Sean Sweetnam
Stanford University
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Featured researches published by Sean Sweetnam.
Nano Letters | 2012
Nichole Cates Miller; Sean Sweetnam; Eric T. Hoke; Roman Gysel; Chad E. Miller; Jonathan A. Bartelt; Xinxin Xie; Michael F. Toney; Michael D. McGehee
We compare the solar cell performance of several polymers with the conventional electron acceptor phenyl-C61-butyric acid methyl ester (PCBM) to fullerenes with one to three indene adducts. We find that the multiadduct fullerenes with lower electron affinity improve the efficiency of the solar cells only when they do not intercalate between the polymer side chains. When they intercalate between the side chains, the multiadduct fullerenes substantially reduce solar cell photocurrent. We use X-ray diffraction to determine how the fullerenes are arranged within crystals of poly-(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and suggest that poor electron transport in the molecularly mixed domains may account for the reduced solar cell performance of blends with fullerene intercalation.
Journal of the American Chemical Society | 2014
Sean Sweetnam; Kenneth R. Graham; Guy Olivier Ngongang Ndjawa; Thomas Heumüller; Jonathan A. Bartelt; Timothy M. Burke; Wentao Li; Wei You; Aram Amassian; Michael D. McGehee
Theoretical and experimental studies suggest that energetic offsets between the charge transport energy levels in different morphological phases of polymer:fullerene bulk heterojunctions may improve charge separation and reduce recombination in polymer solar cells (PSCs). In this work, we use cyclic voltammetry, UV-vis absorption, and ultraviolet photoelectron spectroscopy to characterize hole energy levels in the polymer phases of polymer:fullerene bulk heterojunctions. We observe an energetic offset of up to 150 meV between amorphous and crystalline polymer due to bandgap widening associated primarily with changes in polymer conjugation length. We also observe an energetic offset of up to 350 meV associated with polymer:fullerene intermolecular interactions. The first effect has been widely observed, but the second effect is not always considered despite being larger in magnitude for some systems. These energy level shifts may play a major role in PSC performance and must be thoroughly characterized for a complete understanding of PSC function.
Advanced Materials | 2012
Nichole Cates Miller; Eunkyung Cho; Matthias J. N. Junk; Roman Gysel; Chad Risko; Dongwook Kim; Sean Sweetnam; Chad E. Miller; Lee J. Richter; Regis J. Kline; Martin Heeney; Iain McCulloch; Aram Amassian; Daniel Acevedo-Feliz; Christopher Knox; Michael Ryan Hansen; Dmytro Dudenko; Bradley F. Chmelka; Michael F. Toney; Jean-Luc Brédas; Michael D. McGehee
The molecular packing in a polymer: fullerene bimolecular crystal is determined using X-ray diffraction (XRD), molecular mechanics (MM) and molecular dynamics (MD) simulations, 2D solid-state NMR spectroscopy, and IR absorption spectroscopy. The conformation of the electron-donating polymer is significantly disrupted by the incorporation of the electron-accepting fullerene molecules, which introduce twists and bends along the polymer backbone and 1D electron-conducting fullerene channels.
Macromolecular Rapid Communications | 2015
Dominik Gehrig; Ian A. Howard; Sean Sweetnam; Timothy M. Burke; Michael D. McGehee; Frédéric Laquai
The effect of donor-acceptor phase separation, controlled by the donor-acceptor mixing ratio, on the charge generation and recombination dynamics in pBTTT-C14:PC70 BM bulk heterojunction photovoltaic blends is presented. Transient absorption (TA) spectroscopy spanning the dynamic range from pico- to microseconds in the visible and near-infrared spectral regions reveals that in a 1:1 blend exciton dissociation is ultrafast; however, charges cannot entirely escape their mutual Coulomb attraction and thus predominantly recombine geminately on a sub-ns timescale. In contrast, a polymer:fullerene mixing ratio of 1:4 facilitates the formation of spatially separated, that is free, charges and reduces substantially the fraction of geminate charge recombination, in turn leading to much more efficient photovoltaic devices. This illustrates that spatially extended donor or acceptor domains are required for the separation of charges on an ultrafast timescale (<100 fs), indicating that they are not only important for efficient charge transport and extraction, but also critically influence the initial stages of free charge carrier formation.
Advanced Energy Materials | 2015
Timothy M. Burke; Sean Sweetnam; Koen Vandewal; Michael D. McGehee
Advanced Energy Materials | 2015
Jonathan A. Bartelt; David W. Lam; Timothy M. Burke; Sean Sweetnam; Michael D. McGehee
Advanced Energy Materials | 2012
Nichole Cates Miller; Eunkyung Cho; Roman Gysel; Chad Risko; Veaceslav Coropceanu; Chad E. Miller; Sean Sweetnam; Alan Sellinger; Martin Heeney; Iain McCulloch; Jean-Luc Brédas; Michael F. Toney; Michael D. McGehee
Advanced Functional Materials | 2014
I. T. Sachs-Quintana; Thomas Heumüller; William R. Mateker; Darian E. Orozco; Rongrong Cheacharoen; Sean Sweetnam; Christoph J. Brabec; Michael D. McGehee
Journal of Physical Chemistry C | 2016
Sean Sweetnam; Rohit Prasanna; Timothy M. Burke; Jonathan A. Bartelt; Michael D. McGehee
Advanced Energy Materials | 2016
Kenneth R. Graham; Guy Olivier Ngongang Ndjawa; Sarah M. Conron; Rahim Munir; Koen Vandewal; John J. Chen; Sean Sweetnam; Mark E. Thompson; Alberto Salleo; Michael D. McGehee; Aram Amassian