Planar Organic Photovoltaic Devices

Field: Materials Science (Physical Sciences)

Organic Photovoltaic devices (OPV) are considered to be attractive candidates for clean and renewable energy source because of their potential for low cost of fabrication, easy processing, and their mechanical flexibility. The device efficiency of OPV cells are limited by several factors. Among them are: (i) donor-acceptor interface, (ii) morphology of the materials, (iii) electrode-organic semiconductor (OSC) interface and (iv) device architecture such as active material thickness and electrode separation. Although, the donor-acceptor interface has been studied in detail, the commonly prevalent vertical OPV device structure does not allow a good understanding of the other key issues as the vertical structure limits one of the electrode to be a transparent electrode as well as introducing inseparable relation between the electrodes separation and the active material thickness. In addition, it is also well known that the charge transport in OSC is anisotropic and the charge mobility is better in lateral direction rather than vertical direction. In order to address some of these issues, we fabricated OPV devices in a planar device structure where cathode and anode of dissimilar metals are in-plane with each other and their photovoltaic behaviors were studied. We used poly(3-hexylthiophene) and [6,6]- pheny1 C61-butyric acid methy1 ester (P3HT:PCBM) blend as an active material. In particular, we present a detailed study about the effects of the structural parameters such as the channel length, the active layer thickness, and the work function of the electrodes on the open circuit voltage (Voc), short circuit current (Isc), fill factor (FF) and the power conversion efficiency (PCE). In order to determine the suitable anode and cathode for the planar organic photovoltaic (P-OPV) structure, we first fabricated and measured organic field effect transistor (OFET) devices with different contacts and studied the effect of barrier height at the iv P3HT:PCBM/electrode interface on the device output and transport properties. The study showed a clear effect of varying the contact material on the charge injection mechanism and on the carriers mobilities. The results have also shown that Au with high hole mobility and on current in the p-channel can be used as an anode (holes extractor) in the P-OPV device while In, Cr, and Ti that showed a reasonable value of electron mobility can be good candidates for cathode (electron extractor). We also found that, Ag, Al, and Mg showed large barrier which resulted in large threshold voltage in the I-V curve making them undesired cathode materials in the P-OPV device. We then fabricated P-OPV devices with Au as an anode material and varied the cathode material to study the effect of the interface between the P3HT:PCBM layer and the cathode material. When Al, Mg, or Ag used as a cathode material no PV behavior was observed, while PV behavior was observed for In, Cr, and Ti cathode materials. The PV behavior and the characteristic parameters including Voc, Isc, FF and PCE were affected by varying the cathode material. The results have shown that the P-OPV device performance can be affected by the cathode material depending on the properties and the work function of the metal. We have also studied the effect of varying the P3HT:PCBM layer thickness at a fixed channel length for Cr and Ti cathode materials and Au as anode. While Voc and FF values do not change, Isc and PCE increase with increasing the layer thickness due to the increase of the light absorption and charges generation. Moreover, we studied the effect of varying the channel length at a fixed film thickness; and showed that the values of Isc and PCE increase with decreasing channel length while Voc and FF maintain the same value. In this thesis we will also present the results on experimentally defining and testing the illuminated area in the P-OPV device by using different measurement set-ups and different v electrodes patterns. The results prove that the illuminated area in the P-OPV device is the area enclosed between the two electrodes. Lastly, we will present the effect of the P3HT:PCBM ratio on the P-OPV device performance. We show that 1:2 ratio is the optimized ratio for the P-OPV device. The detailed results in this thesis show a potential opportunity to help improving and understanding the design of OPV device by understanding the effects of the device structural parameters.