报告人：Mats Fahlman 教授

Department of Physics, Chemistry and Biology

  Linköping University, SE-58183, Linköping, Sweden

Website: http://www.liu.se/ifm/surfphys/?l=en

题目：Molecule-Doped Conjugated Polymer, Conjugated Polyelectrolyte/Electrode and Donor-Acceptor Interfaces in Organic Electronics

时间：2015年4月8日（星期三）2:30pm

地点：独墅湖校区909号楼一楼B会议室

摘要：

Interlayers based on molecule-doped organic semiconductors or polyelectrolytes are promising concepts for improving charge injection/extraction by tailoring bare electrode electronic structures as well as effectively suppressing unwanted charge recombination [1]. Their influences upon interface energetics are critical for understanding the process, in turn a prerequisite for enhancing e.g. power conversion efficiency. Here, we explore and establish a unified model for the energetics of conjugated interlayer / electrode interfaces featuring molecule-doped conjugated polymers [2] or conjugated polyelectrolytes [3]. The interface energetics are found to follow a scheme with (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as described by integer charge transfer model [4] and additionally (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex or polyelectrolyte ions that cause a shift of the work function for both pinned and unpinned interfaces [2,3]. Such behavior is expected to hold for all low to intermediate level doped polymer systems. The results further reveal tangible design rules of conjugated polyelectrolyte/electrode modifiers to achieve the smallest charge barrier and break through the current interlayer thickness limitation [3].

Organic photovoltaic is under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here we show that for a series of regioregular-poly(3-hexylthiophene):fullerene bulk heterojunction organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at the bulk heterojunction have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed and tested against a series of donor polymers and solution-processed small molecules [5].

References

[1] F. Deschler, E. D. Como, T. Limmer, et al. Phys. Rev. Lett. 107 (2011) 127402.

[2] Q. Y. Bao, X. J. Liu, S. Braun, F. Gao and M. Fahlman, Adv. Mater. Interfaces, 2 (2015) 1400403, DOI: 10.1002/admi.201400403

[3] Q. Y. Bao, X. J. Liu, E. G. Wang, S. Braun and M. Fahlman (Submitted)

[4] M. Fahlman, et al, J. Phys.: Condens. Matter 19 (2007) 183202.

[5] Qinye Bao, et al, Adv. Funct. Mater. 24 (2014) 6309

个人简介：

Prof. Fahlman received his PhD in 1995 at Linköping University, started his own group and became an associated professor in Norrköping in 2000, promoted to full professor in 2005 and received the Chair in Surface Physics and Chemistry in Linköping 2008. In 2003 he was a co-recipient of the Descartes Prize awarded by the European Union and in 2013 the Royal Swedish Academy of Science awarded him the Göran Gustafsson Prize in Physics, the largest and most prestigious national prize in natural sciences in Sweden, for his work on organic semiconductors.

联络人：唐建新 教授