Doping and interlayer are two promising technologies for improving charge injection/extraction through tailoring bare electrode electronic structures as well as effectively suppressing charge recombination even at donor-acceptor interface for organic photovoltaic operation.[1] Their influences upon interface energetics are critical for understanding the process, in turn a prerequisite for enhancing power efficiency conversion. Here, we explore and establish for the first time a unified energetics of conjugated modifier /electrode interfaces featuring molecule-doped conjugated polymer and conjugated polyelectrolyte. We find regular plots with a constant thickness-independent displacement away from the original energy level alignment behavior of the pristine polymer at low to intermediate level for molecule-doped conjugated polymer interfaces, and that at high doping level are substrate-independent. We propose a universal model for controlling the interfaces [2]: (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as described by integer charge transfer model and additionally (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex that cause a shift of the work function for both pinned and unpinned interfaces. Such behavior is expected to hold for all low to intermediate level doped polymer systems except intermolecular hybridization. The results further reveal tangible design rules of conjugated polyelectrolyte/electrode interlayer combinations to achieve the smallest charge barrier and break through the current thickness limitation [3].
[1] F. Deschler, E. D. Como, T. Limmer, et al. Phys. Rev. Lett. 2011, 107, 127402. [2] Q. Y. Bao, X. J. Liu, S. Braun, F. Gao and M. Fahlman, Adv. Mater. Interfaces.2014, DOI: 10.1002/admi.201400403 [3] Q. Y. Bao, X. J. Liu, E. G. Wang, S. Braun and M. Fahlman (Submitted)