Organic Solar Cell Near-Infrared Response Testing: Methods, Standards, and Applications

Near-infrared (NIR) response testing is a critical characterization procedure for organic solar cells (OSCs), quantifying how efficiently these thin-film photovoltaic devices convert wavelengths beyond 700 nm into electrical current. As non-fullerene acceptors have pushed OSC absorption deep into the NIR region, accurate measurement of external quantum efficiency, spectral responsivity, and photocurrent generation in this band has become essential for device optimization, tandem cell design, and commercial validation. This article outlines the sample types, testing items, standardized methods, and application scenarios for organic solar cell NIR response evaluation in accredited third-party laboratories.

Technical Overview

Organic solar cells, also known as organic photovoltaic (OPV) devices, are thin-film solar cells that use organic semiconducting materials—typically conjugated polymers or small molecules—as the light-absorbing layer. One of the most significant advances in OPV technology over the past decade has been the extension of spectral absorption into the near-infrared region (approximately 700 nm to 1100 nm, and in some low-bandgap systems up to 1400 nm or beyond). This progress has been driven primarily by the development of non-fullerene acceptors (NFAs), which enable narrow optical bandgaps while maintaining low energy losses.

Near-infrared response testing measures how effectively an organic solar cell harvests photons in this long-wavelength region. Because photons in the NIR band carry lower energy per photon but represent a substantial fraction of the standard AM1.5G solar spectrum—roughly 40 percent of terrestrial solar energy arrives beyond 700 nm—extending and verifying NIR photocurrent generation is directly linked to higher short-circuit current density (Jsc) and overall power conversion efficiency (PCE). Modern high-performance OSC systems now demonstrate external quantum efficiency values exceeding 80 percent in selected NIR wavelength windows.

Testing in this domain presents unique technical challenges compared with conventional silicon photovoltaics. Organic semiconductors exhibit sub-bandgap absorption tails, charge-transfer state absorption, and nonlinear behavior under varying illumination intensity, all of which must be carefully accounted for during measurement. Consequently, NIR response characterization must be performed with calibrated instrumentation, appropriate reference detectors such as silicon or indium gallium arsenide photodiodes, and strict adherence to international measurement protocols.

Testing Samples

Small-Area Laboratory Cells

Standard research-scale devices with active areas typically between 0.04 cm² and 1 cm², fabricated on patterned indium tin oxide (ITO) glass substrates with layer structures such as electrode/hole transport layer/bulk heterojunction/electron transport layer/electrode. These samples are the most common format for material screening and NIR efficiency optimization studies.

Low-Bandgap and NFA-Based Devices

Devices employing narrow-bandgap donor polymers or non-fullerene acceptor systems specifically engineered for NIR harvesting, including materials with absorption edges approaching or beyond 1000 nm. These samples require measurement ranges that extend past the cutoff of standard silicon detectors.

Semi-Transparent Organic Solar Cells

Devices with transparent electrodes and thin active layers designed for window integration. For these samples, NIR response testing is combined with transmittance measurement to evaluate the trade-off between visible light transparency and long-wavelength power generation.

Tandem and Multi-Junction Devices

Stacked architectures in which a wide-bandgap front cell harvests visible light and a narrow-bandgap rear cell absorbs NIR photons. Characterizing the NIR subcell response often requires bias illumination and bias voltage techniques to isolate the current-limiting junction.

Flexible and Large-Area Modules

Roll-to-roll or sheet-coated devices on flexible substrates such as PET or metal foil, including serially interconnected modules. NIR response verification at the module level supports technology scale-up and commercial qualification.

Testing Items

Comprehensive NIR response evaluation for organic solar cells typically encompasses the following measurement parameters:

• External quantum efficiency (EQE) spectrum across the visible and near-infrared range, typically 300 nm to 1100 nm or extended to 1400–1700 nm using InGaAs detection
• Internal quantum efficiency (IQE), derived from EQE combined with reflectance and transmittance measurements using an integrating sphere
• Spectral responsivity (A/W) as a function of wavelength in the NIR band
• Integrated short-circuit current density calculated by convolving the EQE spectrum with the AM1.5G reference spectrum (IEC 60904-8 methodology)
• NIR absorption onset and optical bandgap determination from the EQE absorption edge using Tauc-type analysis
• Sub-bandgap EQE measurements under sensitive detection modes to probe charge-transfer states and energetic disorder
• Ultraviolet-visible-near-infrared (UV-Vis-NIR) transmittance and reflectance spectra for semi-transparent device evaluation
• Current density–voltage (J–V) characteristics under standard test conditions (1000 W/m², 25 °C, AM1.5G) with special attention to the NIR-driven photocurrent contribution
• Bias-light-dependent EQE to assess nonlinear spectral response and injection effects common in organic devices
• NIR photostability assessment, monitoring EQE and Jsc evolution under prolonged long-wavelength illumination

Testing Methods

Measurement of near-infrared response in organic solar cells follows internationally recognized photovoltaic characterization standards, principally IEC 60904-8 for spectral responsivity measurement, ASTM E1021 for spectral response testing, and companion standards IEC 60904-1, IEC 60904-2, IEC 60904-9, and IEC 60904-10 covering current-voltage characteristics, reference devices, solar simulator classification, and linearity of measurement. The general test workflow is described below.

Spectral Response and EQE Measurement

The sample is illuminated with monochromatic light generated by a grating monochromator or filter wheel coupled to a xenon or tungsten-halogen source. The monochromatic beam is typically chopped at a low frequency, and the resulting modulated photocurrent is recovered using a lock-in amplifier to achieve high sensitivity—essential for resolving weak sub-bandgap and deep-NIR signals in organic devices. The device current response is normalized against a calibrated reference detector, typically a traceable silicon photodiode for wavelengths up to approximately 1100 nm and an indium gallium arsenide photodiode for longer wavelengths. The resulting spectral responsivity is converted to EQE using the photon energy at each wavelength.

Bias Illumination and Corrected EQE

Because organic semiconductors can exhibit intensity-dependent charge generation and recombination, an unfiltered white-light bias source is applied simultaneously with the chopped monochromatic probe. The bias light drives the device close to its operating condition under one-sun illumination, and the small AC signal from the monochromatic light is measured differentially. Where the bias illumination itself contributes to the measured signal, correction procedures described in IEC 60904-8 are applied. For tandem devices, selective bias illumination and applied bias voltages are used to ensure the target subcell is current-limiting during its spectral scan.

Integration with the AM1.5G Spectrum

The measured EQE curve is mathematically integrated with the photon flux of the AM1.5G (IEC 60904-3) reference spectrum to calculate the short-circuit current density. This integrated Jsc is cross-validated against the Jsc obtained from J–V measurements under a calibrated Class AAA (or better) solar simulator, with agreement typically required within a few percent. Significant deviation between the two values indicates measurement artifacts such as spectral mismatch, device hysteresis, or insufficient bias correction.

Optical Characterization

Total transmittance and diffuse plus specular reflectance of the complete device stack are measured with a UV-Vis-NIR spectrophotometer equipped with an integrating sphere. These optical data enable separation of absorption from interference and allow calculation of internal quantum efficiency, providing insight into whether performance limits arise from photon management or charge collection.

Sample Conditioning and Test Conditions

All measurements are performed at a controlled temperature of 25 °C ± 1 °C in a light-tight, electrically shielded enclosure. Devices are characterized through the substrate side as defined by the device architecture, electrical contacts are made with low-resistance probing, and samples are typically allowed to equilibrate after any preconditioning or light-soaking steps so that metastable performance drift does not contaminate the spectral data.

Applications

Near-infrared response testing supports a broad range of research, development, and commercialization activities in the organic photovoltaic field:

• Development and benchmarking of new NIR-absorbing donor polymers and non-fullerene acceptor materials, where EQE spectra directly reveal how molecular design translates into long-wavelength photocurrent
• Optimization of bulk heterojunction morphology and layer thickness, using IQE analysis to distinguish optical absorption losses from charge collection losses
• Design and characterization of tandem solar cells, in which precise knowledge of the narrow-bandgap subcell's NIR response is essential for current matching between junctions
• Semi-transparent photovoltaic windows and building-integrated photovoltaics (BIPV), where simultaneous NIR harvesting and visible transparency are quantified to balance power output against daylighting requirements
• Agricultural photovoltaics and greenhouse applications, in which NIR-selective organic films generate electricity while transmitting photosynthetically active radiation to crops
• Indoor energy harvesting and internet-of-things (IoT) devices, where matching the device spectral response to artificial light sources improves delivered power in low-illumination environments
• Wearable and flexible electronics, validating that solution-processed NIR-responsive devices retain spectral performance on unconventional substrates
• Quality control and certification support during pilot production and module scale-up, ensuring batch-to-batch consistency of NIR performance metrics
• Academic publication and IP documentation, where standardized, traceable EQE data from accredited third-party laboratories strengthens the credibility of reported efficiencies

FAQ

Why is near-infrared response so important for organic solar cells?

A large share of the solar energy reaching the earth's surface lies in the NIR region beyond 700 nm. Traditional fullerene-based organic cells absorbed little light beyond this point, capping their achievable current. Modern low-bandgap and non-fullerene acceptor systems can convert NIR photons efficiently, and every increment of usable NIR spectrum directly increases short-circuit current density and therefore power conversion efficiency. NIR response testing is the only way to quantify and verify this capability quantitatively.

What wavelength range can be measured?

Standard measurements cover approximately 300 nm to 1100 nm using calibrated silicon reference detectors. For deeper-NIR organic systems, extended scans from roughly 1100 nm to 1700 nm are available using indium gallium arsenide detectors and appropriate long-pass filtering to suppress higher-order diffraction from the monochromator.

How is EQE different from spectral responsivity?

Spectral responsivity expresses the photocurrent generated per incident optical power (amperes per watt) at each wavelength, while EQE expresses the fraction of incident photons converted to collected electrons. The two are mathematically interchangeable, with EQE being the more commonly reported figure of merit in organic photovoltaic literature because it normalizes out photon energy.

Why is bias illumination needed when testing organic devices?

Organic semiconductors often show intensity-dependent charge generation, carrier mobility, and recombination. Measuring EQE with only weak monochromatic light can therefore underestimate—or in some cases overestimate—the responsivity at realistic operating conditions. Applying white-light bias replicates one-sun operating conditions and, following IEC 60904-8 correction procedures, yields EQE spectra consistent with actual device performance under sunlight.

Can the integrated current from EQE be used to verify solar simulator results?

Yes. The EQE spectrum integrated with the AM1.5G reference spectrum yields a spectral-mismatch-free short-circuit current density. Comparing this value with the Jsc measured under a solar simulator is a widely accepted cross-validation practice; agreement within approximately two to five percent confirms measurement integrity for both systems.

What sample preparation is required before testing?

Samples should have clearly defined active areas (verified by aperture masks during J–V measurement), stable electrical contacts, and no encapsulation or sealing that would obstruct the optical path unless it is part of the final device architecture. Devices should be delivered light-shielded, and any preconditioning history—such as light soaking or thermal annealing—should be documented, as organic devices can exhibit metastable behavior.

How long does NIR response testing take?

A routine EQE scan over the 300–1100 nm range is typically completed within about thirty minutes per sample once contacts and alignment are established. Extended deep-NIR scans, sub-bandgap sensitive measurements, IQE derivation with integrating-sphere optics, and tandem-device characterization with bias techniques require additional time, often one to several hours per device depending on the test matrix.