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Riken claims 10% efficiency with organic photovoltaic devices

To achieve the 10% efficiency Riken made improvements to the semiconductor polymer as well as the structures of the charge-generation layer and device to be formed

29 May 2015 | Editor

It has been reported on the Tech-on news website that Riken has announced that it has achieved a power conversion efficiency of 10% for an organic photovoltaic (OPV).

To achieve the 10% power conversion efficiency with its OPV device, Riken made improvements to the semiconductor polymer as well as the structures of the charge-generation layer and device to be formed.

More specifically, it increased the thickness of the charge-generating layer made by mixing:

  1. a semiconductor polymer that carries positive charges (holes)
  2. fullerene derivative that carries negative charges (electrons)

The thickness was increased from about 150nm to 300nm, and current density was increased, improving power conversion efficiency from about 6% to 8.5%.

Furthermore, Riken employed an inverted structure element whose positive electrodes are located in the position of negative electrodes and vice versa, to achieve the 10% efficiency.

When the power-generating layer of a solar cell increases in thickness, it absorbs more light, increasing the amount of electric charge. However, the hole drift mobility of semiconductor polymer is lower than that of silicon. As a result, holes are recombined with electrons before reaching the electrodes and cannot be easily extracted as an electric current, resulting in a lower power conversion efficiency.

To overcome this problem, Riken employed an organic semiconductor polymer that has a high crystallinity, which improves hole drift mobility and enables holes to reach the electrodes even with a thick charge-generating layer.

When the structure of the charge-generating layer of the new OPV device was analysed at SPring-8 - a large radiation height facility - it was found that the molecular orientation of the semiconductor polymer near the upper electrodes is different from that near the lower electrodes, meaning that there is a difference in the flow of electric charges between the electrodes.

Riken will continue to engage in further research and development of both materials and devices with the aim of achieving a power conversion efficiency of 15% - making the OPV device viable for commercialisation.

This recent development was achieved via joint research between Japan Advanced Institute of Science and Technology (JAIST) and Japan Synchrotron Radiation Research Institute (JASRI). The research was conducted as part of the JST Strategic Basic Research Programs of Japan Science and Technology Agency (JST).

techon.nikkeibp.co.jp/english/    www.riken.jp   

About Riken

RIKEN is Japan's largest comprehensive research institution renowned for high-quality research in a diverse range of scientific disciplines. Founded in 1917 as a private research foundation in Tokyo, RIKEN has grown rapidly in size and scope, today encompassing a network of world-class research centers and institutes across Japan.

Source: Riken

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