Transparent conducting graphene electrodes for photovoltaic applications
Luca Ortolani1*, Caterina Summonte1, Rita Rizzoli1, Meganne Christian1, Isabella Concina2,3, Gurpreet S. Selopal2,3, Riccardo Milan2,3, Alberto Vomiero3,4, Vittorio Morandi1
- CNR-IMM, Via Gobetti 101, 40129, Bologna, Italy
- SENSOR Lab, Department of Information Engineering, University of Brescia, Via Valotti 9, 25133 Brescia, Italy
- CNR-INO SENSOR Lab, Via Branze 45, 25123 Brescia, Italy
- Luleå University of Technology, 971 98 Luleå, Sweden
* E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
The use of few-layer graphene membranes as front transparent conductive layer (TCL) is discussed, for both nanostructured thin film solar cells, requiring high temperature processing for the formation of silicon nano-dots in the absorbing layer, and dye sensitized solar cells obtained on glass and on flexible substrates. These applications require the use of a TCL front contact, which is usually made of metal oxides, such as Indium Tin Oxide (ITO), Al-doped ZnO, MgO, F-doped Tin Oxide (FTO) [1]. The use of these materials for high processing temperatures, or for the fabrication of flexible devices, it is an open issue. ITO contact is brittle and it rapidly degrade after bending, so that it is unsuitable for flexible electronics applications. Moreover, the TCL represents one of the most expensive parts of solar cells, and many strategies are under investigation to replace these materials with lower cost but still effective layers.
In the case of silicon nano-dot based solar cells, the fabrication involves processing temperatures up to 1100°C, preventing the use of transparent conductive metal oxides (TCOs) in p-i-n devices [2]. Indeed, while ITO contacts degrade above 900°C, we demonstrated that chemical vapour deposition (CVD) grown graphene sheets of several square centimetres, having a sheet resistance of about 500 ohm/sq with transmittances around 94 %, maintain their optoelectronic properties even if annealed up to 1100 °C [3]. Moreover, the careful control of the graphene growth parameters, as well as a tailored approach to the transfer process on functional substrates, provides the suitable control of the physical and chemical properties of the front contact over large area, which is mandatory in photovoltaic devices. The results on a-Si:H p-i-n Si nanodot-based solar cells [3] and dye-sensitized solar cells [4] fabricated on graphene TCL front contacts will be discussed. The major limitation in solar cells application arise from the high sheet resistance of state-of-the-art wet transferred graphene with respect to standard ITO coating. In order to address this issue, doped graphene sheets [5-7] will be studied as TCL.
References:
[1] L.G. De Arco et al., ACS Nano 4 (2010) 2865.
[2] P. Löper et al., Appl. Phys. Lett. 102 (2013) 033507.
[3] G. P. Veronese et al, Solar Energy Materials & Solar Cells, 138 (2015) 35.
[4] G. S. Selopal et al, Solar Energy Materials & Solar Cells 135 (2015) 99.
[5] S. Bae et al., Nature Nanotechnology 5 (2010) 574.
[6] G.V. Bianco et al., Phys.Chem.Chem.Phys., 16 (2014) 3632.
[7] J. H. Bong et al., Nanoscale, 6 (2014) 850.
