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

Abstract

Multi-exciton-generation (MEG) process is critical due to its potential to push solar cell photoconversion efficiencies (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.