🔬 physics

Ammonium thiocyanate enables in-situ structural templating of MAPbI3 via quasi-2D perovskite scaffold

This study reveals that ammonium thiocyanate enhances MAPbI₃ perovskite solar cell performance not by facilitating charge transport through 2D layers, but by kinetically controlling quasi-2D phase formation to create a lattice-matched scaffold that structurally templates the growth of highly ordered 3D perovskite crystals.

Original authors: Sophie Tucker, Isaac Metcalf, Rachel C. Kilbride, Elena J. Cassella, Jessica H. Persaud, Faiz Mandani, Niranjana Raj, Max Allen, Charlie H. G. Nicholls, David G. Lidzey, Alice L. B. Pyne, Robert D. J.

Published 2026-10-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sophie Tucker, Isaac Metcalf, Rachel C. Kilbride, Elena J. Cassella, Jessica H. Persaud, Faiz Mandani, Niranjana Raj, Max Allen, Charlie H. G. Nicholls, David G. Lidzey, Alice L. B. Pyne, Robert D. J. Oliver, Silvia Motti, Jacky Even, Aditya D. Mohite, 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

Solar cells made from a class of materials called perovskites hold great promise for the future of clean energy. These materials are cheap to make and can be printed onto flexible surfaces, but they face a significant hurdle: they are often unstable when exposed to moisture and air. To solve this, scientists have developed a variation known as quasi-two-dimensional perovskites. Imagine a sandwich where layers of a highly efficient, light-absorbing material are separated by thick, water-repelling organic sheets. These organic sheets act as a protective barrier, shielding the inner layers from humidity and making the solar cell much more durable. However, this protection comes with a cost. The thick organic layers can block the flow of electricity, and the material often forms in a messy mix of different layer thicknesses, which traps energy and lowers the power the cell can generate. For years, researchers have struggled to find a way to keep the stability of these layered structures without sacrificing their ability to conduct electricity efficiently.

In a recent study, a team of researchers investigated a common chemical additive called ammonium thiocyanate, which had been used to improve these solar cells but whose exact function remained a mystery. The prevailing belief was that this additive worked by forcing the protective organic layers to stand up vertically, like a stack of books on their edges, allowing electricity to flow more easily through the material. The researchers set out to test this idea by creating thin films of the perovskite material using two different types of organic molecules, one with a slightly longer chain than the other, and observing how the material formed with and without the additive. They watched the films form in real-time, measured how they absorbed light, and used powerful X-ray beams to see the arrangement of atoms inside the material.

What they discovered turned the previous understanding on its head. The additive did not simply reorient the organic layers to stand up. Instead, it acted as a guide that helped the material form a highly ordered, three-dimensional crystal structure right from the start. The researchers found that the additive slowed down the process of crystallization, giving the atoms more time to arrange themselves into a perfect pattern rather than rushing into a messy, disordered state. In the samples using the longer-chain organic molecule, the additive successfully prevented the formation of a specific, problematic layer thickness that acts as a roadblock for electricity. It encouraged the growth of intermediate layers that served as a temporary scaffold, allowing the final, highly efficient three-dimensional crystal to grow in a perfectly aligned orientation. This resulted in a material that was not only more stable but also much better at moving electricity.

The evidence for this new mechanism was found in the way the atoms were arranged. Using X-ray scattering, the team saw that the additive caused the three-dimensional crystal planes to align such that the (110) plane was parallel to the substrate, a configuration that is ideal for extracting power. This alignment was so precise that it resembled a perfectly tiled floor rather than a pile of rubble. Crucially, this effect only happened when the additive interacted with the specific organic molecule that had the longer chain. When the researchers used the shorter-chain molecule, the additive failed to stop the formation of the problematic layers, and the material remained disordered. This proved that the success of the additive depended on a specific chemical match between the additive and the organic molecule, rather than a general effect on the material.

The impact of this discovery was immediately visible in the performance of the solar cells. The devices made with the longer-chain molecule and the additive produced significantly more electricity than those without it. The amount of current they could generate increased substantially, rising from approximately 9.2 to 17.1 mA cm-2, and the overall efficiency of converting sunlight into power jumped from a modest level to a much more competitive figure. The improvement came because the new, ordered structure allowed charges to move freely without getting stuck at grain boundaries or blocked by the unwanted thin layers. The researchers also confirmed that the additive did not simply patch up defects in the existing material; it fundamentally changed how the material grew. By extending the time window for the atoms to find their correct positions, the additive allowed the formation of a high-quality crystal that was previously difficult to achieve in these complex, layered systems.

This work suggests that the key to better solar cells lies not just in adding chemicals to fix problems, but in carefully controlling the timing and conditions of how the material forms. The study demonstrates that by slowing down the crystallization process and using the right combination of molecules, scientists can guide the material to build itself into a highly efficient structure. While the additive did not work for every type of organic molecule tested, the success with the longer-chain version provides a clear blueprint for designing future solar cells. It shows that with the right chemical partners, it is possible to create perovskite materials that are both stable against the elements and efficient at harvesting energy, bringing us closer to a future where solar power is both reliable and affordable.

Technical Summary: Ammonium Thiocyanate Enables In-Situ Structural Templating of MAPbI3 via Quasi-2D Perovskite Scaffold

Problem Statement
Quasi-two-dimensional (quasi-2D) Ruddlesden-Popper perovskites offer improved moisture stability compared to their three-dimensional (3D) counterparts but suffer from lower power conversion efficiencies (PCE). This performance gap is attributed to reduced out-of-plane charge mobility, exciton binding energy increases, and phase inhomogeneity. Specifically, quasi-2D films often contain a mixture of phases with different layer numbers ( $n$ ), where low- $n$ domains (e.g., $n=1$ ) act as charge blocking layers and recombination sites. While ammonium thiocyanate (NH $_4$ SCN) is widely used as an additive to improve device performance and induce vertical orientation in quasi-2D perovskites, the underlying mechanism remains poorly understood. Previous reports suggest NH $_4$ SCN induces vertical orientation of 2D layers or facilitates charge transfer through them, but a detailed structural mechanism has not been established.

Methodology
The authors investigated the role of NH $_4$ SCN in = 4 Ruddlesden-Popper perovskites using three aromatic spacer cations: 4-fluoro-phenethylammonium (FPEA), 4-fluoro-benzylammonium (FBZA), and 4-fluoro-phenylammonium (FPA). Films were fabricated via one-step spin-coating with varying concentrations of NH $_4$ SCN (optimized at 25 mol% relative to Pb).

The study employed a multi-modal characterization approach:

In-situ Photoluminescence (PL): To monitor crystallization kinetics and phase evolution during spin-coating.
Optical Spectroscopy: UV-Vis absorption and steady-state PL to identify phase composition and bandgap changes.
Time-Resolved PL (TRPL) and PL Quantum Yield (PLQY): To assess carrier lifetimes and radiative recombination efficiency.
Atomic Force Microscopy (AFM): To analyze surface morphology and grain size.
Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS): To determine crystal structure, phase purity, domain orientation, and lattice parameters in both surface and bulk regions.
Device Fabrication: p-i-n solar cells (ITO/MeO-2PACz/Perovskite/C60/BCP/Ag) were constructed to correlate structural findings with photovoltaic performance metrics (J-V curves, EQE, PCE).

Key Findings and Results

Mechanism of Action: Contrary to the prevailing view that NH $_4$ SCN primarily induces vertical orientation of 2D layers, the authors demonstrate that the additive enables the in-situ structural templating of the dominant 3D MAPbI $_3$ phase by quasi-2D scaffold phases.
Phase Control and Kinetics:
- In FPEA-based systems, NH $_4$ SCN significantly delays nucleation, suppressing the thermodynamically unfavorable but kinetically favored $n=1$ phase.
- This delay promotes the formation of high- $n$ quasi-2D intermediate phases ( $n \ge 2$ ).
- These high- $n$ phases possess lattice parameters that closely match the (110) plane of MAPbI $_3$ (6.131 Å for $n=2$ vs. 6.12 Å for MAPbI $_3$ ), acting as a scaffold for the epitaxial growth of highly ordered, (110)-oriented 3D perovskite domains.
- In contrast, FBZA-based systems failed to undergo this templating. The FBZA spacer stabilized the $n=1$ phase and prevented the formation of stable high- $n$ intermediates, likely due to steric hindrance and stronger DMF coordination, resulting in no significant structural improvement or templating.
Structural Evidence:
- GIWAXS data for FPEA + NH $_4$ SCN showed the extinction of the $n=1$ (002) reflection and a shift in the MAPbI $_3$ (110) peak, accompanied by a transition from Debye-Scherrer rings to Bragg spots, indicating a transition from polycrystalline to highly preferentially oriented growth.
- The absence of low-Q (002) reflections for high- $n$ phases in GIWAXS, despite their presence in absorption spectra, suggests these phases exist as ultra-small, isolated, or translationally disordered templates within the bulk, rather than as large, oriented 2D layers.
- The redshift in PL emission observed in FPEA samples is attributed to bandgap narrowing in the 3D phase due to lattice matching with the scaffold, rather than simple SCN $^-$ passivation (as pure MAPbI $_3$ with NH $_4$ SCN showed no such shift).
Morphological and Electronic Improvements:
- NH $_4$ SCN treatment increased grain sizes and reduced surface roughness in FPEA films.
- PLQY increased significantly for FPEA (from ~0.6% to ~1.1%), and carrier lifetimes were extended, indicating reduced defect density and trap states.
Photovoltaic Performance:
- FPEA Devices: The structural improvements translated to a substantial increase in PCE from 4.6% (pristine) to 13.2% (NH $_4$ SCN-treated). This was driven by a large increase in short-circuit current ( $J_{SC}$ ) and fill factor (FF), attributed to the removal of charge-blocking $n=1$ phases and improved crystallinity.
- FBZA Devices: Performance remained low (2.3% with additive) due to the persistence of the $n=1$ phase and lack of templating.
- VOC Trade-off: A slight reduction in open-circuit voltage ( $V_{OC}$ ) was observed in treated devices, attributed to the bandgap narrowing of the 3D phase.

Significance and Claims
The paper claims to establish that the primary benefit of NH $_4$ SCN in quasi-2D perovskites is not the vertical alignment of 2D layers, but rather the kinetic control of phase distribution that facilitates the in-situ templated growth of highly crystalline, oriented 3D perovskite.

The authors assert that this templating mechanism is cation-dependent, requiring two specific conditions:

The additive must effectively suppress the $n=1$ phase and promote high- $n$ quasi-2D intermediates.
There must be close lattice matching between the high- $n$ quasi-2D scaffold and the target 3D phase.

This work challenges previous interpretations of additive effects in 2D perovskites and provides a structural basis for rational additive design. The authors conclude that optimizing quasi-2D photovoltaics relies on engineering the precursor solution to bypass kinetic traps (like $n=1$ formation) and utilize quasi-2D phases as transient scaffolds for high-quality 3D growth, rather than relying on the 2D layers themselves for charge transport.

More like this