Carbazole Core-Based Small Molecule as an Effective 4- <i>tert</i> -Butylpyridine Replacement for Perovskite Solar Cells

4-tert-Butylpyridine (TBP), commonly employed in spiro-OMeTAD-based hole transport layers (HTLs), suffers from volatility and triggers lithium salt precipitation and perovskite degradation, compromising device stability. To address these issues, we develop a solid small-molecule additive CN-CZ to take the place of TBP. Featuring an extended π-conjugated framework and an electron-rich pyridine moiety, CN-CZ exhibits stronger Li+ coordination, significantly enhancing Li-TFSI solubility in chlorobenzene and improving HTL film quality. Furthermore, CN-CZ optimizes the energy level alignment, facilitating efficient hole extraction and transport. Correspondingly, the resulting perovskite solar cell achieves a power conversion efficiency (PCE) of 25.8%, surpassing that of the TBP-based reference device (23.8%) while demonstrating markedly enhanced long-term stability.