Solar cells constructed of organic materials are becoming increasingly efficient due to the discovery of the bulk heterojunction concept. This review provides an overview of organic solar cells. Topics covered include: a brief history of organic solar cell development; device construction, definitions, and characteristics; and heterojunction morphology and its relation to device efficiency in conjugated polymer/fullerene systems. The aim of this article is to show that researchers are developing a better understanding of how material structure relates to function and that they are applying this knowledge to build more efficient light-harvesting devices.

Contents 1. General Experimental Methods 2. Experimental Procedures 2.1 Monomer Syntheses 2.2 Polymerization Procedures 2.3 Optimization of "Click-Chemistry" for Ethynyl-Terminated Poly(3-hexylthiophene) 2.4 Model Compound for End "Clicking" Reactions 2.5 Synthesis of Naphthalenediimides 2.6 Application of "Click-Chemistry" Conditions to Post-Polymerization Side-Chain Functionalization 3. NMR Spectra 4. Supplementary GPC Data 5. Supplementary UV-vis Data S2 General Experimental MethodsAll chemicals were purchased from commercial sources (Acros, Aldrich, Alfa Aesar, TCI, and Strem) and used without further purification, unless otherwise noted. THF was distilled from sodium-benzophenone ketyl. NBS was recrystallized from water. t-BuMgCl (nominally 1.0 M in THF) and ethynyl-MgBr (nominally 0.50 M in THF) were titrated against salicylaldehyde phenylhydrazone according to the method of Love (J. Org. Chem. 1999, 64(10), 3755). Common solvents were purchased from EMD (through VWR). Routine monitoring of reactions was carried out on glass-supported EMD silica gel 60 F 254 TLC plates. Flash chromatography was performed using silica gel from Sorbent Technologies (Standard Grade, 60 Å, 32-63 μm). All 1 H and 13 C{ 1 H} NMR spectra were recorded on a Bruker Avance400 spectrometer. 19 F NMR spectra were recorded on a Bruker DPX300 spectrometer. Chemical shifts and coupling constants (J) are reported in parts per million (δ) and Hertz, respectively. Deuterated solvents were obtained from Cambridge Isotope Laboratories, Inc. Chloroform-D (CDCl 3 ) contained 0.05% v/v tetramethylsilane (TMS) and this peak was set to 0.00 ppm on all proton spectra. In the case of 1,1,2,2-tetrachloroethane-D 2 (TCE), which did not contain TMS, the residual solvent peak was set to 6.00 ppm. Gel permeation chromatography (GPC) was done in the NSF-sponsored MaterialsResearch Science and Engineering Center on Polymers (MRSEC) at the University of Massachusetts Amherst. GPC analyses were performed on a Polymer Laboratories GPC50 integrated system with tetrahydrofuran (1.0 mL/min, 35 ºC) elution, 3 x Mixed C (300 x 7.5 mm) columns, and RI detection. Molecular weights were obtained based on polystyrene standards, with toluene as the flow rate marker, and may be overestimated by a factor of 1.5-2.0.UV-vis absorption spectra were measured with a Shimadzu UV 2600PC spectrometer in the laboratory of Prof. Paul M. Lahti (University of Massachusetts Amherst, Department of Chemistry). Stock solutions of polymers (c = 1 mg/10 mL) were prepared in spectrophotometric grade chloroform (Fisher, Optima) and diluted 10x (c = 1 mg/100 mL) in order to attain absorbances < 1.0. Films for UV-vis experiments were prepared by drop-casting solutions of polymer (c = 1 mg/mL) onto glass cover slips (Fisher Cat. No. 12-548-B) and allowing the solvent to freely evaporate.

We report a general strategy for the spontaneous segregation of electron-rich and electron-poor pi-conjugated moieties using mutually phobic aliphatic fluorocarbon-hydrocarbon interactions.

Solar cells require symmetry-breaking features such as built-in electrostatic fields and/or effective fields. We examine an organic heterojunction structure with no built-in field and explore the origins of its current-voltage characteristics and open circuit voltage (VOC). Two behaviors are found: (1) VOC=VI+m[(HOMO(D) - (LUMO(A)] where m≈1, the intercept (VI) is determined by interface recombination kinetics, HOMO(D) is the donor highest occupied molecular orbital, and LUMO(A) is the acceptor lowest unoccupied molecular orbital; (2) if interface recombination is suppressed, VOC is controlled by bulk/contact recombination and is not dependent upon HOMO(D) - LUMO(A).