Ultrafast Intramolecular Exciton Splitting Dynamics in Isolated Low-Band-Gap Polymers and Their Implications in Photovoltaic Materials Design
- 6 February 2012
- journal article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 134 (9), 4142-4152
- https://doi.org/10.1021/ja209003y
Abstract
Record-setting organic photovoltaic cells with PTB polymers have recently achieved ∼8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2–8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular (“pseudo”) charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C61-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole–electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.This publication has 47 references indexed in Scilit:
- Photoinduced Carrier Generation and Decay Dynamics in Intercalated and Non-intercalated Polymer:Fullerene Bulk HeterojunctionsACS Nano, 2011
- A quantum-chemical perspective into low optical-gap polymers for highly-efficient organic solar cellsChemical Science, 2011
- Efficiency Enhancement in Organic Photovoltaic Cells: Consequences of Optimizing Series ResistanceAdvanced Functional Materials, 2009
- X‐ray structural and crystallinity studies of low and high molecular weight poly(3‐hexylthiophene)Physica Status Solidi (a), 2008
- Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption SpectroscopyJournal of the American Chemical Society, 2008
- Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiolsNature Materials, 2007
- Toward Cost-Effective Solar Energy UseScience, 2007
- Intramolecular Charge Transfer Assisted by Conformational Changes in the Excited State of Fluorene-dibenzothiophene-S,S-dioxide Co-oligomersThe Journal of Physical Chemistry B, 2006
- Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network MorphologyAdvanced Functional Materials, 2005
- Electron transfers in chemistry and biologyBiochimica et Biophysica Acta (BBA) - Reviews on Bioenergetics, 1985