| 초록 |
Organic metal halide perovskites (CH3NH3PbX3) have been widely studied for various applications such as light-emitting diodes (LEDs), photovoltaics, lasers or sensors. In particular, organic metal halide perovskites have emerged as a next generation light emitter in LED application because of their outstanding color purity (full width at half maximum of electroluminescence spectra as narrow as 20 nm), easily tunable bandgap through adjustment of the halide element ratio, and low materials cost,. However, methylammonium lead bromides (CH3NH3PbBr3) which are suitable for green light emission, have a low exciton binding energy and have a tendency to form cubic shaped crystals, making it difficult to obtain a uniform film. Therefore, the device performance of perovskite LEDs has not yet approached that of OLEDs or QLEDs. Here, we studied the effect of varying molar ratio of the perovskite precursors (MABr and PbBr2) on the properties of synthesized MAPbBr3 thin films as well as on LED device performance. The molar ratio (MABr/PbBr2) was adjusted from 1 (stoichiometry) to 2 (excess MABr) in precursor solution. Structural characterization revealed that the excess MABr condition led to the formation of a new crystalline phase, which appeared to be one of methylammonium compounds, within the MAPbBr3 films. The excess MABr sample also exhibited highest PL intensity, which suggested less occurrence of non-radiative recombination compared to other molar ratios. Based on the PL, the excess MABr was expected to produce the best LED performance, however, it was when MABr/PbBr2 is 1.05 found that led to the best performance. We explain these results by other electronic properties such as Urbach tail energy that we found relevant to the operation of LEDs. With further optimization of hole injection layer, we achieved highly efficient green perovskite LEDs with luminance = 1032.1 cd/m2, current efficiency = 0.905 cd/A and external quantum efficiency (EQE) = 0.198%. Details of characterization results with varying molar ratios and optimization of LED device structures will be presented. |
