Dipole Moment Difference Research Articles

Synthesis, photophysical and solvatochromic properties of diacetoxyboron complexes with curcumin derivatives.

Five novel diacetoxyboron complexes of β-diketones incorporated curcumin moiety were designed and synthesized. Their photophysical behaviors were investigated by UV-vis absorption and fluorescence spectroscopy in different solvents and solid state. It was observed that their fluorescence spectra yielded a blue to yellow-green emission in solution and emitted a yellow to red emission in solid state. Especially, the complex 3b displayed the strongest emission and the highest quantum efficiency (Φu = 0.98) in toluene among these complexes. The CIE coordinate of the complex 3b in solid state was positioned in an ideal red region of the chromaticity diagram. The maximal emission of these complexes exhibited a large wavelength shift and Stokes shift increased with the increase of the solvent polarity. Their dipole moment differences between the ground and excited states were also estimated by using the Lippert-Mataga equation. Meanwhile, their HOMO, LUMO energy levels and energy band gaps were calculated by cyclic voltammetry.

Interfacial charges and electroluminescence in bilayer organic light-emitting diodes with different hole transport materials

The performance of organic light-emitting diodes (OLEDs) and their interfacial charge density have been studied in tris-(8-hydroxyquinolate) aluminum (Alq3)-based OLEDs with four different hole transport layers (HTLs). The four HTLs have almost identical highest occupied molecular orbital (HOMO) energy levels. The performance of the four OLEDs is not greatly changed. The interfacial charge density is determined from the hole injection voltage in the capacitance—voltage characteristics of the OLEDs. Determination of the interfacial charge density is verified by device simulation. The interfacial charge density is dependent on the HTL materials in Alq3-based OLEDs, and is proportional to the difference in dipole moments between the HTLs and electron transport layers including Alq3.

Extended kinetic method study of the effect of ortho‐, meta‐, and para‐chlorination on the proton affinity of phenylalanine with full entropy analysis

AbstractAbsolute proton affinities (PAs) of phenylalanine, ortho‐chlorophenylalanine, meta‐chlorophenylalanine, and para‐chlorophenylalanine were determined using the extended kinetic method with full entropy analysis to investigate the effect of chlorination on the PA of phenylalanine. The measured absolute PAs of the molecules are 907.9 ± 2.3, 901.9 ± 2.2, 889.0 ± 2.8, and 904.9 ± 3.1 kJ·mol−1, respectively. The decrease and variations in the PAs upon chlorination are investigated based on the electronegativity of chlorine, polarizability, dipole moment, and the substituent effect. The effect of electronegativity of chlorine fails to explain differences in the PAs of the chlorine derivatives, and the ordering of the PAs is also not in line with polarizability. However, the ordering of dipole moments and substituent effects are, in part, consistent with that of the measured PAs except for the para and ortho derivatives, respectively. Thus, while no one molecular property fully explains the decrease in the absolute PA upon chlorination and its variation depending on the chlorination site, differences in dipole moments and the field effect provide the most plausible explanation for the observed changes.

Substituent effects on linear and nonlinear optical properties of fluorescent (E)-2-(4-halophenyl)-7-arlstyrylimidazo[1,2-A]pyridine: spectroscopic and computational methods

Abstract Effects of alkylamino and bromo substituents on imidazo[1,2-a]pyridines containing donor-π-acceptor type groups were comprehensively investigated for their linear and nonlinear optical properties by solvatochromic and DFT (CAM-B3LYP and BHandHLYP) methods. The difference between the ground and excited dipole moments as well as their ratios obtained by solvatochromic analysis indicate that the excited state is more polar than the ground state for both the bromo and diethyl amino derivative. More than twofold enhancement in the excited state dipole moments was observed as revealed by the difference and ratio of dipole moment upon the introduction of alkylamino donor group and these suggest large intramolecular charge transfer in the dyes. Stabilization energy above 20 kJ/mol was observed for large number of electron donor–acceptor interactions in Natural Bonding Orbital (NBO) analysis. Bond length alternation (BLA) and Bond order alternation (BOA) values tend to zero suggesting a high degree of polarization in the dyes. Enhancement in mean polarizability (α0), first hyperpolarizability (β0) and second static hyperpolarizabilities ( $\bar \gamma$) were observed by the introduction of alkylamino and bromo group in place of chloro in spite of the fact that Hammett constant of chloro and bromo are the same. The dyes have fundamental and intrinsic properties within the Hamiltonian limits. The two-photon absorption cross section value (≈100 GM) is comparable with LDS-698, a commercial TPA dye. This investigation is important for understanding the electronic structure of imidazo[1,2-a]pyridine with active functional groups and extending the potential for optical applications.

NLOphoric benzyl substituted BODIPY and BOPHY: A comprehensive linear and nonlinear optical study by spectroscopic, DFT and Z-scan measurement.

BOPHY (BPY) and BODIPY (BDY) dye bearing benzyl group (Bn) at 4,4' and 2,6 position respectively were synthesized and characterized. The fluorescence decay measurements were performed which reveal that benzyl BOPHY (Bn-BPY) has shorter fluorescence lifetime compared to benzyl BODIPY (Bn-BDY). The difference in transition dipole moment is found to be 6.93 and 11.3 D for Bn-BDY and Bn-BPY respectively in toluene. The molecular electrostatic potential (MEP) plot shows Bn-BDY is more polarised compared to Bn-BPY. The nonlinear optical (NLO) property was evaluated using Z-scan measurement. The molecular electronic arrangement of Bn-BPY significantly affects the nonlinear absorption properties resulting into reverse saturable absorption (nonlinear absorption coefficient β = 0.256 × 10-11 m/W). In contrast, the Bn-BDY displays saturable absorption character. The calculated third-order nonlinear susceptibility χ(3) value is 12.23 × 10-13 esu and 2.49 × 10-13 esu for Bn-BDY and Bn-BPY respectively. The power limiting behaviour of Bn-BPY displays limiting threshold energy around 70 Jcm-2 with clamped output at ~35 Jcm-2.

Influence of acceptors in NLOphoric aacenaphthene and morpholine-thiourea hybrid dyes: Photophysical, viscosity, DFT and Z-Scan study

Three novel fluorescent NLOphoric push-pull fluorophores (2a-c) constituted by different cyano containing acceptors linked to morpholine-thiourea (donor) and acenaphthene (rotor) hybrid were synthesized and characterized. They exhibit positive absorption and emission solvatochromism. The solvent polarity function based Lippert-Mataga plot provides the validation of charge transfer (CT) characteristics whereas, Rettig plot furnishes an alternative relaxation channel due to twisting around the σ-bonds between donor and acceptor on photoexcitation leading to the twisted intramolecular charge transfer (TICT) state in dyes 2a-c. Viscosity induced emission studies show 4.18, 17.61, and 5.30 fold increase in the emission intensity for dyes 2a, 2b, and 2c respectively which recommends these dyes as fluorescent molecular rotors (FMR). Density Functional Theory (DFT) calculations [(B3LYP/6-311++G(d,p)] give complete information of structural as well as electronic properties of dyes 2a-c. The difference in dipole moment (ca. 4.86–8.23 D) results in a strong non-linear optical (NLO) properties which increases in the order 2a<2b < 2c. Z-scan studies reveal that the dyes possesses reverse-saturable i.e. positive type of absorption non-linearity resulting in a positive values of β (2c > 2b > 2a). The lower optical limiting threshold values for dyes 2a-c, made them suitable for the use in the application of NLO materials.

Molecular Dynamics Simulations of the c-kit1 Promoter G-Quadruplex: Importance of Electronic Polarization on Stability and Cooperative Ion Binding.

G-quadruplexes (GQs) are guanine-rich, noncanonical nucleic acid structures that play fundamental roles in genomic stability and the regulation of gene expression. GQs are enriched in promoter sequences of growth regulatory genes and proto-oncogenes such as c-kit, which is linked to gastrointestinal stromal tumors, mast cell disease, and leukemia. While GQs have become a popular subject for experimental and computational research, the forces governing GQ dynamics are not fully understood. To gain insights into cation interactions and base-dipole moments of these highly ordered nucleic acid structures, we performed molecular dynamics simulations on the c-kit1 GQ using the CHARMM36 additive and Drude-2017 polarizable force fields. These simulations are the first of their kind to investigate the role of electronic polarization on interactions dictating GQ conformational sampling and cation interactions. Use of a polarizable model revealed differences in base dipole moments between GQs and B-form duplex DNA, force field-dependent ion binding pathways, and allowed for quantification of multibody contributions of water to ion-GQ interactions. These results emphasize the importance of electronic polarization as a contribution to the forces underlying nucleic acid dynamics.

The effect of polarizable environment on two-photon absorption cross sections characterized by the equation-of-motion coupled-cluster singles and doubles method combined with the effective fragment potential approach.

We report an extension of a hybrid polarizable embedding method incorporating solvent effects in the calculations of two-photon absorption (2PA) cross sections. We employ the equation-of-motion coupled-cluster singles and doubles method for excitation energies (EOM-EE-CCSD) for the quantum region and the effective fragment potential (EFP) method for the classical region. We also introduce a rigorous metric based on 2PA transition densities for assessing the domain of applicability of QM/MM (quantum mechanics/molecular mechanics) schemes for calculating 2PA cross sections. We investigate the impact of the environment on the 2PA cross sections of low-lying transitions in microhydrated clusters of para-nitroaniline, thymine, and the deprotonated anionic chromophore of photoactive yellow protein (PYPb). We assess the performance of EOM-EE-CCSD/EFP by comparing the 2PA cross sections against full QM calculations as well as against the non-polarizable QM/MM electrostatic embedding approach. We demonstrate that the performance of QM/EFP improves when few explicit solvent molecules are included in the QM subsystem. We correlate the errors in the 2PA cross sections with the errors in the key electronic properties-identified by the analysis of 2PA natural transition orbitals and 2PA transition densities-such as excitation energies, transition moments, and dipole-moment differences between the initial and final states. Finally, using aqueous PYPb, we investigate the convergence of 2PA cross sections to bulk values.

Effect of electric field on migration of defects in oxides: Vacancies and interstitials in bulk MgO

Dielectric layers composed of metal oxides are routinely subjected to external electric fields during the course of normal operation of electronic devices. Many phenomenological theories suggest that electric fields strongly affect the properties and mobilities of defects in oxide films and can even facilitate the creation of new defects. Although defects in metal oxides have been studied extensively both experimentally and theoretically, the effect of applied electric fields on their structure and migration barriers is not well understood and still remains subject to speculations. Here, we investigate how static, homogeneous electric fields affect migration barriers of canonical defects---oxygen vacancies and interstitial ions---in a prototypical oxide, MgO. Using the modern theory of polarization within density functional theory (DFT), we apply electric fields to defect migration pathways in three different charge states. The effect of the field is characterized by the change of the dipole moment of the system along the migration pathway. The largest changes in the calculated barriers are observed for charged defects, while those for the neutral defects are barely significant. We show that by multiplying the dipole moment difference between the initial and the transition states, which we define as the effective dipole moment, by the field strength, one can obtain an estimate of the barrier change in excellent agreement with the DFT calculated values. These results will help to assess the applicability of phenomenological models and elucidate linear and nonlinear effects of field application in degradation of microelectronic devices, electrocatalysis, batteries, and other applications.

Fluorescent star ATRP initiators and fluorescent star poly(methyl methacrylate)s: Synthesis and photophysical properties

Fluorescent star ATRP initiators containing four and six initiating sites, comprised with stilbene and α-cynostilbene units were synthesized and their photophysical properties were studied thoroughly. These multifunctional ATRP initiators showed solvatochromism. The dipole moment difference between ground state and excited state of the molecules were calculated using Lippert-Mataga equation and found that the molecules were more polar at excited state. The absorption coefficient, absorption and emission wavelength of the initiators prepared with α-cyanostilbene unit were found high compared to that prepared without -CN substituent. On the other hand, fluorescent lifetime of α-cyanostilbene based initiators was found low. The optical band gap energy calculated for the initiators were considerably low in the range of 2.56 eV–2.83 eV. Based on the photophysical properties, the hexa functional ATRP initiator containing α-cyanostilbene unit was used for ATRP of methyl methacrylate monomer to prepare fluorescent star PMMA; CuBr/PMDETA and CuBr/2,2′-bpy were used as catalysts. The molecular weight of star PMMAs ranged between 1.79 × 104 g/mol and 8.42 × 104 g/mol and the polydispersity index were in the range of 1.12–1.48. The linear increase of molecular weight against the monomer conversion indicated the living nature of polymerization. The absorption and emission spectra of polymer solution and film, and observation of polymer in bulk under UV light confirmed the polymer as green fluorescent; the average fluorescence lifetime of the polymer was found to be 0.92 ns.

Sequential Fluorination on Napthaleneamide-Based Conjugated Polymers and Their Impact on Charge Transport Properties

Improving π-conjugated polymer electron transport and injection efficiency are important to realize high performance n-channel organic field-effect transistors (OFETs). This paper reports a series of naphthalene diimide (NDI-) based n-type π-conjugated polymers, NDI-T-1FP-T, NDI-T-2FP-T, and NDI-T-4FP-T, with varioulis amounts of fluorine atoms (1F, 2F, and 4F) in their backbones. We found notable differences in energy levels, ground and excited state dipole moments (Δμge), exciton lifetimes, dihedral angles, and charge transport properties by varying the fluorine content: Δμge and charge transport properties reduced with increasing fluorine content. Electron mobility of optimized top gate bottom contact OFETs using the n-type π-conjugated polymers were 0.35, 0.18, and 0.16 cm2 V–1 s–1, respectively. OFET performances were further improved further by low concentration polyethylenimine (PEI) doping, significantly improving field-effect mobility to (maximum) 0.51, 0.34, and 0.26 cm2 V–1 s–1, respectively. D...

Influence of electron donors in fluorescent NLOphoric D-π-A derivatives with acenaphthene rotor: Photophysical, viscosity, and TD-DFT studies

The novel fluorescent NLOphoric push-pull fluorophores (2a-c) constituted by the different N-substituted donors linked via π-bridge as a spacer to 1,1′-dicyanomethylidene group as an acceptor with acenaphthene rotor were synthesized and characterized. These dyes exhibit positive absorption and emission solvatochromism in solvents of different polarities. Solvent polarity plots viz. Lippert-Mataga, McRae, and Weller models provide the validation of charge transfer (CT) characteristics whereas, the Rettig model furnishes an alternative relaxation channel due to twisting around the σ-bonds between donor and acceptor on photoexcitation leading to the twisted intramolecular charge transfer (TICT) state in 2a-c. Viscosity induced emission studies show 4.40, 8.78, and 5.63 fold increase in the emission intensity for dyes 2a, 2b, and 2c respectively which recommends the titled dyes as a fluorescent molecular rotor (FMR). Density Functional Theory (DFT) calculations [(B3LYP/6-311++G(d,p)] give complete information of structural as well as electronic properties of dyes 2a-c. The large difference in dipole moment (ca. 7.92–20.0 D) results in a strong non-linear optical (NLO) properties. The NLO properties were estimated theoretically as well as spectroscopically in solvents of different polarities. The computed values for these dyes show high first-order hyperpolarizability (β) in the range of 210–577 × 10−3° esu and second-order hyperpolarizability (γ) in the range of 506-2,325 × 10-36 esu.

Tuning ‘Stokes Shift’ and ICT Character by Varying the Donor Group in Imidazo[1,5 a]pyridines: A Combined Optical, DFT, TD‐DFT and NLO Approach

AbstractImidazo[1, 5a]pyridine based novel donor‐acceptor dyes (2a‐e) with the large Stokes shift (ca. 140–200 nm) were synthesized and characterized. We report on the steady‐state and nanosecond time‐resolved emission studies of 2a‐e in different microenvironments which reveal that the dyes 2a‐c (average lifetime of 0.65, 1.36 and 1.63 ns respectively) perform reversible proton motion in chloroform due to variable donor group present at the para position of substituted phenyl group attached to imidazopyridine core as an acceptor group whereas 2d and 2e (average lifetime of 3.50 and 4.06 ns respectively) compete with 2a‐c in agreement with considerable intramolecular charge transfer (ICT) characteristics. Solvent polarity plots viz. Lippert‐Mataga, McRae, and Bakhshiev equation furnish the confirmation of ICT character. Density functional theory (DFT) calculations [(B3LYP/6‐311++G(d, p)] provide the information of structural as well as the electronic properties of dyes 2a‐e. Natural bond orbital (NBO) analyses enlighten the nature of the charge or proton transfer within the selected donor‐acceptor orbital interaction with the large energy of stabilization (the donor‐acceptor orbital interactions increases in the order 2a &gt; 2b &gt; 2c &gt; 2d &gt; 2e). The large difference in dipole moment (ca. 1.57‐21.55 D) results in a strong non‐linear optical (NLO) properties. The NLO properties were evaluated theoretically as well as spectroscopically in solvents of different polarities and the resulting hyperpolarizabilities fall within the expected limiting values. These results bring new findings in coupled ICT and proton transfer reactions which will be useful for further research and promising applications in sensing polarity or H‐bonding of microenvironments similar to that of the biological ones.

NLO properties of ester containing fluorescent carbazole based styryl dyes – Consolidated spectroscopic and DFT approach

The linear and nonlinear optical (NLO) properties of new fluorescent styryl dyes based on anchoring ester containing carbazole as donor appended to different acceptor groups to have a conjugated π-system with push-pull geometry are studied. The NLO properties have been determined using solvatochromic and computational methods. Three different TD-DFT functional are used namely, B3LYP, BHandHLYP, and CAM-B3LYP, with aim of elucidating better functional for NLOphores. Further, the two photon properties (σ2PA) have been described theoretically by two level model considering the dipole moment difference between the ground and the final electronic states and bypassing the intermediated resonance state. The compounds with a high charge transfer from the acceptor group to the carbazole ring have relatively high two-photon absorption cross-sections (60–317 GM). The linear polarizability (αCT), first order hyperpolarizability (β) and second order hyperpolarizability (ɣ) for 4c dye was the highest among the studied dyes which is attributed to the lesser energy gap evident by both the methods. But in contrary, the σ2PA cross-section value was low for dye 4c which is due to the presence of freely rotatable twisted phenyl ring in the conjugation path, pulling the electron density towards itself and thus lead to decrease in σ2PA cross-section. Structure-property relationship is better understood by the correlation of bond length alternation/bond order alternation (BLA/BOA) with NLO properties of dyes. Thus by simple solvatochromic method and computational method, we have screened the carbazole styryls as NLO candidates with good first order hyperpolarizability and good two photon cross-section.

Influence of External Electric Field on Vibrational Spectrum of Imidazolium-Based Ionic Liquids Probed by Molecular Dynamics Simulation

In this study, the influence of an external electric field (EEF) on the vibrational spectra of an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium hexfluorophosphate (EMIMPF6), in the wavenumber range from 0 to 4000 cm(-1) was probed by molecular dynamics (MD) simulation at 350 K. The results showed that the experimentally obtained spectrum could be reproduced by the calculated vibrational bands (VBs) in the wavenumber range from 400 to 4000 cm(-1) using MD simulation without any EEF. When the EEF applied increased from 0 to 9 V.nm(-1), the VB intensities at 50.0 and 199.8 cm(-1) increased continuously and then tended to be saturated, while the VB intensities from 400 to 4000 cm(-1) decrease and eventually disappear. Moreover, the VB at 50.0 cm(-1) was red-shifted to similar to 16.7 cm(-1) and then increased to 33.3 cm(-1) as the EEF was increased from 0 to 2 and then to 3 V.nm(-1) and higher. The VB at 3396.6 cm(-1) was redshifted to similar to 16.7 cm(-1) and then increased to 33.3 cm(-1) as the EEF was increased from 0 to 3 and then to 4 V.nm(-1) and higher; however, the position of other VBs from 0 to 4000 cm(-1) remain almost unchanged. Based on further analysis of the simulation results and previously reported studies, for the VB at 50.0 cm(-1), the increasing EEF enhances the polarity between cations and anions; thus, the difference in dipole moment between the cations and the anions increases, which continually increases the VB intensity until saturation is reached. For the VB at 199.8 cm(-1), the increasing EEF intensifies the twisting of the ethyl chain, which enhances the VB intensity until saturation. For the other VBs from 400 to 4000 cm(-1), the increasing EEF makes the orientation of the cations and anions in EMIMPF6 more consistent; thus, it can be conjectured that such consistent orientation may weaken the VB intensities and can even make them disappear. The redshift of VB at 50.0 cm(-1) may occur because the EEF breaks the distribution of the electrostatic field inside EMIMPF6 and then weakens the interactions between cations and anions. The redshift of VB at 3396.6 cm(-1) may be attributed to the EEF weakening the stretching vibration of the hydrogen bonds formed between the N atoms and the acidic hydrogen atoms on the cationic imidazolium rings. The EEF does not change the positions of the other VBs because the inherent stretching, bending, and rocking vibration of functional groups are not affected by the EEF.

Solar Cells: Design of Cyanovinylene‐Containing Polymer Acceptors with Large Dipole Moment Change for Efficient Charge Generation in High‐Performance All‐Polymer Solar Cells (Adv. Energy Mater. 3/2018)

In article number 1701436 by Bumjoon J. Kim and co-workers, a series of naphthalenediimide-based polymer acceptors with superior electron mobility and large dipole moment difference is developed by incorporating electron-withdrawing cyanovinylene groups into a polymer backbone. All-polymer solar cells based on these polymers generate outstanding power conversion efficiency of 7.4% with high fill factor (65%), by virtue of the high electron transport and efficient exciton dissociation with greatly suppressed charge recombination.

Wurtzite BAlN and BGaN alloys for heterointerface polarization engineering

The spontaneous polarization (SP) and piezoelectric (PZ) constants of BxAl1-xN and BxGa1-xN (0 ≤ x ≤ 1) ternary alloys were calculated with the hexagonal structure as reference. The SP constants show moderate nonlinearity due to the volume deformation and the dipole moment difference between the hexagonal and wurtzite structures. The PZ constants exhibit significant bowing because of the large lattice difference between binary alloys. Furthermore, the PZ constants of BxAl1-xN and BxGa1-xN become zero at boron compositions of ∼87% and ∼74%, respectively, indicating non-piezoelectricity. The large range of SP and PZ constants of BxAl1-xN (BAlN) and BxGa1-xN (BGaN) can be beneficial for the compound semiconductor device development. For instance, zero heterointerface polarization ΔP can be formed for BAlN and BGaN based heterojunctions with proper B compositions, potentially eliminating the quantum-confined Stark effect for c-plane optical devices and thus removing the need of non-polar layers and substrates. Besides, large heterointerface polarization ΔP is available that is desirable for electronic devices.

Design of zinc porphyrin‐perylene diimide donor‐bridge‐acceptor chromophores for large second‐order nonlinear optical response: A theoretical exploration

AbstractThe electronic structures and second‐order nonlinear optical (NLO) properties of a series of zinc porphyrin (ZnP)‐perylene diimide (PDI) donor‐bridge‐acceptor (D‐π‐A) molecules have been investigated using density functional theory (DFT) and time‐dependent DFT (TDDFT). The results show that these compounds possess excellent second‐order NLO properties and large static first hyperpolarizabilities (β0) values on the order of 103 – 104 esu−30. A DFT benchmark calculation of β0 value was first performed, confirming the optimally tuned range‐separated functionals (LC‐BLYP* and ωB97X*) can produce similar magnitudes for β0 as Møller–Plesset second‐order perturbation (MP2) calculations. The magnitudes of β0 values were studied as a function of different types of bridges: oligo‐p‐phenylenevinylene (OPV), carbon‐bridged oligo‐p‐phenylenevinylene (COPV), oligo‐thienylenevinylene (OTV), carbon‐bridged oligo‐thienylenevinylene (COTV), and also the length of π‐bridges. The calculated β0 values were found to be inversely related to the bond length alternation (BLA) values of various π‐bridges. Next, we found that the static β0 value of designed ZnP‐COTV1‐PDI molecule can be significantly enhanced by finely tuning the dihedral angles between the π‐bridge and ZnP donor or PDI acceptor. The calculated β0 value is also sensitive to the substitution positions of PDI, that is, ortho‐(α‐), bay‐(β‐), nitrogen‐(N‐). A two‐level model was proven to be useful for the qualitative description of the β0 values and further the substantial oscillator strength and the difference in transition dipole moments between the ground and excited state mainly contribute to the large β0 values. The presented work will be beneficial for potential applications of the ZnP‐PDI‐based chromophores in the optoelectronic devices and high‐performance NLO materials.

Polymer-dielectric molecular interactions in defect-free poly(3-hexylthiophene): dependence and consequences of regioregularity on transistor charge transport properties

We investigate the molecular interaction of poly(3-hexylthiophene-2,5-diyl) (P3HT) molecules with polar functional groups of the dielectric surface, and its dependence on the regioregularity of P3HT. With this aim, we consider thickness-dependent molecular order of 100% regioregular defect-free P3HT (DF-P3HT) and 93% regioregular P3HT (LT-P3HT), deposited on top of cross-linked poly(vinyl alcohol) (cr-PVA) substrates. Intimate contact of P3HT molecules and cr-PVA surface defects affects the molecular order of P3HT differently, depending on the regioregularity. Consequently, these molecular order changes on the charge transport properties of organic field-effect transistors (OFETs) are investigated using four thicknesses (20, 40, 80 and 120 nm) of P3HT. As compared to other thicknesses, μsat for 20 nm DF-P3HT OFETs shows further improvement, while the opposite occurs for 20 nm LT-P3HT OFETs. Depending on the regioregularity (and thus the chain orientation), P3HT molecules exhibit a difference in dipole moments. Consequently, the interaction of edge-on or face-on P3HT molecules with cr-PVA surface dipoles has different contributions towards the electrostatic energetic disorder at cr-PVA/P3HT interface. This subtle difference of behavior helps one to understand the huge spread of characteristics of P3HT based transistors found in literature.

The Impact of Sequential Fluorination of π‐Conjugated Polymers on Charge Generation in All‐Polymer Solar Cells

The performance of all‐polymer solar cells (all‐PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors (P1–P3) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all‐PSCs is investigated. Sequential fluorination of the polymer backbones increases the dipole moment difference between the ground and excited states (Δµge) from P1 (18.40 D) to P2 (25.11 D) and to P3 (28.47 D). The large Δµge of P3 leads to efficient exciton dissociation with greatly suppressed charge recombination in P3‐based all‐PSCs. Additionally, the fluorination lowers the highest occupied molecular orbital energy level of P3 and P2, leading to higher open‐circuit voltage (VOC). The power conversion efficiency of the P3‐based all‐PSCs (6.42%) outperforms those of the P2 and P1 (5.00% and 2.65%)‐based devices. The reduced charge recombination and the enhanced polymer exciton lifetime in P3‐based all‐PSCs are confirmed by the measurements of light‐intensity dependent short‐circuit current density (JSC) and VOC, and time‐resolved photoluminescence. The results provide reciprocal understanding of the charge generation process associated with Δµge in all‐PSCs and suggest an effective strategy for designing π‐conjugated polymers for high performance all‐PSCs.