Origin of the Kink in Current-Density Versus Voltage Curves and Efficiency Enhancement of Polymer-C $_{\bf 60}$ Heterojunction Solar Cells

Current-voltage (J-V ) curves of photovoltaic devices can reveal important microscopic phenomena when parameterization is properly related to physical processes. Here, we identify a pronounced effect of thermal annealing on the organic-cathode metal interface and show that this interface is related to the origin of the kink often observed in J-V curves close to the open circuit. We propose that isolated metal nanoclusters that form upon cathode evaporation lead to defect states close to the interface and change the electric field distribution in the device. We express this scenario with a modified equivalent circuit and consistently fit J- V curves as a function of the annealing process. The developed model is general in the sense that any physical process that leads to the change in electric potential as described in this paper will possibly lead to a kink in the J- V curves. Knowing the origin of the kink allowed us to largely increase the device efficiency of the archetypal material combination Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) -C. We fabricated solar cells with an efficiency of 1.85% under 100 mW/cm AM1.5 illumination by using a deliberately designed interpenetrating bilayer film morphology, aluminium as cathode and thermal annealing. This is so far the highest reported efficiency for this particular combination of materials.