25 ORGANIC NANOMATERIALS FOR EFFICIENT BULK HETEROJUNCTION SOLAR CELLS

Solar energy has primary importance among all renewable energy resources. It is a remarkable fact that sun gives to the earth surface in one hour so much energy that it would be enough to cover the annual need of the present population of our planet. Photovoltaic devices allow for the most simple and efficient conversion of the solar energy to electricity. The power conversion efficiencies of the best laboratory devices based on silicon reach 24.4% [1]. The first generation of solar cells based on crystalline silicon is known since the middle of the last century (1954 by AT&T Bell Labs). However, for a long time, wide-scale distribution of such devices has been limited to aerospace applications by their extremely high cost. Today, the crystalline silicon modules can be installed at the cost of 2–3 USD per every watt of nominal power generated at maximal (peak) (denoted as watt-peak, Wp) [2]. The second generation of solar cells is based mainly on the amorphous and multicrystalline silicon and metal chalcogenides such as CdTe, CuInxGa(1-x)Se2 (CIGS) and other similar thin-film absorbers. The best devices of the CIGS type yield power conversion efficiencies of 15–20% [3]. The power conversion efficiencies of such cells in general stay below the level achieved for the first-generation solar cells. However, second-generation solar cells are less expensive than the devices of the first generation. Therefore, the resulting economical effect is quite significant. The price of the generated energy becomes much lower, which stimulated growth of the thin-film photovoltaic technologies market [4]. Unfortunately, toxicity of cadmium telluride or gallium arsenide and expected shortage of indium and gallium world production will have to be considered for these technologies as an energy solution for the future [5]. At the same time, the efficiency of the cells based on amorphous silicon remained stuck at around 10.1%, which resulted in decrease of their market share since 2009–2010 [6]. The tandem and triple-junction solar cells are expected be able to operate at potentially ultrahigh light power conversion efficiencies (>50%) for concentrated sun light. Power conversion efficiencies of the best tandem solar cells are near 43% while their price remains much higher than the target 100–150 USD/m2 in spite of using inexpensive solar light concentrators [7]. Organic solar cells offer another alternative that yields moderate power conversion efficiencies of 8–10% at very low module costs (40–60 USD/m2) and lifetimes of 5–10 years. Indeed, laboratory prototypes of organic solar cells with the active areas as large as 1–8 cm2 demonstrated certified power conversion efficiencies of 8–10% (see Table 25.1 below). Lifetimes of 7 years were projected for conjugated polymer-based devices using accelerated tests [8]. At the same time, Heliatek confirmed continuous operation lifetimes of 16,000 hr (100 mW/cm2, AM1.5) for their small molecular-based double-junction devices [9]. Further improvements of organic solar cells in terms of performance, lifetime, module design and production technologies might lead to a breakthrough in the renewable energies. Ultimately, the energy generated by solar light conversion should become less expensive than the energy produced by combustion of fossil fuels.