Hole Transport Material Research Articles

Unveiling the Role of BODIPY Dyes as Small‐Molecule Hole Transport Material in Inverted Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have become a research hotspot since their dramatic increase in power conversion efficiency (PCE), surpassing 26% due to advances in cell engineering and interfacial layers. Within the last factor, hole transporting materials play a crucial role in enhancing device performance and stability. Among several molecular building blocks, BODIPYs are attractive for the design of novel hole transporting material (HTMs) due to their outstanding photophysical and charge transport properties easily tuned by synthetic modifications. Herein, the synthesis of five new BODIPY‐based HTMs PyBDP 1–5 are reported, functionalized at the meso‐ and α‐ positions with pyrenyl and arylamino units, respectively. The resulting compounds exhibit broad absorption in the visible region, remarkable thermal stability, narrow bandgaps, suitable energy levels, and good hole extraction capability, as subtracted from experimental and computational characterizations. The performance of the BODIPY derivatives as HTMs is evaluated in planar inverted (p‐i‐n) PSCs and compared to commonly used PTAA, resulting in highly efficient systems, reaching PCEs very close to that obtained with the reference polymer (21.51%). The incorporation of these BODIPY‐based HTMs result in an outstanding PCE of 20.37% for devices including PyBDP‐1 and 19.97% for devises containing PyBDP‐3, thus demonstrating that BODIPY derivatives are a promising alternative to obtain simple and efficient organic HTMs.

Halogenated Hole Selective Contact Enhances Interfacial Weak Bonding of Perovskite Solar Cells

AbstractWeak bonding between the perovskite and charge transport layers can lead to interfacial defects, hindering charge transfer and limiting the efficiency and stability of perovskite solar cells (PSCs). To address this issue, two halogenated spiro[fluorene‐9,9′‐xanthene]‐based molecules (SFX‐DM‐F and SFX‐DM‐Cl) are designed as an interfacial layer between perovskite and hole transport materials (HTMs). Both first‐principles simulations and experimental results are used to demonstrate that these halogenated interfacial layers improve the contact stability between perovskite's Pb(II) and HTMs, increasing the efficiency of charge transfer. The similar structure of interlayer to HTM also enhances the interfacial hole transfer integral, favoring effective hole transport. The PSCs based on SFX‐DM‐Cl achieve power conversion efficiencies of 24.8% (0.0625 cm2) and 23.1% (1 cm2). Even after 2000 h at a relative humidity of 15–20%, the unencapsulated PSC retains 94% of its initial efficiency. This work proposes the halogenated homologous HTMs as interfacial molecular bridges to optimize weak chemical bonds and hole transfer, thereby enabling efficient and stable PSCs.

Arylsilane derivatives of 1,3-diphenylisobenzofuran (DPBF) for better organic transistor applications and photophysical properties

We have proposed and carefully investigated arylsilanes based on 1,3-diphenylisobenzofuran (DPBF) as potential new organic semiconductor candidates. To determine their band gaps, excitation energies, hole- (λH) and electron- (λE) reorganization energies, etc., of the modeled arylsilanes, the DFT calculations are performed at B3LYP/6-311++G level. The results reveal that these suggested arylsilanes have superior properties compared to several other well-known semiconductor materials. The geometric and electronic structures and absorption/emission spectra of these 42 molecules are calculated at PBE0/Def2-TZVP level. Ground and excited state properties of these arylsilanes in gas phase and in different organic solvents are studied. Compounds with λH>λE are better electron transport materials (ETMs), and those with λH<λE are better hole transport materials (HTMs). Furthermore, the study shows that these molecules can be employed as effective trifunctional materials, i.e., emitter, hole, and electron transporters in OLEDs due to their improved electron injection and transport balance.

3D Conjugated Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells and Modules.

The orthogonal structure of the widely used hole transporting material (HTM) 2,2',7,7'-tetrakis(N, N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD)imparts isotropic conductivity and excellent film-forming capability. However, inherently weak intra- and inter-molecular π-π interactions result in low intrinsic hole mobility. Herein, a novel HTM, termed FTPE-ST, with a twist conjugated dibenzo(g,p)chrysene core and coplanar 3,4-ethylenedioxythiophene (EDOT) as extended donor units, is designed to enhance π-π interactions, without compromising on solubility. The three-dimensional (3D) configuration provides the material multi-direction charge transport as well as excellent solubility even in 2-methylanisole, and its large conjugated backbone endows the HTM with a high hole mobility. Moreover, the sulfur donors in EDOT units coordinate with lead ions on the perovskite surface, leading to stronger interfacial interactions and the suppression of defects at the perovskite/HTM interface. As a result, perovskite solar cells (PSCs) employing FTPE-ST achieve a champion power conversion efficiency (PCE) of 25.21% with excellent long-time stability, one of the highest PCEs for non-spiro HTMs in n-i-p PSCs. In addition, the excellent film-forming capacity of the HTM enables the fabrication of FTPE-ST-based large-scale PSCs (1.0cm2) and modules (29.0cm2), which achieve PCEs of 24.21% (certificated 24.17%) and 21.27%, respectively.

Indolo[3,2-a]carbazoles as Engineered Materials for Optoelectronic Applications: Synthesis, Structural Insights, and Computational Screening.

The biological and medicinal importance of indolocarbazoles has been known for the past several decades. However, in recent times, these compounds have been emerging as potential candidates for optoelectronic applications, although several challenges are associated with their synthesis. We report here a Pd(II)-catalyzed process for the synthesis of indolo[3,2-a]carbazoles. The reaction proceeded under neat conditions and in the presence of aqueous nonmetallic oxidant TBHP, and the products were purified directly after the completion of the reaction. Also, the possibility of employing the present method for reaction with gram-scale feed was investigated. A detailed single-crystal analysis of several indolo[3,2-a]carbazoles revealed how the molecular arrangement can be tuned by altering the functionalization. Finally, the developed molecules were screened computationally to assess their potential for possible use as hole transport materials (HTMs) for organic light-emitting diodes (OLEDs).

Inverted device architecture for high efficiency single-layer organic light-emitting diodes with imbalanced charge transport

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.

Asymmetrical benzothiadiazole core-based hole transport materials for planar perovskite solar cell

For the purpose of fabricating high performance hole transport materials (HTMs) for n-i-p planar perovskite solar cells (PSCs), two asymmetric and one symmetric structured donor-acceptor-donor (D-A-D) type small molecules (a-BTC, a-BTBC and BTDC) were synthesized by introducing two different peripheral groups toward benzothiadiazole core. The a-BTBC with longer π conjugation lengths shows good film formation property, higher hole mobility and conductivity, and achieves power conversion efficiency (PCE) of 23.5 %. It can retain 80 % of the initial PCE after aging 30 days in an ambient condition with relative humidity of 40–60 %. Our results show the potential and provides reference for molecular design of asymmetric structured HTMs.

Enhanced performance of HTM-free perovskite solar cells with free-standing carbon electrode via surface treatment and conductive support

This study explores the optimization of hole transport material (HTM)-free perovskite solar cells featuring laminated free-standing carbon film as a back electrode. Surface treatment of perovskite film using different types of solvent (chlorobenzene, toluene, and ethyl acetate) was carried out to improve the electrical contact at the carbon/methylammonium lead iodide interface. Overall, the surface treatment improved mechanical adhesion between the perovskite and carbon film. The conductivity of the free-standing carbon electrode was further improved by attaching the carbon film to various types of conductive support. Our results show that the selection of both solvents used during surface treatment and the type of conductive substrate has a considerable impact on the resulting performance, wherein all modifications gave rise to substantial improvement over the benchmark unmodified carbon electrode. A power conversion efficiency (PCE) of 7.74% was achieved by the perovskite solar cell with laminated carbon electrode on Al foil combined with the application of chlorobenzene as surface treatment, demonstrating a tremendous increase in performance compared to that of untreated free-standing carbon electrode that produced PCE less than 1%. The main contributor to the improved performance is presumably due to the low carrier recombination rate and improved charge carrier extraction as implied by the electro-impedance spectroscopy and photoluminescence analysis. This work demonstrates a facile approach for resolving the challenges in developing low-cost HTM-free perovskite solar cells with a free-standing carbon film electrode.

Fluorinated hole transport material resistant to high annealing temperature applied in inverted perovskites solar cells

A dopant-free and low-cost hole transport material (HTM) with dibenzo[b,d]thiophene as the center unit, bis(4-methoxyphenyl)amine as the terminal unit, and fluorine substitution adjacent to the methoxyl (named DBT-4F-MOP) is designed and applied in inverted organic-inorganic hybrid perovskite solar cells (OIH-PSCs). The synthetic cost of DBT-4F-MOP in the lab is just as low as 115 CNY/g, which is much lower than the benchmark HTM poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA, 2500 CNY/g). Due to the introduction of the fluorine atoms, the annealing temperature of the HTM can be increased by over 150 ℃. The highest efficiency of the inverted device based on it can reach 16.8 %, with an open circuit voltage of 1.04 V, a short circuit current of 21.57 mA cm−2, and a fill factor of 74.69 %, while the reference device based on PTAA can reach a PCE of 17.54 %. This work provides an avenue for the design of low-cost, dopant-free, and high-annealing temperature HTMs for inverted OIH-PSCs.

Non-halogenated solvent processed new Spiro-type hole transport material for efficient perovskite solar cells

Perovskite solar cells (PSCs) are the fastest developing solar cells and pull in a part of attention due to the prevalent advantages such as solution process, light weight, flexibility, and low cost. In common, Spiro-OMeTAD is the most used hole transport material (HTM) in PSCs. Thanks to its good merits, such as forming smooth surface films, matching energy levels with perovskites, and high tolerance with dopants. Therefore, Spiro-P was designed and synthesized by using an end group strategy (Propargyloxy). Additionally, the thermal, optical, and electrochemical properties of the new HTM and device performance were investigated. As a result, the Spiro-P HTM based device showed high power conversion efficiency (PCE) of 22.36% with an open circuit voltage (Voc ) of 1.102 V, a short-circuit density (Jsc ) of 26.20 mA/cm2 and a fill factor (FF) of 77.43%. This investigation shows that the new synthesized Spiro-P is a potential HTM to achieve good performance in the field of PSCs.

An end-capped acceptor modification of anthracene-based hole transport materials to study optoelectronic properties for high-performance perovskite solar cells

The present study employed Schiff base chemistry to develop a set of seven highly effective organic hole-transporting materials (HTMs) developed by modification of anthracene-based core having dimethoxydiphenylamine group by end-capped engineering technique. Different acceptors were attached at terminals through thiophene spacers. The DFT methodology is utilized for investigating various important electrical, optical, and efficiency-related features of molecules. Enhanced planarity, along with greater attractive forces, was demonstrated by the proposed HTMs (A1D1-A1D7), which made them suitable for faster hole mobility. Furthermore, they demonstrated lower band gap energies (3.51 eV to 3.97 eV) and deep HOMO levels (-6.18 eV to -6.00 eV), minimum excitation energy (2.43 eV), 82 % intrinsic charge transport, lower reorganization energy of electrons which improves the charge transfer capabilities of the PSCs. The exceptional solubility of the HTMs, as shown by their high dipole moments and more negative values of solvation-free energies, makes them appropriate for the manufacturing of multilayered films. Moreover, all newly proposed HTMs' acceptor terminals exhibited great conjugation as well as efficient π-π stacking, as evidenced by the dihedral angle across the spacer thiophene and EWD acceptors ranging from 1.09◦ to 21.26◦ and proved to be more potential candidates for future research in PSCs.

Exploring Acceptor-Functionalized Perylenes as Hole Transport Materials for Solution Process Organic Light-Emitting Diodes

Exploring Acceptor-Functionalized Perylenes as Hole Transport Materials for Solution Process Organic Light-Emitting Diodes

Improving the operational stability of perovskite solar cells with cesium-doped graphene oxide interlayer

Perovskite solar cells (PSCs) have made great advances in terms of power conversion efficiency (PCE), yet their subpar stability continues to hinder their commercialization. The interface between the perovskite layer and the charge-carrier transporting layers plays a crucial role in undermining the stability of PSCs. In this work, we propose a strategy to stabilize high-performance PSCs with PCE over 23% by introducing a cesium-doped graphene oxide (GO-Cs) as an interlayer between the perovskite and hole-transporting material. The GO-Cs treated PSCs exhibit excellent operational stability with a projected T80 (the time where the device PCE reduces to 80% of its initial value) of 2143 h of operation at the maximum powering point under one sun illumination.

Exploring the impact of tailored π-linkers on perovskite solar cell efficiency: a computational study in triphenylamine-based hole transport material design

Exploring the impact of tailored π-linkers on perovskite solar cell efficiency: a computational study in triphenylamine-based hole transport material design

Molecular engineering of benzothiadiazole core based non-fullerene acceptors to tune the optoelectronic properties of perovskite solar cells

Non-fullerene acceptors exhibit durable absorption capacity, and exceptional power conversion efficiency that make them appealing photovoltaic material for use as hole transport materials in perovskite solar cells (PSCs). To interpret the benzothiadiazole as the central-core unit, five novel acceptors (U1-U5) have been successfully fabricated by end-capped engineering. Their electrical and optical performance have been investigated theoretically. Utilizing density functional theory (DFT) and time-dependent DFT computations, we have examined the photo-physical and optoelectronic characteristics. U4 exhibits 3.80 eV optical band gap with an absorption (λmax) of 670.58 nm. Lowest excitation energy (1.85 eV), greater LHE (0.98) and improved PCE (18.31 eV) validate the U4 designed molecule as a brilliant photovoltaic material for PSCs. Outstanding charge transfer at the donor–acceptor junction has also been revealed by thorough analysis of U4/PTB7-Th. Therefore, substitution of alternate functional groups in terminal moieties is efficient for effective PSCs that can be used as photon harvesting materials.

Interface Defects Dependent on Perovskite Annealing Temperature for NiOX-Based Inverted CsPbI2Br Perovskite Solar Cells.

Nickel oxide (NiOX) is an ideal inorganic hole transport material for the fabrication of inverted perovskite solar cells owing to its excellent optical and semiconductor properties. Currently, the main research on developing the performance of NiOX-based perovskite solar cells focuses on improving the conductivity of NiOX thin films and preventing the redox reactions between metal cations (Ni3+ on the surface of NiOX) and organic cations (FA+ or MA+ in the perovskite precursors) at the NiOX/perovskite interface. In this study, a new type of interface defects in NiOX-based CsPbI2Br solar cells is reported. That is the Pb2+ from CsPbI2Br perovskites can diffuse into the lattice of NiOX surface as the annealing temperature of perovskites changes. The diffusion of Pb2+ increases the ratio of Ni3+/Ni2+ on the surface of NiOX, leading to an increase in the density of trap state at the interface between NiOX and perovskites, which eventually results in a serious decline in the photovoltaic performance of solar cells.

Low-Temperature Cross-Linked Hole Transport Layer for High-Performance Blue Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs), a kind of promising optoelectronic device, demonstrate potential superiority in next-generation display technology. Thermal cross-linked hole transport materials (HTMs) have been employed in solution-processed QLEDs due to their excellent thermal stability and solvent resistance, whereas the unbalanced charge injection and high cross-linking temperature of cross-linked HTMs can inhibit the efficiency of QLEDs and limit their application. Herein, a low-temperature cross-linked HTM of 4,4'-bis(3-(((4-vinylbenzyl)oxy)methyl)-9H-carbazol-9-yl)-1,1'-biphenyl (DV-CBP) with a flexible styrene side chain is introduced, which reduces the cross-linking temperature to 150 °C and enhances the hole mobility up to 1.01 × 10-3 cm2 V-1 s-1. More importantly, the maximum external quantum efficiency of 21.35% is successfully obtained on the basis of the DV-CBP as a cross-linked hole transport layer (HTL) for blue QLEDs. The low-temperature cross-linked high-mobility HTL using flexible side chains could be an excellent alternative for future HTL development.

A Fluorination Strategy and Low-Acidity Anchoring Group in Self-Assembled Molecules for Efficient and Stable Inverted Perovskite Solar Cells.

Herein, we synthesized two donor-acceptor (D-A) type small organic molecules with self-assembly properties, namely MPA-BT-BA and MPA-2FBT-BA, both containing a low acidity anchoring group, benzoic acid. After systematically investigation, it is found that, with the fluorination, the MPA-2FBT-BA demonstrates a lower highest occupied molecular orbital (HOMO) energy level, higher hole mobility, higher hydrophobicity and stronger interaction with the perovskite layer than that of MPA-BT-BA. As a result, the device based-on MPA-2FBT-BA displays a better crystallization and morphology of perovskite layer with larger grain size and less non-radiative recombination. Consequently, the device using MPA-2FBT-BA as hole transport material achieved the power conversion efficiency (PCE) of 20.32 % and remarkable stability. After being kept in an N2 glove box for 116 days, the unsealed PSCs' device retained 93 % of its initial PCE. Even exposed to air with a relative humidity range of 30±5 % for 43 days, its PCE remained above 91 % of its initial condition. This study highlights the vital importance of the fluorination strategy combined with a low acidity anchoring group in SAMs, offering a pathway to achieve efficient and stable PSCs.

Development of Superior Nickel Oxide Layer for Boosted Hole‐Transport Performance in Perovskite Solar Cells

Although revolutionary power conversion efficiency progress has been realized for inverted perovskite solar cells (PSCs), the selection and optimization of the hole‐transporting materials (HTMs) are still crucial for fabricating high‐efficiency, stable, and low‐cost PSCs. Nickel oxide (NiOx) is widely used in the PSCs community for material design and modification. However, various NiOx synthesis processes lead to kinds of NiOx, and it is difficult to determine which NiOx is suitable for application in PSCs. Herein, two kinds of NiOx HTMs are synthesized using sol–gel processed and chemical‐calcination‐processed (CCP) routes for the preparation of PSCs. Based on the physical properties of two kinds of NiOx HTMs as well as the corresponding photovoltaic parameters of PSCs, it is demonstrated that superior NiOx HTMs is obtained by the CCP method with the following properties: high transmittance to ensure sufficient sunlight capture of perovskite, appropriate energy levels to promote hole extraction and transfer, high conductivity to support charge transfer, and a good wetting surface to boost perovskite spreading and film formation. Thus, in this study, the direction for the reasonable selection of HTMs is pointed out to prepare efficient PSCs.

Facile synthesis and optimization of graphene oxide reduction by annealing in hydrazine vapour in ambient air for potential application in perovskite solar cells

Graphene oxide (GO) was synthesized from graphite via a modified Hummer's method, followed by thermal and chemical reductions to produce reduced graphene oxide (RGO) samples at various temperatures. A suite of characterization techniques including Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), UV-Visible Spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Hall effect measurements were employed to assess the structural, morphological, optical, and electrical properties of the samples. FTIR analysis confirmed the successful functionalization of graphite to GO and subsequent reduction to reduced graphene oxide, with peak intensities decreasing as the reduction temperature increased. UV-visible spectroscopy of GO showed a maximum absorption at 235 nm which confirmed the synthesis of GO while the reduction revealed a notable red shift in absorption peaks with increasing annealing temperature, and that signified a reduction in bandgap. XRD analyses demonstrated the removal of oxygen functional groups. The X-ray diffraction (XRD) analysis of GO showed diffraction at 2θ = 10.74° which revealed a fully oxidized graphene oxide with oxygen-containing functional groups, and hence an increase in interlayer spacing (d002) from 3.341 Å (graphite) to 8.228 Å (GO). Upon reduction, there is a gradual decrease in d002 from 8.228 Å (GO) to 3.387 Å (HRGO300), suggesting the gradual removal of intercalated oxygen molecules, and hence the gradual restoration of sp2 hybridisation in graphene. The EDS analysis revealed an increase in the carbon-to-oxygen (C/O) ratio from 1.78 in GO to 2.75 in HRGO300 as the annealing temperature for the reduction process increased which further confirmed the removal of oxygen functional groups. The Hall effect data showed hole mobility of 4.634 x101 (GO), 4.831 x101 (HRGO200), and 5.462 x100 (HRGO300) with conductivities of 8.985 x10-5 (GO), 1.087 x100 (HRGO200) and 1.791 x101 1/Ω cm, suggesting an increase in conductivity as the annealing temperature increased as revealed in the EDS. Out of the three samples identified as hole transport materials, the sample HRGO300 with the highest C/O ratio of 2.75 has the highest conductivity, and hence most suitable for application as hole transport material in perovskite solar cell.