Probing the formation of polarons in organic semiconductors and their optoelectronic properties
Creators: name: Nightingale, James
Publisher: name: Imperial College London
Year: 2019

description: Solution processable organic semiconducting polymers show favourable properties for applications in low-cost printed electronics, which offer unique capabilities and low-cost alternatives to conventional devices. The performance of devices is critically dependent on the formation and transport of charges in the organic semiconducting material. Electron-phonon coupling in conjugated polymers leads to a structural deformation when it becomes charged (so called ‘polaron’). Polaron formation has profound implications on the transport of charge carriers in conjugated polymers. In this work we develop a powerful structural probe that allows direct observation of polaron formation in situ using electrochemical resonant Raman spectroscopy (ERRS). We were able to elucidate the fundamental relationships between polaron formation and chemical composition/structure, polymer conformation and optoelectronic properties for a range of conjugated polymers. First, for the model semiconducting polymer, poly(3-hexyl)thiophene (P3HT) we find that molecular order plays a significant role in determining conformational changes induced during hole polaron formation. Our study provides direct spectroscopic evidence for a lower degree of lattice reorganisation in crystalline (and therefore more planarised) polymers than in conformationally disordered polymers. This observation is consistent with higher charge carrier mobility and better device performance commonly found in crystalline polymer materials. Blending P3HT with PCBM (a model solar cell system) disturbs polymer crystallinity and reduces the density of polarons that can form, whilst post-deposition thermal annealing partly restores this lost capacity for charge. Second, donor-acceptor type copolymers are an important class of materials, which display superior optoelectronic properties compared to homopolymers such as P3HT. We consider a series of copolymers that incorporate fused rings and acceptor units into the P3HT backbone and how their chemical design affects polaron formation. We find that fused thienothiophene units in poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) results in a large lattice reorganisation during polaron formation. By incorporating diketopyrrolopyrrole (DPP) units in poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene) (DPPT-TT) the lattice reorganisation can be completely reduced owing to the highly planar conformation of the polymer backbone. This contributes to the high hole mobility commonly reported for DPP based polymers. We investigate another planar donor-acceptor copolymer, indacenodithiophene–benzothiadiazole (IDTBT), but with much stronger D-A strength. We find this copolymer displaying much stronger D-A strength results in a redistribution of π-electron density which generates polarons with a relatively higher lattice reorganisation. The IDTBT shows the photo-generation of free charges in neat films; which is not observed for the other polymers studied here. Furthermore, we demonstrate that the degree of lattice reorganisation governs the polymers stability to electrochemical oxidative stress. The DPPT-TT displaying the lowest degree of lattice reorganisation shows the highest stability to electrochemically induced oxidative stress of all the polymers studied here. Our results are highly relevant to improve performance and operational stability in optoelectronic devices, in which charges are injected/ extracted and transported. Finally, we investigate the effect of a small molecule additive, DBSA, on polaron formation in conjugated polymers. For the case of P3HT we directly probe structural changes in the polymer when DBSA is added to the electrolyte. We find adding DBSA enhances the electrochemical doping effect of the electrolyte by lowering the onset of oxidation and allowing the generation of a higher polaron density. We employ DBSA by adding it to the gating electrolyte in an organic electrochemical transistor (OECT) to improve the device performance for a range of p-type polymers, including P3HT, PBTTT and DPPT-TT. This study demonstrates the application of DBSA for low-power accumulation mode OECTs, and opens the possibility to utilise otherwise unsuitable polymers, with deep HOMO levels, in OECT devices for biosensing applications.