  Energetic disorder has been known for decades to limit the performance of
structurally disordered semiconductors such as amorphous silicon and organic
semiconductors. However, in the past years, high performance organic solar
cells have emerged showing a continuously reduced amount of energetic disorder.
While searching for future high efficiency material systems, it is therefore
important to correctly characterize this energetic disorder. While there are
several techniques in literature, the most common approaches to probe the
density of defect states are using optical excitation as in external quantum
efficiency measurements or sequential filling of the tail states by applying an
external voltage as in admittance spectroscopy. A metanalysis of available
literature as well as our experiments using four characterization techniques on
two material systems reveal that electrical, voltage-dependent measurements
frequently yield higher values of energetic disorder than optical measurements.
With drift-diffusion simulations, we demonstrate that the approaches probe
different energy ranges of the subband-gap density of states. We further
explore the limitations of the techniques and find that extraction of
information from a capacitance-voltage curve can be inhibited by an internal
series resistance. Thereby, we explain the discrepancies between measurements
techniques with sensitivity to different energy ranges and electronic
parameters.
