🔬 applied physics

Understanding surface potential dynamics of passivated perovskites via Kelvin Probe Force Microscopy

This study utilizes Kelvin probe force microscopy to demonstrate that AEAPTMS molecular passivation homogenizes surface potential, reduces grain-boundary barriers, and enhances photovoltage stability in mixed-cation perovskite films by mitigating electronic disorder.

Original authors: Rehmat Sood-Goodwin, Xue-Li Cao, Benjamin C. Kinvig, Robert D. J. Oliver, Yen-Hung Lin, Nic Mullin, Alexandra J. Ramadan

Published 2026-07-09

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CC BY 4.0

Original authors: Rehmat Sood-Goodwin, Xue-Li Cao, Benjamin C. Kinvig, Robert D. J. Oliver, Yen-Hung Lin, Nic Mullin, Alexandra J. Ramadan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a solar cell made of a special material called a "perovskite." Think of this material not as a smooth sheet of glass, but as a patchwork quilt made of thousands of tiny, irregular tiles (grains) stuck together. Ideally, sunlight should flow smoothly through this quilt, turning into electricity. However, in reality, the seams where these tiles meet (the grain boundaries) and the surface of the quilt are often messy and full of "potholes" (defects). These potholes act like speed bumps or traffic jams, trapping the energy and causing it to leak away before it can be used.

This paper is about a new way to smooth out those potholes using a special "molecular glue" called AEAPTMS. Here is how the researchers tested it and what they found, using simple comparisons:

The Problem: A Bumpy, Unpredictable Ride

The researchers looked at these perovskite films using a super-sensitive tool called a "Kelvin Probe Force Microscope." You can think of this tool like a very delicate finger that can feel the electrical "height" of the surface, even when it's too small to see with the naked eye.

Without the glue (Unpassivated): The electrical landscape was very bumpy. Some spots were high, some were low, and the difference between them was huge (about 45.7 mV). It was like driving over a road where the speed limit changed randomly every few feet. The seams between the tiles were especially problematic, acting as deep valleys where energy got stuck.
With the glue (Passivated): After applying the AEAPTMS molecule, the road became incredibly smooth. The difference between the high and low spots dropped significantly (to about 14.6 mV). The "molecular glue" didn't change how the tiles looked (the physical shape was the same), but it fixed the invisible electrical bumps underneath.

The Test: Turning on the Lights

The team then turned on a bright light (simulating the sun) to see how the material reacted. They measured something called "Surface Photovoltage" (SPV), which is basically a measure of how much electrical pressure builds up when light hits the material.

Speed of Reaction: When the light turned on, the unpassivated film was slow to wake up. It took a long time (about 840 seconds) to reach its full power. The passivated film, however, woke up much faster (about 470 seconds). It was like the difference between a sluggish car taking minutes to reach highway speed versus a sports car that hits it almost instantly.
Peak Power: Once the light was on, the passivated film didn't just wake up faster; it also generated a stronger electrical push (about 21% higher). This suggests that the "glue" helped separate the energy carriers more efficiently, allowing more of them to reach the finish line.

Peeking Under the Hood: Different Colors of Light

To understand where the fix was happening, the researchers used different colors of light, like shining a flashlight through a stained-glass window.

High-Energy Light (365 nm): This light gets absorbed right at the surface. The passivated film showed a steady, strong rise in power, proving the surface was working well.
Low-Energy Light (850 nm): This light usually passes right through a perfect material without doing anything. However, the unpassivated film still reacted to this light, which is a sign of "electronic disorder" or hidden defects deep inside or on the surface. The passivated film barely reacted to this low-energy light, indicating that the "molecular glue" had successfully filled in those hidden potholes and reduced the disorder.

The Grain Boundaries: Fixing the Seams

Finally, the researchers zoomed in specifically on the "seams" between the tiles (grain boundaries).

Before: In the unpassivated film, the seams were like deep trenches where electrical charges piled up, creating a barrier that blocked the flow of energy.
After: In the passivated film, those trenches were much shallower. The "glue" smoothed out the potential barriers, allowing the energy to flow across the seams much more easily.

The Bottom Line

By using this specific amino-silane molecule, the researchers didn't change the physical look of the solar material, but they completely transformed its electrical personality. They turned a bumpy, slow, and disordered surface into a smooth, fast, and stable one. This suggests that the "molecular glue" effectively patches up the defects that usually steal energy, making the solar cell more efficient and stable.

Technical Summary: Understanding Surface Potential Dynamics of Passivated Perovskites via Kelvin Probe Force Microscopy

Problem Statement
While molecular passivation is established as a critical strategy for reducing photovoltage losses in metal-halide perovskite solar cells, its electronic mechanisms are frequently inferred from macroscopic device metrics rather than directly resolved at the nanoscale. Interfacial defects, particularly at grain boundaries and surfaces, act as electronic shunts that narrow the Quasi-Fermi Level Splitting (QFLS), thereby limiting the open-circuit voltage ( $V_{OC}$ ). There is a need to correlate these macroscopic efficiency gains with localized changes in surface potential and carrier dynamics to fully understand how passivation molecules, such as amino-silanes, modify the electronic landscape of polycrystalline perovskite films.

Methodology
The authors employed Amplitude-Modulated Kelvin Probe Force Microscopy (AM-KPFM) to investigate mixed-cation, mixed-halide perovskite thin films ( $FA_{0.83}Cs_{0.17}Pb(I_{0.9}Br_{0.1})_3$ , ~1.60 eV bandgap) treated with the passivation molecule [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPTMS). The study utilized a dual-pass AM-KPFM setup under ambient conditions, with illumination applied through the glass substrate to mimic working device conditions while mapping the top perovskite surface.

The investigation involved three primary analytical approaches:

Dark and Illuminated CPD Mapping: Comparing Contact Potential Difference (CPD) distributions in dark and steady-state illuminated conditions to assess surface potential homogeneity and Surface Photovoltage (SPV).
Temporal Dynamics: Measuring the evolution of SPV over several hours under continuous white-light illumination (~1 sun) and post-illumination recovery to determine stabilization time constants ( $\tau$ ).
Wavelength-Dependent SPV: Utilizing sub-bandgap (850 nm) and above-bandgap (365 nm) illumination to decouple surface-specific dynamics from bulk contributions and assess electronic disorder.
Spatial Decoupling: Developing a custom analysis pipeline to mask grain boundaries (GB) from grain interiors (GI) based on topographic data, allowing for the statistical comparison of CPD values and the calculation of potential barriers ( $\Delta CPD$ ) across the film.

Key Results

Homogenization of Surface Potential: AEAPTMS passivation significantly reduced the spatial heterogeneity of the dark CPD. The distribution width (Full-Width Half-Maximum) narrowed from ~45.7 mV in unpassivated films to ~14.6 mV in passivated films, without inducing observable morphological changes to the grain structure.
Enhanced Surface Photovoltage (SPV): Under steady-state illumination, passivated films exhibited a 21% increase in SPV magnitude (~345 mV to ~417 mV). This increase correlates with enhanced hole accumulation at the surface and improved carrier separation, suggesting a higher potential $V_{OC}$ .
Accelerated Stabilization: The time constant ( $\tau$ ) for SPV stabilization decreased by 44% (from ~840 s to ~470 s) in passivated films. This indicates that AEAPTMS effectively passivates slow-acting defect states that typically govern long-lived transients.
Reduction of Electronic Disorder: Wavelength-dependent measurements revealed that unpassivated films exhibited significant SPV responses under 850 nm (sub-bandgap) illumination, indicative of high electronic disorder and defect states. Passivated films showed a negligible response at this wavelength, suggesting a reduction in sub-bandgap electronic disorder.
Grain Boundary Suppression: Analysis of $\Delta CPD$ (the potential difference between grain interiors and boundaries) demonstrated that passivation significantly reduced the magnitude of carrier accumulation at grain boundaries. The polarity and magnitude of $\Delta CPD$ confirmed that AEAPTMS suppresses the potential barriers at grain boundaries, leading to a more uniform carrier landscape.

Significance and Claims
The paper claims that AEAPTMS passivation creates a more homogeneous and stable carrier landscape by specifically suppressing grain-boundary potential barriers and reducing surface electronic disorder. By linking the amino-silane passivation to these nanoscale improvements, the authors provide direct evidence that the observed macroscopic performance gains are driven by the mitigation of non-radiative recombination pathways at the surface and grain boundaries.

The authors position their work as a demonstration of how AM-KPFM can resolve the specific electronic actions of passivation molecules, moving beyond device-level inference. They further contribute to the field by providing open-source analysis code for temporal and spatial KPFM measurements, asserting that their analytical approach is broadly applicable to other perovskite systems and passivation chemistries. The study modestly concludes that these findings help explain the mechanisms behind improved device performance and stability in passivated perovskite solar cells.

The Problem: A Bumpy, Unpredictable Ride

The Test: Turning on the Lights

Peeking Under the Hood: Different Colors of Light

The Grain Boundaries: Fixing the Seams

The Bottom Line

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