Metal Halide Perovskites for Light Emitting Diodes

My research has been focusing on the development and processing of metal halide perovskites, an emerging class of semiconductor materials, for various types of light emitting diodes (LEDs). In this dissertation, introduction to metal halide perovskites will be presented in the chapter 1, followed by three chapters detailing the work on the use of metal halide perovskites as hole transport layer for organic LEDs (chapter 2), as light emitting layer for electrically driven LEDs (chapter 3), and down conversion LEDs (chapter 4). A new type of hole transport layer based on metal halide perovskite, CH3NH3PbCl3 has been demonstrated for highly efficient OLEDs. Two types of hole transport layer have been fabricated, including PIP-CH3NH3PbCl3 composite thin films and neat CH3NH3PbCl3 thin films. Solution processed multilayer green phosphorescent OLEDs based on this new PIP- CH3NH3PbCl3 nanograss HTLshowed superior performance over devices using conventional PEDOT: PSS HTL with lower turn-on and operating voltages, as well as higher brightness. The improved device performance is primarily attributed to the high conductivity of CH3NH3PbCl3 and large interpenetrating interfaces between the hole transporting perovskite nanopillars and the emitting layer. In order to further enhance efficiencies of OLEDs based on CH3NH3PbCl3 as HTL, the solvent passivation approach has been adapted for the formation of smooth neat CH3NH3PbCl3 perovskite thin films with great surface coverage. Solution-processed multilayer green phosphorescent OLEDs based on this new perovskite HTL showed superior performance over the device using conventional PEDOT:PSS HTL, with lower turn-on and operating voltages, as well as higher brightness, EQE, power efficiency and luminous efficiency. The improved device performance is primarily attributed to the wide band gap of CH3NH3PbCl3, suitable energy levels, and efficient the hole injection and transport from ITO to CH3NH3PbCl3 and light emitting layer. This work demonstrates a new pathway toward highly efficient solution processed multilayer OLEDs, and further establishes organic-inorganic halide perovskites as a new class of semiconductors with highly desirable characteristics for thin film optoelectronic devices. Perovskite LEDs have recently attracted great research interest for their narrow emissions and solution processability. Remarkable progress has been achieved in green emitting perovskite LEDs in recent years, but not blue or red ones. Highly efficient and spectrally stable red perovskite LEDs with quasi-2D perovskite/poly (ethylene oxide) (PEO) composite thin films as the light-emitting layer have been successfully demonstrated. By controlling the molar ratios of organic salt (benzyl ammonium iodide) to inorganic salts (cesium iodide and lead iodide), luminescent quasi-2D perovskite thin films are obtained with tunable emission colors from red to deep red. The perovskite/polymer composite approach enables quasi-2D perovskite/PEO composite thin films to possess much higher photoluminescence quantum efficiencies and smoothness than their neat quasi-2D perovskite counterparts. Electrically driven LEDs with emissions peaked at 638, 664, 680, and 690 nm have been fabricated to exhibit high brightness and external quantum efficiencies (EQEs). For instance, the perovskite LED with an emission peaked at 680 nm exhibits a brightness of 1392 cd/m2 and an EQE of 6.23%. Both the maxima brightness and external quantum efficiencies of our devices are among the highest reported to date for red perovskite LEDs. Moreover, exceptional electroluminescence spectral stability under continuous device operation has been achieved for these red perovskite LEDs. This work clearly shows the effectiveness of processing and device engineering to realize high performance perovskite optoelectronic devices. Recently, our lab developed a new class of luminescent materials, zero-dimensional perovskites. These novel luminescent materials show very high photoluminescence quantum efficiencies (PLQEs) with a very broad emission. For these new 0D perovskites, their excellent optical properties enable us to fabricate down conversion white LEDs. By combing these new 0D perovskites with commercialized blue phosphor, I have successfully fabricated down conversion white LEDs with high color quality. For example, a new type of mixed halide 0D perovskite, (C4N2H14Br)4SnBrxI6–x (x = 3), with a PL emission peak of 582 nm, a larger FWHM of 126 nm and a high PLQEs of 85% have been used as yellow phosphors in down conversion LEDs. By overcoming the issue of deficiency in the red emission present in most yellow phosphors, this 0D tin mixed-halide perovskite enabled optically pumped WLEDs with high color rendering index (CRIs) of up to 85.