Organic Solar Cell Surface Roughness Analysis: Testing Methods, Parameters and Applications

Organic solar cell surface roughness analysis is a critical morphology characterization technique that quantifies the nanoscale topography of active layers, transport layers, and electrodes, directly impacting device efficiency, stability, and manufacturing yield. This article provides a comprehensive technical overview of surface roughness measurement parameters, testing methods compliant with ISO and ASTM standards, sample types, and practical applications in organic photovoltaic research and quality control.

Technical Overview

Organic solar cells (OSCs) are thin-film photovoltaic devices built from conjugated polymer donors and small-molecule or non-fullerene acceptors arranged in a bulk heterojunction (BHJ) architecture. Because the functional layers in these devices are typically only 10–200 nanometers thick, nanometer-scale variations in surface topography can have an outsized influence on device physics. Surface roughness analysis therefore serves as one of the most important characterization tools in OSC development and manufacturing.

The root-mean-square (RMS) roughness of a well-optimized BHJ active layer generally falls between 1 nm and 5 nm. Excessive roughness at the active layer interface can cause partial penetration of the top electrode through the thin film, creating shunt paths that reduce fill factor and open-circuit voltage. Conversely, a degree of textured roughness can enhance light absorption through weak scattering and increase the effective interfacial area for charge extraction. Quantitative roughness data allow researchers to correlate processing conditions with morphology and, ultimately, with power conversion efficiency.

Surface roughness analysis in the OSC context is typically performed by accredited third-party laboratories or in-house characterization facilities using high-resolution instrumentation such as atomic force microscopy (AFM) and optical profilometry, following internationally recognized standards including ISO 25178 for areal surface texture and ISO 21920 for profile-based measurement.

Testing Samples

Active Layer Thin Films

Spin-coated, blade-coated, or slot-die-coated BHJ films such as PM6:Y6, PTB7-Th:PC71BM, and P3HT:O-IDTBR blends deposited on glass, ITO, or silicon substrates. These are the most frequently submitted samples for roughness evaluation, as active layer morphology dominates device performance.

Charge Transport Layers

Hole transport layers (PEDOT:PSS, NiOx, MoO3) and electron transport layers (ZnO, SnO2, PFN-Br) coated on ITO/glass or flexible substrates. Interlayer roughness determines interface quality and contact resistance in the finished device stack.

Electrode Materials

Patterned ITO transparent conductive oxides, thermally evaporated metal electrodes (Ag, Al, Au), and solution-processed silver nanowire or carbon-based electrodes. Electrode roughness affects both optical transmittance and the risk of electrical shorting across ultra-thin active layers.

Flexible Substrates and Encapsulation Layers

PET and PEN polymer foils, barrier coatings, and encapsulant films used in flexible OSC modules. Substrate roughness propagates upward through all subsequently deposited layers, while encapsulation surface texture influences adhesion and moisture barrier performance.

Complete Device Stacks and Aged Samples

Finished devices before and after damp-heat, thermal cycling, or continuous illumination aging, evaluated per IEC 61215-style stability protocols to track morphology evolution and degradation-induced roughening.

Testing Items

Surface roughness analysis for organic solar cells typically covers the following measurable quantities:

• Profile roughness parameters: Ra (arithmetic mean deviation), Rq/RMS (root-mean-square deviation), Rz (maximum height of profile), Rt (total height), and Rsk/Rku (skewness and kurtosis), defined in ISO 21920 (superseding ISO 4287).
• Areal (3D) roughness parameters: Sa, Sq, Sz, Ssk, Sku, Sdr (developed interfacial area ratio), and Str (texture aspect ratio) per ISO 25178.
• Film thickness and step-height determination alongside roughness mapping.
• Surface uniformity and large-area roughness mapping for coated areas up to wafer or module dimensions.
• Grain size, domain size, and fibrillar feature characterization derived from topographic images.
• Defect and particle detection, including spikes, pinholes, scratches, and coating streaks.
• Step height and roughness evolution after thermal annealing, solvent vapor annealing, or additive treatment.
• Comparative roughness studies between processing routes (spin coating versus blade coating versus slot-die coating).

Testing Methods

Atomic Force Microscopy (AFM)

AFM in tapping (intermittent contact) mode is the reference technique for OSC surface roughness analysis because organic films are soft and the relevant morphological features occur at the nanometer scale. Typical scan areas range from 1 µm × 1 µm for fine phase-separated texture to 20 µm × 20 µm for large-domain evaluation, with scan rates of 0.5–2 Hz. Measurements follow ISO 25178 and the AFM-specific procedures of ASTM E1813. AFM can also provide phase images that distinguish donor-rich and acceptor-rich domains in BHJ blends.

Stylus Profilometry

Contact stylus profilometry with low-force diamond tips (typically ≤ 1 mgf to avoid film damage) measures line profiles and step heights across larger traverse lengths. Data are processed with Gaussian filters and evaluated according to ISO 21920 and ASME B46.1. This method is well suited for film thickness measurement, coating uniformity over centimeter scales, and repeatable production-line quality control.

White-Light Interferometry and Optical Profilometry

Non-contact vertical scanning interferometry and phase-shifting interferometry deliver rapid, large-field 3D topography maps with sub-nanometer vertical resolution. These techniques are ideal for electrode roughness, substrate texture, and large-area module inspection where AFM scanning would be impractically slow, and they comply with ISO 25178 areal parameter definitions.

Confocal Laser Scanning Microscopy

Confocal microscopy provides extended z-range topography for strongly textured or transparent multilayer samples, bridging the gap between profilometry and electron microscopy. It is useful for encapsulated devices and thick transport layers.

Data Processing and Reporting

All measurements apply appropriate S-filter, L-filter, and F-operator nesting indexes as defined in ISO 25178, with leveling (plane fit) performed before parameter extraction. Reports typically include height and phase images, quantitative Sa/Sq values, cross-sectional profiles, and statistical comparison across sample sets. Testing can be arranged through accredited third-party laboratories to ensure traceable, standards-compliant results.

Applications

Surface roughness analysis supports organic photovoltaic research and industrialization across multiple stages:

• Blend morphology optimization: correlating RMS roughness and phase-image domain size with donor–acceptor miscibility, charge generation, and fill factor in new material systems.
• Processing engineering: evaluating the effects of solvent choice, processing additives (such as 1,8-diiodooctane or 1-chloronaphthalene), thermal annealing, and solvent vapor annealing on film smoothness and phase separation.
• Scalable coating development: comparing spin-coated laboratory films with blade-coated, slot-die coated, or roll-to-roll processed films to transfer morphology control from lab to pilot line.
• Interface and electrode engineering: verifying that transport layer deposition smooths or planarizes the underlying ITO, minimizing series resistance and suppressing shunts through the active layer.
• Stability and degradation studies: tracking roughness evolution during damp-heat, UV, and operational aging, where morphological coarsening and electrode diffusion often precede efficiency loss.
• Encapsulation assessment: evaluating barrier film texture and adhesion-related topography to support moisture-barrier performance in flexible modules.
• Quality control in manufacturing: inline and offline roughness specification limits for incoming ITO substrates, coated transport layers, and finished active layers, ensuring consistent device yield.

FAQ

Q1: What is the typical surface roughness of an organic solar cell active layer?

Well-optimized BHJ active layers usually show RMS (Sq) roughness of approximately 1–5 nm. Values significantly above this range often indicate excessive phase separation or aggregation, while extremely smooth, featureless surfaces may indicate over-mixing that limits charge transport.

Q2: Why is AFM preferred over stylus profilometry for OSC films?

AFM offers lateral resolution in the nanometer range, matching the scale of donor–acceptor domains, and applies minimal force so soft organic films are not deformed or scratched. Stylus profilometry remains valuable for larger-area profiles and step-height measurement.

Q3: How does surface roughness affect device efficiency?

Excessive roughness in ultra-thin stacks can create electrical shorts between the active layer and the counter electrode, reducing fill factor and open-circuit voltage. Moderate, controlled texture can enhance light absorption and interfacial contact area for charge collection.

Q4: What scan size should be used for roughness measurement?

Scan areas of 1–10 µm are standard for active layer morphology, while 10–50 µm scans or larger interferometry fields are used for substrates, electrodes, and coating uniformity. The chosen area must exceed the largest morphological feature of interest to ensure statistically valid parameters.

Q5: Which standards govern these measurements?

ISO 25178 defines areal surface texture parameters, ISO 21920 (replacing ISO 4287/ISO 4288) covers profile-based roughness measurement, ASME B46.1 provides the American national framework, and ASTM E1813 addresses AFM measurement practice. Device-level reliability testing references IEC 61215.

Q6: Can roughness analysis be performed on complete encapsulated devices?

Yes. Non-contact optical profilometry and confocal microscopy can measure topography through or above encapsulation layers, which is useful for degradation tracking, although nanoscale active layer detail is best captured on unencapsulated witness samples.