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

Effect of 1,8-Diiodooctance additive on the charge carriers behavior in the PCPDTBT:PC71BM BHJ films investigated by using electric-field-induced optical second-harmonic generation measurement

Electric-field-induced optical second harmonic generation (EFISHG) measurement was conducted to investigate the effect of 1,8-diiodooctane (DIO) additive, which is relevant to improve the efficiency of bulk heterojunction (BHJ) solar cells, on carrier behaviors in the BHJ layer, where the BHJ layer is a blend of [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and regioregular poly [2,6-(4,4-bis-(2-Ethylhexyl)-4H-cyclopentane [2,1-b;3,4-b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Using the laser wavelength of 1000 nm, we could selectively observe the electric field in the PCPDTBT phase of BHJ layer, which allowed us to determine the difference in charge accumulation at the interface with and without DIO. With adding DIO, a decrease in interfacial stored charge was observed, and this plays a role in improving the efficiency by about 25%. The DIO modified the internal field distribution so that the electric field promotes the electrons toward the ITO electrode and holes toward the PEDOT:PSS/Au electrode.

Degradation analysis of AlGaN/GaN high electron mobility transistor by electroluminescence, electric field-induced optical second-harmonic generation, and photoluminescence imaging

The degradation positions in a Schottky gate AlGaN/GaN high electron mobility transistor device were visualized systematically by the complementary use of electroluminescence (EL), electric field-induced optical second-harmonic generation (EFISHG), and photoluminescence measurements in a single apparatus. The EL measurement clearly observed the degradation position, owing to its luminescence by the injection of hot carriers through the damaged AlGaN layer. On the other hand, EFISHG measurements suggested the weakening of the electric field at the degradation position. The degradation position was the gate edge on the drain side, and Ni residue like the particle was identified approximately 800 nm from the gate edge. The direct reason to degrade the device performances is the concentration of the electric field at the gate edge, and Ni residue would be the trigger of the degradation. Hot electrons could then easily tunnel through the inside of the damaged AlGaN barrier due to the formation of surface defects.

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.

Direct observation of trapped charges under field-plate in p-GaN gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Trapped charges underneath the field-plate (FP) in a p-gallium nitride (GaN) gate AlGaN/ GaN high electron mobility transistor device were visualized by using electric field-induced optical second-harmonic generation imaging. Second-harmonic (SH) signals in the off-state of the device with FP indicated that the electric field decreased at the p-GaN gate edge and concentrated at the FP edge. Nevertheless, SH signals originating from trapped charges were slightly observed at the p-GaN gate edge and were not observed at the FP edge in the on-state. Compared with the device without FP, reduction of trapped charges at the p-GaN gate edge of the device with FP is attributed to attenuation of the electric field with the aid of the FP. Negligible trapped charges at the FP edge is owing to lower trap density of the SiO2/AlGaN interface at the FP edge compared with that of the SiO2/p-GaN sidewall interface at the p-GaN gate edge and attenuated electric field by the thickness of the SiO2 passivation layer on the AlGaN surface.

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.

Determination of carrier mobility of semiconductor layer in organic metal-insulator-semiconductor diodes by displacement current and electric-field-induced optical second-harmonic generation measurements

A novel method for determining carrier mobility of semiconductor layer in thin-film organic metal-insulator-semiconductor (MIS) diodes is proposed, where displacement current measurement (DCM) is used in combination with electric-field-induced optical second-harmonic generation (EFISHG) measurement. EFISHG signals generated from the semiconductor layer probe the electric field caused by carriers moving in the semiconductor layer of MIS diodes. On the other hand, DCM signals generated in accordance with the time derivative of induced charge on metal electrode well identify the transit time of carriers across the semiconductor layer. By using Au/pentacene/polyimide (PI)/indium-tin-oxide (ITO) diodes, we experimentally determined the carrier mobility of the pentacene layer. Results and analysis showed that step-voltage application to MIS diodes is suitable for the use of this proposed method.

Three-dimensional current collapse imaging of AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Three-dimensional (3D) current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistor devices was achieved by a combination of two-dimensional (2D) and depth directional electric field-induced optical second-harmonic generation (EFISHG) measurements. EFISHG can detect the electric field produced by trapped carriers, which causes the current collapse. In the 2D measurement, the strong second-harmonic (SH) signals appeared within 1 μm from the gate edge on the drain side at 0.8 μs after the transition from the off- to no bias- state in both unpassivated and passivated samples. In the depth measurement, the SH signals were generated mainly from the AlGaN surface region of the unpassivated sample due to the presence of high-density trap sites in the AlGaN layer, and SH signals from bulk GaN region were also detected at 50 μs after the transition from the off- to no bias- state in the passivated sample. The origin of the traps is presumably the nitrogen vacancies in the GaN buffer layer.

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.

Optical second-harmonic generation measurement for probing organic device operation.

We give a brief overview of the electric-field induced optical second-harmonic generation (EFISHG) technique that has been used to study the complex behaviors of organic-based devices. By analyzing EFISHG images of organic field-effect transistors, the in-plane two-dimensional distribution of the electric field in the channel can be evaluated. The susceptibility tensor of the organic semiconductor layer and the polarization of the incident light are considered to determine the electric field distribution. EFISHG imaging can effectively evaluate the distribution of the vectorial electric field in organic films by selecting a light polarization. With the time-resolved technique, measurement of the electric field originating from the injected carriers allows direct probing of the carrier motion under device operation, because the transient change of the electric field distribution reflects the carrier motion. Some applications of the EFISHG technique to organic electronic devices are reviewed.

A way for studying the impact of PEDOT:PSS interface layer on carrier transport in PCDTBT:PC71BM bulk hetero junction solar cells by electric field induced optical second harmonic generation measurement

Electric-field-induced optical second-harmonic generation (EFISHG) measurement was employed to study the impact of poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) interface layer on the carrier transport mechanism of the PCDTBT:PC71BM bulk heterojunction (BHJ) organic solar cells (OSCs). We revealed that the electric fields in the PCDTBT and PC71BM were allowed to be measured individually by choosing fundamental laser wavelengths of 1000 nm and 1060 nm, respectively, in dark and under illumination. The results showed that the direction of the internal electric fields in the PCDTBT:PC71BM BHJ layer is reversed by introducing the PEDOT:PSS layer, and this results in longer electron transport time in the BHJ layer. We conclude that TR-EFISHG can be used as a novel way for studying the impact of interfacial layer on the transport of electrons and holes in the bulk-heterojunction OSCs.

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.

Current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation measurement

Two-dimensional current collapse imaging of a Schottky gate AlGaN/GaN high electron mobility transistor device was achieved by optical electric field-induced second-harmonic generation (EFISHG) measurements. EFISHG measurements can detect the electric field produced by carriers trapped in the on-state of the device, which leads to current collapse. Immediately after (e.g., 1, 100, or 800 μs) the completion of drain-stress voltage (200 V) in the off-state, the second-harmonic (SH) signals appeared within 2 μm from the gate edge on the drain electrode. The SH signal intensity became weak with time, which suggests that the trapped carriers are emitted from the trap sites. The SH signal location supports the well-known virtual gate model for current collapse.

Detection of pre-electrical breakdown of IZO/α-NPD/Alq3/Al light-emitting diodes by electric-field-induced optical second-harmonic generation measurement

By using the electric-field-induced optical second-harmonic generation (EFISHG), we measured the EFISHG–time (EFISHG–t) characteristics to study pre-electrical breakdown of 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 organic light-emitting diodes (OLEDs). A series of EFISHG pulses were identified as a pre-electrical breakdown phenomenon, before OLEDs were electrically broken. Analyzing the results revealed the additional negative charge accumulation at the α-NPD/Alq3 interface which caused by the generation of EFISHG pulses. We concluded that the EFISHG–t measurement is available as a method for detecting pre-electrical breakdown phenomena of OLEDs.

Analysis of carrier behavior in α-NPD/P(VDF–TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation

The effect of ferroelectric polymer on carrier behaviors in indium–tin oxide (ITO)/poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)]/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/Au structure was investigated. The internal electric field in α-NPD introduced by the dipole moments from ferroelectric P(VDF–TrFE) together with the α-NPD/P(VDF–TrFE) interface accumulated charges plays an important role in the carrier motion in α-NPD. In displacement current measurement (DCM), two asymmetric peaks and reduced polarization resulting from a large injection barrier from Au electrode to α-NPD layer were observed. Coupled with the electric-field-induced optical second-harmonic generation (EFISHG) measurement, the electric field in α-NPD layer was directly probed and gave a support that only part of the dipoles in P(VDF–TrFE) was polarized. We proposed that the formation of an amorphous insulating layer between amorphous α-NPD layer and partially crystallized P(VDF–TrFE) layer markedly decreased the voltage directly applied on the P(VDF–TrFE) film and caused a reduced polarization. Our findings here will be helpful in analyzing the carrier behavior in organic electronic devices using ferroelectric layers.

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.

Selective observation of photo-induced electric fields inside different material components in bulk-heterojunction organic solar cell

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement at two laser wavelengths of 1000 nm and 860 nm, we investigated carrier behavior inside the pentacene and C60 component of co-deposited pentacene:C60 bulk-heterojunctions (BHJs) organic solar cells (OSCs). The EFISHG experiments verified the presence of two carrier paths for electrons and holes in BHJs OSCs. That is, two kinds of electric fields pointing in opposite directions are identified as a result of the selectively probing of SHG activation from C60 and pentacene. Also, under open-circuit conditions, the transient process of the establishment of open-circuit voltage inside the co-deposited layer has been directly probed, in terms of photovoltaic effect. The EFISHG provides an additional promising method to study carrier path of electrons and holes as well as dissociation of excitons in BHJ OSCs.

Analysis of carrier behavior in C60/P(VDF-TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation measurement

By using displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the carrier behavior in the indium-tin oxide (ITO)/Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/C60/Au(or Al) capacitors. Two DCM peaks appeared asymmetrically at around −35.5 V and +30.0 V in the dark. Correspondingly, the EFISHG response from the C60 layer was observed, but the peak positions were different with respect to DCM ones. The results show that the spontaneous polarization of the ferroelectric P(VDF-TrFE) polymeric layer directly affects the electric field in the C60 layer, and thus governs the carrier motion in this layer. As a result, the C60 layer serves like an insulator in the dark, while electrons and holes are captured and released at the interface in response to the turn-over of spontaneous polarization of ferroelectric layer. On the other hand, under white light illumination, C60 layer serves like a conductor due to the increase of photogenerated mobile carriers, and these carriers dominate the carrier motions therein. Our findings here will be helpful for analyzing carrier behaviors in organic electronic devices using ferroelectric polymers.

Study of effect of inserted pentacene layer in ITO/P(VDF-TrFE)/α-NPD/Au capacitor using electric-field-induced optical second-harmonic generation and displacement current

The effect of inserted pentacene layers on the interface optimization was investigated in the indium-tin oxide (ITO)/Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/N, N′-bis(1-naphthyl)-N,N′-diphenyl-1, 1′-biphenyl-4,4′-diamine (α-NPD)/Au capacitors, by using displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement. Results show that the inserted pentacene layer between the α-NPD and P(VDF-TrFE) layers effectively improves the contact between the semiconductor and the ferroelectric P(VDF-TrFE) and it further enhances dipolar switching, resulting in three peaks in the DCM, while the insertion of a pentacene layer between the α-NPD and the Au electrode can effectively acquire holes from the Au electrode to be conductive, only leading to peak shift in the DCM with only two peaks. It is concluded that pentacene is a good candidate available for the interface optimization of organic devices.