Andreas Fell

A Free and Fast Three-Dimensional/Two-Dimensional Solar Cell Simulator Featuring Conductive Boundary and Quasi-Neutrality Approximations

Details of Quokka, which is a freely available fast 3-D solar cell simulation tool, are presented. Simplifications to the full set of charge carrier transport equations, i.e., quasi-neutrality and conductive boundaries, result in a model that is computationally inexpensive without a loss of generality. Details on the freely available finite volume implementation in MATLAB are given, which shows computation times on the order of seconds to minutes for a full I-V curve sweep on a conventional personal computer. As an application example, the validity of popular analytical models of partial rear contact cells is verified under varying conditions. Consequently, it is observed that significant errors can occur if these analytical models are used to derive local recombination properties from effective lifetime measurements of test structures.

Input Parameters for the Simulation of Silicon Solar Cells in 2014

Within the silicon photovoltaics (PV) community, there are many approaches, tools, and input parameters for simulating solar cells, making it difficult for newcomers to establish a complete and representative starting point and imposing high requirements on experts to tediously state all assumptions and inputs for replication. In this review, we address these problems by providing complete and representative input parameter sets to simulate six major types of crystalline silicon solar cells. Where possible, the inputs are justified and up-to-date for the respective cell types, and they produce representative measurable cell characteristics. Details of the modeling approaches that can replicate the simulations are presented as well. The input parameters listed here provide a sensible and consistent reference point for researchers on which to base their refinements and extensions.

Laser-doped silicon solar cells by Laser Chemical Processing (LCP) exceeding 20% efficiency

The introduction of selective emitters underneath the front contacts of solar cells can considerably increase the cell efficiency. Thus, cost-effective fabrication methods for this process step would help to reduce the cost per Wp of silicon solar cells. Laser Chemical Processing (LCP) is based on the waterjet-guided laser (LaserMicroJet®) developed and commercialized by Synova S.A., but uses a chemical jet. This technology is able to perform local diffusions at high speed and accuracy without the need of masking or any high-temperature step of the entire wafer. We present experimental investigations on simple device structures to choose optimal laser parameters for selective emitter formation. These parameters are used to fabricate high-efficiency oxide-passivated LFC solar cells that exceed 20% efficiency.

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.

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.

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.

Simplified Device Simulation of Silicon Solar Cells Using a Lumped Parameter Optical Model

An optical model for solar cell device simulations providing a computational rapid alternative or extension to ray tracing is presented. Its lumped input parameters are mainly the wavelength-dependent external front surface transmission Text and pathlength enhancement Z, which can be derived by measurements and/or ray tracing of finished devices. A way to calculate the generation profile G from those inputs is described, showing negligible error for typical silicon solar cell properties compared to G from ray tracing. Including a recently proposed parameterization of Z, it is shown that the lumped input parameters are, to good approximation, independent of 1) the incident spectrum, 2) the device thickness, and 3) the device temperature. The latter mainly assumes that the temperature influence of the silicon bulk dominates over the thin films one, which is shown experimentally for a few typical thin-film materials. The model is successfully applied to accurately predict optical characteristics of high efficiency laboratory solar cells with two different thicknesses and temperatures. It is thus useful to simplify and speed up optical modeling relative to ray tracing alone, without significant error for typical silicon solar cell properties.

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.