The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4f(n) inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.

Efficient quantum cutting by the Pr3+–Yb3+ couple is demonstrated for the conversion of blue/green to near‐infrared (NIR) radiation with quantum efficiencies close to 200%. The resonant two‐step energy transfer process, which involves one higher‐energy photon becoming two NIR photons, may lead to the reduction of energy loss in solar cells, revealing how spectral conversion is a promising avenue for boosting solar‐cell efficiency.

Downconversion is a promising avenue to boost the efficiency of solar cells by absorbing one higher energy visible photon and emitting two lower energy near-infrared ͑NIR͒ photons. Here the efficiency of downconversion for the ͑Er 3+ ,Yb 3+ ͒ couple is investigated in NaYF 4 , a well-known host lattice for efficient upconversion with ͑Er 3+ ,Yb 3+ ͒. Analysis of the excitation and emission spectra for NaYF 4 doped with 1% Er 3+ and codoped with 0%, 5%, 10%, or 30% Yb 3+ show that visible to NIR downconversion is inefficient. Downconversion by the scheme based on the reverse of the upconversion process is hampered by fast multiphonon relaxation from the 4 F 7/2 level ͑the starting level for downconversion͒ to the 4 S 3/2 level. Energy transfer from the 4 S 3/2 level of Er 3+ to Yb 3+ is shown to be inefficient. Efficient downconversion from the 4 G 11/2 of Er 3+ level is observed, resulting in emission of two photons ͑one around 980 nm and one around 650 nm͒ after absorption of a single 380 nm photon.

Energy losses inherent to the conversion of sunlight to electricity in solar cells are mainly due to the so-called spectral mismatch: low energy photons are not absorbed while the energy of high energy photons is only partly used by the solar cell. The losses can be significantly reduced by adapting the solar spectrum. A promising avenue is the use of a downconversion material where one higher energy visible ͑blue-green͒ photon is "cut" into two lower-energy near-infrared photons that both can be used by the solar cell.

No abstract