Duty-cycle dependence of photo-induced displacement current in MISIM photocells

Abstract

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)|insulator (I)|semiconductor (S)|insulator (I)|metal (M)] structure (MISIM photocells) that can effectively induce photo-induced displacement current without any passage of carriers through the S|I and I|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 μs, 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 ≈ 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. • MISIM photocells: Unique optoelectrical conversion suitable for organic materials. • Pure polarization current without any passage of carriers through the S|I and I|M interfaces • Optimization of conversion efficiency and speed by tuning duty cycle. • Enhancement of conversion speed by 30%. • Successful theoretical fit to the photocurrent as a function of the light illumination and blocking times.