2011.12.16 Literature seminar Yuna Kim

Organic field-effect transistor Source (S), through which the majority carriers enter the channel. Conventional current entering the channel at S is designated by IS. Drain (D), through which the majority carriers leave the channel. Conventional current entering the channel at D is designated by ID. Drain to Source voltage is VDS. Gate (G), the terminal that modulates the channel conductivity. By applying voltage to G, one can control ID.

Higher switching speed Downscale geometry Mobility Ion/Ioff LINEAR REGIME SATURATION REGIME

OFETs with specific functionality organic phototransistors, organic memory transistors, light-emitting organic transistors, OFET-based sensors and other stimuli-responsive transistors mimic the functions of biological systems and conversion of external inputs to useful signals applications; light detection, signal storage, laser emission, environmental monitoring, and medical diagnostics installing desired functionalities into OFETs remains a major challenge

Purpose of this work Neutral colorless form (SP-closed) zwitterionic, colored form (SP-open) Purpose of this work OFET structure electric dipole moment SP-closed SP-open electrical characteristics of OFETs can be reversibly and optically controlled for the first time through engineering of the interfacial properties induced by the effect of the molecular dipoles. charge transport occurs through at most the first few layers of molecules at the semiconductor/dielectric interface. ->tailoring the charge mobility, the contact resistance and the trap density at the interface.

Synthesis of spiropyran carboxylic acid Formation of SP SAMs 1. SiO2 substrates underwent a hydrophilic treatment by heating to 110℃ in a Piranha solution for 0.5 h. 2. (3-aminopropyl)-trimethoxysilane (APTMS) SAMs were formed by immersing cleaned substrates in freshly mixed 94% acidic methanol 5.0% H2O, and 1.0% APTMS for 15 min at room temperature and then rinsing 3. baked on a hotplate at 120℃ for 5 min. 4. immersed in the fresh 1mM SP /toluene solution, which contains 0.1% DCC inside, at room temperature for 3 days.

Device Fabrication and Characterization bottom-gate top-contact Monolayer formation Pentacene films (40 nm) fabrication on top of the SAMs by thermal evaporation 3. Drain and source electrodes (50 nm Au) deposition on the surface of the semiconductor layer through a shadow mask (The resulting channel length and width were 60 μm and 7 mm, respectively.) Carrier mobilities (μ, in the saturation regime) ID = WCiμ(VG-VT)2/(2L) where ID is the source-drain saturation current, Ci is the gate dielectric capacitance (per area), VG is the gate voltage, and VT is the threshold voltage. VT can be estimated as the x intercept of the linear section of the plot of VG vs. (ID)1/2. Light irradiations (~ 10 μW/cm2, λ = 365 nm) and with a 150 W Halogen incandescent lamp (Imax = ~ 30 mW/cm2, λ > 520 nm).

Immobilization of SPs on the SiO2 surface covalent amide bond formation with the aid of the well-known carbodiimide dehydrating/activating agent DCC. carboxyl acids of SPs and the surface-bound amino groups produces the amide linkage FT-IR of SAM XPS

Determine the thickness of SAMs Parratt formalism to a box model with three layers Low-angle X-ray scattering of SP SAMs coverage ; ∼1.8 molecules in a 1x1 nm2 optimized conformation

Photoactivity of SP SAMs The calculated percent conversion (xe) of SP molecules from SP-closed to SP-open at the photostationary state; ∼84.4% UV/visible absorption spectra of a SP SAM UV (λ = 365 nm) and visible light (λ > 520 nm) visible under UV

The average saturation carrier mobility (μmax) ∼0.03 cm2 V-1 s-1, <on bare silicon substrates (∼0.2 cm2 V-1 s-1), Threshold voltages (VTh) ; shifted to more negative values (∼50 V). +Possibilities; increase of the interface trap density in the presence of SP molecules and unreacted amine groups on the surface formation of relatively small pentacene crystal domains during thermal deposition

Device characteristics and photo-responses of a pentacene device one full switching cycle VD = -100 V w/o SP SAMs rate contants By photoexitation of pentacene photoisomerization of SP molecules in SAMs is responsible for the switching effect in device characteristics the percent conversion (xe) of SPs from SP-closed to SP-open; ∼55.3% (<∼84.4% in sln, larger steric hindrance effect in SP SAMs covered by a thick pentacene layer)

reversibility of the switching Responsivity (R) and photosensitivity (P) ∼3 h VD =-30 V; VG =-15 V L ;channel length, W;channel width Best value (VG scans from 10 to 100 V under UV, 7.4 μW/cm2 ) ; R =~ 400 A/W and P =~ 450 amorphous silicon; R = 300 A/W and P = 1000

organic memory device Reversible shifts in Vth threshold voltages programming with UV light, reading electrically and erasing with visible light

SAMs modulate… Electronic states at the dielectric interface electrode work function switching mechanism UV-> increase in local electric field -> increased band bending -> increased hole density in the channel-> VG shift

expected electric field inside SAM layers using Ein = N(μmol/εdmol) N; areal density dmol ; height of SP-SAM molecules ε ; effective dielectric constant inside the SP-SAM molecules assumed N is ∼1.8x 1014 cm-2 ε; 2 ~ 3 μmol values; 6.4 D for SP-closed, 13.9 D for SP-open dmol; ∼2.2 nm Obtained difference in the net voltage between SP-open SAMs and SP-closed SAMs; 1.1~1.7 V corresponding to the internal field Ein of 5.2~ 7.9 MV/ cm thickness of the SiO2 gate insulator; 300 nm difference in the external gate voltage of 156~237 V, which is 7~10 times larger than the results obtained experimentally (ΔVG = ∼22 V on average). -> inaccuracy in the density and the tilted angle of SAM dipoles, and in the crystallinity of SP SAMs -> possible charge transfer between organic layers and the photogenerated phenoxide ion groups in SP-open forms

Photogenerated phenoxide ion can behave like a charge trap (quench p-type carriers) -> decrease of carrier mobility and the VG shift in the negative direction? similar photoswitching properties!

Conclusion The photochemical bistable SAMs triggered by SP photoisomerizations produce two distinct built-in electric fields on the OFET that can reversibly modulate the channel conductance and consequently VTh values, thus leading to a bifunctional OFET and a low-cost noninvasive memory device. Decreasing the thickness of the insulating SiO2 layer using a high dielectric constant (high-k) layer, in combination of the significant positive shifts in VG, the high operating voltage needed for programming and erasing can be dramatically reduced. ability to realize nonvolatile memory effects is hampered by the thermal back-conversion process of SPs