Graduate Thesis Or Dissertation
 

Light soaking in metal-halide perovskites

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  • Despite climate change requiring the cutback of fossil fuel consumption, the world continues to increase its energy demands each year. To meet the ambitious goals outlined during the Paris Agreement in 2015, which set a goal to limit global warming to 1.5°C [1], changes in energy production methods are necessary. Energy sources that produce minimal carbon emissions, such as solar, wind, hydroelectric, or nuclear energy, must be prioritized. Solar energy is the most abundant of all these resources, as it is available anywhere with a clear view of the sky. Recent advances in solar technology are dominated by organohalide perovskites (OHPs). Interest in this class of materials is driven by a number of factors including being easily solution processible at low temperatures [2], possessing a high absorption coefficient [3], having a low recombination rate [4], [5], exhibiting high mobility [6] and possessing a tunable band gap which can easily be modified by changing the halide composition [7]–[9]. In addition to these properties, one of the earliest MAPbI3 solar cells produced in study in 2012 reported an astounding power conversion efficiency (PCE) of 10.9% [10]. The current record for a single layer perovskite solar cell is now up to 25.5% as of 2021 [11]. This is an astonishing 134% increase in PCE in just 9 years. Although commercialization is in progress, it has not yet found widespread adoption. This is because there are several issues plaguing perovskite devices such as environmental concerns due to lead, instability in humid environments, and a poorly understood effect known as the light soaking effect (LSE), which is an increase or decrease in PCE that occurs under constant illumination [12]. In this work, an understanding of how the LSE in OHPs react to different external stimuli is developed to identify possible causes of the LSE. Because steady-state microwave conductivity (SSMC) is a contactless method of conductivity measurement, no electron/hole blocking layers are necessary. The sample therefore has no metal interfaces of which it can interact with, so any effects arising in the material can therefore be attributed to the material itself rather than contact effects. The SSMC technique provides information on the mobility of the material and can be obtained over long periods (>18 hours) with constant monitoring of the mobilitylifetime product (ϕƩµτ). The effects of external stimuli such as light and oxidation can be observed as changes in the ϕƩµτ as time progresses. By illuminating the sample under 405 nm UV light and 525 nm green light it was found that a wavelength dependence does exist in MAPbI3. In MAPbI3, carrier generation is believed to be directly proportional to the number of photons. Therefore, by holding the number of photons constant, the effect of incident wavelength in the role of degradation in MAPbI3 can be observed. This resulted in the observation that MAPbI3 degrades under UV illumination in contrast with the green illuminated sample, which experienced a LSE reflected in the rise in mobility-lifetime product even after 24 hours. Encapsulating the sample in polymethylmethacrylate (PMMA) protects the surface of MAPbI3 from the ambient oxygen. Repetition of the wavelength dependent study using encapsulated samples, resulted in the UV illuminated sample exhibiting no visible degradation and undergoing the LSE. This suggests that oxidation is possibly in competition with the LSE. A study by Chen et al, suggested that the PMMA polymer could interact with the surface lead in MAPbI3[13] causing a passivation of trap states, a result which would result in an increase in ϕƩµτ. Therefore, experimentation using a second encapsulant without oxygen in the polymer is necessary. The chosen polymer, parylene-C, upon illumination under UV light exhibited the LSE, a response which confirms the role of oxidation in the breakdown of MAPbI3. A model of carrier generation in the material is presented to determine the effect of intermittent-light exposure from the chopper wheel on generated carriers, although long computations are resource-intensive and can take many years to perform. With this work, a more detailed understanding of the LSE in OHPs can be obtained.
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