Role of ions and interfaces for efficient and stable perovskite solar cells

Perovskite Solar Cells (PSCs) have grabbed global attention of the researchers due to their outstanding Photovoltaic (PV) performance. PSCs have the potential to be the future of the PV technology as they can generate power with performance being comparable with the leading Silicon solar cells, with the cost being lower than Silicon solar cells. The enormous potential of PSCs is evident from the fact that the efficiency of these cells has risen from 3.8% to 25.5% within a decade, and it is continuously rising to date. In my thesis, I investigate the impact of ions in the bulk of perovskite and also look at the ETL-perovskite interface to create a simpler, more efficient, more reproducible and more stable system for PSCs. At first, I managed to achieve high efficiency devices with the standard triple cation perovskite on both planar and mesoporous n-i-p ETL device architecture. Adding KI to the already established compositions showed a boost in performance of the solar cells while significantly reducing hysteresis. We then performed a systematic study of KI on various perovskite compositions comprised of FA, MA, Cs, Rb, I and Br ions. We demonstrated that KI reacts with the bromide ions in perovskite passivating the grains leading to a red shift hence and improvement in current density as well as efficiency. MAPbBr3 is generally used is small proportions along with FAPbI3 to stabilise the phase of FAPbI3. But the effect of Br- ions on the perovskite performance was relatively unknown. The introduction of Br- ions in the lattice causes a blue shift in the band gap pushing it further away from the ideal Shockley-Quessier limit. On top of this, the effect of MAPbBr3 on phase stability of FAPbI3 is similar to the effect of CsPbI3. This led me to examine the impact of reduced bromine concentration of the device performance. Interestingly, reducing the bromine concentration not only leads to a red shift in band gap, hence increasing the current density but also causes a reduced voltage loss resulting in much superior performance. Moreover, the Br- ions cause increased halide diffusion in the bulk leading to decreased stability with higher the bromide concentration. While perovskite composition plays an important role in defining the device characteristics, it could be argued that the interfaces play an equally important role in doing the same. With the rise in usage of SnO2 as viable substitute for TiO2 as ETL, I explore the effect of using TiO2 particles that exhibit similar properties to the SnO2 particles. These smaller size TiO2 nanoparticles can form a very thin, compact, uniform and pin-hole free layer with a morphology that follows the morphology of FTO. This results in a reduced recombination in the interface of TiO2 and perovskite causing the open circuit voltage and hence the device performance to improve significantly.