In recent years, various organic compounds have attracted increasing interest as new types of energy materials with illumination efficiencies of 150 lm/W, photovoltaic conversion efficiencies of over 11%, and thermoelectric figures of merit surpassing 0.4. However, carrier transport in these materials is affected by complex factors, such as charge localization and disorder/impurity scattering, resulting in unclear energy conversion mechanisms and many related disputes which have hindered effective engineering and design of the materials.

Taking aim at the above problems, the iChEM researcher Prof. Zhigang Shuai and his group from Tsinghua University this year made important progress on several fronts. They proposed a super-exchange mechanism for charge transport in organic donor (D)-acceptor (A) materials, and using a method of first-principles calculation, investigated the energy level alignments and electronic couplings for hole transport, electron transport and ambipolar transport via super-exchange couplings. The result was published in the journal Advanced Materials as “Understanding the charge transport and polarities in organic donor-acceptor mixed-stack crystals: molecular insights from the super-exchange couplings”. (Advanced Materials, 2015, 27(8), 1443-1449).

The group also applied the Boltzmann transport equation to consider the competition between ionized impurity scattering and acoustic phonon scattering. Again with the aid of first-principles calculation, they investigated the effect of doping on the charge and thermoelectric transport in PEDOT. The result was published in the Journal of the American Chemical Society as “Unravelling Doping Effects on PEDOT at the Molecular Level: From Geometry to Thermoelectric Transport Properties”. (Journal of the American Chemical Society, 2015, 137(40), 12929-12938).

Finally, based on the localized charge nuclear tunneling model proposed previously together with Prof. Yi Zhao from Xiamen University, the two groups investigated the competitive relationship between charge delocalization and the nuclear tunneling effect by developing the theory of time-dependent wave packet diffusion, and quantitatively compared the two kinds of efficiency of five organic semiconductors with high mobilities. Their results suggested that the quantum nuclear tunneling effect of localized charge is the dominant mechanism of charge transport in many organic materials, and the charge delocalization effect depends on the particular system: it is not significant for some, while for others it needs to be considered. This research bridges the gap between two extreme models, the semiclassical Marcus theory and the fully delocalized band like theory, and applies more universally for organic materials. The result was published in the journal Nanoscale Horizon as “Nuclear tunnelling and carrier delocalization to bridge the gap between hopping and bandlike behaviors in organic semiconductors”. (Nanoscale Horizon, 2015，DOI: 10.1039/c5nh00054h)


Paper links: http://onlinelibrary.wiley.com/doi/10.1002/adma.201404412/pdf

http://pubs.acs.org/doi/abs/10.1021/jacs.5b06584

http://pubs.rsc.org/en/content/articlepdf/2016/NH/C5NH00054H