Multi exciton generation and recombination of semiconductor nanocrystals : fundamental understanding and applications

Abstract

Semiconductor nanocrystal quantum dots (QDs) have been found to be very promising for important application areas in optoelectronics and photonics. Their energy band-gap tunability, high performance band-edge emission, decent temperature stabilities, and easy material processing make the QDs attractive for these applications ranging from photovoltaic devices to photodetectors and lasers to light-emitting diodes. For these QDs, the concepts of multi exciton generation (MEG) and recombination (MER) have recently been shown to be important especially because they possibly enable efficiency levels exceeding unity using these QDs in various device configurations. However, understanding multi exciton kinetics in QD solids has been hindered by the confusion of MER with the recombination of carriers in charged QDs. This understanding lacks to date and the spectral-temporal aspects of MER still remain unresolved in solid QD ensembles. In this thesis, we reveal the spectral-temporal behavior of biexcitons (BXs) in the presence of photocharging using near-unity quantum yield core/shell CdSe/CdS QDs. The spectral behavior of BXs and that of excitons (Xs) were obtained for the QD samples with different core sizes, exhibiting the strength-tunability of the X-X interaction energy in these QDs. The extraction of spectrally resolved X, BX, and trion kinetics, which would be spectrally unresolved using conventional approaches, is enabled by our approach introducing the integrated time-resolved fluorescence. Besides the fundamental understanding of MEG and MER concepts, we also explored the possibility of utilizing multi excitons in these QDs for optical gain. In this part of the thesis, tunable, high performance, two-photon absorption (TPA) based amplified spontaneous emission (ASE) from the same QDs is presented. Here, for the first time, in addition to the absolute spectral tuning of the ASE, on the single material system of CdSe/CdS, the relative spectral tuning of ASE peak with respect to spontaneous emission was demonstrated. With the core and shell size adjustments, it was shown that Coulombic X-X interactions can be tuned to be either attractive leading to the red-shifted ASE peak or repulsive leading to the blue-shifted ASE peak and that non-shifting ASE can be achieved with the right core-shell combinations. It was further found here that it is possible to obtain ASE at a specific wavelength from both Type-I-like and Type-II-like CdSe/CdS QDs. In addition to the CdSe/CdS QDs, we showed ASE and Type-tunability features on CdSe/CdS nanorods (NRs), which are particularly promising with their extremely high TPA cross-sections and independent emission/absorption tunabilities. In the final part of the thesis, we report the observation of MEG on CdHgTe QDs, for the first time in the literature, and a novel application of MEG concept in a photosensor device, one of the first examples of real-life photosensing application of MEG concept. We believe that the results provided in this thesis do not only contribute to the fundamental understanding of MEG and MER concepts in the QDs, but also pave the way for the utilization of these concepts in the QD-based lasers, photodetectors and photovoltaic devices.