Organic solar cells : materials, nano-morphology and device engineering

The production of electrical energy from solar radiation has always been dominated by photovoltaic panels, whose conversion layer is made of silicon. Organic photovoltaics, on the other hand, feature a conversion layer made of organic semiconductors, and have very specific properties. These specific properties give organic photovoltaics real potential to complement current offerings for sustainable, renewable electricity production. This article presents the state of the art in organic photovoltaics, and the key points that still need to be developed to make this technology fully mature. The first section of the article describes the operating principles of organic photovoltaic cells. This first section introduces the notion of exciton photogeneration, exciton dissociation, electron-donating and electron-accepting organic semiconductors, and volume heterojunction. The key points for achieving efficient organic solar cells are discussed in detail, with emphasis on the importance of the boundary energy levels of the active layer semiconductors and the dominant role of active layer morphology at the nanometer scale. In a second section, the engineering of materials to adjust the boundary energy levels of organic semiconductors while retaining their good electronic and processing properties is detailed. Molecular engineering makes it possible to consider a multitude of variations in the chemical structure of organic semiconductors in order to adjust the boundary energy levels of these materials. Ultimately, organic semiconductors must also have good light absorption properties (range of wavelengths absorbed, but also amplitude of absorption coefficient), sufficient charge carrier mobility to extract free charges efficiently, and adequate solubility for wet deposition. It is often necessary to make trade-offs, favouring one property over another. In a third section, the focus is on controlling the morphology of the active layer at the nanoscale, a parameter that is particularly important if we hope to develop high-efficiency organic photovoltaic cells. Nanoscale active layer morphology is highly dependent on the molecular structure of the organic semiconductors concerned, and anticipating the morphology of a nanoscale mixture for given molecular structures is a particularly perilous exercise. In a volume heterojunction with domains rich in electron-donating semiconductor material and others rich in electron-accepting semiconductor material, domain size is important, but not the only parameter of interest. Domain purity and percolation of the domains to the electrodes are parameters that have a considerable impact on the performance of organic solar cells. In the fourth section, the basic engineering principles of organic photovoltaic devices are outlined. The various possible architectures for organic solar cells are...