Photovoltaic systems, aspects

R. Whisnant, S. Johnston, and J. Hutchby, Economic analysis and environmental aspects of photovoltaic systems, Ch. 21, Handbook of Photovoltaic Science and Engineering, Ed. by A. Luque and S. Hegedus, John Wiley and Sons, Ltd., 2003. 

There are some further aspects which must be considered when photochemical diodes are used. When a metal (catalyst) is deposited on a semiconductor, then frequently a Schottky barrier instead of an ohmic contact is formed at the semiconductor-metal interface. In this case, the latter junction behaves as a photovoltaic system by itself, which may determine or essentially change the properties of the photochemical diode. The consequences have been discussed in detail in [14, 27]. Frequently, colloidal semiconducting particles have been used, their size being much smaller than the thickness of the space charge region expected. 

The development of amorphous silicon solar cells has progressed so well in recent years that they threaten to dominate the field of solar energy conversion. It is difficult to see how wet systems can compete with these cells, the only limitations of which arise from design problems. In the near future, it seems probable that their only failure wOl be the material used to support the cells. The progress and status of this subject does not come within the scope of this review and the interested reader should consult journals directed more towards solid-state physics than photochemistry. In this Section, we consider only aspects of photovoltaics that are directly relevant to photochemistry. 

In photochemical processes for solar energy storage, photon absorption either creates a molecular excited state or stimulates an interband electronic transition in a semiconductor, which induces a molecular change. Comprehensive reviews, including those by Gratzel (1980), Kalyanasundaram (1982) and Harriman (1986-7), have discussed various aspects of photochemical energy conversion. A photoactivated molecular excited state can drive either i) photodissociation ii) photoisomerisation or iii) photoredox reactions. Processes based on semiconductors may involve photovoltaic or photoelectrochemical systems. 

Photonics is a rapidly emerging field that uses the quantum interpretation that light has both wave and particle aspects that generate, detect, and modify it. Photonics covers the full range of the electromagnetic spectrum, but most applications are in the visible and infrared. Photonic systems are replacing electricity in the transmission, reception, and amplification of telecommunication information. Photonic applications include lasers, photovoltaic solar cells, sensors, detectors, and quantum computers. 

In recent years there has been an increasing interest in systems which enable the conversion of solar energy into electrical or chemical energy. Many types of systems have been proposed and studied experimentally, the fundamentals of which extend from solid state physics to photo- and electrochemistry. For most of the systems considered excitation of an electron by absorption of a photon is followed by charge separation at an interface. It follows that the different fields involved (photovoltaics, photoelectrochemistry, photogalvanics, etc.) have several essential aspects in common. 

The high aspect ratio of nanorods can facilitate charge transport, while the handgap can he tuned by vaiying the nanorod radius. This enables the absorption spectmm of the devices to be tailored to overlap with the solar emission spectmm, whereas traditionally polymer absorption has been limited to only a small fraction of the incident solar irradiation. At present, the nanorods in polymer solar cells are typically incorporated into a homopolymer matrix. An alternative to this approach is to incorporate the nanorods into either a polymer blend or diblock copolymer system. The photovoltaic properties of nanorod polymer composites could potentially be improved due to the percolation of nanorods, and the presence of continual electrical pathways, from the DA interfaces to the electrodes. To test this hypothesis, we use the distribution of nanorods from the self-assembled stmcture in Figure 1(b) as the input into a drift-diffusion model of polymer photovoltaics. 

Several aspects must be considered for a comparison of energy sources that generate electricity. The first one is the quantity of carbon dioxide equivalent/kWh rejected by energy sources. Figure 1.3 shows the relationship between the CO2 production and the reaction (response time) for a number of electric power suppliers [11]. It is evident that the response time of nuclear plant (48 h) is bigger than hydro-electric dam or renewable systems such as wind mill and photovoltaic (few seconds). In the middle are the electric power stations using coal, oil or gas. 

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