Toray Industries Inc announced Sept 20, 2013, a polymer organic thin-film photovoltaic (PV) cell that has the world's highest level of conversion efficiency of 10.6%.
On the same day, the company explained the details of the cell at the 74th JSAP Autumn Meeting 2013, which took place from Sept 16 to 20, 2013.
For the battery, Toray used the "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, as an electron acceptor (n-type semiconductor), it employed the "PCBM70," which is a C70 derivative.
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, the company did not disclose the detailed structural formula of the Polymer-1 due to "some patent reasons."
When Toray made a 4mm2 organic thin-film PV cell whose active layer film is 130nm thick by using the materials, its conversion efficiency was 9.4%.
Then, Toray made a 4mm2 organic thin-film PV cell whose active layer film is 300nm thick by using the same materials in the aim of achieving a higher light absorption rate. As a result, the absorption rate for light with a relatively short wavelength increased, improving the conversion efficiency to 10.6%. In this case, the short-circuit current density, open voltage and fill factor (FF) were 21.7mA/cm2, 0.762V and 0.641, respectively.
There are few organic thin-film PV cells whose active layer film is as thick as 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.
"The Polymer-1 is a polymer having a high carrier mobility," Toray said. "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."
Also, the company expects that the leak breakdown caused by short circuit between electrodes is prevented, improving durability, in addition to a higher efficiency.