engleski

Materials for third generation solar cells

Materials for third generation solar cells Davor Gracin Ruđer Bošković Institute, Bijenička 54, Zagreb, Croatia Third-generation photovoltaic cells (as identified by Martina Green) are solar cells that are expected to overcome the theoretical limit of 31–41% power efficiency for single band-gap solar cells (Shockley–Queisser limit). This assumes a range of alternatives to cells made of semiconducting p-n or p-i-n junctions (Si-"first generation") and thin film cells (number of binary or ternary inorganic semiconductors - "second generation"). Common third-generation systems include several routes as non-semiconductor technologies (including polymers), quantum dots, tandem/multi-junction cells, intermediate band solar cell, hot-carrier cells, frequency conversion, (i.e. changing the frequencies of light from those for which conversion efficiency is low or zero to the frequencies for which the cell's efficiency is higher), hot-carrier effects and other multiple- carrier ejection techniques and solar thermal technologies. The most part of talk will be devoted to multy-layer structures and some organic and perovskite solar cells with emphasis on examples that are promising for commercial application. The majority limit of a cell's theoretical efficiency is due to the difference in energy between the band gap and absorbed photon. Any photon with more energy than the band gap can cause photo excitation, but all energy above the band gap energy is lost due to thermalisation. On the other hand, for photons lower than band gap, the material is transparent. So, it is possible to improve efficiency of a single- junction cell by stacking thin layers of material with varying band gaps on top of each other, starting with lowest gap – the "tandem cell" or "multi-junction" approach. Numerical analysis shows that the "perfect" single-layer solar cell should have a band gap of 1.13 eV with maximum theoretical power conversion efficiency of 33.7% because the solar power in the infrared is lost, and the extra energy of the higher energies is also lost. For a two layer cell, one layer should be tuned to 1.64 eV and the other at 0.94 eV, with a theoretical performance of 44%. A three-layer cell should be tuned to 1.83, 1.16 and 0.71 eV, with an efficiency of 48%. A theoretical "infinity- layer" cell would have a theoretical efficiency of 68.2% for diffuse light. For multi-layer ("tandem") cells the key property of material is possibility of adjusting the band gap. In the moment, most tandem-cell structures are based on higher performance semiconductors, as gallium arsenide (GaAs). Three-layer GaAs cells achieved 41.6% efficiency for experimental examples and four layer cell reached 44.7 % efficiency. The actual promising combinations of materials for large production are amorphous/crystalline silicon, perovskite/crystalline silicon and some combination of organic materials and perovskites. As alternative materials for single solar cells the most popular are some organic compounds and particularly metal halide perovskites largely due to a combination of their high-quality optoelectronic properties that are unexpected for materials processed at low temperatures and from solution. Some advantages and disadvantages of these materials in comparison with crystalline Si, which dominates on the market, will be discussed. The work has been supported in part by Croatian Science Foundation under the project IP-2018-01- 5246 and by the European Union funds under the project "Improvement of solar cells and modules through research and development"

solar cells ; efficiency

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