Nanostructured quantum well and quantum dot III–V solar cells provide a pathway to implement advanced single-junction photovoltaic device designs that can capture energy typically lost in traditional solar cells. To realize such high-efficiency single-junction devices, nanostructured device designs must be developed that maximize the open circuit voltage by minimizing both non-radiative and radiative components of the diode dark current. In this work, a study of the impact of barrier thickness in strained multiple quantum well solar cell structures suggests that apparent radiative efficiency is suppressed, and the collection efficiency is enhanced, at a quantum well barrier thickness of 4 nm or less. The observed changes in measured infrared external quantum efficiency and relative luminescence intensity in these thin barrier structures is attributed to increased wavefunction coupling and enhanced carrier transport across the quantum well region typically associated with the formation of a superlattice under a built-in field. In describing these effects, a high efficiency (>26% AM1.5) single-junction quantum well solar cell is demonstrated in a device structure employing both a strained superlattice and a heterojunction emitter.

We present a low-cost and scalable approach for the synthesis of wafer-scale InAs nanowire (NW) arrays on photolithographically patterned, reusable Si wafers using a localized selfassembly (LSA) epitaxial growth technique. Conventional i-line lithography is used to define arrays of 500 nm diameter pores through 50 nm thick SiO 2 layers, which serve as the LSA mask. A two-step, flowrate-modulated growth sequence is implemented to optimize selective-area self-assembly of NW arrays with over 80% yield and excellent control over the placement of one NW, with a mean diameter of 130 nm, inside each 500 nm pore. As-grown NW arrays are delaminated from the growth substrate, enabling fabrication of flexible membrane devices as well as reuse of Si wafers and growth masks while preserving the template pattern fidelity. Reuse of Si substrates for III−V epitaxy is demonstrated with and without pre-growth substrate restoration treatments. In both cases, the yield of NWs on reused wafers is comparable to that achieved in the original growth run. Without substrate restoration procedures, the remnant base segments of NWs on parent wafers act as preferential sites for regrowth of vertical NWs. Transmission electron microscopy analysis reveals that the InAs lattice is coherently extended from the remnant NW base segments during regrowth. The delaminated InAs NW arrays are transferred to carrier wafers for the fabrication of substrate-free photodetectors through the use of an anchoring procedure, which preserves the original NW position and orientation. Under broadband illumination, the NW array-based photodetectors produce a photo-to-dark current ratio of 10 2 , demonstrating the utility of the fabrication procedure employed. This work establishes a low-cost route toward III−V semiconductor-based flexible optoelectronics via LSA epitaxial growth of NW arrays on reusable Si wafers.

We demonstrate the fabrication and characterization of high-efficiency, singlejunction p-in GaAs solar cells, on flexible metal foil with epi-ready buffer via roll-toroll fabrication. Single-junction p-in GaAs solar cells were fabricated using metalorganic chemical vapor deposition (MOCVD). An efficiency greater than 13% was obtained at 1 sun, which is the highest reported efficiency on GaAs photovoltaics directly deposited on metal tapes. This exceeds our previously reported study showcasing 11.5% efficiency on single-junction p-n solar cell structure. Improved morphology of p-in structure compared with p-n is explained by atomic force microscopy (AFM), scanning electron microscopy (SEM), and helium ion microscopy (HIM) measurements. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis showed reduced Zn diffusion in p-in cell compared with the p-n cell. We attribute the improvement in efficiency of p-in cells on flexible metal tapes to the quality of the junction, surface morphology, controlled diffusion of species within the active layers, and increase in absorption due to the optimized intrinsic layer thickness.

Intermediate band solar cells promise improved efficiencies beyond the Shockley-Queisser limit by utilizing an intermediate band formed within the bandgap of a single junction solar cell. InP quantum dots (QDs) in an In0.49Ga0.51P host are a promising material system for this application, but two-step photon absorption has not yet been demonstrated. InP QDs were grown via metalorganic chemical vapor deposition, and a density, a diameter, and a height of 0.7 × 1010 cm−2, 56 ± 10 nm, and 18 ± 2.8 nm, respectively, were achieved. Time-resolved photoluminescence measurements show a long carrier lifetime of 240 ns, indicating a type-II band alignment of these InP quantum dots. Several n-i-p In0.49Ga0.51P solar cells were grown with both 3 and 5 layers of InP QDs in the i-region. While the solar cells showed an overall loss in short circuit current compared to reference cells due to emitter degradation, a sub-bandgap enhancement of 0.11 mA/cm2 was clearly observed, due to absorption and collection from the InP QDs. Finally, two-step photon absorption experiments have shown unambiguous photocurrent generation involving an intermediate band within the bandgap at temperatures up to 250 K.

Self-assembly of vertically aligned III-V semiconductor nanowires (NWs) on two-dimensional (2D) van der Waals (vdW) nanomaterials allows for integration of novel mixed-dimensional nanosystems with unique properties for optoelectronic and nanoelectronic...