Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology , Thuwal 23955-6900, Kingdom of Saudi Arabia.
J Am Chem Soc. 2014 May 14;136(19):6952-9. doi: 10.1021/ja413254g. Epub 2014 Feb 25.
Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C61-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion.
量子点 (QD) 太阳能电池作为现有光伏技术的低成本替代品而备受关注。在这里，我们使用飞秒宽带瞬态吸收 (TA) 光谱和稳态光致荧光猝灭测量相结合的方法，研究了 PbS QD 和苯基-C61-丁酸甲酯 (PCBM) 界面处的电荷转移和分离。我们分析了四种不同 QD 尺寸和不同 PCBM 浓度下 PbS QD/PCBM 界面的超快电子注入和电荷分离。结果表明，通过量子尺寸效应调谐的能带排列是高效电子注入和电荷分离过程的关键因素。更具体地说，稳态和时间分辨数据表明，只有带隙大于 1eV 的小尺寸 PbS QD 才能在吸收光后将电子转移到 PCBM。我们表明，这些趋势是由于 QD 尺寸分布导致形成了 II 型界面能带排列所致。瞬态吸收数据表明，对于带隙达到 II 型排列的 QD，光激发的 PbS QD 向 PCBM 的电子注入发生在我们 120fs 的时间分辨率内，而在较小带隙 QD 样品中观察到的几乎所有信号都来自尺寸分布中的大带隙异常值。总之，我们的结果清楚地表明，通过工程化 QD 尺寸分布，可以将 QD 界面的电荷转移速率调谐几个数量级。本工作将推进量子点界面在太阳能转换中的设计和理解。
