Category Solar Cell Materials

Lanthanides for UC Devices

An alternative group of materials that give long-lived intermediate states are those based on the lanthanide (or rare-earth) group of elements, because of their narrow and suitably spaced energy transitions. The lanthanides have a valence shell of 4f electrons that are shielded by full outer 6s and 5p shells. Hence, their electronic transitions tend to remain similar to those of an isolated atom and fairly discrete. However, in a host material (or phosphor) the absorption and emission levels are broadened somewhat by the combination with the allowed phonon energies of the phosphor.

Several lanthanides have demonstrated upconversion, including Pr and europium (Eu), but this has usually been for laser applications, [Xie, 1993]...

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Singlet-Triplet Coupling in Organic Molecules

One such long-lived energy state can be achieved in the transfer of electrons from ‘allowed’ excited singlet states to ‘forbidden’ excited triplet states in some organic molecules (S-T transitions).[7] Exploitation of this for upconversion has been demonstrated by [Baluschev et al., 2007]. The spin-orbit coupling that can occur due to the heavy-metal atoms in some complexed molecules, such as porphyrins, can allow a mixing of singlet (S) and triplet (T) states, such as to transfer the excited electron in the first excited singlet state (S1) to the excited triplet state (T1) where it can have an extremely long lifetime (up to 100 |as). This is illustrated in Figure 9.11.

These transitions are strictly ‘allowed’ or ‘forbidden’ only for pure electric dipole tran­sitions, but in real m...

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Upconversion of Below-Bandgap Photons

An UC device is designed to absorb subbandgap photons in an UC layer behind a bifacial solar cell. This layer radiatively absorbs two or more long-wavelength photons and emits a photon of higher energy above the bandgap of the bifacial solar cell. Thus, the current in the device is boosted by photons that would not normally be absorbed. As the UC does not interrupt the incidence of photons on the front surface, even a very low efficiency of UC gives a small current boost and hence an efficiency increase.

For application to photovoltaics there are two broad possibilities. Either the current in a silicon solar cell can be boosted through the application of a simply applied thick-film upconverting layer on the back surface of a bifacial silicon cell...

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Reverse Auger Generation

An alternative approach is to absorb a short-wavelength photon high up in the conduction band of various semiconductors. There is then the possibility of an impact ionisation event (i. e. reverse Auger recombination) in which the high-energy electron excites an additional electron to the conduction band, thus creating two or more electron-hole pairs at the bandgap energy, [Berkowitz and Olsen, 1991]. Luminescent recombination of these electron-hole pairs is then usually enhanced by choice of an appropriate doping level within the bandgap and an increased number of photons emitted. The quantum efficiency (QE) of such impact ionisation depends on the energy of the initial photon and is reduced by nonradiative thermalisation and recombination...

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Lanthanide-Based DC

Praseodymium (Pr3 +) is a good choice because of its widely dispersed energy levels well matched for photon cutting, [Dieke, 1968], see Figure 9.10. The 3P2, % and P0 levels at between 440 and 490 nm can absorb blue photons that can then radiatively recombine via the :G4 level at 1010 nm – at just greater than twice this wavelength – thus emitting two photons at just above the silicon bandgap, although nonradiative recombination via the other levels at longer wavelengths than 1G4 is also likely. Experiments indicating such photon cutting have been carried out on Pr3 + embedded in various phosphors, [Meijerink et al., 2006], and also for other lanthanide-doped materials, [Wegh et al., 2002, Michels et al., 2002]...

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Downconversion, QE > 1

A downconverting device (DC) must be placed in front of a standard cell and can boost current by converting a UV photon to more than one photon just above the bandgap of the solar cell – thus boosting the current. However, the DC does require that more lower-energy photons are emitted than high-energy photons absorbed, i. e. its quantum efficiency (QE) must be greater than 100%. Hence, there must be at least as many photons emitted at the lower energy as are absorbed at the higher, or else the DC layer will decrease the number of photons absorbed by the cell...

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MODIFICATION OF THE SOLAR SPECTRUM

As an alternative to modifying the structure of a solar cell, another approach is to modify the spectrum incident on the cell to narrow its bandwidth and make it closer to optimal for a single-bandgap cell. This involves a limiting quantum efficiency (QE) of conversion of photons to electron-hole pairs in the cell not limited to unity. (The physics of such nonunity quantum efficiency processes is discussed further in Section 2.4.3.) For short-wavelength photons – at least twice the energy of the cell bandgap – this means a QE > 1. This ‘downconversion’ (DC) or ‘quantum cutting’ (QC) approach means that more electron-hole pairs are generated than the number of blue photons in the spectrum...

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Intermediate-Band Solar Cells (IBSC)

The IBSC has an additional energy level (the intermediate band) within the bandgap of a single-junction cell such that this level absorbs photons below the bandgap energy in parallel with the normal absorption of above bandgap photons in the cell. Photons with less energy than the primary bandgap can be absorbed by transitions from the VB to the IB and from the IB to the CB. This is shown schematically in Figure 9.9 and is also an implementation of strategy (b), absorption of below-bandgap photons to produce carriers at the bandgap energy. The semiparallel operation of the direct band-to-band and transition via the IB processes, offers the potential to be much less spectrally sensitive than a tandem cell, but still has the potential to give high efficiencies...

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