Flexible Colloidal Quantum Dot Photodetection with Cellulose Structures

Thesis (Ph.D.)--University of Washington, 2016-08This thesis will outline a new way of fabricating flexible photodetectors. Solution-processable colloidal quantum dots (QDs, or nanocrystals(NCs)) are incorporated into cellulose structures to form a composite structure that can be used for photodetection. This enables new ways of device fabrication and also makes ultrathin, ultraflexible and even transparent optoelectronic devices possible. Inkjet printing with an office inkjet printer is introduced and applied towards PEDOT:PSS transparent electrode deposition. This offers a low cost method for material deposition. Flexible photoconductors are fabricated with these electrodes and CdSe quantum dot embedded tracing paper by utilizing the porous cellulose structure. Consistent photoresponse is achieved with such a structure under 550nm light illumination. After further realizing the shortcomings of tracing paper for its large thickness and low porosity, which both deteriorate the performance of these devices, natural plant-membranes are chosen as an alternative and offer superb properties for optoelectronic device fabrication. Visible-blind self-powered ultra-violet detectors are designed and fabricated with the incorporation of ZnO NCs on reed membrane. Schottky junction devices are fabricated with the use of gold and aluminum as the electrodes. Sub-second responses are observed at a bias of zero, which is superior than most of the flexible photoconductors in the literature. An external quantum efficiency of over 3% is discovered with the device at 350nm light illumination under zero bias. A great performance enhancement is also observed on the devices fabricated on reed membrane comparing to the ones on tracing paper. Nanofibrillated cellulose(NFC) can be readily used to fabricate transparent papers. ZnO NC-NFC composite structure is prepared and fabricated into ultrathin transparent papers with a thickness less than 1 micrometer. Self-powered Schottky photodiodes are fabricated on such papers and relatively fast response is observed at zero bias