Molecular versus crystallite PCBM diffusion in P3HT:PCBM blends

Abstract

The diffusion of PCBM in P3HT:PCBM blend films has been investigated using multi-wavelength scanning absorption microscopy (MWSAM). By studying the depletion of PCBM in the vicinity of the phase segregated PCBM-rich regions that form upon thermal annealing, we are able to measure the diffusion constant and activation energy for PCBM diffusion in P3HT:PCBM blend films. The measured kinetic parameters are consistent with the diffusion of nanoscale PCBM crystallites rather than molecular PCBM. We show that the presence of two distinct diffusion processes in these blend materials provides an explanation for the large differences that have been reported for PCBM diffusion in P3HT:PCBM blends. This insight allows us to develop a unified model for PCBM mass transport in these materials.

Stability of Polymer:PCBM Thin Films under Competitive Illumination and Thermal Stress

“…We find that a single diffusion coefficient D ≈ 5 × 10 −9 cm 2 s −1 satisfactorily describes all datasets, in line with the single k parameter found with the descriptive fit. This D value is of the same order of magnitude, albeit larger, than previously reported values for PCBM within various polymer matrices (0.3–1.3 10 −9 cm 2 s −1 ) at these temperatures . Similarly, we find that a single β ≈ 0.09 cm s −1 and γ ≈ 18 Å accounts for the attractive surface potential (from PEDOT:PSS), which seems sensible since the interface remains the same for all blend compositions and light exposure conditions.…”

Bridging the thermodynamics and kinetics of temperature-induced morphology evolution in polymer/fullerene organic solar cell bulk heterojunction

“…The calculated activation energy, 1.32 AE 0.4 eV, is similar to the value reported for poly(3hexylthiophene) (P3HT):PCBM blends, 1-1.5 eV, which was assigned to PCBM aggregation by coalescence of PCBM crystallites in a diffusion-limited process. 61 Therefore, based on the literature for P3HT:PCBM and the high activation energy found here, we speculate that PCBM aggregate growth in PCE11:PCBM BHJs during annealing in the 125-140 1C temperature range is also diffusion limited. It should be noted that the energy barrier for the nucleation of the crystalline PCBM phase also contributes to the activation energy for k.…”

X-Ray Nanoscopy of a Bulk Heterojunction

“…This indicates that the dynamics of the PCBM diffusion out of the depletion zone is similar across the sample. A rough estimate of the diffusion coefficient, yielding D = 7 x 10 −12 cm 2 s -1 was performed using analysis similar to that reported by Dastoor et al [ 8 , 37 ], see short discussion on diffusion analysis in the Methods section.…”

“…Here χ is the PCBM concentration, D is the diffusion coefficient, x and t denote position and time. The PCBM diffusion coefficient can be obtained by fitting the PCBM volume profile to a one-dimensional solution to Fick’s second law as demonstrated by Dastoor et al [ 8 , 37 ]. Under the assumptions of an initially uniform concentration of PCBM, and effective sinks at x = 0 and x = L , standard Fourier analysis gives the equation …”