The Impact of Local Work Function Variations on Fermi Level Pinning of Organic Semiconductors

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• 作者：Stefanie Winkler ; Johannes Frisch ; Raphael Schlesinger ; Martin Oehzelt ; Ralph Rieger ; Joachim R盲der ; J眉rgen P. Rabe ; Klaus M眉llen ; Norbert Koch
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：October 31, 2013
• 年：2013
• 卷：117
• 期：43
• 页码：22285-22289
• 全文大小：261K
• 年卷期：v.117,no.43(October 31, 2013)
• ISSN：1932-7455

文摘

This photoemission study shows that the work function (桅) of indium鈥搕in-oxide (ITO) can be increased from 4.2 up to 6.5 eV upon the deposition of the molecular electron acceptors tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and hexaazatriphenylene-hexacarbonitrile (HATCN). The evolution of sample 桅 and the hole injection barrier upon subsequent deposition of the hole transport material N,N鈥?bis(1-naphthyl)-N,N鈥?diphenyl-1,1鈥?biphenyl-4,4鈥?diamine (伪-NPD) was studied for different acceptor precoverages of ITO, corresponding to different initial 桅 values. When 桅 of the acceptor covered substrate exceeds a critical value 桅b>critb>, the highest occupied molecular level of multilayer 伪-NPD is found to be pinned 0.5 eV below the Fermi level (Eb>Fb>). Noteworthy, 桅b>critb> is found at 5.2 eV, which is 0.4 eV higher than expected for 伪-NPD (4.8 eV), and vacuum level alignment does not apply even before Eb>Fb>-pinning sets in. An electrostatic model that accounts for nonuniformity of the substrate at acceptor submonolayer coverages and the associated local work function changes explains the origin of 鈥渄elayed鈥?Eb>Fb>-pinning.