Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

Abstract

Here, studies on the evolution of photophysics and device performance with annealing of blends of poly(3‐hexylthiophene) with the two polyfluorene copolymers poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2′′‐diyl) (F8TBT) and poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100 °C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140 °C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing‐incidence small‐angle X‐ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140 °C‐annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as‐spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200 °C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize.

Solvent Additive Control of Morphology and Crystallization in Semiconducting Polymer Blends

“…[40] In addition to the V oc , the EQE and J sc also improved after post-thermal annealing. For P3HT:PCBM and P3HT:F8TBT, thermal annealing alone can effectively increase P3HT crystalline order similar to using additives, [2,14] and the EQE with only post-thermal annealing here was slightly higher than with only additive processing. However, a combination of solvent additives and post-annealing led to even greater improvement than applying either process alone, and the hole mobility in the near saturation at negative biases, where charge separation and extraction are facilitated by a high internal field, suggests that more efficient exciton dissociation is an important factor for the ordering in this blend.…”

Temperature Dependence of Charge Carrier Generation in Organic Photovoltaics

“…For the unannealed device, charge carrier density starts to decrease at relatively high temperatures, indicating less efficient charge separation. For the P3HT∶F8TBT device, the charge carrier density already decreases from RT, meaning that charge separation limits V OC even at RT [38].…”

Conjugated Polymer Photovoltaic Materials

“…In particular, a PCE up to 1.8 % and extremely high V oc over 1 V were recorded for the optimized F8TBT/ P3HT-based PSC device. Further studies by McNeill et al [177] demonstrated that the relatively low efficiency of the P3HT/F8BT system can be attributed to poor charge generation and separation efficiencies which result from the failure of P3HT reorganization.…”