The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

Conjugated polymer-based organic solar cells

Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.

Organic Thin Film Transistors for Large Area Electronics

Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buckminsterfullerene, C(60), is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C(60) molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C(60) anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C(60), the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.

https://science.sciencemag.org/content/sci/258/5087/1474.full.pdf

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene.

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer-based organic photovoltaic systems hold the promise for a cost-effective, lightweight solar energy conversion platform, which could benefit from simple solution processing of the active layer. The function of such excitonic solar cells is based on photoinduced electron transfer from a donor to an acceptor. Fullerenes have become the ubiquitous acceptors because of their high electron affinity and ability to transport charge effectively. The most effective solar cells have been made from bicontinuous polymer–fullerene composites, or so-called bulk heterojunctions. The best solar cells currently achieve an efficiency of about 5 %, thus significant advances in the fundamental understanding of the complex interplay between the active layer morphology and electronic properties are required if this technology is to find viable application.

Polymer–Fullerene Composite Solar Cells

-phenylenevinylene)s L4. Fluorene-Based Conjugated Polymers L4.1. Fluorene-Based Copolymers ContainingElectron-Rich MoietiesM4.2. Fluorene-Based Copolymers ContainingElectron-Deﬁcient MoietiesN4.3. Fluorene-Based Copolymers ContainingPhosphorescent ComplexesQ5. Carbazole-Based Conjugated Polymers R5.1. Poly(2,7-carbazole)-Based Polymers R5.2. Indolo[3,2-

https://ir.nctu.edu.tw:443/bitstream/11536/6508/1/000271856900020.pdf

Synthesis of Conjugated Polymers for Organic Solar Cell Applications

The properties of an organic molecular ferromagnet [C(60)TDAE(086); TDAE is tetrakis(dimethylamino)ethylene] with a Curie temperature ;T(c) = 161 kelvin are described The ferromagnetic state shows no remanence, and the temperature dependence of the magnetization below ;T(c) does not follow the behavior expected of a conventional ferromagnet These results are interpreted as a reflection of a three-dimensional system leading to a soft ferromagnet

https://science.sciencemag.org/content/sci/253/5017/301.full.pdf

Organic molecular soft ferromagnetism in a fullerene c60.

The toluene extract of the fluffy carbon material produced by resistive heating of graphite contains a variety of molecules larger than C(60) and C(70) in a total amount of 3 to 4% by weight. Repeated chromatography of this material on neutral alumina has led to the isolation of stable solid samples of C(76), C(84), C(90), and C(94). The characterization, which includes mass spectrometry, (13)C nuclear magnetic resonance, electronic absorption (ultraviolet/visible) and vibrational (infrared) spectroscopy identifies these all-carbon molecules as higher fullerenes. In addition, C(70)O, a stable oxide, has been isolated that is structurally and electronically closely related to D5h-C(70). This compound forms during the resistive heating process and probably has an oxygen atom inserted between two carbon atoms on the convex external surface of the C(70) skeleton.

https://science.sciencemag.org/content/sci/252/5005/548.full.pdf

The Higher Fullerenes: Isolation and Characterization of C76, C84, C90, C94, and C70O, an Oxide of D5h-C70

Two different fullerenes have the same cyclic voltammetry

The Unusual Electron Spin Resonance of Fullerene C60.

J Am Chem SOC 1991, 113, 6698-6700 Table I Yields of Products of Reactions of Magnesium with Bromocyclopropane and 1-Bromohexane, Separately and Together, in Diethyl Ether, under Nitrogen, and a t Reflux (=34 'C)O HxBr CpBr CPH CpMgBr HxMgBr ss CpHx HxS W P m x 1 o los M-I s-1 in T H F for reaction 2 with R = isobutyl, R' = ethyl, and X = BrIob According to D-model calculations,I3 a rate constant of 105 M-1 s-l w ould not be sufficient to make the reaction very significant during the formation of a 1 M solution of a Grignard reagent A rate constant as large as lo6 M-l PI, however, could lead to significant reaction The rate constant for the reaction of Cp' with S H is probably near lo6 s-12,14 The present data indicate that reaction 2 (R = Cp, R' = Hx) is competitive with solvent attack by Cp' when [HxMgBr] = 02 M so the rate constant for reaction 2 probably approaches lo7 M-' s-l Rate constants for related reactions of alkylborons, -aluminums, - mercuries, etc can have similar value^'^ Historically, entrainment has been most often used with aryl halides7 Cyclopropyl and aryl radicals, which are u radicals, are more electronegative than typical alkyl ( 7 ) radicals Since R'IR'MgX exchange can be regarded as a net electron transfer, reaction of u radicals with alkylmagnesium halides such as ethyl- and hexylmagnesium halides are expected to be thermodynam- ically favored Conversely, the Hx*/CpMgBr exchange reaction is expected to be thermodynamically unfavored and therefore relatively slow Consistent with this, no effects on the yields of (8) Scheme I uses the D model, in which intermediate radicals diffuse freely in solution For a recent summary of another view of the mechanism of RMgX formation, see ref 9 (9) Walborsky, H M Acc Chem Res 1990, 23, 286 (IO) (a) Lawler, R G; Livant, P J  Am Chem Soc 1976,98,3710 (b) Lehr, G F; Lawler, R G J Am Chem Soc 1984, 106, 4048 (11) (a) Kharasch, M S; Fuchs, C F J  Org Chem 1945,10,292 (b) Reference 3, p 130 (12) (a) Schaart, B J; Bodewitz, H W H J; Blomberg, C; Bickelhaupt, F J  Am Chem Soc 1976, 98, 371 2 (b) The possible relevance of this work was pointed out by an anonymous referee (13) (a) Garst, J F; Swift, B L J  Am Chem Soc 1989, 111, 241 (b) Garst, J F; Swift, B L; Smith, D W J Am Chem SOC 1989, 1 1 1 , 234 (c) Garst, J F; Deutch, J M; Whitesides, G M J  Am Chem SOC 1986, 108, 2490 (d) Garst, J F Acc Chem Res 1991, 24, 95 (14) Johnston, L J; Scaiano, J C; Ingold, K U J  Am Chem Soc 1984 (IS) Ingold, K U; Roberts, B P Free Radical Substitution Reactions; Wiley-Interscience: New York, 1971 See especially pp 18-20, 41, 51-52, and references cited therein CpMgBr, CpH, and SS are found when magnesium reacts with HxBr in the presence of CpMgBr, and no CpHx or HxS is formed The scope of R'/R'MgX exchange reactions, their mechanistic details, accurate values of their rate constants, the extents of their contributions to entrainment in other examples, solvent, halogen, and other effects on R'/R'MgX exchange, and the possible roles of transition-metal impurities all remain to be established16 Acknowledgment is made to the National Science Foundation (Grant CHE-8721866) and to the donors of the Petroleum Re- search Fund, administered by the American Chemical Society, for partial support of this work (16) Allen, R B; Lawler, R G; Ward, H R Tetrahedron Lett 1973, Coo- with Coordination Compounds (Tetraphenylporphinato)chromium(III) Fulleride Alain Pinicaud,' John Hsu, and Christopher A Reed* Department of Chemistry University of Southern California Los Angeles, California 90089-0744 A Koch, K C Khemani, P-M Allemand, and F Wudl Institute for Polymers and Organic Solids Departments of Physics and Chemistry University of California Santa Barbara, California 93106 Received June 26, 1991 Recent reports of the reductive electrochemistry of Cm,14 the isolation of the first fulleride salt,' the conductivity and super- conductivity of alkali metal doped fullerenes,6s7 and the lithiation of Cm6 suggest that the anions of the buckminsterfullerenes ( bucky balls ) have a very rich chemistry With the charac- 'On leave from the Laboratoire de Physique des Solides, UA 2 CNRS, Universit6 de Paris-Sud, 91405 Orsay, France (1) Haufler, R E; Conceicao, J; Chibante, L P F; Chai, Y; Byrne, N E; Flanagan, S; Haley, M M; O'Brien, S C; Pan, C; Xiao, Z; Billups, W E; Ciufolini, M A; Hauge, R H; Margrave, J L; Wilson, L J; Curl, R F; Smalley, R E J Phys Chem 1990, 94, 8634 (2) Allemand, P M; Koch, A; Wudl, F; Rubin, Y; Diederich, F; Al- varez, M M; Anz, S J; Whetten, R L J Am Chem Soc 1991,113, 1050 (3) Cox, D M; Behal, S; Disko, M; Gorun, S M; Greaney, M; Hsu, C S; Kollin, E B; Millar, J; Robbins, J; Robbins, W; Sherwood, R D; Tindall, P J  Am Chem SOC 1991, 113, 2940 (4) Dubois, D; Kadish, K M; Flanagan, S; Haufler, R E; Chibante, L P F; Wilson, L J J  Am Chem SOC 1991, 113, 4364 (5) Allemand, P-M; Srdanov, G; Koch, A; Khemani, K; Wudl, F; Rubin, Y; Diederich, F; Alvarez, M M; Anz, S J; Whetten, R L J  Am Chem SOC 1991, 113, 2780 (6) Haddon, R C; Hebard, A F; Rosseinsky, M J; Murphy, D W; Duclos, S J; Lyons, K B; Miller, B; Rosamilia, J M; Fleming, R M; Kortan, A R; Glarum, S H; Makhija, A V; Muller, A J; Elick, R H; Zahurak, S M; Tycko, R; Dabbagh, G; Teiel, F A Nature 1991,350,320 (7) Hebard, A F; Rosseinsky, M J; Haddon, R C; Murphy, D W; Glarum S H; Palstra, T T M; Ramirez,, A P; Kortan, A R Nature 1991, (8) Bausch, J W; Surya Prakash, G K; Olah, G A,; Tse, D S; Lorents, D C; Bae, Y K; Malhotra, R J  Am Chem SOC 1991, 113, 3205 0 1991 American Chemical Societv

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A. Koch

Papers

Organic molecular soft ferromagnetism in a fullerene c60.

The Higher Fullerenes: Isolation and Characterization of C76, C84, C90, C94, and C70O, an Oxide of D5h-C70

Two different fullerenes have the same cyclic voltammetry

The Unusual Electron Spin Resonance of Fullerene C60.

C60.bul.- with coordination compounds. (Tetraphenylporphinato)chromium(III) fulleride