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 Alq₃ 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.     

Indolo[3,2-b]carbazole: Promising building block for highly efficient electroluminescent materials

Two luminescent materials based on indolo[3,2-b]carbazole have been designed and synthesized. They were highly fluorescent both in solution and in the solid state. High-performance electroluminescent devices with indolo[3,2-b]carbazole luminescent derivatives as the emissive materials were fabricated for the first time with low turn-on voltage of 2.65 V, high luminescence efficiency of 7.92 lm W^?1, and high brightness of 68729 cd m^?2. The results demonstrated that indolo[3,2-b]carbazole has great potentials as promising building block for highly efficient electroluminescent materials.     

Poly(2-(N-carbazolyl)ethyl acrylate) as a host for high efficiency polymer light-emitting devices

Poly(2-(N-carbazolyl)ethyl acrylate), a poly(acrylate) comprised of carbazole-side groups attached via a flexible chain to the polymer backbone (PVAK) has been tested as host for solution-processed polymer light-emitting devices (PLEDs). This non-conjugated polymer proved to be an excellent candidate to host wide-bandgap phosphors. Notably, this polymer exhibited a high thermal stability (T_d = 322 °C), a glass transition temperature (T_g) of 91 °C and a wide bandgap corresponding to the pendent carbazole units and the disrupted π-conjugation of the polymer main chain, making this polymeric host a suitable candidate for wide bandgap triplet emitters. When tested as a host for FIrpic and Ir(ppy)₃, the resulting blue and green light-emitting devices showed a maximum luminous efficiency of 18.25 and 17.74 cd/A, respectively, which are comparable to recent reports of devices made using other carbazole-based oxygen-rich polymeric hosts. The polymer was also characterized by UV–visible absorption, photoluminescence spectroscopy as well as cyclic voltammetry.     

Functionalized terfluorene for solution-processed high efficiency blue fluorescence OLED and electrophosphorescent devices

A new multifunctional blue-emitting terfluorene derivative (TFDPA) featured with triphenylamine groups for hole-transportation and long alkyl chains for solution processability on the conjugation inert bridge centers was reported. TFDPA can give homogeneous thin film by solution process and exhibits high hole mobility (μ_h ≈ 10^?3 cm² V^?1 s^?1) and suitable HOMO for hole injection. Particularly, TFDPA performs efficient deep-blue emission with high quantum yield (～100% in solution, 43% in thin film) and suitable triplet energy (E_T = 2.28 eV), making solution-processed OLED devices of using TFDPA as blue emitter and as host for iridium-containing phosphorescent dopants feasible. The solution-processed nondoped blue OLED device gives saturated deep-blue electroluminescence [CIE = (0.17, 0.07)] with EQE of 2.7%. TFDPA-hosted electrophosphorescent devices performed with EQE of 6.5% for yellow [(Bt)₂Ir(acac)], 9.3% of orange [Ir(2–phq)₃], and 6.9% of red [(Mpq)₂Ir(acac)], respectively. In addition, with careful control on the doping concentration of [(Bt)₂Ir(acac)], a solution-processed fluorescence–phosphorescence hybrided two-color-based WOLED with EQE of 3.6% and CIE coordinate of (0.38, 0.33) was successfully achieved.     

Photoluminescent and electroluminescent properties and ultra-fast spectrometric studies of dihydroheptacenes

Photophysical and electrochemical properties of 7,16-dihydroheptacenes (1–3) were investigated in detail. Although their HOMO–LUMO gaps are higher than 3 eV, the organic light-emitting diodes containing 1 as the emitting dopant showed green electroluminescence (λ_max ∼ 515 and 550 nm) even at a concentration of the dopant as low as 1%. The green electroluminescence of 1 appears to be originating from an electromeric state (an intermolecular ion pair). The maximum brightness of 190 cd/m² was observed at a current density of 34 mA/cm² at operating voltage of 19 V for the device containing 1% 1, and overall performance of the devices decreased with an increase in the doping concentration of 1.     

Efficient solution processed D1-A-D2-A-D1 small molecules bulk heterojunction solar cells based on alkoxy triphenylamine and benzo[1,2-b:4,5-b′]thiophene units

Two molecules denoted as VC96 and VC97 have been synthesized for efficient (η = 6.13% @ 100 mW/cm² sun-simulated light) small molecule solution processed organic solar cells. These molecules have been designed with the D₁-A-D₂-A-D₁ structure bearing different central donor unit, same benzothiadiazole (BT) as π-acceptor and end capping triphenylamine. Moreover, the optical and electrochemical properties (both experimental and theoretical) of these molecules have been systematically investigated. The solar cells prepared from VC96:PC₇₁BM and VC97:PC₇₁BM (1:2) processed from CF (chloroform) exhibit a PCE (power conversion efficiency) of η = 4.06% (J_sc = 8.36 mA/cm², V_oc = 0.90 V and FF = 0.54) and η = 3.12% (J_sc = 6.78 mA/cm², V_oc = 0.92 V and FF = 0.50), respectively. The higher PCE of the device with VC96 as compared to VC97 is demonstrated to be due to the higher hole mobility and broader IPCE spectra. The devices based on VC96:PC₇₁BM and VC97:PC₇₁BM processed with solvent additive (3 v% DIO, 1,8-diiodooctane) showed PCE of η = 5.44% and η = 4.72%, respectively. The PCE device of optimized VC96:PC₇₁BM processed with DIO/CF (thermal annealed) has been improved up to 6.13% (J_sc = 10.72 mA/cm², V_oc = 0.88 V and FF = 0.61). The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive plus the thermal annealing.     

Zero-valent nanophase iron and nitrogen co-modified titania nanotube arrays: Synthesis,characterization, and enhanced visible-light photocatalytic performance

We prepared nano-zero-valent iron (nZVI) and N co-modified TiO₂ (nZVI/N–TiO₂) nanotube arrays as an enhanced visible-light photocatalyst. The TiO₂ nanotube arrays were synthesized by electrochemical anodization of Ti foil in a two-electrode system. Amorphous TiO₂ nanotube arrays were immersed in ammonia and then annealed to produce crystalline N-doped TiO₂ (N–TiO₂) nanotube arrays. nZVI spheres were directly deposited on the N–TiO₂ nanotube arrays by borohydride reduction. The photocatalysts were characterized by field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (UV–vis DRS), and electrochemical impedance spectroscopy (EIS). The environmental applicability and photocatalytic activity of the proposed nZVI/N–TiO₂ nanotube arrays were tested by phenol degradation in an aqueous system under UV and visible light irradiation. The phenol degradation rate constants of each sample under visible light irradiation were in the following order: nZVI/N–TiO₂ (k_obs=0.006 min⁻¹⁾>N–TiO₂ (k_obs=0.002 min⁻¹) ⪢ nZVI/TiO₂ (k_obs=0.0003 min⁻¹)>TiO₂ (k_obs=0.0001 min⁻¹). This result can be attributed to the synergistic effect of the N–TiO₂ nanotubes with lower energy band gap and the electron transfer from the conduction band (CB) of N–TiO₂ to nZVI spheres highly-dispersed on the N–TiO₂ for enhanced separation of photogenerated electrons and holes.     

Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J–V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).     

Spiro-fused N-phenylcarbazole-based host materials for blue phosphorescent organic light-emitting diodes

Two host materials, SFCA and SFCC, consist of a diphenylamine or carbazole unit linking to spiro-fused phenyl carbazole (SFC) backbone, were designed and synthesized. By choosing the meta linkage way between diphenylamine/carbazole units and SFC ring, higher triplet energies could be easily achieved for the two new materials, which mean that they could be used as effective host material for popular blue phosphorescent material Iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C^2′] picolinate (FIrpic, E_T = 2.65). Besides that, the steric SFC structure could guarantee their good thermal stabilities. Their thermal, photophysical and electroluminescent properties were systematically investigated. The blue phosphorescent OLEDs with the two materials as hosts and FIrpic as a dopant exhibited excellent performance with maximum current efficiencies of 33.9 and 40.8 cd/A, respectively.     

Carbazole based A-π-D-π-A dyes with double electron acceptor for dye-sensitized solar cell

Three novel carbazole-based A-π-D-π-A-featured dyes (CSG1–CSG3) have been designed, synthesized for applications in dye-sensitized solar cells and fully characterized with NMR, MS, IR, UV–vis and electrochemical measurements. These dyes share the same donor (N-hexylcarbazole) and acceptor/anchoring group (cyanoacrylic acid), but differs in conjugated linkers incorporated, such as benzene, furan or thiophene, to configure the novel A-π-D-π-A framework for effective electron flow. The power conversion efficiencies were observed to be sensitive to the π-bridging linker moiety. The photovoltaic experiments showed that dye with a benzene linker exhibited a higher open-circuit voltage (0.699 V) compared to thiophene and furan linker. Among all dyes, CSG2 containing a thiophene linker exhibited the maximum overall conversion efficiency of 3.8% (J_SC = 8.90 mA cm⁻², V_OC = 584 mV, FF = 0.74) under standard global AM 1.5 G solar condition. Under similar fabrication conditions, champion dye N719 exhibited the maximum overall conversion efficiency of 6.4% (J_SC = 14.74 mA cm⁻², V_OC = 606 mV, FF = 0.716).     

High-efficiency red electroluminescence from europium complex containing a neutral dipyrido(3,2-a:2′,3′-c)phenazine ligand in PLEDs

In order to obtain high-efficiency monochromatic red emission in polymer light-emitting devices, a tris(dibenzoylmethanato)(dipyrido(3,2-a:2′,3′-c)phenazine) europium [Eu(DBM)₃(DPPZ)] doped single-emissive-layer devices were fabricated using a blend of poly(9,9-dioctyl-fluorence) and 2-tert-butyl-phenyl-5-biphenyl-1,3,4-oxadiazole as a host matrix by solution process. Significantly improved electro-luminescent properties with sharp red emission at 611.5 nm were displayed in the Eu(DBM)₃(DPPZ)-doped devices at dopant concentrations from 1 to 8 wt.%. The highest luminance up to 1783 cd/m² at 2 wt.% dopant concentration, as well as the maximum external quantum efficiency of 2.5% and current efficiency of 3.8 cd/A were obtained at 1 wt.% dopant concentration.     

2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

Controlling singlet–triplet splitting in carbazole–oxadiazole based bipolar phosphorescent host materials

A rational molecular design strategy for carbazole–oxadiazole based bipolar host materials was developed to improve the device efficiency of blue phosphorescent organic light-emitting diodes (PHOLED). Steric effects of strategically placed methyl groups led to an increase of triplet energies (o-2MPCzPOXD: 2.66 eV and o-3MPCzPOXD: 2.73 eV versus the initial host material o-PczPOXD: 2.62 eV) while less pronouncedly affecting singlet energies and, therefore, retaining low driving voltages, high power efficiencies and remarkably low efficiency roll-offs in PHOLEDs. The maximum quantum efficiencies (EQE) for blue devices (FIrpic) were significantly raised for o-2MPCzPOXD (13.6%) and o-3MPCzPOXD (11.5%) versus o-PCzPOXD (9.0%) although yielding comparable values for green devices (Ir(ppy)₃; 12.9% and 15.4% versus 13.2%). Supported by theoretical calculations a structure–property relationship was established from photo-physical properties, PHOLED performance measurements and structural characterization from single crystal data.     

Effects of annealing temperature of aqueous solution-processed ZnO electron-selective layers on inverted polymer solar cells

We investigate the effects of ZnO annealing temperature (T_A) on the performance of inverted polymer solar cells with ZnO electron-selective layers deposited by spin coating aqueous solutions of an ammine-hydroxo zinc complex. The inverted solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester with T_A as low as 80 °C exhibit power-conversion efficiencies of 3.6%, which is equal to those of devices with higher T_A. Characterizations of the ZnO films using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, grazing incidence wide-angle X-ray scattering, and optical transmittance measurements show that the abrupt improvement of device performance from T_A = 60 to 80 °C is due to the improvement of energy-level alignment arising from the increases in the relative amount and the crystallinity of ZnO.     

Synthesis and photovoltaic properties of organic dyes containing N-fluoren-2-yl dithieno[3,2-b:2′,3′-d]pyrrole and different donors

New organic dyes containing fluorene appended dithienopyrrole as electron rich linker, different arylamine/heterocyclic units as conjugating donors and cyanoacrylic acid as acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. The effect of different conjugated donors such as triarylamine, carbazole and phenothiazine on the photophysical, electrochemical and photovoltaic properties is investigated. The optical and electrochemical properties of the dyes are strongly influenced by conjugating donors. The dye containing phenothiazine donor exhibited longer wavelength absorption and low oxidation potential. The time dependent density functional calculations performed on the dye models reveal charge transfer character for the longer wavelength absorption. The dye-sensitized solar cells fabricated using a dye containing fluorenyldiphenylamine donor displayed highest power conversion efficiency (6.81%) in the series originating from the high short circuit current density (J_SC = 14.01 mA cm⁻²) and high open circuit voltage (V_OC = 738 mV).     

Investigation of benzo(1,2-b:4,5-b′)dithiophene as a spacer in organic dyes for high efficient dye-sensitized solar cell

Two benzo(1,2-b:4,5-b′)dithiophene (denoted as BDT) based organic dyes, Dye 1 and Dye 2, containing triphenylamine and carbazole in the molecular frameworks respectively, were synthesized, characterized and applied in dye-sensitized solar cells (DSSCs). The photo-physical, photovoltaic, and electrochemical properties of the two dyes were analyzed in this work. The two dyes exhibit strong charge transfer absorption bands in the visible region. The dyes were applied in dye sensitized solar cells obtaining 11.34 mA/cm², 0.75 V and 0.74, for the short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc), and fill factor (FF) respectively, corresponding to an overall power conversion efficiency of 6.3%. These results revealed that BDT-based dyes are promising dyes for DSSCs.     

Fluorene trimers with various 9,9′-substituents: The synthesis,characteristics, condensed state structures,and electroluminescence properties

The four fluorene-based trimers with various aromatic and alkyl substituents (T1–T4) are synthesized and characterized. These oligomers show the similar electronic absorption and emission characteristics (e.g., absorption peak at 351 nm, and highly efficient deep blue emission at 394 nm in solution), indicating that the major electronic properties of the core chromophore are essentially independent of the substituents. However, the condensed state structures and thermal properties of four trimers are found to be different from each other, from crystalline (full alkyl (T1) or full aromatic (T2) substituted trimers) to amorphous (mixed aromatic and alkyl (T4) substituted trimers). The effect of different condensed state structures on electroluminescence device properties is presented: The blue light-emitting devices with accordant structure of ITO/PEDOT:PSS/TCTA (40 nm)/trimers (40 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF/Al exhibit different EL efficiency (2.9% of T2, 1.8% of T3 and 2.7% of T4). Using amorphous T4, the white light-emitting device of ITO/TCTA (40 nm)/rubrene (0.1 nm)/T4 (8 nm)/Alq₃(52 nm)/LiF/Al is fabricated with high efficiency (6.15 cd A⁻¹), high brightness (9500 cd m⁻²) and good white light CIE coordinates (0.32, 0.37).     

The locally twisted thiophene bridged phenanthroimidazole derivatives as dual-functional emitters for efficient non-doped electroluminescent devices

A series of locally twisted dual-functional materials namely PIPT, PITT and PIFT have been designed and synthesized by introducing different polyaromatic hydrocarbon groups to a phenanthroimidazole backbone through a thiophene bridge. In these molecules, the thiophene bridge and phenanthroimidazole platform are nearly coplanar and this endows these materials with relatively shallow HOMO levels (−5.35 to −5.21 eV). On the other hand, the bulky polyaromatic hydrocarbon units introduce non-planar twisty structures which reduce molecular aggregations. These three materials show color-tunable emission (emission peak from 468 to 532 nm in film) and high thermal stability (T_g > 160 °C). Simple trilayer devices using these three phenanthroimidazole derivatives as non-doped emitting layers exhibit low turn-on voltages (2.3–2.7 V) and high maximum efficiencies of 3.74, 6.15 and 6.89 cd/A for PIPT, PITT and PIFT, respectively. Above all, owing to their shallow HOMO levels for enabling efficient hole-injection, even simpler bilayer devices employing these materials as hole-transporting emitters show low turn-on voltages (2.6–2.8 V) and high efficiencies of 5.77 cd/A for PIPT, 6.03 cd/A for PITT and 6.04 cd/A for PIFT, respectively. These comparable performances with those of the trilayer configurations show the efficient hole-injection/transport ability of these three newly developed emitters.     

Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet–triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO–LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (E_T = 3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A⁻¹ and a power efficiency of 11.5 lm W⁻¹, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.     

Development of a new diindenopyrazine–benzotriazole copolymer for multifunctional application in organic field-effect transistors,polymer solar cells and light-emitting diodes

A new donor–acceptor (D?A) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^?1 s^?1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light with a maximum brightness of 385 cd m^?2 at the turn-on voltage of 6 V.