Regioregular Research Articles

Using Bulky Dodecaborane-Based Dopants to Produce Mobile Charge Carriers in Amorphous Semiconducting Polymers.

Conjugated polymers are a versatile class of electronic materials featured in a variety of next-generation electronic devices. The utility of such polymers is contingent in large part on their electrical conductivity, which depends both on the density of charge carriers (polarons) and on the carrier mobility. Carrier mobility, in turn, is largely controlled by the separation between the polarons and dopant counterions, as counterions can produce Coulombic traps. In previous work, we showed that large dopants based on dodecaborane (DDB) clusters were able to reduce Coulombic binding and thus increase carrier mobility in regioregular (RR) poly(3-hexylthiophene-2,5-diyl) (P3HT). Here, we use a DDB-based dopant to study the effects of polaron-counterion separation in chemically doped regiorandom (RRa) P3HT, which is highly amorphous. X-ray scattering shows that the DDB dopants, despite their large size, can partially order the RRa P3HT during doping and produce a doped polymer crystal structure similar to that of DDB-doped RR P3HT; Alternating Field (AC) Hall measurements also confirm a similar hole mobility. We also show that use of the large DDB dopants successfully reduces Coulombic binding of polarons and counterions in amorphous polymer regions, resulting in a 77% doping efficiency in RRa P3HT films. The DDB dopants are able to produce RRa P3HT films with a 4.92 S/cm conductivity, a value that is ∼200× higher than that achieved with 3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the traditional dopant molecule. These results show that tailoring dopants to produce mobile carriers in both the amorphous and semicrystalline regions of conjugated polymers is an effective strategy for increasing achievable polymer conductivities, particularly in low-cost polymers with random regiochemistry. The results also emphasize the importance of dopant size and shape for producing Coulombically unbound, mobile polarons capable of electrical conduction in less-ordered materials.

Influence of thermal annealing on microstructure, energetic landscape and device performance of P3HT:PCBM-based organic solar cells

Thermal annealing alters the morphology of organic donor-acceptor bulk-heterojunction thin films used in organic solar cells. Here, we studied the influence of thermal annealing on blends of amorphous regio-random (RRa) and semi-crystalline regio-regular (RR) poly (3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester. Since the P3HT:PCBM blend is one of the most studied in the OPV community, the existing research provides a solid foundation for us to compare and benchmark our innovative characterization techniques that have been previously under-utilized to investigate bulk heterojunction organic thin films. Here, we combine advanced novel microscopies and spectroscopies, including polarized light microscopy, photo-deflection spectroscopy, hyperspectral photoluminescence imaging, and energy resolved-electrochemical impedance spectroscopy, with structural characterization techniques, including grazing-incidence wide-angle x-ray scattering, grazing-incidence x-ray diffraction, and Raman spectroscopy, in order to reveal the impact of thermal annealing on the microstructural crystallinity and morphology of the photoactive layer in organic solar cells. Coupled transfer matrix and drift-diffusion simulations were used to study the impact of the density of states on the solar cells’ device performance parameters, namely the short-circuit current (J SC), open circuit voltage (V OC), fill factor (FF), and power conversion efficiency (PCE).

Study on the Diffusion Rate of the Charge Carrier Transport in Regio-Random and Regio-Regular P3HT

Muon spin relaxation experiment has been conducted to probe the hopping mechanism in the poly(3-hexylthiophene-2,5-diyl) (P3HT) for both types of regio-random (Rdm) and regio-regular (RR). In this study we have performed calculations over the collected data to obtain the parallel and perpendicular diffusion rates, at temperatures of 10 K and 300 K. The calculation is based on the fitting method to the empirical function that relates the relaxation rate with the diffusion rates. For Rdm-P3HT, we have obtained the parallel diffusion rate to be 5.43 x 1013 rad/s at 300 K and 4.90 x 1014 rad/s at 10 K. While the perpendicular diffusion rates are 5.29 x 108 rad/s at 300 K and 1.88 x 106 rad/s at 10 K. For RR-P3HT, we have obtained the parallel diffusion rate to be 1.04 x 1014 rad/s at 300 K and 1.28 x 1015 rad/s at 10 K. While the perpendicular diffusion rates are 6.10 x 108 rad/s at 300 K and 5.35 x 105 at 10 K. The diffusion rates of RR-P3HT are higher than that of Rdm-P3HT, especially in the parallel direction. In both types of material, the parallel diffusion rate decreased with temperature, while the perpendicular diffusion rate increased with temperature, showing a change of behavior from 1D to 3D direction of charge transport.

Spontaneous Phase Separation of Poly(3‐hexylthiophene)s with Different Regioregularity for a Stretchable Semiconducting Film

AbstractHighly regioregular (RR) poly(3‐hexylthiophene)s PHTs are known to exhibit excellent electrical properties in comparison to chemically identical but regiorandom (rr) PHTs. In this study, distinct RR (97% and 55%)‐graded PHTs are subjected to solution blending to spontaneously separate the high‐RR PHT chains from the low‐RR PHT media and develop highly conjugated nanodomains in both solution and film. In the spun‐cast blend films, the rr PHT matrix imparts sufficient deformability of the channel layer required for stretchable organic thin‐film transistors (OTFTs), compared to neat RR PHTs and blends with a deformable polymer. OTFTs including RR PHT/rr PHT blend films show excellent hole mobility (µ) values up to 0.13 cm2 V−1 s−1, surpassing that of the best RR PHT films (0.026 cm2 V−1 s−1) fabricated by ultrasound solution pretreatment. Furthermore, a 50% stretched RR PHT/rr PHT film maintains ≈55% of its µ value at no strain, while RR PHT films show a sudden decrease in µ even at 10% stretch. The simple blending approach imparts deformability to π‐conjugated polymer films for application in stretchable OTFTs.

Effect of Backbone Regiochemistry on Conductivity, Charge Density, and Polaron Structure of n-Doped Donor-Acceptor Polymers.

We investigated the influence of backbone regiochemistry on the conductivity, charge density, and polaron structure in the widely studied n-doped donor–acceptor polymer poly[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) [P(NDI2OD-T2)]. In contrast to classic semicrystalline polymers such as poly(3-hexylthiophene) (P3HT), the regioirregular (RI) structure of the naphthalenediimide (NDI)-bithiophene (T2) backbone does not alter the intramolecular steric demand of the chain versus the regioregular (RR) polymer, yielding RI-P(NDI2OD-T2) with similar energetics and optical features as its RR counterpart. By combining the electrical, UV–vis/infrared, X-ray diffraction, and electron paramagnetic resonance data and density functional theory calculations, we quantitatively characterized the conductivity, aggregation, crystallinity, and charge density, and simulated the polaron structures, molecular vibrations, and spin density distribution of RR-/RI-P(NDI2OD-T2). Importantly, we observed that RI-P(NDI2OD-T2) can be doped to a greater extent compared to its RR counterpart. This finding is remarkable and contrasts benchmark P3HT, allowing us to uniquely study the role of regiochemistry on the charge-transport properties of n-doped donor–acceptor polymers.

Excited State Behavior of Single Strand and Bulk P3HT in Contact with a Au-Nanowire Array

Pump–probe transient absorption and single molecule fluorescence microscopy reveal multifaceted excited state behavior of regioregular (RR) P3HT interacting with a hexagonal array of Au nanowires. An average 5-fold increase of fluorescence intensity has been detected for single strands of P3HT in the presence of the nanowire array, with the strongest emitters exceeding the brightness of their counterparts on glass by a factor of 40. The single molecule trajectories show that the same polymer strand is capable of exhibiting a range of enhancement levels. The “off” periods on the Au substrate are exceptionally long and erratic. This behavior is attributed to sporadic electron transfer events between the strand and the metal. Bulk P3HT deposited on the same Au-nanowire array shows a dramatic reduction of the yield and lifetime of singlet excitons in comparison with the glass support, indicating that the proximity of the metal greatly accelerates both exciton dissociation and charge recombination. Interesting...

The molecular regioregularity induced morphological evolution of polymer blend thin films

The polymer regioregularity has a dramatic effect on device performance of organic photovoltaics in terms of material crystallinity and phase separated morphology. In this study, we investigated the molecular regioregularity induced morphological evolution of P3HT and PF12TBT blend thin films processed by solvents with different solubility and boiling point. After cast by good solvents chloroform (CF) and o-dichlorobenzene (oDCB), the regioregular (RR) and regiorandom (RA)-P3HT/PF12TBT blend films showed the similar phase separated morphology, namely fast solidification induced unsharp morphology for CF blend films and long-range demixing via spinodal decomposition within extending drying process for oDCB blend films, while the domain size for RA blend films were larger than those of RR blend films which could be attributed to easily disentanglement and diffusion of RA-P3HT molecules facilitated by smaller aggregation size in RA blend solutions. In addition, the morphology of RR blend films exhibited micro change while the RA blend films developed into long-range phase separation after thermal annealing for CF blend films which also confirmed the molecular diffusion induced morphological evolution. For marginal solvents p-xylene (XY), the RR P3HT showed obvious molecular aggregation in XY solution due to the intensive solvent–polymer interaction (Δδ) and high regioregularity and therefore the value of aggregation size (Ra) for RR-P3HT is as three times as that of RA-P3HT. After cast from XY solution, the RA blend films displayed droplet and bicontinuous morphology based on various blend ratios via spinodal decomposition. Meanwhile, the nanofiber morphology could be observed when the weight fraction of RR-P3HT beyond 10% in RR blend films, which resulting from the continuous growth of P3HT aggregations formed anterior of the phase separation in the film solidification process.

Synergistic Effect of Regioregular and Regiorandom Poly(3‐hexylthiophene) Blends for High Performance Flexible Organic Field Effect Transistors

Decreased conjugated polymer regioregularity can provide for mechanical flexibility, which is favorable for the fabrication of flexible electronics; however, concomitant decreased thin‐film crystallinity severely deteriorates charge transport performance. Here, the advantage of this inverse relationship has been taken to fabricate flexible organic field effect transistors using ultraviolet irradiated regioregular (RR) and regiorandom (RRa) poly(3‐hexylthiophene) (P3HT) blend thin‐films as the device active layer. The blend films exhibit prominent charge carrier mobility up to 0.22 cm2 V‐1 s‐1. Even when the RR‐P3HT component comprises only a small proportion of the blend, vertical phase separation of the components ensures formation of effective charge transport pathways. The underlying mechanism is correlated to molecular self‐assembly of RR‐P3HT and its unfavorable intermolecular interactions with RRa‐P3HT. Transistor performance is retained upon application of high external strain and bending 1000 times. The top‐gate architecture enables the dielectric to serve as a self‐encapsulation layer to protect the underlying semiconductor from atmospheric oxygen and moisture, resulting in over 700 h ambient device stability. This facile blend approach is expected to be an alternative strategy for the fabrication of high performance flexible electronics.

Effect of Backbone Regioregularity on the Structure and Orientation of a Donor–Acceptor Semiconducting Copolymer

A regioregular (RR) donor–acceptor conjugated copolymer based on cyclopenta[2,1-b:3,4-b′]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units was prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the asymmetric PT heterocycle along the copolymer backbone. Compared to its regioirregular (RI) counterpart, the RR polymer exhibits a 2 orders of magnitude increase in hole mobility from 0.005 to 0.6 cm2 V–1 s–1. To probe the reason for this difference in mobility, we examined the crystalline structure and its orientation in thin films of both copolymers as a function of depth via grazing incidence wide-angle X-ray scattering (GIWAXS). In the RI film, the π–π stacking direction of the crystallites is mainly perpendicular to the substrate normal (edge-on orientation) while in the RR film the crystallites adopt a mixed π–π stacking orientation in the center of the film as well as near the interface between the polymer and the dielectric layer. These results demonstrate that control of backbone regularity is another important design criterion to consider in the synthesis and optimization of new conjugated copolymers with asymmetric structural units.

Spectroelectrochemical characterization of anionic and cationic polarons in poly(3-hexylthiophene)/fullerene blend. Effects of morphology and interface

To investigate the effect of film morphology on the electronic structures of anionic and cationic polarons of poly(3-hexylthiophene), P3HT•− and P3HT•+, we have characterized the subgap optical absorption of these polaronic species by means of spectroelectrochemistry. For the pristine films of regiorandom (RRa) P3HT or its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), both the ground-state bleaching and the subgap absorption of RRa-P3HT•− resemble closely to those of RRa-P3HT•+. For the solvent vapor annealed neat film of regioregular (RR) P3HT, RR-P3HT•− shows distinctly different absorption spectra from RR-P3HT•+, and the difference is substantially enhanced by mixing up with PC61BM. The spectral differences are ascribed to the effects of the boundaries among the polymer phases and the polymer–fullerene interfaces. With respect to the neat RR-P3HT film, the RR-P3HT/PC61BM blend exhibits a 0.05–0.06eV downshift of the SOMO levels for both types of polarons, implying the anisotropic hole and electron conductivities of crystalline RR-P3HT phases, which may play important roles in the transport and photogeneration of free charge carriers.

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

Despite the central role of light absorption and the subsequent generation of free charge carriers in organic and hybrid organic-inorganic photovoltaics, the precise process of this initial photoconversion is still debated. We employ a novel broadband (UV-Vis-NIR) transient absorption spectroscopy setup to probe charge generation and recombination in the thin films of the recently suggested hybrid material combination poly(3-hexylthiophene)/silicon (P3HT/Si) with 40 fs time resolution. Our approach allows for monitoring the time evolution of the relevant transient species under various excitation intensities and excitation wavelengths. Both in regioregular (RR) and regiorandom (RRa) P3HT, we observe an instant (<40 fs) creation of singlet excitons, which subsequently dissociate to form polarons in 140 fs. The quantum yield of polaron formation through dissociation of delocalized excitons is significantly enhanced by adding Si as an electron acceptor, revealing ultrafast electron transfer from P3HT to Si. P3HT/Si films with aggregated RR-P3HT are found to provide free charge carriers in planar as well as in bulk heterojunctions, and losses are due to nongeminate recombination. In contrast for RRa-P3HT/Si, geminate recombination of bound carriers is observed as the dominant loss mechanism. Site-selective excitation by variation of pump wavelength uncovers an energy transfer from P3HT coils to aggregates with a 1/e transfer time of 3 ps and reveals a factor of 2 more efficient polaron formation using aggregated RR-P3HT compared to disordered RRa-P3HT. Therefore, we find that polymer structural order rather than excess energy is the key criterion for free charge generation in hybrid P3HT/Si solar cells.

Morphology dependent exciton formation in regioregular poly(3‐alkyl)thiophenes

AbstractThe photophysics of the regioregular (RR) poly(3‐alkylthiophene) (P3AT) films is investigated by cw‐photoinduced absorption (PIA) measurements based on bleaching signals. The photoexcitation intensity dependence of the cw‐PIA signals demonstrates that both photogenerated localized and delocalized polarons obey a bimolecular recombination process under cw photoexcitation. It is demonstrated that the lineshape of cw‐PIA bleaching spectra for the P3AT films varies with the length of the alkyl substituents of P3AT. The bleaching spectra reveal a trend that the electron‐phonon couplings of final exciton migration sites become strong with increasing the interlamella distance in the P3AT film. This indicates that interlamella couplings affect the electron‐phonon couplings of the migration sites locating at two‐dimensional lamellas. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Poly(3-dodedyl-2,5-thienylenevinylene)s from the Stille coupling and the Horner–Emmons reaction

Regioregular (RR) conjugated polymers are critical for electronic and optoelectronic properties of polymer based semiconducting devices. Monosubstituted polythienylvinylene (PTV), a relatively low band-gap conjugated polymer, has been reportedly synthesized using the Stille coupling reaction between 3-dodecyl-2,5-dibromothiophene and (E)-1,2-bis(tributylstannyl)ethylene. Although a small fraction of the product shows good (∼90%) regioregularity, the whole product (S-C12-PTV) consists of mostly unidentified structures. To gain better control of the polymer structure and regioregularity, a difunctionalized C12-thiophene monomer, 3-dodecyl-5-formyl-thiophen-2-ylmethyl)-phosphonic acid diethyl ester, has been polymerized in near quantitative yield via the Horner–Emmons reaction. The full (100%) regioregularity of the resulting polymer (HE-C12-PTV) has been confirmed by its 1H and 13C NMR spectra. The full regioregularity is also reflected in its strong tendency to crystallize and practically no solubility in boiling hexane, in sharp contrast to S-C12-PTV. UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltammetry, thermal analysis (DSC & TGA) and X-ray diffraction have been used to characterize both polymers. UV-vis absorption spectra of the HE-C12-PTV chloroform solutions of different concentrations show well-resolved vibronic structures with an absorption maximum at 577 nm and a prominent shoulder at 614 nm. The optical bandgaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO energy levels of the HE-C12-PTV film were found to be at −4.98 eV and −2.88 eV, respectively. The electrochemical bandgap of the polymer in the film is estimated to be 2.1 eV. The DSC curve shows a pronounced melting peak at 205 °C. XRD study shows that a decent crystalline structure is formed without any annealing of the as-cast films.

Ultrafast coherent vibronic oscillations in regioregular poly(3‐alkylthiophene)

AbstractUltrafast degenerate four‐wave mixing (DFWM) signals of the regioregular (RR) poly(3‐hexylthiophene) (P3HT) film have been investigated by the experiments using sub‐20 fs pulses generated from a noncollinear optical parametric amplifier (NOPA) system. Strong DFWM signals were observed owing to a large third‐order nonlinear susceptibility χ(3) of the RR‐P3HT film. The time profile of the DFWM signals exhibits clear coherent oscillation on a decaying signal. The oscillation turn out to be caused by the C=C stretching mode that gives strong Raman signal. The time profile of the DFWM signal is simulated by a numerical calculation. The result of simulation reveals that decay constants of electronic population and of vibronic oscillation are 200 fs and 210 fs, respectively. This is the first determination of the decay constants for the RR‐P3HT film. The obtained result demonstrates that the χ(3) signal of the RR‐P3HT film decays very rapidly. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Polaron Dynamics of Heavily Doped Regioregular and Regiorandom Poly(3-alkylthiophenes) Revealed by Electron Spin Resonance Spectroscopy

Electron spin resonance (ESR) features in heavily doped conjugated polymers are investigated through the comparison of temperature dependences of ESR spectra between head-to-tail coupled regioregular (RR) and regiorandom (RRa) poly(3-octylthiophenes) (P3OTs). RR-P3OT, used as a model of having crystalline grains in the solid film, is found to exhibit anisotropic ESR spectra, whereas RRa-P3OT gives almost isotropic ESR spectra similar to those of usual heavily doped conjugated polymers. This difference in the degree of spectral anisotropy primarily arises from a difference in their film morphology. Spectral simulations show the anisotropy observed in RR-P3OT to be caused by g-anisotropy. The presence of the g-anisotropy in RR-P3OT indicates that its polarons spend most of the time within a single crystalline grain that has some domains with a common direction of the g-tensor. The g-anisotropy turns out to decrease with increasing temperature. This result is explained by thermally activated hopping motions between crystalline grains. We emphasize that the decrease in the g-anisotropy with temperature should be associated with its activated type of temperature dependence of conductivity. In RRa-P3OT, its isotropic ESR spectra are suggested to be caused by the interchain motion as well as the intrachain one.

Fabrication of Oriented and Periodic Hybrid Nanostructures of Regioregular Poly(3‐hexylthiophene) and CdSe Nanocrystals by Directional Epitaxial Solidification

Hybrid nanostructures of regioregular (RR) P3HT and colloidal CdSe nanocrystals (spheres and rods, see figure) have been prepared by directional epitaxial solidification. The hybrid nanostructured films involve the periodic alternation of crystalline P3HT lamellae separated by amorphous interlamellar zones harboring the confined CdSe nanocrystals. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2007/c0311_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Intrachain photoluminescence properties of conjugated polymers as revealed by long oligothiophenes and polythiophenes diluted in an inactive solid matrix

The intrinsic intrachain photoluminescence (PL) dynamics of conjugated polymers in the solid state is investigated. We focus on the PL properties of long $\ensuremath{\beta}$-substituted oligothiophenes (8-mer, 12-mer, and 16-mer) and regio-regular (RR) and regio-random poly(3-octylthiophenes) (P3OTs) diluted in the inactive solid matrix polypropylene (PP). The oligothiophenes have well-resolved 0-0 PL and absorption peaks at $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and show a good linear relationship with the reciprocal of the ring number. Franck-Condon analyses on the PL spectra reveal that the strength of the electron-phonon coupling represented by the Huang-Rhys factor becomes weak with increasing chain length in the oligomers. In contrast, the presence of stronger electron-phonon coupling is confirmed in RR P3OT despite it being of a much longer chain length than oligomers. This is probably due to its stereoregular chain conformation. The materials used in this work diluted in PP exhibit single-exponential PL decays with lifetimes that decrease with increases in the chain length. The presence of intrachain exciton migration is shown by the time-resolved PL measurements, except for the case of RR P3OT. The energy shifts attributed to intrachain migration are much smaller than those ascribed to interchain migration. We conclude that a change in the torsion angle around the thiophene ring bonds occurs within $50\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ following the absorption transition in thiophene derivatives and that the torsional change gives rise to a constant Stokes shift $(60--70\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$, regardless of the chain length. We propose that the Stokes shift can be utilized for estimating the magnitude of exciton migration in thiophene derivatives, even in the case of undiluted films.