Solid-state Ionization Potentials Research Articles

Design, synthesis and theoretical simulations of novel spiroindane-based enamines as p-type semiconductors.

The search for novel classes of hole-transporting materials (HTMs) is a very important task in advancing the commercialization of various photovoltaic devices. Meeting specific requirements, such as charge-carrier mobility, appropriate energy levels and thermal stability, is essential for determining the suitability of an HTM for a given application. In this work, two spirobisindane-based compounds, bearing terminating hole transporting enamine units, were strategically designed and synthesized using commercially available starting materials. The target compounds exhibit adequate thermal stability; they are amorphous and their glass-transition temperatures (>150°C) are high, which minimizes the probability of direct layer crystallization. V1476 stands out with the highest zero-field hole-drift mobility, approaching 1 × 10-5 cm2 V s-1. To assess the compatibility of the highest occupied molecular orbital energy levels of the spirobisindane-based HTMs in solar cells, the solid-state ionization potential (Ip) was measured by the electron photoemission in air of the thin-film method. The favourable morphological properties, energy levels and hole mobility in combination with a simple synthesis make V1476 and related compounds promising materials for HTM applications in antimony-based solar cells and triple-cation-based perovskite solar cells.

Mixed-Metal Cu-Zn Thiocyanate Coordination Polymers with Melting Behavior, Glass Transition, and Tunable Electronic Properties.

The solid-state mechanochemical reactions under ambient conditions of CuSCN and Zn(SCN)2 resulted in two novel materials: partially Zn-substituted α-CuSCN and a new phase CuxZny(SCN)x+2y. The reactions take place at the labile S-terminal, and both products show melting and glass transition behaviors. The optical band gap and solid-state ionization potential can be adjusted systematically by adjusting the Cu/Zn ratio. Density functional theory calculations also reveal that the Zn-substituted CuSCN structure features a complementary electronic structure of Cu 3d states at the valence band maximum and Zn 4s states at the conduction band minimum. This work shows a new route to develop semiconductors based on coordination polymers, which are becoming technologically relevant for electronic and optoelectronic applications.

Interfacial versus Bulk Properties of Hole-Transporting Materials for Perovskite Solar Cells: Isomeric Triphenylamine-Based Enamines versus Spiro-OMeTAD.

Here, we report on three new triphenylamine-based enamines synthesized by condensation of an appropriate primary amine with 2,2-diphenylacetaldehyde and characterized by experimental techniques and density functional theory (DFT) computations. Experimental results allow highlighting attractive properties including solid-state ionization potential in the range of 5.33–5.69 eV in solid-state and hole mobilities exceeding 10–3 cm2/V·s, which are higher than those in spiro-OMeTAD at the same electric fields. DFT-based analysis points to the presence of several conformers close in energy at room temperature. The newly synthesized hole-transporting materials (HTMs) were used in perovskite solar cells and exhibited performances comparable to that of spiro-OMeTAD. The device containing one newly synthesized hole-transporting enamine was characterized by a power conversion efficiency of 18.4%. Our analysis indicates that the perovskite–HTM interface dominates the properties of perovskite solar cells. PL measurements indicate smaller efficiency for perovskite-to-new HTM hole transfer as compared to spiro-OMeTAD. Nevertheless, the comparable power conversion efficiencies and simple synthesis of the new compounds make them attractive candidates for utilization in perovskite solar cells.

Triphenylethylene-based emitters exhibiting aggregation induced emission enhancement and balanced bipolar charge transport for blue non-doped organic light-emitting diodes

New triphenylethylene-based derivatives, containing 2,6-di-tert-butylcarbazole or 9,9-dimethyl-9,10-dihydroacridine as donor moieties and phenylene or tetrafluorophenylene linking bridges were synthesised by Heck coupling and characterized. The synthesized compounds emitted blue light in the solid state with emission maxima in the range of 478–487 nm and photoluminescence quantum yields of 16–29%. In thermogravimetric analysis they demonstrated sublimation with 5% weight loss temperatures in the range of 307–355 °C. They exhibited glass-forming properties with glass-transition temperatures of 89–123 °C. The solid-state ionization potentials and electrochemical band gaps ranged from 5.88 to 6.37 eV and from 3.30 to 3.57 eV, respectively. Three compounds demonstrated bipolar semiconducting properties. Derivative of triphenylethylene and 2,6-di-tert-butylcarbazole with phenylene bridge showed highest hole and electron drift mobilities reaching 3.07 × 10−3 cm2/Vs and 4.2 × 10−3 cm2/Vs respectively at high electric field. Utilizing compounds in organic light‐emitting diodes as emitters external quantum efficiency of 2% was observed.

Differently substituted benzothiadiazoles as charge-transporting emitters for fluorescent organic light-emitting diodes

Carbazolyl- and 1,2,3,4-tetrahydrocarbazolyl-substituted benzothiadiazoles were prepared by Pd-catalyzed Buchwald-Hartwig and Heck reactions. Photophysical, electrochemical, thermal, charge-transporting properties, and performance in organic light emitting diodes (OLEDs) of the derivatives with donor-π-acceptor-π-donor and donor-acceptor-donor structures are discussed. Photo- and electroluminescent properties of the studied compounds of carbazole and benzothiadiazole are very sensitive to their molecular structures. Their glass transition temperatures range from 145 to 191 °C. Investigation of photophysical properties revealed importance of intramolecular twisting on excited state deactivation. Emission of dilute solutions of the compounds in nonpolar solvents appears in the orange region with fluorescence quantum yields up to 0.6.4,7-(Di (1,2,3,4-tetrahydro)carbazolyl)-[2,1,3]-benzothiadiazole demonstrates phosphorescence and delayed fluorescence at room temperature. Solid-state ionization potentials of the materials range from 5.64 to 5.82 eV. The layer of 4,7-(di (tert-butyl)carbazolyl)-[2,1,3]-benzothiadiazole shows high hole drift mobility of 1.7 × 10−3 cm2/V·s at an electric field of 5.3 × 105 V/cm and relatively high electron drift mobility of 5.7 × 10−5 cm2/V·s at an electric field of 5.8 × 105 V/cm. Red OLEDs based of singlet emission with maximum external quantum efficiency values of 1.3 and 0.13% were achieved using new emitters. Much better electroluminescent performance was observed for the compound having donor-acceptor-donor structure relative to the compound with donor-π-acceptor-π-donor structure.

Phenanthroimidazole-based monomers: synthesis, properties and self-polymerization

Phenanthroimidazole-based monomers with reactive vinyl groups were synthesized, and their thermal, optical, photophysical and electrochemical properties were investigated. The monomers exhibited high thermal stability with 5% weight loss temperatures (Td) ranging from 378 to 409 °C. Thermal degradation of the polymerization products apparently takes place in this temperature range. The solutions of the monomers exhibit emission peaks in the range from 388 to 398 nm. In the solid state, the emission of these molecules shows red shift which is coherent with the similar red shifts of the corresponding absorption spectra. Ionization potential values of the compounds estimated by cyclic voltammetry were found to be close and varied in the range from 5.44 to 5.63 eV. Solid-state ionization potentials estimated by photoelectron emission spectrometry varied in the range from 5.54 to 5.66 eV. Self-polymerization of the synthesized monomers was demonstrated by differential scanning calorimetry. The number average molecular weights of the polymerization products of monomers containing substituents at phenyl rings linked to C-2 and N-1 positions of imidazole ring were found to be 100,100 and 196,000, respectively. The apparent activation energy and pre-exponential factor of self-polymerization were found to be dependent on conversion degree. The values of activation energy for self-polymerization of monomers varied in the range from 78.7 to 136.0 kJ/mol (estimated by Ozawa method) and from 78.3 to 139.0 kJ/mol (estimated by Kissinger method).

Photophysics, electronic structure and solar cell performance of a donor-acceptor poly(N-dodecyl-2,7-carbazole-alt-benzothiadiazole) copolymer

Abstract In this contribution, we investigate the optoelectronic properties of a donor-acceptor poly(N-Dodecyl-2,7-carbazole-alt-benzothiadiazole (CBZ-BT) copolymer in solutions and thin films, by a combination of complementary optical and electronic spectroscopy techniques including stationary absorption and fluorescence, femtosecond time-resolved and ultraviolet/x-ray photoelectron spectroscopy. Absorption spectroscopy revealed two bands at 322/338 and 445/475 nm for CBZ-BT in solutions/films attributed to the carbazole and benzothiadiazole groups respectively. Photoexcitation either to the absorption band of the carbazole or benzothiadiazole group led to a broad and structureless fluorescence spectrum due to large torsional disorder in the excited state, originating from intramolecular energy transfer between carbazole and benzothiadiazole. Time resolved spectroscopy in solutions reveals a transient red-shift of the emission spectrum within less than 5 ps due to exciton migration and/or conformational relaxation of the polymer backbone. In films, this relaxation is faster accompanied by a quenching of the exciton lifetime. Fluorescence depolarization in solutions follows the rate of spectral relaxation. In films, the overall depolarization is faster leading to a reduced limiting anisotropy, due to efficient energy transfer to adjacent chains with different polarization of the transition dipoles and increased disorder in the solid state. An almost complete quenching of the copolymer fluorescence, taking place on ∼150 fs, was observed upon blending with a fullerene PC70BM acceptor pointing to an efficient electron transfer. CBZ-BT shows a large solid-state ionization potential of 6.2 eV and an electron affinity of 3.7 eV. Its potential as an electron donor in polymer solar cells (PSCs) was unveiled upon fabrication of relatively high open circuit voltage (∼0.85 V) but low power conversion efficiency (∼1.7%) bulk-heterojunction single layer PSCs with PC70BM as electron acceptor.

High-triplet-energy derivatives of indole and carbazole as hosts for blue phosphorescent organic light-emitting diodes

The synthesis and characterization of a series of electro-active compounds containing carbazolyl and indolyl moieties are reported. All the synthesized derivatives were capable of glass formation with the glass transition temperatures ranging from 89 to 130 °C. They showed high thermal stability with the 5% weight loss temperatures ranging from 370 to 412 °C. The obtained compounds emit light in the deep blue region (350–366 nm) and their solutions exhibit moderately high fluorescence quantum yields in the range of 22.9–44.2%. The compounds exhibited high triplet-energy levels ranging from 2.70 to 3.06 eV. The solid-state ionization potentials of the synthesized compounds were found to be comparable (5.68–5.72 eV). They are characterized by equal values of the optical band gaps (3.53 eV). Cyclic voltammetry revealed two irreversible oxidation peaks with the onsets at 0.95 V and 1.35 V in the repeated cycles. 9-{4-[Bis(1-ethyl-2-phenyl-1H-indol-3-yl)methyl]phenyl}-9H-carbazole exhibited well balanced hole and electron mobilities reaching 1.3 × 10−3 cm2/V × s at electric fields higher than 8.1 × 105 V/cm. The compounds were tested as hosts in electrophosphorescent devices. The best device showed turn-on voltage of 2.4 V (at 1 cd/m2); maximum current efficiency of 44.7 cd/A, maximum brightness of 4500 cd/m2 (at 7 V), maximum power efficiency of 38 l m/W, and external quantum efficiency of ca. 18%.

Synthesis, properties and self-polymerization of 1,8-naphthalimide-based vinyl monomer

ABSTRACT1,8-Naphthalene imide with reactive vinyl group was synthesized by Suzuki reaction. Self-polymerization of the compound starts at 177°C as confirmed by differential scanning calorimetry. The electron affinity values of the monomer and the polymer were found to be −3.30 eV and −3.05 eV, respectively. The solid state ionization potential values of the monomer and the polymer were recorded to be 6.42 eV and 6.17 eV, respectively. The activation energy of the process was estimated using Ozawa method. It depends on the conversion degree and was found to be in the range of 136-195 kJ/mol. Pre-exponential factor values decrease during polymerization with decreasing chain mobility due to increasing molecular weight of the polymer.

Derivative of oxygafluorene and di-tert-butyl carbazole as the host with very high hole mobility for high-efficiency blue phosphorescent organic light-emitting diodes

Derivative of oxygafluorene and di-tert-butyl carbazole was synthesized as the host material for high-efficiency blue phosphorescent organic light-emitting diodes. It exhibited moderate thermal stability with 5% weight loss temperature of 337 °C and glass-forming ability with the glass-transition temperature of 65 °C. The solid-state ionization potential of di-tert-butyl carbazolyl-substituted oxygafluorene was found to be 5.54 eV as established by electron photoemission spectrometry. It showed low-dispersity hole transport with time-of-flight hole mobilities of 10−2 cm2 V−1 s−1 observed at high electric fields. Blue devices based on the synthesized compound showed high external quantum efficiency up to 16.5% and low efficiency roll-off.

Carbazole-based hole transporting material for solid state dye-sensitized solar cells: Influence of the purification methods

The synthesis and properties of a glass-forming carbazole compound 9-(ethyl)-3,6-bis(4,4′-dimethoxydiphenylaminyl)-carbazole are reported. The thermal, optical and electrochemical properties of the hole-transporting molecule were studied by differential scanning calorimetry, thermogravimetric analysis, UV/Vis spectroscopy and cyclic voltammetry. The molecular glass exhibits a thermal stability as high as 370°C and a glass transition temperature of 68°C. The synthesized coumpound absorbs in the 250–400nm range and possesses an optical band gap of 2.76eV, avoiding any screening effect with the dye. The solid state ionization potential (IPss) of the molecule, estimated by cyclic voltammetry is around 4.77eV, higher than the standard spiro-OMeTAD hole-transporting material. The compound was finally assessed as hole-transporting material in the solid state dye-sensitized solar cells with (5-(1,2,3,3a,4,8b-hexahydro-4-[4-(2,2-diphenylvinyl)phenyl]-cyclopenta[b]indole-7-ylmethylene)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid (D102) as sensitizer. The effect of the purity of the glassy molecule on photovoltaic performances is discussed and showed a two-fold increase of the power conversion efficiency after purification by sublimation, going from 0.82% to 1.62% under standard AM 1.5G solar irradiation (100mW/cm2).

Effect of Ethynyl Linkages on the Properties of the Derivatives of Triphenylamine and 1,8-Naphthalimide

Three new derivatives of triphenylamine containing different number of 1,8-naphthalimide moieties linked via ethynyl linkages were obtained by a Sonogashira cross-coupling reaction and their properties were studied. The glass-forming derivatives showed glass transition temperatures ranging from 73 to 96 °C. They showed high thermal stabilities with the temperatures of the onset of thermal degradation ranging from 421 to 462 °C. Fluorescence quantum yields of the dilute solutions in nonpolar solvents of the compounds ranged from 0.063 to 0.94 while those of the solid films were in the range of 0.011–0.25. Small singlet–triplet gap was predicted by DFT calculations, which was confirmed by the similarity between the prompt and the delayed fluorescence energies. Cyclic voltammetry measurements revealed close values of the solid state ionization potentials ranging from 5.48 to 5.61 eV and electron affinities ranging from −3.29 to −3.16 eV. The layer of 4,4′-((di(N-(2-ethylhexyl)-1,8-naphthalimide-4-yl)phenyl)e...

Influence of methoxy substitution on the properties of 9,9-fluorenylidene-linked triphenylamine derivatives

Triphenylamine derivatives, comprised of two methoxy-substituted triphenylamine moieties linked through 9,9-fluorenylidene unit, were synthesized via Ullmann-type coupling reaction from primary aromatic diamine, 9,9-bis(4-aminophenyl)fluorene, and respective aryl iodides. Thermal behaviour, photophysical, electrochemical and charge-transporting properties of the materials were investigated. Ortho- and meta-methoxy-substituted derivatives readily form molecular glasses which exhibit glass-transition temperatures of 114°C and 86°C, respectively. Solid state ionization potentials of methoxy-substituted derivatives are set in the range of 5.35–5.58eV. The materials exhibit fluorescence in UV-violet light region with the maxima situated in the range of 360–389nm. The phosphorescence spectra of all the compounds are almost identical and not influenced by methoxy-substitution. The materials exhibit high triplet-energy levels of 2.88eV. Hole mobilities in solid amorphous films of the neat compounds exceed 10−3cm2V−1s−1 at the electric field of 1MVcm−1.

Structure-properties relationship of the derivatives of carbazole and 1,8-naphthalimide: Effects of the substitution and the linking topology

Nine compounds having electron-accepting 1,8-naphthalimide and electron-donating carbazole moieties were synthesized employing palladium-catalyzed C–N and C–C coupling reactions and characterized by the thermal methods, absorption and emission spectrometry, electrochemical and photoelectrical tools. The synthesized compounds possess high thermal stability with the 5% weight loss temperatures being in the range of 351–476 °C. Most of the synthesized compounds are capable of glass formation with glass transition temperatures ranging from 30 to 87 °C. The cyclic voltammetry measurements showed that the solid state ionization potentials values of the carbazole and 1,8-naphthalimide derivatives range from 5.46 eV to 5.76 eV and the electron affinities values range from −3.04 eV to −2.92 eV. Dilute solutions of the 3- and 3,6-naphthalimide-substituted derivatives of carbazole in polar solvents were found to emit in the green region with quantum yields ranging from 0.66 to 0.83, while in the solid state fluorescence quantum yields were found to be in the range of 0.01–0.45. ((E)-9-(((N-(2-ethylhexyl)-1,8-naphthalimide)-4-yl)ethenyl)-9H-carbazole) exhibited efficient fluorescence in the solid state with quantum yield as high as 0.45. The effects of the linking topology of the chromophores and of the incorporated alkyl substituents on the thermal, optical, and photoelectrical properties of the synthesized donor-acceptor compounds are analyzed. The impact of the ground state intramolecular twisting of the carbazole and naphthalimide moieties induced by the substituents resulting in significant variation in the rates of radiative and nonradiative excitation deactivation is revealed.

Hole-transporting thiophene-based hydrazones with reactive vinyl groups

Synthesis and optical, thermal, electrochemical, and photoelectrical properties of new thiophene-based hydrazones with reactive vinyl groups are reported. The synthesized compounds were characterized by 1H, 13C NMR, IR, UV/vis, fluorescence, mass spectrometries, and cyclic voltammetry. The synthesized thiophene-based hydrazones form glasses with the glass transition temperatures ranging from 15°C to 72°C, as it was elucidated by DSC. Cyclic voltammetry measurements showed that all the synthesized dihydrazones are electrochemically stable up to 0.8V. Their oxidation potentials are in the range of 0.02–0.55V. Solid state ionization potentials of these compounds were found to be in the range of 4.82–5.35eV. Time-of-flight hole-drift mobility in the layer of di-(2-(4-vinylbenzyl)-2-phenyl-1-((thiophene-2-yl)methylene))hydrazone approached 10−5cm2/Vs at high electric field.

Star-shaped triphenylamine-based molecular glass for solid state dye sensitized solar cell application

The synthesis by Suzuki cross-coupling and properties of a glass-forming star-shaped compound tris(9-(3-methylphenyl)carbazol-3-yl)-triphenylamine are reported. The thermal, optical, photoelectrical and electrochemical properties of the hole-transporting compound were studied by differencial scanning calorimetry, thermogravimetric analysis, UV/vis spectroscopy, electron photoemission, time-of-flight technique and cyclic voltammetry. The compound exhibits high thermal stability with a the temperature of the onset of the thermal degradation of 510°C. The compound absorbs in 200–400nm range and possesses an optical band gap of 3.15eV, avoiding any screening effect with the dye. The solid state ionization potential (IPss) of the molecule, measured by electron photoemission and cyclic voltammetry is around 5eV similar to the standard spiroOMeTAD hole-transporting material. The hole drift mobility in the amorphous layer of reported compound reach 6.4×10−5cm2/Vs under high electrical field (6.4×105V/cm). This synthesized derivative was finally assessed as hole transporting material in the solid state dye-senstized solar cells with (5-(1,2,3,3a,4,8b-hexahydro-4-[4-(2,2-diphenylvinyl)phenyl]-cyclopeanta[b]indole-7-ylmethylene)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid (D102) as sensitizer and showed a power conversion efficiency of 0.63% under standard solar irradiation (100mW/cm2, AM1.5).

Hole-transporting phenothiazine-based hydrazones with reactive vinylbenzyl groups

Synthesis and optical, thermal, electrochemical, photoelectrical properties of new phenothiazine-based hydrazones with reactive vinyl groups are reported. The synthesized compounds were characterized by 1H NMR, IR, UV/VIS, fluorescence, mass spectrometries, and cyclic voltammetry. The oxidation potentials of the phenothiazine-based mono- and dihydrazones are 0.17V and 0.19V, respectively. Solid-state ionization potentials of these compounds are 4.97eV and 4.99eV, respectively. Hole-drift mobilities in the layer of the phenothiazine-based monohydrazone exceed 10−5cm2/Vs at the electric field of 1×106V/cm, at 25°C. The self-polymerization in the solid state of the synthesized monomers was demonstrated by differential scanning calorimetry.

Synthesis and Properties of the Derivatives of 2,4,6-Tris(Phenoxy)-1,3,5-Triazine

Five derivatives of 2,4,6-tris(phenoxy)-1,3,5-triazine were synthesized and characterized by 1H NMR, IR and mass spectrometries. The optical and electrochemical properties of the synthesized compounds were studied. Dilute solutions of 1,3,5-triazines compounds in acetonitrile absorb electromagnetic radiation in the region of 200-400 nm with the band gaps of 3.96–4.58 eV. Cyclic voltammetry studies revealed that the electron affinities of the synthesized compound range from −3.24 eV to −3.58 eV, and solid state ionization potentials range from 7.20 eV to 8.16 eV.

Star-shaped carbazole derivative based efficient solid-state dye sensitized solar cell

Two new star-shaped carbazole molecules, including tri(9-(methoxyphenyl)carbazol-3-yl)amine named TMPCA having molecular glasses properties and hole transport properties were synthesized. Their thermal, optical, photophysical and electrochemical properties were studied. The carbazole based molecules exhibit high thermal stability with 5% weight loss temperatures over 480 °C with higher glass temperature transitions 164–175 °C than the classical spiro-OMeTAD reference molecule. Their optical band gaps (2.76 eV) are low enough not to hinder neither the absorption of the indoline sensitizer (D102) nor its photoexcitation and charge transfer. Solid state ionization potential (IPs) of TMPCA is well adapted to that of D102 and ensure a driving force ΔrG >0.2 eV for an efficient transfer and regeneration of the photo-oxidized dye. Solid-state dye sensitized solar cells ITO/TiO2/D102/T4MPCA/Au showed a power conversion efficiency of 2.23% with Jsc of 8.85 mA cm−2 under standard AM 1.5 simulated solar irradiation.

Electron-transporting naphthalenetetracarboxy diimide based monomers and polymers

Electron-transporting polymers containing naphthalenetetracarboxy diimide moieties in the main and side chains were prepared by polyaddition and cationic polymerization respectively from the newly synthesized epoxy monomers. DSC measurements showed that the macromolecular compounds exhibit glass transition temperatures reaching 150°C. Cyclic voltammetry measurements revealed the electrochemical stability of the macromolecular derivatives obtained. The electrochemically established solid-state ionization potential values of the synthesized polymers are comparable and range from 7.23eV to 7.28eV. The electron affinity values are also close (−4.05 and −4.10eV). Electron drift mobilities of one macromolecular derivative in air at 22°C approach 10−5cm2V−1s−1 at high electric fields.