Latest JACS from Liu Yongsheng and Chen Yongsheng at Nankai University: Dipole Moment Tuning of Self-Assembled Monolayer Diphosphonic Acid Molecules Boosts Organic Solar Cell Efficiency Beyond 19.7%

PEDOT:PSS has been widely used as a hole-extraction layer in organic solar cells. However, over time, its acidic nature can corrode the ITO electrode, thereby adversely affecting the device’s lifetime. In light of this, on May 8, 2024, Yongsheng Liu and Yongsheng Chen from Nankai University published in JACS a study demonstrating that tuning the dipole moment of self-assembled monolayers of diphosphonic acid molecules can boost the power conversion efficiency of organic solar cells to over 19.7%. They developed a series of self-assembled monolayer (SAM) diphosphonic acid molecules with tunable dipole moments—namely, 3-BPIC(i), 3-BPIC, and 3-BPIC-F—which exhibit progressively increasing dipole moments in that order. Compared with the centrosymmetric 3-BPIC(i), the axially symmetric 3-BPIC and 3-BPIC-F display higher adsorption energies (Eads) on ITO, shorter interfacial spacings, more uniform coverage of the ITO surface, and better interfacial compatibility with the active layer. The introduction of fluorine atoms further deepens the highest occupied molecular orbital (HOMO) energy level and increases the dipole moment of 3-BPIC-F relative to 3-BPIC, leading to a higher work function (WF) at the ITO/3-BPIC-F interface. These advantages of 3-BPIC-F not only enhance hole extraction within the device but also reduce interfacial impedance and suppress non-radiative recombination at the interface. As a result, organic solar cells employing a SAM based on 3-BPIC-F achieved a record-high power conversion efficiency of 19.71%, surpassing those based on 3-BPIC(i) (13.54%) and 3-BPIC (19.34%). Importantly, compared with organic solar cells using PEDOT:PSS as the hole-extraction layer, the 3-BPIC-F–based devices exhibit markedly improved stability. This work provides guidance for the future design of functional SAM molecules aimed at achieving higher performance in organic solar cells.