Role of Postdeposition Thermal Annealing on Intracrystallite and Intercrystallite Structuring and Charge Transport in Poly(3-hexylthiophene)

Abstract

The performance of electronic devices comprising conjugated polymers as the active layer depends not only on the intrinsic characteristics of the materials but also on the details of the extrinsic processing conditions. In this study, we examine the effect of postdeposition thermal treatments on the microstructure of poly­(3-hexylthiophene) (P3HT) thin films and its impact on their electrical properties. Unsurprisingly, we find thermal annealing of P3HT thin films to generally increase their crystallinity and crystallite coherence length while retaining the same crystal structure. Despite such favorable structural improvements of the polymer active layers, however, thermal annealing at high temperatures can lead to a net reduction in the mobility of transistors, implicating structural changes in the intercrystallite amorphous regions of these semicrystalline active layers take place on annealing, and the simplistic picture that crystallinity governs charge transport is not always valid. Our results instead suggest tie-chain pullout, which occurs during crystal growth and perfection upon thermal annealing to govern charge transport, particularly in low-molecular-weight systems in which the tie-chain fraction is low. By demonstrating the interplay between intracrystallite and intercrystallite structuring in determining the macroscopic charge transport, we shed light on how structural evolution and charge-transport properties of nominally the same polymer can vary depending on the details of processing.

Critical Factors of Diarylethene for a High-Performance Photoprogrammable Polymer Transistor: Crystallographic Compatibility, Quantum Yield, and Fatigue Resistance

“…To realize highly efficient photoprogrammable devices, DAE needs to be incorporated within the intercrystallite amorphous region where tie chains communicate between crystallites. DPPDTT, which has a stiffer and more planar backbone compared to conventional homopolymer semiconductors such as poly­(3-hexylthiophene), tends to contain more tie chains in pristine condition without annealing, which is why we did not use thermal annealing . Therefore, we speculate that the DPPDTT/DAE-316 blend system had a higher fraction of charge trap sites generated by DAE-316_c near the tie chains of the intercrystallite amorphous regions, resulting in optimal crystallographic compatibility for effectively enhancing the performance of photoprogrammable switching devices.…”

“…DPPDTT, which has a stiffer and planar backbone compared to conventional homopolymer semiconductors such as poly(3-hexylthiophene), tends to contain more tie chains in pristine condition without annealing, which is why we did not use thermal annealing. 27 Therefore, we speculate that the DPPDTT/DAE-316 blend system had a higher fraction of charge trap sites generated by DAE-316_c near the tie chains of the intercrystallite amorphous regions, resulting in optimal crystallographic compatibility for effectively enhancing the performance of photoprogrammable switching devices. NEXAFS spectroscopy is a soft X-ray technique used to investigate the molecular orientation of organic films based on the energy differences between the resonant X-ray excitations of core-level (1s) electrons and unoccupied σ* or π* orbitals.…”

“…In a typical semicrystalline polymer semiconductor, it is well known that tie chains located at intercrystallite amorphous regions are responsible for electrical communication between crystallites. The tie chain fraction is thus a crucial factor for macroscopic charge transport. − Therefore, the preferential location of DAE at such intercrystallite amorphous regions is a very important requirement for effectively modulating the charge transport of polymer semiconductors. As such, the crystallographic compatibility between DAE and matrix polymer OSCs is of primary importance, although this issue has not yet been thoroughly considered.…”

Gaining control over conjugated polymer morphology to improve the performance of organic electronics

“…38 Finally, the role of polymer ties chains on charge-transporting properties of semicrystalline CPs was brought forward in Section 2.1. 74 However, even though prior research implied a certain relationship between the tie-chain assisted alignment of the neighboring crystallites and charge mobility increase in the strain direction, 89,90 no proof of the tie-chain effect on the mechano-electrical CP characteristics has been reported. With this motivation in mind, Gu et al chose two samples of regioregular P3HT with known tie-chain fractions, namely P3HT_40 and P3HT_5, to investigate the microstructural changes of the films upon strain and underpin the role of tie chains on the relationship between their electrical and mechanical characteristics.…”

Recent Advances of Film–Forming Kinetics in Organic Solar Cells

“…Here, we summarize in Table 1 the critical works on film-forming kinetics in OSCs, and the cartoon illustration is shown in Figure 2. Due to the semicrystalline character of the conjugated molecules, the crystalline phase and amorphous phase always coexist in the active layer [78][79][80][81][82][83]. For the crystalline phase, the degree of intermolecular coupling is high, which reduces the energy barrier of carrier transport and is of benefit for carrier mobility [84].…”