Two different types of S-shaped J-V characteristics in organic solar cells

In this paper, S-shaped characteristics of organic solar cells (OSCs) are analyzed by a drift–diffusion model which includes charge carrier surface recombination and thermal injection on the anode and cathode through boundary conditions. By varying surface recombination velocities (SRVs) for electrons and holes on both contacts and the injection barrier heights for majority carriers, two different S-shaped deviations in OSCs J-V characteristics were observed. The first type of S-shaped J-V characteristic manifests the S-shape bending in the vicinity of the voltage axis, after which it rises almost exponentially. This kind of S-shape deformation was found to arise from the finite SRVs. The second type of the S-shaped J-V characteristic also makes a kink near the voltage axis but proceeds to grow monotonically, having only one saddle point. The S-shaped J-V curve of this kind turned out to be the consequence of the electron injection barrier height larger than 0.2 eV. The validity of our model is confirmed by comparing the simulated J-V curves with the experimentally obtained data. The model has been applied to the ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/PEDOT:PSS/P3HT:ICBA/Al solar cells. For the P3HT:PCBM based solar cells, the regular J-shaped J-V curves were measured, while for the P3HT:ICBA solar cells, the J-V curves with anomalous S-shape behavior were recorded. All experimentally obtained J-V curves were reproduced very well with our model. It seems that the ITO/PEDOT:PSS/P3HT:ICBA/Al solar cell S-shaped J-V curve originates from the electron barrier height on the cathode contact, rather than from the low surface recombination on the electrodes.