Rare-earth doped materials enhance silicon solar cell efficiency

Abstract

In silicon solar cells, the mismatch between the incident solar spectrum and the spectral absorption frequencies results in a major energy loss. Photons with energies smaller than the silicon band gap are not absorbed, and their energy is totally wasted. Photons with energies larger than twice the silicon band gap are absorbed, but the excess energy is lost to heating. Down-conversion and up-conversion mechanisms are usually exploited to modify the incident solar spectrum. Figure 1 illustrates the process schematically. In down conversion, multiple low-energy photons are generated to exploit the energy of one incident high-energy photon. In up conversion, two or more incoming photons generate at least one photon with a higher energy than the incoming photons. We focus on the down-conversionmechanism. Rare earth (RE) ions, in which absorption and emission occur through a number of energy levels, allow down-conversion processes, as shown in Figure 2. Different mechanisms based on energy transfer between a RE3C (absorbing ion) and Yb3C (emitting ion) were investigated recently: Pr3C/Yb3C; Tb3C/Yb3C; Tm3C/Yb3C. Most studies of down conversion to date have been performed on single crystals characterized by a low phonon cut-off energy to minimize non-radiative transitions from the RE ions to the host matrix. Recent studies, however, have demonstrated that some glasses and transparent glass ceramics (GCs) could be valid alternative systems for supporting an effective quantum cutting process.1 Figure 1. Solar spectrum showing the band gap and twice the band gap of silicon.2 Down conversion shifts photons from a high-energy band (blue) to the maximum absorption band of silicon (white). Up conversion shifts photons from a low-energy band (red) to the maximum absorption band of silicon.