Colloidal quantum dot photodetectors

Applications of top-surface photodetectors Optical detection encompasses a vast number of applications, including optical communications, remote sensing, spectroscopy, and metrology. A major part of the sensor market is associated with imaging applications, hence image sensors have become ubiquitous. This has largely been due to the advent of digital imaging. In an image sensor a photosensitive material is required to absorb optical signals in the visible range (i.e., wavelengths of 400–700 nm) and transform them into electronic signals. Visible imaging applications also include surveillance, machine vision, industrial inspection, spectroscopy, and fluorescent biomedical imaging. Sensitive photodetection in the short wavelength infrared (SWIR), on the other hand, enables passive night vision [1, 2] from 1 μm to 1.7 μm and biomedical imaging for tumor detection [3] exploiting the tissue transparent windows around 900 and 1100 nm [4, 5]. Additional applications of SWIR imaging can be found in astrophysics [6], remote environmental sensing [7], quality control and product inspection [8] in the food and pharmaceutical industries, and identification [9]. Imaging in the visible is largely facilitated by silicon photodiodes. Early imaging arrays were based on charge-coupled devices (CCD). This approach involves a photoactive sensor array with several stages of photocharge transfer to the read-out circuit for electronic processing. CCD cameras offer a fill-factor – the ratio of the optically active area of the chip to its total area – that may vary from 100% to 50% depending on the chip design [10]. CCDs are prone to high fabrication and integration cost due to multiple chip interconnections and the incompatibility of CMOS with the process required for the CCD platform. In 1997, an integrated CMOS image sensor was reported on a single chip for optical sensing and signal processing [11]. Its fill-factor was limited to 30% due to the coexistence of photoactive elements and read-out circuitry on the same chip. Amorphous silicon photodetectors have been proposed as a top-surface photodetector with promise of a 100% fill-factor [12]. However, these suffer from long time constants [13] and material instabilities under illumination [14].