Antimony Selenide Thin Film Solar Cells:Studies On N-type Buffer Layer And Back Surface Field

New inorganic compound semiconductors like hybrid halide perovskite and copper zinc tin sulfoselenide thin film solar cells have recently gained a wide range of research attention.Compared with the above-mentioned star photovoltaic materials,antimony selenide(Sb2Se3)not only consists of relatively environment friendly chemical elements,but also has the advantagest of appropriate band gap(1.17 eV),large absorption coefficient(105 cm-1),simple composition(binary compound with single phase),cheap and non-toxic raw materials,low crystal growth temperature,thus are very promising for thin-film solar cell application.According to the theory of Shockley-Queisser calculation,the efficiency of single-cell Sb2Se3 thin film solar cells can reach above 30%.This thesis focuses on the N-type buffer layer and the back field of Sb2Se3 photovoltaics where the Sb2Se3 absorption layer was prepared by rapid thermal evaporation(RTE).The main contents of this thesis include:In our previous study on Sb2Se3 thin film photovoltaics,the N-type CdS buffer is synthesized by chemical bath deposition.The surface of CdS film contains impurities,thereby would introduce a lot of defects on the CdS surface.We developed an ambient CdCl2 thermal treatment to improve the quality of CdS buffer layer and consequently the heterojunction quality between CdS buffer layer and Sb2Se3 absorption layer.We compared the quality factor(A),dark saturation current(J0)and performance data statistics of devices with and without treatment,and studied surface roughness(AFM),X-ray photoelectron spectroscopy(XPS)and transmittance of CdS thin film with and without treatment.It is concluded that the introduction of additional oxygen and chlorine is advantageous to the CdS surface defects passivation.Finally,the devices using ambient CdCl2 thermal treatment got a certified efficiency of 5.6%.Sb2Se3 film has low doping density(1013 cm-3)in our CdS/Sb2Se3 solar cells,and low doping density would lead to a lower device open circuit voltage and built-in electric field.In view of this,we proposed to add a hole transport layer(HTL)at the back of Sb2Se3 photovotlacs.Through the SCAPS software simulation,the basic parameters of the HTL were determined.We thus chose PbS quantum dots as the HTL,and by optimizing the dot size and film preparation process,device performance was significantly improved to a certified efficiency of 6.5%.The HTL not only reduced the barrier and recombination loss at the back of Sb2Se3 solar cells,but also improved film uniformity of the device.We also tried non-toxic Cu-Sb-S nanocrystals instead of PbS quantum dot as the HTL,and found that Cu-Sb-S nanocrystals s can also greatly increase the Sb2Se3 device performance.We further explored the Cu(NO3)2 treatment of the Sb2Se3 film and observed substantial efficiency improvement.So far,the best superstrate Sb2Se3 thin film solar cell rely on N-type CdS as the buffer layer,but the CdS film are poisonous.We thus explored non-toxic zinc oxide(ZnO)to replace the toxic CdS layer.ZnO thin films were fabricatd by spray pyrolysis of zinc nitrate aqueous solution.Through a series of spaying process optimization,high quality ZnO thin films were obtained.We found that the orientation of ZnO thin films could be regulated by spraying process,and[221]oriented Sb2Se3 film could be induced by randomly oriented ZnO thin films,which was confirmed by XRD and TEM characterization.Sb2Se3 solar cell with randomly oriented ZnO showed low interface defects and high device efficiency,which was interpreted by an interfacial atom model and confirmed by device physics characterization.Through a series of process optimization and cleaning of Sb2Se3 thin film,a fully non-toxic ZnO/Sb2Se3 cell with a certified efficiency of 5.93%was obtained.As-prepared solar cell exhibits excellent damp-heat,light-soaking,UV light and thermal cycling stability,nearly fully passing the IEC61646 industrial standard.We attributed the improved device stability to suppressed interfacial diffusion and weak absorption of ZnO through HADDF-EDX characterization and first principle simulation,as well as monochromatic light illumination study.We believe our work summaried in this thesis has almost solved the problem of buffer layer toxicity and device stability,and promoted the further development of Sb2Se3 thin film solar cells.