It has been reported by Tech-on that Toray Industries Inc has announced a polymer organic thin-film photovoltaic (OPV) cell that is claimed to have world's highest level of power conversion efficiency of 10.6%. Details of the device where presented at the recently held 74th JSAP Autumn Meeting 2013, at Doshisha University, Kyoto, Japan.
In construction of the solar cell Toray researchers used a material they have named "Polymer-1" - a thiophene-based polymer material developed by the company - as an electron donor (the equivalent of the p-type semiconductor of inorganic semiconductor). Also the device uses "PCBM70" - which is a C70 derivative - as an electron acceptor (n-type semiconductor).
The HOMO (highest occupied molecular orbital) level and band gap of the Polymer-1 are -5.1eV and 1.58eV, respectively. Its carrier mobility is about 1.0 x 10-2cm2/Vs, which is higher than that of the electron donor material that Toray previously used.
However, for undisclosed "patent reasons" the company did not disclose the detailed structural formula of Polymer-1.
The Toray researchers initially made a 4mm2 OPV cell whose active layer film was just 130nm thick by using Polymer-1 and it produced a power conversion efficiency of 9.4%. Next the researcher made similar size OPV cells but this time they increased the active layer thickness to 300mm with the aim to achieve higher light absorption. What the researchers found was that absorption rates increased for short-wavelengths - and improved the pwer conversion efficiency to 10.6%.
The short-circuit current density, open voltage and fill factor (FF) were 21.7mA/cm2, 0.762 V and 0.641, respectively.
According to Toray there are have been few OPV cells whose active layer film thickness measures 300nm. It is because, in general, a thick active layer film of an organic material makes it difficult to efficiently extract electrons and holes generated by receiving light from electrodes.
A Toray spokesperson said, "The Polymer-1 is a polymer having a high carrier mobility." The spokesperson added, "Therefore, a path through which a carrier travels to an electrode can be easily formed, making it possible to increase the thickness of the film."
The company expects that the leak breakdown caused by short circuit between electrodes is prevented, improving durability, in addition to a higher efficiency.
