UC San Diego

PbS quantum dots (QDs) are promising materials for optoelectronic devices due to their size-tunable band gap, strong light absorption and low cost solution processing.[1,2] However, efficient charge collection is a major issue for QDs films due to the poor film mobility and short diffusion length (~200 nm). If QDs film thickness is beyond diffusion length, electron and hole pair generation occurs too far from the junction, charge carriers will recombine before producing any photocurrent. Thus, there is a compromise of QDs thickness between efficient charge extraction and light absorption, the thickness of absorbing layers should not exceed the carrier diffusion length plus depletion width. This has restricted the practical QDs film thickness to ~200-300 nm due to the limitation that the diffusion length imposes on film thickness in order to keep efficient charge collection. Such thin films result in a significant decrease in quantum efficiency for >700 nm in QDs optoelectronic devices, causing a reduced photoresponsivity and a poor absorption towards the infrared part of the sunlight spectrum, wasting almost half of solar energy.

In this work, we present a novel architecture that incorporates multiple intercalated graphene layers inside the QD film to enhance charge extraction, breaking the restriction that short diffusion length imposes on the QDs film thickness. The intercalated graphene layers ensure a faster and more efficient carrier collection to enhance the performance while increasing the QDs film thickness, especially in the near-infrared range. The intercalated Gr/QD devices were made by sequential Gr wet transfer and QD spin coating on SiO2/Si substrate with predefined gold electrodes. The intercalating graphene layers ensure the efficient charge extraction despite the thickness increment, breaking the limitation that diffusion length imposes on QDs film thickness. We also added the vertical interconnect access between each graphene layers by depositing Au electrodes on each graphene with E-beam evaporator and shadow mask. Furthermore, the optimal graphene interspace, which is the QDs film thickness between two graphene layers, is studied to ensure the efficient charge extraction as well as fabrication efficiency. At last, we fabricated 1-miron meter thick device by employing this intercalated Gr/QD configuration.

We demonstrate high quantum efficiency (~90% to 70%) from λ = 600 nm to 950 nm with this 1-miron meter thick Gr/QDs device, avoiding the drastic drop at λ =700 nm. We achieved device photoresponsivity of 107AW-1, indicating corresponding gains of 108. We also demonstrate this technology is flexible substrate compatible, showing 70% of the original performance after 1000 times bending test on flexible PET substrate. This work demonstrates the first intercalated Gr/QD hybrid photodetectors, introducing a new approach to achieve high light absorption and efficient charge collection in high-response photodetectors. Our approach allows breaking the restriction that diffusion length imposes on the thickness of QD layers and paves the way for the development of multi-band photodetector, high efficiency photovoltaics, high resolution CMOS camera, as well as wearable sensors.