Short Survey of Chalcogenide Based Thin-Film Solar Cells
With increasing environmental concerns and greenhouse effects, almost every country is looking for ways to mitigate these concerns. Solar energy reaching the Earth’s surface provides an energy supply potential surpassing the power consumption of our civilization by three orders of magnitude. Still, this immense source of energy is unused because of certain very specific problems such as high cost, low throughput, available only at morning, etc. There are three major developments that need to be done in a reasonably short timeframe to achieve the terawatt-scale power generation capacities using solar cells. These three major developments include high and durable power conversion efficiency, sufficient and low-cost material availability and low-cost processing.
Thin-film chalcogenide materials, in particular, CuInGa(S, Se)2 (CIGS) and CdTe, have attracted considerable attention in past few years and are already commercialized in the market for large-scale PV manufacturing. Chalcogenide-based materials offer stable and efficient (above 10%) photovoltaic modules fabricated by scalable thin-film technologies. Still, these materials hold just 7% of solar-cell market share that is led by Si-based solar cells with almost 90% of market share. Indium and Tellurium are the rare materials that certainly act as a limiting factor in the mass production of these kinds of solar cell but Indium-free kesterite-related materials such as Cu2ZnSnS4 and Cu2ZnSn(S, Se)4 (collectively referred to as CZTSSe) are under R&D. In kesterite materials, indium is replaced with earth-abundant zinc and tin metals and sulphur is substituted at for sulphur to tune the band gap from 1.0 to 1.5 eV.
How a thin film solar cell work?
As the name suggest, unlike Si-solar cells in a thin-film solar cell the absorber material is deposited over a relatively cheap substrate such as glass, plastic or metal foil that significantly reduces the cost. These kind of thin-film structures have many advantages over Si-based solar cells. Some of the materials which are generally deposited over the substrate for solar cells fabrication include Cadmium Telluride (CdTe), Cadmium Sulphide (CdS), Copper Indium Gallium Selenide (CIGS), Amorphous Si, Tandem cells based on Si, Polycrystalline Si and GaAs. In all of these materials, CIGS and CdTe based solar cells are most popular in thin-film solar cells market. The basic concept of working is similar to Si-based solar cells except direct band materials are used as a base for fabrication of these kinds of solar cell. Once the thin film is deposited a heterojunction or a p-n junction is created by deposition of suitable material which helps in the extraction of photo generated electrons and holes. These electrons and holes are then drifted or diffused toward corresponding metal contacts so that the current can be generated. In these kinds of devises, the I-V curve starts in the fourth quadrant and ends in the first quadrant, unlike normal transistors where I-V is restricted to the first quadrant. As can be understood, the power would be negative for the fourth quadrant so we can harness the energy and store it for further use.
What makes Chalcogenide better than Si-based solar cells?
Si-based solar cells are most famous among people, therefore, let’s compare Si solar cells Vs. Chalcogenide solar cells.
- Si-based solar cell modules are fabricated on wafers that are almost 35-microns thick whereas thin-film solar cells use only a few microns of material to absorb complete visible range sunlight.
- The band gap engineering in Si-based solar cells cannot be attained easily whereas band gap in chalcogenide based solar cells can be engineered just by changing concentration of Sulphur and Selenium in the CIGS materials.
- In general, heavy and costly machinery are used for making of Si-based solar cells whereas chalcogenide based solar cells can be fabricated with ultrahigh-throughput coating techniques such as liquid-based printing or casting.
- Both CdTe and CIGSSe technologies yield efficiencies greater than 15% (16.7% for CdTe and 20.1% for CIGSSe that is not far behind of crystalline Si technology.
Overall Chalcogenide based solar cells seem to a better option than Si-based solar cells but there are other factors such as low efficiency, durability, use of rare earth materials, lack of R&D, etc. which needs to be solved before this material can compete with Si-based solar cells market.
Current status of R&D
The quest to develop thin-film solution processing approaches that offer low-cost and preferably low-temperature large scale deposition, while simultaneously providing quality semiconductor characteristics, has become an important area of research among solar cells material scientists. While inorganic compounds offer the potential for outstanding electronic properties when compared to organic systems, the very nature of these materials rendering them good electronic materials—namely strong covalent bonding—also leads to poor solubility. Poor solubility makes the band gap engineering tough. So, material scientists are looking for ways in which the solubility of metals in these chalcogenide compounds can be increased. In addition to it, there is the lot of research going on to find the replacement of rare earth materials such as In, Ga and Te, which are used for making efficient chalcogenide based solar cells.
There is no doubt that chalcogenide materials have shown excellent overall performance, both in device and commercial viability, and in future can be a good replacement for Si-based solar cells. The present efficiency record already shows promise for practical commercial applications, though there is still room for further improvement by changing the solution processing chemistry and using different technologies such as heterojunction, tandem, light trapping etc.
Mitzi, D. B., Gunawan, O., Todorov, T. K., Wang, K., & Guha, S. (2011). The path towards a high-performance solution-processed kesterite solar cell. Solar Energy Materials and Solar Cells, 95(6), 1421–1436. doi:10.1016/j.solmat.2010.11.028
Todorov, T., Gunawan, O., Chey, S. J., de Monsabert, T. G., Prabhakar, A., & Mitzi, D. B. (2011). Progress towards marketable earth-abundant chalcogenide solar cells. Thin Solid Films, 519(21), 7378–7381. doi:10.1016/j.tsf.2010.12.225