4-tert-Butylpyridine (4-tBP) is an organic compound with the molecular formula C9H13N. It is a substituted pyridine derivative featuring a bulky tert-butyl group at the 4-position. This structure imparts it with strong basicity and steric hindrance, making it a non-coordinating organic base. Its most prominent and critical application is as a key additive in the fabrication of perovskite solar cells (PSCs). It is primarily used in the hole-transporting layer (HTL), often in conjunction with spiro-OMeTAD. 4-tBP functions as a Lewis base additive that passivates surface defects and traps on the perovskite layer, particularly by interacting with uncoordinated Pb²⁺ ions. This passivation significantly reduces charge recombination, improves the open-circuit voltage (Voc), and enhances the overall power conversion efficiency and long-term stability of the solar cell device. Beyond its role in photovoltaics, 4-tert-butylpyridine serves as a versatile organic base and building block in synthetic organic chemistry. Its sterically hindered nature makes it useful in reactions where a strong, non-nucleophilic base is required. It is also employed as a ligand precursor and finds use in the synthesis of more complex heterocyclic compounds and coordination complexes.