High-mobility Polymer Research Articles

Marked Consequences of Systematic Oligothiophene Catenation in Thieno[3,4-c]pyrrole-4,6-dione and Bithiopheneimide Photovoltaic Copolymers.

As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress charge recombination. Here we investigate two series of in-chain donor-acceptor copolymers, PTPDnT and PBTInT, based on thieno[3,4-c]pyrrole-4,6-dione (TPD) or bithiopheneimide (BTI) as electron acceptor units, respectively, and oligothiophenes (nTs) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar TPD polymer backbones, however backbone torsion yields greater open-circuit voltages for BTI polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that PTPDnT and PBTInT conjugation lengths scale with nT catenation up to n = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The 3T with "outward" alkylation in PTPD3T and PBTI3T affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm(2)/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for PTPD3T-based PSCs with PCE = 7.7%. The results demonstrate that 3T is the optimal donor unit among nTs (n = 1-4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based PTPD3T blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance.

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.

High-mobility polymeric semiconductors

As an important part of organic field-effect transistors (OFETs), organic semiconductor plays an important role on the device performance. Compared with organic small semiconductors, polymeric semiconductors possess better solution processability and room-temperature-manufacturing ability, which make them be widely investigated. Since 1970s, significant advances have been achieved both on the development of high-mobility polymer semiconductors and construction of high-performance polymer FETs (PFETs). To date, the field-effect mobility of PFETs has been significantly increased from the initial value of 10 –5 cm 2 V –1 s –1 to the state-of-art value of 36.3 cm 2 V –1 s –1 due to the both advances of developing high-performing polymeric semiconductors and optimizing device fabrication. Therefore, there have accumulated rich experience on molecular design for high-mobility polymer semiconductors and new insights of understanding charge transport physics have also been gained from the experiment. With molecular design as the entry point, this review systematically summarizes the recent progresses on polymeric semiconductors, including p-type, n-type and ambipolar semiconductors, then try to briefly analyze the nature of charge transport in polymeric semiconductors and the corresponding optimization processes for high performance PFETs, hopefully providing valuable guideline for the further scientific works in this field.

Utilization of a Non-Ionic Surfactant in the Fabrication of Water-Borne Polymeric Semiconductor Nanoparticles for High-Performance, Green Organic Electronics

We report a novel method for synthesizing water-borne nanoparticles of semiconducting polymers for use in environmentally benign processes involving organic electronics, without compromising the high charge carrier mobility of the polymeric semiconductors. Non-ionic surfactants were utilized as a key material to fabricate aqueous nanoparticles of semiconducting polymers via a miniemulsion process for the first time. To maximize the charge transport between polymer nanoparticles, the surface adsorption density and polarity of the non-ionic surfactant were carefully controlled. By introducing such non-ionic surfactants onto a donor-acceptor type polymeric semiconductor with an inherently high charge carrier mobility, we could realize a high-mobility (>0.5 cm2/Vs) water-borne polymer field effect transistor. Structural analyses based on X-ray diffraction showed that films consisting of polymer nanoparticles still maintained some degree of edge-on crystalline orientation, which was enough to render high charge carrier mobility.

Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors.

Since the report of the first diketopyrrolopyrrole (DPP)-based polymer semiconductor, such polymers have received considerable attention as a promising candidate for high-performance polymer semiconductors in organic thin-film transistors (OTFTs). This Progress Report summarizes the advances in the molecular design of high-mobility DPP-based polymers reported in the last few years, especially focusing on the molecular design of these polymers in respect of tuning the backbone and side chains, and discussing the influences of structural modification of the backbone and side chains on the properties and device performance of corresponding DPP-based polymers. This provides insights for the development of new and high-mobility polymer semiconductors.

(Invited) Development of High Mobility Polymer Semiconductors for p-Channel, n-Channel, and Ambipolar Thin Film Transistors

Polymer semiconductors are enabling materials for a wide range of printed electronics. As channel semiconductors for organic thin film transistors (OTFTs), charge carrier mobility on par with that of amorphous silicon (~0.5 cm2V-1s-1) or higher is required for many applications. The development of polymer semiconductors with high mobility and good ambient stability has been a great challenge. To improve the charge carrier transport, a combination of donor and acceptor units in an alternating fashion in the polymer backbone has been recently found very effective since the intermolecular donor-acceptor attraction can shorten the interchain distance. By utilizing this strategy, we have developed a number of donor-acceptor polymers semiconductors with very high mobility. One particular class of such polymers are based on a diketopyrrolopyrrole (DPP) acceptor building block. By carefully choosing the electron donor blocks, we have successfully developed p-type polymers with high mobility up to 6.9 cm2V-1s-1 and amibipolar polymers with balanced electron and hole mobility of 6.30 cm2V-1s-1 and 2.78 cm2V-1s-1, respectively. By using a ultra-thin layer (~2-5 nm) of polyethylenimine (PEI) to lower the work function of the source and drain contacts, we were able to block the hole injection and successfully transform ambipolar and even p-type transistors unipolar n-type transistors. This general approach to converting the transistor polarity broadens the scope of available polymer semiconductors for constructing n-channel transistors. References Li, Y.; Singh, S. P.; Sonar, P. Adv. Mater. 2010, 22, 4862-4866.Li, Y.; Sonar, P.; Singh, S. P.; Soh, M. S.; van Meurs, M.; Tan, J. J. Am. Chem. Soc. 2011, 133, 2198-2204.Li, Y.; Sonar, P.; Murphy, L.; Hong, W. Energy Environ. Sci. 2013, 6,1684-1710.Sun, B.; Hong, W.; Aziz, H.; Abukhdeir, N. M.; Li, Y. J. Mater. Chem. C 2013, 1, 4423-4426.Chen, S.; Sun, B.; Hong, W.; Aziz, H.; Meng, Y.; Li, Y. J. Mat. Chem. C. 2014, 2, 2183-2190.Sun, B.; Hong, W.; Yan, Z.; Aziz, H.; Li, Y. Adv. Mater. 2014, 26, 2636–2642.Sun, B.; Hong, W.; Thibau, E.; Aziz, H.; Lu, Z.H.; Li, Y. Organic Electronics 2014, 15, pp. 3787-3794.

High Bulk Electron Mobility Diketopyrrolopyrrole Copolymers with Perfluorothiophene

The question of designing high electron mobility polymers by increasing the planarization using diffusive nonbonding heteroatom interactions in diketopyrrolopyrrole polymers is addressed in this. For this, three different diketopyrrolo[3,4‐c]pyrrole (DPP) derivatives with thienyl‐, 2‐pyridinyl‐, and phenyl‐flanked cores are copolymerized with an electron‐rich thiophene unit as well as an electron‐deficient 3,4‐difluorothiophene unit as comonomer to obtain diverse polymeric DPPs which vary systematically in their structures. The crystallinity differs significantly with clear trends on varying both flanking unit and comonomer. The optical gap and energy levels depend more on the nature of the flanking aryl units rather than on fluorination. Additionally, the charge transport properties are compared using different methods to differentiate between interface or orientation effects and bulk charge carrier transport. In organic field effect transistor devices with very high electron as well as hole mobilities (up to 0.6 cm2 V−1 s−1) are obtained and fluorination leads to a more pronounced n‐type nature in all polymers, resulting in ambipolar behavior in otherwise p‐type materials. In contrast, space‐charge limited current measurements show a strong influence of the flanking units only on electron mobilities. Especially, the elegant synthetic strategy of combining pyridyl flanking units with difluorothiophene as the comonomer culminates in a record bulk electron mobility of 4.3 × 10−3 cm2 V−1 s−1 in polymers.

Room-temperature bandlike transport and Hall effect in a high-mobility ambipolar polymer

The advent of a new class of high-mobility semiconducting polymers opens up a window to address fundamental issues in electrical transport mechanism such as transport between localized states versus extended state conduction. Here, we investigate the origin of the ultralow degree of disorder $({E}_{a}\ensuremath{\sim}16\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ and the ``bandlike'' negative temperature ($T$) coefficient of the field effect electron mobility: ${\ensuremath{\mu}}_{\mathrm{FET}}^{e}(T)$ in a high performance $({\ensuremath{\mu}}_{\mathrm{FET}}^{e}>2.5\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$ diketopyrrolopyrrole based semiconducting polymer. Models based on the framework of mobility edge with exponential density of states are invoked to explain the trends in transport. The temperature window over which the system demonstrates delocalized transport was tuned by a systematic introduction of disorder at the transport interface. Additionally, the Hall mobility $({\ensuremath{\mu}}_{\mathrm{Hall}}^{e})$ extracted from Hall voltage measurements in these devices was found to be comparable to field effect mobility $({\ensuremath{\mu}}_{\mathrm{FET}}^{e})$ in the high $T$ bandlike regime. Comprehensive studies with different combinations of dielectrics and semiconductors demonstrate the effectiveness of rationale molecular design, which emphasizes uniform-energetic landscape and low reorganization energy.

High charge mobility polymers based on a new di(thiophen-2-yl)thieno[3,2-b]thiophene for transistors and solar cells

A novel random copolymer of di(thiophen-2-yl)thieno[3,2-b]thiophene (DTTT) with DPP achieved a high hole mobility of 0.627 cm2 V−1 s−1.

Synthesis, characterization, and field-effect transistor properties of tetrathienoanthracene-based copolymers using a two-dimensional π-conjugation extension strategy: a potential building block for high-mobility polymer semiconductors

Novel polymer semiconductors with 2D π-extended tetrathienoanthracene units for field-effect transistor applications, exhibiting a high mobility of 0.15 cm2V−1s−1.

Transferred metal electrode films for large-area electronic devices

The evaporation of metal-film gate electrodes for top-gate organic field-effect transistors (OFETs) limits the minimum thickness of the polymer gate dielectric to typically more than 300 nm due to deep hot metal atom penetration and damage of the dielectric. We show here that the self-release layer transfer method recently developed for high-quality graphene transfer is also capable of giving high-quality metal thin-film transfers to produce high-performance capacitors and OFETs with superior dielectric breakdown strength even for ultrathin polymer dielectric films. Dielectric breakdown strengths up to 5–6 MV cm−1 have been obtained for 50-nm thin films of polystyrene and a cyclic olefin copolymer TOPAS® (Zeon). High-quality OFETs with sub-10 V operational voltages have been obtained this way using conventional polymer dielectrics and a high-mobility polymer semiconductor poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene-2,5-diyl]. The transferred metal films can make reliable contacts without damaging ultrathin polymer films, self-assembled monolayers and graphene, which is not otherwise possible from evaporated or sputtered metal films.

Impact of the Doping Method on Conductivity and Thermopower in Semiconducting Polythiophenes

The development of organic semiconductors for use in thermoelectrics requires the optimization of both their thermopower and electrical conductivity. Here two fundamentally different doping mechanisms are used to investigate the thermoelectric properties of known high hole mobility polymers: poly 3‐hexylthiophene (P3HT), poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT‐C14), and poly(2,5‐bis(thiphen‐2‐yl)‐(3,7‐diheptadecantyltetrathienoacene)) (P2TDC17‐FT4). The small molecule tetrafluorotetracyanoquinodimethane (F4TCNQ) is known to effectively dope these polymers, and the thermoelectric properties are studied as a function of the ratio of dopant to polymer repeat unit. Higher electrical conductivity and values of the thermoelectric power factor are achieved by doping with vapor‐deposited fluoroalkyl trichlorosilanes. The combination of these data reveals a striking relationship between thermopower and conductivity in thiophene‐based polymers over a large range of electrical conductivity that is independent of the means of electrical doping. This relationship is not predicted by commonly used transport models for semiconducting polymers and is demonstrated to hold for other semiconducting polymers as well.

Analysis of charge transport in high-mobility diketopyrrolopyrole polymers by space charge limited current and time of flight methods

The hole mobility of the widely studied diketopyrrolopyrole-based polymers (PDPPDTSE) was examined using space charge limited current (SCLC) and time of flight (TOF) methods. The mobility of the hole-only device based on PDPPDTSE was found to be dependent upon the e-field over the range of 10−3 to 10−2 cm2 V−1 s−1 with nearly identical Poole–Frenkel coefficients. In addition, we found that the mobility strongly depended on the thickness of the PDPPDTSE. By analyzing the temperature dependence of transport characteristics, we argued that the charge transport in this polymer was greatly influenced by trap distribution at the electrode/semiconductor interface.

Solution-processable low-voltage and flexible floating-gate memories based on an n-type polymer semiconductor and high-k polymer gate dielectrics.

High-performance low-voltage flash memories based on organic floating-gate field-effect transistors are prepared by a solution process for the first time. Transistors with a high-mobility n-type polymer semiconductor, poly{[N,N(')-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, and a high-k polymer gate dielectric, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (k ≈ 60), are successfully fabricated on flexible substrates. A thin layer of Au nanoparticles is embedded in the gate dielectric, which can store injected charge from the channel and result in a memory effect. The organic memories demonstrate high carrier mobilities (>0.3 cm(2)/(V s)), low program/erase voltages (±6 V), little degradation after 10(5) program/erase cycles, and good retention after 10(5) s, which suggest great promise in the application of nonvolatile memories in flexible electronics.

High mobility polymer based on a π-extended benzodithiophene and its application for fast switching transistor and high gain photoconductor.

Here we present synthesis and electronic properties of a new alternating copolymer composed of dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT) and diketopyrrolopyrrole units, poly dithienobenzodithiophene-co-diketopyrrolopyrrolebithiophene (PDPDBD). The resulting polymer showed hysteresis free, fast switching and highly reliable organic thin-film transistor properties comparable to a-Si. Hole mobility of the polymer is about 2.7 cm2V−1s−1, which is remarkably improved compared with its benzodithiophene (BDT)-analougue that contains a smaller aromatic ring of BDT in the place of DTBDT. This is mainly due to much increased intramolecular charge transport originated from PDPDBD's rigid molecular backbone. Furthermore, photoconductor devices fabricated by using PDPDBD as an active layer showed a high performance with the highest photoconductive gain of ~105. Taken together, the successful PDPDBD's transistor and photoconductor performances with high device stability demonstrated practical applicability of PDPDBD in low-cost and flexible optoelectronic devices.

Charge Transport Anisotropy in Highly Oriented Thin Films of the Acceptor Polymer P(NDI2OD‐T2)

The nanomorphology of the high mobility polymer poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} P(NDI2OD‐T2) in thin films is explored as a function of different annealing conditions and correlated to optical and electrical properties. While nanofibrils with face‐on orientation in form I are obtained directly after spin‐coating and annealing below the melt transition temperature, clear evidence of lamellar structures is found after melt‐annealing followed by slow cooling to room temperature. Interestingly these structural changes are accompanied by distinct changes in the absorption patterns. Electron diffraction measurements further show clear transitions towards predominant edge‐on oriented chains in form II upon melt‐annealing. Large‐scale alignment with dichroic ratios up to 10 and improved order is achieved by high temperature rubbing and subsequent post‐rubbing annealing. These highly oriented morphologies allow anisotropic in‐plane charge transport to be probed with top‐gate transistors parallel and perpendicular to the polymer chain direction. Mobilities up to 0.1 cm2 V‐1 s‐1 are observed parallel to the polymer chain, which is up to 10 times higher than those perpendicular to the polymer chain.

High mobility polymer gated organic field effect transistor using zinc phthalocyanine

Organic thin film transistors were fabricated using evaporated zinc phthalocyanine as the active layer. Parylene film prepared by chemical vapour deposition was used as the organic gate insulator. The annealing of the samples was performed at 120 °C for 3 h. At room temperature, these transistors exhibit p-type conductivity with field-effect mobilities ranging from 0·025–0·037 cm2/Vs and a (I on/I off) ratio of ∼103. The effect of annealing on transistor characteristics is discussed.

Electrospun Polymer Fibers for Electronic Applications.

Nano- and micro- fibers of conjugated polymer semiconductors are particularly interesting both for applications and for fundamental research. They allow an investigation into how electronic properties are influenced by size confinement and chain orientation within microstructures that are not readily accessible within thin films. Moreover, they open the way to many applications in organic electronics, optoelectronics and sensing. Electro-spinning, the technique subject of this review, is a simple method to effectively form and control conjugated polymer fibers. We provide the basics of the technique and its recent advancements for the formation of highly conducting and high mobility polymer fibers towards their adoption in electronic applications.

“Conformation Locked” Strong Electron-Deficient Poly(p-Phenylene Vinylene) Derivatives for Ambient-Stable n-Type Field-Effect Transistors: Synthesis, Properties, and Effects of Fluorine Substitution Position

The charge carrier mobility of p-type and ambipolar polymer field-effect transistors (FETs) has been improved substantially. Nonetheless, high-mobility n-type polymers are rare, and few can be operated under ambient conditions. This situation is mainly caused by the scarcity of strong electron-deficient building blocks. Herein, we present two novel electron-deficient building blocks, FBDOPV-1 and FBDOPV-2, with low LUMO levels down to -4.38 eV. On the basis of both building blocks, we develop two poly(p-phenylene vinylene) derivatives (PPVs), FBDPPV-1 and FBDPPV-2, for high-performance n-type polymer FETs. The introduction of the fluorine atoms effectively lowers the LUMO levels of both polymers, leading to LUMO levels as low as -4.30 eV. Fluorination endows both polymers with not only lower LUMO levels, but also more ordered thin-film packing, smaller π-π stacking distance, stronger interchain interaction and locked conformation of polymer backbones. All these factors provide FBDPPV-1 with high electron mobilities up to 1.70 cm(2) V(-1) s(-1) and good stability under ambient conditions. Furthermore, when polymers have different fluorination positions, their backbone conformations in solid state differ, eventually leading to different device performance.

Solubility‐Limited Extrinsic n‐Type Doping of a High Electron Mobility Polymer for Thermoelectric Applications

The thermoelectric properties of a highperformance electron-conducting polymer, (P(NDIOD-T2), extrinsically doped with dihydro-1H-benzoimidazol-2-yl (NDBI) derivatives, are reported. The highest thermoelectric power factor that has been reported for a solution-processed n-type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.