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

• Published in: Advanced functional materials 2012-04, Vol.22 (8), p.1741-1748
• Main Authors: Pal, Bhola N., Robel, Istvan, Mohite, Aditya, Laocharoensuk, Rawiwan, Werder, Donald J., Klimov, Victor I.
• Format: Article
• Language: English
• Subjects: detectivity; mid-gap band; nanocrystal quantum dot; p-n junction; PbS; photodetector
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201102532

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. A schematic structure of a p–n junction photodiode, which comprises a nearly fully depleted p‐type layer of PbS nanocrystal quantum dots and an n‐type layer of ZnO nanoparticles, is shown. This device architecture allows us to significantly reduce noise current and obtain high detectivity of more than 1012 cm Hz1/2 W−1.