This is a sponsored post by KYOCERA Document Solutions Inc.
The performance of perovskite solar cells is not determined by the material itself alone. How we engineer the "buried interface" between the electrode and the perovskite at the molecular level can make a decisive difference—one that directly impacts charge extraction, reproducibility, and ultimately stability. Kyocera Document Solutions and the National Institute for Materials Science (NIMS) focused on the "number" and "arrangement" of methoxy groups introduced into self-assembled monolayers (SAMs). By elucidating how differences in molecular design—namely C21 versus C22—affect everything from work-function control to defect density, film formation, and device performance, the team presented design guidelines for SAMs in inverted (p–i–n) perovskite solar cells (PSCs).
The study, which focuses on methoxy-functionalized conjugated SAMs as hole-selective contacts, has been published in the international scientific journal ACS Applied Materials & Interfaces (DOI: 10.1021/acsami.6c04007).
About Kyocera Document Solutions and NIMS
Kyocera Document Solutions has been actively expanding its research and development activities in functional organic materials for perovskite solar cells, with a particular focus on hole-transport and hole-selective materials across both regular (n–i–p) and inverted (p–i–n) device architectures.
In the regular (n–i–p) structure, the company has established proprietary synthesis and manufacturing technologies for widely used hole-transport materials (HTMs) such as Spiro-OMeTAD and PTAA, leveraging decades of experience in organic photoconductor (OPC) materials for office equipment. Building on this foundation, Kyocera Document Solutions has also been developing next-generation HTMs aimed at improving energy-level alignment, stability, and lot-to-lot consistency for perovskite photovoltaics.
At the same time, recognizing the growing importance of inverted (p–i–n) perovskite solar cells for low-temperature processing, reduced hysteresis, and tandem compatibility, the company has intensified its R&D efforts to develop new hole-selective materials specifically designed for inverted architectures. These efforts include not only polymer-based HTMs intended to overcome the wettability and interface issues associated with conventional PTAA, but also the exploration of self-assembled monolayer (SAM) materials as an emerging and highly promising class of ultrathin hole-selective contacts.
In parallel to Kyocera Document Solutions' materials development, the National Institute for Materials Science (NIMS) has been conducting world-leading research on perovskite photovoltaics, with particular strengths in interfacial science, energy-level characterization, defect analysis, and device physics. NIMS has played a central role in revealing how buried interfaces govern charge extraction, recombination, and long-term stability in inverted perovskite solar cells, thereby providing a rigorous scientific framework for molecular level interface engineering.
Within this context, Kyocera Document Solutions and NIMS have collaborated on the design and evaluation of methoxy-functionalized conjugated self-assembled monolayers (SAMs) as hole-selective contacts for inverted perovskite solar cells. The present study focuses on how molecular structure—specifically the number and position of methoxy groups in triphenylamine-based conjugated SAMs—affects work-function tuning, perovskite film formation, buried interfacial defect density, and overall device performance.
Image 1: Kyocera Document Solutions and the National Institute for Materials Science (NIMS)
Why SAMs for inverted PSCs?
Inverted PSCs have gained strong momentum because they can be processed at relatively low temperatures, tend to show reduced hysteresis, and are compatible with tandem architectures.
Yet the choice of hole-selective contact remains critical: widely used options can suffer from interface reactivity, acidity, or insufficient wettability—factors that can degrade the buried interface and compromise reproducibility.
By contrast, SAMs offer an ultrathin, highly transparent contact with tunable dipoles/work functions and strong potential for interfacial passivation, while using minimal material and allowing straightforward deposition.
Methoxy-functionalized conjugated SAMs: C21 vs. C22
In this study, two triphenylamine-based methoxy-functionalized conjugated self-assembled monolayers (SAMs), denoted as C21 and C22, were synthesized and evaluated as hole-selective contacts for inverted perovskite solar cells. The key molecular design difference between C21 and C22 lies in the number and positional arrangement of methoxy groups on the terminal unit. C21 has three methoxy groups on the terminal triphenylamine unit, whereas C22 has four methoxy groups.
Density functional theory (DFT) calculations revealed that C22 exhibits a larger molecular dipole moment than C21, highlighting the importance of methoxy group positioning in controlling interfacial electrostatics.
Both SAMs were successfully deposited on fluorine-doped tin oxide (FTO) substrates. Surface analysis showed that C22 forms a denser and more uniform monolayer on the rough FTO surface, which is a critical requirement for suppressing leakage current in ultrathin hole-selective contacts. Ultraviolet photoelectron spectroscopy (UPS) measurements demonstrated that SAM modification increases the FTO work function from –4.35 eV for bare FTO to –4.71 eV for C21 and –4.76 eV for C22. This dipole-induced work-function shift improves energy-level alignment for hole extraction and simultaneously suppresses interfacial electron backflow, directly contributing to higher device efficiency and improved operational stability.
Perovskite growth on SAM: coverage, crystallinity, and reduced buried defects
A practical concern for SAM-based HTLs is whether they enable robust perovskite film growth. Here, good coverage of the perovskite layer was observed on both C21 and C22.
Notably, methoxy placement at ortho and para positions, with a total of four methoxy groups (C22) was found favorable for forming high-quality films, with slightly improved crystallinity and reduced lattice strain relative to C21.
To probe defect density at the buried interface, the team used hole-only devices and extracted trap-filled limit voltages (V_TFL), yielding defect densities of 3.91×10¹⁵ cm⁻³ for C21 and 3.10×10¹⁵ cm⁻³ for C22, confirming more effective buried interface passivation with C22.
The lower defect density for C22 points to more effective buried-interface quality, consistent with dense monolayer coverage and possible methoxy-mediated passivation.
Image 2: SEM top views and trap-density analysis (hole-only device) comparing C21 vs. C22
Devices were fabricated with an inverted architecture on FTO using the SAM as the hole-selective layer, followed by a triple-cation perovskite and an evaporated electron-transport stack (C60/BCP/Ag).
Under reverse scan, the C22-based champion device reached a power conversion efficiency of 21.58%, with V_OC = 1.07 V, J_SC = 24.12 mA/cm², and FF = 83.63%.
For comparison, the C21-based champion device achieved 21.10% (V_OC 1.05 V, J_SC 24.06 mA/cm², FF 83.62%).
Stability remains a central hurdle for PSC commercialization. In maximum power point tracking (MPPT) tests under open-air conditions (~27°C, RH ~70%), C22-based unencapsulated devices showed improved stability relative to C21-based devices.
The authors attribute the stability gain primarily to the improved perovskite film quality on C22 and the reduced density of buried interfacial defects.
Image 3: PCE and MPPT data of the unencapsulated PSCs for C21 vs. C22 devices
Toward scalable inverted PSCs via molecular interface engineering
Kyocera Document Solutions and the National Institute for Materials Science (NIMS) will continue to advance SAM-centered materials development for inverted perovskite solar cells, aiming to translate molecular design principles into scalable, reproducible, and stable device platforms.
Through continued collaboration spanning fundamental science and practical application, the partners aim to enhance interfacial stability and charge-transport efficiency, accelerating efforts toward higher performance and practical implementation of inverted-structure PSCs.
Toward Mass Production and Sustainability
Kyocera Document Solutions has completed preparations for the mass production of hole-transport materials (HTMs) for perovskite solar cells, including commercially established materials such as Spiro-OMeTAD and PTAA. Stable, high-quality prototype materials are available upon request, and the company also offers custom material development tailored to specific customer requirements. Through continued innovation in materials design and manufacturing, Kyocera Document Solutions supports the global adoption of perovskite solar cells and contributes to the realization of a more sustainable society.
This was a sponsored post by KYOCERA Document Solutions Inc.