Morphology control of polymer: fullerene solar cells by nanoparticle self-assembly
Date
2016
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Publisher
University of Delaware
Abstract
During the past two decades, research in the field of polymer based solar cells has attracted great effort due to their simple processing, mechanical flexibility and potential low cost. A standard polymer solar cell is based on the concept of a bulk-heterojunction composed of a conducting polymer as the electron donor and a fullerene derivative as the electron acceptor. Since the exciton lifetime is limited, this places extra emphasis on control of the morphology to obtain improved device performance. In this thesis, detailed characterization and novel morphological design of polymer solar cells was studied, in addition, preliminary efforts to transfer laboratory scale methods to industrialized device fabrication was made.
Magnetic contrast neutron reflectivity was used to study the vertical concentration distribution of fullerene nanoparticles within poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2- b]thiophene (pBTTT) thin film. Due to the wide space between the side chains of polymer, these fullerene nanoparticles intercalate between them creating a stable co-crystal structure. Therefore, a high volume fraction of fullerene was needed to obtain optimal device performance as phase separated conductive pathways are required and resulted in a homogeneous fullerene concentration profile through the film. Small angle neutron scattering was used to find there is amorphous fullerene even at lower concentration since it was previously believed that all fullerene formed a co-crystal. These fullerene molecules evolve into approximately 15 nm sized agglomerates at higher concentrations to improve electron transport. Unfortunately, thermal annealing gives these agglomerates mobility to form micrometer sized crystals and reduce the device performance.
In standard poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCMBM) solar cells, a higher concentration of PCBM at the cathode interface is desired due to the band alignment structure. This was achieved by having a sparse mono-layer of Fe3O4 nanoparticles (NPs) that formed a polymer depletion zone excluding P3HT away from cathode interface. Convective outflow and surface energy ordering were hypothesized to promote the NPs toward the cathode interface. By proper tuning of the NPs volume fraction added to the films the distance between two NPs can be made to be smaller than the P3HT radius of gyration to form the polymer depletion zone. PCBM molecules can then fill the space left by P3HT and help build electron transport pathways improving electron collection at cathode. The addition of NPs does not affect the PCBM agglomerate morphology, but does decrease the degree of P3HT crystallinity, so a balance between this NP effect and P3HT crystallinity has to be reached to obtain optimum device performance.
To assess this NP effect in industrialized device fabrication, the rod coating method was used for a preliminary study. It is found that, the NPs do not move upward and are kinetically trapped with random a distribution within the film. In addition, the P3HT crystallinity was also reduced by adding NPs, so the device performance actually is even lower. It is hyposized that the slower drying rate in rod coating compared to spin coating traps the NPs in the film perhaps due to les convective flow. Hence, further effort is needed to realize this NP effect in large scalable device fabrication.
Considering the low cost of NPs and the simple process applied to achieve this improvement, it is remarkably beneficial to organic photovoltaic industry. Further study could combine light management by using colloid particles and this NP effect to further modify morphology to obtain better solar cells. It is believed that this NP effect could be broadly applicable to other organic electronic devices like light emitting diodes, and batteries for lighting and energy storage.