Transport Properties Of Novel Si-Based High-Efficiency Solar Cells

The long-term stable development of humans is under threat due to the global energy crisis andenvironmental degradation. Energy and environment are two major issues in the world during the21stcentury. Solar photovoltaic technology is an effective way of resolving the energy exhaustion andenvironmental pollution and realizing the sustainable development of human society. However,compared to the traditional power generation, the cost of solar photovoltaic technology is still veryhigh, which limits the large-scale application of solar photovoltaic technology. Therefore, it is anurgent task to search for new materials and to develop new technologies for further improving cellefficiency and reducing cell cost.This work has been carried out under the above-mentioned background and by the support of thenational "863" project. The transport properties of β-FeSi₂solar cells and impurity photovoltaic (IPV)solar cells have been investigated by numerical simulation and theory analysis. These research resultscan provide a theory guide for preparing the relating high-efficiency solar cells. Main contents andresults of this study are as follows.(1)The numerical model of β-FeSi₂solar cell has been constructed and β-FeSi₂homojunctionsolar cell has been simulated. The optimal structure parameters of β-FeSi₂homojunction solar cell areas following: emitter thickness20nm, emitter concentration2×1018cm^-3, base thickness500nm, baseconcentration1×1016cm^-3. The corresponding cell performance is η=16.32%, Jsc=45.88mA/cm2,FF=78.8%, and Voc=0.451V. The maximum theoretical efficiency of the cell can reach21.11%.(2)The influences of emitter parameters, light incidence surface, interface states andrecombination mechanism on the β-FeSi₂/c-Si heterojunction solar cell performance have beenstudied. The results show that p-β-FeSi₂/n-c-Si is a good structure configuration and the lightincidence from β-FeSi₂surface is better than that from Si surface. Interface states should beminimized since large interface states can cause more recombination of photocarriers and largereverse saturation current. The very large light absorption coefficient of β-FeSi₂results in the fact thatβ-FeSi₂solar cell performance is very sensitive to surface recombination velocity. Moreover, theAuger recombination and radiative recombination have less impact on device performance. When theSRH recombination lifetime is greater than1μs, the SRH recombination has little effect on cellefficiency. The optimal thickness and concentration for the emitter of p-β-FeSi₂/n-c-Si are350nm and2×10¹⁷cm^-3, respectively. The conversion efficiency of the optimized cell can achieve19.49%. The theoretical limit of efficiency for β-FeSi₂/c-Si heterojunction solar cells can attain28.12%.(3)β-FeSi₂has been applied to the bottom absorber of the tandem solar cell for widening thespectrum response above1400nm. The effects of sub-cell parameters, μc-Si bandgap, spectralirradiance and operating temperature on the a-Si/μc-Si/β-FeSi₂triple-junction solar cell performancehave been investigated. The optimal absorber thickness of sub-cell for a-Si/μc-Si/β-FeSi₂is260,900and40nm, respectively. The efficiency of the optimized cell can achieve19.80%. The optimalbandgap of μc-Si cell is1.30eV. For a-Si/μc-Si/β-FeSi₂tandem cell, when spectral irradiance is AM0,the conversion efficiency is highest, then AM1.0, and the smallest one AM1.5G. Moreover, thetemperature coefficient of conversion efficiency for the tandem cell is-0.308%/K. This value is lowerthan that of μc-Si single-junction cell, and it is only higher than that of a-Si single-junction cell. Whenmaterial quality of a-Si and μc-Si is improved, the short-circuit current density of a-Si/μc-Si/β-FeSi₂can reach around16mA/cm2, and the conversion efficiency may achieve24.50%.(4)μc-3C-SiC material has been proposed as the emitter of the HIT solar cell. The resultindicates that the cell efficiency is improved by using a μc-3C-SiC emitter. The origin of theimprovement is the lower absorption loss of μc-3C-SiC emitter in short-wavelength region.(5)The influences of the Te and In IPV impurities on the crystalline silicon (c-Si) cellperformance have been studied. The short-circuit current density can be increased by5.38mA/cm2and the efficiency can be increased by2.79%due to the incorporation of IPV impurity Te. In addition,IPV effect can extend near-infrared spectral response, and a good light trapping can result in the widerextension of the near-infrared spectral response. Furthermore, the thermal capture cross-section ofelectron (hole) is crucial to the device performance and that of hole (electron) has few influences onthe cell property for acceptor-type (donor-type) impurity level near the valence (conduction) bandedge. These results may help to evaluate the potential of the IPV effect for improving cell efficiencyaccording to the thermal capture cross-sections of the impurity in host semiconductor.(6)The effects of the energy level, and Mg, In, Tl impurity on the IPV c-Si cell performancehave been studied. The optimal energy level for donor-type IPV impurity is at0.20-0.25eV below theconduction band edge. When two levels work, the energy level, which is not at optimal location,would divert some incident sub-band photons, making efficiency less than those with an optimalsingle-level. When two different IPV impurities are introduced into cells, the concentration of oneimpurity can be changed for increasing cell efficiency.