Abstract
Several parameters of the fabrication process of inverted polymer bulk heterojunction solar cells based on titanium oxide as an electron selective layer and molybdenum oxide as a hole selective layer were tested in order to achieve efficient organic photovoltaic solar cells. Thermal annealing treatment is a common process to achieve optimum morphology, but it proved to be damageable for the performance of this kind of inverted solar cells. We demonstrate using Auger analysis combined with argon etching that diffusion of species occurs from the MoO3/Ag top layers into the active layer upon thermal annealing. In order to achieve efficient devices, the morphology of the bulk heterojunction was then manipulated using the solvent annealing technique as an alternative to thermal annealing. The influence of the MoO3 thickness was studied on inverted, as well as direct, structure. It appeared that only 1 nm-thick MoO3 is enough to exhibit highly efficient devices (PCE = 3.8%) and that increasing the thickness up to 15 nm does not change the device performance.
Highlights
Organic photovoltaic solar cells (OSC) have been the subject of increasing attention around the world for the last 20 years as a potential source of renewable energy
In an inverted OSC, electrons are collected by the indium tin oxide (ITO) bottom electrode through transparent electron selective layers made of materials such as calcium [7,8], titanium oxide (TiOx) [9,10], zinc oxide (ZnO) [11,12,13], cesium carbonate (Cs2CO3) [14,15] or aluminum-doped zinc oxide [9]
The aim of this paper is to study the influence of two techniques, namely thermal annealing and solvent annealing, combined with the influence of the MoO3 thickness
Summary
Organic photovoltaic solar cells (OSC) have been the subject of increasing attention around the world for the last 20 years as a potential source of renewable energy. Since 2005, so-called inverted OSC have been developed, which exhibit improved air stability compared to conventional direct structures [3,4,5,6]. In an inverted OSC, electrons are collected by the indium tin oxide (ITO) bottom electrode through transparent electron selective layers made of materials such as calcium [7,8], titanium oxide (TiOx) [9,10], zinc oxide (ZnO) [11,12,13], cesium carbonate (Cs2CO3) [14,15] or aluminum-doped zinc oxide [9]. The holes are collected from the evaporated top electrode, generally across a thin film of hole selective layer, such as molybdenum oxide (MoO3) or poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT-PSS). The top electrode is generally made of silver, since air exposure leads to the formation of silver oxide; the latter exhibits a higher work function, enhancing hole collection [16]
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