Quantum efficiency (QE) testing measures the wavelength-dependent ability of a photovoltaic module or cell to convert incident photons into collectable electrical current, making it one of the most powerful diagnostic tools in solar device characterization. Accredited third-party laboratories perform QE measurements in accordance with IEC 60904-8, ASTM E1021, and IEC 61853-2 to support cell development, production quality control, degradation analysis, and precise power rating. This article explains the technical principles of photovoltaic module quantum efficiency testing, typical sample types, testing items, measurement methods, and practical applications across the solar industry.
Technical Overview
Quantum efficiency (QE) is defined as the ratio of charge carriers collected at the terminals of a photovoltaic device to the number of photons incident upon it at a specified wavelength. Because different semiconductor materials absorb and convert light with strongly wavelength-dependent behavior, a QE measurement effectively produces a spectral fingerprint of a cell or module, revealing how the device performs from the ultraviolet range through the visible spectrum to the near-infrared absorption edge. For crystalline silicon devices, this range typically spans approximately 300 nm to 1200 nm, while thin-film and multi-junction devices may require measurement between 300 nm and 1400 nm or beyond.
Two complementary quantities are commonly reported in quantum efficiency characterization:
- External Quantum Efficiency (EQE): the fraction of all incident photons that generate collectable electrons, including the effects of front-surface reflection, metal grid shading, and parasitic absorption. EQE represents the real-world conversion behavior of the device as manufactured.
- Internal Quantum Efficiency (IQE): the fraction of absorbed photons that generate collectable electrons, obtained by correcting EQE for reflectance and transmittance losses. IQE isolates the electronic quality of the absorber material and junction structures from purely optical losses.
Quantum efficiency is mathematically related to spectral responsivity (SR), which is expressed in A/W, through the relation QE(λ) = 1240 × SR(λ) / λ, where the wavelength λ is given in nanometers. This relationship allows laboratories to measure spectral responsivity directly and report the result in quantum efficiency terms. One of the most important analytical outputs is the short-circuit current density calculated by integrating the EQE spectrum with a standard reference solar spectral irradiance distribution, such as the AM1.5G spectrum defined in IEC 60904-3 and ASTM G173. Agreement between the integrated current and the current measured by current-voltage characterization is a widely accepted cross-validation of both measurements.
Quantum efficiency testing is governed primarily by IEC 60904-8 (measurement of spectral responsivity of photovoltaic devices), IEC 60904-8-1 for multi-junction devices, ASTM E1021 (spectral responsivity measurement of photovoltaic cells), and IEC 61853-2, which incorporates spectral responsivity measurement into the module energy rating procedure. Spectral mismatch corrections are performed according to IEC 60904-7, while linearity verification follows IEC 60904-10.
Testing Samples
Crystalline Silicon Cells and Mini-Modules
Individual crystalline silicon solar cells, including monocrystalline, multicrystalline, PERC, TOPCon, and heterojunction architectures, are the most frequently tested samples. Mini-modules assembled from full or cut cells (half-cut, quarter-cut) are also accepted for evaluating interconnection and encapsulation effects. Samples are typically required to have accessible busbars or tabbed leads for four-wire electrical contact.
Full-Size Photovoltaic Modules
Complete laminated modules, either glass-fabricate or flexible constructions, can be characterized at the module level. Module testing evaluates the combined spectral effects of the encapsulant, front glass, and anti-reflective coatings in addition to the cell response. Depending on laboratory equipment, testing is performed either with a large-area monochromatic beam illuminating a representative region or by scanning the beam across multiple cell positions to assess spatial uniformity.
Thin-Film Devices
Thin-film technologies such as cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous or microcrystalline silicon are tested with particular attention to metastability and spectral splitting behavior. These samples usually require extended wavelength coverage, controlled light soaking history, and continuous bias illumination during measurement to ensure representative operating conditions.
Emerging and Multi-Junction Devices
Perovskite solar cells, perovskite-silicon tandems, III-V multi-junction concentrator cells, and other advanced architectures demand specialized protocols. Each sub-junction must be measured selectively using appropriate light biasing and voltage biasing so that the sub-junction under test limits the overall device current, as prescribed in IEC 60904-8-1.
Testing Items
A standard quantum efficiency test campaign for photovoltaic cells and modules generally includes the following deliverables:
- Absolute external quantum efficiency (EQE) as a function of wavelength over the applicable spectral range.
- Internal quantum efficiency (IQE) spectrum, calculated from simultaneous or supplementary reflectance and, where relevant, transmittance measurements.
- Absolute spectral responsivity curve in A/W across the measured wavelength range.
- Integrated short-circuit current density under the AM1.5G reference spectrum, with comparison to values obtained from current-voltage measurements.
- Spectral mismatch factor between the test device and a calibrated reference device under a specified light source, calculated per IEC 60904-7.
- Absorption edge position, effective bandgap determination, and long-wavelength cut-off characterization.
- Spatial non-uniformity mapping of quantum efficiency across a cell or module area, where scanning capability is available.
Reporting normally includes the measurement configuration, calibration reference traceability, ambient and device temperature, bias conditions, wavelength step and bandwidth, and a stated measurement uncertainty, which for well-controlled systems is typically within a few percent relative over most of the spectral range.
Testing Methods
Sample Preparation and Mounting
Samples are electrically connected using a four-wire (Kelvin) configuration to eliminate lead resistance errors, and are mounted on a temperature-controlled vacuum chuck or stage maintained at 25 °C ± 1 °C. Devices are stabilized, and where thin-film metastability is a concern, samples are preconditioned with defined light soaking before measurement. Full modules are secured in vertical or horizontal fixtures with the test plane accurately aligned to the optical axis.
Monochromatic Illumination and Signal Acquisition
A quartz tungsten halogen lamp or xenon source is coupled to a grating monochromator or a filter wheel to generate monochromatic light, which is chopped at a low frequency, typically in the range of tens of hertz. Order-sorting filters suppress higher-order diffraction. The resulting modulated photocurrent is detected with a lock-in amplifier, providing excellent signal-to-noise ratio even at low photon flux levels in the ultraviolet and near-infrared regions. Step sizes are selected to resolve fine spectral features, commonly 5 nm to 10 nm for silicon devices and finer steps near the band edge.
Bias Light and Bias Voltage Application
A spectrally continuous bias light source, such as white-light LEDs or a filtered halogen lamp approximating one-sun intensity, is superimposed on the chopped monochromatic probe beam. The bias light fixes the device at an operating point representative of real illumination, which is essential for thin-film devices and multi-junction structures exhibiting light-dependent collection behavior. For sub-junction-selective measurements in tandem devices, dedicated bias light sources and adjustable bias voltages are applied to saturate the complementary sub-junctions so that only the junction under test responds to the probe wavelength.
Reference Detector Calibration
The monochromatic power at each wavelength is determined using calibrated reference photodiodes, typically traceable silicon and indium gallium arsenide detectors covering the short and long wavelength regions respectively. Reference devices are measured immediately before or after the sample under identical optical conditions, and the ratio of sample photocurrent to reference signal yields the absolute spectral responsivity, from which quantum efficiency is computed. Traceability to national metrology institutes is documented in the test report.
Data Processing and Reporting
Raw responsivity data are corrected for detector calibration values, amplifier gains, and chopper geometry, then converted to EQE. IQE is derived as EQE divided by the absorptance, that is, one minus reflectance minus transmittance. The integrated short-circuit current density is computed by convolving the EQE spectrum with the AM1.5G photon flux distribution, and results are presented as graphical spectra with tabulated key values. Measurement uncertainties are evaluated considering wavelength accuracy, reference detector calibration, stray light suppression, temperature stability, and repeatability.
Applications
Quantum efficiency testing serves a wide range of technical and commercial purposes throughout the photovoltaic value chain:
- Research and development: quantifying the effectiveness of anti-reflection coatings, surface texturing, passivation layers, and emitter designs by separating optical from recombination losses, and guiding the optimization of novel absorber materials such as perovskites and their tandem configurations.
- Production quality control: monitoring batch-to-batch consistency of cell fabrication processes and detecting process drifts that manifest as characteristic QE deficits in specific spectral regions.
- Failure and degradation analysis: identifying ultraviolet-induced encapsulant degradation, potential-induced degradation, light-induced degradation, cell cracking, and corrosion, each of which leaves a distinct signature in the spectral response.
- Energy yield prediction: enabling spectral mismatch correction when modules are measured under simulators or deployed under real-world spectra that differ from the standard reference spectrum, thereby improving energy rating in accordance with the IEC 61853 series.
- Reference device calibration: establishing the spectral responsivity of primary and secondary reference cells and modules used for simulator-based power measurement in accredited third-party laboratories.
- Bankability and certification support: supplying independent spectral performance data that underpin technical due diligence for module procurement, project financing, and independent verification programs.
FAQ
What is the difference between external and internal quantum efficiency?
External quantum efficiency counts every photon incident on the device surface and therefore includes losses from reflection, grid shading, and parasitic absorption. Internal quantum efficiency excludes these optical losses by considering only photons actually absorbed in the device, making it a direct indicator of the electronic quality of the junction and absorber. A high IQE combined with a lower EQE points to optical optimization opportunities, whereas a depressed IQE indicates recombination or collection problems.
How does quantum efficiency relate to module power output?
The short-circuit current of a module is proportional to the integral of its quantum efficiency weighted by the incident solar spectrum. A device with reduced QE in the blue or near-infrared region will deliver correspondingly lower current and thus lower power. QE data are also used to compute spectral mismatch factors, which correct power measurements made under simulator spectra that differ from the reference spectrum.
What wavelength range is covered in a typical test?
For crystalline silicon devices, measurements commonly cover 300 nm to 1200 nm. Thin-film and tandem devices may require coverage from approximately 300 nm to 1400 nm or beyond, depending on the bandgaps of the constituent absorber layers. The specific range is selected to fully capture the absorption edge and any relevant sub-bandgap response.
Can full-size modules be tested, or only individual cells?
Both can be tested. Cell-level measurements offer the highest accuracy and spatial resolution, while module-level measurements capture the spectral influence of glass, encapsulant, and interconnects. Large-aperture systems or scanning beam techniques allow representative regions or multiple cells of a full module to be characterized, including spatial uniformity assessment.
Which standards govern quantum efficiency testing?
The principal standards are IEC 60904-8 for spectral responsivity measurement of photovoltaic devices, IEC 60904-8-1 for multi-junction devices, ASTM E1021 for cell-level spectral responsivity, IEC 61853-2 for spectral responsivity within the module energy rating framework, IEC 60904-3 and ASTM G173 for reference spectra, and IEC 60904-7 for spectral mismatch computation.
What sample preparation is required before testing?
Samples should be clean, free of shading objects, and equipped with accessible electrical contacts for four-wire connection. Thin-film and metastable devices may require documented light soaking or dark storage history. Clients are typically advised to confirm sample dimensions, contact layout, and any special handling requirements with the testing laboratory in advance to ensure efficient measurement.
