16.2.1
Single-junction solar cells have the limitation that every photon can only generate one
collected electron-hole pair at most. This limitation can be tackled by spectral conversion.
The main idea of spectral conversion is to add an additional layer to the solar cell
consisting of a material that can alter the incident spectrum. Materials that are considered
for spectral conversion are organic dyes, quantum dots, lanthanide ions, and transition
metal ion systems [119].
Spectral upconversion
Spectral upconversion works by combining the energies of two absorbed low-energy
photons into one higher energy state, which can then radiate an upconverted photon into
the solar cell active layer, as illustrated in Figure 16.1.
Figure 16.1: Illustrating the principle of spectral upconversion.
Two distinct material systems have been employed to produce photon upconversion
for solar cells: firstly, triplet-triplet annihilation upconverters (TTA-UC), which use pairs
of organic chromophores with rationally ordered energy levels [120], and secondly, rare
earth upconverters, which make use of atomic transitions within lanthanide ions, from
elements such as erbium and ytterbium [121].
TTA-UC systems, which are reliant on molecular absorbers, work better at shorter
wavelengths between about 500 to 800 nm, while rare earths work in the region around
1,500 nm, where the ions are absorbed. Hence, the choice of the upconverter technology
also depends on the bandgap of the solar cell to which it is connected. For both
approaches, enhancement in solar cell efficiency has been demonstrated [122–124].
The upconverting layer should be placed at the back of the solar cell as low energy
photons can pass through the solar cell absorber to this layer and can be converted there to
high energy photons that are absorbed by the solar cell in a second pass [119]. In this way,
upconverting layers with low efficiencies might also help to increase the photocurrent of
the solar cell. Further, placing the upconverting layers at the back prevents them from
being irradiated by high energy photons which might reduce their lifetime. Due to
parasitic absorption it is not advised to put these layers at the front of the solar cell.
Single-junction solar cells have the limitation that every photon can only generate one
collected electron-hole pair at most. This limitation can be tackled by spectral conversion.
The main idea of spectral conversion is to add an additional layer to the solar cell
consisting of a material that can alter the incident spectrum. Materials that are considered
for spectral conversion are organic dyes, quantum dots, lanthanide ions, and transition
metal ion systems [119].
Spectral upconversion
Spectral upconversion works by combining the energies of two absorbed low-energy
photons into one higher energy state, which can then radiate an upconverted photon into
the solar cell active layer, as illustrated in Figure 16.1.
Figure 16.1: Illustrating the principle of spectral upconversion.
Two distinct material systems have been employed to produce photon upconversion
for solar cells: firstly, triplet-triplet annihilation upconverters (TTA-UC), which use pairs
of organic chromophores with rationally ordered energy levels [120], and secondly, rare
earth upconverters, which make use of atomic transitions within lanthanide ions, from
elements such as erbium and ytterbium [121].
TTA-UC systems, which are reliant on molecular absorbers, work better at shorter
wavelengths between about 500 to 800 nm, while rare earths work in the region around
1,500 nm, where the ions are absorbed. Hence, the choice of the upconverter technology
also depends on the bandgap of the solar cell to which it is connected. For both
approaches, enhancement in solar cell efficiency has been demonstrated [122–124].
The upconverting layer should be placed at the back of the solar cell as low energy
photons can pass through the solar cell absorber to this layer and can be converted there to
high energy photons that are absorbed by the solar cell in a second pass [119]. In this way,
upconverting layers with low efficiencies might also help to increase the photocurrent of
the solar cell. Further, placing the upconverting layers at the back prevents them from
being irradiated by high energy photons which might reduce their lifetime. Due to
parasitic absorption it is not advised to put these layers at the front of the solar cell.