Description: Firefly, in collaboration with Rochester Institute of Technology, proposes developing a space solar cell having record efficiency exceeding 40% (AM0) by the introduction of nanowires within the active region of the current limiting sub-cell. The introduction of these nanoscale features will enable realization of an intermediate band solar cell (IBSC), while simultaneously increasing the effective absorption volume that can otherwise limit short-circuit current generated by thin quantized layers. The triple junction cell follows conventional designs comprised of bottom Ge cell (0.67eV), a current-limiting middle GaAs (1.43eV) cell, and a top InGaP (1.90eV) cell. The GaAs cell will be modified to contain InAs nanowires to enable an IBSC, which is predicted to demonstrate ~45% efficiency under 1-sun AM0 conditions. The InAs nanowires will be implemented in-situ within the epitaxy environment, which is a significant innovation relative to conventional semiconductor nanowire generation using ex-situ gold nanoparticles. Successful completion of the proposed work will result in ultra-high efficiency, radiation-tolerant space solar cells that are compatible with existing manufacturing processes. Significant cost savings are expected with higher efficiency cells, enabling increased payload capability and longer mission durations.

Benefits: The high-efficiency of the proposed device represents a significant competitive advantage for any space-based power generation application. This proposed device offers a significant increase in efficiency which corresponds to a significant cost savings in terms of photovoltaic array size, array weight, and launch costs. The proposed cells would represent a disruptive technology within the space photovoltaic marketplace. Additionally, a path is proposed for commercial, low-cost nanowire growth with broad market implications. Nanowires of highly mismatched systems (>7%) have been demonstrated in the literature and, more importantly, in the Phase 1 of this proposal. This flexibility in substrate/nanowire combinations enables more optimum bandgap and material combinations for novel devices. One exciting possibility is the integration of III-V nanostructures on low-cost silicon substrates for photovoltaic applications. In addition, the use of core-shell geometry for photovoltaic applications decouples the absorption length from the carrier collection length, which allows low diffusion length material to be effective PV materials in the core-shell configuration.

The high efficiency (>40%) of the proposed PV cell will make it the obvious choice for NASA space-based applications. Another potential application revisits a NASA research thrust on virtual substrates. One important aspect of nanowires is the demonstrated capability to integrate widely mismatched nanowires and substrates. The restricted cross-sectional area of the nanowire reduces the opportunity for mismatch defect generation. Nanowires of highly mismatched systems (>7%) have been demonstrated in the literature and, more importantly, in the Phase 1 of this proposal. This flexibility in substrate/nanowire combinations can enable more optimum bandgap and material combinations for novel devices. Incorporating nanowires onto a recrystallized Ge/metal foil substrate would potentially solve the problem of grain boundary shunting of generated carriers by restricting the cross-sectional area of the nanowire (10s of nms diameter) to sizes smaller than the recrystallized grains (0.5-1 um2). In this approach, the nanowire PV device integrated with a low-cost foil substrate would have potential for high weight-specific power (W/kg).