Influence Of Illumination On Exciton Dynamics In CdSe Coupled Quantum Dot Thin Films

Solar cell, after more than half a century of development, has entered the thirdgeneration of nanostructure research stage. Because the external quantum yield ofquantum dot solar cells can be more than100%and thus draw a widespread attention.However, studies on the active layer of quantum dot solar cell are not deep enough,especially for the exciton dynamics in coupled quantum dot films which underillumination. In fact, it is meaningful to investigate the process of exciton separation,surface trapping, exciton annihilation, carrier migration and space charge distributionunder optical radiation, which is beneficial for improving the efficiency of solar cells.In this paper, we focus on the processes of exciton dynamics in CdSe coupled quantumdot multilayer films and the effects of light on the exciton dynamics, which providingtheoretical and experimental basis for improving the photoelectric conversionefficiency of solar cells. The corresponding works as follows:Firstly, we prepare two different kinds of CdSe coupled quantum dot film samples,one with a single size and the other with two different sizes mixed. Then wecharacterize the absorption and emission spectra of the samples and compare with thespectra of corresponding solution, which confirmed the film qualities.Secondly, we investigate the mechanisms that the effect of illumination on theexciton dynamics in single size CdSe coupled quantum dot films. We put the samplesunder picosecond laser for prolonged exposure, at the same time we monitor thefluorescence spectrum of the samples. Therefore, we obtain the changes offluorescence spectra with illumination time. Namely, the spectra shift graduallytowards blue with the illumination time, while the fluorescence intensity increases untilstabilized. However, we do not observe this phenomenon in corresponding quantumdot solution. In order to understand the mechanism of the spectral changes in CdSecoupled quantum dot films that under illumination, we analyzed the transient decayproperties of fluorescence. The results indicate that the evolution of exciton processesoccurred in CdSe coupled quantum dot films have three distinct relaxation time scalesand with different center wavelength. According to the results, we explain this phenomenon as follows: due to the space charge accumulation that caused by surfacetrapping, eventually leading to the spectral line shift towards blue and the fluorescenceintensity enhancement, consequently the generation of charged exciton resulted in thatCdSe coupled quantum dots having different center wavelength of fluorescence.Moveover, we study the influence of illumination on exciton dynamic processesfor CdSe coupled quantum dot films with two different sizes. We compare thefluorescence of the films with the corresponding solution and also analyze the transientdecay processes of the two kinds of films, we conclude that the F rster resonanceenergy transfer process occurred in the two different sizes mixed CdSe sample and theenergy transfer direction is from the small size quantum dots to the larger ones. Bycomparing ofFRET efficiency before and after long time illumination, we concludethat due to space charge accumulation,the F rster resonance energy transfer is blockedand lead to lower FRET efficiency.