Low bandgap polymers with benzo [1,2-b:4,5-b′] dithiophene and bisthiophene-dioxopyrrolothiophene units for photovoltaic applications

New donor/acceptor polymers PBDTTPT1 and PBDTTPT2 with alternating benzodithiophene (BDT) and bisthiophene-dioxopyrrolothiophene (TPT) units were synthesized by Stille coupling reaction. The polymers had optical bandgaps of 1.78 and 1.82 eV, and HOMO energy levels of −5.30 and −5.35 eV for PBDTTPT1 and PBDTTPT2, respectively. Polymeric solar cell devices based on these copolymers as donors and PC₇₁BM as acceptor showed the highest open circuit voltage of 0.95 V and power conversion efficiency of 2.68% under the illumination of AM 1.5, 100 mW/cm².     

Dinaphtho-s-indacene-based copolymers for inverted organic solar cells with high open-circuit voltages

We report on the synthesis and characterization of a series of donor–acceptor copolymers (PF1, PF2, PF3 or PF4) based on a dinaphtho-s-indacene (DNI) donor unit and four different acceptor units. The molecular weights of the copolymers were determined by using gel permeation chromatography, and their electrochemical properties were investigated by cyclic voltammetry. All four copolymers showed deep-lying highest occupied molecular orbital energy levels. Inverted bulk heterojunction solar cells were fabricated by using the synthesized copolymers as the electron donor material and 6,6-phenyl-C71-butyric acid methyl ester (PC₇₁BM) as the electron acceptor material. Inverted solar cells based on PF1:PC₇₁BM (1:4, w/w) exhibited a power conversion efficiency (PCE) of 3.07%, a high open-circuit voltage (V_oc) of 0.99 V, a short-circuit current density (J_sc) of 7.85 mA/cm², and a fill factor of 39.5% under the AM1.5G illumination. With the same fabrication method, the inverted devices based on PF2, PF3 and PF4 showed PCEs of 2.62, 1.18 and 1.32%, and V_oc values of 0.97, 0.91 and 0.80 V, respectively.     

Synthesis and photovoltaic behaviors of benzothiadiazole- and triphenylamine-based alternating copolymers

A series of donor–acceptor (D–A) alternating copolymers (P1, P2 and P3) with thiophene–benzothiadiazole–thiophene–triphenylamine main chain have been synthesized by Suzuki polycondensation. P1, P2, and P3 possess medium optical band gaps of 1.99, 1.97 and 1.93 eV, respectively. Bulk heterojunction polymer solar cells (BHJ PSCs) with these polymers as donor and PC₇₁BM as acceptor showed power conversion efficiency (PCE) in the range of 2.1–2.8%. The highest PCE of 2.8 % was achieved for P1 with short circuit current (J_sc) of 7.8 mA/cm². This study offers a useful and important insight for designing triphenylamine derivative-based polymers used for efficient PSCs.     

Enhancement of photovoltaic properties in supramolecular polymer networks featuring a solar cell main-chain polymer H-bonded with conjugated cross-linkers

Stille polymerization was employed to synthesize a low-band-gap (LBG) conjugated main-chain polymer PBTH consisting of bithiazole, dithieno[3,2-b:2′,3′-d]pyrroles (DTP), and pendent melamine derivatives. Novel supramolecular polymer networks PBTH/C and PBTH/F were developed by mixing proper molar amounts of polymer PBTH (containing melamine pendants) to be hydrogen-bonded (H-bonded) with complementary uracil-based conjugated cross-linkers C and F (i.e., containing two symmetrical uracil moieties connected with carbazole and fluorene units through triple bonds). The formation of multiple H-bonds between polymer PBTH and cross-linkers C or F was confirmed by FT-IR measurements. In contrast to polymer PBTH, the supramolecular design with multiple H-bonds can enhance the photovoltaic properties of polymer solar cell (PSC) devices containing H-bonded polymer networks PBTH/C and PBTH/F by tuning their light harvesting capabilities, HOMO energy levels, and crystallinities. Initially, the power conversion efficiency (PCE) values of PSC devices containing supramolecular polymer networks PBTH/C and PBTH/F as electron donors and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) as an electron acceptor (polymer:PC₇₀BM = 1:1 w/w) are found to be 0.97 and 0.68%, respectively, in contrast to 0.52% for polymer PBTH. The highest PCE value of 1.56% with a short-circuit current densities (J_sc) value of 7.16 mA/cm², a open circuit voltages (V_oc) value of 0.60 V, and a fill factor (FF) of 0.36 was further optimized in the PSC device containing a supramolecular polymer network PBTH/C as polymer:PC₇₀BM = 1:2 w/w. These results indicate that supramolecular design is an effective route towards better photovoltaic properties of V_oc, J_sc, and PCE values in polymer solar cells.     

A copolymer based on benzo[1,2-b:4,5-b′]dithiophene and quinoxaline derivative for photovoltaic application

A D–A–D copolymer (PBDTQx) with a bandgap of 1.78 eV, containing alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and quinoxaline derivative (Qx) as acceptor, was synthesized by Stille coupling reaction. In order to study the photovoltaic property of PBDTQx, polymer solar cells (PSCs) were fabricated with PBDTQx as the electron donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) as the electron acceptor. The power conversion efficiency (PCE) of PSC was 1.01% for an optimized PBDTQx: PC₆₁BM ratio of 1:5, under the illumination of AM 1.5, 100 mW/cm². The results indicated that PBDTQx was a promising donor candidate in the application of polymer solar cells.     

The influence of side chains on solubility and photovoltaic performance of dithiophene–thienopyrazine small band gap copolymers

Three small band gap copolymers based on alternating dithiophene and thienopyrazine units were synthesized via Yamamoto coupling and applied in bulk heterojunction solar cells as donor together with PCBM ([6,6]-phenyl C₆₁ butyric acid methyl ester) as acceptor. The polymers have an optical band gap of about 1.3 eV in the solid state and only vary by the chemical nature of the solubilizing side chains. The nature of the side chain has a major effect on solubility and processability of the polymer. Using n-butoxymethyl side chains a soluble, easy to process polymer was obtained that gave the best photovoltaic performance. With short-circuit currents up to 5.2 mA/cm² an efficiency of 0.8% was achieved under estimated standard solar light conditions (AM1.5G, 100 mW/cm²) with spectral response up to 950 nm.     

Synthesis and photovoltaic properties of D-π-A copolymers based on thieno[3,2-b]thiophene bridge unit

Three D-π-A copolymers containing thieno[3,2-b]thiophene (TT) bridge and BDT, carbazole, fluorene as D units and benzothiadiazole as A unit were synthesized and characterized. These copolymers of PBDT-tt-BT, PC-tt-BT and PF-tt-BT exhibited enough high thermal stabilites and good solubilites in chloroform and dichlorobenzene. Among the copolymers, with the increase of the electron-donating abilities of the D units from fluorene to carbazole further to BDT, the absorption spectra of PF-tt-BT shows blue shift and that of PBDT-tt-BT shows red shift comparing to that of PC-tt-BT in their solutions and films. Meanwhile, by electrochemical cyclic voltammetry measurements we found the HOMO levels vary in the same trench according to their electron-donating abilities. Under the illumination of AM 1.5G, 100 mW/cm², power conversion efficiency (PCE) of the PSCs based on these copolymers as donors and PC₇₀BM as acceptor were measured and PBDT-tt-BT shows a higher efficiency of 4.91% than PC-tt-BT and PF-tt-BT based devices mostly due to its higher hole mobility and broader absorption range. These results indicate that PBDT-tt-BT is a promising photovoltaic polymer donor material for efficient PSCs.     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.     

Synthesis and application of H-Bonded cross-linking polymers containing a conjugated pyridyl H-Acceptor side-chain polymer and various carbazole-based H-Donor dyes bearing symmetrical cyanoacrylic acids for organic solar cells

A series of novel hydrogen-bonded (H-bonded) cross-linking polymers were generated by complexing various proton-donor (H-donor) solar cell dyes containing 3,6- and 2,7-functionalized electron-donating carbazole cores bearing symmetrical thiophene linkers and cyanoacrylic acid termini with a proton-acceptor (H-acceptor) side-chain homopolymer carrying pyridyl pendants (with 1/2 M ratio of H-donor/H-acceptor). The supramolecular H-bonded structures between H-donor dyes and the H-acceptor side-chain polymer were confirmed by FTIR measurements. The effects of the supramolecular architecture on optical, electrochemical, and organic photovoltaic (OPV) properties were investigated. From DFT (density functional theory) calculations, the optimized geometries of organic dyes reflected that the carbazole cores of H-donor dyes were coplanar with the conjugated thiophenes and cyanoacrylic acids, which is essential for strong conjugations across the donor-acceptor units in D1-D4 dyes. Under 100 mW/cm² of AM 1.5 white-light illumination, bulk heterojunction (BHJ) OPV cell devices containing an active layer of H-bonded polymers (PDFTP/D1-D4) as an electron donor blended with [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) as an electron acceptor in a weight ratio of 1:1 were explored. From the preliminary investigations, the OPV device containing 1:1 weight ratio of H-bonded polymer PDFTP/D2 and PCBM showed the best power conversion efficiency (PCE) value of 0.31% with a short-circuit current (J_sc) of 1.9 mA/cm², an open-circuit voltage (V_oc) of 0.55 V, and a fill factor (FF) of 29%, which has a higher PCE value than the corresponding H-donor D2 dye (PCE = 0.15%) or H-acceptor PDFTP homopolymer (PCE = 0.02%) blended with PCBM in 1:1 weight ratio.     

Synthesis and photovoltaic properties of low-bandgap polymers based on N-arylcarbazole

Low-bandgap poly(2,7-carbazole) derivatives with variable N-substituent of ethyl (PEtCzBT), phenyl (PPhCzBT) and 4-diphenylaminophenyl (PTPACzBT) on the carbazoles, were synthesized through Suzuki coupling reaction. The polymers show excellent solubility in organic solvents (readily soluble in chloroform, THF and toluene etc.), good thermal stability (5% weight loss temperature of more than 417 °C), and electrochemical properties (reversible redox process with narrow bandgap), and deep HOMO energy levels (∼5.1 eV), allowing them promising candidates in the solar cell fabrication. Bulk-heterojunction solar cells with these polymers as electron donor and (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as electron acceptor exhibit high V_oc (0.91-0.95 V) and good power conversion efficiency (PCE) of 1.69% for PEtCzTB, 2.01% for PPhCzTB, and 2.42% for PTPACzTB.     

Tailoring side chains of low band gap polymers for high efficiency polymer solar cells

High efficiency organic solar cells (OSCs) require conjugated polymers with a low band gap, broad absorption in visible and IR region, high carrier mobility, and relatively high molecular weight as p-type donor materials. Flexible side chains on the rigid polymer backbone are crucial for the solubility of conjugated polymers. In this work, four polymers with the main chain structure of fluorene-thiophene-benzothiadiazole-thiophene and flexible side chains located on fluorene, thiophene, and benzothiadiazole moiety, respectively, have been synthesized by Suzuki-Miyaura-Schlüter polycondensation. Photovoltaic device measurements with a device configuration of ITO/polymer:PC₇₁BM blends/LiF/Al show that P1 carrying octyloxy chains on benzothiadiazole rings gives the best performance, with a power conversion efficiency of 3.1%.     

Design and synthesis of carbonyl group modified conjugated polymers for photovoltaic application

In this article, a simple and common electron-withdrawing moiety, carbonyl group, is applied in the molecular design of conjugated polymers for high-performance polymer solar cells (PSCs). Two series of donor–acceptor (D–A) copolymers are synthesized through alternating copolymerization of the electron-donating (D) benzodithiophene and dithienopyrrole with various electron-accepting (A) units containing carbonyl groups. The absorption range and the band gap of copolymers can be tuned by changing the molecular structure of A unit and the number of carbonyl groups. Moreover, by introducing the carbonyl group, the highest occupied molecular orbital energy level of the copolymer is lowered efficiently, leading to the improvement of the open-circuit voltage (V _OC) of PSCs. The best photovoltaic performance is obtained while poly(benzodithiophene-alt-thiophene-3-carboxylate) is functioned as the electron donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester as electron acceptor in a bulk heterojunction solar cell with a power conversion efficiency of 4.13%, a V _OC of 0.80 V, a short-circuit current of 8.19 mA/cm², and a fill factor of 63.2%.     

Fluorene‐based copolymers with donor–acceptor units on the side chain and main chain: Synthesis and application in polymer solar cells

Two narrow band gap fluorene‐based copolymers with donor–acceptor (D–A) structure on the polymer side chain and/or main chain are synthesized by Pd‐catalyzed Stille coupling reactions. The two copolymers have excellent thermal stability. The effects of D–A structure on the main and side chains on the absorption and electrochemical properties are studied. The copolymer PF‐BTh‐DBT with D–A structure both on the main and side chains has broader and stronger absorption and narrower band gap than the copolymer PF‐BTh with only a pendent D–A structure. The power conversion efficiency of the assembled solar cell using PF‐BTh‐DBT as donor and PC₇₁BM as acceptor is 1.6% with open‐circuit voltage (V_oc) 0.84 V under simulated AM 1.5 G solar irradiation (100 mW/cm²). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3276–3281, 2013     

Low bandgap EDOT-quinoxaline and EDOT-thiadiazol-quinoxaline conjugated polymers: Synthesis, redox, and photovoltaic device

Three new low bandgap conjugated copolymers with 3,4-ethylenedioxythiophene (EDOT) as donor and 2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline (P2, P3) as acceptors were synthesized by Stille cross-coupling reaction, and their optical and electrochemical properties were studied. These polymers exhibited optical bandgap of 1.77, 1.29 and 1.13 eV, for P1, P2 and P3, respectively. Photovoltaic cells with device configuration of ITO/PEDOT: PSS/Copolymer: PCBM (1:4 w/w)/LiF/Al were fabricated. The measurements revealed an open-circuit voltage (V_oc) of 0.52 V, short-circuit current density (J_sc) of 3.24 mA/cm² and power conversion efficiency (PCE) of 0.60% for P1, and showed a V_oc of 0.33 V, J_sc of 2.11 mA/cm², PCE of 0.39% for P2.     

Molecular structure–property engineering of low‐band‐gap copolymers,based on fluorene,for efficient bulk heterojunction solar cells: A density functional theory study

A comprehensive theoretical study of new donor (D)–acceptor (A) copolymers properties has been developed through quantum chemical calculations, on the basis of density functional theory (DFT). Fluorene and 2,1,3‐benzothiadiazole units were used as donor and acceptor units, respectively, in alternating D–A structures. The efficiency of D–A strength in the polymer was characterized, and their electronic properties are modulated through the insertion of thiophene, vinylene, or ethynylene as π‐spacer groups, in an attempt to reach the critical values, required for organic solar cells. Optoelectronic properties of these copolymers are rationalized on the basis of the patterns of their frontier orbitals. The analysis of the corresponding transition provides an efficient way for tracing the origin of various optical transitions of these copolymers and their photophysical properties. Bulk heterojunction photovoltaic cells, designed with the based fluorene‐copolymer as electron donor, blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester as an electron acceptor, were energetically elucidated. A model band diagram was established, simulating the energy behavior of this active layer. POLYM. ENG. SCI., 2013. © Society of Plastics Engineers     

D-A copolymers based on 5,6-difluorobenzotriazole and oligothiophenes: Synthesis,field effect transistors,and polymer solar cells

Two new 5,6-difluorobenzotriazole (FBTA)-oligothiophene copolymers PFBTA-3T and PFBTA-4T, comprising terthiophene (3T) and quaterthiophene (4T) on the backbone, respectively, were successfully synthesized. A new route to synthesize FBTA monomer was established. Polymers PFBTA-3T and PFBTA-4T exhibited good solubility in common organic solvents and good thermal stability. In comparison to poly (3-hexylthiophene), the incorporations of the FBTA as in PFBTA-3T and PFBTA-4T could result in smaller band gaps around 1.83 eV for the two copolymers. The HOMO levels of PFBTA-3T and PFBTA-4T were −5.49 and −5.31 eV, respectively, while their LUMO levels were −3.65 and −3.90 eV, respectively. In field-effect transistors fabricated without high temperature thermal annealing, PFBTA-3T and PFBTA-4T could display hole mobilities of 1.68 × 10⁻³ and 1.31 × 10⁻² cm² V⁻¹ s⁻¹, respectively. The mobility for PFBTA-4T is the highest among the reported FBTA-based polymers, suggesting that FBTA is a promising heterocycle to construct polymers with high mobility. Polymer solar cells were also fabricated with PFBTA-3T and PFBTA-4T as the donor and PC₆₁BM as the acceptor. With copolymer: PC₆₁BM = 1:1.5 as the active layers, polymer solar cells showed power conversion efficiencies of 3.0% and 2.51% for PFBTA-3T and PFBTA-4T, respectively.     

Hydrogen-bond induced side-chain liquid crystalline polymers based on nicotinic acid derivatives

Supramolecular side-chain liquid crystalline poly(acrylate)s have been prepared by self-assembly of H-bond donor and acceptor complexes through intermolecular complementary hydrogen bond formation. Poly[4-(m-acryloyloxyalkyloxy)benzoic acid]s [m = 6 (P1) and 8 (P2)] were employed as polymer components. Liquid crystalline nicotinic acid derivatives (C1, C2, C3, and C4) were used as complementary H-bond donor/acceptor counterparts. The liquid crystalline properties of the nicotinic acid derivatives, the polymers and their complexes were investigated by DSC and POM. Methylene spacers present at the terminal position of the nicotinic acid derivatives played a key role in mesophase arrangements. The columnar phase exhibited by the nicotinic acid derivatives completely disappeared in the H-bonded complexes to afford a nematic phase, thereby substantiating the complex formation.     

A series of lanthanide (III) coordination polymers derived via in situ hydrothermal decarboxylation of quinoline-2,3-dicarboxylic acid

A series of one-dimensional coordination polymers assembled from LnNO₃ · 6H₂O (Ln = Sm(1), Eu(2), Tb(3), La(4), Ce(5), Pr(6), Nd(7), Dy(8)), quinoline-2,3-dicarboxylic acid (2,3-H₂qldc) and 1,10-phenanthroline (phen) formulated as [Ln(2,3-qldc)(3-qlc)(phen)]_n (3-Hqlc = quinoline-3-carboxylic acid) were obtained under hydrothermal conditions. It is remarkable that in situ hydrothermal decarboxylation was observed during preparing these polymers. Complexes 1–8 were characterized by elemental analyses, IR spectroscopy and single crystal X-ray diffraction analyses. The thermal stabilities and photoluminescence properties of these complexes have been investigated.     

Synthesis and characterization of donor-acceptor type 4,4′-bis(2,1,3-benzothiadiazole)-based copolymers

A series of novel donor-acceptor type polymers based on 4,4′-bis(2,1,3-benzothiadiazole) were synthesized and characterized. Two soluble regioregular tail-to-tail and head-to-head coupled polymers, poly[7,7′-bis(3-octyl-2-thienyl)-4,4′-bis(2,1,3-benzothiadiazole)] poly[3TBB3T], and poly[7,7′-bis(4-octyl-2-thienyl)-4,4′-bis(2,1,3-benzothiadiazole)] poly[4TBB4T] were synthesized by FeCl₃-mediated oxidative polymerization. To further decrease the band gap of the polymers, vinylene spacers were incorporated into the polymer backbone by Stille coupling of the corresponding monomers and (E)-1,2-bis(tributylstannyl)ethene. A crystal structure of a monomer analog shows near planar arrangement of the aromatic units in the solid state. The optical properties of the monomers and polymers were investigated by steady-state absorption and photoluminescence spectroscopy. Cyclic voltammetry measurements indicate that the polymers could be employed as acceptor materials in polymer-polymer bulk heterojunction solar cells due to their low LUMO energy of about −4.0 eV. A maximum photovoltaic power conversion efficiency of about 0.3% was observed for a 1:1 blend of regioregular poly(3-hexylthiophene) (rr-P3HT) and poly[4TBB4T] and the origin of the moderate efficiency is discussed by interpreting the device current-voltage characteristics, external quantum efficiency and incident light intensity dependence of the power conversion efficiency.     

Synthesis and characterization of thieno[3,4-c]pyrrole-4,6-dione and pyrrolo[3,4-c]pyrrole-1,4-dione-based random polymers for photovoltaic applications

New random poly{benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-c]pyrrole-4,6-dione-pyrrolo[3,4-c]pyrrole-1,4-dione} (PBDT-TPD-DPP) based on benzo[1,2-b:4,5-b']dithiophene (BDT) as donor and thieno[3,4-c]pyrrole-4,6-dione (TPD, 60–90%), pyrrolo[3,4-c]pyrrole-1,4-dione (DPP, 10–40%) as acceptors were synthesized through Stille coupling reaction. The photophysical, electrochemical and photovoltaic properties of random polymers were investigated. The random polymers with high molecular weight (M_n = 33.5–41.7 kDa) exhibited broad and strong absorption covering the spectra range from 350 nm up to 922 nm with absorption maxima at around 700 nm, the relatively deep highest occupied molecular orbital (HOMO) energy levels vary between ?5.25 and ?5.42 eV and suitable lowest unoccupied molecular orbital (LUMO) energy levels ranging from ?3.85 to ?3.91 eV. Polymer solar cells (PSC) based on these new random polymers were fabricated with device structures of ITO/PEDOT: PSS/random polymers: PC₇₁BM (1:2, w/w)/Ca/Al. The photovoltaic properties of random polymers were evaluated under AM 1.5G illumination (100 mW/cm²). Devices based on the random polymers showed open circuit voltage (V_oc) of 0.71–0.83 V, and power conversion efficiency (PCE) of 0.82–1.80%.