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

Li-TFSI free carbazole-based hole transport materials enable highly stable perovskite solar cells

It is crucial to fabricate highly stable perovskite solar cells for further commercialization. Herein, a novel cationic salt OY(TFSI)2 as an effective p-dopant to replace Li-TFSI is synthesized and introduced into the hole transport layer to improve the long-term stability of the device.

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.

Stable Perovskite Solar Cells Using Molecularly Engineered Functionalized Oligothiophenes as Low-Cost Hole-Transporting Materials

Triarylamine-substituted bithiophene (BT-4D), terthiophene (TT-4D), and quarterthiophene (QT-4D) small molecules are synthesized and used as low-cost hole-transporting materials (HTMs) for perovskite solar cells (PSCs). The optoelectronic, electrochemical, and thermal properties of the compounds are investigated systematically. The BT-4D, TT-4D, and QT-4D compounds exhibit thermal decomposition temperature over 400 °C. The n-i-p configured perovskite solar cells (PSCs) fabricated with BT-4D as HTM show the maximum power conversion efficiency (PCE) of 19.34% owing to its better hole-extracting properties and film formation compared to TT-4D and QT-4D, which exhibit PCE of 17% and 16%, respectively. Importantly, PSCs using BT-4D demonstrate exceptional stability by retaining 98% of its initial PCE after 1186 h of continuous 1 sun illumination. The remarkable long-term stability and facile synthetic procedure of BT-4D show a great promise for efficient, stable, and low-cost HTMs for PSCs for commercial applications.

Fluorene-terminated hole transporting materials with a spiro[fluorene-9,9′-xanthene] core for perovskite solar cells

SFX-FM with four FPA groups (two meta-substituted FPA groups of the xanthene unit) exhibited a maximum PCE of 17.29%.

Influences of Structural Modification of Naphthalenediimides with Benzothiazole on Organic Field-Effect Transistor and Non-Fullerene Perovskite Solar Cell Characteristics

Developing air-stable high-performance small organic molecule-based n-type and ambipolar organic field-effect transistors (OFETs) is very important and highly desirable. In this investigation, we designed and synthesized two naphthalenediimide (NDI) derivatives (NDI-BTH1 and NDI-BTH2) and found that introduction of 2-(benzo[d]thiazol-2-yl) acetonitrile groups at the NDI core position gave the lowest unoccupied molecular orbital (LUMO; -4.326 eV) and displayed strong electron affinities, suggesting that NDI-BTH1 might be a promising electron-transporting material (i.e., n-type semiconductor), whereas NDI-BTH2 bearing bis(benzo[d]thiazol-2-yl)methane at the NDI core with a LUMO of -4.243 eV was demonstrated to be an ambipolar material. OFETs based on NDI-BTH1 and NDI-BTH2 have been fabricated, and the electron mobilities of NDI-BTH1 and NDI-BTH2 are 14.00 × 10^-5 and 8.64 × 10^-4 cm²/V·s, respectively, and the hole mobility of NDI-BTH2 is 1.68 × 10^-4 cm²/V·s. Moreover, a difference in NDI-core substituent moieties significantly alters the UV-vis absorption and cyclic voltammetry properties. Thus, we further successfully employed NDI-BTH1 and NDI-BTH2 as electron transport layer (ETL) materials in inverted perovskite solar cells (PSCs). The PSC performance exhibits that NDI-BTH2 as the ETL material gave higher power conversion efficiency as compared to NDI-BTH1, that is, NDI-BTH2 produces 15.4%, while NDI-BTH1 gives 13.7%. The PSC performance is comparable with the results obtained from OFETs. We presume that improvement in solar cell efficiency of NDI-BTH2-based PSCs is due to the well-matched LUMO of NDI-BTH2 toward the conduction band of the perovskite layer, which in turn increase electron extraction and transportation.

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.

Influence of Nonfused Cores on the Photovoltaic Performance of Linear Triphenylamine-Based Hole-Transporting Materials for Perovskite Solar Cells

The core plays a crucial role in achieving high performance of linear hole transport materials (HTMs) toward the perovskite solar cells (PSCs). Most studies focused on the development of fused heterocycles as cores for HTMs. Nevertheless, nonfused heterocycles deserve to be studied since they can be easily synthesized. In this work, we reported a series of low-cost triphenylamine HTMs (M101-M106) with different nonfused cores. Results concluded that the introduced core has a significant influence on conductivity, hole mobility, energy level, and solubility of linear HTMs. M103 and M104 with nonfused oligothiophene cores are superior to other HTMs in terms of conductivity, hole mobility, and surface morphology. PSCs based on M104 exhibited the highest power conversion efficiency of 16.50% under AM 1.5 sun, which is comparable to that of spiro-OMeTAD (16.67%) under the same conditions. Importantly, the employment of M104 is highly economical in terms of the cost of synthesis as compared to that of spiro-OMeTAD. This work demonstrated that nonfused heterocycles, such as oligothiophene, are promising cores for high performance of linear HTMs toward PSCs.

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.