High-Speed Photothermal Patterning of Doped Polymer Films.

Organic semiconductors (OSCs) offer a new avenue to the next-generation electronics, but the lack of a scalable and inexpensive nanoscale patterning/deposition technique still limits their use in electronic applications. Recently, a new lithographic etching technique has been introduced that uses molecular dopants to reduce semiconducting polymer solubility in solvents and a direct-write laser to remove dopants locally, enabling rapid OSC etching with diffraction limited resolution. Previous publications postulated that the reaction that enables patterning is a photochemical reaction between photoexcited dopants with neutral solvent molecules. In this work, we analyze the photoinduced dissolution kinetics of F4TCNQ doped P3HT films using time-resolved in situ optical probing. We find two competing mechanisms that control de-doping and dissolution: the first is the photochemical reaction posited in the literature, and the second involves direct heating of the polymer by the laser, inducing increased solubility for both the polymer and dopant. We show that the wavelength-specific photochemical effect is dominant in low photon doses while the photothermal effect is dominant with high excitation rates regardless of laser wavelength. With sufficiently high optical intensity input, the photothermal mechanism can in principle achieve a high writing speed up to 1 m/s. Our findings bring new insights into the mechanisms behind laser direct writing of OSCs based on dopant induced solubility control and enable ultraprecise fabrications of various device configurations in large-scale manufacturing

Molecular Dynamics Study of the Local Structure of Photovoltaic Polymer PCDTBT

To meet the huge demand for renewable energy, significant research effort focuses on creating efficient organic photovoltaic (OPV) devices. In comparison to silicon-based semiconductors, OPV materials have many superior properties such as cost effectiveness, being lightweight, and flexibility, which lead to a high potential for the replacement of silicon-based semiconductors. Recently, a large number of new alternating copolymer materials have demonstrated high power conversion efficiency (PCE). These successful polymers typically have low long-range order but a high hole mobility which directly affects the PCE which depends on polymer structure. In this study, a solution molecular model for poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl (PCDTBT) is developed and subsequently a molecular dynamics simulation conducted in order to understand the structure of the polymer solution. The simulation results are consistent with a low-solubility polymer that requires long equilibration times to planarize. The structural addition of side chains to inhibit rotation of thiophene rings could improve the conjugation and processability of PDCTBT leading to further improvements in OPV efficiency or hole mobility

Photochemical Charge Separation in Poly(3-hexylthiophene) (P3HT) Films Observed with Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap (<2.0 eV) and super-band-gap (>2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model. © 2013 American Chemical Society

Quantum motion of charges in ππ-conjugated polymers: kinkiness induces localization

We develop a model for charge trapping on conjugated polymer chains using a continuous representation of the polymer. By constraining the motion to along the chain, we find that kinks along the chain serve as points of attraction and can induce localization and spontaneous buckling of the chain. We implement this in a model system of a conjugated polymer donor and fullerene acceptor to mimic a polymer based photovoltaic system. We find that geometric fluctuations of the polymer give rise to deep potential energy traps at kinks which are often stronger than the Coulomb interaction between the electron and hole

Controlling Molecular Doping in Organic 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

Quantum motion of charges in ππ-conjugated polymers: kinkiness induces localization

We develop a model for charge trapping on conjugated polymer chains using a continuous representation of the polymer. By constraining the motion to along the chain, we find that kinks along the chain serve as points of attraction and can induce localization and spontaneous buckling of the chain. We implement this in a model system of a conjugated polymer donor and fullerene acceptor to mimic a polymer based photovoltaic system. We find that geometric fluctuations of the polymer give rise to deep potential energy traps at kinks which are often stronger than the Coulomb interaction between the electron and hole