GaAs Nanowire Array Solar Cells Research Articles

Inverse design of GaAs nanowire array solar cell structures with nonuniform diameters

Non-uniform diameter GaAs nanowire array solar cell structures are designed using the particle swarm optimized inverse design method. The results show that unlike the uniform diameter structure, which has a series of absorption valleys, the absorption spectra of the non-uniform structures are much flatter, which is attributed to a combination of absorption spectra from nanowires of different diameters. For the same height, the optimized structure with non-uniform diameter yields a remarkable ultimate efficiency of 34.3%, 1.1% higher than that the structure with uniform diameter, mainly due to the enhanced absorption at long wavelengths. For the same volume, an even higher ultimate efficiency of 34.6% is obtained, due to the enhanced absorption over almost the entire wavelength range. This work could pave the way for the design of low-cost high-efficiency nanoscale solar cells and other photoelectric conversion devices.

GaAs/AlGaAs Nanowire Array Solar Cell Grown on Si with Ultrahigh Power-per-Weight Ratio

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.

Performance Enhancement of Ultra-Thin Nanowire Array Solar Cells by Bottom Reflectivity Engineering.

A bottom-reflectivity-enhanced ultra-thin nanowire array solar cell is proposed and studied by 3D optoelectronic simulations. By inserting a small-index MgF2 layer between the polymer and substrate, the absorption is significantly improved over a broad wavelength range due to the strong reabsorption of light reflected at the polymer/MgF2 interface. With a 5 nm-thick MgF2 layer, the GaAs nanowire array solar cell with a height of 0.4–1 μm yields a remarkable conversion efficiency ranging from 14% to 15.6%, significantly higher than conventional structures with a much larger height. Moreover, by inserting the MgF2 layer between the substrate and a part of the nanowire, in addition to between the substrate and polymer, the absorption of substrate right below the nanowire is further suppressed, leading to an optimal efficiency of 15.9%, 18%, and 5.4% for 1 μm-high GaAs, InP, and Si nanowire solar cells, respectively. This work provides a simple and universal way to achieve low-cost high-performance nanoscale solar cells.

An Optimized Structure for Enhancing Optical Absorption of Solar Energy in Elliptical GaAs Nanowire Array Solar Cell

The enhanced optical absorption of solar energy in a solar cell using nanostructured materials is a very demanding and important research area. Hence, an optimized structure to increase the optical absorption of solar energy in an elliptical GaAs (Gallium Arsenide) nanowire array solar cell was proposed in this paper. The influence of geometric parameters on the optical absorption of elliptical GaAs nanowire solar cells was investigated by exploiting the finite difference in time domain simulations. Based on the design and analysis, the elliptical GaAs nanowire array performed better than any circular GaAs nanowire. It was found that the enhanced performance was due to the reduced reflection from the top surface and a reduced transmission from the bottom surface of the elliptical GaAs nanowire array. The structural parameters of the elliptical GaAs nanowire were optimized by calculating the short circuit current density (Jsc) for different geometries over the solar spectrum. It was demonstrated that there was an increase of 9.4% in the Jsc compared with the circular GaAs nanowire under the optimized conditions.

Evolution of microstructures during rapid crystallization of liquid GaAs

The technological importance of compound semiconductor GaAs are increasing because of their use in optoelectronic and microelectronic applications. Due to the high conversion efficiency and carrier mobility, GaAs can also be applied in solar cells and the recent study upon GaAs nanowires and their heterostructures has revealed that the conversion efficiency of GaAs nanowire array solar cells conversion is high up to 15.3%. Early the liquid and amorphous properties of GaAs were investigated by employing the first-principles calculations. The emergence of semi-empirical potential and the improvement of computer level have promoted the research and application of molecular dynamics (MD) simulation. MD simulation has now become one of the typical modeling methods at the molecular scale. The simulation is based on the known physical approximation of all particles in the system to solve the equation of motion, and obtain the atomic motion trajectory. Analytical potentials is very important in MD simulation as it is not feasible to solve the Hamiltonian by means of quantum-mechanical methods with huge computational complexity. Abell-Tersoff potential function is a short-ranged bond-order algorithm, which depends on bond lengths and bond angles and hence accesses information about the atomic structure. So it is suitable for simulating covalent bond species. Generally used for the IV elements and compounds like silicon, carbon, and others, but for the III-V compound semiconductor it is not very accurate due to the ionic bonds. Usually the modified tersoff potential, by the addition of Coulomb term, the modified exclusion potential and the truncation parameter, is used to simulate such semiconductor materials. Many studies on the bulk, surface and elastic properties of GaAs by means of MD method, are in good agreement with the experimental results. In this paper Karsten Albe’s Tersoff potential is adopted as it allows one to model a wide range of properties of GaAs compound structure. GaAs has two kinds of tetrahedral crystal structure, namely, Zinc-blende and Wurtzite, the former structure is more stable under normal conditions. But when reduced to a nanoscale scale, Wurtzite structure becomes stable. Different structures have distinct properties, similar to carbon and grapheme. But so far, there is no report on the evolution of the microstructure and the specific crystalline structure of GaAs during crystallization under rapid cooling. In this study, MD simulation was performed for liquid GaAs at the cooling rate 1×1010 K/s. The pair distribution function, the total energy per atom, the bond angle distribution function, the dihedral angle distribution and visualization method were used to analyze the variations of microstructure during the solidification process. Results show that the onset temperature of crystallization of GaAs liquid is 1460 K. The random network is the essential structural feature of liquid. The rapidly cooled crystallization is Zinc-blende based polycrystalline structure, with the grain boundary in a eutectic twin structure is a layer of wurtzite structure. At temperature below 520 K, part of As atoms segregate into simple cubic structure As8.

Performance of GaAs Nanowire Array Solar Cells for Varying Incidence Angles

Nanowire array solar cells show intrinsic light trapping and absorption enhancement properties due to the diffraction and optical resonances. Here, we report the effect of varying incidence angle on the performance of GaAs nanowire array solar cells. We provide experimental evidence that nanowire array solar cells are highly efficient at gathering diffuse or tilted incident light, even at very high incidence angles; hence, the performance of the nanowire solar cell is retained up to these high incident light angles. Specifically, the measured efficiency at an incidence angle of 60° is 95% of the efficiency at normal incidence. Moreover, our measurements show that a nonzero incidence angle is beneficial for wavelengths above 600 nm, which results in an efficiency improvement by 0.5% absolute points. This increase is so large that we even measure a small increase in absolute output power at 15° tilt, thus, more than fully compensating for the reduced incoming power over the cell with increasing tilt. We show how this gain arises from an enhanced absorption in the part of the nanowire with a high probability of charge extraction. Thus, nanowires show great promise for the delivery of high efficiency in practical nontracking positioning conditions, as well as under diffuse light illumination.

Characterization of a Ga-Assisted GaAs Nanowire Array Solar Cell on Si Substrate

A single-junction core-shell GaAs nanowire (NW) solar cell on Si (1 1 1) substrates is presented. A Ga-assisted vapor-liquid-solid growth mechanism was used for the formation of a patterned array of radial p-i-n GaAs NWs encapsulated in AlInP passivation. Novel device fabrication utilizing facet-dependent properties to minimize passivation layer removal for electrical contacting is demonstrated. Thorough electrical characterization and analysis of the cell is reported. The electrostatic potential distribution across the radial p-i-n junction GaAs NW is investigated by off-axis electron holography.

A GaAs Nanowire Array Solar Cell With 15.3% Efficiency at 1 Sun

A GaAs nanowire array solar cell with an independently verified solar energy conversion efficiency of 15.3% and open-circuit voltage of 0.906 V under AM1.5g solar illumination at 1-sun intensity has been fabricated. This is the highest published efficiency for nanowire array solar cells and is twice the prior record for GaAs nanowire array solar cells. The solar cell has been fabricated by substrate-based epitaxy but is structurally compatible with substrate-less aerotaxy fabrication, providing a path to high-volume manufacturing. The short-circuit current of 21.3 mA/cm2 was generated with axial p-n junction GaAs cores covering 13% of the surface area, which is a volume of GaAs equivalent to a 370-nm-thick planar layer.

GaAs纳米线阵列太阳能电池的设计与优化

GaAs纳米线阵列太阳能电池的设计与优化

Tandem Solar Cells Using GaAs Nanowires on Si: Design, Fabrication, and Observation of Voltage Addition

Multijunction solar cells provide us a viable approach to achieve efficiencies higher than the Shockley-Queisser limit. Due to their unique optical, electrical, and crystallographic features, semiconductor nanowires are good candidates to achieve monolithic integration of solar cell materials that are not lattice-matched. Here, we report the first realization of nanowire-on-Si tandem cells with the observation of voltage addition of the GaAs nanowire top cell and the Si bottom cell with an open circuit voltage of 0.956 V and an efficiency of 11.4%. Our simulation showed that the current-matching condition plays an important role in the overall efficiency. Furthermore, we characterized GaAs nanowire arrays grown on lattice-mismatched Si substrates and estimated the carrier density using photoluminescence. A low-resistance connecting junction was obtained using n(+)-GaAs/p(+)-Si heterojunction. Finally, we demonstrated tandem solar cells based on top GaAs nanowire array solar cells grown on bottom planar Si solar cells. The reported nanowire-on-Si tandem cell opens up great opportunities for high-efficiency, low-cost multijunction solar cells.

Influence of Electrical Design on Core–Shell GaAs Nanowire Array Solar Cells

In this paper, we perform coupled optical and electrical simulation for core–shell junction GaAs nanowire array solar cells to obtain the important physical insights of how the cell efficiency is affected by various key parameters, including core–shell doping, junction position, carrier lifetime, and surface effect. Our study reveals that junction design in core–shell nanowires in terms of doping and geometry is largely restricted and affected by the small nanowire dimension, requiring different optimization from those of conventional planar solar cells. To take advantage of the radial p-n junction, high core and shell doping are essential to achieve effective radial carrier collection. Moreover, maintaining thin nanowire shell could effectively offset the detrimental surface effect for improved efficiency.

GaAs nanowire array solar cells with axial p-i-n junctions.

Because of unique structural, optical, and electrical properties, solar cells based on semiconductor nanowires are a rapidly evolving scientific enterprise. Various approaches employing III-V nanowires have emerged, among which GaAs, especially, is under intense research and development. Most reported GaAs nanowire solar cells form p-n junctions in the radial direction; however, nanowires using axial junction may enable the attainment of high open circuit voltage (Voc) and integration into multijunction solar cells. Here, we report GaAs nanowire solar cells with axial p-i-n junctions that achieve 7.58% efficiency. Simulations show that axial junctions are more tolerant to doping variation than radial junctions and lead to higher Voc under certain conditions. We further study the effect of wire diameter and junction depth using electrical characterization and cathodoluminescence. The results show that large diameter and shallow junctions are essential for a high extraction efficiency. Our approach opens up great opportunity for future low-cost, high-efficiency photovoltaics.

Theoretical analysis and modeling of light trapping in high efficicency GaAs nanowire array solar cells

A rigorous analysis and design of efficient light trapping for Gallium arsenide nanowire array (NWA) solar cells is presented. The influence of the geometric parameters on the optical absorption of NWA has been thoroughly analyzed by using finite-difference time-domain simulations. It is found that the NWA have superior absorption characteristic over thin-film due to the combined effects of intrinsic anti-reflection and efficient excitation of resonant modes. Optimized optical designs are obtained by maximizing the ultimate photocurrent under AM1.5G illumination. The photogeneration profiles under optimized conditions are incorporated into electrical modeling, in which the core-shell NWA solar cells exhibit 22.3% efficiency.