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 (NIR) photons. Here the efficiency of downconversion for the (Er3+,Yb3+) couple is investigated in NaYF4, a well-known host lattice for efficient upconversion with (Er3+,Yb3+). Analysis of the excitation and emission spectra for NaYF4 doped with 1% Er3+ and codoped with 0%, 5%, 10%, or 30% Yb3+ 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 F47/2 level (the starting level for downconversion) to the S43/2 level. Energy transfer from the S43/2 level of Er3+ to Yb3+ is shown to be inefficient. Efficient downconversion from the G411/2 of Er3+ level is observed, resulting in emission of two photons (one around 980 nm and one around 650 nm) after absorption of a single 380 nm photon.

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