Near-Infrared Photodetector Consisting of J-Aggregating Cyanine Dye and Metal Oxide Thin Films

We demonstrate a photodetector structure that employs metal-oxide charge transport layers and that is sensitized at near-infrared wavelengths by a thin film of a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of 16.1 $\pm $ 0.1{\%} ($\lambda $ = 756 nm) to be achieved at zero-bias in a device consisting of an 8.1 $\pm $ 0.3 nm-thick dye film. The specific detectivity (D*) and response speed (f$_{3dB})$ of the fully-optimized device are measured to be (4.3 $\pm $ 0.1)$\times $10$^{11}$ cm Hz$^{1/2}$ W$^{-1 }$and 91.5 kHz, respectively. Modeling of our structure reveals that the photocurrent is limited by the diffusion of photo-generated excitons to the metal oxide/J-aggregate hetero-interface and we determine the exciton diffusion length in the J-aggregate film to be L$_{D}$ = 2.0 $\pm $ 0.4 nm. This work provides insights relevant to the use of J-aggregating cyanine dyes in photodetector and photovoltaic applications and highlights the importance of engineering the optical field profile within such structures in order to maximize performance.