Synthesis Of MoS₂ Thin Film And Its Application In Solar Cells

Molybdenum disulfide?MoS₂?,a new type of graphene-like two-dimensional material,attracts great attentions because of its unique electrical,optical,mechanicalandcatalytic properties.Compared with zero bandgapsemimetal graphene,MoS₂ has a bandgap of about 1.2¹.8 eV,in particular,monolayer MoS₂ is a direct bandgap semiconductor,and possesses high carrier mobility and excellent light absorption properties,which make it very broad applications prospects in the field of optoelectronic devices.In this paper,we prepared large area continuous MoS₂ thin film by ambient chemical vapor deposition with MoO₃ and S as precursor and reactant.We also explored the influence of different experimental conditions on the thin film growth,which include reaction times,precursor mass,temperature-rising rate of the system,reaction atmosphere and so on,to make surethe best growth conditions to prepare MoS₂thin films on SiO₂/Si substrate by ambient CVD.Controlling and optimizing the MoS₂ film growth conditions have a great influence on the film purity and crystallinity.When applied to the solar cell,the high quality MoS2 film can effectively keep the light generated electrons and holes from capturing by the defective state and hence improve the performance of the solar cell.The structures and optical properties of the films were characterized by Raman spectroscopy and photoluminescence spectroscopy analyses.The results showed that the Raman spectra of and located at 390.0cm^-1 and 408.0cm^-1 respectively,the frequency difference between two modes was 18 cm^-1,verifying monolayer MoS₂.The peaks of photoluminescence spectra of MoS₂were located at 679.8nm?1.82eV?and627.5nm?1.97eV?respectively,which were mainly caused by spin-orbit coupling in monolayer MoS₂ in valence band.The optical microscopy,atomic force microscopy and scanning electron microscope were also used to observe the surface morphology and thickness,identified the formation of MoS₂ monolayer structure.The growth mechanism of MoS₂ thin films was studied.The synthesis processwas found to beconsist of evaporation,diffusion of molybdenum source and sulfur source,sulfurization of MoO₂,the formation ofMoS₂ nanoparticles and last the merging of MoS₂ nanoparticles to form continuous films.Understanding thegrowth mechanism of MoS₂ thin film is suggestive to the controllable synthesis of continuous large-area MoS₂ thin films.On the basis of the above experiments,the prepared MoS₂ films on SiO₂/Si was transferred to crystalline silicon through PMMA-assisted transfer method.Optical microscopy,Raman and photoluminescence spectroscopy were carried out to identify the quality of transferred films.The results showed that the main peak of PL has a blue shift of about 49 meV due to the stress-release between the films and substrates during the transferring.We analyzed the MoS₂/p-Si heterojunction photovoltaic principle from the perspective of band structure,and simulated the structure of MoS₂/p-type silicon heterojunction solar cell,whichshowed a high efficiency of about 21.3%.Wedeposited upper and lower electrodes using thermal evaporation to form a Al/MoS₂/p-Si/Al heterojunction solar cell,the solar cell test platform based on Keithley 2400 unit was built and the J-V characteristics of the prepared cells were measured under the standard light source at AM1.5.The test result of this heterojunction solar cell showed a photovoltaic effect,the photoelectric conversion efficiency was about 0.039%.We analyzed the existing problems in the fabrication of solar cell and its structure,and laid the foundation to further improve the solar cell efficiency based on MoS₂ films.