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efficiency for a two-junction cell. Most of the previous work in such a structure focused on III-V top cells that were lattice-mismatched to the silicon bottom cell. Under the best conditions, this leads to a high density of threading dislocations in the III-V top cell. This dislocation density decreases the electronic quality of the top cell to the point that a lattice-matched tandem solar cell with a less optimal bandgap combination, such as GaInP/GaAs, is much more efficient. The III-V alloy GaNxP1-x-y Asy (GaNPAs) is a direct-gap that can be grown lattice-matched to Si with very low structural defect densities. We have proposed the use of lattice-matched GaNPAs on silicon for high- efficiency multijunction solar cells. During this year, we have fabricated this monolithic III-V-on- silicon tandem solar cell. The cell is functionally comprised of an n-on-p silicon bottom homojunction, III-V tunnel junction, and 1.8-eV n-i-p GaNPAs top junction. The epitaxial portions of the structure are composed mainly of GaNPAs and GaNP layers that are lattice-matched to the single-crystal silicon. Fig. 4. Composite spectral absolute external quantum efficiency (solid lines) and spectral reflectance (dotted line) data for an ultra-thin, handle-mounted GaInP/GaAs/GaInAs series- connected tandem solar cell. The electrical quality of GaNP(As) is highly dependent on growth conditions. In particular, growth conditions that minimize unintentional carbon and hydrogen contamination provide the highest carrier lifetimes. Thus, the GaNPAs top junction was grown at 700°C and a growth rate of 1 μm/h. Similar to the case of most GaInNAs Photovoltaic R&D Fundamental Research material, GaNPAs grown to date appears to have very short diffusion lengths, even when special care is taken to minimize H and C contamination. Figure 5 shows how the spectral response of GaN0.02P0.98 grown on silicon increases dramatically with the depletion width. Thus, an n-i-p device employing field-aided collection was used to maximize quantum efficiency (QE). Unlike GaInNAs material grown by metal organic chemical vapor deposition, intrinsic GaNPAs has relatively low carrier concentrations so that high QEs can be achieved in an n-i-p structure. Fig. 5. Spectral response of GaN0.02P0.98 grown on silicon for a variety of n-type doping levels. The fit implies short diffusion lengths. We have demonstrated the first lattice-matched III-V-on-silicon tandem solar cell. It reached only a Voc of 1.53 V, FF of 54%, and Jsc of 6.3 mA/cm2, resulting in an AM1.5G efficiency of 5.2% without any AR coatings. For such a tandem solar cell to achieve efficiencies greater than current state-of- the-art silicon cells and compete with GaInP/GaAs/Ge solar cells, the performance of the top GaNPAs junction and tunnel junction must be further improved. These improvements will require better control over nucleation of III-V on silicon substates, doping of GaP and GaNP(As) materials, and material quality of GaNP(As). 4. Planned FY 2006 Activities During FY 2006, much of our attention will be focused on relocating facilities and building new tools in the new Science and Technology Facility. This is an important milestone for the PV Subprogram, requiring that most of the research 20PDF Image | DOE Solar Energy Technologies Program
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