Impact of Thermal Stress on Device Physics and Morphology in Organic Photodetectors

Abstract

Organic photodetectors (OPDs) capable of detecting visible to near-infrared light provide a ubiquitous platform for emerging flexible and wearable electronics. In the process of implementing OPDs into a Si-based manufacturing process, organic semiconductors undergo ≥ 200 °C thermal stress, leading to the deterioration of photosensing capability. Here, we combine multiscale characterization and device physics to unravel the impact of thermal stress on the optoelectronics characteristics of PTB7-Th:non-fullerene acceptor blends (NFAs: SiOTIC-4F, COTIC-4F, CO1-4F, and CO1-4Cl). For as-cast devices, favorable intermixing and phase separation between PTB7-Th and the NFA facilitate charge generation and extraction. Reductions in the OPD performance after thermal annealing (200 °C for 5–120 min) are observed due to the morphological degradation, regardless of the NFA choice, but the reduction is more severe for the PTB7-Th:SiOTIC-4F blend. Thermally induced morphological changes are examined using atomic force microscopy, wide-angle X-ray scattering, and solid-state NMR spectroscopy. This study provides essential insights into morphology-driven deteriorations, which will help in developing structure–stability–performance relationships in high detectivity OPDs.

Importance of Short-Range Order in Governing Thin Film Morphology and Electronic Properties of Polymeric Organic Semiconductors

“…Figure compares 19 F ssNMR spectra of the PTB7-Th powder and thin film obtained after spin-casting on a glass substrate. In both cases, the narrow feature at −110 ppm is due to the locally ordered 19 F atoms in FTT moieties involved in CH···F hydrogen bonding interactions with T and BDT groups, as discussed below. , Integration of the respective 19 F peak intensities reveals that the powder and spin-cast PTB7-Th polymers contain ≈84 and ≈99% locally ordered 19 F sites (−110 ppm), respectively, indicating that the thin film deposition and thermal annealing processes influence the local morphology. It is noteworthy that the majority of FTT moieties are locally ordered in the PTB7-Th π-conjugated chains.…”

“…In both cases, the narrow feature at −110 ppm is due to the locally ordered 19 F atoms in FTT moieties involved in CH•••F hydrogen bonding interactions with T and BDT groups, as discussed below. 45,46 Integration of the respective 19 F peak intensities reveals that the powder and spin-cast PTB7-Th polymers contain ≈84 and ≈99% locally ordered 19 F sites (−110 ppm), respectively, indicating that the thin film deposition and thermal annealing processes influence the local morphology. It is noteworthy that the majority of FTT 47 To confirm that the large difference between the 19 F chemical shifts of ordered and disordered regions of PTB7-Th not only originates from the C−F•••H hydrogen bonding interactions but also with the backbone/side-chain conformational disorder and the other intermolecular packing interactions, we carried out liquid-state NMR measurements and DFT modeling studies and analysis.…”

Additive-free molecular acceptor organic solar cells processed from a biorenewable solvent approaching 15% efficiency

“…Next, impedance spectroscopy was carried out to determine the charge carrier density in the active layer upon varying operational biases and light intensities. 45–52 To maintain the steady-state properties of the system during measurement under different illumination conditions, a DC bias in a range from −2 V to V oc , and a small AC signal (40 mV) were applied during the scanning. To account for the impact of the device series resistance and parasitic inductance of the connecting cables, a correction factor is included in the measured capacitance of the BHJ layer as represented in eqn (2):…”

Green-Solvent-Processed High-Performance Broadband Organic Photodetectors

“…Light sensing is a fundamentally important technology in modern optoelectronics and is widely applied in various applications, including self-driving vehicles, medical diagnosis, optical communications, security, and chemical analysis . Over the recent years, research on organic photodetectors (OPDs) has demonstrated continuously enhanced performance in light sensing as a result of innovation of the photoactive materials and device engineering. − Consequently, we are seeing more reports of solution-processed bulk heterojunction (BHJ) OPDs that achieve broadband activity spanning from ultraviolet to short-wave infrared spectra, ,− high responsivity approaching that of commercial photodetectors, impressive detectivity enabled by low dark/noise current, , highly spectrally selective response, ,− mega-hertz working bandwidth, and initial success in the actual integrated sensor panels . However, the vast majority of the OPDs are processed with halogenated solvents, such as chloroform (CF), chlorobenzene (CB), and dichlorobenzene (DCB), which are highly environmentally hazardous and risky to health.…”