Acceptor Polymers Research Articles

Resolving the Molecular Origin of Mechanical Relaxations in Donor–Acceptor Polymer Semiconductors

AbstractThe thermomechanical behavior of polymer semiconductors plays an important role in the processing, morphology, and stability of organic electronic devices. However, donor–acceptor‐based copolymers exhibit complex thermal relaxation behavior that is not well understood. This study uses dynamic mechanical analysis (DMA) to probe thermal relaxations of a systematic set of polymers based around the benzodithiophene (BDT) moiety. The loss tangent curves are resolved by fitting Gaussian functions to assign and distinguish different relaxations. Three prominent transitions are observed that correspond to: i) localized relaxations driven primarily by the side chains (γ ), ii) relaxations along the polymer backbone (β ), and iii) relaxations associated with aggregates (α ). The side chains are found to play a clear role in dictating Tγ, and that mixing the side chain chemistry of the monomer to include alkyl and oligo(ethylene glycol) moieties results in splitting the γ ‐relaxation. The β relaxations are shown to be associated with backbone elements along with the monomer. In addition, through processing, it is shown that the α‐relaxation is due to aggregate formation. Finally, it is demonstrated that the thermal relaxation behavior correlates well with the stress–strain behavior of the polymers, including hysteresis and permanent set in cyclically stretched films.

Selenium-Containing Organic Photovoltaic Materials.

ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (JSC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a JSC of approximate 27.5 mA cm-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.

Effects of the Selective Alkoxy Side Chain Position in Quinoxaline-Based Polymer Acceptors on the Performance of All-Polymer Solar Cells.

The effects of the position of alkoxy side chains in quinoxaline (Qx)-based polymer acceptors (PAs) on the characteristics of materials and the device parameters of all-polymer solar cells (all-PSCs) are investigated. The alkoxy side chains are selectively located at the meta, para, and both positions in pendant benzenes of Qx units, constructing PAs denoted as P(QxCN-T2)-m, P(QxCN-T2)-p, and P(QxCN-T2), respectively. Among them, P(QxCN-T2)-m exhibits the deepest energy levels owing to the enhanced electron-withdrawing effect of meta-positioned alkoxy chains, which is in contrast to P(QxCN-T2)-p where para-positioned alkoxy chains have an electron-donating property. In addition, the meta-positioned alkoxy chains induce good electron-conducting pathways, while the para-positioned ones significantly interrupt crystallization and intermolecular interactions between the conjugated backbones. Thus, when the PAs are applied to all-PSCs, a power conversion efficiency (PCE) of 5.07% is attained in the device using P(QxCN-T2)-m with efficient exciton dissociation and good electron-transporting ability. On the contrary, the P(QxCN-T2)-p-based counterpart has a PCE of only 1.62%. These results demonstrate that introducing alkoxy side chains at a proper location in the Qx-based PAs is crucial for their application to all-PSCs.

A Y6-based polymerized small-molecule acceptor with non-conjugated alkyl linkages for efficient all-polymer solar cells

A polymerized small-molecule acceptor, NC-PY6, has been newly designed and successfully synthesized. NC-PY6 contains a rigid conjugated Y6-derived fused-ring small acceptor SY6 and a non-conjugated alkyl chain as the linkage. Similar to SY6, NC-PY6 exhibits the narrow bandgap of 1.44 eV and the low-lying lowest unoccupied molecular orbital energy level (−3.92 eV), which match with the known wide-bandgap polymer donor D18. The D18:SY6 based polymer solar cells showed a power conversion efficiency (PCE) of 13.1% with a voltage loss of 0.58 V. The all-polymer solar cells (all-PSCs) based on D18:NC-PY6 blend film gave an encouraging PCE of 12.3% with a voltage loss of 0.57 V. The results demonstrate the potential of molecular design strategy of polymerizing small-molecule acceptor with the non-conjugated alkyl chain as the linkage for new type of polymer acceptors for high-performance all-PSCs.

Ester side chains engineered quinoxaline based D-A copolymers for high-efficiency all-polymer solar cells

The simple ester side-chains strategy has been an effective approach in fine regulating electronic levels, intermolecular interaction, and aggregation morphology of organic photovoltaic materials, which makes for promoting the photovoltaic performance of organic solar cells (OSCs). Herein, two novel D-A conjugated polymers, namely PBQxBE and PBQxTE, are designed and synthesized, derived from an alkylthiothiophene benzodithiophene (BDTTS) as donor moiety, thiophene as the π-bridges, and an identical molecular skeleton but different side chains based on quinoxaline (Qx) as acceptor units. The PBQxBE with ester engineered benzene side chains on the Qx unit exhibits better molecular flatness, stronger intermolecular interaction, and higher hole mobility than the analogous PBQxTE with ester engineered thiophene side chains. After combined with polymer acceptor PIDTC-T, the blend film of PBQxBE:PIDTC-T presented higher and balanced hole/electron mobility, better favorable aggregation morphology, as well as less charge carrier recombination. Thus, the all-polymer solar cells (all-PSCs) based on PBQxBE:PIDTC-T delivered a power conversion efficiency (PCE) of 10.17% with simultaneously increased basic parameters, which is much higher than that of 7.40% for the PBQxTE-based ones. The PCE of 10.17% is by far the highest values reported for all-PSCs from the Qx-based polymers as donor. The results demonstrate that the ester side-chains engineering of Qx-based polymers is an effective strategy to improve their photovoltaic performance of all-PSCs.

Hybrid donor-acceptor polymer nanoparticles and combination antibiotic for mitigation of pathogenic bacteria and biofilms

Biofilms pose a significant clinical problem in skin and soft tissue infections. Their resistance to antibiotics has spurred investigations into alternative treatments, such as nanoparticle-mediated photothermal ablation. Non-toxic Hybrid Donor- Acceptor (DA) Polymer nanoParticles (H-DAPPs) were developed for fluorescence imaging (using poly(3-hexylthiophene-2,5 diyl) (P3HT)) and rapid, near-infrared photothermal ablation (NIR- PTA) (using poly[4,4-bis(2-ethylhexyl)-cyclopenta[2,1-b;3,4-b’]dithiophene-2,6-diyl-alt−2,1,3-benzoselenadiazole-4,7-diyl] (PCPDTBSe)). H-DAPPs were evaluated alone, and in combination with antibiotics, against planktonic S. aureus and S. pyogenes, and S. aureus biofilms. H-DAPPs NIR-PTA (15–700 μg/ mL) can generate rapid temperature changes of 27.6–73.1 °C, which can eradicate planktonic bacterial populations and reduce biofilm bacterial viability by more than 4- log (> 99.99%) with exposure to 60 s of 800 nm light. Reductions were confirmed via confocal analysis, which suggested that H-DAPPs PTA caused bacterial inactivation within the biofilms, but did not significantly reduce biofilm polysaccharides. SEM imaging revealed structural changes in biofilms after H-DAPPs PTA. S. aureus biofilms challenged with 100 μg/mL of H-DAPPs (H-DAPPs-100) to induce an average temperature of 55.1 °C, and the minimum biofilm eradication concentration (MBEC) of clindamycin, resulted in up to ~3- log decrease in bacterial viability compared to untreated biofilms and those administered H-DAPPs-100 PTA only, and up to ~2- log compared to biofilms administered only clindamycin. This study demonstrates that polymer nanoparticle PTA can mitigate biofilm infection and may improve antimicrobial efficacy.

How to Choose an Interfacial Modifier for Organic Photovoltaics Using Simple Surface Energy Considerations.

Organic photovoltaics are typically composed of at least four different materials, including the donor and acceptor components of the bulk heterojunction, the interfacial layers at each electrode, the electrodes themselves, and solution additives that may persist in the final sandwich structure. The interplay of surface energies between these different layers is profoundly complex, as the deposition and annealing of one layer on top of another may be influenced by the surface energy of these interfaces. While the energy levels of one layer with respect to adjacent layers are important to facilitate charge separation and collection at the electrodes, the relative surface energies of the interfaces in contact with the multicomponent bulk heterojunction can be beneficial or disadvantageous, or be neutral, with respect to the performance of the OPV device. Because the bulk heterojunction is a mixture of donor and acceptor polymers and/or small molecules, the accumulation of one of the components on the underlying electrode interface can be driven by surface energy considerations. A donor- or acceptor-rich interface may affect charge carrier flow to the electrode, thus affecting the overall efficiency. Here, ITO/PEDOT:PSS electrodes in forward organic photovoltaic devices are treated with five different thin interfacial layers to change the relative surface energy of this electrode with respect to the adjacent bulk heterojunction. Contact angle measurements with four probe liquids enable calculation of the surface energies, and the results are compared with the performance of forward-biased organic photovoltaic devices. Time-of-flight secondary ion mass spectrometry results substantiate the predictions of gradients in the bulk heterojunction layers, and grazing-incidence wide-angle X-ray scattering measurements show the impact on the polymer crystallites. Thus, a simple algorithm based on surface energy considerations may inform which interfacial layer for a given bulk heterojunction in an organic photovoltaic device can be the most appropriate.

Ternary polymerization strategy to approach 12% efficiency in all-polymer solar cells processed by green solvent and additive

The terpolymers based on indacenodithiophene dicyanindenone derivative, named as PTer-N10, PTer-N25 and PTer-N50, were designed and synthesized by incorporating naphthalenediimide derivative with 10%, 25% and 50% molar ratios as another electron-withdrawing unit, respectively. These terpolymers were used as the acceptors to prepare all-polymer solar cells (all-PSCs) with o-xylene as the solvent and 2-methylnaphthalene as the additive without any annealing treatment, which showed high power conversion efficiency (PCE). The terpolymer PTer-N25 possesses the highest light harvest, the best morphology, the highest and most balanced charge mobilities in blend film, so the all-PSC based on PM6: PTer-N25 achieved the highest PCE of 11.94%. This study indicates that ternary polymerization is an efficient strategy to obtain green-solvent processing polymeric acceptors, which can achieve high-performance all-PSCs.

All-polymer indoor photovoltaic modules

SummaryIndoor photovoltaic (IPV) with power output over 100 μW is promising to power the numerous sensor nodes in the Internet of Things (IoT) ecosystem. All polymer photovoltaic has the advantages of excellent thermal stability and superior mechanical properties. In this work, we fabricate the first all-polymer indoor photovoltaic module with the active area of 10 cm2. The module uses polymer donor CD1 and new polymer acceptor PBN-21 with medium optical band gap of 1.9 eV as the active layer. It is processed with eco-friendly solvent tetrahydrofuran and the morphology can be improved by blade coating at 55°C. Under light emitting diode illumination at 1000 lux, the module exhibits a power conversion efficiency of 12.04% and a power output of 367.2 μW. The sufficient power output, high efficiency, excellent stability, and eco-friendly processing indicate that all-polymer indoor photovoltaic is a promising approach to achieve the self-powered of sensor nodes in the IoT ecosystem.

Polymerized small molecular acceptor based all-polymer solar cells with an efficiency of 16.16% via tuning polymer blend morphology by molecular design

All-polymer solar cells (all-PSCs) based on polymerized small molecular acceptors (PSMAs) have made significant progress recently. Here, we synthesize two A-DA’D-A small molecule acceptor based PSMAs of PS-Se with benzo[c][1,2,5]thiadiazole A’-core and PN-Se with benzotriazole A’-core, for the studies of the effect of molecular structure on the photovoltaic performance of the PSMAs. The two PSMAs possess broad absorption with PN-Se showing more red-shifted absorption than PS-Se and suitable electronic energy levels for the application as polymer acceptors in the all-PSCs with PBDB-T as polymer donor. Cryogenic transmission electron microscopy visualizes the aggregation behavior of the PBDB-T donor and the PSMA in their solutions. In addition, a bicontinuous-interpenetrating network in the PBDB-T:PN-Se blend film with aggregation size of 10~20 nm is clearly observed by the photoinduced force microscopy. The desirable morphology of the PBDB-T:PN-Se active layer leads its all-PSC showing higher power conversion efficiency of 16.16%.

N-Type Quinoidal Polymers Based on Dipyrrolopyrazinedione for Application in All-Polymer Solar Cells.

Conjugated molecules and polymers with intrinsic quinoidal structure are promising n-type organic semiconductors, which have been reported for application in field-effect transistors and thermoelectric devices. In principle, the molecular and electronic characteristics of quinoidal polymers can also enable their application in organic solar cells. Herein, two quinoidal polymers, named PzDP-T and PzDP-ffT, based on dipyrrolopyrazinedione were synthesized and used as electron acceptors in all-polymer solar cells (all-PSCs). Both PzDP-T and PzDP-ffT showed suitable energy levels and wide light absorption range that extended to the near-infrared region. When combined with the polymer donor PBDB-T, the resulting all-PSCs based on PzDP-T and PzDP-ffT exhibited a power conversion efficiency (PCE) of 1.33 and 2.37 %, respectively. This is the first report on the application of intrinsic quinoidal conjugated polymers in all-PSCs. The photovoltaic performance of the all-PSCs was revealed to be mainly limited by the relatively poor and imbalanced charge transport, considerable charge recombination. Detailed investigations on the structure-performance relationship suggested that synergistic optimization of light absorption, energy levels, and charge transport properties is needed to achieve more successful application of intrinsic quinoidal conjugated polymers in all-PSCs.

Fine Tuning Miscibility of Donor/Acceptor through Solid Additives Enables All‐Polymer Solar Cells with 15.6% Efficiency

Optimization of the photovoltaic performance of all‐polymer solar cells (all‐PSCs) includes delicate control of the film morphology of the light‐harvesting layer. Although miscibility of polymer donors and polymer acceptors plays a critical role in the description of film morphology of all‐PSCs, the mixing thermodynamics is unrevealed. Herein, we demonstrate that by incorporating 1% weight ratio of PC71BM as the solid additive into the blends of electron‐donating polymer PTzBI‐oF and electron‐accepting polymer PFA1, the miscibility of donor/acceptor can be improved by virtue of forming a favorable phase separation, which leads to an increased charge carrier transport and simultaneously enhanced fill factor. The maximum power conversion efficiency is thereby improved from 14.6% to 15.6%. The miscibility of two components in the photoactive layer can be quantitatively described using the Flory−Huggins interaction parameter (χ). In particular, a correlation between the Flory−Huggins parameters of the two components, in terms of phase separation morphology and device performance of all‐PSCs, is established and the mechanism by which PC71BM is added to this system is explored. This study establishes guidelines for the selection of solid additives when optimizing the efficiency of all‐PSCs and promotes the integration and development of polymer physics and organic photovoltaics.

N-alkyl chain modification in dithienobenzotriazole unit enabled efficient polymer donor for high-performance non-fullerene solar cells

Molecular design of either polymer donors or acceptors is a promising strategy to tune the morphology of the active layer of organic solar cells, enabling a high-performance device. Thereinto, developing novel polymer donors is an alternative method to obtain high photovoltaic performance. Herein, we present a facile side-chain engineering on the dithiophenobenzotriazole (DTBTz) unit of newly-designed polymer donors (named pBDT-DTBTz-EH and pBDT-DTBTz-Me) to boost the performance of non-fullerene solar cells. Compared with pBDT-DTBTz-EH with long N-alkyl side chains, pBDT-DTBTz-Me with a short methyl exhibits stronger molecular aggregation, higher absorption coefficient, and preferred face-on orientation packing. As a consequence, pBDT-DTBTz-Me based devices achieve an optimal power conversion efficiency of 15.31% when donors are paired with the narrow bandgap acceptor Y6, which is superior to that of pBDT-DTBTz-EH based devices (9.17%). Additionally, the pBDT-DTBTz-Me based devices manifest more effective charge separation and transfer than pBDT-DTBTz-EH based devices. These results indicate that fine-tuning side chains of polymer donors provide new insights for the design of high-performance polymer donors in non-fullerene solar cells.

Modulating Chlorination Position on Polymer Donors for Highly Efficient Nonfullerene Organic Solar Cells

Introducing substituent groups has been regarded as an effective method to construct highly efficient polymer donors. However, the correlation between the position of substituent groups and the device performance of polymer donors has rarely been carefully studied and compared. Herein, Cl atoms are introduced into the BDT‐based thienyl side chains and BDD‐based thienyl π‐bridges to obtain two chlorinated donor−acceptor (D−A) polymer donors H1 and H2, respectively. By systematically comparing the photovoltaic properties of H1 and H2, it is found that the device performance of polymer donors is sensitive to the position of chlorine atoms. The nonfullerene organic solar cells (OSCs) based on H1:IT‐4F and H1:Y6 display a superior power conversion efficiency (PCE) of 12.34 and 15.62%, whereas the PCE of H2:IT‐4F and H2:Y6 is 11.04 and 13.80%. As the H1‐based blend shows more desirable aggregation morphology, more preferential face‐on orientation, and more efficient extraction dissociation occurs. The current work demonstrates that the position of chlorine substitution can be reasonably optimized for state‐of‐the‐art polymer donors in the highly efficient nonfullerene OSCs.

Tailoring polymer acceptors by electron linkers for achieving efficient and stable all-polymer solar cells.

The trade-off between efficiency and stability is a bit vague, and it can be tricky to precisely control the bulk morphology to simultaneously improve device efficiency and stability. Herein, three fused-ring conducted polymer acceptors containing furan, thiophene and selenophene as the electron linkers in their conjugated backbones, namely PY-O, PY-S and PY-Se, were designed and synthesized. The electron linker engineering affects the intermolecular interactions of relative polymer acceptors and their charge transport properties. Furthermore, excellent material compatibility was achieved when PY-Se was blended with polymer donor PBDB-T, resulting in nanoscale domains with favorable phase separation. The optimized PBDB-T : PY-Se blend not only exhibits maximum performance with a power conversion efficiency of 15.48%, which is much higher than those of PBDB-T : PY-O (9.80%) and PBDB-T : PY-S (14.16%) devices, but also shows better storage and operational stabilities, and mechanical robustness. This work demonstrates that precise modification of electron linkers can be a practical way to simultaneously actualize molecular crystallinity and phase miscibility for improving the performance of all-polymer solar cells, showing practical significance.

Polymerized small-molecule acceptors based on vinylene as π-bridge for efficient all-polymer solar cells

Two near-infrared (NIR) polymer acceptors (PAs, PYV, and PYV-Tz) based on fused-ring 2,1,3-benzothiadiazole (BT)- or benzotriazole (BTz)-based A′-DAD-A′ structure as electron-deficient-core and vinylene as the linking units, were designed and synthesized. The structure-property relationships of the all-polymer solar cells (all-PSCs) with a wide bandgap PBDB-T as donor were investigated systematically. Compared to the BT-based PA PYV, using BTz unit as the central core in PA PYV-Tz leads to the red-shifted absorption spectra and higher absorption coefficient in the PYV-Tz neat film. In addition, PYV-Tz also exhibits a higher lowest unoccupied molecular orbital (LUMO) energy level and better electron mobility. Consequently, the PBDB-T:PYV-Tz all-PSCs displayed a PCE of 13.02%, under the illumination of AM 1.5G, 100 mW cm-2, which is much higher than that of the PBDB-T:PYV device (11.51%). The measurements of relevant physical dynamics and blend morphologies further demonstrated the photovoltaic performance differences between PBDB-T:PYV and PBDB-T:PYV-Tz. This result indicates that PYV-Tz is a promising polymer acceptor material for the potential application of all-PSCs.

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.

Active Layer Morphology Engineering of All-polymer Solar Cells by Systematically Tuning Molecular Weights of Polymer Donors/Acceptors

In all-polymer solar cells (APSCs), number-average molecular weights (Mn) of polymer donors and polymer acceptors play an important role in active layer morphology and photovoltaic performance. In this manuscript, based on a series of APSCs with power conversion efficiency of approaching 10%, we study the effect of Mn of both polymer donor and polymer acceptor on active layer morphology and photovoltaic performance of APSCs. We select poly[4-(5-(4,8-bis(5-((2-butyloctyl)thio)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole] (CD1) as the polymer donor and poly[4-(5-(5,10-bis(2-dodecylhexadecyl)-4,4,9,9-tetrafluuoro-7-methyl-4,5,9,10-tetrahydro3a,5,8,10-tetraaza-4,9-diborapyren-2-yl)thiophen-2-yl)-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PBN-14) as the polymer acceptor. The Mn of polymer donor CD1 are 14.0, 35.5 and 56.1 kg mol-1, respectively. The Mn of polymer acceptor PBN-14 are 32.7, 72.4 and 103.4 kg mol-1, respectively. To get the desired biscontinueous fibrous network morphololgy of the polymer donor/polymer acceptor blends, at least one polymer should have high or medium Mn. Moreover, when the Mn of polymer acceptor is high, the active layer morphology and APSC device performance are insensitive to the Mn of polymer donor. The optimal APSC device performance is obtained when the Mn of both the polymer donor and the polymer acceptor are medium. These results provide a comprehensive and deep understanding on the interplay and the effect of Mn of polymer donors and polymer acceptors in high-performance APSCs.

Improvement in power conversion efficiency of all-polymer solar cells enabled by ultrafast channels for charge dynamics

Despite the rapid breakthrough of polymerized fused-ring small molecular acceptors, developing accessible and inexpensive polymer acceptors for high performance all-polymer solar cells (all-PSCs) and revealing the fundamental work mechanism of all-PSCs is highly desirable. In addition, the isomeric issue of polymerized fused-ring small molecular acceptors still needs to be concerned. In this work, we report an accessible and inexpensive polymer acceptor P2TF by polymerization of 2,2'-((2Z,2Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b']dith-iophene-2,7-diyl)bis (me-thanylylidene))bis (3-oxo-2,3-dihydro-1H-indene-2,1-diylide-ne)) dimalononitrile-based small molecular. Molecular electrostatic potentials and permanent dipole moments are evaluated by theoretical calculations, revealing that structural isomerization has a negligible impact on their optoelectronic properties. Femtosecond-resolved transient absorption, device charge dynamic measurements and space-charge limited current method are performed to systematically investigate charge dynamics in all-PSCs. The PM6:P2TF-based as-cast device exhibits poor power conversion efficiency of 4.51%, while the optimized device achieves a superior power conversion efficiency of 10.65%. The significantly improved performance of the optimized device originates from ultrafast channels of charge dynamics for ultrafast charge transfer, efficient charge transport and suppressed charge recombination, which is strongly related to the well-established morphology induced by optimal treatment. Furthermore, the PM6:P2TF also has excellent bending property, indicating its great potential for application in flexible all-PSCs.

Fine-tuned crystallinity of polymerized non-fullerene acceptor via molecular engineering towards efficient all-polymer solar cell

Despite remarkable advancement made by virtue of “polymerized non-fullerene acceptor” strategy in all-polymer solar cells (all-PSCs) recently, the tuning of polymer crystallinity via molecular design to optimize the nanostructured blend morphology remains challenging for boosting the short-circuit current density (JSC). Herein, through systematically optimizing the central core and π-spacer, we present a facile method to regulate the solid-state crystallinity of these emerging polymer acceptors. Specifically, we have synthesized a new family of polymerized non-fullerene acceptors named PY-2T and PY-2T2Cl by copolymerizing the Y5-derivative with bithiophene or chlorinated bithiophene. Compared to the previously used IDIC-based polymer named PIDIC-2T, the extended D-A-D fused ring core renders PY-2T with significantly red-shifted optical absorption and up-shifted energy levels, leading to simultaneously improved JSC and open circuit voltage (VOC) in the resultant all-PSCs. More importantly, the chlorinated PY-2T (PY-2T2Cl) endows the desirable phase separated blend morphology with favorable film crystallinity when paired with polymer donor PBDB-T, thus PY-2T2Cl based all-PSCs delivers a promising power conversion efficiency of approaching ~10% with a greatly enhanced JSC of 16.3 mA/cm2 and high VOC of 0.87 eV. This systematic study provides an insight into the effect of central core and π-spacer on the film crystallinity for developing high-performance polymerized non-fullerene acceptors, and also highlights the importance of both absorption and morphology in boosting the desired JSC in all-PSCs.