Electron‐Transport Layers Employing Strongly Bound Ligands Enhance Stability in Colloidal Quantum Dot Infrared Photodetectors

Abstract

circuits. [4][5][6][7] PbS CQDs are well-suited to SWIR photodetection in light of their sizetuned bandgap and strong absorption in the IR region. The latest CQD photodetectors have adopted the photodiode architecture consisting of a CQD-based active layer, a CQD-based hole-transport layer (HTL), and a metal oxide-based electrontransport layer (ETL). [7][8][9][10] High-efficiency photodetectors require accurate control of the optoelectronic properties of each layer. To reach a peak sensitivity in the wavelength range of 1400-1500 nm, small bandgap PbS CQDs are used as active layers, and their relatively deep conduction band position limits the selection of ETs. Known metal oxides such as ZnO or TiO 2 do not offer the combination of properties needed for SWIR CQD detectors: the requisite union of band alignment at the interface, and stable performance at the operating wavelengths. [8,10] CQD-based transport layers benefit from tunable optoelectronic properties through surface ligand engineering and quantum-size effect tuning. While CQD HTLs have been extensively studied and improved, CQD ETLs are rarely explored. [11][12][13] Here we develop n-type CQDs as ETLs and tailor their band-edge positions for efficient charge extraction at the active layer/ETL interface. We fabricate all-CQD photo detectors in which CQDs of the same composition but with different sizes and ligand passivation were used to produce functionalities optimized separately for the active layer and charge-transport layers. The CQD photodetectors operating at 1450 nm exhibit a high external quantum efficiency (EQE) of 66% and a low dark current of ≈1 × 10 −3 mA cm −2 at 1 V, which has not been realized simultaneously in the prior works that used metal oxides as ETLs.We find that the surface passivation of CQDs is crucial to suppressing ion migration and preventing performance degradation in CQD photodetectors. Therefore, we develop a strategy to improve the passivation of ETL CQDs using strongly bound organic ligands. Specifically, we demonstrate that ETLs employing a strong organic ligand trans-4-(trifluoromethyl) cinnamic acid (TFCA) improve the dark current stability of CQD photodetectors by 50× compared to ETLs employing a weakly bound inorganic ligand tetrabutylammonium iodide (TBAI). Solution-processed photodetectors based on colloidal quantum dots (CQDs)are promising candidates for short-wavelength infrared light sensing applications. Present-day CQD photodetectors employ a CQD active layer sandwiched between carrier-transport layers in which the electron-transport layer (ETL) is composed of metal oxides. Herein, a new class of ETLs is developed using n-type CQDs, finding that these benefit from quantum-size effect tuning of the band energies, as well as from surface ligand engineering. Photodetectors operating at 1450 nm are demonstrated using CQDs with tailored functionalities for each of the transport layers and the active layer. By optimizing the band alignment between the ETL and the active layer, CQD photodetectors that combine a low d...

Control over Charge Carrier Mobility in the Hole Transport Layer Enables Fast Colloidal Quantum Dot Infrared Photodetectors

“…19−22 Solution processed (sol−gel) NiOx HTL has been used with PbS CQD active layers in the past to achieve an EQE of 24% at 1 V reverse bias. 23 The reported valence band position of the sol− gel NiOx (5.4 eV) is deep and close to the valence band energy reported for the PbS CQD based absorber layer (5.2−5.5 eV 2,3 ). This band alignment mismatch obstructs efficient charge extraction, leading to low EQE values.…”

“…To investigate the temporal response of PbS CQD photodetectors, we fabricated p-i-n devices operating at 1370 nm based on prior approaches. 2,3 The structure of this device is shown in Figure 1a PbS CQDs were utilized in this work, one having an excitonic peak at 1350 nm and the other at 930 nm. Smaller CQDs were used for HTL and HBL deposition, while larger dots were used for the absorber layer.…”

Tailored Band Edge Positions by Fractional Ligand Replacement of Nonconductive Colloidal Quantum Dot Films

“…Due to their tunable optoelectronic properties, solution-processable colloidal quantum dots (CQDs) have been successfully incorporated as efficient semiconductors in various optoelectronic devices including solar cells, − light-emitting diodes, − and photodetectors. − Over the past decades, control over the electronic properties of CQD films has been intensively studied to efficiently inject or extract charge carriers to/from the CQD layer in solid-state devices. ,, …”

Improved Operation Stability of Colloidal Quantum Dots Photodiodes via Suppressing Ion Migration

“…As only one known commercial application, CdSe CQDs have been used as a color conversion layer in QDTV, which could operate at high temperature with no electric field . The PbS CQDs solar cells, operating at zero bias, have been reported on CQDs film stability under ambient air and humid atmosphere. − As one of the highest potential applications, PbS CQDs photodiodes for infrared imaging with ROIC-compatible structures have yet to be explored on operation stability under strong electric field at high temperature, which is required for actual applications …”

“…11−14 As one of the highest potential applications, PbS CQDs photodiodes for infrared imaging with ROIC-compatible structures 15 have yet to be explored on operation stability under strong electric field at high temperature, which is required for actual applications. 16 In this work, we took PbS CQDs photodiodes with a ROICcompatible structure of ITO/NiO x /CQDs/C 60 /ZnO/ITO for stability monitoring. The CQDs photodiodes showed good storage stability at high temperature, bad operation stability at room temperature, and sharply deteriorated operation stability under strong electric field at high temperature.…”