Phase behavior and state diagrams of push-pull conjugated systems for organic optoelectronics: importance of intercalation and homocoupling

Organic optoelectronics such as organic photovoltaics (OPV) and organic photodetectors (OPD) have gained a boost in development during the past decades. Although it is known that the complex morphology of active layers in OPV/OPD directly influences the device performance, the current optimization of the morphology remains a time-consuming trial-and-error approach. Therefore, this thesis focuses on a rational methodology to guide the control of the morphology for active layers, i.e. state diagrams, which directly reveal the composition-temperature-phase behavior (morphology) relationships in optoelectronic devices. In this PhD, accurate state diagrams (i.e. out of thermodynamic equilibrium) were constructed for a benchmark system, PBTTT:PC61BM, and two novel derivatives mixed with PC61BM, using RHC combined with T-resolved synchrotron SAXS/WAXS. PBTTT is known to show unique co-crystals when mixed with PC61BM, by intercalating the fullerene acceptor into the side chains of the polymer. Such co-crystals exhibit an optimal interface between the polymer and the fullerene and are crucial for the promotion of the weak CT absorption in optoelectronic devices (especially important in cavity based NIR-OPD). The two novel derivatives of PBTTT, i.e. PBTTT-OR-R and PBTTT-(OR)2, were developed to have improved electronic properties while also forming intercalated co-crystals with PC61BM. Investigating this potential co-crystal formation was one of the main goals of this work. In addition, homocoupling, a common defect in high-performance donor polymers, may influence the intercalated co-crystals. Therefore, the three polymer:PC61BM systems with and without homocoupling were investigated, in order to systematically construct their state diagrams, with a particular focus on the potential formation of cocrystals and the influence of homocoupling defects. In this thesis, the homocoupling-free PBTTT, and its two novel derivatives as well as their 45:55 mixtures (approaching the theoretical 1:1 molar mixing ratio) with PC61BM, are studied by RHC combined with T-resolved synchrotron SAXS/WAXS. The three polymers all show the side chain melting/crystallization at lower temperature and a backbone melting/crystallization at higher temperature. It is seen that the more flexible alkoxy side chains exhibit a lower Tm,endset than alkyl side chains, while the presence of alkoxy side chains increases the Tm,endset of backbone melting to much higher temperature. The desired co-crystals are found for all three 45:55 mixtures, showing a comparable Tm, endset and SAXS/WAXS diffractions which are completely different from the pure polymers and PC61BM. Pure co-crystals were observed to melt incongruently forming C61BM crystals. Besides the studied pure polymers and pure co-crystals, a wider range of mixtures were investigated by RHC combined with T-resolved synchrotron SAXS/WAXS, to systematically construct complete state diagrams for homocoupling-free PBTTT and its two novel derivatives mixed with PC61BM. It was shown that state diagrams of homocoupling-free PBTTT:PC61BM and PBTTT-OR-R:PC61BM both show a eutectic between the polymer and co-crystals at the polymer-rich side and incongruent melting of co-crystals at higher PC61BM contents. For homocoupling-free PBTTT-(OR)2:PC61BM, the state diagram shows a eutectic between polymer and PC61BM. Based on a theoretical phase diagram, the differences between the three systems were linked to Tm,endset of the backbone melting, and the incongruent melting of co-crystals was confirmed to take place. In analogy to the homocoupling-free systems, the same three polymer:PC61BM systems, this time containing homocoupling defects in the polymers, were also investigated. It was shown that the effect of homocoupling defects on the co-crystal formation is related to the amount of such defects in the materials. The co-crystal formation in hc-PBTTT:PC61BM is hardly influenced, and in hc-PBTTT-OR-R:PC61BM is markedly affected, while hc-PBTTT-(OR)2:PC61BM cannot form co-crystals. This follows the trend of the amount of homocoupling defects in the three polymers, i.e. hc-PBTTT < hc-PBTTT-OR-R < hc-PBTTT-(OR)2. The state diagrams were successfully constructed for hc-PBTTT:PC61BM and hc-PBTTT-OR-R:PC61BM, but not for hc-PBTTT-(OR)2:PC61BM as no clear transitions were seen, likely related to the very high amount of homocoupling in hc-PBTTT-(OR)2. The state diagram of hc-PBTTT:PC61BM is similar to that of homocoupling-free PBTTT:PC61BM, while the state diagram of hc-PBTTT-OR-R:PC61BM is significantly different from the homocoupling-free equivalent, which again seems to follow the trend of amount of defects present. The latter exhibits a eutectic between the polymer and PC61BM, which is reminiscent of the homocoupling-free PBTTT-(OR)2:PC61BM. Based on the theoretical phase diagram, the differences between the two systems were linked to the downward shift of the co-crystals liquidus line. Last but not least, the effect of homocoupling on phase behavior in these polymer:PC61BM mixtures was preliminarily linked to the device performance. The EQE, a crucial parameter reflecting the device performance, was measured for homocoupling-free and homocoupling-containing PBTTT:PC61BM and PBTTT-(OR)2:PC61BM 20:80 mixtures. Consistent with the observed trends in the state diagrams, which reflected the minimal amount of homocoupling in hc-PBTTT and the much more significant amount in hc-PBTTT-(OR)2, similar EQE spectra were obtained for the homocoupling-free and homocoupling containing PBTTT:PC61BM, while the EQE spectra of hc-PBTTT-(OR)2:PC61BM showed obviously lower EQE at the CT absorption region than for the homocoupling-free version. In this thesis, RHC combined with T-resolved synchrotron SAXS/WAXS proved an excellent tool for the successful construction of state diagrams. The obtained state diagrams for the three polymer:PC61BM systems offer new insights regarding the phase behavior, and how this is impacted by the very common homocoupling defects, which may be valuable knowledge for the performance optimization of future organic optoelectronic devices.