Theoretical Simulation And Experimental Study Of Hydrogenated Microcrystalline Silicon Germanium Solar Cell

Hydrogenated microcrystalline silicon germanium (μc-Si1-xGex:H) which exhibits narrow variable band gap and high absorption coefficientis a promising candidate for the absorber of solar cells and has been applied in different solar cell structures after continous constantimprovement of its quality in recent years. However, it is necessary to make a theoreticallysystematic study of its solar cellstructure in order to maximize itpotential. In this thesis, radio frequency plasma enhanced chemical vapor deposition (RF-PECVD)is used for the preparation of μc-Si1-xjGex:H thin films and solar cells. Based on parameters of the μ-Si1-xGex:H thin films, systematic photoelectric simulation of its solar cells were carried out and discussed in detail. The main content of this work are as follows:First, after optimization,fundamental parameters suitable to prepare μc-Si1-xGex:H thin films for solar cells were determined. Total gas flux, pressure, substrate temperature,glow discharge power and electrode separationare350sccm、300Pa、200℃、10W and10mm respectively. The concentration of silane and germaneis kept between1%and2%. In order to abstract the optical constants of μc-Si1-xGex:H thin films from the transmission spectra, a Matlab method is proposed which is based on multi-layer model and the Cauchy dispersion formula. Compared with traditional fringe pattern methods such as Swanepeol method, PUMA (Pointwise Unconstrained Minimization Approach), the Matlab method determines the refractive index not from the amplitude of the interference fringe but from the position of the interference fringe, so it can avoid the negative influence owing to the deviation of the amplitude and can offer more precise optical constants, finally it can promote the fitting precision to one order of magnitude. The results shows that the absorption coefficients and the refractive indexes ofμc-Si1-xGex:H are higher than μc-Si:H in the whole spectrum, and increase as the Ge content increases.Second, in order to simulate the light trapping structure inuc-Si1-xGex:Hsolar cells, a complete Matlab methodis proposed to model the light scattering properties of different kinds of morphologies, which is based on the framework of the Reyleigh-Sommerfeld diffraction algorithm.Four types of light scattering (transmission scattering at normal and oblique incidence, reflection scatteringat normal and oblique incidence) for three types morphologies:(random morphologies,periodic morphologies,superimposing morphologies) were modeled and analysed respectively. The results indicate that the superimposing morphology can offer better light trapping capacity, owing to the coexistence of the random scattering mechanism and the periodic scattering mechanism. Its scattering property can be dominated by the individual nanostructures whose geometrical features take play the leading role.Third, based on the optical constants and the scattering models, uc-Si1-xGex:H solar cellswas studied theoretically and experimentally, the result shows that under the restriction of optical structures, the conversion efficiency limit of μ-Si1-x.xGex:H solar cellscan overpass14%.The P/I and I/N interfaces ofμc-Si1-xGex:Hwith high Ge content of0.35were well optimized with different structures. The result shows that an seed layer and fine profiling layers can help to mitigate the bandgap and the lattice mismatch ofμc-Si1-xGex:H and the p-μc-Si:H film while the SC profiling layers is the best choice to modify the I/N interface. Finally, an initial efficiency of4.63%in μc-Si0.65Ge0.35:H single junction structure (Jsc=19.17mA/cm2, Voc=0.43V, FF=0.56) with the thickness of only650nm and an initial efficiency of10.36%(Jsc=11.72mA/cm2, Voc=1.27V, FJF=0.69)with an total thickness of around0.9μm stucture were obtained. Our results suggest that the application ofμc-Si1-xGex:H can at least extend to the high Ge content of35%.Fourth, In order to take thea-Si:H/a-SiGe:H/μc-SiGe:H triple-junction solar cell in the stacked structure as a complete single device into simulation, we developed aTRJ-F/TRJ-M/TRJ-Btunnel-recombination junction model. It is found that three key profiles should be kept. The bandgap gradingis1.7eV/1.1eV/1.7eV. Thedonor-like Gauss gap state density densitiesprofile islex1015cm-3/5ex1021cm-3/l ex21020cm-3, and the acceptor-like Gauss gap state density densitiesprofile wasl ex1015cm-3/5ex1021cm-3/5ex1021cm-3.After the calculation, it is found that the optimization bandgap of the a-Si:H/a-SiGe:H/μc-SiGe:H triple-junction solar cell is1.85eV/1.50eV/1.0eV,and the highest conversion efficiency can reach18.56% (Jsc=10.12mA/cm-2, Voc=2.21V, FF=0.83).