Multiple Exciton Effects In Nano-semiconductor PbS And Their Applications

Energy depletion crisis and the gradual deterioration of the environment has brought a opportunit for the development of clean energy and solar energy has received extensive attention currentlly.The important issue in the current solar cell is how to improve its energy conversion efficiency,and even break the theoretical limit of the traditional single-junction solar cells.On the other hand,with the development of electrification of society,the importance of fabricating highperformance electronic components has gradually emerged.The recently found multiple exciton generation in nanosized semiconductor systems will provide a new approach to enhance the efficiency of nanocrystalline solar cells and to fabricate the new generation optoelectronic devices with high performance.Although the multiple exciton generation has been only a decade or so since its discovery,a large number of studies have been published.In this paper,multiple exciton generation,including the research progress,the generation in different dimensions,the experimental and theoretical explanations,and the application of optoelectronic devices,is reviewed in detail.And its application prospect is also prospected.At present,although the ?-? semiconductor PbS has many advantages,such as easier to prepare and more stable with the direct narrow bandgap?0.4 eV?,and many applications in remote sensing and environmentalmonitoring,there are few studies on the multiple exciton generation in PbS nanocrystals.In this paper,the multiple exciton generation in PbS quantum dots and the enhancement of solar cell efficiency is explored in detail.The multiple exciton generation in the nano-PbS was calculated by using the statistical model.The results show that the efficiency I_QE of multiple exciton generation in PbS quantum dots is affected by the energy hv of photon absorbed and the diameter d?or band gap E_g?of quantum dots.The efficiency I_QE increases with the increase of photon energy hv,and the "step" character is found on the I_QE curve which is the most characteristic pattern of multiple exciton generation.And I_QE first increases and then decreases with the increasing of the quantum dot diameter d.The threshold energy Eth of multiple exciton generation in PbS quantum dot is controlled by it's diameter d,which is varied from 4.00E_g?for d=0.5 nm?to 2.35E_g?for d=8.0 nm with t_S=50 fs?or2.50 E_g?for d=5.2 nm with t_S =150 fs?.The effect of multiple exciton generation on the efficiency of PbS nanocrystal solar cells was explored by the improved detailed balance theory.The results show that the ultimate efficiency u?E_g?and the energy conversion efficiency ? of PbS quantum dot with large diameter has enhancedby multiple exciton generation.The multiple exciton generation can increase the maximum energy conversion efficiency of the PbS nanocrystal solar cells from 49.0% to 52.5%,with the optimum diameter changed from 4.4 nm?E_g = 1.12 eV?to 11.6 nm?E_g = 0.56 eV?.And the "ideal" multiple exciton generation will even increase the maximum conversion efficiency to 84.9%.However,the effect of multiple exciton generation on the conversion efficiency of nano-PbS semiconductor solar cell with small diameter?or large E_g?is not obvious.