Photovoltaic Cell Quantum Efficiency Measurement: Principles, Standards, and Test Methods

Quantum efficiency (QE) measurement is the definitive technique for characterizing how effectively a photovoltaic cell converts incident photons into collectable charge carriers across the solar spectrum. This article explains the principles of external and internal quantum efficiency testing, the applicable IEC and ASTM standards, the sample types and test items involved, and the typical measurement procedures used by accredited third-party laboratories. It also covers practical applications in solar cell research, development, production control, and failure analysis.

Technical Overview

Quantum efficiency (QE) describes the ratio of charge carriers collected by a photovoltaic device to the number of photons of a given wavelength incident upon it. Because the QE spectrum reveals exactly where in the wavelength range a cell performs well or poorly, it is one of the most information-rich characterizations available in photovoltaics. Two complementary quantities are routinely measured:

• External Quantum Efficiency (EQE): counts all photons incident on the cell front surface, including those reflected, shaded by the metal grid, or absorbed without carrier generation. EQE therefore captures both optical and recombination losses.

• Internal Quantum Efficiency (IQE): removes the optical reflection term by combining EQE with a hemispherical reflectance measurement, IQE(λ) = EQE(λ) / (1 − R(λ)). IQE isolates the electronic quality of the absorber, junction, and passivation layers.

QE is directly related to the spectral responsivity SR(λ) (in A/W) through the relation EQE(λ) = 1240 × SR(λ) / λ, with the wavelength λ expressed in nanometers. Integrating the EQE spectrum over a reference spectral irradiance, such as the AM1.5G spectrum defined in IEC 60904-3, yields the short-circuit current density Jsc. This integrated Jsc provides an independent cross-check of the Jsc obtained from current–voltage (I–V) measurements and is essential for diagnosing measurement discrepancies.

QE measurements are governed internationally by IEC 60904-8 (measurement of spectral responsivity of photovoltaic devices) and ASTM E1021 (standard test method for spectral response of photovoltaic devices), with supporting requirements from IEC 60904-2 for reference devices, IEC 60904-10 for linearity, and ISO/IEC 17025 for laboratory competence. These standards ensure that results reported by different accredited third-party laboratories are comparable and traceable to national metrology institutes.

Testing Samples

Crystalline Silicon Solar Cells

Conventional and advanced silicon architectures, including monocrystalline, multicrystalline, PERC, TOPCon, heterojunction (HJT), and interdigitated back contact (IBC) cells, are routinely characterized. Typical measurement ranges span 300 nm to 1200 nm, with extended sweeps to 1250 nm for high-quality material. Both bare cells and encapsulated or laminated samples can be measured, provided the encapsulant optical effects are accounted for.

Thin-Film Photovoltaic Cells

Amorphous and microcrystalline silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) devices are tested over ranges matched to their absorber bandgaps, typically 300–900 nm for a-Si and 300–1300 nm for CIGS. Substrate transparency allows both reflection-mode and transmission-mode configurations when required.

III-V and Multi-Junction Solar Cells

Concentrator and space photovoltaic stacks such as GaInP/GaInAs/Ge or GaInP/GaAs/InGaAsP require spectral sweeps from approximately 300 nm to 1800 nm, together with selective bias illumination to isolate individual subcells. These samples demand the most sophisticated measurement protocols described below.

Emerging Photovoltaic Technologies

Perovskite cells, perovskite/silicon tandem devices, organic photovoltaics (OPV), dye-sensitized solar cells (DSSC), and quantum-dot cells are frequently submitted for QE characterization. Special precautions address slow transient effects, metastability, and light soaking sensitivity inherent to some of these materials.

Mini-Modules, Cut Cells, and Research Coupons

Half-cut cells, shingled strips, small-area research coupons, and minimodules can be accommodated. Practical sample requirements normally include accessible contacts (busbars or pads suitable for four-wire probing), a defined aperture area, and a front surface free of opaque fixturing within the illuminated spot.

Testing Items

• External quantum efficiency spectrum EQE(λ) over the applicable wavelength range (typically 300–1200 nm for silicon, extended to 1800 nm for multi-junction devices).

• Internal quantum efficiency spectrum IQE(λ), derived from simultaneous or sequential hemispherical reflectance measurement.

• Absolute spectral responsivity SR(λ) in A/W, traceable to national metrology standards.

• Hemispherical reflectance R(λ) and, for transparent samples, transmittance T(λ).

• Integrated short-circuit current density Jsc under the AM1.5G reference spectrum, with comparison against I–V measured values.

• Individual subcell EQE for multi-junction and tandem devices, including current-matching analysis of limiting subcells.

• Optional studies: response linearity versus light intensity, response dependence on bias voltage, and temperature-dependent QE sweeps.

Testing Methods

Measurement System Configuration

A standard QE measurement system per IEC 60904-8 and ASTM E1021 combines a broadband xenon short-arc lamp and a quartz tungsten-halogen lamp to cover the ultraviolet, visible, and near-infrared ranges. A double-grating monochromator or a set of narrow-band interference filters produces quasi-monochromatic light, which is modulated by a mechanical chopper at a frequency typically between 30 Hz and 400 Hz. The resulting alternating photocurrent is recovered with a lock-in amplifier, providing excellent rejection of ambient light and a high signal-to-noise ratio even at deep-UV and near-band-edge wavelengths where cell response is weak.

Reference Detectors and Traceability

The monochromatic power is established using calibrated reference detectors, such as silicon photodiodes for 300–1100 nm and indium gallium arsenide detectors for the 900–1800 nm region. These detectors are calibrated by national metrology institutes, giving the measurement absolute traceability. The test device is measured under identical geometric conditions, and its spectral responsivity is computed as the ratio of the device photocurrent to the calibrated photon flux. Well-maintained systems achieve a typical relative expanded uncertainty of 2–3% for EQE and slightly higher for IQE once reflectance uncertainty is included.

Measurement Procedure

• Sample mounting on a temperature-controlled vacuum or stage fixture at 25 ± 1 °C, with four-wire contact to busbars or pads to eliminate series resistance artifacts.

• Verification of monochromatic spot size, uniformity, and alignment so that the illuminated area falls within the designated aperture of the cell.

• Determination of the chopper-frequency response, and, where the device exhibits slow transients (common in thin-film and perovskite samples), reduction of the chopping frequency to avoid underestimating the response.

• Wavelength sweep with signal acquisition at each point, applying a specified dwell time to allow detector settling.

• Reflectance measurement with an integrating sphere for IQE calculation where required.

• Data validation, smoothing of high-frequency noise, and computation of the integrated Jsc against the AM1.5G spectral irradiance distribution.

Special Techniques for Multi-Junction and Tandem Devices

In series-connected multi-junction stacks, the measured photocurrent is limited by the subcell that responds to the incident monochromatic light only when all other subcells are saturated. IEC 60904-8 therefore prescribes the use of spectrally selective bias lights: intense auxiliary illumination tuned to each subcell other than the one under test drives those junctions into current saturation, allowing the subcell of interest to be characterized independently. In addition, an appropriate DC voltage bias may be applied to keep the device operating point within the correct regime during the AC measurement. These protocols are indispensable for perovskite/silicon tandems and III-V concentrator cells, where current-matching optimization directly determines fill factor and efficiency.

Data Analysis and Reporting

A complete test report issued in accordance with ISO/IEC 17025 typically includes the EQE and, where applicable, IQE and reflectance spectra in tabular and graphical form, the measurement wavelength range and step, the calculated integrated Jsc, measurement conditions (temperature, spot size, chopping frequency, bias configuration), the standards applied, and a stated measurement uncertainty. Comparison of the integrated Jsc with the Jsc from a calibrated simulator I–V curve is provided as a consistency check, with deviations beyond about 2% triggering investigation.

Applications

• Research and development: diagnosing front-surface recombination, emitter quality, back-surface field effectiveness, anti-reflection coating optimization, and light-trapping performance from the shape of the QE spectrum.

• Tandem and multi-junction development: quantifying subcell currents, identifying the current-limiting junction, and guiding thickness and bandgap engineering for current matching.

• Production quality control: tracking process drift in texturing, passivation, and metallization through statistical monitoring of QE features.

• Failure and degradation analysis: distinguishing UV-induced degradation, potential-induced degradation (PID), light-induced degradation (LID), encapsulant yellowing, and moisture ingress, each of which imprints a characteristic signature on the QE spectrum.

• Reference cell calibration: absolute spectral responsivity measurement is the primary method for calibrating reference devices used in I–V performance measurement per IEC 60904-2.

• Energy rating and yield modeling: spectral responsivity data feed the energy rating procedures of the IEC 61853 series and site-specific spectral yield simulations.

• Academic and publication support: providing traceable, standards-compliant spectra for peer-reviewed studies and thesis work.

FAQ

What is the difference between EQE and IQE?

EQE measures the fraction of all incident photons that generate collected carriers, so it includes losses from reflection and grid shading. IQE removes the reflection loss using a measured reflectance spectrum, revealing only losses caused by incomplete absorption or carrier recombination inside the device. A high IQE combined with a low EQE points to an optical problem, whereas a depressed IQE indicates an electronic one.

Which standards apply to quantum efficiency measurement?

The primary standards are IEC 60904-8 for the measurement of spectral responsivity of photovoltaic devices and ASTM E1021 for spectral response measurement. Supporting documents include IEC 60904-2 (reference devices), IEC 60904-3 (reference spectral irradiance), IEC 60904-10 (linearity), the IEC 61853 series (energy rating), and ISO/IEC 17025 for laboratory quality management. Regional standards such as JIS C 8917 also address spectral response measurement for crystalline silicon cells.

What wavelength range can be measured?

For crystalline silicon cells, the standard range is approximately 300–1200 nm. CIGS and other thin-film devices are typically measured from 300 nm to 1300 nm, while III-V multi-junction and tandem devices require sweeps extending to 1700–1800 nm to capture germanium or silicon bottom-cell response. Custom ranges can be configured to match the absorber bandgap.

Can encapsulated cells or minimodules be tested?

Yes. Laminated cells with glass or polymer front sheets can be characterized, although the encapsulant introduces wavelength-dependent reflection and absorption, particularly in the UV below about 400 nm. Reports for such samples note these effects, and IQE interpretation is adjusted accordingly.

How are tandem and multi-junction cells measured?

Each subcell is measured separately using spectrally selective bias lights that saturate all other junctions, often combined with a DC voltage bias, as prescribed by IEC 60904-8. This allows the EQE of each subcell to be reconstructed and the current-matching ratio of the stack to be quantified.

What sample preparation is required?

Samples should provide electrically accessible contacts (busbars, pads, or ribbon leads), a definable aperture area, and a front surface not obstructed by fixtures. Typical accepted sample dimensions range from small research coupons of about 1 cm² to full-size 210 mm cells and minimodules; four-wire probing is used to avoid contact resistance errors.

How long does a QE measurement take?

A standard EQE sweep on a silicon cell takes roughly 15–30 minutes, including mounting and contact verification. Full multi-junction characterization with bias-light configuration for each subcell may require one to two hours, and reflectance or linearity add-ons extend the session further.

How does quantum efficiency relate to cell efficiency?

QE does not directly yield efficiency, but it determines the photogenerated current: integrating the EQE spectrum with the AM1.5G reference spectrum gives the short-circuit current density, one of the three factors (along with open-circuit voltage and fill factor) that define conversion efficiency. Because the QE spectrum exposes the physical origin of current losses, it is the principal diagnostic tool for improving overall device performance.