Perovskite-based tandem solar cells, with their compelling performance and cost-effective fabrication, stand at the forefront of photovoltaic technologies. Nevertheless, reduction in energy loss and enhancements in robustness of the front wide-bandgap perovskite sub-cell remain crucial, aiming for an efficient and stable perovskite-based tandem device. Surface passivation using large organic spacer cations on the top of perovskite films is a promising strategy to address the issues mentioned above, as it can effectively diminish the defect density. Simultaneously, the introduction of low-dimensional phases into perovskites has been demonstrated to decrease the minority concentration at the interface, effectively reducing nonradiative recombination and mitigating the mismatch between the open-circuit voltage (V_OC) and the quasi-Fermi level splitting (QFLS), particularly for wide-bandgap-based perovskites.[1-4] In some other instances, the enhancement in VOC following surface passivation was attributed to the dipole moment of specific spacer cations that did not give rise to low-dimensional phases.[5,6] Moreover, the reduction in energy offsets of conduction bands (CBs) between the perovskite layer and the transporting layers also directly results in a V_OC enhancement.[6,7] Uniquely separating passivation from energy alignment remains challenging, especially for multifunctional cations that can form 2D phases but also generate a dipole moment. Therefore, to achieve further enhancements in V_OC and fill factor (FF) for wide bandgap perovskite sub-cells, it is essential to develop a comprehensive understanding of the underlying mechanisms.

As it will be discussed at the conference, the impacts of surface treatments with guanidinium bromide (GABr), 4-fluorophenylammonium iodide (F-PEAI) and their mixture on the formation of low-dimensional phases, device performances, as well as underlying loss mechanisms. Based on a hybrid experimental (QFLS) - theoretical (drift-diffusion simulation) approach, it reveals that the reductions in surface recombination velocity and energy level offsets between perovskite and electron transporting layer are the major contributors to a record V_OC × FF production for 1.80 eV perovskite solar cell, which exhibited notably reproducibility transferring to another laboratory. After integrating with a narrow-bandgap perovskite rear cell, we demonstrated an efficient all-perovskite tandem solar cell with stable output of 27.2% under maximum power point.