To address these problems, SANYO Electric Co Ltd has developed the HIT (Heterojunction with Intrinsic Thin-layer) structured solar cell – marketed under the trade name of HIT Power 21. This is a novel hybrid-type solar cell, constructed using thin layers of amorphous silicon (a-Si) for junction formation on a single crystal silicon (c-Si) wafer. This structure makes it possible to form junctions under lower temperatures and to simplify production of the cells.
HIT solar cell structure
Junction formation for conventional c-Si cells is carried out using thermal diffusion processes. These make dopant sources diffuse from the substrate’s surfaces under high temperatures of around 1,000¡C. The process forms junctions on both sides of the substrate, so additional processes are needed to remove a junction from one of the surfaces.
However, junctions for the HIT cell are more simply formed by depositing thin a-Si layers on the substrate through plasma chemical vapour deposition (CVD) processes at low temperatures of 200¡C or less. This is an artificial “heterojunction” composed of a-Si and c-Si. Its characteristic is that a thin i-type a-Si layer which has no dopant is inserted between the junction formed with a-Si and c-Si. These layers each have a different conductivity. This method of junction construction causes no thermal damage to a cell, forms steeper junctions and prevents a p-type dopant and an n-type one from mixing with each other (see Figure 1).
To make a conventional solar cell with the back surface field (BSF) effect, a highly doped layer is formed in the area of the substrate bordering on the rear-side electrode. The highly doped layer should be the same type as the substrate (ie, in the case of the n-type substrate, a n+ layer). This highly doped region prevents carriers generated by light in the vicinity of the rear side from recombining on the electrode interface. To achieve this BSF effect for the HIT solar cell, thin layers of i-type a-Si and n-type thin a-Si (ie the same type as the substrate) are deposited on the n-type c-Si substrate. Figure 2 shows the structure of the HIT solar cell compared with that of SANYO’s single crystal silicon cell produced by conventional thermal diffusion.
Owing to the 17.3% conversion efficiency of the production HIT solar cell, a power generation efficiency of 15.2% is achieved, the world’s highest for production modules in practical use. Table 1 shows the specifications of the HIT solar cell modules.
The HIT solar cell has other excellent features. Firstly, degradation of performance by light irradiation, which is characteristic of a-Si layers, does not occur. The efficiency of the cell showed excellent stability in light-soaking tests for five hours under the irradiation condition of air mass (AM) 1.5, 5,000 W/m2 at 50oC. Owing to the very thin i-type a-Si layers, the properties of a-Si layers do not affect those of the HIT cells to the same degree as in amorphous solar cells.
The most important feature of the HIT solar cell is the smaller decrease in power generation efficiency. On a hot summer’s day in Japan, rooftop solar cells can reach 70–80oC. At 75oC, conventional c-Si cells manufactured by thermal diffusion may lose more than 20% of their power output at the standard temperature of 25oC, with efficiency decreasing by 0.45% per 1¡C rise in temperature. The rate of efficiency decrease in the HIT solar cell is 0.33% per 1oC rise in temperature.
In a test conducted on a summer’s day, the HIT system yielded 10% more electricity at 75¡C at midday than SANYO’s c-Si system with the same output at the standard temperature. The total enhanced daytime output of the HIT system is 8.8% greater than SANYO’s c-Si system of the same capacity. Consequently, this feature enables the HIT solar cell to generate electricity equivalent to that generated by conventional crystalline solar cells with an efficiency as high as 18%.
For more information contact the CADDET Japanese National Team in Tokyo.
The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.
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