AFM Characterization of Perovskite Solar Cells

How KPFM and ResiScope™ reveal grain boundary potentials, nanoscale charge transport, and defect distribution in perovskite and organic photovoltaic films — from lab-scale optimization to degradation analysis.

KPFM · Surface PotentialResiScope™ · Resistance MappingGrain Boundary AnalysisHD-KFM IIIOrganic PVSilicon Solar Cells

The Challenge

Why Macroscopic Measurements Are Not Enough

Perovskite solar cell efficiency is governed by nanoscale phenomena that bulk measurements cannot resolve: grain boundary recombination, local ion migration, and interfacial charge trapping occur at length scales of 5–500 nm — precisely the domain of AFM-based electrical characterization.

Despite impressive certified efficiencies above 26%, perovskite devices suffer from spatial heterogeneity at the grain level. Two grains separated by 10 nm can differ by 100–300 mV in surface potential, act as current-blocking barriers, or provide preferential ion migration pathways. These variations determine whether grain boundaries are benign, beneficial, or detrimental to device performance.

Techniques like EQE, J-V characterization, and impedance spectroscopy yield area-averaged responses. They cannot localize the source of Voc loss, FF degradation, or hysteresis. KPFM and conductive AFM provide the missing spatial dimension, mapping potential and current distributions with sub-10 nm resolution across the exact grain structures responsible.

Technique 1 · HD-KFM III

Surface Potential Mapping of Grain Boundaries

Kelvin Probe Force Microscopy (KPFM) measures the contact potential difference (VCPD) between a conductive AFM tip and the sample surface at each pixel, producing a quantitative map of local work function and surface potential. In perovskite films, this directly reveals the electrostatic landscape governing charge separation and transport.

CSInstruments' HD-KFM III operates in true single-pass mode with bimodal dual-frequency excitation at an effective tip-sample distance of 0.1–0.5 nm — dramatically shorter than conventional lift-mode KFM (20–100 nm). This proximity delivers 100–10,000× greater electrostatic sensitivity and sub-10 nm spatial resolution, enabling visualization of individual grain boundary widths and intragranular potential gradients that lift-mode KPFM cannot resolve.

Grain Boundary Potential

Identify whether grain boundaries act as charge recombination centers (negative potential barriers) or charge transport highways (positive potential accumulation).

Built-in Electric Field

Map the built-in potential across ETL/perovskite and HTL/perovskite interfaces. Direct measurement of local band bending under dark and illuminated conditions.

Illuminated KPFM

Surface photovoltage mapping under controlled illumination reveals photogenerated carrier separation, local Voc variations, and charge recombination hotspots.

Ion Migration Tracking

Time-resolved KPFM captures ion redistribution under applied bias, correlating surface potential drift with J-V hysteresis behavior.

Technique 2 · ResiScope™ III

Resistance Mapping of Photovoltaic Films

Conductive AFM techniques reveal the current and resistance distribution across the photoactive layer — information critical for understanding shunting, current collection uniformity, and layer-to-layer charge transport. The challenge in photovoltaic samples is the enormous dynamic range: conductive grain cores and resistive grain boundaries or interfacial layers may differ by 4–8 orders of magnitude in resistance within a single scan area.

Standard C-AFM amplifiers saturate when encountering this range, forcing researchers to choose between resolving high-resistance or low-resistance regions — never both simultaneously. ResiScope™ III solves this with real-time DSP-driven AutoGain, covering 10² to 10¹² ohms (50 fA to 1 mA) in a single continuous scan with no manual adjustment.

Technique 3 · Soft ResiScope™

Organic Solar Cells: Electrical Mapping Without Damage

Organic photovoltaic films — P3HT:PCBM, PM6:Y6, and related blends — present a particular challenge for conductive AFM: they are mechanically soft, and continuous tip-sample contact in standard C-AFM rapidly damages the organic layer, introduces friction-induced artifacts, and degrades tip conductivity. Traditional contact-mode measurements on organic PV films are therefore inherently unreliable.

Soft ResiScope™ combines interaction contron (Soft-IC) mode with ResiScope current/resistance detection. The tip makes brief, controlled vertical contacts with constant force at each pixel, retracting fully between measurements — eliminating lateral friction entirely. The result is quantitative resistance and current mapping on fragile organic films with the same 10-decade dynamic range as standard ResiScope, but without sample degradation.

Comparison

Choosing the Right AFM Technique for Solar Cell Research

Technique	Measures	Best For	Key Advantage
HD-KFM III Single-pass	Surface potential, work function (mV)	Grain boundary potential, illuminated KPFM, band bending	Sub-10 nm resolution; 0.1–0.5 nm tip distance
Standard KPFM	Surface potential (mV)	Larger-area surface potential mapping	Available on all CSI systems; simpler setup
ResiScope™ III 10 decades	Current (fA–mA) and resistance (10²–10¹² Ω)	Perovskite, silicon, semiconductor grain conductivity	Full dynamic range in one scan; no saturation
Soft ResiScope™	Current and resistance (friction-free)	Organic PV, fragile films, repeated measurements	No sample damage; quantitative on soft materials
EFM	Electrostatic force gradient (qualitative)	Fast screening of charge domains	Rapid; no reference calibration needed
PFM / Soft PFM	Piezoelectric response	Ferroelectric domains in hybrid perovskites	Simultaneous with ResiScope III on Nano-Observer II

Workflow

Recommended Measurement Protocol

1 Sample Preparation in Glove Box

For air-sensitive perovskite compositions (e.g., Cs-FA mixed-halide), prepare and load samples under N₂ or Ar atmosphere. The Nano-Observer II is compatible with standard glove-box pass-through configurations. Use conductive substrates (ITO, FTO, gold) for reliable electrical contact.

2 Topography Survey (Resonant Mode)

Begin with a standard resonant (tapping) mode topography scan to map grain size, film roughness, and identify regions of interest. Target scan areas of 2–10 µm for grain boundary analysis. Identify representative grains of varying orientation or composition for correlation studies.

3 HD-KFM III — Dark Condition Surface Potential

Acquire simultaneous topography and surface potential map in single-pass HD-KFM III mode. Calibrate tip work function using a freshly cleaved HOPG reference. Record at least 3 areas per sample for statistical grain boundary potential analysis.

4 Illuminated KPFM — Surface Photovoltage

Repeat HD-KFM III scan on the same area under controlled white light illumination (use the Nano-Observer II optical access port). The difference map (light − dark) yields the surface photovoltage, directly measuring local charge separation efficiency at each grain and boundary.

5 ResiScope™ III — Resistance / Current Map

Switch to ResiScope III mode on the same or adjacent scan area. Apply a small DC sample bias (typically 0.1–0.5 V). AutoGain ensures no saturation across grain boundaries and grain interiors regardless of the resistance contrast. Correlate resistance features with KPFM potential features.

6 Data Correlation in NanoSolution Software

NanoSolution software enables overlay and direct pixel-by-pixel correlation of topography, surface potential, and resistance maps acquired on the same area. Export for grain boundary statistics, current pathway mapping, and comparison with macroscopic J-V device data.

Instrument

Nano-Observer II — Configured for Photovoltaic Research

The Nano-Observer II is the only AFM system that combines HD-KFM III, ResiScope III, Soft ResiScope, and Soft PFM in a single platform with glove-box compatibility and optical access for illuminated experiments. Key photovoltaic-specific capabilities:

Glove Box Compatible

Full N₂/Ar atmosphere operation for air-sensitive perovskite, organic, and lithium-based samples.

Optical Coupling

Open design accepts UV, visible, and IR light sources for illuminated KPFM, surface photovoltage, and photo-conductive AFM.

Temperature Control

–40°C to 300°C with optional Peltier or heating stage — enabling stability and degradation studies under thermal stress.

Electrochemical Cell

Three-electrode EC-AFM configuration for in-situ ionic transport, corrosion, and solid-state battery interface studies.