Research On The Influence From Rare Earth Elements And Related Upconversion Materials On The Performance Of Cu₂ZnSn?S,Se?₄ Solar Cell

With the rapid development of economy,peoples'demand for energy has been keep increasing for the last two decades.The fast consume of fossil energy not only threatens the sustainability of current development,but impacts our environment with inevitable pollution.Other than fossil energy,soalr energy is clean and renewable.As a result,solar cells attract lots of attention for their ability to convert solar energy to electricity without producing any pollution.Among numerous absorber materials,Cu₂ZnSn?S,Se?₄ is considered as one of the most ideal absorber materials for its earth-aboudant constituent elements,suiable band gap and high absorption coefficient.As we know,the top effiency of CZTSSe solar cells has arrived 12.6%since its birth in 1988,but it's still a long to go to be industrialized?at least 15%?,let alone to catch up with its theoretical efficiency?32.8%?.The main factor restricted the improvement of CZTSSe solar cell is the open-circult voltage deficiency from massive anti-site defects.It is one of the most important topics in improving CZTSSe solar cells that finding reasonable soulution to minish the loss in open-circult voltage.It is reported that replacing the Sn in CZTSSe lattice with trivalent elements could depress the deficiency of open-circuit voltage.View that the rare earth element Nd is a stable trivalent element with great photoelectric properties,we deduce that Nd doping could be a promising way to improve open-circuit voltage of CZTSSe solar cells.In addition,the up-conversion materials,prepared with rare earth elements,could improve the devices'photogenerated current with converting the near infrared photons to visible light.Even it have played an important role in many solar cell systems,there is no application in CZTSSe solar cells yet.Therefore,incorporating up-conversion materials is a feasible way to improve the performance of CZTSSe solar cells.Therefore,the research work of this paper mainly focused on the two essential issues.We systematically investigated the feasibility of Nd doping in CZTSSe and the influence from up-conversion NaErF₄@NaYF₄ on CZTSSe devices by combining experiments and first-principles calculations.The results are as follows:1.The formation energy of defects in Nd doping CZTSSe and supplemental forms of Nd were investigated by combining experiments and first-principles calculations.The results confirmed that the Nd _Cu has the lowest formation energy and the formation energy of Nd_Cu is more than 1.5eV.We predicted that the Nd can't be doped in the lattice of CZTSSe from mentioned results.The Nd incorporated CZTSSe thin films were fabricated with sol-gel solution,and their XRD characterization confirmed our prediction.2.TheCZTSSe:NEYF?dopingNaErF₄@NaYF₄inabsorber?and CZTSSe/NEYF?inserting NaErF₄@NaYF₄ in Mo/CZTSSe interface?solar cells were fabricated with sol-gel solution.The devices were characterized by different methods.The results confirmed that doping in absorber is a better applying mode for NaErF₄@NaYF₄ in CZTSSe solar cells.The NaErF₄@NaYF₄ doped in absorber improved devices'J_SC from 28.94mA/cm² to 33.51mA/cm²,enhanced devices'V_OCC from 368mV to 378mV,increased devices'FF from 37.8%to 56.4%,and thus improved devices'efficiency from 4.03%to 7.10%.The mechanisms of influence were further studied by different characterization methods.The influences of NaYF₄ doping on CZTSSe were investigated by combining experiments and first-principles calculations.We fabricated NaYF₄ doped CZTSSe solar cells,together with NaErF₄ doped CZTSSe solar cells and CZTSSe solar cells for reference.The influences from NaErF₄ doping on CZTSSe solar cells were found to be the same as Na doping,while NaY₄ doping could decrease absorbers'hole concentration,broaden depletion area and thus enhance photo-generated current.It is indicated that the main cause for decreasing hole concentration in absorber is Y's passivation to accepter defects in grain boundary,from the defects formation energy calculation of Na and Y doping CZTSe.