Novel cost-effective light emitters based on metal halide perovskites

Resumen

In the last decade, metal halide perovskites have attracted huge attention and remarkable progress has been accomplished owing to efforts from researchers all over the world. In photovoltaic applications, perovskites with narrow band gap could realize solar cells as efficient as silicon based solar cells since perovskites show high charge carrier mobility and especially lead iodide and tin iodide perovskites possess high absorption coefficient in visible region. In case of light-emitting applications, the emission peak can be tuned by halide anion exchange and the width of emission is the narrowest among the emerging materials, organics emitters and inorganic quantum dots. In addition, the low material-cost and processing-cost make this material group attractive, hence, it might be a game changer in semiconductor market. Since the use of display system is getting ubiquitously, the development of cost-effective light emitting system is indispensable. In chapter 2, a down-conversion light emitter is realized by lead bromide precursor solute and polyethylene glycol solvent by one-pot synthesis. This solution shows 23 % of photoluminescent quantum yield with emission peak at 610 nm, originates from PbBr3-, which is superior to reported value (0.5 % with acetonitrile). The emission colour is also tunable by introducing alkylammonium halides in dimethyl sulfoxide as solid metal halide perovskites. For instance, introducing 2-folds of tetrabutylammonium bromide results in peak shift to 560 nm, originates from PbBr42-, (yellowish green), and all chloride solution of tetrabutylammonium chloride has emission peak at 510 nm (bluish green). To achieve white emissions for lighting, equimolecular amount of lead bromide and tetrabutylammonium bromide are dissolved for yellow emission which could be completed with a blue emissive dye, 4,4'-Bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl (BCzVBi). Corelated colour temperature of two solutions satisfy CIE D55 (5503K, Mid-morning / Mid-afternoon daylight) and CIE D75 (7504K, North sky daylight), respectively. These values are proper to the desktop publishing industry (monitor's colour temperature). In chapter 3, light-emitting electrochemical cells are developed with a 3D metal halide perovskite material, CsPbBr3, mixed with polyethylene oxide. The perovskite-polymer precursors are dissolved in dimethyl sulfoxide and spin-coated on a ITO/PEDOT:PSS coated substrate that functions as the anode. The crystallinity of the CsPbBr3: Poly-ethylene oxide (PEO) light-emitting layer is enhanced via a vacuum treatment and thermal annealing of the slightly wet coated film. After annealing a metal cathode is deposited on the dried light-emitting layer without inserting an electron transport material. The resulting devices showed good luminance (2200 cd/m2) and a rather fast turn-on time of 5 min when driven with a constant current (100 A/m2). This light emitting device consists of cost-effective ionic metal halide perovskite material in polymer electrolyte. Besides the simple structure of this device makes it suitable for low-cost fabrication. Using these perovskite-based light-emitting electrochemical cells (LECs) a fundamental study on anion interdiffusion at perovskite heterojunction interfaces was performed. For that heterostructures were prepared involving the CsPbBr3:PEO layer and vacuum deposited 3D CsPbCl3 perovskite and 0D Cs4PbCl6 perovskite-related material. In the CsPbBr3-CsPbCl3 heterojunctions the emission spectrum shifts, and it emits a cyan-blue colour which is evidence of instant halide mixing taking place in the as-deposited films. On the other hand, there was no significant halide anion intermixing in the CsPbBr3-Cs4PbCl6 heterojunction. Hence, it is confirmed that 0D inorganic caesium lead halide compounds only marginally donate halide anions to the adjacent CsPbBr3 layer. Exploiting this point, light-emitting electrochemical cells employing 0D Cs4PbCl6 layers are characterized. By sandwiching the CsPbBr3:PEO light-emitting layer in 0D Cs4PbCl6 layers a two-fold increase in device stability was obtained. This improvement in device operation might arise from the limited and balanced ion movement of the Cs4PbCl6 buffer layers. In chapter 4, a co-evaporated halide perovskite material, CsPbCl3, is used as a hole transport layer in organic light emitting diodes. First, the perovskite film is passivated by depositing a thin film of the precursor salt CsCl at the interfaces. This passivation method increased the photoluminescent of the CsPbCl3 layer 3 times. This modified perovskite hole transport layer is used in phosphorescent organic light emitting diodes comprising ultrathin (sub-nanometre) undoped iridium complex. I refer to these electroluminescent devices as perovskite-ultrathin emitter LEDs (PUE-LEDs). Employing ultrathin emissive layer can reduce the material-cost by almost 70 %. In addition, this makes the fabrication process more facile. However, since such very thin devices are prone to have high leakage currents, I inserted thick perovskite layers to overcome this issue. As a result, 200 nm thick perovskite hole transport layer with interface passivation and a light-emitting layer of nominally 0.05 nm reach an efficiency of 31 cd/A whereas the efficiency of a co-evaporated counterpart is 20 cd/A. The work presented in this thesis, present several strategies that can help to decrease the complexity of LED and as such lead to more economic devices. It is shown that metal halide perovskites are promising candidates as down-conversion light emitters, ionic material in light-emitting electrochemical cells, and charge transport material in light emitting diodes. However, the perovskites used in this thesis contain toxic lead, which implies that rigorous encapsulation is needed to prevent leaching of the toxic salts. Additionally, an end-of-life reuse protocol must be put in place. Such a re-use protocol is mandatory for an ever increasing amount of devices. In this light, simplifications as proposed in this thesis by reducing the number of layers in LEDs can help to make the reuse of these materials more sustainable. There are promising developments related to lead-free perovskites which may also be studied in the same way as I did with the lead containing perovskites in this thesis. This can assist in the realization of cost-effective light-emitters and spreading perovskite-based display system all over the world in this Internet-of-Things (IoT) era.