Benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐terthiophene Copolymers Containing Styryl‐Triphenylamine Side Chains: Synthesis and Photovoltaic Performance Optimization with Fullerene Acceptors

A new two‐dimensional‐conjugated polymer (PBDTT3‐TPA) containing benzodithiophene (BDT) and a side chain isolation comonomer is designed and synthesized. Interestingly, PBDTT3‐TPA is compatible with higher lowest unoccupied molecular level (LUMO) acceptors of indene‐C₆₀ bisadduct (ICBA), and polymer solar cells based on PBDTT3‐TPA/ ICBA show an open‐circuit voltage (V_OC) of ca. 0.80 V and a power conversion efficiency of 2.48% under AM1.5G illumination of at 100 mW cm^?2. Furthermore, the energy loss in the corresponding fullerene acceptor devices is discussed, and the increase in the observed V_OC is explained quantitatively by the up‐shifted LUMO energy of ICBA (0.17 eV) and the reduced saturation current (J_SO) in the blends.

     

Synthesis and Characterization of Isoindigo‐Based Polymers with Thermocleavable Side Chains

Stability of bulk heterojunction films for organic solar cells is a critical factor for commercial viability. One method to stabilize these films is to include cleavable side chains, which reduce the solubility of the polymers when removed. In order to study the stabilizing effect of cleavable side chains, a series of random copolymers using isoindigo with 0, 10, 20, 50, and 100% thermally cleavable side chains based on the tert‐butyloxycarbonyl (t‐BOC) group are synthesized. The polymers show a distinct one‐step thermal cleavage of the side chains, with no separable dealkylation and decarboxylation steps. The thermal stability in film is studied with transmission electron microscopy and atomic force microscopy. The polymer with all t‐BOC side chains on isoindigo significantly improves thermal stability with regard to crystal growth and phase separation in film. These results suggest BOC‐substitution can be used for large scale processing to produce insoluble polymer films with a high degree of thermal stability.     

Spectroelectrochemical and Photovoltaic Characterization of a Solution‐Processable n‐and‐p Type Dopable Pyrrole‐Bearing Conjugated Polymer

D‐A‐D‐type polymers are of high interest in the field of photovoltaics and electrochromism. In this study we report the synthesis and electrochemical properties of PPyBT along with its photophysical properties and photovoltaic performance. PPyBT is soluble in common organic solvents and both n‐ and p‐type dopable, which is a desired property for conjugated polymers. During electrochemistry studies, the onset potentials of the polymer were determined as +0.2 V for oxidation and ?1.4 V for reduction. Using these values, HOMO and LUMO energy levels were calculated. The photovoltaic properties of PPyBT were also studied. PL studies showed that there is a charge transfer between PPyBT (donor) and PCBM (acceptor).

     

n‐Type Copolymers with Fluorene and 1,3,4‐Heterodiazole Moieties

Summary: The successful realization of n‐channel field‐effect transistors requires the application of semiconducting polymers with high electron mobility (n‐type). However, reports on n‐type polymers are rather scarce in the literature. Therefore, the development of polymers with suitable electron transport properties is particularly challenging for the synthetic chemistry. Main chain polymers with strong acceptor units, such as 1,3,4‐heterodiazoles, are potential candidates for electron transport materials in electronic devices. The fluorene unit is another ring system with interesting physical and chemical properties, which is often used in rigid‐rod, main chain polymers. The present work introduces the synthesis of organo‐soluble copolymers consisting of alternating fluorene‐, 1,3,4‐heterodiazole, and, in some cases, additional 2,5‐dialkoxyphenylene units in the main chain. The reported synthesis involves modified classical polycondensation as well as the tetrazole route. We demonstrate the possibility of exchanging oxygen in the heteroaromatic ring with sulfur using Lawesson's reagent during the ring closure reaction. An alternative structure of the heterocyclic ring with N‐phenyl in the oxygen position is feasible using the tetrazole synthetic route. The chemical and electrochemical properties of the copolyfluorenes are investigated in detail. Some of the synthesized copolyfluorenes have also been used for the preparation of “electron‐only” devices enabling the calculation of the electron mobilities. Further, an organic field‐effect transistor (OFET) characteristic was shown.

OFET characteristics of the polymer 12b .     

Efficient Polymer Solar Cells Based on New Random Copolymers with Porphyrin‐Incorporated Side Chains

Two new wide bandgap block copolymers (PL1 and PL2) with porphyrin‐incorporated side chains are designed and used as electron donors for solution‐processed bulk heterojunction polymer solar cells. The photophysical, electrochemical, and photovoltaic properties, charge transport mobility and film morphology of these two block copolymers are investigated. Detailed investigations reveal that the different alkyl groups and electron‐withdrawing substituents on the porphyrin pendant units have significant influence on the polymer solubility, absorption energy level, band gap, and charge separation in the bulk‐heterojunction thin films, and thus the overall photovoltaic performances. Organic photovoltaic devices derived from these copolymers and ([6,6]‐phenyl‐C₇₁‐butyric acid methyl ester) (PC₇₁BM) acceptor show the best power conversion efficiencies of 5.83% and 7.14%, respectively. These results show that the inclusion of a certain proportion of side chain porphyrin group as a pendant in the traditional donor‐acceptor (D‐A) type polymer can broaden the molecular absorption range and become a full‐color absorbing molecule. The size of the porphyrin pendant also has an obvious effect on the properties of the molecule.     

Simultaneous Substitution of Silicon and Fluorine Atom on Donor–Acceptor Copolymers for Photovoltaic Applications

To investigate the effect of silole‐containing and fluorination on the photovoltaic performance of polymer solar cells, two conjugated donor–acceptor copolymers have been synthesized via Stille coupling reaction. Both donor–acceptor alternating copolymers consisted of 4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b :4,5‐b′ ]dithiophene as the donor unit and nonfluorinated or fluorinated 2,1,3‐benzooxadiazole (BO) as the acceptor unit, along with a thiophene group as the π‐bridge between the donor and acceptor unit. For comparison, the copolymer composed of the 4,4‐diethylhexyl‐cyclopenta[2,1‐b :3,4‐b′ ]dithiophene (CPDT) donor unit and the BO acceptor unit has also been prepared. UV–vis spectra of the three copolymers display a panchromatic absorption ranging from 300 to 1100 nm. The optical band gap obtained from the Tauc relation for the CPDT‐containing polymer is 1.54 eV. With regard to the silole‐containing copolymers, the optical band gaps for nonfluorinated copolymer and fluorinated copolymer are reduced to 1.46 and 1.42 eV, respectively. In comparison with both nonfluorinated polymers, the photovoltaic performance of fluorinated polymer is significantly improved due to the increased J _sc and enhanced V _oc. Based on the indium tin oxide (ITO) coated glass/poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate)/polymer:phenyl‐C₆₁‐butyric acid methyl ester/LiF/Al device structure, the optimal device efficiency obtained from the fluorinated copolymer at room temperature under the illumination of AM 1.5 is 3.74%, with a high V _oc up to 0.87 V.

     

Novel Cyclopentadithiophene‐Based D–A Copolymers for Organic Photovoltaic Cell Applications

Here, the synthesis, characterization, and photovoltaic properties of four new donor–acceptor copolymers are reported. These copolymers are based on 4,4‐difluoro‐cyclopenta[2,1‐b:3,4‐b′] dithiophene as an acceptor unit and various donor moieties: 4,4‐dialkyl derivatives of 4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene and its silicon analog, dithieno[3,2‐b:2′,3′‐d]‐silol. These copolymers have an almost identical bandgap of 1.7 eV and have a HOMO energy level that varies from ?5.34 to ?5.73 eV. DSC and X‐ray diffraction (XRD) investigations reveal that linear octyl substituents promote the formation of ordered layered structures, while branched 2‐ethylhexyl substituents lead to amorphous materials. Polymer solar cells based on these copolymers as donor and PC₆₁BM as acceptor components yield a power conversion efficiency of 2.4%.

     

Efficient Polymer Solar Cells Employing Solution‐Processed Conjugated Polyelectrolytes with Differently Charged Side Chains

Poly(6‐(4,7‐dimethyl‐2H‐benzo[d][1,2,3]triazol‐2‐yl)‐N,N,N‐trimethylhexan‐1 aminium iodide) (PBTz‐TMAI) and poly(sodium 4‐(4,7‐dimethyl‐2H‐benzo[d] [1,2,3]triazol‐2‐yl)butane‐1‐sulfonate) (PBTz‐SO₃Na) based on the same benzotriazole‐conjugated backbone but with ammonium and sulfonated side chains are designed and synthesized through side‐chain functionalization and Yamamoto polymerization, respectively, and are used as the cathode interlayers in fullerene‐ and non‐fullerene‐based polymer solar cells. The interfacial modification of PBTz‐TMAI and PBTz‐SO₃Na onto the active layer achieves good energy alignment at cathode electrodes and optimized exciton‐dissociation efficiency from the active layer. Consequently, the power conversion efficiencies (PCEs) of 7.8% and 9.6% are obtained for the fullerene PTB7:PC₇₁BM‐based and non‐fullerene PBDB‐T:ITIC‐based polymer solar cells (PSCs) with PBTz‐SO₃Na interlayer. The PCS devices based on PTB7:PC₇₁BM and PBDB‐T:ITIC active layers with PBTz‐TMAI interlayer achieved a remarkably improved performance with PCEs of 8.2% and 10.2%, respectively.     

A New Dithienylbenzotriazole‐Based Poly(2,7‐carbazole) for Efficient Photovoltaics

A new dithienyl benzotriazole‐based conjugated polymer was synthesized by Suzuki coupling reaction. The polymer was found to be soluble in common organic solvents, such as chloroform, tetrahydrofuran and chlorobenzene, with excellent film‐forming properties. The structure of the polymer was confirmed by ¹H NMR, the molecular weights determined by GPC and the thermal properties investigated by TGA and DSC. The polymer films exhibited an absorption band in the wavelength range 300 to 610 nm. Preliminary photovoltaic cells based on the composite structure of indium tin oxide (ITO)/PEDOT:PSS/ PCDTBTz:PC ₆₀ BM (1:2 w/w)/Al showed an open‐circuit voltage of 0.92 V, a power conversion efficiency of 2.2% and a short circuit current of 5.33 mA cm^?2.

     

New Alternating Copolymers of 3,6‐Carbazoles and Dithienylbenzothiadiazoles: Synthesis,Characterization, and Application in Photovoltaics

Three new low‐bandgap copolymers containing 3,6‐linked carbazole units are synthesized and characterized using a combination of spectroscopic and electrochemical techniques and DFT calculations. The electron‐withdrawing effect of the BTD units leads to a significant decrease of the optical bandgap from 2.59 eV for PBTC to 1.81 eV for PCDTBT, whereas pendant octyl chains on the thiophene ring lead to an increase of the gap to 2.05 eV for PCDOTBT due to steric hindrance. PBTC:PCBM photovoltaic devices exhibit a PCE of 0.35% with an EQE reaching 35% in the blue region of the solar spectrum whereas PCDTBT:PCBM blends are efficient over a wider spectral range due to the lower bandgap of PCDTBT.

     

Bandgap and Molecular‐Energy‐Level Control of Conjugated‐Polymer Photovoltaic Materials Based on 6,12‐Dihydro‐diindeno[1,2‐b;10,20‐e]pyrazine

Six conjugated polymers based on the indenopyrazine ( IPY) unit are designed and synthesized by copolymerization with different electron‐deficient and electron‐rich building blocks. All of the polymers show good solubility, excellent film‐forming ability, and low‐lying highest occupied molecular orbit (HOMO) energy levels. The effects of the different copolymerized units on the optical, electrochemical, and photovoltaic properties are investigated. Results indicate that their bandgaps and molecular energy levels are readily tuned by copolymerizing with electron‐deficient and electron‐rich units. Polymer solar‐cell devices are fabricated utilizing the polymers as electron donors and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) as an electron acceptor. The best power conversion efficiency of the cell based on PIPY‐DTBTA , one of the IPY‐ based polymers, reaches 0.77%, with a relatively high V_oc up to 0.78 V.

     

Effect of Side Chains on Charge Transport of Anthracene‐Based PPE–PPV Copolymers

The variation of the drift mobility of positive and negative charge carriers in films of anthracene‐containing poly(p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene)s ( AnE‐PVs ), differently substituted, is investigated as a function of the applied electric field. Branched 2‐ethylhexyl and linear alkoxy side chains of different lengths are considered, as well as well‐defined and random distributions of lateral substituents. The same conditions are used both for the deposition of the polymer films and for their characterization, which allows for the establishment of a clear relationship between the chemical structure and the charge carrier mobility.

     

Synthesis and Photovoltaic Properties of a D–A Copolymer Based on the 2,3‐Di(5‐hexylthio­phen‐2‐yl)quinoxaline Acceptor Unit

A new D–A copolymer ( PBDT‐DTQx) based on the 2,3‐di(5‐hexylthiophen‐2‐yl)quinoxaline acceptor unit and a bithienyl‐substituted benzodithiophene (BDT) donor unit is designed and synthesized for application as the donor material in polymer solar cells (PSCs). The polymer film shows a broad absorption band covering the wavelength range from 300 to 720 nm and a low highest occupied molecular orbital (HOMO) energy level at ?5.35 eV. A device based on PBDT‐DTQx :PC₇₀BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester) (1:2.5, w/w) with chloronaphthalene as a solvent additive displays a power conversion efficiency (PCE) of 3.15%. With methanol treatment, the PCE of the PSCs is further improved to 3.90% with a significant increase of the short‐circuit current density, J_sc, from 10.10 mA cm^?2 for the device without the methanol treatment to 11.71 mA cm^?2 for the device with the methanol treatment.

     

Polymers with Alkylsulfanyl Side Chains for Bulk Heterojunction Solar Cells: Toward a Greener Strategy

The synthesis and properties of three copolymers obtained through a Knoevenagel polycondensation are reported. They are soluble in organic solvents, filmable, and solvatochromic. They are characterized through gel permeation chromatography, NMR and UV–vis spectroscopies, cyclic voltammetry, and atomic force microscopy. They display some proneness to form π‐stacks, broad absorptions up to about 700 nm, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies that fit those of [6,6]‐phenyl‐C61‐butyric acid methyl ester ([60]PCBM). The polymers are incorporated in blends with PCBM within bulk heterojunction devices. The best results are found for polymer solar cells obtained from P1 (2.7% power conversion efficiency). The insertion of alkylsulfanyl chains improves the photovoltaic performances, with respect to those of alkyl‐substituted analogous polymers. The J _sc of the devices are greatly enhanced, supporting the employment of (alkylsulfanyl)thienyl units as base units in photoactive materials.

     

Optimization of Polymer Solar Cells Based on the Conjugated Copolymer of Poly(phenylenevinylene‐alt‐4,7‐diphenyl‐2,1,3‐benzothiadiazole) (PP‐DBT)

Polymer solar cells based on poly(phenylenevinylene‐alt‐4,7‐diphenyl‐2,1,3‐benzothiadiazole) (PP‐DBT):[6,6]‐phenyl C₇₁ butyric acid methyl‐ester (PC₇₁BM) blend films are optimized. The results show that the solar cell has a better performance at a 1:4 blend ratio than at 1:1, 1:2, 1:3, and 1:5 blend ratios, due to the higher hole mobility of active layer and the optimized morphology. The device prepared from chlorobenzene (CB) has a higher power conversion efficiency (PCE) than the devices prepared from chloroform and o‐dichlorobenzene, because the former has a much higher short‐circuit current density (J_sc) resulting from the morphology with proper phase separation. The solar cell modified with two layers of ZnO nanocrystals achieves the best performance with an open‐circuit voltage of 0.9 V, a J_sc of 7.46 mW cm^?2, fill factor (FF) of 0.50, and PCE of 3.36%.

     

Effect of Branched Side Chains on the Physicochemical and Photovoltaic Properties of Poly(3‐hexylthiophene) Isomers

Two P3HT isomers with branched alkyl side chains, P3EBT and P3MPT, are synthesized. The HOMO energy levels of P3EBT and P3MPT are ?5.35 and ?5.24 eV, respectively, which are significantly lower than that of P3HT with a linear side chain. The absorption edges of the two P3HT isomer films, especially those of P3EBT, are blue‐shifted in comparison with that of P3HT. A PSC based on P3EBT:IC₆₀BA (2:1 w/w) shows a high open‐circuit voltage of 0.98 V, which is the highest V_oc reported so far for polythiophene‐based PSCs. A PSC based on P3MPT:IC₇₀BA (2:1 w/w) exhibits a power conversion efficiency of 3.62% with a V_oc of 0.91 V. P3MPT is suitable for the application in tandem PSCs.     

Electroluminescent and Photovoltaic Properties of the Crosslinkable Poly(phenylene vinylene) Derivative with Side Chains Containing Vinyl Groups

Summary: The photoluminescence (PL), electroluminescence (EL), and photovoltaic properties of a poly(phenylene vinylene) derivative with its side chains containing vinyl groups, poly[(2,5‐bis‐allyloxyphenylene vinylene)‐co‐(2‐methoxy‐5‐(2′‐ethylhexyloxy)phenylene vinylene)] (A‐PPV) are reported. A‐PPV with the active vinyl units in the end of its side chains can be crosslinked either by UV light irradiation or heating. The crosslinking does not change the color of light emitted from the A‐PPV polymer light‐emitting diode (PLED). FTIR confirms that the photo/heating‐induced crosslinking does not damage the conjugation of polymer at all. However, the PL and EL efficiency of A‐PPV decreased upon photoirradiation due to the existence of residual radicals in the solid‐state film. The relatively stable radicals function as a trap that quenches the luminescence. The blend of A‐PPV and noncrosslinkable polymer, poly(2‐methyloxy‐5‐ethylhexyloxyl‐1,4‐phenylene vinylene) (MEH‐PPV) was employed in the fabrication of PLEDs, and the PLED shows improved performance with a luminous efficiency of >1 cd · A⁻¹. The blend keeps the advantage of crosslinkability of the polymers and lessens the effects of the photo‐produced radicals. The photovoltaic cells comprised of A‐PPV and C₆₀ were fabricated and the energy conversion efficiency of the devices with and without UV treatment was similar, both around 0.3%. The UV irradiation on the blend film with C₆₀ seems resistant to the residual radicals.

The I‐V, L‐V plot of PLEDs with A‐PPV treated by UV light.     

Synthesis and Photovoltaic Properties of New Multifused Anthradithiophene‐Based Narrow‐Bandgap D–A Copolymers

Three structurally novel donor–acceptor copolymers are synthesized and fully characterized using an anthradithiophene (ADT) derivative as the donor block. All of them exhibit broad absorption ranges and moderate hole and electron mobilities. The multifused ADT donor block makes them exhibit lower highest occupied molecular orbital (HOMO) energy levels than many reported benzodithiophene‐based polymers. Copolymer blends with [6,6]‐phenyl C71 butyric acid methyl ester (PC₇₁BM) as active layers are used to fabricate polymer solar cells (PSCs) and a variety of post‐treatment methods are employed to optimize the device performance. The conventional and inverted configuration devices are prepared to evaluate the photovoltaic properties. A maximum power conversion efficiency of 1.66% is achieved for the inverted‐configuration device. The improved efficiency is caused by the close energy alignment between the work function of MoO₃ and the HOMO energy levels of the copolymers, facilitating better charge transport and increased short‐circuit current density (J_sc) with the inverted devices.

     

Bisfuran‐s‐Tetrazine‐Based Conjugated Polymers: Synthesis,Characterization, and Photovoltaic Properties

Bisfuran‐s‐tetrazine (FTz) and its copolymers with cyclopenta[2,1‐b:3,4‐b′]dithiophene (CPDT) and benzo[1,2‐b:4,5‐b′]dithiophene (BDT) (PCPDTFTz and PBDTFTz) are prepared with the alternating s‐tetrazine and CPDT or BDT units bridged by a furan ring. Their optical and electrochemical properties are studied and compared with their thiophene analogs (PCPDTTTz‐out and PCPDTTTz‐in), in which the bridging unit is 4‐hexylthiophene or 3‐hexylthiophene, respectively. Bulk heterojunction (BHJ) solar cells fabricated from these polymers with PC₇₁BM have a power conversion efficiency (PCE) of 0.8%, which is much lower than that from the PCPDTTTz‐out analog (3.2%), due to the low steric hindrance of the furan polymers in the absence of alkyl substitution on the furan ring.     

Thermochromic and Photovoltaic Properties of an Alternating Copolymer of Dithieno[3,2‐b:2′,3′‐d]thiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione

The synthesis of a new alternating conjugated polymer, PDTTTPD, based on electron‐donating dithieno[3,2‐b:2′,3′‐d]thiophene (DTT) and electron‐withdrawing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) units is reported. This polymer shows strong thermochromic effect in chlorobenzene with a peak shift up to 170 nm. This phenomenon is studied with fluorescence spectroscopy and related to the steric hindrance along the polymer main chain. Polymer solar cells are fabricated from its blends with PC₇₁BM, and shows modest power conversion efficiency up to 2.1%. The low efficiency is due to the low short‐circuit current, which is also attributed to the steric effect. Based on these results, a general design rule for polymer photovoltaic material with controlled backbone conformation is proposed.