The miscibility and depth profile of PCBM in P3HT: thermodynamic information to improve organic photovoltaics

Abstract

Recent work has shown that poly(3-hexylthiophene) (P3HT) and the surface-functionalized fullerene 1-(3-methyloxycarbonyl)propyl(1-phenyl[6,6])C(61) (PCBM) are much more miscible than originally thought, and the evidence of this miscibility requires a return to understanding the optimal morphology and structure of organic photovoltaic active layers. This manuscript describes the results of experiments that were designed to provide quantitative thermodynamic information on the miscibility, interdiffusion, and depth profile of P3HT : PCBM thin films that are formed by thermally annealing initial bilayers. It is found that the resultant thin films consist of a 'bulk' layer that is not influenced by the air or substrate surface. The composition of PCBM in this 'bulk' layer increases with increased PCBM loading in the original bilayer until the 'bulk' layer contains 22 vol% PCBM. The introduction of additional PCBM into the sample does not increase the amount of PCBM dispersed in this 'bulk' layer. This observation is interpreted to indicate that the miscibility limit of PCBM in P3HT is 22 vol%, while the precise characterization of the depth profiles in these films shows that the PCBM selectively segregates to the silicon and near air surface. The selective segregation of the PCBM near the air surface is ascribed to an entropic driving force.

Mixing in PCBM/P3HT bilayers, using in situ and ex situ neutron reflectivity

“…We adopt the robust methodology that we have used in our previous studies of PCBM/ polymer bilayers 26 and use a combination of in situ annealing (annealing a sample in the neutron beam) and ex situ annealing (annealing a sample before measuring the reflectivity). Our results add to the strong body of evidence that there is rapid intermixing between PCBM and P3HT, and support the work of H. Chen et al 17 -giving a maximum loading of PCBM in P3HT of around 20%. However, we find contrasting (and rather subtle) results in comparison with the reported findings of H. Chen et al, in relation to PCBM-enrichment at the free surface.…”

“…These values correspond reasonably closely to previously reported values. 17,31 The interfacial roughness, r, between the two layers is 1.2 nm. This SLD profile corresponds closely to that of an as-fabricated PCBM/polymer bilayer (see e.g., Môn et al 26 ), and shows that little (if any) interdiffusion has occurred during the temperature step from room temperature to 80°C.…”

“…14,17,30 The calculated value for PCBM was 4.4 Â 10 À6 Å À2 and for P3HT was 0.62 Â 10 À6 Å À2 . These values were used as initial guesses when fitting the reflectivity curves of unannealed bilayers and ex situ annealed bilayers.…”

“…21,22 In addition to the measurement of layer composition offered by ion beam methods and ellipsometry, 23,24 this increased resolution allows characterization of roughness at buried polymer interfaces, which is typically on the order of a few nanometers. 25 Neutron reflectivity was performed on several P3HT/PCBM bilayers with different ratios of PCBM to P3HT by H. Chen et al 17 They reported only partial mixing in bilayers after annealing for 60 min at 150°C, a maximum loading of ;22% PCBM in the P3HT-rich layer, and surface enrichment of PCBM within the P3HT-rich (top) layer.…”

“…[11][12][13][14][15][16][17][18][19][20] These bilayers initially consisted of pure P3HT and PCBM layers, with thermal annealing used as a means to study the intermixing of the PCBM and P3HT. A number of studies have shown that PCBM and P3HT can rapidly intermix, 11,14,18,19 and that this mixing involves diffusion of PCBM into amorphous regions of the P3HT.…”