Category Solar Cell Materials

Multiple-Exciton Generation (MEG)

Carriers generated from high-energy photons (at least twice the bandgap energy) absorbed in a semiconductor can undergo impact ionisation events resulting in two or more carriers close to the bandgap energy. This approach is an implementation of approach (b) to produce more carriers per incident photon and thus boost current, as is discussed in Section 2.4.1.1. In order to achieve this, the rates of photogenerated carrier separation, transport, and interfacial transfer across the contacts to the semiconductor must all be fast compared to the rate of carrier cooling [Nozik et al., 1980]. But impact ionisation has a vanishingly small probability in bulk materials because of restrictions imposed by energy and momentum conservation

and the extremely fast carrier cooling rate...

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Si Nanostructure Tandems

It is possible to retain both the advantages of crystalline material and of thin-film deposition but to avoid the high costs of epitaxial III-Vs by use of thin-film crystalline Si, which is crystallised by a postgrowth solid-phase crystallisation anneal [Aberle, 2006]. Such single­junction cells are now in production at efficiencies of 10% [Basore, 2006].

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Thin film Si cell

Eg = 1.1 eV
or another QD cell

To boost the efficiencies of silicon-based cells in a tandem and retain the other advantages of third-generation approaches, wider bandgaps in Si-based materials can be realised using quantum confinement in nanostructures. Such engineered bandgap material fabricated in a cell can be used as an element on top of a thin film bulk Si cell, as shown in Figure 9.5 [Cho et al...

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Amorphous Si and Micromorph Tandems

Amorphous silicon (a-Si) cells are used for single-junction cells, but tend to give efficiencies of only about 4-5% because of high defect concentrations associated with the lack of crystallinity [Meier et al., 2004]. These efficiencies can be boosted in tandem cells with amorphous silicon (a-Si:H) as a top cell[6] with one or two lower cells of an alloy with Ge (a-Si:Ge), which lowers the bandgap. These cells are in-series devices that are grown by thin-film processes such as chemical vapour deposition (CVD) or other vacuum deposition techniques. The lack of a need for crystallisation and the vapour-phase deposition mean that much less energy is required for the process, and the use of raw materials tends to be low for the thin layers deposited...

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Thin-Film Tandems

An alternative approach to reducing the cost per Watt is to use material that is not of as high a quality as epitaxial III-V materials and hence has a higher defect density and lower efficiency, but that can be produced by much cheaper, low energy intensity deposition methods and uses elements and compounds that are not scarce or toxic. This thin-film approach thus tackles the twin requirements of third-generation devices, namely low cost per Watt and the use of nontoxic and abundant materials.

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Concentrator Systems

The expense of the growth techniques and of the compounds used means that such devices are usually designed for use in optical concentrator systems operating at a few hundred suns. This means that only a small area of the very efficient but also very expensive cell material is required at the optical focus of a relatively cheap concentrator. Potentially this can bring the cost per Watt of electricity generated down to low levels [King et al., 2006]. Concentration also gives the higher limiting efficiencies mentioned above because the sun effectively fills a larger fraction of the sky as far as the cell is concerned compared to no concentration.

This increases efficiency because the cell must clearly be able to absorb photons from the sun and hence must have an acceptance angle at least as ...

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III-V Tandems

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The highest-quality and hence highest-efficiency tandem devices are those made by epitaxial single-crystal III-V growth. A simplified generic III-V tandem device is shown in Figure 9.4. Such structures are grown monolithically by epitaxial growth process such as metal organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). These epitaxial techniques are very expensive but give very high quality crystalline material. The epitaxial growth requires control of the lattice parameter to a constant value; and bandgap control is required for a tandem cell. It is the flexibility of the III-V group of compounds that lends it to the growth of such cells, usually lattice matched on a germanium substrate...

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Tandem Cells

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The tandem or multicolor cell is conceptually the easiest configuration to understand. It belongs to strategy (a) of increasing the number of energy levels. Solar cells consisting of p-n junctions in different semiconductor materials of increasing bandgap are placed on top of each other, such that the highest bandgap intercepts the sunlight first (see Figure 9.3). This approach was first suggested by Jackson in 1955 using both spectrum splitting and photon

Figure 9.3 A tandem cell with the bandgap of each subcell decreasing from the front to the back, giving both spectrum splitting and photon selectivity.

selectivity [Jackson, 1955]. The particle balance limiting efficiency depends on the number of subcells in the device. For 1, 2, 3, 4, and to subcells, the efficiency n is 31...

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MULTIPLE-ENERGY-LEVEL APPROACHES

The concept of using multiple energy levels to absorb different sections of the solar spectrum can be applied in many different device structures. The ideal limiting efficiencies for these are often very similar and sometimes identical for a given number of energy levels. Hence their differences are manifest in the degree to which each overcomes nonidealities. This includes any inability of a particular cell design to select photon absorption at its optimum energy level in the cell, the presence of parasitic processes (usually associated with defects), and the ease of manufacture and the abundance of appropriate materials.

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