Solar Cell Characterisation Laboratories

The PV characterisation laboratories at SERIS are equipped with a comprehensive suite of measurement tools to examine both material and device properties. The characterisation of optical and passivation layers, bulk materials, and solar cell metallisations allows better understanding of their influence on the PV efficiency. This complements device level measurements of silicon wafer solar cells, tandem cells, and single-cell mini-modules. Combining judicious characterisation with specialised simulation techniques permits deeper solar cell analysis to quantify performance limiting factors as well as predict the room for further efficiency improvements.

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 the 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.

  • The effective charge carrier lifetime directly influences the open-circuit voltage and the voltage at the maximum power, two of the most important solar cell parameters. Microwavedetected photoconductance decay (μ-PCD) is a time-resolved method to determine and map the effective carrier lifetime of silicon samples with a spatial resolution of 5 mm. Differential lifetime measurement at up to 20 Suns bias light intensity can be performed in this tool.

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

  • This is a non-contact yet very sensitive technique which detects the surface photovoltage signal with UV and blue wavelength excitations, in order to determine the surface recombination velocity of a thin-film coated silicon wafer and infer the surface passivation quality of the coating. For samples with a p-n junction below the probed surface, the output metric is related to the sample’s short-wavelength spectral response.

  • The Sinton Instruments WCT-120 system is a standardised carrier lifetime tester for silicon samples that is widely used by research laboratories and PV companies around the world. The system measures the effective carrier lifetime of a silicon sample from its impedance and the incident light intensity. From the measured effective carrier 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) are the “X-ray scanners” of the silicon PV industry, capable of producing quick scans for routine inspections, or detailed two-dimensional data sets amenable to sophisticated computational analysis. PL and EL images are maps of the excess charge carrier density, which in turn are influenced by the local junction voltage and effective carrier lifetime. These maps can be obtained in the order of seconds, 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 as the sample goes through the individual processing steps.

    PL as a tool to track the evolution of the cell voltage potential at different processing steps for n-type bifacial cells. The accompanying simulated images to the experimental data illustrate the approach to use computational device models to autofit to data and extract important device parameters.

  • External quantum efficiency (EQE) and total reflectance (R) measurements on the active area of the solar cell (i.e., between the 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 integrates a microscope, spectrometer unitand other sampling accessories. It extracts 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. A Raman spectrum as a function of wavenumber can be computed, where known peak positions from the literature allow the identification of materials. The relative peak intensities and peak shifts provide important information with regards to the measured material, such as strain and stress information.

  • An FTIR spectrometer measures the amount of infrared light a sample absorbs at each wavelength due to change in dipoles of molecules during vibration. It is used to characterise the chemical composition of materials. Its purpose-built accessories and integrated software allow surface analysis of materials (i.e. solid, liquid or gas) using attenuated total reflection (ATR) as well as fully automated mapping analysis of samples with relatively high optical 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.

  • Four-point-probe-based measurement is the industry standard for the measurement of the sheet resistance of conductive materials. It is commonly used 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 in the mapping mode.

  • Among the various contributors to the total series resistance of 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. 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 transmission line method (TLM) enables metal-semiconductor contact resistance measurements down to 1 mΩ-cm2. Different probe heads are available for a wide range of metallisation finger pitches, for measuring both screen-printed cells as well as evaporated metal contacts on test structures. Busbarto-busbar resistance and line resistance measurements are complementary techniques to determine metal grid resistance.

  • This surface profiler measures surface steps, variation and roughness as a function of position by monitoring the vertical displacement of a stylus that is moved laterally across the surface of a sample. With a vertical resolution of 10 nm, vertical range of 524 µm, and lateral scan length range of 55 mm, it is an excellent tool for the determination of material etch rates and deposition rates, and the profiles of microstructures.

  • Light and dark conductivity characterisation provides very important parameters describing 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.

    Silicon wafer bulk resistivity (Left) and thickness (Right) measurement maps .

  • 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 wafers and silicon thin-films. Active dopant densities in the range of 1012 – 1021 cm-3 can be detected with a depth resolution of 1 nm.

    Phosphorus dopant profiles in silicon measured with the ECV technique

  • As the standard method for the measurement 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 to 10 MΩ).

  • An analytical technique that is used to measure elemental concentrations down to the parts per trillion (ppt) level, 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 device manufacturing process. Localised analysis can be performed by laser ablation of the sample in spot sizes ranging froom 1 µm to 400 µm. Here, the sample surface is ablated with a high-powered laser, creating an aerosol that is swept by a carrier gas into the ICP-MS system.

    Detection of trace metal contamination in silicon wafers by ICPMS

  • Modulated PL is capable of measuring the charge carrier lifetime as a function of injection level for a variety of silicon wafer 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. The tool has also been cross-calibrated with µ-PCD and eddy current based carrier lifetime measurement methods.

  • Time-resolved fluorescence spectroscopy is a technique used to study various transient events in fluorescent and semiconductor samples, e.g. charge carrier transfer and recombination, down to sub-nanosecond time resolution. For solar cell applications, this capability makes it ideal for the study of carrier lifetimes in direct-bandgap semiconductors like InGaP, GaAs, InGaN and perovskites. The setup presently employs two pulsed lasers providing optical excitation at 520 and 760 nm. It may be upgraded with other excitation laser wavelengths to take advantage of the highly capable monochromator (spectral range about 250 to 1700 nm, with sub-nm resolution) and photomultiplier tube (detection range about 250 to 920 nm).

    Time-resolved photoluminescence (PL) of a GaAs thin film showing the instrument response function (IRF), PL decay, and fitted model decay. The residual trace (bottom panel) indicates good agreement between the fitted and measurement data

Microscopy Laboratory

  • This field emission 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 micro- and nanopatterned structures.

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

  • EBIC is routinely used to scan a solar cell cross-section to determine 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 carrier generation, to infer the emitter’s carrier collection efficiency.

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

  • Ellipsometry measures the change in polarisation of light reflected by a sample surface. By comparing the measurements with an optical model, the technique enables the determination of the complex refractive index (n, k) and the thickness of thin optical coatings. Further details like interface roughness, interface oxide, and layer stack resolution can also be obtained. The SE-2000 has an additional tilted sample stage, which is ideal for measurements of the pyramid facets of textured monocrystalline silicon solar cells.

  • The Zeta optical profiler is a 3D true colour imaging tool that can image large areas of a sample surface and provide accurate topography information contactlessly. It allows measurements of lateral dimensions, step heights and wall angles and is ideally suited to obtain high-resolution 3D shapes of metallisation lines and pyramid textures on silicon wafer solar cells.

    The Zeta optical profiler is capable of obtaining detailed topographic information of structures on a solar cell, for example printed metallisation lines.

    Scanning electron microscope

    Spectroscopic ellipsometer

Solar Cell Measurement Laboratory

  • This xenon lamp based solar simulator meets the specifications of the highest solar simulator class (‘AAA’), with a spectrum that achieves better than 12.5% spectral match to the AM1.5G solar spectrum. It provides 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 under standard test conditions.

  • Two Class AAA solar simulators featuring state-of-the-art LED array technology provide spectrum tunability across 300- 1100 nm. The built-in spectrometer and photodetector 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 measurements to be performed from 1.2 Suns down to 0.1 Suns. Together, these solar simulators are capable of measuring solar cells with metallisation grid patterns ranging from traditional H-patterns to newer busbarless designs.

    LED solar simulator for solar cell I-V measurements.

  • This system projects a large monochromatic beam which overfills the silicon wafer area for differential spectral response measurement up to 1-Sun bias light intensity. Solar cells with an area of up to 160 mm × 160 mm can be measured. The external quantum efficiency (EQE) extracted from the spectral response curve enables the determination of the 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, antireflection 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. The tool allows measurement of parameters such as the area and dimensions of the sample, as well as features (e.g. busbars and fingers of the 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 1 Sun and at 25°C device temperature. 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 heated surface to achieve a higher temperature. Light soaking is important for solar cells that suffer from light-induced degradation (LID) effects, such as amorphous silicon thin-film solar cells (‘Staebler-Wronski effect’), boron-doped Cz silicon wafer solar cells (boron-oxygen related defects), and other degradation modes found in multicrystalline silicon solar cells.

  • A custom-built LED-based light soaking system extends the capabilities of light soaking up to 5 Suns intensity and up to 300°C device temperature. The functionalities are particularly relevant for studying light and elevated temperature induced degradation (LeTID) - also known as carrier-induced degradation - in multicrystalline silicon solar cells employing the passivated emitter and rear cell (PERC) architecture.

    SERIS’ customised large-area light soaking tool is capable of steady-state light intensities of up to 5 Suns and device temperatures of 25 - 300°C.

For further information, please contact:

Dr HO Jian Wei
jw.ho@nus.edu.sg