Enhancing Interfacial Properties via Ionic Exchange for the Highly Efficient Sn-Pb Mixed Perovskite Based Solar Cells

Sn-Pb mixed perovskite is considered as an ideal light absorber not only for single-junction solar cells, but also for bottom cells of muti-junction solar cells due to its narrow-band gap feature. The p-orbital hybridization of Sn²⁺ and Pb²⁺causes an upward shift of the valence band maximum (VBM) and conduction band minimum (CBM) in addition to the bandgap reduction. This results in the highest occupied molecular orbital level of poly[3,4-ethylenedioxythiophene]:poly[styrene sulfonate] (PEDOT:PSS) having an ideal band alignment with the VBM of Sn-Pb mixed perovskite. Therefore, the PEDOT:PSS is the most widely used hole transport layer (HTL) in p-i-n structured Sn-Pb mixed perovskite solar cells (PSCs).<br/><br/>However, PEDOT:PSS showed several critical drawbacks that affected the device performance such as their acidic condition, inferior molecular structure, and nonideal absorption property. Among them, the inappropriate molecular structure composed of hydrophilic PEDOT molecule and hydrophobic PSS molecule does not form a uniform and ideal micro structured surface when it has been coated. Therefore, it has been induced undesirable interface like voids between PEDOT:PSS and perovskite. Various studies have been conducted to obtain better interfacial properties with respect to the perovskite precursor, like adding additives. However, the origin of the undesirable interface was due to the structural limit of the PEDOT:PSS molecule. Therefore, fundamental research is needed to address these issues from the perspective of the PEDOT:PSS surface.<br/><br/>This study introduces an effective ionic exchange for PEDOT:PSS surface modification to drastically enhance the interfacial properties between PEDOT:PSS and perovskite. The surface modified PEDOT:PSS by ionic exchange brought the conformal interfacial contact and enhanced charge extraction confirmed via time-resolved photoluminescence (TRPL) analysis. As a result, we achieved more than 20% efficiency of p-i-n structured Sn-Pb mixed PSCs. The mechanism of ionic exchange also had been elucidated through various surface analysis and density functional theory (DFT) calculations. In addition, we confirmed that this ionic exchange was also effective for tandem devices. In conclusion, we fabricated all-perovskite tandem solar cells with an approaching 24% efficiency through this ionic exchange