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Title: Role of Crystallization in the Morphology of Polymer: Non-fullerene Acceptor Bulk Heterojunctions

Abstract

Many high efficiency organic photovoltaics use fullerene-based acceptors despite their high production cost, weak optical absorption in the visible range, and limited synthetic variability of electronic and optical properties. To circumvent this deficiency, non-fullerene small-molecule acceptors have been developed that have good synthetic flexibility, allowing for precise tuning of optoelectronic properties, leading to enhanced absorption of the solar spectrum and increased open-circuit voltages ( V OC). We examined the detailed morphology of bulk heterojunctions of poly(3-hexylthiophene) and the small-molecule acceptor HPI-BT to reveal structural changes that lead to improvements in the fill factor of solar cells upon thermal annealing. The kinetics of the phase transformation process of HPI-BT during thermal annealing were investigated through in situ grazing incidence wide-angle X-ray scattering studies, atomic force microscopy, and transmission electron microscopy. The HPI-BT acceptor crystallizes during film formation to form micron-sized domains embedded within the film center and a donor rich capping layer at the cathode interface reducing efficient charge extraction. Thermal annealing changes the surface composition and improves charge extraction. In conclusion, this study reveals the need for complementary methods to investigate the morphology of BHJs.

Authors:
 [1];  [2];  [3];  [1];  [4];  [3];  [1]
  1. Univ. of California, Santa Barbara, CA (United States)
  2. Univ. of Colorado, Boulder, CO (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
  3. Colorado School of Mines, Golden, CO (United States)
  4. Univ. of Colorado, Boulder, CO (United States)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
1364166
Report Number(s):
NREL/JA-5K00-68704
Journal ID: ISSN 1944-8244; TRN: US1701983
Grant/Contract Number:
AC36-08GO28308
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
ACS Applied Materials and Interfaces
Additional Journal Information:
Journal Volume: 9; Journal Issue: 22; Journal ID: ISSN 1944-8244
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; crystallization; non-fullerene acceptor; organic photovoltaics; small molecules; transmission electron microscopy; x-ray scattering

Citation Formats

O’Hara, Kathryn A., Ostrowski, David P., Koldemir, Unsal, Takacs, Christopher J., Shaheen, Sean E., Sellinger, Alan, and Chabinyc, Michael L. Role of Crystallization in the Morphology of Polymer: Non-fullerene Acceptor Bulk Heterojunctions. United States: N. p., 2017. Web. doi:10.1021/acsami.7b03529.
O’Hara, Kathryn A., Ostrowski, David P., Koldemir, Unsal, Takacs, Christopher J., Shaheen, Sean E., Sellinger, Alan, & Chabinyc, Michael L. Role of Crystallization in the Morphology of Polymer: Non-fullerene Acceptor Bulk Heterojunctions. United States. doi:10.1021/acsami.7b03529.
O’Hara, Kathryn A., Ostrowski, David P., Koldemir, Unsal, Takacs, Christopher J., Shaheen, Sean E., Sellinger, Alan, and Chabinyc, Michael L. Mon . "Role of Crystallization in the Morphology of Polymer: Non-fullerene Acceptor Bulk Heterojunctions". United States. doi:10.1021/acsami.7b03529. https://www.osti.gov/servlets/purl/1364166.
@article{osti_1364166,
title = {Role of Crystallization in the Morphology of Polymer: Non-fullerene Acceptor Bulk Heterojunctions},
author = {O’Hara, Kathryn A. and Ostrowski, David P. and Koldemir, Unsal and Takacs, Christopher J. and Shaheen, Sean E. and Sellinger, Alan and Chabinyc, Michael L.},
abstractNote = {Many high efficiency organic photovoltaics use fullerene-based acceptors despite their high production cost, weak optical absorption in the visible range, and limited synthetic variability of electronic and optical properties. To circumvent this deficiency, non-fullerene small-molecule acceptors have been developed that have good synthetic flexibility, allowing for precise tuning of optoelectronic properties, leading to enhanced absorption of the solar spectrum and increased open-circuit voltages (VOC). We examined the detailed morphology of bulk heterojunctions of poly(3-hexylthiophene) and the small-molecule acceptor HPI-BT to reveal structural changes that lead to improvements in the fill factor of solar cells upon thermal annealing. The kinetics of the phase transformation process of HPI-BT during thermal annealing were investigated through in situ grazing incidence wide-angle X-ray scattering studies, atomic force microscopy, and transmission electron microscopy. The HPI-BT acceptor crystallizes during film formation to form micron-sized domains embedded within the film center and a donor rich capping layer at the cathode interface reducing efficient charge extraction. Thermal annealing changes the surface composition and improves charge extraction. In conclusion, this study reveals the need for complementary methods to investigate the morphology of BHJs.},
doi = {10.1021/acsami.7b03529},
journal = {ACS Applied Materials and Interfaces},
number = 22,
volume = 9,
place = {United States},
year = {Mon May 22 00:00:00 EDT 2017},
month = {Mon May 22 00:00:00 EDT 2017}
}

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  • Formation, relaxation and dynamics of polarons and methanofullerene anion radicals photoinitiated in poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′, 3′-benzothiadiazole)]:-[6,6]-phenyl-C{sub 61}-butyric acid methyl ester (PCDTBT:PC{sub 61}BM) bulk heterojunctions were studied mainly by light-induced EPR (LEPR) spectroscopy in wide photon energy and temperature ranges. Some polarons are pinned by spin traps whose number and depth are governed by the composite morphology and photon energy. The proximity of the photon energy and the polymer bandgap reduces the number of such traps, inhibits recombination of mobile charge carriers, and facilitates their mobility in polymer network. Spin relaxation and charge carrier dynamics were studied by the steady-state saturation method atmore » wide range of temperature and photon energy. These processes were shown to be governed by spin exchange as well as by the photon energy. Charge transfer in the composite is governed by the polaron scattering on the lattice phonons of crystalline domains embedded into amorphous polymer matrix and its activation hopping between polymer layers. The energy barrier required for polaron interchain hopping exceeds that of its intrachain diffusion. Anisotropy of polaron dynamics in the PCDTBT:PC61BM composite is less than that of poly(3-alkylthiophenes)-based systems that evidences for better ordering of the former. Lorentzian shape of LEPR lines of both charge carriers, lower concentration of spin traps as well as behaviours of the main magnetic resonance parameters were explained by layer ordered morphology of polymer matrix.« less
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