Role of Interfacial Potential Drops on Redox‐Couple Dependent Voltages Using Hybridized Si(111)–(Bis)Anthracene Photoelectrodes

Abstract We investigate the flat band voltage ( V FB ) of silicon (Si) surfaces functionalized with methyl (Me), 9‐anthracene (Anth), 1,8‐anthracene (DiAnth; two attachment points), and 9‐bianthracene (BiAnth) on n ‐type and p ‐type Si substrates. Flat band potential ( E FB , by Mott‐Schottky) provided V FB (or V BI ) dependent on the contacted redox couple ( E Redox ). On p ‐type Si, V FB increased linearly until a limiting value was reached; similarly, the n ‐type Si V FB decreased linearly until it plateaued at more negative potentials. Notably, the slope of V FB depended on the surface modifier, exhibiting opposite trends for p ‐type and n ‐type Si. Curiously, anthracene‐functionalized p ‐Si exhibited an unexpectedly more shallow (and beneficial) slope than ‐methyl, attributed to the polarizability of the anthracene π electron cloud and a potential drop across the molecular interface. On n ‐type Si, anthracene‐functionalized surfaces displayed a higher slope than ‐methyl, suggesting a gradual cancellation of the voltage shift effect due to a fixed surface dipole. We also quantified the interfacial potential drop across p‐Si–Anth as 275 mV using variable frequency (10 kHz vs 1 kHz) Mott‐Schottky analysis. The interfacial potential drop and dipoles that result from molecular functionalization are thus critical design parameters for PEC cells that utilize moderate‐potential redox couples or reactions; however, such effects are negligible with redox couples that reside at or beyond the semiconductor band‐edge.