Book Description

In this work, we first proposed the tandem architecture for solution-processed near infrared PbSe colloidal quantum dot (CQD)-based photodetectors to address the high dark current issue. The tandem architecture not only absorbs the virtue of tandem solar cell by means of efficient photon-to-current conversion, but also functions as the effective barrier that can block the leakage current. More than three orders of magnitude reduction in dark current has been observed, along with an elevated photocurrent. The low temperature current-voltage characteristics revealed that the tandem architecture posed a high energy barrier which effectively blocks the dark current. Our results suggest that tandem architecture can be employed to developing high-performance solution-processed photodetector. The application of tandem photodetectors was further extended to sensors on flexible substrates where little study has been reported to date. Our results on flexible tandem photodetectors validate the high efficiency and detectivity of the tandem architecture. Two different bending states have been studied which revealed the small critical bend radii of ~8mm and ~3mm for tensile and compressive bending, respectively. The photodetector performance remains stable under mechanical stress which offers great potential of CQDs-based tandem photodetectors for flexible device applications. Furthermore, we have demonstrated the chip level integration of flip-chip light emitting diode (LED) with current rectifying GaN Schottky barrier diodes constituting the Wheatstone bridge circuitry for alternating current (AC) driving. The flip-chip LED scheme offers better p-contact, high light extraction efficiency and fast heat dissipation. The reflectance and turn-on voltage were investigated under various p-contact annealing conditions. The flip-chip alternating current LEDs (ACLEDs) demonstrated more than ~23% improvement in terms of energy conversion efficiency over top-emissive ACLEDs and offer the potential of using such device for high brightness, high power, high efficiency and high reliability solid state lighting applications. Finally, built on our studies of LEDs and photodetectors, and of chip level integration of LEDs and GaN Schottky barrier diodes. we, for the first time, proposed the integration of visible LEDs and UV GaN photodetectors for bi-directional optical wireless communication (OWC) applications. The LEDs function as transmitters to emit visible light signal whereas the photodetectors as receivers to collect UV signals. The crosstalk can be neglected due to the superior visible-blind property of GaN UV photodetector. The experimental results demonstrated that the LEDs and photodetectors can work together efficiently which opens up a new avenue for using such device for bi-directional OWC applications.