Below Band-Gap IR Response of Substrate-Free GaAs Solar Cells Using Two-Photon Up-Conversion

We have developed a device based upon the concept of two-photon up-conversion to use a part of the IR photons otherwise lost by transparency in a GaAs cell. An ultra thin GaAs cell has been fabricated using the technique of epitaxial lift-off (ELO). This thin cell is placed on top of a 100 µ m thick vitroceramic doped with Yb³⁺ and Er³⁺. The two photon upconversion process involved here is based on sequential absorption and energy transfer of two IR photons from Yb³⁺ to Er³⁺, which then emit one photon in the green. This green light then produces a photoresponse in the GaAs cell. This cell coupled to the vitroceramic was lighted by an Ti-sapphire IR laser at 1.391 eV, a photon energy below the band gap of GaAs, with an input power able to reach ∼1 W. The GaAs cell photoresponse increases quadratically with the input excitation. For an input excitation of 1 W at 1.39 eV on a 0.039 cm² substrate-free GaAs cell, the measured efficiency was 2.5%.