Hole Transport Properties Research Articles

Solution-processed, ultrasensitive, high current density vertical phototransistor using porous carbon nanotube electrode

Multi-wall carbon nanotube (MWCNT) thin film has been identified as the ideal electrode material due to their inherent electronic and optoelectronic properties. We have successfully fabricated a vertical field-effect phototransistor (vFEpT) using MWCNT as a porous source electrode. The optimized MWCNT electrodes showed an electrical conductivity of 3.92 × 103 S/m and a sheet resistance of about 490 Ω/□, with a layer thickness of about 520 nm. The as-fabricated transistor has p-type conductivity because of the unintentional doping of the PbS layer and the intrinsic hole transport property of the MWCNT network. Additionally, the device shows excellent photoelectric performance at 30 mWcm−2 of illumination, with a high current density of 2.3 × 10−3 Acm−2. With a high conductivity network for carrier transfer and separation, the special characteristics of MWCNT nanoporous thin film enable improved photoelectric properties, such as a high photoresponsivity of 3.16 AW−1 and a specific detectivity of 2.33 × 1012 Jones when illuminated at 808 nm light irradiance with an intensity of 0.6 µW/cm2. Furthermore, the device demonstrated a steady dynamic response to an optical signal, with rising and falling times of 31.8 ms and 32.9 ms, respectively. This work presents a new method for creating high-performance vertical phototransistors with MWCNT as the source electrode.

Nonplanar Nanographene: A Hydrocarbon Hole-Transporting Material That Competes with Triarylamines.

Hole-transporting materials (HTMs) are essential for optoelectronic devices, such as organic light-emitting diodes (OLEDs), dye-sensitized solar cells, and perovskite solar cells. Triarylamines have been employed as HTMs since they were introduced in 1987. However, heteroatoms or side chains embedded in the core skeleton of triarylamines can cause thermal and chemical stability problems. Herein, we report that hexabenzo[a,c,fg,j,l,op]tetracene (HBT), a small nonplanar nanographene, functions as a hydrocarbon HTM with hole transport properties that match those of triarylamine-based HTMs. X-ray structural analysis and theoretical calculations revealed effective multidirectional orbital interactions and transfer integrals for HBT. In-depth experimental and theoretical analyses revealed that the nonplanarity-inducing annulative π-extension can achieve not only a stable amorphous state in bulk films, but also a higher increase in the highest occupied molecular orbital level than conventional linear or cyclic π-extension. Furthermore, an in-house manufactured HBT-based OLED exhibited excellent performance, featuring superior curves for current density-voltage, external quantum efficiency-luminance, and lifetime compared to those of representative triarylamine-based OLEDs. A notable improvement in device lifetime was observed for the HBT-based OLED, highlighting the advantages of the hydrocarbon HTM. This study demonstrates the immense potential of small nonplanar nanographenes for optoelectronic device applications.

Nonplanar Nanographene: A Hydrocarbon Hole‐Transporting Material That Competes with Triarylamines

AbstractHole‐transporting materials (HTMs) are essential for optoelectronic devices, such as organic light‐emitting diodes (OLEDs), dye‐sensitized solar cells, and perovskite solar cells. Triarylamines have been employed as HTMs since they were introduced in 1987. However, heteroatoms or side chains embedded in the core skeleton of triarylamines can cause thermal and chemical stability problems. Herein, we report that hexabenzo[a,c,fg,j,l,op]tetracene (HBT), a small nonplanar nanographene, functions as a hydrocarbon HTM with hole transport properties that match those of triarylamine‐based HTMs. X‐ray structural analysis and theoretical calculations revealed effective multidirectional orbital interactions and transfer integrals for HBT. In‐depth experimental and theoretical analyses revealed that the nonplanarity‐inducing annulative π‐extension can achieve not only a stable amorphous state in bulk films, but also a higher increase in the highest occupied molecular orbital level than conventional linear or cyclic π‐extension. Furthermore, an in‐house manufactured HBT‐based OLED exhibited excellent performance, featuring superior curves for current density–voltage, external quantum efficiency–luminance, and lifetime compared to those of representative triarylamine‐based OLEDs. A notable improvement in device lifetime was observed for the HBT‐based OLED, highlighting the advantages of the hydrocarbon HTM. This study demonstrates the immense potential of small nonplanar nanographenes for optoelectronic device applications.

Spin-Coated and Vacuum-Processed Hole-Extracting Self-Assembled Multilayers with H-Aggregation for High-Performance Inverted Perovskite Solar Cells.

We report a highly crystalline self-assembled multilayer (SAMUL) that is fundamentally different from the conventional monolayer or disordered bilayer used for hole-extraction in inverted perovskite solar cells (PSCs). The SAMUL can be easily formed on ITO substrate to establish better surface coverage to enhance the performance and stability of PSCs. A detailed structure-property-performance relationship of molecules used for SAMUL is established through a systematic study of their crystallinity, molecular packing, and hole-transporting properties. These SAMULs are rationally optimized by varying their molecular structures and deposition methods through thermal evaporation or spin-coating for fabricating PSCs. The CbzNaphPPA-based SAMUL was chosen for fabricating inverted PSCs due to it exhibiting the highest crystallinity and hole mobility which is derived from the ordered H-aggregation. This resulted in a remarkably high fill factor of 86.45 %, which enables a very impressive power conversion efficiency (PCE) of 26.07 % to be achieved along with excellent device stability (94 % of its initial PCE retained after continuous operation for 1200 h under 1-sun irradiation at maximum power point at 65 °C). Additionally, a record-high PCE of 23.50 % could be achieved by adopting a thermally evaporated SAMUL. This greatly simplifies and broadens the scope for SAM to be used for large-area devices on diverse substrates.

Dimeric Carbazole Core Based Dopant‐Free Hole Transport Material for n‐i‐p Planar Perovskite Solar Cell

AbstractA profitable approach for improving the stability of n‐i‐p planar perovskite solar cell (PSC) is developing dopant‐free hole transport materials (HTMs) with desirable intrinsic charge transport properties. In this work, by adopting a metal‐free catalytic intramolecular C─C coupling reaction, a carbazole dimer‐based fused‐ring is synthesized, and further developed a small molecular dopant‐free HTM abbreviated as NCz‐DM with appropriate energy level, excellent hole transport and film formation properties. Applied in n‐i‐p planar PSC, an impressive power conversion efficiency (PCE) of 24.0% is achieved, together with enhanced humidity, thermal and light stability, which is among the high level of ever‐reported results for PSCs utilizing small molecular dopant‐free HTM.

Polypyrrole as photo-thermal-assisted modifier for BiVO4 photoanode enables high-performance photoelectrochemical water splitting

Herein, iron nickel oxide (FeNiOx) and conductive polymer polypyrrole (PPy) with excellent hole transport property and catalytic oxidation ability combine with bismuth vanadate (BiVO4) based photoanode to obtain a remarkable improvement of photocurrent density. More importantly, due to the photo-thermal effect of PPy, the operating temperature of the photoanode reaches 323.15 K under AM 1.5G illumination without infrared light source, thus activating the photoanode surface and accelerating the water splitting reaction. As a result, when the temperature of electrolyte reaches 323.15 K, the photocurrent density of BiVO4/FeNiOx/PPy photoanode achieves 6.91 mA cm−2 at 1.23 V versus reversible hydrogen electrode (vs. RHE), which is 3.04 folds of pristine BiVO4 (2.27 mA cm−2). Such a photo-thermal-assisted enhancement strategy based on the temperature sensitivity of BiVO4 provides a new way for the breakthrough of the performance bottleneck of BiVO4 photoanode and the design of the next generation PEC catalyst materials.

Facile Doping of 2,2,2-Trifluoroethanol to Single-Walled Carbon Nanotubes Electrodes for Durable Perovskite Solar Cells

Perovskite solar cells with an indium tin oxide (ITO)/SnO2/CH3NH3PbI3/Spiro-OMeTAD/2,2,2-trifluoroethanol (TFE) doped single-walled carbon nanotube (SWCNT) structure were developed by dropping TFE onto SWCNTs, which replaced the metal back electrode, and a conversion efficiency of 14.1% was achieved. Traditionally, acidic doping of the back electrode, SWCNT, has been challenging due to the potential damage it may cause to the perovskite layer. However, TFE has facilitated easy doping of SWCNT as the back electrode. The sheet resistance of the SWCNTs decreased and their ionization potential shifted to deeper levels, resulting in improved hole transport properties with a lower barrier to carrier transport. Furthermore, the Seebeck coefficient (S) increased from 34.5 μV/K to 73.1 μV/K when TFE was dropped instead of EtOH, indicating an enhancement in the behavior of p-type charge carriers. It was observed that hydrophilic substances adhered less to the SWCNT surface, and the formation of PbI2 was suppressed. These effects resulted in higher conversion efficiency and improved solar cell performance. Furthermore, the decrease in conversion efficiency after 260 days was suppressed, showing improved durability. The study suggests that combining SWCNTs and TFEs improves solar cell performance and stability.

Styrene monomer as potential material for design of new optoelectronic and nonlinear optical polymers: density functional theory study.

Using density functional theory, we have studied the intrinsic properties of styrene. First, we determine the optimized structures, structural parameters and thermodynamic properties to make our simulations more realistic to experimental results and check the stability. Second, we investigate optoelectronic, electronic and global descriptors, transport properties of holes and electrons, natural bond orbital analysis, absorption and fluorescence properties. Finally, we study nonlinear optical (NLO) properties: first- and second-order hyperpolarizability, second and third-order optical susceptibilities, hyper-Rayleigh scattering hyperpolarizability, electro-optical Pockel effect, direct current Kerr effects and quadratic refractive index. The bandgap energy E g = 5.146 eV and dielectric constant show that styrene is a good insulator with an average electric field value of 4.43 × 108 Vm-1. Thermodynamic findings show that our molecule is thermodynamically and chemically stable. Electron and hole reorganization energies of 0.393 and 0.295 eV, respectively, show that styrene is more favourable to hole transport than electron transport. Styrene is transparent with linear refractive index n = 1.750 and quadratic n 2 = 1.748 × 10-20 m2 W-1. At the NLO, styrene has a non-zero value of which confirms the existence of first-order NLO activity. Globally the study shows that the styrene monomer is suitable for the architecture design of new polymer materials for NLO applications and optoelectronic by functionalization.

Boosting Hole Mobility: Indenofluorene-Arylamine Copolymers and Their Impact on Solution-Processed OLED Performance.

We introduce three newly designed thermally cross-linkable hole transport copolymers (PIF-TPD, PIF-F2PCz, and PIF-TPAPCz) for improving the performance of solution-processed organic light-emitting diodes (s-OLEDs). These copolymers, designed through a strategic molecular approach with benzocyclobutene (BCB) and styrene-based cross-linking monomers, show high solvent resistance at a low cross-linking temperature (150 °C). Furthermore, these conjugated copolymers based on planar indenofluorene with three different hole transport (HT) units, exhibit outstanding charge carrier mobility (1.61 × 10-2 cm2 V-1s-1), demonstrated by comparing hole reorganization energy and electronic coupling strength of HT units. Despite these copolymers showing the overall vertical orientation in the horizontal dipole moment measurement results, we demonstrated that the HT units can exhibit the preferred orientation, contributing to high hole transport properties. As a result, they perform exceptionally well as hole transport layers in green phosphorescent s-OLEDs, achieving a maximum external quantum efficiency of 15.3% and a maximum current efficiency of 53.9 cd A-1 with a small efficiency roll-off despite their relatively low triplet energy levels. These results are comparable to vacuum-deposited OLEDs, highlighting the potential of these copolymers in advancing OLED technology for display panels and lighting applications.

High‐Performance Tin‐Halide Perovskite Transistors Enabled by Multiple A‐Cation Engineering

AbstractMetal‐halide perovskite semiconductors have garnered great attention in optoelectronic devices due to their remarkable properties and processing capabilities. Among them, tin (Sn2+) perovskites stand out as promising candidates for use as channel layers in high‐performance p‐channel thin‐film transistors (TFTs) owing to their excellent hole transport property. In this study, a multi A‐cation approach is applied to pure Sn2+ perovskite, incorporating a small amount (7 mol%) of bulky PEA+ cation. This addition greatly improved the crystallinity and orientation of 3D FA0.9Cs0.1SnI3 perovskite, leading to optimized p‐channel Sn2+‐perovskite TFT with a hole mobility of ≈18 cm2 V−1 s−1, a high on/off current ratio of 108, and a small subthreshold swing of 0.5 V dec−1. These TFTs also exhibit a low contact resistance of 87 Ω·cm, attributed to the improved electrode/perovskite channel interface. Furthermore, the TFTs are utilized as an electrical characterization platform to study the charge transport properties and interfacial defects using the low‐frequency noise method, providing insights into the interface properties of perovskite electronic devices.

Bi(III)‐based oxide semiconductors with ambipolar doping: (Na, K)BiO2

AbstractBi(III)‐based oxides are emerging as important semiconductors for future electronic applications. Through hybrid density functional theory calculations and defect analysis, we demonstrate that ABi(III)O2 (A = Na or K) is a promising wide‐band‐gap bipolar semiconductor. Our calculations predict ABiO2 to have a large band gap of 1.97 eV for NaBiO2 and 2.77 eV for KBiO2. Unlike widely‐used oxide semiconductors such as In2O3 and ZnO, the spatially extended cation states (Bi 6s) of ABiO2 significantly contribute to the valence band maximum, enhancing p‐type dopability. We indeed show that potential acceptors, such as SrBi, produce shallow levels. In addition, n‐type doping is found to be possible through the extrinsic doping of heterovalent elements such as Te and F. The band‐structure analysis reveals that ABiO2 has small effective masses for electrons and holes (< 1m0 where m0 is the electron rest mass), indicating favorable electron and hole transport properties.

An inorganic-blended p-type semiconductor with robust electrical and mechanical properties

Inorganic semiconductors typically have limited p-type behavior due to the scarcity of holes and the localized valence band maximum, hindering the progress of complementary devices and circuits. In this work, we propose an inorganic blending strategy to activate the hole-transporting character in an inorganic semiconductor compound, namely tellurium-selenium-oxygen (TeSeO). By rationally combining intrinsic p-type semimetal, semiconductor, and wide-bandgap semiconductor into a single compound, the TeSeO system displays tunable bandgaps ranging from 0.7 to 2.2 eV. Wafer-scale ultrathin TeSeO films, which can be deposited at room temperature, display high hole field-effect mobility of 48.5 cm2/(Vs) and robust hole transport properties, facilitated by Te-Te (Se) portions and O-Te-O portions, respectively. The nanosphere lithography process is employed to create nanopatterned honeycomb TeSeO broadband photodetectors, demonstrating a high responsibility of 603 A/W, an ultrafast response of 5 μs, and superior mechanical flexibility. The p-type TeSeO system is highly adaptable, scalable, and reliable, which can address emerging technological needs that current semiconductor solutions may not fulfill.

Superwetting Nanofluids of NiOx-Nanocrystals/CsBr Solution for Fabricating Quality NiOx-CsPbBr3 Gradient Hybrid Film in Carbon-Based Perovskite Solar Cells.

The wettability of precursor solution on substrates is the critical factor for fabricating quality film. In this work, superwetting nanofluids (NFs) of non-stoichiometric nickel oxide (NiOx) nanocrystals (NCs)-CsBr solution are first utilized to fabricate quality NiOx-CsPbBr3 hybrid film with gradient-distributed NiOx NCs in the upper part for constructing hole transport ladder in carbon-based perovskite solar cells (C-PSCs). As anticipated, the crystalline properties (improved crystalline grain diameters and reduced impurity phase) and hole extraction/transport of the NiOx-CsPbBr3 hybrid film are improved after incorporating NiOx NCs into CsPbBr3. This originates from the superb wettability of NiOx-CsBr NFs on substrates and the excellent hole-transport properties of NiOx. Consequently, the C-PSCs with the structure of FTO/SnO2/NiOx-CsPbBr3/C displays a power conversion efficiency of 10.07%, resulting in a 23.6% improvement as compared with the pristine CsPbBr3 cell. This work opens up a promising strategy to improve the absorber layer in PSCs by incorporating NCs into perovskite layers through the use of the superwettability of NFs and by composition gradient engineering.

Ternary Polymer Solar Cells: Impact of Non-Fullerene Acceptors on Optical and Morphological Properties

Ternary organic solar cells contain a single three-component photoactive layer with a wide absorption window, achieved without the need for multiple stacking. However, adding a third component into a well-known binary blend can influence the energetics, optical window, charge carrier transport, crystalline order and conversion efficiency. In the form of binary blends, the low-bandgap regioregular polymer donor poly(3-hexylthiophene-2,5-diyl), known as P3HT, is combined with the acceptor PC61BM, an inexpensive fullerene derivative. Two different non-fullerene acceptors (ITIC and eh-IDTBR) are added to this binary blend to form ternary blends. A systematic comparison between binary and ternary systems was carried out as a function of the thermal annealing temperature of organic layers (100 °C and 140 °C). The power conversion efficiency (PCE) is improved due to increased fill factor (FF) and open-circuit voltage (Voc) for thermal-annealed ternary blends at 140 °C. The transport properties of electrons and holes were investigated in binary and ternary blends following a Space-Charge-Limited Current (SCLC) protocol. A favorable balanced hole–electron mobility is obtained through the incorporation of either ITIC or eh-IDTBR. The charge transport behavior is correlated with the bulk heterojunction (BHJ) morphology deduced from atomic force microscopy (AFM), contact water angle (CWA) measurement and 2D grazing-incidence X-ray diffractometry (2D-GIXRD).

Performance enhancement of 2D tin-halide perovskite transistors via molecule-assisted grain boundary passivation and hole doping

Tin (Sn2+)-based halide perovskites have garnered considerable interest for potential applications in field-effect transistors, owing to their low-cost solution processing capability and favorable hole transport properties. However, the polycrystalline nature of halide perovskite films necessitates efficient grain boundary passivation for reliable and stable device operation. Additionally, as a p-type semiconductor, controllable hole-doping is desired for modulating electrical properties. In this study, we demonstrate the dual functionality of doping the small molecule tetrafluoro-tetracyanoquinodimethane (F4TCNQ) into (C6H5C2H4NH3)2SnI4 (abbreviated as (PEA)2SnI4) through a cost-effective solution process. This doping introduces F4TCNQ as both a grain boundary passivator and a charge transfer p-dopant, resulting in effective improvements in transistor performance with a 6-fold enhancement in mobility, a substantial reduction in hysteresis, an enhanced current ratio, and improved stabilities.

Role of Bridging Groups in Regulating the Luminescence and Charge Transfer Properties of Thermally Activated Delayed Fluorescence Molecules: A Theoretical Perspective.

Organic emitters with a simultaneous combination of aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in great demand due to their excellent comprehensive performances toward efficient organic light-emitting diodes (OLEDs), biomedical imaging, and the telecommunications field. However, the development of efficient AIE-TADF materials remains a substantial challenge. In this work, light-emitting properties of two AIE-TADF molecules with different bridging groups ICz-BP and ICz-DPS are theoretically investigated in the solid state with the combined quantum mechanics/molecular mechanics (QM/MM) method and the thermal vibration correlation function (TVCF) theory. The research indicates that the C═O bridging bond in ICz-BP is more favorable than the S═O bridging bond in ICz-DPS for enhancing the planarity of the acceptor, increasing conjugation, and thereby elevating the transition dipole moment density. Simultaneously, the stacking pattern of ICz-BP in the solid facilitates a reduction in energy gap between S1 and T1 (ΔEST), achieving rapid reverse intersystem crossing rate (kRISC). Furthermore, compared to toluene, the stacking patterns of ICz-BP and ICz-DPS in the solid effectively suppress the out-of-plane wagging vibration of the acceptor, thereby inhibiting the loss of nonradiative energy in the excited state and realizing aggregation-induced emission. Moreover, the charge transport properties of both electrons and holes in ICz-BP are found to be higher than the corresponding rates in ICz-DPS, attributed to the smaller internal reorganization energy of ICz-BP in the solid state. Additionally, the calculations reveal a more balanced charge transport characteristic in ICz-BP, contributing to efficient exciton recombination and emission and ultimately mitigating efficiency roll-off. Based on these computational results, we aim to unveil the relationship between molecular structure and light-emitting properties, aiding in the design and development of efficient AIE-TADF devices.

Fluorination effects on bithiophene bridged hole transporting materials for perovskite solar cells

ABSTRACT The use of hole transporting material (HTM) remains indispensable for efficient charge extraction in perovskite solar cells. In this work, a series of small-molecule HTMs are designed by introducing fluorine atoms into the central part of 2,2′-bithiophene core. The effect of fluorination on the photophysical, electrochemical, and hole transport properties is systematically explored using Quantum chemistry calculation. The Calculated results reveal that all the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of the studied molecules are well matched with the energy band structure of perovskite. Three fluorine substituted hole-transporting materials possess lower reorganisation energy than the reference molecule BT-MTP. The higher mobility and matched energy levels of predicted HTMs are beneficial for improving the performance of PSCs. All the designed HTMs display higher hole mobility than those of the BT-MTP. Especially, molecule DFBT-p-MTP exhibits better optical properties and stability, indicating that fluorination is an effective strategy to improve the performance of the device in real application. This work provides an avenue for designing low-cost small molecule HTMs for high-performance perovskite solar cells.

Derivatives of Phenyl Pyrimidine and of the Different Donor Moieties as Emitters for OLEDs.

Two derivatives of phenyl pyrimidine as acceptor unit and triphenylamino or 4,4'-dimethoxytriphenylamino donor groups were designed and synthesized as emitters for organic light-emitting diodes (OLEDs) aiming to utilize triplet excitons in the electroluminescence. Thermogravimetric analysis revealed high thermal stability of the compounds with 5% weight loss temperatures of 397 and 438 °C. The theoretical estimations and photophysical data show the contributions of local excited and charge transfer states into emission. The addition of the methoxy groups led to the significant improvement of hole-transporting properties and the bathochromic shift of the emission from blue to green-blue spectral diapason. It is shown that mixing of the compounds with the organic host results in facilitation of the delayed emission. The singlet-triplet energy splitting was found to be too big for the thermally activated delayed fluorescence. No thermal activation of the long-lived emission was detected. No experimental evidence for triplet-triplet annihilation and room temperature phosphorescence were detected making the hot exciton mechanism the most probable one. The OLEDs based on the compounds reached the maximum external quantum efficiency of up to 10.6%.

Non-Equivalent Donor-Acceptor Type Polymers as Dopant-Free Hole-Transporting Materials for Perovskite Solar Cells.

Electron donor (D)-electron acceptor (A) type conjugated polymers present bright prospects as dopant-free hole-transporting materials (HTMs) for perovskite solar cells (PVSCs). Most of the reported D-A polymeric HTMs contain equivalent amounts of D and A units, while the appropriate excess proportion of D units could optimize the aggregation state of polymer chains and improve the hole transport properties of the polymers. Herein, a non-equivalent D-A copolymerization strategy was utilized to develop three indacenodithiophene-benzotriazole-based polymeric HTMs for PVSCs, named as F-10, F-15, and F-20, and the equivalent D-A polymer F-00 was studied in parallel. Effects of D : A ratio on the hole transport properties of these D-A type polymeric HTMs, including energy level, molecular stacking, hole mobility, and surface morphology, were investigated by theoretical simulation and test analysis. F-15 performed best due to the appropriate D : A ratio, endowing the PVSCs a champion power conversion efficiency of 20.37 % with high stability, which confirms the fine-tuning D : A ratio via non-equivalent D-A copolymerization strategy is very helpful to construct D-A type polymeric HTMs for high-performance PVSCs.

Monte Carlo simulation of mobility enhancement in multilayer graphene with turbostratic structure

The transport properties of electrons and holes in the multilayer graphene with a turbostratic structure are investigated by calculating the electrostatic potential induced by ionized impurities on the substrate and integrating the potential profile into Monte Carlo simulation. It is shown that the potential fluctuation caused by the impurities decreases exponentially with the distance from the substrate. The decrease in the potential fluctuation almost nullifies the effect of the impurities on the carrier mobility, showing the carriers to move as fast as the case in the suspended graphene. It is also shown that regardless of impurity density, there is an almost one-to-one correspondence between the mobility and the potential fluctuation. Therefore, it is shown that the use of multilayer graphene with a turbostratic structure is a viable approach to diminish the effects of impurities on the transport properties of graphene, even in systems with high impurity density.