Low Band Gap Copolymers Research Articles

The enhanced performance of fluorinated quinoxaline-containing polymers by replacing carbon with silicon bridging atoms on the dithiophene donor skeleton

The bridging atom effect shows a great influence on the thermal stability, absorption, energy levels, carrier mobility and photovoltaic performance of fluorinated quinoxaline-dithiophene based low band gap copolymers.

A low band-gap copolymer composed of thienyl substituted anthracene and diketopyrrolopyrrole compatible with multiple electron acceptors for high efficiency polymer solar cells

A low band-gap polymer composed of thienylanthracene and DPP exhibits promising PCEs of 7.02% with PC71BM, and 4.23% with non-fullerene acceptor di-PBI, demonstrating the potential for universal electron donor polymer of polymer solar cells.

Enabling high-mobility, ambipolar charge-transport in a DPP-benzotriazole copolymer by side-chain engineering.

In this article we discuss the synthesis of four new low band-gap co-polymers based on the diketopyrrolopyrrole (DPP) and benzotriazole (BTZ) monomer unit. We demonstrate that the BTZ unit allows for additional solubilizing side-chains on the co-monomer and show that the introduction of a linear side-chain on the DPP-unit leads to an increase in thin-film order and charge-carrier mobility if a sufficiently solubilizing, branched, side chain is attached to the BTZ. We compare two different synthetic routes, direct arylation and Suzuki-polycondensation, by a direct comparison of polymers obtained via the two routes and show that direct arylation produces polymers with lower electrical performance which we attribute to a higher density of chain Furthermore we demonstrate that a polymer utilizing this design motif and synthesized via Suzuki-polycondensation ((l-C18)-DPP-(b-C17)-BTZ) exhibits exceptionally high and near balanced average electron and hole mobilities >2 cm2 V-1 s-1 which are among the highest, robustly extracted mobility values reported for DPP copolymers in a top-gate configuration to date. Our results demonstrate clearly that linear side chain substitution of the DPP unit together with co-monomers that allow for the use of sufficiently long or branched solubilizing side chains can be an attractive design motif for solution processable, high mobility DPP copolymers.

Low bandgap copolymers based on monofluorinated isoindigo towards efficient polymer solar cells

Low bandgap copolymers based on fluorinated isoindigo afford 5.0% efficiency in polymer solar cells.

Enhanced Photovoltaic Performance of Amorphous Copolymers Based on Dithienosilole and Dioxocycloalkene-annelated Thiophene

Organic photovoltaics (OPVs) have attracted considerable attention due to their potential for generating renewable energy. The power conversion efficiency (PCE) of the OPVs largely depends on the organic semiconducting materials. Thus, the elucidation of structure–property OPV performance relationships is important for the rational improvement of OPVs. Here, low-bandgap copolymers comprising dithieno[3,2-b:2′,3′-d]silole as a donor unit and dialkyl-substituted naphtho[2,3-c]thiophene-4,9-dione as an acceptor unit were synthesized to investigate the influence of the polymer molecular weight and the alkyl chain length in the acceptor unit on the polymer properties and photovoltaic performance. All the prepared copolymers are amorphous in the solid state. Both the increase of polymer molecular weight and variation of the alkyl side chains in the acceptor unit subtly affected molecular properties. However, these structural modifications showed significant impact on the photovoltaic performance in bulk heteroj...

The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71BM Bulk Heterojunction Systems

The best performing low bandgap copolymers PTB series to date which is based on thieno[3,4‐b]thiophene‐alt‐benzodithiophene units blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM), have been the focus of polymer‐based solar cells. Here, novel fluorinated polymers PTB7‐Fx (fluorine units coupled with submonomer thieno[3,4‐b]thiophene) with varied degree of fluorination are used as electron donor materials. The PTB7‐Fx:PC71BM bulk heterojunction (BHJ) films spin‐coated from the host solvent chlorobenzene without and with solvent additive 1,8‐diiodooctane (DIO) and the corresponding solar cell devices are systematically investigated to address the morphology‐efficiency relationship. Self‐assembled BHJ morphology is already observed for as‐spun blend films. After adding the solvent additive DIO, the pronounced ordered structures are suppressed and better intermixed films with much smaller domain sizes result. Full fluorination of the third C‐atom of thienothiophene gives rise to the highest power conversion efficiency. As the absorption properties, film morphology and crystallinity remain similar for different degrees of fluorination, the main influence of the photovoltaic performance is ascribed to the different lowest unoccupied molecular orbital (LUMO) of each polymer instead of the film morphology. Thus the device performance can be efficiently improved by tuning the energy level of the polymer without necessarily changing either the film nanomorphology or crystallinity dramatically.

Enhanced Performance of the Nanostructured Zinc Oxide-Conjugated Polymer Based Hybrid Solar Cell with the Application of a Low Band Gap Co-Polymer

Enhanced Performance of the Nanostructured Zinc Oxide-Conjugated Polymer Based Hybrid Solar Cell with the Application of a Low Band Gap Co-Polymer

Germanium‐ and Silicon‐Substituted Donor–Acceptor Type Copolymers: Effect of the Bridging Heteroatom on Molecular Packing and Photovoltaic Device Performance

The effects of heteroatom substitution from a silicon atom to a germanium atom in donor‐acceptor type low band gap copolymers, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PSiBTBT) and poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PGeBTBT), are studied. The optoelectronic and charge transport properties of these polymers are investigated with a particular focus on their use for organic photovoltaic (OPV) devices in blends with phenyl‐C70‐butyric acid methyl ester (PC70BM). It is found that the longer C‐Ge bond length, in comparison to C‐Si, modifies the molecular conformation and leads to a more planar chain conformation in PGeBTBT than PSiBTBT. This increase in molecular planarity leads to enhanced crystallinity and an increased preference for a face‐on backbone orientation, thus leading to higher charge carrier mobility in the diode configuration. These results provide important insight into the impact of the heavy atom substitution on the molecular packing and device performance of polymers based on the poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole) (PCPDTBT) backbone.

Fluorinated benzothiadiazole-based low band gap copolymers to enhance open-circuit voltage and efficiency of polymer solar cells

Two donor–acceptor (D–A) copolymers containing dithienosilole (DTS) donor unit and unsubstituted benzothiadiazole (BT) or fluorinated benzothiadiazole (ffBT) acceptor unit, PDTSBT and PDTSffBT, were synthesized by Stille cross coupling polymerization and tested for application in polymer solar cells (PSCs). The new alternating copolymer (PDTSffBT) possesses both a low optical bandgap (Eg=1.54eV) and a deep highest occupied molecular orbital energy level (HOMO) (−5.46eV). It was found that the introduction of the two electron-withdrawing fluorine atoms on the benzothiadiazole unit results in a decrease of the HOMO energy level with slight effect on the lowest unoccupied molecular orbital (LUMO) as observed through cyclic voltammetry (CV) analysis. Inclusion of fluorine atoms also leads to an increase in the interchain interaction in the PDTSffBT copolymer with respect to its analogue PDTSBT copolymer according to the X-ray diffraction measurements (XRD). When PDTSffBT:PC71BM blends are tested in bulk heterojunction (BHJ)-polymer solar cells, the cells display a short-circuit current (JSC) of 8.82mA/cm2, a high open circuit voltage (VOC) of 0.74V, and a Fill Factor (FF) of 45%, giving an overall power conversion efficiency (PCE) of 2.93%, compared to 1.64% for the BT-containing cells prepared in parallel under identical conditions. The significant performance enhancement results from the higher VOC and JSC in the ffBT-containing cells. These results unambiguously indicate that the fluorination is an efficient method to modulate the energetic levels and improve the photovoltaic performances of the widely used benzothiadiazole-based low bandgap copolymers.

Relationship between interchain interaction, exciton delocalization, and charge separation in low-bandgap copolymer blends.

We present a systematic study of the roles of crystallinity, interchain interaction, and exciton delocalization on ultrafast charge separation pathways in donor-acceptor copoloymer blends. We characterize the energy levels, excited state structures, and dynamics of the interchain species by combined ultrafast spectroscopy and computational quantum chemistry approaches. The alkyl side chain of a highly efficient donor-acceptor copolymer for solar cell applications, PBDTTT (poly(4,8-bis-alkyloxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-(alkylthieno[3,4-b]thiophene-2-carboxylate)-2,6-diyl), is varied to tune the molecular packing and interchain interaction of the polymers in order to elucidate the charge separation pathways originating from intrachain and interchain species. Polymers with linear side chains result in more crystalline polymer domain that lead to preferential formation of interchain excitons delocalizing over more than one polymer backbone in the solid state. Our results demonstrate that the higher polymer crystallinity leads to slower charge separation due to coarser phase segregation and formation of the interchain excited states that are energetically unfavorable for charge separation. Such energetics of the interchain excitons in low-bandgap copolymers calls for optimized solar cell morphologies that are fundamentally different from those based on homopolymers such as P3HT (poly-3-hexylthiophene). A long-range crystalline polymer domain is detrimental rather than beneficial to solar cell performance for a low-bandgap copolymer which is in direct contrast to the observed behavior in P3HT based devices.

Improved photovoltaic performance of two-dimensional low band-gap conjugated polymers with thieno[3,2-b]thiophene and diketopyrrolopyrrole units by altering pendent position of conjugated side chain

Two-dimensional low band-gap copolymers of P1 and P2 with a framework of (2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene)-alt-(2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DTTT-alt-DTPP) were synthesized, in which two 2,3-dioctylthiophene-5-yl groups were pended into the 3-positions of two thiophenyl rings close to the thieno[3,2-b]thiophene (TT) unit for P1 and the 3, 6-positions of the TT unit for P2, respectively. The influence of pending positions on optophysical and photovoltaic properties were studied. Similar broad absorption spectra from 350 to 950 nm were observed with a low band-gap of 1.25 eV. However, significantly different photovoltaic properties were obtained in the bulk heterojunction polymer solar cells with a device structure of ITO/PEDOT:PSS/active layer/LiF/Al. The P1-based devices showed a power conversion efficiency (PCE) of 0.79%, but the P2-based devices displayed a notably increasing PCE up to 2.12% under an AM 1.5G irradiation with an intensity of 100 mW cm−2. Our results indicate that altering pendent position of alkyl thiophene is effective way to improve the photovoltaic properties for the two-dimensional copolymers.

Tuning the Polarizability in Donor Polymers with a Thiophenesaccharin Unit for Organic Photovoltaic Applications

This paper describes the synthesis of low bandgap copolymers incorporating an artificial sweetener derivative, N‐alkyl, 3‐oxothieno[3,4‐d]isothiazole 1,1‐dioxide (TID). This new TID unit is identical to the well‐known thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) unit except that one carbonyl has been replaced by a sulfonyl group. Semi‐empirical calculations on the local dipole moment change between ground and excited states (Δμge) in the repeating units of the new polymer indicate that the replacement of the carbonyl by a sulfonyl group leads to larger Δμge values. The resulting polymers exhibit a diminished power‐conversion efficiency (PCE) compared to a bulk heterojunction (BHJ) solar cells with PC71BM as an acceptor, which extends the correlation between PCE and Δμge of single repeating units in p‐type polymers to a new regime. Detailed studies show that the strongly electron‐withdrawing sulfonyl group is detrimental to charge separation in alternating copolymers containing a TID unit.

Computational atomistic blueprinting of novel conducting copolymers using particle swarm optimization

A proficient metaheuristic approach viz., particle swarm optimization coupled with negative factor counting technique and inverse iteration method has been employed for designing novel binary and ternary copolymers based on thiophene, pyrrole and furan skeletons. A comparative study of the electronic structures and conduction properties of neutral heterocyclic copolymers and their benzene substituted analogues is inferred using the band structure results derived from ab-initio Hartree-Fock crystal orbital calculations. The band gap value decreases as a result of substitution on the polymer backbone due to increased quinoid contributions which is expected to enhance the intrinsic conductivity of the resulting copolymers. In general, it has been found that HOMO energies have a more decisive influence than LUMO energies on the relative fraction of constituents of the respective low band gap copolymers. The trends in the electronic properties of the respective copolymers are also verified and discussed with the help of density of states. These results can help streamline scrupulous synthetic efforts providing a potent route for molecular engineering of sustainable and efficient electronic materials.

Low band-gap copolymers derived from fluorinated isoindigo and dithienosilole: synthesis, properties and photovoltaic applications

Fluorination of isoindigo affords a dithienosilole-based low band-gap copolymer with low-lying energy levels, strong and broad absorption, high carrier mobility as well as efficient power conversion efficiency.

Fluorinated low band gap copolymer based on dithienosilole–benzothiadiazole for high-performance photovoltaic device

The introduction of fluorine atom into a copolymer of dithienosilole–benzothiadiazole causes significant enhancement in the PSCs with the PCE increasing from 4.68% to 6.70%.

Synthesis and photovoltaic performance of low band gap copolymers based on diketopyrrolopyrrole and tetrathienoacene with different conjugated bridges

Using tetrathienoacene (TT) and diketopyrrolopyrrole (DPP) units as donor and acceptor blocks, three new low band gap (LBG) conjugated copolymers (PTTDPO, PTTDPS and PTTDPSe) with different heterocycle bridges (furan, thiophene and selenophene) were designed and synthesized by Stille coupling polymerization reactions. Their structures were verified by 1HNMR spectroscopy and elemental analysis. All these copolymers exhibit broad absorption bands with small band gaps. UV-vis absorption spectra and cyclic voltammetry (CV) measurements indicated that the selenophene inclusion resulted in a reduction in the band gap, which could be attributed to a reduction of oxidation potentials and an increase of the electron affinities of the related copolymer. The density functional theory (DFT) calculations showed that PTTDPSe with selenophene bridges favoured a much more planar conformation than PTTDPO and PTTDPS, which afforded a higher hole mobility of 7.9 × 10−4 cm2 V−1 s−1. Photovoltaic properties of the copolymers blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor were investigated. The polymer solar cell (PSC) based on the structure of ITO/PEDOT:PSS/PTTDPSe:PC71BM (1 : 1.5, w/w)/Ca/Al exhibited a high power conversion efficiency (PCE) of 5.68% with an improved short circuit current density (Jsc) of 15.62 mA cm−2. The primary results show that changing heterocycle bridges with different electron-donating ability can easily and finely tune the optical absorptions, band gaps and energy levels of DPP-containing copolymers. The results also demonstrate that the TT unit is a new and promising electron-donating donor block for constructing highly efficient LBG photovoltaic materials.

Effect of spacer insertion in a commonly used dithienosilole/benzothiadiazole-based low band gap copolymer for polymer solar cells

Dithienosilole-benzothiadiazole based low bandgap copolymers remain promising material for organic photovoltaics. A new copolymer, poly[(4,4′-dioctyldithieno[3,2-b:2′,3′-d]silole-2,6-diyl)-alt-{4,7-bis[2-(3-hexyl)thienyl]-2,1,3-benzothiadiazole-5,5′-diyl}] (PDTSDTBT) was designed by introducing a thiophene spacer bearing a hexyl chain at β-position in the main backbone and compared to its analog poly[(4,4′-dioctyldithieno[3,2-b:2′,3′-d]silole-2,6-diyl)-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PDTSBT). In PDTSDTBT, linear alkyl chains on silicon were chosen due to facile and cheap access and the inserted 3-hexylthiophene units were chosen to increase solubility and molar mass, a weak point with PDTSBT. The two parameters are important to optimize photovoltaic performances. To compare characteristics, PDTSDTBT of molar masses greater than, and equal to a sample of PDTSBT, were prepared. Pd-catalyzed Stille cross-coupling reactions in a micro-wave reactor to promote an efficient copolymerisations. A strong absorption ranging from 370nm to 800nm and a good thermal stability were observed. PDTSDTBT showed better solubility and higher degree of crystallinity. Facile synthesis of high molar masses meant that higher efficiencies, around 40% greater, could be obtained with PDTSDTBT. The polymer was demonstrated to be susceptible to improvement through the use of device-additives. For example, under initial optimisations using PDTSDTBT:PC60BM blend at a ratio of 1:1 delivered a power conversion efficiency of 2.13% with JSC=7.73 (mA/cm2), under AM 1.5 (100mW/cm2) illumination.

Side-chain effects on the solution-phase conformations and charge photogeneration dynamics of low-bandgap copolymers

Solution-phase conformations and charge photogeneration dynamics of a pair of low-bandgap copolymers based on benzo[1,2-b:4,5-b(')]dithiophene (BDT) and thieno[3,4-b]thiophene (TT), differed by the respective carbonyl (-C) and ester (-E) substituents at the TT units, were comparatively investigated by using near-infrared time-resolved absorption (TA) spectroscopy at 25 °C and 120 °C. Steady-state and TA spectroscopic results corroborated by quantum chemical analyses prove that both PBDTTT-C and PBDTTT-E in chlorobenzene solutions are self-aggregated; however, the former bears a relatively higher packing order. Specifically, PBDTTT-C aggregates with more π-π stacked domains, whereas PBDTTT-E does with more random coils interacting strongly at the chain intersections. At 25 °C, the copolymers exhibit comparable exciton lifetimes (~1 ns) and fluorescence quantum yields (~2%), but distinctly different charge photogeneration dynamics: PBDTTT-C on photoexcitation gives rise to a branching ratio of charge separated (CS) over charge transfer (CT) states more than 20% higher than PBDTTT-E does, correlating with their photovoltaic performance. Temperature and excitation-wavelength dependent exciton∕charge dynamics suggest that the CT states localize at the chain intersections that are survivable up to 120 °C, and that the excitons and the CS states inhabit the stretched strands and the also thermally robust orderly stacked domains. The stable self-aggregation structures and the associated primary charge dynamics of the PBDTTT copolymers in solutions are suggested to impact intimately on the morphologies and the charge photogeneration efficiency of the solid-state photoactive layers.

A new low band gap donor–acceptor alternating copolymer containing dithienothiophene and fluorenone unit

A low bandgap copolymer poly(3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene-alt-9-fluorenone) (PDTTFO) consisting of dithieno[3,2-b:2′,3′-d]thiophene (DTT) and 9-fluorenone (FO) was synthesized as the donor material for the polymer solar cells via Stille coupling polymerization. Both donor and acceptor units were confirmed by FT-IR and 1H-NMR. Optoelectronic properties of the PDTTFO copolymer were investigated and observed by UV-vis, photoluminescence (PL) spectrum, and cyclic voltammogram (CV). UV-vis spectrum of the film exhibited two absorption peaks centered at 358, 474 nm with a broad absorption band in the range of 300–700 nm and showed a low bandgap of 1.68 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be −5.09 and −3.41 eV, respectively. Based on the ITO/PEDOT:PSS/PDTTFO:PCBM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm2) was 0.374 %. The effects of annealing temperature on the device performance were studied. At annealing temperature of 175 °C/30 min, the device demonstrated an optimal efficiency of 0.923 %. The improved device efficiency under the optimal condition was confirmed by the higher light harvest in UV-vis spectra, the enhanced quenching of photoluminescence (PL) emission, the improved nanoscale morphology by atomic force microscopy (AFM) examination, and the increase in external quantum efficiency.

Donor–acceptor copolymers incorporating polybenzo[1,2-b:4,5-b′]dithiophene and tetrazine for high open circuit voltage polymer solar cells

Two new low band gap conjugated polymers containing a benzo[1,2-b:4,5-b′]dithiophene donor unit and a tetrazine acceptor unit were synthesized by Stille cross-coupling polymerization. The structural and thermal properties of copolymers were characterized using nuclear magnetic resonance, gel permeation chromatography and thermogravimetric analysis. The results show that the donor–acceptor copolymers thus developed have good thermal stability with decomposition temperature of 294°C and 305°C. Cyclic voltammetric study revealed that the copolymers possess a deep-lying highest occupied molecular orbital energy level, which is desired for high open circuit voltage polymer solar cells (PSCs) and is also favorable for stable device operation in air. Bulk-heterojunction PSCs based on blend of low band gap copolymers: [6,6]-phenyl-C71-butyric acid methyl ester on indium tin oxide/glass substrates were fabricated. This work yielded a promising power conversion efficiency of >5.0%, with a high open circuit voltage of ∼1.0V, measured under air mass 1.5 global irradiation of 100mW/cm2.