Microscopic View of Charge Injection in an Organic Semiconductor

Silveira, William R.; Marohn, John A.

doi:10.1103/physrevlett.93.116104

Cited by 88 publications

(71 citation statements)

References 24 publications

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“…Meanwhile, the capacitance signal (∂ 2 C/ ∂z 2 ) is a measure of mobile carrier density N m (density of free moving carriers in response to the tip's AC bias with ω = 2kHz). 18 The simultaneously recorded topography and surface potential images of a FET channel area is shown in Figure 1c,d. The images show that the QD film has submonolayer coverage, with a porous structure connecting the source and drain electrodes.…”

Section: Nano Lettersmentioning

confidence: 99%

Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors

et al. 2015

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Noncrystalline semiconductor materials often exhibit hysteresis in charge transport measurements whose mechanism is largely unknown. Here we study the dynamics of charge injection and transport in PbS quantum dot (QD) monolayers in a field effect transistor (FET). Using Kelvin probe force microscopy, we measured the temporal response of the QDs as the channel material in a FET following step function changes of gate bias. The measurements reveal an exponential decay of mobile carrier density with time constants of 3−5 s for holes and ∼10 s for electrons. An Ohmic behavior, with uniform carrier density, was observed along the channel during the injection and transport processes. These slow, uniform carrier trapping processes are reversible, with time constants that depend critically on the gas environment. We propose that the underlying mechanism is some reversible electrochemical process involving dissociation and diffusion of water and/or oxygen related species. These trapping processes are dynamically activated by the injected charges, in contrast with static electronic traps whose presence is independent of the charge state. Understanding and controlling these processes is important for improving the performance of electronic, optoelectronic, and memory devices based on disordered semiconductors.

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Section: Nano Lettersmentioning

confidence: 99%

Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors

et al. 2015

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“…[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.…”

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confidence: 99%

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

Duhm¹,

Glowatzki²,

Rabe³

et al. 2007

Applied Physics Letters

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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...

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“…As can be seen in Figure 6.7, the structure shows a characteristic close to ohmic for voltages between 1 V and 10 V. An exponential dependence between I and V , previously observed in m-DNB films doped with resorcinol, has been evidenced for PTCDA films at voltages < 1 V suggesting a Poole-Frenkel conduction mechanism (Stanculescu, 2006 b;Silveira, 2004) with a field dependent mobility of the charge carriers. At voltages > 10 V the effect of the spacecharge limited current became significant.…”

Section: Mis Type Structures: Metal/organic Semiconductor/inorganic Smentioning

confidence: 73%

“…where μ 0 is the zero-field mobility, E is the electric field strength and β is associated with the Poole-Frenkel-like field dependence of the mobility in the organic (Silveira, 2004). For field dependent mobility the SCL current density becomes:…”

Section: Theoretical Approachmentioning

confidence: 99%