A Quantitative Study of PCBM Diffusion during Annealing of P3HT:PCBM Blend Films

Abstract

Scanning transmission X-ray microscopy has been used to quantitatively map the composition of P3HT:PCBM blend films in the vicinity of PCBM crystals formed during annealing at 140 °C. The observed PCBM concentration profiles around these crystals have been fitted to Fick’s second law of diffusion and the diffusion constant found to be 2.5 × 10−14 m2 s−1. The PCBM concentration at the crystal boundary was found to be 19% (v/v) and is interpreted, together with the annealing temperature of 140 °C, as a point on the bimodal line of the composition−temperature phase diagram. The diffusion of PCBM through P3HT is observed to be bulk-dominated, in contrast to the surface/interface-dominated diffusion observed in MDMO-PPV:PCBM blend films by Yang et al. ,

Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend

“…[ 21 ] Since the interdiffusion process is complete at 150 ° C for 30 sec, it is clear that the interdiffusion coeffi cient for this system must be greater than 3 × 10 − 10 cm 2 /s (See Supporting Information) at 150 ° C. Diffusion constants for PCBM within a P3HT matrix have also been measured by studying the depletion of PCBM from the P3HT matrix surrounding a PCBM crystalline domain and shown to be 2.5 × 10 − 10 cm 2 /s, which is consistent with our estimate for the lower limit of the interdiffusion coeffi cient. [ 14 ] While it is often assumed that phase separation between P3HT and PCBM to form the BHJ microstructure is due to the immiscibility of these components in liquid or amorphous states, the observation of interdiffusion demonstrates signifi cant miscibility and complete mixing in our bilayer system at typical annealing temperatures (i.e. > 120 ° C).…”

“…This rapid interdiffusion rate gives evidence that PCBM mixes as molecular species or small aggregates, thus supporting the idea that the BHJ morphology is very dynamic with signifi cant molecular mobility under widely-used annealing conditions. [ 14 ] Furthermore, PCBM diffuses within the fi lm without affecting the crystal size, structure, or orientation of P3HT, even at an equal P3HT:PCBM weight ratio. This result suggests that diffusion occurs only through the disordered regions of P3HT.…”

On the morphology of polymer‐based photovoltaics

“…It is also known, from dynamic secondary ion mass spectroscopy studies on bilayers, that PCBM diffuses in P3HT and its mixtures with PCBM with a diffusion coefficient in excess of 10 À11 cm 2 /s. 42,46 Therefore, it is reasonable to conclude that the characteristic size of the domain formed from the growth and impingement of ordered P3HT will be on the tens of nanometer size scale, as observed experimentally. An alternate origin of the small size of the crystalline P3HT and PCBM-rich domains is that there are numerous nucleation sites to initiate crystallization of P3HT and it is simply high nucleation density that gives rise to the small scale domains.…”

Molecular versus crystallite PCBM diffusion in P3HT:PCBM blends

“…7,8 Understanding the formation of the optimal film morphology therefore requires knowledge of the key factors that govern mass transport in these materials. 9 For the highly studied P3HT:PCBM system, 10 the diffusion of PCBM in the P3HT matrix is recognised as a key parameter, since it governs the formation of an intricate film structure that incorporates both crystalline and amorphous P3HT regions. 11 In particular, the phase segregation of PCBM is known to influence ordering of the polymer regions and hence device function.…”