William Hullinger Huber

The renaissance of CdTe-based photovoltaics

Over the past several years, CdTe-based photovoltaics have undergone a revolution. CdTe is a mature PV technology, spanning over 60 years. For a decade, CdTe record cell efficiencies remained dormant at 16.7%, far behind multi-crystalline Silicon (mc-Si) as well as the competing thin film PV technology, CuInxGa1-xSe2 (CIGS). Recently, the CdTe field reawakened as multiple fundamental breakthroughs pushed the record cell efficiencies from 16.7% in 2011 to 21.0%, announced recently1. CdTe now stands as the most efficient, polycrystalline, thin-film PV system with ample room for further optimization.

Diameter distribution of thermally evaporated indium metal islands on silicon substrates

Although many groups have studied the initial growth stages of various metals, including indium, there is little information in literature on diameter distributions of indium in relation to film thickness or annealing conditions. This paper reports island size distributions of thermally evaporated In islands on Si (100) and Si (111) substrates for nominal film thicknesses ranging from 5 to 50 nm. Because indium has a low melting temperature, and therefore a high homologous temperature at room temperature, 3-dimensional islands form during deposition with no subsequent heat treatments needed. Island diameters were calculated using commercial image analysis software in conjunction with SEM images of the samples. It is found that there is a bimodal island diameter distribution for nominal indium thicknesses greater than 5 nm. While the diameters of the larger islands increase exponentially with nominal thickness, those of the smaller islands increase linearly, and therefore more slowly, with nominal thickness. For nominal thickness of 50 nm, the average diameters of the small and large islands differ by almost an order of magnitude. Anneal conditions were studied in an attempt to narrow diameter distributions. Samples of each nominal thickness were annealed at temperatures ranging from 360°C to 550°C and the diameters again measured. The range of island diameters become narrower with 360°C anneal and volume average island diameter increases by ~30-50%. This narrowing of the distribution occurs due to smaller islands being absorbed by the larger in a process akin to Ostwald ripening, which is facilitated by higher surface diffusivities at higher homologous temperatures.