Eco-Friendly Process to Control the Assembly of Functional Photovoltaic Polymers

An eco-friendly method which uses surfactants to regulate photovoltaic polymer assembly has been discovered. The study used optical probes and neutrons to track the molecular dynamics of the polymer.  

A surfactant template guides the self-assembly of functional polymer structures in an aqueous solution. Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; image by Youngkyu Han and Renee Manning.

Solar cell efficiency mainly relies on the precise assembly of polymers. These polymers organize themselves into thin films that are many times thinner than a human hair. Presently, environmentally unfriendly solvents are used in the driving of polymer assembly.

Surfactants, which are detergent-like molecules, are far greener and have been shown to also direct polymer assembly.

In order to produce the self-assembling polymers, the scientists utilized several DOE Office of Science User Facilities. These included the Advanced Photon Source (APS) at Argonne National Laboratory, the SNS, and the Center for Nanophase Materials Sciences (CNMS) at ORNL.

Self-assembly of polymers using surfactants provides huge potential in fabricating nanostructures with molecular-level controllability. Scattering of neutrons and X-rays is a perfect method to investigate these structures.

                                                                         Changwoo Do - ONRL

“We would like to create very specific polymer stacking in solution and translate that into thin films where flawless, defect-free polymer assemblies would enable fast transport of electric charges for photovoltaic applications. We demonstrated that this can be accomplished through understanding of kinetic and thermodynamic mechanisms controlling the polymer aggregation.

                                                                             Ilia Ivanov - ONRL

This latest breakthrough generates molecular building blocks which can be used for designing sensory and optoelectronic materials. In this novel approach, a semiconducting polymer needs to be designed with a hydrophilic, or water-loving, chains and hydrophobic, or water-fearing backbone.

In the case of water-soluble side-chains, green processing can be performed if this results in a polymer that is capable of self-assembling into an organic photovoltaic (PV) material.

The researchers used an aqueous solution containing amphiphillic surfactant, i.e. one with both hydrophilic and hydrophobic regions. The polymer was added to this solution, and soon afterwards, the surfactant self-assembled into different templates based on the concentration and temperature.

The polymer was then guided by this surfactant template to pack into different types of nanoscale shapes, such as spherical micelles, hexagons, and sheets.

Atoms in the semiconducting polymer are arranged in such a way that electrons are shared easily. The latest study provides a better understanding of the polymer system and its different structural phases. It also sheds light on how functional crystals are formed through growth of assemblies of repeating shapes.

Such functional crystals form the basis of PV thin films, which can be used to provide power in outer space, deserts and other demanding environments.

Rationally encoding molecular interactions to rule the molecular geometry and inter-molecular packing order in a solution of conjugated polymers is long desired in optoelectronics and nanotechnology. “The development is essentially hindered by the difficulty of in situ characterization.

                                                                            Jiahua Zhu - CNMS

While the change in molecular morphology was taking place in situ measurements were taken. These measurements differed from those measurements that were taken after the material was separated from the system.

The team used SNS’ equipment which provided the powerful pulsed neutron beams to help in discovering that a functional PV polymer is actually capable of self-assembling in an eco-friendly solvent.

Also, a selective deuteration technique was used to further improve the efficiency of the neutron scattering. In this technique, heavier deuterium atoms replace certain hydrogen atoms in the polymers. This phenomenon has a heightening contrast effect in the structure. CNMS is quite familiar with the former method.

We needed to be able to see what’s happening to these molecules as they evolve in time from some solution state to some solid state. This is very difficult to do, but for molecules like polymers and biomolecules, neutrons are some of the best probes you can imagine

                                                                      Bobby Sumpter - CNMS

With a combined expertise in high-throughput data analysis, neutron scattering, theory, simulation and modeling, the researchers successfully developed a unique test chamber to track phase transitions as they occurred.

This test chamber subsequently helped them to use optical probes whilst such changes are taking place. It monitors molecules under conditions of varying pressure, temperature, light, humidity, solvent composition and more. This made it possible to study how functioning materials change over time, and also made it easy to enhance their performance.

In order to make measurements, the researchers positioned a sample in the chamber and transported the same to a different set of instruments. A transparent face in the chamber allows laser beams to enter and probe the materials.

Helped by high-performance computing, probing modes such as electrical charge, photons, magnetic spin and calculations can simultaneously work to define matter within a variety of conditions. The test chamber is designed to utilize X-rays and neutrons as complementary and additional probes in the future.

Incorporation of in situ techniques brings information on kinetic and thermodynamic aspects of materials transformations in solutions and thin films in which structure is measured simultaneously with their changing optoelectronic functionality. It also opens an opportunity to study fully assembled photovoltaic cells as well as metastable structures, which may lead to unique features of future functional materials.

                                                                             Ilia Ivanov - ONRL

Whilst the latest study assessed phase transitions at rising temperatures the subsequent in situ diagnostics defines them at high pressure. Neural networks will also be used by the research team to study complicated nonlinear processes with various feedbacks.

The study was headed by Zhu, Do and Ivanov. Optical measurements and synchrotron X-ray scattering were performed by Youngkyu Han, Zhu and Ivanov and Youngkyu Han. Theory calculations were conducted by Sumpter, Sean Smith and Rajeev Kumar.

The water-soluble polymer was developed by Kunlun Hong and Youjun He, and the thermal nuclear magnetic resonance study on the water-soluble polymer was performed by Peter Bonnesen. Neutron measurement and scattering result analysis were performed by Do, Han and Greg Smith.

The study was performed at SNS and CNMS. In addition, synchrotron X-ray scattering measurements were performed on the polymer solution using the Advanced Photon Source, a DOE Office of Science User Facility at Argonne National Laboratory. The work was partly funded by Laboratory Directed Research and Development.

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