Department of Electrical and Computer Engineering, University of Pittsburgh, 1140 Benedum Hall, Pittsburgh, Pennsylvania 15261, USA.
Nano Lett. 2013 May 8;13(5):2086-91. doi: 10.1021/nl400395c. Epub 2013 Apr 18.
Carbon nanotubes have been proven to play significant roles in polymer-based solar cells. However, there is intensive debate on whether carbon nanotube behaves as a donor or acceptor in the semiconducting polymer:carbon nanotube composite. In this paper, we report a direct observation via Kelvin probe force microscopy (KPFM) that single walled carbon nanotubes (SWNTs) behave as hole transporting channels in poly(3-hexylthiophene-2,5-diyl) (P3HT)/SWNT heterojunctions. By comparing the surface potential (SP) change of SWNT in dark and under illumination, we observed that electrons are blocked from SWNT while holes are transferred to SWNT. This observation can be well-explained by our proposed band alignment model of P3HT/SWNT heterojunction. The finding is further verified by hole mobility measurement using the space charge limited current (SCLC) method. SCLC results indicate that the existence of small amount of SWNT (wt 0.5%) promotes device hole mobility to around 15-fold, indicating SWNT act as hole transfer channel. Our finding of hole transporting behavior of SWNT in P3HT/SWNT blend will provide a useful guidance for enhancing the performance of polymer solar cells by carbon nanotubes.
碳纳米管已被证明在基于聚合物的太阳能电池中发挥重要作用。然而，关于碳纳米管在半导体聚合物：碳纳米管复合材料中是作为供体还是受体存在，仍存在激烈的争论。在本文中，我们通过 Kelvin 探针力显微镜（KPFM）直接观察到，单壁碳纳米管（SWNTs）在聚（3-己基噻吩-2,5-二基）（P3HT）/SWNT 异质结中充当空穴传输通道。通过比较 SWNT 在黑暗中和光照下的表面电势（SP）变化，我们观察到电子被阻挡在 SWNT 之外，而空穴则转移到 SWNT 上。这一观察结果可以通过我们提出的 P3HT/SWNT 异质结能带排列模型得到很好的解释。通过空间电荷限制电流（SCLC）方法进行空穴迁移率测量进一步验证了这一发现。SCLC 结果表明，少量 SWNT（wt 0.5%）的存在将器件的空穴迁移率提高到约 15 倍，表明 SWNT 充当空穴传输通道。我们发现 SWNT 在 P3HT/SWNT 共混物中的空穴传输行为，将为通过碳纳米管来提高聚合物太阳能电池的性能提供有用的指导。
