Tailored ZnS/Ag/TiOx transparent and conductive electrode for organic solar cells

Abstract

Organic photovoltaic cells (OPVCs) attract high interest for solar energy harvesting. They are based on organic thin films sandwiched between two electrodes, one of them being transparent and conductive. Nowadays, ITO remains the most widely used transparent conductive electrode (TCE) because of its excellent optical and electrical properties compared to other TCEs. However, it has some drawbacks such as scarcity of indium, high fabrication cost, and mechanical properties poorly adapted to use as flexible substrates. To keep these performances without indium, several materials can replace ITO such as MoO3, ZnO, ZnS, TiO2,… as dielectric and Ag, Cu,... as metal inside a dielectric/metal/dielectric three-layer structure. A Transfer Matrix Method (TMM) based numerical model is used to predict the optical behavior of the considered electrodes. ZnS/Ag/TiOx electrodes are manufactured by a vacuum electron beam evaporator on glass substrates, then characterized by UV-Visible spectrophotometer for obtaining transmittance and reflectance and by a four-point method for the measurement of sheet resistance. It is found that the simulation and experimental curves are quite similar. The transmittance is measured to be higher than 80% on a wide spectral band that can be tailored by the thickness of the upper dielectric material. The optical window Δλ, for T > 80%, can be tuned in the 400–800 nm spectral band, according to the thickness of TiOx in the 25–50 nm range. This variation allows us to adapt our electrode to organic materials in order to optimize the performance of organic solar cells. The sheet resistance obtained is around to 7 Ω/sq, which gives our electrodes the transparent and conductive character simultaneously. A typical parameter to compare the electrodes is the merit figure, which questions the average optical transmission T av in the visible range and the sheet resistance R sq. By applying this figure to many manufactured electrodes, the obtained optimal structure of our TCEs is demonstrated to be ZnS (40 nm)/Ag (10 nm)/TiOx (30 nm).