Photovoltaic Materials and Processing

Many materials with photovoltaic capability are known but only the following have been developed for commercial applications. 

Single-Crystal Silicon. Silicon is still the dominant material in photovoltaic. It has good efficiency, which is 25% in theory and 15% in actual practice. Silicon photovoltaic devices are made from wafers sliced from single crystal silicon ingots, produced in part by CVD (see Ch. 8, Sec. 5.1). However, silicon wafers are still costly, their size is limited, and they cannot be sliced to thicknesses less than 150 im. One crystalline silicon wafer yields only one solar cell, which has an output of only one watt. This means that such cells will always be expensive and can only be used where their high efficiency is essential and cost is not a major factor such as in a spacecraft applications. 

Amorphous Silicon (a-Si). Amorphous silicon is considered a promising new material.As mentioned above, only a very thin coating is necessary, since the amorphous structure is much better at absorbing sunlight than is the crystalline material. The most common process to produce a-Si is the decomposition of silane by plasma CVD (see Ch. 8). Thicknesses of a few micrometers can be deposited and, 

A thin film of tin oxide with a rough texture, produced by MOCVD from tetramethyl tin, (CH3)4Sn, deposited on an amorphous silicon cell provides a light-trapping surface, which enhances the efficiency of the device. 

Deposition has been carried out on architectural glass yielding single-junction amorphous silicon with an efficiency of 13% in the laboratory, but with lower efficiency in production devices. An atmospheric-pressure deposition system in shown in Fig. 15.5. The gas injection device is shown in Fig. 15.6. 

---