Multiple Exciton Generation In Nano-semiconductor And Its Application

With the energy crisis aggravated and the conception of greendevelopment popularized, solar energy has attracted the world-wideattention currentlly. The widely use of photovoltaic depends on the furtherlowering of the cost of solar cells and increasing of their conversionefficiency. Furthermore, nano-semiconductor material and technology havenot only the core of modern information technology, but also the drivingforce and engine to promote the further development of informationtechnology. In particular the observation of multiple exciton generation(MEG), more than one electron-hole pairs generated in semiconductornanostructures by absorption of one photon, has drawn considerable interestdue to its improvement of quantum dots solar cells efficiency by reducing ofthermal loss in conventional PV. On the other hand, MEG has offeredprospects for the development of advanced optoelectronic devices, such ashigh-sensitivity photodetectors, high-quality optical amplifiers,high-performance lasers, as well as avalanche photodiodes.However, the history of MEG from discovery to date is less than adecade, and the research is still in the primary stage of exploration. Therehas been a lack of consensus on experimental/theoretical results of the MEGeffects. These can be summarized as follows:(i) does the MEG effects occur in semiconductor quantum dots?, i.e., some claimed it does but others said itdoes not, and, if so,(ii) does the MEG efficiency of quantum dots be reallylarger than that of bulk materials? and (iii) is there any universal behavior ofthe MEG threshold energy to explain observed different results for the samequantum dots? And even the effect of MEG on new types of quantum dotsolar cell performance is various from person to person.It is very important to establish a theoretical model for explorationMEG within a large range of nano-semiconductors in order to clarify thecontroversial issues in MEG research field and to explore the application ofMEG on optoelectronic debices. In this thesis, a statistical theory model ofMEG has been created based on the impact ionization, a process throughwhich more than one electron-hole pairs produed by collision and generallyrecognized as the physical mechanism of MEG in semiconductornanostructures, and Fermi statistical theory model about the formation ofmultiple elementary particles through collisions of protons.The band gap Egof semiconductor nanostructures, characteristic time tSof multiple exciton generation and photon energy hν have been used asthree parameters in this statistical model of MEG. Results of relativeprobability of n-pairs generated by one photon, MEG efficiency andthreshold energy can be calculated from this statistical theory model.Thorugh comparison of experimental data and the calculated results in PbSequantum dots, it has been demonstrated that it‘s a simple but effectivestatistical model to explore the MEG effect in semiconductor quantum dots.This simple yet powerful model has been able to resolve perhaps all thenon-consensus on the MEG and its effects in PbSe nanostructures. With themodel, we have showed and concluded that the MEG actually occurs. Also,we have found out a critical radius Rc(～9nm), with which we have been able to conclude that the MEG efficiency of quantum dots is smaller thanthat of bulks if R <Rc, but it is larger if R> Rc. Moreover, we have obtainedan MEG threshold energy as a function of the quantum dot radius, seeminglyshowing a universal behavior of the MEG threshold in semiconductor PbSematerials.We have also carried out a detailed investigation of the MEG efficiencyand threshold energy in various Si quantum dots with a photonenergy-dependent characteristic time tS(EP/Eg). We have shown that theMEG effect in Si quantum dots can improve only~1%of the powerconversion efficiency for single stage solar cells as a result of large energyband gap (>1.15eV) and high threshold energy (ET~2.2-3.1Eg), however,MEG in Si quantum dots is likely more prospective for its application inultraviolet detectors due to the high internal quantum efficiency under shortincident light...