A Highly Stable Organic–Inorganic Hybrid Electron Transport Layer for Ultraflexible Organic Photodiodes

Flexible organic photodiodes (OPDs) are used to detect light in system-scale demonstrations of skin-conformable devices. However, the detectivity of OPDs deteriorates under various environmental conditions, such as light irradiation, air exposure, and heating. This decrease in detectivity is observed in OPDs with a widely used sol-gel ZnO (ZnO SG) electron transport layer (ETL), where the dark current at the reverse bias increased by several orders of magnitude. In this study, a low dark current and stable detectivity with respect to the aforementioned external changes are achieved. The enhanced stability stems from the suppression of the increase in dark current realized by using a mixture of an organic polymer, polyethyleneimine (PEIE), and inorganic crystals (ZnO nanoparticles) to create a nanoparticle-based, Zn-chelated PEIE (PEI-Zn NP) as the ETL of the OPDs. The detectivities of OPDs with PEI-Zn NP are 89%, 84%, and 93% of their original values after light irradiation, air storage, and thermal heating, respectively. In contrast, their ZnO SG counterparts exhibited stabilities of only 9.9%, 55%, and 2.6%, respectively, in the same tests. Furthermore, the use of PEI-Zn NP ETL in ultraflexible OPDs is demonstrated by the maintained detectivity after 5000 cycles of device bending.