Green-Light-Selective Organic Photodiodes with High Detectivity for CMOS Color Image Sensors

Abstract

Stacked structures employing wavelength-selective organic photodiodes (OPDs) have been studied as promising alternatives to the conventional Si-based image sensors because of their color constancy. Herein, novel donor (D)−π–acceptor (A) molecules are designed, synthesized, and characterized as green-light-selective absorbers for application in organic-on-Si hybrid complementary metal-oxide-semiconductor (CMOS) color image sensors. The p-type molecules, combined with two fused-type heterocyclic donors and an electron-accepting unit, exhibit cyanine-like properties that are characterized by intense and sharp absorption. This molecular design leads to improved absorption properties, thermal stability, and higher photoelectric conversion compared to those of a molecular design based on a nonfused ring. A maximum external quantum efficiency of 66% (λmax = 550 nm) and high specific detectivity (D*) of 8 × 1013 cm Hz1/2/W are achieved in an OPD consisting of a bulk heterojunction blend with two transparent electrodes on both sides. Finally, the green-light-detection capability of the narrow-band green-selective OPD is demonstrated by the optical simulation of an organic-on-Si hybrid, stacked-type, full-color photodetector comprising the green-light-selective OPD and a bottom Si photodiode with only blue and red color filters. Based on this molecular design, further optimization of the OPDs can allow the development of various optoelectronic sensors including 3D-stacked image sensors with enhanced sensitivities to replace the conventional Si-based CMOS image sensors.

Wavelength‐Selective Organic Photodetectors

“…Figure shows D * values extracted from 43 papers reporting narrow‐band OPDs from the year 2012 onwards with an FWHM value of a maximum of 150 nm. [ 4,17–21,25,28,30–34,38–67 ] To get an overview of the current status of the field, it is instructive to plot D * as a function of peak EQE value, J d , and design wavelength. It is important to mention that not in all of these papers the noise currents have been directly measured and D * has often been calculated using , given by Equation (2).…”

“…In literature, there is, however, some inconsistency in the approach to calculate the LDR. In most papers, [ 18,19,28,32,33,42–44,46–48,50,53,55–57,59–61 ] the authors convert the ratio between the maximum and minimum intensity over which the OPD behaves linearly to dB by taking the log (base 10) and multiplying it by a factor 20, while a few authors multiply by a factor 10. [ 33,42 ] This should be taken into account when comparing LDR values.…”

“…[ 93 ] Lee and co‐workers compared this molecule with the naphthalenedione analog (NPBIDDTSe) and it was shown to perform better upon optimization of the ratio of C 60 (FWHM = 97 nm, D * = 4.37 × 10 13 Jones). [ 45 ] The influence of the donor moiety in these types of structures was investigated by Lim et al [ 43 ] A comparison was made between a non‐fused diphenylamine donor (DP) and the ring‐closed analogs with S (PS) or Se (PSe), both close to the cyanine limit (Table 1). The fused variants exhibited a hypsochromic shift compared to the pristine material.…”

“…Most of the stability studies originate from NBA OPDs and some from internally (optically) filtered devices. Narrow‐band OPD stability studies can be categorized into thermal, [ 41,43,44,47,92 ] thermal/humidity (85 °C/85% relative humidity (RH)), [ 19,40,44,92 ] oxidative, [ 46,50 ] bias, [ 58 ] and light pulse studies. [ 60 ] Since the utilization of cavity device concept is rather new, stability studies have yet to be reported.…”

“…For example, Lim et al concluded that a glass‐encapsulated green OPD was more stable when the diphenylamine moiety in the donor part of a push‐pull molecule was fused via a sulfur or selenium atom (Figure 6a). [ 43 ] Exposure to 170 °C for 3 h had no significant impact on the EQE and dark current for the PSe and PS devices, while the EQE dropped by 9% for the device made from the non‐fused DP. On the contrary, thermal testing can also result in annealing effects on the blend, potentially increasing device performance.…”