Direct Solar Energy
cell is 24.2% for 155.1 cm2 (Bunea et al., 2010). Commercial module efficiencies for wafer-based silicon PV range from 12 to 14% for multi- crystalline Si and from 14 to 20% for monocrystalline Si.
Commercial thin-film PV technologies include a range of absorber material systems: amorphous silicon (a-Si), amorphous silicon-germa- nium, microcrystalline silicon, CdTe and CIGS. These thin-film cells have an absorber layer thickness of a few m or less and are deposited on glass, metal or plastic substrates with areas of up to 5.7 m2 (Stein et al., 2009).
The a-Si solar cell, introduced in 1976 (Carlson and Wronski, 1976) with initial efficiencies of 1 to 2%, has been the first commercially successful thin-film PV technology. Because a-Si has a higher light absorption coef- ficient than c-Si, the thickness of an a-Si cell can be less than 1 m—that is, more than 100 times thinner than a c-Si cell. Developing higher effi- ciencies for a-Si cells has been limited by inherent material quality and by light-induced degradation identified as the Staebler-Wronski effect (Staebler and Wronski, 1977). However, research efforts have success- fully lowered the impact of the Staebler-Wronski effect to around 10% or less by controlling the microstructure of the film. The highest stabi- lized efficiency—the efficiency after the light-induced degradation—is reported as 10.1% (Benagli et al., 2009).
Higher efficiency has been achieved by using multijunction technologies with alloy materials, e.g., germanium and carbon or with microcrystal- line silicon, to form semiconductors with lower or higher bandgaps, respectively, to cover a wider range of the solar spectrum (Yang and Guha, 1992; Yamamoto et al., 1994; Meier et al., 1997). Stabilized efficiencies of 12 to 13% have been measured for various laboratory devices (Green et al., 2010b).
CdTe solar cells using a heterojunction with cadmium sulphide (CdS) have a suitable energy bandgap of 1.45 electron-volt (eV) (0.232 aJ) with a high coefficient of light absorption. The best efficiency of this cell is 16.7% (Green et al., 2010b) and the best commercially available modules have an efficiency of about 10 to 11%.
The toxicity of metallic cadmium and the relative scarcity of tellurium are issues commonly associated with this technology. Although several assessments of the risk (Fthenakis and Kim, 2009; Zayed and Philippe, 2009) and scarcity (Green et al., 2009; Wadia et al., 2009) are available, no consensus exists on these issues. It has been reported that this poten- tial hazard can be mitigated by using a glass-sandwiched module design and by recycling the entire module and any industrial waste (Sinha et al., 2008).
The CIGS material family is the basis of the highest-efficiency thin-film solar cells to date. The copper indium diselenide (CuInSe2)/CdS solar cell was invented in the early 1970s at AT&T Bell Labs (Wagner et al., 1974). Incorporating Ga and/or S to produce CuInGa(Se,S)2 results in the benefit of a widened bandgap depending on the composition (Dimmler and Schock, 1996). CIGS-based solar cells have been validated at an
efficiency of 20.1% (Green et al., 2010b). Due to higher efficiencies and lower manufacturing energy consumptions, CIGS cells are currently in the industrialization phase, with best commercial module efficiencies of up to 13.1% (Kushiya, 2009) for CuInGaSe2 and 8.6% for CuInS2 (Meeder et al., 2007). Although it is acknowledged that the scarcity of In might be an issue, Wadia et al. (2009) found that the current known economic indium reserves would allow the installation of more than 10 TW of CIGS-based PV systems.
High-efficiency solar cells based on a multijunction technology using III-V semiconductors (i.e., based on elements from the III and V columns of the periodic chart), for example, gallium arsenide (GaAs) and gallium indium phosphide (GaInP) , can have superior efficiencies. These cells were originally developed for space use and are already commercial- ized. An economically feasible terrestrial application is the use of these cells in concentrating PV (CPV) systems, where concentrating optics are used to focus sunlight onto high efficiency solar cells (Bosi and Pelosi, 2007). The most commonly used cell is a triple-junction device based on GaInP/GaAs/germanium (Ge), with a record efficiency of 41.6% for a lattice-matched cell (Green et al., 2010b) and 41.1% for a metamorphic or lattice-mismatched device (Bett et al., 2009). Sub-module efficien- cies have reached 36.1% (Green et al., 2010b). Another advantage of the concentrator system is that cell efficiencies increase under higher irradiance (Bosi and Pelosi, 2007), and the cell area can be decreased in proportion to the concentration level. Concentrator applications, how- ever, require direct-normal irradiation, and are thus suited for specific climate conditions with low cloud coverage.
Emerging photovoltaic technologies
Emerging PV technologies are still under development and in laboratory or (pre-) pilot stage, but could become commercially viable within the next decade.They are based on very low-cost materials and/or processes and include technologies such as dye-sensitized solar cells, organic solar cells and low-cost (printed) versions of existing inorganic thin-film technologies.
Electricity generation by dye-sensitized solar cells (DSSCs) is based on light absorption in dye molecules (the ‘sensitizers’) attached to the very large surface area of a nanoporous oxide semiconductor electrode (usu- ally titanium dioxide), followed by injection of excited electrons from the dye into the oxide. The dye/oxide interface thus serves as the separator of negative and positive charges, like the p-n junction in other devices. The negatively charged electrons are then transported through the semi- conductor electrode and reach the counter electrode through the load, thus generating electricity.The injected electrons from the dye molecules are replenished by electrons supplied through a liquid electrolyte that penetrates the pores of the semiconductor electrode, providing the elec- trical path from the counter electrode (Graetzel, 2001). State-of-the-art DSSCs have achieved a top conversion efficiency of 10.4% (Chiba et al., 2005). Despite the gradual improvements since its discovery in 1991 (O’Regan and Graetzel, 1991), long-term stability against ultraviolet light