Up Conversion for Photovoltaics
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Up Conversion for Photovoltaics Gavin Conibeer1, Avi Shalav2, Thorsten Trupke1, and Martin Green1 1 ARC Photovoltaics Centre of Excellence, University of New South Wales, UNSW, Sydney, NSW 2052, Sydney, 2052, Australia 2 Australian National University, The Research School of Physical Sciences and Engineering, Building 60, Canberra, 0200, Australia ABSTRACT An up-converter (UC) absorbs two or more low-energy photons and emits a single high-energy photon. A down-converter (DC) absorbs a single high-energy photon and emits two or more low energy photons. The current work extends previous limiting efficiency analysis to a combination of UC and DC; an up converter with two levels; and to compare analyses using real air mass data with that modelling the sun as a blackbody. Analysis has been carried out both with the band gap of the cell as an optimized parameter and at a fixed value of 1.1eV. All of UC, DC and two level UC are shown to improve efficiencies for both spectra. Combined UC/DC improves the efficiency further for the 1.1eV band gap but gives a lower efficiency for the optimised band gap. The explanation for this unexpected result is presented based on the small coupling losses that result from absorption/re-emission in the DC. The limiting efficiencies of such an approach are very similar to several other third generation concepts such as impurity PV, Intermediate Band solar cells or three level tandems. However in practice the UC (or DC) approach has the advantage that the optical properties of the UC are decoupled from the electrical properties of the PV cell, and hence each can be optimised independently. This means that it may be the simplest third generation approach to implement using existing PV cells, if a reasonable UC efficiency can be obtained. Nonetheless experimental work on realising UC is at an early stage. Some of the work on rare earth doped UC is reviewed together with the potential to improve the spectral sensitivity to below band gap radiation. INTRODUCTION The application of up- or down-converters to photovoltaics is clear, sub-band-gap photons, which are normally not absorbed, can be up-converted to above band gap photons and be absorbed by the cell. Alternatively, a very high-energy photon can be down converted to two photons at energies close to that of the band gap, thereby creating two photons that can produce electron hole pairs upon absorption, thus reducing lattice thermalisation losses. It has been shown theoretically by some of the current authors that both up [1] and down [2] conversion of sunlight can in principle increase the conversion efficiency of sunlight. In the first section we extend the theoretical analysis to a combination of up and down converters on a single device and to an up-converter with two intermediate levels. In the second section we concentrate on experimental UC using rare earth doped phosphors.
THEORETICAL EFFICIENCIES The operation of a solar cell with a single level up- converter attached to the rear has been described in [1] and the oper
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