New Helicene-Type Hole-Transporting Molecules for High-Performance and Durable Perovskite Solar Cells

Aza[5]helicene-Derived Semiconducting Polymers for Improved Performance in Perovskite Solar Cells: Exploring Energetic and Morphological Influences

The strategic design of solution-processable semiconducting polymers possessing both matched energy levels and elevated glass transition temperatures is of urgent importance in the progression of thermally robust n-i-p perovskite solar cells with efficiencies exceeding 25 %. In this work, we employed direct arylation polymerization to achieve the high-yield synthesis of three aza[5]helicene-derived copolymers with distinct HOMO energy levels and exceptional glass transition temperatures. Upon integration of these semiconducting polymers into formamidinium lead triiodide-based perovskite solar cells, marked disparities in photovoltaic parameters manifest, primarily stemming from variations in the electrical conductivity and film morphology of the hole transport layers. The p-A5HP-E-POZOD-E copolymer, featuring a main chain comprising alternating repeats of aza[5]helicene, ethylenedioxythiophene, phenoxazine, and ethylenedioxythiophene, attains an initial average efficiency of 25.5 %, markedly surpassing reference materials such as spiro-OMeTAD (23.0 %), PTAA (17.0 %), and P3HT (11.6 %). Notably, p-A5HP-E-POZOD-E exhibits a high cohesive energy density, resulting in enhanced Young's modulus and diminished external species diffusion coefficients, thereby conferring perovskite solar cells with exceptional 85 °C tolerance and operational stability.

Beyond Planar Structure: Curved π-Conjugated Molecules for High-Performing and Stable Perovskite Solar Cells

Perovskite solar cell (PSC) shows a great potential to become the next-generation photovoltaic technology, which has stimulated researchers to engineer materials and to innovate device architectures for promoting device performance and stability. As the power conversion efficiency (PCE) keeps advancing, the importance of exploring multifunctional materials for the PSCs has been increasingly recognized. Considerable attention has been directed to the design and synthesis of novel organic π-conjugated molecules, particularly the emerging curved ones, which can perform various unmatched functions for PSCs. In this review, the characteristics of three representative such curved π-conjugated molecules (fullerene, corannulene and helicene) and the recent progress concerning the application of these molecules in state-of-the-art PSCs are summarized and discussed holistically. With this discussion, we hope to provide a fresh perspective on the structure-property relation of these unique materials toward high-performance and high-stability PSCs.

A Curious Case of Two-Photon Absorption in n-Helicene and n-Phenylene, n=6-10: Why n=7 is Different?

n-Helicenes and n-Phenylenes are interesting examples of twisted molecules, where although the atoms are connected through conjugated π ${\pi }$ -bonds, the π ${\pi }$ -conjugation is largely hindered by the twisted nature of the bonds. Such structures provide a unique opportunity to study the effect of twisted π ${\pi }$ -system on non-linear optical properties. In this work, we studied the two-photon absorption in donor-acceptor substituted n-helicenes and n-phenylenes employing the state-of-the-art RI-CC2 method and reported a unique feature we observed in n=7 systems. We found that both 7-helicene and 7-phenylene systems exhibit largest two-photon absorption than other members in their respective classes. Furthermore, using generalized few-state model, we provided a detailed microscopic mechanism of this unique observation involving participation of different transition dipole moment vectors and their relative orientations.

Molecular engineering of nitrogen-rich helicene based organic semiconductors for stable perovskite solar cells

Polycyclic heteroaromatics play a pivotal role in advancing the field of high-performance organic semiconductors. In this study, we report the synthesis of a pyrrole-bridged double azahelicene through intramolecular oxidative cyclization. By incorporating bis(4-methoxyphenyl)amine (OMeDPA) and ethylenedioxythiophene-phenyl-OMeDPA (EP-OMeDPA) into the sp3-nitrogen rich double helicene framework, we have successfully constructed two organic semiconductors with ionization potentials suitable for application in perovskite solar cells. The amorphous films of both organic semiconductors exhibit hole density-dependent mobility and conductivity. Notably, the organic semiconductor utilizing EP-OMeDPA as the electron donor demonstrates superior hole mobility at a given hole density, which is attributed to reduced reorganization energy and increased centroid distance. Moreover, this organic semiconductor exhibits a remarkably elevated glass transition temperature of up to 230 °C and lower diffusivity for external small molecules and ions. When employed as the p-doped hole transport layer in perovskite solar cells, TMDAP-EP-OMeDPA achieves an improved average efficiency of 21.7%. Importantly, the solar cell with TMDAP-EP-OMeDPA also demonstrates enhanced long-term operational stability and storage stability at 85 °C. These findings provide valuable insights into the development of high-performance organic semiconductors, contributing to the practical application of perovskite solar cells.

Recent Progress of Helicene Type Hole-Transporting Materials for Perovskite Solar Cells

Perovskite solar cells have emerged as one of the most promising photovoltaic technologies for future clean energy sources to replace fossil fuels. Among the various components in a perovskite solar cell, the hole-transporting materials play significant roles in boosting device performance and stability. Recently, hole-transporting materials with helicene cores have received much attention due to their unique properties and ability to improve the performance and stability of the perovskite solar cells. The focus of this review is on the emerging special class of HTMs based on helicenes for perovskite solar cells. The optical, electrochemical, thermal and photovoltaic properties of helicene-based small molecules as HTMs or interfacial layer materials in n-i-p or p-i-n type perovskite solar cells are summarized. Finally, perspectives for the future development of helicene type hole-transporting materials are provided.

Synthesis, Characterization, and Thin-Film Transistor Response of Benzo[i]pentahelicene-3,6-dione

Organic semiconductors hold the promise of simple, large area solution deposition, low thermal budgets as well as compatibility with flexible substrates, thus emerging as viable alternatives for cost-effective (opto)-electronic devices. In this study, we report the optimized synthesis and characterization of a helically shaped polycyclic aromatic compound, namely benzo[i]pentahelicene-3,6-dione, and explored its use in the fabrication of organic field effect transistors. In addition, we investigated its thermal, optical absorption, and electrochemical properties. Finally, the single crystal X-ray characterization is reported.

Chrysene-Based Azahelicene π-Linker of D-π-D-Type Hole-Transporting Materials for Perovskite Solar Cells

Chrysene is a readily available material for exploring new polycyclic aromatic hydrocarbons (PAHs). In this study, two chrysene based azahelicenes, nine-membered BA7 and ten-membered DA6, are constructed by intermolecular oxidative annulation of 6-aminochrysene and intramolecular annulation of N⁶ ,N¹² -bis(1-chloronaphthalen-2-yl)chrysene-6,12-diamine, respectively. The hexylated BA7 and DA6 and their brominated products were undoubtedly characterized by single crystal XRD. Subsequent amination with bis(9-methyl-9H-carbazol-3-yl)amine (BMCA) electron donor afforded D-π-D-type semiconductors BA7-BMCA and DA6-BMCA with beneficial properties to act as hole transport materials for perovskite solar cell. Compared with 19.4 % champion power conversion efficiency (PCE) of BA7-BMCA based device, a higher PCE of 20.2 % for DA6-BMCA counterpart may be attributed to its S-shaped double helicene-like linker with extended π-conjugated system.

Toward Large-Area and Fully Solution-Sheared Perovskite Solar Cells

The solution shearing technique was used to prepare the various layers involved in perovskite solar cells (PSCs), with a device structure of FTO/c-TiO₂/mp-TiO₂/CH₃NH₃PbI₃/Spiro-OMeTAD/Ag, in an area as large as 6 × 10 cm². The film morphology and thickness of each layer were optimized by varying respective shearing parameters. The fully solution-sheared PSCs exhibited a champion power conversion efficiency (PCE) of 15.89%. In comparison, the PSCs with only perovskite layer solution-sheared and other layers spin-coated showed a high PCE of 17.27%. These results demonstrate the potential of a simple, rapid, cost-effective, and scalable solution shearing process to fabricate large-area PSCs and modules.

Synthesis, Structure, and Chiroptical Properties of Indolo- and Pyridopyrrolo-Carbazole-Based C2 -Symmetric Azahelicenes

Treatment of 11,12-bis(1,1'-biphenyl-3-yl or 6-phenylpyridin-2-yl)-substituted 11,12-dihydro-indolo[2,3-a]carbazole with an oxidizing system of Pd(II)/Ag(I) induced effective double dehydrogenative cyclization to afford the corresponding π-extended azahelicenes. The optical resolutions were readily achieved by a preparative chiral HPLC. It was found that the pyridopyrrolo-carbazole-based azahelicene that contains four nitrogen atoms exhibits ca. 6 times larger dissymmetry factors both in circularly dichroism (CD) and circularly polarized luminescence (CPL), |g_CD | and |g_CPL | values being 1.1×10^-2 and 4.4×10^-3 , respectively, as compared with the parent indolocarbazole-based azahelicene. Theoretical calculations at the RI-CC2 level were employed to rationalize the observed enhanced chiroptical responses. The (chir)optical properties of the former helicene was further tuned by a protonation leading to remarkable red-shift with a considerable enhancement of the |g_CPL | value.

Low-Cost Hole-Transporting Materials Based on Carbohelicene for High-Performance Perovskite Solar Cells

Two hole-transporting materials (HTMs) based on carbohelicene cores, CH1 and CH2, are developed and used in fabricating efficient and stable perovskite solar cells (PSCs). Owing to the rigid conformation of the helicene core, both compounds possess unique CH-π interactions in the crystalline packing pattern and good phase stability, which are distinct from the π-π intermolecular interactions of conventional planar and spiro-type molecules. PSCs based on CH1 and CH2 as HTMs deliver excellent device efficiencies of 19.36 and 18.71%, respectively, outperforming the control device fabricated with spiro-OMeTAD (18.45%). Furthermore, both PSCs exhibit better ambient stability, with 90% of initial performance retained after aging with a 50-60% relative humidity at 25 °C for 500 h. Due to the low production cost of both compounds, these newly designed carbohelicene-type HTMs have the potential for the future commercialization of PSCs.

Dopant-Free All-Organic Small-Molecule HTMs for Perovskite Solar Cells: Concepts and Structure–Property Relationships

Since the introduction of Perovskite Solar Cells, their photovoltaic efficiencies have grown impressively, reaching over 25%. Besides the exceptional efficiencies, those solar cells need to be improved to overcome some concerns, such as their intrinsic instability when exposed to humidity. In this respect, the development of new and stable Hole Transporting Materials (HTMs) rose as a new hot topic. Since the doping agents for common HTM are hygroscopic, they bring water in contact with the perovskite layer, thus deteriorating it. In the last years, the research focused on “dopant-free” HTMs, which are inherently conductive without any addition of dopants. Dopant-free HTMs, being small molecules or polymers, have still been a relatively small set of compounds until now. This review collects almost all the relevant organic dopant-free small-molecule HTMs known so far. A general classification of HTMs is proposed, and structure analysis is used to identify structure–property relationships, to help researchers to build better-performing materials.

Thiophene-Fused Butterfly-Shaped Polycyclic Arenes with a Diphenanthro[9,10-b:9',10'-d]thiophene Core for Highly Efficient and Stable Perovskite Solar Cells

Two polycyclic heteroarene derivatives, namely, V-1 and V-2, with a diphenanthro[9,10-b:9',10'-d]thiophene (DPT) core tethered with two diphenylaminophenyl or diphenylamino groups were first synthesized and used as hole-transporting materials (HTMs) in perovskite solar cell (PSC) fabrication. The novel HTMs exhibit appropriate energy-level alignment with the perovskite so as to ensure efficient hole transfer from the perovskite to HTMs. V-2 with the diphenylamino substituent on DPT exhibited impressive photovoltaic performance with a power conversion efficiency of 19.32%, which was higher than that of V-1 (18.60%) and the benchmark 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-OMeTAD) (17.99%), presumably because of a better hole extraction, higher hole mobility, and excellent film-forming ability, which were supported by steady-state photoluminescence (PL), time-resolved PL, the hole mobility experiment, scanning electron microscopy, and atomic force microscopy measurements. Meanwhile, V-2-based PSCs exhibited better long-term durability than that with V-1 and the state-of-the-art spiro-OMeTAD, which is ascribable to the excellent surface morphology and hydrophobicity of the film. This systematic study suggests that DPT-based molecules are good potential candidates as HTMs for achieving high-performance PSCs.

An Oxa[5]helicene-Based Racemic Semiconducting Glassy Film for Photothermally Stable Perovskite Solar Cells

Attaining the durability of high-efficiency perovskite solar cells (PSCs) operated under concomitant light and thermal stresses is still a serious concern before large-scale application. It is crucial to maintain the phase stability of the organic hole-transporting layer for thermostable PSCs across a range of temperatures sampled during device operation. To address this issue, we propose a racemic semiconducting glassy film with remarkable morphological stability, exemplified here by a low-molecular symmetry oxa[5]helicene-centered organic semiconductor (O5H-OMeDPA). The helical configuration of O5H-OMeDPA confers the trait of multiple-dimension charge transfer to the solid, resulting in high hole mobility of 6.7 × 10^-4 cm² V^-1 s^-1 of a solution-processed glassy film. O5H-OMeDPA is combined with a triple-cation dual-halide lead perovskite to fabricate PSCs with power conversion efficiencies of 21.03%, outperforming the control cells with spiro-OMeTAD (20.44%). Moreover, the cells using O5H-OMeDPA exhibit good long-term stability during full-sunlight soaking at 60°C.

Core Structure Engineering in Hole-Transport Materials to Achieve Highly Efficient Perovskite Solar Cells

In this work, the thiadiazolopyridine (PT) unit was introduced as the core structure, with N³ ,N³ ,N⁶ ,N⁶ -tetrakis(4-methoxyphenyl)-9-phenyl-9 H-carbazole-3,6-diamine as the peripheral group, to obtain a new compound, JY8, for use as a hole-transport material (HTM) in planar perovskite solar cells (PSCs). Compared with the previously reported JY5 with benzothiadiazole as the core structure, the PT unit with stronger electron-withdrawing ability enhanced the intermolecular dipole-dipole interaction. Moreover, the introduction of the PT unit made the central part in JY8 more planar than its analogue JY5, which is conducive to charge transport. Field-emission (FE)-SEM images suggested a smooth and condense morphology of the JY8 film, which could improve the contact between the perovskite layer and the metal electrode. Space-charge limitation of current results, steady-state, and time-resolved photoluminescence decay curves indicated that JY8 as HTM facilitated hole extraction and hole transport. Consequently, planar PSCs fabricated with JY8 as the HTM exhibited a decent efficiency of 19.14 % with a high fill factor of 81 %.