Materials Chemistry to Advance the Speed and Sensitivity of Infrared Solution-Cast Photodetectors

There has been considerable interest in solution-processed semiconductors for use in near-infrared (NIR) and short-wavelength infrared (SWIR) photodetectors. In this regard, perovskites and colloidal quantum dots (CQDs) are two prominent materials. I aim, in this thesis, to explore the chemistry of these material systems to enable efficient, stable, and fast solution-based infrared photodetectors. In Pb-Sn binary perovskite photodetectors, I uncover a low charge collection efficiency that results from the chemical incompatibility between tin perovskite material processing and metal oxide transport layers. I develop a new approach, revisiting the method used in chemically reducing the tin precursor. I report photodetectors with an external quantum efficiency of 85% at 850 nm, a dark current below 10-8 A/cm2, and a response time faster than 100 picoseconds. I showcase the application of these detectors in LiDAR, demonstrating sub-millimeter distance resolution. I turn my attention to SWIR InAs CQD photodiodes, finding that there is a non-optimal band alignment at the interface between the QD layer and the hole transport layer. I utilize functionalized molecular ligands to engineer the energy levels of QDs. The incorporation of molecularly tuned QDs as hole transport layers improves charge extraction efficiency from 20% to 31% at 1120 nm and leads to a two-orders-of-magnitude improvement in the dark current, reaching 4×10-7 A/cm2. I then use energy level tuning in QDs to design an electron transport layer that replaces conventional metal oxides in SWIR PbS CQD photodetectors. The favorable band alignment and improved passivation of the QD-based transport layer results in a one-order-of magnitude suppression of the dark current down to ~1×10-6 A/cm2 while maintaining an external quantum efficiency ~70% at 1450 nm. I then apply the knowledge gained on CQD surface modification to devise an efficient ligand exchange route for small-bandgap PbS CQDs. In devices, the CQD solids give rise to an external quantum efficiency greater than 80% at 1550 nm, a measured detectivity of 8×1011 Jones, and a 10 ns response time. The studies in this thesis suggest future routes to sensitive, fast, and stable photodetectors that bring out the full potential of solution-processed materials in advanced light-sensing applications.

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