Solar materials interface studies

An overview of the most important phenomena in interface science related to studying solar materials is presented in this section. The methods for characterizing interfaces and those deemed likely to have the largest near-term impact on solving the problems of interface degradation are then mentioned. 

Table 1 of a paper by Murr (2) lists problems and/or concerns related to specific interface materials and specific components of SECS. In Table 2 of the same work, he related topical study areas and/or research problems to S/S, S/L, S/G, L/L, and L/G interfaces. It is also useful to divide interface science into specific topical areas of study and consider how these will apply to interfaces in solar materials. These study areas are thin films grain, phase, and interfacial boundaries oxidation and corrosion adhesion semiconductors surface processes, chemisorption, and catalysis abrasion and erosion photon-assisted surface reactions and photoelectrochemistry and interface characterization methods. The actual or potential solar applications, research issues and/or concerns, and needs and opportunities are presented in the proceedings of a recent Workshop (4) and summarized in a recent review (3). 

The three phases of interest are the solid (S), liquid (L), and gas (G) phases, none of which is infinite. The boundary region between the S, L, and G phases has fundamentally different properties from the bulk. The S/S, S/G, and S/L surfaces, in that order, are of greatest interest to the solar materials scientist (4). Some of the broad topical areas of study at the interfaces of SECS are listed in Table 2. An understanding of these topics is enhanced by applying the methodologies of interface science. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

The Se-capped Cu(In,Ga)Se2 films used for the present studies were prepared at the Zentrum fur Sonnenenergie und Wasserstoffforschung in Stuttgart, Germany with 30% of the In substituted by Ga. The films are also used for solar cell preparation and yield an energy conversion efficiency of-14% [36,123]. Good conversion efficiencies are obtained from films, which are prepared with a slight Cu deficiency (—22% Cu instead of the nominal 25 % of Cu in stoichiometric chalcopyrites) [124]. Surfaces of such materials are, however, considerably depleted of Cu and show a surface composition that corresponds to the Cu(In,Ga Ses vacancy compound with a typical Cu concentration of 11 — 13% [36,123,125]. The importance of this compound for the Cu(In,Ca)Se2 surfaces and interfaces has been pointed out first by Schmid et al. [126,127]. 

Critical properties of TCO coatings are electrical resistance and transparency [3], but for solar cell applications very often texture and large haze factors, i.e., ratio of diffuse to total transmission, have similar importance. Large haze factors have been shown to influence positively the efficiency of silicon solar cells, because the reflection at the TCO-silicon interface is reduced and the scattering increases the pathway of light inside the active material. The preparation and characteristics of several TCO materials have been reviewed by Chopra et al. [92] and Dawar and Joshi [93]. The optical and electrical properties of ITO and aluminum doped zinc oxide have been studied in detail by Granqvist and coworkers [94, 95], but these films were prepared by sputtering and not by CVD. Very recently they also published an overview of transparent conductive electrodes for electrochromic devices [7]. 

The toxicology of the Cd-based materials increases as CdS < Si < CdTe < Cd < CdO. That is, Cd compounds such as CdS and CdTe are less toxic than Cd, while CdO is more toxic than Cd. Accordingly more systematic studies are needed on the toxicology of the materials described in this chapter for thin-film solar cell appUcations because the increase of their use in PV systems wiU involve extensive human interfaces. 

---