The perovskite/Cu(In,Ga)Se₂ (CIGS) tandem architecture (theoretical efficiency >40%) combines flexibility and lightweight advantages, making it ideal for wearable devices and building-integrated photovoltaics. However, current efficiencies (highest 26.7%) remain far lag behind theoretical limits. The primary bottleneck lies in fabricating wide-bandgap (1.6–1.75eV) perovskite top cells: mixed-halide compositions complicate crystallization, inducing grain defects, open-circuit voltage loss, and halide phase segregation, ultimately degrading device performance.

Surface defect passivation in wide-bandgap perovskites (WBG PSCs) is critical for improving photovoltaic performance. Current strategies using phenethylammonium (PEA⁺) and similar aromatic ammonium passivators, combined with Me-4PACz self-assembled monolayers (SAMs) as hole transport layers, have pushed perovskite/CIGS tandem efficiencies to 24.2%. Yet significant limitations persist: open-circuit voltage only 1.77V, fill factor 71.2%, mainly due to (1) incomplete passivation (single-target/weak adsorption defects), (2) impeded charge transport (resistive passivation layers/electronic structure defects), and (3) dynamic instability (photothermal stress-induced passivator desorption). These factors collectively prevent devices from reaching theoretical efficiency and lifetime expectations.

Professors Chen Qi and Jiang Yan from Beijing Institute of Technology and their team demonstrated that the widely used passivator PEA⁺ desorbs under photothermal stress, leading to passivation failure. They introduced a bulk aromatic ammonium cation passivator (4-(2-aminoethyl)benzenesulfonyl fluoride) with carefully engineered functional groups and high electric dipole moment. This passivator achieves multi-site defect passivation independent of perovskite termination. Crucially, strong multi-site bonding suppresses passivator desorption, enhances resistance to photothermal stress, and effectively mitigates phase segregation in WBG perovskites. Additionally, the resulting perovskite films exhibit no low-conductivity products (e.g., low-dimensional phases) and form surface electronic structures favorable for charge transport. The resulting p–i–n WBG (1.68eV) PSC achieved a champion PCE of 23.50% with negligible degradation after 1,000 hours of continuous 1-sun illumination. The corresponding perovskite/CIGS tandem solar cell delivered a steady-state PCE of 27.93% (certified 27.35%) and retained initial efficiency after 420 hours of maximum power point tracking under continuous illumination at ~38°C.

The paper, titled "Inhibiting defect passivation failure in perovskite for perovskite/Cu(In,Ga)Se₂ monolithic tandem solar cells with certified efficiency 27.35%," was published in Nature Energy.

Developed a robust passivator (TAR 3): A novel bulk passivator, 4-(2-aminoethyl)benzenesulfonyl fluoride (TAR 3), with high dipole moment and tailored functional groups, solving desorption issues common with conventional passivators like PEA.

Suppressed passivator desorption and enhanced photothermal stability: Strong multi-site bonding of TAR 3 effectively prevents desorption and significantly improves perovskite resistance to combined light and heat stress, a marked improvement over PEA-induced passivation failure.

Effectively mitigated phase segregation and ion migration: TAR 3 significantly alleviates phase segregation in wide-bandgap perovskites and suppresses ion migration, both critical for device stability.

Achieved high PCE and long-term stability: TAR 3 enabled WBG PSC PCE of 23.50% (negligible degradation after 1,000h) and perovskite/CIGS tandem certified PCE of 27.35% (steady-state 27.93%), stable for over 420 hours.

This work demonstrates that aromatic ammonium cations with engineered functional groups (sulfonyl fluoride) can suppress passivation failure, enable multi-site defect passivation, and facilitate carrier transport in WBG perovskites. This pushes VOC and FF of 1.68eV perovskites near theoretical limits, significantly boosting efficiency of WBG PSCs (>23%) and perovskite/CIGS tandems (certified PCE >27%). Most importantly, TAR 3-treated films eliminate phase segregation even under 200-sun illumination, ensuring stable long-term operation of both single-junction and tandem devices under continuous photothermal stress. These findings highlight the potential of this passivation strategy in advancing highly durable and efficient perovskite-based photovoltaics.