Review Of High Efficiency Silicon Solar Cells

High efficiency is the key to large scale applicability of photovoltaic systems. Detailed cost analysis done by Electric Power Research Institute (EPRI) indicates that greater than 15% efficient modules will be required at a cost of less than 50c/watt in order for PV to compete with conventional energy sources. There are various approaches being investigated for a commercially viable PV product including low cost, low efficiency thin film approach using amorphous silicon and CnInSe2, and high cost, high efficiency approach using GaAs material and sophisticated tandem and concentrator cell designs. Silicon solar cells, which are the focus of this paper, provide an intermediate path for achieving the cost and efficiency goals. It has been more than thirty years since silicon solar cells were first discovered, but the drive for high efficiency has only recently become intense due to the balance of system (BOS) cost. Figure 1 shows a road map of silicon cell efficiency improvement since 1950, when the cell efficiency was only 5%. During the period 1960-1975, only a small improvement in cell efficiency was realized because major emphasis was on radiation hardness and space applications. Drive for terrestrial applications started in 1975 with the initial emphasis on low cost, therefore, only a moderate improvement in cell efficiency was observed during 1975-1980. In the early 1980s, when the importance of balance of system cost was recognized, the emphasis shifted to high efficiency and since then the progress in silicon cell efficiency has been phenomenal (Figure 1). Five years ago, 20% efficient cells seemed unattainable, but today we have already seen 22% (AM1) efficient silicon cells. Major advances in cell efficiency in the 1980s came from the emphasis on improving V c and optical design of I silicon cells. Improved material quality also contributed a ot to the current rapid progress. The purpose of this paper is to review and summarize some of those approaches that resulted in AM1 silicon cell efficiencies in the range of 18-22%.