A Study of the Effects of Thermal Annealing/Pressure and stretching on Performance of Organic & Hybrid Perovskite Solar Cells

Oyewole, Deborah O.

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A Study of the Effects of Thermal Annealing/Pressure and stretching on Performance of Organic & Hybrid Perovskite Solar Cells

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There has been increasing interest in improving the performance characteristics (power conversion efficiencies, charge transport, and light absorption) of organic and perovskite solar cells to levels that are comparable to, or beyond those of the conventional silicon solar cells. There is also an interest in the development of stretchable or flexible solar cells for applications in homes and a wide range of portable devices. This thesis explores the effects of processing parameters (annealing temperature and pressure) that can be used to enhance the performance characteristics (PCEs, light absorption and charge transport) of organic and hybrid perovskite solar cells. The failure mechanisms of stretchable organic solar cells are also elucidated using a combination of models and experiments. First, a combined experimental and theoretical approach is used to study the effects of pressure and thermal annealing on the PCEs of organic solar cells (OSCs). The PCEs of the OSCs increased from ~2.3% (for the unannealed devices) to ~3.7% for devices annealed at ~150oC. Further increase in thermal annealing temperatures (beyond 150oC) resulted in lower PCEs. Further improvements in the PCEs (from ~3.7% to ~ 5.4%) were observed with pressure application between 0 - 8 MPa. However, a decrease in PCEs was observed for pressure application beyond 8 MPa. The improved performance associated with thermal annealing is attributed to changes in active layer microstructure and texture, which also enhance the optical absorption, mobility and lifetime of the optically-excited charge carriers. The beneficial effects of applied pressure are attributed to the increased interfacial surface contacts that are associated with pressure application. Subsequently, a systematic study of the effects of annealing and pressure is carried out on formamidinium iodide perovskite solar cells. Annealing is shown to promote the interdiffusion of constituent elements in efficient formamidinium-based (FAI-based) PSCs, while the application of pressure is shown to promote the compaction of microvoids and the closure of cracks within the layers and the interfaces in the FAI-based PSCs. These changes are shown to be associated with improvements in PCEs and solar cell stability. Annealing-induced interdiffusion is shown to result in significant changes in layer microstructure, local surface strains, and changes in the optoelectronic properties of the layers in the FAI-based PSCs. We also observe a dramatic upward diffusion of tin (Sn) and titanium (Ti) from fluorine-doped tin oxide (FTO), and Titanium dioxide (TiO2), into the perovskite films. In contrast, a downward diffusion of lead (Pb) and iodine (I), is observed from the perovskite films into the mesoporous layer of the electron transport layer (ETL), after annealing at temperatures between 100 and 150oC. The diffused I substitutes for Ti in the ETL, which improves the optoelectronic properties of the films, for annealing temperatures between 100oC and 130oC. The annealing-induced interdiffusion that occurs at higher temperatures (between 140 – 150oC) results in higher levels of interdiffusion, along with increased local strains that lead to the nucleation of pores and cracks. For the devices processed at 130oC, a range of mechanical pressures was applied to reduce the sizes of pre-existing grain-boundary voids and interfacial cracks within the devices. This resulted in enhanced PCEs, for applied pressures between ~ (0 – 7) MPa. Unlaminated device stability increased by 33%, falling to 80% of initial PCE in 1800 hrs without compression, as compared to 2400 hrs with compression. Finally, the underlying failure mechanisms associated with stretchable organic solar cells (SOSCs) are elucidated for deformation and cracking under monotonic and cyclic loading. The associated changes in the optical transmittance of the anodic layer and the PCEs of the multilayered SOSC structures are also elucidated as functions of loading. An increase in the transmittance of the anodic layer is observed as strain is applied to flatten the wrinkled structures. This enhances the power conversion efficiencies of the SOSCs, as the strains increase from 0% to 32%. However, beyond this initial strain regime, the onset of overstretching decreases the optical transmittance and photoconversion efficiencies. The fatigue lifetimes of the layered SOSCs also decrease with increasing fatigue strain ranges between 10% and 25%. The decrease in the fatigue lifetimes is associated with a higher incidence of cracking and delamination. The implications of the results are discussed for the design of efficient and stable rigid/ stretchable organic and hybrid perovskite solar cells.

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