Abstract's Details

Understanding P3HT Crystallite Orientation and Morphology in P3HT:PCBM Blends
Abstract ID	MAT-27
Presenter	Alex Mayer
Presentation Type	Poster
Full Author List	A. Mayer (1), M. Topinka (1), M. Rowell (1), M. F. Toney (2), M. McGeHee (1)
Affiliations	(1) Stanford University (2) SSRL
Category	Materials Science
Abstract	Organic bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-c61-butyric acid methyl ester (PCBM) have demonstrated a drastic increase in their power conversion efficiency as researchers gain control of the nanostructured morphology. In order to improve the efficiency further, a detailed understanding of the crystallographic orientation and morphology is required. Recently, researchers have found that decreasing the evaporation rate of the solvent in P3HT:PCBM solar cells greatly enhances power conversion efficiency of the device due to an increase in the hole mobility in the polymer phase. The mechanisms behind the mobility increase remain a puzzle. We have employed 2D grazing incidence x-ray scattering (2D GIXS) and rocking curves of films spun from a slow and a fast drying solvent at 11-3. We show for the first time that the orientation of the P3HT domains is highly dependent on the evaporation rate of the solvent and that the crystallographic orientation increases through the use of a slowly drying solvent. Interestingly, we find that the polymers preferentially orient their insulating alkyl chains, normally thought to be the direction of slow transport, within ~30° of the direction of charge transport. We rationalize this observation by noting that a percolating charge encounters many grain boundaries while traversing the cell that affect the charge to a varying degree. In a randomly oriented sample, the angle between adjacent grains can be higher than that found in an oriented sample, leading to a larger hopping barrier and thus a decreased mobility. Further knowledge of the nucleation profile is gained through the use of depth-resolved GIXD performed at 7-2. Another puzzle that remains to be solved in order to have the blend solar cells increase to their limit is that the P3HT photoluminescence is quenched in blends. But the internal quantum efficiency is <80% even at a large reverse bias. This is a huge loss mechanism that needs to be explained if further optimization is possible. In order to explain this phenomenon, small angle x-ray scattering, surface x-ray scattering, and device modeling are employed with the aim of determining the true 3D structure.
Footnotes
Funding Acknowledgement