Chalcopyrite thin-film materials

Spray CVD of Single-Source Precursors for Chalcopyrite I—III—Vl2 Thin-Film Materials... 

Banger, K. K. Cowen, J. E. Hepp, A. F. 2001. Synthesis and characterization of the first liquid single-source precursors for the deposition of ternary chalcopyrite (CuInS2) thin film materials. Chem. Mater. 13 3827-3829. 

CuInSi (and, even more, CuInSei) are strong candidates for thin-film photovoltaic cells. For this purpose, the chalcopyrite structure (which is an ordered lattice) is preferred over the disordered, zincblende form. Due to the large absorption coefficients of these materials, a 1-iJim-thick film is more than enough to absorb almost all the suprabandgap radiation. Somewhat thicker films are generally used, due to problems of pinholes, which commonly occur in thinner films. A number of methods have been used to deposit these films. Surprisingly, very few (published) attempts have been made to deposit them by CD. 

Fig. 9.1. Schematic cross-section of a chalcopyrite-based thin-film solar cell. Typical materials for the individual parts of the cell are given in square brackets...

This book is devoted to the properties, preparation and applications of zinc oxide (ZnO) as an transparent electrode material. It focuses on ZnO for thin film solar cell applications and hopefully inspires also readers from related fields. The book is structured into three parts to serve both as an overview as well as a data collection for students, engineers and scientists. The first part, Chaps. 1-4, provide an overview of the application and fundamental material properties of ZnO films and their surface and interfaces properties. Chaps. 5-7 review thin film deposition techniques applied for ZnO preparation on lab scale but also for large area production. Finally, Chaps. 8 and 9 are devoted to applications of ZnO in silicon- and chalcopyrite-based thin film solar cells, respectively. One should note that the application of CVD grown ZnO in silicon thin film cells is discussed earlier in Chap. 6. 

Another important strategy is the adaptation of efficient PV semiconductor thin-films and nanostructures for effective use in PEC applications. Recent research in this area has focused on material classes with inherent bandgap tuning capabilities such as the amorphous silicon compounds (including silicon carbides and nitrides) [109, 112-114, 131, 132], and polycrystalline copper chalcopyrite compounds [133-137]. 

Can cost-effective thin film PV materials (e.g., amorphous silicon or more efficient copper chalcopyrites) and multijunction devices be adapted through surface modifications to split water If so, a wealth of PV manufacturing experience would be available for enabling commercial scale deployment... 

Some ternary compounds are also technologically important. The I-lll-Vlj chalcopyrite compounds are included here because the anion is from group VI and the average cation valence is 2. CulnSej is an important candidate material for highly efficient thin film solar cells. 

Another class of heterojunction solar cells are CuInSei-based devices, formed from p-n junctions with CdS thin films. CuInSei is a ternary compound with a band gap of 2.4 eV and is stable as a chalcopyrite or sphalerite structure.Chalcopyrite (lattice constant a = 0.5789 and c = 1.162 A) is stable at room temperature up to 810 °C. The band gap is direct and approximately 1.02 eV at room temperature, with an absorption coefficient above 5 x 10" cm . On substitution of Ga for In or S for Se, the band gap increases up to 1.68 eV. For high efficiency devices, band gaps between 1.20 eV and 1.25 eV are used with [Ga]/[In- -Ga] ratios between 25% and 30%. Different stoichiometries give rise to different intrinsic defects and hence electronic properties, e.g. Cu and In (acceptor) vacancies (excess Se) give rise to p-type character and Se vacancies lead to n-type material (for solar cells a slightly Cu-deficient material is used). 

A champion CIGS-based thin film solar cell was prepared and characterised at NREL by Repins et al A record efficiency of 19.9% was measured for a 0.41 cm device in which, during the latter stages of vacuum deposition (100 A), the chalcopyrite was deposited without gallium, improving the quality of the material near the surface by reducing the number of surface defect states. An improved (5.5 ns) charge carrier lifetime was measured and a maximum carrier density of 2 x 10 cm was measured. 

Siebentritt, S. 2002. Wide gap chalcopyrites material properties and solar cells. Thin Solid Films 403-404 1-8. 

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