Talking to the conversion
of light to electricity, photoelectric effect is the most possible to be
mentioned, however, the operating principle of solar cells, so called “photovoltaic
effect” is a different concept. Photoelectric (PE) effect was discovered by Albert
Einstein in 1905, but photovoltaic (PV) effect was demonstrated by Edmond
Becquerel much earlier in 1839. PE and PV are quite related and similar. In both
effects, an electron is excited to higher energy state by light irradiation.
The difference is that, the electron is ejected in PE effect, but remain in the
semiconducting material in PV effect. Now assuming there are two different types
of semiconductors, one is p-type and the other is n-type. By joining the two
semiconductors and exposing the p-n junction structure under sunlight, an
electric field can be formed between the positive p-side and negative n-side.
Therefore, voltage or electric current can be generated by PV effect in this so
called “solar cell”.
energy is relatively new-born industry, there have been several generations of
solar cells, classifying by the semiconducting materials they are made of. The first-generation
material, also most mature and popular used, for solar cell is based on
crystalline silicon, including monocrystalline silicon and polycrystalline
silicon. By the year of 2016, polycrystalline silicon solar cell takes account for
70% of the global market, and monocrystalline silicon takes about 24%. 3 The
monocrystalline silicon solar cell has better conversion efficiency by using
very high grade of silicon, however, the cost is also much higher than
polycrystalline silicon solar cell.
second-generation cell is thin-film solar cell, which takes around 6% in the
market, due to its lower conversion efficiency compared to traditional solar cells.
But as the name suggests, thin-film solar cell may have wider application because
of the possibility to make transparent or flexible solar cell. The materials to
make thin-film solar cell includes amorphous silicon, CdTe, and CIGS. Amorphous
silicon solar cell is only used in small-scale application, since its output
power and the conversion efficiency are very low. CdTe solar cell has very good
potential due to its high cost-efficiency rate. Also, remarkable progress has
been made on the efficiency improvement in recent decade. As reported in 2017, a
CdTe thin-film power generating glass production line in China with annual
output of 100 megawatts has entered the equipment commissioning phase. 4
However, the element tellurium is rare in mineral deposit, and cadmium is toxic;
either of the disadvantages limits massive applications of the CdTe solar cell.
Copper Indium Gallium Selenide (CIGS) is another mainstream thin-film PV
material, and it shows best efficiency compared to the other two thin-film technologies.
A higher efficiency means thinner film is needed, therefore CIGS has more
potential applications, such as on glass or transparent substrates, flexible
solar cell, curved surfaces, etc. It is also worth noting that there is less toxic
material cadmium in CIGS solar cells.