Junction Engineering of Organic Photodiodes for Color Filter-Free Image Sensors

Abstract

The industrially first CMOS image sensor consisting of organic photodiodes (OPDs) was reported by Panasonic on Feb. 14, 2018 (http://www.panasonic.com/global). As such, OPDs are now moving to the commercializing stage. However, here we want to address that even this advanced organic CMOS image sensor technology still relies on panchromatic OPDs with no color readability. They only utilize organic semiconductor’s high extinction coefficient and thus high degree-of-integration of thin film OPDs, which means they still use color filters with large volume and expensive processing cost. In other words, the development of a photodiode capable of maintaining the thin film form (total thickness <1 μm) while excluding the R/G/B color filters has emerged as a key issue for improving the integration of the image sensor. In this Theses, I will suggest three approaches to realize color-selective high performance OPDs without using color filters and their image sensor arrays by employing various methods. In the first part, various poly-electrolytes were used for realizing high-performance color filter-free OPDs. By introducing polyelectrolytes as interlayers, not only surface traps of sol-gel-derived ZnO were passivated, but also work function (WF) was shifted toward the preferred direction, so that dark current could be suppressed and external quantum efficiency (EQE) could be enhanced, simultaneously. In the second part, I will suggest Schottky junction approach with color-selective donor materials to realize color filter-free color-selective OPDs. To enhance the OPD performance, such as dark current, noise current, photocurrent, EQE, specific detectivity, linear dynamic range, and -3 dB frequency, various methods such as the introduction of a small amount of acceptor materials, or optical design for suppressing the absorption of the unwanted wavelength range. By adding a proper amount of fullerene acceptor on the donor photoactive layer, the EQE was significantly increased due to the newly created donor-acceptor interfaces. Furthermore, this boost in EQE occurred without sacrificing dark current, and therefore, specific detectivity could be significantly enhanced by nearly an order of magnitude. In case of optical design, by optimizing each constituting layer in organic Schottky photodiode structure (donor and ZnO layers), the absorption of unwanted wavelength range could be effectively minimized In the last part, I will suggest a new strategy to realize color filter-free, full-color organic image sensor pixel with a new diode architecture that combines a dual functional thin-film etalon-electrode and a thin-film panchromatic organic photodiode. Furthermore, because this new “etalon-electrode” strategy enables facile patterning of RGB pixel arrays simply by adjusting the thickness of constituent layers within etalon structure, we could demonstrate a full-color image capturing ability of organic image sensor without using color filters.