ICMCTF2008 Session H4: Thin Films for Photovoltaics: Synthesis and Characterization

Several III-VI body-centered tetragonal layered compounds such as β-In₂S₃ have been a subject of interest recently because of their applications as photovoltaic materials. Here we have studied four compounds β-In₂X₃ (X = O, S, Se, Te), having space group I4₁/amd. We calculated lattice constants a and c, bulk moduli B, local electronic density of states (LDOS), total density of states (DOS), and band gap E_g of these phases. Our computations were performed using first-principles total energy calculations within the local density approximation (LDA) to density functional theory (DFT). Core electrons were implicitly treated by ultra soft Vanderbilt type pseudopotentials.

The lattice constants in Å of the β-In₂X₃ (X = O, S, Se, Te) were found to be a = 6.32, 7.50, 7.95, and 8.71, and c = 27.20, 32.20, 33.17, and 34.28 respectively. The 10 internal structural parameters for each of these complex structures were also calculated. Experimental values for lattice constants, which are available only for β-In₂S₃, match our computed lattice structural parameters within an error of 2%. Our internal parameters for β-In2S3 also match with experiment closely. Values of B in GPa are predicted to be 120.60, 62.14, 46.72, and 32.87 for (X = O, S, Se, Te) respectively. Going down group VI from O to Te values of a, c, 1/B, and a/c increased. The peak in LDOS, of the In p electronic states that hybridize with X p (X = O, S, Se, Te) states, increases monotonically as we go down group VI from O to Te.

The historical dominance of the photovoltaics (PV) market by crystalline silicon and cast multicrystalline Si is now being challenged by thin-film PV. In the past two years, thin-film solar modules based on cadmium telluride, amorphous silicon, and copper indium gallium diselenide have gained a strong foothold in the market. Furthermore, there are now good data indicating that thin films have a decided manufacturing cost advantage over wafer silicon. This presentation will cover some of the underlying reasons for the historically slow penetration of thin-film PV into the market and how several factors have changed. Discussion will touch on the substantial number of materials issues that have been overcome as well as the considerable range of materials challenges and opportunities that remain for photovoltaics based on thin-film coatings on inexpensive substrates.

Work supported in part by: NREL, AFRL-Kirtland, NSF, and the State of Ohio

Vertically aligned Si/SiO₂ nanowhiskers were synthesized over a large area by an ion-enhanced Vapor-Liquid-Solid (VLS) method at low temperatures. The whisker arrays exhibit enhanced light trapping and are thus promising candidates for solar energy conversion applications. Growth of nanowhiskers is initiated by a self-organized molten indium seed layer predeposited on a Si or SiO₂ surface. In contrast to conventional VLS growth that uses a Si-containing gaseous precursor, atomic Si was in this case supplied to the metal seed layer by magnetron sputtering. Concurrent vigorous ion bombardment aligned the whisker vertically and enabled a state of supersaturation within the seeds due to enhanced mixing of the two insoluble phases, indium and silicon.

Whisker growth occurred normal to the substrate surface at rates up to 200 nm/min in an ambient containing argon, hydrogen, and traces of water vapor at about 190⁰C. Whisker diameters were on the order of 10-100 nm and the whisker aerial density was found to be between 10⁹ and 10¹⁰ cm^-2. Ion-enhanced VLS growth offers a promising route for array-growth of high aspect-ratio semiconducting nanostructures. Careful control of the ion bombardment however is essential due to physical sputtering of the elevated metal seed. One seed material of choice is In due to its low eutectic temperature with low Si solubility and its common use as p-type dopant for Si. It is expected that the growth process also applies to other potential seed materials. The use of an atomic Si source offers furthermore technological advantages due to replacement of the gaseous precursors. Concurrent and subsequent ion-assisted Si deposition has been shown to allow a great deal of choice over the size, shape, and coverage of the final nanostructures. Technological implementation of shape-controlled nanostructures for point-contact photovoltaic devices is currently under investigation and will be discussed.