Enhanced and Anisotropic Charge Transport in Polymer-Based Thin-Film Transistors by Guiding Polymer Growth

Ideal molecular features and microstructural properties of organic semiconducting thin films are being explored to achieve high-performance organic thin-film transistors (OTFTs). We prepared and processed hexamethylbenzene (HMB)/poly­(3-hexylthiophene) (P3HT) mixtures using a thermal gradient system to fabricate P3HT-based OTFTs. In the thermal gradient system, the HMB separated from the HMB/P3HT mixtures and crystallized along the sample movement direction. The crystallized HMB affected and guided the growth behavior of P3HT at the molecular level. Observations from joint microscopic and spectroscopic analyses revealed that the HMB-processed P3HT (H-P3HT) thin film possessed anisotropic and improved microstructures, particularly in crystalline domains. The improved molecular features and microstructural properties of the H-P3HT thin film enhanced the intramolecular and intermolecular charge transport by extending the π-conjugation, decreasing the reorganization energy, and strengthening the π–π overlaps. The electrical performance of the H-P3HT OTFT was augmented significantly with respect to that of the spin-coated P3HT OTFT. In addition, the H-P3HT OTFT exhibited an anisotropic charge transport property, correlating with microstructure directionality and resulting from the difference in the directions of the π–π overlaps. This effective and simple technique can be applied to other device types and has the potential to achieve high-performance organic electronic/photonic devices

Synergistic Effects of Binary-Solvent Annealing for Efficient Polymer–Fullerene Bulk Heterojunction Solar Cells

Conjugated polymer–fullerene-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted tremendous attention over the past two decades because of their potential to develop low-cost and easy methods to produce energy from light. The complicated microstructure and morphology with randomly organized architecture of these polymer–fullerene-based active layers (ALs) is a key factor that limits photovoltaic performance. In this study, a binary-solvent annealing (BSA) approach was established to improve the poly­(3-hexylthiophene):indene-C₆₀ bisadduct-based AL for efficient BHJ-type OSCs by varying the second solvents with different boiling points (BP). Thus, we were able to change the evaporation behavior of cosolvents and consequently obtain the various microstructural properties of the AL. An in-depth study was conducted on the solvent-evaporation driven morphology of the active layer under various cosolvent conditions and its effect on the photovoltaic parameters of OSCs. Under the BSA processes, we found that the specimens with low-BP second solvents allows us to observe a more ideal AL for increasing photon absorption and efficient charge transport and collection at the respective electrodes, resulting in enhanced PCE of the corresponding OSCs. By contrast, the specimens with high-BP second solvents exhibit random microstructures, which are detrimental to charge transport and collection and lead to diminished PCE of the corresponding OSCs. By appropriately selecting the composition of a binary solvent, BSA can be employed as an easy method for the effective manipulation of the microstructures of ALs. BSA is a promising technique for the performance enhancement of not only OSCs but also other organic/polymeric-based electronic devices