Hole Transport Material Research Articles

Spiro[fluorene-9,9′-xanthene]-Based Hole-Transporting Materials for Photovoltaics: Molecular Design, Structure–Property Relationship, and Applications

Spiro[fluorene-9,9′-xanthene]-Based Hole-Transporting Materials for Photovoltaics: Molecular Design, Structure–Property Relationship, and Applications

Cost-effective high-performance quantum dot photodetectors with dual polythiophene hole transporting layers

The strong aggregation of Poly(3-hexylthiophene) (P3HT) severely limits its use as the hole-transport material in emerging quantum dot photodetectors and photovoltaics. Herein, we propose a facile and cost-effective strategy to control the solution-state aggregation of hole transporting layers by designing a dual polythiophene blend based on P3HT and its alkylthio-substituted analogue named Poly(3-hexylthiothiophene) (P3HTT). In our photodetector device, we have used the dual polythiophene as the hole transport layer and achieved a specific detectivity (D*) on the order of 1012 Jones. In particular, by incorporating a small amount of P3HTT into the dual polythiophene mixture, we observed a remarkable 28% performance enhancement. This study provides a comprehensive analysis of the solution structure of the dual polythiophene blend, elucidates the evolution of the condensed matter structure, and ultimately presents a promising avenue for enhancing the performance of low-cost quantum dot photodetectors.

Self-Assembled Monolayer-Based Hole-Transporting Materials for Perovskite Solar Cells.

Ever since self-assembled monolayers (SAMs) were adopted as hole-transporting layers (HTL) for perovskite solar cells (PSCs), numerous SAMs for HTL have been synthesized and reported. SAMs offer several unique advantages including relatively simple synthesis, straightforward molecular engineering, effective surface modification using small amounts of molecules, and suitability for large-area device fabrication. In this review, we discuss recent developments of SAM-based hole-transporting materials (HTMs) for PSCs. Notably, in this article, SAM-based HTMs have been categorized by similarity of synthesis to provide general information for building a SAM structure. SAMs are composed of head, linker, and anchoring groups, and the selection of anchoring groups is key to design the synthetic procedure of SAM-based HTMs. In addition, the working mechanism of SAM-based HTMs has been visualized and explained to provide inspiration for finding new head and anchoring groups that have not yet been explored. Furthermore, both photovoltaic properties and device stabilities have been discussed and summarized, expanding reader's understanding of the relationship between the structure and performance of SAMs-based PSCs.

π-Conjugated Polymer with Pendant Side Chains as a Dopant-Free Hole Transport Material for High-Performance Perovskite Solar Cells.

Dopant-free polymeric hole transport materials (HTMs) have attracted considerable attention in perovskite solar cells (PSCs) due to their high carrier mobilities and excellent hydrophobicity. They are considered promising candidates for HTMs to replace commercial Spiro-OMeTAD to achieve long-term stability and high efficiency in PSCs. In this study, we developed BDT-TA-BTASi, a conjugated donor-π-acceptor polymeric HTM. The donor benzo[1,2-b:4,5-b']dithiophene (BDT) and acceptor benzotriazole (BTA) incorporated pendant siloxane, and alkyl side chains led to high hole mobility and solubility. In addition, BDT-TA-BTASi can effectively passivate the perovskite layer and markedly decrease the trap density. Based on these advantages, dopant-free BDT-TA-BTASi-based PSCs achieved an efficiency of over 21.5%. Furthermore, dopant-free BDT-TA-BTASi-based devices not only exhibited good stability in N2 (retaining 92% of the initial efficiency after 1000 h) but also showed good stability at high-temperature (60 °C) and -humidity conditions (80 ± 10%) (retaining 92 and 82% of the initial efficiency after 400 h). These results demonstrate that BDT-TA-BTASi is a promising HTM, and the study provides guidance on dopant-free polymeric HTMs to achieve high-performance PSCs.

Complex Metal Oxides as Emerging Inorganic Hole-Transporting Materials for Perovskite Solar Cells.

Perovskite solar cells (PSCs) have achieved revolutionary progress during the past decades with a rapidly boosting rate in power conversion efficiencies from 3.8% to 26.1%. However, high-efficiency PSCs with organic hole-transporting materials (HTMs) suffer from inferior long-term stability and high costs. The replacement of organic HTMs with inorganic counterparts such as metal oxides can solve the above-mentioned problems to realize highly robust and cost-effective PSCs. Nevertheless, the widely used simple metal oxide-based HTMs are limited by the low conductivity and poor light transmittance due to the fixed atomic environment. As an emerging family of inorganic HTMs, complex metal oxides with superior structural/compositional flexibility have attracted rapidly increasing interest recently, showing superior carrier conductivity/mobility and superb light transmittance. Herein, the recent advancements in the design and development of complex metal oxide-based HTMs for high-performance PSCs are summarized by emphasizing the superiority of complex metal oxides as HTMs over simple metal oxide-based counterparts. Consequently, several distinct strategies for the design of complex metal oxide-based HTMs are proposed. Last, the future directions and remaining challenges of inorganic complex metal oxide-based HTMs for PSCs are also presented. This review aims to provide valuable guidelines for the further advancements of robust, high-efficiency, and low-cost PSCs.

Cross-linkable fluorene-based hole transporting materials for perovskite solar cells

Cross-linkable hole transporting materials (HTMs) based on fluorene core containing vinyl groups were synthesized and analyzed. It has been demonstrated that these HTMs are cross-linkable at temperatures above 200 °C and they are thermally stable at up to 400 °C. Cross-linking has little effect on energy level (HOMO) and causes only slight changes in the hole drift mobility values at room temperature, due to changes in spatial and energetic disorder. While employing cross-linked fluorene-diphenylamine derivative (V1145) as an HTM layer between perovskite and indium-tin oxide (ITO) in inverted perovskite solar cells (PSCs), the power conversion efficiency (PCE) of the cell was 11.42% (under AM 1.5G 100 mW cm−2 illumination), demonstrating the potential of the presented class of dopant-free organic HTMs.

Hole-Transporting Materials based on Oligo(hetero)aryls with a Naphthodithiophene Core - Succinct Synthesis by Twofold Direct C-H Olefination.

This work demonstrated the first synthetic application of direct C-H olefinations in the step-saving preparation of various hole-transporting materials (HTM) for efficient perovskite solar cells (PSC). Cross-dehydrogenative couplings of naphthodithiophene (NDT) with vinyl arenes under palladium-catalysis facilely generated various new oligo(hetero)aryls with internal alkenes. Reaction conditions were optimized, which gave the product isolated yields of up to 71 % with high (E)-stereoselectivity. These readily accessible NDT core-based small molecules involving olefin as π-spacers displayed immediate power conversion efficiencies of up to 17.2 % without a device oxidation process that is required for the commercially available spiro-OMeTAD and most other existing HTMs while fabricated in corresponding PSC devices.

2D Polymers with Lead Anchoring Groups Enable Perovskite Solar Cells with Over 24% Efficiency

The hygroscopic dopants used in Spiro‐OMeTAD hole‐transport materials (HTMs) in n–i–p perovskite solar cells (PSCs) inevitably cause device degradation. Herein, two polymer interface materials based on lead anchoring groups are developed. It is found that 2D polymers 2DP‐BT and 2DP‐Por can form dense films and exhibit excellent hydrophobicity. Importantly, 2DP‐Por can passivate the surface defects through noncovalent interactions, reducing nonradiative recombination loss. After introducing these polymer interface materials between the perovskite layer and the HTM layer, the optimized devices using 2DP‐Por and 2DP‐BT achieve champion power conversion efficiency of 24.12% and 23.29%, respectively, and the stability is significantly improved. These results indicate that developing polymer interface materials containing lead anchoring groups can improve PSC efficiency and stability and elucidate critical molecular design rules for interface materials.

Charge Carrier Dynamics at Carbon/Perovskite Interface: Implications on Carbon‐Based HTM‐Free Solar Cell Stability

Carbon‐based hole transport material (HTM)‐free perovskite solar cells (CPSCs) are an innovative device architecture that mitigates inherent challenges associated with record‐breaking perovskite solar cells (PSCs), which rely on metal/HTMs, including instability, manufacturing intricacy, and elevated costs. The photovoltaic efficiency and stability of CPSCs are profoundly influenced by the charge carrier dynamics at the interfaces. Herein, the charge carrier dynamics at the carbon(C)–perovskite interface in CPSCs and its implications on photovoltaic performances and stability, an aspect that has received limited exploration thus far are probed, are investigated using transient surface photovoltage (Tr‐SPV) and transient photoluminescence measurements. The study reveals that the C‐electrode effectively acts as a selective barrier, impeding electrons while facilitating the extraction of holes at the C–perovskite interface. This selective blocking mechanism holds significant implications for improving the performance and stability of CPSCs over HTM‐free PSCs with gold(Au) electrodes. The stability of CPSCs is evaluated by measuring shelf life, maximum power point tracking, Tr‐SPV, and X‐Ray diffraction measurements. By delving into these pivotal aspects, this work aims to contribute to the advancement and understanding of CPSCs for sustainable and efficient energy conversion.

Improving the performance of perovskite solar cells by extending π-conjugation system.

In perovskite solar cells (PSCs), hole transporting materials (HTMs) play a critical role in determining the stability and efficiency of the devices. However, the high cost and complex synthesis processes associated with conventional HTMs can hinder their widespread applications. This work presents a low-cost and efficient HTM, namely N,N'-(naphthalene-1,5-diyl)bis(1-(dibenzo[a,c]phenazin-11-yl)-1-phenylmethanimine) (PEDN), based on a naphthalene core with an extended π-conjugation system for improving the performance of PSCs. The PEDN was synthesized via a facile two-step condensation method, eliminating the need for expensive catalysts such as BINAP. The newly developed HTM with an extended π-conjugation length was compared with BEDN and spiro-OMeTAD as the benchmark HTM, in terms of their optical, electrochemical, hole mobility properties, and efficiency in PSCs. The PEDN showed suitable highest occupied molecular orbital levels (HOMOs), good hole mobilities, as well as strong hydrophobicities. The extended π-conjugation system in PEDN contributes to the stability of the solar cells. The PSCs fabricated with PEDN achieved a high efficiency of 18.61%, comparable to the efficiency obtained using the commonly used HTM spiro-OMeTAD (19.68%). Furthermore, the cost-effectiveness of PEDN makes it a suitable alternative to spiro-OMeTAD for PSC applications.

Effect of heterocyclic and non-heterocyclic units on FDT-based hole transport materials for efficient perovskite solar cells: a DFT study.

The development of a new structure is one of the important approaches for the advancement of efficient hole-transporting materials (HTMs). In this work, novel and efficient HTMs are designed based on the experimentally reported fluorene-dithiophene (FDT) system which shows the effect of four different units phenyl, pyridine, thiane, and oxane in the FDT unit. The structural, optoelectronic, and charge transport properties of the newly developed HTMs are probed using density functional theory (DFT) and time-dependent DFT (TD-DFT) methodologies. The calculated highest occupied molecular orbital (HOMO) energies for all HTMs are higher compared to the valence band energy level of the perovskite which exhibits outstanding hole extraction ability of all HTMs at the charge buffer interface. In addition, the designed HTMs have red-shifted absorption spectra compared to FDT. The computed hole mobilities of newly designed HTMs are faster compared to that of FDT. Moreover, newly tailored HTMs demonstrate improved solubility. The results indicate that a one thiane and one phenyl unit-based system among all materials is the most suitable for HTM design.

Effect of substituting donors on the hole mobility of hole transporting materials in perovskite solar cells: a DFT study.

Several hole-transporting materials (HTMs) have been designed by incorporating different types of π-conjugation group such as long chain aliphatic alkenes and condensed aromatic rings of benzene and thiophene and their derivatives on both sides between the planar core and donor of a reference HTM. Various electronic, optical, and dynamic properties have been calculated by using DFT, TDDFT, and Marcus theory. In this study, all the designed HTMs show a lower HOMO energy level and match well with the perovskite absorbers. Inserting condensed rings results in better hole mobility compared to aliphatic double bonds. It is found that the charge transfer integral is the dominant factor which mainly influences the hole mobility in our studied HTMs. Other factors such as hole reorganization energy, hole hopping rate, and centroid distance have a minor effect on hole mobility. Thus, this study is expected to provide guidance for the design and synthesis of new HTMs with increased hole mobility.

De Novo Design of Spiro-Type Hole-Transporting Material: Anisotropic Regulation Toward Efficient and Stable Perovskite Solar Cells.

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenyl)-amine-9,9'-spirobifluorene (Spiro-OMeTAD) represents the state-of-the-art hole-transporting material (HTM) in n-i-p perovskite solar cells (PSCs). However, its susceptibility to stability issues has been a long-standing concern. In this study, we embark on a comprehensive exploration of the untapped potential within the family of spiro-type HTMs using an innovative anisotropic regulation strategy. Diverging from conventional approaches that can only modify spirobifluorene with single functional group, this approach allows us to independently tailor the two orthogonal components of the spiro-skeleton at the molecular level. The newly designed HTM, SF-MPA-MCz, features enhanced thermal stability, precise energy level alignment, superior film morphology, and optimized interfacial properties when compared to Spiro-OMeTAD, which contribute to a remarkable power conversion efficiency (PCE) of 24.53% for PSCs employing SF-MPA-MCz with substantially improved thermal stability and operational stability. Note that the optimal concentration for SF-MPA-MCz solution is only 30mg/ml, significantly lower than Spiro-OMeTAD (>70mg/ml), which could remarkably reduce the cost especially for large-area processing in future commercialization. This work presents a promising avenue for the versatile design of multifunctional HTMs, offering a blueprint for achieving efficient and stable PSCs.

Spirobifluorene-based hole-transporting materials for RGB OLEDs with high efficiency and low efficiency roll-off.

In this work, we designed and synthesized three spirobifluorene (SBF)-based hole-transporting materials (HTMs) by incorporating the di-4-tolylamino group at different positions of the SBF skeleton. These materials demonstrate excellent thermal stability with thermal decomposition temperatures (T d) up to 506 °C and outstanding morphological stability with a glass transition temperature (T g) exceeding 145 °C. The meta-linkage mode between the conjugated skeleton and functional groups in the molecular structure results in electronic decoupling, giving these 3,6-substituted SBFs higher triplet energies (E T) compared to 2,7-substituted SBFs. This makes the 3,6-substituted SBFs suitable as universal HTMs for red, green, and blue (RGB) organic light emitting diodes (OLEDs). Among the three HTMs, 3,3',6,6'-tetra(N,N-ditolylamino)-9,9'-spirobifluorene (3,3',6,6'-TDTA-SBF) exhibits the best device performance, achieving maximum external quantum efficiencies (EQEmax) of 26.1%, 26.4%, and 25.4% for RGB phosphorescent OLEDs, with extremely low efficiency roll-off in both green and blue devices. Utilizing 3,3',6,6'-TDTA-SBF as the HTM, we have also fabricated narrowband blue OLEDs based on the widely used multiple resonance emitter BCz-BN, which exhibits a EQEmax of 29.8% and low efficiency roll-off.

Electroluminescence efficiency and stability of near ultraviolet organic light-emitting diodes based on BCPO luminous materials

To date, in the traditional method of obtaining near-ultraviolet (NUV) light, mercury atoms, which can create a highly toxic heavy metal contaminant, have been used. Therefore, it is an important issue to obtain NUV light by using new environmentally friendly devices. In the last decade, the fabrication of near ultraviolet organic light-emitting diodes (NUV-OLEDs) has become a research hotspot in the field of organic electronics. However, when the electroluminescence wavelength is extended to shorter than 400 nm, higher requirements are put forward for the materials used for each functional layer in these devices. In this work, a wide bandgap small molecule material of BCPO is used as the luminescent layer. The electron-transporting and hole-transporting materials are determined based on the overlaps between absorption spectra of these materials and emission spectrum of BCPO. And NUV-OLEDs with electroluminescent peak wavelength at 384 nm are prepared. By using the optimal device structure, the maximum external quantum efficiency of the device reaches 2.98%, and the maximum radiance of the device reaches 38.2 mW/cm<sup>2</sup>. In the electroluminescence spectrum, NUV light with wavelengths below 400 nm accounts for 57% of the light emission. In addition, the device demonstrates good stability when biased at two different constant voltage modes. The multiple key factors which affect the stability of the device are analyzed in detail. Firstly, it is found that the high glass transition temperature (<i>T</i><sub>g</sub>) of hole-transporting material is very important for the long-time stability of this device. The poor device stability is closely related to the low <i>T</i><sub>g</sub> temperature of hole-transporting material. Secondly, due to the widespread use of PEDOT:PSS as hole injection material in OLEDs, the electron leakage from the hole-transpor layer into the PEDOT:PSS layer may cause significant damage to the conducting polymer. When bombarded with low energy electrons, bond breakage occurs on the surface of PEDOT:PSS, followed by the release of oxygen and sulfur, resulting in changes in conductivity and oxidation reactions with molecules of hole transport material. Thirdly, the photoelectrical stability of organic molecules is the most fundamental reason that restricts the device lifetime. The aging process of material or device is directly relevant to the bond dissociation energy (BDE) of organic molecule. Generally, the BDE value of organic molecule is not high enough. As a result, molecules are prone to chemical bond breakage during electrochemical or photochemical aging. In summary, highly stable NUV-OLEDs should be fabricated by using hole-transporting materials with high <i>T</i><sub>g</sub> temperature, sufficient electron-blocking capacity, and large BDE value.

NiOx/PANI nanocomposite doped carbon paste as electrode for long-term stable and highly efficient perovskite solar cells.

Carbon-based perovskite solar cells (PSCs) have emerged as a hopeful alternative in the realm of photovoltaics. They are considered promising due to their affordability, remarkable durability in humid environments, and impressive electrical conductivity. One approach to address the cost issue is to use affordable counter electrodes in PSCs that do not require organic hole transport materials (HTMs). This study utilized an innovative and economical method to create NiOx/PANI nanocomposites. Later, these nanoparticles were integrated into a carbon paste to act as an HTM. This incorporation is intended to optimize charge extraction, improve interfacial contact, align energy levels, reduce energy loss, minimize charge recombination, and protect the perovskite (FAPbI3) surface from degradation. The optoelectronic properties of these devices were investigated, and all cells showed improved efficiency compared to control cells. The NiOx/PANI doped carbon (NiOx/PANI+CE) exhibited excellent performance due to strong hole conductivity, well-aligned energy levels, and the formation of stepwise band alignment at the perovskite interface.

Organic hole transport materials for high performance PbS quantum dot solar cells.

We developed a triazatruxene-based hole transport material (HTM), 3Ka-DBT-3Ka, aiming to enhance band alignment and augment charge generation and collection in devices, as an alternative for 1,2-ethanedithiol (EDT). The PbS CQD solar cells employing 3Ka-DBT-3Ka as the HTM achieve a peak efficiency of 11.4%, surpassing devices employing the conventional PbS-EDT HTM (8.9%).

Solid‐State Monolithic Dye‐Sensitized Solar Cell Exceeding 10% Efficiency Using a Copper‐Complex Hole Transport Material and a Carbon Counter‐Electrode

Solid‐State Monolithic Dye‐Sensitized Solar Cell Exceeding 10% Efficiency Using a Copper‐Complex Hole Transport Material and a Carbon Counter‐Electrode

Design, synthesis and characterization of indolo[3,2-a]carbazole-based low molecular mass organogelators as hole transport materials in perovskite solar cells

Low-molecular-mass indolocarbazole-based HTMs (CRICs), with hydrophobicity, water gelation properties, and suitable electronic structures, were developed as alternatives to the expensive benchmark HTM spiro-OMeTAD for PSC devices.

Assessing the Electronic and Optical Properties of Lanathanum Diselenide: a computational study

The lanthanide polychalcogenides are a diverse family of compounds with numerous tuneable properties, but only recently has stoichiometric LaSe2 been proposed as a hole transport material. Clarification over its electronic,...