Organic solar cell transmittance measurement is a critical optical characterization process that quantifies how much light passes through photovoltaic films and devices across the ultraviolet, visible, and near-infrared spectra. Accurate transmittance data is essential for evaluating semitransparent organic photovoltaics (OPVs) used in building-integrated photovoltaics, smart windows, and agrivoltaic applications. This article provides a comprehensive guide to transmittance testing samples, measurable parameters, standard methods, and frequently asked questions for organic solar cell development and quality control.
Technical Overview
Transmittance measurement for organic solar cells (OSCs) determines the fraction of incident light that passes through the device or material stack at specific wavelengths, typically expressed as a percentage. Unlike opaque conventional silicon photovoltaics, organic solar cells can be engineered as semitransparent devices, making optical transmittance a defining performance attribute alongside power conversion efficiency (PCE). The key figure of merit for semitransparent OSCs is the light utilization efficiency (LUE), calculated as the product of average visible transmittance (AVT) and PCE, which quantifies the trade-off between light harvesting and transparency.
Transmittance characterization is typically performed using ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometry over the wavelength range of 300–2500 nm. Because organic photoactive layers, transparent electrodes, and flexible substrates each contribute to the overall optical response, comprehensive characterization requires separating specular and diffuse transmittance components, applying photopic and solar spectral weightings, and referencing appropriate international standards such as ISO 9050, ISO 13468, ASTM D1003, and ASTM E424. Accredited third-party laboratories routinely conduct these measurements to support research validation, product certification, and quality control for commercial deployment.
Testing Samples
Photoactive Layer Films
Standalone organic semiconductor blend films, such as polymer:non-fullerene acceptor systems coated on quartz or glass substrates, are measured to isolate the intrinsic optical absorption and transparency of the active layer. These samples help researchers tune the visible transmittance and near-infrared absorption characteristics during material development.
Complete Semitransparent Devices
Fully fabricated device stacks, including the substrate, transparent conducting electrode, hole transport layer, active layer, electron transport layer, and top transparent electrode, are measured to determine the transmittance of the finished product. This reflects real-world optical performance and supports the calculation of light utilization efficiency.
Transparent Electrodes and Substrates
Transparent conductive oxides, silver nanowire networks, ultrathin metal films, conductive polymers, and flexible polymer substrates such as PET and PEN are tested individually. Substrate-referenced measurements subtract the contribution of the bare substrate to reveal the optical properties of the deposited functional layers.
Encapsulated Cells and Modules
Encapsulation foils, barrier films, and cover glasses alter the spectral transmittance of the finished module. Testing encapsulated samples confirms whether the optical transmittance meets design specifications after all protective layers are applied and verifies long-term optical stability after damp-heat and UV aging exposures.
Testing Items
The following optical parameters are typically reported for organic solar cell transmittance characterization:
- Average visible transmittance (AVT or T_lum): integrated transmittance over 380–780 nm weighted by the photopic luminosity function of the human eye
- Solar transmittance (T_sol): spectrally weighted transmittance over 300–2500 nm using the AM1.5G reference solar spectrum defined in ASTM G173
- Ultraviolet transmittance (T_UV): transmittance integrated over the UV region, relevant for photostability and agrivoltaic applications
- Near-infrared transmittance (T_NIR): transmittance in the 780–2500 nm region, important for heat management in smart windows
- Spectral transmittance curve: full wavelength-dependent transmittance profile across the measured range
- Haze: the ratio of diffuse to total transmittance, indicating light scattering within the film
- Color coordinates: CIE 1931 chromaticity values (x, y) and color rendering performance calculated from the transmittance spectrum
- Light utilization efficiency (LUE): combined metric derived from AVT and power conversion efficiency
Testing Methods
UV-Vis-NIR Spectrophotometry
A double-beam UV-Vis-NIR spectrophotometer measures spectral transmittance at normal incidence, typically scanning from 300 nm to 2500 nm. Baseline correction is performed using air or, preferably, an identical bare substrate to obtain substrate-referenced values for the deposited layers. This method forms the basis of most transmittance measurements and is consistent with the general procedures of ISO 9050 and ASTM E424.
Integrating Sphere Measurement
For films exhibiting light scattering, such as thick active layers or nanowire electrodes, an integrating sphere accessory captures both specular and diffuse transmitted light to determine total transmittance. Haze is calculated as the difference between total transmittance and specular transmittance divided by total transmittance, following the principles of ASTM D1003 and ISO 14782. Using only a standard detector without an integrating sphere can significantly underestimate the transmittance of hazy samples.
Spectrally Weighted Calculations
The measured spectral transmittance data are integrated with weighting functions to yield application-relevant parameters. Visible transmittance is computed by weighting with the photopic response function per ISO 13468 and ISO 9050, while solar transmittance is weighted by the AM1.5G spectral irradiance distribution per ASTM G173. CIE color coordinates are derived from the transmittance spectrum combined with standard illuminant D65 data.
Reference Standards and Conditions
- ISO 9050 — Determination of light transmittance, solar direct transmittance, and total solar energy transmittance of glazing materials
- ISO 13468-1 — Determination of the total luminous transmittance of transparent materials using a single-beam spectrophotometer
- ASTM D1003 — Haze and luminous transmittance of transparent plastics
- ASTM E424 — Solar energy transmittance and reflectance of sheet materials
- ISO 14782 — Determination of haze for transparent materials
- ASTM G173 — Reference solar spectral irradiance tables used for spectral weighting
Measurements are conducted under controlled laboratory conditions at room temperature, with each sample typically measured at multiple positions to assess spatial uniformity. Sample size requirements generally range from 25 mm × 25 mm to full substrates of 50 mm × 50 mm, and repeat measurements are recommended to evaluate reproducibility.
Applications
Transmittance measurement data directly support the design, certification, and deployment of semitransparent organic photovoltaic products across multiple industries:
- Building-integrated photovoltaics (BIPV): semitransparent window and façade modules where visible transmittance and color neutrality must balance daylighting requirements against power generation
- Smart windows and electrochromic hybrids: near-infrared transmittance control for thermal management and energy-saving glazing
- Agrivoltaics and greenhouse films: wavelength-selective films that transmit photosynthetically active radiation while converting unused spectral bands into electricity
- Wearable and portable electronics: flexible, lightweight semitransparent power sources integrated into textiles and accessories
- Automotive glazing: transparent photovoltaic coatings for sunroofs and vehicle windows requiring strict optical clarity and low haze
- Research and development: benchmarking new active layer materials, transparent electrodes, and device architectures against published performance metrics
FAQ
What is a good transmittance value for a semitransparent organic solar cell?
Most practical applications require an average visible transmittance between 25% and 40%, combined with sufficient power conversion efficiency to achieve a competitive light utilization efficiency. Architectural glazing often demands AVT above 30% to satisfy daylighting requirements, while lower transmittance values may be acceptable for power-optimized applications.
Why must an integrating sphere be used for some samples?
Organic films, nanowire electrodes, and textured substrates scatter a significant portion of transmitted light away from the straight-through optical path. An integrating sphere collects this diffuse light, ensuring that total transmittance and haze are measured accurately rather than systematically underestimated.
What is the difference between visible transmittance and solar transmittance?
Visible transmittance considers only the 380–780 nm range weighted by human eye sensitivity and determines perceived transparency, while solar transmittance spans the full 300–2500 nm solar spectrum and determines how much total solar energy passes through the device, which governs both photocurrent generation and heat gain.
How does the substrate affect the measurement results?
Glass, PET, and electrode layers each absorb and reflect light before it reaches the active layer. Reporting substrate-referenced transmittance isolates the contribution of the functional organic layers, whereas absolute device transmittance reflects the complete product performance required for end-use specifications.
How long does a typical transmittance measurement take?
A full spectral scan with integrating sphere data acquisition across 300–2500 nm generally takes 10 to 30 minutes per sample position, excluding sample preparation. Multi-point mapping and additional haze or color calculations extend the total turnaround, and accredited third-party laboratories typically report results within several working days.