Cascaded Förster Energy Transfer in ZnO QDs/F8BT/TIPS-P Hybrid Heterojunction for Broadband Photodetection

This study reports the design and investigation of a hybrid heterojunction photodetector based on cascaded energy transfer among ZnO colloidal quantum dots (QDs), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), and 6,13-bis(tri-isopropylsilyl-ethynyl) pentacene (TIPS-pentacene ~ TIPS-P). ZnO QDs were synthesized using a hot-injection method to achieve size-controlled, the nanocrystals with an average diameter of 2.21 nm, smaller than the exciton Bohr's radius of ZnO (~ 2.87 nm). The hybrid thin film was fabricated via sequential spin-coating to form a ZnO QDs/F8BT: TIPS-P multilayer on ITO substrates, followed by Ag electrode deposition. The optical absorption and photoluminescence analyses reveal strong spectral overlap among the components, facilitating efficient Förster resonance energy transfer (FRET). The estimated Förster critical distances (R₀) of ~ 33 Å for ZnO QDs → F8BT and ~ 34 Å for F8BT → TIPS-P confirm the feasibility of cascaded non-radiative energy transfer. Time-resolved photoluminescence (TRPL) decay studies demonstrate a multiexponential behaviour indicative of energy migration and exciton transfer across the interfaces. Electrical characterization under illumination shows a photocurrent enhancement at 660 nm, validating the extended visible response through organic sensitization. Capacitance-voltage (C-V) and Nyquist analyses reveal frequency-dependent interfacial charge dynamics, with illumination inducing higher impedance and charge relaxation behaviour. The results establish the ZnO QDs/F8BT: TIPS-P system as an efficient cascaded heterojunction, combining the high mobility of TIPS-P, the strong visible absorption of F8BT, and the UV sensitivity of ZnO QDs. This work demonstrates a promising strategy for broadband, solution-processable hybrid photodetectors with enhanced exciton dissociation and charge transport efficiency.