This master thesis deals with the spectroscopic characterization of Lead Bromide Perovskite Nanocrystals (MAPbBr3-NCs), a material which has been widely investigated over recent years due to its promising applications in LEDs, solar cells and LCD backlit screens. For the purposes of this work, the experiments were conducted at the facilities of the Centre for Nano Science and Technology (CNST) at POLIMI. Many samples of MAPbBr3 were synthesized, in the form of either colloidal NCs suspensions or thin films, varying some synthesis parameters such as temperature, humidity contamination and ligand concentration (i.e. the quantity of molecules added during the synthesis process to facilitate the NCs formation, so-called ligands). Such samples were characterized through spectroscopy experiments such as the determination of photoluminescence quantum yield (PLQY) and time-resolved photoluminescence (tr-PL) spectroscopy, to understand the effect of the synthesis parameters on their optical properties and to address their stability over time. The colloidal MAPbBr3 NCs were found to have a period of instability (1-2 months) after their synthesis, during which the PLQY grew, until a stable configuration was reached with values up to 92%. The capability to retain a high PLQY after a long time moreover appeared to be dependent on the ligand concentration, and the good luminescence properties to be conserved after deposition of the NCs, forming thin films. Finally, the tr-PL spectroscopy measurements showed a redshift in the PL emission in the first nanoseconds after pump excitation.
Questa tesi tratta della caratterizzazione spettroscopica di nanocristalli Methylammonium-Lead-Bromide (MAPbBr3-NCs), un materiale che è stato ampiamente studiato negli ultimi anni per via delle sue promettenti applicazioni in LED, celle solari e schermi LCD retroilluminati. Ai fini di questo lavoro, gli esperimenti sono stati condotti presso le strutture del Centre for Nano Science and Technology (CNST) presso POLIMI. Molti campioni di MAPbBr3 sono stati sintetizzati, sotto forma di sospensioni colloidali o di thin film, variando alcuni parametri di sintesi come temperatura, contaminazione da umidità e concentrazione di leganti (cioè la quantità di molecole aggiunte durante il processo di sintesi per facilitare la formazione di NC, “leganti”). Tali campioni sono stati caratterizzati mediante esperimenti di spettroscopia come la determinazione della efficienza esterna di fotoluminescenza (PLQY) e la spettroscopia di fotoluminescenza risolta in tempo (tr-PL), per comprendere l'effetto dei parametri di sintesi sulle loro proprietà ottiche e per studiare la loro stabilità nel tempo. I nanocristalli colloidali di MAPbBr3 sono risultati instabili per un periodo di 1-2 mesi dopo la loro sintesi, durante la quale il PLQY è cresciuto, fino a quando si è raggiunta una configurazione stabile con valori fino al 92%. La capacità di mantenere un alto PLQY dopo molto tempo risulta inoltre dipendere dalla concentrazione di leganti, e le buone proprietà di luminescenza si conservano dopo la deposizione dei NC in film sottili. Infine, le misure di tr-PL hanno mostrato un red-shift nell'emissione di fotoluminescenza nei primi nanosecondi dopo l'eccitazione.
Characterization of lead bromide perovskite colloidal nanocrystals : stability and optical properties
FORNAROLI, MICHELE
2017/2018
Abstract
This master thesis deals with the spectroscopic characterization of Lead Bromide Perovskite Nanocrystals (MAPbBr3-NCs), a material which has been widely investigated over recent years due to its promising applications in LEDs, solar cells and LCD backlit screens. For the purposes of this work, the experiments were conducted at the facilities of the Centre for Nano Science and Technology (CNST) at POLIMI. Many samples of MAPbBr3 were synthesized, in the form of either colloidal NCs suspensions or thin films, varying some synthesis parameters such as temperature, humidity contamination and ligand concentration (i.e. the quantity of molecules added during the synthesis process to facilitate the NCs formation, so-called ligands). Such samples were characterized through spectroscopy experiments such as the determination of photoluminescence quantum yield (PLQY) and time-resolved photoluminescence (tr-PL) spectroscopy, to understand the effect of the synthesis parameters on their optical properties and to address their stability over time. The colloidal MAPbBr3 NCs were found to have a period of instability (1-2 months) after their synthesis, during which the PLQY grew, until a stable configuration was reached with values up to 92%. The capability to retain a high PLQY after a long time moreover appeared to be dependent on the ligand concentration, and the good luminescence properties to be conserved after deposition of the NCs, forming thin films. Finally, the tr-PL spectroscopy measurements showed a redshift in the PL emission in the first nanoseconds after pump excitation.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/142738