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Polymer tandem solar cells

(2007) Hadipour, Afshin

The global demand for energy is expanding continually. Therefore, realization of green power sources are needed since combustion of fossil fuels will have serious consequences for the climate on the Earth. With a photovoltaic device, the solar
light can be converted into electricity which is the most useful forms of energy. For this reason, solar cells have attracted much attention in the last decades as most clean, sustainable and renewable energy sources. In order to produce low-cost
and large-area solar cells, organic materials provides many possibilities. Especially semiconducting polymers combine the favorable opto-electronic properties, such a high absorption coefficients, of organic materials with the excellent processing and mechanical properties of plastic materials. This implies that an organic solar cell can be processed from solution at room temperature onto (flexible) substrate using simple and, therefore, much cheaper methods such as spin (or blade) coating and inkjet printing. However, the formation of a bound-electron-hole pair that needs to be separated, the low mobility of charge carriers (or the mobility difference
between holes and electrons) together with relatively narrow absorption spectra of the organic materials lead to relatively low performance (typically amounts to 4-6%). To improve the absorption of the solar radiation by organic solar cells, materials with a broad absorption band have to be designed or different narrow band absorbers have to be stacked in multiple junctions. When two (or more) donor materials with non-overlapping absorption spectra are used in a tandem (or multi-junction) solar cell, a very broad range (from visible to infrared) of the solar radiation can be absorbed by such a device.