Evan Franklin

3-D Simulation of Interdigitated-Back-Contact Silicon Solar Cells With Quokka Including Perimeter Losses

An interdigitated-back-contact (IBC) version of Quokka, a recently developed free and fast solar cell simulation program, is presented. It is capable of simulating IBC unit cells with a variety of interdigitated contact and diffusion patterns in both 2-D and 3-D. The program is evaluated by comparing simulated and experimental current-voltage (I-V) curves of high-efficiency IBC solar cells. The simulations include the perimeter effects of edges and busbars by simulating the inner unit cell in 3-D, and accounting for the edges and busbars by 2-D unit cell approximations. The simulation agrees well with the experiment under 1-sun conditions with different aperture areas. Furthermore, simulations of the inner unit cell are successfully validated against Sentaurus Device, for both the I-V curve and detailed free energy losses at maximum power point. The results evidence the validity of the quasi-neutral and conductive-boundary approximations employed by Quokka for fast simulation of IBC solar cells.

The Impact of Silicon CCD Photon Spread on Quantitative Analyses of Luminescence Images

Commercial and R&D photoluminescence imaging systems commonly employ indirect bandgap silicon charge-coupled device (CCD) imaging sensors. Silicon is a weak absorber of the near-infrared band-to-band emission of silicon, and significant lateral spreading of the luminescence signal can occur within the sensor. Uncorrected, this effect reduces image contrast, introduces artificial signal gradients, and limits the minimum feature size for which accurate quantitative measurements can be derived. Empirical quantification of the spreading effect defined in terms of the point-spread function (PSF) for the imaging apparatus allows for postprocessing deconvolution, which quantitatively improves image accuracy and contrast. Assessment of the impact of a photon spread indicates that signal smear under commonly occurring high contrast ratio scenarios is sufficient to warrant the application of deconvolution to improve the accuracy of quantitative data in calibrated luminescence images. With a carefully defined PSF, corrections to within ± 10% of the true signal ratio for small-area features can be achieved. Short-pass filtering provides partial correction of the photon spread with the advantage of reduced experimental complexity but, nonetheless, with limitations on the minimum feature size for which accurate signal ratios can be measured.

Contrast enhancement of luminescence images via point-spread deconvolution

We investigate the impact of point-spread in the silicon CCD sensor of a BT Imaging LIS-R1 luminescence imaging system. It is found that an experimental definition of the point-spread function allows for a significant restoration of CCD point-spread to be achieved. A comparison with short-pass filtering is performed, demonstrating that point-spread will still have a measurable influence on image quality while reducing the luminescence signal and increasing the relative noise level. An implementation of point-spread deconvolution is presented at a multi-crystalline silicon grain boundary, illustrating a practical enhancement of resolution in a typical high-contrast scenario. The characteristics of the point-spread presented here are specific to the experimental apparatus investigated, but the procedure described is universally applicable.

Sliver solar cells: high efficiency, low cost PV technology

Sliver cells are thin, single-crystal silicon solar cells fabricated using standard fabrication technology. Sliver modules, composed of several thousand individual Sliver cells, can be efficient, low-cost, bifacial, transparent, flexible, shadow tolerant, and lightweight. Compared with current PV technology, mature Sliver technology will need 10% of the pure silicon and fewer than 5% of the wafer starts per MW of factory output. This paper deals with two distinct challenges related to Sliver cell and Sliver module production: providing a mature and robust Sliver cell fabrication method which produces a high yield of highly efficient Sliver cells, and which is suitable for transfer to industry; and, handling, electrically interconnecting, and encapsulating billions of sliver cells at low cost. Sliver cells with efficiencies of 20% have been fabricated at ANU using a reliable, optimised processing sequence, while low-cost encapsulation methods have been demonstrated using a submodule technique.

Dislocations in laser-doped silicon detected by micro-photoluminescence spectroscopy

We report the detection of laser-induced damage in laser-doped layers at the surface of crystalline silicon wafers, via micron-scale photoluminescence spectroscopy. The properties of the sub-band-gap emission from the induced defects are found to match the emission characteristics of dislocations. Courtesy of the high spatial resolution of the micro-photoluminescence spectroscopy technique, micron-scale variations in the extent of damage at the edge of the laser-doped region can be detected, providing a powerful tool to study and optimize laser-doping processes for silicon photovoltaics.

Sliver Cells - A Complete Photovoltaic Solution

Sliver technology was invented and developed at the Australian National University, with support from the Australian company Origin Energy. The Sliver process uses standard materials and techniques in novel ways to create thin single crystalline solar cells with superior performance and reduced cost. Sliver cells are made from very thin single crystalline silicon, and are highly efficient. Sliver technology offers reductions in silicon consumption by a factor of 10-15 and reductions in wafer throughput per Megawatt by a factor of 20-50. This paper examines the economic potential of Sliver cells. We show that, with careful engineering, a reduction in the cost of PV modules of up to three quarters is possible in the medium term, without the need for any breakthroughs. Sliver technology, with its low cost and multiple attributes, could be a long-term solution for photovoltaics

Characterization of Laser-Doped Localized p-n Junctions for High Efficiency Silicon Solar Cells

To further increase the efficiency of industrial crystalline silicon solar cells, a point-contact solar cell concept with localized p-n junctions is considered a promising candidate if implemented by a low cost processing technique like laser doping. For efficient development and optimization of such a processing technique, we present a dedicated test structure to derive the fundamental diode characteristics specific to the localized p-n junction, namely the contact resistance to the metal and the recombination properties, i.e., the dark saturation current. Those properties are fitted to measured dark current-voltage curves by 3-D device simulations using Quokka. We show that in particular, the contact resistance can be accurately extracted and that the method is robust against uncertainties of other device properties of the test structure. Simulations of an idealized point-contact solar cell are performed to judge the usefulness of the extractable value range with respect to the efficiency potential. Furthermore, we apply the method to laser doping experiments. We successfully characterize the recombination and contact resistance and identify a ~24% efficiency potential of a nonoptimized two-step laser doping process. Other single step processes show a very high recombination (J0pn ≫ 1e-10 A/cm2) likely due to imperfections around the perimeter of the laser processed area.

Secondary Electron Microscopy Dopant Contrast Image (SEMDCI) for Laser Doping

Laser doping has been the subject of intense research over the past decade, due to its potential to enable high-efficiency, low-cost silicon solar cell fabrication. Information about the doping profile that is created by the process is critical for process optimization but is generally difficult to obtain. We apply the technique of secondary electron image (SEI) contrast to the characterization of the cross sections of laser-doped lines. We demonstrate that this technique can be used for a large range of different dopant sources and different laser doping methods and that good dopant contrast can be obtained under a relatively wide range of microscope parameters. Comparison of dopant contrast and doping density profiles shows that the substrate doping is an important parameter that can significantly influence the dopant contrast, particularly at low (~1018 cm-3) and high (~10 20 cm-3 ) dopant densities. When suitable calibration samples are used, the technique can be employed to obtain quantitative dopant density images for p-type laser-doped regions, albeit currently over a limited range of dopant densities and with relatively large error. Furthermore, the technique can be used to evaluate the risk of metallization shunts near the edges of dielectric film windows that are opened by the laser.

Perimeter Recombination Characterization by Luminescence Imaging

Perimeter recombination causes significant efficiency loss in solar cells. This paper presents a method to quantify perimeter recombination via luminescence imaging for silicon solar cells embedded within the wafer. The validity of the method is discussed and verified via 2-D semiconductor simulation. We demonstrate the method to be sufficiently sensitive in that it can quantify perimeter recombination even in a solar cell where no obvious deviation from ideality is observed in the current-voltage (J-V) curve.

The Impact of SiO

Laser doping of semiconductors has been the subject of intense research over the past decades. Previous work indicates that the use of SiO2/SiNx stacks instead of a single dielectric film as the anti-reflection coating and passivation layer results in laser doped lines with superior properties. In this paper, the impact of the SiNx layer thickness in the SiO2/SiNx stacks on the properties of laser doped lines is investigated through resistance measurements of the laser doped line and the silicon-metal contact and the doping profile near the edge of the dielectric window, the latter being an important factor in determining the likelihood of high recombination or even shunting from the subsequent metallization process. Fundamentally, a problem of exposed and undoped silicon near the dielectric window is identified for most of the investigated parameter range. However, optimization of the laser parameters and dielectric film conditions is shown to be capable of preventing or at least minimizing this problem. The results indicate that for the used laser system, samples with thick dielectric stack processed using a low pulse energy and pulse distance yield the most favorable properties, such as low line resistance and low contact resistivity. Under these conditions, the laser doped regions laterally extend underneath the dielectric films, thus reducing the likelihood of high surface recombination.