Heterometal Chalcogenides

Ternary chalcopyrite materials such as copper indium dis-elenide (CuInSe2), copper indium disulfide (CuInS2), and copper gallium diselenide (CuGaSe2) are potential solar cell materials, given that their direct bandgaps are nearly optimal for the absorption of the solar spectrum. High-quality thin films of solar-cell materials with high specific power (W kg-1) are required to fabricate efficient devices. Nanocrystalline films are an attractive alternative to single-crystalline devices that are currently used. Since the microstructure and secondary phases (surface layers) may have a large influence on the transport properties of the films [577], optimal control over the purity of films is necessary. Moreover, the current thin-film technologies are limited by the requirement of

Figure 53. (a) Molecular structure of Ba-Ti oxo-alkoxide. (b) Comparison of the core structure of monoclinic barium-titanium oxo-isopropoxide with the BaTiO3 structure viewed along the 111 direction. Reprinted with permission from [576], B. C. Gaskins and J. J. Lannutti, J. Mater. Res. 11, 1953 (1996). © 1996, Materials Research Society.

Figure 53. (a) Molecular structure of Ba-Ti oxo-alkoxide. (b) Comparison of the core structure of monoclinic barium-titanium oxo-isopropoxide with the BaTiO3 structure viewed along the 111 direction. Reprinted with permission from [576], B. C. Gaskins and J. J. Lannutti, J. Mater. Res. 11, 1953 (1996). © 1996, Materials Research Society.

high-temperature deposition processes that are incompatible with all polyimide or other polymer substrate materials. To eliminate or minimize the problems associated with differences between the coefficients of thermal expansion of the substrate and functional film, new approaches to depositing chalcopyrite materials at reduced temperatures are desired. One interesting solution to this problem is the use of ternary SSPs to obtain single-phase materials at low temperatures.

Banger et al. have tested molecular precursors with the general formula [{ER3}2Cu(YR')2In(YR')2] (E = P, As, Sb; Y = S, Se; and R = alkyl, aryl). Besides the appropriate elemental ratio required to form the chalcopyrite phase, another key feature of this class of precursors is the number of tunable sites within the complex that allow engineering of the molecular architecture to direct properties (liquid state, high volatility, thermal stability) that best meet the requirements of the deposition process. An important illustration of the flexibility of the [{ER3}2Cu(YR')2In(YR')2] architecture is the phosphine stabilized complexes [{P(Bu")3}2- Cu(SEt)2In(SEt)2] (A) and [{P(Bu")3}2Cu(SPr")2In(SPr")2] (B) that represent the first liquid precursor for the deposition of CuInS2. The low-temperature differential scanning calorimetry (DSC) profiles of the two compounds (Fig. 54) show an absence of an endotherm assignable to a melting phase transition, thus confirming their liquid state. The exothermic peaks at 250 and 268 °C correspond to the decomposition of the samples.

[{PPh3}2Cu(SEt)2In(SEt)2] was used to deposit well-adherent mirror-like films of CuInS2 at 390 °C [252] (Scheme 27). The films were dense, with a columnar grain growth and grain size of ca. 500 nm. No evidence of phosphorus or carbon contamination was obtained, indicating a clean decomposition of the precursor [250].

[(CjH^CuInSJ Scheme 27.

CuInS2

[(CjH^CuInSJ Scheme 27.

CuInS2

Vittal et al. have synthesized and structurally characterized molecular precursors (Fig. 55) to ternary silver indium sulfide phases, AgInS2 and AgIn5S8. Pyrolysis of [{PPh3}2AgIn(SCOR)4] (R = Me, Ph) compounds results in the formation of AgInS2, whereas aerosol-assisted CVD (AACVD) produced dark red films of AgIn5S8.

Mixed-metal perovskites are well known for their technological significance to electrochemical applications. In view of their unique ferroelectric, pyroelectric, piezoelectric, and dielectric properties, these materials are finding applications in transducers, light modulation, charge storage, and nonvolatile memory devices [46, 559, 560]. The columbite-like phase of MgNb2O6 (MN) has attracted interest because of

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