Katsuaki Tanabe

Plasmonic nanoparticle enhanced light absorption in GaAs solar cells

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.

Direct-bonded GaAs∕InGaAs tandem solar cell

A direct-bonded GaAs/InGaAs 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+GaAs/n+InP 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.

High efficiency InGaAs solar cells on Si by InP layer transfer

InP/Si substrates were fabricated through wafer bonding and helium-induced exfoliation of InP, and InGaAs solar cells lattice matched to bulk InP were grown on these substrates using metal-organic chemical-vapor deposition. The photovoltaic characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epiready InP substrates, thus providing a demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.

Improved electrical properties of wafer-bonded p-GaAs/n-InP interfaces with sulfide passivation

Sulfide-passivated GaAs and InP wafers were directly bonded to explore the efficiency of sulfide passivation on the bonded interfacial properties. We find that the bonded GaAs/InP interfaces after sulfide passivation contain sulfur atoms and a decreased amount of oxide species relative to the pairs bonded after conventional acid treatment; however, the residual sulfur atoms have no effect on the bonding strength. The electrical properties of the bonded p-GaAs/n-InP heterojunctions were studied for different acceptor concentrations in p-GaAs. A reduced interfacial trap state density enhances the tunnel current flow across the depletion layer in the sulfide-passivated case. A directly bonded tunnel diode with a heavily doped p-GaAs/n-InP heterojunction was achieved when the wafers were sulfide passivated and then bonded at temperatures as low as 300 °C. This sulfide-passivated tunnel diode can be used for fabrication of lattice-mismatched multijunction solar cells in which subcells are integrated via direct bonding.

Lattice-Mismatched Monolithic GAAS/INGAAS Two-Junction Solar Cells by Direct Wafer Bonding

Direct bonded interconnect between subcells of a lattice-mismatched III-V compound multijunction cell would enable dislocation-free active regions by confining the defect network needed for lattice mismatch accommodation to tunnel junction interfaces, while metamorphic growth inevitably results in less design flexibility and lower material quality than is desirable. The first direct-bond interconnected multijunction solar cell, a two-terminal monolithic GaAs/InGaAs two-junction solar cell, is reported and demonstrates viability of direct wafer bonding for solar cell applications. The tandem cell open-circuit voltage was approximately the sum of the subcell open-circuit voltages. This achievement shows direct bonding enables us to construct lattice-mismatched III-V multijunction solar cells and is extensible to an ultrahigh efficiency InGaP/GaAs/InGaAsP/InGaAs four-junction cell by bonding a GaAs-based lattice-matched InGaP/GaAs subcell and an InP-based lattice-matched InGaAsP/InGaAs subcell. The interfacial resistance experimentally obtained for bonded GaAs/InP smaller than 0.10 Ohm-cm2 would result in a negligible decrease in overall cell efficiency of ~0.02%, under 1-sun illumination

Surface plasmon enhanced photocurrent in thin GaAs solar cells

We report fabrication of size-controlled plasmonic nanoparticle arrays by which optically thin GaAs single junction solar cells are decorated. Ordered Ag and Al nanoparticles with average diameters of 60-150 nm and interparticle spacings of 100-300 nm were templated onto the window layers of the GaAs solar cells using nanoporous anodic aluminum oxide membrane templates. Near the surface plasmon resonances, 60nm-diameter Ag and Al nanoparticles serve as light-absorbers so that non-radiative surface plasmon resonances reduce the photocurrent of the cells, which is improved by increasing the nanoparticle size. Photocurrent enhancements are seen at wavelengths longer than surface plasmon resonance which is maximized near the band gap edge of GaAs. These enhancements can be attributed to the increased optical path in the photovoltaic layers resulting from multi-angle scattering by the nanoparticles, while high scattering efficiency nanoparticles in turn increase the back scattering light out of the cell reducing the photocurrent.

Plasmon-enhanced absorption and photocurrent in ultrathin GaAs solar cells with metallic nanostructures

Metallic nanostructures can excite surface plasmons and can dramatically increase the optical path length in thin active photovoltaic layers to enhance overall photoabsorption. This effect has potential for cost and weight reduction with thinned layers and also for efficiency enhancement associated with increased carrier excitation level in the absorber layer.

Photocurrent Enhancement in In0.53Ga0.47As Solar Cells Grown on InP/SiO2/Si Transferred Epitaxial Templates

InP/Si engineered substrates formed by wafer bonding and layer transfer have the potential to significantly reduce the cost and weight of III-V compound semiconductor solar cells. InP/Si substrates were prepared by He implantation of InP prior to bonding to a thermally oxidized Si substrate and annealing to exfoliate an InP thin film. Following thinning of the transferred InP film to remove surface damage caused by the implantation and exfoliation process, InGaAs solar cells lattice-matched to bulk InP were grown on these substrates using metal-organic chemical vapor deposition. The photovoltaic current-voltage characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epi-ready InP substrates, and had a ~20% higher short-circuit current which we attribute to the high reflectivity of the InP/SiO2/Si bonding interface. This work provides an initial demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.