Flexible and robust organic photovoltaics could be a step closer to mainstream use after researchers developed a way to deposit a thin gold electrode layer directly onto plastic cells.
One of the key barriers preventing anything but small demonstrator organic photovoltaic devices has been the electrode material.
Generally, indium tin oxide (ITO)-coated glass has been used as the transparent electrode — a material essentially borrowed from the liquid crystal display (LCD) industry.
However, it is inflexible and chemically unstable, presenting a problem for photovoltaics, which must be robust and able to cope with variable outdoor conditions.
‘Organic solar cells are very thin, typically a few hundred nanometres in thickness, and the last thing you want is any kind of chemical reaction between the organic layers and the electrode, and you don’t want atoms or ions leeching out of the electrode and into the device,’ said project lead Dr Ross Hatton of Warwick University’s chemistry department.
In addition, because the ITO-manufacturing process is sensitive to slight variations in temperature and conditions, reproducibility is a major issue.
‘A batch of ITO made in one company won’t be exactly the same as a batch of ITO made in another company — in fact, there may be batch-to-batch variations within the same company,’ Hatton said.
In their latest work, the researchers succeeded in depositing a layer of gold around 7–8nm in thickness directly onto a plastic substrate. As a single metal element, the advantage of gold is that it is well characterised, highly conductive and already used widely in the electronics industry. In fact, the team found that its cells retained conductivity after test treating them with an aggressive solvent and ultrasonic agitation.
Crucially, Hatton said the method is fully scalable for producing whole rolls of photovoltaics several square metres in size.
‘We already have the infrastructure from the preparation of thin metal films on plastic substrates. If you ever open a packet of crisps and look at the lining, there’s a thin metal film supported on a plastic substrate that is cheap and has been mass produced,’ Hatton said.
Although gold prices are currently at a historic high, he said that, due to the very thin layer required, the cost to fabricate 1m2 of electrode is only around £4.50. Meanwhile, at the end of a cell’s life, the gold can be recovered fairly easily for reuse.