High-resolution microscopy of carbon nanotubes/polymer composites for highly efficient organic solar cells

Marco Notarianni1, Andrea Capasso1, Ernesto Placidi 2, John Bell1, Claudia Caddeo3, Alessandro Mattoni3 and Nunzio Motta1

Scenario

1School of Chemistry Physics and Mechanical Engineering , Queensland University of Technology, Brisbane, Australia 2CNR-ISM, University of Rome Tor Vergata, Rome, Italy

3CNR-IOM SLACS, University of Cagliari, Cagliari, Italy

Goals

Università di Roma Tor Vergata

Theoretical Model Many research efforts have been recently pursued in the field of thin film and organic photovoltaics, with the goal of reaching a power-conversion efficiency that will make such solar cells commercially viable. CNTs, because of their electrical and mechanical properties, are considered a valid material to replace the fullerene derivative (PCBM) in organic solar cells (OSCs). OSCs are expected to be commercialized soon.

Characterization

Why CNTs in OSCs: • Low percolation threshold • High aspect ratio • High conductivity • Matching work function with fullerene derivative (PCBM) Bulk heterojunction OSCs: • ITO glass • PEDOT:PSS buffer layer (spin coating) • Blend of SWCNTs:P3HT (spin coating) • LiF buffer layer (thermal evaporation) • Al cathode (thermal evaporation)

This research has been partially funded by the Queensland government throught the Smart State NIRAP grant Solar Powered Nano Sensors.

Scanning Tunneling Microscopy at QUT

Background

OSCs

Coiling angle=50°

STM image confirms the presence of: • SWCNT (7,6) through the chiral angle of 57° • Longitudinal polymer backbone along the tube axis (in green) plus a wrapping polymer backbone (in red) is similar to the theoretical model

Future Research

• The SWCNTs and the P3HT were investigated by Molecular Dynamics (MD) simulations and their interaction is described by the sum of two body Lennard Jones • The equations of motion of atoms were integrated by using the velocity Verlet algorithm

Band diagram of a bulk heterojunction OSC

The model confirms the helical wrapping of the P3HT on SWCNT

• Better dispersion of the composite in different solutions • STM studies of the composite on Au substrate • STM studies of SWCNTs/PCPDTBT composite • Preparation and test of a solar cell in and outside a glovebox

Università di Cagliari

• UHV-STM study of the interaction and charge transfer between CNTs/conducting polymers • Proper dispersion of the CNTs in order to form an ordered composite with conducting polymers • Development and testing of a solar cell based on these composites

• Replacement of the fullerene derivative (PCBM) with carbon nanotubes (CNTs) as electron acceptor material in organic solar cells • Nanoscale studies of the composites P3HT/CNTs to understand the physical interaction • Improvement in the efficiency of the organic solar cells

• Solution of SWCNTs (15,0)/rr-P3HT 3% wt in DCB • Drop cast on HOPG + oven 65°C for 2h

• Solution of SWCNTs (7,6)/rr-P3HT 3% wt in DCB • Add cyclohexanone (10% in volume to DCB) • Drop cast on HOPG + oven 65°C for 2h

• STM studies of SWCNTs wrapped with polymers allow to understand the electronic properties of composites for solar cells

Conclusions

CRICOS No. 00213J

EXAM

PLE

PresenterPresentation NotesAbstract: We report on the direct synthesis of carbon nanotubes (CNTs) on two transparent conductive oxides commonly used as electrodes in organic photovoltaics: indium tin oxide (ITO) and fluorine doped tin oxide (FTO). The addition of an intermediate layer of CNTs is beneficial for the charge collection at the electrode, by providing additional and highly-conductive percolation paths for the charges [1]. A specific synthesis procedure has been established, allowing a good degree of control on the CNT properties without decreasing the conductivity and the optical transmittance of the oxide film.Introduction: Improvement in the power conversion efficiency of organic and dye-sensitized solar cells can be achieved by optimizing the materials used for the electrodes. CNTs in particular have been often proposed as suitable material to increase the photovoltaic performance, due to their excellent charge transport properties combined with very high aspect ratios [2].Experimental: Multi-walled carbon nanotubes (MWCNTs) were grown by CVD on FTO-glass (Pilkington, 8 Ω/sq) and on patterned ITO-glass (Kintec Company, 15 Ω/sq). The substrates were cleaned by ultrasonic baths in acetone, ethanol and de-ionized water. Thin layers of Ni (3 nm) were deposited as catalyst by thermal evaporation. After the metal deposition, the substrates were loaded into a CVD quartz tube, where the synthesis is performed under a constant flow of Ar/ H2/C2H2 between 500 and 600°C in 10 minutes, cooling the substrates in less than one minute. After CVD, the substrates were used to create test Organic and Dye-sensitized Solar Cells (OSC and DSSC) (Fig.1). Bulk heterojunction OSC were built in a nitrogen atmosphere glove-box by using a solution (1:0.7) of regio-regular poly(3-hexylthiophene) (rr-P3HT, Sigma-Aldrich) and phenyl-C61-butyric acid methyl ester (PCBM, from Solenne BV) diluted in ortho-dichlorobenzene and spin-coated at 400 rpm on our CNT-enhanced ITO coated glass. A 100 nm thick Al cathode was then thermally evaporated in high vacuum (2x10-6 mbar), by using a shadow-mask with 3 mm wide stripes. The final device has an active area of 25 mm2. The DSSCs have been created on CNT-enhanced FTO-glass substrates (2x2 cm2, 8 Ω/sq) cleaned by ultrasonic baths in acetone and ethanol. TiO2 films were distributed by squeegee printing, using a TiO2 paste (DSL 90T, Dyesol). The substrates were oven dried for 10 min at 100 °C to evaporate the solvent, and sintered raising the T up to 500°C for 30 min at 10°C/min. After sintering, the TiO2 substrates were placed into a 0.5 mM dye-ethanol solution (N719, Dyesol) overnight and then rinsed in ethanol. The cells were completed by sealing with a Dyesol Pt counter-electrode by a 60 µm-thick Surlyn gaskets. An electrolyte (EL-HSE, Dyesol) was inserted into the cell via vacuum backfilling. All the devices were tested under a calibrated sun simulator (Solar constant 1200 KH) at AM 1.5, 1000 W/m2. Reference cells with bare ITO and FTO coated-glass were also made for comparison with the same procedure.Results and Discussion: MWCNTs grew on all the substrates after CVD, as confirmed by SEM, EDX and Raman spectroscopy. The created CNT-enhanced electrodes preserve their transparency and provide a significant increase of the cell Voc (40%) and Isc (30%) for the OSC. For the DSSC the increase in Voc is of 350 mV and the overall PCE of the device is 3.7%, an excellent result for such an experimental cell. We believe that our fast CVD synthesis method is the key to obtain a well-distributed synthesis of MWCNTs and to preserve the properties of the underlying tin oxide films. We have been able to produce controlled mats of MWCNTs on ITO and FTO, increasing substantially the superficial area of the electrode and protruding into the active layer of the cell, with a significant improvement in the charge collection and extraction. In the case of DSSCs this approach could be exploited further, if one considers that FTO-glass is reported to withstand even higher temperature without increasing its sheet resistivity [3]. For the more temperature-sensitive OSCs, higher care during the CNTs synthesis have to be taken, but it is worth noting that as-treated electrodes can also better matching the Highest Occupied Molecular Orbital (HOMO) level of the polymer, helping the hole transport in the matrix and the charge collection to the anode.Acknowledgements�This work has been supported by the Queensland State Government through the NIRAP funding “Solar powered nanosensors“ and by the project ”Polo Solare Organico - Regione Lazio”. References�A.J. Miller et al. Applied Physics Letters, 90(2), 023105 (2007).M.C. Scharber et al. Advanced Materials, 18(6), 789 (2006).

All the devices (25 mm2 ) were tested under a calibrated sun simulator at AM 1.5, 1000 W/m2.OSCs showed:

Reference cells with bare ITO and FTO coated-glass were also made for comparison with the same procedure.

V. Zardetto V, T.M. Brown, A. Reale, A. Di Carlo. “Substrates for flexible electronics: a practical investigation on the electrical, film flexibility, optical, temperature, and solvent resistance properties”, Journal of Polymer Science Part B: Polymer Physics, 49(9): 638-648 (2011).

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