On the Impact of Linear Siloxanated Side Chains on the Molecular Self-Assembling and Charge Transport Properties of Conjugated Polymers

Herein reported is the impact of the functionalization of four different semiconducting polymer structures by a linear siloxane-terminated side-chains. The latter is tetrasiloxane (Si₄) or trisiloxane (Si₃) chains, substituted at their extremity to a pentylene linker. The polymer structure is based on 5,6-difluorobenzothiadiazole comonomer (PF2), a diketopyrrolopyrrole unit (PDPP-TT), a naphtalediimide unit (PNDI-T₂), and a poly[bis(thiophen-2-yl)thieno[3,2,b]thiophene (PBTTT). The properties of these siloxane-functionalized polymers are scrutinized and compared with the ones of their alkyl-substituted polymer analogues. The impact of the alkyl-to-siloxane chain substitution clearly depends on the molecular section of the side chains. When a branched 2-octyldodecyl chain (C₂₀) is replaced by a Si₄ chain of same molecular section, the greatest impact is the strong increase of the π-stacking overlap of the polymer backbones. This effect leads to a significative enhancement of the charge mobility values of the polymers. As in-plane and out-of-plane mobility are increased simultaneously, this π-overlap enhancement effect happens to be preponderant over the polymer orientation variations. When a linear tetradecyl chain (C₁₄) is replaced by a linear Si₃ chain of twice larger molecular section, the polymer structure is profoundly affected. While PBTTT-C₁₄ is crystalline and purely edge-on, PBTTT-Si₃ is mesomorphic and shows a mixed face-on/edge-on orientation.     

Optically Tunable Field Effect Transistors with Conjugated Polymer Entailing Azobenzene Groups in the Side Chains

Semiconducting conjugated polymers with photoswitching behavior are highly demanded for field‐effect transistors (FETs) with tunable electronic properties. Herein a new design strategy is established for photoresponsive conjugated polymers by incorporating photochromic units (azobenzene) into the flexible side alkyl chains. It is shown that azobenzene groups in the side chains of the DPP (diketopyrrolopyrrole)‐quaterthiophene polymer ( PDAZO ) can undergo trans/cis photoisomerization in fully reversible and fast manner. Optically tunable FETs with bistable states are successfully fabricated with thin films of PDAZO . The drain‐source currents are reduced by 80.1% after UV light irradiation for ≈28 s, which are easily restored after further visible light irradiation for ≈33 s. Such fast optically tunable FETs are not reported before. Moreover, such current photomodulation can be implemented for multiple light irradiation cycles with good photofatigue resistance. Additionally, thin film charge mobility of PDAZO can be reversibly modulated by alternating UV and visible light irradiations. On the basis of theoretical calculations and GIWAXS data, it is hypothesized that the dipole moment and configuration changes associated with the trans‐/cis‐photoisomerization of azobenzene groups in PDAZO can affect the respective intra‐chain and inter‐chain charge transporting, which is responsible for the optically tunable behavior for FETs with thin films of PDAZO .     

Enhancing Field‐Effect Mobility of Conjugated Polymers Through Rational Design of Branched Side Chains

The design of polymer semiconductors possessing effective π–π intermolecular interactions coupled with good solution processability remains a challenge. Structure‐property relationships associated with side chain structure, π–π intermolecular interactions, polymer solubility, and charge carrier transport are reported for a donor–acceptor(1)‐donor–acceptor(2) polymer: 5‐Decylheptadecyl (5‐DH), 2‐tetradecyl (2‐DT), and linear n‐octadecyl (OD) chains are substituted onto a polymer backbone consisting of terthiophene units (T) between two different electron acceptors, benzothiadiazole (B), and diketopyrrolopyrrole (D), pTBTD, to afford pTBTD‐5DH, pTBTD‐2DT, and pTBTD‐OD, respectively. In the 5‐DH side chain, the branching position is remote from the polymer backbone, whereas it is proximal in 2‐DT. This study demonstrates that incorporation of branched side chains where the branching position is remote from the polymer backbone merges the advantages of improved solubility from branched units with effective π–π intermolecular interactions normally associated with linear chains on conjugated polymers. pTBTD‐5DH exhibits superior qualities with respect to the degree of polymerization, solution processability, π–π interchain stacking, and charge carrier transport relative to the other analogs. pTBTD‐5DH exhibits a field‐effect hole mobility of up to 2.95 cm² V^–1 s^–1, a factor of 3–7 times that achieved with pBDT6‐DT and pBDT6‐OD.     

Effect of Spacer Length of Siloxane‐Terminated Side Chains on Charge Transport in Isoindigo‐Based Polymer Semiconductor Thin Films

A series of isoindigo‐based conjugated polymers (PII2F‐C_mSi, m = 3–11) with alkyl siloxane‐terminated side chains are prepared, in which the branching point is systematically “moved away” from the conjugated backbone by one carbon atom. To investigate the structure–property relationship, the polymer thin film is subsequently tested in top‐contact field‐effect transistors, and further characterized by both grazing incidence X‐ray diffraction and atomic force microscopy. Hole mobilities over 1 cm² V^?1 s^?1 is exhibited for all soluble PII2F‐C_mSi (m = 5–11) polymers, which is 10 times higher than the reference polymer with same polymer backbone. PII2F‐C₉Si shows the highest mobility of 4.8 cm² V^?1 s^?1, even though PII2F‐C₁₁Si exhibits the smallest π–π stacking distance at 3.379 Å. In specific, when the branching point is at, or beyond, the third carbon atoms, the contribution to charge transport arising from π–π stacking distance shortening becomes less significant. Other factors, such as thin‐film microstructure, crystallinity, domain size, become more important in affecting the resulting device's charge transport.     

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

Here, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 10²² m^?3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.     

Competition between Exceptionally Long‐Range Alkyl Sidechain Ordering and Backbone Ordering in Semiconducting Polymers and Its Impact on Electronic and Optoelectronic Properties

Intra‐ and intermolecular ordering greatly impacts the electronic and optoelectronic properties of semiconducting polymers. The interrelationship between ordering of alkyl sidechains and conjugated backbones has yet to be fully detailed, despite much prior effort. Here, the discovery of a highly ordered alkyl sidechain phase in six representative semiconducting polymers, determined from distinct spectroscopic and diffraction signatures, is reported. The sidechain ordering exhibits unusually large coherence lengths (≥70 nm), induces torsional/twisting backbone disorder, and results in a vertically multilayered nanostructure with ordered sidechain layers alternating with disordered backbone layers. Calorimetry and in situ variable temperature scattering measurements in a model system poly{4‐(5‐(4,8‐bis(3‐butylnonyl)‐6‐methylbenzo[1,2‐b:4,5‐b′]dithiophen‐2‐yl)thiophen‐2‐yl)‐2‐(2‐butyloctyl)‐5,6‐difluoro‐7‐(5‐methylthiophen‐2‐yl)‐2H‐benzo[d][1,2,3]triazole} (PBnDT‐FTAZ) clearly delineate this competition of ordering that prevents simultaneous long‐range order of both moieties. The long‐range sidechain ordering can be exploited as a transient state to fabricate PBnDT‐FTAZ films with an atypical edge‐on texture and 2.5× improved field‐effect transistor mobility. The observed influence of ordering between the moieties implies that improved molecular design can produce synergistic rather than destructive ordering effects. Given the large sidechain coherence lengths observed, such synergistic ordering should greatly improve the coherence length of backbone ordering and thereby improve electronic and optoelectronic properties such as charge transport and exciton diffusion lengths.     

Impact of Polystyrene Oligomer Side Chains on Naphthalene Diimide–Bithiophene Polymers as n‐Type Semiconductors for Organic Field‐Effect Transistors

A series of naphthalene diimide‐based conjugated polymers are prepared with various molar percentage of low molecular weight polystyrene (PS) oligomer of narrow polydispersity as the side chain. The PS side chains are incorporated through preparation of a macromonomer by chain termination of living anionic polymerization. The effects of the PS side chains amount (0–20 mol%) versus overall sidechain on the electrical properties of the resulting polymers as n‐type polymer semiconductors in field‐effect transistors are investigated. We observe that all the studied polymers show similarly high electron mobility (≈0.2 cm² V^?1 s^?1). Importantly, the polymers with high PS side chain content (20 mol%) show a significantly improved device stability under ambient conditions, when compared to the polymers at lower PS content (0–10 mol%). By comparing this observation to the physical blending of the conjugated polymer with PS, we attribute the improved stability to the covalently attached PS side chains potentially serving as a molecular encapsulating layer around the conjugated polymer backbone, rendering it less susceptible to electron traps such as oxygen and water molecules.     

High Electron Mobility and Ambipolar Charge Transport in Binary Blends of Donor and Acceptor Conjugated Polymers

High electron mobility and ambipolar charge transport are observed in phase‐separated binary blends of n‐type poly(benzobisimidazobenzophenanthroline) (BBL) with p‐type polymer semiconductors, poly[(thiophene‐2,5‐diyl)‐alt‐(2,3‐diheptylquinoxaline‐5,8‐diyl)] (PTHQx) and poly(10‐hexylphenoxazine‐3,7‐diyl‐alt‐3‐hexyl‐2,5‐thiophene) (POT). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) show phase‐separated domains of 50–300 nm in the binary blend thin films. The TEM images and electron diffraction of BBL/PTHQx blends show the growth of single‐crystalline phases of PTHQx within the BBL matrix. A relatively high electron mobility (1.0 × 10^–3 cm² V^–1 s^–1) that is constant over a wide blend‐composition range is observed in the PTHQx blend field‐effect transistors (FETs). Ambipolar charge transport is observed in both blend systems at a very high concentration of the p‐type semiconductor (≥90 wt % PTHQx or ≥80 wt % POT). Ambipolar charge transport is exemplified by an electron mobility of 1.4 × 10^–5 cm² V^–1 s^–1 and a hole mobility of 1.0 × 10^–4 cm² V^–1 s^–1 observed in the 98 wt % PTHQx blend FETs. These results show that ambipolar charge transport and the associated carrier mobilities in blends of conjugated polymer semiconductors have a complex dependence on the blend composition and the phase‐separated morphology.     

Precise Control of Lamellar Thickness in Highly Oriented Regioregular Poly(3‐Hexylthiophene) Thin Films Prepared by High‐Temperature Rubbing: Correlations with Optical Properties and Charge Transport

Precise control of orientation and crystallinity is achieved in regioregular poly(3‐hexylthiophene) (P3HT) thin films by using high‐temperature rubbing, a fast and effective alignment method. Rubbing P3HT films at temperatures T_R ≥ 144 °C generates highly oriented crystalline films with a periodic lamellar morphology with a dichroic ratio reaching 25. The crystallinity and the average crystal size along the chain axis direction, l_c, are determined by high‐resolution transmission electron microscopy and differential scanning calorimetry. The inverse of the lamellar period l scales with the supercooling and can accordingly be controlled by the rubbing temperature T_R. Uniquely, the observed exciton coupling in P3HT crystals is correlated to the length of the average planarized chain segments l_c in the crystals. The high alignment and crystallinity observed for T_R > 200 °C cannot translate to high hole mobilities parallel to the rubbing because of the adverse effect of amorphous zones interrupting charge transport between crystalline lamellae. Although tie chains bridge successive P3HT crystals through amorphous zones, their twisted conformation restrains interlamellar charge transport. The evolution of charge transport anisotropy is correlated to the evolution of the dominant contact plane from mainly face‐on (T_R ≤ 100 °C) to edge‐on (T_R ≥ 170 °C).     

Linking Glass-Transition Behavior to Photophysical and Charge Transport Properties of High-Mobility Conjugated Polymers

The measurement of the mechanical properties of conjugated polymers can reveal highly relevant information linking optoelectronic properties to underlying microstructures and the knowledge of the glass transition temperature (T_g) is paramount for informing the choice of processing conditions and for interpreting the thermal stability of devices. In this work, we use dynamical mechanical analysis to determine the T_g of a range of state-of-the-art conjugated polymers with different degrees of crystallinity that are widely studied for applications in organic field-effect transistors. We compare our measured values for T_g to the theoretical value predicted by a recent work based on the concept of effective mobility ζ. The comparison shows that for conjugated polymers with a modest length of the monomer units, the T_g values agree well with theoretically predictions. However, for the near-amorphous, indacenodithiophene–benzothiadiazole family of polymers with more extended backbone units, values for T_g appear to be significantly higher, predicted by theory. However, values for T_g are correlated with the sub-bandgap optical absorption suggesting the possible role of the interchain short contacts within materials’ amorphous domains.     

Highly Efficient Microscopic Charge Transport within Crystalline Domains in a Furan‐Flanked Diketopyrrolopyrrole‐Based Conjugated Copolymer

Elucidating the interrelation between the molecular structure and charge transport properties in conjugated polymer thin films is an essential issue in developing the design principle of high‐performance polymer materials for application in organic electronics. In particular, the backbone planarity is suggested to be a key element that governs the transport performance, especially in recently developed donor–acceptor (D–A)‐type copolymers exhibiting high mobility, whereas the direct evaluation of the intrinsic transport performance, usually realized only within the small crystalline domains, is difficult by using conventional macroscopic measurements. Here, it is demonstrated that a D–A type copolymer, PDPPF‐DTT, which consists of furan‐flanked diketopyrrolopyrrole (DPP) and dithienothiophene (DTT) units in the conjugated backbone, exhibits a highly efficient charge transport performance within the crystalline domains with a remarkably low activation energy of less than 8 meV, based on microscopic measurements using field‐induced electron spin resonance spectroscopy. This high transport performance is primarily caused by the high backbone planarity realized by introducing furan‐flanked DPP and fused dithienothiophene units, which is demonstrated from the density functional theory calculations. This result provides a microscopic indication of the effectiveness of the present molecular design to produce a planar backbone and realize highly efficient charge transport performance.     

The Effects of Crystallinity on Charge Transport and the Structure of Sequentially Processed F4TCNQ‐Doped Conjugated Polymer Films

The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) into poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X‐ray data suggest that F₄TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π‐stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F₄TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm² V^?1 s^?1 in the most crystalline P3HT samples. Temperature‐dependent conductivity measurements show that polaron mobility in SqP‐doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature.     

Double Doping of Semiconducting Polymers Using Ion-Exchange with a Dianion

The interactions between counterions and electronic carriers in electrically doped semiconducting polymers are important for delocalization of charge carriers, electronic conductivity, and thermal stability. The introduction of a dianions in semiconducting polymers leads to double doping where there is one counterion for two charge carriers. Double doping minimizes structural distortions, but changes the electrostatic interactions between the carriers and counterions. Polymeric ionic liquids (PIL) with croconate dianions are helpful to investigate the role of the counterion in p-type semiconducting polymers. PILs prevent diffusion of the cation into the semiconducting polymers during ion exchange. The redox-active croconate dianions undergo ion exchange with doped semiconducting polymers depending on their ionization energy. Croconate dianions are found to reduce doped films of poly(3-hexyl thiophene), but undergo ion exchange with a polythiophene with tetraethylene glycol side chains, P(g₄2T-T), that has a lower ionization energy. The croconate dianion maintains crystalline order in P(g₄2T-T) and leads to a lower activation energy for the electrical conductivity than PF₆⁻ counterions. The control of the doping level with croconate allows optimization of the thermoelectric performance of the semiconducting polymer. The thermal stability of the doped films of P(g₄2T-T) is found to depend strongly on the nature of the counterion.     

Nature of Charge Carriers in a High Electron Mobility Naphthalenediimide Based Semiconducting Copolymer

The nature of charge carriers in recently developed high mobility semiconducting donor‐acceptor polymers is debated. Here, localization due to charge relaxation is investigated in a prototypal system, a good electron transporting naphthalenediimide based copolymer, by means of current‐voltage I‐V electrical characteristics and charge modulation spectroscopy (CMS) in top‐gate field‐effect transistors (FETs), combined with density functional theory (DFT) and time dependent DFT (TDDFT) calculations. In particular, pristine copolymer films are compared with films that underwent a melt‐annealing process, the latter leading to a drastic change of the microstructure. Despite the packing modification, which involves also the channel region, both the electron mobility and the energy of polaronic transitions are substantially unchanged upon melt‐annealing. The polaron absorption features can be rationalized and reproduced by TDDFT calculations for isolated charged oligomers. Therefore, it is concluded that in such a high electron mobility copolymer the charge transport process involves polaronic species which are intramolecular in nature and, from a more general point of view, that interchain delocalization of the polaron is not necessary to sustain charge mobilities in the 0.1 to 1 cm² V^{– 1} s^–1 range. These findings contribute to the rationalization of the charge transport process in the recently developed class of donor‐acceptor π‐conjugated copolymers featuring high charge mobilities and complex morphologies.     

Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors

Polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. The recent development of semicrystalline polymers with strongly improved charge transport properties forces a review of the current understanding of the charge transport mechanisms and how they relate to the polymer's chemical and structural properties. Here, the charge density dependence of field effect mobility in semicrystalline polymer semiconductors is studied. A simultaneous increase in mobility and its charge density dependence, directly correlated to the increase in average crystallite size of the polymer film, is observed. Further evidence from charge accumulation spectroscopy shows that charges accumulate in the crystalline regions of the polymer film and that the increase in crystallite size affects the average electronic orbitals delocalization. These results clearly point to an effect that is not caused by energetic disorder. It is instead shown that the inclusion of short range coulomb repulsion between charge carriers on nanoscale crystalline domains allows describing the observed mobility dependence in agreement with the structural and optical characterization. The conclusions that are extracted extend beyond pure transistor characterization and can provide new insights into charge carrier transport for regimes and timescales that are relevant to other optoelectronic devices.     

Band-Like Charge Transport in Phytic Acid-Doped Polyaniline Thin Films

We explore the charge transport properties of phytic acid (PA) doped polyaniline thin films prepared by the surfactant monolayer-assisted interfacial synthesis (SMAIS). Structural and elemental analysis confirms the inclusion of PA in the thin films and reveals a progressive loss of crystallinity with the increase of PA doping content. Charge transport properties are interrogated by time-resolved terahertz (THz) spectroscopy. Notably, independently of doping content and hence crystallinity, the frequency-resolved complex conductivity spectra in the THz region can be properly described by the Drude model, demonstrating band-like charge transport in the samples and state-of-the-art charge carrier mobilities of ≈1 cm²V⁻¹s⁻¹. A temperature-dependent analysis for the conductivity further supports band-like charge transport and suggest that charge carrier mobility is primarily limited by impurity scattering. This work highlights the potential of PA doped polyaniline for organic electronics.     

Enhanced Vertical Charge Transport in a Semiconducting P3HT Thin Film on Single Layer Graphene

The crystallization and electrical characterization of the semiconducting polymer poly(3‐hexylthiophene) (P3HT) on a single layer graphene sheet is reported. Grazing incidence X‐ray diffraction revealed that P3HT crystallizes with a mixture of face‐on and edge‐on lamellar orientations on graphene compared to mainly edge‐on on a silicon substrate. Moreover, whereas ultrathin (10 nm) P3HT films form well oriented face‐on and edge‐on lamellae, thicker (50 nm) films form a mosaic of lamellae oriented at different angles from the graphene substrate. This mosaic of crystallites with π–π stacking oriented homogeneously at various angles inside the film favors the creation of a continuous pathway of interconnected crystallites, and results in a strong enhancement in vertical charge transport and charge carrier mobility in the thicker P3HT film. These results provide a better understanding of polythiophene crystallization on graphene, and should help the design of more efficient graphene based organic devices by control of the crystallinity of the semiconducting film.     

Bipolar Charge Transport in PCPDTBT‐PCBM Bulk‐Heterojunctions for Photovoltaic Applications

An experimental study of the transport properties of a low‐bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (E_g‐opt ～ 1.35 eV) is presented. Using a organic thin film transistor geometry, we demonstrate a relatively high in‐plane hole mobility, up to 1.5 · × 10^?2 cm² V^?1 s^?1 and quantify the electron mobility at 3 × · 10^?5 cm² V^?1 s^?1 on a SiO₂ dielectric. In addition, singular contact behavior results in bipolar quasi‐Ohmic injection both from low and high workfunction metals like LiF/Al and Au. X‐ray investigations revealed a degree of interchain π‐stacking that is probably embedded in a disordered matrix. Disorder also manifests itself in a strong positive field dependence of the hole mobility from the electric field. In blends made with the electron acceptor methanofullerene [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM), the transistor characteristics suggest a relatively unfavorable intermixing of the two components for the application to photovoltaic devices. We attribute this to a too fine dispersion of [C60]‐PCBM in the polymer matrix, that is also confirmed by the quenching of the photoluminescence signal measured in PCPDTBT [C60]‐PCBM films with various composition. We show that a higher degree of phase separation can be induced during the film formation by using 1,8‐octanedithiol (ODT), which leads to a more efficient electron percolation in the [C60]‐PCBM. In addition, the experimental results, in combination with those of solar cells seem to support the correlation between the blend morphology and charge recombination. We tentatively propose that the drift length, and similarly the electrical fill factor, can be limited by the recombination of holes with electrons trapped on isolated [C60]‐PCBM clusters. Ionized and isolated [C60]‐PCBM molecules can modify the local electric field in the solar cell by build‐up of a space‐charge. The results also suggest that further improvements of the fill factor may also be limited by a strong electrical‐field dependence of the hole transport.     

Temperature‐Dependent Electrical Transport in Polymer‐Sorted Semiconducting Carbon Nanotube Networks

The temperature dependence of the electrical characteristics of field‐effect transistors (FETs) based on polymer‐sorted, large‐diameter semiconducting carbon nanotube networks is investigated. The temperature dependences of both the carrier mobility and the source‐drain current in the range of 78 K to 293 K indicate thermally activated, but non‐Arrhenius, charge transport. The hysteresis in the transfer characteristics of FETs shows a simultaneous reduction with decreasing temperature. The hysteresis appears to stem from screening of charges that are transferred from the carbon nanotubes to traps at the surface of the gate dielectric. The temperature dependence of sheet resistance of the carbon nanotube networks, extracted from FET characteristics at constant carrier concentration, specifies fluctuation‐induced tunneling as the mechanism responsible for charge transport, with an activation energy that is dependent on film thickness. Our study indicates inter‐tube tunneling to be the bottleneck and implicates the role of the polymer coating in influencing charge transport in polymer‐sorted carbon nanotube networks.