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

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, Ea, versus applied field, F. Ea 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 PooleFrenkel (PF)-like decrease in Ea at low F, which then plateaus at higher F. The low defect material does not exhibit PF behavior and Ea remains approximately constant. Upon addition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), however, both materials show a large increase in Ea 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 Ea, near the open circuit photovoltage. This causes a decrease in fill factor, which may be a general limitation in such solar cells.