Study On The Controllable Conductivity Of Cuprous Oxide Thin Films Fabricated By Differential Potentiostatic Deposition

Cuprous oxide?Cu₂O?is a kind of promising material for solar energy conversion such as photovoltaic cell and photoelectrochemical water splitting.The mechanism of conductivity transforming from p-type to n-type for Cu₂O films during a long-term potentiostatic deposition is waiting to be revealed.In order to study the conductivity transforming mechanism of Cu₂O films in this work,differential potentiostatic deposition?DPD?was developed to simulate different stages in the process of preparing Cu₂O films by traditional potentiostatic deposition?TPD?.The experimental results and conclusions are as follows:?1?The effect of deposition time on the conductivity of Cu₂O films fabricated by TPD was studied.The conductivity of Cu₂O?TPD?films transformed from p-type to n-type with prolonged deposition time.The photocurrent responses showed that the cathodic photocurrent density of the p-type film gradually decreased while the anodic photocurrent of the n-type film increased with deposition.The Cu₂O?TPD?films behaved as p-type semiconductors as deposition time was less than 5 h.While the Cu₂O?TPD?films behaved as n-type semiconductors as deposition time was more than 7 h.The deposition curve showed that the current density decreased with the increase of the deposition time.The initial deposition current density of Cu₂O?TPD?film was ca.4.5 m A cm^-2,but the current density steeply decreased 95%in the first 2 h deposition,then decreased to 0.41%of the initial current density after deposited for 12 h.The results showed that the growth rate of the film slowed down continuously.The conclution was also confirmed by the SEM characterization on the cross-section of Cu₂O?TPD?films.Therefore,the conductivity transforming was related with the growth rate of Cu₂O?TPD?films.?2?In order to study the conductivity transforming mechanism of Cu₂O films formed by TPD,a novel electrochemical technology,differential potentiostatic deposition,was developed by coupling a 3-electrode setup with a high value resistor connected in series with the counter electrode through a potentiostat.By this approach,the current density was adjusted with controlling the deposition potential constant and some certain deposition stages in a TPD were simulated.Employing the DPD method,conductivity transformation of Cu₂O films from p-type to n-type with decreasing current density was observed.Photoelectrochemical characterization showed that when the deposition current density was more than 0.12 m A cm^-2?resistance in current circuit was less than 30 k??,the Cu₂O?DPD?films behaved as p-type semiconductor.When the deposition current density was less than0.07 m A cm^-2?resistance in current circuit was more than 50 k??,the Cu₂O?DPD?films behaved as n-type semiconductor.The electrodeposited Cu₂O film grew layer by layer.If the growth rate of Cu₂O film was fast,the Cu vacancy defects in obtained films would much more than O vacancy defects,because Cu vacancy defects in each layer were covered rapidly by next layer of film.When the growth rate of film became slow,it woukd take a long time for growing each layer of film,then the copper vacancies in this layer of film were made up by reduced Cu²⁺,resulting in the number of copper vacancies in the film gradually less than that of oxygen vacancies.Therefore,when the O vacancy density in film was much more than Cu vacancy density,the obtained film behaved as n-type semiconductor.The results indicated that the DPD successfully simulated different stages in a TPD process,the DPD method would effectively reduce the time occupied by performing experiments and enhance the efficiency.