Exploring the Stability of Planar-Architecture Perovskite Solar Cells

Abstract

The aim of this thesis is to study the stability of lead halide perovskites when used in planar (n-i-p) photovoltaic (PV) devices. Since their first use in solar cells, a tremendous improvement has been witnessed in the efficiency of perovskite devices which now compete with established single crystalline silicon technologies. Notably, the low costs associated with the fabrication of perovskite devices from solution makes them a very promising challenger PV technology. For perovskite solar cell manufacture to become commercially viable, device stability needs to be increased towards that of silicon that typically has an operational lifetime in excess of 25 years. This issue is addressed in Chapter 4, where the role of perovskite composition is explored using a device structure based on ITO/SnO2/perovskite/HTL/Au. Devices based on the perovskite MAPbI3 are explored alongside those based on the triple-cation perovskite Cs0.05FA0.79MA0.15Pb I2.45Br0.55. While both types of device have shown power conversion efficiencies (PCE), [16% and 19.5% respectively], it is found that triple-cation perovskites have substantially better structural and optoelectronic properties; a feature that is reflected by enhanced film and device stability. Using optimised triple-cation perovskites, the effect of the hole transport material (HTM) used in PV devices is then explored. Here, devices were fabricated using the HTMs Spiro-OMeTAD and PTAA, and it is found that PTAA devices demonstrate higher thermal and dark- storage stability compared to Spiro-OMeTAD. This result suggests that further investigation of HTMs and other charge-selective interlayers are crucial to create devices that combine both high efficiency and long-term operational stability.

In Chapter 5, the effect of incorporating potassium iodide (KI) into a triple-cation perovskite device is explored. Here, it is shown that this additive (that is often used a defect passivating agent), can in fact reduce film and device stability. This result is explained by a small amount of KOH contained within a SnO2 colloidal solution used to prepare the electron transfer layer (ETL). It is proposed that the KOH negates the benefits resulting from the KI passivation, leading to the formation of an undesirable secondary KBr phase.