Solar cells light trapping

Gordon, R. G, Proscia, J., Ellis, F., and Delahoy, A., Texture Tin Oxide Films Produced by Atmospheric Pressure Chemical Vapor Deposition from Tetramethyltin and Their Usefulness in Producing Light Trapping in Thin Film Amorphous Silicon Solar Cells, >/or Energy Materials, (18) 263-281 (1989)... 

Possibility for preparation of tailored surfaces with suitable light scattering properties for light trapping, which is particularly important for Si thin film solar cells... 

The effect of light trapping on solar cell performance is demonstrated by ZnO Al films with different surface textures. Figure 8.8 shows scanning electron microscope (SEM) images of these ZnO Al films. Sputtered ZnO Al is initially smooth (Fig. 8.8a). By a simple wet-chemical etching step in diluted hydrochloric acid (typically 0.5% HC1) one can roughen the ZnO Al surface which is shown after a short dip (Fig. 8.8b) and after an optimized etch... 

Improved Carrier Extraction by Intercalating Membranes. With light trapping, the condition for good extraction of electrons and holes requiring Le,h l, is also relaxed, since a smaller thickness of the solar cell is possible. For low mobility organic materials, this condition is still a problem. It ensures that electrons and holes generated in the absorber reach the membrane within their recombination lifetime. They can then pass into the external circuit. The distance of the membranes, however, is not limited by the thickness l of the absorber, as can be seen in Fig. 4.11, and can be made arbitrarily short. 

As a result of light trapping and intercalation, thin film solar cells can be made of a thickness /, which apparently violate the condition Le,h l 1 /a. Even with light trapping, the thickness l must be of the order of the penetration depth /a of the light. The diffusion length, on the other hand, can be arbitrarily small, if caused by a small diffusion coefficient. The recombination lifetime should always be as large as possible and should approach the radiative lifetime. 

Staebler and Wronski (1977) were the first to observe light-induced changes in the properties of a-Si H. The Staebler-Wronski effect is now known to affect the performance of a-Si H solar cells through the creation of recombination centers and charged traps (Carlson et al., 1983b). These light-induced centers are metastable and can be annealed out at temperatures of 150 - 200,C. 

Deckman et al. (1983) have shown that the long-wavelength response of a-Si H solar cells can be increased by using textured surfaces to promote a light-trapping effect. They demonstrated that texturing can enhance the short-circuit current density of p-i-n cells by —25%. 

Fig. 1 Buried contact solar cell structure, an important wafer-based commercial silicon cell technology. Features include surface texturing for light trapping, diffusions front and rear and the front current grid buried in laser grooves. (Courtesy of UNSW Centre for Photovoltaic Engineering Image Library.)...

Figure 6.21 Illustration of light trapping in a solar cell with an embedded porous layer sequence, light enters nearly vertically and is scattered at the layer interfaces. When reflective back contacts are assumed, only a small fraction of the scattered light wthin the cone 5 from the normal can escape from the film. With an embedded absorber, light absorption in the region of small absorption coefficients is significantly enhanced.

Abstract In solar applications microstructured polymer surfaces can be used as optically functional devices. Examples are antireflective surfaces, dayUghting, sun protection systems, concentrator photovoltaic modules and light trapping structures in organic solar cells. The examples and the principles of function of the respective microstmctures are described in detail. The suitability of different manufacturing methods is discussed. Two of them, ultraprecision machining and interference lithography are described. For the latter experimental results are shown. Finally, the opportunities and the risks of the shown approaches are discussed. 

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