Mid-infrared HgTe colloidal quantum dot photovoltaic devices previously achieved background-limited infrared photodetection at cryogenic temperatures but also decreased from 20 to 1% efficiency from 150 to 300 K. The reduced quantum efficiency was tentatively attributed to the carrier diffusion length being much shorter than the device thickness of ∼400 nm at room temperature. Here, the carrier diffusion length is measured and is found to peak at 215 nm at 200 K and decrease only to 180 nm at 295 K. It is therefore not the cause of the much larger reduced quantum efficiency. Instead, it is shown that the efficiency drops due to the series resistance. With the device size reduced to 50 by 50 μm, the room-temperature quantum efficiency reaches 10 and 15% for HgTe colloidal quantum dot devices with 2400 cm-1 (4.2 μm) and 2675 cm-1 (3.7 μm) cutoff, respectively. These small-area devices achieve background-limited photodetection at 150 K and a detectivity above 109 Jones at room temperature with a cutoff at 2675 cm-1 (3.7 μm).
Keywords: HgTe; midwave infrared; photodetector; photovoltaic; quantum dots.