Electronically Monodisperse Single‐Walled Carbon Nanotube Thin Films as Transparent Conducting Anodes in Organic Photovoltaic Devices

Abstract

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...

ITO‐Free and Flexible Organic Photovoltaic Device Based on High Transparent and Conductive Polyaniline/Carbon Nanotube Thin Films

“…Contact angle measurements have shown that the PEDOT:PSS solution has a better wettability with the PANI:CNT electrode than with ITO (as can be seen in Figure S4, Despite the great advantages of the PANI:CNT fi lms, their properties do not seem to affect other device characteristics. The fi ll factors (FF), [ 41 ] which are ultimately related to the series resistance of the device (limited in these cases by the anode), are very similar for devices built using ITO, FTO, and PANI:CNT. The V OC value of the PANI:CNT device is also very similar to that observed for ITO device.…”

Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing

“…373 Since nitric acid treated semiconducting SWCNTs exhibit lower sheet resistance than comparably treated metallic SWCNTs, 382,383 the dramatically reduced PCE for semiconducting SWCNTs appears counterintuitive. However, it was shown that the films were dedoped by the hole transport layer PEDOT:PSS in the fabricated OPVs, 381 thus compromising the sheet resistance of the semiconducting SWCNTs to a much greater degree than the metallic SWCNTs. Further improvements in sheet resistance and thus OPV power conversion efficiency can be achieved with longer CNTs as is evident in Fig.…”

“…418 Doping strategies have been attempted by many groups to achieve higher carrier concentrations and thus lower R s values in graphene. Following early reports on CNTs, 381,382 doping of graphene has been achieved through chemical treatments with nitric acid and gold chloride. 329,[419][420][421][422] However, even these doped CVD graphene anode devices fail to surpass the performance of control ITO anode devices.…”

“…It is also well known that metallic and semiconducting CNTs interact differently with adsorbates and dopants. 381,494,495 Monodisperse CNTs thus present an opportunity for improved sensing as explored in two recently published reports. 496,497 In a paper by Nakano et al,…”

Carbon Nanotube Transparent Electrode

“…In our homes or offices, they are found in flat panel displays such as in TVs, laptops and in touch panels, of phones, tablet computers, E-readers and digital cameras [1]. Besides, they are also used as the electrodes for photovotaic devices such as solar cells [2] and organic light-emitting diodes (OLEDs) [3]. Liquid crystal display (LCD) is by far the largest user of transparent conductive films but many devices are showing rapid growth in popularity such as touch panels (362 million units in 2010 with annual growth of 20% through 2013), E-paper (30 fold growth expected from 2008 to 2014), and thin film solar cells (expected sales of over $13 billion by 2017) [4].…”