Radial junction amorphous silicon solar cells on PECVD-grown silicon nanowires

Constructing radial junction hydrogenated amorphous silicon (a-Si:H) solar cells on top of silicon nanowires (SiNWs) represents a promising approach towards high performance and cost-effective thin film photovoltaics. We here develop an all-in?situ strategy to grow SiNWs, via a vapour-liquid-solid (VLS) mechanism on top of ZnO-coated glass substrate, in a plasma-enhanced chemical vapour deposition (PECVD) reactor. Controlling the distribution of indium catalyst drops allows us to tailor the as-grown SiNW arrays into suitable size and density, which in turn results in both a sufficient light trapping effect and a suitable arrangement allowing for conformal coverage of SiNWs by subsequent a-Si:H layers. We then demonstrate the fabrication of radial junction solar cells and carry on a parametric study designed to shed light on the absorption and quantum efficiency response, as functions of the intrinsic a-Si:H layer thickness and the density of SiNWs. These results lay a solid foundation for future structural optimization and performance ramp-up of the radial junction thin film a-Si:H photovoltaics.     

Single wire radial junction photovoltaic devices fabricated using aluminum catalyzed silicon nanowires

Single nanowire radial junction solar cell devices were fabricated using Si nanowires synthesized by Al-catalyzed vapor-liquid-solid growth of the p(+) core (Al auto-doping) and thin film deposition of the n(+)-shell at temperatures below 650 °C. Short circuit current densities of 11.7 mA cm(-2) were measured under 1-sun AM1.5G illumination, showing enhanced optical absorption. The power conversion efficiencies were limited to < 1% by the low open circuit voltage and fill factor of the devices, which was attributed to junction shunt leakage promoted by the high p(+)/n(+) doping. This demonstration of a radial junction device represents an important advance in the use of Al-catalyzed Si nanowire growth for low cost photovoltaics.     

Fluorine doped indium oxide films for silicon solar cells

The effect of conditions of preparation of the In₂O₃:F(IFO)/(pp⁺)Si solar cell (SC) by pyrosol method was systematically studied with the goal to maximize its photovoltage. Heterojunction IFO/(pp⁺)Si SC was obtained with the efficiency of 16.6% and photovoltage of 617 mV as well as the IFO/(n⁺pp⁺)Si SC with the efficiency of 19.2% using the following obtained optimal conditions: film-forming solution: 0.2 M InCl₃ + 0.05 M NH₄F + 0.1 M H₂O in methanol; carrier gas — Ar + 5% O₂; deposition temperature — 480 °C; duration of deposition — 2 min; two-minute annealing in argon with sprayed methanol at a temperature of 380 °C.     

Selective plating for junction delineation in silicon nanowires

The in situ growth of p-n junctions in silicon nanowires enables the fabrication of a variety of nanoscale electronic devices. We have developed a method for selective coating of Au onto n-type segments of silicon nanowire p-n junctions. Selective plating allows for quick verification of the position of p-n junctions along the nanowire using electron microscopy and allows for measurement of segment length.     

Synthesis and characterization of silicon nanowires using tin catalyst for solar cells application

Tin-catalyzed silicon nanowires were synthesized for solar cells application. Voluminous silicon nanowires were fabricated on single crystalline silicon wafer. Optical reflectance and solar cell efficiency of the synthesized silicon nanowires were explored. The reflectance of as-synthesized silicon nanowires was obtained approximately 5% in the short wavelength region (λ < 500 nm). A short circuit current of 2.3 mA/cm² and open circuit voltage of 520 mV for 1 cm² SiNWs solar cell was obtained.     

Increasing the efficiency of polymer solar cells by silicon nanowires

Silicon nanowires have been introduced into P3HT:[60]PCBM solar cells, resulting in hybrid organic/inorganic solar cells. A cell efficiency of 4.2% has been achieved, which is a relative improvement of 10% compared to a reference cell produced without nanowires. This increase in cell performance is possibly due to an enhancement of the electron transport properties imposed by the silicon nanowires. In this paper, we present a novel approach for introducing the nanowires by mixing them into the polymer blend and subsequently coating the polymer/nanowire blend onto a substrate. This new onset may represent a viable pathway to producing nanowire-enhanced polymer solar cells in a reel to reel process.     

Large-area silicon nanowires from silicon monoxide for solar cell applications

Large-area upstanding silicon nanowires (SiNWs) were synthesized by hot-filament chemical vapor deposition (HFCVD) using silicon monoxide (SiO) powder as Si source under high vacuum (1.2 x 10(-5) Torr). Gold nanoparticles (AuNPs) were employed as catalyst, which were formed on Si substrate by in-situ reduction of gold chloride (AuCl3). The size and distribution of the Au nanoparticles can be easily controlled through chemical reaction conditions. Consequently, the diameter, length and density of SiNWs could be varied in certain range. The SiNWs obtained are single crystalline with growth directions predominantly along [01-1]. Silicon nanowires in large-scale and diameter less than 10 nm can be grown on different Si substrates with this method. Organic inorganic hybrid solar cells based on SiNWs arrays have been demonstrated.     

Device fabrication with solid-liquid-solid grown silicon nanowires

High quality, single-crystal silicon nanowires were successfully grown from silicon wafers with a nickel catalyst by utilizing a solid-liquid-solid (SLS) mechanism. The nanowires were composed of a crystalline silicon core with an average diameter of 10?nm and a thick outer oxide layer of between 20 and 30?nm at a growth temperature of 1000?°C. When utilizing the SLS growth mechanism, the diameter of the silicon nanowire is dependent solely upon the growth temperature, and has no relation to either the size or the shape of the catalyst. The characteristics of the silicon nanowires are highly dependent upon the properties of the silicon substrate, such as the crystal phase of silicon itself, as well as the doping type. The possibility of doping of silicon nanowires grown via the SLS mechanism without any external dopant source was demonstrated by measuring the electrical properties of a silicon nanowire field effect transistor.     

Novel fabrication of copper nanowire/cuprous oxidebased semiconductor-liquid junction solar cells

Nano Research - A Cu nanowire (NW)/cuprous oxide (Cu2O)-based semiconductor-liquid junction solar cell with a greatly enhanced efficiency and reduced cost was assembled. The Cu NWs function as a...     

Controlable diffusion parameters with chemical vapour deposition for p-n junction silicon solar cells

The chemical vapour deposition technique for the fabrication of p-n junction silicon solar cells is reported. This technique involves the use of native oxide on silicon to limit the diffusion flux and yields lower surface concentrations of impurities and shallow p-n junctions. Photolithography is used for cell fabrication. Data are given to demonstrate the effects of technological parameters on solar cell performance and the controllability of the diffusion parameters obtained by this technique.     

Doping-free fabrication of silicon thin films for schottky solar cell

Thin film Schottky solar cells were fabricated without doping processes, which may provide an alternative approach to the conventional thin film solar cells in the n-i-p configuration. A thin Co layer was coated on a substrate, which worked as a back contact metal and then Si film was grown above it. Deposition condition may modulate the Si film structure to be a fully amorphous Si (a-Si) or a mixing of microcrystalline Si (mc-Si) and a-Si. A thin Au layer was deposited above the grown Si films, which formed a Schottky junction. Two types of Schottky solar cells were prepared on a fully a-Si film and a mixing of mc-Si and a-Si film. Under one sun illumination, the mixing of mc-Si and a-Si device provided 35% and 68.4% enhancement in the open circuit voltage and fill factor compared to that of the amorphous device.     

Confinement effects and hyperfine structure in se doped silicon nanowires

We report a density functional study of the electronic properties and hyperfine structure of substitutional selenium in silicon nanowires using plane-wave pseudopotential techniques. We simulated hydrogen passivated [001] oriented nanowires with a diameter up to 2 nm, analyzing the effect of quantum confinement on the defect formation energy and on the hyperfine parameters as a function of the diameter and of the defect position. We show that substitutional Se in silicon has favorable configurations for positions near the surface with possible formation of chalcogen-hydrogen complexes. We also show that hyperfine interactions increase at small diameters, as long as the nanowire is large enough to prevent surface distortion which modifies the symmetry of the donor wave function. Moreover, surface effects lead to strong differences in the hyperfine parameters depending on the Se location inside the nanowire, allowing the identification of an impurity site on the basis of electron paramagnetic resonance spectra.     

Roll to roll fabrication of thin film silicon solar cells on nano-textured substrates

ECN is developing a novel fabrication process for thin film silicon solar cells on steel foil. Key features in this process are: (1) application of an insulating barrier layer which enables monolithic interconnection and texturization of the rear contact with submicron structures for light trapping; (2) Si deposition with remote, linear PECVD; (3) series interconnection by laser scribing and printing after deposition of all layers, which reduces the total number of process steps. The barrier layer is essential for the monolithic series interconnection of cells, but we show that it also enables optimum light trapping in the solar cells. We can fabricate any arbitrary sub-micron surface profile by hot embossing the barrier layer. For deposition of doped and intrinsic silicon layers we use novel remote, linear plasma sources, which are excellently suited for continuous roll-to-roll processing. We have been able to fabricate device-quality amorphous and microcrystalline silicon layers with these sources. The first nip a-Si cells were made on steel substrates with flat barrier layer and had initial efficiencies of 6.3%, showing the potential of the concept.     

Segregation behaviors and radial distribution of dopant atoms in silicon nanowires

Gaining an understanding the dynamic behaviors of dopant atoms in silicon nanowires (SiNWs) is the key to achieving low-power and high-speed transistor devices using SiNWs. The segregation behavior of boron (B) and phosphorus (P) atoms in B- and P-doped SiNWs during thermal oxidation was closely observed using B local vibrational peaks and Fano broadening in optical phonon peaks of B-doped SiNWs by micro-Raman scattering. Electron spin resonance (ESR) signals from conduction electrons were used for P-doped SiNWs. Our results showed that B atoms preferentially segregate in the surface oxide layer, whereas P atoms tend to accumulate in the Si region around the interface of SiNWs. The radial distribution of P atoms in SiNWs was also investigated to prove the difference segregation behaviors between of P and B atoms.     

Quantum junction solar cells

Colloidal quantum dot solids combine convenient solution-processing with quantum size effect tuning, offering avenues to high-efficiency multijunction cells based on a single materials synthesis and processing platform. The highest-performing colloidal quantum dot rectifying devices reported to date have relied on a junction between a quantum-tuned absorber and a bulk material (e.g., TiO(2)); however, quantum tuning of the absorber then requires complete redesign of the bulk acceptor, compromising the benefits of facile quantum tuning. Here we report rectifying junctions constructed entirely using inherently band-aligned quantum-tuned materials. Realizing these quantum junction diodes relied upon the creation of an n-type quantum dot solid having a clean bandgap. We combine stable, chemically compatible, high-performance n-type and p-type materials to create the first quantum junction solar cells. We present a family of photovoltaic devices having widely tuned bandgaps of 0.6-1.6 eV that excel where conventional quantum-to-bulk devices fail to perform. Devices having optimal single-junction bandgaps exhibit certified AM1.5 solar power conversion efficiencies of 5.4%. Control over doping in quantum solids, and the successful integration of these materials to form stable quantum junctions, offers a powerful new degree of freedom to colloidal quantum dot optoelectronics.     

Influence of the contacting scheme in simulations of radial silicon nanorod solar cells

Silicon nanorod solar cells were simulated using the Silvaco Technical Computer Aided Design (TCAD) software suite. For reasons of speed optimization the simulations were performed in cylinder coordinates taking advantage of the model's symmetry. Symmetric doping was assumed with a dopant density of 10¹⁸ cm⁻³ in the p-type core and in the n-type shell, and the location of the pn-junction was chosen such that the space charge region was located adjacent to the shell surface. Two contact configurations were explored. In configuration A the cathode contact was wrapped around the semiconductor nanorod, while in configuration B the cathode was assumed just on top of the nanorod. In both cases the anode was located at the bottom of the rod. Cell efficiency was optimized with regard to rod radius and rod length. Optimization was performed in a three-step procedure consisting in radius optimization, length optimization and again radius optimization. A maximum in efficiency with respect to rod length L was visible in configuration A, leading to an optimum value of L = 48 μm. This maximum is explained by the combination of an increase of short-circuit current density J_sc and a decrease of open-circuit voltage U_oc with L. In configuration B, J_sc also increases with L, but U_oc stays rather constant and the maximum in efficiency only appears at very large values of L ≈ 12 mm. We restricted the rod length to L ≤ 100 μm for further optimization, in order to stay in an experimentally feasible range. During the optimization of rod radius R in configuration A the open circuit voltage increased continuously, while short circuit current density stayed rather constant. This leads to an increase in efficiency with R, which only stops at very large radii, where R starts to be comparable with L. In configuration B efficiency is almost independent of R, provided that the radius is large enough to comprise a well-formed space charge region, here only a shallow maximum can be estimated. With the demand of rod lengths being smaller than 100 μm, optimum parameters L = 48 μm, R = 32 μm and L = 96 μm, R = 2 μm were extracted for configuration A and B, respectively.     

Conductance, surface traps, and passivation in doped silicon nanowires

We perform ab initio calculations within the Landauer formalism to study the influence of doping on the conductance of surface-passivated silicon nanowires. It is shown that impurities located in the core of the wire induce a strong resonant backscattering at the impurity bound state energies. Surface dangling bond defects have hardly any direct effect on conductance, but they strongly trap both p- and n-type impurities, as evidenced in the case of H-passivated wires and Si/SiO2 interfaces. Upon surface trapping, impurities become transparent to transport, as they are electrically inactive and do not induce any resonant backscattering.     

Aluminium doped zinc oxide sputtered from rotatable dual magnetrons for thin film silicon solar cells

This study addresses the electrical and optical properties as well as the surface structure after wet-chemical etching of mid-frequency magnetron sputtered aluminium doped zinc oxide (ZnO:Al) films on glass substrates from rotatable ceramic targets. Etching of an as-deposited ZnO:Al film in acid leads to rough surfaces with various feature sizes. The influence of working pressure and substrate temperature on the surface topography after etching was investigated. It was found that the growth model which Kluth et al. applied to films sputtered in radio frequency mode from planar ceramic target can be transferred to film growth from tube target. Furthermore, the influence of Ar gas flow and discharge power on the film properties was investigated. We achieved low resistivity of about 5.4 × 10^− 4 Ω·cm at high growth rates of 120 nm·m/min. Finally, surface textured ZnO:Al films were applied as substrates for microcrystalline silicon solar cells and high efficiencies of up to 8.49% were obtained.