Electrodeposition Preparation Of Copper-iron-sulfur Photoelectric Thin Films And Their Properties

Solar cells as a green and sustainable energy converter,can directly use sunlight to generate electricity,and its absorb layer material has attracted wide attention in recent years.CuFeS₂,a typical chalcopyrite structure material,as a direct band-gap semiconductor,has 0.6 e V band gap,high light absorption coefficient,no photodegradation effect and abundant reserves,can be used as an excellent absorbtion layer material.The electrodeposition can be uniformly deposited on the matrix surface with complex structure at room temperature or slightly higher than room temperature.The thickness,chemical composition and crystal structure of the films can be precisely controlled by controlling parameters.In this paper,Cu-Fe-S precursor film was prepared by one-step electrodeposition method,and CuFeS₂photoelectric thin film was prepared by sulfur annealing treatment after electrodeposition,and film deposition mechanism was analyzed by cyclic voltammetry experiment.In the first part,Cu-S precursor films were prepared by one-step electrodeposition,and the preparing technology of Cu-Fe-S precursor films was explored.The findings are as follows.(1)In the preparation of Cu-S precursor films,Cu appears in the chloride system at the set potential gradient,and CuS and CuO mixture exists in the films;The morphology of the precursor film deposited by sulfate system is spherical aggregated particles with uniform particle distribution and good dispersion,and the size of large particles is between 100nm-400nm.(2)In the preparation of Cu-Fe-S precursor films,various iron,copper and sulfur sources were screened,and finally the solution was determined to be sulfate system.The control variable method was used to investigate the pH value,Cu²⁺concentration and deposition time of the precipitation solution of deposition conditions at-1.0V,respectively.It was found that the film was amorphous with irregular morphology,and belonged to the precursor film of the target product,and was the intermediate product.(3)Heat treatment is helpful to improve the crystallinity of the film's target product,and the appropriate amount of sulfur powder added in the annealing process(sulfur annealing treatment)is helpful to the target product CuFeS₂'s phase.The prepared grains showed a three-dimensional cross-stacked lamellar structure,resembling cauli flower.In the untreated group,the electrical conductivity was low and electrical resistivity was high due to the quasi-amorphous mixture.After crystal nucleation and growth,the overall electrical conductivity of the film increases and the electrical resistivity decreases.In the second part,CuFeS₂film was prepared by sulfur annealing treatment after electrodeposition.The copper source,iron source,sulfur source,complexing agent and pH value in the precipitation solution were analyzed to determine the formulation,the deposition time and deposition potential were analyzed to determine the deposition process parameters,and the curing time and temperature were analyzed to determine CuFeS₂.The phase formation,morphology,composition and electrical properties of the films were characterized respectively.The results showed as follows.(1)The optimal preparation parameters were CuSO₄·5H₂O 0.01mol/L as copper source,(NH₄)₂Fe(SO₄)₂·6H₂O 0.01mol/L as iron source,Na₂S₂O₃·H₂O 0.02 mol/L as sulfur source,sodium citrate 0.01mol/L and ascorbic acid 0.01 mol/L.The prepared solution's pH value is 4,and it was deposited potentiostatic at-1.0V for 20 min.The solution was placed in a reaction kettle and annealed at 220?for 30 h.(2)Proper control of the amount of sulfur powder in the reactor is conducive to the successful preparation of CuFeS₂photoelectric thin film.The morphology of CuFeS₂photoelectric thin film is a three-dimensional cross-stacked sheet structure with grain size between 10nm and 60nm and thickness between 2?m and 5?m.The composition of CuFeS₂photoelectric thin film conforms to the target product CuFeS₂atom ratio of 1:1:2 roughly and forms a preferred orientation at the(112)crystal plane.(3)Sulfide samples can absorb ultraviolet light well,orange light and red light well in visible light range,but the absorption effect of near infrared light is poor.With the increase of deposition time,the conductivity of the film increases,but the resistivity decreases.When the deposition time is 30min,the conductivity is the highest.When the solution is acidic,the conductivity of the film decreases and the resistivity increases with the increase of pH value.When the pH value is 4,the best conductivity is 1.667 S/cm.With the increase of the amount of sulfur source,the conductivity of the film decreases and the resistivity increases.When the amount of sulfur source input is 0,the highest conductivity is 3.33 S/cm.In the third part,the cyclic voltammetry experiment was carried out based on the external three-electrode system of the electrochemical work station,and the electrodeposition mechanism was gradually explored in the mono,binary and ternary solution systems,in order to guide the preparation of the experiment.The findings are as follows.(1)The whole process of the deposition of the unitary solution containing Cu²⁺consists of two steps of single electron transfer.The complexing agent will make the reduction potential of copper ions shift negatively,and the negative shift degree of the combination of two complexing agents will be greater than that of a single complexing agent.The best complexing effect is sodium citrate.In the unitary solution containing Fe²⁺,the phenomenon of water electrolysis is more serious.In the unitary solution containing S,the reduction peak position is-0.65V,and the oxidation peak position is-0.80V.(2)The redox peak potential of the binary copper-sulfur co-deposition solution was positively shifted compared with that of the corresponding unary solution,and the absolute value of the potential difference of the redox peak of each group was smaller than that of the corresponding unitary solution.The reduction potential of the two-element co-deposition solution of"iron-sulfur"shifted negatively compared with that of the corresponding one-element solution.When the two ions co-deposition,they will affect each other's deposition potential.(3)In ternary co-deposition solution,reduction peaks of S and Cu²⁺have no obvious change in potential value compared with its corresponding unitary solution.The deposition potential of Fe²⁺is not obvious,and the oxidation peak has a positive trend compared with the binary solution.Increasing Fe²⁺concentration makes the precipitated potential of Cu²⁺shifted positively.Since the third reduction peak appeared at-1.11V during co-deposition,the potential range was-0.70V to-1.20V when exploring the optimal deposition process.