Efficient Bifacial Passivation with Crosslinked Thioctic Acid for High-Performance Methylammonium Lead Iodide Perovskite Solar Cells

Defects, inevitably produced within bulk and at perovskite-transport layer interfaces (PTLIs), are detrimental to power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). It is demonstrated that a crosslinkable organic small molecule thioctic acid (TA), which can simultaneously be chemically anchored to the surface of TiO2 and methylammonium lead iodide (MAPbI(3)) through coordination effects and then in situ crosslinked to form a robust continuous polymer (Poly(TA)) network after thermal treatment, can be introduced into PSCs as a new bifacial passivation agent for greatly passivating the defects. It is also discovered that Poly(TA) can additionally enhance the charge extraction efficiency and the water-resisting and light-resisting abilities of perovskite film. These newly discovered features of Poly(TA) make PSCs herein achieve among the best PCE of 20.4% ever reported for MAPbI(3) with negligible hysteresis, along with much enhanced ultraviolet, air, and operational stabilities. Density functional theory calculations reveal that the passivation of MAPbI(3) bulk and PTLIs by Poly(TA) occurs through the interaction of functional groups (-COOH, -C-S) in Poly(TA) with under-coordinated Pb2+ in MAPbI(3) and Ti4+ in TiO2, which is supported by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.