High efficiency photovoltaic semiconductors

However, the last few years have also seen a growing awareness of the problems inherent in using the semiconductor-electrolyte interface as a means of solar-energy conversion. Very long-term stability may not be possible in aqueous electrolytes and no oxide material has been identified that has properties suitable for use as a photoanode in a photoelectrolysis cell. Highly efficient photovoltaic cells are known, both in aqueous and non-aqueous solutions, but it is far from clear that the additional engineering complexity, over and above that required for the dry p-n junction photovoltaic device, will ever allow the "wet photovoltaic cells to be competitive. These, and other problems, have led to something of a pause in the flood of papers on semiconductor electrochemistry in the last two years and the current review is therefore timely. I have tried to indicate what is, and is not, known at present and where future lines of development may lie. Individual semiconductors are not treated in detail, but it is hoped that most of the theoretical strands apparent in the last few years are discussed. 

Significantly more progress has been made in the electrochemical formation of ll-Vl compound semiconductors such as CdTe. High efficiency photovoltaics have been formed commercially using electrodeposited CdTe. and several reviews have been published concerning the electrodeposition of II-VI compounds [7-12]. 

Unlike many semiconductor devices, solar cells require production in large areas with high uniformity. The syndiesis of nanoparticles with controlled size distributions, chemical composition, and chemical termination is critical to produce large area, high efficiency photovoltaics. There are several nKthods of... 

These fields, where photoalignment technology happens to be useful, may cover the ordering of thin semiconductor layers, thin layers in solar cells, optical data storage, and holographic memory. New, highly efficient photovoltaic, optoelectronic, and photonic devices thus become possible. 

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. 

The incident monochromatic photon-to-current conversion efficiency (IPCE), also called external quantum efficiency, is defined as the number of electrons generated by light in the external circuit divided by the number of incident photons as a function of excitation wavelength. It is expressed in Equation (7).29 In most cases, the photoaction spectrum overlaps with the absorption spectrum of the sensitizer adsorbed on the semiconductor surface. A high IPCE is a prerequisite for high-power photovoltaic applications, which depends on the sensitizer photon absorption, excited state electron injection, and electron transport to the terminals ... 

Another requirement for efficient photovoltaic energy conversion is that the solar cells have a semiconductor junction with a large built-in potential (Vbi). The doped layers in a p-i-n cell are mainly responsible for determining VH. Ideally, one would want wide-band-gap doped layers that are degenerate or highly conductive. In such a case, the Fermi levels would lie very close to the band edges, and the built-in potential would approach the band gap. 

Small molecule semiconductors for high-efficiency organic photovoltaics 12CSR4245. 

The drive toward high efficiency requires the use of the tandem cell (or cascade cell) concept in which solar cells based on photovoltaically active semiconductors having different energy gaps are... 

The drives toward high efficiency and low cost may intersect by the fabrication of thin film tandem cell stacks in which the thin films consist of four-and five-element alloys of the ternary semiconductors. It has already been demonstrated that thin films of such alloys can be deposited by rf-sputtering7 and chemical spray pyrolysis.8 It has also been shown that large grained specimens of these four- and five-element alloys can be used to fabricate solar cells having efficiencies in excess of 10%, i.e. these semiconductor alloys are promising photovoltaic materials. 

It is evident from our discussion that realizing the promise of high efficiency cells requires photovoltaically active semiconductors with band gaps in the range 1.0 to 2.4eV. Figure 5 tabulates the band gaps of some binary and ternary semiconductors which could serve as the photovoltaically active materials in the cells. These materials have discrete values of E, and while it may be possible to identify a group of them for a specific tandem cell system. 

First consider the kind of universal cell structure which would satisfy our requirements. To conserve material, the photovoltaic-ally active semiconductors must be direct gap materials. If the solar cells are to be of the p/n homojunction type, excessive surface recombination losses, which are usually encountered at the light receiving surface, must be eliminated if high efficiencies are to be attained. One way to eliminate surface recombination losses is to use the direct gap semiconductor as the light absorbing, photovoltaically active part of a p/n heterojunction in which the other is a semiconductor whose band gap is so large that it cannot absorb any significant fraction of photons from the solar spectrum A heterojunction device of this type is illustrated in Fig. 6 [15]. ... 

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