• Interface energetics critically govern charge transport, recombination, and device performance in organic and perovskite solar cells.
• Unmatched energy level alignment at interfaces leads to injection or extraction barriers, causing thermionic loss and charge recombination loss, affecting Voc and FF.
• Charge accumulation due to poor energetics can induce hysteresis and stability issues in perovskite solar cells.
• Charge transport layers (CTLs) must be solvent resistant, thermally stable, and thickness tolerant for roll-to-roll fabrication, with requirements differing between OSCs and PSCs.