Structural Characterization of the Active Layer in Organic Solar Cells

Open Access
Author:
Kozub, Derek
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 07, 2011
Committee Members:
  • Enrique Daniel Gomez, Thesis Advisor
Keywords:
  • P3HT
  • PCBM
  • GISAXS
  • EFTEM
  • Teubner-Strey
  • PBTTT
  • Organic Solar Cells
  • Elemental Mapping
Abstract:
Organic solar cells belong to a class of devices where the morphology of the active layer has a large impact on device performance. However, characterization of the morphology of organic semiconductor mixtures remains a challenge. We have utilized Grazing-Incidence Small-Angle X-Ray Scattering (GISAXS) and Energy Filtered Transmission Electron Microscopy (EFTEM) to characterize the morphology of polythiophene:fullerene mixtures as a function of processing conditions. GISAXS has been used to determine the domain spacing within the active layer, whereas EFTEM has been used to generate images with high contrast between domains. We have applied these methods to two promising polythiophene:fullerene systems, 1:1 mixtures of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and 1:4 mixtures of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) and PC71BM. In 1:1 P3HT:PCBM systems, domain compositions are found via EFTEM analysis, which are used in conjunction with DSC experiments to develop a theory for the mechanism of structural formation in polythiophene:fullerene blends. Previously, it was thought that P3HT and PCBM were immiscible, but we show that a significant amount of the non-crystallized polymer forms a mixed amorphous phase with PCBM. In 1:4 PBTTT/PC71BM mixtures, we show how larger domains form at long annealing times, which leads to poor performance in solar cell devices. Using the fact that large domains lead to poor performance as a guiding principle, quantitative structure-function relationships are explored for the 1:4 PBTTT/PC71BM and 1:1 P3HT/PCBM mixtures based on GISAXS results. Furthermore, EFTEM and GISAXS have been useful in guiding our attempts to perturb the structure of the active layer in organic solar cells. Preliminary results show that the electric field alignment of crystalline P3HT fibers in an amorphous P3HT/PCBM matrix is promising. Finally, we have discussed the limitations of EFTEM as it is currently used and how it can be expanded upon using a Focused Ion Beam (FIB) to study cross sections of solar cells.