Role of Sub-Nanometer Dielectric Roughness on Microstructure and Charge Carrier Transport in α<i>,</i>ω‑Dihexylsexithiophene Field-Effect Transistors

The effect of dielectric roughness on the microstructure evolution of thermally evaporated α,ω-dihexylsexithiophene (α,ω-DH6T) thin films from a single molecular layer to tens of monolayers (ML) is studied. Thereby, the surface roughness of dielectrics is controlled within a sub-nanometer range. It is found that the grain size of an α,ω-DH6T ML is affected by dielectric roughness, especially for 1.5 ML, whereby the transistor performance is barely influenced. This can be attributed to a domain interconnection in the second layer over a long-range formed on the rough surface. With deposition of more layers, both microstructure and charge carrier transport exhibit a roughness-independent behavior. The structural characterization of α,ω-DH6T 10 ML by grazing-incidence wide-angle X-ray scattering reveals that the interlayer distance is slightly decreased from 3.30 to 3.15 nm due to a higher roughness, while an unchanged π-stacking distance is in excellent agreement with the roughness-independent hole mobility. This study excludes the influence of molecular-solvent interaction and preaggregation taking place during solution deposition, and provides further evidence that the microstructure of the interfacial layer of organic semiconductors has only minor impact on the bulk charge carrier transport in thicker films

Positive Magneto-LC Effect in Conjugated Spin-Bearing Hexabenzocoronene

The first neutral spin carrying hexabenzocoronene (HBC) derivative is described. The conjugated phenyl nitroxide substituted HBC with five alkyl chains exhibits a positive magneto-LC effect in columnar hexagonal liquid crystalline phase as probed by differential scanning calorimetry and electron paramagnetic resonance spectroscopy. Surprisingly, at 140 K the Δ<i>M</i>_S = 2 transition can be observed indicating a thermally accessible triplet state between the neighboring molecules in the columnar arrangements

Modulation of Domain Size in Polycrystalline n‑Type Dicyanoperylene Mono- and Bilayer Transistors

A single molecular layer (monolayer) of organic semiconductors is proven to be sufficient to create a conducting channel for charge carriers in field-effect transistors, which is an ideal platform to investigate the correlation between molecular self-assembly and device performance. Herein, ultrathin films including mono- and bilayers of an n-type dicyanoperylene (PDI₈-CN₂) are solution-processed by dip-coating. The domain size of the polycrystalline layers is modulated <i>via</i> the surface roughness of the dielectric within an extremely narrow window from 0.15 to 0.39 nm. When the surface roughness is varied from smooth to rough, the domain size and molecular order in the monolayer are significantly decreased, leading to the reduction in electron mobility by 3 orders of magnitude. On the contrary, a lower roughness dependence is observed in the case of the bilayers, with only a slight difference in domain size and charge carrier transport. On the smooth surface, the bilayers exhibit a transistor performance identical to that of the bulk film, confirming that the first few layers near the dielectric dominate the charge carrier transport. Additionally, these results provide insights into the intrinsic role of the interfacial microstructure of small molecular organic semiconductors

Solution-Processable n‑Type Organic Semiconductors Based on Angular-Shaped 2‑(12<i>H</i>‑Dibenzo­fluoren-12-ylidene)malononitrilediimide

The angular-shaped n-type semiconductors 2-(12<i>H</i>-dibenzo­fluoren-12-ylidene)­malono­nitrile­diimide <b>2a</b> and <b>2b</b> were successfully designed, synthesized, and fully characterized by optical absorption and fluorescence, cyclic voltammetry, X-ray crystal structure analysis, XRD, and OFET device performance. The varying alkyl chain lengths of <b>2a</b> and <b>2b</b> caused different molecular orientations with respect to the substrate. Thus, <b>2a</b> presents an electron mobility of 0.01 cm² V^–1 s^–1, whereas <b>2b</b> resulted in poor device performance with a much lower electron mobility of 5 × 10^–4 cm² V^–1 s^–1

Balanced Ambipolar Organic Field-Effect Transistors by Polymer Preaggregation

Ambipolar organic field-effect transistors (OFETs) based on heterojunction active films still suffer from an imbalance in the transport of electrons and holes. This problem is related to an uncontrolled phase separation between the donor and acceptor organic semiconductors in the thin films. In this work, we have developed a concept to improve the phase separation in heterojunction transistors to enhance their ambipolar performance. This concept is based on preaggregation of the donor polymer, in this case poly­(3-hexylthiophene) (P3HT), before solution mixing with the small-molecular-weight acceptor, phenyl-C61-butyric acid methyl ester (PCBM). The resulting heterojunction transistor morphology consists of self-assembled P3HT fibers embedded in a PCBM matrix, ensuring balanced mobilities reaching 0.01 cm²/V s for both holes and electrons. These are the highest mobility values reported so far for ambipolar OFETs based on P3HT/PCBM blends. Preaggregation of the conjugated polymer before fabricating binary blends can be regarded as a general concept for a wider range of semiconducting systems applicable in organic electronic devices

Combination of Two Diketopyrrolopyrrole Isomers in One Polymer for Ambipolar Transport

Combination of Two Diketopyrrolopyrrole Isomers in One Polymer for Ambipolar Transpor

Thiadizoloquinoxaline-Based Low-Bandgap Conjugated Polymers as Ambipolar Semiconductors for Organic Field Effect Transistors

Two novel conjugated polymers with high molecular weight, <b>PBDTTQ-3</b> and <b>PAPhTQ</b>, were synthesized by tuning alkyl chains and alternating the electron-donating ability of the thiadiazoloquinoxaline (TQ) moiety. Both polymers have excellent solubility in common organic solvents. UV–vis–NIR absorption and cyclic voltammetry indicate a bandgap of (0.76 eV) and high electron affinity level (−4.08 eV) for <b>PBDTTQ-3</b>. Two dimensional wide-angle X-ray scattering shows that both polymers are only poorly ordered in the bulk but possess a close π-stacking distance of 0.36 nm. Despite the disorder in thin film observed by grazing incidence wide-angle X-ray scattering, <b>PBDTTQ-3</b> exhibits good ambipolar transport, with a maximum hole mobility of 0.22 cm² V^–1 s^–1 and comparable electron mobility of 0.21 cm² V^–1 s^–1

Tuning Packing and Solubility of Donor (D)–Acceptor (A) Polymers by <i>cis</i>–<i>trans</i> Isomerization within Alkenyl Side Chains

The impact of alkenyl substituents on the behavior of cyclopentadithiophene–benzothiadiazole (CDT–BTZ) donor (D)–acceptor (A) polymers in organic field-effect transistors (OFETs) and on the supramolecular organization was investigated. Linear <i>cis</i>- and <i>trans-</i>alkenes were attached to the donor unit of CDT–BTZ polymers to demonstrate the dependence of supramolecular ordering and solubility in organic solvents on chemical conformation. The layer interdigitation of the substituents differed due to shape disparities between <i>cis-</i> and <i>trans-</i>alkenes. While <i>trans-</i>alkenes exhibit zigzag structures that are beneficial for close packing, <i>cis</i>-alkenes are curved and thus possess a less regular shape that is disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies, which revealed shorter intermolecular distances for the polymer with <i>trans-</i>alkene substituents even in comparison to analogous polymers with saturated alkyl substituents. Furthermore, the isomerization of the <i>cis</i>-substituents toward their <i>trans-</i>conformers allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies

Tertiary Carbonate Side Chains: Easily Tunable Thermo-labile Breaking Points for Controlling the Solubility of Conjugated Polymers

We present a new class of solubilizing groups for conjugated polymers that enable solution processing of multilayer devices. Conjugated polymers in organic devices are sometimes difficult to process, because of their limited solubility. Well-soluble polymers decorated with alkyl side chains, however, introduce new challenges for thin-film deposition. Using the same solvent for multiple layers can dissolve the already applied layers. In this work, we introduce a new class of thermo-labile groups, which reduce the solubility of conjugated polymers after thermal treatment. Following a very modular approach, we can tune the temperature of the thermo-cleavage between 140 °C and 200 °C. This enables the fabrication of organic solar cells and field-effect transistors (FETs) with robust, solvent-resistant active layers

Solvent Annealing Effects in Dithieno[3,2‑<i>b</i>:2′,3′‑<i>d</i>]pyrrole–5,6-Difluorobenzo[<i>c</i>][1,2,5]thiadiazole Small Molecule Donors for Bulk-Heterojunction Solar Cells

Low-bandgap small molecule (SM) donors that can be solution-processed with fullerene acceptors (e.g., PC₆₁/₇₁BM) are proving to be particularly promising in bulk-heterojunction (BHJ) solar cells. Compared to their π-conjugated polymer counterparts, SM donors are well-defined (monodisperse) and more synthetically modular, with relatively wide ranges of bandgaps that can be achieved in stepwise couplings of various donor and acceptor motifs. However, the optimization of SM–fullerene morphologies and BHJ device efficiencies relies more specifically on the use of processing additives, postprocessing thermal, or solvent vapor annealing (SVA) approaches, and achieving adequate interpenetrating networks and structural order in BHJ thin films can be challenging. In this report, we examine the correlated effects of molecular structure and postprocessing SVA on the BHJ solar cell performance of a set of π-extended SM donors composed of dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole (DTP) and 5,6-difluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole ([2F]­BT) units. In these systems (<b>SM1–SM3</b>), the introduction of additional alkyl substituents and unsubstituted thiophene rings on the peripheral unit groups critically impacts the effects of SVA steps on BHJ solar cell efficiency. We show that the more π-extended and alkyl-substituted analogue <b>SM3</b> stands out, with BHJ device efficiencies of ∼6% obtained from SVA with CS₂, while SVA-treated <b>SM3</b>-based active layers also show the most favorable ordering and carrier mobility patterns. However, unlike numbers of SM donors reported in recent years, DTP–[2F]­BT SM analogues are in general not prone to dramatic performance variations in BHJ thin films cast with processing additives. Our results indicate that the role of SVA steps is not independent of the molecular structure of the SM donors used in the BHJ solar cells