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.
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.Go To Solar Energy