High Bulk Electron Mobility Diketopyrrolopyrrole Copolymers with Perfluorothiophene

Abstract

The question of designing high electron mobility polymers by increasing the planarization using diffusive nonbonding heteroatom interactions in diketopyrrolopyrrole polymers is addressed in this. For this, three different diketopyrrolo[3,4‐c]pyrrole (DPP) derivatives with thienyl‐, 2‐pyridinyl‐, and phenyl‐flanked cores are copolymerized with an electron‐rich thiophene unit as well as an electron‐deficient 3,4‐difluorothiophene unit as comonomer to obtain diverse polymeric DPPs which vary systematically in their structures. The crystallinity differs significantly with clear trends on varying both flanking unit and comonomer. The optical gap and energy levels depend more on the nature of the flanking aryl units rather than on fluorination. Additionally, the charge transport properties are compared using different methods to differentiate between interface or orientation effects and bulk charge carrier transport. In organic field effect transistor devices with very high electron as well as hole mobilities (up to 0.6 cm2 V−1 s−1) are obtained and fluorination leads to a more pronounced n‐type nature in all polymers, resulting in ambipolar behavior in otherwise p‐type materials. In contrast, space‐charge limited current measurements show a strong influence of the flanking units only on electron mobilities. Especially, the elegant synthetic strategy of combining pyridyl flanking units with difluorothiophene as the comonomer culminates in a record bulk electron mobility of 4.3 × 10−3 cm2 V−1 s−1 in polymers.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

“…These two features could be of interest to design new organic n-type semiconducting materials. Along these lines, Jo and Thelakkat groups have shown that Diketopyrrolopyrrole-based polymers using fluorine-flanked π-linkers between two adjacent DPP moieties lead to higher electron mobilities in comparison to non-fluorinated ones [29,94]. Interestingly, Jo and co-workers have shown, on the DPPPhF n copolymer series (Figure 21), that increasing the number of fluorine substitutions does not only impact the LUMO level, which decreases slightly, but it also modifies the polymer chain orientation in thin films, with a face-on orientation becoming more favorable.…”

“…For instance, if we consider two of the most common building blocks in the conjugated polymer field, namely the thiophene electron-donating group [26][27][28][29][30][31][32] and the 2,1,3-benzothiadiazole Possible physical mechanisms that may contribute to the remarkable increase in performances after backbone fluorination of different chemical systems have already been discussed in the literature. However, due to the large variety of molecular systems investigated so far and due to the complexity of the physics underlying the photovoltaic effect in organic BHJ devices, a general understanding of the impact of fluorination on the device performances is still lacking.…”

“…A number of recent reports have shown that fluorination of conjugated polymer backbones can have a strong impact on one or several of these properties, often leading to a more favorable morphology and to improved photovoltaic performances. Adding fluorine heteroatoms has, for instance, been found to enhance the domain purity (lowering the charge carrier recombination rate) [25,59,67], reduce the domain size [28,32,59,67], enhance the structural order [24,25,28,29,59,61,[69][70][71] and promote polymer face-on orientation at the interface with the bottom electrode [28,32,59,61,67,69,72] (increasing the out-of-plane hole mobility) or induce a face-on orientation at the D/A interface [69,73]. However, these effects can be more or less pronounced or even reversed [27,33,67], depending on the molecular structure of the conjugated building blocks as well as on the nature and positioning of the solubilizing side-chains.…”

“…Indeed, the fluorine atoms need to be introduced before the functionalization step [10,[19][20][21][22][23][24][25]. For instance, if we consider two of the most common building blocks in the conjugated polymer field, namely the thiophene electron-donating group [26][27][28][29][30][31][32] and the 2,1,3-benzothiadiazole [11,[32][33][34][35][36][37][38][39][40][41][42] electron-accepting group, their fluorinated derivatives requires a minimum of three and one additional steps, respectively (see Scheme 1).…”

π‐Conjugated Donor Polymers: Structure Formation and Morphology in Solution, Bulk and Photovoltaic Blends

“…These preferential alignments directly correlate with the charge transport properties as measured in space charge limited diodes. [100][101][102][103] …”