Aligned carbon nanotube webs as a replacement for indium tin oxide in organic solar cells
Highlights
► Drawable carbon nanotube webs were used as an anode in bulk heterojunction cells. ► One and two layers of carbon nanotube webs were compared. ► A thick active layer of ~ 530 nm was needed to avoid shunting through nanotubes. ► Two layers of web gave the better efficiency of 1.6%. ► Flexible devices on Mylar were demonstrated with 1.2% efficiency.
Introduction
Organic solar cells are attractive for low cost, flexible devices due to their compatibility with plastic substrates and high throughput, reel to reel processing. However, to achieve fully flexible organic solar cells, alternative materials are needed to replace Indium Tin Oxide (ITO, In2O3:Sn) which conventionally serves as the transparent electrode for hole collection. ITO suffers from a number of drawbacks. It is expensive, cannot be solution processed and is brittle making it unsuitable for applications requiring high flexibility. Conductive, flexible films fabricated from carbon nanotubes (CNTs) are a promising alternative to ITO and have attracted considerable interest with a number of devices demonstrated [1], [2], [3], [4], [5], [6], [7], [8], [9]. Ago et al. was one of the first to use CNTs as a hole collecting electrode [1]. These devices consisted of a spin-coated layer of CNTs and an active layer of poly(phenylenevinylene) (PPV) but gave poor efficiency [1]. More recently a number of devices based on the more efficient phenyl-C61-butyric acid methyl ester (PCBM)/poly-(3-hexylthiophene) (P3HT) system have been reported with efficiencies of up to 2.7% [4], [5], [6], [7].
These previous studies focused primarily on single-walled CNT (SWNT) films formed by first purifying and dispersing the carbon nanotubes in solution and then applying the dispersion to the substrate by a variety of methods (e.g. spin coating, transfer printing or spraying). This work investigates the use of multi-walled CNT (MWNT) webs, which are drawn in a dry state directly from the edge of a forest of aligned, ‘spinnable’ MWNTs (Fig. 1c) [10], [11], avoiding the often time consuming steps of purification and dispersion. ‘Spinnability’ is not fully understood but is known to be related to properties of the CNT forest such as purity, diameter, forest height, degree of entanglement, and adhesion to the substrate [11]. Once spinnability is achieved, a continuous web of free standing CNTs can be drawn from the forest edge (Fig. 1c). The MWNT webs are readily applied to a variety of substrates and are amenable to reel–reel fabrication. Working solar cells using similar MWNT webs were demonstrated by Ulbricht et al. with a power conversion efficiency of 1.3% [8], [9]. Here we further extend this work to investigate (i) the effect of active layer thickness and (ii) the trade off between CNT sheet resistance and transparency by varying the number of MWNT web layers.
Section snippets
Experimental details
Device structures were fabricated as shown in Fig. 1(a, b) consisting of unpatterned poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and P3HT:PCBM layers sandwiched between crossed bottom and top stripe electrodes. This device layout is commonly used in the organic solar cell community to screen and compare new materials and processing conditions. Although the 5 device pixels on each substrate are not strictly isolated, a high resistivity PEDOT:PSS formulation is used to
Trade-off between transparency and sheet resistance
High transparency and low sheet resistance are two of the most important characteristics for transparent electrodes but are also conflicting requirements. For our MWNT webs, the transparency follows Beer–Lambert's law. Thus, the optical absorbance is directly proportional to the number of layers and hence total thickness. Conversely, the sheet resistance is inversely proportional to the number of MWNT web layers (Fig. 2a). Increasing the number of web layers to reduce sheet resistance results
Conclusions
P3HT/PCBM bulk heterojunction solar cells were fabricated with MWNT webs as the hole collecting electrodes. To investigate the trade-off between transparency and resistivity, either a single or two (double) layers of MWNT web were used for the electrode. While the double MWNT web has lower transparency, this was compensated for by its higher conductivity. The double web device led to a device efficiency of 1.66%, one of the highest reported for MWNT electrodes. Even higher efficiencies should
Acknowledgements
The authors would like to thank Katalin Hegedus and Dr. Lynn Rozanski for assistance with device fabrication. Marta Redrado Notivoli is also kindly acknowledged for her expert advice on MWNT webs. This work was partially supported by the Victorian Organic Solar Cells Consortium (Victorian Department of Primary Industries, Sustainable Energy Research and Development Grant). K. Sears would also like to acknowledge the financial support of a CSIRO OCE post-doctoral fellow scholarship. P3HT for
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