In this review we present an overview of the different organic solar cells families. After recalling shortly the specificities of organic materials, the band structure, the electronic properties and the charge separation process in organic materials are shortly described. Then the new organic solar cell concepts are presented. Plastic organic solar cells consist either of two organic layers or a homogeneous mixture of two organic materials. One of them -either an organic dye or a semiconducting polymer -donates the electrons. The other component serves as the electron acceptor. Principles of these multi-layers and bulk heterojunctions are presented and discussed. Then some typical examples are presented, showing the fast evolution of the cells performances. Finally, a specific attention is devoted to the interfaces electrodes/organics. Indeed recent results show that, at least in the case of multi-layers cells, the introduction of thin buffer layers at the interfaces cathode/organic acceptor and/or anode/organic donor, can strongly improve the efficiency of the organic solar cells. About the interface organic acceptor/cathode, we report the influence of an exciton-blocking layer and/or an Al 2 O 3 thin layer on the efficiency of CuPc/C 60 based photovoltaic cells. The presence, or not, of a thin Al 2 O 3 layer depends on the encapsulating process of the devices. In the case of glass/ITO/CuPc/C 60 /Al cells, the presence of an Al 2 O 3 thin layer at the interface "organic acceptor/aluminium" increases strongly the open circuit voltage of the cells but decreases slightly their short circuit current and fill factor. In the case of glass/ITO/CuPc/C 60 /Alq 3 /Al cells, the open circuit voltage is systematically higher than without Alq 3 . However, in that case, the presence of Al 2 O 3 does not improve significantly the cell performances. All these results are discussed in terms of series and shunt resistance values related to possible oxygen contamination and organic covalent action with the Al films. The effectiveness of these different phenomena depends on the presence, or not, of Alq 3 and/or Al 2 O 3 layers.About the interface anode/organic donor, it is shown that an ultra thin metallic film improves significantly the short circuit current and the cell performances. The anode in plastic solar cells, which is a transparent conductive oxide (TCO), is usually an indium tin oxide film (ITO). Indeed, when a ZnO anode is used, cells performances are far from those achieved with ITO. However, strong improvement of the cells efficiency is encountered when an ultra thin buffer layer is introduced between the ZnO and the organic film. The presence of this ultra thin buffer layer at the surface of the TCO allows decreasing the performance difference between the cells using ITO and those using ZnO. More generally such ultra thin buffer layer improves the solar cells performances.

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International audienceDepending on their resistivity and their transmittance, the thin films of transparent conductive oxide (TCO) are widely used in optoelectronic devices. In2O3:Sn (ITO) is the most widely used TCO in optoelectronic devices. As indium is scarce and ITO is limited in flexibility due to its ceramic structure, many studies have been dedicated to new transparent conductive electrodes. This review article presents the state-of-the-art concerning the dielectric/metal/dielectric structures and their application as transparent electrodes in organic photovoltaic cells (OPVCs). First, TCO/Ag/TCO structures were created to achieve higher conductivity than ITO films. Then others dielectrics have been used such as transition-metal oxides (WO3, MoO3, V2O5, etc.), ZnS, etc. Such structures exhibit excellent flexibility, high conductivity, and good transparency. They can be deposited onto substrates at room temperature by simple evaporation under vacuum. Moreover, it is possible to manage the anode work function through the choice of the dielectric, which can allow them to be used as cathodes or anodes and as intermediate electrodes in tandem solar cells. The properties of the dielectric/metal/dielectric (D/M/D) structures depend on the thickness of the different layers. The threshold thickness value of the metal film is usually around 10 nm, where the structures change from an insulating state to a highly conductive state. This is attributed to the percolation of conducting metal paths. The transmittance of the films increases when the metal thickness increases up to the percolation thickness, while further increase induces a decrease in transmittance. Finally, the nature and the thickness of the dielectric layers can be chosen as a function of the device properties requested, which is illustrated through different examples of OPVCs