Preparation Of Cu₂O And Study Of Cu₂O-Based Heterojunction Photoelectrical And Photoelectrochemical Devices

Cuprous oxide(Cu2O)is a traditional semiconductor material that shows p-type conductivity due to the Cu vacancy of the crystal.With the band gap of 2.1eV,it can reach the solar conversion efficiency of 21%theoretically,which makes it a promising photovoltaic material.Moreover,with the negative conductive band edge that can drive the water reduction,Cu2O is also used to fabricate the photocathode to get hydrogen from water.Recently,more and more Cu2O-related photovoltaic(PV)and photoelectrochemical(PEC)devices were reported.Due to the intrinsic property,it is difficult to get stable and high quality n-type Cu2O.An alternative strategy is fabricating heterojunction device with p-type Cu2O and another n-type semiconductor.However,some troubles exist in these systems.In this thesis,we studied the Cu2O-related PV and PEC devices.This thesis focus on the Cu2O.We used several different methods to get high quality Cu2O film which is eligible to fabricate PV or PEC devices.The content includes:1.The pure Cu2O films were successfully obtained by pulsed laser deposition(PLD),magnetron sputtering(MS),electrodeposition(ED)and thermal oxidation(TO)methods respectively.Even though the film quality and electrical property of Cu2O grown by PLD and MS are good,the high deposition temperature will hinder its application in heterojunction device.The ED method is a low-temperature method that makes it much convenient to control the thickness and morphology of Cu2O,which is the most promising method.The TO method gives the best quality Cu2O sheet which make the efficiency highest,up to now.2.We fabricated a Cu2O based heterojunction solar cell with a sulfur-doped ZnO(ZnOS)film as the buffer layer.The electric and optical properties of ZnOS films deposited under different oxygen pressures were investigated.A ZnOS film with suitable S content was inserted between the Cu2O absorbing layer and TCO layer.It is demonstrated that the open circuit voltage is improved extremely from 0.34 V to 0.63 V,corresponding to the PCE of 0.94-1.48%,respectively.The dark current-voltage curve shows the reverse saturation current was extremely reduced,thereby enhancing the VOC.To understand the mechanism of ZnOS buffer layer,the band alignment of Cu2O/ZnOS heterojunction,together with Cu2O/ZnO as a reference,was measured by XPS.The elevation of both conduction band minimum and valence band maximum was found,forming an electron barrier between the Cu2O layer and TCO layer which decreases the recombination between the electrons from ZnO and holes from Cu2O.3.In conclusion,we have fabricated a photocathode with the structure of Cu2O/Ga2O3/AZO/TiO2 using the PLD technique for the first time.The effect of factors such as the thickness of Cu2O and the thickness and deposition temperature of TiO2 on the PEC response was examined.Increases in photocurrent and photovoltage were obtained in our experiment.The stability of the photocathode was improved to some extent but still not sufficiently.The properties of the Ga2O3 layer,which were affected by the oxygen pressure,played a vital role in the improvement.The band diagram illustrated that the band bowing found in the CBM of Ga2O3(deposited at 1Pa)would cause electrons to accumulate,which hindered carrier transport and resulted in deactivation of the electrode.The use of a higher oxygen pressure for the deposition of Ga2O3 is necessary for better PEC performance.Analysis by XPS depth profiling revealed that Cu+ was reduced to Cu0 at the Cu2O/Ga2O3 interface,which hindered the migration of electrons.4.A Cu2O/Ga2O3/TiO2/RuOx photocathode was fabricated.Attributed to the high quality of the thermal oxidized Cu2O and the preferable band alignment of the Ga2O3 buffer layer,this photocathode reached a photocurrent of 6 mA cm-2 at 0V vs.RHE and a positive onset potential of 0.9V vs.RHE.The IPCE curve showed a peak at 500-600nm and a drop below 500nm.Analysing the photoresponse behavioe under differernt white light intensity or colored light intensity and with or with H2O2(electon acceptor)electrolyte,we concluded that the limiations in the TO-Cu2O photocathode.Based on the feature of IPCE in TO-Cu2O photocathode,a tandem photocathode containing a transparent ED-Cu2O photocathode and TO-Cu2O photocathode was designed to make a better use the photons can be absorbed.A new ABPE record of 1.9%was obtained,with the photocurrent of 7 mA cm-2 at 0V vs.RHE and the onset potential of0.9Vvs.RHE.