Isothianaphthene-Based Polymers

Polyisothianaphthene (PITN, P88) is a polythiophene derivative where all the thiophene rings are fused to a benzene ring at the 3,4-positions of thiophene. The first PITN was reported by Wudl et al. in 1984 and prepared by electrochemical polymerization (Scheme 41).⁴² The PITN main chain tends to favor the quinoid form to preserve the benzene aromaticity at the expense of the thiophene aroma-ticity. As a consequence, PITN is the first conjugated polymer to have a band gap as low as 1 eV, which is 1 eV smaller than that of polythiophene. The monomer precursor isothia-naphthene (ITN, 138) was prepared by dehydration of the sulfoxide 137, which was made by the NaIO4oxidation of 136 (Scheme 41).²²⁶

PITN syntheses have been described using various elec-trochemical or oxidative polymerizations of isothianaphthene.

Straightforward chemical polymerization routes have also been developed to make substituted PITNs with sufficient solubilities and tunable band gaps. As shown in Scheme 42, the thermal polymerization of dithiophthalide 140^227,228in the presence of catalytic amounts of toluenesulfonic acid afforded poly(5,6-bis(octylmercapto)isothianaphthene) (P89).

Its band gap of 1.16 eV is too low to obtain sufficient photovoltaic performance.²²⁹

Incorporation of different aromatic units into PITN allows greater structural versatility for the fine-tuning of the intrinsic properties. However, the use of these low band gap PITN derivatives for solar cell device applications encountered difficulties such as inefficient polymerization, low molecular weights, poor film-forming ability, and the unstable nature of both the isothianaphthene monomer and the polymer.

Thiophene-isothianapthene copolymers P90, P91, and P92, synthesized by FeCl3oxidative polymerization (Scheme 43) with band gaps of 1.8, 1.5, and 1.4 eV, respectively, cannot form homogeneous pinhole-free thin films by spin-coating due to their low molecular weights, which is a result of an inefficient oxidative coupling reaction.^230,231

In addition to PCBM and a donor polymer, poly(methyl methacrylate) (PMMA) was required to serve as the host

matrix in the active layer to improve the film-forming property. The PSC based on PMMA/P90/PCBM (1:2:6, w/w/

w) showed the highest device characteristic with a PCE of 0.31%.²³¹

Hillmyer and co-workers reported a new distannyl-isothianaphthene compound (144) which was synthesized by the n-BuLi lithiation of isothianaphthene (138) in tetram-ethylethylenediamine (TMEDA) and THF followed by reaction with trimethyltin chloride.²³² This molecule is chemically stable under an inert atmosphere at low temper-atures and can be used as a useful building block to prepare the alternating ITN-fluorene copolymer P93 or the ITN–

thiophene copolymer P94 by a Stille coupling with 9,9-dihexyl-2,7-dibromofluorene (60) or bis(bromothienyl)-isothianaphthene 145, respectively (Scheme 44). It is particularly noteworthy that 145 can also be synthesized from 144 by a Stille coupling reaction with 2-bromo-3-decylth-iophene (146) followed by NBS bromination. This is a more facile way for making dithienylisothianaphthene derivatives without using the traditional route, which requires harsh conditions involved with using Lawesson’s reagent.^233,234 Although when blended with PCBM (1:4, w/w) P93 has a larger band gap of 2.3 eV, it showed a higher PCE of 0.46%

than P94 (band gap 1.66 eV) with a PCE of 0.23% under the same testing conditions. This could be ascribed to its better film-forming properties.

6.5. Cyclopenta[2,1-b:3,4-b′]dithiophene-Based Polymers

Structurally analogous to fluorene, 4H-cyclopenta[2,1-b:

3,4-b′]dithiophene (CPDT) derivatives, where two thiophene units are tied and rigidified by a covalent carbon, have attracted considerable research interest.²³⁵Due to its fully Scheme 41. Synthesis of PITN (P88) by Electrochemical Polymerization and Its Mesomeric Structure

Scheme 42. Synthesis of P89 by Thermal Polymerization of Dithiophthalide

Scheme 43. Synthesis of P90-P92 by FeCl3Oxidative Polymerization

coplanar structure, many intrinsic properties based on bithiophene can be altered, leading to extended conjugation, lower HOMO-LUMO energy band gaps, and stronger intermolecular interactions. Furthermore, the option of func-tionalization at the bridging carbon allows greater structural variations for fine-tuning both the electronic and steric properties.

As reported by Ferraris and co-workers,^236,237 poly(cyclo-penta[2,1-b:3,4-b′]dithiophen-4-one) (P95) or poly(4-(dicya-nomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene) (P96) were electropolymerized from their parent monomers 148 and 149, respectively (Scheme 45). Owing to the electron-withdrawing effect of the ketone or dicyano groups for reducing the aromaticity of bithiophene and stabilizing the quinoid form, the band gaps of P95 and P96 can be markedly reduced to as low as 1.2 and 0.8 eV, respectively. To make solution-processable polymers, 2,6-dibromo-CPDT 150 with two 2-ethylhexyl side chains was homopolymerized by a Ni-catalyzed coupling reaction to afford P97.²³⁸The resulting polymer showed a much lower band gap of ca. 1.7-1.8 eV compared to its analogous poly(3-alkylthiophene) or poly-fluorene. It should be noted that the absorption spectra of

this polymer both in solution and in the solid state are essentially identical. This suggests that self-assembly of the interchain interactions in the solid state is not established.

To further decrease the band gap and improve the absorption coverage, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cy- clopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothia-diazole)] (PCPDTBT) (P98) containing alternating CPDT units as the donor and benzothiadiazole units as the acceptor was developed and synthesized by Zhu and co-workers via a Stille coupling (Scheme 46).²³⁹With two ethylhexyl groups, PCPDTBT is very soluble in organic solvents and has good miscibility with PCBM, thus facilitating its processability.

The HOMO and LUMO values, from electrochemical measurements, were calculated to be -5.3 and -3.57 eV, respectively. The optical band gap, from solid-state absorp-tion, is approximately 1.4 eV, which is regarded as ideal for polymer-fullerene BHJ solar cells. Unlike the homopolymer of CPDT, PCPDTBT showed a much greater red shift in the absorption spectrum of the thin film than in solution.

This indicates more interchain interactions when benzothia-diazole units are incorporated. When blended with PC71BM, the BHJ device achieves a PCE of up to 3.5% with a Jscof 11.8 mA/cm²and a Vocof 0.65 V. This is together with an elevated EQE higher than 25% over the spectral range from 400 to 800 nm: the maximum of 38% is around 700 nm.

Moreover, the photocurrent production is extended to wavelengths even longer than 900 nm.²⁴⁰The high perfor-mance of PCPDTBT can be attributed to its broad, strong absorption spectrum and high mobility of charge carriers.

The planar structure facilitates carrier transport between the polymer chains, allowing its hole mobility to reach up to 1

× 10^-3cm²/(V s). By incorporating a small amount of 1,8-octanedithiols into the PCPDTBT/PC71BM solution prior to spin coating, the solar cell efficiency was further improved to 5.5% through the formation of an optimal bulk morphol-ogy which enhances both the photoconductivity and charge carrier lifetime.²⁴¹Without the need for thermal annealing during fabrication, this approach offers easier control over tailoring the morphology and is especially applicable to the more amorphous p-type polymers with low degrees of crystallinity where thermal annealing is less effective.^242,243 Scheme 44. Synthesis of P93 and P94 by a Stille Coupling Reaction

Scheme 45. Synthesis of Homopolymers P95-P97

Heeger and co-workers also reported an efficient tandem cell with two active layers. This was aimed at maximizing the sunlight harvesting by smartly integrating a complementary pair comprised of P3HT with the wider band gap and PCPDTBT with the smaller band gap. The result led to the realization of a record-high PCE of over 6%.²⁴⁴ Because PCPDTBT is the first breakthrough on the way to developing efficient low band gap D-A polymers, extensive analysis of the physical behavior of PCPDTBT/PCBM blends was carried out, namely, in terms of photoconductivity,²⁴⁵electron transfer,²⁴⁶and charge transport.²⁴⁷

About the same time, Mu¨llen and co-workers reported a polymer (P99) analogous to PCPDTBT (P98) but with 4,4-dihexyldecyl substituents instead of the ethylhexyl groups (Scheme 47). P99 shows a markedly high hole mobility of 0.17 cm²/(V s) in FET. This can be attributed to the enhanced packing order as a result of the introduction of the straight side chains.²⁴⁸

By decreasing the content of the acceptor 4,7-(2,1,3-benzothiadiazole) molecule 152 and introducing 5,5′ -[2,2′]bithiophene 154 as the donor, a series of random copolymers P100 were also synthesized (Scheme 48).²³⁹ Through adjusting the D:A ratio, the absorption character-istics can be tuned to cover the whole visible spectrum. Initial cell testing on P100/PCBM has already resulted in PCE values of up to 3% being attained.

Subsequently, CPDT was used as an electron-donating building block for the construction of a series of alternating D-A conjugated polymers. In addition to benzothiadiazole, other electron-accepting moieties such as quinoxaline and dithiophene-ylbenzothiadiazole were also copolymerized with CPDT to form P101 and P102, respectively (Scheme 49).²⁴⁹ By using a 19:1 (v/v) mixture of chlorobenzene and anisole as the solvent to cast the P102/PCBM film, the optimal morphology was attained and the corresponding PSC device reached a PCE of 2.1%.

4,7-Dibromo-2,1,3-benzoselenadiazole (BSe), which is similar to the benzothiadiazole unit, was also coupled with CPDT by a Stille coupling to make the polymer P103.²⁵⁰A

PCE of 0.89% was recorded with a Jscof 5 mA/cm²and a Voc of 0.52 V. The poorer performance of P103 compared to PCPDTBT is attributed to its weaker absorbance and an imbalance in the hole and electron transport of the active layer.

A number of synthetic routes directed toward the prepara-tion of useful CPDT monomers require laborious multistep synthesis and purification.^251-254Considerable interest in the preparation of the ketone 160 as a precursor for low band gap polymers led to a more efficient route developed by Brzezinski and Reynolds using only three steps from starting material 157 (Scheme 50).²⁵⁵ The ketone 160 is further reduced to 161 by a hydrazine Wolff-Kishner reduction.

The methylene proton in 161 is acidic enough to undergo a double alkylation in the presence of potassium hydroxide in dimethyl sulfoxide to yield 163. Lithiation of 163 by tert-butyllithium and sequential quenching using trimethyltin chloride afforded the distannyl monomer 151 ready for polymerization.

6.6. Silafluorene- and Dithieno[3,2-b:2′,3′-d]silole-Based Polymers

Compared with many heterocyclic arenes such as thiophene, furan, or pyrrole, the silole (silacyclopentadiene) ring has the smallest HOMO-LUMO band gap and the lowest lying LUMO level due to the σ*-π* conjugation between the π-symmetrical σ* orbital of two σ exocyclic bonds on silicon and theπ* orbital of the butadiene moiety.²⁵⁶As a result a variety of conjugated small molecules and polymers consist-ing of silole derivatives exhibit extraordinarily unique properties such as high fluorescence efficiency and electron affinity and excellent electron mobility.^257-260 Conjugated polymers containing alternating electron-rich pyrroles or thiophenes and silole units have shown considerably smaller band gaps, which is indicative of the electron-accepting ability of silole to induce intramolecular charge transfer.^261-263 It is envisioned that the silole unit can serve as a useful building block in the molecular design of new conjugated Scheme 46. Synthesis of P98 by a Stille Coupling Reaction

Scheme 47. Synthesis of P99 by a Stille Coupling Reaction

Scheme 48. Synthesis of P100 by a Stille Coupling Reaction

polymers for use in organic solar cell applications. An alternating copolymer P104, derived from 9,9-dihexylfluo-rene and the 1,1-dimethyl-3,4-diphenyl-2,5-bis(2′ -thienyl)-silole unit, was synthesized by a Suzuki coupling (Scheme 51).²⁶⁴The HOMO and LUMO levels and the optical band gap of P104 were determined to be -5.71, -3.6, and 2.08 eV, respectively. Blended with PCBM in the active layer of a bulk heterojunction device, P104 achieved a promising PCE of 2.01%.

A facile synthesis aimed toward accessing 2,5-difunctional silole derivatives has been described by Tamao and co-workers (Scheme 52).²⁶⁵Upon treatment with lithium naph-thalenide, bis(phenylethynyl)dimethylsilane (167) undergoes intramolecular reductive cyclization to form 2,5-dilithiosilole 168, which was then transformed into 2,5-dizincated silole 169. A Negishi Pd-catalyzed cross-coupling of this inter-mediate with 2-bromothiophene (117) afforded 2,5-bis(2′-thienyl)silole 170, which was further brominated by NBS to furnish 164.

The fluorene-containing D-A conjugated polymer P44 and its derivatives have all been shown to have good photovoltaic properties as well as to deliver high PCEs over the 2-3% range. When the carbon atom on the 9-position of the fluorene unit is replaced with a silicon atom, a new type of building block, namely, silafluorene evolves. By fusing a silole ring between two phenyl rings, the silafluorene is expected to combine the advantages of both the silole and fluorene intrinsic properties. The tetravalence of the silicon atom in the silole ring opens up two additional substitution sides for the introduction of solubilizing groups. Cao and co-workers made the copolymer P105, which was synthe-sized by the Suzuki coupling²⁶⁶of 2,7-silafluorene 171 and dithienylbenzothiadiazole unit 42 (Scheme 53).

The solar cells made from a P105:PCBM (1:2, w/w) blend achieved a Vocof 0.9 V, a Jscof 9.5 mA/cm², an FF of 50.7%, and a PCE of up to 5.4%. The high Voccan be attributed to the low-lying HOMO of P105, which is -5.39 eV. From FET measurements, P105 showed a hole mobility of 1× 10^-3cm²/(V s), which is much higher than that of the PFO-DBT polymer (3 × 10^-4 cm²/(V s)). In addition, P105 exhibits a lower optical band gap and a greater red-shifted absorption maximum compared to PFO-DBT P45a (1.82 vs 1.92 eV and 565 vs 544 nm, respectively). These enhanced properties contribute to improved hole transport and broader absorption, leading to a higher Jsc and an acceptable FF.

Around the same time, a parallel study using the same polymer was reported by Leclerc et al.²⁶⁷

The synthetic route for monomer 171 was developed by Holmes and co-workers (Scheme 54).²⁶⁸An Ullmann cou-pling of compound 172 gave the dimerization product 173.

Scheme 49. Synthesis of P101-P103 by a Stille Coupling Reaction

Scheme 50. Synthetic Route toward Monomer 151

Scheme 51. Synthesis of P104 by a Suzuki Coupling Reaction

Reduction of the nitro groups in 173 yielded the amino compound 174. This was followed by iodination via a diazonium pathway to generate 175. Selective lithiation of the iodo groups of 175 by tert-butyllithium to react with dioctyldichlorosilane led to the formation of the central silole ring of 176. Finally, the bromo groups in 176 can be easily transformed to the bis(boronic ester) 171.

In view of the beneficial molecular design on going from fluorene to silafluorene, the exceptional cyclopentadithiophene system is further modified by substituting the carbon atom with a silicon bridge to generate a dithienosilole having a silole ring embedded in the center of dithiophene. A dithienosilole homopolymer (P106) and an alternating dithienosilole and dithienylbenzothiadiazole copolymer (P107) were reported and synthesized by Stille coupling reactions (Scheme 55).²⁶⁹ Due to the incorporation of electron-accepting units, the copolymer P107 has both a lower HOMO level, -5.13 eV, and a smaller band gap, 1.4 eV, than the homopolymer P106 (-4.85 and 1.9 eV, respectively). The best photovoltaic performance after ther-mal annealing at 140 °C was based on P107/PCBM (1:1, w/w) and showed a Voc of 0.44 V, a Jsc of 1.32 mA/cm², and a PCE of 0.18%.

Recently, Yang and co-workers reported another dithienos-ilole-containing copolymer (P108). Here benzothiadiazole is the acceptor and 2-ethylhexyls are the solublizing groups, which makes it structurally similar to PCPDTBT (Scheme 55).²⁷⁰ The HOMO and LUMO of the polymer were estimated to be -5.05 and -3.27 eV, respectively. Although the optical band gap of 1.45 eV is very close to the polymer

PCPDTBT, the substitution of the silicon atom significantly improved the hole-transport properties. The hole mobility of P108 (3× 10^-3cm²/(V s)), determined by a field-effect transistor, is 3 times higher than that for PCPDTBT. The photovoltaic device with an active layer of P108/PC71BM (1:1, w/w) exhibited a Vocof 0.68 V, a Jscof 12.7 mA/cm², and a very impressive PCE of 5.1%. Notably, the EQE as a function of the wavelength showed a broad response ranging from 350 to 800 nm.

The synthesis of the distannyldithienosilole 179 is shown in Scheme 56. By a selective lithium-bromide exchange followed by nucleophilic substitution, the tri-methylsilyl group was introduced into the 5,5′-positions of the bithiophene compound 181 to act as a protecting group. As a consequence, the subsequent lithiation in compound 181 only occurred at the 3,3′-positions of the bithiophene, which then underwent cyclization through double addition to dialkyldichlorosilane and gave 182.

NBS bromination via electrophilic aromatic substitution to replace the trimethylsilyl moiety resulted in the formation of compound 183, which was then converted to the distannyl compound 179 by lithiation and final quenching with trimethyltin chloride.

6.7. Dithieno[3,2-b:2′,3′-d]pyrrole-Based Polymers

After the tricyclic cyclopentadithiophene and dithienos-ilole rings were shown to serve as excellent donor components with various acceptor units in low band gap polymers, dithieno[3,2-b:2′,3′-d]pyrrole attracted attention Scheme 52. Synthetic Route toward Monomer 165

Scheme 53. Synthesis of P105 by a Suzuki Coupling Reaction

Scheme 54. Synthetic Route toward Monomer 171

as another appealing fused bithiopene member with a bridged nitrogen having strong electron-donating ability.

As reported by Rasmussen and synthesized by the oxida-tion polymerizaoxida-tion of 184, the homopolymer poly-(dithieno[3,2-b:2′,3′-d]pyrrole) P109 showed a reduced band gap of ca. 1.7 eV and a high red-emitting quantum efficiency.²⁷¹ Hashimoto and co-workers reported another D-A conjugated copolymer (P110) constituted of alternating dithienopyrrole and dithienylbenzothiadiazole units and synthesized by a Stille cross-coupling reaction (Scheme

57).²⁷²P110 showed a greater red-shifted absorption maxi-mum at 697 nm in a film than in solution at 671 nm. This indicates strong interchain interactions due to its coplanar structure. Compared with the homopolymer dithienopyrrole, P110 also exhibited a more bathochromic shift in the absorption spectrum and a smaller optical band gap of 1.46 eV. Again this demonstrates efficient charge transfer between the donating dithienopyrrole and accepting benzothiadiazole units. The PSC based on P110 as the donor and blended with PCBM (1:1, w:w) exhibited a PCE of 2.18%.

Scheme 55. Synthesis of P106-P108 by a Stille Coupling Reaction

Scheme 56. Synthetic Route toward Monomer 179

Scheme 57. Synthesis of P109 by FeCl3Oxidative Polymerization and P110 by a Stille Coupling Reaction

The dithieno[3,2-b:2′,3′-d]pyrrole ring of 188 was built up by a one-step cyclization through a double Pd-catalyzed amination of 3,3′-dibromobithiophene (187) with the primary amine 186 having a long aliphatic side chain (Scheme 58).

Another efficient way to synthesize the dithieno[3,2-b:2′,3′-d]pyrrole ring involves the Pd-catalyzed amination of 3-bro-mothiophene (157) with a variety of primary amines to form 189. This is followed by a one-pot NBS bromination and Cu cyclization to yield 191(Scheme 58).²⁷³

6.8. Benzo[1,2-b:4,5-b′]dithiophene-Based Polymers

The benzo[1,2-b:4,5-b′]dithiophene (BDT) unit has emerged as an attractive building block for making conjugated polymers due to its large planar conjugated structure ideal for efficientπ-π packing. Alternating BDT and bithiophene copolymers exhibiting a high field-effect transistor mobility of 0.25 cm²/(V s) and enhanced stability has been reported.²⁷⁴

Yang and co-workers reported a series of BDT-based polymers, P111-P117, with different alternating units. They systematically investigated their structure-property relation-ships for PSC applications.²⁷⁵Using a Stille cross-coupling of the relevant monomers, 4,8-bisalkoxy-BDT was copoly-merized with two types of electron-donating units, including thiophene and (ethylenedioxy)thiophene (EDOT), and four electron-deficient units, including thieno[3,4-b]pyrazine (TPZ), benzoselenadiazole (BSe), 2,3-diphenylquinoxaline (DPQ), and benzothiadiazole (BT) (Scheme 59). The absorption spectrum of the homopolymer P111 showed a maximum wavelength at 495 nm. As the donating strength of the incorporated unit increases from thiophene in P112 to EDOT in P113, the maximum absorption shifts to 511 and 532 nm, respectively. However, it was found that the electron-donating power of EDOT in P113 also resulted in an increased HOMO level and a subsequent decrease in Voc. On the other hand, the ability of the four electron-deficient Scheme 58. Synthetic Route toward Monomers 185 and 191

Scheme 59. Dibromo and Distannyl Monomers for the Synthesis of Polymers P111-P117 by a Stille Coupling Using Pd(PPh3)4

as the Catalyst

units to lower the band gap of the corresponding polymer is in the order TPZ (P117)>BSe (P115)>DPQ (P116)>BT (P114). This also reflects their electron-accepting strength as well as their capability to adopt the quinoid structure in the polymer. As a result, P117 containing TPZ showed a very low band gap of 1.05 eV, which matchs the solar spectrum well. However, because of the fact that the HOMO level lies somewhat too high, the Voc of P117 is only 0.21 V. One interesting observation is that, unlike the BSe analogue, the BT unit in the BDT-based polymer P114 can narrow the band gap by depressing the LUMO level without elevating its HOMO level when compared to the standard homopolymer P111. This is reflected in an improved Vocof 0.68 V. When blended with PCBM (1:1, w/w), P112 showed the highest PCE, 1.6%, with a Voc of 0.75 V, a Jscof 3.78 mA/cm², and an FF of 0.56.

The synthesis of the monomer 1,8-bis(dodecyloxy)-BDT 192 is shown in Scheme 60. Lithiation of N,N-diethylth-iophene-3-carboxamide (199) with butyllithium, followed by

The synthesis of the monomer 1,8-bis(dodecyloxy)-BDT 192 is shown in Scheme 60. Lithiation of N,N-diethylth-iophene-3-carboxamide (199) with butyllithium, followed by