Speaker
Description
Molecular doping is key to controlling the electronic and electrical properties of organic semiconductors, lower contact resistance, enhance bulk conductivity and carrier mobility, and create higher performance devices. This talk reviews processes and options for interface doping in molecular and polymer semiconductors, and the roles that electron spectroscopy and carrier transport measurements have played in defining key issues. Various n- and p-type molecular dopants, their doping strength and the challenges they pose are reviewed first. Specific examples will be reviewed. First is the surface/interface doping of polymer-based devices via soft-contact lamination of highly doped interlayers [1,2]. We look at the electrical characteristics of the laminated polymer/polymer interface (with P3HT, PBDTTT-c or poly-TPD), at the problem of dopant diffusion across boundaries, and at the performance of polymer-based solar cells built with laminated hole-extraction layers [3]. We then turn to the issue of improving contacts to very low electron affinity (EA) electron transport layers (ETL), an issue critical to green and blue OLEDs. We look at the air-stable dimer of (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium ([RuCp*Mes]2) [4], and use it to n-dope phenyldi(pyren-2-yl)phosphine oxide (POPy2) (EA = 2.1 eV). We demonstrate that photo-activation of the cleavable dimeric dopant results in kinetically stable and efficient n-doping of the host semiconductor, whose reduction potential is beyond the thermodynamic reach of the dimer’s effective reducing strength [5]. We demonstrate the use of this doped ETL to fabricate high-efficiency organic light-emitting diodes. If time permit, surface n-doping of graphene to decrease its work function, shift its Fermi level and improve electron injection in organic ETLs will be described.
References
[1] A. Shu, A. Dai, H. Wang, Y.-L. Loo, and A. Kahn, Org. Electr. 14, 149 (2013).
[2] A. Dai, A. Shu, H. Wang, S. Barlow, S. Mohapatra, T. Sajoto, Y. Zhou, C. Fuentes-Hernandez, Y.-L. Loo, S. R. Marder, B. Kippelen, and A. Kahn, Adv. Funct. Mat. 24, 2197 (2014).
[3] A. Dai, A. Wan, C. Magee, Y. Zhang, S. Barlow, S. R. Marder and A. Kahn, Org. Electr. 23, 151 (2015)
[4] G. Song, S.-B. Kim, S. Mohapatra, Y. Qi, T. Sajoto, A. Kahn, S. R. Marder, S. Barlow, Adv. Mat. 24, 699 (2012).
[5] X. Lin, B. Wegner, K. M. Lee, M. Fusella, F. Zhang, K. Moudgil, B. Rand, S. Barlow, S. R. Marder, N. Koch and A. Kahn, Nature Materials (2017) DOI: 10.1038/nmat5027.
