Ding Chao, Zhang Yaohong, Liu Feng, Kitabatake Yukiko, Hayase Shuzi, Toyoda Taro, Wang Ruixiang, Yoshino Kenji, Minemoto Takashi, Shen Qing
Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan.
Nanoscale Horiz. 2018 Jul 1;3(4):417-429. doi: 10.1039/c8nh00030a. Epub 2018 Apr 3.
In quantum dot heterojunction solar cells (QDHSCs), the QD active layer absorbs sunlight and then transfers the photogenerated electrons to an electron-transport layer (ETL). It is generally believed that the conduction band minimum (CBM) of the ETL should be lower than that of the QDs to enable efficient charge transfer from the QDs to the collection electrode (here, FTO) through the ETL. However, by employing Mg-doped ZnO (ZnMgO) as a model ETL in PbS QDHSCs, we found that an ETL with a lower CBM is not necessary to realize efficient charge transfer in QDHSCs. The existence of shallow defect states in the ZnMgO ETL can serve as additional charge-transfer pathways. In addition, the conduction band offset (CBO) between the ETL and the QD absorber has been, for the first time, revealed to significantly affect interfacial recombination in QDHSCs. We demonstrate that a spike in the band structure at the ETL/QD interface is useful for suppressing interfacial recombination and improving the open-circuit voltage. By varying the Mg doping level in ZnO, we were able to tune the CBM, defect distribution and carrier concentration in the ETL, which play key roles in charge transfer and recombination and therefore the device performance. PbS QDHSCs based on the optimized ZnMgO ETL exhibited a high power conversion efficiency of 10.6%. Our findings provide important guidance for enhancing the photovoltaic performance of QD-based solar cells.
在量子点异质结太阳能电池（QDHSCs）中，量子点活性层吸收太阳光，然后将光生电子转移到电子传输层（ETL）。一般认为，ETL的导带最小值（CBM）应低于量子点的导带最小值，以便使电荷能够通过ETL从量子点有效地转移到集电极（此处为FTO）。然而，通过在PbS QDHSCs中采用Mg掺杂的ZnO（ZnMgO）作为模型ETL，我们发现对于在QDHSCs中实现有效的电荷转移而言，具有较低CBM的ETL并非必要条件。ZnMgO ETL中浅缺陷态的存在可以作为额外的电荷转移途径。此外，首次揭示了ETL与量子点吸收体之间的导带偏移（CBO）会显著影响QDHSCs中的界面复合。我们证明，ETL/量子点界面处能带结构的尖峰有助于抑制界面复合并提高开路电压。通过改变ZnO中的Mg掺杂水平，我们能够调节ETL中的CBM、缺陷分布和载流子浓度，这些因素在电荷转移和复合过程中起着关键作用，进而影响器件性能。基于优化后的ZnMgO ETL的PbS QDHSCs表现出10.6%的高功率转换效率。我们的研究结果为提高基于量子点的太阳能电池光伏性能提供了重要指导。
