Preparation And Optoelectronic Properties Of P-type Semiconductor CuI Thin Films

Multi-junction structure superimposed semiconductor materials with different band-gap widths from the bottom to the top according to the increasing band-gap width,which is an effective structure to improve the optoelectronic conversion efficiency of solar cells,and has been paid attention to in the field of space solar cell research.In order to ensure that each sub-cell can make full use of the solar spectrum of different bands,the top electrode material with wide band gaps should allow sufficient sunlight transmittance.Therefore,the traditional metal electrode is no longer suitable for the top electrode material of multi-junction solar cells,and the semiconductor thin film electrode material which is transparent to the visible band of sunlight must be selected.ITO is the most commonly used transparent electrode material at present.However,the content of In in ITO is not high in the earth,the price is expensive,and the In element is toxic,and it will cause harm to environment and human body during the preparation process.Therefore,it is necessary to choose environmentally friendly and abundant new wide band-gap semiconductor materials to act as transparent electrodes for multi-junction laminated solar cells.γ-CuI is a transparent semiconductor with direct wide band gap of 3.1 e V.It has much advantages,such as the abundant content,non-toxic,low cost,the good compatibility with a variety of thin film materials,the stable cubic structure also makes it easy to synthesis at room temperature.These characteristics makeγ-CuI semiconductor show potential to replace ITO transparent electrode.Based on the analysis of the research ofγ-CuI thin films,to improve the optoelectronic properties ofγ-CuI thin film,the physical and chemical changes of gas-solid reaction process was study.The evolution of crystallinity,surface morphology,optical band gap,transmittance and conductivity characteristics in the iodization,acceptor ion doping and amorphous films at room temperature are analyzed,which lays a theoretical foundation for the application ofγ-CuI thin film as transparent electrode with excellent photoelectric properties.The dense and uniform surface is obtained in 50 and 100 nm/layer films.The 50nm/layer sample is repeated nucleation,islet generation,islet uniform growth.However,the 100 nm/layer sample was achieved through six main stages of nucleation,islet generation,islet growth,channel generation,channel disappearance and island connection formation.The LBL iodization process has another two advantages:（i）Cu films filled the pores and potholes during deposition;（ii）the last CuI film acts as a seed layer for the next layer.Iodine vapor temperature can also control the gas-solid reaction stage of LBL-CuI film.Post-annealing does not significantly improve the crystallinity of the sample,but facilitates the movement of the CuI grains along the two-dimensional plane,thus promoting the surface density.The layer-by-layer annealing can improve the crystallinity of samples effectively.The I/Cu ratio of LBL-CuI thin films increases gradually with the decrease of the CuI per layer thickness.The high temperature of iodine vapor and annealing result in serious loss of iodine content in the thin film.The transmittance of 50 nm/layer film is 80%,and the resistivityρ,mobilityμand carrier concentration n_preach the optimal values,which are 0.04Ω·cm,8.2cm²/（V·s）and 2.5×10¹⁹cm^-3,respectively.And its figure of merit（FOM）value is4 times that of TOS-CuI thin film.The mobility of LBL-CuI film can be increased to8.7 cm²/（V·s）by the regulation of gas-solid reaction temperature.The post-annealing method optimizes the transmittance of LBL-CuI film to 90%,but the resistivity monotonically increases with the increase of temperature.The resistivity of LBL-CuI sample was optimized to 0.034Ω·cm by LBL-annealing.By using sputtering and spin coating techniques,well controlled S-doping of CuI thin films has been realized.The spin coated samples present a single（111）out-of-plane orientation and very high crystallinit.Substituting S for I can achieve efficient acceptor doping of CuI for both the physical and chemical growth method.When S doping is 2.0%,CuI:S thin film obtains the lowest resistivity of 0.023Ω·cm,which is better than that of CuI sample prepared by LBL iodization method,and the doping efficiency is influenced by the self-compensation effect.By preparing amorphous Cu（S,I）materials,low hole resistivity in the order of10^-3～10^-4Ω·cm has been obtained in p-type a-TCs.These high conductivities are comparable with commercial n-type TCs made of ITO,and are 100 times greater than previously reported amorphous CuI.Responsible for the high hole conduction is the overlap of large p-orbitals of I and S anions,which provide a hole transport pathway insensitive to structural disorder.In addition,the bandgap of amorphous Cu（S,I）can be modulated from 2.6 e V to 2.9 e V by increasing the iodine content.These unique properties demonstrate that the Cu（S,I）film system holds great potential to be a promising p-type amorphous transparent electrode material for optoelectronics.