Electro-Optics and Microelectronics Seminar
Speaker: Himanshu Shekhar
Affiliation: Electrical Eng., Technion
Much effort has been devoted to research and development of organic solar cells with significantly less emphasis on organic photodiodes (OPDs). The increased interest in OPDs is partly motivated by the potential for commercial applications. The offered motivation is that superposing an OPD onto a CMOS circuit would make the sensor more sensitive to light and achieve a wide dynamic range. Moreover, it is expected that these ultrathin organic photosensitive pixels (~100 nm) will have reduced optical cross pixel talk. Despite these advantages, OPDs are still not the first choice for high-end applications. The device physics of OPDs, especially in the context of reverse bias dark leakage current is not well understood and is often attributed to extrinsic factors. The purpose of this research was to gain a better understanding of physical processes which govern organic photodiodes, develop a good quality photodiode and integrate with CMOS ROIC to realize a hybrid CMOS imager. A significant part of this research was spent on addressing the issue of high reverse bias dark leakage current, a common nuisance, in the context of bi-layer small molecule PDs. We identify that, in general, the dark leakage current is several orders higher in magnitude than that of an ideal diode. The conventional Shockley-Read-Hall (SRH) formalism alone is not sufficient to explain this difference. In order to understand it, we fabricated several bi-layer OPDs (where we changed donor material) and conducted electrical, optical, and temperature dependent measurements. Sensitive optical measurements reveal the presence of sub-gap/tail states in the donor material which is in direct correlation with the measured dark leakage current. These tail states are efficient recombination-generation centers which can contribute to the leakage current. In addition to that, we believe that these tail states at the donor-acceptor interface open a new current channel via “Trap assisted Tunneling (TAT).” Our work may act as input for chemists/material scientists to synthesize electronically ordered material and device engineers to think of smart device design in order to achieve low leakage current OPDs.
*PhD student supervised by Prof. Nir Tessler.