Preparation Of Cu₃BiS₃ Thin Films By Solution Method And Its Photovoltaic Applications

Ternary chalcogenide semiconductor Cu₃BiS₃ has been considered as a potential candidate of solar absorber materials in recent years,due to the low-toxic and earth-abundant component elements,and excellent optoelectronic properties.Its direct optical band gap(?1.5 e V)is close to the ideal value for efficient solar energy conversion.It possesses high optical absorption coefficients in the visible region(?>10⁴ cm^-1),which enables full absorption of light within a thin absorber layer.Moreover,Cu3BiS3 film can be obtained under relatively low temperature(200?300?).Nevertheless,sporadic investigations on Cu₃BiS₃ solar cell devices have been reported.There are lots of pending issues in aspects of the fabrication of high-quality film,analysis of basic material properties,structure design and manufacture of photovoltaic device.In this work,the process of fabricating Cu3BiS3 film based on solution spin-coating method was investigated.The effect of parameters of each functional layer in solar cell on the device performance was discussed in detail.In order to further improve the quality of Cu3BiS3 film and the performance of photovoltaic device,the strategy of Ag⁺doping was adopted.The main results are summarized as follows:1.One-step solution spin-coating method based on dimethyl sulfoxide(DMSO)and thiourea(TU)was used to prepare Cu₃BiS₃ thin films.The effects of annealing temperature and TU dosage on the structural,optical and electrical properties of Cu₃BiS₃ film were systematically discerned.The results show that the film of highest quality can be obtained at temperatures within 260?280?.As increasing the concentration of TU in precursor solution,the average grain size of obtained film exhibits the trend of increase followed by decrease.For TU concentration of 3.0 M,the film shows largest average grain size up to 0.98?m.All films have strong absorption in visible region.Particularly,the absorption coefficient(?)can reach up to 10⁵ cm^-1 for films prepared with TU concentration lower than 3.5 M.The optical band gap(1.40?1.68 e V)of Cu₃BiS₃ film fabricated by precursor solution with TU concentration in the range of 1.5 M to 4.0 M,is close to the optimum value for efficient solar energy conversion.The highest carrier mobility of 5.86 cm²/V·s appears at the TU concentration of 3.0 M.2.Based on the device structure of Mo/Cu₃BiS₃/Cd S/Zn O/ITO/Al,the effects of Cd S,Zn O and ITO layers in devices on the performance of solar cell were explored.The mechanisms of the annealing temperature of precursor film,the TU dosage in precursor solution and the thickness of absorption layer impacting the device performance were revealed.It is found that the thickness of ITO layer is not the key factor influencing the device performance.With increasing the sputtering time,the thickness of Zn O layer will be increased,the compactness of film will be improved,and the open–circuit voltage(V_OC)will be enlarged.Increasing the thickness of buffer layer is beneficial to V_OC.However,too thick film will cause severe absorption loss of sunlight in the buffer layer.As a result,the J_SC and device efficiency will decrease.Increasing the thickness of absorption layer will increase J_SC and V_OC,due to the more light absorption and the suppression of electric leakage.Similarly,too thick film will not contribute to the generation and collection of carriers.Whereas,it will block the effective transport of carriers,reduce the J_SC and increase the series resistance of device.The optimal thickness of absorption layer is?1?m.In response to the initial increase of the precursor annealing temperature,J_SC,V_OC and efficiency are all improved.The defects such as holes or pits on absorber surface induced by thermal decomposition at annealing temperatures beyond280?,however,deteriorate the device performance.The influence of increasing TU supply in precursor solutions on device performance exhibits the similar tendency as that on film crystallization.J_SC and FF improves first and then reduces.The V_OC is improved to about 250m V and remains almost constant while TU concentration exceeding 2.5 M.The best conversion efficiency occurs for the precursor solution with TU concentration of 3.0 M.3.Based on the solution spin-coating method,different amounts of Ag⁺was doped into Cu3BiS3 film.The effects of Ag⁺doping on the absorption layer and device performance were explored in detail.The results show that Ag⁺doping in precursor solution with Ag/(Ag+Cu)less than 8%will enlarge grain size and improve the compactness of film.Whereas,high Ag⁺doping will destroy the surface morphology of film,and even cause the phase change.The incorporation of a small amount of Ag⁺can improve the absorption property and increase the optical band gap of film.For the precursor solution with the Ag/(Ag+Cu)value of 8%,the measured value of Ag/(Ag+Cu)in obtained film is 5.37%.This film illustrates the highest light absorption coefficient at the level of 10⁵ cm^-1,and the optical band gap of 1.7 e V.Ag⁺doping with proper amounts improves all device performance factors due to the improvement in Cu3BiS3 material quality.The best device efficiency of 0.481%occurs for the precursor solution with the Ag/(Ag+Cu)value of 8%,and the corresponding V_OC,J_SC and FF values are 263 m V,4.147 m A/cm² and 44.077%,respectively.