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

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.