Activation Energy Spectra: Insights Into Transport Limitations of Organic Semiconductors and Photovoltaic Cells

Ziqi Liang, Alexandre M. Nardes, Jao Van De Lagemaat, Brian A. Gregg

Research output: Contribution to journalArticlepeer-review

7 Scopus Citations


Some mechanisms of charge transport in organic semiconductors and organic photovoltaic (OPV) cells can be distinguished by their predicted change in activation energy for the current, E a, versus applied field, F. E a versus F is measured first in pure films of commercially available regioregular poly(3-hexylthiophene) (P3HT) and in the same P3HT treated to reduce its charged defect density. The former shows a Poole-Frenkel (PF)-like decrease in E a at low F, which then plateaus at higher F. The low defect material does not exhibit PF behavior and E a remains approximately constant. Upon addition of [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM), however, both materials show a large increase in E a and exhibit PF-like behavior over the entire field range. These results are explained with a previously proposed model of transport that considers both the localized random disorder in the energy levels and the long-range electrostatic fluctuations resulting from charged defects. Activation energy spectra in working OPV cells show that the current is injection-limited over most of the voltage range but becomes transport-limited, with a large peak in E a, near the open circuit photovoltage. This causes a decrease in fill factor, which may be a general limitation in such solar cells.

Original languageAmerican English
Pages (from-to)1087-1091
Number of pages5
JournalAdvanced Functional Materials
Issue number5
StatePublished - 2012

NREL Publication Number

  • NREL/JA-5900-5325


  • charge transport
  • conjugated polymers
  • doping
  • organic electronics
  • photovoltaic devices


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