Tungsten oxide interfacial layer for ITO-free organic solar cells with improved performance

Abstract

As one of the alternative technologies for pollution-free energy production, solar cells, which convert energies from sunlight into electricity, are rapidly gaining interests with the help of corporate investment and political encouragement. In particular, organic solar cells made of organic materials have recently drawn much attention due to their inherent advantages: low material cost, ultrathin active layer, and compatibility for solution-processing and low-temp processing. A useful combination of those advantages can open a new area of applications for solar cells such as see-though type, flexible solar cells potentially at a fraction of cost which would be required with the existing technologies. Nonetheless, the highest power conversion efficiency reported to date is still in the range of 5-6%, which is still below the level desired for commercialization. In addition to the demand for higher power conversion efficiency, a recent price hike in indium requires one to develop an alternative electrode technology that can replace expensive ITO electrodes so that organic solar cells can position themselves as truly low-cost photovoltaic technologies. This thesis describes our study on using thin tungsten oxide ($WO_3$) films as an interfacial buffer layer in an effort to improve the power conversion efficiency and, at the same time, to replace ITO electrodes in organic solar cells. That is, thermally evaporated $WO_3$ films are investigated as a buffer layer on anodes to improve the performance of bulk-heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM-60). Characterization of the $WO_3$ film under study shows that it is amorphous with the conductivity on the order of $10^{-6}$ S/cm and can effectively planarize an originally rough ITO surface down to the RMS roughness of 0.88 nm. Insertion of 5 to 30 nm-thick $WO_3$ layers resulted in power conversion efficiency and fill factor as high as 3.1%...