Organic light-emitting diode (OLED) displays using flexible substrates have many attractive features. Since transparent conductive oxides do not fit the requirements of flexible devices, conductive polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) has been proposed as an alternative. The charge injection and accumulation in OLED devices with PEDOT:PSS anodes are investigated and compared with indium tin oxide anode devices. Higher current density and electroluminescence light intensity are achieved for the OLED device with a PEDOT:PSS anode. The electric field induced second-harmonic generation technique is used for direct observation of temporal evolution of electric fields. It is clearly demonstrated that the improvement in the device performance of the OLED device with a PEDOT:PSS anode is associated with the smooth charge injection and accumulation.

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 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 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 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 diode.

By using charge modulated reflectance (CMR) imaging technique, charge distribution in the pentacene organic field-effect transistor (OFET) with a ferroelectric gate insulator [P(VDF-TrFE)] was investigated in terms of polarization reversal of the P(VDF-TrFE) layer. We studied the polarization reversal process and carrier spreading process in the OFET channel. I-V measurement showed a hysteresis behavior caused from the spontaneous polarization of P(VDF-TrFE), but the hysteresis I-V curve changes depending on the applied drain bias, possibly due to the gradual shift of polarization reversal position in the OFET channel. CMR imaging visualized the gradual shift of polarization reversal position, and showed that the electrostatic field formed by the polarization of P(VDF-TrFE) contributes to hole and electron injection into pentacene layer and the carrier distribution is significantly dependent on the direction of the polarization. Polarization reversal position in the channel region is governed by the electrostatic potential and it happens where the potential reaches the coercive voltage of P(VDF-TrFE). Transmission line model developed on the basis of the Maxwell-Wagner effect element analysis well accounts for this polarization reversal process in the OFET channel.