Effects of 1,8-diiodooctane on domain nanostructure and charge separation dynamics in PC71BM-based bulk heterojunction solar cells

Sylvia J. Lou, Nanjia Zhou, Xugang Guo, Robert P. H. Chang, Tobin J Marks, Lin X. Chen

Research output: Contribution to journalArticle

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Abstract

Processing additives are commonly used to optimize phase separation morphology and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) organic photovoltaics (OPVs), however their exact effects are not well understood. A bulk-heterojunction OPV system containing the model bithiophene imide-benzodithiophene copolymer (PBTIBDT):phenyl-C71-butyric-acid-methyl ester (PC71BM) exhibits a maximum PCE of 5.4% with addition of 3.0 vol% 1,8-diiodooctane (DIO). Here the effects of increasing DIO concentrations (0 → 5 vol%) on the PBTIBDT:PC71BM solutions and resulting thin films are studied by X-ray scattering methodologies and transient optical absorption spectroscopy with 0.1 ps time resolution. As the DIO concentration increases, the radius of gyration of the PC71BM aggregates falls from 17 Å to 9 Å in solution, and TEM indicates the formation of increasing PC71BM charge percolation pathways in the thin BHJ films. Increased PBTIBDT + PC71BM intermixing not only affects BHJ film charge transport, but also enhances the initial exciton splitting yield in the PBTIBDT cation population. In contrast, the hole carrier population (as represented by the polymer cation) detected several nanoseconds after the photoexcitation is greatest with 3.0 vol% DIO, agreeing well with the corresponding BHJ composition for maximum OPV short circuit current density (Jsc) and fill factor (FF). The increase in initial polymer cation yield with DIO concentration is attributed to enhanced donor-acceptor interfacial area while the increase in long-lived cation population is attributed to formation of a bicontinuous donor polymer-PC71BM acceptor network that promotes large spatial separation of free charges in the device active layer. These results demonstrate the importance of OPV function for the correct balance, as tuned by processing additives, between a high initial donor cation formation yield and high carrier transport efficiency with minimized charge recombination rate.

Original languageEnglish
Pages (from-to)23805-23818
Number of pages14
JournalJournal of Materials Chemistry A
Volume6
Issue number46
DOIs
Publication statusPublished - Jan 1 2018

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Butyric acid
Butyric Acid
Heterojunctions
Nanostructures
Solar cells
Esters
Cations
Positive ions
Polymers
Conversion efficiency
Imides
Carrier transport
Photoexcitation
Processing
X ray scattering
Absorption spectroscopy
Excitons
Phase separation
Short circuit currents
Light absorption

ASJC Scopus subject areas

  • Chemistry(all)
  • Renewable Energy, Sustainability and the Environment
  • Materials Science(all)

Cite this

Effects of 1,8-diiodooctane on domain nanostructure and charge separation dynamics in PC71BM-based bulk heterojunction solar cells. / Lou, Sylvia J.; Zhou, Nanjia; Guo, Xugang; Chang, Robert P. H.; Marks, Tobin J; Chen, Lin X.

In: Journal of Materials Chemistry A, Vol. 6, No. 46, 01.01.2018, p. 23805-23818.

Research output: Contribution to journalArticle

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abstract = "Processing additives are commonly used to optimize phase separation morphology and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) organic photovoltaics (OPVs), however their exact effects are not well understood. A bulk-heterojunction OPV system containing the model bithiophene imide-benzodithiophene copolymer (PBTIBDT):phenyl-C71-butyric-acid-methyl ester (PC71BM) exhibits a maximum PCE of 5.4{\%} with addition of 3.0 vol{\%} 1,8-diiodooctane (DIO). Here the effects of increasing DIO concentrations (0 → 5 vol{\%}) on the PBTIBDT:PC71BM solutions and resulting thin films are studied by X-ray scattering methodologies and transient optical absorption spectroscopy with 0.1 ps time resolution. As the DIO concentration increases, the radius of gyration of the PC71BM aggregates falls from 17 {\AA} to 9 {\AA} in solution, and TEM indicates the formation of increasing PC71BM charge percolation pathways in the thin BHJ films. Increased PBTIBDT + PC71BM intermixing not only affects BHJ film charge transport, but also enhances the initial exciton splitting yield in the PBTIBDT cation population. In contrast, the hole carrier population (as represented by the polymer cation) detected several nanoseconds after the photoexcitation is greatest with 3.0 vol{\%} DIO, agreeing well with the corresponding BHJ composition for maximum OPV short circuit current density (Jsc) and fill factor (FF). The increase in initial polymer cation yield with DIO concentration is attributed to enhanced donor-acceptor interfacial area while the increase in long-lived cation population is attributed to formation of a bicontinuous donor polymer-PC71BM acceptor network that promotes large spatial separation of free charges in the device active layer. These results demonstrate the importance of OPV function for the correct balance, as tuned by processing additives, between a high initial donor cation formation yield and high carrier transport efficiency with minimized charge recombination rate.",
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