Donor materials fundamentally determine the power conversion efficiency, charge transport, and operational stability of organic solar cells. This article presents a comprehensive performance evaluation framework for organic photovoltaic donor materials, covering molecular, optical, electrochemical, morphological, device-level, and stability testing aligned with IEC, ASTM, ISO, GB, and JIS standards. Learn how accredited third-party laboratories quantify the critical parameters that guide donor material screening, qualification, and commercialization.
Technical Overview
Organic solar cells (OSCs) convert sunlight into electricity using carbon-based semiconducting materials, most commonly in a bulk heterojunction (BHJ) architecture in which an electron-donor material is blended with a non-fullerene acceptor (NFA) such as the Y-series molecules. The donor material, whether a conjugated polymer (for example, PM6, PBDB-T, or PTB7-Th families) or a soluble small molecule, governs light harvesting, exciton generation, hole transport, and the nanoscale phase morphology of the active layer. Consequently, systematic performance evaluation of donor materials is essential for molecular design screening, batch quality control, and device optimization.
A rigorous evaluation program connects intrinsic material properties, such as molecular weight, energy levels, and absorption coefficients, to device-level output metrics measured under standardized conditions. The four primary device performance indicators are:
- Power conversion efficiency (PCE), the overall figure of merit
- Open-circuit voltage (Voc), largely determined by the donor HOMO level and the acceptor LUMO level
- Short-circuit current density (Jsc), driven by absorption breadth and charge generation efficiency
- Fill factor (FF), reflecting charge extraction, recombination, and blend morphology
Testing Samples
Conjugated Polymer Donors
Powder or film samples of donor–acceptor (D–A) type copolymers, such as benzodithiophene- or thiophene-lactam-based backbones. A typical material-level characterization requires 100 to 300 mg, together with solubility information (chlorobenzene, chloroform, o-dichlorobenzene) and expected thermal tolerance for high-temperature gel permeation chromatography.
Small-Molecule Donors
Well-defined oligomeric or small-molecule donor compounds with narrow molecular weight distributions. These samples enable precise structure–property correlation and are analyzed for purity, thermal behavior, and crystallinity.
Donor–Acceptor Blend Formulations and Films
Prepared inks with specified donor:acceptor weight ratios (commonly 1:1.2) and additives, together with spin-coated or blade-coated films on quartz, silicon, and ITO substrates for optical, morphological, and structural analysis.
Fabricated Test Devices
Complete photovoltaic devices in conventional or inverted architectures, for example ITO/PEDOT:PSS/donor:acceptor/PFN-Br/Al or ITO/ZnO/donor:acceptor/MoO3/Ag. Best practice requires at least 6 to 10 devices per condition with aperture-defined active areas of 4 to 10 mm² to ensure statistical reliability.
Testing Items
Material-Level Characterization
- Number-average molecular weight (Mn), weight-average molecular weight (Mw), and dispersity (Đ) by high-temperature gel permeation chromatography
- Thermal decomposition temperature (Td at 5% weight loss) by thermogravimetric analysis; glass transition, melting, and crystallization behavior by differential scanning calorimetry
- UV-Vis absorption in solution and thin film, absorption coefficient, and optical bandgap (Eg, opt) derived from the absorption onset
- HOMO and LUMO energy levels by cyclic voltammetry, with ultraviolet photoelectron spectroscopy to confirm ionization potential
- Hole mobility by space-charge-limited-current (SCLC) hole-only devices or organic field-effect transistor geometry
- Chemical purity by HPLC, NMR, and elemental analysis; residual palladium, tin, and metal catalyst content by ICP-MS; moisture content by Karl Fischer titration
- Surface energy and contact angle; refractive index by spectroscopic ellipsometry for optical modeling
Device-Level Characterization
- Current density–voltage (J–V) characteristics under standard test conditions: Voc, Jsc, FF, PCE, series resistance, and shunt resistance
- External quantum efficiency (EQE) spectra with AM1.5G-integrated Jsc cross-verification against the J–V measurement
- Light-intensity dependence studies, ideality factor extraction, and recombination analysis
- Photoluminescence quenching efficiency and transient photovoltage/photocurrent (TPV/TPC) for charge recombination and lifetime assessment
- Champion and average device statistics across a statistically meaningful device population
Morphological and Structural Analysis
- Atomic force microscopy height and phase images: RMS roughness and phase-separation domain size
- Grazing-incidence wide-angle X-ray scattering (GIWAXS): lamellar packing, π–π stacking distance, coherence length, and face-on versus edge-on orientation
- Transmission electron microscopy for bulk domain visualization
- Film thickness by stylus profilometry or ellipsometry
Stability Assessment
- Dark storage stability, thermal stability at elevated temperature, and operational stability under continuous illumination
- T80 lifetime and initial burn-in loss quantification
- Damp-heat and thermal cycling preconditioning for modules progressing toward IEC 61215 qualification sequences
Testing Methods
Solar Simulation and J–V Measurement
Devices are characterized per IEC 60904-1 using Class AAA solar simulators meeting the spectral match, spatial non-uniformity, and temporal instability requirements of IEC 60904-9, calibrated to the AM1.5G spectrum at 1000 W/m² and 25 ± 1 °C. Reference cells calibrated per IEC 60904-2, with spectral mismatch correction applied in accordance with IEC 60904-7 and ASTM E973, establish traceability. ASTM E948 and the GB/T 6495 and JIS C 8904 series define equivalent measurement principles in other regional frameworks. Aperture masks define the illuminated area, and forward and reverse voltage scans are compared to identify hysteresis.
External Quantum Efficiency
EQE is measured per IEC 60904-8 and ASTM E1021 using chopped monochromatic probe beams and calibrated photodiode standards. The Jsc obtained by convolving the EQE spectrum with the AM1.5G reference spectrum of IEC 60904-3 should agree with the J–V-derived Jsc within approximately 5%; larger discrepancies indicate spectral mismatch, area definition errors, or measurement artifacts.
Optical and Electrochemical Analysis
Film absorption is measured on a UV-Vis spectrophotometer, preferably with an integrating sphere for scatter correction, and the optical bandgap is calculated as 1240 divided by the absorption onset wavelength in nanometers. Energy levels are determined by cyclic voltammetry on thin films in anhydrous, oxygen-free electrolyte using ferrocene/ferrocenium as the internal reference, with the HOMO estimated from the onset oxidation potential; ultraviolet photoelectron spectroscopy provides an independent ionization potential measurement.
Molecular and Thermal Analysis
Molecular weight distributions are obtained by high-temperature GPC following ASTM D5296 with narrow-dispersity polystyrene standards and chlorobenzene or o-dichlorobenzene as eluent. Thermogravimetric analysis is performed under nitrogen per ISO 11358, and differential scanning calorimetry per ISO 11357 determines glass transition, melting, and crystallization events relevant to blend processing and long-term morphological stability.
Charge Transport Measurement
Hole mobility is extracted from SCLC hole-only devices using the Mott–Gurney relationship, in which mobility is proportional to the current, the cube of the film thickness, and the inverse square of the applied voltage. This measurement isolates single-carrier transport from the recombination processes present in full photovoltaic devices.
Stability Testing
Stability is evaluated following the internationally recognized ISOS consensus protocols, including dark storage (ISOS-D), thermal (ISOS-T), laboratory light soaking (ISOS-L), combined light and heat (ISOS-LT), and outdoor exposure (ISOS-O) variants, conducted either under inert atmosphere or ambient humidity as specified by the project. Devices are tracked at the maximum power point or under open-circuit conditions, and results are reported as normalized efficiency evolution, T80 lifetime, and burn-in behavior.
Applications
- Screening of newly synthesized donor polymers and small molecules for efficiency-relevant properties before device integration
- Incoming quality control and batch-to-batch consistency verification for donor material suppliers and module manufacturers
- Shelf-life assessment and molecular weight drift monitoring for stored polymer batches
- Development of tandem devices, all-polymer systems, and ternary blends requiring matched energy-level alignment
- Qualification of donor materials for indoor photovoltaics, flexible electronics, and building-integrated applications
- Generation of certified efficiency data for peer-reviewed publications, patent filings, and investor due diligence
- Process scale-up studies comparing spin-coated laboratory films with blade-coated or slot-die-coated films produced under simulated manufacturing conditions
FAQ
What efficiency level indicates a state-of-the-art donor material?
With optimized Y-series non-fullerene acceptors, leading polymer donors such as the PM6 family now enable single-junction PCE values above 19% in accredited measurements. For material screening purposes, a well-qualified donor should deliver reproducible average efficiencies within roughly 5% of its champion value across a statistically meaningful device set, together with EQE-verified Jsc.
Why is EQE measurement necessary in addition to J–V testing?
J–V measurements alone can be distorted by simulator spectral mismatch and area-definition errors. The EQE spectrum provides a wavelength-resolved, simulator-independent verification of photocurrent generation, confirming that reported Jsc values arise from genuine photon-to-electron conversion across the donor absorption band.
How much donor material is needed for a complete evaluation?
A full material-level characterization, including GPC, TGA, DSC, UV-Vis, cyclic voltammetry, and purity analysis, typically requires 100 to 300 mg. Device fabrication and J–V/EQE testing generally consume an additional 100 to 500 mg depending on the number of conditions and replicates requested.
How are HOMO and LUMO energy levels determined, and why do they matter?
Energy levels are measured by cyclic voltammetry against a ferrocene reference and confirmed by ultraviolet photoelectron spectroscopy. The donor HOMO level sets the achievable Voc together with the acceptor LUMO, while proper energetic offsets govern charge separation, so accurate energy-level data are indispensable for donor–acceptor pairing.
What stability data should accompany efficiency results?
At minimum, thermal stability and operational light-soaking stability following ISOS protocols should be reported as T80 lifetimes with the test atmosphere, temperature, and illumination level clearly specified. Morphological stability indicators, such as GIWAXS and DSC data on aged films, further substantiate long-term reliability claims.
Are the measurement results traceable to international standards?
Yes. Accredited third-party laboratories calibrate reference cells and photodiodes against standards traceable to national metrology institutes and conduct all device measurements in accordance with the IEC 60904 and IEC 60904-9 framework, ASTM E948, ASTM E1021, and related ISO, GB, and JIS procedures, ensuring that reported efficiencies are reproducible and internationally comparable.