High-efficiency solar cells
by the CADDET Japanese National Team

A solar cell developed in Japan has a 17.3% cell conversion efficiency and 15.2% module efficiency, the world’s highest for cells in practical use. It has a hybrid structure composed of a single crystal wafer surrounded by layers of ultra-thin amorphous silicon.

Introduction

Crystalline silicon cells are currently the most commonly used in PV systems because of their high efficiency and, hence, small installation area. Small-area crystalline cells with conversion efficiencies as high as 24% have been produced in laboratories. However, the efficiency of production crystalline cells remains at 14–16% and manufacturing costs are still high. Improvements in efficiency and reductions in cost are imperative to promote the widespread use of PV power generation.

HIT power 21 solar cell modules

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.

Table 1: HIT solar cell module specification

Model number

HIP-J50B1

HIP-H48B1

Maximum ouput (W)

180

170

Optimum operating voltage (V)

50.7

48.6

Optimum operating current (A)

3.55

3.5

Dimensions of module (mm)

1320x895x35

1320x895x35

Weight (kg)

15

15

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).

Figure 1: Concept of the artifically constructed junction

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.

Performance

Figure 2: Structure of the HIT solar cell compared with a single crystal cell
In tests, a small 1 cm2 HIT solar cell achieved a conversion efficiency of 20%. The production cell of about 100 cm2 has a conversion efficiency of 17.3%. In addition to its high power generation efficiency, the HIT solar cell saves energy and resources used in its manufacture. The low junction formation temperature of 200¡C considerably reduces energy consumption. This low temperature also decreases thermal stress on the substrate, making it possible to reduce the thickness of the cell to 250 microns from that of 350 microns for a conventional single crystal cell.

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.

The articles published in the Newsletter reflect the opinions of the authors.  They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44  1235 432968, Fax: +44 1235 433595.