Study On Preparation And Performance Of Ferroelectric Depolarization Field-enhanced CIGS Solar Cells

Among the thin-film solar cells,copper indium gallium selenide（CIGS）solar cells have drawn lots of attention due to the advantages of bandgap tunability for absorption layer,high efficiency,prolonged life,economically feasible and so on.However,it is difficult to further improve the photoelectric conversion efficiency（PCE）of CIGS solar cells due to the limitations of material preparation or device structure.In recent years,the photovoltaic effect in ferroelectric materials has attracted more attention.The charge separation mechanism of ferroelectric photovoltaic effect is different from that on pn-junction,enabling the open circuit voltage(V_OC)of ferroelectric photovoltaic devices to exceed the limitations of bandgap for ferroelectric materials.In order to improve the PCE of CIGS solar cells,this work proposes integrating ferroelectric materials into CIGS solar cells for promoting the separation of photogenerated carriers by coupling a ferroelectric depolarization field with a heterojunction built-in electric field.The main research content and results of this paper are as follows:1.We prepared ZnO thin film as the window layer of CIGS solar cells by pulsed laser deposition,and then a transparent top electrode ITO thin film was deposited on it.We fabricated ITO/ZnO/CIGS/Mo solar cells.The effects of growth temperature,deposition oxygen pressure and deposition time for the ZnO layer on the photovoltaic performance of the device were systematically studied.The results showed that the preparation process of ZnO thin films had a significant impact on the efficiency of solar cells.Firstly,we fixed the deposition oxygen pressure and deposition time of ZnO thin films,and changed their growth temperatures.The results showed that with the increase of the growth temperature for the window layer,the PCE of the device first increased and then decreased.When the growth temperature of ZnO thin films was 100℃,the performance of the device was the best（PCE=6.92%）.Fixing the growth temperature and deposition time of ZnO thin films,we further studied the effect of the deposition oxygen pressure of ZnO thin films on device performance.When the deposition oxygen pressure of ZnO thin films was 5 Pa,the overall performance of the device was the best.Finally,we fixed the growth temperature and deposition oxygen pressure of ZnO thin films,and changed the deposition time of ZnO thin films.The results showed that as the deposition time of ZnO thin films increased,the J_SC and PCE of the device first increased and then decreased.When the thickness of ZnO thin films was about 200 nm,the device had the best performance(J_SC=75.39 m A/cm²,V_OC=0.54 V,FF=35.59%,PCE=14.49%).2.We selected BaTiO₃（BTO）nanoparticles as the ferroelectric layer to prepare ITO/ZnO/BTO/CIGS/Mo devices by spin-coating method,and the effects of the preparation conditions for BTO ferroelectric layer on the performance of the device were optimized such as heat treatment time,solvent type and concentration of BTO nanoparticles solution.The results showed that as the heat treatment time of the BTO layer increased,the device exhibited higher PCE with dispersed by EGMME.It was indicating that the residual organic solvent would affect the performance of the device.Subsequently,we prepare the BTO ferroelectric layer by using ethanol as a solvent.And the performance of the relevant devices was better than that of devices prepared using other solvents The influence of the dosage of BTO nanoparticles on the performance of the CIGS solar cells was systematically investigated.The efficiency of the device decreased as the solution concentration increased.But,the performance of ITO/ZnO/BTO/CIGS/Mo devices were controlled by the polarization state of BTO films.After polarizing the BTO ferroelectric layer,it was found that when the direction of ferroelectric depolarization field inside the ferroelectric layer was the same as the direction of the heterojunction built-in electric field,the J_SC and PCE of the device increased with the polling voltage value increased.When the concentration of solution was 2 mg/m L,the devices exhibited the best performance.3.In order to improve the performance of the device,we further inserted a Cd S buffer layer between the ZnO and CIGS layer.And we prepared Au/ITO/ZnO/Cd S/CIGS/Mo solar cells and Au/ITO/ZnO/BTO/Cd S/CIGS/Mo solar cells.Then,the effect of the particle size for the BTO ferroelectric layer on the performance of Au/ITO/ZnO/BTO/Cd S/CIGS/Mo solar cells was systematically studied.The results showed that when the BTO ferroelectric layer with a smaller particle size of 17 nm,the performance of the solar cell was the best.In addition,the performance of Au/ITO/ZnO/BTO/Cd S/CIGS/Mo solar cells could be controlled by controlling the polarization state of the BTO layer.Compared with the unpolarized state(J_SC=35.96m A/cm²,PCE=15.14%),when the BTO layer was in the upward poling state,the total electric field enhanced.More conductive to enhance the separation of photogenerated carriers and inhibit their recombination which significantly improving the performance of the device(J_SC=37.84 m A/cm²,PCE=15.75%).