Organic optoelectronic conversion possesses various advantages such as low cost and tunable light sensing, but organic semiconductors suffer from significant drawbacks, such as instability in air and low carrier mobility. To overcome them, we recently proposed a [metal (M)vertical bar insulator (I)vertical bar semiconductor (S)vertical bar insulator (I)vertical bar metal (M)] structure (MISIM photocells) that can effectively induce photo-induced displacement current without any passage of carriers through the S vertical bar I and I vertical bar M interfaces. In the present work, we investigated the duty-cycle dependence of MISIM photocells, to determine the guiding principles for improving the speed and efficiency of the optoelectronic conversion toward its practical application. We formed an MISIM photocell consisting of a bilayer of zinc phthalocyanine (ZnPc) and fullerene (C-60) as the S layer and parylene C (PC) as the I layers, and tested the dependence of the photocurrent on the duty cycle D = t(ON)/(t(ON) + t(OFF)), where t(ON) and t(OFF) are the light illumination and blocking times, respectively, in one cycle under light modulation. By independently changing t(ON) and t(OFF) in the range of 0.2-2.0 mu s, the rise time of the transient photocurrent was found to be improved by approximately 30% by the duty cycle. We also revealed that the charge carriers Q generated in one cycle reached their maximum at D approximate to 0.4, and the time average increased gradually with an increase in modulation frequency. This feature of Q was well explained by a theoretical model for the MISIM photocells.

Molecule-based ferroelectrics has attracted much attention because of its advantages, such as flexibility, light weight, and low environmental load. In the present work, we examined an organic metal vertical bar insulator vertical bar semiconductor vertical bar insulator vertical bar metal (MISIM) device structure to stabilize the interfacial polarization in the S layer and to induce polarization hysteresis even without bulk ferroelectrics. The MISIM devices with I = parylene C and S = TMB (=3,3',5,5'-tetramethylbenzidine)-TCNQ (=tetracyanoquinodimethane) exhibited hysteresis loops in the polarizationvoltage (P-V) curves not only at room temperature but also over a wide temperature range down to 80 K. The presence of polarization hysteresis for MISIM devices was theoretically confirmed by an electrostatic model, which also explained the observed thickness dependence of the I layers on the P-V curves. Polarization hysteresis curves were also obtained in MISIM devices using typical organic semiconductors (ZnPc, C-60, and TCNQ) as the S layer, demonstrating the versatility of the interfacial polarization mechanism.

The charge-transfer (CT) complex 3,3 ',5,5 '-tetramethylbendizine-7,7,8,8-tetracyanoquinodimethane was found to display the thermosalient effect, in which crystals exhibit mechanical responses due to temperature changes. Such responses are rarely observed with CT complexes. Furthermore, while this crystalline complex was known to have an interesting phase transition in which the charge distribution changes at low temperatures, some of the physical properties of single crystals of the material were unknown, because the crystal is broken during the thermosalient event. In this study, we used a polymer-coating method to prevent cracking of the crystal, allowing us to observe drastic changes in resistivity and ferroelectric-like dielectric response accompanied by the phase transition at low temperature. This result suggests that organic solids with mechanical response and electric and magnetic functions can be produced, since CT complexes exhibit various electric and magnetic functions and physical properties.

Recently, we proposed a [metalinsulatorsemiconductorinsulatormetal] (MISIM) photocell, as a novel architecture for high-speed organic photodetectors. The electric polarization in the S layer, induced by modulated light illumination, propagates into the outside circuit as a polarization current through the I layers, without any carrier transfer across the interfaces. In the present work, we examined the MISIM photocells consisting of zinc-phthalocyanine(ZnPc)-C60 bilayers for the S layer and Parylene C for the two I layers, to understand the fundamental aspects of the MISIM photocells, such as current polarity and modulation-frequency dependence. It was found that, in such devices, the current polarity was primarily determined by the polarization in the S layer, which was induced by the donor-acceptor charge-transfer upon illumination. Furthermore, the ON and OFF current, which appeared in the periods of illumination-on and -off, respectively, exhibited significantly different dependence on the modulation frequency. This was well-explained by an imbalance between a quick polarization in the S layer during illumination and its slow relaxation in the dark. © 2019 the Owner Societies.