Tailoring The Optoelectronic Properties Of Copper Gallium Chalcogenide-based Core/shell Quantum Dots For Solar Energy Conversion Applications

The progress in the strategies for tuning the optoelectronic properties of semiconductors promotes the development of electronic science and technology.The recently emerging semiconductor colloidal quantum dots(CQDs),which possess excellent tunable optoelectronic properties due to quantum confinement effect,have been widely applied in optoelectronic energy conversion applications.The development of eco-friendly CQDs is a major trend in promoting the commercialization of high-performance CQDs-based optoelectronic energy conversion devices.Among them,CuGaS2(CGS)QDs,as a member of the eco-friendly I-III-VI QDs,have large absorption coefficients and excellent component/size/morphology-dependent tunable optoelectronic properties.This making them potential semiconductor materials in solar-driven optoelectronic energy conversion technologies.However,CGS QDs contains abundant surface defects/trap states,which serve as non-radiative recombination centers,leading to the recombination of photo-generated carriers on their surfaces.This phenomenon impede the transfer and radiative recombination of photo-generated carriers,thereby negatively affecting their optoelectronic properties such as photogenerated exciton lifetime,photoluminescence quantum yield(PLQY),and photo/chemical stability.In addition,the wide band gap of CGS QDs(～2.48 e V)is not suitable for the utilization of photons in the long wavelength band of the solar spectrum.Aiming at the above intrinsic drawbacks of CGS QDs,this thesis takes CGS QDs as the main material and conducts in-depth research on their optoelectronic properties via strategies such as the construction of a core/shell structure,optimization of shell thickness,alloying,design of band alignment,and regulation of donor-acceptor radiation recombination(DAP)related to Cu defect states.Through these effective optimization engineering strategies,CGS-based core/shell QDs with outstanding optoelectronic properties such as effective passivation of surface defects/trapped states,efficient photo-exciton separation,tunable band structure,high PLQY and good photo/chemical stability have been achieved.According to their specific properties,corresponding photoelectrochemical(PEC)cell and luminescent solar concentrator(LSC)were prepared,and a solar-driven tandem LSC self-powered hydrogen generation module of CGS based core/shell QDs was realized.The research contents are as follows:1.The shell passivation effect.The eco-friendly CGS/ZnSe core/shell QDs with controllable shell thickness were synthesized via a facile two-step method by introducing inorganic ZnSe shell.UV-vis spectra shows that with the increase of ZnSe shell thickness,the absorption of CGS/ZnSe core/shell QDs at 300-550 nm can be enhanced due to the absorption of ZnSe shell.The surface defects/trapped states of CGS QDs can be passivated by the ZnSe shell,therefore the non-radiative recombination centers are effectively reduced,and the PL of CGS/ZnSe core/shell QDs is successfully induced.Moreover,there is an optimal relationship between the shell thickness and the PL lifetime of CGS/ZnSe core/shell QDs.An appropriate shell thickness can effectively extend their photo-generated exciton lifetime,which is conducive to charge transfer and separation.However,due to the lattice mismatch between CGS QDs and ZnSe shell,excessively thick shell layers will introduce new defect states at their interfaces,resulting the loss of optical properties.Subsequently,CGS/ZnSe core/shell QDs sensitized PEC cells were prepared.Through ultraviolet photoelectron spectroscopy analysis,indicating the formation of type II band alignment between Ti O2 and CGS/ZnSe core/shell QDs,which is beneficial for improving the electron transmission efficiency of the PEC cell.Due to the passivation effect of the ZnSe shell,the charge transfer separation efficiency is improved.Therefore,the CGS/ZnSe core/shell QDs sensitized PEC cell with the best passivation effect exhibits optimal hydrogen generation performance.Under AM 1.5 G irradiation(100 m W/cm~2)the best devices can achieve a saturated photocurrent density of 3.5 m A/cm~2,and the Faraday efficiency can reach up to 56.1%.2.To further improve the PLQY of CGS-based QDs,an Al alloying strategy and the introduction of a large band gap(3.7 e V)ZnS shell was applied to treat CGS QDs,and alloyed CuGaAl S(GGAS)/ZnS core/shell QDs were prepared by via one-pot synthesis method.The UV-vis and PL spectra show that by adjusting the Cu component ratio,the band gap can be effectively controlled to engineer the absorption and PL range.By analyzing the PL spectra,it was found that CGAS/ZnS core/shell QDs have two major radiation recombination channels,and the Cu defect-related DAP recombination can be regulated by adjusting the Cu component ratio.Further analysis found that as the Cu component decreased,the ratio of Cu defect related DAP recombination gradually increased,resulting in improvement of PLQY.However,excessive DAP recombination centers will produce new defect states that lead to unfavorable non-radiative recombination(Auger recombination),thereby reducing PLQY.The optimized CGAS/ZnS QDs achieved a PLQY of up to 91%,which is nearly twice as high as that of CGS/ZnS core/shell QDs,demonstrating that alloying can effectively reduce the lattice stress on the CGAS surface and reduce related defect states.The optimized sample also exhibits an ultra-long photo-generated carrier lifetime(1.2μs),large Stokes-shift(0.81e V)and long-term photo stability.Due to these excellent properties,the CGAS/ZnS QDs were applied to LSC(with an area of 100 cm~2).Under one standard solar irradiation(100m W/cm~2),the device achieves power conversion efficiency of 4.29%and exhibits excellent thermal stability at a high temperature of 40℃.3.For improving CGS-based QDs’utilization of solar radiation spectrum,In element was introduced to form CuGaIn S(CGIS)/ZnS core/shell QDs via hot injection method to adjust the band gap.Based on optical characterization analysis,it was found that the introduction of In effectively reduced the band gap of CGS.Compared to CGAS/ZnS core/shell QDs(absorption range:UV-500 nm),CGIS/ZnS core/shell QDs expand the absorption range to 700 nm.CGIS/ZnS core/shell QDs also possess Cu defect states related to DAP recombination properties.The optimized CGIS/ZnS core/shell QDs with the widest absorption range exhibit up to high PLQY of 50%and large Stokes-shifts of0.50 e V.Moreover,a large-area tandem LSC(with an area of 144 cm~2)based on CGAS/ZnS and CGIS/ZnS core/shell QDs was prepared.Through device transmission spectrum analysis shows that CGIS/ZnS core/shell QDs-based LSC can effectively utilize solar radiation transmitted through CGAS/ZnS core/shell QDs-based LSC,thereby improving the utilization efficiency of the overall tandem LSC for solar spectrum.Under simulated natural light irradiation(45 m W/cm~2),the tandem LSC achieve overall power conversion efficiency of 0.87%.Subsequently,the tandem LSC was combined with the CGS/ZnSe PEC cell to achieve a solar-driven tandem self-powered hydrogen generation module,which effectively verified the significant potential and commercial prospects of CGS-based core/shell QDs in optoelectronic energy conversion applications.