7.1. Poly(aryleneethynylene)s
Poly(aryleneethynylene)s are a class of conjugated poly-mers where aromatic or heteroaromatic groups are linked by acetylene units. The internal triplet bonds in the polymer main chain provide extended conjugation and rigidified Scheme 82. Synthesis of Regioregular P141 by a Stille Coupling Reaction and P142 with Defects by a Heck Coupling Reaction
Scheme 83. Synthesis of P143 with a Conjugated Side Chain by a Stille Coupling Reaction
Scheme 84. Synthesis of Poly(thienylenevinylene) P144
Scheme 85. Synthesis of Poly(thienylenevinylene) P145
backbones for pronounced interchain interactions. To design low band gap polymers for solar cell applications, donor and acceptor moieties can be regioregularly separated by con-jugated ethyne bridges. Scheme 86 shows the synthesis of two poly(heteroaryleneethynylene) derivatives containing thieno[3,4-b]pyrazine as the acceptor and thiophene (P146) or dialkoxyphenylene (P147) as the donor, respectively, via the palladium-catalyzed Sonogashira coupling reaction.327 Both materials have wide absorption ranges from 390 to 800 nm and low optical band gaps of ca. 1.57 eV due to donor-acceptor push-pull effects through the conjugated triple bonds. A device based on P146:PCBM (1:1, w/w) showed a Jscof 10.72 mA/cm2, a Vocof 0.67 V, and a PCE value of 2.37%. When using P147:PCBM (1:2, w/w) as the active layer, the device exhibited a Jsc of 4.45 mA/cm2, a Vocof 0.7 V, and a PCE value of 1.36%.
Scheme 87 shows the synthesis of a new poly(aryle-neethynylene) copolymer (P148) grafted with tetrathi-afulvalene (TTF) units via a Sonogashira coupling reaction between 278 and 279.328 This polymer has a conjugated main chain serving as the electron-deficient acceptor with an electron-rich TTF side chain as the electron donor, inducing intramolecular charge transfer between the TTF side chains and the benzothiadiazole-containing main chain. X-ray diffraction analysis suggests that strong self-assemby byπ-π stacking through interdigitation due to the coplanarity of the backbone leads to an optical band gap of 1.78 eV from its absorption edge. A polymer BHJ solar cell with the photo-active layer of P148:C60 (2:1, w/w) showed a Jsc of 2.47 mA/cm2, a Vocof 0.42 V, an FF of 24.2%, and a PCE value of 0.25%.
7.2. Triarylamine-and Phenothiazine-Based Polymers
Triarylamine-based small molecules or polymers have been extensively utilized as hole-transporting materials for light-emitting diode applications due to their high hole mobility. It is envisaged that the introduction of electron-donating triarylamine segments into the polymer main chain in conjunction with an electron-deficient unit would form an alternating donor-acceptor arrangement. This should potentially afford a polymer with a lower band gap and improved intrinsic hole mobility. With this in mind, P149, which contains a triarylamine moiety in the main chain, and its PPV derivative P150 were synthesized by a Heck coupling reaction (Scheme 88).329 The optical band gap of P150 is 1.76 eV, which is lower than that of P149 with 1.86 eV.
This can be rationalized by the fact that P150 is a fully conjugated polymer whereas the conjugated length of P149 is disrupted by the amino groups. In spite of a shorter conjugation length, P149 still has a relatively low band gap due to efficient intramolecular charge transfer. The hole mobility, measured by a hole-only device based on the space charge limited current (SCLC), is determined to be 3.07× 10-5 and 4.58 × 10-5 cm2/(V s) for P150 and P149, respectively. This indicates that incorporation of the triary-lamine groups indeed improves the hole mobility. The high mobility for P149 is translated to a higher Jscof 2.85 mA/
cm2and a higher PCE of 0.52% compared to those of P150 with a Jscof 2.16 mA/cm2and a PCE of 0.26%. It is also noteworthy that the nonplanar 3-dimensional structure of the triarylamine moiety makes the polymer more amorphous, which may exert a detrimental influence on the interchain charge carrier transport.
Scheme 89 depicts the Suzuki coupling synthesis of P151 containing tricyclic phenothiazine units from 40 and 286.330The phenothiazine moiety with its electron-rich sulfur and a nitrogen heteroatom incorporated into the polyfluorene skeleton can potentially serve as the donor segment in conjugated polymers and improve their hole-transporting abilities. In addition, attachment of two (phenylcyano)vinyl groups to the phenothiazine exerts an electron-withdrawing influence which reduces the band gap. The HOMO and LUMO of the resulting polymer P151 are -5.26 and -3.12 eV, respectively. The band gap is 2.14 eV. This polymer’s red-emitting luminescence in a thin film can be completely quenched in the presence of PCBM, implying efficient exciton dissociation. The best photovoltaic performance was obtained from a device which used P151:PCBM (1:3 in wt%) as the active layer; it showed a Jscof 2.38 mA/cm2, a Vocof 0.78 V, and a PCE value of 0.53%.
Two other phenothiazine-based copolymers, comprising bithiophene (P152) and thieno[3,2-b]thiophene (P153) moi-eties, were also synthesized by Suzuki coupling reactions (Scheme 90).331The optical band gaps were calculated as 2.47 eV for P152 and 2.54 eV for P153. A device based on P152/PCBM (1:1, w/w) demonstrated the best PCE of 0.24%. This is in contrast to the P153/PCBM device, which only showed a PCE of 0.1% albeit with a high Vocof 0.82 V. Because the absorption of these polymers only covers the 300-500 nm range, the relatively low device efficiencies are mainly due to the lack of sufficient absorption in the red and near-infrared regions.
7.3. Fluorenone-Based Polymers
Fluorenone-containing conjugated polymers have also attracted much interest for solar cell applications. It has been shown that the incorporation of rigid and planar fluorenone subunits into the conjugated polymer not only enhances the intermolecular self-organized packing, but also effectively prevents chain folding. The latter is one of the most likely limiting factors for efficient charge transport.332Furthermore, due to the electron-withdrawing effects of the ketone group, the fluorenone moiety exhibits a wider absorption band covering a large part of the visible spectrum. This effect can be more pronounced when fluo-renone units are conjugated with electron-rich units in the polymer. Two regioregular copolymers (P154333and P155334) were synthesized by oxidative polymerization from the monomers 290 and 291, respectively (Scheme 91). P154 contains alternating fluorenone and tetrathienylenedivinylene Scheme 86. Synthesis of P146 and P147 by a Sonogashira
Coupling Reaction
units, while P155 consists of alternating fluorenone and tetrathiophene units. Compared to the solution state, signifi-cant bathochromic shifts and broadened spectra were
ob-served for both P154 and P155 in the solid state. This is attributed to ordered interchain aggregation. The optical band gaps for P154 and P155 are 1.52 and 2.0 eV, respectively.
Scheme 87. Synthesis of Poly(aryleneethynylene) Copolymer P148
Scheme 88. Synthesis of Triphenylamine-Containing Copolymer P149 and Its PPV Analogue P150
Scheme 89. Synthetic Route toward Phenothiazine-Containing Copolymer P151
Scheme 90. Synthetic Route toward Phenothiazine-Containing P152 and P153
The best photovoltaic performance showed a PCE of 1.1%
and 1.45% for the P154/PCBM- and P155/PCBM-based devices, respectively.
7.4. Porphyrin-Based Polymers
Porphyrin derivatives have attracted considerable attention as useful materials for organic photonic and electronic applications due to their largeπ-conjugation systems as well as their good photochemical and thermal stabilities. The involvement of porphyrin derivatives in many biological processes such as light harvesting and photoinduced electron transfer in the photosynthesis of plants strongly suggests that porphyrin molecules could serve as potential photosensitizers in dye-sensitized solar cells335,336as well as electron donors in BHJ solar cell application. Although organic solar cells utilizing porphyrin-containing polymers as active layers have been reported, only low power conversion efficiencies were obtained. This is presumably due to the limited light absorption of porphyrins insofar that the absorption spectra of porphyrin units exhibit narrow, strong Soret bands (410-430 nm) and weak Q-bands (530-540 nm) with nothing in between.337A soluble porphyrin-dithienothiophene copolymer (P156) was synthesized by a Sonogashira cou-pling reaction of 292 and 293 (Scheme 92).338Introduction of diethynyldithienothiophene into the porphyrin main chain is expected to reduce steric hindrance, extend conjugation, enhance absorption, and improve the charge-transport prop-erty. However, to ensure sufficient solubility of the porphy-rin-containing polymer for solution processing, a large aliphatic chain insulating portion must be incorporated into the porphyrin unit, and this may have a negative effect on the charge transport. Although a rather small PCE of 0.3%
was obtained for the PSC using P156/PCBM (1:3, w/w) as the active layer, this value is among the highest obtained so far for devices based on porphyrin-containing polymers.
7.5. Platinum Metallopolyyne-Based Polymers
In addition to general strategies using organic donor and acceptor segments in the main chain, platinum alkyne organometallic units have attracted a great deal of interest.
They can be incorporated into the conjugated polymers as a result of the fact that the d-orbital of the Pt can overlap with the p-orbital of the alkyne unit, leading to an enhancement in π-electron conjugation and delocalization along the polymer chain.339 Moreover, due to strong spin-orbital coupling, efficient intersystem crossing in such organome-tallic species facilitates the formation of triplet excited states which have longer lifetimes and thus allow extended exciton diffusion lengths. Electron transfer from this triplet-spin donor to the acceptor produces a geminate ion-radical pair which also retains the triplet character, so that charge recombination is spin-forbidden and the probability of efficient charge separation is enhanced compared with those of the system involving only singlet exciton electron transfer.
On the basis of these considerations, Schanze and Reynolds reported a polymer (P157) containing alternate platinum acetylide and thiophene units.340The trimethylsilyl protecting groups in 294 were in situ cleaved by tetrabutylammonium fluoride, and the intermediate was coupled with Pt(PBu3)2Cl2
(295) in the presence of CuI to yield P157 (Scheme 93).341 The phosphorescence of this polymer has a maximum at 609 nm and was quenched upon the addition of PCBM. This is indicative of efficient electron transfer via the triplet excited state of the organometallic polymer. A photovoltaic device made from the 1:4 (w/w) blend of P157 and PCBM gave a PCE of 0.27% with a Voc of 0.6-0.7 V. Due to the high Scheme 91. Synthetic Route toward Fluorenone-Containing P154 and P155
Scheme 92. Synthesis of Porphyrin-Containing P156 by a Sonogashira Coupling Reaction
Scheme 93. Synthesis of P157
band gap of P157, the low efficiency is mainly due to its low coverage of the solar spectrum.
A series of platinum diacetylene-based polymers (P158-P161) containing electron-accepting bithiazole rings with different numbers of electron-donating thiophene units have been synthesized (Scheme 94) from their corresponding monomers 296-299.342As the number of thiophene units increases, the band gap of the polymer decreases from 2.4 to 2.0 eV, due to the extended π-electron delocalization through D-A intramolecular charge transfer. It was also found from emission lifetime measurements that when the content of the heteroaryl units increases to dilute the organometallic influence, these low band gap polymers only exhibit singlet emission instead of triplet-excited-state phos-phorescence. As a result, unlike Pt-monothiophene P157 blends where charge separation occurs via the triplet state of the polymer, it is a charge-transfer excited state which is responsible for the photoinduced charge separation. The photovoltaic performance based on a polymer/PCBM (1:4, w/w) showed an increasing trend for PCE values from 0.21%