Poly(3-alkylthiophene) and Its Derivatives

Polythiophene represents the most important conjugated polymer utilized in a broad spectrum of applications such as conducting polymers, light-emitting diodes, field-effect transistors, and plastic solar cells due to its excellent optical and electrical properties as well as exceptional thermal and chemical stability. Early synthesis of poly(3-alkylthiophene) involved chemical oxidation or electrochemical polymeri-zation in the pursuit of soluble and processable polythio-phenes.^60,182,183However, these processes suffered from the gross defect that the structure of the polymer is somewhat undirected and undefined.¹⁸⁴Because 3-alkylthiophene is an asymmetrical molecule, there are three relative orientations available when the two thiophene rings are coupled between the 2- and 5-positions. The first of these is the 2-5′or head-to-tail coupling (HT), the second is 2-2′ or head-to-head coupling (HH), and the third is 5-5′or tail-to-tail coupling (TT). Containing a mixture of the possible couplings mentioned above results in a regioirregular poly(3-alkyl-Scheme 25. Synthesis of Indolocarbazole-Based P70 by Suzuki Coupling

Scheme 26. Synthetic Route toward Monomer 88

thiophene) in which a large number of thiophene rings twisted out of conjugation in a manner of HH coupling due to steric repulsion between alkyl chains. Overall this reduces the electrical conductivity of the polymer. On the other hand, coupling each thiophene unit in a consecutive head-to-tail manner during the polymerization affords a regioregular poly(3-alkylthiophene) which is capable of adopting a coplanar conformation, resulting in a lower energy. This arrangement produces a highly conjugated polymer having a lower band gap.

The first synthesis of regioregular head-to-tail coupled poly(3-alkylthiophene)s (P3ATs) was reported by Mc-Cullough early in 1992.^185,186As shown in Scheme 28, the regioregular polymerization of 3-alkylthiophene P75 can be controlled by selective lithiation of 2-bromo-3-alkylthiophene 100 with lithium diisopropylamide (LDA) followed by transmetalation using magnesium bromide to yield the organomagnesium intermediate 101. The use of a Ni(dppp)Cl2

catalyst for the polymerization of this intermediate gives the corresponding poly(3-alkylthiophene) P75 with over 90% head-to-tail regioselectivity.

The second synthetic approach to HT-PAT was subse-quently developed by Rieke.^187,188 Highly reactive zinc undergoes a selective oxidative addition to 2,5-dibromo-3-alkylthiophene 102 to yield 2-(bromozincio)-3-alkyl-5-bro-mothiophene intermediate 103 in quantitative yields. In the presence of Ni(dppe)Cl2 as the catalyst this metalated intermediate undergoes regioselective polymerization to yield the desired HT-PAT P76 product (Scheme 29).

Later on, McCullough reported another method for the synthesis of regioregular poly(3-alkylthiophene)s by Grignard metathesis (GRIM).^189,190Treatment of 2,5-dibromo-3-hexy-lthiophene (104) with a variety of alkyl Grignard reagents resulted in two metalated regioisomers (105 and 106) in an 85:15 ratio via a magnesium exchange reaction (Scheme 30).

This ratio appears to be independent of the reaction time, temperature, and Grignard reagent employed. Introduction of a catalytic amount of Ni(dppp)Cl2to this isomeric mixture afforded poly(3-hexylthiophene) (P77), which contained greater than 95% regioregularity.

McCullough and Yokozawa independently demonstrated that the Grignard metathesis polymerization of regioregular 3-alkylthiophene proceeds by a living chain growth mech-anism instead of the traditionally accepted step growth polycondensation. As a result, low polydispersities (ca.

1.2-1.3) and well-defined molecular weights can be con-trolled by the feed ratio of monomer to the Ni catalyst.^191-194 Their proposed mechanism is shown in Scheme 31. First, two Grignard nucleophilic additions to a nickel catalyst generate the intermediate 109. A reductive elimination involving carbon-carbon bond formation in 109 accompa-nied by Ni migration and insertion into the terminal C-Br bond keeps the living chain capable of further reacting with compound 107. Propagation via consecutive coupling be-tween the polymer with a Ni complex at the chain end and compound 107 elongates the conjugated backbone. Because this reaction can be accomplished both at room temperature and on a large scale, the Grignard metathesis/Kumada-Corriu Scheme 27. Synthesis of P71-P74 by Suzuki Coupling from Corresponding Monomers 96-99

Scheme 28. Synthesis of Regioregular Poly(3-alkylthiophene) P75 by the McCullough Route

Scheme 29. Synthesis of Regioregular Poly(3-alkylthiophene) P76 by the Rieke Route

coupling has become the most widely used method for producing poly(3-alkylthiophene)s with predetermined high molecular weights. The beneficial features of GRIM are obtaining monodispersed samples of P3ATs without resorting to time-consuming polymer fractionation techniques and refining the properties of P3ATs by eliminating molecular weight distribution variations. Moreover, it is possible to keep the polydispersity constant when trying to determine if one polymer performs better than another.

Increasing regioregularity in poly(3-hexylthiophene) (P3HT) through these advanced metal-catalyzed reactions leads to various beneficial outcomes including a red shift in absorp-tion in the solid state with an intensified extincabsorp-tion coefficient and an increase in the mobility of the charge carriers.¹⁹⁵ Regioregular poly(3-hexylthiophene) possesses HOMO and LUMO levels at -5.2 and -3.2 eV, respectively, with an optical band gap of ca. 2.0 eV. To date, a combination of poly(3-hexylthiophene) as the electron donor and PCBM as the electron acceptor in the active layer represents the most efficient bulk heterojunction solar cell with power conversion efficiency approaching 5%. The success of the P3HT/PCBM system is largely associated with careful control and opti-mization of the active layer morphology. It has been demonstrated that device performance based on P3HT/PCBM can be dramatically improved by judicious choice of the casting solvent,¹⁹⁶by external treatment of solvent annealing^197-199 and thermal annealing,^200-202or by forming P3HT nanofibers prior to deposition.²⁰³Thermal annealing of the P3HT/PCBM composite provides an external energy to drive reorganization of the polymer chains, which ultimately leads to a nanoscale phase separation with a bicontinuous interpenetrating D-A network.²⁰⁴As a result of this higher degree of nanoscale P3HT crystallinity, better charge transport and maximum interfacial area for efficient charge generation are obtained.

However, prolonged thermal annealing causes regioregular P3HT to further crystallize and thereby tends to destroy the optimal morphology as a result of continuous phase segrega-tion between P3HT and PCBM to form a macroscale domain larger than the exciton diffusion length. Fre´chet and co-workers found that a slight decrease of the regioregularity of P3HT to weaken the crytallization formation not only

retains the device efficiency but also enhances the thermal stability of morphology in the solar cells.²⁰⁵Polythiophene P78 with a regioregularity lower than 91% was synthesized by the copolymerization of 2-bromo-3-hexylthiophene (111) with a small amount of the 2-bromo-3,4-dihexylthiophene (110) unit using a modified McCullough route (Scheme 32).

After 30 min of annealing at 150°C, the resulting devices, using 96% regioregular P3HT and PCBM or P78/PCBM, showed PCEs of 4.3% and 4.4%, respectively. However, the PCE of P3HT/PCBM further fell to 2.6% when the annealing time was increased to 300 min, whereas the PCE of P78/

PCBM remained at 3.5% under identical conditions.

A later more straightforward study, on the influence of the regioregularity of P3HT on polymer-fullerene solar cell performance, was carried out by systematically comparing three samples of P3HT having regioregularities of 86%, 90%, and 96%.²⁰⁶It was found that in blends with PCBM all three polymers were able to achieve solar cells with PCEs of about 4% and exhibit the same order of magnitude of charge mobility (10^-4cm²/(V s)). Most importantly, the P3HT with the lower regioregularity possessed superior thermal stability over those with higher regioregularities. This was ascribed to the suppression of the crystallization-driven phase separa-tion through introducsepara-tion of a controlled amount of disorder into the polymer backbone. This result is consistent with previous observations. Another example illustrating the effect of alkyl substitution patterns in thiophene copolymers was presented by the comparison of polymers P79 and P80, which have identical compositions and similar molecular weights and polydispersities.²⁰⁷Polymer P79, synthesized by the Stille copolymerization of distannyl monomer 114 Scheme 30. Synthesis of Regioregular Poly(3-hexylthiophene) P77 by Grignard Metathesis and Kumada-Corriu Coupling

Scheme 31. Mechanism of Chain Growth Polymerization Proposed by Yokozawa et al

Scheme 32. Synthesis of P78 for Lowering the

Regioregularity by the Incorporation of a Dihexylthiophene Repeat Unit

and dibromo monomer 115, has a perfect regioregular alternating arrangement, whereas P80 made by the Mc-Cullough route has a random sequence distribution (Scheme 33). X-ray diffraction measurements of the polymers were performed after spin-coating and thermal annealing at 100

°C for 30 min. Due to its longer alkyl chain interdigitation and structurally well-defined nature, P79 crystallizes into a three-dimensional highly ordered structure²⁰⁸and exhibits an even higher degree of crystallinity than P3HT, which is known to have disordered noninterdigitated side chains.²⁰⁹ On the contrary, P80 exhibits amorphous character as a result of its random structure. The best device PCE of P80 blended with PCBM (30:70, w/w) was 1.84%. However, the optimal PCE of P79 blended with PCBM (25:75, w/w) only reached 0.54%, which is much lower than that of P3HT. These results indicate that increasing the crystallinity of the polymer to be higher than that of P3HT turns out to have a detrimental effect, because a higher crystallinity makes P79 unable to form bicontinuous donor-acceptor networks at all and the longer alkyl chains further lower the miscibility with PCBM.

Therefore, the random, amorphous polymer with the more favorable active layer morphology outperforms its more highly ordered analogues by more than 3-fold. These results clearly demonstrate that highly crystalline conjugated poly-mers are advantageous for achieving high mobility in field-effect transistors because polymers constitute the only component. However, the addition of another component such as fullerene into a BHJ solar cell renders the morphol-ogy, a bulk property, more important than the polymer crystallinity.

The length of the solubilizing group in the conjugated thiophene unit also plays an important role in determin-ing the balance between crystallinity and miscibility in the bid to achieve optimal morphology. A systematic study of a series of regioregular poly(3-alkylthiophene)s, with different butyl, hexyl, octyl, decyl, and dodecyl side groups, has been conducted.²¹⁰It was found that chain lengths longer than eight carbons facilitate diffusion rates of PCBM in the polymer matrix during the thermal annealing. This leads to a larger scale of phase separation, reduced interfacial area, and thereby lower device performance. Poly(3-butylth-iophene) lacks suitable solubility in chloroform. Ultimately, the hexyl group in P3HT demonstrated an optimal balance with the best device performance in the poly(3-alkylth-iophene) family. Moreover, the efficiency of the BHJ device also greatly depends on the molecular weight^211-213 and polydispersity²¹⁴of the P3HT. It has been shown that only P3HT with a number-average molecular weight greater than 10000 can achieve a power conversion efficiency over 2.5%.²¹¹High molecular weight P3HT exhibits a

bathochro-mic shift of the long-wavelength maxima and a prominent shoulder at 600 nm in the solid-state absorption spectrum.

This indicates the formation of a lamellar structure and interchain aggregates, whereas the lower molecular weight with a shorter conjugation length results in reduced inter-molecular interactions with a concomitant decrease in hole mobility.

The major function of the aliphatic chain substituent is to provide adequate solubility of polythiophenes with weak electronic interactions with the main chain. Nevertheless, alkoxy chains introduced into the 3-position of poly-thiophenes not only serve as solubilizing groups but also greatly perturb the molecular orbitals involved in tuning the band gap.²¹⁵Regioregular poly(3-(decyloxy)thiophene) (P81) was synthesized by a GRIM synthesis of 2,5-dibromo-3-(decyloxy)thiophene (120) (Scheme 34).¹⁵¹Despite the fact that the resultant polymer showed a lower band gap of 1.6 eV, due to electron-donating effects elevating the HOMO level, the resulting poor solubility and film-forming properties and air instability restricted its performance in solar cell devices. Therefore, the high head-to-tail regioregular co-polymer P82 was synthesized by reacting 120 with 2,5-dibromo-3-octylthiophene (121) via the McCullough method.

Compared to that of P3HT, the absorption maximum of P82 in the solid state was dramatically red-shifted to 621 nm.

The electrochemical band gap of P82 was determined to be 1.64 eV. However, P82/PCBM (1:1, w/w) also showed very poor photovoltaic performance with a PCE of 0.054%.

6.2. Poly(3-hexylselenophene)

Poly(3-hexylselenophene) (P3HS, P83) is an analogue of poly(3-hexylthiophene) with selenium replacing the sulfur atom. It can be synthesized successfully in a regioregular manner by employing a McCullough-type Ni coupling reaction of 123 made by bromination of 122 (Scheme 35).²¹⁶ It was found by Heeney and co-workers that poly(3-hexylselenophene) has a smaller band gap of 1.6 eV than P3HT through lowering the LUMO level without elevating the HOMO level. These are promising properties for obtain-ing profound absorption at lower energy while maintainobtain-ing Scheme 33. Synthesis of P79 and P80

Scheme 34. Synthesis of P81 and P82

an open-circuit voltage comparable to that of P3HT. As a result, P3HS shows a much greater red-shifted absorption maximum at 630 nm in the solid state compared to 550 nm for P3HT. It was also found that P3HS is more resistant to photooxidation than P3HT. It has been suggested that one of the photooxidation mechanisms is through electron transfer from excited P3HS to oxygen, generating a superoxide anion that reacts with the conjugated backbone. The low-lying LUMO of P3HS can reduce the electron-transfer rate and therefore accounts for the better photostability of P3HS. The lower LUMO can be attributed to the smaller ionization potential of selenium. Moreover, P3HS displays crystalline morphology and thus has an FET charge mobility similar to that of P3HT under the same measurement conditions. The solar cell device based on P3HS/PCBM (1:1, w/w) produced a PCE of 2.7% after optimal thermal annealing, which is comparable to that of a P3HT/PCBM device.²¹⁷