Performance Of High Efficiency CZTSSe Thin Film Solar Cells Based On Local Electric Field Enhancement

With an urgent requirement of clean renewable energy for future society development,the environmental friendly and low-cost photovoltaic energy sources have attracted much attention and entered into a booming period.Alternatively,the non-toxic kesterite Cu2 Zn Sn(S,Se)4(CZTSSe)material originated from Cu(In,Ga)Se2(CIGS)by substituting In(Ga)with Zn and Sn atoms is particularly interesting due to its abundant element constitutes,high environmental tolerance and long term stability.Currently,solution-processed CZTSSe solar cells achieve higher power conversion efficiencies(PCE)in comparison with the vacuum-processed devices.In 2021,Xin et al.reported a 13% record PCE for CZTSSe solar cells using DMSO system.However,there is still a large efficiency gap for CZTSSe PV from CIGS,and its open circuit voltage(Voc)is about 200 m V lower than that of CIGS.The larger interfacial recombination induced energy losses at Cd S/CZTSSe interface is the main obstacle for the further development of CZTSSe PV.For CZTSSe materials,a large number of Cu Zn antisite defects are generated due to the similar ionic radius and chemical properties of Cu and Zn.Especially,the formation energy of Cu Zn defects gets smaller near p-n junction interface due to the upshift of Fermi level(EF)and its concentration becomes extremely higher than bulk absorber.These surface defects would become recombination centers and have an adverse effect on Voc.After deposition of n-type Cd S buffer layer on top,this p-type micro-area cannot show an effective p-to-n inversion and only a small band bending is formed near p-n junction,even degrading electron extraction.To address these problems,many efforts have been devoted,such as bandgap regulation,heterojunction post-annealing,and ions substitution,to creating electrically benign p-n junction interface,while some unwanted side effects may be induced,making difficulty in practical applications.The fast ions migration during high temperature annealing makes it difficult to confine incorporated ions within surface absorber.The Cd S/CZTSSe interface remains a major obstacle to further improving the efficiency.The plasmon-enhanced local electric field properties of metal nanoparticles have been extensively studied in solution-fabricated solar cells,which is beneficial for the improvement of Voc and short circuit current density(Jsc).Different from the traditional used heterojunction post-annealing and ionssubstitution approaches,the key feature of metal nanoparticles modification is that they can take effects within confined area.In this thesis,the plasmonic local electric field enhancement is introduced into Cd S/CZTSSe interface to improve the charge separation and recombination processes.Different Au NPs modification structures are developed for high quality p-n junction engineering and the effects of local electric field enhancement are further investigated.The main work is divided into the following two parts:Firstly,2D Au@Si O2 NPs array surface modification is developed to improve the electrical properties of Cd S/CZTSSe interface.We successfully incorporated Au@Si O2 core-shell NPs into CZTSSe solar cells through electrostatic assembly of colloidal NPs onto aminosilane modified surface absorber.The electrical properties of the Cd S/CZTSSe interface were tuned by optimizing the concentration of Au@Si O2 solution.Finite time-domain difference simulation(FDTD)demonstrates that the strong near-field enhancement mainly occurs near Cd S/CZTSSe p-n junction interface.Further studies show that the presence of local electric field increases the electrostatic potential(Velec)at the interface and facilitates charge carrier separation and extraction processes.These electrical benefits lead to remarkable improvements in Voc and Jsc,and the PCE of the device is increased from 9.96% to 11.25%.This study opens the door to build plasmonic local electric field enhanced p-n junction interface in future thin film kesterite PV researches.Secondly,Cd S-Au@Cd S-Cd S sandwiched buffer layer structure is developed to improve the electrical properties of Cd S/CZTSSe interface.In our first work,the electrostatic assembled 2D Au@Si O2 NPs array can not realize a continuous closely packed film and the imposed additional local electric field is weak.In this work,a Cd S-Au@Cd S-Cd S sandwiched buffer layer structure is developed to further improve the electrical properties of Cd S/CZTSSe interface through spin-coating a thin layer of Au@Cd S in the middle of Cd S buffer layer.By optimizing the concentration of Au@Cd S NPs,a continuous film was formed.FDTD simulations show fifteenfold increase in electric field strength at the Cd S/CZTSSe interface.Further investigation reveals that the Au@Cd S thin layer leads to remarkable significant improvements in photoconduction and carrier concentration of Cd S buffer layer,thus inducing a widened space charge region on the CZTSSe absorber side and accelerating charge separation.Moreover,the enhanced built-in electric field facilitated Fermi-level EF splitting,thus increasing the Voc.Benefitting from the improved interfacial electric properties,the highest PCE of 11.98% is obtained.This work provides a feasible methodto improve the electric properties of Cd S buffer layer and paves the way for future high efficiency kesterite solar cells researches.