A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1-yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 10(6) cm(-2). The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m(2), AM1.5d ASTM G173-03)

Triple-junction solar cells from III-V compound semiconductors have thus far delivered the highest solar-electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four-junction solar cell architectures with optimum bandgap combination are found for lattice-mismatched III-V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs-based top tandem solar cell structure was bonded to an InP-based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four-junction solar cell with a new record efficiency of 44.7% at 297-times concentration of the AM1.5d (ASTM G173-03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III-V multi-junction solar cells having four and in the future even more junctions.

Silicon is the predominant semiconductor in photovoltaics. However, the conversion efficiency of silicon single junction solar cells is intrinsically constrained to 29.4%, and practically limited around 27%. It is nonetheless possible to overcome this limit by combining silicon with high bandgap materials, such as III-V semiconductors, in a multi-junction device. Despite numerous studies tackling III-V/Si integration, the significant challenges associated with this material combination has hindered the development of highly efficient III-V/Si solar cells. Here we demonstrate for the first time a III-V/Si cell reaching similar performances than standard III-V/Ge triple-junctions solar cells. This device is fabricated using wafer bonding to permanently join a GaInP/GaAs top cell with a silicon bottom cell.The key issues of III-V/Si interface recombination and silicon weak absorption are addressed using polysilicon/SiOx passivating contacts and a novel rear side diffraction grating for the silicon bottom cell. With these combined features, we demonstrate a 2-terminal GaInP/GaAs//Si solar cell reaching a 1sun AM1.5g conversion efficiency of 33.3%.

Improving costs and scale reflect looming opportunities

Procedures for measuring the spectral response of multi-junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non-ideal properties, such as a low shunt resistance or a low reverse breakdown voltage. This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple-junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi-junction cells is developed, specially designed to minimize such measurement artifacts