硅锗量子阱结构在硅异质结太阳电池中应用的数值模拟

利用半导体工艺和器件仿真软件silvaco TCAD(Technology Computer Aided Design),模拟研究了采用硅/硅锗合金(silicon/silicon germanium alloy,Si/Si_(1-x)Ge_x)量子阱结构作为吸收层的薄膜晶体硅异质结太阳电池各项性能.模拟结果显示,长波波段光学吸收随锗含量的增加而增加,而开路电压则因Si_(1-x)Ge_x)层带隙的降低而下降.锗含量为0.25时,短路电流密度的增加补偿了开路电压的衰减,效率提升0.2%.氢化非晶硅/晶体硅(a-Si:H/c-Si)界面空穴密度以及Si_(1-x)Ge_x)量子阱的体空穴载流子浓度制约着空穴费米能级的位置,进而影响到开路电压的大小.随着锗含量增加,a-Si:H/c-Si界面缺陷对开压的影响降低,Si_(1-x)Ge_x)量子阱的体缺陷对开压的影响则相应增加.高效率含Si_(1-x)Ge_x)量子阱结构的硅异质结太阳电池的制备需要a-Si:H/c-Si界面缺陷的良好钝化以及高质量Si_(1-x)Ge_x)量子阱的生长.

Numerical simulation of silicon heterojunction solar cells with Si/Si1-xGex quantum wells

Heterojunction with intrinsic thin-layer (HIT) solar cells attract attention due to their high open circuit voltage and stable performance. However, short circuit current density is difficult to improve due to light losses of transparent conductive oxide and hydrogenated amorphous silicon passivation (a-Si:H) layer and low absorption coefficient of crystalline silicon (c-Si). Silicon germanium alloy (Si/Si_1-xGe_x) quantum wells and quantum dots are capable of improving low light utilization by strong optical absorption in the infrared region. In this article, opto-MoS₂of the HIT solar cells integrated with Si/Si_1-xGe_x quantum wells (HIT-QW) as a surface absorber are investigated by numerical simulation with Technology Computer Aided Design (TCAD). The influences of germanium content on the MoS₂of HIT solar cells with long carrier lifetimes of Si_1-xGe_x layers (τ_p^*) and defect-free a-Si:H/c-Si interface are investigated at first. The simulation results indicate that optical utilization in the infrared region is enhanced with the increase of germanium fraction, while open circuit voltage degrades due to the decreasing of the energy band gap of Si_1-xGe_x, radiative recombination and auger recombination mechanism in the Si/Si_1-xGe_x quantum wells. And the conversion efficiency reaches a maximum value at a germanium fraction of 0.25 then drops distinctly. When the germanium fraction increases from 0 to 0.25, the short circuit current density increases from 34.3 mA/cm² to 34.8 mA/cm², while the open circuit voltage declines from 749 mV to 733 mV. Hence, the conversion efficiency increases from 21.5% to 21.7% due to the fact that the enhancement of short circuit current density compensates for the reduction of open circuit voltage. When the germanium content increases to more than 50%, a serious open circuit voltage loss of more than 130 mV associated with the energy band gap loss of Si_1-xGe_x arises in the HIT-QW solar cells, which indicates that the dominating carrier transport mechanism changes from shockley diffusion to recombination in the Si/Si_1-xGe_x quantum wells. Subsequently, the influences of interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si_1-xGe_x quantum wells are discussed. Both interface holes at a-Si:H/c-Si interface and bulk holes in Si_1-xGe_x quantum wells can be recombined through the interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si_1-xGe_x quantum wells, respectively, which restricts the position of hole fermi level in the open circuit condition. When the germanium fraction increases, the influence of interface defects at a-Si:H/c-Si interface becomes weak on the degradation of open circuit voltage compared with the significant influence of the bulk trap centers. Moreover, τ_p^* of longer than 5×10^-5 s is necessary for the retention of electrical performance of HIT-QW solar cells by the simulation. Based on this research, high-efficiency HIT solar cells can be achieved by incorporating high-quality Si/Si_0.75Ge_0.25 quantum wells, which also requires the impactful passivation of a-Si:H/c-Si interface.