Electric Field-induced Second-harmonic Generation Measurements Research Articles

Detection of voltage pulse width effect on charge accumulation in PSCs using EFISHG measurement

We report the effect of applied pulse width on charge accumulation in triple cation perovskite solar cells (PSCs) during the Electric Field Induced Second Harmonic Generation (EFISHG) measurements. The PSCs were prepared in the n-i-p structure using the layer-by-layer deposition technique. The triple cation perovskite precursor containing FAPbI3, MAPbBr3, and CsPbI3 was used to make the perovskite layer on the FTO/c-TiO2/mTiO2 substrates. Spiro OMeTAD was used as a hole transport material over the perovskite active layer, followed by the deposition of Au electrodes. The average power conversion efficiency (PCE) of the fabricated PSCs was 19.17 ± 0.02%. The electric field in the PSCs was selectively probed by the fundamental laser beam wavelength of 960 nm during the EFISHG measurements. Two different pulse widths (100 µs and 50 ms) of the applied potential were used in this study, with consideration of electrode charging and interface charging of PSCs. It has been observed that the pulse width of the applied electric field has a significant effect on the charge accumulation in the PSCs. Using the transient EFISHG measurements, it has also been found that the charging & discharging processes reversibly happen. However, the charging & discharging processes are influenced by remained charges at the interface in PSCs in the investigated triple cations PSCs. The EFISHG results can be well explained by using the Maxwell–Wagner model.

Stability in 3D and 2D/3D hybrid perovskite solar cells studied by EFISHG and IS techniques under light and heat soaking

Abstract We investigate the effect of temperature and light soaking on the stability and charge carrier behavior in triple cation 3-Dimensional (3D) and 2-Dimensional (2D)/3-Dimensional perovskite solar cells (PSCs) using electric field induced second harmonic generation (EFISHG) and impedance spectroscopy (IS) techniques. To prepare the 2D/3D perovskite structures, a layer-by-layer deposition method was employed. Spiro-OMeTAD based hole transport layer was deposited on the top of the perovskite layer using a spin coating procedure. The average power conversion efficiency (PCE) of the fresh fabricated cells was ∼18%. The electric fields in the PSCs were selectively probed by the EFISHG measurements, using the fundamental laser beam wavelength of 960 nm. The results show that the EFISHG technique could be potentially used to identify the degradation mechanism in the PSCs. Also, the IS technique was performed to investigate the electrical parameters affected under light and heat soaking environment. The 2D/3D hybrid PSCs show relatively good stability and better efficiency as compared to 3D PSCs.

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.

Interfacial charging originated from the conductivity decrease of C60 layer in IZO/pentacene/C60/Al organic double-layer solar cells

The origin of interfacial charging process in double-layer organic solar cells (OSCs) was studied by using the normal structure of Indium–Zinc–Oxide/pentacene/C60/Al and its inverted double-layer system. Optical electric-field-induced second-harmonic generation (EFISHG) measurement was employed and results suggested that interfacial charging in these two kinds of OSCs led to charge accumulation with opposite charge polarity, owing to the conductivity decrease of C60 layer. Applying the EFISHG measurements to the inverted OSCs also showed that the significant charge accumulation on donor–acceptor interface is responsible for the low I–V performance of the inverted OSCs. Thus, Maxwell–Wagner type interfacial charging, which is governed by the conductivity of C60, can cause the degradation of the I–V performance of OSCs. The protection of C60 layer from the conductivity decrease is a way to improve OSCs performance.

Analysis of Interfacial Charging Process in Pentacene/C60/Bathocuproine Triple-Layer Organic Solar Cells Using a Maxwell–Wagner Model

Using the Maxwell–Wagner model, we analyzed the interfacial charging of triple-layer organic solar cells (OSCs). Results showed that the charging processes at the two interfaces depend on each other. The electric-field-induced second-harmonic generation (EFISHG) measurements applying external voltage pulses supported the conclusion. On the other hand, the EFISHG measurements using closed circuits with various external resistances clarified the contribution of interfacial charging to the fill-factor of the OSCs. The dielectric nature of the triple layer of the OSCs governs the working mechanism.

Analysis of interface carrier accumulation and relaxation in pentacene/C60 double-layer organic solar cell by impedance spectroscopy and electric-field-induced optical second harmonic generation

By using the impedance spectroscopy (IS) and optical electric-field-induced second harmonic generation (EFISHG) measurement, interfacial carrier relaxation in pentacene/C60 double-layer organic solar cell (OSC) has been analyzed. The IS measurements showed that the interfacial carrier relaxation time was diminished 70 times under photo illumination, due to the increase of carrier density caused by the photovoltaic effect. The equivalent circuit analysis using a Maxwell-Wagner effect model well accounted for the IS results. On the other hand, the EFISHG measurements directly probed the interfacial carrier behaviors caused by the photovoltaic effect. The results showed the suppression of the Maxwell-Wagner type carrier relaxation in OSCs by applying a voltage corresponding to the open circuit voltage Voc. EFISHG measurement coupled with IS measurement provides a clear physics picture of interfacial phenomena related to the photovoltaic effect in OSCs.

Analyzing interfacial carrier charging in pentacene/C60 double-layer organic solar cells by optical electric field induced second-harmonic generation measurement

Abstract Optical electric field induced second-harmonic generation (EFISHG) measurement was applied to investigate interfacial carrier charging in pentacene/C 60 double-layer organic solar cells (OSCs) in terms of photovoltaic effect. The EFISHG measurements demonstrated that electrode charging and interfacial charging at the pentacene/C 60 interface produce electric fields with opposite directions in the C 60 layer. Making use of OSCs connected to various resistances R e showed that the two charging processes exhibit opposite changing tendency due to the photovoltaic (PV) effect, while the response time of the both two charging processes increase with the increases of R e . The Maxwell–Wagner model analysis well accounted for the results obtained using EFISHG measurements.

Direct Probing of Carrier Behavior in Electroluminescence Indium–Zinc-Oxide/N,N '-Di-[(1-naphthyl)-N,N '-diphenyl]-(1,1'-biphenyl)-4,4'-diamine/Tris(8-hydroxy-quinolinato)aluminum(III)/LiF/Al Diode by Time-Resolved Optical Second-Harmonic Generation

By using electric-field-induced second-harmonic generation (EFISHG) measurement, we probed charging and discharging in an α-NPD/Alq3 organic light-emitting diode [α-NPD, N,N '-di-[(1-naphthyl)-N,N '-diphenyl]-(1,1'-biphenyl)-4,4'-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)] while electroluminescence response was monitored. The EFISHG measurement showed that excessive positive charges accumulated at the α-NPD/Alq3 interface in the charging process, accompanying electroluminescence (EL) radiation, and the accumulated excess positive charges disappeared in the discharging process before the EL decayed. Note that the EL radiation was smooth and strong under the high voltage application, while the EL decayed in a similar way. The Maxwell–Wagner model analysis showed no-dependence of the accumulated excessive positive charge on the applied external voltage, suggesting that electrons and holes injected from the opposite electrodes for EL radiation balanced at the interface. The EFISHG measurement will be useful as a direct way to probe carrier behaviors in organic EL devices.

Direct Observation of Carrier Behavior Leading to Electroluminescence in Tetracene Field-Effect Transistor

By using electric-field-induced second-harmonic generation (EFISHG) measurements, we directly probed carrier motion in tetracene field-effect transistors (FETs), followed by electroluminescence (EL). Under a pulse voltage application, positive charges were injected from the source electrode and accumulated on the gate insulator. After the voltage was removed, positive charges remained and formed a space charge field. As a result, electrons were pulled into the FET to recombine with the remaining holes for radiating EL. The EFISHG measurement provides a way to directly probe the electric field transient that contributes to the EL mechanism of light-emitting transistors.

Direct observation of space charge field in tetracene field-effect transistor using time-resolved microscopic optical second harmonic generation

Using electric field-induced second harmonic generation (EFISHG) measurements, we directly probed a time-varying space charge field responsible for carrier injection followed by radiative recombination (electroluminescence; EL). Experiments using tetracene as the active layer of a top-contact field-effect transistor with application of a pulsed voltage showed that a space charge field remained around the source electrode—caused by injected holes, but not by electrons. Observing the time-varying EFISHG intensity at the electrodes clearly showed that the space charge field formed in the vicinity of the electrodes made a dominant contribution to the opposite-polarity carrier injection, directly leading to EL around the electrodes by carrier recombination. We concluded that EFISHG measurement is useful for probing carrier behavior in luminescent materials, by eliminating two-photon excited photoluminescence from the material induced using an incident laser.

High-resolution electric-field-induced second-harmonic generation with ultrafast Ti:sapphire laser

The technique of electric-field-induced second-harmonic generation (EFISHG) has been extended by use of a femtosecond, quasi-cw laser pump source. The effects of ultrashort pulse widths on the coherent EFISHG process have been examined and the analysis procedure used for nanosecond pulse EFISHG measurements is found to be suitable for femtosecond measurements in off-resonant regions. A convenient method for measurement of the group velocity mismatch between the fundamental and second-harmonic pulses in solutions has also been developed to accurately describe the femtosecond EFISHG process. Phase-sensitive detection of the modulated second-harmonic signals results in improvements in accuracy and sensitivity over existing nanosecond pulse experiments. This has been demonstrated with a measurement of Γ(−2ω; ω, ω, 0) of an organic molecule, 4-nitroanisole.

Optical second-harmonic probe for silicon millimeter-wave circuits

Electric field-induced second-harmonic generation (EFISHG) is shown to be a very powerful method for contactless sampling of high-speed electrical signals on silicon millimeter-wave devices. We present picosecond resolution EFISHG measurements of 2 GHz sinusoidal signals on a photoconductive silicon-on-sapphire antenna. We further demonstrate the application of the EFISHG technique for spatial mapping of free-running microwave signals in silicon.

Supramolecular Second-Order Nonlinearity of Polymers with Orientationally Correlated Chromophores

Nonlinear optical chromophores can be organized as orientationally correlated side groups of polymers with a rigid backbone. In such an organization, each chromophore contributes coherently to the second-order nonlinear response of the polymer structure. A first hyperpolarizability exceeding 5000 × 10 −30 electrostatic units was measured for a poly(isocyanide) structure containing ∼100 chromophores by means of hyper-Rayleigh scattering. Electric field-induced second-harmonic generation measurements confirmed that the product of the permanent dipole moment and the first hyperpolarizability was enhanced for the polymer structure. These results provide guidelines for future efforts to optimize supramolecular structures for applications in second-order nonlinear optics.

Relationship between second-harmonic generation and electric-field-induced second-harmonic generation.

We calculate the electric-field-induced second-harmonic-generation (EFISH) susceptibility of a homogeneously broadened two-level system with permanent dipole moments. This susceptibility has contributions from the second-harmonic-generation (SHG) susceptibility of the same system without the presence of a dc field and the four-wave-mixing (FWM) component of the susceptibility ${\ensuremath{\gamma}}_{\mathit{e}}$(-2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},0). The magnitude and phase of the various contributions to EFISH arising from SHG and from FWM depend on the frequency of the fundamental optical field, the transition dipole moment ${\mathrm{\ensuremath{\mu}}}_{\mathit{a}\mathit{b}}$, the permanent dipole moment of the ground-state level ${\mathrm{\ensuremath{\mu}}}_{\mathit{a}\mathit{a}}$, and the difference of the ground- and excited-state dipole moments \ensuremath{\Delta}\ensuremath{\mu}. We present non-rotating-wave-approximation results for both the quasi-steady-state in systems where the dephasing time ${\mathit{T}}_{2}$ is much shorter than the population decay time ${\mathit{T}}_{1}$, so the polarization is in a steady state but the population is not, and for the complete steady state. Extraction of the SHG susceptibility from EFISH measurements is considered.