Double-layer structure made into efficient solar photovoltaic cell material

A detailed study of the best-performing organic optoelectronic materials has revealed an unusual double-layer flake structure that helps explain their superior performance in converting sunlight into electricity and could also guide the synthesis of new materials with even better performance. The study, published April 24, 2012 in Nature Communications, was conducted by scientists at Brookhaven National Laboratory and in collaboration with researchers at Stony Brook University, Seoul National University, Germany's Max Planck Institute for Polymer Research, and Konarka Technologies.

The material is so famous because of the name PCDTBT (poly(N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)), which is an example of a polycarbazole conjugated polymer, which is a molecule that contains a chain-like carbon backbone with alkyl side chains. It can move electrons around, both emitting and absorbing electrons, making it the best organic photovoltaic material currently in use, converting sunlight into electricity with an efficiency of up to 7.2% for organic solar photovoltaic cells.

Chemical structure of PCDTBT polymer and schematic diagram of X-ray scattering geometry. (a) Molecular structure of PCDTBT. (b) This experimental geometry belongs to the incident wide-angle X-ray scattering geometry. Source: Brookhaven National Laboratory

"In fact, although this material has been studied extensively, no one has reported the detailed structural features that lead to its superior performance," said Benjamin Ocko, a physicist at Brookhaven National Laboratory who led the current research. "Understanding why this material performs so well could help scientists exploit its essential properties to design new materials for a wide range of applications, including displays, solid-state lighting, transistors and possibly improved solar cells," he said.

To probe the structure of the molecule, the team exposed thin films of PCDTBT to a high-intensity X-ray beam using high-resolution X-ray scattering techniques at Brookhaven National Laboratory's National Synchrotron Light Source (NSLS). Unlike previous studies that used less intense X-rays, these studies revealed a crystalline-like phase formed at high temperatures. In addition, the patterns produced by diffracted X-rays showed that the structure contained layers of conjugated backbone pairs, a pattern that is significantly different from the single backbone structure found in all other organic photovoltaic materials studied to date.

Xinhui Lu, the first author of the paper, pointed out that by analyzing these scattering patterns, they found the volatility along these polymer backbones and also found how the fluctuations in neighboring backbones are transferred to each other. Performing molecular model simulations, the researchers were able to predict which polymer backbone configuration is the most stable.

PCDTBT thin film diffraction patterns, unit cell and bilayer features. Source: Brookhaven National Laboratory

In conjugated polymers, the backbone provides the conductive path, while the alkyl side chains, similar to simple oils, provide the solubility needed for processing. Although necessary, these side chains can interfere with the electrical properties of the polymer. PCDTBT is novel, the scientists say, because it is mostly made of the backbone, with very little alkyl material. "Similar to oil and water, this polymer conjugated backbone pair phase separates, leaving their alkyl side chains behind to create a bilayer structure," said David Germack, a co-author on the paper. It is this structural feature that gives the material its excellent electrical properties, an understanding that could guide the design of new organic solar materials.

"While we have a lot of expertise in-house in chemical synthesis and fabrication of organic solar devices, we lack the deep structural characterization tools that we have here at Brookhaven Lab," said Jeff Peet, senior scientist at Conarca Technologies, a leading company developing and commercializing organic solar cells. "This type of tool, along with collaborative research with colleagues at Brookhaven National Laboratory, can clarify the subtle differences between these materials, giving us keen insights into how we should design the next generation of solar photovoltaic cell materials."