Assessment of the composition of Silicon-Rich Oxide films for photovoltaic applications by optical techniques

Abstract The deposition of sub-stoichiometric silicon rich oxide (SRO) is the first step to obtain well ordered silicon Quantum Dots (QDs) in a dielectric matrix. This structure is used also for third generation photovoltaic devices operating in a tandem architecture. A precise control and assessment of the stoichiometry of these films is crucial to tune the electrical and optical properties of the device. In this paper we discuss two optical techniques to assess the composition of such films and we compare their results

Si solid-state quantum dot-based materials for tandem solar cells

The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed

Improvements and gaps in the empirical expressions for the fill factor of modern industrial solar cells

This study assesses and improves the accuracy of commonly used expressions for the fill factor (FF). Parameters that could affect the accuracy of the revised expressions are investigated. Empirical coefficients of the commonly used analytical expressions are first recalculated using a modified fitting approach. Although the predictions of the revised expressions perfectly match the results of theoretical one-diode model simulations, gaps are observed when compared with actual measurements. The different impacts of unaccounted factors in the expressions are then explored. It is shown that adjusting the ideality factor or considering edge recombination improves the accuracy of the predictions. Moreover, the expressions can slightly overestimate the FF of cells with non-uniform implied open-circuit voltage distribution. As methods to extract electrical parameters from luminescence images continuously improve, the findings of this study can aid in developing techniques for extracting FF from luminescence images of industrial solar cells

Experimental testing of SiNx/SiO 2 thin film filters for a concentrating solar hybrid PV/T collector

Achieving high temperature thermal outputs from concentrating photovoltaic/thermal (PV/T) systems presents a challenge in that the performance of the PV cells declines with increasing temperature. Spectral beam splitting is an attractive approach to address this conflict by thermally decoupling the PV and thermal receivers, allowing the PV cells to operate at low temperature and the thermal receiver to operate at high temperature. In this study, SiNx/SiO2 multilayer thin film filters were designed and fabricated to act as beam splitting devices in a 10 sun, linear Fresnel mirror-based, concentrating PV/T solar collector. In this collector, reflected light is directed to a silicon PV cell whilst the transmitted light is directed to a thermal receiver. Plasma-enhanced chemical vapor deposition (PECVD) was used to fabricate the filters which were designed to obtain maximum hybrid output. The resulting devices have high reflectance (greater than 95%) for light between 713 and 1067 nm and high transmittance (greater than 90%) for sunlight outside that reflection window. The concentration of process gases in the PECVD reactor was varied in order to reduce undesired absorption at short wavelengths –lower than 650 nm– by the SiNx layers. Indoor testing was carried out for the filters in a system which consists of a Si PV cell, a thermal sensor, and a solid-state plasma light source (6500 K black body spectrum). This study tested filter performance for various angles of incidence (AOI) between 20 and 45°. The experimental results indicate that the PV cells, illuminated with the reflected light from the filters, operate on average at 9.2% absolute higher efficiency than the same cells without the filter. Furthermore, for the best filter, in terms of relative percentage, the measured hybrid output (weighted by a worth factor of electrical vs. thermal energy) is ∼9% higher than the electrical output of a PV cell stand-alone system exposed to the same light source. This paper represents the first study of a hybrid PV/T solar collector using SiNx/SiO2 thin film filters and demonstrates the feasibility of such systems. This study also indicates that this type of system can utilize 85.6% of the incoming solar spectrum based on the measured optical properties of the filters

Hybrid solar energy harvesting and storage devices: The promises and challenges

Hybrid devices that can harvest solar energy and store that energy electrochemically to provide a source of power are increasingly attracting attention due to their potential to provide autonomous power sources. Of particular interest is their ability to support sensors for the Internet of Things (IoT), wearable electronics and autonomous medical monitoring. Many such hybrid devices have been reported, however challenges exist with respect to electrode arrangements and operating modes, form factors, material compatibility and durability. In this perspective, we review both the application potential and design/fabrication challenges for this class of device. It is proposed that device architecture and material choices need to be carefully selected according to the specific intended application to ensure adequate durability and offer practical outcomes over alternative solutions comprising individual solar harvesting and energy storage devices

Pulsed KrF excimer laser dopant activation in nanocrystal silicon in a silicon dioxide matrix

We demonstrate that a pulsed KrF excimer laser (λ = 248 nm, τ = 22 ns) can be used as a post-furnace annealing method to greatly increase the electrically active doping concentration in nanocrystal silicon (ncSi) embedded in SiO2. The application of a single laser pulse of 202 mJ/cm2 improves the electrically active doping concentration by more than one order of magnitude while also improving the conductivity. It is confirmed that there is no film ablation or significant change in ncSi structure by atomic force microscopy and micro-Raman spectroscopy. We propose that the increase in free-carrier concentration is the result of interstitial P/B dopant activation, which are initially inside the Si crystallites. Evidence of mobility limited carrier transport and degenerate doping in the ncSi are measured with temperature-dependent conductivity

Kinetics studies of thin film amorphous titanium niobium oxides for lithium ion battery anodes

Amorphous titanium niobium oxides (TNOs) with varying ratios of Ti and Nb (Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7) are presented as promising anode materials for Li ion batteries. The capacity of the TNO materials is seen to be equivalent to, or larger than, that of the binary oxides, with average volumetric capacities over the first 10 cycles of 717, 1,039 and 925 mAh cm−3 for amorphous Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7, respectively at a current density of 0.2 A cm−3, compared to 720 mAh cm−3 and 425 mAh cm−3 for amorphous TiO2 and Nb2O5. Using densities estimated with X-ray reflectometry, these are equivalent to gravimetric capacities of 231, 335, 319 mAh g−1 for amorphous Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7, respectively at a current density of ~70 mA g−1, compared to 257 mAh g−1 and 137 mAh g−1 for amorphous TiO2 and Nb2O5 at a current density ~80 mA g−1 and ~50 mA g−1, respectively. We discuss how rate capability varies with varying ratios of Ti and Nb and relate this to electrochemical parameters determined by the potentiostatic intermittent titration technique. Our findings reveal that the rate capability of the films is dominated by the diffusion resistance, RD, a composite parameter linked to the insertion rate and diffusion coefficient of Li, leading to a conclusion that the rate retention of the thin films is dominated by the density of insertion sites and the insertion reaction more generally

Boron doped Si rich oxide/SiO<inf>2</inf> and silicon rich nitride/SiN<inf>x</inf> bilayers on molybdenum-fused silica substrates for vertically structured Si quantum dot solar cells

Vertically structured Si quantum dots (QDs) solar cells with molybdenum (Mo) interlayer on quartz substrates would overcome current crowding effects found in mesa-structured cells. This study investigates the compatibility between boron (B) doped Si QDs bilayers and Mo-fused silica substrate. Both Si/SiO2 and Si/SiNx based QDs bilayers were studied. The material compatibility under high temperature treatment was assessed by examining Si crystallinity, microstress, thin film adhesion, and Mo oxidation. It was observed that the presence of Mo interlayer enhanced the Si QDs size confinement, crystalline fraction, and QDs size uniformity. The use of B doping was preferred compared to phosphine (PH3) doping studied previously in terms of better surface and interface properties by reducing oxidized spots on the film. Though crack formation due to thermal mismatch after annealing remained, methods to overcome this problem were proposed in this paper. Schematic diagram to fabricate full vertical structured Si QDs solar cells was also suggested