• PDF / 1,457,313 Bytes
• 7 Pages / 432 x 648 pts Page_size
• 70 Downloads / 285 Views

DOWNLOAD

REPORT

---

All-Carbon Composite for Photovoltaics Alvin T.L. Tan, Vincent C. Tung, Jaemyung Kim, Jen-Hsien Huang, Ian Tevis, Chih-Wei Chu, Samuel I. Stupp and Jiaxing Huang Department of Materials Science and Engineering, Northwestern University, Evanston, USA ABSTRACT Graphitic nanomaterials such as graphene, carbon nanotubes (CNT), and C60 fullerenes are promising materials for energy applications because of their extraordinary electrical and optical properties. However, graphitic materials are not readily dispersible in water. Strategies to fabricate all-carbon nanocomposites typically involve covalent linking or surface functionalization, which breaks the conjugated electronic networks or contaminates functional carbon surfaces. Here, we demonstrate a facile surfactant-free strategy to create such all-carbon composites. Fullerenes, unfunctionalized single walled carbon nanotubes, and graphene oxide sheets can be conveniently co-assembled in water, resulting in a stable colloidal dispersion amenable to thin film processing. The thin film composite can be made conductive by mild thermal heating. Photovoltaic devices fabricated using the all-carbon composite as the active layer demonstrated an on-off ratio of nearly 106, an open circuit voltage of 0.59V, and a power conversion efficiency of 0.21%. This photoconductive and photovoltaic response is unprecedented among all-carbon based materials. Therefore, this surfactant-free, aqueous based approach to making all-carbon composites is promising for applications in optoelectronic devices. INTRODUCTION Graphitic nanostructures such as carbon nanotubes and fullerenes (C60) can be used to facilitate separation, transport and collection of charge carriers in organic photovoltaic devices.1-8 Fullerenes are highly electron accepting and behave as n-type semiconductors.9 On the other hand, semiconducting carbon nanotubes10,11 and graphene-based sheets12,13 exhibit p-type-like behavior in ambient conditions. Therefore, nanoscopic heterojunctions can be generated if these carbon nanostructures are brought together.1-7,14 Such an approach employing all carbon nanostructures also combines the unique properties of the constituents. Fullerenes are known to exhibit ultra-fast electron transfer9, while semiconducting carbon nanotubes have high absorption in the near IR range and exhibit extraordinary charge carrier mobility.10,11 Therefore, a hybrid material of C60 and carbon nanotubes can be used as a photovoltaic material with good absorption, good charge separation, and efficient carrier transport. However, CNT and C60 nanomaterials tend to aggregate in aqueous solution. To overcome their poor solubility, fabrication of all-carbon nanocomposites typically involves covalently linking of the constituents, extensive surface functionalization, or the use of nonconducting surfactants.1-7 These strategies either break the conjugated carbon networks or contaminate the functional heterojunction interfaces. Recently, we showed that graphene oxide (GO), the chemical exfoliation product of graphite, c