Impact of Initial Bulk-Heterojunction Morphology on Operational Stability of Polymer:Fullerene Photovoltaic Cells

Saurav Limbu, Sebastian Pont, Alexander B. Doust, Sooncheol Kwon, Patrick Fuller, Ellasia Tan, James R. Durrant, Ji Seon Kim

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

Controlling initial bulk-heterojunction (BHJ) morphology of photoactive layer is critical for device efficiency of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long-term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in situ monitoring of devices at a molecular level. Here, a thermal annealing preconditioning step is utilized to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in situ degradation pathways. Direct spectroscopic evidence is reported for molecular-scale phase-segregation temperature (T PS ) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well-mixed blend morphology (no annealing) shows interface instability related to the hole-extracting layer via de-doping. Likewise, initially phase-segregated morphology at a molecular level (annealed above T PS ) shows instability in the photoactive layer via continuous phase-segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. The results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below T PS is critical for improved operational stability while maintaining high efficiency.

Original languageEnglish
Article number1801763
JournalAdvanced Materials Interfaces
Volume6
Issue number6
DOIs
StatePublished - 22 Mar 2019

Keywords

  • bulk-heterojunction morphology
  • interface stability
  • operational stability
  • organic solar cells
  • phase-segregation temperature

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