Solar-Cell Fabrication and Performance

High performance (rj 7%) a-Si H solar cells have been fabricated in three different structures as shown in Figs. 6 and 7. The highest-perform-ance devices ( / = 10.1%) have been fabricated in the structure glass/Sn02/ p-i-n/Ag, in which the p layer is an alloy of a-Si C H (Catalano et al., 1982). The p layer typically contains about 20-30 at. % carbon (Morimoto 

Another structure that has been used to make relatively efficient a-Si H solar cells is shown in Fig. 6b. In this case, the p layer is deposited on a steel substrate, and indium tin oxide (ITO) is electron-beam evaporated onto the flayer. The ITO serves as both a top contact layer and an antireflection coating. The steel substrate may be coated with Cr to improve the back surface reflection. Generally, in all p-i-n cells the top doped layer is thin (— 10 nm) in order to minimize losses due to absorption and recombination in that layer. 

There are other structures that have been used to make a-Si H solar cells such as Schottky-barrier and MIS configurations, but since the conversion efficiencies are generally less than 6%, we will not discuss these structures here (for more information on these structures, see Carlson, 1982b). 

The principal method of characterizing solar-cell performance is the measurement of conversion efficiency while the cell is exposed to 1 sun illumination (—100 mW cm-2). The conversion efficiency is determined by measuring the current - voltage characteristic (see Fig- 8), locating the maximum power point (Pm = JmVm), and also measuring the solar insolation 

The I- V characteristic shown in Fig. 8 is for a cell with the structure glass/Sn02/p-i-n/Ag, where thep layer is alloyed with carbon (Catalano et al., 1982). The conversion efficiency was 10.1% with Vx = 0.84 V, JK = 17.8 mA cm-2, and FF = 0.676. The spectral response (or external quantum efficiency) of the same 10.1% cell is shown in Fig. 9. An integration of the quantum efficiency with the AM 1 solar spectrum gave a current density of 17.6 mA cm-2, in good agreement with the measured value of JK (Catalano et al., 1982). 

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