PbSe nanorod-quantum dot bulk nano-heterojunction solar cells generating multiple excitons with record photo conversion efficiencies

Multi-exciton-generation (MEG) process is critical due to its potential to push solar cell photoconversion effi-ciencies (PCEs) beyond theoretical limits. However, cells with this feature are rare, as the demonstration of this characteristic property in a working cell is challenging. Herein, a donor-acceptor type bulk nano-heterojunction layer, prepared by blending two nanostructures known to have high MEG yields, i.e., PbSe quantum dot-QDs and PbSe nanorods-NRs, is utilized as the light absorbing active layer. Transmission electron microscopy images show that NR-donors are surrounded by QD-acceptors, indicating the formation of optimal donor-acceptor interfaces that allow efficient dissociation of photo-generated excitons. Effective charge separation across the NR/QD interface is also demonstrated with valence and conduction band offsets measured in the range of 0.20-0.25 eV. It is found that the type of the hole transport layer has a profound effect in reducing the electron back injection and increasing the MEG efficiency/yield. Cells with 0.97 eV NR-donor band gap exhibit a PCE of 4.09% with a peak internal quantum efficiency greater than 100%. Cells, outperforming the previously reported MEG-based cells comprising PbSe QDs or NRs, demonstrate a staircase-like response in external quantum efficiency, which is considered as the most obvious indicator of an ideal MEG system.