FeCl3 intercalted graphene electrodes for photovoltaic energy harvesting
Kieran Walsh a, Conor Murphy a, Adolfo De Sanctis a, Christos Melios b, Saverio Russo a, Monica Craciun a
a School of Physics, University of Exeter, Stocker Road, Exeter, Devon EX4 4QL
b National Physical Laboratory, Hampton Road, Teddington, TW11 0LW
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Kieran Walsh, presentation 039
Publication date: 11th February 2019

Over the past decades the efficiency of Organic Photovoltaics (OPVs) and Perovskite Photovoltaic devices has improved at an alarming rate, now both exceeding the efficiency of commercial silicon based photovoltaic devices [1]. However, due to a lack in flexible transparent electrodes, most OPV and perovskite technologies are limited to fabrication on Indium Tin Oxide (ITO) electrodes.  While these are the leading transparent conducting electrode material in industry, ITO lacks the flexibility to truly utilize the advantageous properties of OPVs and perovskite photovoltaic devices.  In addition, ITO has been shown to greatly increase the energy cost of production of OPV devices [2], thereby increasing the energy payback required from them.  In recent years graphene has been heralded as a wonder material with a plethora of uses in optoelectronic devices, photovoltaics and many other areas of scientific interest.  However, the limited quality of graphene produced by Chemical Vapour Deposition (CVD) means that graphene grown by this method still has a relatively high sheet resistance (≈ 1000 Ω/sq).  Graphene also has a relatively low work function (≈ 4.4 eV), necessitating the use of electron/hole transport layers in OPV devices that incorporate graphene electrodes.  These factors would make graphene unsuitable for use in OPV and perovskite photovoltaic devices, however the chemical doping of graphene has been shown to have profound effects on the electrical properties of the material, while leaving its optical properties unchanged.  Here we report the characterization of large area Few Layer Graphene (FLG) electrodes grown by CVD on Ni substrates, functionalized by intercalation with ferric chloride (FeCl3), suitable for flexible photovoltaic devices.  This non-volatile doping method proves to be effective at tuning both the sheet resistance and work function of graphene [3], while being extremely stable to changes in both temperature and humidity [4].  Strong p-doping through charge transfer from the intercalant is characterized by mapping the concomitant shift in G-peak by Raman spectroscopy.  Scanning Kelvin Probe Force Microscopy (SKPFM) was used to characterize variations in the surface potential of the intercalated graphene caused by non-uniform intercalation between the graphene sheets.  This allows the work function of the intercalated graphene to be mapped, increasing up to 4.97 eV due to surface doping from the intercalation process.  Electrical measurements of macroscopic sized hall bar devices shows intercalation to reduce the sheet resistance by an over order of magnitude to ≈ 50 Ω/sq, due to a massive increase in charge carrier concentration to ≈ 1x1015 cm-2.  This low sheet resistance, combined with the transparency and inherent flexibility of graphene makes FeCl3 intercalated graphene a promising material for flexible photovoltaic and optoelectronic technologies.  OPV devices composed of PTB7-th and PC70BM donor/acceptor molecules, fabricated on flexible intercalated graphene electrodes are compared to those on ITO/glass substrates.  This functionalized graphene material has the potential to realize a new generation of fully flexible solar cells, expanding the applicable environments for photovoltaic technologies.

We would like to aknowledge the contributions of those authors mentioned above along with members of our research group who improved understanding in the project through discussions.  I would like to aknowledge the financial support provided by the EPSRC CDT in Metamaterials, who funded project.

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