Abstract
Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current and the CT energy may be regarded as identical. The temperature-dependent analysis of open-circuit voltage and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of and is again found for all material systems with the exception of copper phthalocyanine/C. The general rule of thumb for organic semiconductor heterojunctions, that at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.
2 More- Received 8 October 2013
DOI:https://doi.org/10.1103/PhysRevB.88.235307
©2013 American Physical Society