Postdoctoral student Rui Zhang with a solar cell processed with green solvents. Photo: Olov Planthaber.
Postdoctoral student Rui Zhang with a solar cell processed with green solvents. Photo: Olov Planthaber.

An international team of researchers has shown that a small guest molecule in just the right place makes it possible to produce energy-efficient organic solar cells using eco-friendly solvents. With this technique, the researchers were able to produce large-area organic solar cells with an energy efficiency of more than 17%.

“This is a major step towards large-scale industrial manufacture of efficient and stable organic solar cells,” says Feng Gao, professor in the Department of Physics, Chemistry and Biology (IFM) at Linköping University in Sweden. The researchers report their work in a paper in Nature Energy.

Developments in organic solar cells have been rapid, and the maximum energy efficiency achieved by organic solar cells produced in the laboratory is currently over 18%. Energy efficiency measures how large a fraction of the energy in sunlight can be converted to useful energy by solar cells. The efficiency limit is considered to be around 24% for organic solar cells.

There are, however, several challenges to manufacturing organic solar cells. One is ensuring they are sufficiently stable to function for 10 years or more. Another is that the highest energy efficiency is achieved by solar cells manufactured in solutions containing toxic solvents with a relatively low boiling point. This low boiling point creates problems during manufacture, since the solution evaporates slightly too rapidly. But the use of more eco-friendly solvents with higher boiling points leads to a decrease in energy efficiency.

This dilemma has now been solved by a joint project led by researchers at Linköping University and Soochow University in China. The researchers managed to manufacture a solar cell using a solution with a high boiling point and without any toxic ingredients. They then demonstrated that this solar cell has an energy efficiency of more than 17%.

In addition, using the same technique, they were able to produce a solar module with an area of 36cm2 that had an energy efficiency of over 14%. This is the highest efficiency reported to date for organic solar cell modules with an active area exceeding 20cm2. Both of these advances could help organic solar cell technology to make its commercial breakthrough at a large scale.

“Our results now open for the manufacture of organic solar cells at larger scales for outdoor use,” says postdoc Rui Zhang, who works with Gao in the Electronic and Photonic Materials Division at Linköping University.

When sunlight in the form of photons is absorbed by an organic semiconducting donor, an 'excited state' forms. Electrons in the donor jump to a higher energy level, creating holes at the lower energy level, to which they are still attracted. As a consequence, the electrons are not fully freed, and so a photocurrent does not arise.

To free these electrons, acceptor materials need to be added to the donor. These acceptor materials attract the electrons, fully separating them from the holes and thus allowing them to give rise to a photocurrent.

A couple of years ago, Chinese researchers developed a new acceptor material, called Y6, which can deliver high efficiency for organic solar cells. In this study, the researchers found a guest molecule, known as BTO, that ensures the Y6 molecules in the solar cell are packed in such a close and stable manner by the green solvents that a photocurrent can be generated efficiently. Adding BTO also allows larger areas of the solar cells to be manufactured with high efficiency.

“Our strategy leads to clear design rules for optimizing the interaction between organic donors and acceptors in multicomponent blends, meeting the critical requirements for future development of organic photovoltaic technology,” says Yaowen Li, a professor at Soochow University.

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.