Researchers develop efficient organic solar cells with very low driving force

The researchers achieved PCE of 9.5% for organic photovoltiacs and are confident that they have developed a system with a huge potential to increase the power conversion efficiency in organic solar cells

6 Oct 2016 | Editor

It has been common to believe that efficient operation of organic solar cells requires a large driving force, which limits the efficiency of organic solar cells.

Now researchers led by Feng Gao, lecturer at IFM at LiU, He Yan at the Hong Kong University of Science and Technology, and Kenan Gundogdu at the North Carolina State University, have published research that show they developed organic solar cells with a significantly lower driving force and faster charge separation than previous cells. The results have been published in Nature Energy.

This implication of this research outcome is that the intrinsic limitations of organic solar cells are no greater than those of other photovoltaic technologies, bringing them a step closer to commercialisation.

The researchers said that when photons emitted by the sun are absorbed by organic semiconductors, strongly bound excitons (rather than free carriers) are generated. The driving force is, to put it simply, a measure of the energy that is used to split the excitons into free carriers. The driving force results in the loss of the photovoltage, a key parameter for the solar cell. The lower the driving force, the higher the photovoltage.

Traditionally, efficient organic solar cells are usually composed of semiconducting polymers and carbon balls known as 'fullerene'. In that case, a large driving force over 0.3 eV is usually needed for efficient charge generation. In the present work, the researchers have replaced fullerene with a semiconducting small molecule, and found that that a low driving force is needed for efficient operation of the devices.

Feng Gao, together with his LiU colleagues, including Professor Olle Inganäs, Professor Fengling Zhang, postdoc Jonas Bergqvist and PhD student Deping Qian, describes in the article solar cells with an energy efficiency of 9.5%.

Feng Gao, said, "We have developed a system with a huge potential to increase the power conversion efficiency in organic solar cells."

Fast charge separation in a non-fullerene organic solar cell with a small driving force

Nature Energy 1, Article number: 16089 (2016) | doi:10.1038/nenergy.2016.89

Received: 14 January 2016 | Accepted: 24 May 2016 | Published online: 27 June 2016

Abstract

Fast and efficient charge separation is essential to achieve high power conversion efficiency in organic solar cells (OSCs). In state-of-the-art OSCs, this is usually achieved by a significant driving force, defined as the offset between the bandgap (Egap) of the donor/acceptor materials and the energy of the charge transfer (CT) state (ECT), which is typically greater than 0.3 eV. The large driving force causes a relatively large voltage loss that hinders performance. Here, we report non-fullerene OSCs that exhibit ultrafast and efficient charge separation despite a negligible driving force, as ECT is nearly identical to Egap. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We demonstrate a non-fullerene OSC with 9.5% efficiency and nearly 90% internal quantum efficiency despite a low voltage loss of 0.61 V. This creates a path towards highly efficient OSCs with a low voltage loss.

About Linköping University

Linköping University (LiU) is one of Sweden’s larger academic institutions and among those that offer the largest number of professional degree programmes, in fields such as medicine, education, business, economics and engineering. Research is conducted within a variety of disciplines, with strong internationally recognised research environments, e.g. materials science, information technology and disability studies.

Since gaining university status in 1975 (and before that, too), LiU has worked with innovation in education and research. For example we were first in Sweden to introduce interdisciplinary thematic research, problem-based learning (PBL), graduate schools and several innovative study programmes.

There are four campuses in three cities: Campus Valla and Campus US (both in Linköping), Campus Norrköping (Norrköping) and Campus Lidingö – Carl Malmsten Furniture Studies (Stockholm).

Source: Linköping University