Letter pubs.acs.org/NanoLett
Direct Observation of Hole Transfer from Semiconducting Polymer to Carbon Nanotubes Fei Lan and Guangyong Li* Department of Electrical and Computer Engineering, University of Pittsburgh, 1140 Benedum Hall, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *
ABSTRACT: 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. KEYWORDS: Carbon nanotube, P3HT, charge transfer, Kelvin probe force microscopy
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CNTs, on the other hand, reduces the efficiency due to the increased bimolecular recombination as well as possible circuit shorting.6a Thus, a proper ratio of CNT to polymer is critical for the improvement of power conversion efficiency of CNT based polymer solar cells. Another issue that is critical in enhancing the performance of polymer based solar cells is balancing the carrier mobility considering the relatively low hole mobility in polymers. Thus, understanding the charge transfer mechanism at the polymer/CNT interface, that is, whether CNTs work as donor (transporting holes) or acceptor (transporting electrons), is of great interest to the research community. Many studies concerning the interaction mechanism between CNT and polymer have been reported. Photoluminescence, for instance, has been used in characterizing CNT behavior in polymer blend. The photoluminescence quenching at the presence of CNT indicates that either charge transfer or energy transfer exists in such a blend.9 A study by Geng et al. showed photoluminescence of polymer quenched at the presence of
he incorporation of carbon nanotubes (CNTs) into photovoltaic devices with conjugated polymer was first reported by Kymakis and Amaratunga.1 Since then this type of photovoltaic devices has received a tremendous amount of attention.2 Organic solar cells (OSCs) based on polymer/ CNTs composite were reported to possess high open-circuit voltage (Voc)3 which may reach up to 1.8 V.4 However, these polymer/CNTs based photovoltaic devices usually showed limited efficiency compared with those based on polymer/ fullerene bulk heterojunctions. One possible reason is that interfacial areas between polymer and CNT are insufficient for exciton dissociation considering the insolubility and low concentration of CNT in polymer (
