Due to the high cost, brittle nature, and suboptimal electronic and chemical properties of indium tin oxide (ITO), [ 1 − 4 ] alternative transparent conducting anode materials have played an increasingly important role in organic photovoltaic (OPV) device research. For example, thin fi lms of single-walled carbon nanotubes (SWNTs) have been identifi ed as a promising option due to their excellent electronic properties, solution processability, elemental abundance, environmental stability, and robust mechanical fl exibility. [ 5 − 7 ] Recent reports have demonstrated OPVs incorporating SWNT fi lms as the transparent anode, with the primary barrier to greater effi ciencies being the relatively high sheet resistance of the SWNT fi lm. [ 8 − 10 ] One of the common methods employed to overcome this obstacle has been chemical doping of the fi lms prior to device fabrication, either intentionally or as a byproduct of roughnessreducing acid treatments. In particular, the adsorption of electron-withdrawing species both lowers the SWNT fi lm sheet resistance and bleaches the primary peaks in the optical absorption spectrum, thereby increasing the fi lm transparency. [ 11 − 14 ] However, this chemical doping strategy introduces limited environmental stability, which compromises performance in many device applications. [ 5 , 15 ] Additionally, because previous studies of SWNT-based OPV anodes have employed thin fi lms formulated from electronically polydisperse SWNT mixtures, the role and relative importance of metallic versus semiconducting SWNTs has not been clarifi ed.Herein, we present the use of electronically monodisperse arc discharge SWNTs, sorted via density gradient ultracentrifugation (DGU), [ 7 , 16 ] as the transparent anode material in OPVs. By varying the ratio of semiconducting and metallic species in the SWNT thin fi lms, we fi nd that a composition of 70% or greater metallic SWNTs affords 50× higher OPV power conversion effi ciency (PCE) than monodisperse semiconducting SWNT thin fi lms. Analysis of the stability after chemical doping with nitric acid, which is used to lower the fi lm roughness, indicates that the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electron-blocking layer reverses the effects of the doping process and reduces the SWNT electrical conductivity. X-ray photoelectron spectroscopy (XPS) further reveals that nitric oxide (NO) is the primary adsorbed dopant species that is removed upon PEDOT:PSS deposition. Since the electronic and optical properties of metallic SWNTs are less affected by chemical doping, they remain effective transparent conductors following OPV fabrication, thus explaining the 50× difference in device PCE for metallic versus semiconducting SWNT-enriched anodes. Overall, this study establishes that SWNT chemical doping is incompatible with PEDOT:PSS, thus demonstrating the importance of metallic SWNT-enriched materials in OPV anodes.Previous studies recognized the importance of minimizing the roughness of SWNT thin fi lms in organic electron...

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Abstract

The method by which surfactants selectively interact with particular electronic types of single-walled carbon nanotubes (SWCNTs) and thereby enable the isolation of metallic and semiconducting species is not well understood. While density gradient ultracentrifugation (DGU) has demonstrated its potential as a powerful nanomaterial separation technique, this study utilizes DGU as an analytic tool to probe the interactions between amphiphilic block copolymers, surfactants capable of electronic type extraction, and the SWCNT surface. By modulating the pH during DGU, we find that the linear shaped Pluronic copolymers can extract either metallic or semiconducting SWCNTs at purities in excess of 99%. Furthermore, the first electronic type sorting mechanism is given by which oxygen absorption and subsequent protonation of the SWCNT surface acts to template copolymer adhesion. Detailed characterization reveals the underlying mechanism for pHshifted DGU and is thus likely to enable future development of more efficient and facile SWCNT electronic type sorting methods.

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