We demonstrate an improvement in efficiency of optically thin GaAs solar cells decorated with size-controlled Ag nanoparticles fabricated by masked deposition through anodic aluminum oxide templates. The strong scattering by the interacting surface plasmons in densely formed high aspect-ratio nanoparticles effectively increases the optical path of the incident light in the absorber layers resulting in an 8% increase in the short circuit current density of the cell. The nanoparticle array sheet conductivity also reduces the cell surface sheet resistance evidenced by an improved fill factor. This dual function of plasmonic nanoparticles has potential to enable thinner photovoltaic layers in solar cells.

Monolithic integration of III-V compound semiconductors on silicon is highly sought after for high-speed, low-power-consumption silicon photonics and low-cost, light-weight photovoltaics. Here we present a GaAs/Si direct fusion bonding technique to provide highly conductive and transparent heterojunctions by heterointerfacial band engineering in relation to doping concentrations. Metal- and oxide-free GaAs/Si ohmic heterojunctions have been formed at 300°C; sufficiently low to inhibit active material degradation. We have demonstrated 1.3 μm InAs/GaAs quantum dot lasers on Si substrates with the lowest threshold current density of any laser on Si to date, and AlGaAs/Si dual-junction solar cells, by p-GaAs/p-Si and p-GaAs/n-Si bonding, respectively. Our direct semiconductor bonding technique opens up a new pathway for realizing ultrahigh efficiency multijunction solar cells with ideal bandgap combinations that are free from lattice-match restrictions required in conventional heteroepitaxy, as well as enabling the creation of novel high performance and practical optoelectronic devices by III-V/Si hybrid integration.

We report the fabrication of InAs/GaAs quantum dot solar cells (QDSCs) with enhanced photocurrent and no degradation in open circuit voltage $\left(V_{\text{OC}}\right)$ compared to a solar cell grown without QDs and composed solely of wetting layers. Notably, the achievement of such high $V_{\text{OC}}$ does not require electronic coupling. We report QDSCs with a light absorption range extended up to $1.3\mu \text{m}$ and evidence a trade-off between $V_{\text{OC}}$ and QD ground-state energy. These results are of major significance to the design of high efficiency QDSCs.

Solar cells are a promising renewable, carbon-free electric energy resource to address the fossil fuel shortage and global warming. Energy conversion efficiencies around 40% have been recently achieved in laboratories using III-V semiconductor compounds as photovoltaic materials. This article reviews the efforts and accomplishments made for higher efficiency III-V semiconductor compound solar cells, specifically with multijunction tandem, lower-dimensional, photonic up/down conversion, and plasmonic metallic structures. Technological strategies for further performance improvement from the most efficient (Al)InGaP/(In)GaAs/Ge triple-junction cells including the search for 1.0 eV bandgap semiconductors are discussed. Lower-dimensional systems such as quantum well and dot structures are being intensively studied to realize multiple exciton generation and multiple photon absorption to break the conventional efficiency limit. Implementation of plasmonic metallic nanostructures manipulating photonic energy flow directions to enhance sunlight absorption in thin photovoltaic semiconductor materials is also emerging.

A direct-bonded $\mathrm{Ga}\mathrm{As}∕\mathrm{In}\mathrm{Ga}\mathrm{As}$ solar cell is demonstrated. The direct-bonded interconnect between subcells of this two-junction cell enables monolithic interconnection without threading dislocations and planar defects that typically arise during lattice-mismatched epitaxial heterostructure growth. The bonded interface is a metal-free $n^{+}\mathrm{Ga}\mathrm{As}∕n^{+}\mathrm{In}\mathrm{P}$ tunnel junction. The tandem cell open-circuit voltage is approximately the sum of the subcell open-circuit voltages. The internal quantum efficiency is 0.8 for the GaAs subcell compared to 0.9 for an unbonded GaAs subcell near the band gap energy and is 0.7 for both of the InGaAs subcell and an unbonded InGaAs subcell, with bonded and unbonded subcells similar in spectral response.