Improving organic photovoltaic cells by forcing electrode work function well beyond onset of Ohmic transition

As electrode work function rises or falls sufficiently, the organic semiconductor/electrode contact reaches Fermi-level pinning, and then, few tenths of an electron-volt later, Ohmic transition. For organic solar cells, the resultant flattening of open-circuit voltage (V-oc) and fill factor (FF) leads to a 'plateau' that maximizes power conversion efficiency (PCE). Here, we demonstrate this plateau in fact tilts slightly upwards. Thus, further driving of the electrode work function can continue to improve V-oc and FF, albeit slowly. The first effect arises from the coercion of Fermi level up the semiconductor density-of-states in the case of 'soft' Fermi pinning, raising cell built-in potential. The second effect arises from the contact-induced enhancement of majority-carrier mobility. We exemplify these using PBDTTPD:PCBM solar cells, where PBDTTPD is a prototypal face-stacked semiconductor, and where work function of the hole collection layer is systematically 'tuned' from onset of Fermi-level pinning, through Ohmic transition, and well into the Ohmic regime. Both open-circuit voltage and fill factor of organic solar cells are affected by the metal-organic semiconductor interface. Here, the authors demonstrate that the voltage can continue to rise when the Fermi level is forced up to the semiconductor density-of-states tail.

Automated summary

The study shows that when the work function of an electrode changes significantly, the contact between the organic semiconductor and the electrode goes through Fermi-level pinning and Ohmic transition. This leads to a 'plateau' effect on the open-circuit voltage and fill factor of organic solar cells, maximizing power conversion efficiency.