Understanding the importance of bulk-heterojunction microstructure stability in solution-processed organic solar cells

Title: Understanding the importance of bulk-heterojunction microstructure stability in solution-processed organic solar cells

Speaker: Dr. Ning Li

Venue: 907#--1445    

Time: 2:00 PM   Jan 5, 2018

Dr. Ning Li received his doctor degree in Materials Science and Engineering from Friedrich-Alexander University Erlangen N├╝rnberg (FAU), Germany in 2014, and is now a group leader at the institute of Materials for Electronics and Energy Technology (i-MEET) at FAU with research focus on solution-processed organic photovoltaics and functional optoelectronic devices. He is an author/co-author of more than 50 research papers in ISI-related journals, including Science, Nature Energy, Nature Communications, Energy & Environmental Science, etc. (Citations > 2200 and H-index = 26). He received the Chinese Government Award for Outstanding Self-financed Students Abroad in 2013, the Excellent Doctoral Thesis Award in 2015, the Emerging Talents Initiative (ETI) Funding and the Young Scientist Award (Bronze Award) in IUMRS-ICAM in 2017. He is coordinating a DFG-NSFC international collaborative project and a FAU-HIERN joint research project.



The performance of organic solar cells (OSCs) is governed by the delicate, optimized bulk-heterojunction (BHJ) microstructure, where organic donor and acceptor are fine-mixed in the nanometer regime to facilitate exciton dissociation. For the same material composition, the quantum efficiencies may vary from close to 100 % to even less than 10 % as a function of processing. To push organic photovoltaics towards commercial applications, a module efficiency of >10% in combination with an operational lifetime of >10 years is required. Therefore, the morphology evolution as well as the stability of BHJ microstructure has to be well investigated and analyzed.

In this presentation, I will examine the reliability and stability of BHJ microstructures for various solution-processed OSCs and explore their potential for large-scale mass production. We recently found that selected high performance systems gain their high excellent performance from a meta-stable microstructure, which quickly relaxed to the thermodynamic equilibrium state under external stress, such as light or thermal stress. The so called burn-in degradation is identified as a spinodal de-mixing due to the low miscibility of donor and acceptor, which is turned out to be a major challenge for the development of stable and efficient OSCs. Even though the BHJ microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation even in the solid state.