Investigations on the Stability Issues of Organic-Inorganic Hybrid Perovskite Solar Cells

Abstract

Perovskite solar cells (PSCs) have attracted significant attention in the scientific community due to a fast achievement in efficiencies reaching over 20 % within a few years of development. Despite this incredible enhancement, commercialization of PSCs is still a challenge owing to their rapid degradation. Thus, it is inevitable to understand the degradation mechanisms in PSCs to improve their stability. Methylammonium lead iodide (CH3NH3PbI3) is the most widely studied perovskite structure. This dissertation focuses on identifying the underlying causes related to the degradation of CH3NH3PbI3 based PSC devices and discusses possible ways to improve their stability. This thesis can be divided into four sections. Firstly, we analyzed interfacial interactions and chemical changes directly from complete PSC devices by employing laser desorption/ionization mass spectrometry (LDI-MS). A very prudent and systematic approach was undertaken for the very first time in case of PSCs to compare LDI-MS spectra from fresh and aged devices. We were able to trace molecular interactions and some prominent degradation products within PSC devices. Highlights of the observed transformations are corrosion of metal electrode, decomposition of the metal oxide layer, incorporation of oxygen atoms into perovskite, and formation of charge transfer complex between perovskite with the hole transport layer. LDI-MS method is a soft ionization technique. Thus, it has already become an inevitable analytical tool in life science research. Our results suggest that this method has the potential to make a revolution in the field of solar cell studies as well. Subsequently, we conducted a methodical study by using First-principles calculation within DFT formalism to understand how the choice of different metal oxide layers affect the charge transfer process as well as the stability of CH3NH3PbI3 (MAPI) perovskites. We compared the structure and electronic properties of MAPI based on TiO2, ZnO, and SnO2. Our calculation reveals the decomposition of methylammonium molecules by losing protons at MAPI/TiO2 and MAPI/ZnO interfaces. Bader charge analysis indicates the lowest charge transfer for interfaces of MAPI with ZnO. Although TiO2 based metal oxides contribute to more efficient charge transfer, still SnO2 was found to be a feasible choice in order to balance between efficiency and long-term stability of PSC devices. In the third part, we evaluated the effect of the electron transport layer (ETL) passivation on the stability of PSC devices. Devices were based on SnO2 electron transport layer. The behaviour of PSC devices was compared for three different ETL passivation layers using Indene C60 bisadduct (ICBA), Phenyl-C61-butyric acid methyl ester (PC61BM),Poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) respectively. Further investigations on devices with or without passivation layers revealed notable improvement in stability and efficiency of PSCs upon incorporating ETL passivation layer. Thin film characterizations indicated how might the passivation layer positively influence the performance of PSC devices. ICBA passivated SnO2 ETL showed the best PCE of 16.7%, retaining around 84% of the initial efficiency after being stored in ambient condition for ten days without any encapsulation. Finally, we studied the influence of CH3NH3PbI3 passivation on the device stability of PSCs. A systematic comparison was conducted among devices based on three different perovskite passivation layers named Poly(9-vinylcarbazole) (PVK), Polyvinylpyrrolidone (PVP) and Poly(3-hexylthiophene-2,5-diyl) (P3HT). Our results showed significant improvement in the stability of perovskite passivated PSCs compared with devices without passivation. Additional thin film characterizations explored the beneficial effect of passivation on perovskites. The highest efficiency of 14.88 % was obtained from unencapsulated PVK passivated devices demonstrating around 89.9 % of the original efficiency even after 16 days of exposure in ambient condition.