Ultrasensitive solution-cast quantum dot photodetectors

The best electronic and optoelectronic devices are built via semiconductor crystal growth on a single-crystal substrate. Over 100 papers have been published in recent years in Nature on alternative devices, produced instead from the solution phase. They have some advantages over conventional crystalline semiconductor devices: ease of fabrication, physical flexibility and — most important — low cost. The problem was the poor electronic performance of solution-processed devices, compared with single-crystal counterparts. But that could change now: a team from the University of Toronto reports that one such system — colloidal quantum dots of lead sulphide — can actually outperform the state-of-the-art crystalline alternative. A solution-processed electronic device that uses colloidal quantum dots of lead sulphide outperforms the state-of-the-art crystalline alternatives, with ease of fabrication, physical flexibility, large device areas and low cost among its benefits. Solution-processed electronic1 and optoelectronic2,3,4,5 devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 103 A W-1. The best devices exhibited a normalized detectivity D* of 1.8 × 1013 jones (1 jones = 1 cm Hz1/2 W-1) at 1.3 µm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 1012 jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 1011 jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.