First 3D Images of Polymer Solar Cells
The Eindhoven University of Technology and University of Ulm scientists have successfully developed the first high-resolution 3D images of the internal structure of a hybrid polymer solar cell. This helps them in gaining important insights into the nanoscale structure of polymer solar cells and how much it will be effective when used for energy generation. They also gathered more information on the operational principles of polymer solar cells. Stefan Oosterhout, who is the leader of the team says that the images, prepared using 3D electron tomography, show how the nanoscale structure of the polymer solar cell influences its ability to produce electricity.
Currently their solar conversion efficiency is around two percent only but the main highlight of the hybrid polymer solar cells is their cost effectiveness. Polymer cells can be printed in roll-to-roll processes, at very high speeds which make them commercially viable. They are light in weight and flexible therefore could be used on clothing and vehicles or they could be fixed on other objects too whenever the need arises. In short they will be cost-effective, flexible and lightweight i.e. they have ingredients necessary for marketable success.
Polymer base and metal oxide are two important constituents of hybrid solar cells. Polymer base donates electrons and metal oxide accepts those electrons. Polymer and metal oxides create charges at their interface when sun rays fall on them. The performance efficiency depends on the degree of mixing of two materials. When scientists resort to intimate mixing, it helps in increasing the interface area where charges originated. But it creates hurdles in another way. Movements of the charges are restricted because they have to cover long and winding roads. If we opt for larger domains they give exactly the opposite results. Scientists would love to manipulate the nanoscale structures but chemical nature of polymers and metal oxides vary vastly, hence management becomes difficult. The Eindhoven researchers tried to tackle this difficult problem. They used a precursor compound that mixes with the polymer and is only transformed into the metal oxide after it is integrated in the photo-active layer. This paves the way for superior mixing and enables extracting up to 50% of the absorbed photons as charges in an external circuit.
3D imaging helped Ulm scientists immensely. They were able to examine distances between the two components, the efficiency of charge generation and how much of each component was connected to the electrode. Using 3D electron tomography fixes the problem of mixing with unparalleled detail on a nanoscale. These quantitative analysis of the structure harmonized completely with the observed performance of the solar cells in sunlight.
The researchers are quite hopeful that visualization of the structure will enable improvements to be made to hybrid polymer solar cells. It can come forth from improving the blend of the components, or by creating polymers which can interact with the metal oxides.