Researchers in the US have improved the efficiency of a ‘spray-on’ polymer-based solar cell which moves it closer to practical use.
The North Carolina scientists have hit six per cent efficiency in buckyball-doped polymer solar cells. “We have developed a photovoltaic which is flexible and has a high efficiency,” Professor David Carroll of Wake Forest University told Electronics Weekly.
The material can be solution deposited. “It is just like paint, you just paint it on,” said Carroll.
High efficiency is a relative term, Carroll pointed out. His cells are close to six per cent efficient: double that of similar cells, but low-cost glass substrate amorphous silicon cells achieve 12 per cent.
Starting with a flexible polycarbonate substrate, Wake Forest sputters on ITO (indium tin oxide), spins on the active layer, then tops it off by depositing a thin metal contact.
“The electrons leave through the metal and the holes leave through the ITO,” said Carroll. “It is still very flexible. This thing can be rolled up and taken anywhere you want.”
It is the ease of transport which Carroll thinks will sell this type of cell if the efficiency can be doubled again.
“I strongly believe we can get there [12 per cent] within the next year,” said Carroll. “If you can get close to amorphous silicon, you can replace it completely.”
Increasing efficiency through heat treatment
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David Carroll
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Electron-hole separation in the Wake Forest cells occurs in a polymer layer doped with a fullerene - a compound including C60 buckyballs. In this case the materials are: poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61, called P3HT and PCBM respectively.
There is no junction as such in this kind of cell, instead C60 acts as an electron-acceptor through the bulk of the polymer layer, splitting up photon-induced excitons.
This is sometimes described as a nano-phase material, as individual fullerene molecules are operating in a matrix of polymer.
Crucial to the increase in efficiency at Wake Forest is a novel heat treatment. “We flash-anneal this device by putting a temperature gradient across the material,” said Carroll.
“The nano-phase material begins to form elongated crystals. We get nano-material 0.8nm across, but in a structure hundreds of nanometres across.”
Research teams across the globe are working on C60-doped solar cells. In particular two University of California campuses: Santa Barbara and UCLA, are in friendly rivalry with Carroll.
www.wfu.edu