Downconversion for solar cells with lanthanide ion couples

Abstract

A large part of the losses that limit the maximum theoretical efficiency of solar cells to 30% can be contributed to the spectral mismatch: low energy photons are not absorbed and the energy is lost, while the energy of high energy photons is only partly used while the rest is converted into heat. Lanthanide ions have the potential to reduce the spectral mismatch losses. This can be achieved via downconversion, where one high energy photon is ‘cut’ to obtain two low energy photons, or upconversion, where two low energy photons are ‘added’ to obtain one photon with a higher energy. Theoretical models show that the Shockley-Queisser limit for the efficiency of a single junction solar cell can be raised from 30% to 40% (using downconversion) or even 50% (using upconversion). The unique and rich energy levels structure of lanthanides make efficient up- and downconversion possible through energy transfer processes between lanthanide ions. The Yb3+ ion serves as an ideal acceptor in lanthanide ion couples for downconversion of one visible to two NIR photons: it has a single excited state, just above the bandgap of crystalline Si, the material that dominates the solar cell market. Examination of the energy level diagrams of the lanthanides (the Dieke diagram) shows that (Er3+, Yb3+), (Nd3+, Yb3+), (Ho3+, Yb3+) and (Pr3+, Yb3+) are potential couples for efficient downconversion via resonant two-step energy transfer. (Tm3+, Yb3+) and (Tb3+, Yb3+) are potential couples for downconversion via cooperative or accretive transfer mechanisms. With some of the lanthanide ion couples mentioned visible to NIR conversion effiencies of over 100% have been achieved. The choice of host material has a large influence on the efficiency of downconversion, both by influencing the distribution of the lanthanide ions, as well as by the maximum phonon energy of the lattice. The (Pr3+, Yb3+) downconversion couple has a visible to NIR conversion efficiency up to 140% when doped into SrF2, while in YF3 the efficiency is well below 100%. For the (Er3+, Yb3+) and (Nd3+, Yb3+) couples downconversion is hampered by fast multi-phonon relaxation in host materials where the phonon energy is to high. However, efficient downconversion takes place in CsCdBr3 doped with (Nd3+, Yb3+). The observation of efficient downconversion forms the basis for the development of downconversion materials based on the combinations of host lattice/lanthanide ion couple identified in this work.