Lead sulphide nanocrystal photodetector technologies

Abstract

2 Few other areas of study have uncovered the secrets of the universe like that of the interaction between light and matter, leading to many revolutionary scientific discoveries. The interaction of light, in particular with semiconducting materials, has enabled us to understand the behaviour of various fundamental phenomena and has laid the foundation of the optoelectronic systems that we rely on today. The majority of such systems necessitate the detection of light which is achieved via the use of photodetector devices. A photodetector converts incident photons into an electrical signal, which in the solid-state, is assembled together with an application-oriented readout integrated circuit (ROIC). Present-day commercially available photodetectors are typically made from gallium phosphide/silicon carbide (GaP)/(SiC), silicon (Si) and indium gallium arsenide/germanium (InGaAs)/(Ge) for detection in the ultraviolet (UV), visible and near-infrared (IR) regimes of the electromagnetic spectrum, respectively. For mid and far IR, lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb), indium arsenide (InAs) and mercury cadmium telluride (HgCdTe) based photodetectors are commonly used. Photodetectors based on a photoemissive principle such as photomultiplier tubes (PMTs) are also widely used for ultrasensitive detection in the UV to near-IR spectral regime and are fabricated using alkali metals having a low workfunction. For applications demanding multispectral detection, 'two-colour' detectors are often manufactured with a bi-level structure, for example consisting of an IR transmitting Si photodiode mounted over an IR sensitive PbS photoconductor. Such a device structure has drawbacks that include added cost, increased complexity of device fabrication and associated issues in implementation. Furthermore, a significant majority of applications are based upon UV to near-IR light detection where the indirect bandgap of Si, the InGaAs epitaxial growth process, PMT bulkiness and high bias voltage requirement all present challenges and renders their use with flexible platforms impossible. As a result significant research effort continues to be expended on the development of a singlesystem multispectral photodetector to replace two or more detectors. In the last decade amongst all the candidate materials studied 1 PbS semiconductor nanocrystals (NCs), often referred as 'quantum dots', have emerged as the most promising material for the fabrication of 3 this type of photodetector. Such has been the progress in their development that reported PbS NC based photodetectors have already outperformed conventional state-of-the-art photodetectors in many aspects including low-cost room temperature device fabrication via solution processing, flexible substrate compatibility, broadband spectral sensitivity along with the figures of merit achieved, Fig 1a. In this review article, we present an overview of recent developments in PbS NC based photodetectors. We first provide a brief introduction to PbS NCs and their releva...

Solution-processed semiconductors for next-generation photodetectors

“…7,8 Inorganic CQDs with broader responses up to the short-wave IR have also been demonstrated. [9][10][11] These ultrasensitive photodetectors exhibit either strong intrinsic gain or fast photoresponse, but the simultaneous achievement of both characteristics remains largely unexplored.…”

PbS Colloidal Quantum Dot Photodetectors operating in the near infrared

“…For this reason, significant efforts have been devoted to the reduction of the dark current in PbS CQD photoconductors, resorting to both blending with metal nanoparticles and using different ligands16. However, decreasing the dark current while preserving the photocurrent (or responsivity) proved to be very challenging since the two characteristics typically change unidirectionally.…”

“…(1.3 μm responsivity greater than 10 3  A/W and detectivity exceeding 10 13  cmHz 1/2 W −1 ), several photoconductive detectors were proposed looking for the improvement of the device performance and a suitable balance between sensitivity and speed16.…”

Near-infrared photodetectors based on PbS colloidal quantum dots

“…Thus, photoconductors are best suited for the detection of low-power quasi stationary signals whereas photodiodes are usually employed for the detection of high frequency signals. Even the best performing CQD photodiodes, however, show working frequencies lower by different orders of magnitude if compared with their bulk semiconductor counterparts [3] . Conversely, the extremely high absorption coefficient together with photoconductive gain makes CQD photoconductors the most viable alternative to devices based on classical bulk semiconductors, especially in the NIR and MIR range where such devices are still expensive and are realized by means of complex fabrication processes.…”