利用飛秒瞬時吸收光譜法研究 Formamidinium Tin Iodide 薄膜及其添加物之載子弛豫動態學

Femtosecond Transient Absorption Spectral Studies of the Carrier Relaxation Dynamics of Formamidinium Tin Iodide Thin films and the effects of varied additives.

莫海森(Mahmoud Mohamed Mohyeldin Mostafa Hassan)

指導教授 ： 刁維光

國立交通大學/理學院/應用化學系所/碩士(2018年)

摘要

Our study mainly focuses on the effects of additives on the optical properties and carrier generation and carrier recombination mechanisms of formamidinium tin iodide perovskites. The additives are bulkier organic cations like ethyl diammonium diiodide (EDAI2) and butyl ammonium iodide (BAI), passivate the crystal surface, control the film morphology and improve the crystallinity for the FASnI3 PSC. Though passivation effects can be visualized by optical imaging techniques like scanning emission microscopy and the subsequent photo conversion efficiency improvements of solar cells fabricated with passivated thin films, the actual microscopic picture on the mechanism of charge transport and band gap changes can only be accessed through steady state and ultrafast time-resolved absorption spectroscopy. Here, we present the band gap changes between pristine FASnI3 sample and added additive samples using steady state UV-Vis, PL spectroscopy and carrier generation/relaxation mechanisms by using ultrafast femtosecond transient absorption spectroscopic experiments performed on the all the samples under similar experimental conditions. Exciton formation from hot carriers were detected for all the samples measured due to the so-called phonon bottle neck effect. Laser fluence studies reveal that exciton formation or onset of photoinduced absorption is delayed due to the increase of hot carrier densities and carrier-carrier interactions in conduction and valence bands. Band broadening and blue shifts is also observed in the laser fluence studies confirms band filling effect due to accumulation of charge carriers in the conduction and valence bands. The estimated carrier cooling rates indicates that additives delay the cooling of hot carriers and thereby retard the recombination of carrier relaxation processes. Thus, the reasons for the retardation of fluorescence lifetimes of the samples with additives might be due to retardation of hot carrier cooling and there by re-generation and decays of hot excitons at slower rates. A kinetics study on carrier relaxation and transport processes and their device efficiencies will be presented and compared with those of their Pb-based analogues. Such a time-resolved information obtained in the present study would help us to design Sn-based perovskite solar cells with greater device performance.

關鍵字

Perovskite ； U.V Absorption ； PL ； TCSPC ； Ultrafast spectroscopy ； Femtosecond Transient Absorption Spectroscopy ； FASnI3

並列摘要

Our study mainly focuses on the effects of additives on the optical properties and carrier generation and carrier recombination mechanisms of formamidinium tin iodide perovskites. The additives are bulkier organic cations like ethyl diammonium diiodide (EDAI2) and butyl ammonium iodide (BAI), passivate the crystal surface, control the film morphology and improve the crystallinity for the FASnI3 PSC. Though passivation effects can be visualized by optical imaging techniques like scanning emission microscopy and the subsequent photo conversion efficiency improvements of solar cells fabricated with passivated thin films, the actual microscopic picture on the mechanism of charge transport and band gap changes can only be accessed through steady state and ultrafast time-resolved absorption spectroscopy. Here, we present the band gap changes between pristine FASnI3 sample and added additive samples using steady state UV-Vis, PL spectroscopy and carrier generation/relaxation mechanisms by using ultrafast femtosecond transient absorption spectroscopic experiments performed on the all the samples under similar experimental conditions. Exciton formation from hot carriers were detected for all the samples measured due to the so-called phonon bottle neck effect. Laser fluence studies reveal that exciton formation or onset of photoinduced absorption is delayed due to the increase of hot carrier densities and carrier-carrier interactions in conduction and valence bands. Band broadening and blue shifts is also observed in the laser fluence studies confirms band filling effect due to accumulation of charge carriers in the conduction and valence bands. The estimated carrier cooling rates indicates that additives delay the cooling of hot carriers and thereby retard the recombination of carrier relaxation processes. Thus, the reasons for the retardation of fluorescence lifetimes of the samples with additives might be due to retardation of hot carrier cooling and there by re-generation and decays of hot excitons at slower rates. A kinetics study on carrier relaxation and transport processes and their device efficiencies will be presented and compared with those of their Pb-based analogues. Such a time-resolved information obtained in the present study would help us to design Sn-based perovskite solar cells with greater device performance.

並列關鍵字

Perovskite ； U.V Absorption ； PL ； TCSPC ； Ultrafast spectroscopy ； Femtosecond Transient Absorption Spectroscopy ； FASnI3

參考文獻

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