Talking to the conversionof light to electricity, photoelectric effect is the most possible to bementioned, however, the operating principle of solar cells, so called “photovoltaiceffect” is a different concept.
Photoelectric (PE) effect was discovered by AlbertEinstein in 1905, but photovoltaic (PV) effect was demonstrated by EdmondBecquerel much earlier in 1839. PE and PV are quite related and similar. In botheffects, 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 thesemiconducting material in PV effect. Now assuming there are two different typesof semiconductors, one is p-type and the other is n-type.
By joining the twosemiconductors and exposing the p-n junction structure under sunlight, anelectric 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 socalled “solar cell”. Although solarenergy is relatively new-born industry, there have been several generations ofsolar cells, classifying by the semiconducting materials they are made of.
The first-generationmaterial, also most mature and popular used, for solar cell is based oncrystalline silicon, including monocrystalline silicon and polycrystallinesilicon. By the year of 2016, polycrystalline silicon solar cell takes account for70% of the global market, and monocrystalline silicon takes about 24%. 3 Themonocrystalline silicon solar cell has better conversion efficiency by usingvery high grade of silicon, however, the cost is also much higher thanpolycrystalline silicon solar cell. Thesecond-generation cell is thin-film solar cell, which takes around 6% in themarket, due to its lower conversion efficiency compared to traditional solar cells.
But as the name suggests, thin-film solar cell may have wider application becauseof the possibility to make transparent or flexible solar cell. The materials tomake thin-film solar cell includes amorphous silicon, CdTe, and CIGS. Amorphoussilicon solar cell is only used in small-scale application, since its outputpower and the conversion efficiency are very low. CdTe solar cell has very goodpotential due to its high cost-efficiency rate. Also, remarkable progress hasbeen made on the efficiency improvement in recent decade. As reported in 2017, aCdTe thin-film power generating glass production line in China with annualoutput of 100 megawatts has entered the equipment commissioning phase.
4However, 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 PVmaterial, and it shows best efficiency compared to the other two thin-film technologies.A higher efficiency means thinner film is needed, therefore CIGS has morepotential applications, such as on glass or transparent substrates, flexiblesolar cell, curved surfaces, etc. It is also worth noting that there is less toxicmaterial cadmium in CIGS solar cells.