Vibrational Probe of Electrical Doping in N2200 and Fermi Level Alignment at Polymer Cathode/Metal Cocatalyst/Electrolyte Junction

Hybrid (photo-) cathodes consisted of conjugated polymer and hydrogen evolution reaction (HER) co-catalysts are an emerging platform for low-cost solar fuels generation. The electron-accepting Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene)}, known as P(NDI2OD-T2) or N2200, is a promising material to serve as a conjugated polymer cathode or as the electron acceptor for bulk heterojunction photocathodes. Unlike inorganic semiconductor/metal junctions, much less is known about the energetic alignment of the conjugated polymer electrode/metal junctions, hindering rational development of this emerging class of electrodes. In this work, we investigate the electrical doping behavior in N2200 cathode and its Fermi level alignment with gold nanoparticles, which is used here as a model for hydrogen evolution metal cocatalyst. Through UV/visible, Raman and attenuated total-reflectance infrared spectroelectrochemistry, we observed the impact of electrical doping on the vibrational frequencies of neutral, polaron and dianion species in N2200, which suggests electron density changes within the corresponding NDI units. Upon one-electron reduction, C=O stretching frequency of the polaron unit shows a red shift by ~ 68 cm-1, indicating an increased electron density in the C=O bond. Additionally, the C=O stretching frequency of neutral units in the doped N2200 shows a minor red shift of ~ 5 cm-1, suggesting charge transfer from neighboring polaron units. Surface-enhanced Raman spectroscopy measurements of gold nanoparticle-functionalized N2200 electrode revealed that the Au Fermi level only shifts with that of the N2200 upon the polaron formation. This mechanistic study of the electron transfer from doped N2200 to the metal nanoparticles provides insight for the future design of the HER (photo)cathodes – the formal potential of the polymer polaron formation determines the behavior of the catalyst Fermi level and thus modulates the reaction capability.