Exciton delocalization and optical synaptic functionality in conjugated polymer–In2Se3 heterostructures

Abstract

Investigations into the photoresponses of organic–inorganic heterostructures (OIHs) with similar band alignments have provided conflicting results, with some studies showing ultrafast responses and others reporting slower responses with enhanced photomemory effects. Despite progress, the impact of the organic layer's molecular structure and excitonic coupling on OIH photoresponse times remains unclear. To address this, we fabricated OIHs utilizing poly(3-alkylthiophene) (P3AT) [where alkyl (A) = dodecyl (DD) or hexyl (H)]-based conjugated polymers and indium selenide (In₂Se₃). P3DDT, with its low crystallinity and poor exciton delocalization, exhibits a fast photoresponse, whereas P3HT, with better-ordered crystallites and pronounced exciton delocalization, shows a slower photocurrent rise and photomemory effects. This study uncovers the role of exciton delocalization or the exciton diffusion length of the conjugated polymer in determining the optical response of an OIH device. Further, we investigated the synaptic properties of P3HT/In₂Se₃, revealing short-term memory, long-term memory, and learning-experience behavior. An artificial neural network simulation achieved 88% accuracy in recognizing MNIST handwritten digits, which is significant in the context of OIH synapses. This work provides new insights by revealing the intricate relationships between material properties, exciton dynamics, and OIH charge-transfer pathways, paving the way for the development of more-efficient and -effective neuromorphic devices.