InAs/GaAsSb II类量子点载流子动力学特性及其在太阳电池中应用

Intermediate band solar cells (IBSC) enable broadening the useful range of the solar spectra by absorbing the below band-gap energy photons, realizing high convention efficiency. InAs/GaAs quantum dots are used to make the intermediate band solar cells whereas the open circuit voltages reduce and the efficiencies are lower than the single gap solar cells. One reason is that the addition of multiple InAs QD layers builds up excessive strain and leads to defects, which act as non-radiative recombination centers and reduce the open circuit voltage. Another reason is that the fast electron relaxation from the conduction band (CB) to the intermediate band (IB), associated to the intraband Anger recombination, jeopardizes the quasi-Fermi level separation between CB and IB and open circuit voltage preservation in the IBSC. Base on the research on the photocurrent generation mechanism of the quantum dot solar cells, we use InAs/GaAsSb type II QDs instead of InAs/GaAs type I QDs to make the intermediate band solar cells. With the appropriate band structure engineering, electrons and holes are separated, and the intraband Anger recombination rate reduces. The material is grown by molecular beam epitaxy. We will research on the carrier dynamics of type II QDs and design its band structure to achieve the control of the carrier recombination process. The effect of InAs/GaAsSb QDs on the performance of the solar cells will be evaluated. If the quasi-Fermi level separations between CB and IB and also between IB and valence band (VB) are sustainable upon external excitation, the open circuit voltage is preserved, and ultimately its efficiency is enhanced. The accomplishment of this project will provide reliable theoretical support for the quantum dot intermediate band solar cells.

量子点中间带太阳电池通过对低于带隙能量光子的吸收，拓宽了太阳能光谱的利用范围，具有高的理论效率。然而，电池嵌入量子点后开路电压下降，使其整体效率远低于体材料电池。开路电压的降低，除了多层InAs量子点引入位错造成非辐射复合增强之外；快速的带内俄歇复合，破坏了导带和中间带准费米能级的分离是导致开路电压降低的另一个重要原因。本项目基于对量子点太阳电池光电流产生物理机理的分析，采用分子束外延生长方法，在GaAs电池结构中内嵌InAs/GaAsSb量子点，设计II类能带结构，使得电子和空穴波函数分离；通过对II类量子点载流子动力学以及量子点的存在对电池性能影响的研究，实现对载流子复合过程的控制，使得光偏置条件下电池导带、中间带、价带具有独立的准费米能级，解决电压下降问题；优化器件结构，最终实现中间带太阳电池的效率增加。项目的顺利开展，必为未来量子点中间带电池的广泛应用提供可靠的方案和理论支撑。

InAs/GaAs量子点导带上的电子会通过带内俄歇复合快速弛豫到中间带，破坏了导带和中间带准费米能级的分离，导致量子点中间带太阳电池开路电压降低。本项目利用II型能带结构对电子和空穴的分离作用，抑制载流子复合，提高开路电压。采用八带k•p法计算InAs/GaAsSb量子点能带结构，结果表明当Sb组分为0.14时能带从I型转变为II型，电子空穴波函数交叠迅速减小，复合被抑制，基态发光强度下降到与激发态相比拟，跃迁能量与文献报道一致。利用分子束外延技术生长出高质量InAs/GaAsSb量子点，平均高度9.6nm，直径45.5nm，密度2.15E10cm-2。光谱测试结果表明量子点能带从I型转变为II型，载流子弛豫时间相比InAs/GaAs量子点增加一个量级，带内复合和带间复合被抑制。在GaAs电池结构中内嵌InAs/GaAsSb量子点，利用II类能带结构对电子和空穴的分离作用实现对载流子复合过程的控制，开路电压与内嵌InAs/GaAs的太阳电池相比提高7.5%。通过量子点中掺杂进一步提高太阳电池性能，开路电压增加16%。利用自洽漂移扩散模型计算量子点中间带太阳电池，结果表明短路时导带和价带载流子浓度低，中间带起产生中心的作用，短路电流增加；开路时导带和价带载流子浓度高，中间带起复合中心的作用，开路电压降低。导带上电子快速弛豫回中间带是InAs/GaAs量子点太阳电池开路电压明显降低的主要原因，抑制量子点中载流子的带间和带内复合，可提高电池开路电压。同时指出量子点内光生载流子产生率低限制了中间带太阳电池的效率，要进一步提高效率，需要提高双光子跃迁过程载流子的产生效率。通过本项目的研究，阐明了中间带太阳电池的工作机理，提高了内嵌量子点太阳电池的开路电压，为中间带太阳电池的进一步发展提供技术方案和理论支持。