RESUMEN
In the field of halide perovskite research, the growth of high quality films has been a critical issue. Among the reported growth methods, vacuum processes have attracted much attention due to their accurate controllability and high reproducibility, as proven in the manufacture of vacuum deposited organic-light-emitting-diode industry. In a vacuum process, the major difficulty for growing a perovskite film is control of a precursor, methylammonium iodide (MAI), originating from its uncontrollable behavior i.e., a high working pressure and poor adsorption characteristics. Thus, it is crucial to understand the growth mechanism of MAI vapor for the successful application of vacuum processes in the growth of halide perovskite films. In this paper, we report the growth mechanism and deposition kinetics of MAI in a vacuum. Unlike that of conventional materials evaporated in a vacuum, the deposition rate of MAI was found to be much faster on the reactive surface, PbI2, compared to other non-reactive materials. Surprisingly, a very thin (2 nm-thick) PbI2 layer increased the initial growth rate of MAI 2.7-fold. Based on the real-time monitored data from a quartz microbalance and surface study, we suggest dipole-induced adsorption as the MAI growth mechanism on PbI2 and the perovskite in the vacuum process. We believe that this work will provide meaningful insight into film growth in vacuum processed perovskites.
RESUMEN
Recently, we have addressed that a formation mechanism of a nanolens array (NLA) fabricated by using a maskless vacuum deposition is explained as the increase in surface tension of organic molecules induced by their crystallization. Here, as another research using finite difference time domain simulations, not electric field intensities but transmitted energies of electromagnetic waves inside and outside top-emitting blue organic light-emitting diodes (TOLEDs), without and with NLAs, are obtained, to easily grasp the effect of NLA formation on the light extraction of TOLEDs. Interestingly, the calculations show that NLA acts as an efficient light extraction structure. With NLA, larger transmitted energies in the direction from emitting layer to air are observed, indicating that NLAs send more light to air otherwise trapped in the devices by reducing the losses by waveguide and absorption. This is more significant for higher refractive index of NLA. Simulation and measurement results are consistent. A successful increase in both light extraction efficiency and color stability of blue TOLEDs, rarely reported before, is accomplished by introducing the highly process-compatible NLA technology using the one-step dry process. Blue TOLEDs integrated with a N, N'-di(1-naphthyl)- N, N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine NLA with a refractive index of 1.8 show a 1.55-times-higher light extraction efficiency, compared to those without it. In addition, viewing angle characteristics are enhanced and image blurring is reduced, indicating that the manufacturer-adaptable technology satisfies the requirements of highly efficient and color-stable top-emission displays.
RESUMEN
To date, all deposition equipment has been developed to produce planar films. Thus lens arrays with a lens diameter of <1 mm have been manufactured by combining deposition with other technologies, such as masks, surface treatment, molding etc. Furthermore, a nano-lens array (NLA) with a sufficiently small lens diameter (<1 µm) is necessary to avoid image quality degradation in high resolution displays. In this study, an organic NLA made using a conventional deposition technique - without combining with other techniques - is reported. Very interestingly, grazing-incidence small-angle X-ray scattering (GI-SAXS) experiments indicate that the NLA is formed by the crystallization of organic molecules and the resulting increase in surface tension. The lens diameter can be tuned for use with any kind of light by controlling the process parameters. As an example of their potential applications, we use NLAs as a light extraction film for organic light emitting diodes (OLEDs). The NLA is integrated by directly depositing it on the top electrode of a collection of OLEDs. This is a dry process, meaning that it is fully compatible with the current OLED production process. Devices with NLAs exhibited a light extraction efficiency 1.5 times higher than devices without, which corresponds well with simulation results. The simulations show that this high efficiency is due to the reduction of the guided modes by scattering at the NLA. The NLAs also reduce image blurring, indicating that they increase color stability.