Advanced Technologies for Smart Windows.

LPE graphene ink for printing on organic solar cells

SEM image of graphene flakes
Graphene ink
Scheme of organic solar cell

1 The idea behind

Graphene can be produced by methods like growth on metal substrates by Chemical Vapor Deposition (CVD) or annealing SiC substrates [1]. However, for industrial applications also Liquid Phase Exfoliation (LPE) is a prominent way for a high-yield production of graphene flakes [2] (Figure 1). In this process a dispersion of graphene in a solvent is obtained that can be used as ink for inkjet printing (Figure 2). The top electrode in organic photovoltaic (OPV) solar cells is usually silver deposited by thermal evaporation [3]. Our goal was to replace the silver grid by an inkjet printed graphene pattern.

A common solvent for LPE of graphite is N-methyl-pyrrolidone (NMP) [4]. However, it is forbidden to use with most of the print-heads, obviously, due to its high dissolution power. Moreover the reproduction toxicity of NMP can reduce the working place safety.Several suitable solvents for printing were investigated [5]; however, no organic solvent based graphene ink could combine all requirements: stability, compatibility with the substrate and high conductivity. Therefore it was decided to focus on the printing of graphene as hole-transport-layer (HTL). The HTL is the underlaying layer underneath the top electrode (Figure 3) and enables the transport of holes towards the top electrode and blocks electrons due to the favorable energy level.

2 Advantages

PEDOT:PSS,  a transparent, conductive polymer, is widely used as HTL. However, due to its sensitivity to moisture and oxygen its use leads to limited life times even for encapsulated cells [6]. To improve stability we replace the PEDOT:PSS by inkjet printed graphene. Water can be now chosen as solvent since it is compatible with the underlying active polymer in bulk-heterojunction solar cells.  Furthermore, water is compatible with print heads, low-cost and ecologically harmless.

3 Results

Graphite is successfully exfoliated in water and additionally concentrated to produce a graphene ink with 1 mg/ml. For inkjet printing the graphene flakes have to be smaller than 1 µm and the ink is further optimized for inkjet printing concerning viscosity and boiling point with additives. This is then incorporated into inverted solar cells with an active area of 27 mm² by inkjet printing on Glass/ITO/ZnO/P3HT:PCBM substrates in ambient atmosphere. Silver is used as top electrode.  The cells showed efficiencies of up to 1.8 %.

Up-scaling processes and optimization of the ink are currently under investigation.

4 Contacts

For LPE Graphene ink, please contact:
Andrea .C. Ferrari
University of Cambridge
+44 - 1223 - 748351

For Graphite ink, please contact:
Iurii Gnatiuk
TIGER Coatings GmbH
+43 / (0)664 / 60 400 115


5. References

[1] F. Bonaccorso, A. Lombardo, T. Hasan, Z. Sun, L. Colombo, A.C. Ferrari, Materials Today, 15 (2012) 564-589.
[2] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z. Sun, S. De, I.T. McGovern, B. Holland, M. Byrne, Y. Gun’ko, J. Boland, P. Niraj, G. Duesberg, S. Krishnamurti, R. Goodhue, J. Hutchinson, V. Scardaci, A.C. Ferrari, J.N. Coleman, Nature Nanotechnology, 3 (2008) 563-568.
[3] B. Ma, C.H. Woo, Y. Miyamoto, J.M.J. Fréchet, Chem. Mater., 21 (2009) 1413-1417.
[4] F. Torrisi, T. Hasan, W. Wu, Z. Sun, A. Lombardo, T.S. Kulmala, G.-W. Hsieh, S. Jung, F. Bonaccorso, P.J. Paul, D. Chu, A.C. Ferrari, ACS Nano, 6 (2012) 2992-3006.
[5] J. Kastner, I. Gnatiuk, B. Unterauer, I. Bergmair, O. Lorret, G. Hesser, K. Hingerl, D. Holzinger, M. Mühlberger, Imaginano Conference (2013), Bilbao, Spain.
[6] K. Norrman et al. J. Am. Chem. Soc. 132, 16883 (2010)