Stability studies of organic-inorganic hybrid solar cells

Organic-inorganic hybrid solar cells such as dye-sensitized solar cells (DSSCs) have reached photovoltaic conversion efficiency of 12.3%.¹ Their immediate successor, perovskite solar cells (PSCs), have reached an unprecedented efficiency of 17.9% in 2014, from 3.81% in 2009 ², making them competitive with thin-film silicon based PV technology. However, the most significant concern for the organic-inorganic solar cells is their long-term instability, especially at high temperature, above ca. 60 °C, and in contact with moisture present in the air. Their application will be limited unless they have credited stability performance. Improving the stability of organic-inorganic solar cells and understanding their degradation mechanisms is essential for their commercialization and wide-spread use. In the thesis, the stability and degradation mechanisms of two types of organic-inorganic solar cells: flexible dye-sensitized solar cells (DSSCs) built on ITO/PEN substrates and perovskite solar cells (PSCs) were investigated. The first chapter was a general introduction to the organic-inorganic hybrid solar cells, especially dye-sensitized solar cells and perovskite solar cells, such as the development history, structure and operating principles of the cells and the research objectives of the studies. In Chapter 2, literatures regarding the stability of DSSCs and PSCs were thoroughly reviewed and findings of their degradation mechanisms were summarized, which also pointed out research gaps and critical stability issues that still need to be understood and improved. In Chapter 3, all the experimental details are specified, including the chemicals, materials, device fabrication and characterization techniques, stability testing conditions and data analysis. In Chapter 4, the characteristics of flexible DSSCs with the photoanode produced using cold isostatic pressing technique were studied using electrochemical impedance spectroscopy (EIS). The data obtained were analyzed using a transmission line model of DSSCs, which pointed out that the pressing technique may improve short term performance of the devices, but the long-term performance of devices made on plastic substrates deteriorated with aging as a result of reduced TiO₂ particle-particle and/or particle-conductive substrate contacts. The results obtained from Chapter 4 gave rise to the inspiration of the studies in Chapter 5, where the mechanical and electrochemical stability of compressed photoanodes (TiO₂/ITO/PEN) and counter electrodes (Pt/ITO/PEN) in three different ionic liquid (IL) based electrolyte environment were studied using mechanical bending machine, cyclic voltammetry (CV), EIS, X-ray photoelectron spectroscopy and Inductively coupled plasma mass spectrometry. The results showed that the presence of Li⁺ ions and H₂O molecules in the electrolyte were detrimental to the stability of devices. Li⁺ ions were responsible to the detachment of Pt particles from the Pt/ITO/PEN electrode and the increase in the recombination resistance of compressed TiO₂ films, explaining the gradual increase of Voc with aging observed in chapter 4. The presence of water molecules in the electrolyte can cause degradation of the electrolyte species, forming iodate compounds, and also serious degradation of the ITO substrate. The most stable outcome could be obtained with Li⁺ ions and H₂0 free electrolyte. Efficient and stable flexible DSSCs that take advantage of the negligible vapor pressure of IL electrolytes, with minimal sacrifice of photovoltaic conversion efficiency, were successfully produced and demonstrated in Chapter 6 using an ALD deposited blocking layer of 12 nm and an around 1 µm thick TiO₂ film, sensitized with hydrophobic dye (MK-2). Following the success of making more stable DSSCs, a much improved stability of the perovskite solar cells was demonstrated in Chapter 7 by using a sophisticated sealing method that effectively reduced the attack of water from the environment. The cells showed outstanding stability at 50% humidity and one sun illumination over 16 temperature cycling tests (350 hours) at cell temperatures of 10 to 100 °C, followed by 500 hours constant cell temperature testing at 85 °C. Also, the degradation mechanisms were revealed using post-mortem analysis of these cells using XRD and cross-sectional SEM analysis. We drew some final conclusions in Chapter 8 together with our outlooks in the future research and development of more stable DSSCs and PSCs