Organic solar cell stability: A molecular interaction–diffusion framework and beyond

Harald Ade

Dept. of Physics and Carbon Electronics Laboratories (ORaCEL), NCSU, Raleigh, USA

The rapid increase in the power conversion efficiency of organic solar cells (OSCs) during the last few years has been achieved through the development of non-fullerene small-molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property–function relations and morphological stability have remained elusive. The presentation will discuss the recent discovery that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behavior and that the activation energy scales linearly with the enthalpic Flory-Huggins interaction parameters between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high interaction parameter) are the most kinetically stabilized. In short, unfavorable interactions enable stability. We have also been able to relate the differences in Ea to measured and selectively simulated molecular self-interaction properties of the constituent materials that provide quantitative property–function relations that link thermal characteristics (glass transition) of the NF-SMA and mechanical characteristics of the polymer(elastic modulus) of the polymers [1]. This allows predicting relative diffusion properties and thus morphological stability from simple analytical measurements or molecular dynamic simulations. Unfortunately, the star acceptor Y6 and its analogs have low glass transition temperatures, are highly diffusive and thus yield unstable morphologies. The relationship to the chemical structure is not yet understood except in the most general terms: The side-chains needed to provide solubility are also the enemy of stability. Shorter side-chains are better for stability, but often results in difficulties in processing. A new approach to molecular design or novel stabilization strategies are needed.

[1] Ghasemi, M., Balar, N., Peng, Z. et al. A molecular interaction–diffusion framework for predicting organic solar cell stability. Nat. Mater. (2021). https://doi.org/10.1038/s41563-020-00872-6

Biography

A graduate of Stony Brook University, H. Ade has been a faculty member at NCSU since Nov. 1992, rising through the ranks to full Professor by 2001, and been named Distinguished Professor of Physics in 2014 and Goodnight Innovation Distinguished Professor in 2017. He has had an active and continually funded research program  and served as Director of Graduate Program in Physics from 2006-2013. Recognitions include R&D100 Award, NSF Young Investigator Award, APS Fellow, AAAS Fellow, Alumni Outstanding Research Award (twice, NCSU), Holladay Medal (NCSU), K. F. J. Heinrich Award, and Shirley Price for Outstanding Science and Halbach Award of Innovative Instrumentation (both at the Advanced Light Source). He is a Clarivate Analytics WoS Highly Cited Researcher in the field of Materials Science since 2017. Some of his external engagements include serving on the Scientific Advisory Committee of the Advanced Light Source (2011- 2019) and the BESSY-II Synchrotron Facility in Berlin, Germany (2006-2009), as well as the Scientific Advisory Council of the Helmholz Zentrum Berlin, Germany (2009 – 2012).