Phosphor Materials

Electroluminescent Phosphors

                Electroluminescence (EL) is the phenomenon in which electrical energy is converted to luminous energy without thermal energy generation.  In our research group we explore so-called high field electroluminescent thin film materials.  These materials are different in principle from standard light emitting diode (LED) and diode lasers where electrons and holes recombine to create light.  In these high field EL materials, typically rare earth and transition metal ions are doped in a wide bandgap materials.  This phosphor layer is sandwiched between two insulators to limit the current and driven with an alternating current at high fields.  The figure below shows mechanistically how these alternating current thin film electroluminescent (ACTFEL) devices work.  The four main mechanisms are:  1) tunnel emission of electrons from interface states, 2) acceleration of electrons to high energies, 3) impact excitation or impact ionization of the luminescent center, and 4) de-excitation of the excited electron by radiative (photon generation) or non-radiative recombination.  Also illustrated below is a standard thin film stack for the ACTFEL device (the total film stack is ~ 1 mm thick). 

ACTFEL thin film stack

ACTFEL device mechanisms

Because these devices can be made thin (about the thickness of a glass sheet) this is a flat panel display technology.  The figures below show a few different ACTFEL displays. 

Figures of 2 flat panel ACTFEL displays (from Planar Systems Inc.)

Cathodoluminescent Phosphors

Cathodoluminescence is a process whereby light is created from an energetic electron beam.  This process is what probably powers your TV at home and your computer monitor on your desk (unless you have an LCD screen).  This process is responisible for the light from cathode ray tubes (ie standard TV and computer monitors.  The figure below illustrates how your TV works.  Namely an energetic electron beam is rastered across the screen and bombards individual pixels which have powder materials that give off light when the electron beam hits it. 

Schematic of a standard cathode ray tube (CRT)

Because we always want things bigger and better, CRT displays have grown in size over the years.  The problem is that as the display size increases, the depth also increases because of the electron optics needed to address the larger and larger screens.  To save space, there is a push for flat panel display technologies to replace standard cathode ray tubes.  An alternative technology which is fundamentally similar to CRTs is the so-called Field Emission Display (FED).  A cross-section schematic of an FED is shown below which illustrates that rather than having a single electron source addressing many pixels, an FED has many electron sources addressing a single pixel.  Consequently, the depth needed to "steer" the electron beam is unnecessary and it can be made "flat". 

Schematic of a Field Emission Display (FED)

Field emission displays typically require lower operating voltages than a standard CRT display, however most materials are not as efficient at lower electron beam energies.  Consequently, we are exploring the synthesis of new materials for low voltage applications.