Domain Morphology of Organic Solar Cell Bulk Heterojunctions revealed by GHz Spectroscopy
Suraj Manikandan a, Jens Andreasen a
a Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej 310, DK-2800 Kgs. Lyngby, Denmark
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP23)
Kobe, Japan, 2023 January 22nd - 24th
Organizers: Seigo Ito, Hideo Ohkita and Atsushi Wakamiya
Poster, Suraj Manikandan, 043
Publication date: 21st November 2022

Domain size, phase purity and the mixed network of the bulk heterojunction of organic solar cell active layers of organic solar cells (OSCs) are essential for efficient charge generation and transportation [1]. However, it is challenging to characterize the bulk heterojunction morphology because of the small dimensions and low contrast for many probes of nanoscale structure. One of the most successful methods is Resonant Soft X-ray Scattering (RSoXS) which can only be done at some synchrotron facilities. RSoXS is a small-angle scattering technique leveraging soft X-ray transition dipoles, allowing quantitative characterization of nanoscale molecular orientations [2]. Like RSoXS, GHz spectroscopy uses the molecular orientation dipole moment relaxation, and most organic materials possess a dipole relaxation in the frequency range of 1-110 GHz. In our analysis, we correlated the dielectric relaxation strength of the NFA-polymer blend to the domain length, and through this, we can distinguish mixed and pure domains. A slight limitation is that this method only applies to materials with a known molecular structure and dipole moment.

This presentation first examines the complex permittivity (ϵ*) of the PM6:Y7 blend from 1-70 GHz. In this range, ϵ* is determined by a reflection/transmission measurement on coplanar waveguides using a Vector Network Analyzer. ϵ* is extracted by combining the conformal mapping method and a transmission line distributed element model [3]. We observed two dielectric relaxations at 4.8 (R1) and 35.8 GHz (R2), and the data were fitted with the Havriliak–Negami (HN) function. Relaxation at 4.8 GHz shows Cole-Davidson behaviour (induced relaxation) corresponding to PM6 polymer, while 35.8 GHz is Debye relaxation corresponding to the Y7 NFA molecule [4-5]. Secondly, under biased voltage ϵ* shows butterfly-shaped permittivity hysteresis loops. This allows us to use the double potential well model to fit relaxation strength to extract the domain length [6], which is 90-110 nm.

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