High‐Sensitivity p–n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots

Abstract

Chemically synthesized nanocrystal quantum dots (NQDs) are promising materials for applications in solution‐processable optoelectronic devices such as light emitting diodes, photodetectors, and solar cells. Here, we fabricate and study two types of p‐n junction photodiodes in which the photoactive p‐layer is made from PbS NQDs while the transparent n‐layer is fabricated from wide bandgap oxides (ZnO or TiO2). By using a p–n junction architecture we are able to significantly reduce the dark current compared to earlier Schottky junction devices without reducing external quantum efficiency (EQE), which reaches values of up to ∼80%. The use of this device architecture also allows us to significantly reduce noise and obtain high detectivity (>1012 cm Hz1/2 W−1) extending to the near infrared past 1 μm. We observe that the spectral shape of the photoresponse exhibits a significant dependence on applied bias, and specifically, the EQE sharply increases around 500–600 nm at reverse biases greater than 1 V. We attribute this behavior to a “turn‐on” of an additional contribution to the photocurrent due to electrons excited to the conduction band from the occupied mid‐gap states.

Near-Infrared Dual-Emission Quantum Dots–Gold Nanoclusters Nanohybrid via Co-Template Synthesis for Ratiometric Fluorescent Detection and Bioimaging of Ascorbic Acid In Vitro and In Vivo

“…To further prove this hypothesis, by using differential pulse voltammetry (DPV), we obtained that the oxidation potential of PbS QDs is 0.52 V (vs SCE) in Figure S7C in the SI, which was equal to 0.76 V (vs NHE); thus, the E HOMO was calculated to be −5.26 eV. The calculated results are in accordance with the value of −5.1 eV in a previous study . On the other hand, the redox couple of AA is 0.082 V (vs NHE) .…”

Limiting Factors of Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors

“…65 The high EQE values (80−90%) in visible and ∼30% at 1100 nm are competitive among the previously reported values. 12,25,66,67 It could reflect solid evidence for a well-working device architecture in this work. Different film thicknesses indicate different light absorption capabilities, and optical interference effects (positive and negative) could take place in the photodiode stack.…”

“…As shown in Figure S13, EQE spectra are also provided according to the calculation formula: EQE = Rh ν/ q , where h is Planck’s constant, ν is the frequency of incident light, and q is the elementary charge . The high EQE values (80–90%) in visible and ∼30% at 1100 nm are competitive among the previously reported values. ,,, It could reflect solid evidence for a well-working device architecture in this work. Different film thicknesses indicate different light absorption capabilities, and optical interference effects (positive and negative) could take place in the photodiode stack. , Therefore, points of high and low light intensities are created in the devices.…”

Ultrafast Colloidal Quantum Dot Infrared Photodiode

“…We posit that this higher than unity gain is due to photoconductive gain induced by HTL electron-trapping effect, which also has been reported in other literatures. − The drastic increase in collected photocurrent in visible region leads to a new peak at around 500 nm under −0.5 V bias. Same phenomenon has been reported and its mechanism was explained in detail …”