Fabrication of Cu-doped p-Fe-O/n-Fe-O homojunction solar cells by constant current electrochemical deposition

抄録

1. Introduction

Hematite iron oxide (Fe₂O₃), which is an n-type semiconductor with a bandgap of 2.0~2.2 eV, is harmless for human and extremely abundant on the earth. In previous woks, Cu-doped p-type iron oxide thin films and homojunction solar cells with n-type iron oxide have been successfully fabricated by electrochemical deposition(ECD) at constant-potencial and two-step pulse potencial.¹⁾²⁾ Therefore in this work, Cu-doped p-type iron oxide and homojunction solar cells were fabricated by constant-currrent ECD. The advantage of the constant-current method is that the effects of the first layer on the second layer deposition can be diminished. Moreover, continuous deposition of p-type iron oxide was performed by adding copper sulfate (to dope Cu) to the solution in which n-type iron oxide was deposited.

2. Experimental methods

Tin doped indium oxide (ITO)-coated glass sheet was used as the deposition substrate, platinum as the counter electrode, and Ag/AgCl as the reference electrode. The n-type iron oxide deposition solution contained 50 mM iron(II) sulfate heptahydrate and 100 mM sodium sulfate dissolved in water. The deposition current density was -0.12 mA/cm² and the deposition time was 30 minutes. The p-type iron oxide deposition solution was prepared by adding 5 mM copper sulfate to the n-type iron oxide solution. The deposition current density was -0.64 mA/cm² and the deposition time was 10 minutes. When fabricating pn-junction, the substrate was not removed from the deposition solution after n-type iron oxide deposition, and copper sulfate was added to the solution under the same deposition current and deposition time as above. Then, annealing was performed for 1 hour at 400 °C in an atmospheric atmosphere. Photoelectrochemical (PEC) measurements were performed on each of the p- and n-type iron oxides. Current-voltage (I-V) measurements were performed on the fabricated pn-junction.

3. Results and discussion

The results of the PEC measurements are shown in Fig. 1. If the sample is p-type, a negative photocurrent is observed during a negative potential sweep. For n-type, a positive photocurrent is observed during a positive potential sweep. The figure shows that the iron oxide exhibits n-type photoresponse, while the Cu-doped iron oxide exhibits p-type photoresponse. The results of I-V measurements for the p-n junction are shown in Fig. 2. Although the rectification properties are not good, weak photovoltaic properties were observed during AM1.5 irradiation, resulting in an open circuit voltage of 30.2 mV, a short circuit current of 2.81 μA, and a power generation efficiency of 2.67 × 10^-5 %. In summary, iron oxide homojunction solar cells were successfully fabricated by continuous deposition using the constant current ECD.

References

1) S. Kobayashi and Ichimura, Semicond. Sci. Tecnol. ,33,(2018),105006.

2) R. Takayanagi and Ichimura, Jpn. J. Appl. Phys. ,59,(2020),111002.