Synthesis and characterization of imidazolium-substituted ionic (co)polythiophenes for applications in organic photovoltaics

Abstract

This manuscript is a compilation of the research work conducted over 3 years, mainly focusing on the GRIM polymerization procedure to synthesize bromine-functionalized polythiophene homopolymers, statistical copolymers and block co-polythiophenes, and their applications in organic photovoltaics. The standard poly (3-alkylthiophenes) synthesized in literature are generally soluble in rather apolar organic solvents. Via functionalization of the synthesized (co)polymers with ionic moieties, the materials become (more) soluble in environmentally acceptable solvents. With these ionic groups in place, the polymers also exhibit some special properties which effect the performance of organic solar cells. In a first instance it was demonstrated that the GRIM polymerization procedure is a versatile method to synthesize a broad range of brominefunctionalized random copolythiophenes. Post-polymerization modification of these polymers with N-methylimidazole via microwave activation resulted in a smooth and efficient conversion to the ionic copolymers. These ionic polymers are in general (more) soluble in polar solvents and some of them even in water. Later on, when the polymerization method was optimized, also ionic block copolythiophenes could be synthesized. UV-Vis measurements of the ionic (co)polythiophenes revealed an unusual solution behavior. The idea of aggregate formation was posed and dynamic light scattering (DLS) was used to get more insight in the size of the formed aggregates in solution. In good agreement with the UV-Vis studies, DLS confirmed that the P3HT-P3(MIM)HT-Br 50/50 statistical copolymer forms aggregates in water-rich solvent mixtures. The ionic block copolymers exhibit a similar solution behavior. The P3(MIM)HT-b-P3HT 50/50 and 70/30 block copolymers are soluble in water and have the tendency to form micelle-like structures in solution. Although it is quite clear that structures are formed in solution, the solubility characteristics were too complicated for DLS to give a clear view on the formed aggregates in solution. Cryo-TEM was hence executed to freeze the aggregate solution. In this way the aggregates could be visualized, but even then no clear conclusions could be drawn. From thermal analysis it could be concluded that all the synthesized precursor polymers are semi-crystalline. For all of these materials, a larger amount of bromine-functionalized repeating units lead to melting at lower temperatures. The semi-crystalline behavior changes to fully amorphous for the ionic P3(MIM)HT-Br polymers, where only a glass transition is seen. This lack of order was in agreement with the UV-Vis results on thin films. In both DSC and TGA it could be seen that the latter materials are hygroscopic. A striking change in the thermal behavior of the imidazolium-substituted polythiophenes was seen when the bromine counter ion was replaced by either TFSI or PF6. In the former case, a strong plasticizing effect was observed, while in the latter case the polymer could easily get semi-crystalline (after a coldcrystallization thermal annealing step). The P3BHT-b-P3HT precursor copolymers also showed clear semicrystalline behaviour and a trend was visible in the crystallinity; a larger P3HT block length leads to higher-melting crystals and a higher degree of crystallinity. It was also shown that the P3BHT blocks can crystallize as well, but at a much slower rate than the P3HT blocks. After functionalization one can see a clear Tg for the three ionic P3(MIM)HTb-P3HT block copolymers, which seems to increase with the P3(MIM)HT block length. A melting peak is only observed for the P3(MIM)HT-b-P3HT 30/70 block copolymer at a temperature of 181°C, which is similar to the melting temperature of the precursor block copolymer. It indicates that this transition corresponds to the P3HT block. Some of the novel materials were already applied in OPV. The main difference between a non-ionic and an ionic polythiophene is the fact that the latter one is not soluble in chlorobenzene, thus enabling processing of bi-layer configurations from solution. We have shown that our modified P3(MIM)HT-TFSI polymer shows the same or even slightly superior performance as compared to standard P3HT in a bilayer configuration. We underline that the novel ionic P3HT derivative synthesized in this work shows excellent film-forming properties and devices were highly reproducible. Using it in combination with PC71BM as the acceptor, power conversion efficiencies of 1.6% were achieved for these simple solution-processed bi-layer solar cells. On the other hand, we have also shown that the device performance of polymer solar cells can be remarkably improved by incorporation of a thin electron transport layer (ETL) based on an imidazolium-substituted ionic polythiophene (P3(MIM)HT-Br; 20% increase in PCE up to an average value of 6.2% for PCDTBT:PC71BM). The beneficial effect is notably higher than for previously reported materials such as an analogous trimethylamine-functionalized ionic polythiophene or PFN. Best results were obtained for the highest molecular weight ETL material, pointing to an important influence of polymer chain length on ETL performance. Remaining questions on the exact influence of polymer molecular weight (and its relation to active layer coverage) and the polythiophene backbone need to be addressed in future work.