Colloidal quantum dots provide a powerful materials platform to engineer optoelectronics devices, opening up new opportunities in the thermal infrared spectral regions where no other solution-processed material options exist. This mini-review collates recent research reports that push the technological envelope of colloidal quantum dot-based photodetectors toward mid- and long-wavelength infrared. We survey the synthesis and characterization of various thermal infrared colloidal quantum dots reported to date, discuss the basic theory of device operation, review the fabrication and measurement of photodetectors, and conclude with the future prospect of this emerging technology.
Lately
discovered silver selenide (Ag2Se) colloidal
quantum dots with tetragonal crystal structure exhibit promising optical
properties in the mid-wavelength infrared. Although colloidal synthesis
of uniform sizes and shapes as well as detailed phase transformation
and photoluminescence properties have been studied recently, investigations
of their optoelectronic properties as an active layer in photodetector
devices remain scarce. Herein, we present the fabrication and characterization
of Ag2Se colloidal quantum dot-based photoconductive photodetectors.
We investigate the effect of ligand exchange as well as temperature
and spectral-dependent photoresponses. Our results suggest that further
enhancement in performance could be achieved through accurate control
of carrier concentration. With this improvement, Ag2Se
colloidal quantum dots may serve as a promising mid-wavelength infrared
absorber for the development of thermal infrared sensors and imagers
with low size, weight, power consumption, and cost.
In the past 30 years, scientists
have utilized quantum confinement
to obtain size-tunable interband optical transitions in colloidal
quantum dots (CQDs) and implemented them in various optoelectronic
applications throughout the electromagnetic spectrum. The infrared
(IR) region is particularly important with applications in telecommunications,
night-time surveillance, and satellite imaging for agricultural water
conservation. Nearly all progress with CQDs in the IR region has been
achieved using interband transitions in Pb- and Hg-based heavy metal
compounds with narrow band gaps. An alternative approach is to exploit
intraband optical transitions originating from external- or self-dopants,
which could expand the library of materials for IR-optoelectronic
devices to include nontoxic materials. Herein, we present a simple
two-precursor hot-injection (170 °C) synthesis of 2.6–6.5
nm diameter environmentally benign Ag2Se CQDs that exhibit
a crossover from interband near-infrared (NIR) absorption to intraband
mid-wave infrared (MWIR) absorption above a critical size of 5.1 nm.
CQDs smaller than 5.1 nm are photoactive in the NIR, exhibiting multiple
well-defined excitonic peaks and stable room-temperature emission
in the NIR and short-wave infrared (SWIR) regions of the electromagnetic
spectrum. Films cast from these CQDs and ligand-exchanged with ethanedithiol
exhibit NIR photoconductivity. In contrast, CQDs larger than 5.1 nm
exhibit MWIR absorbance. Compared to other synthetic methods that
generate Ag2Se CQDs over a limited size range, our approach
allows access to both ultrasmall and large Ag2Se CQDs,
enabling a detailed study of the size-dependent interband to intraband
optical transition. We compare the competing effects of quantum confinement,
environmental Fermi level, and particle stoichiometry to provide guidelines
for stable electron occupation of the 1Se state and obtain
tunable intraband MWIR absorption.
Cu 2 Se thin films provide a promising route toward relatively safe, sustainable and solution processed thermoelectric (TE) modules in contrast to more expensive and toxic materials currently on the market such as Bi 2 Te 3. Cu 2 Se is known in the TE community for its high performance at high temperature and has recently attracted attention from its large theoretically predicted figure of merit at room temperature. Unfortunately, one of the main limitations encountered so far in Cu 2 Se thin films is that the carrier concentrations are not optimized for TE operation after solution processing. In this work, we conduct a comprehensive study of the structural, optical, and TE properties of Cu 2 Se thin films and demonstrate that nonoptimized carrier concentrations in these films lead to observations of poor performance at room temperature. Through a simple soaking procedure in a Cu + ion solution for only a few minutes, we demonstrate a 200− 300% increase in power factor. This soaking process pushes the carrier concentration of the Cu 2 Se thin film toward its optimal value for TE operation and marks the highest TE performance for any solution processed Cu 2 Se thin film at room temperature thus far. If the performance can be further optimized at room temperature, Cu 2 Se thin films will be the material of choice to utilize in TE modules for powering miniature electronics and sensors, which has been an increasingly popular and rapidly expanding market.
Electronic and optoelectronic devices fabricated from colloidal quantum dots (CQDs) provide a promising path toward realizing low-cost devices with greatly simplified device fabrication procedure, owing to their solution-processability. The impact that CQD technology would bring is expected to be significant, especially in the mid-infrared application areas, which is currently dominated by epitaxial semiconductor technologies. In this work, we introduce a new generation of infrared CQDs, namely Ag2Se CQDs, which has been recently uncovered to show distinct optical absorption in the mid-infrared. We report on the fabrication of solution-processed photoconductive photodetectors and discuss our analyses on the electronic and optoelectronic characteristics of our devices. We also demonstrate the feasibility of mid-infrared photodetection with a measured peak responsivity of 0.16 mA/W at 4 μm under room temperature operation.
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