pubs.acs.org/cm Published on Web 12/23/2010 r 2010 American Chemical Society Chem. Mater. 2011, 23, 733–758 733 DOI:10.1021/cm102419z π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications † Antonio Facchetti* Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States, and Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States Received August 23, 2010. Revised Manuscript Received November 21, 2010 The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed. 1. Introduction The interest in π-conjugated polymers increased con- siderably after the discovery that their electrical conduc- tivity increases substantially upon electrochemical doping. 1 This discovery led to the 2000 Nobel Prize in Chemistry awarded to Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa. By the mid-1980s, several research teams in both academia and industry were investigating π-conjugated small molecules and polymers to gain benefit of their unique optical and semiconducting properties, paving the way to the emergence of the fields of plastic electronics and photonics. 2 These new technologies are thought to compliment current inorganic-based optoelectronic devices, which greatly impacted our society starting from the second half of the 20th century. The goal of organic- based opto-electronic devices is not that of attaining or exceeding the level of performance of silicon technologies but of enabling the fabrication of certain optoelectronic devices (or part of them) at far reduced costs and/or enabling completely new device functionalities (e.g., mechan- ical flexibility, impact resistance, and optical transparency) that are challenging to achieve with silicon. 3 Besides the discovery of new materials, the develop- ment of organic semiconductor-based opto-electronics requires achieving a much better understanding of the nature of electronic structure and charge transport prop- erties, as well as light-molecule/polymer and charge- charge interactions, in these unusual solids. 4 Although these aspects are fundamental for the optimization of these materials, the goal in this contribution is to review very recent achievements in the development of polymeric semiconductors for charge transport in thin-film transistors (TFTs) and energy production in bulk-heterojunction photovoltaic (PV) cells. Particularly we will first intro- duce basic concepts of organic polymeric semiconductor structure and OTFT/OPV operation and then focus exclusively on the works of the last three years since excellent OTFT 5 and OPV cell 6 review articles cover previous fundamental and evolutionary studies. 2. Polymeric Semiconductors Polymeric semiconductors for OTFT and OPV appli- cations must present two essential structural features (Figure 1). 7 The first is a π-conjugated backbone com- posed of linked unsaturated units resulting in extended π orbitals along the polymer chain, thus enabling proper charge transport and optical absorption. 8 The second is the functionalization of the polymer core with solubiliz- ing substituents, which is essential for inexpensive man- ufacture by solution methods as well as to enhance solid state core interactions. 9 Among the most common un- saturated units there are mono(poly)cyclic aromatic hydro- carbons, heterocycles, benzofused systems, and simple olephinic and acetylinic groups. The extent of conjugation/ interaction between these units determine the polymer solution/solid state electronic structure, which in turn control key polymer properties such as optical absorption/ emission, redox characteristics, and frontier molecular orbital energy levels, to cite just a few properties. Other important polymer architecture parameters are the molecular weight (M w ) and the polydispersity (PD) index since they influence solubility, solution aggregation, and formulation rheology, as well as the thin film formation and morphology for both pristine and blended materials. Since when going from low (oligomers) to high (polymer) molec- ular weights the electronic structure, thermal properties, and microstructure of polymers generally vary considerably, it is important to achieve a M w /PD regime where certain a † Accepted as part of the “Special Issue on π-Functional Materials”. *E-mail: afacchetti@polyera.com or a-facchetti@northwestern.edu.