Stephanie Essig

Fabrication of GaInP/GaAs//Si Solar Cells by Surface Activated Direct Wafer Bonding

GaInP/GaAs//Si solar cells with three active p-n junctions were fabricated by surface activated direct wafer bonding between GaAs and Si. The direct wafer bond is performed at room temperature and leads to a conductive and transparent interface. This allows the fabrication of high-efficiency monolithic tandem solar cells with active junctions in both Si and the III-V materials. This technology overcomes earlier challenges of III-V and Si integration caused by the large difference in lattice constant and thermal expansion. Transmission electron microscopy revealed a 5-nm thin amorphous interface layer formed by the argon fast atom beam treatment before bonding. No further defects or voids are detected in the photoactive layers. First triple-junction solar cell devices on Si reached an efficiency of 23.6% under concentrated illumination.

Phonon-assisted electroluminescence from metallic carbon nanotubes and graphene.

We report on light emission from biased metallic single-wall carbon nanotube (SWNT), multiwall carbon nanotube (MWNT) and few-layer graphene (FLG) devices. SWNT devices were assembled from tubes with different diameters in the range 0.7-1.5 nm. They emit light in the visible spectrum with peaks at 1.4 and 1.8 eV. Similar peaks are observed for MWNT and FLG devices. We propose that this light emission is due to phonon-assisted radiative decay from populated pi* band states at the M point to the Fermi level at the K point. Since for most carbon nanotubes as well as for graphene the energy of unoccupied states at the M point is close to 1.6 eV, the observation of two emission peaks at approximately 1.6 +/- approximately 0.2 eV could indicate radiative decay under emission or absorption of optical phonons, respectively.

Comparison of Direct Growth and Wafer Bonding for the Fabrication of GaInP/GaAs Dual-Junction Solar Cells on Silicon

Two different process technologies were investigated for the fabrication of high-efficiency GaInP/GaAs dual-junction solar cells on silicon: direct epitaxial growth and layer transfer combined with semiconductor wafer bonding. The intention of this research is to combine the advantages of high efficiencies in III-V tandem solar cells with the low cost of silicon. Direct epitaxial growth of a GaInP/GaAs dual-junction solar cell on a GaAsyP1-y buffer on silicon yielded a 1-sun efficiency of 16.4% (AM1.5g). Threading dislocations that result from the 4% lattice grading are still the main limitation to the device performance. In contrast, similar devices fabricated by semiconductor wafer bonding on n-type inactive Si reached efficiencies of 26.0% (AM1.5g) for a 4-cm2 solar cell device.

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

Wafer-Bonded GaInP/GaAs//Si Solar Cells With 30% Efficiency Under Concentrated Sunlight

Highly efficient III-V/Si triple-junction solar cells were realized by a fabrication process based on direct wafer bonding: Ga0.51In0.49P/GaAs dual-junction solar cells were grown inverted by metal organic vapor phase epitaxy on GaAs substrates and bonded to separately fabricated Si solar cells. The fast atom beam activated direct wafer bond between highly doped n-Si and n-GaAs enabled a transparent and electrically conductive interface. Challenges arising from the different thermal expansion coefficients of Si and the III-V semiconductors were circumvented, as the bonding was performed at moderate temperatures of 120 °C. The external quantum efficiency and current-voltage characteristics of the wafer-bonded triple-junction solar cells were thoroughly investigated, and a maximum efficiency of 30.0% was found for a concentration factor of 112.

Interdigitated Back Passivated Contact (IBPC) Solar Cells Formed by Ion Implantation

We describe work toward an interdigitated back passivated contact (IBPC) solar cell formed by patterned ionimplanted passivated contacts. Formation of electron and hole passivated contacts to n-type Cz wafers using a thin SiO₂ layer and ion-implanted amorphous silicon (a-Si) is described. P and B were ion implanted into intrinsic a-Si films, forming symmetric and IBPC test structures. The recombination parameter J₀, as measured by a Sinton lifetime tester after thermal annealing, was J₀ ~ 2.4 fA/cm² for Si:P and J₀ ~ 10 fA/cm² for Si:B contacts. The contact resistivity for the passivated contacts was found to be 0.46 Ω·cm² for the n-type contact and 0.04 Ω·cm² for the p-type contact. The IBPC solar cell test structure gave 1-sun V_oc values of 682 mV and pFF = 80%. The benefits of the ion-implanted IBPC cell structure are discussed.

III-V/Si wafer bonding using transparent, conductive oxide interlayers

We present a method for low temperature plasma-activated direct wafer bonding of III-V materials to Si using a transparent, conductive indium zinc oxide interlayer. The transparent, conductive oxide (TCO) layer provides excellent optical transmission as well as electrical conduction, suggesting suitability for Si/III-V hybrid devices including Si-based tandem solar cells. For bonding temperatures ranging from 100 °C to 350 °C, Ohmic behavior is observed in the sample stacks, with specific contact resistivity below 1 Ω cm2 for samples bonded at 200 °C. Optical absorption measurements show minimal parasitic light absorption, which is limited by the III-V interlayers necessary for Ohmic contact formation to TCOs. These results are promising for Ga0.5In0.5P/Si tandem solar cells operating at 1 sun or low concentration conditions.

Boosting the efficiency of III-V/Si tandem solar cells

We have developed Si-based tandem solar cells with a certified 1-sun efficiency of 29.8% (AM1.5g). The four-terminal tandem devices consist of 1.8 eV rear-heterojunction GaInP top cells and silicon heterojunction bottom cells. The two subcells were fabricated independently in two different labs and merged using an optically transparent, electrically insulating epoxy. Work is ongoing to further improve the performance of each subcell and to push the tandem cell efficiency to > 30%.

Indium zinc oxide mediated wafer bonding for III–V/Si tandem solar cells

Silicon-based tandem solar cells are desirable as a high efficiency, economically viable approach to one sun or low concentration photovoltaics. We present an approach to wafer bonded III-V/Si solar cells using amorphous indium zinc oxide (IZO) as an interlayer. We investigate the impact of a heavily doped III-V contact layer on the electrical and optical properties of bonded test samples, including the predicted impact on tandem cell performance. We present economic modeling which indicates that the path to commercial viability for bonded cells includes developing low-cost III-V growth and reducing constraints on material smoothness. If these challenges can be surmounted, bonded tandems on Si can be cost-competitive with incumbent PV technologies, especially in low concentration, single axis tracking systems.

Development of highly-efficient GaInP/Si Tandem Solar Cells

Dual-junction solar cells consisting of rear-heterojunction GaInP top cells and back-junction, back-contacted crystalline Si bottom cells were fabricated and characterized. Our calculations show that theoretical efficiencies up to 38.9% can be achieved with Si-based tandem devices. In our experiments, the two subcells were fabricated separately and stacked with an index matching fluid. In contrast to conventional mechanically stacked solar cells, that contain two metal grids at the interface, our concept includes a fully back contacted bottom cell which reduces the shadow losses in the device. A 1-sun AM1.5g cumulative efficiency of (26.2 ± 0.6)% has been achieved with this novel GaInP/Si 4-terminal tandem solar cell.