Silicon Nanowire Solar Cells Research Articles

Chemically Doped Radial Junction Characteristics in Silicon Nanowires

We evaluate the boron (B) and phosphorus (P) core-surface codoped radial p-n junction characteristics in silicon nanowires (SiNWs) using density functional theory calculations. We find that the formation of radial p-n junction is energetically favorable. The stability depends on the diameter of SiNWs and the dopant concentration. Generally, a higher concentration of B-P pair dopants results in a more stable nanowire. More importantly, we predict that the radial p-n junction can evolve into a Schottky-like junction in relatively highly doped SiNWs when the diameter increases, attributing to the change of the core p-doping characteristic, that is, the core p-junction becomes metallic, while the n-junction near the surface remains semiconducting. The interfacial contact between the junctions is found to be the key for such change. Our calculated results support an experimental observation in SiNW solar cells.

Direct electrical contact of slanted ITO film on axial p-n junction silicon nanowire solar cells

A novel scheme of direct electrical contact on vertically aligned silicon nanowire (SiNW) axial p-n junction is demonstrated by means of oblique-angle deposition of slanted indium-tin-oxide (ITO) film for photovoltaic applications. The slanted ITO film exhibits an acceptable resistivity of 1.07 x 10⁻³Ω-cm underwent RTA treatment of T = 450°C, and the doping concentration and carrier mobility by Hall measurement amount to 3.7 x 10²⁰ cm⁻³ and 15.8 cm²/V-s, respectively, with an n-type doping polarity. Because of the shadowing effect provided by the SiNWs, the incident ITO vapor-flow is deposited preferentially on the top of SiNWs, which coalesces and eventually forms a nearly continuous film for the subsequent fabrication of grid electrode. Under AM 1.5 G normal illumination, our axial p-n junction SiNW solar cell exhibits an open circuit voltage of VOC = 0.56 V, and a short circuit current of JSC = 1.54 mA/cm² with a fill factor of FF = 30%, resulting in a total power conversion efficiency of PEC = 0.26%.

Anisotropic optical properties of silicon nanowire arrays based on the effective medium approximation

In search of next-generation solar cells, silicon nanowire arrays have attracted great attention since they are cost-effective and may absorb more light compared to current thin-film silicon solar cells. Theoretical studies using finite-difference time-domain and transfer matrix methods have been performed to investigate the optical properties of silicon nanowire (SiNW) arrays. However, these methods are computationally intensive and require periodic conditions, which may not be satisfied with most fabricated samples. In the present study, an effective medium analysis considering the anisotropic nature of vertically aligned SiNWs is performed to study their optical properties in the wavelength range from 310 nm to 1100 nm, which is of the most importance for solar photovoltaic cells. The effective dielectric functions of the SiNW layer for both ordinary and extraordinary waves are obtained from the Bruggeman approximation. Thin-film optics formulae incorporating the anisotropic wave propagation in uniaxial media are employed to calculate the reflectance and absorptance of the SiNWs on silicon substrates for different polarizations. The effect of geometric parameters such as filling ratio and wire length is investigated. In addition to modeling the directional radiative properties at various angles of incidence, the hemispherical properties are also calculated to understand the light absorption and to facilitate the optimal design of high-performance SiNW solar cells.

Numerical Approach to the Investigation of Performance of Silicon Nanowire Solar Cells Embedded in a SiO2 Matrix

The electrical characteristics of silicon nanowire (SiNW) solar cells with p-type hydrogenated amorphous silicon oxide (Eg = 1.9 eV)/n-type SiNWs embedded in a SiO2/n-type hydrogenated amorphous silicon oxide (Eg = 1.9 eV) structure have been investigated using two- and three-dimensional device simulators, taking into account the quantum size effect. The average bandgap of a SiNW embedded in SiO2 increased from 1.15 to 2.68 eV with decreasing diameter from 10 to 2 nm, owing to the quantum size effect. Note that under sunlight of AM1.5G, the open-circuit voltage (Voc) of SiNW solar cells also increased to 1.46 V with decreasing diameter of the SiNWs to 2 nm. This result suggests that it is possible to enhance Voc by applying the quantum size effect, and a SiNW is a promising material for all-silicon tandem solar cells.

Numerical Approach to the Investigation of Performance of Silicon Nanowire Solar Cells Embedded in a SiO2Matrix

Numerical Approach to the Investigation of Performance of Silicon Nanowire Solar Cells Embedded in a SiO₂Matrix

The tradeoff between plasmonic enhancement and optical loss in silicon nanowire solar cells integrated in a metal back reflector

We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhancement and optical loss in periodically aligned, silicon nanowire (SiNW) arrays integrated with a silver back reflector (Ag BR). Optimizing the embedded depth of the wire bottoms into a silver reflector achieved a highly efficient SiNW solar cell. Compared to the SiNW solar cell employing a flat back reflector, the embedded depth of ~20 nm resulted in the relative increase of ~5% in ultimate solar cell efficiency.

Effects of silicon nanowires length on solar cells photovoltaic properties

Silicon nanowires (SiNWs) were produced by an electroless method on FZ-Si (100) wafer, in HF/AgNO3 solution. The influence of etching time and temperature on SiNWs morphology were studied using FESEM images. Optical properties were also investigated by optical absorption spectroscopy and low-temperature photoluminescence at 4.2 K. Considering their role as active regions, photovoltaic properties of SiNWs solar cells were studied for their different lengths. Photovoltaic measurements were taken in 1 sun condition under AM 1.5 illumination supplied by a solar simulator. Measurements indicated a reduction in efficiency as SiNWs length increased, which might be attributed to increased dangling states on nanowires surfaces.

Silicon-nanowire hybrid solar cells fabricated with thin-film absorber

Silicon-nanowire hybrid solar cells fabricated with thin-film absorber

Improved Photovoltaic Performance of Si Nanowire Solar Cells Integrated with ZnSe Quantum Dots

Introducing a ZnSe quantum dot (QD) layer over silicon nanowire (Si NW) solar cells considerably enhances external quantum efficiency (EQE) over broadband wavelengths. This is attributed to the combination of two major benefits of ZnSe QDs: superior light trapping and photon down-conversion. The integration of ZnSe QDs on the Si NW solar cell significantly reduces Fresnel reflection at the silicon/air interface because the refractive index of ZnSe QDs falls between those of Si and air. As a result, the refractive index mismatch at the interface can be alleviated. This decreases the Si NW length required for obtaining superior light absorption over 90%, which consequently leads to a substantial reduction in surface recombination loss. A remarkable enhancement of ∼30% in EQE around the absorption maximum of ZnSe QDs reveals that photon down-conversion by ZnSe QDs significantly contributed to EQE enhancement in a short-wavelength region. Our Si NW/ZnSe QDs hybrid solar cell showed nearly 13% improvement in power conversion efficiency compared to that of a bare Si NW counterpart, highlighting the feasibility of thin-layered semiconductor nanoparticles as a booster for highly efficient Si solar cells.

Preface: Phys. Status Solidi C 6/2012

AbstractThis proceedings issue is a selective collection of papers presented at Symposium B “Amorphous Nanostructured Materials” which was conducted during the E‐MRS Fall Meeting 2011, Warsaw, Poland. The symposium program covered various aspects of amorphous materials science and technology, including the preparation of materials and structures, the characterization and study of their properties (structural, optical, electrical, magnetic), the performance of devices. The objective of the symposium was to put together leading scientists working in the field and to update the research achievements in the area.Among the most substantial issues, discussions at the symposium highlighted the importance of structures and materials that combine amorphous and crystalline phases or that allow phase transitions between such phases. These structures have new physical properties and open the way to new potential applications in the fields of information and energy, in particular for information storage, light emission, and solar cells. Great emphasis was put on silicon‐based materials and structures, either from low temperature (amorphous, polymorphous, microcrystalline silicon) or high temperature processes (silicon nanoparticles and nanocrystals). Recent progress has been achieved in understanding the doping and defect properties of amorphous silicon, as well as the behavior of heterostructures comprising thin amorphous silicon films. Light emission properties of silicon nanoclusters and silicon‐based materials like rare‐earth implanted amorphous silicon dioxide and silicon oxicarbide layers were also underlined. A carrier multiplication effect in silicon nanocrystals embedded in an amorphous silicon oxide matrix was reported, implying the relaxation of photogenerated hot carriers not only at photon absorption sites but also through energy transfer into neighboring nanocrystals with creation of additional electron–hole pairs. Utilization of this effect should improve the efficiency of silicon‐based solar cells. Future trends in materials physics include the study of silicon–germanium nanocrystals and the use for hybrid silicon–organic electronics. In particular, silicon nanocrystals could be used in conjunction with organic semiconductors for hybrid bulk heterojunction solar cells. Studies of other nanostructured silicon‐based devices such as silicon heterojunction and silicon nanowires solar cells were also presented.Considerable attention was paid to the fabrication and properties of metallic nanostructures. In particular, a new method was reported allowing production of amorphous metal nanoparticles that, contrary to their crystalline counterparts, do not coagulate when approaching each other. This unusual property permitted fabrication of thin films consisting of closely packed but still individual nanoparticles. When made of magnetic materials, the characteristics of such films were shown to be determined by magnetic exchange interaction between the particles combined with particle self‐organization. As a result, the blocking temperature of the films was found to be close to room temperature, which is a property being important in magnetic storage application.Related to the above‐mentioned subjects, the issue content is organized as follows. First four articles are devoted to different aspects of light emission from silicon‐based materials (silicon oxicarbide, silicon and metal oxide clusters). The second group of three articles deals with correlation between structure and electrical properties of the materials based on amorphous silicon. Next, four articles consider technology and structure formation mechanisms of amorphous silicon. The following three articles touch upon the problem of silicon‐based solar cells. The last group of four articles deals with fabrication and properties of promising amorphous chalcogenides, metals, metal oxides, and carbon materials.All contributing authors are greatly acknowledged for their input to this issue.The symposium was sponsored by INCATTECH LLC (the company is located in St. Petersburg, Russia, www.incattech.com).

Optical and electrical characteristics of asymmetric nanowire solar cells

We propose an asymmetric radial structure developed via simulation that improves the optical and electrical characteristics of silicon nanowire (SiNW) solar cells. This nanostructure is designed by shrinking the bottom core diameter and holding the top core diameter fixed in the SiNW, which results in a total reflection of the incident light in the outer wall of the shell due to the difference in the refractive index. The reflection enhances light trapping and concentration, which results in a 10 times higher optical generation rate and greater optical absorption in the high energy regime as compared with the fundamental symmetric radial structure. Further, we found that the efficiency is increased by over 10% when the bottom core diameter is decreased. The proposed structure has great potential to effectively improve the efficiency in concert with optimizing the design parameters.

Sn‐catalyzed silicon nanowire solar cells with 4.9% efficiency grown on glass

ABSTRACTWe present a single pump‐down process to texture hydrogenated amorphous silicon solar cells. Mats of p‐type crystalline silicon nanowires were grown to lengths of 1 µm on glass covered with flat ZnO using a plasma‐assisted Sn‐catalyzed vapor‐liquid‐solid process. The nanowires were covered with conformal layers of intrinsic and n‐type hydrogenated amorphous silicon and a sputtered layer of indium tin oxide. Each cell connects in excess of 107 radial junctions over areas of 0.126 cm². Devices reach open‐circuit voltages of 0.8 V and short‐circuit current densities of 12.4 mA cm−2, matching those of hydrogenated amorphous silicon cells deposited on textured substrates. Copyright © 2012 John Wiley & Sons, Ltd.

Fabrication and Photovoltaic Characteristics of Coaxial Silicon Nanowire Solar Cells Prepared by Wet Chemical Etching

Nanostructured solar cells with coaxial p-n junction structures have strong potential to enhance the performances of the silicon-based solar cells. This study demonstrates a radial junction silicon nanowire (RJSNW) solar cell that was fabricated simply and at low cost using wet chemical etching. Experimental results reveal that the reflectance of the silicon nanowires (SNWs) declines as their length increases. The excellent light trapping was mainly associated with high aspect ratio of the SNW arrays. A conversion efficiency of ∼7.1% and an external quantum efficiency of ∼64.6% at 700 nm were demonstrated. Control of etching time and diffusion conditions holds great promise for the development of future RJSNW solar cells. Improving the electrode/RJSNW contact will promote the collection of carries in coaxial core-shell SNW array solar cells.

Effect of the Quantum Size Effect on the Performance of Solar Cells with a Silicon Nanowire Array Embedded in SiO2

ABSTRACTThe electrical characteristics of silicon nanowire (SiNW) solar cells with p-type hydrogenated amorphous silicon oxide (Eg=1.9 eV)/n-type SiNWs embedded in SiO2/n-type hydrogenated amorphous silicon (Eg=1.7 eV) structure have been investigated using a two-dimensional device simulator with taking the quantum size effects into account. The average bandgap of a SiNW embedded in SiO2 increased from 1.15 eV to 2.71 eV with decreasing the diameter from 10 nm to 1 nm due to the quantum size effect. It should be noted that under the sunlight with AM1.5G the open-circuit voltage (Voc) of SiNW solar cells also increased to 1.54 V with decreasing the diameter of the SiNWs to 1 nm. This result suggests that it is possible to enhance the Voc by the quantum size effect and a SiNW is a promising material for the all silicon tandem solar cells.

Photocurrent Mapping in High-Efficiency Radial p–n Junction Silicon Nanowire Solar Cells Using Atomic Force Microscopy

Rapid formation of radial p–n junctions on electroless-etched silicon nanowires (SiNWs) was successfully demonstrated. With a low-cost objective, a homemade nonhazardous diffusion source of high phosphor concentration annealed at a small thermal budget was used. The SiNW solar cell, with Au electrodes, has shown a power conversion efficiency of 8.41%, which is higher by 30% compared with its planar counterpart. The SiNW solar cell incorporates an inherent antireflection property, reduced diffusion length requirement, and broad-band spectral quantum efficiency. The evidence of a successful radial p–n junction formation in the NWs has been revealed through the help of a conducting atomic force microscope (AFM) scanning for the photogenerated currents on the fractured surfaces of the NWs. The demonstrated radial junction fabrication technique is believed to reduce the cost of production and promote widespread use of them.

Self-Assembled Wire Arrays and ITO Contacts for Silicon Nanowire Solar Cell Applications

Self-assembly of silicon nanowire (SiNW) arrays is studied using SF6/O2 plasma treatment. The self-assembly method can be applied to single- and poly-crystalline Si substrates. Plasma conditions can control the length and diameter of the SiNW arrays. Lower reflectance of the wire arrays over the wavelength range 200–1100 nm is obtained. The conducting transparent indium-tin-oxide (ITO) electrode can be fully coated on the self-assembled SiNW arrays by sputtering. The ITO-coated SiNW solar cells show the same low surface light reflectance and a higher carrier collection efficiency than SiNW solar cells without ITO coating. An efficiency enhancement of around 3 times for ITO coated SiNW solar cells is demonstrated via experiments.

Simulation of Symmetrically Doped Silicon Nanowire Solar Cells

ABSTRACTSilicon nanowire solar cells were simulated using the Silvaco TCAD software kit. For optimization of speed the simulations were performed in cylinder coordinates with cylindrical symmetry. Symmetric doping was assumed with a dopant density of 1018 cm-3 in the p-type core and inside the n-type shell. In the implementation a cathode contact was wrapped around the semiconductor nanorod and an anode was assumed at the bottom of the rod. Optimization of cell efficiency was performed with regard to the rod radius and the rod length. In both optimization processes clear maxima in efficiency were visible, resulting in an optimal radius of 66 nm with the pn junction at 43.5 nm and an optimal rod length of about 48 μm. The maximum of efficiency with respect to the rod radius is due to a decrease of short-circuit current density (Jsc) and an increase of open-circuit voltage (Uoc) with radius, while the maximum with respect to the rod length is explained by the combination of an increase of Jsc and a decrease of Uoc. Fill factors stay rather constant at values between 0.6 and 0.8. Further, the influence of a back surface field (BSF) layer was surveyed in simulations. Positioning the BSF next to the cathode contact considerably improved cell efficiency. In addition, simulations with a cathode contact on top of the nanowire structure were undertaken. No severe deterioration of cell performance with increasing radius was observed so far in this configuration. Hence, nanorods with much larger radii can be used for solar cells using this contact scheme. In comparison to simulations with wrapped cathode contacts, Jsc and Uoc and therefore efficiency is considerably improved.

Photovoltaic characteristics of silicon nanowire arrays synthesized by vapor–liquid–solid process

Silicon nanowire (SiNW) arrays were grown directly on a P type Si substrate, pre-deposited with gold catalyst, and then made into solar cell for photovoltaic characteristic measurement. Different growth conditions of SiNWs, including variation of the flow rate ratio of SiH 4 versus N 2, and the thickness of Au film, which can be sputtered into different size of nanoparticles, will be made in order to obtain an optimum photovoltaic conversion efficiency. The morphologies and crystalline structure of the nanowires are studied by SEM, TEM and XRD. The SiNW array surface is shown to have good antireflection property, and is expected to raise light absorption and short circuit current. The photovoltaic performance of the solar cells with SiNWs grown at different conditions is measured and discussed. More effort is still needed to raise the performance of SiNW solar cells.

Single nanowire photovoltaics

This tutorial review focuses on recent work addressing the properties and potential of semiconductor nanowires as building blocks for photovoltaic devices based on investigations at the single nanowire level. Two central nanowire motifs involving p-i-n dopant modulation in axial and coaxial geometries serve as platforms for fundamental studies. Research illustrating the synthesis of these structural motifs will be reviewed first, followed by an examination of recent studies of single axial and coaxial p-i-n silicon nanowire solar cells. Finally, challenges and opportunities for improving efficiency enabled by controlled synthesis of more complex nanowire structures will be discussed, as will their potential applications as power sources for emerging nanoelectronic devices.

Integration of a Silicon Nanowire Array into a Photovoltaic Device

AbstractSilicon nanowire arrays grown by chemical vapour deposition were successfully integrated into functional photovoltaic devices. A crucial planarization step, achieved by embedding the nanowires in a spin-on glass matrix and subsequent polishing of the front surface, allowed to deposit a continuous and uniform conductive film on top of the nanowire array, and thus to form a high-quality front electrical contact. The silicon nanowire array solar cells fabricated using this process exhibited a parasitic series resistance as low as 5 Ω.cm2, which is a clear improvement compared to the recent literature.