The Effects of Ultraviolet Ozone Treated Buffer Layer and Doping of Transition Metal Oxide on the Performance of Organic Photovoltaic Devices


Student thesis: Doctoral Thesis

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Award date29 Aug 2017


Organic photovoltaic (OPV) devices have attracted much interest because of their flexibility, low cost, and ease of device fabrication. The power conversion efficiency (PCE) of small molecule OPV device has reached more than 8 % giving promising hope for commercial applications. Many approaches have been explored for enhancing the PCE of OPV devices. These include the development of new light-harvesting molecules and employment of novel device structure. Besides focusing on the efficiency, device operation degradation is also an important issue as it is currently the major hurdle against the wide application of organic solar cells. Environment conditions, i.e. light, oxygen and moisture in the air, are well known to have major influences on the performance and degradation of OPV devices. However, effective solutions for addressing this issue are yet to be developed. On the other hand, the fundamental physics governing OPV device operation is not fully understood. For example, our understanding of how open circuit voltage (Voc) is controlled by various factors is far from adequate.

In this thesis, the effects of the buffer layer on operation degradation of a boron subphthalocyanine chloride/ fullerene (SubPc/C60) bilayer OPV device were investigated. Although there have been some reports on improving stability by encapsulation or adding anode buffer layers i.e. poly(styrenesulfonate) (PEDOT: PSS), vanadium pentoxide (V2O5), the results still have rooms of improvement. A UV-ozone treated zinc oxide (ZnO) buffer layer prepared by simple solution processing was considered as an alternate candidate for anode buffer layer in this work. It was found that UV-ozone plasma treatment can effectively increase the work function of the ZnO layer from 4.5 to 5.1 eV. The treated ZnO layer also leads to 28 % enhancement in the PCE and substantial degradation improvement. After 60 minutes of continuous irradiation, PCE of the device with the treated buffer layer only decreases by about 5 %.

Secondly, the relationships among dielectric constant, capacitance and Voc in the SubPc/C60 bilayer device were studied. Some researchers have tried to increase the dielectric constant of the active layers in OPV device by doping of polymer, for example, polythiophene (P3HT) etc., into the donor or acceptor layer. However, the resulting increment in dielectric value is relatively small. In this work, a high dielectric constant value was achieved by mixing MoO3 into a SubPc layer from 0 to 50 % (weight %). This leads to a significant change in Voc. The increases in dielectric constant leads to a reduction in binding energy and Voc loss. In addition, the difference of highest occupied molecular orbital of donor and the lowest unoccupied molecular orbital of the acceptor (HOMO donor - LUMO acceptor gap) of SubPc/C60 keeps constant after MoO3 mixing. Comparing the effects of these two Voc controlling factors, the dielectric constant is dominated in the Voc improvement.