Quantification of energy losses in organic solar cells from temperature-dependent device characteristics

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-60 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 Delta E and the CT energy E-CT may be regarded as identical. The temperature-dependent analysis of open-circuit voltage V-OC and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of Delta E and E-CT is again found for all material systems with the exception of copper phthalocyanine/C-60. The general rule of thumb for organic semiconductor heterojunctions, that VOC at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.