Diketopyrrolopyrrole based polymer solar cells: effect of alkyl branching point on device performance (Conference Presentation)

Abstract

Diketopyrrolopyrrole polymers (DPP’s) are an important class of donor materials for organic solar cells owing to their supreme charge carrier mobility and optical absorption which extends into the NIR (until  950-1000 nm). The former allows making efficient solar cells with rather thick active layers while the latter makes them a good candidate to be used in tandem devices. In this study, we synthesized four different DPP polymers with thiophene and thienothiophene conjugation segments in the backbone. For each of the backbones, we changed the branching point of the solubilizing alkyl chains (at 2nd or 6th carbon position). Solar cells were fabricated in the inverted configuration under ambient conditions following the device architecture: ITO/PEIE/active layer/MoOx/Ag. In general, thienothiophene based polymers performed better yielding maximum PCE’s close to 6.5 %. Interestingly, the short-circuit current varied from 7mA/cm2 to around 18mA/cm2 for the best performing system. The morphology was investigated using TEM and grazing incidence wide angle x-ray scattering (GIWAXS). While - stacking was not influenced by the conjugation segments, GIWAXS measurements reveal closer - stacking ( 3.5 A) in polymers with farther alkyl branching (at 6th carbon position) as compared to polymers with branching at the 2nd carbon position (- stacking distance  3.6 A). Alkyl lamellar spacing for branching at the 6th-position was  28 A while for the 2nd- position lamellar spacing was  17 A. Pole figures of the - stacking peak were calculated to get an idea about the distribution of crystallite orientation. For the thiophene substituted DPP’s most of the crystallites had face-on orientation while for thienothiophene substituted DPP’s, population of both face-on and edge-on crystallites were observed. By integrating the peak intensity as a function of polar angle, the relative degree of crystallinity (rdoc) was determined for the four polymer systems. TEM images revealed a fibrillar morphology for the four blended systems. The average polymer fibril width varied among the four polymer systems. For the thiophene-based DPP polymers, fibers widths were 35-50 nm (much larger than the typical exciton diffusion length  10nm). To study the effect of polymer fiber width and fiber purity on charge generation we measured fluorescence quenching in the blend films by selectively exciting polymer domains. To shed further light on phase purity of polymer fibrils, carbon/sulphur elemental maps were obtained using TEM. Overall, we try to correlate the effect of alkyl branching on the formation of mixed-phase morphology and how it affects the device performance.