Photovoltaic cell efficiency

The great energy consumption, limited recources of traditional fuels and environmental problems have lead to intensive research on the conversion of solar energy during the last fifteen years. Conversion into electrical energy has been realized in technical devices consisting of pn-junction photovoltaic cells. Efficiencies of up to 20 % have been obtained with single crystal devices and around 9 % with polycrystalline or amorphous layers. 

Enhanced photovoltaic cell efficiency was achieved via incorporation of highly electron-deficient oxadiazole moieties on side chains of poly(phenylene vinylenejs and poly(fluorene)s <2006SM949>. The synthesis of terminal... 

Forrest, S.R., The limits to organic photovoltaic cell efficiency, MRS Bull. 30, 28-32, 2005. 

The hierarchy of photovoltaic cell efficiencies is presented in Table 1. Semiconductor effects, such as recombination, reduce the power efficiency of a GaAs-based device from a value of 37%, based solely upon band gap, to 15.3%. Reflection losses, with an... 

S.-P. Huang, J.-L. Liao, H.-E. Tseng, T.-H. Jen, J.-Y. Liou, and S.-A. Chen. Enhanced photovoltaic cells efficiency via incorporation of high electron-deficient oxadiazole moieties on side chains of poly(phenylene vinylenejs and poly(fluorene)s. Synth. Met., 156(14-15) 949-953, July 2006. 

Fig. 5. Efficiency improvements in photovoltaic cells where ( ) corresponds to GaAs (—) InP ( ) CdS ( ) CdTe ( ) amorphous siUcon and...

Photovoltaic cells. The selenium photographic exposure meter has already been mentioned it goes back to Adams and Day s (1877) study of selenium, was further developed by Charles Fritt in 1885 and finally became a commercial product in the 1930s, in competition with a device based on cuprous oxide. This meter was efficient enough for photographic purposes but would not have been acceptable as an electric generator. 

For photovoltaic cells made with pure conjugated polymers, eneigy conversion efficiencies were typically I0 3-I0 1%, loo low to be used in practical applications [48, 63, 67]. Thus, pholoinduced charge transfer across a donor/acceptor... 

If for example Ti02, is used to capture sunlight in a photo-catalytic reaction then only about 10% of the available spectrum will be of use, since it requires 3.2 eV to create an electron-hole pair in Ti02. Both the photovoltaic and the photochemical methods are of potential interest, but at present they are too expensive. Also, the production of semiconductors used in photovoltaic cells consumes much energy. Nevertheless, the prospect remains attractive. If cells could be made with an efficiency of say 10 % then only 0.1 % of the earths surface would be required to supply our present energy consumption ... 

As it has been described in various other review articles before, the conversion efficiencies of photovoltaic cells depend on the band gap of the semiconductor used in these systems The maximum efficiency is expected for a bandgap around Eg = 1.3eV. Theoretically, efficiencies up to 30% seem to be possible . Experimental values of 20% as obtained with single crystal solid state devices have been reported " . Since the basic properties are identical for solid/solid junctions and for solid/liquid junctions the same conditions for high efficiencies are valid. Before discussing special problems of electrochemical solar cells the limiting factors in solid photovoltaic cells will be described first. 

Fig. 5.65 Dependence of the solar conversion efficiency (CE) on the threshold wavelength (Ag) for a quantum converter at AM 1.2. Curve 1 Fraction of the total solar power convertible by an ideal equilibrium converter with no thermodynamic and kinetic losses. Curve 2 As 1 but the inherent thermodynamic losses (detailed balance and entropy production) are considered. Continuous line Efficiency of a regenerative photovoltaic cell, where the thermodynamic and kinetic losses are considered. The values of Ag for some semiconductors are also shown (according to J. R. Bolton et al.)... 

The theoretical solar conversion efficiency of a regenerative photovoltaic cell with a semiconductor photoelectrode therefore depends on the model used to describe the thermodynamic and kinetic energy losses. The CE values, which consider all the mentioned losses can generally only be estimated the full line in Fig. 5.65 represents such an approximation. Unfortunately, the materials possessing nearly the optimum absorption properties (Si, InP, and GaAs) are handicapped by their photocorrosion sensitivity and high price. 

Apart from recapture of the injected electrons by the oxidized dye, there are additional loss channels in dye-sensitized solar cells, which involve reduction of triiodide ions in the electrolyte, resulting in dark currents. The Ti02 layer is an interconnected network of nanoparticles with a porous structure. The functionalized dyes penetrate through the porous network and adsorb over Ti02 the surface. However, if the pore size is too small for the dye to penetrate, that part of the surface may still be exposed to the redox mediator whose size is smaller than the dye. Under these circumstances, the redox mediator can collect the injected electron from the Ti02 conduction band, resulting in a dark current (Equation (6)), which can be measured from intensity-modulated experiments and the dark current of the photovoltaic cell. Such dark currents reduce the maximum cell voltage obtainable, and thereby the total efficiency. 

Yu, G., Gao, J., Hummelen, J.C., Wudl, F., and Heeger, A.J. (1995) Polymer photovoltaic cells enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 270, 1789-1791. 

Peumans, P. Uchida, S. Forrest, S. R. 2003. Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films. Nature 425 158-162. 

Bhattacharya, R. N. Contreras, M. A. Egaas, B. Noufi, R. N. Kanevce, A. Sites, J. R. 2006. High efficiency thin-film CuIn1.xGaxSe2 photovoltaic cells using a Cd ZtixS buffer layer. Appl. Phys. Lett. 89 253503(1-2). 

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