Modulating phase segregation during spin-casting of fullerene-based polymer solar-cell thin films upon minor addition of a high-boiling co-solvent

Kuan-Hsun Lu; Wei-Ru Wu; Chun-Jen Su; Po-Wei Yang; Norifumi L Yamada; Hong-Jun Zhuo; Show-An Chen; Wei-Tsung Chuang; Yi-Kang Lan; An-Chung Su; U-Ser Jeng

doi:10.1107/S1600576724010082

Modulating phase segregation during spin-casting of fullerene-based polymer solar-cell thin films upon minor addition of a high-boiling co-solvent

J Appl Crystallogr. 2024 Nov 17;57(Pt 6):1871-1883. doi: 10.1107/S1600576724010082. eCollection 2024 Dec 1.

Authors

Kuan-Hsun Lu¹, Wei-Ru Wu², Chun-Jen Su², Po-Wei Yang², Norifumi L Yamada³, Hong-Jun Zhuo¹, Show-An Chen¹, Wei-Tsung Chuang², Yi-Kang Lan⁴, An-Chung Su¹, U-Ser Jeng^{1

2

5}

Affiliations

¹ Department of Chemical Engineering National Tsing Hua University Hsinchu300044 Taiwan.
² National Synchrotron Radiation Research Center Hsinchu300092 Taiwan.
³ Institute of Materials Structure Science High Energy Accelerator Research Organization Tsukuba Ibaraki305-0801 Japan.
⁴ Materials and Electro-Optic Research Division National Chung Shan Institute of Science and Technology Taoyuan325204 Taiwan.
⁵ College of Semiconductor Research National Tsing Hua University Hsinchu300044 Taiwan.

Abstract

The impact of additives on the nanoscale structures of spin-cast polymer composite films, particularly in polymer solar cells, is a topic of significant interest. This study focuses on the blend film comprising poly(thieno[3,4-b]thio-phene-alt-benzodi-thio-phene) (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), exploring how additives like 1,8-di-iodo-octane (DIO) influence the film structures spin-cast from chloro-benzene solution. Combined results of specular X-ray and neutron reflectivity, grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS), and X-ray photoelectron spectroscopy indicate that DIO could significantly enhance the dispersion of PC₇₁BM and reduce composition inhomogeneity in the film. Time-resolved GISAXS-GIWAXS with 100 ms resolution further captures a rapid spinodal decomposition of the mixture within 1 s in the constant-evaporation stage of spin-casting. Further combined with parallel analysis of time-resolved UV-Vis reflectance, these findings reveal that DIO mitigates the spinodal decomposition process by accelerating solvent evaporation, which, in turn, decelerates phase segregation, leading to a nucleation-driven process. These observations provide mechanistic insights into the role of additives in controlling the nanostructural evolution of spin-cast films by altering the kinetics of solvent evaporation and phase separation during the spin-coating process.

Keywords: GISAXS; GIWAXS; X-ray reflectivity; additive effects; grazing-incidence small/wide-angle X-ray scattering; neutron reflectivity; polymer solar cells; spinodal decomposition.