RESUMEN
Silicon based multi-junction solar cells are a promising approach for achieving high power conversion efficiencies using relatively low-cost substrates. In recent years, 2-terminal triple-junction solar cells using GaInP/GaAs as top cells and Si bottom cell have achieved excellent efficiencies. Epitaxial growth or wafer bonding has been used for the integration of the cells. This requires the top surface of the Si cell to be polished for effective integration, sacrificing the light trapping in the Si cell. The poor long wavelength light absorption in silicon limits the tandem cell efficiency as it is limited by current mismatch. In this work, for the first time, an external surface texturing is attached onto a GaInP/GaAs//Si wafer bonded triple-junction solar cell, using polydimethylsiloxane (PDMS) layers with surface geometries replicated from various pyramidally-textured silicon wafers. With reduced reflection, the short circuit current density is increased by 0.95 mA/cm2, while the overall cell efficiency is boosted by more than 2 % absolute.
RESUMEN
We investigate photon recycling at the top subcell in mechanically stacked multi-junction solar cells with nanometer air gaps between the subcells. We determine the incident-angle-dependence of the reflectivity from the rear surface of the top subcell. The results show that more than 30% of the luminescence at the top subcell is reflected at the air gap even for an air gap thickness of 10 nm. In addition, we demonstrate enhanced luminescence extraction in GaAs//InGaAsP dual-junction devices with nanometer air gaps compared to a device with no gap between the subcells. Our findings indicate that an efficient photon recycling can be realized even for air gaps of a few tens of nanometers.
RESUMEN
This paper describes the way to fabricate two-terminal tandem solar cells using Si heterojunction (SHJ) bottom cells and GaAs-relevant III-V top cells by "smart stack", an approach enabling the series connection of dissimilar solar cells through Pd nanoparticle (NP) arrays. It was suggested that placing the Pd NP arrays directly on typical SHJ cells results in poor tandem performance because of the insufficient electrical contacts and/or deteriorated passivation quality of the SHJ cells. Therefore, hydrogenated nanocrystalline Si (nc-Si:H) layers were introduced between Pd NPs and SHJ cells to improve the electrical contacts and preserve the passivation quality. Such nc-Si:H-capped SHJ cells were integrated with InGaP/AlGaAs double-junction cells, and a certified efficiency of 27.4% (under AM 1.5 G) was achieved. In addition, this paper addresses detailed analyses of the 27.4% cell. It was revealed that the cell had a relatively large gap at the smart stack interface, which limited the short-circuit current density (thereby the efficiency) of the cell. Therefore, higher efficiency would be expected by reducing the interfacial gap distance, which is governed by the height of the Pd NPs.
RESUMEN
Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it's optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72 mA cm(-2) under 1 sun illumination at 1.23 V(RHE) that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56 mA cm(-2) that corresponds to the solar to hydrogen generation efficiency of 8.1%.