Bulk and surface engineering to improve performance and stability of perovskite solar cells

Human society is craving new and renewable energy sources. Indeed, the need for novel sources is an obligation in order to cope with the exponentially increasing global energy demand. Also, environmentally friendly alternatives to fossil fuels are more crucial than ever in order to limit the effects of global warming. Finally, the current gas and oil crisis underlies the importance for each state to have some alternatives in case of shortage, i.e. to gain some energetic independence. In that regard, solar energy represents the best candidate, given its huge potential and its universal character. In particular, perovskite solar cells (PSCs) show the highest promise amongst emerging technologies. From their first report in 2009, the power conversion efficiency rocketed, to cross 25 % recently. Unfortunately, several important issues remain.Chapter 2. Its lower bandgap makes formamidinium lead iodide (FAPbI3) a more suitable candidate for single-junction solar cells than pure methylammonium lead iodide (MAPbI3). However, its structural and thermodynamic stability is improved by introducing a significant amount of MA and bromide, both of which increase the bandgap and amplify trade-off between the photocurrent and photovoltage. Here, we simultaneously stabilized FAPbI3 into a cubic lattice and minimized the formation of photoinactive phases, such as hexagonal FAPbI3 and PbI2, by introducing 5% MAPbBr3, as revealed by synchrotron X-ray scattering. We were able to stabilize the composition (FA0.95MA0.05Cs0.05)Pb(I0.95Br0.05)3, which exhibits a minimal trade-off between the photocurrent and photovoltage. This material shows low energetic disorder and improved charge-carrier dynamics as revealed by photothermal deflection spectroscopy (PDS) and transient absorption spectroscopy (TAS), respectively. This allowed the fabrication of operationally stable perovskite solar cells yielding reproducible efficiencies approaching 22%.Chapter 3. Herein, we successfully engineered a multi-cation halide composition of perovskite solar cells via the two-step sequential deposition method. Strikingly we find that adding mixtures of 1D polymorphs of orthorhombic d-RbPbI3 and d-CsPbI3 to the PbI2 precursor solution induces the formation of porous mesostructured hexagonal films. This porosity greatly facilitates the heterogeneous nucleation and the penetration of FA/MA cations within the PbI2 film. Thus, the subsequent conversion of PbI2 into the desired multication cubic a-structure by exposing it to a solution of formamidinium methylammonium halides is greatly enhanced. During the conversion step, the d-CsPbI3 also is fully integrated into the 3-D mixed cation perovskite lattice, which exhibits high crystallinity and superior optoelectronic properties. The champion device shows a power conversion efficiency (PCE) over 22%, with an open-circuit voltage of 1.15V, a short-circuit photocurrent of 24.82 mA·cm-2, and a fill factor of 78% at a bandgap of 1.55 eV. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining more than 90% of its initial value of PCE under 1-Sun illumination with maximum power point tracking for 400 h.Chapter 4. A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA+) and formamidinium (FA+) as the monovalent cat