Interface charge transfer between metal phthalocyanine and fluorinated hexaazatrinaphthylene molecules
Introduction
Charge transfer (CT) at organic/organic (OO) interfaces is an important issue in low-cost and flexible electronic devices [1,2] such as organic light emitting diodes (OLEDs) [[3], [4], [5]], organic photovoltaics (OPVs) [6,7], and organic field effect transistors (OFETs) [[8], [9], [10]]. The ground-state CT behavior influences electrical conductivity by electrical doping [11], whereas light-induced CT governs charge injection and separation, and even optical characteristics in optoelectronic devices [12]. One of key ingredients to predict the CT behavior at OO interface is the relative energy levels of intrinsic molecules. When acceptor (or dopant) electron affinity (EA) is lower than donor (or host) ionization potential (IP), ion-pair formation takes place. Otherwise, a neutral CT complex between the two molecules can be expected [11]. However, interface energy level alignment is influenced by many factors such as relative molecular orientation [13], interface dipole, electrostatic potential [14], contact doping to electrode [15], and so on. Moreover, actual energy levels are determined by interface OO interaction after putting together into contact.
As promising small molecule derivatives for future optical and electronic devices, metal phthalocyanines (MPcs) have attracted long attention because of their strong orbital conjugation, optical absorption, and crystalline order in solid phase [16]. However, the molecular orientation as well as EA and IP control the CT mechanism in various ways. For example, planar or bent shape of MPc molecules make different energy level alignment at donor/acceptor (D/A) interface and corresponding open circuit voltage, VOC in solar cell devices [17]. Furthermore, even a planar structure molecule, copper phthalocyanine (CuPc) shows IP change by about 0.7 eV according to upstanding-up or lying-down structure in Fig. S1.
Here, to mimic D/A molecular interface, we compare electronic properties of two O/O heterostructures between 2,3,8,9,14,15-hexafluoro-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA-F6) and CuPc or titanyl phthalocyanine (TiOPc) using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The CuPc and HATNA-F6 molecules have a planar shape while the TiOPc molecule has a non-planar shape. As the EA of HATNA-F6 is smaller than the IP of both the MPc, ion pair formation at the interface is not expected. CuPc/HATNA-F6 shows marginal interfacial interaction, whereas TiOPc/HATNA-F6 shows partial CT behavior. The different characteristics stem from nonplanar molecular shape of TiOPc and electronic structure of central Ti–O moiety.
Section snippets
Experimental section
For all experiments, a polycrystalline Au substrate was used after cleaning by sonication bath in acetone, ethanol and deionized water for 10 min each, and subsequent blowing by dry nitrogen gas to avoid contamination. The cleaned substrate was placed in ultrahigh vacuum (UHV) chamber and subjected to Ar+ ion sputtering for 30 min under an accelerating voltage of 3 keV and Ar gas pressure of 10−6 Torr. Organic molecules of HATNA-F6 (99.01%), CuPc (99%), and TiOPc (95%) were purchased from Sigma
Results and discussion
Fig. 1 represents the chemical structure and energy levels of CuPc, TiOPc, and HATNA-F6 molecules. While CuPc and HATNA-F6 have a simple planar shape, the TiOPc has a bent molecular structure with Ti and O atoms being out of plane. The HOMO levels of both the MPcs lie below the HATNA-F6 LUMO level. In other words, the IP of MPcs is higher than the electron affinity (EA) of HATNA-F6. When the two D/A molecules (MPc/HATNA-F6) are in contact, CT from MPcs to HATNA-F6 would be marginal, counting
Conclusions
We have investigated OO interface CT behavior between MPcs (M = Cu, TiO) and HATNA-F6 by photoelectron spectroscopy. As both IPs of CuPc and TiOPc are higher than EA of HATNA-F6, spontaneous CT and ion pair formation at the D/A interface do not take place. However, face-on geometry of both interface generate some degree of interfacial interaction. Remarkable core level and HOMO edge shifts at TiOPc/HATNA-F6 interface indicate partial CT occurring by the interaction of Ti–O moiety and fluorine
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Research Foundation (NRF) (grant No. 2017R1A2B4012086 and 2019M3D1A1078304).
References (24)
- et al.
Oxygen effects on the interfacial electronic structure of titanyl phthalocyanine film: p-Type doping, band bending and Fermi level alignment
Chem. Phys.
(2006) - et al.
Charge transfer between transition metal phthalocyanines and metal substrates: the role of the transition metal
J. Electron. Spectrosc. Relat. Phenom.
(2015) - et al.
Ultrathin and lightweight organic solar cells with high flexibility
Nat. Commun.
(2012) - et al.
Mechanical properties of conjugated polymers and polymer-fullerene composites as a function of molecular structure
Adv. Funct. Mater.
(2014) - et al.
Confinement of charge carriers and molecular excitons within 5-nm-thick emitter layer in organic electroluminescent devices with a double heterostructure
Appl. Phys. Lett.
(1990) - et al.
Organic electroluminescent diodes
Appl. Phys. Lett.
(1987) - et al.
High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer
Nature
(2000) - et al.
Efficiency enhancement in organic solar cells with ferroelectric polymers
Nat. Mater.
(2011) - et al.
The impact of disorder on the energy level alignment at molecular donor-acceptor interfaces
Adv. Mater. Interfaces
(2015) - et al.
Integrated optoelectronic devices based on conjugated polymers
Science
(1998)
Organic field-effect transistors from solution-deposited functionalized acenes with mobilities as high as 1 cm2/V·s
J. Am. Chem. Soc.
Two-dimensional quasi-freestanding molecular crystals for high-performance organic field-effect transistors
Nat. Commun.
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Both authors contributed equally to this work.