Abstract
Domain composition and interfacial structure are critical factors in organic photovoltaic performance. Here, we report neutron reflectivity, grazing-incidence X-ray diffraction and atomic force microscopy measurements of polymer/fullerene thin-films to test a hypothesis that these partially miscible blends rapidly develop composition profiles consisting of co-existing phases in liquid-liquid equilibrium. We study a range of polymer molecular weights between 2 and 300 kg mol−1, annealing temperatures between 120 and 170 oC, and timescales up to 10 min, yielding over 50 distinct measurement conditions. Model bilayers of fullerene-derivatives and polystyrene enable a rigorous examination of theoretical predictions of the effect of polymer mass and interaction parameter on the compositions, ϕ, and interfacial width, w, of the coexistent phases. We independently measure ϕ and w and find that both Flory-Huggins mean-field-theory and key aspects of self-consistent-field-theory are remarkably consistent with experiment. Our findings pave the way for predictive composition and interface design in organic photovoltaics based on simple experimental measurements and equilibrium thermodynamic theory.
| Original language | English |
|---|---|
| Article number | 112 |
| Journal | Communications Physics |
| Volume | 2 |
| Issue number | 1 |
| DOIs | |
| State | Published - Dec 1 2019 |
| Externally published | Yes |
Funding
We thank the ILL and ISIS for the award of beam time (experiment numbers 9–12–387 (Figaro), 9–12–456 (D17) and RB 1620066 (Inter)), and the staff of D17, Figaro and Inter for help during the experiments. We also thank Diamond Light Source for the award of beamtime (SI15486–1) and Tom Arnold and Chris Nicklin at beam-line I07 for their support during the GIXD experiments. E.H. acknowledges Swansea University for funding her PhD studentship, and D.M. acknowledges the UK Engineering and Physical Sciences Research Council (EPSRC) for funding his studentship via the Doctoral Training Grant to Swansea University. A.D. acknowledges the EPSRC for funding through the EPSRC Supergen Solar Challenge Grant: EP/M025020/1. We acknowledge Leeds EPSRC Nanoscience and Nanotechnology Facility (LENNF) and Michael B. Ward for providing access to transmission electron microscopy instrumentation and support. We thank Fabrizia Foglia for help with neutron reflectivity measurements on a couple of samples, and Noura Alhazmi, Samuele Lilliu, Edwin Pindea De La O and Fabio Pon-tecchani for help with the GIXD experiments on beamline I07 at Diamond. A.J.P. thanks Xenocs for their help and ongoing support in the GIXD user program in Sheffield, and we thank EPSRC for funding the purchase of this instrument. Sebastian Pont is thanked for performing the atomic force microscopy measurements shown in Supplementary Fig. 13.
| Funders | Funder number |
|---|---|
| GIXD | |
| Engineering and Physical Sciences Research Council | EP/M025020/1 |
| Engineering and Physical Sciences Research Council | |
| Swansea University |
