Chain Length Dependence of Electron Transport in an n-Type Conjugated Polymer with a Rigid-Rod Chain Topology.

Most currently known n-type conjugated polymers have a semiflexible chain topology, and their charge carrier mobilities are known to peak at modest chain lengths of below 40-60 repeat units. Herein, we show that the field-effect electron mobility of a model n-type conjugated polymer that has a rigid-rod chain topology grows continuously without saturation, even at a chain length exceeding 250 repeat units. We found the mechanism underlying the novel chain length-dependent electron transport to originate from the reduced structural disorder and energetic disorder with the increasing degree of polymerization inherent to the rigid-rod chain topology. Furthermore, we demonstrate a unique chain length-dependent decay of threshold voltage, which is rationalized by decreased trap densities and trap depths with respect to the degree of polymerization. Our findings provide new insights into the role of polymer chain topology in electron transport and demonstrate the promise of rigid-rod chain architectures for the design of future high-mobility conjugated polymers.

Hierarchical Materials from High Information Content Macromolecular Building Blocks: Construction, Dynamic Interventions, and Prediction.

Hierarchical materials that exhibit order over multiple length scales are ubiquitous in nature. Because hierarchy gives rise to unique properties and functions, many have sought inspiration from nature when designing and fabricating hierarchical matter. More and more, however, nature's own high-information content building blocks, proteins, peptides, and peptidomimetics, are being coopted to build hierarchy because the information that determines structure, function, and interfacial interactions can be readily encoded in these versatile macromolecules. Here, we take stock of recent progress in the rational design and characterization of hierarchical materials produced from high-information content blocks with a focus on stimuli-responsive and "smart" architectures. We also review advances in the use of computational simulations and data-driven predictions to shed light on how the side chain chemistry and conformational flexibility of macromolecular blocks drive the emergence of order and the acquisition of hierarchy and also on how ionic, solvent, and surface effects influence the outcomes of assembly. Continued progress in the above areas will ultimately usher in an era where an understanding of designed interactions, surface effects, and solution conditions can be harnessed to achieve predictive materials synthesis across scale and drive emergent phenomena in the self-assembly and reconfiguration of high-information content building blocks.

Hydration of a Side-Chain-Free n-Type Semiconducting Ladder Polymer Driven by Electrochemical Doping.

We study the organic electrochemical transistor (OECT) performance of the ladder polymer poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Regardless of molecular mass, both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance gravimetry, in-operando atomic force microscopy, and both ex-situ and in-operando grazing incidence wide-angle X-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.

Amphiphilic Peptoid-Directed Assembly of Oligoanilines into Highly Crystalline Conducting Nanotubes.

It is reported herein the synthesis of a novel amphiphilic diblock peptoid bearing a terminal conjugated oligoaniline and its self-assembly into small-diameter (D ≈ 35 nm) crystalline nanotubes with high aspect ratios (>30). It is shown that both tetraaniline (TANI)-peptoid and bianiline (BANI)-peptoid triblock molecules self-assemble in solution to form rugged highly crystalline nanotubes that are very stable to protonic acid doping and de-doping processes. The similarity of the crystalline tubular structure of the nanotube assemblies revealed by electron microscopy imaging, and X-ray diffraction analysis of the nanotube assemblies of TANI-functionalized peptoids and nonfunctionalized peptoids showed that the peptoid is an efficient ordered structure directing motif for conjugated oligomers. Films of doped TANI-peptoid nanotubes has a dc conductivity of ca. 95 mS cm-1 , while the thin films of doped un-assembled TANI-peptoids show a factor of 5.6 lower conductivity, demonstrating impact of the favorable crystalline ordering of the assemblies on electrical transport. These results demonstrate that peptoid-directed supramolecular assembly of tethered π-conjugated oligo(aniline) exemplify a novel general strategy for creating rugged ordered and complex nanostructures that have useful electronic and optoelectronic properties.

Ground-state electron transfer in all-polymer donor-acceptor heterojunctions.

Doping of organic semiconductors is crucial for the operation of organic (opto)electronic and electrochemical devices. Typically, this is achieved by adding heterogeneous dopant molecules to the polymer bulk, often resulting in poor stability and performance due to dopant sublimation or aggregation. In small-molecule donor-acceptor systems, charge transfer can yield high and stable electrical conductivities, an approach not yet explored in all-conjugated polymer systems. Here, we report ground-state electron transfer in all-polymer donor-acceptor heterojunctions. Combining low-ionization-energy polymers with high-electron-affinity counterparts yields conducting interfaces with resistivity values five to six orders of magnitude lower than the separate single-layer polymers. The large decrease in resistivity originates from two parallel quasi-two-dimensional electron and hole distributions reaching a concentration of ∼1013 cm-2. Furthermore, we transfer the concept to three-dimensional bulk heterojunctions, displaying exceptional thermal stability due to the absence of molecular dopants. Our findings hold promise for electro-active composites of potential use in, for example, thermoelectrics and wearable electronics.

n-Type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells.

Knowledge of the critical factors that determine compatibility, blend morphology, and performance of bulk heterojunction (BHJ) solar cells composed of an electron-accepting polymer and an electron-donating polymer remains limited. To test the idea that bulk crystallinity is such a critical factor, we have designed a series of new semiconducting naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene random copolymers, xPDI (10PDI, 30PDI, 50PDI), whose crystallinity varies with composition, and investigated them as electron acceptors in BHJ solar cells. Pairing of the reference crystalline (crystalline domain size Lc = 10.22 nm) NDI-selenophene copolymer (PNDIS-HD) with crystalline (Lc = 9.15 nm) benzodithiophene-thieno[3,4-b]thiophene copolymer (PBDTTT-CT) donor yields incompatible blends, whose BHJ solar cells have a power conversion efficiency (PCE) of 1.4%. However, pairing of the new 30PDI with optimal crystallinity (Lc = 5.11 nm) as acceptor with the same PBDTTT-CT donor yields compatible blends and all-polymer solar cells with enhanced performance (PCE = 6.3%, Jsc = 18.6 mA/cm(2), external quantum efficiency = 91%). These photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the best so far for all-polymer solar cells, while the short-circuit current (Jsc) and external quantum efficiency are even higher than reported values for [70]-fullerene:PBDTTT-CT solar cells. The morphology and bulk carrier mobilities of the polymer/polymer blends varied substantially with crystallinity of the acceptor polymer component and thus with the NDI/PDI copolymer composition. These results demonstrate that the crystallinity of a polymer component and thus compatibility, blend morphology, and efficiency of polymer/polymer blend solar cells can be controlled by molecular design.

Beyond fullerenes: design of nonfullerene acceptors for efficient organic photovoltaics.

New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C60]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC60BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm(2)) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60-65% in the 380-620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC60BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials.

Observation of super-Nernstian proton-coupled electron transfer and elucidation of nature of charge carriers in a multiredox conjugated polymer.

Nernstian proton-coupled electron transfer (PCET) is a fundamental process central to many physical and biological systems, such as electrocatalysis, enzyme operation, DNA biosynthesis, pH-/bio-sensors, and electrochemical energy storage devices. We report herein the discovery of super-Nernstian PCET behavior with two protons per electron transferred in the electrochemical doping of a redox conjugated polymer, phenazine-substituted ladder poly(benzimidazobenzophenanthroline) (BBL-P), in aqueous electrolyte. We show that the super-Nernstian response originates from existence of multiredox centers that have a gradient of pKa on the conjugated polymer. Our use of various pH-dependent in operando techniques to probe the nature of charge carriers in n-doped BBL-P found that polarons are the charge carriers at low to intermediate levels of doping (0.1-1.0 electron per repeat unit (eru)) whereas at higher doing levels (1.3 eru), polarons, polaron pairs, and bipolarons co-exist, which evolve into strongly coupled polaron pairs at the highest doping levels (>1.5 eru). We show that PCET-assisted n-doping of BBL-P results in very high redox capacity (>1200 F cm-3) in acidic electrolyte. Our results provide important new insights into PCET in organic materials and the nature of charge carriers in n-doped conjugated polymers while having implications for various electrochemical devices.

High-mobility n-type conjugated polymers based on electron-deficient tetraazabenzodifluoranthene diimide for organic electronics.

High-mobility p-type and ambipolar conjugated polymers have been widely reported. However, high-mobility n-type conjugated polymers are still rare. Herein we present poly(tetraazabenzodifluoranthene diimide)s, PBFI-T and PBFI-BT, which exhibit a novel two-dimensional (2D) π-conjugation along the main chain and in the lateral direction, leading to high-mobility unipolar n-channel transport in field-effect transistors. The n-type polymers exhibit electron mobilities of up to 0.30 cm(2)/(V s), which is among the highest values for unipolar n-type conjugated polymers. Complementary inverters incorporating n-channel PBFI-T transistors produced nearly perfect switching characteristics with a high gain of 107.

All-polymer solar cells with 3.3% efficiency based on naphthalene diimide-selenophene copolymer acceptor.

The lack of suitable acceptor (n-type) polymers has limited the photocurrent and efficiency of polymer/polymer bulk heterojunction (BHJ) solar cells. Here, we report an evaluation of three naphthalene diimide (NDI) copolymers as electron acceptors in BHJ solar cells which finds that all-polymer solar cells based on an NDI-selenophene copolymer (PNDIS-HD) acceptor and a thiazolothiazole copolymer (PSEHTT) donor exhibit a record 3.3% power conversion efficiency. The observed short circuit current density of 7.78 mA/cm(2) and external quantum efficiency of 47% are also the best such photovoltaic parameters seen in all-polymer solar cells so far. This efficiency is comparable to the performance of similarly evaluated [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM)/PSEHTT devices. The lamellar crystalline morphology of PNDIS-HD, leading to balanced electron and hole transport in the polymer/polymer blend solar cells accounts for its good photovoltaic properties.

Applications of light scattering in dye-sensitized solar cells.

Light scattering is a method that has been employed in dye-sensitized solar cells for optical absorption enhancement. In conventional dye-sensitized solar cells, large TiO(2) particles with sizes comparable to the wavelength of visible light are used as scatterers by either being mixed into the nanocrystalline film to generate light scattering or forming a scattering layer on the top of the nanocrystalline film to reflect the incident light, with the aim to extend the traveling distance of incident light within the photoelectrode film. Recently, hierarchical nanostructures, for example nanocrystallite aggregates (among others), have been applied to dye-sensitized solar cells. When used to form a photoelectrode film, these hierarchical nanostructures have demonstrated a dual function: providing large specific surface area; and generating light scattering. Some other merits, such as the capability to enhance electron transport, have been also observed on the hierarchically structured photoelectrode films. Hierarchical nanostructures possessing an architecture that may provide sufficient internal surface area for dye adsorption and meanwhile may generate highly effective light scattering, make them able to create photoelectrode films with optical absorption significantly more efficient than the dispersed nanoparticles used in conventional dye-sensitized solar cells. This allows reduction of the thickness of the photoelectrode film and thus lowering of the charge recombination in dye-sensitized solar cells, making it possible to increase further the efficiency of existing dye-sensitized solar cells.

In Situ Spectroscopic and Electrical Investigations of Ladder-type Conjugated Polymers Doped with Alkali Metals.

Ladder-type conjugated polymers exhibit a remarkable performance in (opto)electronic devices. Their double-stranded planar structure promotes an extended π-conjugation compared to inter-ring-twisted analogues, providing an excellent basis for exploring the effects of charge localization on polaron formation. Here, we investigated alkali-metal n-doping of the ladder-type conjugated polymer (polybenzimidazobenzophenanthroline) (BBL) through detailed in situ spectroscopic and electrical characterizations. Photoelectron spectroscopy and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy indicate polaron formation upon potassium (K) doping, which agrees well with theoretical predictions. The semiladder BBB displays a similar evolution in the valence band with the appearance of two new features below the Fermi level upon K-doping. Compared to BBL, distinct differences appear in the UV-vis-NIR spectra due to more localized polaronic states in BBB. The high conductivity (2 S cm-1) and low activation energy (44 meV) measured for K-doped BBL suggest disorder-free polaron transport. An even higher conductivity (37 S cm-1) is obtained by changing the dopant from K to lithium (Li). We attribute the enhanced conductivity to a decreased perturbation of the polymer nanostructure induced by the smaller Li ions. These results highlight the importance of polymer chain planarity and dopant size for the polaronic state in conjugated polymers.

Influence of Molecular Weight on the Organic Electrochemical Transistor Performance of Ladder-Type Conjugated Polymers.

Organic electrochemical transistors (OECTs) hold promise for developing a variety of high-performance (bio-)electronic devices/circuits. While OECTs based on p-type semiconductors have achieved tremendous progress in recent years, n-type OECTs still suffer from low performance, hampering the development of power-efficient electronics. Here, it is demonstrated that fine-tuning the molecular weight of the rigid, ladder-type n-type polymer poly(benzimidazobenzophenanthroline) (BBL) by only one order of magnitude (from 4.9 to 51 kDa) enables the development of n-type OECTs with record-high geometry-normalized transconductance (gm,norm  ≈ 11 S cm-1 ) and electron mobility × volumetric capacitance (µC* ≈ 26 F cm-1  V-1 s-1 ), fast temporal response (0.38 ms), and low threshold voltage (0.15 V). This enhancement in OECT performance is ascribed to a more efficient intermolecular charge transport in high-molecular-weight BBL than in the low-molecular-weight counterpart. OECT-based complementary inverters are also demonstrated with record-high voltage gains of up to 100 V V-1 and ultralow power consumption down to 0.32 nW, depending on the supply voltage. These devices are among the best sub-1 V complementary inverters reported to date. These findings demonstrate the importance of molecular weight in optimizing the OECT performance of rigid organic mixed ionic-electronic conductors and open for a new generation of power-efficient organic (bio-)electronic devices.

Overcoming excitonic bottleneck in organic solar cells: electronic structure and spectra of novel semiconducting donor-acceptor block copolymers.

A novel class of organic semiconductors aimed at improving the efficiency of organic photovoltaic devices is investigated by an ab initio electronic structure theory. Two conjugated block copolymers composed of chemically bound donor and acceptor blocks show substantial charge transfer upon photoexcitation, suggesting that the optically excited states are separated charge pairs rather than strongly interacting charges forming excitons. In contrast, little charge transfer is seen in the ground electronic state. The optical cross-sections of the charge separated states are quite high due to a good overlap of the tails of the ground and excited state wavefunctions. The absorption spectra of the systems cover visible spectrum and extend to the infrared, suggesting good prospects for light harvesting. The calculation results indicate that the proposed class of semiconducting molecular heterojunctions may overcome the exciton bottleneck problem in organic photovoltaic materials.

Broadband absorbing bulk heterojunction photovoltaics using low-bandgap solution-processed quantum dots.

We describe bulk heterojunction (BHJ) solar cells containing blends of colloidal PbS nanocrystal quantum dots with several new donor-acceptor conjugated polymers. Using photoinduced absorption spectroscopy we found that blends of PbS quantum dots with one polymer, poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-N-octyldithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx), produce significantly more photoinduced charge than blends of PbS with the other donor-acceptor polymers or with traditionally studied polymers like [2-methoxy-5-(3',7'-dimethyloctyloxy)-para-phenylene vinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT). Photovoltaic devices made with PDTPQx/PbS blends exhibit power conversion efficiencies 10-100 times larger than previously reported BHJ blends made with IR-absorbing quantum dots.

Organic Semiconductors at the University of Washington: Advancements in Materials Design and Synthesis and toward Industrial Scale Production.

Research at the University of Washington regarding organic semiconductors is reviewed, covering four major topics: electro-optics, organic light emitting diodes, organic field-effect transistors, and organic solar cells. Underlying principles of materials design are demonstrated along with efforts toward unlocking the full potential of organic semiconductors. Finally, opinions on future research directions are presented, with a focus on commercial competency, environmental sustainability, and scalability of organic-semiconductor-based devices.

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers.

Molecular dopants are often added to semiconducting polymers to improve electrical conductivity. However, the use of such dopants does not always produce mobile charge carriers. In this work, ultrafast spectroscopy is used to explore the nature of the carriers created following doping of conjugated push-pull polymers with both F4 TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and FeCl3 . It is shown that for one particular push-pull material, the charge carriers created by doping are entirely non-conductive bipolarons and not single polarons, and that transient absorption spectroscopy following excitation in the infrared can readily distinguish the two types of charge carriers. Based on density functional theory calculations and experiments on multiple push-pull conjugated polymers, it is argued that the size of the donor push units determines the relative stabilities of polarons and bipolarons, with larger donor units stabilizing the bipolarons by providing more area for two charges to co-reside.

A high-conductivity n-type polymeric ink for printed electronics.

Conducting polymers, such as the p-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have enabled the development of an array of opto- and bio-electronics devices. However, to make these technologies truly pervasive, stable and easily processable, n-doped conducting polymers are also needed. Despite major efforts, no n-type equivalents to the benchmark PEDOT:PSS exist to date. Here, we report on the development of poly(benzimidazobenzophenanthroline):poly(ethyleneimine) (BBL:PEI) as an ethanol-based n-type conductive ink. BBL:PEI thin films yield an n-type electrical conductivity reaching 8 S cm-1, along with excellent thermal, ambient, and solvent stability. This printable n-type mixed ion-electron conductor has several technological implications for realizing high-performance organic electronic devices, as demonstrated for organic thermoelectric generators with record high power output and n-type organic electrochemical transistors with a unique depletion mode of operation. BBL:PEI inks hold promise for the development of next-generation bioelectronics and wearable devices, in particular targeting novel functionality, efficiency, and power performance.

Effects of a Fluorinated Donor Polymer on the Morphology, Photophysics, and Performance of All-Polymer Solar Cells Based on Naphthalene Diimide-Arylene Copolymer Acceptors.

Naphthalene diimide (NDI)-biselenophene copolymer (PNDIBS), NDI-selenophene copolymer (PNDIS), and the fluorinated donor polymer PM6 were used to investigate how a fluorinated polymer component affects the morphology and performance of all-polymer solar cells (all-PSCs). Although the PM6:PNDIBS blend system exhibits a high open-circuit voltage (Voc = 0.925 V) and a desired low optical bandgap energy loss (Eloss = 0.475 eV), the overall power conversion efficiency (PCE) was 3.1%. In contrast, PM6:PNDIS blends combine a high Voc (0.967 V) with a high fill factor (FF = 0.70) to produce efficient all-PSCs with 9.1% PCE. Furthermore, the high-performance PM6:PNDIS all-PSCs could be fabricated by various solution processing approaches and at active layer thickness as high as 300 nm without compromising photovoltaic efficiency. The divergent photovoltaic properties of PNDIS and PNDIBS when paired respectively with PM6 are shown to originate from the starkly different blend morphologies and blend photophysics. Efficient PM6:PNDIS blend films were found to exhibit a vertical phase stratification along with lateral phase separation, while the molecular packing had a predominant face-on orientation. Bulk lateral phase separation with both face-on and edge-on molecular orientations featured in the poor-performing PM6:PNDIBS blend films. Enhanced charge photogeneration and suppressed geminate and bimolecular recombinations with 99% charge collection probability found in PM6:PNDIS blends strongly differ from the poor charge collection probability (66%) and high electron-hole pair recombination seen in PM6:PNDIBS. Our findings demonstrate that beyond the generally expected enhancement of Voc, a fluorinated polymer component in all-PSCs can also exert a positive or negative influence on photovoltaic performance via the blend morphology and blend photophysics.

Phthalimide-based polymers for high performance organic thin-film transistors.

The synthesis and characterization of two new thiophene copolymers with backbone phthalimide units is reported. Thin-film optical and wide-angle X-ray diffraction measurements indicate extended electronic conjugation and close intermolecular pi-stacking for both polymers. Ambient carrier mobility of thin-film transistors prepared from these polymers is as high as 0.28 cm(2)/(V s) with an on/off ratio greater than 10(5).