Overcoming Electrode-Induced Losses in Organic Solar Cells by Tailoring a Quasi-Ohmic Contact to Fullerenes via Solution-Processed Alkali Hydroxide Layers

H. Zhang, R.C. Shallcross, N. Li, T. Stubhan, Y. Hou, W. Chen, T. Ameri, M. Turbiez, N.R. Armstrong, C.J. Brabec

Research output: Contribution to journalArticlepeer-review

Abstract

It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi-ohmic contact via solution-processed alkali hydroxide (AOH) interlayers on top of n-type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C60) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C60 highest occupied molecular orbital for AOH-modified AZO contacts. These novel interfacial gap states are a result of ground-state electron transfer from the metal hydroxide-functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer. Furthermore, the strong "light-soaking" effect is no longer observed in devices with a AOH interface. Solution-processed alkali hydroxides significantly reduce the work function of metal oxides, such as zinc oxide or aluminum zinc oxide (AZO), and are further proven to effectively passivate defect states in these metal oxides. The interface states with alkali hydroxide-modified AZO contacts tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Original languageEnglish
JournalAdvanced Energy Materials
Volume6
Issue number9
DOIs
Publication statusPublished - 2016

Keywords

  • Aluminum oxide
  • Defects
  • Doping (additives)
  • Electric contactors
  • Fermi level
  • Ground state
  • II-VI semiconductors
  • Metals
  • Molecular orbitals
  • Ohmic contacts
  • Organic solar cells
  • Self assembly
  • Solar cells
  • Work function
  • Zinc oxide
  • Alkali hydroxide
  • Aluminum zinc oxides
  • Ground-state electrons
  • Highest occupied molecular orbital
  • Interfacial energetics
  • Interfacial layer
  • Inverted organic solar cells
  • Solution-processing
  • Interface states

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