Narrow Bandgap Polymer Research Articles

18.6% Efficiency All-Polymer Solar Cells Enabled by a Wide Bandgap Polymer Donor Based on Benzo[1,2-d:4,5-d']bisthiazole.

The limited selection of wide bandgap polymer donors for all-polymer solar cells (all-PSCs) is a bottleneck problem restricting their further development and remains poorly studied. Herein, a new wide bandgap polymer, namely PBBTz-Cl, is designed and synthesized by bridging the benzobisthiazole acceptor block and chlorinated benzodithiophene donor block with thiophene units for application as an electron donor in all-PSCs. PBBTz-Cl not only possesses wide bandgap and deep energy levels but also displays strong absorption, high-planar structure, and good crystallinity, making it a promising candidate for application as a polymer donor in organic solar cells. When paired with the narrow bandgap polymer acceptor PY-IT, a fibril-like morphology forms, which facilitates exciton dissociation and charge transport, contributing to a power conversion efficiency (PCE) of 17.15% of the corresponding all-PSCs. Moreover, when introducing another crystalline polymer acceptor BTP-2T2F into the PBBTz-Cl:PY-IT host blend, the absorption ditch in the range of 600-750nm is filled, and the blend morphology is further optimized with the trap density reducing. As a result, the ternary blend all-PSCs achieve a significantly improved PCE of 18.60%, which is among the highest values for all-PSCs to date.

Conjugated [5]Cumulene Polymers Enabled by Condensation Polymerization of Propargylic Electrophiles.

Cumulenes, sp-hybridized carbon motifs featuring consecutive double bonds, have rarely been explored as π-elements for conjugated polymers. Long cumulenic conjugated polymers can serve as models for approaching carbyne, an intriguing yet elusive carbon allotrope. However, their synthesis is notoriously difficult due to intrinsic instability. To date, only few [3]cumulene-based polymers have been synthesized, mostly relying on surface chemistry. Higher cumulene-based polymers remain unknown. Here, we present a "meet in the middle" strategy to overcome this challenge and synthesize high-molecular-weight, stable, and solution-processable conjugated [5]cumulene polymers (Mw up to 67.9 kg/mol). Our approach involves a new polymerization method called step-growth condensation polymerization of propargylic electrophiles (step-growth CPPE). The structures and molecular weights of the cumulenic polymers are established by various spectroscopic methods, including a comparative analysis of a discrete oligomer series. By introducing ortho-substituents on the aryl side groups, we successfully address the stability-conjugation dilemma. Electronic communication between cumulene units is found to be contingent upon the aromaticity of the π-spacers, enabling flexible energy-level adjustment and new narrow band gap polymers. The synthetic methodology and structure-property relationship established in this work serve as the starting points for the exploration of this fascinating family of sp-carbon-rich materials.

Biselenophene Imide: Enabling Polymer Acceptor with High Electron Mobility for High-Performance All-Polymer Solar Cells.

The shortage of narrow band gap polymer acceptors with high electron mobility is the major bottleneck for developing efficient all-polymer solar cells (all-PSCs). Herein, we synthesize a distannylated electron-deficient biselenophene imide monomer (BSeI-Tin) with high purity/reactivity, affording an excellent chance to access acceptor-acceptor (A-A) type polymer acceptors. Copolymerizing BSeI-Tin with dibrominated monomer Y5-Br, the resulting A-A polymer PY5-BSeI shows a higher molecular weight, narrower band gap, deeper-lying frontier molecular orbital levels and larger electron mobility than the donor-acceptor type counterpart PY5-BSe. Consequently, the PY5-BSeI-based all-PSCs deliver a remarkable efficiency of 17.77 % with a high short-circuit current of 24.93 mA cm-2 and fill factor of 75.83 %. This efficiency is much higher than that (10.70 %) of the PY5-BSe-based devices. Our study demonstrates that BSeI is a promising building block for constructing high-performance polymer acceptors and stannylation of electron-deficient building blocks offers an excellent approach to developing A-A type polymers for all-PSCs and even beyond.

Biselenophene Imide: Enabling Polymer Acceptor with High Electron Mobility for High‐Performance All‐Polymer Solar Cells

AbstractThe shortage of narrow band gap polymer acceptors with high electron mobility is the major bottleneck for developing efficient all‐polymer solar cells (all‐PSCs). Herein, we synthesize a distannylated electron‐deficient biselenophene imide monomer (BSeI‐Tin) with high purity/reactivity, affording an excellent chance to access acceptor–acceptor (A–A) type polymer acceptors. Copolymerizing BSeI‐Tin with dibrominated monomer Y5‐Br, the resulting A–A polymer PY5‐BSeI shows a higher molecular weight, narrower band gap, deeper‐lying frontier molecular orbital levels and larger electron mobility than the donor–acceptor type counterpart PY5‐BSe. Consequently, the PY5‐BSeI‐based all‐PSCs deliver a remarkable efficiency of 17.77 % with a high short‐circuit current of 24.93 mA cm−2 and fill factor of 75.83 %. This efficiency is much higher than that (10.70 %) of the PY5‐BSe‐based devices. Our study demonstrates that BSeI is a promising building block for constructing high‐performance polymer acceptors and stannylation of electron‐deficient building blocks offers an excellent approach to developing A–A type polymers for all‐PSCs and even beyond.

Graded-Index Active Layer for Efficiency Enhancement in Polymer Solar Cell

In this paper, narrow-bandgap polymer acceptors combining a benzotriazole (BTz)-core fused-ring segment, named the PZT series, were used with a high-absorption-efficiency polymer (PBDB) compound with branched 2-butyl octyl, linear n-octyl, and methyl to be utilized as a graded-index (GI) active layer of the polymer solar cells (PSCs) to increase the photocurrent and enhance solar efficiency compared to the existing PBDB-T:PZT and PBDB-T:PZT-γ. In addition, a two-dimensional photonic crystal (2D-PhC) structure was utilized as a light-trapping anti-reflection coating (ARC) thin film based on indium tin oxide (ITO) to reduce incident light reflection and enhance its absorption. The dimensions of the cell layers were optimized to achieve the maximum power-conversion efficiency (PCE). Furthermore, the design and simulations were conducted from a 300 nm to 1200 nm wavelength range using a finite difference time-domain (FDTD) analysis. One of the most important results expected from the study was the design of a nano solar cell at (64 µm)2 with a PCE of 25.1%, a short-circuit current density (JSC) of 27.74 mA/cm2, and an open-circuit voltage (VOC) of 0.986 V.

Chlorinated Narrow Bandgap Polymer Suppresses Non-Radiative Recombination Energy Loss Enabling Perylene Diimides-Based Organic Solar Cells Exceeding 10% Efficiency.

The scarcity of narrow bandgap donor polymers matched with perylene diimides (PDI)-based nonfullerene acceptors (NFAs) hinders improvement of the power conversion efficiency (PCE) value of organic solar cells (OSCs). Here, it is reported that a narrow bandgap donor polymer PDX, the chlorinated derivative of the famous polymer donor PTB7-Th, blended with PDI-based NFA boosts the PCE value exceeding 10%. The electroluminescent quantum efficiency of PDX-based OSCs is two orders of magnitude higher than that of PTB7-Th-based OSCs;therefore, the nonradiative energy loss is 0.103eV lower. This is the highest PCE value for OSCs with the lowest energy loss using the blend of PTB7-Th derivatives and PDI-based NFAs as the active layer. Besides, PDX-based devices showed larger phase separation, faster charge mobilities, higher exciton dissociation probability, suppressed charge recombination, elevated charge transfer state, and decreased energetic disorder compared with the PTB7-Th-based OSCs. All these factors contribute to the simultaneously improved short circuit current density, open circuit voltage, and fill factor, thus significantly improving PCE. These results prove that chlorinated conjugated side thienyl groups can efficiently suppress the non-radiative energy loss and highlight the importance of fine-modifying or developing novel narrow bandgap polymers to further elevate the PCE value of PDI-based OSCs.

Generating Long-Lived Triplet Excited States in Narrow Bandgap Conjugated Polymers.

Narrow bandgap conjugated polymers are a heavily studied class of organic semiconductors, but their excited states usually have a very short lifetime, limiting their scope for applications. One approach to overcome the short lifetime is to populate long-lived triplet states for which relaxation to the ground state is forbidden. However, the triplet lifetime of narrow bandgap polymer films is typically limited to a few microseconds. Here, we investigated the effect of film morphology on triplet dynamics in red-emitting conjugated polymers based on the classic benzodithiophene monomer unit with the solubilizing alkyl side chains C16 and C2C6 and then used Pd porphyrin sensitization as a further strategy to change the triplet dynamics. Using transient absorption spectroscopy, we demonstrated a 0.45 ms triplet lifetime for the more crystalline nonsensitized polymer C2C6, 2-3 orders of magnitude longer than typically reported, while the amorphous C16 had only a 5 μs lifetime. The increase is partly due to delaying bimolecular electron-hole recombination in the more crystalline C2C6, where a higher energy barrier for charge recombination is expected. A triplet lifetime of 0.4 ms was also achieved by covalently incorporating 5% of Pd porphyrin into the C16 polymer, which introduced extra energy transfer steps between the polymer and porphyrin that delayed triplet dynamics and increased the polymer triplet yield by 7.9 times. This work demonstrates two synthetic approaches to generate the longest-lived triplet excited states in narrow bandgap conjugated polymers, which is of necessity in a wide range of fields that range from organic electronics to sensors and bioapplications.

Molecular engineering of new electron acceptor for highly efficient solution processable organic solar cells using state-of-the-art polymer donor PffBT4T-2OD

Two molecularly engineered fullerene-based electron acceptors, coded as BITh-C60 and BITh-C70, were designed, synthesized and fully characterized. The molecules have LUMO energy levels at −3.62 and-3.65 eV; and HOMO energy levels at −5.62 and −5.60 eV, respectively as confirmed by cyclic voltammetry. The acceptor BITh-C70 shows comparatively broader and stronger absorption with respect to BITh-C60. A narrow band gap polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl) 2,2′;5′,2″;5″,2′′′-quaterthiophen-5,5′′′-diyl)] (PffBT4T-2OD) was used to construct the solution processed bulk-heterojunction solar cells. Under optimized fabrication and evaluation conditions, BITh-C60 and BITh-C70 showed a highest power conversion efficiency (PCE) of 8.1 % and 8.8 %, respectively, which are comparatively higher than the PCEs obtained from the respective reference solar cells. The PffBT4T-2OD:BITh-C70 based BHJ solar cell shows a nearly flat EQE spectra of 75 % between 380 and 700 nm.

Electrical Conductivities of Narrow-Bandgap Polymers with Two Types of π-Conjugated Post-Crosslinking.

Bandgap energy is one of the most important properties for developing electronic devices because of its influence on the electrical conductivity of substances. Many methods have been developed to control bandgap, one of which is the realization of conducting polymers using narrow-bandgap polymers; however, the preparation of these polymers is complex. In this study, water-soluble, narrow-bandgap polymers with reactive groups were prepared by the addition–condensation reaction of pyrrole (Pyr), benzaldehyde-2-sulfonic acid sodium salt (BS), and aldehyde-containing reactive groups (aldehyde and pyridine) for post-crosslinking. Two types of reactions, aldehyde with p-phenylenediamine and pyridine with 1,2-dibromoethylene, were carried out for the π-conjugated post-crosslinking between polymers. The polymers were characterized by proton nuclear magnetic resonance (1H-NMR), thermogravimetric/differential thermal analysis (TG/DTA), UltraViolet-Visible-Near InfraRed spectroscopy (UV-Vis-NIR), and other analyses. The bandgaps of the polymers, calculated from their absorption, were less than 0.5 eV. Post-crosslinking prevents resolubility and develops electron-conducting routes between the polymer chains for π-conjugated systems. Moreover, the post-crosslinked polymers maintain their narrow bandgaps. The electrical conductivities of the as-prepared polymers were two orders of magnitude higher than those before the crosslinking.

Novel A-π-A-D type perylene diimide acceptor for high-performance fullerene-free organic solar cells

Two novel PDI-based acceptors PDI-SPAc and FPDI-SPAc, which contain perylene diimide (PDI) and a quasi-two-dimensional (quasi-2D) fused PDI (FPDI), respectively, with electron-rich unit 10 H-spiro[acridan-9,9′-fluorene] (SPAc), were designed and synthesized to investigate their application in non-fullerene organic solar cells (NF-OSCs). Expectedly, FPDI-SPAc obtained a fairly twisted conformation in consequence of the quasi-2D configuration of FPDI unit and significant dihedral angle between FPDI and SPAc units. Geometry of PDI-SPAc containing only dihedral angle between PDI and SPAc units was confirmed, too. The optical properties, electron energy levels, charge transport properties, photovoltaic performance, charge dissociation probabilities, and surface morphology were systematically investigated subsequently. By blending with a narrow band-gap polymer PTB7-Th as the polymer donor, NF-OSC based on PTB7-Th:FPDI-SPAc achieved a power conversion efficiency (PCE) of 3.92% with the dominant configuration characteristic, whereas the PDI-SPAc-based NF-OSC exhibited a PCE of only 0.32% with a significantly reduced short-circuit current density (Jsc) of 1.2 mA/cm2. The interesting results indicate that, the quasi-2D structure of FPDI unit produced impressive effect on the photovoltaic performances of the FPDI derivative acceptors, which can demonstrate that quasi-2D structure is a better electron-deficient segment in comparison with PDI for the design of n-type photovoltaic materials.

Phenalene—A New Ring-Locked Vinyl Bridge for Nonfullerene Acceptors With Enhanced Chemical and Photochemical Stabilities

Currently, the two exocyclic vinyl bridges in the acceptor–donor–acceptor (A–D–A)-type nonfullerene acceptors (NFAs) have been widely recognized as one of the most vulnerable sites under external stresses. Embedding the exocyclic vinyl bridges into an aromatic ring could be a feasible solution to stabilize them. Herein, we successfully develop a phenalene-locked vinyl bridge via a titanium tetrachloride—pyridine catalytic Knoevenagel condensation, to synthesize two new A–D–A-type unfused NFAs, EH-FPCN and O-CPCN, wherein malononitrile is used as the electron-deficient terminal group while fluorene and carbazole rings are used as the electron-rich cores, respectively. These two NFAs possess wide bandgaps associated with deep energy levels, and significantly enhanced chemical and photochemical stabilities compared to the analogue molecule O-CzCN with normal exocyclic vinyl bridges. When pairing with a narrow bandgap polymer donor PTB7-Th, the fabricated EH-FPCN- and O-CPCN-based organic solar cells achieved power conversion efficiencies of 0.91 and 1.62%, respectively. The higher efficiencies for O-CPCN is attributed to its better film morphology and higher electron mobility in the blend film. Overall, this work provides a new design strategy to stabilize the vulnerable vinyl bridges of A–D–A-type NFAs.

2+2] Cycloaddition-retroelectrocyclization reactivity and thin film transistor performances of carbazole-based platinum polyyne polymers

Carbazole-based platinum polyyne polymers are a good platform for the postfunctional formation of donor-acceptor type narrow band gap polymers by the [2 + 2] cycloaddition-retroelectrocyclization reaction with tetracyanoethylene (TCNE). The TCNE reactivity depends on the connectivity pattern of the carbazole. The 3,6-cabazole-based polymer shows a higher reactivity than the corresponding 2,7-carbazole-based polymer due to the p-positions with respect to the sp3 nitrogen atom. The connectivity pattern leads to the regioregularity difference in the resulting polymers. When TCNE is added to one of the alkynes of the 3,6-diethynylcarbazole unit, the electron-donating nature of the carbazole is lowered and the second TCNE addition does not occur. Accordingly, the regioregular polymer is formed from the 3,6-carbazole-based polymer, which is revealed from the 1H NMR spectrum. In contrast, the TCNE addition of the 2,7-carbazole-based polymer is mainly governed by the platinum atom, which produces a regiorandom structure. Finally, the charge carrier transporting properties of the precursor polymers are evaluated in thin film transistors. The 3,6-carbazole-based polymer displays a one order higher hole mobility than the corresponding 2,7-carbazole-based polymer. This result suggests that the shallower HOMO level of the former polymer facilitates the hole injection.

Gradual chlorination at different positions of D-π-A copolymers based on benzodithiophene and isoindigo for organic solar cells

Isoindigo (IID) has been widely used as strong acceptor unit (A) to construct narrow bandgap polymers in organic field effect transistors (OFETs) and organic solar cells (OSCs). Combing with IID, we chose benzodithiophene (BDT) as the donor unit (D) and thieno [3,2- b ]thiophene (TT) as the π bridge to construct a new type of D-π-A polymer PE70 . Based on PE70 , we adopt the chlorination strategy to fine-tune photoelectric characteristics and film morphology, and then developed PE74 and PE75 . By blending with non-fullerene acceptor (NFA) Y6 , device based on PE74 with chloride substitution on the BDT unit showed increasing photovoltaic performance. In addition, further chlorine substitution on the IID (PE75) would greatly reduce the non-radiative voltage loss (Δ V 3), and the distorted molecular conformation also took responsible for the excessive recombination. As results, PE74:Y6 -based device achieves a power conversion efficiency (PCE) of 11.06% with open-circuit voltage ( V OC ) of 0.76 ​V, which are higher than those of PE70:Y6 (PCE of 10.40% and V OC of 0.72 ​V) and PE75: Y6 -based device (PCE of 6.24% and V OC of 0.84 ​V). This work demonstrates the regularity of the photovoltaic performance caused by chlorination strategy in polymer in the non-fullerene OSC devices, which provide important insights into high-performance photovoltaic materials.

Metal free benzothiadiazole-diketopyrrolopyrrole-based conjugated polymer/g-C3N4 photocatalyst for enhanced sterilization and degradation in visible to near-infrared region

In recent years, photocatalytic technology has attracted wide attention in environmental treatment, exploring non-toxic and metal-free photocatalysts is imminent to meet sustainable development. However, semiconductors with wide spectral response are rarely studied and applied in the field of photocatalysis. Herein, a new narrow band-gap polymer PFBDT-DPP (P3) with wide absorption from 500 to 860 nm was synthesized and further constructed heterostructure with g-C3N4 for photocatalytic sterilization and degradation of organic pollutant Rhodamine B (RhB). The optimal antibacterial rate for Escherichia coli reached 99.8% after 190 min of light irradiation and for Staphylococcus aureus reached 96.8% after 120 min of irradiation, and the highest degradation efficiency of RhB by P3/g-C3N4 was 98.9% within 60 min light irradiation, while g-C3N4 displayed an unsatisfactory sterilization and photodegradation performance. This is mainly attributed to the broadened light absorption range and enhanced carrier separation efficiency of P3/g-C3N4. This work could provide a new strategy to fabricate metal-free photocatalysts with high utilization of sunlight and excellent photocatalytic performance.

Effect of energy bandgap and sacrificial agents of cyclopentadithiophene-based polymers for enhanced photocatalytic hydrogen evolution

A library of donor-acceptor system consisting of cyclopentadithiophene-based polymer photocatalysts have been designed and synthesized. Among all photocatalysts, the active PCPDTBSO achieved hydrogen evolution rates of 24.6 mmol h–1 g–1 with apparent quantum yields of 8.7 % at 500 nm. More importantly, combined the results of photocatalytic efficiency, apparent quantum yield, the time-resolved fluorescence decay spectra, the steady-state photoluminescence spectra, and the transient absorption spectroscopy, and the oxidation potentials of sacrificial donors and protons reduction potentials in different pH values, we confirmed the concept that ascorbic acid is a suitable sacrificial donor for narrow bandgap polymers and triethylamine is a suitable sacrificial donor for wide bandgap polymers owing to the existence of the optimal thermodynamic driving force. We believed this study would be advantageous for the selection of photocatalysts and sacrificial donors for hydrogen production.

Regioregular Narrow-Bandgap n-Type Polymers with High Electron Mobility Enabling Highly Efficient All-Polymer Solar Cells.

Narrow-bandgap n-type polymers with high electron mobility are urgently demanded for the development of all-polymer solar cells (all-PSCs). Here, two regioregular narrow-bandgap polymer acceptors, L15 and MBTI, with two electron-deficient segments are synthesized by copolymerizing two dibrominated fused-ring electron acceptors (FREA) with distannylated aromatic imide, respectively. Taking full advantage of the FREA and the imide, both polymer acceptors show narrow bandgap and high electron mobility. Benefiting from the more extended absorption, better backbone ordering, and higher electron mobility than those of its regiorandom analog, the L15-based all-PSC yields a high power conversion efficiency (PCE) of 15.2% when blended with the polymer donor PM6. More importantly, MBTI incorporating a benzothiophene-core FREA segment shows relatively higher frontier molecular orbital levels than L15, forming a cascade-like energy level alignment with L15 and PM6. Based on this, ternary all-PSCs are designed where MBTI is introduced as a guest into the PM6:L15 host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 16.2%, which is among the highest values for all-PSCs. The results demonstrate that combining an FREA and an aromatic imide to construct regioregular narrow-bandgap polymer acceptors provides an effective approach to fabricate highly efficient all-PSCs.

Multi-Selenophene-Containing Narrow Bandgap Polymer Acceptors for All-Polymer Solar Cells with over 15 % Efficiency and High Reproducibility.

All-polymer solar cells (all-PSCs) progressed tremendously due to recent advances in polymerized small molecule acceptors (PSMAs), and their power conversion efficiencies (PCEs) have exceeded 15 %. However, the practical applications of all-PSCs are still restricted by a lack of PSMAs with a broad absorption, high electron mobility, low energy loss, and good batch-to-batch reproducibility. A multi-selenophene-containing PSMA, PFY-3Se, was developed based on a selenophene-fused SMA framework and a selenophene π-spacer. Compared to its thiophene analogue PFY-0Se, PFY-3Se shows a ≈30 nm red-shifted absorption, increased electron mobility, and improved intermolecular interaction. In all-PSCs, PFY-3Se achieved an impressive PCE of 15.1 % with both high short-circuit current density of 23.6 mA cm-2 and high fill factor of 0.737, and a low energy loss, which are among the best values in all-PSCs reported to date and much better than PFY-0Se (PCE=13.0 %). Notably, PFY-3Se maintains similarly good batch-to-batch properties for realizing reproducible device performance, which is the first reported and also very rare for the PSMAs. Moreover, the PFY-3Se-based all-PSCs show low dependence of PCE on device area (0.045-1.0 cm2 ) and active layer thickness (110-250 nm), indicating the great potential toward practical applications.

Multi‐Selenophene‐Containing Narrow Bandgap Polymer Acceptors for All‐Polymer Solar Cells with over 15 % Efficiency and High Reproducibility

AbstractAll‐polymer solar cells (all‐PSCs) progressed tremendously due to recent advances in polymerized small molecule acceptors (PSMAs), and their power conversion efficiencies (PCEs) have exceeded 15 %. However, the practical applications of all‐PSCs are still restricted by a lack of PSMAs with a broad absorption, high electron mobility, low energy loss, and good batch‐to‐batch reproducibility. A multi‐selenophene‐containing PSMA, PFY‐3Se, was developed based on a selenophene‐fused SMA framework and a selenophene π‐spacer. Compared to its thiophene analogue PFY‐0Se, PFY‐3Se shows a ≈30 nm red‐shifted absorption, increased electron mobility, and improved intermolecular interaction. In all‐PSCs, PFY‐3Se achieved an impressive PCE of 15.1 % with both high short‐circuit current density of 23.6 mA cm−2 and high fill factor of 0.737, and a low energy loss, which are among the best values in all‐PSCs reported to date and much better than PFY‐0Se (PCE=13.0 %). Notably, PFY‐3Se maintains similarly good batch‐to‐batch properties for realizing reproducible device performance, which is the first reported and also very rare for the PSMAs. Moreover, the PFY‐3Se‐based all‐PSCs show low dependence of PCE on device area (0.045–1.0 cm2) and active layer thickness (110–250 nm), indicating the great potential toward practical applications.

Highly Efficient All-Polymer Solar Cells Processed from Nonhalogenated Solvents.

The remarkable advance of all-polymer solar cells (all-PSCs) achieved in the past decades is primarily powered by the innovation of polymer acceptors. However, most of high-performance all-PSCs are dominantly fabricated with halogenated solvents, which are detrimental to human bodies and the environment. Herein, eco-friendly solvent-processed all-PSCs were developed, based on wide-bandgap polymer poly[4,8-bis(5-(2-ethylhexylthio)thiophen-2-yl)-benzo-[1,2-b;4,5-b']dithiophene-alt-2,5-di(butyloctylthiophen-2-yl) -thiazolo[5,4-d]thiazole] (PSTZ) as donor and newly synthesized narrow-bandgap polymer 5,6-dicyano-2,1,3-benzothiadiazole indacenodithiophene (DCNBT-IDT) as acceptor. When processed with o-xylene and THF, PSTZ : DCNBT-IDT-based all-PSCs yielded remarkable power conversion efficiencies of 7.23 and 8.77 % with high short-circuit currents of 12.94 and 14.12 mA cm-2 , respectively. The results indicated that the utilization of an all-polymer blend based on narrow polymer acceptor and compatible polymer donor is an effective strategy for advancing eco-friendly solvent-processed all-PSCs.

Novel narrow bandgap polymer donors based on ester-substituted quinoxaline unit for organic photovoltaic application

Two narrow bandgap conjugated polymers, PTT-EFQX and PT-DFBT-T-EFQX, comprising a novel bis(2-alkyl) 5,8-dibromo-6,7-difluoroquinoxaline-2,3-dicarboxylate (EF-Qx) unit, are designed and synthesized through palladium-catalyzed Stille coupling reaction. The ester groups and difluorine substituents are introduced into the EF-Qx unit to improve the electron affinity. The polymers containing EF-Qx unit exhibit a relatively narrow bandgap of around 1.6 eV. To optimize the effects of solubility and molecular packing, D-A structure and D-A1-D-A2 structure synthesis strategies are adopted in this work and the structure-property relationship is systematically studied. The results morphology characterizations indicate that the active layer based on D-A structure polymer PTT-EFQX has a superior morphology compared to active layer based on D-A1-D-A2 structure polymer PT-DFBT-T-EFQX, which helps PTT-EFQX-based polymer solar cells (PSCs) to obtain better PCEs of up to 5.4%. Our work demonstrates that the effective design strategy for development of quinoxaline-based polymer donor is necessary for controlling the morphology and thus achieving high-performance PSCs.