High-Performance Nonfullerene Polymer Solar Cells based on Imide-Functionalized Wide-Bandgap Polymers

The Double-Cross of Benzotriazole-Based Polymers as Donors and Acceptors in Non-Fullerene Organic Solar Cells

Organic solar cells (OSCs) are considered a very promising technology to convert solar energy to electricity and a feasible option for the energy market because of the advantages of light weight, flexibility, and roll-to-roll manufacturing. They are mainly characterized by a bulk heterojunction structure where a polymer donor is blended with an electron acceptor. Their performance is highly affected by the design of donor-acceptor conjugated polymers and the choice of suitable acceptor. In particular, benzotriazole, a typical electron-deficient penta-heterocycle, has been combined with various donors to provide wide bandgap donor polymers, which have received a great deal of attention with the development of non-fullerene acceptors (NFAs) because of their suitable matching to provide devices with relevant power conversion efficiency (PCE). Moreover, different benzotriazole-based polymers are gaining more and more interest because they are considered promising acceptors in OSCs. Since the development of a suitable method to choose generally a donor/acceptor material is a challenging issue, this review is meant to be useful especially for organic chemical scientists to understand all the progress achieved with benzotriazole-based polymers used as donors with NFAs and as acceptors with different donors in OSCs, in particular referring to the PCE.

High Polymer Molecular Weight Yields Solar Cells with Simultaneously Improved Performance and Thermal Stability

Simple synthetic routes, high active layer thickness tolerance as well as stable organic solar cells are relentlessly pursued as key enabling traits for the upscaling of organic photovoltaics. Here, the potential to address these issues by tuning donor polymer molecular weight is investigated. Specifically, the focus is on PTQ10, a polymer with low synthetic complexity, with number average molecular weights of 2.4, 6.2, 16.8, 52.9, and 54.4 kDa, in combination with three different non-fullerene acceptors, namely Y6, Y12, and IDIC. Molecular weight, indeed, unlocks a threefold increase in power conversion efficiency for these blends. Importantly, efficiencies above 10% for blade coated devices with thicknesses between 200 and 350 nm for blends incorporating high molecular weight donor are shown. Spectroscopic, GIWAXS and charge carrier mobility data suggest that the strong photocurrent improvement with molecular weight is related to both, improved electronic transport and polymer contribution to exciton generation. Moreover, it is demonstrated that solar cells based on high molecular weight PTQ10 are more thermally stable due to a higher glass transition temperature, thus also improving device stability.

γ-Ester-Functionalized 1,1-Dicyanomethylene-3-indanone End-Capped Nonfullerene Acceptors for High-Performance, Annealing-Free Organic Solar Cells

Modifying the end-capping groups in nonfullerene acceptors (NFAs) is an effective strategy for modulating their properties and that of the entire NFAs. This study reports the synthesis of a novel γ-ester-functionalized IC end-capping group (IC-γe) and its incorporation into the benzothiadiazole-fused central core, yielding isomer-free IC-γe end-capped NFAs, such as Y-IC-γe, Y-FIC-γe, and Y-ClIC-γe. The resultant NFAs exhibited similar absorption profiles but upshifted the lowest unoccupied molecular orbital energy level compared with those of the ester-free analogues, such as Y6 and Y7. Without thermal annealing, an excellent power conversion efficiency (PCE) of 16.4% is realized in the annealing-free OSC based on Y-FIC-γe with the PM6 donor polymer, which outperforms the OSCs based on Y-IC-γe and Y-ClIC-γe. In addition, the OSCs based on asymmetric Y-FIC-γe and Y-ClIC-γe have higher thermal stability with more than 83% PCE retention at an elevated temperature after 456 h than the symmetric Y-IC-γe case. In this study, we not only establish the structure-property relationship regarding the ester functionality and symmetricity tuning on the NFAs but also diagnose the reasons for the best-performing Y-FIC-γe-based OSCs, providing useful information for a novel high-performing NFA design strategy.

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

Highly planar, extended π-electron organic conjugated polymers have been increasingly attractive for achieving high-mobility organic semiconductors. In addition to the conventional strategy to construct rigid backbone by covalent bonds, hydrogen bond has been employed extensively to increase the planarity and rigidity of polymer via intramolecular noncovalent interactions. This review provides a general summary of high-mobility semiconducting polymers incorporating hydrogen bonds in field-effect transistors over recent years. The structural engineering of the hydrogen bond-containing building blocks and the discussion of theoretical simulation, microstructural characterization, and device performance are covered. Additionally, the effects of the introduction of hydrogen bond on self-healing, stretchability, chemical sensitivity, and mechanical properties are also discussed. The review aims to help and inspire design of new high-mobility conjugated polymers with superiority of mechanical flexibility by incorporation of hydrogen bond for the application in flexible electronics.

Efficient Organic Solar Cells Enabled by Simple Non-Fused Electron Donors with Low Synthetic Complexity

Fused-ring electron donors boost the efficiency of organic solar cells (OSCs), but they suffer from high cost and low yield for their large synthetic complexity (SC > 30%). Herein, the authors develop a series of simple non-fused-ring electron donors, PF1 and PF2, which alternately consist of furan-3-carboxylate and 2,2'-bithiophene. Note that PF1 and PF2 present very small SC of 9.7% for their inexpensive raw materials, facile synthesis, and high synthetic yield. Compared to their all-thiophene-backbone counterpart PT-E, two new polymers feature larger conjugated plane, resulting in higher hole mobility for them, especially a value up to ≈10^-4 cm² V^-1 ·s for PF2 with longer alkyl side chain. Meanwhile, PF1 and PF2 exhibit larger dielectric constant and deeper electronic energy level versus PT-E. Benefiting from the better physicochemical properties, the efficiencies of PF1- and PF2-based devices are improved by ≈16.7% and ≈71.3% relative to that PT-E-based devices, respectively. Furthermore, the optimized PF2-based devices with introducing PC₇₁ BM as the third component deliver a higher efficiency of 12.40%. The work not only indicates that furan-3-carboxylate is a simple yet efficient building block for constructing non-fused-ring polymers but also provides a promising electron donor PF2 for the low-cost production of OSCs.

New Organic Materials Based on Multitask 2H-benzo[d]1,2,3-triazole Moiety

Multifunctionality is a desirable aspect in materials science. Indeed, the development of multifunctional compounds is crucial for sustainable chemistry by saving resources and time. In this sense, 2H-benzo[d]1,2,3-triazole (BTz) is an excellent candidate with promising characteristics, including its ability to self-assemble; its acceptor character, which enables the synthesis of donor-acceptor structures; and its facile modulation using standard chemical methods. Thus, due to its interesting properties, it is possible to produce different derivatives with applications in different fields, as summarized in this article, with the correct substitution at the BTz cores. Optoelectronic or biomedical applications, amongst others, are highlighted.

Recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells

Organic semiconductor materials, especially donor-acceptor (D-A) polymers, have been increasingly applied in organic optoelectronic devices, such as organic field-effect transistors (OFETs) and organic solar cells (OSCs). Plenty of high-performance OFETs and OSCs have been achieved based on varieties of structurally modified D-A polymers. As the basic building block of D-A polymers, acceptor moieties have drawn much attention. Among the numerous types, lactam- and imide-functionalized electron-deficient building blocks have been widely investigated. In this review, the structural evolution of lactam- or imide-containing acceptors (for instance, diketopyrrolopyrrole, isoindigo, naphthalene diimide, and perylene diimide) is covered and their representative polymers applied in OFETs and OSCs are also discussed, with a focus on the effect of varied structurally modified acceptor moieties on the physicochemical and photoelectrical properties of polymers. Additionally, this review discusses the current issues that need to be settled down and the further development of new types of acceptors. It is hoped that this review could help design new electron-deficient building blocks, find a more valid method to modify already reported acceptor units, and achieve high-performance semiconductor materials eventually.

The Crystallinity Control of Polymer Donor Materials for High-Performance Organic Solar Cells

Bulk heterojunction (BHJ) organic solar cells (OSCs) can be regarded as one of the most promising energy generation technologies for large-scale applications. Despite their several well-known drawbacks, the devices where polymers are employed as the donor are still leading the OSC universe in terms of performance. Such performance generally depends upon various critical factors such as the crystallinity of the material, the crystallization process during the film formation, and also the final film morphology. Despite a few reviews on the structure of the polymer donor materials and device performance, not enough attention has been paid toward the crystallinity problem. Herein, the structure and crystallinity of the representative polymer donor materials and the corresponding device properties have been briefly reviewed. Furthermore, several typical methods for controlling the crystallinity of materials have been summarized and illustrated as well. Moreover, the obstacles lying in the way of successful commercialization of such polymer solar cells have been systematically discussed. The in-depth interpretation of the crystallinity of the polymer donors in this article may stimulate novel ideas in material design and device fabrication.

n-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications

The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (OPVs) have attracted broad interest because of their solution processability, flexibility, light weight, and potential for large-area processing. The development of OPV materials, especially electron acceptors, has been one of the focuses in recent years. Compared with fullerene derivates, n-type non-fullerene molecules have some unique merits, such as synthetic simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last 5 years, organic solar cells based on n-type non-fullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, which is superior to those of fullerene-based solar cells; meanwhile, n-type non-fullerene molecules have created brand new opportunities for the application of OPVs in some special situations. This Perspective analyzes the key design strategies of high-performance n-type molecular photovoltaic materials and highlights instructive examples of their various applications, including in ternary and tandem solar cells, high-efficiency semitransparent solar cells for power-generating building facades and windows, and indoor photovoltaics for driving low-power-consumption devices. Moreover, to accelerate the pace toward commercialization of OPVs, the existing challenges and future directions are also reviewed from the perspectives of efficiency, stability, and large-area fabrication.

Recent progress in wide bandgap conjugated polymer donors for high-performance nonfullerene organic photovoltaics

This feature article summarizes our recent achievements in the development of wide bandgap polymer donors as high-performance organic photovoltaics.

A Narrow-Bandgap n-Type Polymer Semiconductor Enabling Efficient All-Polymer Solar Cells

Currently, n-type acceptors in high-performance all-polymer solar cells (all-PSCs) are dominated by imide-functionalized polymers, which typically show medium bandgap. Herein, a novel narrow-bandgap polymer, poly(5,6-dicyano-2,1,3-benzothiadiazole-alt-indacenodithiophene) (DCNBT-IDT), based on dicyanobenzothiadiazole without an imide group is reported. The strong electron-withdrawing cyano functionality enables DCNBT-IDT with n-type character and, more importantly, alleviates the steric hindrance associated with typical imide groups. Compared to the benchmark poly(naphthalene diimide-alt-bithiophene) (N2200), DCNBT-IDT shows a narrower bandgap (1.43 eV) with a much higher absorption coefficient (6.15 × 10⁴ cm^-1 ). Such properties are elusive for polymer acceptors to date, eradicating the drawbacks inherited in N2200 and other high-performance polymer acceptors. When blended with a wide-bandgap polymer donor, the DCNBT-IDT-based all-PSCs achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm. Such efficiency greatly outperforms those of N2200 (6.13%) and the naphthalene diimide (NDI)-based analog NDI-IDT (2.19%). This work breaks the long-standing bottlenecks limiting materials innovation of n-type polymers, which paves a new avenue for developing polymer acceptors with improved optoelectronic properties and heralds a brighter future of all-PSCs.

Large-Area Organic Solar Cells: Material Requirements, Modular Designs, and Printing Methods

The printing of large-area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small-area devices is approaching 15%, whereas the PCE for large-area devices has also surpassed 10% in a single cell with an area of ≈1 cm² . Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large-area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large-area fabrication, including knife coating, slot-die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick-film material systems with efficient modular designs exhibiting low-efficiency losses and employing the right printing methods, the fabrication of large-area OSCs will be successfully realized in the near future.

Efficient Non-Fullerene Organic Solar Cells Based on a Wide-Bandgap Polymer Donor Containing an Alkylthiophenyl-Substituted Benzodithiophene Moiety

Two wide-bandgap polymer donors containing an alkylthiophenyl substituted benzo[1,2-b : 4,5-b']dithiophene moiety, namely PTZPO and PTZPS, were designed and synthesized. Both polymers exhibit relatively wide optical bandgap of 1.95 V with similar absorption profiles. The polymer PTZPS with alkylthiophenyl substituted benzo[1,2-b : 4,5-b']dithiophene units showed enhanced light-harvesting capabilities, leading to improved short-circuit current densities. The PTZPS : ITIC film shows more appreciable film morphology and phase separation than the film composed of a blend of ITIC with alkoxyl substitutions containing copolymer PTZPO, which facilitates exciton dissociation and charge transport. The PTZPS : ITIC-based non-fullerene organic solar cells show clearly improved short-circuit current density and an impressively high power conversion efficiency of more than 11 %. These observations demonstrate the great promise of using PTZPS as electron-donating materials for high-performance non-fullerene organic solar cells.

Impact of the Siloxane-Terminated Side Chain on Photovoltaic Performances of the Dithienylbenzodithiophene-Difluorobenzotriazole-Based Wide Band Gap Polymer Donor in Non-Fullerene Polymer Solar Cells

To thoroughly disclose the role of the siloxane-terminated side chain with different substituent positions, three difluorobenzotriazole-dithienylbenzodithiophene (FTAZ-BDTT)-based polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si with the siloxane-terminated side chain on the FTAZ unit (PBZ-1Si), on the BDTT unit (PBZ-2Si), and both on BDTT and FTAZ units (PBZ-3Si), respectively, were synthesized. The different side chain substitutions have slight influences on absorption behavior, thermal stability, and frontier molecular orbitals but have shown a great effect on the aggregation of the polymers. Grazing-incidence wide-angle X-ray scattering measurements reveal that, relative to PBZ-1Si with branched alkyl on the BDTT unit, polymers PBZ-2Si and PBZ-3Si, bearing the siloxane-terminated side chains on the BDTT unit, exhibit smaller π-π stacking distances and larger crystal coherence lengths, suggesting that adopting the siloxane-terminated side chain on the BDTT unit can promote the interchain π-π interaction and the ordering of molecular packing. With IT-M as the non-fullerene acceptor, among the three polymers, the PBZ-2Si-based active layer possesses the highest ordered crystals for both polymers and IT-M as well as the purest domain, which affords efficient exciton dissociation, the most balanced hole-electron transport, and reduced recombination, leading to the highest short-circuit current density (J_sc) and fill factor (FF) and then the highest power conversion efficiency (PCE) of 11.14%. In contrast, PBZ-1Si- and PBZ-3Si-based devices show lower PCEs of 8.98 and 9.92%, respectively. Moreover, PBZ-2Si:IT-M also exhibits good thickness tolerance, and its thick active layer of 240 nm shows the most limited decrease of efficiency after 77 days of storage, supplying good potential for mass fabrication. Our work suggests that the fine pairing of a siloxane-terminated side chain and an alkyl side chain is beneficial for the optimizing of a conjugated polymer donor toward high-performance non-fullerene polymer solar cells.

Effect of Flank Rotation on the Photovoltaic Properties of Dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene-Based Narrow Band Gap Copolymers

Side chain engineering has been an effective approach to modulate the solution processability, optoelectronic properties and miscibility of conjugated polymers (CPs) for organic/polymeric photovoltaic cells (PVCs). As compared with the most commonly used method of introducing alkyl chains, the employment of alkyl-substituted aryl flanks would provide two-dimensional (2-D) CPs having solution processability alongside additional merits like deepened highest occupied molecular orbital (HOMO) energy levels, increased absorption coefficient and charger transporting, etc. In this paper, the triple C≡C bond was used as conjugated linker to decrease the steric hindrance between the flanks of 4,5-didecylthien-2-yl (T) and dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) core. In addition, an alternating CP derived from 4,5-didecylthien-2-yl-ethynyl (TE) flanked DTBDT, and 4,9-bis(4-octylthien-2-yl) naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (DTNT), named as PDTBDT-TE-DTNT, was synthesized and characterized. As compared with the controlled PDTBDT-T-DTNT, which was derived from 4,5-didecylthien-2-yl flanked DTBDT and DTNT, the results for exciton dissociation probability, density functional theory (DFT), time-resolved photoluminescence (PL) measurements, etc., revealed that the lower steric hindrance between TE and DTBDT might lead to the easier rotation of the TE flanks, thus contributing to the decrease of the exciton lifetime and dissociation probability, finally suppressing the short-circuit current density (JSC), etc., of the photovoltaic devices from PDTBDT-TE-DTNT.

Butterfly architecture of NIR Aza-BODIPY small molecules decorated with phenothiazine or phenoxazine

This is the first report on the highest efficiency NIR absorbing Aza-Bodipy small molecules.

Steady Enhancement in Photovoltaic Properties of Fluorine Functionalized Quinoxaline-Based Narrow Bandgap Polymer

To investigate the influence of fluoride phenyl side-chains onto a quinoxaline (Qx) unit on the photovoltaic performance of the narrow bandgap (NBG) photovoltaic polymers, herein, two novel NBG copolymers, PBDTT-DTQx and PBDTT-DTmFQx, were synthesized and characterized. 2-ethylhexylthiothiophene-substituted benzodithiophene (BDTT), 2,3-diphenylquinoxaline (DQx) [or 2,3-bis(3-fluorophenyl)quinoxaline (DmFQx)] and 2-ethylhexylthiophene (T) were used as the electron donor (D) unit, electron-withdrawing acceptor (A) unit and π-bridge, respectively. Compared to non-fluorine substituted PBDTT-DTQx, fluoride PBDTT-DTmFQx exhibited a wide UV-Vis absorption spectrum and high hole mobility. An enhanced short-circuit current (J_sc) and fill factor (FF) simultaneously gave rise to favorable efficiencies in the polymer/PC₇₁BM-based polymer solar cells (PSCs). Under the illumination of AM 1.5G (100 mW cm^-2), a maximum power conversion efficiency (PCE) of 6.40% was achieved with an open-circuit voltage (V_oc) of 0.87 V, a J_sc of 12.0 mA cm^-2 and a FF of 61.45% in PBDTT-DTmFQx/PC₇₁BM-based PSCs, while PBDTT-DTQx-based devices also exhibited a PCE of 5.43%. The excellent results obtained demonstrate that PBDTT-DTmFQx by fluorine atom engineering could be a promising candidate for organic photovoltaics.

Near-Infrared Electron Acceptors with Fluorinated Regioisomeric Backbone for Highly Efficient Polymer Solar Cells

Solar photon-to-electron conversion with polymer solar cells (PSCs) has experienced rapid development in the recent few years. Even so, the exploration of molecules and devices in efficiently converting near-infrared (NIR) photons into electrons remains critical, yet challenging. Herein presented is a family of near-infrared nonfullerene acceptors (NIR NFAs, T1-T4) with fluorinated regioisomeric A-Aπ-D-Aπ-A backbones for constructing efficient single-junction and tandem PSCs with photon response up to 1000 nm. It is found that the tuning of the regioisomeric bridge (Aπ) and fluoro (F)-substituents on a molecular skeleton strongly influences the backbone conformation and conjugation, leading to the optimized optoelectronic and stable stacking of resultant NFAs, which eventually impacts the performance of derived PSCs. In PSCs, the proximal NFAs with varied F-atoms (T1-T3) mostly outperform than that of distal NFA (T4). Notably, single-junction PSC with PTB7-Th:T2 blend can reach 10.87% power conversion efficiency (PCE), after implementing a solvent additive to improve blend morphology. Moreover, efficient tandem PSCs are fabricated through integrating such NIR cells with mediate bandgap nonfullerene-based subcells, to achieve a PCE of 14.64%. The results reveal the structural design of organic semiconductor and device with improved photovoltaic performance.

Fluorinated Thieno[2',3':4,5]benzo[1,2-d][1,2,3]triazole: New Acceptor Unit To Construct Polymer Donors

A new acceptor unit, fluorinated thieno[2',3':4,5]benzo[1,2-d][1,2,3]triazole (fBTAZT), has been designed and synthesized to build two donor-acceptor (D-A) copolymers with the none/fluorinated benzodithiophene (BDT) unit, which have been applied as the electron-donating material with ITIC as an electron-accepting material to fabricate the nonfullerene polymer solar cells (PSCs). It is found that fluorination at the BTAZT unit and BDT unit exerts a significant influence on photophysical properties and photovoltaic performances of the PSCs. As a result, when the fluorine atom is introduced both into the BTAZT unit and the side-chain thiophene ring of the BDT unit, the corresponding polymer PfBTAZT-fBDT exhibits deeper highest occupied molecular orbital-lowest unoccupied molecular orbital energy level and shows stronger interchain interaction with a little broad and red-shift absorption and high charge mobilities as well as good phase-separated morphologies, thus leading to higher power conversion efficiency of 6.59% in nonfullerene PSCs compared with another polymer PfBTAZT-BDT without F atom at the BDT unit, indicating that fBTAZT can be acted as a medium strong organic acceptor to build D-A polymer donor for high efficient PSCs.

Efficient Non-fullerene Organic Solar Cells Enabled by Sequential Fluorination of Small-Molecule Electron Acceptors

Three small-molecule non-fullerene electron acceptors containing different numbers of fluorine atoms in their end groups were designed and synthesized. All three acceptors were found to exhibit relatively narrow band gaps with absorption profiles extending into the near-infrared region. The fluorinated analog exhibited enhanced light-harvesting capabilities, which led to improved short-circuit current densities. Moreover, fluorination improved the blend film morphology and led to desirable phase separation that facilitated exciton dissociation and charge transport. As a result of these advantages, organic solar cells based on the non-fullerene acceptors exhibited clearly improved short-circuit current densities and power conversion efficiencies compared with the device based on the non-fluorinated acceptor. These results suggest that fluorination can be an effective approach for the molecular design of non-fullerene acceptors with near-infrared absorption for organic solar cells.

Improved Efficiency of Polymer Solar Cells by Modifying the Side Chain of Wide-Band Gap Conjugated Polymers Containing Pyrrolo[3,4- f]benzotriazole-5,7(6 H)-dione Moiety

Two novel wide-band gap donor-acceptor-type conjugated copolymers, PTzBI-S and PTzBI-Ph, are designed and synthesized, based on alkylthio-thienyl- or alkylphenyl-substituted benzodithiophene (BDT) derivatives as the electron-donating unit and pyrrolo[3,4- f]benzotriazole-5,7(6 H)-dione as the electron-withdrawing unit. The as-generated copolymers show the comparable optical and electrochemical properties. The alkylthio-thienyl-substituted BDT unit facilities a benign decrease of the highest occupied molecular orbital (HOMO) levels. This consequently enhances open-circuit voltages ( V_OC) over 0.9 V in relevant solar cells with the fullerene acceptor ([6, 6]-phenyl-C71-butyric acid methyl ester, PC₇₁BM) or the nonfullerene acceptor (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene, ITIC). The combination studies of Fourier transform photocurrent spectroscopy and electroluminescence further rationalize the V_OC difference between solar cells with fullerene and nonfullerene acceptors. An impressively high power conversion efficiency of 10.19% is obtained for the device based on PTzBI-Ph:ITIC, outperforming the 8.84% achieved by the PC₇₁BM-based device. Our results demonstrate that the modification of substituents of BDT units can effectively decrease the HOMO level and consequently improve V_OC, ultimately allowing the attainment of high-efficiency polymer solar cells.

Perylene Diimide-Based Zwitterion as the Cathode Interlayer for High-Performance Nonfullerene Polymer Solar Cells

Nonfullerene polymer solar cells (PSCs) have earned widespread and intense interest on account of their properties such as tunable energy levels, potential for low-cost production processes, reduced energy losses, and strong light absorption coefficients. Here, a water-/alcohol-soluble zwitterion perylene diimide zwitterion (PDI-z) consisted of sulfobetaine ion as a terminal substituent and PDI as a conjugated core was synthesized. PDI-z was employed as an electron-transport layer (ETL) for nonfullerene PSC devices, obtaining an optimal power conversion efficiency (PCE) above 11.23%. Moreover, nonfullerene PSCs with the PDI-z cathode interlayer displayed an excellent performance on a large scale of interlayer thickness, which was compatible with printing fabrication techniques. Additionally, the PDI-z interlayer presented good ability of modifying high work function metals (for instance, Au, Cu, and Ag) in nonfullerene devices, and the Ag device displayed a PCE of 9.38%. This work provides a good alternative ETL for high-efficiency nonfullerene PSCs.

Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale-up. Despite the rapid increase in power conversion efficiency (PCE) of spin-coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high-performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a single halogen-free solvent. More importantly, chlorine-free, blade coating of FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X-ray scattering results reveal that large π-π coherence length, high degree of face-on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost, low-toxicity, and high-efficiency OSCs by high-throughput production.

An Unfused-Core-Based Nonfullerene Acceptor Enables High-Efficiency Organic Solar Cells with Excellent Morphological Stability at High Temperatures

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

Environmentally Friendly Solvent-Processed Organic Solar Cells that are Highly Efficient and Adaptable for the Blade-Coating Method

The power conversion efficiencies (PCEs) of state-of-the-art organic solar cells (OSCs) have increased to over 13%. However, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical production. Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this study, a highly efficient PBTA-TF:IT-M-based device processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, is fabricated; a high PCE of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. More importantly, a blade-coated non-fullerene OSC processed with THF/IPA is demonstrated for the first time to obtain a promising PCE of 11.7%; even for the THF/IPA-processed large-area device (1.0 cm² ) made by blade-coating, a PCE of 10.6% can still be maintained. These results are critical for the large-scale production of highly efficient OSCs in future studies.

Cross-conjugated n-type polymer acceptors for efficient all-polymer solar cells

Cross-conjugated polymers with different side chains were developed as the electron accepting material for all-polymer solar cells.

Passivating ZnO Surface States by C60 Pyrrolidine Tris-Acid for Hybrid Solar Cells Based on Poly(3-hexylthiophene)/ZnO Nanorod Arrays

Construction of ordered electron acceptors is a feasible way to solve the issue of phase separation in polymer solar cells by using vertically-aligned ZnO nanorod arrays (NRAs). However, the inert charge transfer between conducting polymer and ZnO limits the performance enhancement of this type of hybrid solar cells. In this work, a fullerene derivative named C60 pyrrolidine tris-acid is used to modify the interface of ZnO/poly(3-hexylthiophene) (P3HT). Results indicate that the C60 modification passivates the surface defects of ZnO and improves its intrinsic fluorescence. The quenching efficiency of P3HT photoluminescence is enhanced upon C60 functionalization, suggesting a more efficient charge transfer occurs across the modified P3HT/ZnO interface. Furthermore, the fullerene modified hybrid solar cell based on P3HT/ZnO NRAs displays substantially-enhanced performance as compared to the unmodified one and the devices with other modifiers, which is contributed to retarded recombination and enhanced exciton separation as evidenced by electrochemical impedance spectra. Therefore, fullerene passivation is a promising method to ameliorate the connection between conjugated polymers and metal oxides, and is applicable in diverse areas, such as solar cells, transistors, and light-emitting dioxides.

Haptacyclic Carbazole-Based Ladder-Type Nonfullerene Acceptor with Side-Chain Optimization for Efficient Organic Photovoltaics

In this research, a haptacyclic carbazole-based dithienocyclopentacarbazole (DTCC) ladder-type structure was formylated to couple with two 1,1-dicyanomethylene-3-indanone (IC) moieties, forming a new nonfullerene acceptor DTCCIC-C17 using a bulky branched 1-octylnonayl side chain at the nitrogen of the embedded carbazole and four 4-octylphenyl groups at the sp³-carbon bridges. The rigid and coplanar main-chain backbone of the DTCC core provides a broad light-absorbing window and a higher-lying LUMO energy level, whereas the bulky flanked side chains reduce intermolecular interactions, making DTCCIC-C17 amorphous with excellent solution processability. The DTCCIC-C17 as an acceptor is combined with a medium band gap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) as the donor in the active layer to obtain suitable highest occupied molecular orbital/lowest unoccupied molecular orbital energy alignments and complimentary absorption. The devices with an inverted configuration (ITO/ZnO/active layer/MoO₃/Ag) without using an aqueous poly(3,4-ethylenedioxythiophene) polystyrene sulfonate layer were fabricated for better device stability. When the diiodooctane-treated PBDB-T:DTCCIC-C17 active layer was thermally annealed at 50 °C for 10 min, the device achieved the highest efficiency of 9.48% with a high V_oc of 0.98 V, a J_sc of 14.27 mA cm^-2, and an FF of 0.68.