Near-Infrared Photodetectors Based on Silver Chalcogenide Colloidal Quantum Dots

Detection of near-infrared (NIR) radiation is essential for a wide variety of applications in industries as varied as healthcare to agriculture. In the healthcare application space in particular, NIR radiation shows good penetration of tissue with low scattering making it ideal for a variety of diagnostic sensors. The primary spectral ranges for this sensing are commonly referred to as NIR-I between 650 nm to 950 nm, and NIR-II from 1000 nm to 1400 nm. The first window can be partially detected by Si based detectors, which are commonplace and low cost, though limited in form factor. The second window typically requires detectors using materials such as InGaAs, which perform well but are expensive to fabricate and also limited to rigid form factors. Therefore the identification of materials capable of photodetetction in these regimes while compatible with solution-based fabrication techniques such as printing is of significant interest, as it could result in low-cost devices which can be integrated into new form factors such as wearable electronics.<br/><br/>Colloidal quantum dots (CQDs) are a class of materials which offer tunable optoelectronic properties and solution processbility. A number of CQD materials have shown promise for NIR detection, however primary materials have all been Pb based, with associated toxicology concerns. This has inspired a search for less toxic alternatives. One such family of materials identified are silver chalcogenides, in particular Ag₂Se and Ag₂Te. To date there has been much interest in these materials, but little reported synthesis optimisation and demonstration in photodiodes, which is essential for low power applications. In this work we investigate the potential for these materials for use in NIR sensitive photodiodes, optimising synthesis and developing novel structures to enable such devices.<br/><br/>We developed Ag₂Se based photodiodes with sensitivity in the NIR-II window. Ag₂Se CQDs were synthesised using a hot injection method. We achieved CQDs with a mean diameter of 3.7 nm and a narrow size distribution by employing a secondary phosphine. These CQDs were then used in a photodiode structure which included a porous TiO₂ scaffold inspired by dye-sensitised solar cells. This structure maximises the absorption of the Ag₂Se CQDs, while minimising the risk of device shorting due to the high conductivity of the Ag₂Se CQDs. The ligand exchange process used for CQD layer fabrication is compatible with low-temperature substrates, and devices demonstrated an EQE of over 1% at a wavelength of 1150 nm and sensitivity up to 1400 nm. We also developed Ag₂Te based photodiodes. Ag₂Te CQDs synthesis was comprehensively optimised by investigation of multiple factors including feed ratio, ligand selection and synthesis method. After optimisation we achieved relatively large Ag₂Te QDs with distinct excitonic absorption peaks (~1050–1450 nm) and a PL emission peak from 1.3–1.7 μm. These Ag₂Te CQDs were used to fabricate photodiode devices which demonstrated sensitivity beyond 1400 nm, with an EQE of 0.14% and a responsivity of ~1.5 mA/W at 1400 nm.<br/><br/>The combination of good material processability, exciting material properties and promising device performance has demonstrated that silver chalcogenide CQDs offer a reduced-toxicity route for low-cost additive fabrication of NIR-II photodiodes. Further work is still needed to improve device performance and unlock the full potential of these materials.<br/><br/>References:<br/>- Graddage, N. & Ouyang, J. et al., Near-Infrared-II Photodetectors Based on Silver Selenide Quantum Dots on Mesoporous TiO2 Scaffolds, ACS Appl. Nano Mater. 2020, 3, 12, 12209–12217 (2020) doi: https://doi.org/10.1021/acsanm.0c02686<br/>- Ouyang, J. et al, Ag₂Te Colloidal Quantum Dots for Near-Infrared-II Photodetectors, ACS Appl. Nano Mater. 2021, 4, 12, 13587–13601 (2021) doi: https://doi.org/10.1021/acsanm.1c03030