P-n junction from solution: Cuprous oxide p-n homojunction by electrodeposition

Inorganic solar cells are traditionally fabricated by vacuum-based semiconductor processing. Although cuprous oxide (Cu₂O) can be synthesized by wet-chemical methods, its natural p-type conductivity has prevented an efficient p-n homojunction solar cell. In this paper we report the effect of solution pH on the conduction type (n- or p-type) of Cu₂O in electrodeposition. It is found that Cu₂O electrodeposited at bath pH below 7.5 is n-type, while Cu₂O deposited at bath pH above 9 is p-type. This is likely because bath pH controls the amount of oxygen incorporated into the growing Cu₂O film and thus the nature of native point defects. Capacitance-voltage and photocurrent measurements confirm that the conduction type of Cu₂O can be controlled by bath pH. X-ray diffraction and scanning electron microscopy reveal that while n-type Cu ₂O deposited on Cu substrate appears amorphous, n-type Cu ₂O deposited on p-type polycrystalline Cu₂O is polycrystalline. Current-voltage measurements indicate that Al makes ohmic contacts to n-type Cu₂O and Cu to p-type Cu₂O. Furthermore, a two-step process is adopted to deposit p-type and n-type Cu ₂O in sequence for the formation of a p-n homojunction in Cu ₂O. Photocurrent and current-voltage characterization confirm that a p-n homojunction of Cu₂O is achieved.