Stepwise Injection of Energetic Electrons and Holes in Plasmon-Induced Charge Separation

Although there are many mechanistic studies for plasmon-induced charge separation (PICS), most of them have been devoted to energetic electron injection (or hot electron injection) from plasmonic metal nanoparticles into electron transport materials (ETMs) including n-type semiconductors. Here we also studied energetic hole injection (or hot hole injection) into five different organic hole transport materials (HTMs) with different ionization energies, by using solid-state photovoltaic cells fabricated by introducing gold nanoparticles (AuNPs) in between TiO₂, which is an ETM, and a HTM. As a result, photocurrents based on PICS are obtained even if the height of the energy barrier at the Au–HTM interface is 0.9 eV. Therefore, the present PICS processes involve both energetic electron injection from AuNPs to TiO₂ and hole injection from AuNPs to the HTMs. In this case, simultaneous electron–hole injection, in which one electron–hole pair gives simultaneous electron injection into TiO₂ and hole injection into a HTM, is often postulated. However, there is another possibility of stepwise electron–hole injection, in which one electron–hole pair leads to electron injection into TiO₂ and another pair gives hole injection into a HTM. The stepwise injection process is accompanied by recombination of the residual hole of the former pair and the electron of the latter. The present cells generated the photocurrents even if the incident photon energy was lower than the sum of the TiO₂–Au and Au–HTM barrier energies (≤2.1 eV). In addition, experimentally obtained photocurrent action spectra of the cells can be reproduced on the basis of plasmonic light absorption and theoretically analyzed carrier injection efficiencies, when the stepwise injection is assumed. It was therefore concluded that the stepwise carrier injection chiefly contributes to the present PICS processes.