Electronegativity‐Guided Molecular Passivation and Bridging for the Enhanced Performance of Carbon‐Based Hole‐Transport‐Layer‐Free CsPbI ₂ Br Solar Cells

Carbon-based hole-transport-layer (HTL)-free CsPbI₂Br solar cells well balance power conversion efficiency (PCE), stability, and cost, but suffer from defects including undercoordinated Pb²⁺ and mobile I^- in CsPbI₂Br, and undercoordinated Sn⁴⁺ and oxygen vacancies (V_O) in the SnO₂ electron transport layers. To address these issues, biphenyl oxyacid additives including [1, 1'-biphenyl]-4, 4'-diphosphonic acid (BDPA), [1, 1'-biphenyl]-4, 4'-dicarboxylic acid, and [1, 1'-biphenyl]-4, 4'-disulfonic acid are investigated. It is found that the para-positioned oxyacid double bonds can coordinate with uncoordinated Pb²⁺ to form stable Pb─O bonds, while hydroxyls can anchor mobile I^- via H-bonding. The opposing oxyacid double bonds can bind with uncoordinated Sn⁴⁺ to form stable Sn─O bonds, thus inhibiting V_O formation. Concurrently, the symmetric oxyacid groups bridge the SnO₂ and CsPbI₂Br layers via coordination, thus enabling the biphenyl structure to function as an electron transport channel. Moreover, the additives increase the CsPbI₂Br grain dimensions alongside enhanced surface density and reduced roughness. BDPA exhibits superior passivation efficacy due to the reduced electronegativity of its central phosphorus atom, strengthening oxygen coordination capability. Consequently, the BDPA-optimized device delivers a leading PCE of 15.55%, ≈24% increment over 11.80% for the control device, as well as the improved operational stability and reduced current-voltage hysteresis.