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Title: Morphological Characterization of a Low-Bandgap Crystalline Polymer: PCBM Bulk Heterojunction Solar Cells

Abstract

Understanding the morphology of polymer-based bulk heterojunction (BHJ) solar cells is necessary to improve device efficiencies. Blends of a low-bandgap silole-containing conjugated polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b;2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (PSBTBT) with [6,6]phenyl-C61-butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the “As-Spun”, “Pre-Annealed”, and “Post-Annealed” films were studied by scanning force microscopy, contact angle measurements, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron reflectivity. The results showed that PSBTBT was enriched at the cathode interface in the “As-Spun” films and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing-incidence X-ray diffraction and small-angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ~10 nm in size, leading to an improvement in photovoltaic performance.

Authors:
; ;
Publication Date:
Research Org.:
Energy Frontier Research Centers (EFRC); Polymer-Based Materials for Harvesting Solar Energy (PHaSE)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1065923
DOE Contract Number:
SC0001087
Resource Type:
Journal Article
Resource Relation:
Journal Name: Advanced Energy Materials; Journal Volume: 1(5); Journal Issue: 5; Related Information: PHaSE partners with University of Massachusetts, Amherst (lead) and Lowell; Oak Ridge National Laboratory; Pennsylvania State University; Renssalaer Polytechnic Institute; University of Pittsburgh
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; solar (photovoltaic), charge transport, materials and chemistry by design, synthesis (novel materials), synthesis (self-assembly)

Citation Formats

Lu, Haiyun, Akgun, Bulent, and Russell, Thomas P. Morphological Characterization of a Low-Bandgap Crystalline Polymer: PCBM Bulk Heterojunction Solar Cells. United States: N. p., 2011. Web. doi:10.1002/aenm.201100128.
Lu, Haiyun, Akgun, Bulent, & Russell, Thomas P. Morphological Characterization of a Low-Bandgap Crystalline Polymer: PCBM Bulk Heterojunction Solar Cells. United States. doi:10.1002/aenm.201100128.
Lu, Haiyun, Akgun, Bulent, and Russell, Thomas P. Fri . "Morphological Characterization of a Low-Bandgap Crystalline Polymer: PCBM Bulk Heterojunction Solar Cells". United States. doi:10.1002/aenm.201100128.
@article{osti_1065923,
title = {Morphological Characterization of a Low-Bandgap Crystalline Polymer: PCBM Bulk Heterojunction Solar Cells},
author = {Lu, Haiyun and Akgun, Bulent and Russell, Thomas P.},
abstractNote = {Understanding the morphology of polymer-based bulk heterojunction (BHJ) solar cells is necessary to improve device efficiencies. Blends of a low-bandgap silole-containing conjugated polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b;2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (PSBTBT) with [6,6]phenyl-C61-butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the “As-Spun”, “Pre-Annealed”, and “Post-Annealed” films were studied by scanning force microscopy, contact angle measurements, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron reflectivity. The results showed that PSBTBT was enriched at the cathode interface in the “As-Spun” films and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing-incidence X-ray diffraction and small-angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ~10 nm in size, leading to an improvement in photovoltaic performance.},
doi = {10.1002/aenm.201100128},
journal = {Advanced Energy Materials},
number = 5,
volume = 1(5),
place = {United States},
year = {Fri Jul 01 00:00:00 EDT 2011},
month = {Fri Jul 01 00:00:00 EDT 2011}
}
  • The impact of the morphological stability of the donor/acceptor mixture under thermal stress on the photovoltaic properties of bulk heterojunction (BHJ) solar cells based on the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']-di-thiophene-2,6-diyl-alt-(4-(2 ethylhexyl)-3-fluorothieno[3,4-b]-thiophene)-2-carboxylate-2,6-diyl]/phenyl-C61-butyric acid methylester (PTB7-Th/PC61BM) blend is extensively investigated. Both optical microscopy and transmission electron microscopy micrographs show that long-term high-temperature aging stimulates the formation of microscale clusters, the size of which, however, is about 1 order of magnitude smaller than those observed in thermally annealed poly(3hexylthiophene)/PC61BM composite film. The multilength-scale evolution of the morphology of PTB7-Th/PC61BM film from the scattering profiles of grazing incidence small-angle and wide-angle X-ray scattering indicates the PC61BM moleculesmore » spatially confine the self-organization of polymer chains into large domains during cast drying and upon thermal activation. Moreover, some PC61BM molecules accumulate into ~30-40 nm clusters, the number of which increases with heating time. Therefore, the hole mobility in the active layer decays much more rapidly than the electron mobility, leading to unbalanced charge transport and degraded cell performance. Importantly, the three-component blend that is formed by replacing a small amount of PC61BM in the active layer with the bis-adduct of PC61BM (bis-P61M) exhibits robust morphology against thermal stress. Accordingly, the PTB7-Th/PC61BM:bis-PC61BM (8 wt %) device has an extremely stable power conversion efficiency.« less
  • Device performances of BHJ solar cells based on poly[(4,4-didodecyldithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzoxadiazole)-4,7-diyl]and PC₇₁BM improve by capping the chain ends with thiophene fragments. This structural modification yields materials that are more thermally robust and that can be used in devices with thicker films – important considerations for enabling the mass production of plastic solar cells.