Issue 5, 2016

Photoinduced charge generation rates in soluble P3HT : PCBM nano-aggregates predict the solvent-dependent film morphology

Abstract

The device efficiency of bulk heterojunction (BHJ) solar cells is critically dependent on the nano-morphology of the solution-processed polymer : fullerene blend. Active control on blend morphology can only emanate from a detailed understanding of solution structures during the film casting process. Here we use photoinduced charge transfer (CT) rates to probe the effective length scale of the pre-formed solution structures and their energy disorder arising from a mixture of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in three different organic solvents. The observed solvent-dependent ultrafast biphasic rise of the transient polaron state in solution along with changes detected in the C[double bond, length as m-dash]C stretching frequency of bound PCBM provides direct evidence for film-like P3HT : PCBM interfaces in solution. Using the diffusive component of the charge transfer rate, we deduce ∼3-times larger functional nano-domain size in toluene than in chlorobenzene thereby correctly predicting the relative polymer nanofiber widths observed in annealed films. We thus provide first experimental evidence for the postulated polymer : fullerene : solvent ternary phase that seeds the eventual morphology in spin-cast films. Our work motivates the design of new chemical additives to tune the grain size of the evolving polymer : fullerene domains within the solution phase.

Graphical abstract: Photoinduced charge generation rates in soluble P3HT : PCBM nano-aggregates predict the solvent-dependent film morphology

Supplementary files

Article information

Article type
Paper
Submitted
18 sep 2015
Accepted
16 dec 2015
First published
22 dec 2015

Nanoscale, 2016,8, 2768-2777

Photoinduced charge generation rates in soluble P3HT : PCBM nano-aggregates predict the solvent-dependent film morphology

P. Roy, A. Jha and J. Dasgupta, Nanoscale, 2016, 8, 2768 DOI: 10.1039/C5NR06445G

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