Radial Pn Junction Research Articles

Fabrication of Vertical Light-Emitting Diodes Using Wurtzite InP/Alinp Core-Multishell Nanowires Towards Yellow Wavelength

Light emitting-diodes (LEDs) using nanostructures has been attracted much attention in the field of next generation solid-state lighting technologies. At present, conventional LEDs are fabricated mainly using nitride-related materials in blue-green region and arsenide/phosphide-related materials in red region. However, both of those materials have issue in significant luminous efficiency drop in yellow wavelength region due to low crystal quality or indirect bandgap. Hence, developing of new candidate materials with high luminous efficiency in yellow region is urgent issue for high color rendering properties. Wurtzite (WZ) phosphide-related III-Vs nanowires (NWs) materials are expected to be the alternative candidate material because crystal phase transitioned WZ changes the band structure from indirect band gap to direct bandgap. Thus, WZ-AlInP alloy would possibly emit the bright yellow color light due to direct gap transition. All WZ InP/AlInP core-multishell (CMS) NWs have been reported in recent study [1]. However, the controllability of Al content for the WZ AlInP shell layer and the performance of WZ InP/AlInP CMS NWs-based LEDs were still uncleared. Here, we report on the selective-area growth (SAG) of WZ InP/AlInP CMS NWs with various Al content and demonstration of vertical NW-LEDs using the WZ InP/AlInP CMS NWs.A 20-nm-thick SiO2 sputtered p-InP (111)A substrates were used for the selective-area growth. Periodical hexagonal openings were formed using electron beam lithography and wet chemical etching. The opening diameter was 30 – 100 nm and pitch was 400 – 2000 nm. Vertical core WZ InP NWs and AlInP shells were grown by low-pressure horizontal metalorganic vapor phase epitaxy (MOVPE). Trimethylindium (TMIn), tertiarybutylphosphine (TBP) and trimethylaluminum (TMAl) were used as source materials, and diethylzinc (DEZn) and monosilane (SiH4) were used for p- and n- type dopants. The carrier concentration of the p-AlInP and n-AlInP were 4×1018 cm-3 and 2×1015 cm-3. The growth temperature for InP NWs was 660ºC with V/III = 24 and growth time was for 400 sec, and AlInP shells were grown at 480 - 600 ºC for 5 min. The Al content in vapor phase varied from 20% - 40%. After the CMS NW growths, we fabricated the vertical NW-LEDs. The NWs were covered with benzocyclobutene (BCB) by spin-coating. The top portion of the NWs was exposed by reactive ion etching (RIE). Then, Ti (2nm)/ITO (200nm) and AuZn were evaporated on the top of NWs and backside of the substrates respectively. The samples were annealed at 420°C in N2 for 3 min.In the NW-LED, we designed tubular AlInP-based double heterostructure with radial pn junction on the sidewalls of WZ InP core NWs. Also, AlInP shell was designed with Al-graded and without graded layer. The growth results of the InP/AlInP CMS NWs without/with Al-graded layers showed both NWs had hexagonal pillar structure surrounded with {-211} facets rotated 30° against to the cleavage {-110} planes, indicating the WZ crystal structure of the AlInP shell was transferred from WZ InP core successfully. NW-LEDs exhibited moderate rectification properties with small series resistance. The ideality factor (n) was 2.69 and 4.09 for the NW-LED without/with Al-graded layer, which were larger than the moderate ideality factor of conventional LED (n = 1 - 2). The large n indicates that the tunneling current through the thin AlInP shells layers. Both NW-LEDs showed EL at room temperature under various current injection. Peak deconvolution of the EL revealed that three peak origins were dominant in both NW-LEDs. The EL peaks were 1.41, 1.48 and 1.54 eV for NW-LEDs without Al-graded layer. And the EL peaks were 1.41, 1.50 and 1.58 eV for NW-LED with Al-graded layer. The main EL peaks at 1.48 eV and 1.50 eV were originated from WZ AlInP shell active layers. The EL intensity from WZ-AlInP shell with Al-graded layer was higher than that of the shell without Al-graded layer. Further investigation of the Al controllability for InP/AlInP CMS NWs and the NW-LED devices toward visible color emission will be discussed.[1]. F. Ishizaka et al., Nano Lett. 17, 1350 (2017).

Optical and electrical characterization of solar cell with nanowires mimicking antireflection coating layers considering axial and radial PN junctions

AbstractIt has been well documented that the usage of a textured cover layer reduces reflection from the air‐solar cell interface, which ultimately enhances the power conversion efficiency (PCE) of a solar cell. The most commonly used patterns, such as pyramids, micropillars, nanowires (NW), and nanoholes have been widely studied and optimized. Besides using such NWs to enhance light absorption, this work also additionally considers the concept of mimicking the antireflection coating of single or multiple layers in minimizing the reflectance and thus enhancing the total absorptance further. It is shown here that at least one order of magnitude shorter multilayer NW pattern of 268 nm total height can outperform a standard NW of 4270 nm height, which needs less material and can also be fabricated at a reduced cost. Furthermore, the proposed design with reduced height has a significantly lower surface‐to‐volume ratio, which also reduces surface recombination loss than the other textured surface patterns. The results presented in this work have been comprehensively analyzed by initially optimizing optical absorption and then completing the electrical simulations. The optimized design in conjunction with a back reflector offers an efficiency as high as 16.434%.

Optical and Electrical Analyses of Solar Cells with a Radial PN Junction and Incorporating an Innovative NW Design That Mimics ARC Layers.

The implementation of a texturing pattern on the surface of a solar cell is well known for reducing reflection, thus increasing the absorption of sunlight by the solar cell. Nanowires (NWs) that are large in their height have been widely used for this purpose. Through rigorous numerical simulations, this work explores the benefits of short but index-matched NWs and how these designs are also affected by surface recombination. Additionally, this work further optimized power conversion efficiency (PCE) by placing two or three NWs of different heights and diameters on top of each other to mimic the performance of two-NW and three-NW ARC designs with PCEs of 16.8% and 17.55%, respectively, when a radial pn junction is considered. These are the highest reported so far for such a thin silicon solar cell. Furthermore, we also show how these designs were impacted by surface recombination velocity and compare these findings to simple NWs of different heights and diameters.

InP nanowire light-emitting diodes with different pn-junction structures

We report on the characterization of wurtzite (WZ) InP nanowire (NW) light-emitting diodes (LEDs) with different pn junctions (axial and radial). The series resistance tended to be smaller in the NW-LED using core–shell InP NWs with a radial pn junction than in the NW-LED using InP NWs with an axial pn junction, indicating that radial pn junctions are more suitable for current injection. The electroluminescence (EL) properties of both NW LEDs revealed that the EL had three peaks originating from the zinc-blende (ZB) phase, WZ phase, and ZB/WZ heterojunction. Transmission electron microscopy showed that the dominant EL in the radial pn junction originated from the ZB/WZ interface across the stacking faults.

Design of hybrid solar cell with GaAs1−xBix (x = 0.01) nanowire core and conformally coated P3HT/ITO shell

The optoelectronic characteristics of the vertically aligned conformally coated radial junction, core–shell nanowire (NW) hybrid solar cell (HSC) having a core active layer of GaAs0.99Bi0.01as acceptor and donor shell layer of poly(3-hexylthiophene-2,5-diyl) (P3HT), have been investigated. Since axial junction solar cells (SCs) have been experimentally showing better efficiency as compared to their radial counterpart, our objective is to improve the efficiency of radial pn junction nanowire solar cells. In order to achieve that, the geometrical parameters, including the core diameter, shell thickness comprising P3HT layer and ITO for a fixed pitch of 480 nm are optimized. A fixed thickness of 10 nm is considered for P3HT due to a diffusion length of 20 nm. 3D finite-difference time-domain (FDTD) method has been employed to examine the effect of geometrical parameter variation on carrier generation and a detailed study has been presented. The core diameter of our proposed structure is 160 nm (Rcore = 80 nm) and the thickness of the surrounding shell is considered to be 90 nm (Rshell = 90 nm) for further optoelectronic performance study. The absorption spectra, generation rate profile and electric field profiles for the wavelength (λ) range of 400–900 nm have been presented for depicting the improvement caused due to incorporation of conformal coating of P3HT on core GaAs0.99Bi0.01 NW. An extensive investigation on the variation of electrical performance parameters such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and power conversion efficiency (PCE) with carrier lifetime and surface recombination velocity (SRV) is performed for a fixed core and shell doping. An efficiency of 19% is achieved for the device featuring minority carrier lifetime of 60 ns and sufficiently high SRV of 105cm/s at the contacts for lightly doped core (∼1 × 1016 cm−3) and heavily doped shell (∼1 × 1019 cm−3) and substrate (∼1 × 1019 cm−3).

Design and fabrication of new wide-band and wide-angle solar cell absorbing layer

Cu–Zn– Sn– S (CZTS) is an ideal material for absorbing layer due to its high optical absorption coefficient and moderate bandgap. But, optical absorption and carrier recombination in thin films may lead to electron losses. Thus, introduction of radial PN junction nanostructures into thin film solar cells make light and current to travel along different paths. In this study, CZTS films were prepared by magnetron sputtering and crystalized at sputtering pressure of 3 mTorr, sputtering power of 120 W, and substrate temperature of 300 °C. The composition of CZTS thin films was closed to the optimal stoichiometry to be used as a solar cell absorption layer. These CZTS films were single-phase and vulcanized at 550 °C. A CZTS absorbing layer with a nanowire array structure had higher absorptivity in visible light region. The wideband optical absorptivity was also significantly higher than that of a planar thin film. The nanowire structure was simulated by the finite-difference time-domain method (FDTD), and its maximum absorption (with good size) reached up to 90% in 300 to ~ 2000 nm wide waveband. The mask was done after determining the ratio of the spreading agent (water to ethanol as 1:2), the surfactant concentration (4% wt.), the equilibrium time (2 min), and other parameters. The best results for normal incidence were evaluated as: D = 0.4 μm, h = 2 μm and f ≈ 0.8. According to calculations, the well-designed nanowires, in the visible light region, have an absorption rate of up to 90%.

Depth-dependent EBIC microscopy of radial-junction Si micropillar arrays

Recent advances in fabrication have enabled radial-junction architectures for cost-effective and high-performance optoelectronic devices. Unlike a planar PN junction, a radial-junction geometry maximizes the optical interaction in the three-dimensional (3D) structures, while effectively extracting the generated carriers via the conformal PN junction. In this paper, we report characterizations of radial PN junctions that consist of p-type Si micropillars created by deep reactive-ion etching (DRIE) and an n-type layer formed by phosphorus gas diffusion. We use electron-beam induced current (EBIC) microscopy to access the 3D junction profile from the sidewall of the pillars. Our EBIC images reveal uniform PN junctions conformally constructed on the 3D pillar array. Based on Monte-Carlo simulations and EBIC modeling, we estimate local carrier separation/collection efficiency that reflects the quality of the PN junction. We find the EBIC efficiency of the pillar array increases with the incident electron beam energy, consistent with the EBIC behaviors observed in a high-quality planar PN junction. The magnitude of the EBIC efficiency of our pillar array is about 70% at 10 kV, slightly lower than that of the planar device (≈ 81%). We suggest that this reduction could be attributed to the unpassivated pillar surface and the unintended recombination centers in the pillar cores introduced during the DRIE processes. Our results support that the depth-dependent EBIC approach is ideally suitable for evaluating PN junctions formed on micro/nanostructured semiconductors with various geometry.

Nanowire photovoltaic efficiency enhancement using plasmonic coupled nano-fractal antennas

We suggest the use of nano-fractal antennas for plasmonic coupling to enhance nanowire (NW) photovoltaic power conversion efficiency. Silicon radial pn junction NWs positioned inside Apollonian and Sierpinski nano-fractal antennas are simulated with different topologies and NW lengths. An enhancement in power conversion efficiency ranging from 12% to up to 24% over the same NW without antenna case is achieved.

Electrostatic model of radial pn junction nanowires

Poisson's equation is solved for a radial pn junction nanowire (NW) with surface depletion. This resulted in a model capable of giving radial energy band and electric field profiles for any arbitrary core/shell doping density, core/shell dimensions, and surface state density. Specific cases were analyzed to extract pertinent underlying physics, while the relationship between NW specifications and the depletion of the NW were examined to optimize the built-in potential across the junction. Additionally, the model results were compared with experimental results in literature to good agreement. Finally, an optimum device design is proposed to satisfy material, optical, and electrostatic constraints in high efficiency NW solar cells.

Spectral and spatial distribution effects on nanowires inside nanoring optical antennas for photovoltaic arrays

The effect of enclosing a nanowire (NW) radial pn junction photovoltaic (PV) element inside a nanoring optical antenna to enhance the electric field in the near field region has been investigated. Five different materials for the NW (Si, Ge, GaAs, GaInP and InGaAs) have been selected to maximize the absorbed solar spectrum. In addition, the position and diameter of the NW are varied through a random distribution to optimize the power conversion efficiency. Results show that the ring antenna geometry and the NW random spatial distribution are effective in both spectral widening and field concentration which result in an increase of the cell conversion efficiency.

Geometrical optimization and contact configuration in radial pn junction silicon nanorod and microrod solar cells

ABSTRACTElectric charge transport simulations of symmetrically doped radial pn junction silicon nanorod solar cells were performed using the Technology Computer‐aided Design software suite by Silvaco. Two schemes of electric contacting were applied, the first one consisting of a cathode wrapped around the cladding of the rod and the second one in a cathode located only on the top rod surface. In both cases, the anode was implemented just below the bottom end of the p‐type rod core. P‐type cores and n‐type shells of the rods were assumed, with dopant densities of 1018 cm− 3 in both regions. The location of the pn junction was chosen such that well‐formed space charge regions could be established with the outer end of the n‐type depletion region being adjacent to the cylindric surface of the nanorod. Rod radii and rod lengths were varied and optimized in a three‐step process for both types of contacting schemes. It was found that inhomogeneous carrier generation profiles diminish the open‐circuit voltage in case of a wrapped cathode configuration. Most realistic is the usage of a top contact configuration with rod radii of 2 µm and lengths of around 100 µm, leading to a cell efficiency of about 15%. Further enhancement of performance is expected, if light trapping of the nanorod layer is taken into account and photonic light harvesting is applied. Copyright © 2012 John Wiley & Sons, Ltd.

Finite difference discretization of semiconductor drift-diffusion equations for nanowire solar cells

We introduce a finite difference discretization of semiconductor drift-diffusion equations using cylindrical partial waves. It can be applied to describe the photo-generated current in radial pn-junction nanowire solar cells. We demonstrate that the cylindrically symmetric (l=0) partial wave accurately describes the electronic response of a square lattice of silicon nanowires at normal incidence. We investigate the accuracy of our discretization scheme by using different mesh resolution along the radial direction r and compare with 3D (x, y, z) discretization. We consider both straight nanowires and nanowires with radius modulation along the vertical axis. The charge carrier generation profile inside each nanowire is calculated using an independent finite-difference time-domain simulation.

Morphology-tunable assembly of periodically aligned Si nanowire and radial pn junction arrays for solar cell applications

Large-area periodically aligned Si nanowire (PASiNW) arrays have been fabricated on Si substrates via a templated catalytic chemical etching process. The diameter, length, packing density, and even the shape of Si nanowires (SiNWs) could be precisely controlled and tuned. A local coupling redox mechanism involving the reduction of H2O2 on silver particles and the dissolution of Si is responsible for formation of SiNWs. With the as-prepared SiNWs as templates, three kinds of PASiNW radial pn junction structures were fabricated on Si substrates via a solid-state phosphorous diffusion strategy and their applications in solar cells were also explored. The PASiNW radial pn junction-based solar cell with big diameter and interspace shows the highest power conversion efficiency (PCE) of 4.10% among the three kinds of devices. Further optimization, including surface passivation and electrode contact, is still needed for the higher efficiency PASiNW radial pn junction-based solar cells in the future.

Growth and Characterization of Radial pn Junction Gaas Nanowire by MOCVD

Growth and Characterization of Radial pn Junction Gaas Nanowire by MOCVD

Growth and Characterization of Radial pn Junction Gaas Nanowire by MOCVD

Radial pn-junction GaAs nanowires were fabricated and investigated in detail. These nanowires were grown on GaAs (111)B substrate by metal-organic chemical vapor deposition via Au-catalyzed vapor-liquid-solid mechanism. Two types of nanowire p-n junctions were fabricated by growing a n(p)-doped GaAs shell outside a p(n) GaAs core. P-type doping was provided by diethyl zinc, while silane was introduced for n-type doping. The morphology, crystal structure and doping characteristics were investigated by FESEM, TEM and EDS. The results showed that both the two structures were of good morphology and both dopants were successfully incorporated into the nanowires.

Three-dimensional nanojunction device models for photovoltaics

A model is developed to describe the behavior of three-dimensionally nanostructured photovoltaic devices, distinguishing between isolated radial pn junctions and interdigitated pn junctions. We examine two specific interdigitated architectures, the point-contact nanojunction and the extended nanojunction, which are most relevant to experimental devices reported to date but have yet to be distinguished in the field. The model is also applied to polycrystalline CdTe devices with inverted grain boundaries. We demonstrate that for CdTe/CdS solar cells using low-quality materials, the efficiency of the extended nanojunction geometry is superior to other designs considered.