Central fluorination strategy of biphosphonic acid molecule for self-assembled monolayer enables efficient organic solar cells

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a primary hole extraction layer (HEL) employed in most state-of-the-art regular organic solar cells (OSCs). However, the acidic nature of PEDOT:PSS could corrode the indium tin oxide (ITO) electrode under prolonged operation, compromising the long-term stability of the devices. Herein, we have designed and synthesized a novel biphosphonic acid molecule, namely 3BPIC-cF, for self-assembled monolayers (SAMs) using a central fluorination strategy. Compared to PEDOT:PSS, the 3BPIC-cF-modified ITO substrate exhibits enhanced light transmittance and improved interfacial compatibility with the organic active layer. Thanks to the introduction of fluorine atoms, 3BPIC-cF exhibits a larger dipole moment and a deeper highest occupied molecular orbital energy level, leading to an increased work function for the ITO/3BPIC-cF substrate compared to the ITO/PEDOT:PSS substrate. These advantages can enhance hole extraction within the device, decrease interfacial impedance and restrain nonradiative recombination at the interface. Consequently, OSCs utilizing 3BPIC-cF as HEL achieved a high efficiency of 19.34%, surpassing the performance of devices based on PEDOT:PSS (power conversion efficiency (PCE) = 18.64%). Importantly, the OSCs based on 3BPIC-cF exhibit significantly improved stability compared to those using PEDOT:PSS. This research provides valuable insights for the development of functional molecules for SAMs, with the potential to enhance the performance of organic solar cells.