Photovoltaic energy types

In evaluating the values of new processes or products, one should consider both their energy payback ratio (EPR) and their carbon emission payback period (CEPP). The EPR is the ratio of energy obtained to energy invested in manufacturing and assembling the product. The EPR of photovoltaic (PV)-type solar collectors is about 10 1. In contrast, the EPR of oil shale production is only 4 1. 

Byvik CE, Smith BT, Reichman B (1982) Layered transition metal thiophosphates (MPX3) as photoelectrodes in photoelectrochemical cells. Sol Energy Mater 7 213-223 Lincot D, Gomez Meier H, Kessler J, Vedel J, Dimmler B, Schock HW (1990) Photoelectrochemical study of p-type copper indium diselenide thin films for photovoltaic... 

Similar photovoltaic cells as those described above can be made with semiconductor/ liquid Junctions. The basic function of such a cell is illustrated in terms of an energy scheme in Fig. 2. The system consists of an n-type semiconductor and an inert metal... 

A galvanic cell is a system that can perform electrical work when its energy is consumed at the expense of chemical or concentration changes that occur inside the system. There are also systems analogous to galvanic cells based on conversion of other types of energy than chemical or osmotic into electrical work. Photovoltaic electrochemical cells will be discussed in... 

At illuminated p-type semiconductors, light energy brings about a shift of the applied cathode potential at which C02 reduction takes place toward a less negative potential by the photovoltaic effect.94 Thus, light energy can be used to reduce the apparent overpotential of C02 reduction. 

This type of sensitizer opens up new avenues for improving the near-IR response of dye-sensitized solar cells. In addition, important applications can be foreseen for the development of photovoltaic windows transmitting part of the visible light. Such devices would remain transparent to the eye, while absorbing enough solar energy photons in the near IR to render efficiencies acceptable for practical applications. 

Wada, T. Nishitani, M. Negami, T. Kohara, N. Ikeda, M. Terauchi, M. 1994. Microstructural characterization of substrate-type and superstrate-type CuInSe2 thin film solar cells. Proc. 12th European Photovoltaic Solar Energy Conf. pp. 1542-1545. 

Electrical cells based on semiconductors that produce electricity from sunlight and deliver the electrical energy to an external load are known as photovoltaic cells. At present most commercial solar cells consist of silicon doped with small levels of controlled impurity elements, which increase the conductivity because either the CB is partly filled with electrons (n-type doping) or the VB is partly filled with holes (p-type doping). The electrons have, on average, a potential energy known as the Fermi level, which is just below that of the CB in n-type semiconductors and just above that of the VB in p-type semiconductors (Figure 11.2). 

Fig. 10-33. Energy diagram for a photovoltaic cell composed of a metal cathode and an n>type semiconductor anode Vpi, = cell voltage in operation at current <.

Figure 10-34 shows the energy diagram of an operating photovoltaic cell which consists of a photoexcited anode of n-type semiconductor and a photoexdted cathode of p-lype semiconductor. The electromotive force, of this type of photovoltaic cell approximately equals the difference between the flat band potential of the n-type anode and the flat band potential fEg, of the p-type cathode as shown in Eqn. 10-62 ... 

Fig. 10-34. Energy diagram for a photovoltaic ceU composed of an n-type semiconductor anode and a p-type semiconductor cathode.

Accordingly, the operation of electrolyzers with intermittent sources of solar energy and the various possibilities for matching photovoltaic current with the characteristics of the electrolyzer was one of the recurrent design issues in such projects. Most of the electrolyzers were of the alkaline type and operated at low pressure. 

Whilst producing energy conversion factors comparable with semi-conducting photovoltaics, the original Gratzel type DSSCs do suffer from some problems around the use of liquid electrolytes. These problems include the need for highly efficient... 

Liu Z, He D, Wang Y et al (2010) Solution-processable functionalized graphene in donor/ acceptor-type organic photovoltaic cells. Solar Energy Mater Solar Cells 94 1196-1200... 

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

Photovoltaic and photoconductive phenomena for various types of CT complexes between saturated polymers and dopant molecules, heterojunctions between polymers and organic and inorganic photoconductors were also investigated in the last few years [86-92]. The quantum efficiency of the energy conversion of 10-3% was obtained for such systems and output power density of 3 x 102 mV cm-2. The mobilities of the heterogeneous polymer systems with despersed inorganic photoconductors reach the value — 10 3-10-4 m2 V 1s 1. 

There are another type of photocells known as barrier layer photocells which work on an entirely different principle. They are semiconductor devices in which impinging photons promote the electrons from the valence band to the conduction band across the energy gap. A photovoltagc is generated which can be measured by a voltmeter. Such photovoltaic devices can have a large surface area and are easy to operate. They are commonly used in many simple colorimeters and fluorimeters and as light Otters for cameras. 

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