New light harvesting architecture towards high performance solid-state mesoscopic perovskite solar cells

Abstract

Solar energy has been considered as very promising in term of a potential cost effective and environmentally friendly technology for energy production. Perovskite solar cells (PSCs) are an exciting new type of solar cell technology that has attracted enormous attention recently. Hence, this study aimed to examine the usage of a new film formation method to control morphology of the perovskite photoactive layer in order to improve the understanding of PSC. Here, the concern is on determining how structured perovskite film with high performance and stability can be achieved by tuning the morphology. In this study, experiments that entail the role of perovskites morphology on the performance and stability of PSCs were conducted. In preparing the novel perovskite solar cell used, this study emphasised a combination of the one-step (1-s) and two-step (2-s) deposition methods in order to obtain the new and versatile 1 & 2-s method. It was found that the 1 & 2-s deposition method was able to produce CH3NH3PbI3 (MAPI) films that have tuneable morphologies, crystal sizes and optoelectronic properties in relation with the conventional 1-s and 2-s film methods. By our method, the final morphology could be systematically changed between the extremes of the 1-s and 2-s morphologies. For example, the key SEM experiments revealed that controlling the PbI2 to MAI composition could lead to tuneable morphology. Higher composition of PbI2 lead to more nucleation site to be formed in which determine the final grain size and morphology. Furthermore, the study showed that the morphology could significantly affect the device properties. The use of the new combined method produces MAPI films that have improved light scattering, stronger absorption and enhanced device performance compared to films prepared using the conventional 1-s or 2-s methods. This study also extended the new combination method by introducing the co-solvent blends and examined how the co-solvent composition affects the properties of the final films. A comparison of the optoelectronic properties, stability and device characteristics of the perovskite absorbing layer between the well-established conventional depositions methods (1-s and 2-s) and the new combined method 1 & 2-s was also conducted. In this regard, an unexpected improvement in thermal and device stability of the MAPI films prepared using the 1 & 2-s method is found to be related with the amount of Br adding to the system. Furthermore, an improved PCE and enhanced stability was achieved by adequately adding Br through this new 1 & 2-s method. The study also investigated how the presence of anion (halide) affects the optoelectronic and device properties of CH3NH3PbI3-xBrx, films and device properties. The study identifies the mechanisms that explain the decoupling of exchange and growth in perovskite-forming stage for these films. This mixed-halide study gave devices with the highest power conversion efficiency of about 13%. From the study, we can conclude that the new 1 & 2-s method is transferable and can be used and improved in other perovskite for future applications in order to obtain a high performance PSCs.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Brian Saunders (Supervisor) & Paul O'Brien (Supervisor)