IEEE JOURNAL OF SELECTED TOPICS INQUANTUM ELECTRONICS, VOL. 30, NO. 3, MAY/JUNE 2024 3801008 Donor-Acceptor Covalent Organic Polymer Framework for High-Performance Broadband Flexible Photodetector Sayan Halder , Keerthi C.J. , Chanchal Chakraborty , and Subhradeep Pal , Member, IEEE Abstract—This paper reports a high-performance, broadband ﬂexible photodetector (PD) based on a novel covalant organic poly- meric framework material, TPATZTPA. One PD on Si-substrate and another PD on ITO-coated PET-based ﬂexible substrate were fabricated. Both PDs exhibited a similar type of photodetection capability ranging between 350 to 950 nm. In the presence of a broadband white light source, the ﬂexible PD exhibited a peak photo-to-dark current ratio of 18.23 × 10 3 with a bias of +2 V. Experimental results indicate that the fabricated ﬂexible PD offers a peak responsivity, external quantum efﬁciency, and speciﬁc detec- tivity of 5.69 A/W at 570 nm, 13.06% at 565 nm, and 1.27 × 10 11 Jones at 570 nm, respectively. The transient study reveals that the fabricated TPATZTPA-based ﬂexible PD offers a rise and fall time of 0.29 s and 0.28 s, respectively. A simple 50 cm long visible lightwave communication (VLC) system utilizing the fabricated ﬂexible PD exhibited its efﬁcacy by successfully demodulating intensity-modulated PN9 PRBS sequence. Index Terms—Flexible, framework material, heterojunction, photodetector, visible lightwave communication (VLC). I. INTRODUCTION W ITH the tremendous development of the internet-of- things (IoT), all sensing components are now being al- tered to support miniaturization and mechanical compliance [1]. Among the various sensors, a photodetector (PD), which per- forms the optical-to-electrical (O/E) conversion, ﬁnds a wide range of applications, including telecommunications, health monitoring, artiﬁcial visions, etc [1], [2], [3], [4], [5], [6]. Most commercial PDs are generally made of inorganic crystalline semiconductors. These PDs are epitaxially grown, expensive, and often demand complex fabrication steps [7]. Owing to their Manuscript received 4 August 2023; revised 3 November 2023; accepted 10 November 2023. Date of publication 14 November 2023; date of current version 15 December 2023. (Corresponding authors: Chanchal Chakraborty; Subhradeep Pal.) Sayan Halder and Chanchal Chakraborty are with the Department of Chem- istry, BITS-Pilani, Hyderabad, Telangana 500078, India (e-mail: p20200425@ hyderabad.bits-pilani.ac.in; chanchal@hyderabad.bits-pilani.ac.in). Keerthi C.J. is with the Department of Electrical and Electronics Engineering, BITS-Pilani, Hyderabad, Telangana 500078, India (e-mail: p20220020@hyderabad.bits-pilani.ac.in). Subhradeep Pal is with the Department of Electrical and Electronics Engi- neering, BITS-Pilani, Hyderabad, Telangana 500078, India, and also with the LAMBDA Lab of BITS-Pilani, Hyderabad Campus, Telangana 500078, India (e-mail: subhradeep@hyderabad.bits-pilani.ac.in). Color versions of one or more ﬁgures in this article are available at https://doi.org/10.1109/JSTQE.2023.3332827. Digital Object Identiﬁer 10.1109/JSTQE.2023.3332827 rigid and fragile texture, applications of these commercial PDs in the next generation soft and wearable optoelectronics are extremely difﬁcult [1], [2], [7]. In this regard, organic photode- tectors have drawn tremendous attention in the last few years. It is expected that a highly efﬁcient organic photodetector will signiﬁcantly boost the development of ﬂexible and wearable de- vices with target applications like artiﬁcial vision, night vision, biomedical imaging, real-time health monitoring, spectroscopy, etc. [1], [7], [8], [9], [10], [11]. Among the various organic materials, covalent organic frame- works (COFs) or the covalent organic polymer frameworks (COPFs) have recently emerged as a new category of porous organic materials supporting ﬂexible skeleton structures and in- trinsic ordered structures with sufﬁcient crystallinity [12], [13], [14]. Generally, the inherent porous COFs with high surface area have been widely used in applications like energy storage and conversion [15], [16], [17], [18], gas adsorptions [19], ionic conductions [20], [21], solid-state electrolyte, [22], etc. Owing to its long-range order, high surface area, and wide range of possible photoactive organic building blocks, COFs exhibit promising optoelectronic properties suitable for pho- todetection applications [23], [24]. However, most COFs are made of relatively small aromatic subunits; thus, they support the absorption of the high-energy part of visible light. In fact, if one can develop a π-conjugated COF structure with stacking phe- nomenon and the arrangement of donor-acceptor (D-A) organic moieties inside the COF building blocks, the COF can facilitate low-bandgap, high charge carrier mobility, and electrical con- ductivity, making them suitable for optoelectronic devices and broadband PDs. However, highly aggregated COF structure can provide restricted solubility with limited solution processibility to prepare a homogeneous ﬁlm of COFs for optoelectronic. The judicious choice of the organic building block to balance all those parameters is crucial for using COFs in optoelectronics. In this paper, we have developed a novel D-A-type COPF-based photodetector with a novel donor-acceptor architecture, denoted as TPATZTPA (Triphenylamine-Thiazolo [5,4-d]thiazole-Triphenylamine). This structure combines thiazolothiazole-derived acceptor units with triphenylamine- based donor components for the purpose of creating a high-performance thin-ﬁlm photodetector with broad spectral sensitivity. The ﬂexible triphenylamine part provides enough solubility to provide a stable thin ﬁlm for ﬂexible devices. The novel COPF material TPATZTPA was synthesized 1077-260X © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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