Photovoltaics for the Next Generation: Organic-Based Solar Cells
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The advancement of renewable energy technology has become increasingly critical as an environmentally sustainable path to world economic and societal development. Large-scale harvesting of energy from sunlight using photovoltaic devices offers tremendous potential as a renewable energy resource. Although the field of photovoltaics has continually evolved over the last several decades, many researchers are looking beyond current device designs to what have been termed third-generation technologies. These technologies are aimed at producing significant improvements in device efficiency-to-cost ratios. This necessitates significantly improving efficiency or reducing cost or ideally both. To realize these goals, many of these technologies will need to utilize nanostructured materials and composite systems that can be tailored to have optimized electronic and optical properties.
Organic/polymeric molecule-based electronics are a promising route to meeting these needs. Organic semiconductors have emerged as a class of materials that can be specifically designed to have a wide range of chemical, optical, and electronic properties, yet they can be processed via low-cost, solution-based techniques. The field has now matured to a point where most of the fundamental photophysical and electronic phenomena of these materials are reasonably well understood. The current challenge is to fully utilize the unique properties of organic semiconductors and to specifically design and fabricate materials and devices that lead to high efficiencies. One critical aspect of these materials is the molecular morphology. It has become apparent that the particular structural arrangement of molecules in an organic semiconductor device can have a large impact on properties such as charge carrier mobility and exciton diffusion. Short-range ordering of the molecules is primarily controlled by intermolecular interactions, as well as by processing conditions such as choice of solvent. Learning how to control the morphology on longer length scales will be a necessity to produce optimized device structures.
Parallel to the development of organic semiconductors has been progress in low-cost synthetic techniques for metal oxide semiconductors. A variety of solution-based and electrochemical routes have been developed that result in materials with well-defined, novel nanostructures that can be easily controlled by varying the fabrication conditions. This then opens up the possibility of low-cost fabrication of organic–inorganic composite structures, in which the long-range morphology of the organic molecules is determined by the three-dimensional structure of the metal oxide semiconductor. Ultimately, the development of efficient photovoltaic devices will require precise control over many parameters, including what wavelengths of light are absorbed to create excitons, the pathways available for exciton migration, the pathways and mobilities for charge carrier transport, and the alignment of energetic levels at interfaces. This article will review existing organic photovoltaic devices and explore initial efforts at more sophisticated, yet low-cost, organic–inorganic composite structures aimed at achieving high power conversion efficiencies.