Ph.D Thesis

Ph.D StudentShekhar Himanshu
SubjectResearch on Leakage Current in Small Molecule Planar
Heterojunction Photodiodes Towards Development of
a Hybrid CMOS Imager
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Nir Tessler


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.

In this thesis, we have studied the issue of high reverse bias dark leakage current, a common nuisance, in the context of bi-layer small molecule PDs. 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 states which is in direct correlation with the measured dark leakage current. These sub-gap states, which are donor mediated new states at the interface underlines the role/importance of junction on the leakage current in heterojunction OPDs. 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)." Furthermore, a high-performance CMOS compatible inverted OPD was fabricated. The obtained dark leakage current of 6 x 10-10 Acm-2 at -0.5V reverse bias is one of the lowest reported for < 150 nm thin films. The calculated specific detectivity is as high as 7.15 x 1012 cm Hz1/2 W-1 at 500 nm, a bandwidth of 400 kHz, and linear dynamic range (LDR) more than 140dB at -0.5V.  In the end, for the first time, small-molecule based OPD was integrated on CMOS ROIC and characterized as an image sensor.

Our work advances the understanding of OPDs in the context of leakage current. Our work may act as input for chemists/material scientists to synthesize electronically ordered material and device engineers to think of smart device design to achieve low leakage current OPDs.