Hybrid and inorganic plumbo-halide perovskites for solar cells

This thesis explores the hybrid and inorganic lead halide perovskite compounds methylammonium lead iodide (MAPI) and caesium lead iodide-bromide (CsPbI2Br) with regard to their synthesis, stability and use in thin film solar photovoltaic devices. MAPI thin films were prepared using two different approaches: the one-step and the two-step solution processes to determine which gives better films, in terms of physical-chemical properties, environmental stability and device performance. The best device efficiency obtained by devices prepared with MAPI (glass/ITO/TiO2/MAPI/spiroOMeTAD/Au) was of 11.8% PCE, however the device stability toward environmental conditions was poor, showing degradation effects in environments containing UV light combined with oxygen. In order to circumvent the instability issue, the fully inorganic CsPbI2Br compound was studied for direct comparison, as an alternative, more stable perovskite compound. Once the synthesis of CsPbI2Br thin films was optimised, devices were fabricated (glass/ITO/TiO2/CsPbI2Br/spiro-OMeTAD/Au) and measured, giving a champion device PCE of 9.1%. However, while the CsPbI2Br devices were found to present higher environmental stability compared to MAPI devices, they showed high susceptibility towards humid conditions. Unlike MAPI, however, the degradation of CsPbI2Br was caused by phase instability, rather than chemical degradation, which was demonstrated to be reversible. Most devices in this work were fabricated using a superstrate n-i-p device architecture commonly used for perovskite solar cells. In addition, a new device structure was created, with the aim of forming a heterojunction between MAPI and silicon. First the device structure was studied (contact/Si/MAPI/transporting material/contact) using both n- and p-doped silicon wafers, to determining the ideal Ohmic contacts, the nature of MAPI deposited onto silicon and by understanding the electrical behaviours of each interface. Solar cells of this type produced efficiencies up to 2.1% PCE. It was demonstrated that these Si/MAPI heterojunction devices form a single-sided junction in the silicon. Recommendations for improvement are given