-VI2 chalcopyrite absorbers: a comparative analysis of structures and stabilities.

Chalcopyrite Cu(In,Ga)Se2 (CIGSe) solar absorbers are renowned for delivering high solar power conversion efficiency despite containing high concentration of lattice defects amounting to copper deficiencies of several atomic percent. The unique ability to incorporate this deficiency without triggering decomposition (i.e. "tolerance to off-stoichiometry") is viewed by many as the key feature of CIGSe. In principle, this property could benefit any solar absorber, but remarkably little attention has been paid to it so far. In this study, we assess the tolerance to off-stoichiometry of thin-film photovoltaic materials by carrying out ab initio analysis of group-I-poor ordered defect compounds (ODCs) in the extended family of I-III-VI systems (where I = Cu, Ag, III = Al, Ga, In, and VI = S, Se, Te). We analyze convex hulls and structural evolution with respect to group-I content, link them with experimental phase diagrams, and determine two empirical principles for the future identification of solar energy materials with high tolerance to off-stoichiometry. Practical implications for the deposition of I-III-VI absorbers are also discussed in light of our computational results and recent experimental findings.

Alkali Dispersion in (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells-Insight from Theory and Experiment.

Silver alloying of Cu(In,Ga)Se2 absorbers for thin film photovoltaics offers improvements in open-circuit voltage, especially when combined with optimal alkali-treatments and certain Ga concentrations. The relationship between alkali distribution in the absorber and Ag alloying is investigated here, combining experimental and theoretical studies. Atom probe tomography analysis is implemented to quantify the local composition in grain interiors and at grain boundaries. The Na concentration in the bulk increases up to ∼60 ppm for [Ag]/([Ag] + [Cu]) = 0.2 compared to ∼20 ppm for films without Ag and up to ∼200 ppm for [Ag]/([Ag] + [Cu]) = 1.0. First-principles calculations were employed to evaluate the formation energies of alkali-on-group-I defects (where group-I refers to Ag and Cu) in (Ag,Cu)(In,Ga)Se2 as a function of the Ag and Ga contents. The computational results demonstrate strong agreement with the nanoscale analysis results, revealing a clear trend of increased alkali bulk solubility with the Ag concentration. The present study, therefore, provides a more nuanced understanding of the role of Ag in the enhanced performance of the respective photovoltaic devices.

Evolution of Na-S(-O) Compounds on the Cu2ZnSnS4 Absorber Surface and Their Effects on CdS Thin Film Growth.

Formation of Na-containing surface compounds is an important phenomenon in the Cu2ZnSnS4 (CZTS) quaternary material synthesis for solar cell applications. Still, identification of these compounds and the understanding of their potential influence on buffer layer growth and device performance are scarce. In this work, we discovered that the evolution of Na-S(-O) compounds on the CZTS surface substantially affect the solution/CZTS interface during the chemical bath deposition of CdS buffer film. We showed that Na2S negatively affects the growth of CdS, and that this compound is likely to form on the CZTS surface after annealing. It was also demonstrated that the Na2S compound can be oxidized to Na2SO4 by air exposure of the annealed CZTS surface or be removed using water dipping instead of the commonly used KCN etching process, resulting in significantly better quality of the CdS layer. Lastly, 6.5% CZTS solar cells were fabricated with air exposure treatment without incorporation of the KCN etching process. This work provides new insight into the growth of the CdS/CZTS interface for solar cell applications and opens new possibilities for improving likewise Cd-free buffer materials that are grown with a similar chemical bath deposition process.

A detrimental reaction at the molybdenum back contact in Cu2ZnSn(S,Se)4 thin-film solar cells.

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.

Thermodynamic aspects of the synthesis of thin-film materials for solar cells.

A simple and useful thermodynamic approach to the prediction of reactions taking place during thermal treatment of layers of multinary semiconductor compounds on different substrates has been developed. The method, which uses the extensive information for the possible binary compounds to assess the stability of multinary phases, is illustrated with the examples of Cu(In,Ga)Se(2) and Cu(2)ZnSnSe(4) as well as other less-studied ternary and quaternary semiconductors that have the potential for use as absorbers in photovoltaic devices.

High throughput X-ray diffraction analysis of combinatorial polycrystalline thin film libraries.

With the increasing demand for new materials, analytical techniques which are able to rapidly characterize a large number of samples are becoming indispensable. Thin film technology has the potential to improve the amount of information contained on as-deposited samples by creating compositionally graded libraries. Conventionally, raster scan methods are used to interrogate such libraries but, in this paper, a different approach is presented to provide a method of high-throughput data collection and analysis using an X-ray diffraction (XRD) probe. An extended X-ray beam was used to illuminate the libraries, and a large area detector was used to collect the data. A new algorithm "Bandit" has been employed to analyze the collected data and extract the crystallographic information. The results of the technique have been compared with the raster scans showing that the algorithm provides reliable data at a significantly increased data acquisition speed.

Triple phase boundary photovoltammetry: resolving rhodamine B reactivity in 4-(3-phenylpropyl)-pyridine microdroplets.

Rhodamine B base is employed as a photoactive redox system in a liquid | liquid | electrode triple phase boundary electrochemical experiment. Microdroplets of the water-immiscible 3-(4-phenylpropyl)-pyridine (PPP) containing rhodamine B are deposited onto a basal plane pyrolytic graphite electrode surface which is then immersed into an aqueous solution containing 0.5 M phosphate buffer solution (pH 12). In cyclic voltammetry experiments, it is shown that dark reduction of rhodamine B to leuco-rhodamine B in PPP occurs in two steps--both one electron processes--and the re-oxidation occurs in a single two-electron process. Voltammetric signals are consistent with Na(+) transfer (for the first reduction step) and proton transfer (for the second reduction step) coupled to the electron transfer. A proton-driven disproportionation reaction (with k(dis)=3 mol(-1) dm(3) s(-1) at pH 12) is observed for the one-electron reduced intermediate. In the presence of light, a new photocatalysed oxidation process for the two-electron conversion of leuco-rhodamine B to rhodamine B is observed. Photoexcitation of rhodamine B is shown to trigger an effective photo-comproportionation mechanism. Quantitative insights into the dark and photomechanisms are obtained by comparison of rhodamine B and rhodamine B octadecylester and by applying an approximate numerical simulation procedure based on DigiSim.