Up-conversion for crystalline silicon photovoltaics: Realistic efficiency limits and enhancement in photonic structures

Up-conversion photovoltaic (UC-PV) devices have been proposed as a simple approach to overcoming the 33% efficiency limit that applies to conventional crystalline silicon solar cells. Existing solar cells may be easily adapted into UC-PV devices by the application of a luminescent material to the rear side. This material absorbs long-wavelength photons to which the solar cell is transparent and converts them to short-wavelength photons that may be absorbed by the cell. Detailed-balance studies report a potential 8% increase in the absolute efficiency of c-Si solar cells when coupled to ideal UC layers. However, real Si solar cells are non-ideal optical absorbers. We propose a detailed-balance model of a realistic Si solar cell. The revised model demonstrates that while peak efficiencies are suppressed and optimum UC layer parameters are altered, the impact of ideal UC on an appropriately-designed solarcell is unchanged.Common UC materials for the c-Si spectral region rely on two-step absorption processes in trivalent erbium (Er). We emphasise that while Er-doped phosphors are efficient photon converters with suitable spectral lines, the Er absorption cross-section is low and its absorption width is narrow. A non-ideal UC layer model is incorporated into the detailed balance analysis. Narrow Er absorption is shown to severely depress UC-PV device efficiency, illustrating that efficiencies much in excess of the single-junction limit are unlikely to be observed with Er-doped phosphors alone. Sensitising Er absorption by co-doping with broad-band absorbers is shown to have a considerable benefit, as would absorption enhancement, e.g., by increasing the optical path length in the phosphor. A means of increasing absorption in Er is examined in a prototype materialEr-doped porous Si (PSi:Er)by structuring it into a one-dimensional photonic crystal. Extraordinary absorption may occur in such structures near the edges of photonic band gaps due to low energy propagation velocities. This effect is simulated: a 22% average increase in the effective absorption coefficient is expected over the broad Er absorption band. An experimental assessment produces enhanced luminescence under laser excitation near the edge of the photonic band gap. We draw preliminary conclusions and suggest future directions of research for UC enhancement of c-Si solar cells