Self-textured transparent conductive oxide film improves efficiency of solar cells

Satisfying the universal goal of improving solar-energy conversion efficiency will require modulating the path taken by light on the surface of solar cells. Greater surface roughness is one way to make light scattering more efficient.1 A rough surface both increases the length of a photon’s light path and also enhances absorption of light by reducing reflection. Typical surface-texturing methods include wet etching the front electrode using a chemical solution.2 However, repeated wet etching increases processing costs and thickens the transparent conductive oxide film. Chemical-vapor deposition represents an alternative method of creating a rough surface. This so-called self-texturing technique also does away with the need for additional chemical-texturing steps. Figure 1 shows schematically the difference in Light scatting between nontextured and textured surfaces. According to the commonly known chemical-vapor-deposition growth model,3 formation of grains (similarly oriented crystalline clusters) depends strongly on process conditions and nucleation densities. For example, if nucleation density decreases, grain size tends to increase and vice versa. Nucleation density is determined by surface mobility and the surface reaction rate of chemical precursors such as diethylzinc. These characteristics, in turn, are controlled by chemical-vapor-deposition process parameters such as substrate temperature, pressure, and impurities. Enhancing surface roughness requires formation of large grains, which is achieved by optimizing process conditions. The resistivity of the film is particularly important in application of transparent conductive oxide in photovoltaics. Low resistivity promotes energy-conversion efficiency owing to improved photocurrent and can be achieved by injecting Figure 1. Schematic diagram showing light absorption as a function of surface morphology of (a) nontextured and (b) textured surfaces.