Semiconducting Block Copolymers for Self-Assembled Photovoltaic Devices
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Semiconducting
Block Copolymers for Self-Assembled Photovoltaic Devices Georges Hadziioannou
Abstract This article focuses on self-assembled photovoltaic materials based on a new class of semiconducting block copolymers for application in photovoltaic devices. Topics discussed include the materials concept for efficient photovoltaic-device structures, their macromolecular design and synthesis, and their performance in relation to their molecular, mesoscopic, and interfacial structures. An ideal organic material for this application would have to compete with amorphous silicon in regard to energyconversion efficiency and fabrication costs. The potential of the semiconducting block copolymers presented in this review lies in the promise of large-area, mechanically flexible, self-structured photovoltaic devices fabricated by inexpensive processing techniques. Keywords: electroactive organic materials, organic semiconductor–metal interfaces, photovoltaic devices, self-assembly, semiconducting block copolymers, superstructures.
An Overview of Polymer Semiconductors: Science and Technology Research on “synthetic metals” has evolved from a rather esoteric occupation to a lively field of activity since the discovery in 1977 of electrical conductivity in the simple hydrocarbon polymer polyacetylene upon oxidation or reduction (doping).1,2 Conjugated polymers, although not the only class of materials known today as “synthetic metals,” now form the most widely investigated group. The dramatic development of this field began in 1990 after the discovery of electroluminescence in a nondoped conjugated-polymer thin film sandwiched between electrodes.3 As a result, during the 1990s, research in this highly interdisciplinary area focused on semiconductor rather than conductor properties. Derivatives of polythiophene (PT), poly(p-phenylene) (PPP), poly(p-phenylene vinylene) (PPV), and polyfluorene (PF) are
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the major candidates for use as the active material in field-effect transistors, lightemitting diodes, photodetectors, photovoltaic cells, sensors, and lasers in solution and solid states.4,5 Besides their semiconductor properties, polymers also provide a way to obtain patterned structures by means of inexpensive techniques such as spin casting, photolithography, ink-jet printing, soft lithography, screen printing, and micromolding onto almost any type of substrate, including flexible ones.4,6 Some applications have already been commercialized (light-emitting devices), while others are certainly technically feasible (plastic solar cells, electronic circuitry). There is good reason to expect that conjugated polymers will play a role in the emerging communications and information technologies that are based on the use of optical signals for data transfer. The main advantage offered by polymers over traditional semiconductor ma-
terials is the versatility of their processing, which allows a polymer to be obtained in virtually any desired shape and in composite form with many other materials. Deposition as a thin film over a
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