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The determination of the band structure along k parallel to the chain direction demonstrates significant electronic delocalization. The small effective mass [m* = 0.106mo ] is consistent with the high measured mobility.

Four isomorphic organic semiconductors are compared to map out how the precision of chemical structures determines solid state molecular organization. Intramolecular electronic structure and intermolecular packing preference in the solid state were shown to be exclusively dependent on the relative location of the electron accepting fragments within a given molecule.

Field-effect transistors fabricated from semiconducting conjugated polymers are candidates for flexible and low-cost electronic applications. Here, we demonstrate that the mobility of high molecular weight (300 kDa) regioregular, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] can be significantly improved by introducing long-range orientation of the polymer chains. By annealing for short periods, hole mobilities of 6.7 cm(2)/(V s) have been demonstrated. The transport is anisotropic, with a higher mobility (approximately 6:1) parallel to the polymer backbone than that perpendicular to the polymer backbone.

Increasing the molecular weight of the low-bandgap semiconducting copolymer, poly[(4,4-didoecyldithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], Si-PDTBT, from 9 kDa to 38 kDa improves both photoresponsivity and charge transport properties dramatically. The photocurrent measured under steady state conditions is 20 times larger in the higher molecular weight polymer (HM{sub n} Si-PDTBT). Different decays of polarization memory in transient photoinduced spectroscopy measurements are consistent with more mobile photoexcitations in HM{sub n} Si-PDTBT relative to the lower molecular weight counterpart (LM{sub n} Si-PDTBT). Analysis of the current-voltage characteristics of field effect transistors reveals an increase in the mobility by a factor of 700 for HM{sub n} Si-PDTBT. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence small angle X-ray scattering (GISAXS) measurements demonstrate that LM{sub n} Si-PDTBT forms a disordered morphology throughout the depth of the film, whereas HM{sub n} Si-PDTBT exhibits pronounced {pi}-{pi} stacking in an edge-on configuration near the substrate interface. Increased interchain overlap between polymers in the edge-on configuration in HM{sub n} Si-PDTBT results in the higher carrier mobility. The improved optical response, transport mobility, and interfacial ordering highlight the subtle role that the degree of polymerization plays on the optoelectronic properties of conjugated polymer based organic semiconductors.

Phosphorescent light-emitting transistors, in which light emission from singlet and triplet energy levels is harvested using solution-processed materials (see figure), are presented. While a green phosphorescent dendrimer exhibits an external quantum efficiency of 0.45% at 480 cd m(-2), a red polymer/phosphorescent small-molecule blend produces a brightness exceeding 30 cd m(-2) with a relatively high hole mobility of 2.5 x 10(-2) cm(2) V-1 s(-1).

We report the controlled nanomorphology of semiconducting polymers on chemically and mechanically stable nanogrooved polymer substrates. By employing silicon dioxide thin films with finely adjusted thicknesses on nanogrooved polymer substrates, semiconducting polymer thin films oriented and aligned along the nanogrooves were obtained. Organic field-effect transistors (OFETs) fabricated from the oriented semiconducting polymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT), yielded saturation hole mobilities as high as 19.3 cm(2) V(-1 )s(-1), and the flexible "plastic" transistors demonstrated excellent mechanical stability under various bending conditions. These results represent important progress for solution-processed flexible OFETs and demonstrate that directed self-assembly of semiconducting polymers can be achieved by soft nanostructures.

The function and utility of inserting an in situ polymerizable triphenylamine-containing polyperfluorocyclobutane (TPA–PFCB) as a hole transport and electron blocking layer into the structure of bulk heterojunction (BHJ) solar cells were demonstrated. The open-circuit voltage (Voc) and short-circuit current (Jsc) increase, leading to BHJ solar cells with improved power conversion efficiency.

A series of narrow-band gap conjugated molecules with specific fluorine substitution patterns has been synthesized in order to study the effect of fluorination on bulk thermal stability. As the number of fluorine substituents on the backbone increase, one finds more thermally robust bulk structures both under inert and ambient conditions as well as an increase in phase transition temperatures in the solid state. When integrated into field-effect transistor devices, the molecule with the highest degree of fluorination shows a hole mobility of 0.15 cm2/V·s and a device thermal stability of >300 °C. Generally, the enhancement in thermal robustness of bulk organization and device performance correlates with the level of C–H for C–F substitution. These findings are relevant for the design of molecular semiconductors that can be introduced into optoelectronic devices to be operated under a wide range of conditions.

Increases in the molecular length of narrow band gap conjugated chromophores reveal potentially beneficial optical and electronic properties, thermal stabilities, and high power conversion efficiencies when integrated into optoelectronic devices, such as bulk heterojunction organic solar cells. With the objective of providing useful information for understanding the transition from small-sized molecules to polymers, as well as providing a general chemical design platform for extracting relationships between molecular structure and bulk properties, we set out to vary the electron affinity of the molecular backbone. Therefore, a series of donor (D)-acceptor (A) alternating narrow band gap conjugated chromophores were synthesized based on the general molecular frameworks: D(1)-A(1)-D(2)-A(2)-D(2)-A(1)-D(1) and D(1)-A(1)-D(2)-A(2)-D(2)-A(2)-D(2)-A(1)-D(1). When the central electron-accepting moiety (A(2)) was varied or modified, two classes of molecules could be compared. First, we showed that the alteration of one single electron-accepting group, while maintaining the shape of the molecular framework, can effectively impact the optical properties and energy levels of the molecules. DFT ground state structure optimizations show similar "U" shape conformations among these molecules. Second, we examined how the site-specific introduction of fluorine atom(s) modifies the thermal properties in the solid state, while maintaining relatively similar optical and electrochemical features of interest. Structure-property relationship of such molecular systems could be rationally evaluated in the aspects of thermal-responsive molecular organizations in the solid state and dipole moments both in the ground and excited states. The impact of molecular structure on charge carrier mobilities in field effect transistors and the performance of photovoltaic devices were also studied.

Polymer light emitting field effect transistors are a class of light emitting devices that reveal interesting device physics. Device performance can be directly correlated to the most fundamental polymer science. Control over surface properties of the transistor dielectric can dramatically change the polymer morphology, introducing ordered phase. Electronic properties such as carrier mobility and injection efficiency on the interface can be promoted by ordered nanofibers in the polymer. Moreover, by controlling space charge in the polymer interface, the recombination zone can be spatially extended and thereby enhance the optical output.

Spectrally dependent steady-state photoconductivity is a convenient method to gain insight into the charge generation and transport processes within a given material. In this work, we report on the photoconductive response of solution-processed neat films and blends of the fullerene, PC(71)BM, and the donor-acceptor small-molecule, p-DTS(PTTh(2))(2), as function of the processing additive, diiodooctance (DIO). The results, when considered in the context of their structural, optical, and electronic properties give insight into the dominant carrier generation and charge transport mechanisms in each of these molecular systems.

π-Conjugated, narrow band gap copolymers containing pyridal[2,1,3]thiadiazole (PT) were synthesized via starting materials that prevent random incorporation of the PT heterocycles relative to the backbone vector. Two regioregular structures could be obtained: in one the PTs are oriented in the same direction, and in the other the orientation of the PTs alternates every other repeat unit. Compared to their regiorandom counterparts, the regioregular polymers exhibit a 2 orders of magnitude increase of the hole mobilites, from 0.005 to 0.6 cm(2) V(-1) s(-1), as determined by field-effect transistor measurements.

The split-gate light emitting field effect transistors (SG-LEFETs) demonstrate a new strategy for ambipolar LEFETs to achieve high brightness and efficiency simultaneously. The SG architecture forces largest quantity of opposite charges on Gate 1 and Gate 2 area to meet in the center of the channel. By actively and independently controlling current injection from separated gate electrodes within transporting channel, high brightness can be obtained in the largest injection current regime with highest efficiency.

The optoelectronic gate light-emitting field-effect transistor (OEG LEFET) containing alternate SiO2 and SiNx dielectric stacks allows enhanced light emission for a designed spectrum range and provides reliable strength for a wide operating voltage. The device can improve the emission efficiency by 4.5 times in comparison to a reference LEFET with a SiNx gate dielectric and can reach a brightness as high as 4500 cd/m2.

A split-gate field effect transistor containing four electrodes, source, drain, two gates allows enhanced transport for specific carrier species and separate control of carrier polarity over two gate regimes. The device can be operated as a transistor or a diode by controlling gate biases.

We have demonstrated a novel sensing strategy employing single-stranded probe DNA, unmodified gold nanoparticles, and a positively charged, water-soluble conjugated polyelectrolyte to detect a broad range of targets including nucleic acid (DNA) sequences, proteins, small molecules, and inorganic ions. This nearly “universal” biosensor approach is based on the observation that, while the conjugated polyelectrolyte specifically inhibits the ability of single-stranded DNA to prevent the aggregation of gold-nanoparticles, no such inhibition is observed with double-stranded or otherwise “folded” DNA structures. Colorimetric assays employing this mechanism for the detection of hybridization are sensitive and convenient—picomolar concentrations of target DNA are readily detected with the naked eye, and the sensor works even when challenged with complex sample matrices such as blood serum. Likewise, by employing the binding-induced folding or association of aptamers we have generalized the approach to the specific and convenient detection of proteins, small molecules, and inorganic ions. Finally, this new biosensor approach is quite straightforward and can be completed in minutes without significant equipment or training overhead.

To develop environmentally friendly solvent processed high performance polymer solar cells (PSCs), an alcohol soluble narrow-band-gap conjugated polymer PCDTBT-N and an alcohol soluble C70 fullerene derivative PC71BM-N, which were functionalized with pendant tertiary amino groups, were developed and used as active layers in PSCs and as buffer layers in metal–semiconductor interfaces. Though solar cells with active layers containing amino groups showed no photovoltaic properties regardless of the processing conditions, both PCDTBT-N and PC71BM-N performed well as buffer layers to improve electron collection in PSCs. Space charge limited current (SCLC), field effect transistor (FET) and cyclic voltammetry (CV) studies revealed that the amino groups act as hole traps and disable hole transport in active layers. Moreover, ultraviolet photoelectron spectroscopy (UPS) indicates electronic structure changes and Fermi level shifts upon complexation of amino groups and the fullerene core, which may diminish the electron-accepting capacity of fullerenes, and may potentially provide a different kind of hole trap in the devices. The underlying mechanisms of the puzzling behavior of PCDTBT-N and PC71BM-N in different layers of PSCs were studied by numerical simulation, which indicates that hole traps distributed in the BHJ layer reduce device performance, conversely hole traps concentrated near the metal electrode can improve device performance. Besides, the formation of an amino group:C70 complex is another important cause of the performance improvements of the resulting solar cells using PCDTBT-N and PC71BM-N as buffer layers, due to the reduced transport loss for efficient electron collection through the n-doping of PC71BM at the interface of the buffer layer and the active layer.

Regioregular conjugated polymers containing alternating pyridyl[2,1,3]thiadiazole (PT) and indacenodithiophene (IDT) structural units were synthesized. In these copolymers, the pyridyl nitrogen atoms on PT are precisely arranged along the backbone so that each one has an adjacent proximal and an adjacent distal counterpart across the two IDT flanking units. We find that despite the absence of obvious differences in orbital energy levels and optical bandgap, the regioregular materials exhibit larger charge carrier mobilities, as determined by using field effect transistor devices, and can yield higher solar cell power conversion efficiencies when mixed with fullerenes in bulk heterojunction active layers.

Solution-processed cross-linkable tetraphenyldiamine-containing material (TPD-BVB) as a highly efficient hole selective transport layer was demonstrated. Polymer solar cells (PSCs) with an inverted structure fabricated with a thin cross-linked TPD-BVB film show comparable efficiency and superior long-term air stability when compared to devices fabricated with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Thus, solution-processed TPD-BVB is an attractive alternative to PEDOT:PSS as a hole extraction layer in inverted structure PSCs.

We show that p- and n-type light emitting field-effect transistors (LEFETs) can be made using “superyellow” as a light-emitting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as a p-type material and a naphthalene di-imide as an n-type material. By connecting two of these LEFETs, we have demonstrated a light emitting complementary inverter (LECI). The LECI exhibited electrical and optical characteristics in the first and third quadrant of the transfer characteristics with voltage gain of 6 and 8, respectively.