| Abstract [eng] |
Metal halide perovskites are a relatively recently created soft-lattice semiconductor. Over the past decade, scientists have made an astounding breakthrough in regards to the material’s applications in optoelectronics. Perovskites stand-out given their properties, such as a tunable bandgap, a higher mobility than organic semiconductors in addition to compatibility with solution-processing methods for device fabrication, notable examples being solar cells, laser diodes and LED’s. Published works on the latter topic showcasing external luminescence quantum yields over 20% can be found. Still, to achieve technological repeatability perovskite manufacturing specifics must be clarified and optimised. Typical bulk perovskites (like FAPbBr3 ) possess a high carrier mobility owing to low exciton binding energy, although slow recombination rates at low excitation densities aren’t optimal for LED’s. In addition, their inherent instability from ambient factors (humidity, oxygen, UV radiation etc.) prompted the development of a more stable perovskite structure, incorporating large organic cation spacers, leading to formation of quasi-2D perovskite crystallites. In this system, charge carriers are more confined, exciton energy increases as well as the rate of carrier recombination - all beneficial for LED’s. Among the methods of fabricating quasi-2D perovskites, co-evaporation methods produce films of high quality, yet the process itself is expensive, so more cost-effective solution-based methods are frequently considered. One among them - spin coating - produces quasi-2D perovskite films with a distribution of crystallite dimensionalities n, leading to potentially unwanted non-radiative energy transfer. Therefore development of technological methods for quasi-2D perovskites is an ever-relevant topic, since it impacts the optical and electrical properties of the films. In this Master’s thesis quasi-2D lead bromide perovskites based on organic cations (small - formamidinium FA+ , large spacer - butylammonium BA+ ) - BA(2)FA(n−1)Pb(n)Br(3n+1) - are investigated in their feasibility for LED production using spin-coating techniques, since the particular material isn’t well described for this specific application in literature. As a result, LED’s emitting in the 440-570 nm spectral range have been produced. Some of the key technological factors in making feasible prototype devices included spin-coating of the PEDOT:PSS hole transport layer, working in an inert ”MBraun” gas environment, preparation and spin-coating of perovskite precursors as well as thermal evaporation electron transfer (bathocuproine BCP) and insulator (LiF) layers with the correct thickness. After investigating the affects on the photophysical properties of simple glass substrate spin-coated perovskite layers from different technological conditions, it was concluded that the best photoluminescence quantum yield is achieved when spin coating from concentrated 0.4 M solutions of medium stoichiometry s = 4.5 − 8 as well as drop-casting a generous volume of the anti-solvent chlorobenzene during the process (at least 160 μl for 20 μl of precursor solution in DMSO, to ensure layer homogeneity). The same conclusions about the best produced batch of PeLED’s were made. The latter were tested using electroluminescence and voltamperimetry procedures, revealing LED turn on voltages of 5-7.5 V, appreciable EL intensity as well as ohmic conductivity of (0.71-3.46)·10−10 Ω−1 m-1 and space-charge limited region width of 202, 71-96 nm. Despite promising results, further optimisation of PeLED fabrication is needed, such as improving perovskite layer uniformity, perovskite/PEDOT:PSS interface and finding a way to reduce the anisotropy of n in perovskite layers produced from low s precursors. |
