Plasmon induced NIR response of thin-film a-Si:H solar cells

Abstract

The defect absorption in hydrogenated amorphous silicon (a-Si:H) photosensitive devices in the presence of resonant absorbing silver nanoparticles (Ag NPs) is investigated. Defect states are created in the a-Si:H network by the incorporated Ag NPs in their direct environment. The strong electromagnetic fields accompanied by the localized surface plasmon (LSP) resonance of the Ag NPs enable high transition rates between the defect states and the conduction band. This results in an observable signal for near infrared (NIR) photon energies in external quantum efficiency (EQE) measurements. By applying different Ag NP size distributions to the devices the LSP resonance is shifted together with a shift of the EQE peak observed for NIR energies. This indicates that the available defect states in the a-Si:H bandgap are addressed by the LSP resonance. Dominant transitions take place for electrons occupying defect levels having an energetic position equal to the LSP resonance energy. Boron doping of the a-Si:H environment shifts the Fermi level towards or below the introduced defect states. These are depleted and cannot contribute to sub bandgap photon absorption. This is an indication for the consistence of the proposed model for resonant defect absorption of the bandgap defect states.