Controlled radical polymerizations as versatile synthetic routes for

Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: August 13, 2009 | doi: 10.1021/bk-2009-1023.ch017

Chapter 17

Controlled radical polymerizations as versatile synthetic routes for conjugated rod-coil block copolymers and their use as active polymer semiconducting materials in flexible organic electronic devices and systems Cyril Brochon and Georges Hadziioannou Laboratoire d’Ingénierie des Polymères pour les Hautes Technologies Université de Strasbourg Ecole Européenne de Chimie, Polymères et Matériaux (ECPM) 25 rue Becquerel 67087 Strasbourg, France

Controlled Radical Polymerization (CRP) such as Atom Transfer Radical Polymerization (ATRP) or Nitroxide Mediated Radical Polymerization (NMRP), can be used to obtain various well defined rod-coil conjugated diblock or triblock copolymers having well controlled opto-electronic properties. These copolymers are soluble in common organic solvents and can be self-organized giving rise at various nm length scale and thermodynamically stable structures. They show promising opto-electronic properties and fulfil the basic morphological requirements towards the fabrication of efficient flexible organic electronic devices.

Introduction Conjugated polymers are promising as functional polymer materials for the development of low cost flexible organic electronic devices and systems1. In this field, the processability (solubility) of the material, the control of their morphology and design and control of the optoelectronic functionality are of a © 2009 American Chemical Society In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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244 crucial importance. The design of semiconducting polymer materials for solution-processed photovoltaic devices has been extensively developed during the last years.2 Relatively high energy conversion efficiencies (up to 5.2%)3 were achieved with blends of poly-(3-alkyl) thiophenes (P3ATs) and fullerene derivatives as photoactive layer where electron-donating (donor) polymers and electron-accepting (acceptor) fullerenes form an interpenetrated network the so called donor-acceptor bulk heterojunction (BHJ) system. Device performances and stability strongly depend on the active layer morphology which can be dramatically affected by the process (spin coating, annealing, solvent quality …). Conjugated rod-coil block copolymers are constituted of a conjugated polymer (based on sp2 carbon), the “rod” block having semiconducting properties, and a classical polymer (based on sp3 carbon) “coil” block covalently bonded. Such materials are suitable to be used as active materials for flexible organic electronic devices and systems in particular in solar cells. Their suitability results from the fact that they can be designed so as to be soluble in common organic solvents and allow control of stable nm size structures as a consequence of their equilibrium mesomorphic phase separation. Moreover, by the judicious choice of the formation of carbon-carbon bonds in the polymerisation process involving sp2 and or sp3 carbons the optoelectronic properties of the rod-coil block copolymers can be designed/controlled adequately. Over the last decade, several groups have studied the self-organising properties and structure of this new class of rod-coil block copolymers in bulk and in thin film configurations. 4,5 The fact that the size, stability and structure of donor and acceptor phases is crucial in organic photovoltaic devices for their optimal performance, recently donor-acceptor block copolymers have been developed where each microphase has the required size and optoelectronic properties. These materials have been obtained from well-defined linear conjugated rod-coil block copolymers,6 but also from grafted or comb-like rodcoil copolymers. 7,8 Also, block copolymers with architectures such as rod-rod conjugated donor-acceptor multiblock copolymers 9 or coil-coil diblock copolymers grafted with donor and acceptor side groups10 have been developed. These novel materials can be used as active layer, to replace the P3AT/fullerene blends but also as surfactant/compatibilizer of the semiconducting polymer/fullerene blends. Indeed, block copolymers when used as surfactants/compatibilizers in blends reduce the interfacial tension and thus stabilise thermodynamically the phase separated materials.7 In this chapter, we will focus on the synthetic procedures of conjugated rodcoil block copolymers. Theses copolymers have semiconducting properties and have been subject of various experimental and theoretical studies regarding their phase separation properties. They can be obtained easily by controlled radical polymerization (CRP) or anionic polymerization. Anionic polymerization of the coil block allows the synthesis of such system in a very controlled way, either by using a conjugated macro-initiator11 or by quenching the living polymer with an end functionalized conjugated block.12,13,14,15,16,17 Rod-coil copolymers can be also obtained from a coil-coil copolymer by sequential anionic polymerization, where one of the two blocks is a precursor of a conjugated sequence.18 However, CRP is a more suitable method for the synthesis of block copolymers with a

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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variety of structures, architectures and tolerant synthetic process conditions. Thus CRP offers an important advantage for the design of optoelectronic properties and the synthesis of functional polymer materials to be used in flexible organic electronic systems especially for solar cells. Indeed, in the later case the introduction of all functionalities on the block copolymer (for example electron-accepting and donating groups) requires functional monomer units in the coil and rod blocks respectively. In the following sections, an overview of recent results regarding the synthesis by CRP of conjugated rod-coil suitable for flexible organic electronic systems, more specifically for photovoltaic devices, is presented. We will be particularly focusing on model semiconducting block copolymer materials and on small band gap copolymers.

Synthesis of well defined conjugated rod-coil block copolymers via controlled radical polymerization Well-defined rod-coil diblock or triblock copolymers can be obtained from conjugated macro-initiators by CRP. In each case the synthetic route requires the mono- (for diblock) or di-functionnalization (triblock) of a previous conjugated polymer. For this purpose, this last one needs to be properly end capped with a high conversion and further transformed into a macro-initiator. Many examples can be found in the literature, such as poly(para phenylene vinylene) (PPV)6,19,20,21, Polythiophene22,23,24 Polyfluorene,25,26, Poly(thienylene vinylene) (PTV)27 or other designed oligomers.28 CRP allows the polymerization of functional monomers for the coil block which is suitable to tune the materials properties, for example it is possible to obtained amphiphilic conjugated triblocks copolymers with neutral25 or ionic26 blocks. The used CRP technique depends on the functionalization of the endcapping conjugated polymer, indeed conjugated macro-initiators can be tuned for Atom Transfer Radical Polymerization21,22,23,25,26,28 (ATRP), Nitroxide Mediated Radical Polymerization6,19,20,23,24,27 (NMRP) or Reversible Addition Fragmentation Transfer polymerization23 (RAFT). Various coil blocks have been grown from these macro-initiators: polystyrene, polyacrylates and polymethacrylates derivatives, including functional monomers, and polyisoprene. ATRP seems to be a more versatile technique because it allows the polymerization of a wide range of functional monomers; however, considering the application in optoelectronic, the presence of residual catalyst could affect the material final properties. However, NMRP allows the synthesis of polymers without by-products and makes possible the incorporation of functional monomers, such as chloromethylstyrene, in the coil block. Polyparaphenylenevinylene based block copolymers as model materials Polyparaphenylenevinylene based block copolymers are very interesting as model materials. Indeed poly((2,5-dialkyloxy)-1,4-phenylenevinylene) (DEHPPV) can be obtained by Seigrist condensation 29 in order to obtained low polydispersity polymer with one aldehyde function. From this route, NMRP and

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

246 ATRP macro-initiators have been obtained easily from poly ((2,5di(2’ethyl)hexyloxy)-1,4-phenylenevinylene) (DEH-PPV), a very soluble PPV derivative. The synthetic routes are schematically presented in Figure 1. OR

OR O

(i)

OR

RO

RO

OR

(ii)

N

RO

OR

(1)

OR

LiAlH4

(1)

THF / 0°C


RO

OR

1) THF, 40°C, 3h +

2) H2O/HCl

RO

O (2)

Br

(2)

OH

OH O N

RO

O N

BrMg

n

RO

OR

(1)

O

n

RO

R= 2-ethylhexyl

n (3)

OR

OR

Br

Br CH2Cl2 / Et3N

RO

RO

n

O O

(4)

Figure 1. Syntheses of DEH-PPV macro-initiators for NMRP(3) and ATRP (4). (i) aniline, 60°C, 50 mmbar, 3 h (ii) tbuOK, DMF, 50°C, 1h then HCl/H2O Aldehyde end-functionalized DEH-PPV 1 is obtained easily by Siegrist condensation with low polydispersity index (PDI

Controlled radical polymerizations as versatile synthetic routes for

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