Material design and synthesis for high-performance organic solar cells enhanced open-circuit voltage and good stability

Abstract

Organic Solar Cells have been attracting attentions from both academia and industry owing to its reduced fabrication cost, light weight and mechanical flexibility. Rational material design and feasible synthetic routes are critical to realize the solar cell technology in real-life applications, though the morphological control, the structure-property relationship, synthetic methodology are not yet fully explored. Thus in this thesis, I will show my efforts in developing novel donor and acceptor materials with good performance and good stability. To approach high efficiencies in device, one of the critical pathway is to achieve an ideal morphology in blend film. Based on the temperature-dependent-aggregation strategy, we developed a novel low bandgap polymer PffBTT2-DPPT2, which contains a D-A1-D-A2 structure. With the introduction of a second electron-withdrawing unit, ffBT, PffBTT2-DPPT2 possessed a much deeper HOMO energy level and stronger temperature-dependent aggregation property than its analogue PDPP4T, resulting in an enhanced Voc as well as PCE up to 8.6%. Owing to its deeper energy levels as well as stronger aggregation property, PffBTT2-DPPT2 based devices possessed excellent stability, not only in air stability and thermal stability, but also in light-soaking stability. Over 80% of PCE maintained after being illuminated under light for 283h. An environmental-friendly method to synthesis the photo-active materials is of vital important to bringing organic solar cells into real-life application. We developed a novel large band-gap polymer PFFTAZ-4FTVT with direct hetero-arylation polymerization method, which was more straightforward and would not generate any toxic organometallic compound. Synthetic methods were detailed explored, including temperature selection, solvent selection and etc. Mesitylene stood out owing to the excellent dissolving ability both monomers and the resulting polymers as well as less reactivity in the polymerization process. The temperature-depend aggregation strategy was also applied to the polymer design to achieve an ideal blend-film morphology, as a result, the devices based on PFFTAZ-4FTVT:PC₇₁BM achieved a good PCE of 6.91%. To achieve fine-tune energy levels as well as stronger absorption, small molecular acceptors (SMA) was greatly developed. However, the molecular ordering in SMA based systems are not yet fully understood. We developed a novel small molecular acceptor TS-5 by replacing the benzene ring in the ending group with thiophene ring. The resulting molecular exhibited a clear hypsochromatic shift in either solution and solid state once diiodooctane was applied. By tuning the molecular packing, a significant enhancement of Voc (0.1V enhancement) was achieved without any negative impact on the overall PCE. This is the first report of this unique phenomenon in organic solar cells.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology