ORGANIC PHOTOVOLTAIC CELLS: HISTORY, PRINCIPLE AND TECHNIQUES

Abstract

In this review we present an overview of the different solar cells families. After recalling shortly the specificities of materials, the band structure, the electronic properties and the charge separation process in materials are shortly described. Then the new solar cell concepts are presented. Plastic solar cells consist either of two layers or a homogeneous mixture of two materials. One of them - either an dye or a semiconducting polymer - donates the electrons. The other component serves as the electron acceptor. Principies 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 solar cells. About the interface acceptor/cathode, we report the influence of an exciton-blocking layer and/or an A1203 thin layer on the efficiency of CuPc/C60 based photovoltaic cells. The presence, or not, of a thin A1203 layer depends on the encapsulating process of the devices. In the case of glass/ITO/CuPc/C60/Al cells, the presence of an A1203 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/C60/Alq3/Al cells, the open circuit voltage is systematically higher than without Alq3. However, in that case, the presence of A1203 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 covalent action with the Al films. The effectiveness of these different phenomena depends on the presence, or not, of Alq3 and/or A1203 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 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