Understanding organic solar cells: Journal Club with Oleksandra Korychenska

When compared to the silicon solar cells used in many photovoltaic devices today, organic solar cells based on polymer technology appear to have many advantages. Grantham Scholar Oleksandra Korychenska led a Journal Club session to discuss with our other first year PhD students how this technology could be used to make solar cells even more sustainable. In this blog post, she sets out the challenges.

This week’s paper

Efficient organic solar cells processed from hydrocarbon solvents‘ by J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma and H. Yan

Oleksandra Korychenska
Oleksandra Korychenska

Developing organic solar cells is vitally important to progress in the field of sustainable energy. There is much interest in applying polymers to support this progress for three reasons:

  • the suitable electronic structure of polymers
  • the possibility of processing conductive layers from the solution under ambient temperatures (this is not possible with silicon based solar cells, because high temperature is needed for obtaining specific crystalline structure)
  • the possibility of increasing light conversion yields of solar cells.

Therefore, using polymers in solar cells could potentially make the light capturing technology the use cheaper and more sustainable.

Polymer solar cells are devices made up from a blend of organic molecules that are sandwiched between two electrodes. The paper we discussed in this week’s Journal Club focused on problems during the creation of these devices. Usually organic solar cells are made from the solution of organic materials in halogenated solvents (organic liquids that contain fluorine, chlorine, bromine or iodine atom), which are toxic to the environment. In this paper it was suggested that using alternatives to these not only makes the production process more sustainable, but can improve a solar cell’s light conversion yields too.

The main drop in a polymer solar cell’s efficiency is caused by the structural defects in its layers, since the capability of organic molecules to transport charges (electrons and holes, which are crucial for enabling electrical current) depends on the orientation of organic molecules. It is this structure that affects the efficiency of an electrical current, which makes it important to the overall efficiency of the device. But according to the paper, it appeared that the alternative solvents had a positive effect on the orientation and assembly formation of molecules, which resulted in a superior light conversion rate for the polymer-based device (11.48% – though this still compares to an efficiency rate of 25% for silicon-based devices used currently, so there is still some way to go).

During the Journal Club session, we tried to understand the main principles of solar cells’ architecture, and what improvements could make organic solar cells more efficient. We discussed the main aspects of ‘green chemistry’ and its application in solar cells production.

We also questioned whether this technology is sustainable now, taking into account the complex multistep synthesis of the polymer (the main and most important component of the solar cell), as well as the difficulties of processing it. It seemed that, currently, organic solar cells are far from sustainable, since a lot of toxic solvents and precursors need to be used during synthesis and a lot of energy is used for synthesis. In addition, the long-term stability of polymer solar cells currently doesn’t exceed a few thousand hours, which means industrial implementation is unfeasible. However, the biggest problem is scaling up this technology from a small laboratory experiment to a real industrial fabrication – a problem that still needs to be solved.

Nevertheless, organic solar cells are a promising technology, which give us hope for greener production and high light conversion efficiencies. To solve the problems mentioned above, we need a deeper understanding of the fundamental processes in organic electronics.