Optoelectronic conversion and polarization hysteresis in organic MISM and MISIM devices with DA-type single-component molecules.

Organic electronic devices offer various advantages, such as low cost and tunability. However, the organic semiconductors used in these devices have significant drawbacks, including instability in air and low carrier mobility. To address these challenges, we recently introduced organic MISM and MISIM (M = metal, I = insulator, S = semiconductor) devices, which effectively generate photo-induced displacement current and exhibit ferroelectric behavior. In previous studies, the S layer consisted of an organic donor-acceptor (DA) bilayer. In the present research, we fabricated MISM and MISIM devices using DA-type single-component molecules as the S layer and examined their photocurrent and polarization hysteresis. While the performance of these devices does not surpass that of DA bilayer devices, we discovered that DA-type single-component molecules can be utilized for photoelectric conversion and polarization trapping.

Stabilization of Interfacial Polarization and Induction of Polarization Hysteresis in Organic MISIM Devices.

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|insulator|semiconductor|insulator|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 polarization-voltage (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, C60, and TCNQ) as the S layer, demonstrating the versatility of the interfacial polarization mechanism.

Rate-determining process in MISIM photocells for optoelectronic conversion using photo-induced pure polarization current without carrier transfer across interfaces.

Recently, we proposed a [metal|insulator|semiconductor|insulator|metal] (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.