Spontaneous charge transfer at organic-organic homointerfaces to establish thermodynamic equilibrium

Abstract

The energy level alignment of ␣ , -dihexylsexithienyl ͑DH6T͒ mono-and multilayers on tetrafluorotetracyanoquinodimethane ͑F4-TCNQ͒ precovered Ag͑111͒ and polycrystalline Au substrates was investigated with ultraviolet photoelectron spectroscopy. For certain F4-TCNQ precoverages molecular level pinning at DH6T monolayer-multilayer homointerfaces was observed. The pinning behavior shows that thermodynamic equilibrium can be established across hexyl chains via charge transfer, indicating the limited use of these short alkyl chains for insulation in the field of molecular electronics. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2715042͔The charge carrier injection barrier height across organic/metal interfaces plays a crucial role in virtually all devices in the field of "organic electronics." [1][2][3] Energy levels at organic/metal interfaces can be tuned by precovering the metal substrate with strong electron accepting molecules such as tetrafluorotetracyanoquinodimethane ͑F4-TCNQ͒ ͓chemical structure shown in Fig. 1͑a͔͒. Such molecules can undergo a charge-transfer-like interaction on the metal substrate, 4,5 which increases the substrate work function. Energy levels of subsequently deposited molecules are aligned to this new effective work function. ␣ , -dihexylsexithienyl ͑DH6T͒ ͓Fig. 1͑a͔͒ on Ag͑111͒ and polycrystalline Au undergoes a structural transition from a flat lying monolayer ͑L1͒ to inclined multilayers ͑L2͒. 6 Most likely changes in the polarization energy, as suggested to explain similar changes in orientation for pentacene, 7,8 lead to a decrease of the ionization energy of the molecules and consequently to a rigid shift of molecular levels towards the Fermi level ͑E F ͒ ͓i.e., lowering of the hole injection barrier ͑HIB͔͒. In our work we investigated the energy level alignment of DH6T deposited on F4-TCNQ precovered Ag͑111͒ and polycrystalline Au with ultraviolet photoelectron spectroscopy ͑UPS͒. A transition from vacuum level alignment to molecular level pinning-reminiscent of Fermi-level pinning 9 -at the homointerface between DH6T monolayer and multilayers was observed, which depended on the amount of predeposited F4-TCNQ.UPS experiments were carried out at the FLIPPER II end station at HASYLAB ͑Hamburg, Germany͒. 10 The interconnected sample preparation chambers ͑base pressure of 2 ϫ 10 −9 mbar͒ and analysis chamber ͑base pressure of 2 ϫ 10 −10 mbar͒ allowed sample transfer without breaking ultrahigh vacuum conditions. The Ag͑111͒ single crystal was cleaned by repeated Ar-ion sputtering and annealing cycles ͑up to 550°C͒ until a clear low energy electron diffraction pattern was observed. The polycrystalline Au substrates were at least 400 Å thick Au films on a Cu foil, evaporated in situ and Ar-ion sputtered before use. DH6T ͑H. C. Starck GmbH͒ and F4-TCNQ ͑Fluka͒ were evaporated using resistively heated pinhole sources at evaporation rates of about 1 Å / min. The film mass thickness was monitored with a quartz crystal microbalance. Spectra were recorded with a double-pass cylindric...

Influence of intramolecular polar bonds on interface energetics in perfluoro-pentacene on Ag(111)

“…A similar spontaneous charge transfer at an organic/ organic homointerface due to an orientational transition has been observed before, 44 however with opposite direction. For PFP in the herringbone arrangement the small transport gap ͑estimated to be ϳ2.10 eV͒ ͑Ref.…”

Controlling energy level offsets in organic/organic heterostructures using intramolecular polar bonds

“…1,2 Intensive research efforts are being carried out to understand the energy level alignment in organic heterostructures with the aim of achieving full control over interface energetics. [3][4][5][6] Recently, the importance of intramolecular polar bonds ͑IPBs͒ on the electronic structure in organic/organic heterostructures was realized and it was demonstrated that IPBs can significantly impact the ionization energy ͑IE͒ of ordered molecular assemblies. 7,8 The IE is commonly believed to be a key parameter for energy level alignment.…”

Interdiffusion of molecular acceptors through organic layers to metal substrates mimics doping-related energy level shifts

“…[1][2][3][4] They yield improved device performance due to organic/organic ͑O/O͒ charge transfer complex ͑CTC͒ formation in the codeposited thin films, which increases the layer conductivity and lowers the hole injection barrier ͑HIB; defined as energy difference between E F and the highest occupied molecular orbital onset͒ at the anode ͑p-type doping͒. Strong electron acceptors can also be used to precisely adjust the energy levels at organic/metal ͑O/M͒ interfaces, 5,6 even in the absence of doping. In this case, an O/M CTC ͑Ref.…”

“…7͒ is formed, which increases the effective substrate surface potential and thus reduces the HIB for any subsequently deposited organic material. 5,6 Note that HIB lowering due to O/M CTC formation may be misinterpreted as p-type doping of the organic layer ͑O/O CTC͒ because a shift of the donor energy levels toward the substrate Fermi level ͑E F ͒ occurs in both cases. Acceptors are often small molecules like tetrafluoro-tetracyanoquinodimethane ͑F4-TCNQ͒ ͓chemical structure shown in Fig.…”