Renewable Energy Global Innovations features: Electrochemical and photocurrent characterization of polymer solar cells with improved performance after graphene oxide addition to PEDOT:PSS hole transporting layer

Significance Statement

Extensive knowledge exists regarding the predominant role that in bulk heterojunction solar cells donor and acceptor interfaces play in charge carrier formation and separation. Interface improvements by addition of surfactants and by active layer annealing has already been attempted. Interfacial layers have also been optimized so as to avoid charge recombination at the collecting electrodes. These interfacial layers however must qualify as efficient carriers, possess a reduced resistance and be characterized by low light absorption capacities. Consequently, graphene oxide in polymer solar cells possess such qualities that defend their application, contrary to the popularly used tin-doped indium oxide and aluminum electrodes, so as to improve the photovoltaic performance and stability of the devices. Herein, chemically fabricated graphene oxide is added so as to modify the performance and electrochemical properties of bulk heterojunction solar cells of varying architecture.

In a recent research collaboration between Polish and Mexican researchers, Agnieszka Iwan, Felipe Caballero-Briones and colleagues investigated the photocurrent and electrochemical characterization of polymer solar cells with improved performance, after addition of graphene oxide to the PEDOT:PSS hole transporting layer. They focused on applying chemically synthesized graphene oxide in polymer solar cells of varying architectures while altering the placement and amount of the graphene oxide in the polymer solar cells. Their aim was to establish a knowledge base that enlightens on graphene oxide addition in different layers of the same device. Their research work is now published in Solar Energy.

First, the research team obtained the graphene oxide by modified Hummers method and fully characterized by Raman spectroscopy, Fourier Transform Infrared Spectroscopy, X-ray diffraction as well as with cyclic voltammetry. They then constructed bulk heterojunction polymer solar cells with P3HT:PC61BM or PTB7:PC71BM active layers and PEDOT:PSS as hole transport layers. The constructed layers were then subject to investigation relative to: the concentration of graphene oxide in hole transport layer, the acidity of the graphene oxide, the type of polymer used in the active layer, the annealing temperatures of the active layer and the place where the graphene oxide is incorporated in the devices.

The authors observed that the best performance for the polymer solar cells was obtained for the devices with the ITO/PEDOT: PSS:GO/PTB7:PC71BM/Al architecture and at the point where the volume ratio of the graphene oxide to PEDOT:PSS was 1:1. Under these conditions, the researchers noted that higher power conversion efficiency was obtained. They also observed a better active layer performance of the polymer solar cells with the graphene oxide annealed at 130⁰ C.

Herein, the positive effects of incorporation of graphene oxide in bulk heterojunction polymer solar cells, as additive to the hole transport layer PEDOT:PSS with the volume ratio 1:1 are demonstrated. Improved performance of the polymer solar cells is notably achieved in both photocurrent and electrochemical characterization. In totality, the improvement of the polymer solar cells performance upon graphene oxide addition can therefore be comprehended in terms of hole movement and better HOMO-LUMO matching within the structure.

About The Author

Dr. Agnieszka Iwan, assoc. prof. has completed her Ph.D. from Technical University in Silesia (Poland) and postdoctoral studies from Centre National De La Recherche Scientifique in Grenoble (France). She received Ph.D., D.Sc. in Technical University in Wroclaw (Poland). She formerly worked at the Centre of Polymer and Carbon Materials, PAS (Zabrze, Poland) and next at the Electrotechnical Institute (Wroclaw, Poland) as head of the New Technologies Lab., in October 2016 moved to Military Institute of Engineer Technology (Wroclaw, Poland) and has professor position in Institute.

Her research focuses on the organic/polymer/perovskite solar and fuel cells, flexible electronics, nanomaterials such as graphene, TiO₂ or ZnO, liquid crystals and acid-base interactions.

She is author and co-author of more than 260 articles, including 8 book chapters, 3 books and more than 135 presentations in scientific conferences.

About The Author

Dr. Felipe Caballero-Briones, Full Professor, has completed his PhD at the University of Barcelona (Spain) in 2009 and did a postdoctoral stay at Institute of Engineering of Catalonia (IBEC) and Department of Chemical Physics-UB in 2010-2011. From 1999 to 2009 was appointed as associate professor and from 2010 became full professor at the Center for Applied Science and Advanced Technology (CICATA Unidad Altamira) of the Instituto Politecnico Nacional (Mexico) where he is Leader of the Materials and Technologies for Energy, Health and Environment Group (GESMAT).

His research is directed to design and develop graphene-based and semiconducting materials and oxides for photovoltaics, microbial and polymeric fuel cells, supercapacitors, thermoelectrics, and photocatalyts. Other current research interests are graphene-based materials for water remediation, cancer treatment and desalination.

Dr. Caballero-Briones advised or is advising 5 PhD, 14 MSc and 13 BSc thesis and has authored or coauthored 55 articles and more than 200 presentations in scientific conferences; he has 601 cites in Google Scholar (H index 16).

Reference

Agnieszka Iwan, Felipe Caballero-Briones, Michal Filapek, Bartosz Boharewicz, Igor Tazbir, Agnieszka Hreniak, Jesus Guerrero- Contreras. Electrochemical and photocurrent characterization of polymer solar cells with improved performance after graphene oxide addition to the tin-doped indium oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) hole transporting layer. Solar Energy volume 146 (2017) page 230–242.

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Renewable Energy Global Innovations features: Molecular insights into water vapor absorption by aqueous lithium bromide and lithium bromide/sodium formate solutions

Significance Statement

A commonly used liquid desiccant, lithium bromide faces some shortcomings such as corrosion, crystallization at high concentrations and their need for high energy inputs to regenerate the absorbent.

Molecular dynamic simulations have recently been applied in understanding of molecular driving forces such as vapor pressure and absorption rate that influence performance of liquid desiccants. Despite knowledge provided by molecular dynamic simulations on solution behavior, little efforts have been made on microscale kinetics of liquid desiccants including current ternary working fluids.

Researchers led by Professor Gloria D. Elliott from University of North Carolina at Charlotte, published an article in Applied Thermal Engineering conducted a series of molecular dynamic simulations of the absorption of water vapor into aqueous lithium bromide and lithium bromide /sodium formate mixtures at various temperatures.

The molecular dynamic simulation was able to provide a quasi-static absorption process as lithium bromide solution absorbs water vapor at a nearly constant rate. Large number of water molecules was absorbed by 60wt% lithium bromide solution at a temperature of 443K after a simulation time of 20ns which yielded an approximate absorption rate of 38Kg·m²/s but decreased as temperature decreases. It was also seen that absorption of water molecules increases as concentration of lithium bromide increases at a minimum temperature of 373K.

Analysis on mass density profiles of lithium and bromide ions with water molecules showed a decrease in interfacial thickness layer of lithium bromide solution as its concentration increases when observed at 383K and 408K, but no effect was found with respect to temperature. It was also observed that the interfacial dipoles were most likely to lie in a plane parallel to the interface.

Effects of addition of sodium formate into the LiBr + H₂O system when simulated at a temperature of 373K showed good correlation of density profile of lithium ions Li⁺ to that of formate anion COOH^– in all the mass ratios which maintains local proximity throughout the solution.

“What we found the most interesting was that the strong interaction between Li⁺ and COOH^– created cavities of various sizes that can accommodate water molecules,” said Dr. Lindong Weng, a former postdoctoral researcher in Elliott’s lab and the first author of the study. “Such fascinating morphology of ion placement provides a geometrical explanation for the increase in absorption capacity with added formate.”

The study also found that when the molar ratios of LiBr to NaCOOH are about 1.5:1 and 0.8:1, respectively, Li+-COOH^– clusters size mainly 5< (cluster size: defined as number of lithium ions that can be linked together with each other via COOH^–). This result showed that more addition of sodium formate led to more extended and interwoven lithium and formate ion clusters. The effect of addition of sodium formate also led to a decrease in water vapor absorption rate despite increase in absorption capacity.

The authors successfully optimized the advantages of lithium bromide and lowered crystallization temperature (minimal thermal energy needed via including formate salt). The molecular design and simulation methods presented in this study can aid in improved defined compositions to undergo more detailed experimental studies.  

About The Author

Dr. Gloria D. Elliott

Professor and Associate Chair, Research.
Department of Mechanical Engineering & Engineering Science- University of North Carolina at Charlotte

Dr. Gloria Elliott is the founding Director of the Charlotte Banks Research Initiative, an academic think tank integrating economics, technology, and policy to address logistical challenges in organ and tissue transport, with the aim of accelerating growth in regenerative medicine, transplantation, and the tissue engineering sector.

Dr. Elliott completed post-doctoral training at Harvard Medical School and Massachusetts General Hospital.  She received her BS degree in Applied Chemistry at the University of Waterloo in Canada, and her MS and PhD in Mechanical Engineering from Michigan State University.

Dr. Elliott currently directs the Biostability Lab at the University of North Carolina at Charlotte (UNCC). Her research area is experimental thermodynamics with applications to living systems.  Dr. Elliott’s group has been developing technology and investigating the underlying science of biopreservation, and she holds several patents in this area.  Her research program has included the development of stabilization technologies for biomolecules, viruses, cells, gametes, and tissues, for diagnostic and therapeutic use.

Dr. Elliott has over a decade of experience as a pioneer in engineering education at UNCC, with a focus on training in thermodynamics and energy transport phenomena. She has created numerous new courses with biomedical engineering and energy production themes, and has also been an architect of several new programs, including a Research Experience for Undergraduates program and two separate degree concentrations in Energy Engineering and Biomedical Engineering.  She currently provides academic integrity oversight as a member of the Chancellor’s Advisory Council on Inter-collegiate Athletics, and she has also previously served on the UNCC Academic Integrity Board.

Dr. Elliott currently serves as a scientific advisor to the Organ Preservation Alliance and is a co-organizer of the upcoming Organ Banking Summit.  She is a member of the Bioengineering Technology Advisory Panel for the American Society of Mechanical Engineers. Dr. Elliott’s strategic planning experience also includes service on the executive committee of the Society of Cryobiology.

She currently serves on several research advisory committees at the university, most notably the Standing Committee on Conflicts of Interest and Commitment, the university’s highest research integrity oversight and advisory committee.  

Journal Reference

Lindong Weng ¹, Wei Song^2, ,  Donald J. Jacobs², Gloria D. Elliott¹. Molecular insights into water vapor absorption by aqueous lithium bromide and lithium bromide/sodium formate solutions, Applied Thermal Engineering 102 (2016) 125-133.

Show Affiliations

1. Department of Mechanical Engineering and Engineering Sciences, University of North Carolina at Charlotte, Charlotte, NC 28223, United States.
2. Department of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, NC 28223, United States.

 

 

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