Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, United States.
ACS Nano. 2010 Nov 23;4(11):6599-606. doi: 10.1021/nn1018297. Epub 2010 Oct 28.
Charge transfer at the interface of conjugated polymer and nanoscale inorganic acceptors is pivotal in determining the efficiency of excitonic solar cells. Despite intense efforts, carbon nanotube/polymer solar cells have resulted in disappointing efficiencies (<2%) due in large part to poor charge transfer at the interface. While the interfacial energy level alignment is clearly important, the self-assembly and the interface structure also play a major role in facilitating this charge transfer. To understand and control this effect to our advantage, we study the interface of commonly used conductive polymer poly-3-hexylthiophene (P3HT) and single-walled carbon nanotubes (SWNTs) with a combination of molecular dynamics simulations, absorption spectra experiments, and an analysis of charge transfer effects. Classical molecular dynamics simulations show that the P3HT wraps around the SWNTs in a number of different conformations, including helices, bundles, and more elongated conformations that maximize planar π-π stacking, in agreement with recent experimental observations. Snapshots from the MD simulations reveal that the carbon nanotubes play an important templating role of increasing the π-conjugation in the system, an effect deriving from the π-π stacking interaction at the interface and the 1-dimensional (1D) nature of the SWNTs, and independent of the SWNT chirality. We show how this increase in the system conjugation could largely improve the charge transfer in P3HT-SWNT type II heterojunctions and support our results with absorption spectra measurements of mixtures of carbon nanotubes and P3HT. These findings open possibilities for improved preparation of polymeric solar cells based on carbon nanotubes and on 1D nanomaterials in general.
在共轭聚合物和纳米级无机受体的界面处发生电荷转移对于确定激子太阳能电池的效率至关重要。尽管付出了巨大的努力，但碳纳米管/聚合物太阳能电池的效率却令人失望（<2%），这在很大程度上是由于界面处的电荷转移不良所致。虽然界面能级排列显然很重要，但自组装和界面结构在促进这种电荷转移方面也起着重要作用。为了理解和控制这种效应，我们使用分子动力学模拟、吸收光谱实验以及对电荷转移效应的分析，研究了常用导电聚合物聚 3-己基噻吩（P3HT）和单壁碳纳米管（SWNTs）的界面。经典分子动力学模拟表明，P3HT 以多种不同构象缠绕在 SWNTs 周围，包括螺旋、束和更伸长的构象，这些构象最大限度地增加了平面π-π堆叠，与最近的实验观察结果一致。MD 模拟的快照显示，碳纳米管在增加系统中的π共轭方面起着重要的模板作用，这种效应源于界面处的π-π堆叠相互作用和 SWNTs 的 1 维（1D）性质，并且与 SWNT 手性无关。我们展示了这种系统共轭度的增加如何在很大程度上改善 P3HT-SWNT 型 II 异质结中的电荷转移，并通过碳纳米管和 P3HT 混合物的吸收光谱测量来支持我们的结果。这些发现为基于碳纳米管和一般 1D 纳米材料的聚合物太阳能电池的改进制备开辟了可能性。
