🔬 materials science

Resolving the energy alignment between methylammonium lead iodide and C60: an in-situ photoelectron spectroscopy study

Using in-situ photoelectron spectroscopy on cleaved MAPbI3 single crystals, this study reveals that C60 consistently undergoes a downward energy shift relative to the perovskite with a stable 0.52 eV HOMO-valence band offset at high coverage, while highlighting that minor surface variations significantly impact interfacial energetics and necessitate precise surface control for optimizing perovskite device performance.

Original authors: Alberto García-Fernández, Karen Radetzky, Stefania Riva, Birgit Kammlander, Brian Rydgren, Evelyn Johannesson, Rahul Mahavir Varma, Håkan Rensmo, Ute B. Cappel

Published 2026-06-18

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Alberto García-Fernández, Karen Radetzky, Stefania Riva, Birgit Kammlander, Brian Rydgren, Evelyn Johannesson, Rahul Mahavir Varma, Håkan Rensmo, Ute B. Cappel

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 you are trying to build a very efficient solar cell. Think of the solar cell as a factory that catches sunlight and turns it into electricity. Inside this factory, there are two main workers: the Perovskite (the absorber that catches the light) and C60 (a soccer-ball-shaped molecule that acts as a conveyor belt to carry the electricity away).

For the factory to work perfectly, these two workers need to stand at just the right height relative to each other. If the conveyor belt (C60) is too high or too low compared to the worker (Perovskite), the electricity gets stuck, and the machine runs poorly.

This paper is like a team of scientists using a super-powerful microscope (called photoelectron spectroscopy) to measure exactly how high or low these two workers are standing when they meet.

Here is the story of what they found, broken down simply:

1. The "Clean Room" Experiment

In the past, scientists tried to measure this meeting, but they were often looking at messy surfaces. Imagine trying to measure the height of two people standing on a floor covered in dust, crumbs, and sticky tape. It's hard to know if the height difference is real or just because of the mess.

In this study, the scientists did something special. They took a giant, perfect crystal of the Perovskite and snapped it open inside a vacuum chamber (a room with no air). This is like snapping a fresh cookie in half in a sterile lab. The inside surface is perfectly clean, with no dust or dirt. They then carefully placed the C60 "conveyor belt" right onto this clean surface. This allowed them to see the true relationship between the two materials without any outside interference.

2. No Chemical Explosions

When scientists put metals on Perovskite, it's like throwing a match into a gas tank—the materials react violently and change. But when they put C60 on the Perovskite, nothing exploded. Both materials stayed exactly as they were, like two strangers shaking hands without changing their clothes. This was a relief; it meant the scientists could trust their measurements of the energy levels.

3. The "Height" Problem

The main goal was to see if the C60 was positioned correctly to grab the electricity.

The Ideal Scenario: The C60 should be slightly lower than the Perovskite so electricity can slide down easily, like a ball rolling down a gentle hill.
What They Found: In almost every test, the C60 actually shifted downward toward the Perovskite. Imagine the conveyor belt suddenly sinking into the ground. This is actually the opposite of what you want for easy movement; it's like trying to roll a ball up a small hill. This "downward shift" makes it harder for the electricity to get out.

4. The "Messy Floor" Effect

Here is the most interesting part: The amount the C60 sank varied from experiment to experiment.

In one spot on the crystal, the C60 sank a little bit.
In another spot, it sank a lot.

The scientists realized this was because of tiny, invisible specks of dust (contaminants) or slight differences in how the crystal surface ended. It's like trying to stack blocks: if the table has a tiny pebble under one corner, the whole stack tilts differently. Even though the crystal was "perfect," these microscopic differences changed the energy alignment significantly.

5. The Final "Stable" Height

The scientists kept adding more layers of C60, like stacking more blankets on a bed.

Thin layers (1–3 blankets): The height was unpredictable and wobbly.
Thick layers (5+ blankets): Once they added enough C60 (more than 5 layers), the height stopped changing. It settled at a specific, constant difference.

They calculated that once the C60 layer was thick enough, there was a fixed "gap" of 0.52 electron-volts between the two materials.

6. Why This Matters for Solar Cells

The paper concludes that because of this "downward shift" and the sensitivity to tiny surface details, the natural meeting point between Perovskite and C60 isn't perfect for moving electricity efficiently.

Think of it like this: If you try to build a solar cell with just these two materials, the electricity might get stuck at the doorway. This explains why real-world solar cells often need extra helpers (like special passivation layers or blocking layers) to fix the height difference and make the electricity flow smoothly. Without these extra helpers, the solar cell won't be as efficient as it could be.

In short: The scientists found that while C60 and Perovskite get along without reacting chemically, their energy levels don't line up perfectly on their own. The alignment is very sensitive to tiny surface details, and the C60 tends to shift in a way that makes it harder for electricity to escape, which is why solar cell makers need to add extra layers to fix the problem.

Problem Statement
Optimizing perovskite solar cells (PSCs) requires a precise understanding of the energy level alignment at the interface between the lead halide perovskite absorber and the electron transport layer (ETL). Fullerene ( $C_{60}$ ) is a widely used ETL material, particularly in $p$ - $i$ - $n$ architectures and multijunction devices, where it is typically deposited via thermal evaporation. However, despite numerous studies investigating the energetic alignment between methylammonium lead iodide ( $MAPbI_3$ ) and $C_{60}$ , there is no scientific consensus. Previous reports have yielded conflicting models: some suggest close alignment of the $C_{60}$ LUMO with the perovskite conduction band, while others report a $C_{60}$ LUMO significantly above the perovskite conduction band or a $n$ - $n$ junction. These discrepancies are often attributed to variations in sample preparation, such as the use of polycrystalline thin films with grain boundaries, substrate effects, surface contaminants, and differences in surface termination. Consequently, the intrinsic energetic properties of the $MAPbI_3$ / $C_{60}$ interface remain unclear, complicating the rational design of efficient devices.

Methodology
To resolve these inconsistencies, the authors employed synchrotron-based soft X-ray photoelectron spectroscopy (PES) to investigate the interface in a highly controlled, in-situ environment.

Sample Preparation: Instead of thin films, the study utilized in-situ cleaved $MAPbI_3$ single crystals. This approach provides a clean, well-defined surface free from grain boundaries, adventitious carbon, and substrate-induced effects.
Deposition: $C_{60}$ layers were deposited via thermal evaporation in ultra-high vacuum (UHV) onto the cleaved crystals.
Measurement Protocol: The study involved three separate experimental series (Exp 1, 2, and 3) conducted over different beamtimes to assess reproducibility. Measurements were taken at multiple spots on the same crystal and with varying photon energies (130, 535, and 758 eV) to distinguish between surface and bulk effects and to minimize X-ray exposure damage.
Data Analysis: The authors avoided relying solely on absolute binding energies, which can be affected by sample charging. Instead, they utilized relative core level positions, specifically the energy difference between the $Pb$ 4f core level of the perovskite and the $C$ 1s core level of $C_{60}$ . The thickness of the $C_{60}$ layer was calculated in monolayers (ML) based on the attenuation of the $Pb$ 4f signal. The valence band maximum (VBM) of $MAPbI_3$ and the HOMO onset of $C_{60}$ were determined using specific analysis methods (logarithmic representation for perovskites and linear extrapolation for $C_{60}$ ) to calculate the energy offset ( $\Delta E_{VB-HOMO}$ ).

Key Results

Chemical Stability: Unlike the deposition of metals on perovskites, which often causes chemical damage, the formation of the $MAPbI_3$ / $C_{60}$ interface via thermal evaporation resulted in no significant chemical reaction. Both materials remained chemically stable, and the electronic structure of $C_{60}$ remained largely intact, evidenced by the preservation of shake-up satellite peaks in the $C$ 1s spectra.
Energy Shift Trends: The study observed a consistent downward energy shift of the $C_{60}$ layer toward the $MAPbI_3$ interface. This manifests as a shift to lower binding energies for the $C$ 1s core level relative to the $Pb$ 4f level as $C_{60}$ coverage increases.
Variability: While a general trend was observed, the magnitude of the energy shift varied between different sample spots and experimental runs. This variability was attributed to subtle differences in surface chemistry, such as the presence of trace adventitious carbon or slight variations in surface termination (e.g., MAI-terminated vs. $PbI_2$ -terminated), even on single crystals.
Constant Offset at High Coverage: At higher $C_{60}$ coverages (greater than 5 monolayers), the energy offset between the $MAPbI_3$ valence band and the $C_{60}$ HOMO level stabilized at a constant value of 0.52 eV. This indicates that the interface energetics reach a steady state once a bulk-like $C_{60}$ layer is formed.
Band Alignment Implications: Using the measured 0.52 eV offset and assuming a $C_{60}$ HOMO-LUMO gap of 1.86 eV, the $C_{60}$ LUMO is positioned 0.25 eV below the $MAPbI_3$ conduction band, which is favorable for electron extraction. However, if a larger gap of 2.3 eV (often cited in literature) is used, the $C_{60}$ LUMO would sit above the perovskite conduction band, hindering charge extraction.

Significance and Claims
The authors claim that their study provides a definitive reference for the intrinsic energy alignment at a clean $MAPbI_3$ / $C_{60}$ interface, free from the complexities of thin-film processing. The findings underscore that:

Surface Sensitivity: Interfacial energetics are extremely sensitive to minute variations in surface composition and contamination, explaining the spread of values reported in previous literature.
Device Performance: The observed downward energy shift of $C_{60}$ toward the perovskite is unfavorable for charge extraction and may contribute to charge trapping. This provides a physical rationale for why high-efficiency devices often require additional interface modifications, such as charge blocking layers (e.g., BCP, LiF) or surface passivation, to optimize the energetic landscape.
Methodological Rigor: The use of relative core level analysis on single crystals allows for the isolation of intrinsic phenomena from external factors, offering a more reliable basis for comparing different experimental conditions than absolute binding energy measurements alone.

The paper concludes that while a favorable alignment is possible, the variability inherent in surface conditions necessitates careful control and accurate measurement of surface properties to ensure consistent device performance.