Near-Infrared-II Photodetectors Based on Silver Selenide Quantum Dots on Mesoporous TiO₂ Scaffolds

Abstract

Biosensing applications are driving an increase in demand for low-cost photodetectors with sensitivity between 1000 and 1400 nm. Recently, the sensitivity of Ag2Se colloidal quantum dot based devices in this wavelength range has been observed. In this work we make the first demonstration of an Ag2Se photodiode device with sensitivity in this entire range. By employing secondary phosphine to elevate the precursor reactivity, we achieved accurate size control of the Ag2Se nanocrystals with a distinct excitonic absorption peak. These nanocrystals were deposited from solution into a mesoporous TiO2 scaffold, similar to that used in dye-sensitized solar cells, to increase the light absorption and charge separation and reduce the exciton diffusion length. By incorporation of a suitable hole-transporting layer between the active layer and Ag anode, the resulting devices showed a responsivity of 4.17 mA/W at a wavelength of 1200 nm. These results demonstrate that Ag2Se colloidal quantum dots offer a low-toxicity route for low-cost fabrication of near-infrared photodetectors.

Origin of Intraband Optical Transitions in Ag2Se Colloidal Quantum Dots

“…Prior studies and literature on Ag 2 Se can be collected into two groups, one group focusing on small (<5 nm) CQDs yielding interband NIR absorptions ,,− and the other focusing on larger CQDs (6–12 nm) that exhibit intraband MWIR absorptions. ,,, The intraband absorption is thought to be due to transitions from the filled 1S e states to the unoccupied 1P e levels; the 1S e electrons can arise from self-doping via deviations from the Ag 2 Se stoichiometry. Specifically, excess Ag on the surface or metallic Ag(0) within the Ag 2 Se CQDs dope them n - type .…”

“…Although HgTe remains the dominant CQD infrared material for wavelengths longer than 3 μm, HgSe and HgS were also investigated as potential candidates, opening up the possibility for intraband transitions in infrared detectors. , These transitions can only be observed when electrons from external or self-dopants are stabilized in the conduction band at specific energies that avoid reaction with the environment but can still enable higher temperature operation with reduced Auger recombination rates. ,− Including HgSe and HgS quantum dots, most photoactive intraband materials overcome the challenge of stabilizing an electron in the 1S e state, capable of an optical transition to the 1P e state that is smaller in magnitude compared to the interband gap (band gap). − Utilizing intraband transitions in materials with wider band gaps can open the opportunity to use environmentally friendly materials to replace common narrow band gap semiconductors, which typically comprise heavy metals. , Neither Hg nor Pb is compliant with the Restriction of Hazardous Substances (RoHS) and pose severe health and environmental risks if improperly handled, stored, or disposed . While Hg and Pb chalcogenides are still being developed toward commercialization, Ag 2 Se has thoroughly been investigated as a heavy metal free alternative NIR and MWIR-active material. ,,− Additionally, a cytotoxicity study on Ag 2 Se concluded that Ag 2 Se is a much less toxic material, allowing large-scale MWIR detector implementation or NIR biomarker use in the human body. ,− Currently, Ag 2 Se lags behind its Pb and Hg counterparts in performance metrics of infrared detectors, but there has been significant progress in the past 3 years. For example, recently reported Ag 2 Se MWIR devices have shown rapid progress, a 70-fold increase in responsivity from 0.3 to 21 mA/W within 1 year. , The improved responsivity is promising, but while bearing an uncanny resemblance to HgSe CQDs in terms of IR absorption, the performance of Ag 2 Se CQD detectors is orders of magnitude lower than their HgSe counterpart .…”

Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals

“…In the case of Ag 2 Se, substantial changes in physical properties accompany solid–solid phase transitions . For example, the orthorhombic phase of Ag 2 Se is known to be a narrow-band-gap semiconductor in the bulk ( E g = 0.15 eV) and orthorhombic Ag 2 Se nanocrystals are promising for near-infrared detection and imaging applications and as topological insulators. − In contrast, the high-temperature cubic phase is electrically and ionically conductive, with highly mobile Ag + cations that move through a rigid-body-centered Se 2– sublattice. ,, These properties of cubic Ag 2 Se are desirable for solid-state electrolytes, ,, and the differences in electrical and thermal conductivity between orthorhombic and cubic Ag 2 Se have been used to optimize thermoelectric responses at temperatures near the boundary of the orthorhombic to cubic phase transition . Significantly less is known about the “tetragonal” phase of Ag 2 Se nanocrystals, in part because the crystal structure remains unresolved.…”

Toward Green Optoelectronics: Environmental-Friendly Colloidal Quantum Dots Photodetectors

“…In addition to MIR photodetection, Ag 2 Se QDs demonstrated NIR light harvesting. Similarly, Graddage et al (2020) reported the first Ag 2 Se QD-based photodiode (Figure 8D) with high sensitivity ranging from 1,000 to 1,400 nm. To improve the precursor reactivity, the authors applied secondary phosphine and realized accurate QD size control.…”