Fluorinating Dopant-Free Small-Molecule Hole-Transport Material To Enhance The Photovoltaic Property

For the stability and commercial development of the perovskite solar cells (PVK-SCs), synthesizing high-efficiency dopant-free hole-transport materials (DF-HTMs) and exploring how the DF-HTM structure affects the photovoltaic performance is inevitable. Two small-molecule DF-HTMs based on 2,2'bithiophene as a central part (denoted by BT-MTP and DFBT-MTP) were designed and synthesized. DFBT-MTP, with two more fluorine atoms substituted on the 2,2'-bithiophene group, exhibited enhanced photovoltaic property as DFHTMs, including larger backbone planarity, declining highest occupied molecular orbit (HOMO) energy level, increasing hole transportation, more effective passivation, and efficient charge extraction. With fluorinated DFBT-MTP being applied as DF-HTMs in p-i-n PVK-SCs, an efficiency of 20.2% was achieved, showing similar to 35% efficiency increase compared with the nonfluorinated BT-MTPbased devices. The leading power conversion efficiency (PCE) indicates that the fluorinated compounds should be a promising direction for exploring high-performance DF-HTMs in the p-i-n PVK-SCs. KEYWORDS: fluorinated small-molecules, dopant-free, hole-transport materials