Recombination Suppression in PbS Quantum Dot Heterojunction Solar Cells by Energy-Level Alignment in the Quantum Dot Active Layers
Recombination Suppression in PbS Quantum Dot Heterojunction Solar Cells by Energy-Level Alignment in the Quantum Dot Active Layers
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DOI：
10.1021/acsami.7b06552
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发表时间：
2018-08-08
影响因子：
9.5
通讯作者：
Shen, Qing
中科院分区：
文献类型：
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作者：
Ding, Chao;Zhang, Yaohong;Shen, Qing
利用不同尺寸量子点的空间能级梯度工程来增加量子点异质结太阳电池结处的载流子收集，是提高能量转换效率的一条有希望的途径。然而，目前相关研究的结果表明，QDHSC中的可变带隙结构将产生明显的增加，不是在照明电流密度上，而是在填充因子上。此外，对于这些梯度结构对QDHSCs光伏性能影响的机理还缺乏研究。本研究通过利用不同尺寸的量子点(四个量子点尺寸)的有序排列来设计有源层的能级排列(ELA)，从而开发了大气溶液处理的二氧化钛/硫化铅量子点/金QDHSCs。与未分级的器件(不含ELA)相比，优化的梯度结构(含ELA)太阳电池的短路电流密度(J(Sc))显著提高(21.4%)。结果表明，在平面、较薄的PbS QDHSCs中实现了J(Sc)值大于30 mA/cm(2)，开路电压(V-oc)和功率转换效率(PCE)也得到了提高。通过J(Sc)和V-oc的光强依赖关系和瞬时光电压衰减的表征，我们发现：(I)ELA结构作为电子阻挡层，减少了PbS/阳极界面的界面复合；(Ii)ELA结构可以驱动更多的载流子走向理想的收集电极，额外的载流子可以填充陷阱态，减少了PBS QDHSCs中陷阱辅助的复合。这项工作清楚地阐明了梯度QDHSC中复合抑制的机制，并证明了ELA结构对J(Sc)的改善作用。这项工作中描述的电荷复合机制将能够为QDHSCs的进一步改进提供线索，这甚至可能有利于其他类型的太阳能电池。
Using spatial energy-level gradient engineering with quantum dots (QDs) of different sizes to increase the generated carrier collection at the junction of a QD heterojunction solar cell (QDHSC) is a hopeful route for improving the energy-conversion efficiency. However, the results of current related research have shown that a variable band-gap structure in a QDHSC will create an appreciable increase, not in the illumination current density, but rather in the fill factor. In addition, there are a lack of studies on the mechanism of the effect of these graded structures on the photovoltaic performance of QDHSCs. This study presents the development of air atmosphere solution-processed TiO2/PbS QDs/Au QDHSCs by engineering the energy-level alignment (ELA) of the active layer via the use of a sorted order of differently sized QD layers (four QD sizes). In comparison to the ungraded device (without the ELA), the optimized graded architecture (containing the ELA) solar cells exhibited a great increase (21.4%) in short-circuit current density (J(sc)). As a result, a J(sc) value greater than 30 mA/cm(2) has been realized in planar, thinner absorption layer (similar to 300 nm) PbS QDHSCs, and the open-circuit voltage (V-oc) and power-conversion efficiency (PCE) were also improved. Through characterization by the light intensity dependences of the J(sc) and V-oc and transient photovoltage decay, we find that (i) the ELA structure, serving as an electron-blocking layer, reduces the interfacial recombination at the PbS/anode interface, and (ii) the ELA structure can drive more carriers toward the desirable collection electrode, and the additional carriers can fill the trap states, reducing the trap assisted recombination in the PbS QDHSCs. This work has clearly elucidated the mechanism of the recombination suppression in the graded QDHSCs and demonstrated the effects of ELA structure on the improvement of J(sc). The charge recombination mechanisms characterized in this work would be able to shed light on further improvements of QDHSCs, which could even benefit other types of solar cells.