Fabrication And Characterization Of CdS/Sb₂Se₃ And CdS/Sb₂S₃ Heterojunction Thin Film Solar Cells

Till now fossil energy is still the main source of energy,however,the overuse of fossil fuels makes energy scarce and brings about serious environmental pollution with the development of modern industrialization.It is urgent to explore new and renewable energy sources.As a clean and renewable energy source,the utilization of solar energy can effectively alleviate the problem of energy shortage.Solar cells is a device use photovoltaic effect to convert light directly into electricity.The conversion efficiency of silicon solar cells is high and mature in large-scale production.Although less cost of CdTe compound solar cells is regarded,it has environmental problems for its highly toxic of Cd.GaAs is expensive,and usually applied to aeronautical area.CIGS solar cell has a scarce of In element.In recent year,the perovskite solar cell has been rapidly developed,but it is limited due to its poor environmental friendliness and stability.People are still exploring new materials and novel structure solar cells to reach the higher efficiency and better stability.Sb₂Se₃ and Sb₂S₃ materials are attracting people’s attention because of their simple structure,rich content of elemental crust,almost non-toxicity,proper optical band gap and good charge transport properties.In this paper,based on the traditional CIGS and CdTe thin-film solar cell structures,the absorption layers of CdS/Sb₂Se₃ and CdS/Sb₂S₃heterojunction thin-film solar cells were prepared by thermal evaporation method and magnetron sputtering method,and their materials and device characteristics were also studied.The comprehensive analysis and results are as follows.Firstly,the Se content decreasing as well as deduced the internal defects and interface recombination on the evaporated Sb₂Se₃ thin film is the main problems in our study.We improved the interface by Se treatment of Mo substrate.The effect of substrate temperature on the growth direction of the film was studied.The diffraction peaks of the samples deposited at 200°C and 250°C were very weak,while the peaks of（221）were appearsed at280°C,300°C and 330°C.As the substrate temperature rised to 350°C,the（221）peak became weaken and the（120）peak was increased.The best solar cell was obtained at 330°C due to its better grain boundary and higher carrier transport capability,which reduced the composite loss and promoted carrier transport.A MoSe₂ layer was formed by Se treatment on Mo substrate which was benefit to improve the Sb₂Se₃/Mo interface and reduce the transport barriers of the carriers.The optimized conditions for Se treatment was at 620°C in 20 minutes.After the Se treatment,the（hk1）orientation of Sb₂Se₃ thin film and the carrier transport ability were both enhanced,resulting in the increase of J_SC.MoSe₂ thin film also changed the contact between Sb₂Se₃ and Mo substrate which affected the collection of carriers,leading an increase of V_OC and FF.The Sb₂Se₃ film deposited on the MoSe₂ layer had a higher carrier concentration,indicating that the formation of the MoSe₂ layer can improve the quality of the Sb₂Se₃ film.As a result,the CdS/Sb₂Se₃ heterojunction thin film solar cell with a maximum efficiency of 4.25%was achieved.Secondly,hetetrojunction CdS/Sb₂S₃ device structure had better stability than mesoscopic device structure,we prepared hetetrojunction CdS/Sb₂S₃ thin film solar cells on CdS/FTO substrates at different substrate temperatures.As the reference,the films were also prepared directly on the glass.The preferred orientation of the Sb₂S₃ thin film on the glass substrate was（hk0）,while the one on the CdS/FTO substrate was（hk1）.The texture coefficient（TC）was calculated indicating the（Sb₄S₆）_n band was likely grown in parallel on the glass substrate,while the（Sb₄S₆）n band on the CdS/FTO substrate was favorable in vertical growth mode.The Sb₂S₃ thin film began to be crystallized above 280°C.The best crystallization temperature was 320°C.By Raman analysis,it was found that higher laser intensity induced oxidization and phase change of films.The Sb₂S₃ films prepared at higher temperatures（280°C,300°C,320°C,and 350°C）had a band gap of 1.7 eV to 1.8 eV.S conent deficiency was found at all temperatures.The highest fluorescence lifetime of the film was at320°C from TRPL test compared with other temperatures.A maximum conversion of 3.01%is obtained.After stored in air for three months without packaging,the efficiency was slightly decreased of 0.2%（from 3.01%to 2.84%）.The QE test showed that the applied bias voltage has less influence on the cell prepared at 320°C,indicating that the internal recombination was small.These defects can be solved by doping,post-processing,element replacement,etc.to increase device efficiency in the futher.Finally,due to the magnetron sputtering method has higher purity and better uniformity than the thermal evaporation method,we studied Sb₂Se₃ films prepared by magnetron sputtering under different pressures.The grain orientation increased with increasing pressure.The films deposited at 0.3 Pa and 2 Pa exhibited a preferred orientation of（hk0）,while a preferred orientation of（hk1）at 5 Pa,8 Pa,and 10 Pa,indicating that formation of（hk1）orientation was suppressed on lower pressure and be enhanced at higher pressures,respectively.All films prepared at differernt pressures exhibited Se-loss after depositing and post annealing,indicating the presence of varying degrees of defects in the films.The photocurrent response of the sample under low pressure（0.3Pa and 2Pa）was poor.The influence of the applied bias voltage was small at 8Pa,and the best CdS/Sb₂Se₃ heterojunction thin film solar cell was obtained with efficiency of 0.79%.