Molecularly engineered ultrathin films enhance efficiency and stability in perovskite solar cells by aligning energy levels precisely, offering an alternative to conventional electron transport layers.
(Nanowerk Spotlight) Solar cell performance depends as much on what happens at the microscopic interfaces inside the device as on how much sunlight reaches its surface. Beneath their dark, orderly surfaces is a complex internal structure that determines how efficiently they convert sunlight into electricity, how long they last, and how much they cost to produce. These internal layers control the behavior of electrical charges as they move through the device. Even minor inefficiencies in these layers can reduce performance significantly.
Halide perovskite solar cells are emerging as a strong alternative to silicon. Their appeal lies in a combination of high efficiency, relatively simple processing, and compatibility with lightweight and flexible substrates. But their internal architecture requires precise control. Layers that manage the movement of electrons and holes—the carriers of electrical charge generated by sunlight—remain a key constraint.
These charge-selective contacts are typically made from metal oxides or organic semiconductors. Many of these materials are chemically unstable, difficult to process at low temperatures, or poorly suited to large-scale manufacturing.
Self-assembled monolayers offer a different approach. These are molecular films only a few nanometers thick that can spontaneously organize on surfaces and be chemically tailored for specific electronic functions. Because they can be applied from solution and require no high-temperature treatment, they offer a more controllable and potentially lower-cost alternative.
Most efforts so far have focused on monolayers that extract holes, with less work done on those that selectively extract electrons. This leaves open important questions about how to design molecular layers that support efficient electron transport in perovskite devices.
By systematically varying the chemical structure of the molecules, the authors showed how electronic properties at the interface could be tuned to improve charge extraction and overall device performance. Their results establish key design rules for using monolayers to replace conventional electron-transport materials in perovskite cells.
a) Schematic illustration of the perovskite solar cell configuration used in this work that details the component structure of the self-assembled monolayers molecules, and b) the molecular structures with electrostatic potential values of the self-assembled monolayers investigated. (Image: Reprinted from DOI:10.1002/aenm.202502789, CC BY) (click on image to enlarge)
Each monolayer in the study was based on a core naphthalimide structure. This group of organic molecules is known for high electron mobility and thermal stability. The molecules were functionalized with a phosphonic acid anchoring group to attach to the conductive oxide substrate, and with terminal groups designed to modify their electronic properties.
The researchers tested three types of terminal groups: cyano (strongly electron-withdrawing), bromo (electronegative), and methoxy (electron-donating). They also varied the length of the alkyl chain connecting the anchoring group to the functional core, which affects how the molecules pack on the surface.
These monolayers were evaluated as the only electron-selective layer in a typical n–i–p perovskite solar cell configuration. The cells used a mixed-cation, mixed-halide perovskite absorber and a standard hole transport material. No additional electron-transport layer such as tin oxide was included, which allowed the researchers to isolate the role of the monolayers in governing electron extraction.
Among the six molecular variants tested, the cyano-functionalized monolayer with a short ethyl linker—identified as NI-CN—produced the best performance. Solar cells using this monolayer achieved a power conversion efficiency of 20.6 percent. This is comparable to devices using conventional metal oxide layers, despite relying on a single self-assembled monolayer only a few nanometers thick. In contrast, the unmodified naphthalimide monolayer reached only 5.8 percent efficiency.
The improved performance was attributed to better energy level alignment between the monolayer and the perovskite. For electrons to move efficiently from the perovskite layer to the electrode, the energy level of the monolayer’s lowest unoccupied molecular orbital (LUMO) must be well matched to the perovskite’s conduction band. The cyano group lowers the LUMO energy, making it easier for electrons to transfer without energy loss. This was supported by measurements of surface potential, photoluminescence quenching, and ultraviolet photoelectron spectroscopy.
The choice of terminal group strongly affected device behavior. The methoxy-substituted monolayer had a higher LUMO energy and acted as a barrier to electron flow, leading to an efficiency of only 2.6 percent. The bromo-substituted variant offered a small improvement over the control, but still performed far below NI-CN. These results highlight the importance of using electron-withdrawing groups to align energy levels for efficient electron extraction.
The length of the alkyl chain also influenced performance. Longer linkers such as propyl and butyl introduced insulating character and disrupted molecular packing. This led to reduced surface uniformity and increased variation in local work function, which in turn caused less consistent charge transport. Measurements using Kelvin probe force microscopy showed that monolayers with longer chains produced patchy surface potentials. These irregularities can hinder device performance even when the average energy alignment appears acceptable.
Devices built with the NI-CN monolayer also demonstrated better operational stability. Under continuous illumination and thermal stress without encapsulation, cells using NI-CN maintained their efficiency more consistently over 500 hours than those using tin oxide or unmodified monolayers.
The improved stability may be due to stronger interfacial interactions between the cyano group and the perovskite surface, which can reduce chemical degradation. Optical measurements of perovskite films exposed to heat confirmed that those deposited on NI-CN remained more stable than films on other surfaces.
Additional photoluminescence data showed that perovskite layers on NI-CN had lower emission intensity, consistent with more efficient charge transfer. Computational analysis supported the experimental results, with density functional theory calculations showing that the cyano group shifted the molecule’s energy levels downward and improved alignment with the perovskite conduction band.
These results establish a clear connection between molecular structure, interfacial energy alignment, and device performance. Monolayers that combine a strong electron-withdrawing group with a short linker produce deeper energy levels, more uniform surfaces, and better charge extraction. The use of cyano-substituted naphthalimide molecules illustrates how fine-tuning the molecular design of a monolayer can replace more complex and less stable materials while maintaining high efficiency.
The efficiency achieved in this work represents one of the highest reported for perovskite solar cells using a standalone self-assembled monolayer as the electron-selective contact. By removing the need for additional metal oxide layers, this approach offers a simpler path to high-performance devices that can be manufactured with fewer steps and greater flexibility. The findings also provide a framework for designing future molecular layers with tailored electronic properties.
Rather than relying on bulk material properties or complex multilayer stacks, this strategy uses single-molecule design to control charge behavior at critical interfaces. As perovskite solar technologies move toward commercial viability, improvements in stability, scalability, and manufacturing will require this kind of precision. Monolayers like those developed in this study show how molecular engineering can directly address those needs.
Han-Hee Cho (Ulsan National Institute of Science and Technology)
, 0000-0003-2491-4619 corresponding author
Jong-Woon Ha (Gyeongsang National University)
, 0000-0003-3753-7947 corresponding author
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