Morphology Controlling And Interface Optimization Of Superstrate-Typed CZTS Solar Cells

At present,with the Chinese major strategic goal of "carbon peaking in 2030 and carbon neutrality in 2060" proposed,more and more sights are pouring into the new energy source development.With that in mind,the photovoltaics,as one major branch of new energy technologies,have also received tremendous attention.It can be predicted that the photovoltaics will be seeing dramatical development in the next few decades,and particularly developing new generation of high-efficiency and low-cost solar cells is becoming a hot topic.Due to their abundant reserves and non-toxic of constituent elements,suitable bandgap（1.5 eV）,high light absorption coefficient（>104 cm-1）,no light-induced degradation,and low fabrication cost,copper-zinc-tin-sulfur（Cu₂ZnSnS₄,CZTS）thin film solar cells,are deemed to a promising new type of solar cells.In this paper,a novel superstrate-type of CZTS solar cells using transparent conductive glass and carbon as electrodes has been designed.Compared with the conventional substratetyped CZTS solar cells,the superstrate-typed CZTS solar cells can be directly fabricated on commercial transparent conductive glass,and thus the fabrication cost can be reduced.The CZTS light absorbing films have been prepared by sol-gel method and sequential sulfidation annealing.The morphologies of CZTS films were regulated by adjusting the concentration of monoethanolamine in the CZTS precursors,and thus the high quality CZTS films could be obtained;meanwhile,the CZTS/electrode interfaces were optimized by designing electron transport layer（ETL）and hole transport layer（HTL）to enhance the carrier extraction capability.TiO₂，In₂S₃,and SnO₂ films had been explored for the ETL of CZTS solar cells and ultimately TiO₂ was determined as the ETL;the bipolar semiconductor CsPbBr₃ perovskite,which exhibits excellent environmental stability and hole transport capability,was employed as the HTL,providing the experimental basis for fabricating superstrate-typed CZTS solar cells.The main research contents and results are as follows:（1）Using copper acetate monohydrate,zinc acetate dihydrate,stannous chloride dihydrate,thiourea and ethylene glycol methyl ether as metal source,sulfur source and solvent,respectively,CZTS sol was successfully prepared.The particle size of CZTS nanoparticles in the precursor solution was controlled by adding different volume ratios of monoethanolamine（0%,2.5%,5%,7.5%,10%）.By testing it by ultraviolet-visible spectroscopy,it was found that the light absorption intensity of the CZTS precursor solution increased with the increase of the concentration of monoethanolamine.（2）The CZTS films were prepared by sol-gel method and sequential sulfidation annealing.The grain size,surface morphology and phase structure were characterized by XRD and SEM.The device with the structure of FTO/TiO₂/CZTS/carbon was prepared,and the optoelectronic properties of the device derived from different volume ratios of monoethanolamine were studied.It was found that with the optimal volume concentration of monoethanolamine of 5%,the CZTS films with appropriate grain size,pure phase and smooth surface morphology could be successfully prepared.The as-prepared film was proved to be kesterite structure with preferential orientation of（112）crystal planes,and the optical band gap was determined to be 1.501 eV.The CZTS solar cell achieved the highest photovoltaic conversion efficiency（PCE）of 0.54%,with open circuit voltage（Voc）,short-circuit current density（Jsc）,and fill factor（FF）of 0.38 V,5.41 mA/cm²,and 26%,respectively.（3）TiP₂,In₂S₃ and SnO₂ were selected as ETLs to prepare CZTS solar cells.Through the photoelectric performance test,it was found that In₂S₃ and SnO₂ failed to show efficiency except TiO₂,therefore TiO₂ was determined to be the optimal ETL.（4）The perovskite CsPbBr₃ HTLs were prepared by a multi-step solution method using PbBr₂-DMF and CsBr-methanol solutions.The effect of PbBr₂ solution concentrations and CsBr solution spin-coating cycles on the CsPbBr₃ nucleation,crystal growth,and film thickness were investigated.It was found that when the PbBr₂ solution concentration was 0.25 M and the CsBr layer was spin-coated twice,the CsPbBr₃ HTL showed better crystallinity,uniform grains,and smooth surface without holes and cracks.By comparing the photovoltaic performance of FTO/TiO₂/CZTS/CsPbBr₃/C and FTO/TiO₂/CZTS/C devices,it was found that by adding the CsPbBr₃ HTL,the Voc of the whole device was increased from 0.23 V to 0.27 V,Jsc from 3.02 mA/cm² to 3.90 mA/cm²,the PCE from 0.18%to 0.28%,indicating that the CsPbBr₃ HTL greatly improves the CZTS/C interface contact.