半导体带尾和带填充对光伏效率极限的影响
Impact of Semiconductor Band Tails and Band Filling on Photovoltaic Efficiency Limits
摘要：
太阳能电池的理论最大效率通常由半导体在单位辐射效率的极限中的光吸收和发射的细致平衡以及半导体带边缘处的状态密度和吸收的理想阶跃函数响应（称为肖克利-奎瑟极限）来表征。然而，真实材料具有非突变的能带边缘，其典型特征在于状态的指数分布，称为Urbach尾。在这里，我们开发了一个修改后的细致平衡限制的太阳能电池与不完善的带边，使用光电互易关系。我们发现，对于半导体的带边比热能，kT更宽，有一个有效的重整化带隙内的太阳能电池的准费米能级分裂。这种重整带隙在吸收和光致发光发射能量的开始之间产生斯托克斯位移，这显著降低了最大可实现效率。因此，带边态密度的不稳定性对于最大可实现的光伏效率具有重要意义。
英文摘要：
The theoretical maximum efficiency of a solar cell is typically characterized by a detailed balance of optical absorption and emission for a semiconductor in the limit of unity radiative efficiency and an ideal step-function response for the density of states and absorbance at the semiconductor band edges, known as the Shockley-Queisser limit. However, real materials have non-abrupt band edges, which are typically characterized by an exponential distribution of states, known as an Urbach tail. We develop here a modified detailed balance limit of solar cells with imperfect band edges, using optoelectronic reciprocity relations. We find that for semiconductors whose band edges are broader than the thermal energy, kT, there is an effective renormalized bandgap given by the quasi-Fermi level splitting within the solar cell. This renormalized bandgap creates a Stokes shift between the onset of the absorption and photoluminescence emission energies, which significantly reduces the maximum achievable efficiency. The abruptness of the band edge density of states therefore has important implications for the maximum achievable photovoltaic efficiency.
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