Three-dimensional imaging leads to greater solar-cell efficiency, study shows

German and Dutch researchers have produced the first three-dimensional images of nanoscale structures inside a polymer solar cell. Published in the journal Nature Materials, the findings give new insights into the structure of solar cells and may lead to their performance being enhanced.

Polymer solar cells are plastic solar cells with unique properties that have the potential to outperform traditional silicon solar cells at lower costs. Until now, polymer solar cells have not been very efficient at capturing light and transforming it into energy.

Traditionally, solar cells have been based on a refined, highly purified silicon crystal, similar to those used in the manufacture of integrated circuits and computer chips. The high cost of these silicon solar cells and their complex production process is a driving force behind the development of alternative photovoltaic technologies.

Polymer solar cells, called ‘third-generation solar cells’, are lightweight, low-cost, flexible, and can be modified on the molecular level. Until now, visualising polymers at the nanoscale level has been problematic. Researchers at the Institute of Stochastics in Ulm, Germany, have obtained highly detailed images of the nanoscale structures in polymer solar cells using three-dimensional electron tomography (3DET).

3DET is a popular tool for measuring structures of cellular components at the nanometre resolution. Using a transmission electron microscope, the researchers were able to visualise ways of enhancing the power-conversion efficiency of the polymer solar cells by changing the compounds used in its manufacture.

Hybrid solar cells are made by mixing of two different materials, a polymer and a metal oxide. When sunlight shines on the resulting solar cell, charges are created where the materials meet. The precise relationship between the polymer and the metal oxide determine the solar cell’s efficiency. If the two mix closely, then this creates a very dense cell structure, which means more obstruction and lower efficiency. By making a cell with a more spaced structure, the researchers are hoping to cut down on obstacles to the energy’s flow to the collection point. This would give the solar cell greater energy-generating efficiency.

The Dutch researchers have been able to create this larger structure by using a precursor compound which mixes with the polymer. This is converted into the metal oxide at a later stage.

The use of the three-dimensional visualisation showed the importance of the mixing and solar cell morphology. This allowed the researchers to measure a consistent and quantitative correlation between solar cell performance and the underlying structure of solar cells with various cell structures.

The report concludes that the polymer solar cells produced during the experiment were among the most efficient third-generation cells produced. However, they still have a conversion efficiency of only 2% as compared to the 30% level frequently found in first-generation cells.

Further study of polymers, metal oxide and their morphology in polymer solar cells is needed in order to realise the further promise of the technology. Using three-dimensional imaging of nanostructures may bring more rapid progress in that area of research.

For more information, please visit:

Nature Materials:

http://www.nature.com/naturematerials

Technical University of Eindhoven:

http://www.tue.nl/