Tetraphenylmethane-Arylamine Hole-Transporting Materials for Perovskite Solar Cells

The evolution of triphenylamine hole transport materials for efficient perovskite solar cells

In recent years, the dramatic increase in power conversion efficiency (PCE) coupled with a decrease in the total cost of third-generation solar cells has led to a significant increase in the collaborative research efforts of academic and industrial researchers. Such interdisciplinary studies have afforded novel materials, which in many cases are now ready to be brought to the marketplace. Within this framework, the field of perovskite solar cells (PSCs) is currently an important area of research due to their extraordinary light-harvesting properties. In particular, PSCs prepared via facile synthetic procedures, containing hole transport materials (HTMs) with versatile triphenylamine (TPA) structural cores, amenable to functionalization, have become a focus of intense global research activity. To optimize the efficiency of the solar cells to achieve efficiencies closer to rival silicon-based technology, TPA building blocks must exhibit favourable electrochemical, photophysical, and photochemical properties that can be chemically tuned in a rational manner. Although PSCs based on TPA building blocks exhibit attractive properties such as high-power efficiencies, a reduction in their synthetic costs coupled with higher stabilities and environmental considerations still need to be addressed. Considering the above, a detailed summary of the most promising compounds and current methodologies employed to overcome the remaining challenges in this field is provided. The objective of this review is to provide guidance to readers on exploring new avenues for the discovery of efficient TPA derivatives, to aid in the future development and advancement of TPA-based PSCs for commercial applications.

Effect of Thiophene Insertion on X-Shaped Anthracene-Based Hole-Transporting Materials in Perovskite Solar Cells

In this work, two novel tetra-substituted X-shaped molecules X1 and X2 that were constructed with anthracene as the central core and arylamine as the donor groups have been synthesized. The HTMs X1 and X2 were synthesized in two steps from industrially accessible and moderately reasonable beginning reagents. These new HTMs are described in terms of utilization of light absorption, energy level, thermal properties, hole mobility (µ_h), and film-forming property. The photovoltaic performances of these HTMs were effectively assessed in perovskite solar cells (PSCs). The devices based on these HTMs accomplished an overall efficiency of 16.10% for X1 and 10.25% for X2 under standard conditions (AM 1.5 G and 100 mW cm⁻²). This precise investigation provides another perspective on the use of HTMs in PSCs with various device configurations.

Charge transport materials for mesoscopic perovskite solar cells

An overview on recent advances in the fundamental understanding of how interfaces of mesoscopic perovskite solar cells (mp-PSCs) with different architectures, upon incorporating various charge transport layers, influence their performance.

Polycyclic Arenes Dihydrodinaphthopentacene-based Hole-Transporting Materials for Perovskite Solar Cells Application

In this paper, two D-π-D type compounds, C1 and C2, containing dihydrodinaphthopentacene (DHDNP) as a π-bridge, p-methoxydiphenylamine and p-methoxytriphenylamine groups as the donor groups were synthesized. The four 4-hexylphenyl groups at the sp³ -carbon bridges of DHDNP were acquainted with control morphology and improving solubility. The light absorption, energy level, thermal properties, and application as hole-transporting materials in perovskite solar cells of these compounds were fully investigated. The HOMO/LUMO levels and energy gaps of these DHDNP-based molecules are suitable for use as hole-transporting materials in PSCs. The best power conversion efficiencies of the PVSCs based on the C1 and C2 are 15.96% and 12.86%, respectively. The performance of C1 is comparable to that of the reference compound spiro-OMeTAD (16.38%). Compared with spiro-OMeTAD, the C1-based PVSC device showed good stability, which was slightly decreased to 98.68% of its initial efficiency after 48 h and retained 81% of its original PCE after 334 h without encapsulation. These results reveal the potential usefulness of the DHDNP building block for further development of economical and highly efficient HTMs for PVSCs.

Novel adamantane-based hole transport materials for perovskite solar cells: a computational approach

Adamantane derivatives have been subjected to quantum mechanical calculations to determine their capabilities as potential hole transport materials (HTMs) in perovskite solar cells (PSCs). Adamantane has been modified in two ways: multiple arm substitution and outermost substituent variation. It has been shown that tetra-substitution of adamantane gave the best characteristics as a HTM. Further modification showed tetra-ethyl substituted adamantane (ad-EtTPA) has the lowest HOMO, a small hole reorganization energy (λh), absorption in the UV region, and good stability. These appropriate properties mean that ad-EtTPA could be a promising HTM in PSCs. In addition, the relationship between λh and the electrostatic potential (ESP) maps of the cationic geometry has been studied. Three outcomes were drawn from the investigation: (1) the high positive potential in the ESP map is the region where more geometric distortions are occurring in going from the neutral to the cationic state, (2) large geometric distortions in this region lead to high λh, and (3) for tetra-substituted adamantane derivatives, delocalization of the positive potential leads to lower λh. The results showed that ESP maps can give insight into the molecular engineering of HTMs.

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

Three azahelicene derivatives with electron-rich bis(4-methoxyphenyl)amino or bis( p-methoxyphenyl)aminophenyl groups at the terminals were deliberately designed, synthesized, and characterized as hole-transporting materials (HTMs) for perovskite solar cells (PSCs). Optical and thermal properties, energy level alignments, film morphologies, hole extraction ability, and hole mobility were studied in detail. PSCs using the newly synthesized molecules as HTMs were fabricated. A maximum power conversion efficiency (PCE) of 17.34% was observed for the bis( p-methoxyphenyl)amino-substituted derivative (SY1) and 16.10% for the bis( p-methoxyphenyl)aminophenyl-substituted derivative (SY2). Longer-chain substituent such as hexyloxy group greatly diminishes the efficiency. In addition, the dopant-free devices fabricated with SY1 as the HTM shows an average PCE of 12.13%, which is significantly higher than that of spiro-OMeTAD (7.61%). The ambient long-term stability test revealed that after 500 h, the devices prepared from SY1 and SY2 retained more than 96% of its initial performance, which is much improved than the reference device with standard spiro-OMeTAD as the HTM under the same conditions. Detailed material cost analysis reveals that the material cost for SY1 is less than 8% of that for spiro-OMeTAD. These results provide a useful direction for designing a new class of HTMs to prepare highly efficient and more durable PSCs.

Reducing the Universal "Coffee-Ring Effect" by a Vapor-Assisted Spraying Method for High-Efficiency CH3NH3PbI3 Perovskite Solar Cells

Organic-inorganic perovskite solar cells (PSCs) are one of the most attractive and efficient burgeoning thin-film photovoltaics. The perovskite films have been fabricated via lots of deposition methods, but these laboratory-based fabrication methods are not well-matched with large-area manufacture. Herein, spray coating as a deposition technique was explored to prepare perovskite films and break the bottleneck that plagued large-scale production. However, it is hard to reduce the notorious "coffee-ring effect" during the process of spraying perovskite films especially in a one-step spraying method. Thus, the vapor-assisted spraying method (VASM), namely, fabricating perovskite films through a vapor-solid in situ reaction between CH₃NH₃I vapor and sprayed PbI₂ films, was creatively applied to the preparation of dense and uniform perovskite films. The surfaces of the sprayed PbI₂ films were optimized by adjusting the wettability, viscosity, and contact quality via various methods such as the selection of solvent, solution concentration, and substrate temperature to inhibit the capillary flow and release the pinning contact line. The application of a component solvent could effectively crush the dense structure of the PbI₂ film, optimizing the morphology of PbI₂ films and reducing the influence of the coffee-ring effect. Integrating the above aspects, the optimized PbI₂ films could form uniform perovskite films via an in situ reaction, and a best power conversion efficiency of 17.56% was achieved for planar structure PSCs, which is high among the PSCs fabricated by the spraying method. In addition, the VASM could be applied in the actual conditions for mass production, exhibiting excellent optical and electrical properties and paving the way of the commercialization of PSCs.

Molecular Engineering of Zinc-Porphyrin Sensitisers for p-Type Dye-Sensitised Solar Cells

Design of novel efficient light-harvesters for p-type dye-sensitised solar cells (DSSCs) is indispensable for further advances in this photovoltaic technology. Herein, a novel D-π-A (D=donor, π=π-conjugated linker, A=acceptor) sensitiser, ZnP1, featuring an electron acceptor, perylenemonoimide (PMI), connected to an electron donor, di(p-carboxyphenyl)amine (DCPA), through fluorene and a zinc(II) porphyrin with alkyl chains as a π-conjugated bridge is introduced. Spectroscopic and electrochemical characterisation of this dye along with a newly synthesised PMI-free reference dye ZnP0 has been undertaken to demonstrate strong electron coupling between the DCPA donor and PMI acceptor subunits through the porphyrin ring in ZnP1, which redshifts the light absorption onset to the near-IR region. When integrated into p-DSSCs based on a mesoporous nickel(II) oxide semiconductor electrode and a tris(acetylacetonato) iron(III/II) redox mediator, ZnP1 exhibits an onset of the incident photon-to-current conversion efficiency at 800 nm and a power conversion efficiency of up to 0.92 % under simulated 100 mW cm^-2 AM 1.5 G irradiation. This is the highest efficiency of the porphyrin-based p-DSSCs hitherto reported.

Incorporating C60 as Nucleation Sites Optimizing PbI2 Films To Achieve Perovskite Solar Cells Showing Excellent Efficiency and Stability via Vapor-Assisted Deposition Method

To achieve high-quality perovskite solar cells (PSCs), the morphology and carrier transportation of perovskite films need to be optimized. Herein, C₆₀ is employed as nucleation sites in PbI₂ precursor solution to optimize the morphology of perovskite films via vapor-assisted deposition process. Accompanying the homogeneous nucleation of PbI₂, the incorporation of C₆₀ as heterogeneous nucleation sites can lower the nucleation free energy of PbI₂, which facilitates the diffusion and reaction between PbI₂ and organic source. Meanwhile, C₆₀ could enhance carrier transportation and reduce charge recombination in the perovskite layer due to its high electron mobility and conductivity. In addition, the grain sizes of perovskite get larger with C₆₀ optimizing, which can reduce the grain boundaries and voids in perovskite and prevent the corrosion because of moisture. As a result, we obtain PSCs with a power conversion efficiency (PCE) of 18.33% and excellent stability. The PCEs of unsealed devices drop less than 10% in a dehumidification cabinet after 100 days and remain at 75% of the initial PCE during exposure to ambient air (humidity > 60% RH, temperature > 30 °C) for 30 days.

Forming Intermediate Phase on the Surface of PbI2 Precursor Films by Short-Time DMSO Treatment for High-Efficiency Planar Perovskite Solar Cells via Vapor-Assisted Solution Process

Morphology regulation is vital to obtain high-performance perovskite films. Vapor-assisted deposition provides a simple approach to prepare perovskite films with controlled vapor-solid reaction. However, dense PbI₂ precursor films with large crystal grains make it difficult for organic molecules to diffuse and interact with inner PbI₂ frame. Here, a surface modification process is developed to optimize the surface layer morphology of PbI₂ precursor films and lower the resistance of the induced period in crystallization. The vapor optimization time is shortened to several seconds, and the intermediate phase forms on the surface layer of PbI₂ films. We achieve porous PbI₂ surface with smaller grains through dimethyl sulfoxide vapor treatment, which promotes the migration and reaction rate between CH₃NH₃I vapor and PbI₂ layer. The PbI₂ precursor films undergo dramatic morphological evolution due to the formed intermediate phase on PbI₂ surface layer. Taking advantage of the proposed surface modification process, we achieve high-quality uniform perovskite films with larger crystal grains and without residual PbI₂. The repeatable perovskite solar cells (PSCs) with modified films exhibit power conversion efficiency of up to 18.43% for planar structure. Moreover, the devices show less hysteresis because of improved quality and reduced defect states of the films. Our work expands the application of morphology control through forming intermediate phase and demonstrates an effective way to enhance the performance of the PSCs.

Dopant-Free Tetrakis-Triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells

Developing dopant-free hole transporting layers (HTLs) is critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance, which adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTL's suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL material in low-cost high-performance tin-based perovskite solar cells.

Hole-Transporting Materials for Printable Perovskite Solar Cells

Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in their power conversion efficiency from less than 5% to over 22% in just a few years. Hole-transporting materials (HTMs) are an essential building block of PSC architectures. Currently, 2,2',7,7'-tetrakis-(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene), better known as spiro-OMeTAD, is the most widely-used HTM to obtain high-efficiency devices. However, it is a tremendously expensive material with mediocre hole carrier mobility. To ensure wide-scale application of PSC-based technologies, alternative HTMs are being proposed. Solution-processable HTMs are crucial to develop inexpensive, high-throughput and printable large-area PSCs. In this review, we present the most recent advances in the design and development of different types of HTMs, with a particular focus on mesoscopic PSCs. Finally, we outline possible future research directions for further optimization of the HTMs to achieve low-cost, stable and large-area PSCs.

Molecular Engineering of Simple Benzene-Arylamine Hole-Transporting Materials for Perovskite Solar Cells

Three benzene-arylamine hole-transporting materials (HTMs) with different numbers of terminal groups were prepared. It is noted that the molecule with three arms (H-Tri) shows a lower highest occupied molecular orbital level and a better film morphology on perovskite layer than the molecules with two or four arms (H-Di, H-Tetra). When these molecules were applied to the perovskite solar cells, the H-Tri-based one showed better performance compared with the H-Di- or H-Tetra-based ones. Photoluminescence and impedance spectroscopy demonstrate that H-Tri can improve the hole-electron separation efficiency and decrease the charge recombination, thus leading to a better performance. Moreover, the H-Tri-based device shows a comparable performance and a much less materials cost than the conventional spiro-OMeTAD. Therefore, we have presented a new low-cost and high-performance HTM through simple molecular engineering.

Anthracene–arylamine hole transporting materials for perovskite solar cells

Novel anthracene–arylamine hole transporting materials achieved a power conversion efficiency over 17% for perovskite solar cells.