Modeling free-carrier absorption in ultrathin III-V solar cells with light management.

Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x1019 cm-3. Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x1019 cm-3, respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.

Mixed-dimensional InAs nanowire on layered molybdenum disulfide heterostructures via selective-area van der Waals epitaxy.

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 device applications. Here, selective-area vdW epitaxy (SA-vdWE) of InAs NWs on isolated 2D molybdenum disulfide (MoS2) domains is reported for the first time. The MOCVD growth parameter space (i.e., V/III ratio, growth temperature, and total molar flow rates of metalorganic and hydride precursors) is explored to achieve pattern-free positioning of single NWs on isolated multi-layer MoS2 micro-plates with one-to-one NW-to-MoS2 domain placement. The introduction of a pre-growth poly-l-lysine surface treatment is highlighted as a necessary step for mitigation of InAs nucleation along the edges of triangular MoS2 domains and for NW growth along the interior region of 2D micro-plates. Analysis of NW crystal structures formed under the optimal SA-vdWE condition revealed a disordered combination of wurtzite and zinc-blend phases. A transformation of the NW sidewall faceting structure is observed, resulting from simultaneous radial overgrowth during axial NW synthesis. A common lattice arrangement between axially-grown InAs NW core segments and MoS2 domains is described as the epitaxial basis for vertical NW growth. A model is proposed for a common InAs/MoS2 sub-lattice structure, consisting of three multiples of the cubic InAs unit cell along the [21̄1̄] direction, commensurately aligned with a 14-fold multiple of the Mo-Mo (or S-S) spacing along the [101̄0] direction of MoS2 hexagonal lattice. The SA-vdWE growth mode described here enables controlled hybrid integration of mixed-dimensional III-V-on-2D heterostructures as novel nanosystems for applications in optoelectronics, nanoelectronics, and quantum enabling technologies.

Design and Demonstration of High-Efficiency Quantum Well Solar Cells Employing Thin Strained Superlattices.

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.