Bifacial n-type silicon solar cells for upconversion applications

Upconversion of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly, but requires modification of the solar cells. In this paper, we present a calculation framework to assess the efficiency of a combined bifacial silicon solar cell upconverter device, which is then used to optimize the solar cell׳s front and rear side anti-reflection coatings. Our calculations show that an upconverter can increase the efficiency of an optimized solar cell by 3.0% relative. Subsequently, planar bifacial n-type silicon solar cells were fabricated with optimized anti-reflection coatings. An upconversion layer – containing the upconverter phosphor β-NaY_0.8Er_0.2F₄ embedded in the polymer perfluorocyclobutyl – was attached to the rear side of the solar cells and an external quantum efficiency arising from the upconversion of sub-band-gap photons of 1.69% was measured under 1508 nm monochromatic excitation with an irradiance of 1091 W/m². This corresponds to a value of 0.15 (W/cm²)⁻¹ when normalized to the irradiance, constituting a five-fold increase compared to the previously best published normalized values that were achieved without optimized solar cells.