Investigation On The Preparation Of Cu₂O Based Thin Films And Their Heterojunctions Properties

Energy crisis and environment pollution are two major challenges facing the world today.In photovoltaic technology fields,the investigation on potential efficient materials with suchs advantages as low cost,simple structure and nontoxicity is extremely a important for developing the third generation solar cell.Cuprous oxide?Cu₂O?is a p-type semiconductor with direct band gap,and posses high optical absorption in the visible region.It is regarded as a promising absorber layer material for new type thin film solar cell due to its high elemental abundance in the earth crust,nontoxicity and low-cost production.However,the intrinsic Cu2 O has low carrier concentration and high resistivity,and the contaminating from CuO is often a serious problem in the growth of Cu2 O thin films,that can decrease the absorption of Cu2 O in the visible region.In addition,the deteriorative interface states are introduced easily in the preparation of Cu2 O thin film solar cells,which will affect the effective separation of the photo-generated carriers.The above shortcomings affect seriously the photoelectric properties of Cu2 O thin film solar cells.Therefore,it is urgent that study on the peraparation of pure phase Cu2 O thin films with free CuO phase and good surface state,and the method of reducing the resistivity of Cu2 O thin films by p-type doped effectively.It is also great significance for the development of high performance of Cu2 O thin film solar cell.In this dissertation,N-doped Cu₂O?Cu₂O:N?,ZnO and Al-doped ZnO?AZO?thin films were fabricated based on magnetron sputtering and rapid thermal annealing technologies,and the structure and photoelectric properties of thin films were investigated.In particular,the effects of key process parameters on the growth behavior,phase structure and photoelectric properties of thin films were studied in detail.Finally,the controllable preparation technique of single phase Cu2 O thin film was established,the p-type doping of Cu2 O was realized effectively,and two kinds of Cu₂O:N/AZO and Cu₂O:N/ZnO/AZO heterojunction thin films were prepared successfully.The performances of the heterojunctions were also calculated and analyzed by using the 1D-SCAPS simulation software,and the results can provide a theory guide on the design and fabrication of Cu2 O thin film solar cells.The main study contents and results of this dissertation are listed as follows:1.The Cu2 O thin films were deposited by pluse magnetron sputtering technology by using a metallic copper target in O2 and Ar mixture atmosphere.Effects of different deposited conditions on the growth behavior of the thin films were investigated.The possible formation mechanism of different properties for the thin films has been discussed.The Cu2 O thin films were deposited by radio frequency?RF?magnetron sputtering technology by using a Cu2 O ceramics target in O2 and Ar mixture atmosphere,and the as-deposition thin films were then annealed by rapid thermal annealing?RTA?treatment in N2 atmosphere.Effects of different deposited conditions and annealing temperatures on growth behavior of the thin films were investigated.It is shown that the as-deposited thin films contain only single phase of Cu₂O?111?.The results show that the crystallite temperature of thin film was about 300?.The RTA process under below 600? has obviously influence on the crystallinity of as-deposition thin films.2.The N-doped Cu₂O?Cu₂O:N?thin films were prepared by RF magnetron sputtering technology by using a Cu2 O ceramics target in N2 and Ar mixture atmosphere,and the as-deposition thin films were then annealed by RTA.Effects of N2 flow rates and annealing temperatures on growth behaviour of Cu₂O:N thin films were investigated detailedly.The results show that the thin films deposited with low N2 flow rate?<12 sccm?consist of CuO and Cu2 O phases,but when the N2 flow rate increase to 12 sccm,the thin films will be single phase Cu2 O.The growth process of thin films deposited under different N2 flow rates obey 3D growth mode.The RTA process has obviously influence on the crystallinity of as-deposition thin films.The resistivity???of thin films decreased with the increase of annealing temperatures,and thin films show good electrical properties.3.The ZnO thin films have been deposited by RF magnetron sputtering technology by using a ZnO ceramics target in O2 and Ar mixture atmosphere.Effects of different substrate temperaturs on the growth behaviour of ZnO thin films were investigated.The results show that the thin films deposited at room temperature?RT?were signal phase ZnO?002?,the average transmittance of the thin films was about 85 %,and they can be used as buffer layer for thin film solar cells.But the thin films deposited above RT consist of?002?and?103?phases of ZnO.The AZO thin films have been deposited by RF magnetron sputtering technology by using a AZO ceramics target in Ar atmosphere.Effects of different substrate temperaturs on the growth behaviour of AZO thin films were investigated,and the possible formation mechanism of different properties for thin films has been discussed.The results show that the thin films deposited at 150 ? substrate temperature show good surface morphology and photoelectric properties,and the thin films can be used as window layer for thin film solar cells.4.The Cu₂O:N/AZO heterojunctions were fabricated by deposition AZO thin film on Cu₂O:N thin film by using RF magnetron sputtering technology,and the photoelectric properties of the heterojunctions were investigated.It can be seen that the heterojunctions show obvious rectifying behavior,the turn-on voltage?Vth?was 0.68 V,and the current density?J?was 2.5×10-4 mA/cm2.The heterojunction thin films have strong absorption below 580 nm wavelength range for the solar spectrum.The Cu₂O:N/ZnO/AZO heterojunctions were fabricated by inserting a ZnO buffer layer deposited at RT temperature between Cu₂O:N and AZO thin films by using RF magnetron sputtering technology,and the photoelectric properties of the heterojunctions were investigated.The results show that the diffusion of heterojunction interface can be effectively prevented,and the surface state can be protected.Compared to Cu₂O:N/AZO heterojunctions,the Vth of the Cu₂O:N/ZnO/AZO heterojunctions increase 0.08 V,and the external quantum efficiency?EQE?of the heterojunction thin filme increase below 580 nm wavelength range.Owing to the increase of the series resistance of heterojunctions,the current density?J?of Cu₂O:N/ZnO/AZO heterojunctions decrease by the inserting of ZnO layer.5.The performances of Cu₂O:N heterojunctions were simulated by using the 1D-SCAPS simulation software.The effects of the thickness and defect concentration Cu₂O:N thin film and the ZnO buffer layer thickness on the performance of the Cu₂O:N/ZnO/AZO heterojunction were investigated.The results show that when the defects concentrations of Cu₂O:N thin films is higher than 1.0×1016 cm-3,the performance of Cu₂O:N/AZO and Cu₂O:N/ZnO/AZO heterojunctions decrease sharply.The offset of Cu₂O:N and AZO conduction bands will decrease when inserting a ZnO buffer layer between Cu₂O:N and AZO thin films,and the energy band matching degree of Cu₂O:N/AZO heterojunction is improved,but the short circuit current density?Jsc?of Cu₂O:N/ZnO/AZO heterojunction decreases with an increasing ZnO layer thickness.