CIGS Solar cell:
CIGS solar cell (or photovoltaic) is basically a p-n diode. Typically, the Cu(In,Ga)Se2 and CdS are p- and n-types, respectively. This structure can convert sunlight directly into electricity. When the junction is illuminated, the material absorbs the incoming photons.
The absorbed photons are converted into electron-hole pairs which are separated by the internal electric field in the p-n diode where holes drift to one electrode (Mo) and electrons move to the other one (ZnO). The electricity produced by a solar cell is direct current and can be converted to alternating current by using an inverter for feed-in tariff, or stored in the battery for later use.
Advantages of CIGS solar cell :
Superior Electricity Output
The spectrum of light spans from the ultraviolet, visible to the near-infrared zone. CIGS, a direct band gap material, can absorb sunlight efficiently from 400nm to 1200nm; thus it can generate electricity even in low-light conditions. This gives CIGS the ability to generate electricity throughout the day from sunrise to sunset, and under various weather conditions such as cloudy and snowy days.
Despite its lower module efficiency compared to the crystalline Si counterpart, the total power generated from CIGS solar systems is actually higher. These findings have been confirmed in numerous studies conducted in different regions throughout the world.
An independent study conducted by the University of Stuttgart in 2006, which compares the performance of 12 different solar technologies, CIGS had outperformed other contestants by as much as 13%. The solar technologies selected in this study includes mono-Si, Multi-Si, Sunpower’s back-contact cell, Sanyo’s HIT, amorphous-tandem, CdTe, and CIGS.
Our recent installation in Hsinchu, Taiwan also observed more energy generation from CIGS system. Although the total recording time in our study is relatively short, we have observed 5-10% difference depending on the weather conditions. Based on these results, it is evident that CIGS systems offer superior electricity output than Multi-Si systems, especially in high temperature regions.
Energy efficient production
Compared to crystalline silicon technology, although CIGS technology requires higher capital investment and faces greater technical challenge to achieve mass production, the production process of CIGS is relatively short. A module can be produced through the processing steps of sputtering, selenization, annealing, and lamination. On the other hand, crystalline silicon technology, which has a very long value chain – from conversion of silica to silicon, purification to polysilicon, crystal growth, wafer dicing, cell production, to module lamination. The value chain is distributed among several companies.
Energy Saving and CO2 Reduction
The making of Multi-Si solar cell not only requires an enormous amount of capital investment, but also is a very energy intensive process, especially in the making of polysilicon material: more than 100 kWh of electricity is consumed in the making of 1 kg . For module, every 1,000 Wp of solar module that is produced, Multi-Si consumes 1,160 kWh, whereas CIGS consumes merely 300 kWh of electricity.
Given the currently worldwide production capacity of 30GW, 16 million tons of CO2 emission could be saved if we chose to produce CIGS instead of Multi-Si.
Great Potential for Efficiency Improvement
Although the typical efficiency for a CIGS module is 11~12%, a CIGS solar cell has already demonstrated greater than 20% efficiency. This means that CIGS has not yet reached its full potential in terms of the module efficiency.