CdTe类光伏半导体中点缺陷的载流子俘获截面及少子寿命的计算和测量

The short lifetime of minority carriers is one important factor limiting the efficiency of CdTe, Cu2ZnSnS4 and related solar cells. There are many deep-level point defects in these photovoltaic semiconductors which may induce the non-radiative recombination of electrons and holes, and thus decrease the lifetime of minority carriers. However, it is difficult to measure the carrier capture cross-section of each defect experimentally and identify the most effective recombination-center defect, so there are still controversies about how to control the defect concentration in order to increase the lifetime of minority carriers. Therefore the computation of the carrier capture cross-section becomes very important. Because the carrier capture is a multiphonon assisted electronic transition, the first-principles calculation of the cross-section was very challenging in the past 60 years. Recently a new method was developed for the quick calculation of electron-phonon coupling constants of a defect supercell, which made the calculation of the cross-section in GaN possible. We will adopt and improve this new method for calculating the electron and hole capture cross-section of the deep-level defects in these photovoltaic semiconductors. Through including the contribution of all defects, we can calculate the lifetime of minority carriers in the bulk region of the semiconductor samples and study its change as a function of the chemical composition and the temperature. Meanwhile, the carrier capture cross-section of these defects will be measured by the deep-level transient spectroscopy, and the lifetime of the minority carriers in the bulk region of the semiconductor samples will be measured by the two-photon-excitation time-resolved photoluminescence which is not influenced by the surface recombination. Then the calculated and measured results can be compared directly. With the direct comparison, a reliable calculation method will be developed, and utilized to identify the most detrimental recombination-center defect in these photovoltaic semiconductors and then search for methods to increase the lifetime of the minority carriers.

少数载流子寿命短是CdTe、Cu2ZnSnS4等太阳能电池效率偏低的一个重要原因。这些光伏半导体中存在多种深能级点缺陷，可能诱导电子-空穴的非辐射复合，降低少子寿命，但是，由于实验上难以确定哪种缺陷的载流子俘获截面大，如何调控缺陷浓度以提高少子寿命缺乏明确方向。因此俘获截面的计算非常重要。缺陷俘获载流子是多声子辅助的电子跃迁过程，60年来直接计算非常困难，直到最近缺陷态电声耦合常数的快速计算方法取得突破后，才在GaN体系取得成功。本项目拟推广该方法，计算光伏半导体中深能级点缺陷的电子、空穴俘获截面，综合各种缺陷的影响，计算半导体体内少子寿命随化学成分和温度的变化。同时，采用深能级瞬态谱测量俘获截面；采用双光子激发的时间分辨荧光技术，避免表面复合等影响，测量体内少子寿命，使计算和测量结果可直接对比，从而确定可靠的计算方法，探明这类光伏半导体中最关键的复合中心缺陷，寻找提高少子寿命的有效方法。

提升少数载流子寿命是改善CdTe, Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4类薄膜太阳能电池性能的重要途经，但是，这些薄膜光伏半导体中可能存在众多深能级点缺陷，诱导电子-空穴的非辐射复合，降低少子寿命。由于多种缺陷共存，并且能级可能接近简并，实验上难以确定哪种缺陷的载流子俘获截面大、是有效的复合中心，因此，难以厘清各种缺陷对少子寿命的定量影响，导致通过缺陷调控以提高少子寿命的研究缺乏明确方向。.本项目将近年来GaN体系发展的俘获截面计算方法推广应用到带隙1.5 eV左右的光伏半导体上，发展了基于静态耦合理论的点缺陷对载流子俘获截面和非辐射复合速率的计算方法和流程，并在开源的第一性原理计算软件包Quantum Espresso上实现了相关程序。将上述计算方法应用到CdTe, Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4类薄膜光伏半导体和近年来新兴的CH3NH3PbI3和Sb2Se3等光伏半导体上，计算了其中高浓度的深能级点缺陷对电子和空穴的俘获截面，确定了有效的复合中心缺陷。结合缺陷浓度和离化能级位置的计算，计算了Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4和CH3NH3PbI3三种体系中少子寿命随样品成分、温度和费米能级位置的变化关系，提出了调控缺陷以延长少子寿命、提高光伏效率的理论建议。提出了一种快速辨别缺陷是否有效复合中心的经验判据：被强化学键包围、不同价态之间结构弛豫较大的深能级缺陷可能具有较大的载流子俘获截面，是潜在的非辐射复合中心。将Cu2ZnSn(S,Se)4体系的计算结果与深能级瞬态谱测量的俘获截面、时间分辨荧光技术测量的薄膜内部少子寿命进行了对比，实现了计算和测量结果在数量级上的相符。探明了Cu2ZnSn(S,Se)4中最关键的复合中心缺陷和带尾态的来源缺陷，从理论上提出了抑制带尾态和复合中心缺陷的方法，并与实验组合作制备了不同组分的Cu2ZnSn(S,Se)4薄膜，实现了带尾态和复合中心缺陷的抑制，提高了少子寿命和电池效率。.半导体缺陷的浓度、能级和载流子俘获截面是电子器件和光电器件设计必需的关键参数，本项目发展的计算方法和程序可以定量计算出这些性质，为器件设计提供参数。