Molecular electrical doping is of central technological relevance for organic (opto-) electronics since it allows control of charge carrier density and Fermi level position in organic semiconductors (OSCs). Here, we chose to investigate the doping capability of the n-dopant 1,2,3,4,1′,2′,3′,4′-octaphenylrhodocene (OPR). Using the bulky, strongly reducing metallocene to dope the electron-transport polymer poly{[N,N-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} [P(NDI2OD-T2)] leads to an increased bulk conductivity and decreased contact resistance. While the former is due to low-level n-doping of the polymer, trap filling and concomitant charge carrier mobility increase, the latter is caused by an accumulation of OPR at an indium tin oxide (ITO) substrate.
Polyethylenimine (PEI) layers are used as cathode interlayer to reduce the work function of electrode materials and are frequently applied to organic or perovskite opto-electronic devices. PEI was applied from solution on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate mixture, zinc oxide and graphite. Choice of solvent and residual solvent removal ensure the superior functionality. [1] Furthermore, a single-step formation of a low work function cathode interlayer and n‑type bulk doping from semiconducting polymer/PEI blend solution was observed. [2]
[1] S. Bontapalle et al., Adv. Mater. Interface 7 (2020) 2000291.
[2] K. Seidel et al., ACS Appl. Mater. Interfaces 12 (2020) 28801.
KEYWORDS: Solar cells, Electrodes, Diodes, Molecules, Interfaces, Organic semiconductors, Electron transport, Photoemission spectroscopy, Fullerenes, Solar energy
One material system of interest for photovoltaic cells is the combination of the p-conducting copper-phthalo-cyanine
(CuPc) and the n-conducting fullerene (C60) as donor and acceptor materials, respectively. Therefore the
transport properties for diodes containing neat and blended organic films are analysed in the space charge limited
current regime. The charge carrier mobilities are found to decrease upon dilution of the respective conducting
phase by the other species. Photovoltaic cells can be realised with bilayered or blended organic donor/acceptor
films. The influence of both photo-active layer types on the electronic structure and the open circuit voltage is
investigated. From photoelectron spectroscopy a higher open circuit voltage is predicted for bilayered solar cells.
Due to mixing of the organic materials the intermolecular gap between the highest occupied molecular orbital
of the donor and the lowest unoccupied molecular orbital of the acceptor is reduced. This prediction is proven
true by photocurrent measurements.
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