CPOS Seminar: "Organic Photodetectors under High Temperature for Manufacturing Processes: Device Performance and Morphology Evolution"

Speaker: HOANG LUONG, PhD, Center for Polymers and Organic Solids, UCSB

Abstract: Organic semiconductors amenable for visible to the near-infrared (NIR) detection provide ubiquitous platform for flexible and wearable electronics. Adopting the organic photodetectors (OPDs) fabrication conditions into the existing Si-based OPD devices necessitates the resistance towards high thermal stress, whereby the deterioration of photosensing capability occurs due to the undesirable morphological changes. The underlying morphology-stability-property relationships are difficult to establish due to the compositional and structural heterogeneities associated the OPDs. Here, we combine OPD processing with multiscale characterization and device physics to systematically investigate the role of thermal stress on the thin film morphology of PTB7-Th:nonfullerene acceptor blends (NFAs: SiOTIC-4F, COTIC-4F, CO1-4F, and CO1-4Cl). Our results indicate that the bulk heterojunction (BHJ) morphology exhibits optimal intermixing and phase separation between the PTB7-Th and NFA moieties, facilitating the charge generation and extraction in the pristine OPD devices. However, a quick drop in the efficiency of thermally annealed OPDs at 200 ºC is observed, regardless of the NFA choice, but is more severe in the case of PTB7-Th:SiOTIC-4F blend. The photoactive layer degeneration by means of the bulk and interfacial morphology changes as a function of annealing time is examined at different length scales with atomic-force microscopy (pc- AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS) and solid-state NMR spectroscopy. This study provides deep new understandings of morphology-driven variations in the devices, and is likely to be an essential feature of developing structurestability- property relationships in high detectivity OPDs.