Intrinsic degradation factors of perovskite semiconductors in optoelectronic devices

Aygüler, Fatma Meltem (2018): Intrinsic degradation factors of perovskite semiconductors in optoelectronic devices. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Semiconductors have had a monumental impact on our society, being at the heart of every electronic device in our daily lives. Undoubtedly, they have changed the world beyond anything that could have been imagined before them. Otherwise, we would be using computers like a monster of a machine, weighing more than 30 tons and consuming several kilowatts of electrical power that when it is turned on, the lights in a few cities would be dimmed. Among all other semiconductor materials, so-called hybrid organic-inorganic perovskites have drawn a great deal of attention in the last decade due to dramatic strides in power conversion efficiencies (PCEs) as photoabsorbers in solar cells. Their similarities to gold standards of crystalline silicon have carried them to be some of the most intensely researched semiconductors by producing a plethora of optoelectronic devices. However, despite their versatility, there are several hurdles, holding their further improvement back. In this thesis, intrinsic properties of this family of materials and their limitations are profoundly investigated and solutions are provided for further developments. In the first part of this thesis, a performance difference in the current-voltage scans of solar cells, so-called anomalous hysteresis, is unveiled. This phenomenon emerges from a combination of ion migration and charge recombination at the charge transport layer-perovskite interface and hampers the device performance severely. It is found that deep trap states at the interface likely trigger charge accumulation due to the Fermi level offset of SnOx from the perovskite, which in turn leads to enhanced charge recombination, causing a higher degree of hysteresis in solar cells. It is suggested that if the band alignment between the perovskite absorber and the SnOx layer is improved, the resulting device excels due to greatly reduced hysteresis and much better performance. Charge transport characteristics and their limiting factors are of utmost importance in photovoltaic devices in order to extract the charges efficiently. The charge carrier mobilities in CH3NH3PbI3 thin films extracted from lateral time-of-flight measurements are found to be 6 cm2/Vs, whereas similar measurements performed on a solar cell architecture, i.e. in the vertical direction, show effective mobilities that are reduced by 3 orders of magnitude. By varying the thickness of the charge extraction layers, it is revealed that the limiting factors of the charge carrier transport time in working devices are the electron and hole transport layers rather than the perovskite material itself. In chapters 5 and 6, the versatility of perovskite semiconductors was extended by synthesizing both hybrid and all-inorganic perovskite nanocrystals (NCs) with compositional engineering. It is found that exchanging cation and mixing halide ions in the perovskite structure not only altered their charge recombination rates as well as photoluminescence spectra but also the photoluminescence quantum yield (PLQY). The implementation of mixed halide systems into lighting devices, so-called light-emitting electrochemical cells (LECs), revealed another major intrinsic concern of perovskites: halide segregation, forming bromide- and iodide-rich phases upon application of voltage in the devices. However, this problem was tackled by adding the salt KCF3SO3 in the active layer of LECs with mixed halide NCs. This addition not only suppresses the halide segregation by further stabilization of the perovskite lattice with potassium ions but also improves the brightness of the devices with low injection voltage. In the last part of thesis, four-terminal perovskite/CIGS tandem solar cells were presented. In order to boost the efficiency of the CIGS bottom-cell in tandem configuration, the transparency of the top-cell is of paramount importance. Thus, wide-band perovskite top-cells with different transparent conductive oxides used for both substrates and back electrodes were investigated, revealing that the transparency of the substrate is more critical than the back electrode’s transmittance to increase the performance of CIGS solar cells. Additionally, the reason of low voltage output in the top-cell is attributed to the halide segregation caused by application of voltage and illumination, where the formation of bromide- and iodide-rich phases was demonstrated by X-ray diffraction measurements. Finally, methods to improve the performance of perovskite top-cells were suggested as an outlook.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 26. November 2018
• 1. Bericht­erstatter:in: Bein, Thomas