Recent advance in multiple exciton generation in semiconductor nanocrystals

Abstract

The multiple exciton generation (MEG), a process in which two or even more electron-hole pairs are created in nanostructured semiconductors by absorbing a single high-energy photon, is fundamentally important in many fields of physics, e.g., nanotechnology and optoelectronic devices. Many high-performance optoelectronic devices can be achieved with MEG where quite an amount of the energy of an absorbed photon in excess of the band gap is used to generate morei additional electron-hole pairs instead of rapidly lost heat. In this review, we present a survey on both the research context and the recent progress in the understanding of MEG. This phenomenon has been experimentally observed in the 0D nanocrystals, such as PbX (X=Se, S, and Te), InX (X=As and P), CdX (X=Se and Te), Si, Ge, and semi-metal quantum dots, which produce the differential quantum efficiency as high as 90%10%. Even more remarkably, experiment advances have made it possible to realize MEG in the one-dimensional (1D) semiconductor nanorods and the two-dimensional (2D) nano-thin films. Theoretically, three different approaches, i.e., the virtual exciton generation approach, the coherent multiexciton mode, and the impact ionization mechanism, have been proposed to explain the MEG effect in semiconductor nanostructures. Experimentally, the MEG has been measured by the ultrafast transient spectroscopy, such as the ultrafast transient absorption, the terahertz ultrafast transient absorption, the transient photoluminescence, and the transient grating technique. It is shown that the properties of nanostructured semiconductors, e.g., the composition, structure and surface of the material, have dramatic effects on the occurrence of MEG. As a matter of fact, it is somewhat hard to experimentally confirm the signature of MEG in nanostructured semiconductors due to two aspects:i) the time scale of the MEG process is very short; ii) the excitation fluence should be extremely low to prevent the multi-excitons from being generated by multiphoton absorption. There are still some controversies with respect to the MEG effect due to the challenge in both the experimental measurement and the explanation of signal data. The successful applications of MEG in practical devices, of which each is composed of the material with lower MEG threshold and higher efficiency, require the extraction of multiple charge carriers before their ultrafast annihilation. Such an extraction can be realized by the ultrafast electron transfer from nanostructured semiconductors to molecular and semiconductor electron acceptors. More recently, an experiment with PbSe quantum dot photoconductor has demonstrated that the multiple charge extraction is even as high as 210%. It is proved that MEG is of applicable significance in optoelectronic devices and in ultra-efficient photovoltaic devices. Although there are still some challenges, the dramatic enhancement of the efficiency of novel optoelectronic devices by the application of MEG can be hopefully realized with the rapid improvement of nanotechnology.