Photovoltaic devices efficiency

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

There is experimental consensus on the most important parameters of singlelayer polymer photovoltaic devices, the short circuit current / , the open circuit voltage V c, and the filling factor FF. From these parameters the efficiencies of PPV based devices were typically calculated to be around 0.1% under monochromatic low light intensities. Efforts to extend the classical semiconductor picture of... 

The excellent photosensitivity and relatively high energy conversion efficiencies obtained from the bulk hetcrojunction materials arc promising. The monochromatic power efficiencies for conjugated polymer photovoltaic devices are around... 

Single-Crystal Silicon. Silicon is still the dominant material in photovoltaic. It has good efficiency, which is 25% in theory and 15% in actual practice. Silicon photovoltaic devices are made from wafers sliced from single crystal silicon ingots, produced in part by CVD (see Ch. 8, Sec. 5.1). However, silicon wafers are still costly, their size is limited, and they cannot be sliced to thicknesses less than 150 im. One crystalline silicon wafer yields only one solar cell, which has an output of only one watt. This means that such cells will always be expensive and can only be used where their high efficiency is essential and cost is not a major factor such as in a spacecraft applications. 

Kruger J, Bach U, Plass R, Cevey L, PiccireUi M, Gratzel M (2001) High efficiency solid-state photovoltaic device due to inhibition of interface charge recombination. Appl Phys Lett 79 2085-2087... 

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. 

With the D S SCs containing Hibiscus surattensis extract, the best performances were probably obtained because of the large amount of dye adsorbed on Ti02 (lxl cm2 active surface) in comparison with the other extracts from tropical flowers. Incident photon to current conversion efficiency (IPCE) values of 76% were calculated (2 — 590 nm). Jsc was 5.45 mAcm-2, Voc = 392mV, FF — 54%, and efficiency = 1.14%. Also, the stability of the photovoltaic devices was the best in the case of Hibiscus surattensis, even though it needs to be improved to achieve real long-term stability, especially as far as the sealing quality is concerned. 

Mitzi, D. B. Yuan, M. Liu, W. Kellock, A. Chey, S. J. Deline, V. Schrott, A. G. 2008. A high-efficiency solution-deposited thin-film photovoltaic device. Adv. Mater. 20 3657-3662. 

Figure 7.1. The current-voltage plot for an 18.5%-efficient CBD ZnS(O.OH)/CIGS-based photovoltaic device. Device temperature 25.0 + 1°C Device area = 0.402cm2 Irradiance 1000.0W/m2 V

The addition of In and Ga and selenization at high temperature were crucial in obtaining the 13.4%-efficient device. Photovoltaic device fabrication was completed by a CBD of about 500 A of CdS, followed by RF sputtering of 500 A of intrinsic ZnO and 3500 A of Al203-doped conducting ZnO. Bilayer Ni/Al top contacts were deposited in an e-beam system. The final step in the fabrication sequence is the deposition of lOOnm of MgF2 as an antirellection coating. 

Gratzel, M. Highly efficient nanocrystalUne photovoltaic devices. Plat. Met. Rev., 1994, 38, 151-159 Gratzel, M. Dye-sensitized solar cells, J. Photochem. Photobiol. C-Photochem. Rev., 2003, 4(2), 145-153. 

Curve P in Fig. 2 represents the ideal thermodynamic limit for conversion efficiency and cuarve C sets an approximate limit for the efficiency of conversion to stored chemical energy. There are, however, other loss factors to be considered which vaary according to the device. These loss factors have been considered in considerable detail for photovoltaic devices (15, 19) and estimates for the ultimate achievable conversion efficiency to electricity vaary from 25 - 2 8% however, only recently have efficiencies been considered for conversion to stored chemical energy (5). In this case... 

Heterogeneous mixing of fullerenes and fullerene derivatives with Ji-conjugated polymers has been used to produce excellent materials for photovoltaic devices [141], Upon irradiation of fullerene/polymer blends, charge transfer from the polymer to occurs, resulting in efficient photoconductivities. Better behavior of fullerene derivatives than with pristine Cgg has been observed, and attributed to the improved miscibility of the functionalized species. 

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