Optoelectronic performance of Au/FMDD/p-Si/Al structure: photodiode and photovoltaic analysis

Abstract In this study, to improve the performance of conventional silicon (Si)-based optoelectronic devices, an Au/FMDD/p-Si/Al Metal-Interlayer-Semiconductor (MIS) structure was successfully fabricated by introducing a new organic interlayer material at the metal-semiconductor interface. In this context, a new fluorene-based aldodioxime derivative, (1E,1’E)-4,4’-((9 H -fluoren-9-ylidene)methylene)dibenzaldehyde dioxime (FMDD), was designed, synthesized, and spectroscopically characterized. The target compound was successfully obtained in three steps-bromination, Suzuki coupling, and condensation reactions-starting from commercially available 9 H -fluoren-9-one ( 1 ). The structure of the product was confirmed by 1 H NMR, 13 C NMR, and MS analyses. The morphological, structural and optical properties of the FMDD interlayer film were investigated in detail by SEM, AFM, XRD and UV-Vis analyses. The analyses revealed that the FMDD film was not amorphous but had a nanogranular (20–50 nm) crystalline structure and a rough surface. Optical analysis confirmed that FMDD has a wide band gap of ~ 2.25 eV and exhibits high optical transparency to visible/near-infrared light above 550 nm. The electrical properties (I-V) of the device were investigated under darkness and illumination. In darkness, the diode exhibited good rectifying characteristics with an ideality factor (n) of 1.21 and a rectification ratio (RR) of 144.3. However, under illumination, the series resistance (R s ) increased sharply up to 3386 Ω and n increased to 3.14, severely limiting the photovoltaic performance of the device. Consequently, the device exhibited a very low fill factor (FF ~ 22.9%) and a maximum efficiency (η) of 0.34%. Analyses performed in photodiode mode (under V = -2 V) revealed a much more interesting behavior: the photocurrent (I ph ), photosensitivity (S), and detectability (D * ) of the device remained constant at low levels up to 60 mW/cm² light intensity, but showed a sharp and sudden increase above this threshold. This superlinear behavior was attributed to a photoconductive mechanism triggered by the saturation of the trap states at the interface under high photon flux. These results indicate that the FMDD interlayer is a potential candidate for achieving nonlinear and tunable photoresponse properties in Si-based devices.