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).
Highlights
The excessive use of fossil fuels causes the release of large amounts of CO2, leading to global warming of our planet
In order to limit the emission of greenhouse gases, fossil energy sources are being replaced by renewable sources such as sun, wind, etc
We use an algorithm based on the Transfer Matrix Method (TMM) [14,15], which assumes that the layers deposited are flat, massive and homogeneous; it estimates that the light comes from a semiinfinite substrate in normal incidence and that the stack is surrounded by air [16]
Summary
The excessive use of fossil fuels causes the release of large amounts of CO2, leading to global warming of our planet. Solar energy based on the photovoltaic effect can be an efficient alternative. It is a well-adapted answer for the. The photosensitive layer differs according to the nature of the organic material used, one of the two electrodes being metallic (Al, Ag,...) [3,4] while the other. The photovoltaic effect is based on the light conversion into electrical energy This conversion involves a set of physical processes namely the absorption of the incident photons, exciton generation, exciton diffusion and dissociation, transport and collection of charges carriers. A second, rather important objective is to have the possibility of controlling the bandwidth (for transmission greater than 80%) and its spectral position with respect to the absorption band of the organic material likely to be used in as an active layer in an organic solar cell
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