Silicon Nanowire Solar Cells Research Articles

Improved power conversion efficiency of silicon nanowire solar cells based on transition metal oxides

An n-type Si nanowire (SiNW) solar cell based on transition metal oxides V2O5 and TiO2 has been fabricated. The cell is demonstrated in the ITO/V2O5/n-SiNWs/TiO2/Al heterojunction structure, in which the V2O5 and TiO2 work as the hole- and the electron-selective layers, respectively. Parameters of cells with different lengths of nanowires have been investigated. Owing to the excellent light trapping effect of nanowire arrays and superior carrier transporting properties of thin V2O5 and TiO2 layers, an improved power conversion efficiency of 12.7% has been achieved in a cell with 1.45-μm-long nanowires. Due to the large work function difference between V2O5 and n-Si, a build-in potential of 0.75 V was obtained from the capacitance-voltage measurement, which implies the possibility of achieving a large open circuit voltage for these nanowire solar cells. The efficiency loss is mainly caused by the recombination via the defects on the unpassivated surface of SiNWs. By solving this problem, the cell efficiency may be improved further and realize the application in PV industries.

Characteristics of highly efficient star-shaped nanowires solar cell

The optical characteristics of a star-shaped silicon nanowires (Si NWs) solar cell (SC) are numerically studied using three-dimensional (3-D) finite-difference time-domain (FDTD). The particle swarm optimization technique is used to optimize the geometrical parameters of the NW to maximize its light absorption. The optimized star-shaped Si NWs offer high ultimate efficiency (η) of 43.7% while its complementary design achieves η of 40.1%. This is due to the star surface textures that allow multiple light scattering between the NWs. This will increase the optical path length through the NW and improve its light absorption. The electrical properties of the proposed design are also calculated using the finite-element method. In this investigation, the doping level of the star-shaped Si NWs with p − i − n axial and radial configurations is simulated to quantify the optoelectronic performance of the reported design. The p − i − n axial doped design offers power conversion efficiency (PCE) of 21%; however, the p − i − n radial doping gives PCE of 21.3%. Therefore, the star-shaped design violates PCE of 17.3% and 15.9% of the conventional Si NWs with axial and radial doping, respectively.

Antireflection and passivation property of aluminium oxide thin film on silicon nanowire by liquid phase deposition

To improve the silicon nanowires solar cell of conversation efficiency is one of the most important and challenging problems, the antireflection coatings and surface passivation technique were very important. In this investigation, aluminium oxide coatings were deposited on silicon nanowires by using liquid phase deposition. The deposition solution of aluminium sulfate and sodium bicarbonate were used for aluminium oxide deposition. The Al2O3 film can cause different surface passivation at different annealing temperatures, allowing the Al2O3/SiNWs interface to form different amounts of negatively charged AlO4 and positively charged SiOx, that the mutually suppressing action of the two oppositely charged structure. Under the optimal condition, the reflectance and effective minority carrier lifetime of liquid phase deposited aluminium oxide film were 0.97% and 31 μs, respectively. The aluminium oxide films were used herein to fabricate antireflection coating and passivation film to ensure low cost, good uniformity, favorable adhesion, mass producibility, and the formation of large-area thin film; thus, the liquid phase deposition-antireflection coating film was highly favorable for silicon-based solar cells.

Efficiency enhancement of silicon nanowire solar cells by using UV/Ozone treatments and micro-grid electrodes

Silicon nanowire solar cells were fabricated by metal catalyzed electroless etching (MCEE) followed by thermal chemical vapor deposition (CVD). In this study, we investigated two effects, a UV/ozone treatment and the use of a micro-grid electrodes, to enhance light absorption and reduce the optic losses in the solar cell device. The UV/ozone treatment successfully improved the conversion efficiency. The micro-grid electrodes were then applied in solar cell devices subjected to a back surface field (BSF) treatment and rapid thermal annealing (RTA). These effects improved the conversion efficiency from 9.4% to 10.9%. Moreover, to reduce surface recombination and improve the continuity of front electrodes, we optimized the etching time of the MCEE process, giving a high efficiency of 12.3%.

Fabrication of silicon nanowire based solar cells using TiO2/Al2O3 stack thin films

ABSTRACTTo improve conversion efficiency of silicon nanowire (SiNW) solar cells, it is very important to reduce the surface recombination rate on the surface of SiNWs, since SiNWs have a large surface area. We tried to cover SiNWs with aluminum oxide (Al2O3) and titanium oxide (TiO2) by atomic layer deposition (ALD), since Al2O3 grown by ALD provides an excellent level of surface passivation on silicon wafers and TiO2 has a higher refractive index than Al2O3, leading to the reduction of surface reflectance. The effective minority carrier lifetime in SiNW arrays embedded in a TiO2/Al2O3 stack layer of 94 μsec was obtained, which was comparable to an Al2O3 single layer. The surface reflectance of SiNW solar cells was drastically decreased below around 5% in all of the wavelength range using the Al2O3/TiO2/Al2O3 stack layer. Heterojunction SiNW solar cells with the structure of ITO/p-type hydrogenated amorphous silicon (a-Si:H)/n-type SiNWs embedded in Al2O3 and TiO2 stack layer for passivation/n-type a-Si:H/back electrode was fabricated, and a typical rectifying property and open-circuit voltage of 356 mV were successfully obtained.

Electrical characteristics of funnel-shaped silicon nanowire solar cells

The electrical characteristics of funnel-shaped silicon nanowire (SiNW) solar cells are introduced and numerically analyzed. The funnel-shaped NW consists of a cylinder over a conical unit. Its aim is to maximize the optical absorption over a large wavelength range and hence the electrical efficiency by increasing the number of resonance wavelengths or by enlarging the resonance wavelength range. The conical part has different radii in the axial direction, which increases the number of resonance wavelengths. Further, the coupling between the supported modes by the upper cylinder and the lower tapered cone offers multiple optical resonances required for broadband absorption. The optical characteristics and generation rates through the studied design are obtained using 3-D finite difference time domain. However, the electrical properties are calculated using finite element via the Lumerical device software package. In this regard, radial and axial junctions are examined for the suggested design and compared with the conventional cylindrical SiNW counterpart. In this investigation, short circuit current density, open circuit voltage, fill factor, and power conversion efficiency (PCE) are simulated to quantify the optoelectronic performance of the reported design. Furthermore, the effects of the doping concentration and carrier lifetime on the performance of the funnel-shaped design are reported. The proposed SiNWs offer PCE and short circuit density of 12.7% and 27.6 mA/cm2, respectively, for the axial junction. However, the funnel design with core–shell junction shows an efficiency and short-circuit current (Jsc) of 14.13% and 31.94 mA/cm2, respectively. Therefore, the suggested design has higher efficiency than 6.4% and 9.6% of the conventional cylindrical SiNWs according to the axial and core shell junctions, respectively.

Optimal design of vertical silicon nanowires solar cell using hybrid optimization algorithm

An approach to enhance the ultimate efficiency of the silicon nanowires (Si NWs) solar cell is proposed based on a hybrid population-based algorithm. The suggested technique integrates the ability of exploration in a gravitational search algorithm (GSA) with the exploitation capability of particle swarm optimization (PSO) to synthesize both algorithms’ strengths. The hybrid GSA-PSO algorithm in MATLAB® code is linked to finite-difference time-domain solution technique based on Lumerical-software to simulate and optimize the Si NWs’ geometrical parameters. The suggested GSA-PSO algorithm has advantages in terms of better convergence and final fitness values than that of the PSO algorithm. Further, the Si NWs lattice with optimized diameters and heights shows a high ultimate efficiency of 42.5% with an improvement of 42.8% over the Si NWs lattice with the same diameters and heights. This enhancement is attributed to the different generated optical modes combined with multiple scattering and reduced reflection due to the different heights and different diameters, respectively.

Pencil-shaped silicon nanowire synthesis and photovoltaic application

Pencil-shaped silicon nanowires (SiNWs) were synthesized by colloidal lithography and inductively coupled plasma reactive ion etching for photovoltaic application. Light reflectance of below 10% could be obtained by the asymmetric pencil-shaped SiNW array. The pencil-shaped SiNW structure and properties were investigated and compared with metal catalyzed electroless etching (MCEE) and nanoimprinted SiNWs. Single-junction solar cells consisting of pencil-shaped n-SiNW substrates and p-Si shell layer grown by chemical vapor deposition were fabricated. A combination of two-step H2 annealing and back surface field formation was applied to improve the solar cell properties. Good crystal quality and surface of pencil-shaped SiNWs provided an excellent solar cell junction interface. The great light trapping of the pencil-shaped SiNW solar cell could enhance the conversion efficiency by more than 0.6% compared to the solar cells using MCEE and nanoimprinted SiNWs.

Comparison of Light Trapping in Silicon Nanowire and Surface Textured Thin-Film Solar Cells

The optics of axial silicon nanowire solar cells is investigated and compared to silicon thin-film solar cells with textured contact layers. The quantum efficiency and short circuit current density are calculated taking a device geometry into account, which can be fabricated by using standard semiconductor processing. The solar cells with textured absorber and textured contact layers provide a gain of short circuit current density of 4.4 mA/cm2 and 6.1 mA/cm2 compared to a solar cell on a flat substrate, respectively. The influence of the device dimensions on the quantum efficiency and short circuit current density will be discussed.

Stability study: Transparent conducting oxides in chemically reactive plasmas

Abstract Effect of plasma treatment on transparent conductive oxides (TCOs) including indium-doped tin oxide (ITO), fluorine-doped tin oxide (FTO) and aluminium-doped zinc oxide (AZO) are discussed. Stability of electrical and optical properties of TCOs, when exposed to plasma species generated from gases such as hydrogen and silane, are studied extensively. ITO and FTO thin films are unstable and reduce to their counterparts such as Indium and Tin when subjected to plasma. On the other hand, AZO is not only stable but also shows superior electrical and optical properties. The stability of AZO makes it suitable for electronic applications, such as solar cells and transistors that are fabricated under plasma environment. TCOs exposed to plasma with different fabrication parameters are used in the fabrication of silicon nanowire solar cells. The performance of solar cells, which is mired by the plasma, fabricated on ITO and FTO is discussed with respect to plasma exposure parameters while showing the advantages of using chemically stable AZO as an ideal TCO for solar cells. Additionally, in-situ diagnostic tool (optical emission spectroscopy) is used to monitor the deposition process and damage caused to TCOs.

Improvement of silicon nanowire solar cells made by metal catalyzed electroless etching and nano imprint lithography

Silicon nanowires were fabricated by metal catalyzed electroless etching (MCEE) and nano imprint lithography (NIL), then a shell p-type layer was grown by thermal chemical vapor deposition (CVD) techniques. To reduce back surface recombination and also to activate the dopant, we used two techniques, back surface field (BSF) treatment and rapid thermal annealing (RTA), to improve device performance. In this study, we investigated BSF and RTA treatments in silicon nanowire solar cells, and improved the device performance and efficiency from 4.1 to 7.4% (MCEE device) and from 1.1 to 6.6% (NIL device) after introducing BSF and RTA treatments. Moreover, to achieve better metal contact without sacrificing the reflectance after the shell formation, the selective-area etching method was investigated. Finally, after combining all processes, silicon nanowire solar cells fabricated via the MCEE process exhibited 8.7% efficiency.

Diffused back surface field formation in combination with two-step H2 annealing for improvement of silicon nanowire-based solar cell efficiency

Silicon nanowire (SiNW)-based solar cell fabrication was developed using a combination of diffused back surface field (BSF) formation and two-step H2 annealing for greater efficiency enhancement. Two different n-SiNW structures synthesized by metal-catalyzed electroless etching and nanoimprinting followed by Bosch process were used as substrates to fabricate single-junction solar cells with chemical vapor deposition grown p-Si shell layer. Without BSF formation, shell layer coverage of nanoimprinted SiNW solar cells fabricated using two-step H2 annealing was optimized and the H2 annealing effect on the shell layer crystallinity was investigated using transmission electron microscopy. Then, the effects of combined H2 annealing process for BSF formation together with shell layer treatment were observed under various annealing times. Enhancement of ∼1.5% absolute efficiency of both SiNW-based solar cells was achieved. Reduction of carrier recombination by BSF layer accompanied with the reduction of the defect density on n-SiNW surfaces and inside p-Si layer could be accomplished.

Antireflection and passivation property of titanium oxide thin film on silicon nanowire by liquid phase deposition

To improve a high-efficiency silicon nanowires solar cell, using the antireflection coatings and surface passivation technique were very important. In this investigation, titanium oxide antireflection coatings were deposited on silicon nanowires by using liquid phase deposition. The deposition solution of (NH4)2TiF6 and H3BO3 were used for titanium oxide deposition. The concentration of the H3BO3 in the deposition solution play important roles in the formation of Ti–Si1−xOy interface layer between the titanium oxide/silicon nanowires interface and to control the trace amount of hydrofluoric acid in the solution. The titanium oxide films modification decreases and increases the reflectance and effective minority carrier lifetime of the silicon nanowires arrays. Under the optimal condition, the reflectance and effective minority carrier lifetime of liquid phase deposited titanium oxide film were 3.6% and 1.29μs, respectively. The titanium oxide films were used herein to fabricate antireflection coating and passivation film to ensure low cost, good uniformity, favorable adhesion, mass producibility, and the formation of large-area thin film; thus, the liquid phase deposition-antireflection coating film was highly favorable for silicon-based solar cells.

Novel approach for fabrication of buried contact silicon nanowire solar cells with improved performance

Generally, a selective SiNW-type structure is used to avoid resistive loss in SiNW-based solar cells. However, the performance of these selective SiNW-based solar cells is lower than that of conventional Si solar cells, due to their low collection efficiency and high series resistance. Herein, a novel process is developed to enhance the collection efficiency of photogenerated charge carriers, and hence the performance of SiNW solar cells. Self-aligned single-step lithography is used to fabricate buried contact SiNW (SiNWBC) solar cells. The effectiveness of the SiNWBCs is manifested in the conversion efficiency (η≈15.02%) of the solar cell, which is improved by ∼7.82% compared to that of the control selective SiNW cell (η≈13.93%). The performance and PV cell parameters of the SiNWBCs are analyzed and compared with those of this control cell. Losses due to the PV cell parameters of the SiNWBC solar cell are lower than those of the control cell. The reduced number of front surface recombinations lowers the n and J0 values, resulting in enhanced SiNWBC cell performance.

Omnidirectional broadband absorption enhancement in laterally assembled quadrangle silicon nanowire solar cells

In this paper, effects of geometry and spatial placement of quadrangle silicon nanowires (NWs) on the performance of laterally assembled NW solar cells are investigated. Two different approaches are proposed to reach the broadband absorption enhancement in these types of solar cells. In the first method, a semi-periodic array is used to have a better utilization of the optical antenna effect. The current density can be enhanced as a result of a compromise between the diffraction and optical antenna effects. In the second approach, multiple NWs with different geometries are employed to use the cavity modes of the various NWs. The best current density is achieved for the combination of both of these methods. The different parameters in each structure are selected using the optimization algorithm. Finally, a multilayer structure with the optimized dimensions is proposed to obtain the maximum achievable current density in ultrathin solar cells. The broadband absorption enhancement in the proposed structure is preserved for a wide range of incident angles. Using the multilayer NW arrays, it is possible to improve the absorption enhancement of solar cells without introducing more absorbing material.

Erratum to: Field-effect passivation on silicon nanowire solar cells

Surface recombination represents a handicap for high-efficiency solar cells. Surface recombination represents a handicap for high-efficiency solar cells. This is especially important for nanowire array solar cells, where the surface-to-volume ratio is greatly enhanced. Here, the effect of different passivation materials on the effective recombination and on the device performance is experimentally analyzed. Our solar cells are large area top-down axial n–p junction silicon nanowires fabricated by means of Near-Field Phase-Shift Lithography (NF-PSL). We report an efficiency of 9.9% for the best cell, passivated with a SiO2/SiNx stack. The impact of the presence of a surface fixed charge density at the silicon/oxide interface is studied.

Improving the performance of radial n-i-p junction Si nanowire solar cells by catalyst residue removal

It is hoped that silicon nanowire (SiNW)-based solar cells will provide the basis for a new generation of photovoltaics. However, metal-catalyzed SiNWs contain metal residues (such as indium) which may degrade the performance of solar cells. In this study, we prepared SiNW solar cells by plasma-enhanced chemical vapor deposition using indium as the catalyst to grow the SiNWs. The SiNWs were treated with hydrochloric acid to reduce the indium contamination at different concentrations, CHCl (1–5%). We found the decreasing the indium contamination improved the performance of the solar cells at optimum CopHCl. However, the performance of the solar cells decreased when CHCl exceeded CopHCl. This was attributed to the variation in the conduction-band offset ΔEc between the n type amorphous silicon layer (Ec n-a-Si) and the n type crystalline silicon nanowires (Ec n-c-SiNWs). Finally, a conversion efficiency (Eff) improvement from 2.11% to 6.18% was obtained with the optimized CHCl.

Optimal design of laterally assembled hexagonal silicon nanowires for broadband absorption enhancement in ultrathin solar cells

Design approaches to carry out broadband absorption in laterally assembled hexagonal silicon nanowire (NW) solar cells are investigated. Two different methods are proposed to improve the current density of silicon NW solar cells. It is observed that the key to the broadband absorption is disorder and irregularity. The first approach to reach the broadband absorption is using multiple NWs with different geometries. Nevertheless, the maximum enhancement is obtained by introducing irregular NWs. They can support more cavity modes, while scattering by NWs leads to broadening of the absorption spectra. An array of optimized irregular NWs also has preferable features compared to other broadband structures. Using irregular NW arrays, it is possible to improve the absorption enhancement of solar cells without introducing more absorbing material.

Photovoltaic properties of axial-junction silicon nanowire solar cells with integrated arrays

Wide-gap Si nanowire (SiNW) solar cells that use the quantum size effect require a p–n junction to be formed inside wires with a diameter smaller than 4 nm. To realize this, we developed a device integration process and confirmed power generation, for the first time, by an axial-junction SiNW solar cell with a diameter of 30 nm and a device area of 1 mm2. We confirmed that such a SiNW solar cell had almost the same open-circuit voltage and low short-circuit current density as a bulk Si solar cell. We also investigated the problems related to SiNW solar cells by measuring the functioning SiNW density.

High efficiency silicon solar cell based on asymmetric nanowire

Improving the efficiency of solar cells through novel materials and devices is critical to realize the full potential of solar energy to meet the growing worldwide energy demands. We present here a highly efficient radial p-n junction silicon solar cell using an asymmetric nanowire structure with a shorter bottom core diameter than at the top. A maximum short circuit current density of 27.5 mA/cm2 and an efficiency of 7.53% were realized without anti-reflection coating. Changing the silicon nanowire (SiNW) structure from conventional symmetric to asymmetric nature improves the efficiency due to increased short circuit current density. From numerical simulation and measurement of the optical characteristics, the total reflection on the sidewalls is seen to increase the light trapping path and charge carrier generation in the radial junction of the asymmetric SiNW, yielding high external quantum efficiency and short circuit current density. The proposed asymmetric structure has great potential to effectively improve the efficiency of the SiNW solar cells.