Polymer solar cells are very promising for making inexpensive photovoltaics on flexible plastic substrates. These devices work by absorbing energy from the Sun to produce photogenerated charge carriers (electrons and holes). Because it is the electrons and holes that carry the current in such cells, a hole transporting layer is needed to improve the devices' solar power-conversion efficiencies (defined by the amount of solar radiation falling on the material divided by the delivered power that comes out).

The most commonly employed hole transporting layer in polymer solar cells is semiconducting PEDOT:PSS sandwiched between an indium tin oxide anode (ITO) and the active layer. However, the problem here is that PEDOT:PSS is usually deposited from highly acidic aqueous suspensions that corrode the ITO at high temperatures. The process can also introduce water into the active layer, which degrades device performance.

Graphene oxide thin films
To overcome these challenges, Chun-Wei Chen's group at the National Taiwan University and co-workers at Imperial College London decided to look at using graphene oxide thin films as a possible replacement for PEDOT:PSS. The graphene oxide thin films are deposited from neutral aqueous suspensions and solar cells incorporating the material show light-power conversion efficiencies that are as high as those fabricated using PEDOT:PSS.

So why is graphene oxide so good in such applications? For one, it is an insulator and acts an effective hole transporting layer, explains Chen. Its electronic band structure allows holes to effectively travel toward the electrode of the solar cell while hindering electron transport. This reduces the possibility of electron and holes recombining, which ultimately improves device performance.

"The most exciting thing in this study is that our device made of graphene oxide is as efficient as those containing PEDOT:PSS," Chen told nanotechweb.org. "What's more, contrary to other hole transporting layers made by vacuum deposition techniques, our method provides a simple solution processable method that is compatible with polymer solar cells or LEDs for large-area applications. The graphene oxide device is also more stable than the PEDOT:PS ones."

Using graphene oxide as the hole transport layer in photovoltaics means that future polymer solar-cell devices could be "all-carbon". "This could be important for future large-area and low-cost organic photovoltaics on flexible substrates," insisted Chen.

The results were reported in ACS Nano.