Studies on Green-sensitive Organic Optoelectronic Materials with High Spectral Sensitivity and Frequency Response for the Application of CMOS Image Sensors

Abstract

The progresses in photodetector technology, converting light into electric signal, has boosted up the development of various fields, for example in medicine imaging with x-ray detector, in the military and security domains with infrared sensors, or in image processing with sensors for the visible range. Conventional silicone-based photodiodes (Si PD) used in complementary metal oxide semiconductor (CMOS) image sensors have led the imaging technology market thanks to their high photo-electric conversion efficiency and low dark currents. However, their low absorption coefficient and broad absorption in the visible region require thick photodiodes with additional image processing systems, which might restrict their high sensitivity and ultra-high resolution. Organic semiconductor materials that selectively absorb visible colors with a high absorption coefficient are promising material candidates to replace silicon in PD, making obsolete the use of color filters. In particular, green-sensitive organic photodetectors (OPD) can be integrated into a stacked PD on the Si-based blue and red PDs, which might lead to a significant increase in the sensitivity because of the doubled sensing area. This thesis therefore focuses on the development and characterization of new green-sensitive organic small molecules for CMOS image sensors applications. To date, fullerene has been the material of choice for acceptor materials in most OPDs because of its high electron mobility. However, fullerene is not the ideal material for green-selective OPDs due to its absorption characteristics in the blue range of the visible spectrum. Non-fullerene donor and acceptor materials with narrower absorption band widths are still needed. In this regard, this dissertation addresses the following three topics: i) high performing non-fullerene small molecules for donor/acceptor bulk-heterojunction (BHJ) OPD devices, ii) spectral sensitivity and color crosstalk with these organic materials, and iii) dynamic characterization of frequency response based on their molecular structure.

In chapter II, fullerene-free BHJ OPD with high efficiency and green-color selectivity are presented. It is demonstrated that, by choosing N,N-dimethyl quinacridone (DMQA) as a donor and dibutyl-substituted dicyanovinyl-terthiophene (DCV3T) as an acceptor, a maximum external quantum efficiency (EQE) of larger than 67 % at 540 nm could be achieved at -5 V bias. The overall OPD device performance, including the electrical and optical response, as well as the charge carrier generation and charge transport characteristics are discussed. In particular it is shown that the material compositions rich in DMQA exhibited a high yield of photogenerated charge carriers and a low absorption intensity, whereas the compositions rich in DCV3T had a high absorption intensity and low yield of charge carriers. A 1:1 ratio is optimal for device performance as a result of the relatively high absorption and efficient photogeneration of the charge carriers. This composition ensures the balance between electron and hole mobilities, which is essential to enhance the EQE. Chapter III focuses on green-sensitive OPDs with high sensitivity and spectral selectivity, which employ boron subphthalocyanine chloride (SubPc) derivatives as either the donor or acceptor material. A maximum EQE of 62.6 % at an applied voltage of -5 V is achieved by combining SubPc with DCV3T, at the value of a large full-width-at-half-maximum (FWHM) of 211 nm, however. Considering spectral selectivity, the optimized performance is obtained by combining DMQA and SubPc with a high specific detectivity (D*) of 2.34 × 1012 cm Hz1/2/W, an EQE value of 60.1% at -5 V and a narrow FWHM of 131 nm. It is further shown that, in spite of the sharp absorption of SubPc at the maximum wavelength (λmax) of 586 nm, the EQE spectrum is smooth and favorably centered in the green region at λmax of 560 nm, assisted by the high reflectance of SubPc centered at 605 nm. The photoresponsivity of the OPD devices is found to be consistent with their absorptance. With this DMQA / SubPc composition, promising green-sensitive OPD device are finally obtained, which is characterized by low value blue crosstalk (0.42) and moderate red crosstalk (0.37). In chapter IV, the dynamic characterization of two high performaing green-sensitive OPDs introduced in chapter II and III is further investigated by analyzing the electrical parameters based on experimental and simulation data. The two OPDs comprise DMQA as the common donor and DCV3T or SubPc as the respective acceptors. At the applied voltage of -5 V, the device composed of DMQA/SubPc shows a higher frequency response at 148.3 kHz, by 55 kHz higher than the device based on DMQA/DCV3T. The impedance spectroscopic results indicate that the former device exhibits the lower resistance due to the higher mobility and the lower capacitance attributed to the lower dielectric constant. The calculated reorganization energy and polarizability of these two different acceptors, which are theoretical parameters related to charge mobility and dielectric constant, are consistent with the experimental results. The OPD device comprising SubPc, with the dynamic response surpassing the commercialization level of 100 kHz, is presented as an interesting candidate for potential applications as image sensors, together with its good static performance with external quantum efficiency of 60.1 % at the wavelength of 540 nm. The specific approach toward green-sensitive OPDs elaborated in this dissertation, presented sequentially in Chapter II, III, and IV, can serve as a guideline for developing the blue- or red-sensitive OPDs.