Lead sulfide quantum dots (QDs) have been widely used for various optoelectronic devices due to their high absorption coefficient and tunable bandgap. However, the low mobility of QD films results in poor charge collection and device performance. By combining QDs with graphene into hybrid graphene/QD photodetectors, photocarriers from QDs are transferred to graphene, improving charge collection and transport, drastically increasing the photoresponsivity. Herein, we carry a systematic analysis on how critical tuning parameters such as QD size and QD film thickness affect responsivity, spectral response, and time response. We report the absorption coefficient, refractive index (n, k), penetration depth, and energy bandgap of PbS QDs of different sizes. We study systematically how the photocurrent, photoresponsivity, time response, and power density dependence vary with QD size in hybrid Gr/QD. The bandgap of lead sulfide quantum dots was size-tuned between 0.86 and 1.39 eV. The time response shows that subsecond modulation can be achieved for different QD sizes with a responsivity up to 10⁷ A/W at power densities of 10⁻⁵ mW/cm². We also studied how the performance of the photodetectors is affected by the thickness, discussing the limitations on the thickness by the compromise between light absorption and charge collection. We describe how the optical response shifts toward the infrared as QD films get thicker. Time responses below 1 s are obtained for graphene/QD devices with thickness from 150 nm to 1 μm. This systematic study provides important guidelines to design hybrid graphene/QD photodetectors and tune their spectral response and performance.

We use a top-graphene layer to protect QD surface ligands during the lithography process, allowing reliable patterning of hybrid Gr/QD photodetectors via lithography and dry etching.

Quantum dots (QDs) offer several advantages in optoelectronics such as easy solution processing, strong light absorption and size tunable direct bandgap. However, their major limitation is their poor film mobility and short diffusion length ( 700 nm in QDs photodetector and photovoltaic devices, causing a reduced photoresponsivity and a poor absorption towards the near-infrared part of the sunlight spectrum. Herein, we demonstrate 1 μm thick QDs photodetectors with intercalated graphene charge collectors that avoid the significant drop of quantum efficiency towards λ > 700 nm observed in most QD optoelectronic devices. The 1 μm thick intercalated QD films ensure strong light absorption while keeping efficient charge extraction with a quantum efficiency of 90%–70% from λ = 600 nm to 950 nm using intercalated graphene layers as charge collectors with interspacing distance of 100 nm. We demonstrate that the effect of graphene on light absorption is minimal. We achieve a time-modulation response of <1 s. We demonstrate that this technology can be implemented on flexible PET substrates, showing 70% of the original performance after 1000 times bending test. This system provides a novel approach towards high-performance photodetection and high conversion photovoltaic efficiency with quantum dots and on flexible substrates.