High-mobility Polymer Research Articles

Photocarrier recombination of polymer/fullerene composites in surface-type photocells

Carrier recombination kinetics is studied by photocurrent measurements of surface-type photocells of polymer/fullerene composites. A quadrimolecular recombination (QR) in the time range of 0.1 to 100 s, which was found for regioregular poly(3-hexylthiophene), is observed in another semicrystalline polymer, poly{2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene} (PBTTT). By measuring photocurrent and transistor action in the same device, suppression of the recombination in the ambipolar region at a high fullerene content is demonstrated. No QR but bimolecular recombination is observed in cells using amorphous-like poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). These results provide further support that the QR is characteristic of high-mobility semicrystalline polymers in which doubly charged states are formed owing to the lamellar structure.

Toward Printed Integrated Circuits based on Unipolar or Ambipolar Polymer Semiconductors

For at least the past ten years printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had already been fabricated by printing techniques at industrial scale, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications despite their great potential. In the case of logic integrated circuits (ICs), which constitute the focus of this Progress Report, the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector. Solution-processable polymeric semiconductors are the best candidates to fulfill the requirements for printed logic ICs on flexible substrates, due to their superior processability, ease of tuning of their rheology parameters, and mechanical properties. One of the strongest limitations has been mainly represented by the low charge carrier mobility (μ) achievable with polymeric, organic field-effect transistors (OFETs). However, recently unprecedented values of μ ∼ 10 cm(2) /Vs have been achieved with solution-processed polymer based OFETs, a value competing with mobilities reported in organic single-crystals and exceeding the performances enabled by amorphous silicon (a-Si). Interestingly these values were achieved thanks to the design and synthesis of donor-acceptor copolymers, showing limited degree of order when processed in thin films and therefore fostering further studies on the reason leading to such improved charge transport properties. Among this class of materials, various polymers can show well balanced electrons and holes mobility, therefore being indicated as ambipolar semiconductors, good environmental stability, and a small band-gap, which simplifies the tuning of charge injection. This opened up the possibility of taking advantage of the superior performances offered by complementary "CMOS-like" logic for the design of digital ICs, easing the scaling down of critical geometrical features, and achieving higher complexity from robust single gates (e.g., inverters) and test circuits (e.g., ring oscillators) to more complete circuits. Here, we review the recent progress in the development of printed ICs based on polymeric semiconductors suitable for large-volume micro- and nano-electronics applications. Particular attention is paid to the strategies proposed in the literature to design and synthesize high mobility polymers and to develop suitable printing tools and techniques to allow for improved patterning capability required for the down-scaling of devices in order to achieve the operation frequencies needed for applications, such as flexible radio-frequency identification (RFID) tags, near-field communication (NFC) devices, ambient electronics, and portable flexible displays.

Organic photovoltaics with thick active layers (∼800 nm) using a high mobility polymer donor

Rather consistent power conversion efficiencies have been achieved in organic solar cells throughout a wide range of active layer thicknesses (∼100nm to ∼800nm) by using a p-type polymer semiconductor with high hole transport mobility (∼1cm2V−1s−1) as the donor, and PC61BM as the acceptor. A comparison between standard and inverted device configurations demonstrates that the inverted configuration is more suitable for achieving thicker active layers when a high hole mobility donor is used. We attribute this to the longer hole collection path in the inverted structure, which can benefit from using a high hole mobility material. Our results will be very useful for the development of new semiconductor materials and the design of device structures for more feasible manufacturing of large area OPV devices via high speed roll-to-roll printing processes.

Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer

The molecular orientation and microstructure of films of the high-mobility semiconducting polymer poly(N,N-bis-2-octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene) (P(NDI2OD-T2)) are probed using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. In particular a novel approach is used whereby the bulk molecular orientation and surface molecular orientation are simultaneously measured on the same sample using NEXAFS spectroscopy in an angle-resolved transmission experiment. Furthermore, the acquisition of bulk-sensitive NEXAFS data enables a direct comparison of the information provided by GIWAXS and NEXAFS. By comparison of the bulk-sensitive and surface-sensitive NEXAFS data, a distinctly different molecular orientation is observed at the surface of the film compared to the bulk. While a more "face-on" orientation of the conjugated backbone is observed in the bulk of the film, consistent with the lamella orientation observed by GIWAXS, a more "edge-on" orientation is observed at the surface of the film with surface-sensitive NEXAFS spectroscopy. This distinct edge-on surface orientation explains the high in-plane mobility that is achieved in top-gate P(NDI2OD-T2) field-effect transistors (FETs), while the bulk face-on texture explains the high out-of-plane mobilities that are observed in time-of-flight and diode measurements. These results also stress that GIWAXS lacks the surface sensitivity required to probe the microstructure of the accumulation layer that supports charge transport in organic FETs and hence may not necessarily be appropriate for correlating film microstructure and FET charge transport.

Charge transport study of high mobility polymer thin-film transistors based on thiophene substituted diketopyrrolopyrrole copolymers

In this paper, we report on the device physics and charge transport characteristics of high-mobility dual-gated polymer thin-film transistors with active semiconductor layers consisting of thiophene flanked DPP with thienylene-vinylene-thienylene (PDPP-TVT) alternating copolymers. Room temperature mobilities in these devices are high and can exceed 2 cm(2) V(-1) s(-1). Steady-state and non-quasi-static measurements have been performed to extract key transport parameters and velocity distributions of charge carriers in this copolymer. Charge transport in this polymer semiconductor can be explained using a Multiple-Trap-and-Release or Monroe-type model. We also compare the activation energy vs. field-effect mobility in a few important polymer semiconductors to gain a better understanding of transport of DPP systems and make appropriate comparisons.

Charge transport characteristics of a high-mobility diketopyrrolopyrrole-based polymer

In this study, we attempt to unveil the charge-transport abnormality of the widely studied diketopyrrolopyrrole (DPP)-based polymers with exceptionally high charge carrier mobility [>5 cm(2) V(-1) s(-1)]. Based on the electric field and temperature dependence of the charge-transport characteristics of the field effect transistor (FET) geometry of one of the highly conductive DPP derivatives, namely, (poly[2,5-bis(7-decylnonadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-(1,2-bis(5-(thiophen-2-yl)selenophen-2-yl)ethene) (PDPPDTSE), we show that the high gate-source bias drew the carriers closer to the interface of the semiconductor/dielectric layers where the density of state (DOS) of the charge carrier is significantly broader than the bulk. We argue that the intrinsically narrow DOS in the PDPPDTSE bulk resulted in significantly different charge-transport behavior between the semiconductor bulk and the semiconductor/dielectric interface, which was not visible in the other low-mobility organic semiconductors that contain intrinsically high density of trap states in their bulk. To avoid these charge transport abnormalities, we try to operate the FETs under low gate bias without compromising the accumulated charge carrier density. By carefully employing a thin metal oxide covered with a self-assembled monolayer (SAM) as a dielectric layer, we can demonstrate low-voltage PDPPDTSE FETs with near-ideal performance both in terms of hysteresis-free operation and operating reliability while maintaining a high charge carrier mobility of ~2.8 cm(2) V(-1) s(-1).

Recent Progress in High Mobility Polymer Semiconductors for Organic Thin Film Transistors

Recent Progress in High Mobility Polymer Semiconductors for Organic Thin Film Transistors

Charge-Carrier Velocity Distributions in High-Mobility Polymer Dual-Gate Thin-Film Transistors

We report charge-carrier velocity distributions in high-mobility polymer thin-film transistors (PTFTs) employing a dual-gate configuration. Our time-domain measurements of dual-gate PTFTs indicate higher effective mobility as well as fewer low-velocity carriers than in single-gate operation. Such nonquasi-static (NQS) measurements support and clarify the previously reported results of improved device performance in dual-gate devices by various groups. We believe that this letter demonstrates the utility of NQS measurements in studying charge-carrier transport in dual-gate thin-film transistors.

Charge carrier velocity distributions in high mobility polymer field-effect transistors

In this letter, the velocity distributions of charge carriers in high-mobility polymer thin-film transistors (TFTs) with a diketopyrrolopyrrole-naphthalene copolymer (PDPP-TNT) semiconductor active layer are reported. The velocity distributions are found to be strongly dependent on measurement temperatures as well as annealing conditions. Considerable inhomogeneity is evident at low measurement temperatures and for low annealing temperatures. Such transient transport measurements can provide additional information about charge carrier transport in TFTs which are unavailable using steady-state transport measurements.

From Monolayer to Multilayer N‐Channel Polymeric Field‐Effect Transistors with Precise Conformational Order

Monolayer field-effect transistors based on a high-mobility n-type polymer are demonstrated. The accurate control of the long-range order by Langmuir-Schäfer (LS) deposition yields dense polymer packing exhibiting good injection properties, relevant current on/off ratio and carrier mobility in a staggered configuration. Layer-by-layer LS film transistors of increasing thickness are fabricated and their performance compared to those of spin-coated films.

Highly efficient indacenodithiophene-based polymeric solar cells in conventional and inverted device configurations

Highly efficient bulk-heterojunction polymer solar cells based on two indacenodithiophene-containing low bandgap, high mobility polymers in both of their conventional and inverted device configurations are demonstrated. The highest power conversion efficiency obtained from the conventional structure device is 6.3%, while the PCE of the inverted device reaches 4.9%. These results are very encouraging for further material and device optimization.

Polymer Blend Solar Cells Based on a High‐Mobility Naphthalenediimide‐Based Polymer Acceptor: Device Physics, Photophysics and Morphology

AbstractA high electron mobility polymer, poly{[N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5’‐(2,2’‐bithiophene) (P(NDI2OD‐T2)) is investigated for use as an electron acceptor in all‐polymer blends. Despite the high bulk electron mobility, near‐infrared absorption band and compatible energy levels, bulk heterojunction devices fabricated with poly(3‐hexylthiophene) (P3HT) as the electron donor exhibit power conversion efficiencies of only 0.2%. In order to understand this disappointing photovoltaic performance, systematic investigations of the photophysics, device physics and morphology of this system are performed. Ultra‐fast transient absorption spectroscopy reveals a two‐stage decay process with an initial rapid loss of photoinduced polarons, followed by a second slower decay. This second slower decay is similar to what is observed for efficient P3HT:PCBM ([6,6]‐phenyl C61‐butyric acid methyl ester) blends, however the initial fast decay that is absent in P3HT:PCBM blends suggests rapid, geminate recombination of charge pairs shortly after charge transfer. X‐ray microscopy reveals coarse phase separation of P3HT:P(NDI2OD‐T2) blends with domains of size 0.2 to 1 micrometer. P3HT photoluminescence, however, is still found to be efficiently quenched indicating intermixing within these mesoscale domains. This hierarchy of phase separation is consistent with the transient absorption, whereby localized confinement of charges on isolated chains in the matrix of the other polymer hinders the separation of interfacial electron‐hole pairs. These results indicate that local, interfacial processes are the key factor determining the overall efficiency of this system and highlight the need for improved morphological control in order for the potential benefit of high‐mobility electron accepting polymers to be realized.

Surface and Bulk Structural Characterization of a High-Mobility Electron-Transporting Polymer

This publication investigates the microstructure of the high electron mobility polymer poly([N,N′-bis(2-octyldodecyl)-11 naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-12 biothiopene)) (P(NDI2OD-T2)) using a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption fine structure (NEXAFS), and optical spectroscopies. GIXD measurements confirm extended in-plane lamellae ordering and out-of-plane π−π stacking. Angle-resolved surface-sensitive NEXAFS measurements, however, indicate a lack of preferred in-plane or out-of-plane orientation of either the conjugated backbone or side chains at the P(NDI2OD-T2) film surface. This lack of pronounced orientation effects is attributed to a significant proportion of amorphous content in the film, with NEXAFS measurements sensitive to both amorphous and crystalline content while GIXD is sensitive only to coherent ordering. Furthermore, it is found that increased crystallinity in annealed thin films is not beneficial to the p...

Revelation of the spatial structure of a highly crystalline and conducting polyaniline membrane constructed by epitaxial growth

A single crystal polyaniline membrane was deposited on a crystalline HDPE (high density polyethylene) via an in-situpolymerization/deposition method. Both crystalline and amorphous polymers were used as substrates to prove that the deposition process is an epitaxial growth. Single crystal polyaniline membranes have not only high conductivity but also good conducting and doping homogeneity. The epitaxial growth of the conducting polymer on the flexible polymeric substrate is reported for the first time and the mechanism for the epitaxial growth of polyaniline chains on HDPE was proposed. Being able to epitaxially grow highly crystalline and good conducting homogeneity conjugated polymer membranes on flexible polymeric substrates provides a very promising step towards preparing high conducting polymer films or high mobility polymer dielectrics for flexible organic electronic applications.

High-mobility polymer space–charge-limited transistor with grid-induced crystallinity

The vertical mobility of a polymer film prepared on a textured substrate is enhanced from 10 −5 cm 2/Vs to 10 −3 cm 2/Vs by a slow drying process which is unattainable in planar substrate. Highly ordered structure is observed for polymer film treated with high-boiling-point solvent and slow solvent annealing. The enhanced vertical mobility enables the polymer space–charge-limited transistor (SCLT) capable of outputting current density as high as 100 mA/cm 2 while maintaining good current gain and on–off ratio. Such vertical transistor is able to drive high-power devices or mechanical actuator in large-area and flexible array.

Organic Thin-Film Transistors Processed from Relatively Nontoxic, Environmentally Friendlier Solvents

Poly(2,6-bis(3-alkylthiophen-2-yl)dithieno- [3,2-b;2′,3′-d]-thiophene)s represent a new class of solution-processable polymer semiconductors for organic thin-film transistors. This class of polymer semiconductors offers not only high field-effect transistor mobility, but also processability from relatively nontoxic, environmentally friendlier solvents. Thin-film transistors utilizing these polymer semiconductors fabricated even from environmentally friendlier organic solvents of generally low solvency such as hexane and toluene have exhibited high field-effect mobility and current on/off ratio as well as excellent thermal stability and high resistance to photoinduced oxidative doping. This is in sharp contrast to most high-mobility polymer semiconductors, which require toxic chlorinated aromatic solvents for fabrication to achieve high mobility. The use of relatively nontoxic processing solvent would eliminate the safety concerns associated with use of chlorinated aromatic solvents, potentially enabling l...

Patterning of Solution‐Processed Semiconducting Polymers in High‐Mobility Thin‐Film Transistors by Physical Delamination

Patterning of high-mobility semiconducting polymer thin films is essential to prevent undesirable crosstalk in realization of integrated circuits. We present here two simple selective physical delamination methods that allow clean and high-resolution patterning of semiconducting polymers for thin-film transistors, leading to a significant reduction in OFF current while retaining the high field-effect mobilities of the unpatterned polymer films.

A high-mobility electron-transporting polymer for printed transistors

Printed electronics is a revolutionary technology aimed at unconventional electronic device manufacture on plastic foils, and will probably rely on polymeric semiconductors for organic thin-film transistor (OTFT) fabrication. In addition to having excellent charge-transport characteristics in ambient conditions, such materials must meet other key requirements, such as chemical stability, large solubility in common solvents, and inexpensive solution and/or low-temperature processing. Furthermore, compatibility of both p-channel (hole-transporting) and n-channel (electron-transporting) semiconductors with a single combination of gate dielectric and contact materials is highly desirable to enable powerful complementary circuit technologies, where p- and n-channel OTFTs operate in concert. Polymeric complementary circuits operating in ambient conditions are currently difficult to realize: although excellent p-channel polymers are widely available, the achievement of high-performance n-channel polymers is more challenging. Here we report a highly soluble ( approximately 60 g l(-1)) and printable n-channel polymer exhibiting unprecedented OTFT characteristics (electron mobilities up to approximately 0.45-0.85 cm(2) V(-1) s(-1)) under ambient conditions in combination with Au contacts and various polymeric dielectrics. Several top-gate OTFTs on plastic substrates were fabricated with the semiconductor-dielectric layers deposited by spin-coating as well as by gravure, flexographic and inkjet printing, demonstrating great processing versatility. Finally, all-printed polymeric complementary inverters (with gain 25-65) have been demonstrated.

High Performance Amorphous Metallated π-Conjugated Polymers for Field-Effect Transistors and Polymer Solar Cells

Both high charge carrier mobility in OFETs and high power conversion efficiency in solar cells were demonstrated in a series of amorphous metallated π-conjugated polymers. The best solar cell performance is based on a high field-effect mobility (μh = 1.0 × 10−2 cm2 V−1 s−1) polymer blended with PC71BM leading to a power conversion efficiency of PCE = 4.13%.

A High-Mobility Electron-Transport Polymer with Broad Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells

An electron-transport polymer with good solution processibility, excellent thermal stability, and high electron affinity based on alternating perylene diimide and dithienothiophene units has been synthesized. Electron mobilities as high as 1.3 × 10-2 cm2 V-1 s-1 have been measured in field-effect transistor geometry. The polymer shows broad absorptions throughout the visible and extending into the near-IR. A power conversion efficiency of over 1%, under simulated AM 1.5, 100 mW/cm2, was measured for a single-layer solar cell using this polymer as an acceptor and a polythiophene derivative as a donor.