ORGANIC PHOTOVOLTAIC CELLS: HISTORY, PRINCIPLE AND TECHNIQUES

J. Chil. Chem. Soc., 53, Nº 3 (2008) ORGANIC PHOTOVOLTAIC CELLS: HISTORY, PRINCIPLE AND TECHNIQUES J. C. BERNÈDE LAMP, FSTN, Université de Nantes, 2 Rue de la Houssinière, BP 92208, Nantes CEDEX 3, 44322, France. (Received: December 4, 2007 - Accepted: SUMMARY: 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 Al2O3 thin layer on the efficiency of CuPc/C60 based photovoltaic cells. The presence, or not, of a thin Al2O3 layer depends on the encapsulating process of the devices. In the case of glass/ITO/CuPc/C60/Al cells, the presence of an Al2O3 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 Al2O3 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 Alq3 and/or Al2O3 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. I: Introduction to the photovoltaic energy 1: I-1. About the energy in the world: Availability to all citizens of safe and renewable energy in sufficient quantities is a prerequisite for a sustainable society. A clean energy is necessary to decrease the atmosphere contamination and the greenhouse gas (GHG) emissions to ensure people safety and security. A renewable energy is necessary, meaning that the use of finite fossil resources has to be gradually replaced. Therefore it is necessary to diversify the energy sources, mainly the renewable energies. It is a very urgent goal. Presently almost one thirds of the world population does not have access to electricity, while 20% of the world population of the developed countries use 80% of the world energy production. The very fast increase of the demand of the emerging countries highlights the urgency to develop renewable energies. Up to day photovoltaic energy is the most expensive source (Table I), which implies investigation in the field of cheap materials, low processing costs, ease of large scale manufacture… Hydroelectricity Bio-energy Wind energy Geothermal energy Marine energy Solar thermal energy Photovoltaic energy Renewable energies Total electrical energy Wold energy production in 2003 (TWh) Electricity costs in 2003 (€ cents/ kWh) 3000 175 75 50 0.5 0.8 2.5 3300 15000 2-8 5-6 4-12 2-10 8-15 12-18 26-65 2-3.5 Table I: Energy production and cost [1]. I-2. Current status of photovoltaic energy: The total solar power production reach 1.7 GW for 2005, that is to say less than à.01% of the total global power demand. However this is a big hike on the 2004 figure of 1.2 GW. In 2006, it is around 2.4 GW, representing year-on year growth of nearly 50% 2. Commercial photovoltaic modules are mainly based on silicon. First, the wafer based crystalline silicon either monocrystals or multicrystals. Efficiencies of these modules are near 15%. The energy pay-back time of such modules is typically around three years or 10% of the operational life time. The other technology is based on thin films: amorphous silicon, cadmium telluride, copper-indium/gallium-selenide/sulphide (CIGS). Here also silicon is dominant. However, as amorphous silicon did not fulfil stability expectations, over the past decade the thin film market reduced from 15% (1995) to 5% to day. The actual 40 to 50% annual expansion of production might reduce prices, however if crystalline silicon is still more than 90% of the cells production, the demand will exceed supply through the end of the decade and prices are likely to remain high. By the end of the decade solar power generation will have hit the 10 GW mark. The disparity between this increase and the available crystalline silicon suggests that a substantial, widening gap in the market will exist for other technologies. At the moment these only account for 9% of solar power generation. In 2010, the proportion of solar power that comes from nonsilicon technologies will grow to 20%, i.e. around 2 GW. To day the use of compound semiconductor Copper indium gallium diselenide (CIGS) is spreading fast, it appears that 2007 is the year of CIGS. In 2006 the production was 8 MWp. The production capacity at the end of 2007 is around 350 MWp with an effective production of 130 MWp for the year 3. Therefore, if as shown above, the increase of the photovoltaic market induces price reduction, competitive price production requires transition from crystalline silicon to thin film technology. Researches in two categories of technologies are under investigation: - Option 1: primarily aimed at very high efficiency, while optimization cost: multi-junction cells, use of concentrators….. - Option 2: primarily aimed at very low cost, while optimizing efficiency: organic solar cells, hybrid solar cells… micro or nano structured materials. Our c (...truncated)