Hole-transport Performance Research Articles

Direction Modulation of Intramolecular Electric Field Boosts Hole Transport in Phthalocyanines for Perovskite Solar Cells.

Tuning the strength of intramolecular electric field (IEF) in conjugated molecules has emerged as an effective approach to boost charge transfer. While direction manipulation of IEF would be a potential way that is still unclear. Here, we leverage the control of peripheral substituents of conjugated phthalocyanines to chemically tune the spatial orientation of IEF. By analyzing the spatial swing of side chains using the Kolmogorov-Arnold representation and least squares algorithm, a comprehensive mathematical-physical model has been established. This model enables rapid evaluation of the IEF and maximum hole transport performance induced by spatial swings. The champion phthalocyanine as dopant-free hole transport material in perovskite solar cell realizes a record performance of 23.41%. Greatly device stability is also exhibited. This work affords a new way to enhance hole transport capabilities of conjugated molecules by optimizing their IEF vector for photovoltaic devices.

Direction Modulation of Intramolecular Electric Field Boosts Hole Transport in Phthalocyanines for Perovskite Solar Cells

Tuning the strength of intramolecular electric field (IEF) in conjugated molecules has emerged as an effective approach to boost charge transfer. While direction manipulation of IEF would be a potential way that is still unclear. Here, we leverage the control of peripheral substituents of conjugated phthalocyanines to chemically tune the spatial orientation of IEF. By analyzing the spatial swing of side chains using the Kolmogorov‐Arnold representation and least squares algorithm, a comprehensive mathematical‐physical model has been established. This model enables rapid evaluation of the IEF and maximum hole transport performance induced by spatial swings. The champion phthalocyanine as dopant‐free hole transport material in perovskite solar cell realizes a record performance of 23.41%. Greatly device stability is also exhibited. This work affords a new way to enhance hole transport capabilities of conjugated molecules by optimizing their IEF vector for photovoltaic devices.

Enhanced Carrier Transport Performance of Monolayer Hafnium Disulphide by Strain Engineering.

For semiconducting two-dimensional transition metal dichalcogenides (TMDs), the carrier transport properties of the material are affected by strain engineering. In this study, we investigate the carrier mobility of monolayer hafnium disulphide (HfS2) under different biaxial strains by first-principles calculations combined with the Kubo-Greenwood mobility approach and the compact band model. The decrease/increase in the effective mass of the conduction band (CB) of monolayer HfS2 caused by biaxial tensile/compressive strain is the major reason for the enhancement/degradation of its electron mobility. The lower hole effective mass of the valence bands (VB) in monolayer HfS2 under biaxial compressive strain improves its hole transport performance compared to that under biaxial tensile strain. In summary, biaxial compressive strain causes a decrease in both the effective mass and phonon scattering rate of monolayer HfS2, resulting in an increase in its carrier mobility. Under the biaxial compressive strain reaches 4%, the electron mobility enhancement ratio of the CB of monolayer HfS2 is ~90%. For the VB of monolayer HfS2, the maximum hole mobility enhancement ratio appears to be ~13% at a biaxial compressive strain of 4%. Our results indicate that the carrier transport performance of monolayer HfS2 can be greatly improved by strain engineering.

Surface-enhanced p-type transparent conducting CuI−Ga2O3 films with high hole transport performance and stability

CuI is a wide bandgap p-type transparent conductor with promising optoelectronic properties. However, conventional CuI films prepared through the iodination of Cu films exhibit rough surface, poor stability and excessively high hole concentration, hindering its application. Herein, we introduce CuI−Ga2O3 composite films as a substitution to overcome the shortcomings of pure CuI. During the iodination process, Ga2O3 hindered the mobility of CuI grain boundaries, resulting in small grain size and low surface roughness. After the charge re-equilibrium and low-energy carrier filtering at the interface between Ga2O3 and CuI, the hole concentration in the films decreased by an order of magnitude. Due to the interface effect of Ga2O3 at the grain boundaries of CuI, the hole mobility increased by 5 times, and the conductivity improved by 53%. The ultra-wide band gap of Ga2O3 enhanced the transmittance of CuI films in short wavelength region. Ga2O3 served to passivate the grain boundaries of CuI, preventing the oxidation of CuI and the loss of iodine, consequently improving the stability of the films. This work will deepen the understanding of the failure and hole transport mechanisms of CuI films.

Ion‐Dipole Interaction for Self‐Assembled Monolayers: A New Strategy for Buried Interface in Inverted Perovskite Solar Cells

AbstractNickel oxide (NiOX) has a crucial role in enhancing the efficiency and stability of p‐i‐n inverted perovskite solar cells (PSCs), which hold great potential for commercialization. However, improving contact passivation between perovskites and NiOX is a challenge due to a hindered buried interface. In order to address this issue, self‐assembled monolayers (SAMs) are introduced as a buffer layer to prevent direct contact and non‐radiative recombination. While, the large dipole moment of SAMs increases the work function of NiOX, which is crucial for enhancing hole transport performance, given the low interfacial potential barrier for hole transfer. By a combination of the first‐principles calculations, drive‐level capacitance profiling, and transient absorption spectrum characterization, the understanding of the ion‐dipole interactions and interface passivation mechanism of potassium fluoride (KF) ultra‐thin buffer layer between SAMs and perovskites is provided. The efficiency of inverted PSCs as high as 23.25% is obtained, and the unencapsulated devices kept 90% of initial efficiency following 1400 h aging under nitrogen, which demonstrate remarkable long‐term stability as well. This novel strategy highlights the significance of SAMs dipole moment at the NiOX/perovskites interface and provides a new approach to address buried interfaces for high‐efficiency and long‐term stability in inverted PSCs.

Theoretical study on photophysical properties of twisted D-A interaction TPA-BSM derivatives

Luminogens with aggregation-induced emission (AIE) properties have received considerable attention as excellent candidates for optoelectronics, chemical/biosensors, and bioimaging. Recently, molecule TPA-BSM with the twisted intramolecular charge transfer, with a twist angle of 40.96° between the donor TPA and acceptor BSM units, has exhibited unique photophysical properties, especially the AIE properties in the aggregate and solid state. However, the relationship between their structure and performance has not been fully explored, which will further expand their application areas. Herein, based on the reported TPA-BSM, four new derivatives are designed. Their electronic structure, electron absorption, hole reorganization energy and second-order NLO response have been systematically studied using DFT and TD-DFT theories. It is found that they are all narrow bandgap derivatives. The introduction of donor units can reduce the energy gap, red-shift the first absorption peak, increase the dipole moment and the static first hyperpolarizability, while the introduction of acceptor units is just the opposite. However, the introduction of donor or acceptor units cannot further enhance the hole transport performance, but except for molecule TB-N(CH3)2, the other four derivatives are potential hole transport materials. Due to have large first hyperpolarizabilities of the investigated derivatives, they are promising candidates for excellent second-order NLO materials.

Effect of fluorine substitution on properties of hole-transporting materials for perovskite solar cells

In photovoltaic technologies, hole-transporting materials (HTMs) play a critical role in realizing highly efficient performance of perovskite solar cells (PSCs). Herein, a series of fluorine substituted small molecules based on triphenylamine backbone are designed by controlling fluorine atoms number and position. A comprehensive study on the structural modification is carried out to reveal the effect of fluorine substitution of HTMs. The energy level alignment, reorganization energy and the charge-transport properties are explored based on Quantum-chemical calculations. It is found that all difluorinated molecules have lower HOMO levels, which indicates that perovskite solar cells with two fluorine substituted hole-transporting materials may have higher open-circuit voltages. Moreover, the UV–vis absorption spectra of the studied molecules are slightly red-shifted with the increase of fluorine atoms. To further understand the effect of fluorine substitution on the hole transport properties, the hole mobilities of all HTMs are evaluated quantitatively using the Marcus theory and Einstein relation. The calculated results show that difluorinated molecules TPB-m-2F exhibit relatively higher hole mobility owing to efficient intermolecular interactions. As for monofluorinated molecules TPB-1F-a and TPB-1F-b, TPB-1F-a with fluorine atoms substituted on the inside position exhibit higher hole-transporting performance. Overall, we conclude that suitable modification of fluorine substitution is expected to improve the electrochemical and hole transport property of HTMs. We hope our theoretical investigation provide useful information for further improving the photovoltaic performance of PSCs.

Improving the hole transport performance of perovskite solar cells through adjusting the mobility of the as-synthesized conjugated polymer

The as-fabricated PSC with the as-synthesized HTL P5NH maintained a very high fill factor of 83.1% with an efficiency of 18.1%.

Designing Hole Transport Materials with High Hole Mobility and Outstanding Interface Properties for Perovskite Solar Cells.

Organic-inorganic halide perovskite solar cells (PSCs) have attracted much attention due to their rapid increase in power conversion efficiencies (PCEs), and many efforts are devoted to further improving the PCEs. Designing highly efficient hole transport materials (HTMs) for PSCs may be one of the effective ways. Herein we theoretically designed three new HTMs (FDT-N, FDT-O, and FDT-S) by introducing a nitrogen-phenyl group, an oxygen atom, and a sulfur atom into the spiro core of an experimentally synthesized HTM (FDT), respectively. And then we performed quantum chemical calculation to study their application potential. The results show that the devices with FDT-O and FDT-S instead of FDT may have higher open circuit voltages owing to their lower highest occupied molecular orbital (HOMO) energy levels. Moreover, FDT-S exhibits the best hole transport performance among the studied HTMs, which may be due to the significant HOMO-HOMO overlap in the hole hopping path with the largest transfer integral. Furthermore, the results on interface properties indicate that introducing oxygen and sulfur atoms can enhance the MAPbI3 /HTM interface interaction. The present work not only offers two promising HTMs (FDT-O and FDT-S) for PSCs but also provides theoretical help for subsequent research on HTMs.

Efficient non-fullerene polymer solar cells enabled by side-chain conjugated thieno[3,4-c]pyrrole-4,6-dione-based polymer and small molecular acceptors

The application of non-fullerene (NF) acceptors in bulk-heterojunction (BHJ) polymer solar cells (PSCs) is a promising approach to overcome the inherent drawbacks of fullerene derivatives-based acceptors. In PSCs, complementary absorption as well as matched molecular energy levels between the low bandgap acceptor-donor-acceptor (A-D-A) small molecular acceptor and medium/wide bandgap polymer donor is crucial to achieve high power conversion efficiency (PCE). Alternating polymers based on benzodithiophene (BDT) electron-donating segment and thieno[3,4-c]pyrrole-4,6-dione (TPD) electron-withdrawing segment own medium bandgap and low-lying highest occupied molecular orbital (HOMO) energy level, leading to presentable photovoltaic properties with fullerene derivatives. To probe into the performances of TPD-based polymers in NF-PSCs, two TPD-based polymers containing alkoxy or alkylthienyl modified benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized and adopted as electron-donors and blended with A-D-A-type electron-acceptor 2,2′-[[6,6,12,12-tetrakis(4- hexylphenyl)-s-indacenodithieno[3,2-b]thiophene]methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis(propanedinitrile) (ITIC) to fabricate the corresponding photovoltaic devices. The two-dimensional conjugated polymer PBDTT-TPD shows enhanced extinction coefficient, deeper HOMO energy level and better hole transport performance, resulting in improved PCE of 6.17%. To further boost the performances of the polymers, a small molecular acceptor 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl) bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) with down-shifted energy level was also used to blend with the two polymers in PSCs. Despite the open-circuit voltage (VOC) of the PBDTT-TPD:IDIC-based device is slightly decreased, the short-circuit current density (JSC) and fill factor (FF) are simultaneously improved, yielding an promising PCE of 7.15%. These results indicate that two-dimensional conjugated TPD-based polymers can be potential application as medium bandgap polymeric donor to match with small molecular acceptors having suitable molecular energy levels to get high efficiency in PSCs.

Nonlinear Nonacenes with a Dithienothiophene Substructure: Multifunctional Compounds that Act as Columnar Mesogens, Luminophores, π Gelators, and p-Type Semiconductors.

Board-like liquid-crystalline semiconductors based on the dithieno[3,2-b;2',3'-d]thiophene (DTT) substructure were synthesized and their thermal, self-assembly, optical, and semiconducting properties investigated. These sanidic compounds, bearing eight peripheral chains, are mesomorphic and spontaneously self-assemble into columnar hexagonal mesophases (Colhex ) over broad temperature ranges, as confirmed by polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering. Strong blue photoluminescence with absolute quantum yields up to 33 % were measured. These compounds also form blue-light emitting gels in various organic solvents. The fibrillar-like morphology of these gels, reminiscent of the columnar structure, and stabilized by efficient intermolecular interactions, was characterized by scanning electron microscopy. The charge carrier mobility of these π-extended mesogens was measured by time-of-flight transient photocurrent technique in the Colhex mesophase, and showed good hole-transport performance with charge carrier mobility of 10-3 cm2 V-1 s-1 .

Synthesis of Poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] Copolymer and Its Potential as Hole-Transporting Material to Solid-State Dye-Sensitized Solar Cells

In this study, thermally stable and solution-processable conjugated poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer containing alternating carbazole and triarylamine moieties in the polymer backbone was synthesized in a good yield by Pd-catalyzed polycondensation. The polymer exhibited maximum UV-vis absorption peak at 309 nm and maximum photoluminescence peak at 452 nm. From the optical and electrochemical characterization, the band gap, HOMO, and LUMO energy levels of the polymer were measured to be 2.91 eV, -5.19 eV, and -2.28 eV, respectively. The polymer was used to examine the hole-transporting performance to a solid-state dye-sensitized solar cell (ssDSSC). The ssDSSC fabricated with poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer showed a PCE of 1.50% with Jsc = 3.76 mA/cm2, Voc = 538 mV, and FF = 74.3%, implying the potential of the synthesized polymer as a hole-transport material (HTM) to ssDSSC.

Pyrimido[4,5-g]quinazoline-4,9-dione as a new building block for constructing polymer semiconductors with high sensitivity to acids and hole transport performance in organic thin film transistors

Polymers based on pyrimido[4,5-g]quinazoline-4,9-dione (PQ) building block are sensitive to acids and show good hole transport performance in thin film transistors.

Enhancement of the p-channel performance of sulfur-bridged annulene through a donor–acceptor co-crystal approach

By introducing naphthalene diimide into the meso-diphenyl tetrathia[22]annulene[2,1,2,1], a natural donor–acceptor co-crystal was prepared. In this co-crystal, donor and acceptor alternatingly stack into one-dimensional columns along the π–π stacking direction. With π–π interaction among the donor molecules, no effective π–π overlapping between DPNDIs in the acceptor columns was observed. Microcrystal transistor devices based on this D–A co-crystal displayed higher hole transport performance compared to the pristine crystals of donor molecules. Quantum calculation confirmed the absence of electron transport pathways and the super exchange between the adjacent donor and acceptor. From the diversity and versatility of the donor and acceptor used, we believe that the strategy developed here would be readily employed to tune the charge transport of organic semiconductors.

Synthesis and photoluminescence properties of novel copolymers containing salenAlQ-complex moieties and N-vinylcarbazole segments

Two novel salenAlQ-complex monomers and the corresponding copolymers are synthesized and characterized by 1H NMR, 13C NMR, FT-IR spectroscopy, and elemental analysis. The properties of the copolymers are investigated by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), UV–vis absorption and photoluminescence (PL) spectra. The results indicate that the copolymers exhibit appropriate molecular weight, good solubility-processability, perfect thermal stability, and improved hole-transporting performance. Particularly, since two ligands (salen and quinoline) coordinate with aluminum ions simultaneously, the copolymers exhibit exciting photoluminescence phenomenons. The solution emission is mainly salen-centered, while the solid emission is mainly quinoline-centered. In addition, the PL emission intensities, efficiencies and decay lifetimes of the copolymers are higher than those of the corresponding monomers. These results demonstrate that the incorporated NVK segments are used as multiple roles (hole-transport group, light-harvesting groups and luminescence protection) in the copolymers.

Bipolar Alq3-based complexes: Effect of hole-transporting substituent on the properties of Alq3-center

Two bipolar Alq3-based complexes, tris{5-[(carbazole-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(CzHQ)3) and tris{5-[(phenothiazine-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(PHQ)3), involving an Alq3-center and three hole-transporting substituents (carbazole or phenothiazine), were prepared and characterized. Effects of hole-transporting substituent on the properties of Alq3-center were investigated in detail. It is found that the two complexes have improved hole-transporting performance and appropriate thermal stability (the 5%-weight-loss temperatures T5%>260oC). Photoluminescence (PL) spectra indicate that both energy transfer and electron transfer can take place simultaneously in the PL process of these complexes. Both thermodynamics and dynamics of the electron transfer were studied and corresponding parameters were calculated. Energy transfer is favorable for the PL of Alq3-center, while electron transfer is unfavorable for the PL of Alq3-center. These results will be useful to explore novel OLEDs material with increased efficiency.

Synthesis of Hole-Transporting Materials Containing Triarylamine and its Properties

Two hole-transporting materials containing triphenylamine unit, 4-(4-(di-p- tolylamino)styryl)-N,N-di-p-tolylbenzenamine (TM 1) and 4-(4-(4-(di-p-tolylamino)styryl) styryl)-N,N-di-p-tolylbenzenamine(TM 2), have been designed and synthesized in this paper, which using conjugated bonds as the bridge to connect two molecules of triphenylamine. The optical spectrum, thermo-stabilization and electrochemical properties of two compounds were studied. The results show that these compounds have blue emission, proper HOMO levels and high thermal stability. Furthermore, EL devices with the configuration of ITO/HTL/Alq3/Al show the activating voltages are 11V for TM 1 and 5.5V for TM 2, respectively. The maximal luminancce efficiencies are 1.135cd/A for TM 1 and 2.068cd/A for TM 2, respectively. The difference of EL performance indicates that TM 2 possessing better hole-transporting performance than that of TM 1.

DFT/TDDFT Study on the electronic structures and optoelectronic properties of several red-emitting osmium(ii) complexes with different P∧P ancillary ligands

The ground and excited state geometries of several red-emitting phosphors (N^N)(2)Os(P^P) [where N^N = 5-(1-isoquinolyl)-1,2,4-triazoles, P^P = bis(dimethylphosphino)methylene(dmpm) (1); P^P = cis-1,2-bis-(dimethylphosphino)ethene(dmpe) (2); P^P = 1,2-bis(dimethylphosphino)benzene(dmpb) (3); P^P = 1,2-bis(dimethylphosphino)naphthalene(dmpn) (4); P^P = 1,2-bis(dimethylphosphino)-4-cyano-benzene(dmpcb) (5)] have been investigated by using the density functional theory (DFT) methods. The calculated results indicate that, for the studied complexes, the electron-transporting performance is better than the hole-transporting performance. The alteration of cis-P^P ancillary ligands with different conjugation lengths and substituents has an impact on the optoelectronic properties of these complexes, especially the electron-withdrawing group -CN in 5. The calculated energy gaps are nearly the same for complexes 1 to 4 (3.34 eV), while for 5, the HOMO and LUMO energies are lowered and the energy gap increases (3.42 eV). The absorption of 1 is red shifted, while that of 5 is blue shifted compared with the absorptions of 2, 3, and 4, which have similar absorptions. Complexes 2, 3, and 4 have almost identical emission wavelength 699 nm, while 1 (715 nm) and 5 (735 nm) are red shifted. The calculated electron affinities and reorganization energies indicate that complex 5 is the easiest for electron injection and has the best electron-transporting performance.

Planarity of triphenylamine moieties of a typical hole-transport material for OLEDs, N,N′-diphenyl- N,N′-di( m-tolyl)benzidine (TPD), in the amorphous state

N,N′-Diphenyl- N,N′-di( m-tolyl)benzidine (TPD) is a well-known hole-transport material for organic light-emitting diodes (OLEDs). Here, we studied the planarity of triphenylamine moieties of TPD in the amorphous state, by the combined use of solid-state 15N NMR experiments and density functional theory (DFT) calculations. It was shown that the planarity is of crucial importance for hole-transport performance of TPD, because of the strong effects for the shape of HOMO and for the intermolecular repulsion energy. It was also found that the molecular structure around the nitrogen in its amorphous state is not pyramidal, but is instead planar, which is favorable for hole-transport. Natural bond orbital calculation indicates that the nitrogen atom is sp 2 hybridized. This is unlike most amine compounds, which have pyramidal structures with sp 3 hybridized nitrogens.

Effect of substituents on the properties of indolo[3,2- b]carbazole-based hole-transporting materials

Three new compounds based on indolo[3,2- b]carbazole,2,8-bis(4-diphenylaminophenyl)-5,11-di- n-octylindolo[3,2- b]carbazole ( BTOICZ), 2,8-bis(9,9′-di- n-butylfluorenyl)-5,11-di- n-octylindolo[3,2- b]carbazole ( BFOICZ) and 2,8-bis[ N-( n-butyl)carbazolyl]-5,11-di- n-octylindolo[3,2- b]carbazole ( BCOICZ), as hole-transporting materials have been synthesized by Suzuki coupling reaction. The effects of substituents on the optical, thermal and electrochemical properties of indolo[3,2- b]carbazole derivatives have been studied carefully. Electroluminescent (EL) devices using these compounds as hole-transporting layer in combination with Alq 3 as electron-transporting and emitting layer have been fabricated and characterized. The devices based on BTOICZ or BFOICZ showed green emission at 526 nm, while the device based on BCOICZ emitted very weak light. The turn-on voltages are 4.1 and 5.4 V for BTOICZ and BFOICZ, respectively. The maximal luminance efficiencies are 0.738 and 0.767 lm/W for BTOICZ and BFOICZ, respectively. The difference of the EL performances of these compounds reveals that the substituent effects have great effect on the hole-transporting performance of the indolo[3,2- b]carbazole-based compounds.