Photoconducting CdSe powder for flat panel displays
The initial aim of this work was the preparation of sensitized photoconductor layers for use in conjunction with d.c. electroluminescent panels to develop an optically triggered bistable system for sustaining pixel states during the frame time of dynamically addressed flat panel displays. More than 200 CdSe powder samples and devices were prepared in attempts to achieve the required response speed of approximately 1 millisecond and photocurrent density of at least 10mA cm(^-2) under expected operating conditions. Powder samples were sensitized by various dopant incorporation and heat treatment processes. Copper and halogen ions were introduced in the form of dilute solutions. Correlations between preparation conditions and photoelectronic behaviour were sought, additional information being obtained from various physical and chemical analytical techniques. Devices were prepared from sensitized powder samples by compression, sintering or mixing with a nitrocellulose binder. A screen printing process was employed to deposit powder-binder layers 40-70 microns in thickness. Although an optically triggered bistable element was not experimentally demonstrated, sensitized powder characteristics were successively improved to within an order of magnitude of those required. Response times of 2 - 10 milliseconds were recorded, and photo-response in a sandwich type powder-binder device was finally achieved with photocurrent density of approximately 100mA cm(^-2). Simple power law relationships between conductivity and applied voltage, pressure and photoexcitation intensity were experimentally established. Prolonged ball milling of CdSe powder was found to cause a wurtzite to sphalerite phase transition which was reversible by firing in air for 2 hours at 500ºC. The estimated sphalerite lattice parameter was 6.051A. Long term variations in sample conductivities and photosensitivies were attributed primarily to oxygen adsorption in the presence of water vapour during storage. Photoelectronic characteristics indicative of both oxygen photoadsorption and photodesorption were observed, and explained in terms of dopant levels. Photoconductive processes in sensitized CdSe powder were shown to be dictated principally by surface chemistry and inter particle contacts.