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Orientation of conjugated polymers is increasingly important in organic photovoltaics (OPV) to achieve high power conversion efficiency (PCE). The optimized orientation of conjugated backbones for photo-generated charge carriers in OPV devices is in contrast to organic semiconductor devices, demanding new strategies to control and realize face-on orientation of conjugated systems onto substrates. Here we report new conjugated polymers composed of electron-accepting benzothienoisoindigo (BTIDG), an asymmetric unit of isoindigo and thienoisoindigo. BTIDG was coupled with weakly electron-donating thiazolothiazole or benzobisthiazole, concurrently leading to moderate optical band gaps (1.41–1.52 eV) and the highest occupied molecular orbital (−5.35 to −5.50 eV). The alkylthiophene spacer between BTIDG and the donor unit provided a marked control over the orientation of polymers, among which the degree of face-on orientation as high as 95% was revealed by grazing incidence X-ray diffraction. The maximum PCE was improved up to 4.2% using the system with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). We present a useful basis on the structure (orientation)–property (OPV output) relationship to lay down new guidelines for the design of efficient solar cell materials.

New donor–acceptor-type copolymers comprised of benzobisthiazole (BBTz) as a weak donor rather than acceptor are proposed. This approach can simultaneously lead to deepening the HOMO and LUMO of the polymers with moderate energy offset against fullerene derivatives in bulk heterojunction organic photovoltaics. As a proof-of-concept, BBTz-based random copolymers conjugated with typical electron acceptors: thienopyrroledione (TPD) and benzothiadiazole (BT) based on density functional theory calculations are synthesized. Laser-flash and Xe-flash time-resolved microwave conductivity (TRMC) evaluations of polymer:[6,6]-phenyl C61 butyric acid methyl ester (PCBM) blends are conducted to screen the feasibility of the copolymers, leading to optimization of processing conditions for photovoltaic device application. According to the TMRC results, alternating BBTz-BT copolymers are designed, exhibiting extended photoabsorption up to ca. 750 nm, deep HOMO (–5.5 to –5.7 eV), good miscibility with PCBM, and inherent crystalline nature. Moreover, the maximized PCE of 3.8%, the top-class among BBTz-based polymers reported so far, is realized in an inverted cell using TiOx and MoOx as the buffer layers. This study opens up opportunities to create low-bandgap polymers with deep HOMO, and shows how the device-less TRMC evaluation is of help for decision-making on judicious molecular design.

We present herein the synthesis and properties of the largest hitherto unknown graphyne fragment, namely trigonally expanded tetrakis(dehydrobenzo[12]annulene)s (tetrakis-DBAs). Intramolecular three-fold alkyne metathesis reactions of hexakis(arylethynyl)DBAs 9 a and 9 b using Fürstner's Mo catalyst furnished tetrakis-DBAs 8 a and 8 b substituted with tert-butyl or branched alkyl ester groups in moderate and fair yields, respectively, demonstrating that the metathesis reaction of this protocol is a powerful tool for the construction of graphyne fragment backbones. For comparison, hexakis(arylethynyl)DBAs 9 c-g have also been prepared. The one-photon absorption spectrum of tetrakis-DBA 8 a bearing tert-butyl groups revealed a remarkable bathochromic shift of the absorption cut-off (λcutoff ) compared with those of previously reported graphyne fragments due to extended π-conjugation. Moreover, in the two-photon absorption spectrum, 8 a showed a large cross-section for a pure hydrocarbon because of the planar para-phenylene-ethynylene conjugation pathways. Hexakis(arylethynyl)-DBAs 9 c-e and 9 g and tetrakis-DBA 8 b bearing electron-withdrawing groups aggregated in chloroform solutions. Comparison between the free energies of 9 e and 8 b bearing the same substituents revealed the more favorable association of the latter due to stronger π-π interactions between the extended π-cores. Polarized optical microscopy observations, DSC, and XRD measurements showed that 8 b and 9 e with branched alkyl ester groups displayed columnar rectangular mesophases. By the time-resolved microwave conductivity method, the columnar rectangular phase of 8 b was shown to exhibit a moderate charge-carrier mobility of 0.12 cm(2) V(-1) s(-1) . These results indicate that large graphyne fragments can serve as good organic semiconductors.

State-of-the-art low band gap conjugated polymers have been investigated for application in organic photovoltaic cells (OPVs) to achieve efficient conversion of the wide spectrum of sunlight into electricity. A remarkable improvement in power conversion efficiency (PCE) has been achieved through the use of innovative materials and device structures. However, a reliable technique for the rapid screening of the materials and processes is a prerequisite toward faster development in this area. Here we report the realization of such a versatile evaluation technique for bulk heterojunction OPVs by the combination of time-resolved microwave conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash lamp. Xe-flash TRMC allows examination of the OPV active layer without requiring fabrication of the actual device. The transient photoconductivity maxima, involving information on generation efficiency, mobility, and lifetime of charge carriers in four well-known low band gap polymers blended with phenyl-C(61)-butyric acid methyl ester (PCBM), were confirmed to universally correlate with the PCE divided by the open circuit voltage (PCE/V(oc)), offering a facile way to predict photovoltaic performance without device fabrication.

Si-based inorganic electronics have long dominated the semiconductor industry. However, in recent years conjugated polymers have attracted increasing attention because such systems are flexible and offer the potential for low-cost, large-area production via roll-to-roll processing. The state-of-the-art organic conjugated molecular crystals can exhibit charge carrier mobilities (μ) that nearly match or even exceed that of amorphous silicon (1-10 cm(2) V(-1) s(-1)). The mean free path of the charge carriers estimated from these mobilities corresponds to the typical intersite (intermolecular) hopping distances in conjugated organic materials, which strongly suggests that the conduction model for the electronic band structure only applies to μ1 cm(2) V(-1) s(-1) for the translational motion of the charge carriers. However, to analyze the transport mechanism in organic electronics, researchers conventionally use a disorder formalism, where μ is usually less than 1 cm(2) V(-1) s(-1) and dominated by impurities, disorders, or defects that disturb the long-range translational motion. In this Account, we discuss the relationship between the alternating-current and direct-current mobilities of charge carriers, using time-resolved microwave conductivity (TRMC) and other techniques including field-effect transistor, time-of-flight, and space-charge limited current. TRMC measures the nanometer-scale mobility of charge carriers under an oscillating microwave electric field with no contact between the semiconductors and the metals. This separation allows us to evaluate the intrinsic charge carrier mobility with minimal trapping effects. We review a wide variety of organic electronics in terms of their charge carrier mobilities, and we describe recent studies of macromolecules, molecular crystals, and supramolecular architecture. For example, a rigid poly(phenylene-co-ethynylene) included in permethylated cyclodextrin shows a high intramolecular hole mobility of 0.5 cm(2) V(-1) s(-1), based on a combination of flash-photolysis TRMC and transient absorption spectroscopy (TAS) measurements. Single-crystal rubrene showed an ambipolarity with anisotropic charge carrier transport along each crystal axis on the nanometer scale. Finally, we describe the charge carrier mobility of a self-assembled nanotube consisting of a large π-plane of hexabenzocoronene (HBC) partially appended with an electron acceptor. The local (intratubular) charge carrier mobility reached 3 cm(2) V(-1) s(-1) for the nanotubes that possessed well-ordered π-stacking, but it dropped to 0.7 cm(2) V(-1) s(-1) in regions that contained greater amounts of the electron acceptor because those molecules reduced the structural integrity of π-stacked HBC arrays. Interestingly, the long-range (intertubular) charge carrier mobility was on the order of 10(-4) cm(2) V(-1) s(-1) and monotonically decreased when the acceptor content was increased. These results suggest the importance of investigating charge carrier mobilities by frequency-dependent charge carrier motion for the development of more efficient organic electronic devices.

Two new alternating copolymers P1 and P2, of bithiazole (BT) and benzothiadiazoles (BTZ), differing in their side chain positioning at the thiophene units which sandwich the BT unit, were designed and synthesized. Both polymers exhibited broad absorption ranging from 300 to 700 nm with a narrow optical bandgap in the film state. Control over structural ordering of polymer chains was achieved in P1 by treating with a small amount of additive (1,8-octanedithiol, ODT) as evident by a large red shift of absorption peak and also from the XRD measurements. In contrast, no such effects were observed in the case of P2 in the presence of additive. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) experiments revealed that the transient photoconductivity of P1 is far superior to that of P2, which is further increased when processed with ODT. The charge carrier mobility, as determined by the space-charge-limited current (SCLC) technique, indicates that P1 exhibits both electron and hole mobilities with...

We performed kinetic study on the photoconductivity of coassembled nanotubes consisting of an amphiphilic hexa-peri-benzocoronene (HBC) and a HBC derivative appended with an electron-accepting trin...

The charge carrier mobility in films of regioregular polythiophenes substituted by n -alkyl side-chains, of butyl, hexyl, octyl, decyl, and dodecyl, was investigated by flash-photolysis time-resolved microwave conductivity (FP-TRMC), and transient photoabsorption spectroscopy excited at 355 nm. The charge carrier concentration was estimated from inspecting radical anions of perylenecarboxydiimide (PDI), which also acts as an efficient electron acceptor to improve the yield of photocarrier generation. We discuss the dependence of side-chain length in terms of decay dynamics, quantum yield of charge carrier generation, and alternating current (ca. 9 GHz) mobility, which exceeded 0.12 cm 2 /(V s).

Time-resolved microwave conductivity (TRMC) measurement has been performed for fullerene-doped thin films of oligo (dimethylgermane) at different excitation energies to evaluate the intra-molecular mobility of holes along their Ge backbones. Photo-induced electron transfer reaction between oligogermane and fullerene has been observed in the solution with a variety of solvent polarity using transient optical spectroscopy (TOS). The transient spectrum at 391 nm can be attributed to the radical cation of the oligomer under an excitation at 532-nm light, whereas the same spectrum (391 nm) is the overlapping of absorptions of radical cations and neutral radicals of oligogermanes upon exposure of 355-nm light in polar solvent. A combined TRMC and TOS experiments on the solutions of oligomer confirms the conductive transients originate from the radical cations on the backbone chains.

The transient absorption of radical anions and cations of 9,9′-di-n-hexyl-oligofluorene was measured using pulse radiolysis and low-temperature γ-radiolysis techniques. The infrared absorption maxima of both radical anions and cations exhibit red-shift continuously upon elongation of the chain length. The absorption bands are blue-shifted by 0.04–0.07 eV with increasing the temperature from 80 to 106 K. The extinction coefficients were determined by scavenging technique, demonstrating an increase with the elongation of the chain length. The optimized geometry of fluorene trimers, calculated using density functional theory, shows that the oligofluorene molecules are more planar in its charged state than in its neutral state.

For the investigation of photoconductivity and charge carrier dynamics in regioregular poly(3-hexylthiophene) (RR-P3HT), an in situ flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient absorption spectroscopy (TAS) was performed using 6.4 and 3.5 eV photon excitation. Since the former energy is sufficiently higher than ionization potential of RR-P3HT, a direct ionization and consequent bulk charge recombination was observed. The dynamics of charge carriers of transient conductivity and photoabsorption kinetics are discussed in terms of the X-ray diffraction pattern and film morphology of RR-P3HT observed by tapping mode atomic force microscope.

The photocarrier generation mechanism and mobility in poly[bis(p-n-butylphenyl)silane] (PBPS) thin films doped with a variety of electron acceptors are studied by time-resolved microwave conductivity (TRMC) measurements. It was found that fullerene is a suitable electron acceptor for PBPS as it provides the highest product of photocarrier generation yield phi and mobility Sigmamu under excitation at 532 and 355 nm. The observed high phiSigmamu value of 4.5 x 10(-3) cm(2)/(V s) under excitation at 193 nm (6.39 eV) can be attributed to the direct ionization of PBPS molecules. The photoinduced electron transfer between C(60) and PBPS was investigated in a solution sample by laser flash photolysis under excitation at 532 nm. On the basis of the extinction coefficient of PBPS(*+), transient absorption of PBPS(*+) provides a maximum value of phi of 0.83% for the electron-transfer reaction from PBPS to (3)C(60). On the basis of this value of phi, the intrinsic intrachain mobility of holes on the PBPS backbone is estimated to be higher than 1.7 x 10(-2) cm(2)/(V s), suggesting the presence of a high conducting path along the Si backbone of PBPS.

Anion and cation radicals of substituted thiophene oligomers were generated and isolated from respective solutions at low temperature by gamma irradiation at a total dose of 4 kGy. The absorption spectra of these radicals were investigated using low temperature matrix isolation technique. Absorption bands in two typical regions: visible and infrared (IR) were observed experimentally for both anion (HOMO → SOMO and SOMO → LUMO) and cation (SOMO → LUMO and HOMO → SOMO) radicals of all molecules. The values of oscillator strength ( f ) decreases with the increase in the number of phosphate bridges in thiophene molecule for both the anion and cation radicals, suggesting the breaking conjugation by the dihedral angle of thiophene rings at centre of the molecule. The optical transitions in substituted thiophene oligomers were also investigated on the basis of a theoretical approach using ZINDO CI at B3LYP/6-31G(d) DFT geometry. The most stable backbone geometries of the anion and cation radicals are the planner ones in spite of twisted helical structures calculated for the steady-state molecules as the plausible structures. Transformation from twisted (helical) to planner structure plays a crucial role in the determination of energy levels of the molecules bearing negative and positive charge carriers. Incomplete transformation due to the low temperature matrix effects causes considerable decrease in the value of f .

We report the increase in the mobility of charge carriers in regioregular poly 3-hexyl thiophene (RR-P3HT) films by mixing them with tetracyanoethylene (TCNE), which is examined by in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS). TCNE acts not only as an electron acceptor which increases the number of charge carriers on photoexposure but also as a functional additive which enhances the mobility of the charge carriers. This conclusion was deduced from the results of fluorescence quenching, transient optical absorption and photobleaching, and comparison of the TRMC signal with the TOS signal. The combination of the TRMC and TOS techniques represents a comprehensive and fully experimental approach to the determination of the intrinsic carrier mobility in conjugated polymers.

The charge carrier photo generation in fullerene (C60) doped polysilane (PS) is a multistage process including, photo induced electron transfer between polysilane to 3 C 60 ∗ and relaxation of the charge-transfer state which results in polaron pair (PS + and C 60 - ) formation. To study the mobility of charge carriers in polysilane films (PS1-8) doped with fullerene (C60), the flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements have been performed. Second harmonic generation (SHG), laser of wavelength 532 nm has been used as excitation source. The highest ϕ∑μ value (7 × 10−5 cm2/V s) obtained for PS6 among all polysilanes due to its highly ordered structure. The photo carrier generation efficiency of polysilane films have increased due to fullerene doping. The fast decay kinetics of TRMC signal is due to backward electron transfer between PS + and C 60 - where as slow decay can be attributed to charge recombination after diffusion process.

The transient absorption of radical cations of a variety of substituted polysilanes is discussed quantitatively in terms of the molar extinction coefficient and oscillator strength by nanosecond pulse radiolysis. Oxygen-saturated polysilane solutions in benzene exhibit a strong transient absorption band ascribed to the polysilane radical cation. The transient species react with N,N,N',N'-tetramethyl-p-phenylene-diamine (TMPD) to produce TMPD radical cations. On the basis of the molar extinction coefficient of the TMPD radical cation, the molar extinction coefficients for the radical cations of polysilanes are found to increase in the range 3.3 x 10(4) to 2.0 x 10(5) M(-)(1) cm(-)(1) with increasing polymer segment length. The stepwise increase in the total oscillator strength with an increase in the number of phenyl rings directly bonded to the Si skeleton suggests the delocalization of the positive polaron state and/or the SOMO state over the phenyl rings, indicating the importance of phenyl rings in intermolecular hole transfer processes.

The impacts of solvent and thermal annealing on the mobility and photogeneration efficiency of free charge carriers in regioregular poly(3-hexylthiophene) were investigated by flash-photolysis time...

π-Extension of tetrathiafulvalene by hybridization with an indacenetetraone leads to an electron acceptor that can harvest visible light efficiently and serve as an electron-transport material for ...

Self-condensation of the amphiphilic fluorene trimer occurs in the solvent mixture, giving pure “green” fluorescent nanoparticles with controlled radii of 20−100 nm. The origin of the green fluorescence is attributed to the excimer emission via excitation of aggregated dimer structures in the nanoparticles rather than the keto-defects. The present study also suggests the possibility of spectral tuning of emissions from oligofluorene aggregates.

The electronic structure and optical properties of charged oligofluorenes were studied experimentally and theoretically. Measurements of the optical absorption spectra of charged oligofluorenes in dilute solutions have been performed by using the pulse radiolysis technique. In addition, optical absorption spectra of radical cations and anions in a solid matrix were measured after gamma-irradiation at 77 K. The optical absorption spectra were measured in the range of 440-2100 nm (0.6-2.8 eV) and compared with results from time-dependent density functional theory (TDDFT) calculations. The calculated charge induced deformations and charge distribution do not indicate the occurrence of polaronic effects. The potential energy profiles for rotation around the inter-unit bond show that oligofluorenes are nonplanar in their neutral state, while they tend to more planar structures in their charged state. The optical absorption spectra of charged oligofluorenes are dependent on the angle between neighboring units. TDDFT absorption energies shift to lower values with increasing chain length, which suggests that the charge delocalizes along the oligomer chain.

Oligofluorenes with well-defined chain length (n = 1–5) have been synthesized by step-by-step Suzuki coupling reaction as the basis sets of conjugated segments in polyfluorenes. Cation and anion ra...

Herein we report the systematic study of the structure–property relationship of a few dithienothiophene (DTT) based donor–acceptor conjugated polymers using various techniques such as UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry (CV), flash-photolysis time-resolved microwave conductivity (FP-TRMC), density functional theory (DFT), X-ray diffraction (XRD), and field-effect transistor (FET). A medium donor, DTT, was coupled in an alternating fashion with thiazole-based weak, medium, and strong acceptors. Though the optical properties showed good correlation with the donor–acceptor strength, the FET properties indicated significant deviation. The XRD analysis and DFT calculations revealed that the deviation is caused by the difference in structural ordering of the polymers in the film state. Since the FP-TRMC analysis reflects the properties of semiconducting organic materials at the molecular level such as the donor–acceptor strength and structural ordering in the film state, it showed good correlation with FET properties. Thus the present work illustrates that the study of charge carrier generation and mobility dynamics by FP-TRMC is a valuable addition to the conventional structure–property analysis methods, and is reliable to find the suitability of conjugated polymers for electronic device applications.

Direct nanopatterning of polyfluorene derivative is demonstrated using electron-beam lithography. Polyfluorene, which has attracted much attention because of its strong fluorescence and application in organic light emitters, is a negative (crosslinking) type material upon exposure to radiation, and requires a large amount of exposure dose ∼2300μC∕cm2 to be patterned in the present case. This extremely large dose would lead to radiation damage of the polymer. To address this issue, a polyfluorene derivative for acid-catalyzed chemical amplification was synthesized to realize nanopatterning of π-conjugated polymer without degrading its optical property. The synthesized polyfluorene derivative was investigated in terms of sensitivity, optical absorption, and spatial resolution. Sensitivity of 4μC∕cm2 was achieved, and lines of 70 nm width were fabricated after optimization of the process conditions.

The photoconductive properties of a series of oligogermanes with dendrite or linear skeleton have been studied by flash photolysis time-resolved conductivity (FP-TRMC) technique. The values of mobi...