Performance Of All-polymer Solar Cells Research Articles

Isomerization effect of benzothiophene-substituted benzodithiophene-based donor polymer on the blend morphology and photovoltaic performance of all-polymer solar cells

All-polymer solar cells (all-PSCs) hold great promise to be used as portable and wearable electronic power sources. It is well recognized that a gradient distribution of the two components of...

19% Efficiency All-Polymer Solar Cells via Introducing Pincer-Shaped Non-covalent Bond Interactions

19% Efficiency All-Polymer Solar Cells via Introducing Pincer-Shaped Non-covalent Bond Interactions

A-A Strategy Enables Desirable Performance of All-Polymer Solar Cells Fabricated with Nonhalogenated Solvents.

Most polymer acceptors have been designed by applying a D (electron-rich unit)-A (electron-deficient unit) strategy, which are principally processed with halogenated solvents to fabricate all-polymer solar cells (all-PSCs). Two novel polymer acceptors, containing an A-A type backbone, were designed and synthesized, which can be readily dissolved in o-xylene. The polymer PY-FBTA, comprising a Y6 derivative as the first A unit and a benzotriazole derivative as the second A unit, shows smaller dihedral angles in the backbone, stronger molecular interactions, higher LUMO level, more complementary absorption spectrum, and better morphology with PM6 than the polymer PY-DPP comprising a diketopyrrolopyrrole derivative as the second A unit. Accordingly, the PM6:PY-FBTA all-PSC achieves a higher PCE of 13.95% than the all-PSC based on PM6:PY-DPP (9.51%) for thoroughly improved Jsc (22.34 mA cm-2), Voc (0.963 V), and FF (64.84%) values, which are fabricated with o-xylene as the solvent. This work demonstrates that the A-A structure is a desirable strategy for designing polymer acceptors for efficient all-PSCs prepared with nonhalogenated solvents.

Improving Photovoltaic Performance of All-Polymer Solar Cells by Adding An Amorphous B←N Embedded Polymer as the Third Component.

Currently, most of the disclosed ternary strategies to improve photovoltaic performance of all-polymer solar cells commonly focus on the guest polymers having similar structures with the host polymer donors or acceptors. Herein, we develop a distinctive ternary method that adding an amorphous B←N embedded polymer named BN-Cl-2fT to a crystallized host polymer blend of PM6 and PY-TT (a copolymer of Y6 and thieno[3,2-b]thiophene). Although the structures between BN-Cl-2fT and PM6 and PY-TT are completely different, excellent miscibility is found between BN-Cl-2fT and both of the host PM6 and PY-TT, which can be interpreted by the crowded phenyl groups anchoring along the backbone of BN-Cl-2fT, leading to weak self-aggregation. GIWAXS measurements explicitly confirm the crystallization of PM6 and PY-TT and amorphous feature of BN-Cl-2fT. Furthermore, adding 10 wt% BN-Cl-2fT to PM6:PY-TT could significantly enhance the crystallization of the host polymers. Thus the ternary devices based on PM6:PY-TT:BN-Cl-2fT afford promoted JSC (23.29versus 21.80mA/cm2 ), FF (62.4%versus 60.0%), and PCE (13.70%versus 12.23%) in contrast to these parameters of binary devices based on PM6:PY-TT. We provide a unique and enlightening avenue to design high performance all-PSCs by adding amorphous B←N embedded polymers as guest component to enhance host-crystallization. This article is protected by copyright. All rights reserved.

Suppressing nongeminate recombination with two well-compatible polymer donors enables 16.6% efficiency all-polymer solar cells

The power conversion efficiency of all-polymer solar cells is rising quickly due to the development of polymerized small molecule acceptors. However, optimizing the active layer morphology remains challenging owing to the long and entangled polymer chains; therefore, the power conversion efficiency still lags behind that of organic solar cells composed of small molecule acceptors. Herein, we adopted a ternary strategy to improve the performance of all-polymer solar cells consisting of PM6:PY-IT by introducing D18-Cl as the third component to fine-tune the nanoscale morphology of the active layer. Because D18-Cl has good compatibility with PM6 and more robust crystalline features, adding D18-Cl regulates the donor and acceptor aggregations in the ternary blend films, leading to stronger crystallinity than the PM6:PY-IT films. Consequently, the ternary devices attained a champion power conversion efficiency of 16.6% (16.5% averaged), outperforming their binary counterparts. The improved performance was associated with enhanced fill factor and short circuit current density, resulting from significantly suppressed nongeminate charge recombination. Our results provide an in-depth understanding of the relationship between the active layer morphology and the nongeminate recombination in the ternary system composed of two well-compatible donors.

New Type of Polymerized Small A-D-A Acceptors Constructed by Conjugation Extension in the Branched Direction

The current high-performance polymerized small molecular acceptors (PSMAs) are mainly polymerized through terminal groups of small molecular acceptors (SMAs), which will inevitably damage the preferred stackings of terminal groups and render an inferior photoelectric performance of all-polymer solar cells (all-PSCs). In order to liberate terminal groups for more efficient intermolecular stackings, a new pathway to construct PSMAs is explored by extending molecular conjugation in branched directions, thereby affording two PSMAs of SP-T and SP-TT with thiophene and bithiophene as linkers, respectively. Benefiting from more compact/ordered π–π stackings and superior morphology, the SP-T-based all-PSC exhibits facilitated charge generation/transport and suppressed recombination, thus yielding a greatly improved efficiency of 13.54% compared with 9.76% for the SP-TT-based one. Our work not only develops a new type of PSMA featuring much enhanced molecular packings but also demonstrates its great potential for achieving state-of-the-art all-PSCs through the delicate engineering of linkers.

An improved performance of all polymer solar cells enabled by sequential processing via non-halogenated solvents

Polymer acceptors based on the polymerization of small-molecule acceptor (PSMAs) have developed rapidly in recent years. All-polymer solar cells (all-PSCs) composed of PSMAs and polymer donors not only retain the excellent mechanical properties, but also have better morphological properties than traditional polymer acceptors such as N2200. However, since PSMAs are generally not highly polymerized, their electron mobilities are generally lower than the hole mobilities of the high-molecular-weight polymer donors they are used with. In this work, we apply the sequential processing (SqP) method and use toluene as the processing solvent to prepare PSMA-based all-PSCs, which effectively improves the electron mobility and shows much higher efficiency (15.8 %) than the SqP device prepared from chloroform (13.9 %) – the conventional solvent used for high-performance PSMAs. Not only that, the efficiency of the toluene-processed SqP device is also higher than that of the device prepared by the traditional blending-casting (BC) method (14.2 %). In addition, for the first time, we reveal the film formation process of all-polymer active layers prepared by the SqP method using the in situ UV-Vis absorption technique, and investigate the vertical phase separation and the rate of exciton generation across the device using film-depth-dependent absorption spectroscopy, which systematically explained the effect of increased mobility on device parameters. This methodology is applied in two different all-polymer systems, namely, PM6:PYF-T-o and PM6:PY-IT, and both systems manifest the same conclusion.

Fused perylene diimide-based polymeric acceptors with different π-conjugation and molecular conformation in all polymer solar cells

Two perylene diimide (PDI) based polymeric acceptors, PFPDIBT-TT2F and PFPDIT-TT2F, which consist of fused-PDI moieties (FPDIBT and FPDIT) and 3,3′-difluoro-2,2′-bithiophene (TT2F) units, has been designed and synthesized. Compared with the unfused-PDI polymer (PPDI-TT2F) as the control molecule, the influence of structural conformation and π-conjugation extension on the performance of all-polymer solar cells has been investigated. Due to both the extended π-conjugation and the torsional polymer skeleton, the device adopting PFPDIT-TT2F achieved an obviously improved PCE of 5.11% comparing with the devices using the other two polymeric acceptors. This study demonstrates that the fused PDIs bridged by a thiophene enables both extended π-conjugation and twisted molecular conformation, and the latter plays a dominant role in the improvement of device performance.

A Vinylene-Linker-Based Polymer Acceptor Featuring a Coplanar and Rigid Molecular Conformation Enables High-Performance All-Polymer Solar Cells with Over 17% Efficiency.

State-of-art Y-series polymer acceptors are typically based on a mono-thiophene linker, which can cause some twisted molecular conformations and thus limit the performance of all-polymer solar cells (all-PSCs). Here, a high-performance polymer acceptor based on vinylene linkers is reported, which leads to surprising changes in the polymers' molecular conformations, optoelectronic properties, and enhanced photovoltaic performance. It is found that the polymer acceptors based on thiophene or bithiophene linkers (PY-T-γ and PY-2T-γ) display significant molecular twisting between end-groups and linker units, while the vinylene-based polymer (PY-V-γ) exhibits a more coplanar and rigid molecular conformation. As a result, PY-V-γ demonstrates a better conjugation and tighter interchain stacking, which results in higher mobility and a reduced energetic disorder. Furthermore, detailed morphology investigations reveal that the PY-V-γ-based blend exhibits high domain purity and thus a better fill factor in its all-PSCs. With these, a higher efficiency of 17.1% is achieved in PY-V-γ-based all-PSCs, which is the highest efficiency reported for binary all-PSCs to date. This work demonstrates that the vinylene-linker is a superior unit to build polymer acceptors with more coplanar and rigid chain conformation, which is beneficial for polymer aggregation and efficient all-PSCs.

Reassessing the Significance of Reduced Aggregation and Crystallinity of Naphthalene Diimide-Based Copolymer Acceptors in All-Polymer Solar Cells

Synthesizing copolymer acceptors based on a mix of three co-monomers is a facile and effective strategy to control the aggregation and crystallinity of semiconducting polymers which has been exploited to improve the photovoltaic performance of all-polymer solar cells (all-PSCs). Applying this strategy to the well-studied electron-transporting polymer acceptor PNDI2OD-T2, different amounts of 3-octylthiophene (OT) are used to partially replace the bithiophene (T2) unit, resulting in three copolymer acceptors PNDI-OTx where x = 5, 10, or 15%. Another polymer, namely PNDI2OD-C8T2, consisting of naphthalene diimide (NDI) polymerized with 3-octyl-2,2′-bithiophene (C8T2) is also synthesized for comparison. It is found that the solution aggregation and thin-film crystallinity of PNDI-OTx are systematically tuned by varying x, evidenced by temperature-dependent UV–vis and grazing incidence wide-angle X-ray scattering measurements. PNDI2OD-C8T2 is also found to have reduced solution aggregation and thin-film crystallinity relative to PNDI2OD-T2. However, the photovoltaic performance of all-PSCs based on J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends are much lower than that of the reference J71:PNDI2OD-T2 system. Extensive morphological studies indicate that reduced aggregation and crystallinity do not guarantee a more favorable blend morphology, with coarser phase separation found in J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends compared to J71:PNDI2OD-T2 blends. Furthermore, the OT-modified copolymers with reduced crystallinity are found to have reduced electron mobilities. The results here suggest that reduced aggregation and less crystallinity of random copolymer acceptors do not always produce favorable morphology in polymer/polymer blends and do not guarantee for improvement in the photovoltaic performance.

Conquering the morphology barrier of ternary all-polymer solar cells by designing random terpolymer for constructing efficient binary all-polymer solar cells

The ternary strategy based on two electron donors and one electron acceptor can effectively expand the absorption and photovoltaic performance of all-polymer solar cells (all-PSC). However, it is still challenging to realize the ideal morphology of the ternary blend film through reasonable molecular design. To overcome the morphology barrier of ternary all-PSC, here we replaced physical blending of two donors (P1 and P2) by chemical random ternary copolymerization obtaining a terpolymer namely P1-co-25%P2. The binary all-PSCs constructed by integrating P1-co-25%P2 with a polymerized non-fullerene acceptor PYFT exhibited an impressively high power conversion efficiency (PCE) of 14.67%, which obviously outperformed those of obtained from ternary all-PSCs based on P1:P2:PYFT. Further morphology characterization of these blend films revealed that the incorporation of the P2 unit in the P1 backbone can optimize the π-π stacking, miscibility and phase separation that beneficial to exciton diffusion and charge transport, illustrating the unparalleled superiority of the random ternary copolymerization strategy compared to the ternary blending strategy.

Effects of fluorination position on all-polymer organic solar cells

In organic solar cells, fluorination has a great influence on the performance of non-fullerene acceptors, however there are very few studies on the effects of fluorination position on polymerized small molecule acceptors. In this article, two polymer acceptor materials (PAL1 and PAL2) with different fluorine substitution positions are investigated to study the influence of fluorinated forms on polymerized small molecule acceptor materials and the all-polymer solar cell performance. The end group fluorinated polymer acceptor material PAL1 and the π-bridge fluorinated polymer acceptor material PAL2 have similar absorption spectra and energy levels. While the blend film based on the polymer donor PM6:PAL1 has smaller roughness, finer phase separation, and tighter molecular packing, compared to the blend film of PM6:PAL2. Thus PM6:PAL1-based all-polymer organic solar cells (all-PSCs) have longer carrier life, faster carrier mobilities, and less bimolecular recombination.These polymer properties can simultaneously enhance the Jsc and FF of PM6:PAL1 based devices, finally achieving a device efficiency of 13.53%, which is significantly better than PM6:PAL2 based devices (9.16%). More importantly, the device based on PM6:PAL1 has excellent stability. When stored in a nitrogen atmosphere for 90 days, the PCE attenuation is only 4.2%. This work enriches the types of polymer acceptor materials and provides new ideas for the design of high-performance all-PSCs.

Improving the fill factor of N2200-based all polymer solar cells by introducing EPPDI as a solid additive

A cheap and commercially available small molecule (namely EPPDI) is introduced to the active layer of N2200-based all polymer solar cells as a solid additive. EPPDI at the optimal ratio can improve the D-A nano-scale morphology and reduce trap density of the active layer by filling morphological spaces. As a result, the photovoltaic performance of the resulting devices based on PF2:N2200 are increased from 6.28% to 7.03% with significantly enhanced fill factor. This work demonstrates a facile approach for improving the performance of all polymer solar cells. • A cheap and commercially available small molecular solid additive. • No influence on the original polymer thin film order. • Reduce the morphological traps in active layer. • Improve the fill factor and power conversion efficiency of all polymer solar cells.

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.

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.

Regio-Regular Polymer Acceptors Enabled by Determined Fluorination on End Groups for All-Polymer Solar Cells with 15.2 % Efficiency.

Polymerization sites of small molecule acceptors (SMAs) play vital roles in determining device performance of all-polymer solar cells (all-PSCs). Different from our recent work about fluoro- and bromo- co-modified end group of IC-FBr (a mixture of IC-FBr1 and IC-FBr2), in this paper, we synthesized and purified two regiospecific fluoro- and bromo- substituted end groups (IC-FBr-o & IC-FBr-m), which were then employed to construct two regio-regular polymer acceptors named PYF-T-o and PYF-T-m, respectively. In comparison with its isomeric counterparts named PYF-T-m with different conjugated coupling sites, PYF-T-o exhibits stronger and bathochromic absorption to achieve better photon harvesting. Meanwhile, PYF-T-o adopts more ordered inter-chain packing and suitable phase separation after blending with the donor polymer PM6, which resulted in suppressed charge recombination and efficient charge transport. Strikingly, we observed a dramatic performance difference between the two isomeric polymer acceptors PYF-T-o and PYF-T-m. While devices based on PM6:PYF-T-o can yield power conversion efficiency (PCE) of 15.2 %, devices based on PM6:PYF-T-m only show poor efficiencies of 1.4 %. This work demonstrates the success of configuration-unique fluorinated end groups in designing high-performance regular polymer acceptors, which provides guidelines towards developing all-PSCs with better efficiencies.

Optimized active layer morphology via side-chain atomic substituents to achieve efficient and stable all-polymer solar cells

The device performance of all-polymer solar cells can be improved via optimizing phase separation morphology through side-chain atomic substituents.

Effect of Alkyl Side Chains of Polymer Donors on Photovoltaic Performance of All-Polymer Solar Cells

In this manuscript, we investigate the effect of alkyl side chains of polymer donors on photovoltaic performance of all-polymer solar cells (all-PSCs) based on polymer acceptors containing a boron–...

Impact of Chlorination Patterns of Naphthalenediimide-Based Polymers on Aggregated Structure, Crystallinity, and Device Performance of All-Polymer Solar Cells and Organic Transistors.

The aggregation properties of conjugated polymers can play a crucial role in their thin film structures and performance of electronic devices. Control of these aggregated structures is particularly important in producing efficient all-polymer solar cells (all-PSCs), considering that strong demixing of the polymer donor and polymer acceptor typically occurs during film formation because of the low entropic contribution to the thermodynamics of the system. Here, three naphthalenediimide (NDI)-based polymer acceptors with different backbone chlorination patterns are developed to investigate the effect of the chlorination patterns on the aggregation tendencies of the polymer acceptors, which greatly influence their crystalline structures, electrical properties, and device performances of the resultant all-PSCs and organic field-effect transistors (OFETs). The counterparts of NDI units, dichlorinated bithiophene (Cl2T2), monochlorinated bithiophene (ClT2), and dichlorinated thienylene-vinylene-thienylene (Cl2TVT), are employed to synthesize a series of P(NDIOD-Cl2T2), P(NDIOD-ClT2), and P(NDIOD-Cl2TVT) polymers. The P(NDIOD-Cl2T2) polymer takes advantage of strong noncovalent bonding induced by its chlorine substituents, resulting in the formation of optimal face-on oriented crystalline structures which are suitable for efficient all-PSC devices. In comparison, the P(NDIOD-Cl2TVT) polymer forms bimodal crystalline structures in thin films to yield optimal performances in the resultant OFETs. When the three chlorinated polymers are applied to all-PSCs with the PBDTTTPD polymer donor, P(NDIOD-Cl2T2) achieves a maximum power conversion efficiency (PCE) of 7.22% with an appropriate blend morphology and high fill factor, outperforming P(NDIOD-ClT2) (PCE = 4.80%) and P(NDIOD-Cl2TVT) (PCE = 5.78%). Our observations highlight the effectiveness of the chlorination strategy for developing efficient polymer acceptors and demonstrate the important role of polymer aggregation in modulating the blend morphology and all-PSC performance.

Influence of 3D morphology on the performance of all-polymer solar cells processed using environmentally benign nonhalogenated solvents

Advancing the photovoltaic performance of all-polymer solar cells (APSCs) requires a comprehensive understanding of the dependence of the properties of photoactive blend films on the solvents used to process them. We have studied the correlation between the morphology of PTB7-Th:PNDI2OD-T2 blend films prepared using six different processing solvents. Three-dimensional (3D) transmission electron microscopy tomography (TEMT) revealed the influence of the 3D morphology of the PTB7-Th:PNDI2OD-T2 blend films on their photovoltaic performance. The high-performing toluene (TOL)- and o -xylene (XY)-processed blend films displayed power conversion efficiencies (PCEs) of ~4%, which contained nanoscopically networked fibrous PNDI2OD-T2 polymer domains. The films processed using the other solvents exhibited partly disconnected domains or unfavorable vertical phase separation. In addition, the use of N -dimethylformamide (DMF), N , N -dimethylacetamide (DMAc), or N -methyl-2-pyrrolidone (NMP) as a solvent additive in XY solution resulted in a substantial improvement in the PCEs to 6.1%. This large enhancement was attributed to the formation of the better-interconnected narrower crystalline fibrillary structure of PNDI2OD-T2 chains with the aid of solvent additives, as clearly elucidated by 3D-TEMT imaging. This work highlights that APSCs processed from benign nonhalogenated solvents with appropriate additives can outperform APSCs processed from toxic halogenated solvents by affording a favorable blend morphology and that 3D TEMT imaging is a powerful and essential tool for characterizing the true morphology of photoactive layers. • Three-dimensional transmission electron microscopy revealed nanoscopic morphology. • 3D morphology is a key in determining the performance of all-polymer solar cells. • Environmentally benign nonhalogenated solvents performed better than halogen solvents. • The use of DMAc solvent additive greatly improved the photovoltaic.