ALLPCB
Introduction
To improve understanding of solar cells, this article introduces three types: 1. compound thin-film solar cells, 2. polymer multilayer modified-electrode solar cells, and 3. nanocrystalline dye-sensitized solar cells. Continue reading for technical details.
1. Compound Thin-Film Solar Cells
As alternatives to monocrystalline silicon cells, researchers developed polycrystalline silicon and amorphous silicon thin-film cells and continued to explore other materials. Major candidates include GaAs and other III-V compound semiconductors, cadmium sulfide, cadmium telluride, and copper indium selenide thin-film cells. Although cadmium telluride and cadmium-based polycrystalline thin-film cells can have higher efficiencies than amorphous silicon and lower cost than monocrystalline silicon while being amenable to large-scale production, cadmium is highly toxic and causes serious environmental contamination, so these are not ideal replacements for crystalline silicon cells.
III-V compounds such as GaAs and copper indium selenide thin-film cells have attracted attention because of their relatively high conversion efficiencies. GaAs is a III-V compound semiconductor with a bandgap of about 1.4 eV, which matches well the high-absorption region of sunlight, making it an attractive cell material. Preparation of III-V thin-film cells like GaAs typically uses MOVPE and LPE techniques; the MOVPE method for GaAs is influenced by many parameters including substrate dislocations, reaction pressure, III-V ratio, and total flow rate. Other III-V materials such as GaSb and GaInP have also been developed. In 1998, the Solar Energy Systems Research Institute in Freiburg reported a GaAs solar cell with 24.2% conversion efficiency, a European record. The first GaInP cells achieved 14.7% efficiency. Using stacked structures, the institute fabricated a GaAs/GaSb tandem cell, stacking two independent cells with GaAs as the top cell and GaSb as the bottom cell, reaching an overall efficiency of 31.1%.
Copper indium selenide, CuInSe2, abbreviated CIS, has a bandgap of about 1.1 eV, suitable for solar photoelectric conversion. CIS thin-film solar cells do not suffer from light-induced degradation, so CIS has drawn interest as a high-conversion-efficiency thin-film material.
CIS films are mainly prepared by vacuum evaporation and selenization. Vacuum evaporation uses separate sources to evaporate copper, indium, and selenium, while selenization uses H2Se to selenize stacked films, though selenization can make it difficult to obtain uniform CIS composition. CIS thin-film cell efficiency has developed from about 8% in the 1980s to roughly 15% today. Panasonic Electric Industrial Co. developed gallium-doped CIS cells with 15.3% photoelectric conversion efficiency measured on a 1 cm2 area. In 1995, a U.S. renewable energy laboratory reported a CIS cell with 17.1% conversion efficiency, the highest reported for this cell type. CIS offers low cost, good performance, and simple processing, making it an important direction for solar cell development. The main limitation is material availability, since indium and selenium are relatively scarce elements, which will constrain large-scale development.
2. Polymer Multilayer Modified-Electrode Solar Cells
Replacing inorganic materials with polymers in solar cells is an emerging research direction. The idea is to use polymers with different redox potentials to form multilayer composites on conductive electrode surfaces, creating a unidirectional charge-transport device analogous to an inorganic p-n junction. One electrode is modified with an inner polymer layer of lower reduction potential and an outer polymer layer of higher reduction potential, so electron transfer is constrained to proceed from the inner layer toward the outer layer. The other electrode is modified in the opposite order. When the two modified electrodes are placed in an electrolyte containing a photosensitizer, the photosensitizer absorbs light and transfers electrons to the electrode with the lower reduction potential. Electrons accumulated on that lower-potential electrode cannot pass to the outer polymer layer and must return to the electrolyte via the other, higher-potential electrode through the external circuit, producing a photocurrent in the external circuit.
Organic materials offer flexibility, ease of fabrication, abundant sources, and low cost, which are significant advantages for large-scale, low-cost solar power. However, polymer-based solar cells are at an early stage; neither lifetime nor efficiency currently matches inorganic materials, especially silicon. Whether they can become practical products requires further research.
3. Nanocrystalline Dye-Sensitized Solar Cells
Silicon-based solar cells are the most mature, but their high cost limits widespread adoption. Researchers have therefore explored new processes, materials, and thin-film approaches. Among recent developments, nanocrystalline TiO2 dye-sensitized solar cells have attracted attention from scientists in China and abroad since Professor Gr?tzel successfully demonstrated them. Some institutions in China are also conducting research in this area.
Nanocrystalline dye-sensitized solar cells, often called DSCs, consist of a narrow-bandgap semiconductor sensitized with organic transition-metal dyes (for example ruthenium or osmium complexes) assembled on a wide-bandgap semiconductor electrode made of nanocrystalline porous TiO2, together with a suitable redox electrolyte. Working principle: dye molecules absorb sunlight and are promoted to an excited state; the excited dye rapidly injects an electron into the conduction band of adjacent TiO2. The dye, having lost an electron, is quickly regenerated by the redox electrolyte. Electrons injected into the TiO2 conduction band travel to the conductive coating, pass through the external circuit, and return to the electrolyte, producing photocurrent.
Nanocrystalline TiO2 dye-sensitized cells are notable for low cost, simple processing, and stable performance. Their photoelectric efficiency is stable above 10%, production cost can be only one-fifth to one-tenth that of silicon cells, and lifetimes can reach more than 20 years.
