First Principle Calculation Of Interfacial Properties At The Si/PEDOT:PSS Interface In Silicon-based Hybrid Solar Cell

Silicon-based hybrid solar cells integrate the excellent conductivity and the high stability of silicon and the low cost and easy preparing of organic material.Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT:PSS)is the most commonly used organic material in silicon-based hybrid solar cells.At present,the power conversion efficiency(PCE)of the silicon-based hybrid solar cell can reach 14.1%.The problem of Si/organic interface is one of the key technologies in current research.Meanwhile the methyl passivation and oxygen passivation are the most commen methods to passivate silicon surface.While there is a lack of in-depth research on the mechanism of interfacial passivation at present.The charge transfer mechanism can be studied in atomic level by first-principles calculation,and the effects of different passivation methods on the silicon and the interface charge transfer can be reflected,such as the the driving force,type,path,speed and so on of the charge transfer.This can provide theoretical basis and innovative ideas for experiment.The main contents of this paper are as follows:(1)We build the reasonable calculation models at the first.Here we selected three common treatment methods:p(2×2)surface reconstruction,methyl passivation and oxygen passivation as the upper surface of silicon layer.The bottom silicon atoms were hydrotreated and then we build three silicon substrate models and three silicon/PEDOT:PSS models.The contact surface of PEDOT:PSS was determined by Forcite molecular dynamics and here the PEDOT.PSS crystal model was simplified.The electric properties of the three silicon substrates and silicon/PEDOT:PSS systems were calculated by the first-principles calculation code CASTEP.(2)The effects of different passivation methods on the silicon substrate are analyzed from four aspects,including the deep defect level,charge distribution,carrier effective mass and the work function.Passivation can eliminate deep defect levels and reduce carrier recombination by eliminating surface dangling bonds.According to the surface charge distribution of passivated silicon substrate,it is speculated that the charge transfer between silicon layer and organic layer may be realized through charge tunneling.The effective mass of carriers in different directions for the silicon substrate were calculated by using quadratic difference method,and the effective mass will reduce after passivation so that the rate of charge transfer will improve.Further more,the work functions of silicon substrate were calculated using dipole correction,whhis means that the energy barrier for electrons to escape the silicon is the lowest.Therefore,it is necessary to adopt appropriate passivation method for silicon substrate and methyl passivation is superior to oxygen passivation.(3)The influence of different passivation methods on the interfacial charge transfer of silicon/PEDOT:PSS is analyzed from three aspects,including direction and driving force,type and path,and the speed of charge transfer:We calculated the number of charge transfer,distribution of the charge transfer,potential curve and the level arrangement.In the methyl passivation system,there is one electric field from the silicon substrate to PEDOT:PSS and the amount of charge transfer is more.The dipole moments derived from passivation resulte in the energy level bending at the interface.And the bending direction in methyl passivation system is favorable for hole transfer.Therefore under the light conditions,holes can smoothly transfer from methyl silicon substrate to PEDOT:PSS.We compared the DOS diagram and wave function to analyze the type and path of charge transfer in silicon/PEDOT:PSS function.The charge transfer in reconstructed silicon system is adiabatic process.Whereas in the passivation system,electrons can transfer smoothly but holes cannot,so the charge transfer in the passivation system may be a non-adiabatic process,which needs through charge tunneling.The orbital coupling coefficients of carrier separation and recombination were calculated.The methyl passivation system is faster than the oxygen passivation system.Therefore the methyl passivation is better for the silicon/PEDOT:PSS system.