Electric-field-induced Optical Second-harmonic Generation Measurement Research Articles

Study of carrier-mobility of organic thin film by dark-injection time-of-flight and electric-field-induced optical second-harmonic generation measurements

By using dark-injection time-of-flight (ToF) and time-resolved electric-field-induced optical second-harmonic generation (EFISHG) measurements, we studied carrier mobility μ of pentacene (Pen) thin film of ITO/Pen/Al and Au/Pen/polyimide/ITO diodes where pentacene film is ∼100nm in thickness. ToF showed that determination of transit time tr from trace of transient currents is difficult owing to large capacitive charging current. On the other hand, optical EFISHG is free from this charging current, and allows us to calculate hole and electron mobility as μh=1.8×10-4cm2/Vs and μe=7.6×10-7cm2/Vs, respectively, by using the relation tr=d/μ∫tctrE(0)dt (d: Pen thickness, E(0): electric field across Pen), instead of the conventional relationship tr=d2/μV (V: voltage across Pen). Time-resolved EFISHG measurement is useful for the determination of carrier mobility of organic thin film in organic devices.

Study of carrier energetics in ITO/P(VDF-TrFE)/pentacene/Au diode by using electric-field-induced optical second harmonic generation measurement and charge modulation spectroscopy

By using electric-field-induced optical second harmonic generation (EFISHG) measurement and charge modulation spectroscopy (CMS), we studied carrier behavior and polarization reversal in ITO/ poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE))/pentacene/Au diodes with a ferroelectric P(VDF-TrFE) layer in terms of carrier energetics. The current-voltage (I–V) characteristics of the diodes showed three-step polarization reversal in the dark. However, the I–V was totally different under illumination and exhibited two-step behavior. EFISHG probed the internal electric field in the pentacene layer and accounted for the polarization reversal change due to charge accumulation at the pentacene/P(VDF-TrFE) interface. CMS probed the related carrier energetics and indicated that exciton dissociation in pentacene molecular states governed carrier accumulation at the pentacene/ferroelectric interface, leading to different polarization reversal processes in the dark and under light illumination. Combining EFISHG measurement and CMS provides us a way to study carrier energetics that govern polarization reversal in ferroelectric P(VDF-TrFE)/pentacene diodes.

Observation of turnover of spontaneous polarization in ferroelectric layer of pentacene/poly-(vinylidene-trifluoroethylene) double-layer capacitor under photo illumination by optical second-harmonic generation measurement

The details of turnover process of spontaneous polarization and associated carrier motions in indium-tin oxide/poly-(vinylidene-trifluoroethylene)/pentacene/Au capacitor were analyzed by coupling displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement. A model was set up from DCM results to depict the relationship between electric field in semiconductor layer and applied external voltage, proving that photo illumination effect on the spontaneous polarization process lied in variation of semiconductor conductivity. The EFISHG measurement directly and selectively probed the electric field distribution in semiconductor layer, modifying the model and revealing detailed carrier behaviors involving photo illumination effect, dipole reversal, and interfacial charging in the device. A further decrease of DCM current in the low voltage region under illumination was found as the result of illumination effect, and the result was argued based on the changing of the total capacitance of the double-layer capacitors.

Study of Electrical Conduction Mechanism of Organic Double-Layer Diode Using Electric Field Induced Optical Second Harmonic Generation Measurement.

By using electric field induced optical second harmonic generation (EFISHG) and current voltage (I-V) measurements, we studied the electrical transport mechanism of organic double-layer diodes with a structure of Au/N, N'-di-[(1-naphthyl)-N, N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine (a-NPD)/poly(methyl methacrylate) (PMMA)/indium zinc oxide (IZO). Here the α-NPD is a carrier transport layer and the PMMA is an electrical insulating layer. The current level was very low, but the I-V characteristics showed a rectifying behavior. The EFISHG measurement selectively and directly probed the electric field across the α-NPD layer, and showed that the electric field across the a-NPD layer is completely relaxed owing to the charge accumulation at the a-NPD/PMMA interface in the region V > 0, whereas the carrier accumulation was not significant in the region V < 0. On the basis of these experimental results, we proposed a model of the rectification. Further, by coupling the I-V characteristics with the EFISHG measurement, the I-V characteristics of the diodes were well converted into the current-electric field (I-E) characteristics of the α-NPD layer and the PMMA layer. The I-E characteristics suggested the Schottky-type conduction governs the carrier transport. We conclude that the I-V measurement coupled with the EFISHG measurement is very useful to study carrier transport mechanism of the organic double-layer diodes.

Analysis of interfacial energy states in Au/pentacene/polyimide/indium–zinc-oxide diodes by electroluminescence spectroscopy and electric-field-induced optical second-harmonic generation measurement

By using electroluminescence (EL) spectroscopy and electric-field-induced optical second-harmonic generation (EFISHG) measurement, we analyzed interfacial energy states in Au/pentacene/polyimide/indium–zinc-oxide (IZO) diodes, to characterize the pentacene/polyimide interface. Under positive voltage application to the Au electrode with reference to the IZO electrode, the EFISHG showed that holes are injected from Au electrode, and accumulate at the pentacene/polyimide interface with the surface charge density of Qs = 3.8 × 10−7 C/cm2. The EL spectra suggested that the accumulated holes are not merely located in the pentacene but they are transferred to the interface states of polyimide. These accumulated holes distribute with the interface state density greater than 1012 cm−2 eV−1 in the range E = 1.5–1.8 and 1.7–2.4 eV in pentacene and in polyimide, respectively, under assumption that accumulated holes govern recombination radiation. The EL–EFISHG measurement is helpful to characterize organic–organic layer interfaces in organic devices and provides a way to analyze interface energy states.

Degradation analysis and current collapse imaging of AlGaN/GaN HEMTs by measurement of electric field-induced optical second-harmonic generation

On the basis of electric field-induced optical second-harmonic generation (EFISHG) measurements, the degradation of AlGaN/GaN high electron mobility transistor (HEMT) devices was analyzed and the current collapse phenomenon was imaged. Strong second-harmonic (SH) signals were observed at the gate edge at the site of a small defect during the application of a high drain stress voltage (300V), thus indicating that this technique can identify degradation of HEMTs. The EFISHG technique also generated images of the trapped carriers that lead to current collapse in these devices during the on-state. Immediately following the completion of drain-stress voltage application in the off-state (that is, 1μs), SH signals were observed at the gate edge on the drain electrode. This signal location is in good agreement with the well-known virtual gate model of current collapse. The EFISHG measurement technique is therefore capable of detecting pre-breakdown phenomena and producing images of current collapse in AlGaN/GaN HEMT devices.

Study of carrier blocking property of poly-linalyl acetate thin layer by electric-field-induced optical second-harmonic generation measurement

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the carrier-blocking property of poly-linalyl acetate (PLA) thin layers sandwiched in indium-zinc-oxide (IZO)/PLA/C60/Al double-layer diodes. Results showed that the PLA layer totally blocks electrons crossing the C60 layer, and also blocks holes entering from the IZO layer. The EFISHG measurement effectively substantiates the hole-blocking electron-blocking property of the PLA layer sandwiched in double layer diodes.

Study of interface layer effect in organic solar cells by electric-field-induced optical second-harmonic generation measurement

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the effect of the use of bathocuproine (BCP) interface layer. The EFISHG measurements of indium–zinc–oxide (IZO)/C60/Al diodes showed that the BCP layer inserted between C60 and Al formed an electrostatic field |Ei|=2.5×104V/cm in the C60 layer, pointing in a direction from the Al to the IZO. Accordingly, in the IZO/pentacene/C60/BCP/Al organic solar cells (OSCs), holes (electrons) move to the IZO (Al) electrode, enhancing the short-circuit current. The EFISHG measurement is capable of directly probing internal fields in the layers used for OSCs, and is helpful for studying the contribution of the interface layer in OSCs.

Analyzing hysteresis behavior of capacitance–voltage characteristics of IZO/C60/pentacene/Au diodes with a hole-transport electron-blocking polyterpenol layer by electric-field-induced optical second-harmonic generation measurement

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we analyzed hysteresis behavior of capacitance–voltage (C–V) characteristics of IZO/polyterpenol (PT)/C60/pentacene/Au diodes, where PT layer is actively working as a hole-transport electron-blocking layer. The EFISHG measurement verified the presence of interface accumulated charges in the diodes, and showed that a space charge electric field from accumulated excess electrons (holes) that remain at the PT/C60 (C60/pentacene) interface is responsible for the hysteresis loop observed in the C–V characteristics.

Direct Probing of Internal Electric-fields in Fullerene Diodes Using Electric-field-induced Second-harmonic Generation Measurement

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we directly probed internal electrostatic fields formed in indium-zinc-oxide (IZO)/fullerene (C60)/Al diodes, which are electrically shorted. Results showed that an internal electric-field is formed in the direction from the IZO to Al electrode, whereas the electric-field points in the opposite direction by the use of an interlayer of bathocuproine (BCP) between C60 and Al. We concluded that the EFISHG measurement directly probes internal electric-fields formed in organic devices, and it is thus helpful for understanding the effect of an interlayer in diodes.

Probing carrier behavior in organic semiconductor device by electric field induced optical second harmonic generation measurement

Electric field induced optical second harmonic generation (EFISHG) measurement is capable of probing carrier motions in organic devices. By measuring nonlinear polarization induced in active layer of organic devices, the carrier motion is visualized. After summarizing the conventional SHG measurement as a tool for material characterization, we show that the EFISHG is a way for probing carrier motions, and thus extends the potentiality of the conventional SHG measurement. We employ the EFISHG measurement to probe carrier behaviors in double-layer metal-insulator–metal diodes, e.g., Au/TIPS-pentacene/PI/ITO diodes, and demonstrate how the time-resolved SHG measurement probes carrier motion in the diodes, in terms of the I–V and C–V characteristics. Results give an insightful picture on the carrier injection, and the succeeding carrier transport.

Probing charging effects induced in ITO/polyimide/6,13-Bis(triisopropylsilylethynyl)-pentacene/Au diodes by time-resolved optical second harmonic generation measurement

By using the electric field induced optical second harmonic generation (EFISHG) measurement, we studied the charging effects induced in double-layer ITO/polyimide (PI)/6,13-Bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/Au diodes. Fresh samples showed EFISHG response due to electrode charging and the Maxwell–Wagner type interfacial hole charging at the TIPS-pentacene/PI. On the other hand, aged samples showed the SHG response due to the electrode charging only. We concluded that using the EFISHG measurement is an effective way to probe degradation induced in organic devices.

Probing of carrier behavior in organic electroluminescent diode using electric field induced optical second-harmonic generation measurement

By using the electric field induced optical second-harmonic generation (EFISHG) measurements, we probed the transient electric field in a double-layer indium zinc oxide (IZO)/N, N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4, 4′-diamine(α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al electroluminescent (EL) diode. Results evidently showed that EL was initiated by the injected hole transport across α-NPD layer, and holes accumulated at the α-NPD/Alq3 interface while EL was enhanced. Analysis based on the Maxwell–Wagner effect model well accounted for the hole accumulation. EFISHG measurement is useful as a tool for probing carrier behavior in organic EL devices.