The mechanisms behind the threshold-voltage shift in organic transistors due to functionalizing of the gate dielectric with self-assembled monolayers (SAMs) are still under debate. We address the mechanisms by which SAMs determine the threshold voltage, by analyzing whether the threshold voltage depends on the gate-dielectric capacitance. We have investigated transistors based on five oxide thicknesses and two SAMs with rather diverse chemical properties, using the benchmark organic semiconductor dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene. Unlike several previous studies, we have found that the dependence of the threshold voltage on the gate-dielectric capacitance is completely different for the two SAMs. In transistors with an alkyl SAM, the threshold voltage does not depend on the gate-dielectric capacitance and is determined mainly by the dipolar character of the SAM, whereas in transistors with a fluoroalkyl SAM the threshold voltages exhibit a linear dependence on the inverse of the gate-dielectric capacitance. Kelvin probe force microscopy measurements indicate this behavior is attributed to an electronic coupling between the fluoroalkyl SAM and the organic semiconductor.

The one‐step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3‐hexylthiophene) (P3HT) as well as P3HT:fullerene bulk–heterojunction blends can be spin‐coated from a mixture of the crystallizable solvent 1,3,5‐trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square‐centimeter‐sized domains that are composed of one spherulite‐like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite‐like structures. Moreover, grazing‐incidence wide‐angle X‐ray scattering reveals an increased relative degree of crystallinity and predominantly flat‐on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip‐coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi‐crystalline conjugated polymer systems is established. Those include other poly(3‐alkylthiophene)s, two polyfluorenes, the low band‐gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends.

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance–voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency.

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We address the impact of the relative orientation between donor (D) and acceptor (A) molecules at the D/A heterojunction on the exciton dissociation. For this purpose, two-dimensional heterojunctions of diindenoperylene (DIP) and N,N′-dioctyl-3,4,9,10-perylene tetracarboxylicdiimide (PTCDI-C8) deposited onto SiO2/Si are grown, which exemplify two model interfaces with the π-staking direction either perpendicular or parallel to the interface. Aspects related to the morphology of the heterojunctions and charge photogeneration are studied by scanning probe force methods and photoluminescence (PL) spectroscopy. Results from PL spectroscopy indicate that the exciton dissociation is influenced by the different relative molecular orientations of A and D. For the configuration with stronger orbital overlap between A and D at the interface, the exciton dissociation is dominated by recombination from an interfacial charge-transfer state.