Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymer-based Solar Cells. Time-Resolved Microwave Conductivity and Theory

Journal Article · Mon Sep 23 04:00:00 EDT 2013 · Advanced Functional Materials (Online)

DOI:https://doi.org/10.1002/adfm.201301757· OSTI ID:1227050

Aguirre, Jordan C. ^[1]; Arntsen, Christopher D. ^[1]; Hernandez, Samuel ^[1]; Huber, Rachel ^[1]; Nardes, Alexandre M. ^[2]; Halim, Merissa ^[1]; Kilbride, Daniel ^[1]; Rubin, Yves ^[1]; Tolbert, Sarah H. ^[1]; Kopidakis, Nikos ^[2]; Schwartz, Benjamin J. ^[1]; Neuhauser, Daniel ^[1]

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1. Univ. of California, Los Angeles, CA (United States)
2. National Renewable Energy Lab. (NREL), Golden, CO (United States)

The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well-connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash-photolysis time-resolved microwave conductivity (TRMC) experiments, and space-charge-limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so-called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self-assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl-C60 butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near-spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1227050
• Journal Information: Advanced Functional Materials (Online), Journal Name: Advanced Functional Materials (Online) Journal Issue: 6 Vol. 24; ISSN 1616-3028
• Publisher: Wiley
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
Environmental Molecular Sciences Laboratory