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Molecular doping of conjugated polymers causes bleaching of the neutral absorbance and results in new polaron absorbance transitions in the mid and near infrared. Here, the concentration dependent changes in the spectra for a series of molecularly doped diketopyrrolopyrrole (DPP) co‐polymers with a series of ultra‐high electron affinity cyanotrimethylenecyclopropane‐based dopants is analyzed. With these strong dopants the polaron mole fraction (Θ) reaches saturation. Analysis of the full spectrum enables separation of neutral and polaron signals and quantification of the polaron mole fraction using a simple noninteracting site model. The peak ratios for both neutral and polaron peaks change systematically with increasing polaron mole fraction for all measured polymers. Analysis of the spectral changes indicates that the polaron mole fraction can be quantified to within 5%. While the total change in the absorbance spectrum with increasing polaron mole fraction is linear, the lowest energy polaron peak (P1) grows nonlinearly, which indicates increased polarization/delocalization. Molecular doping of polymers that form either H‐ or J‐aggregates shows systematically different spectral changes in the vibronic peak ratios of the neutral spectra and provides insights into the polymer configuration at undoped sites in the film.

 

Patterned semiconductors are essential for the fabrication of nearly all electronic devices. Over the last two decades, semiconducting polymers (SPs) have received enormous attention due to their potential for creating low‐cost flexible electronic devices, while development of scalable patterning methods capable of producing sub‐μm feature sizes has lagged. A novel method for patterning SPs termed Projection Photothermal Lithography (PPL) is presented. A lab scale PPL microscope is built and it is demonstrated that rapid (≈4 cm²h⁻¹) and large single exposure area (≈0.69 mm²) sub‐μm patterns can be obtained optically. Polymer domains are selectively removed via a photo‐induced temperature gradient that enables dissolution. It is hypothesized that commercial‐scale patterning with a throughput of≈5 m²h⁻¹and resolution of<1μm could be realized through optimization of optical components.

 

Recent development of dopant induced solubility control (DISC) patterning of polymer semiconductors has enabled direct‐write optical patterning of poly‐3‐hexylthiophene (P3HT) with diffraction limited resolution. Here, the optical DISC patterning technique to the most simple circuit element, a wire, is applied. Optical patterning of P3HT and P3HT doped with the molecular dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) wires with dimensions of 20–70 nm thickness, 200–900 nm width, and 40 μm length is demonstrated. In addition, optical patterning of wire patterns like “L” bends and “T” junctions without changing the diameter or thickness of the wires at the junctions is demonstrated. The wires themselves show up to 0.034 S cm^‐1conductance when sequentially doped. It is also demonstrated that a P3HT nanowire can be doped, de‐doped, and re‐doped from solution without changing the dimension of the wire. The combined abilities to optically pattern and reversibly dope a polymer semiconductor represents a full suite of patterning steps equivalent to photolithography for inorganic semiconductors.

 

The field of organic electronics thrives on the hope of enabling low‐cost, solution‐processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution‐processed p‐type doped polymeric semiconductors. Highlighted topics include how solution‐processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication—applications beyond those directly analogous to inorganic doping.

 

p-Type molecular dopants are a class of high electron affinity (EA) molecules used to ionize organic electronic materials for device applications. It is extremely challenging to ionize high-performance, high-ionization energy (IE) polymers because the dopant molecule needs to be compatible with solution processing. Here, we describe the synthesis and characterization of two new solution processable molecular dopants with the highest EA values yet reported. These molecules, based on the parent hexacyanotrimethylenecyclopropane (CN6-CP) structure, achieve solubility by the substitution of one or more of the cyano groups with esters, which both reduces the volatility relative to CN6-CP and allows for solution processing. The efficacy of these new molecular dopants, which have EA values up to 5.75 eV with respect to vacuum, was tested by performing sequential solution doping experiments with a series of thiophene and alternating diketopyrrolopyrrole polymers with IEs ranging from 5.10 eV to 5.63 eV. For completeness, the new dopant results are compared to a previously reported tri-ester substituted CN6-CP analogue with an EA of 5.50 EV. The increased EA of these stronger dopants induces a 10–100 fold increase in film conductivity and saturation of the conductivity at 15–100 S cm −1 for almost all polymers tested. These new dopant structures enable controlled solution doping at high doping levels for most alternating co-polymers of interest to the organic electronics community. 

There is a critical need to develop a method to pattern semiconducting polymers for device applications on the sub-micrometer scale. Dopant induced solubility control (DISC) patterning is a recently published method for patterning semiconductor polymers that has demonstrated sub-micron resolution. DISC relies on the sequential addition of molecular dopants (here 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)) to the conjugated polymer. In doped areas, the conjugated polymer is protected from dissolution while in undoped areas, the polymer dissolves into solution. Here we examine factors that affect the resolution of the developed pattern. Two factors are determined to be critical to pattern resolution, the initial crystallinity of the polymer, here poly(3-hexylthiophene) (P3HT), and the quality of the development solvent. We find that dopants diffuse more readily in highly crystalline films than in amorphous films of P3HT and that dopant diffusion reduces the fidelity of the resulting pattern. We also find that the choice of development solvent affects both the fidelity of the pattern and dopant distribution within the patterned polymer domains. Finally, we show that a dopant that diffuses more slowly than F4TCNQ in the P3HT film can be used to pattern the film with higher fidelity. These results together provide a road map for optimizing additive DISC patterning for any polymer/dopant pair.