The development of charge-selective monolayers is pivotal for advancing the performance and stability of perovskite photovoltaics. In this contribution, we will report on the latest developments by the team at Karlsruhe Institute of Technology on self-assembled monolayers (SAMs) with carbazole-based compounds for charge transport layers in perovskite-based single junction solar cells and tandem devices. This contribution encompasses results on interfacial optimization, evaporated SAMs, device stability as well as performance.

In 2022, we reported for the first time that evaporation is a suitable method to realize high performance SAM hole transport layers (SAM-HTLs) for perovskite photovoltaics [1]. Here, we will present a follow-up study that reports on the critical role of SAM-HTLs for the growth of co-evaporated perovskite thin films and the amount of organic materials required for stoichiometric film growth [2]. Through simulations, material analysis and X-ray spectroscopy, we demonstrate how exposed phosphonic acid functional groups impact the perovskite crystal structure and material usage. Our findings pave the way for the controlled design of substrates to optimize interfacial interactions and material consumption while enhancing the electrical properties of the films.

Additionally, we will discuss a study that reports on the use of SAM-HTLs and a bilayer passivation strategy for reducing interfacial non-radiative recombination at the perovskite/electron-transport-layer interface. This interfacial design strategy leads to improved charge extraction and a high power conversion efficiency (>31%) as well as good stability in perovskite/Si tandem solar cells after 1,000 hours of maximum power point tracking at 1 Sun [3].

[1] Farag, A.; et al. Evaporated Self‐Assembled Monolayer Hole Transport Layers: Lossless Interfaces in p‐i‐n Perovskite Solar Cells. Adv. Energy Mater. 2023, 13.

[2] Feeney, T.; et al. Understanding and Exploiting Interfacial Interactions Between Phosphonic Acid Functional Groups and Co-Evaporated Perovskites. Matter 2024, 7, 2066–2090.

[3] Fang, L.; Interfacial Design Strategies for Stable and High-Performance Perovskite/Silicon Tandem Solar Cells on Industrial Silicon Bottom Cells. Under Review.

Organic Photovoltaics (OPVs) have received widespread attention and research, owing to their unique advantages like inexpensive, light weight, flexible, processable and less environmental pollution. In the past decades, due to the efforts made in device engineering and material replacement, bulk heterojunction (BHJ) OPVs has developed rapidly with power conversion efficiency (PCE) exceeding 20 % in recently developed non-fullerene small-molecules acceptors (NFAs) to replace fullerene-counterpart¹.

To further improve the performance of NFA-OPV, a ternary strategy was evolved by adding a proper third component to the binary system. High potential PCEs are offered by ternary system where either the third component acts as an additional donor or additional acceptor that serves to extend the range of absorption and can also tune material properties. Interestingly, the incorporation of the third counterpart that serves to extend the range of absorption and can also tune material properties. It can regulate the accumulation and orientation of the molecule, as well as the phase separation of donor and acceptor, providing high crystallinity and ordered molecular stacking that can improve the charge transport and inhibit the bimolecular recombination through well optimized phase separation^2,3.

However, most research on ternary strategies is based on the bulk hetero junction (BHJ) system. This makes it difficult to control other important morphological parameters, such as molecular orientation and domain purity as it further complicates the morphological regulation. Accordingly, to tailor vertical phase distribution efficiently, the Layer-by-Layer (LBL) deposition approach of the layers is considered as a promising alternative to the BHJ. The sequential deposition method allows the carefully controlled arrangement and orientation of the organic molecules, leading to improved photovoltaic performance. It avoids the difficulty of controlling the bulk morphology through forming a proper vertical phase separation that can be controlled, which is efficient for the charge transportation and collection at the corresponding electrodes. Furthermore, p–i–n-like bilayer structure enables easier exciton dissociation at the D/A interface and can reduce charge carrier recombination loss^4,5.

Hence, in this piece of work, I have developed a novel OPV structure, introducing a perovskite quantum dots (PQDs) interlayer sandwiched between the organic semiconductor donor and the NFA layers using LBL deposition approach, resulting in enhancement in the performance of  QDs based OPV devices by 11% ( from PCE of 16.6 % for the pristine Binary OPV to PCE = 18.8%  for the QDs-based Ternary OPV) along with 99 % performance retention after 3 months of storage compared to only 30% for the bilayer devices without PQDs. This unique structure allows for the LBL preferred vertical phase separation and well-controlled D/A interface film morphology, exhibiting efficient transport and extraction properties. Moreover, the incorporation of PQDs to create alloy structure with the NFA was beneficial for reducing bimolecular and trap-assisted recombination at the interface, improving exciton separation, and charge transfer, resulting in the higher, V_OC, J_SC, and FF of the PQDs based device. Finally, our findings evidence that introducing the PQDs as a third component in the interface between the donor and the NFA via LBL deposition approach is an effective strategy to improve the interfacial microstructure of the active layers, resulting in high-performance OPV devices.

Several recently reported high-efficiency devices incorporate 2D layers to passivate 3D perovskites, leading to improved performance metrics such as open-circuit voltage (Voc) and long-term stability. Motivated by these advances, we investigated the growth of oriented 2D Ruddlesden–Popper metal halides by pulsed laser deposition on various substrates. While oriented growth is found to be substrate-independent, the stability of the 2D phase is strongly influenced by the underlying substrate. Specifically, (PEA)₂PbI₄ films grown on strained epitaxial MAPbI₃ remain stable for over 184 days, showing no signs of cation exchange. In contrast, when deposited onto single crystals of MAPbI₃, the (PEA)₂PbI₄ films undergo a phase transition from n = 1 to n > 3, driven by cation exchange. The optoelectronic properties of these 2D/3D interfaces were analyzed both in situ and ex situ, offering insights into the growth mechanisms and material characteristics required to achieve stable vapor-phase heterojunctions. We discuss how these findings can be extended to a broader range of 2D/3D material systems and explore their implications for future device applications.

Ref. https://doi.org/10.1038/s41699-025-00571-3

Hole-selective monolayers have become a versatile tool for fabricating both single-junction perovskite and tandem perovskite/silicon solar cells. Although these monolayer materials and their formation methods have been employed for several years, aspects of their behavior and design remain poorly understood.

In this presentation, I will discuss the questions of the molecular design principles of monolayer-based selective contact layers. First, I will examine the success of materials featuring a carbazole chromophore backbone, highlighting how variations in linker fragments, functional groups, and anchoring moieties influence performance of the devices. Next, I will explore mixed-monolayer strategies, demonstrating how co-adsorption of complementary molecules enables tuning of interfacial properties such as wettability. Finally, I will present our efforts in developing electron-selective monolayers, describing the design strategies and synthetic routes we have employed to achieve electron selectivity. Through these insights, I aim to highlight the unresolved questions and suggest future advances in selective-contact engineering for perovskite photovoltaics.

Tin-lead perovskite solar cells have emerged as an alternative to pure lead analogues, reducing the toxicity and allowing for ideal bandgaps (~1.3 eV). The optimization of the interface between the perovskite and the charge carrier-selective layer plays a pivotal role in improving the device performance and stability, the main bottlenecks towards commercialization. In this regard, self-assembled monolayers (SAMs) have gained popularity as hole-selective layers to adjust the band energy alignment. In our research group, we have demonstrated that tin-lead perovskite solar cells with carbazole-based SAMs exhibit higher power conversion efficiencies (PCE) than those with commonly used PEDOT:PSS [1]. However, the stability of SAMs remains a debate in the research field. Herein, we investigate the role of the halogen atom of the SAMs into the device life-shelf stability and photostability by employing different halogenated SAMs. In particular, we show that Cs_0.25FA_0.75Pb_0.5Sn_0.5I₃ solar cells with iodide-2-(9H-carbazol-9-yl)ethyl)phosphonic acid (I-2-PACz) retain ~90% of the initial PCE after 7944 hours of storage in nitrogen atmosphere and without encapsulation. This outstanding life-shelf stability is attributed to defect passivation at the buried perovskite interface related to the halogen atom.

The buried interface between the perovskite absorber and the hole-transporting layer (HTL) has emerged as a critical bottleneck that limits both the power conversion efficiency (PCE) and long-term operational stability of inverted perovskite solar cells (PSCs)[1,2]. We propose a rational interfacial engineering strategy involving the incorporation of a thin interlayer composed of arylphosphonic acids at the buried perovskite/HTL interface. This molecular layer facilitates chelating interactions with undercoordinated Pb²⁺ ions at the perovskite surface, thereby effectively passivating interfacial defects. Additionally, the bulky aryl groups and multidentate coordination introduce steric hindrance, which modulates perovskite crystallization dynamics and promotes the formation of uniform, defect-suppressed films. The resultant inverted PSCs demonstrate remarkable photovoltaic performance, achieving a PCE of 26.8% on a small-area device (0.053 cm²) with an exceptional fill factor (FF) exceeding 87%. On a larger active area of 1.08 cm², the device maintains a high PCE of 25.6% with an open-circuit voltage (Voc) of 1.21 V and a FF of 80.3%. Furthermore, these devices exhibit high operational stability under both indoor conditions (including room and elevated temperatures) and prolonged outdoor maximum power point tracking, highlighting this interfacial modification approach in advancing the practical deployment of high-efficiency inverted PSCs.

 Perovskite solar cells (PSCs) have attracted worldwide attention due to their high power conversion efficiency (PCE) and low-cost solution processing. Besides the film and interfacial engineering of the perovskite layer, the development of hole-collecting materials (HCMs) is a critical key to boosting the performance of PSCs.

 In p-i-n PSCs, the use of chemically adsorbed monolayers that can efficiently collect photo-generated holes from the perovskite layer and transport them to a transparent conducting oxide (TCO) electrode as the hole-collecting layer (HCL) has become one of the mainstream directions.^[1]

 Recently, we developed a tripodal molecule composed of a triazatruxene core connected with three phosphonic acid anchoring groups (PATAT).^[2] We demonstrated that after being chemically adsorbed on the TCO surface, PATAT molecules tend to form a monolayer with a face-on orientation, resulting in improved hole-collection compared to their monopodal counterpart, and the corresponding p-i-n PSCs using the PATAT monolayer as HCL exhibited PCEs up to 23%. However, PATAT still has its own drawbacks that need to be improved, such as its difficulty in further functionalization and the hydrophobicity of its monolayer.

 In this work, we designed a series of isotriazatruxene derivatives bearing three phosphonic acid anchoring groups, iso-PATAT and its halogen-substituted analogues, as hole-collecting monolayer materials. Isotriazatruxene is an isomer of triazatruxene, which has two indole moieties facing each other, leading to a steric hindrance when bulky substituents such as alkyl phosphonic acid groups were introduced into –NH positions. Taking advantage of this point, after being chemically adsorbed, instead of binding to the TCO surface, some phosphonic acid groups would point upward, leading to a hydrophilic monolayer and improved surface wettability.

 P-i-n PSC devices using these iso-PATAT molecules as hole-collecting monolayers were fabricated and evaluated. The single-junction and monolithic all-perovskite tandem solar cells fabricated with iso-PATAT derivatives showed a champion efficiency approaching 26 and 29%, respectively.

 In this presentation, molecular design, characterization, and device evaluation will be discussed in detail.

Perovskite based solar cells (single- and multijunction) have emerged as highly promising candidates for next-generation photovoltaic technologies, offering low-cost fabrication, high efficiencies, and tunable bandgaps. In particular, perovskites with a bandgap of 1.68 eV are ideally suited as top cells in perovskite/silicon tandem solar cells, which gained relevance in both research and industry.

A critical challenge on the path to further efficiency improvements and industrial implementation lies at the interface between the perovskite absorber and the electron transport layer, typically C₆₀. In this talk, I will present a detailed analysis of a loss mechanism that occurs when C₆₀ is exposed to air. This degradation process not only impacts device performance but also has important implications for the design of manufacturing environments and for the accurate evaluation of interfacial engineering strategies: For example, the apparent effectiveness of an interlayer can be misinterpreted if C₆₀ degradation is not properly accounted for.

Furthermore, I will demonstrate two approaches to reduce interfacial losses:
(1) The incorporation of functional interlayers between perovskite and C₆₀, reduce non-radiative recombination by passivation and improved bandalignment.
(2) The use of an evaporable industrially relevant alternative to C₆₀ that shows reduced interfacial losses.

The results presented are relevant for both p-i-n single-junction and perovskite-based multi-junction solar cells and offer concrete strategies for improving device architectures and manufacturing processes-paving the way for stable, scalable, and high-efficiency perovskite photovoltaic technologies.

For highest efficiency potential and compatibility with industrial silicon surface texture, our work focusses on the fabrication of perovskite top solar cells that conformally coat µm-sized random pyramid silicon texture. For that purpose, the hybrid co-evaporation/ wet chemical route is applied to ensure controlled perovskite thin film formation over these relatively large silicon pyramids. Similarly, suitable deposition methods for selective contacts are chosen. Resulting perovskite interfaces to the hole and electron selective contacts are studied in detail and suitable passivation schemes are proposed. Supported by opto-electronic simulation and device characterization, we gain insights to the underlying working mechanisms and find that the large improvement in V_OC and FF of the applied passivation approach at the perovskite/C₆₀ interface is driven by a field-effect reducing interfacial recombination and increasing electron concentration within the perovskite bulk which enhances conductivity. Based on these findings, we were able to yield high open circuit voltage, closing the gap to one-step wet chemically processed tandem devices, and reach a certified power conversion efficiency of 31.6% for fully-textured perovskite/silicon tandem solar cells.

In recent years, inverted (p-i-n) perovskite solar cells (PSCs) employing self-assembled monolayers (SAMs) at their bottom interface have been widely investigated owing to their low-temperature processing, high fabrication reproducibility, ever-increasing power conversion efficiency (PCE) and long-term damp-heat stability.[1,2] In comparison to the functionalization of the bottom contact interface by SAMs, the top interface presents more challenging issues because solution-processed perovskite thin films tend to lost volatile species from the upward side during high-temperature annealing. These compositional variations lead to defect generation and, in turn, to non-ideal charge carrier dynamics. This, in addition to the formation of energetic barriers due to poor energy level alignment, hinder efficient electron extraction from the perovskite to the electron transport layer (ETL). Therefore, tailored interlayers at the top interface between the perovskite and the ETL, C60 or its derivatives (e.g., [6,6]-phenyl-C₆₁-butyric acid methyl ester, PCBM), are crucial for improving the performance and stability of inverted perovskite solar cells (PSCs).

Indeed, surface modification plays a pivotal role in developing state-of-the-art inverted perovskite solar cells (PSCs).[3-5] However, the functional specificity of amine-based compounds often results in selective passivation, leading to inconsistent device performance improvements. In this study, by combining the analysis of the quasi-Fermi level splitting (QFLS) [6] and surface photovoltage (SPV) by absolute photoluminescence and Kelvin probe measurements, we systematically investigate three cyclic-structured amine salts (benzene-, thiophene-, and piperazine-based) to elucidate their distinct enhancement mechanisms via defect passivation and thus suppression of non-radiative recombination. Our results reveal that the benzene and thiophene functional groups effectively passivate defects at grain interiors and boundaries owing to their conjugated structure, thereby improving the interfacial contact and reducing the defect density. Meanwhile, the piperazine-based molecules exhibit a superior hole-blocking capability, effectively repelling minority carriers back into the perovskite bulk and thus improving charge transfer kinetics. These mechanistic insights inspired the development of a bimolecular passivation strategy that simultaneously addresses surface defect mitigation and charge carrier management. By implementing this bifunctional approach combining piperazine- and thiophene- iodate, we achieved a champion device efficiency exceeding 25% (mask area: 0.152 cm²) with exceptional operational stability. The encapsulated devices maintained over 85% of their initial performance after 600 hours of continuous operation at 50°C under ambient conditions (ISOS-L-1 protocol).

Perovskite solar cells have attracted substantial attention due to their promising efficiency, but their operational stability, particularly at the interfaces with charge transport layers, remains a key limitation. This issue is especially critical in inverted (p-i-n) architectures, where fullerene-based electron transport layers often compromise long-term device stability. In response, the use of low-dimensional perovskite interlayers with hydrophobic organic spacers was explored to improve stability by templating the perovskite structure. ^[1] However, conventional organic cations used in these interlayers are typically electronically inert, which hampers charge extraction and reduces overall device efficiency. In this work, we introduce low-dimensional perovskites incorporating electroactive napthalimide- and napthalenediimide-based spacers.^[2,3] These functional organic moieties also enable modification or replacement of the fullerene-based electron transport layers, producing passivated interfaces that support efficient charge transport. This strategy led to improved photovoltaic performance, achieving power conversion efficiencies exceeding 20% along with enhanced device stability, underlining the value of electroactive interlayers in optimizing inverted perovskite solar cells.

Commercialization of perovskite photovoltaics (PPVs) hinges on the successful transition from laboratory-scale fabrication to industrial-scale manufacturing. Gas-quenching-assisted blade coating (GABC) has emerged as a promising strategy for high-throughput deposition of perovskite films. However, challenges remain in controlling crystallization kinetics and achieving uniform film morphology. Particularly, the formation of voids at the substrate/perovskite interface hinders the fabrication of thick perovskite layers compatible with fully printable devices with non-reflective carbon electrodes. This talk focuses on advancing fab-scale production of PPVs, showcasing a successful transition from lab-scale fabrication to GABC. By integrating in situ optical spectroscopy with doctor-blade coating, we first reveal the critical role of gas quenching in modulating nucleation kinetics and enabling compact, dense perovskite layers. Furthermore, we identify the gas-quenching stage as pivotal for α-phase FAPbI₃ nanocrystal nucleation under ambient conditions, which is essential for producing highly crystalline and stable perovskite films after annealing. Spontaneous cesium ion incorporation from cesium chloride further facilitates nucleation and nanocrystal growth, enabling fully printed devices with power conversion efficiencies (PCEs) of up to 19.36% and minimodules reaching 16.23%. Finally, to address the optical limitations of carbon electrodes in light recycling, we introduce a two-dimensional perovskite layer-assisted growth (2D-LAG) strategy. This approach mitigates void formation in perovskite films over 1 μm thick by promoting heterogeneous nucleation and suppressing solvent entrapment. The resulting monolithic, void-free films achieve PCEs of 19.9% on rigid substrates and 17.5% on flexible substrates in fully printed perovskite solar cells with non-reflecting carbon electrodes.

Lead-Sn (Pb-Sn) perovskites are highly promising for single and tandem solar cells owing to their reduced toxicity and narrower bandgap. However, achieving high-quality films remains a challenge due to the rapid crystallization and self-doping of tin, especially under scalable fabrication techniques. Here, we present a scalable two-step blade coating technique involving solvent and spacer engineering to enhance Pb-Sn perovskite crystallization. In the second step, a mixed solvent system of isopropanol and 2-methyl-2-butanol was employed to facilitate precursor diffusion and promote complete conversion to perovskite. Introducing organic spacers such as phenylethylammonium (PEA) is an effective strategy to enhance passivation of grain boundaries and improve stability by forming low-dimensional/3D hybrid structures. This approach enabled power conversion efficiencies (PCE) exceeding 16% with good operational stability.^[1] In this work we show that organic cations based on the thiophene moiety can reach PCE of 17.7%. Our results demonstrate substantial improvements in both efficiency and stability, marking a significant step toward the industrial-scale fabrication of high-performance Pb–Sn perovskite solar cells.

Perovskite-based multijunction solar cells are rapidly approaching the stage of commercial deployment, exhibiting extraordinary advancements in their energy conversion efficiencies over the past five years. For example, the most efficient two-terminal configuration that has been developed to date is a tandem structure combining perovskite materials with silicon (Si). Despite these impressive efficiency gains, the long-term reliability and stability of these solar devices remain significant concerns.

One of the primary reasons for this uncertainty is the relatively brief period since the inception and development of perovskite-based multijunction solar cells. This short timeframe poses substantial challenges for conducting long-term, decade-spanning reliability tests, primarily due to the inherent time constraints associated with such extensive evaluations. Additionally, there is a notable lack of established accelerated aging protocols, which are essential for predicting the long-term performance and durability of these devices. This gap exists because the fundamental failure mechanisms of perovskite-based multijunction solar cells are not yet fully understood, making it difficult to develop standardized testing methods that can reliably simulate prolonged operational conditions.

In the upcoming presentation, I will elaborate on our recent research findings that investigate the behavior of perovskite cells when integrated into multijunction configurations. First, I will explore how defects at the nanometric scale can initiate and propagate degradation within the solar cells (starting from single-junctions), ultimately impacting their overall performance and longevity. Furthermore, I will discuss the various strategies we have developed to mitigate these nanometric defects, thereby enhancing the stability and reliability of the devices.[1,2]

Beyond the microscopic level, I will also address the effects observed at larger scales, such as the micrometer (µm) and centimeter (cm) levels. For instance, the choice of texturing applied to the silicon cells plays a crucial role in determining both the performance and stability of the solar cells, however by adjusting fabrication parameters, µm-sized textures can be covered without performance loss. [3]  Moreover, I will highlight how precise control of passivation over various length scales has enabled us to fabricate wafer-scale tandem devices with high performance [4]. Finally, we extend our reach beyond tandem devices and I will showcase our latest developments in perovskite-perovskite-Si triple junction solar cells. [5]

Over the past decade, perovskite photovoltaics have made remarkable progress, with the best lab-scale cells now achieving close to 27% power conversion efficiency. These gains come from a deeper ability to engineer and passivate the interfaces inside the cell. Yet high efficiency is only half the story: the very passivation strategies that unlock record voltages and fill factors can introduce new routes to degradation, leaving stability lagging behind. In this talk, I will trace how light- and heat-induced stresses activate instabilities at critical interfaces. One example is the thermal decomposition of ammonium-based passivation ligands that were initially chosen to suppress non-radiative recombination. Another example is halide ions migrating across the perovskite/transport-layer junction and penetrating as far as the electrode. I will also examine chemical mismatches—such as acidic hole-transport layers contacting lead-halide perovskites—that accelerate interface breakdown. Finally, I will outline emerging strategies to arrest these pathways, including diffusion barriers, thermally robust molecular passivants, and interface-benign transport materials.

Perovskite solar cells (PSCs) have rapidly gained prominence in the field of photovoltaics due to their outstanding optoelectronic properties and ease of fabrication. In less than a decade, these devices have evolved from modest efficiencies of 3.8% to rivalling state-of-the-art silicon-based cells at 27.0%, positioning them as a leading candidate for next generation solar energy technologies. As performance gains begin to plateau near the thermodynamics efficiency threshold, the need to innovative light management techniques becomes increasingly critical to achieve further improvements. To overcome the optical losses, nanoscale surface structuring has emerges as a promising strategy to increase photo harvesting. However, traditional approaches often involve intricate, multi-step processes that can damage the perovskite material and hinder large-scale production. [1][2]

This work presents a novel solution based on femtosecond laser technology for surface nanopatterning of metal halide perovskite (MHP) films. The technique explores the formation of Laser-Induced Periodic Surface Structures (LIPSS) which is a rapid, single-step, and scalable solution compatible with industrial fabrication [3]. These periodic line structures, with tuneable periods, could significantly enhance light trapping and absorption within the perovskite thin film. We report the successful fabrication of LIPSS with periodicities around 300 nm in CsPbI₃ thin films. Detailed optical, electronic and structural characterization reveals enhanced properties, including increased photoluminescence intensity, longer carrier lifetimes, and improved stability of the black phase compared to pristine surface.

These findings highlight a powerful route toward integrating nanostructures into PSCs to improve light absorption and reduce losses. The proposed laser-based technique holds strong potential for boosting efficiency while meeting the demands of scalable, low-cost solar cell manufacturing.

Fully printed flexible perovskite solar modules (f-PSMs) offer strong potential for the commercialization of perovskite photovoltaics (PV) due to their compatibility with roll-to-roll (R2R) manufacturing. However, challenges remain in suppressing interfacial recombination losses and improving the mechanical reliability of microcrystalline perovskite films, leading to significant efficiency losses in large-scale production. Here, self-assembled monolayers (SAMs) are employed to modify or replace SnO₂, effectively reducing interfacial energy barriers and recombination losses. The resulting flexible devices achieve a PCE of 17.0% with negligible hysteresis, retain 95% of initial efficiency after 3000 bending cycles, and exhibit a T₉₅ lifetime of 1200 hours under 1 sun at 65 °C. Furthermore, thiol vapor annealing is introduced to treat freshly printed perovskite films in a fully R2R process. Thiol molecules facilitate solvent removal, passivate under-coordinated Pb²⁺ ions, and promote grain growth, relieving lattice strain and enhancing mechanical resilience. Treated films show reduced modulus, increased fracture toughness, and minimal cracking, with devices retaining 93% efficiency after 25000 bending cycles. This process also improves film uniformity and interfacial contact, yielding record PCEs of 12.1% (20.25 cm²) and 10.1% (100 cm²) for fully R2R-printed f-PSMs.

A straightforward, solution-processable method facilitates the production of high-performance perovskite solar cells at a relatively low temperature and cost. Nevertheless, defect-induced charge traps are unavoidable in three-dimensional (3D) active-layer perovskite films, particularly at their surfaces and grain boundaries, which can affect their long-term stability. A comprehensive understanding of the fundamental characteristics of 3D perovskite films and their interfaces is essential for their effective fabrication and device engineering. In this talk, we examine strategies to mitigate defects by transforming defective surfaces, such as residual non-active phases δ-FAPbI3 and PbI2, into low-dimensional perovskite (LDP) structures. The incorporation of LDP into both interfaces of 3D perovskite films is anticipated to reduce non-radiative interface recombination and enhance their formation energy. For instance, large alkyl-amine ligands from an LDP layer can passivate a 3D perovskite film through field-effect passivation and robust chemical bonding. Nonetheless, the development of a pure-phase homogeneous LDP capping layer with an adjusted crystal orientation remains challenging. To address these challenges, we successfully demonstrated that by designing the ligand chemistry and optimizing the fabrication engineering of LDP, it is possible to achieve the desired high-quality LDP capping layers with high phase purity, good orientation, and full coverage. This approach also considers how our findings can improve the long-term stability of devices using scaled-up technologies. Ultimately, we expect that our methods will contribute to meeting the industrial stability criteria for photovoltaic modules, considering industrial aspects.

Understanding and controlling interface chemistry is key to enhancing the performance and stability of halide perovskite (MHP) semiconductor devices. However, analyzing buried interfaces remains challenging due to their complex chemical reactivity and sensitivity to external stimuli. In this work, we employ a combination of hard X-ray photoelectron spectroscopy (HAXPES) and operando X-ray photoelectron spectroscopy (opXPS) to investigate the chemical and electronic structure of MHP interfaces with adjacent functional layers.[1]

Focusing on atomic layer deposited (ALD) SnO₂ and NiO layers on mixed-cation mixed-halide perovskite films, we demonstrate how advanced photoemission techniques enable the detection of buried chemical species and subtle changes in energy level alignment that are critical to device operation.[2] In particular, our HAXPES measurements reveal the formation of new interfacial species that can detrimentally affect charge transport, while opXPS provides insights into the dynamic chemical evolution of these interfaces under light and bias. Furthermore, we explore the impact of introducing protective organic interlayers between the perovskite and oxide layers, which can mitigate interfacial reactions and preserve favorable band alignment.

In this framework, we further investigate nickel oxide (NiOₓ), a widely used inorganic hole transport layer (HTL) in inverted MHP solar cells. Despite its advantageous optical and electronic properties, NiOₓ suffers from high surface reactivity and defect formation at the NiOₓ/MHP interface, limiting device performance. We show that ultraviolet-ozone (UVO) treatments, commonly employed to improve wettability and surface activation, can in fact enhance NiOₓ surface reactivity and introduce additional defect states. By systematically comparing pristine and UVO-treated NiOₓ interfaces, we identify the chemical nature of these defects and demonstrate that the insertion of a MeO-2PACz organic interlayer effectively mitigates interfacial reactions. Importantly, neither the UVO treatment nor the grafting of the organic interlayer significantly alters the bulk properties of NiOₓ, emphasizing the interfacial specificity of these modifications.

This methodological approach highlights the strength of combining synchrotron-based and lab-based XPS to gain a comprehensive understanding of interface stability and defect formation in MHP-based optoelectronics.

Identification and development of various novel charge transport and photoactive materials, additive engineering, and controlling the absorber’s morphology mainly helped to achieve efficient perovskite and organic solar cell (PSC and OSC) devices. However, one of the dominant factors in improving cell Power Conversion Efficiency (PCE) is the charge-carrier extraction from the perovskite or organic photo-absorber material. It has been observed that the improvement in PCE of the PSCs and OSCs depends very much on the hole-transport layer (HTL) of the device. Recently, self-assembled monolayers (SAMs), notably [2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz) and its derivatives have gained attention as interfacial modifiers for tuning the work function (WF) of indium tin oxide (ITO) substrate in both inverted PSC and conventional OSC configurations.

This work explores the interfacial behaviour of several donor polymers- PFO, P3HT, PM6, PCE10, and PCE13 used as hole-injection layers, in combination with blended SAMs of varying dipole strengths. Through ultraviolet photoelectron spectroscopy (UPS), we examined that the strategy of blending SAMs altered the ITO work function by more than 1 eV. Our results, however, demonstrate that the optimal behaviour predicted by the existing integer charge transfer (ICT) model [1,2] is not always shown by polymer/SAM/ITO interfaces. Rather, large interfacial dipoles predominate, which hinders alignment of the vacuum level and reduces the impact of the underlying SAM/ITO WF on the polymer WF. When donor polymer generations are compared, clear patterns emerge that the older materials, such PFO and P3HT, have greater SAM compatibility and are less sensitive to variations in ITO WF. The surface potential of the electrode is more important for newer donor polymers (such PM6, PCE10, and PCE13), hence SAMs with higher intrinsic dipole moments are required to minimize energy losses. This knowledge is particularly important for perovskite devices, as the interface between the HTL and the perovskite layer has a big impact on the effectiveness of hole extraction, determining V_OC, [3,4] and the overall functionality of the device.

Overall, this study underscores the need for deliberate interface design in both OSCs and PSCs. By understanding the interfacial energetics shaped by blended SAMs and donor materials, we can more effectively engineer contacts to enhance charge transfer, reduce recombination, and support the advancement of efficient next-generation solar technologies.

Electrical SPM operation modes like Kelvin probe force microscopy (KPFM) or conductive atomic force microscopy are ideally suited to do nanoscale photovoltaic measurements. In this presentation, I will present some of our recent activities in the development of specialized scanning probe microscopy methods to study hybrid perovskite materials. Solar cells based on metal halide perovskite (MHP) materials will enable cheaper and more energy-efficient photovoltaic and optoelectronic devices compared to current silicon-based technologies. To advance MHP technology further, however, will require a better understanding of the fundamental processes leading to energy losses, unstable operation conditions and premature aging. The macroscopically observed properties of optoelectronic MHP materials and devices are the result of the complicated interplay between nanoscale structure and function. Thus, the key to understanding MHP materials is the micro- and nanoscale characterization of the many nano- and microscale structures; from sub-granular twin domains, over grain boundaries and interfaces to lateral variations in crystal grains orientations and facets.

Using static and time-resolved Kelvin probe force microscopy (KPFM), we have developed unique techniques for mapping the surface potential and photovoltage. Using cross sectional measurements, we map and record the potential distribution across different layers of solar cell devices under operating conditions [1–3]. Here, the key to a reliable cross-sectional KPFM measurement is the connection of a high-resolution quantitative KPFM operation mode [4] and the preparation of a smooth surface through the solar cell. We use a combination of mechanical fracturing of the cells with a complex polishing process using ion beams. Using a pointwise spectroscopy technique, we can record and map the surface photovoltage (SPV) dynamics with 10-20 nm lateral resolution. With this Nano-SPV technique, we revealed local SPV overshoots in the vicinity of grain boundaries following an illumination pulse [5]. The overall aim of our research is to address some of the key challenges of MHP research, such as phase segregation, degradation and interface heterogeneity, to enable a deeper understanding of the different loss mechanisms and intrinsic instabilities that currently limit the application of MHP solar cells.

[1]         V. W. Bergmann, S. A. L. Weber, F. Javier Ramos, M. K. Nazeeruddin, M. Grätzel, D. Li, A. L. Domanski, I. Lieberwirth, S. Ahmad, and R. Berger, Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell, Nat Commun 5, 6001 (2014).

[2]         S. A. L. Weber, I. M. Hermes, S.-H. Turren-Cruz, C. Gort, V. W. Bergmann, L. Gilson, A. Hagfeldt, M. Graetzel, W. Tress, and R. Berger, How the formation of interfacial charge causes hysteresis in perovskite solar cells, Energy Environ. Sci. 11, 2404 (2018).

[3]         I. M. Hermes, Y. Hou, V. W. Bergmann, C. J. Brabec, and S. A. L. Weber, The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells, J. Phys. Chem. Lett. 9, 6249−6256 (2018).

[4]         A. Axt, I. M. Hermes, V. W. Bergmann, N. Tausendpfund, and S. A. L. Weber, Know your full potential: Quantitative Kelvin probe force microscopy on nanoscale electrical devices, Beilstein J. Nanotechnol. 9, 1809 (2018).

[5]         Y. Yalcinkaya, P. N. Rohrbeck, E. R. Schütz, A. Fakharuddin, L. Schmidt-Mende, and S. A. L. Weber, Nanoscale Surface Photovoltage Spectroscopy, Adv. Opt. Mater. 12, 2301318 (2024).