Solar Cell Characterisation Laboratory

A comprehensive set of characterisation tools operated by competent scientists fully complements the state-of-the-art solar cell processing lines at SERIS. These tools are used on a routine basis to characterise optical and passivation layers, bulk materials, metallisation grids, and to separate a whole host of cell device parameters that are critical for identification of areas for improvement. Silicon wafer solar cells, thin-film cells, tandem cells, and single-cell modules are measured in the characterisation laboratories on a daily basis. Cells and materials from the processing lines at SERIS, and from other labs across the world, go through the comprehensive characterisation routines to obtain a breakdown of the optical, resistive and recombination losses, as well as other, more specific, analyses.

Materials Characterisation Laboratory

Materials Characterisation Laboratory

  • This technique provides fast, contactless measurement of interface parameters that affect the passivation quality of dielectric films for PV applications. The technique uses incremental corona charging of dielectrics and subsequent measurement of the surface potential with a vibrating capacitive electrode (‘Kelvin probe’). The metrological capabilities include a mapping of contact potential (in the dark or under illumination), band bending at the semiconductor/dielectric interface, fixed charge in the dielectric, and interface defect density.

    Interface defect density determined by the corona-voltage measurement tool.

  • The effective carrier lifetime directly influences the open-circuit voltage and the voltage at maximum power, two of the most important solar cell parameters. μ-PCD is a time resolved method to determine the effective lifetime in silicon samples with 5 mm spatial resolution.

  • An alternative to the four-point probe, this non-contact probe provides mapping of the emitter sheet resistance of solar cells in the range of 10 to 1000 Ohm/square.

  • This is a non-contact, yet very sensitive technique to determine the front surface recombination velocity in a silicon wafer, in order to infer the surface passivation quality of the front coating.

  • The Sinton Instruments WCT-120 system is a standardised lifetime tester that is widely used by research laboratories and PV companies around the world. The system measures the effective lifetime of a silicon sample from its impedance and the incident light intensity. Besides measuring the effective lifetime, the system also provides the implied Voc and the emitter saturation current density j0. There is also the option to control the sample temperature in the range of 30-190°C.

  • Photoluminescence (PL) and electroluminescence (EL) have proven themselves as some of the most versatile imaging tools in the PV industry. PL and EL images are maps of the excess carrier lifetime, which in turn are influenced by the junction voltage and the effective carrier lifetime. These maps can be obtained in the order of a second, on both partially processed silicon wafers as well as finished solar cells. For cells, combinations of images enable the separation of factors that influence device voltage, such as series resistance and the saturation current density. Because PL can be applied to a wafer at any stage of processing, it is also an ideal tool to track the evolution of the cell voltage potential at different processing steps.

    (Left) Simulated and experimental effective carrier lifetime for rear side locally laser-ablated structures. (Right) Photoluminescence image derived lifetime maps at 1 Sun.

  • External quantum efficiency (EQE) and total reflectance (R) measurements on the active area of the solar cell (i.e., between metallisation fingers) enable detailed current loss analysis and the identification of areas of improvement in diffusion lengths and light management. The PVE-300 allows quick and localised measurements of both EQE and R over a wavelength range of 300 – 1700 nm for various types of solar cells.

  • This Raman system, which is compatible with microscopes, spectrometers and sampling accessories, is capable of extracting material information by observing the inelastic scattering of laser light in the visible range. The laser light interacts with the crystal lattice vibrations within the material, causing losses or gains in energy (and hence wavelength changes in the scattered light) during the scattering process. It results in a Raman spectrum as a function of wavenumber, where known peak positions from the literature allow the identification of materials. The relative peak intensities and peak shifts give in-depth information with regards to the materials, such as strain and stress information.

  • The FTIR spectrometer, which measures how much a sample absorbs infrared light at each wavelength where dipoles of the molecules change during vibration, is used to characterise the chemical composition of materials. Its purpose-built accessories and integrated software allow a surface analysis of materials (i.e. solid, liquid or gas) using attenuated total reflection (ATR) as well as a fully automated mapping analysis of samples with a relatively high resolution (~ 0.07 cm-1). It is ideally suited for the analysis of the composition of thin films like amorphous silicon, silicon oxide, silicon nitride and aluminium oxide.

  • The four-point probe is the industry standard for the measurement of the sheet resistance of thin doped layers. It is the most routine measurement tool to characterise the dopant density in solar cell emitter layers as well as transparent conductive oxides. The AIT tool is automated to provide four-point probe measurements with mapping mode.

  • Among the various contributors to series resistance in a solar cell, the contact resistance between the metal electrode and the highly doped semiconductor layers figures prominently because it often has a large impact on the device efficiency, and also because its magnitude varies widely depending on the cell architecture, the metallisation technology used to form the contact, the carrier concentration in the highly doped semiconductor layer, the metallisation material used, and the processing conditions. The tried and tested transmission line method (TLM) enables metal-semiconductor contact resistance measurements down to 1 mΩ-cm2, with different probe heads available for a wide range of front metallisation finger pitch, suitable for measuring both screen printed cells as well as metal evaporated contacts on test structures. Busbar-to-busbar resistance and line resistance measurements are complementary techniques to determine metal finger resistance.

  • The Veeco Dektak 150 Surface Profiler measures surface steps, variation and roughness as a function of position by monitoring the vertical displacement of a stylus that is moved across the sample surface. Vertical resolution 10 nm; Vertical range 524 mm; Scan length range: 55 mm. An excellent tool for the determination of etch rates, deposition rates, and photolithographically defined structures.

  • Light and dark conductivity constitute very important parameters for semiconductor thin films used in solar cell devices, such as the amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) films used in heterojunction silicon wafer solar cells. The activation energy can be extracted from dark conductivity measurements performed at different temperatures, enabling the determination of the Fermi energy for both undoped (or intrinsic) and doped films. Furthermore, the photosensitivity (a quality parameter of amorphous and microcrystalline silicon films) can be determined from the ratio of the light and dark conductivities.

  • ECV allows the extraction of the active doping concentration of doped semiconductors. It can be used to measure the phosphorus or boron doping profile of silicon wafer solar cells and silicon thin-film solar cells. Active dopant densities in the range of 1012 – 1021 cm-3 can be detected with a depth resolution of 1 nm.

  • This class-AAA flash tester with 10-millisecond I-V curve trace-time can perform simultaneous I-V measurements at three light intensities (1, 0.5 and 0.1 Suns) at a throughput of 1200 solar cells per hour, making it ideal for the measurement of pilot line solar cells for statistical analysis. After the I-V testing, an automatic sorter bins the solar cells according to their short-circuit current (Isc), current at maximum power (Imp), or efficiency. For high-efficiency cells with large junction capacitance, a calibration of the I-V curve to the SERIS high-precision solar simulator is necessary to achieve the desired accuracy.

  • As the standard measurement method for the determination of majority carrier concentration and mobility, the Hall system is routinely used to characterise transparent conductive oxides (TCOs) and semiconductor films. It is suitable for samples with a wide variety of resistances (0.5 mΩ to10 MΩ).

  • An analytical technique that is used to measure elemental concentrations down to parts per trillion (ppt), ICP-MS is especially useful for measuring metallic impurities (e.g. Fe, Cu, Cr, Co, Ni, Mo, Zn etc) in silicon wafers or identifying impurities introduced during the manufacturing process. The sample can be prepared by laser ablation of the wafer in spot sizes ranging from 1 µm to 400 µm. In this method, the surface of the sample is ablated with a high-powered laser, creating an aerosol that is swept by a carrier gas into the ICP-MS system.

  • Modulated PL can infer the carrier lifetime as a function of injection level for a variety of silicon wafer related samples, including midstream processed samples, lifetime samples, metallised cells, and even single-cell modules. It is a versatile tool that can track the evolution of carrier lifetimes in samples that are processed into cells.

  • TRPL is a versatile tool used to study various transient events in fluorescent and semiconductor samples like charge transfer and recombination, down to sub-nanosecond time resolution. For solar cell applications, this specification makes it ideal for the study of carrier lifetimes in direct-bandgap semiconductors like InGaP, GaAs, GaN and perovskites. The excitation wavelengths vary from ~500 to 750 nm, and detection in the range of 600 to 950 nm is possible.

Microscopy Laboratory

  • This field emission tip SEM can achieve a resolution of 1 nm at acceleration voltages of below 1 kV. It is ideal for imaging sub-micron morphologies, cell front textures, and for patterning.

    Scanning electron microscope

  • This feature enables the mapping of the crystal orientation of semiconductor films and multicrystalline wafers. It also allows the type of grain boundary of neighbouring crystalline grains to be inferred.

  • EBIC is routinely used to scan a cell cross-section to find the depth, location and uniformity of the p-n junction. It can also be used with the electron beam impinging normal to the sample surface (top view), with the beam energy varied to create different depths of generation, to infer the emitter collection efficiency.

  • Upgraded to do energy dispersive spectroscopy (EDS) measurements of elemental mapping, which is useful for the determination of metal layers in the vicinity of solar cell contacts.

  • The classical technique of comparing the reflectance of s and p polarised light incident on the sample surface. It enables the extraction of the complex refractive index (n, k) and thickness of thin optical coatings. With additional modelling, further details like interface roughness, interface oxide, and layer stack resolution can be obtained as well. The SE-2000 has an additional tilted sample stage, which is ideal for measurements of the pyramid facets of textured monocrystalline silicon solar cells.

    Spectroscopic ellipsometer

  • The Zeta optical profiler, as a well-defined 3D true colour imaging tool, can image large areas and provide accurate topography information by calculating 2D and 3D roughness parameters from millions of data points without contacting the sample surface. It allows measurements of lateral dimensions, step heights and wall angles from a single scan, and is ideally suited to obtain high-resolution 3D shapes of metallisation lines and pyramid textures on silicon wafer solar cells.

  • The Veeco WYCO interferometry based optical profiler with 5x objective is a versatile tool that complements the stylus profiler. It is equipped with the phase shifting interferometry (PSI) for scanning of smooth samples with 1 nm resolution, as well as vertical scanning interferometry (VSI) for scanning of coarse samples that may have large topographies, with < 10 nm resolution. An excellent tool for the determination of etch rates, deposition rates, and photolithographically defined structures.


Solar Cell Measurement Laboratory

  • This xenon lamp based solar simulator meets the specifications of the highest simulator class (‘AAA’), with a spectrum that achieves better than 12.5% spectral match to the AM1.5G spectrum. It provides a uniform illumination intensity across an area of 300 mm × 300 mm, making it well suited to the measurement of silicon cells, single-cell modules, and small thin-film modules.

  • Featuring state-of-the-art LED array technology, this is a solar simulator with a tunable spectrum across 300-1100 nm. The built-in spectrometer and photodiode are designed to give real time feedback to maintain intensity and spectral stability. The LED intensity can be changed over a very large range, enabling I-V and Suns-Voc to be performed from 1.2 Suns down to 0.1 Suns.

    LED solar simulator for solar cell I-V measurements.

  • This system projects a large monochromatic beam which overfills the silicon wafer area to perform differential spectral response under a bias light intensity of up to 1 Sun. Solar cells with an area of up to 156 mm × 156 mm can be measured. The external quantum efficiency (EQE) extracted enables the determination of the so-called spectral mismatch correction factor to refine the prediction of a test solar cell’s short-circuit current under the AM1.5G spectrum.

  • As a solar cell converts light into electricity, knowledge of the interaction of light with the various layers and the bulk material in the solar cell device is crucial. The CARY-7000 enables the determination of the specular and diffuse reflectance and transmittance of materials/devices in the 190 ‑ 2500 nm wavelength range. These measurements are routinely used to assess cell front texture quality, anti-reflection layer properties, and transparency of TCOs. The CARY-7000 is also equipped with a sophisticated angular resolved reflectance accessory, which is useful for determining the angular distribution in reflectance in solar module components or cell front texture.

  • This is a 2D and 3D measurement tool for mono- and multicrystalline silicon wafers and finished solar cells. Measured parameters include the area and width of the sample, as well as the width of busbars and fingers of a metal electrode on the sample’s surface.

  • Light soaking is the pre-conditioning of a solar cell sample prior to device testing. It is usually conducted at an intensity of one sun and at 25°C, but in some cases (for example amorphous silicon thin-film solar cells) a higher light intensity may be used, or the sample may be placed on a hot plate to achieve a higher temperature. Light soaking is important for solar cells that suffer from light-induced degradation effects, such as amorphous silicon thin-film solar cells (‘Staebler-Wronski effect’), boron-doped Cz silicon wafer solar cells (boron-oxygen related defects), and even multicrystalline silicon solar cells.

For further information, please contact:

Dr Johnson WONG
johnson.wong@nus.edu.sg