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The power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) has ascended rapidly arising from the development of polymerized small-molecule acceptor materials. However, numerous insulating long alkyl chains, which ensure the solubility of the polymer, result in inferior aggregation and charge mobility. Herein, this study proposes a facile random copolymerization strategy of two small molecule acceptor units with different lengths of alkyl side chains and synthesizes a series of polymer acceptors PYT-EHx, where x is the percentage of the short alkyl chain units. The aggregation strength and charge mobility of the acceptors rise linearly with increasing the proportion of short alkyl chain units. Thus, the PYT-EH20 reaches balanced aggregation with the star polymer donor PBDB-T, resulting in optimal morphology, fastest carrier transport, and reduced recombination and energy loss. Consequently, the PYT-EH20-based device yields a 14.8% PCE, a 16% improvement over the control PYT-EH0-based device, accompanied by an increase in open-circuit voltage (V oc ), short-circuit current density (J sc ), and fill factor (FF). This work demonstrates that the random copolymerization strategy with short alkyl chain insertion is an effective avenue for developing high-performance polymer acceptors, which facilitates further advances in the efficiency of all-PSCs., (© 2022 Wiley-VCH GmbH.)

Nonfullerene polymer solar cells (PSCs) are developed based on a fluorinated thienyl-based wide-bandgap (WBG) polymer PBBF as the electron donor and nonfullerene small molecule IDIC as the electron acceptor. PBBF exhibits a strong absorption in the range of 300-605 nm with a wide optical bandgap of 2.05 eV, which is complementary with that of IDIC. Meanwhile, it possesses a deeper highest occupied molecular orbital energy level of  -5.52 eV and a higher hole mobility of 7.3 × 10 -4  cm 2 V -1  s -1 compared to the nonfluorinated polymer PBDTT. The PSCs based on PBBF:IDIC without extra treatment show a power conversion efficiency (PCE) of 8.5% with a V oc of 0.95 V, a J sc of 15.3 mA cm -2 , and an FF of 58.8%, which is much higher than that of the devices based on PBDTT:IDIC (a PCE of 5.3% with a V oc of 0.88 V, a J sc of 13.7 mA cm -2 , and an FF of 43.9%). These results indicate that PBBF is a promising WBG polymer donor material for the photovoltaic applications in nonfullerene PSCs., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new broad bandgap and 2D-conjugated D-A copolymer, PBDTBTz-T, based on bithienyl-benzodithiophene donor unit and bithiazole (BTz) acceptor unit, is designed and synthesized for the application as donor material in polymer solar cells (PSCs). The polymer possesses highly coplanar and crystalline structure with a higher hole mobility and lower HOMO energy level which is beneficial to achieve higher open circuit voltage (Voc ) of the PSCs with the polymer as donor. The PSCs based on PBDTBTz-T:PC71 BM blend film with a lower PC71 BM content of 40% demonstrate a power conversion efficiency (PCE) of 6.09% with a relatively higher Voc of 0.92 V. These results indicate that the lower HOMO energy level of the BTz-based D-A copolymer is beneficial to a high Voc of the PSCs. The polymer, with highly coplanar and crystalline structure, can effectively reduce the content of fullerene acceptor in the active layer and can enhance the absorption and PCE of the PSCs., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synergistic effect of fluorination on molecular energy level modulation is realized by introducing fluorine atoms onto both the donor and the acceptor moieties in a D-A polymer, and as a result, the polymer solar cell device based on the trifluorinated polymer, PBT-3F, shows a high efficiency of 8.6%, under illumination of AM 1.5G, 100 mW cm(-) (2) ., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new copolymer PIDTDTQx based on indacenodithiophene and quinoxaline is synthesized and characterized. The correlation between the D/A ratio, mobility, and photovoltaic properties, as well as morphology of the D/A blend based on a PIDTDTQx:PC(70) BM system is investigated. The power conversion efficiency of the polymer solar cells based on PIDTDTQx/PC(70) BM (1:4, w/w) reaches 7.51%., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Understanding effect of morphology on charge carrier transport within polymer/fullerene bulk heterojunction is necessary to develop high-performance polymer solar cells. In this work, we synthesized a new benzodithiophene-based polymer with good self-organization behavior as well as favorable morphology evolution of its blend films with PC(71)BM under improved processing conditions. Charge carrier transport behavior of blend films was characterized by space charge limited current method. Evolved blend film morphology by controlling blend composition and additive content gradually reaches an optimized state, featured with nanoscale fibrilla polymer phase in moderate size and balanced mobility ratio close to 1:1 for hole and electron. This optimized morphology toward more balanced charge carrier transport accounts for the best power conversion efficiency of 3.2%, measured under simulated AM 1.5 solar irradiation 100 mW/cm(2), through enhancing short circuit current and reducing geminate recombination loss.

The determination of molecular conformations of oligomeric acceptors (OAs) and their impact on molecular packing are crucial for understanding the photovoltaic performance of their resulting polymer solar cells (PSCs) but have not been well studied yet. Herein, we synthesized two dimeric acceptor materials, DIBP3F‐Se and DIBP3F‐S, which bridged two segments of Y6‐derivatives by selenophene and thiophene, respectively. Theoretical simulation and experimental 1D and 2D NMR spectroscopic studies prove that both dimers exhibit O‐shaped conformations other than S‐ or U‐shaped counter‐ones. Notably, this O‐shaped conformation is likely governed by a distinctive "conformational lock" mechanism, arising from the intensified intramolecular π–π interactions among their two terminal groups within the dimers. PSCs based on DIBP3F‐Se deliver a maximum efficiency of 18.09 %, outperforming DIBP3F‐S‐based cells (16.11 %) and ranking among the highest efficiencies for OA‐based PSCs. This work demonstrates a facile method to obtain OA conformations and highlights the potential of dimeric acceptors for high‐performance PSCs. [ABSTRACT FROM AUTHOR]

Due to their high transparency, electrodes fabricated from conductive polymers are often implemented in semitransparent organic solar cells. Opaque solar cells usually employ metal back electrodes with high reflectivity for best photon confinement in the light‐harvesting layer. Herein, a bilayer back electrode comprising conductive polymers and nanofoamed poly(methyl methacrylate) (PMMA) is investigated, the latter of which creates diffuse reflection of the incoming light. By tuning the thickness of the nanofoamed PMMA layer, absorption and transmission of the solar cells can be tailored from opaque to vastly transparent. Due to its diffusive character, this versatile electrode enhances the light absorption in the wavelength regimes with lower absorption coefficient. The solar cells are particularly suited for deployment in frosted window applications. [ABSTRACT FROM AUTHOR]

Main observation and conclusion: Recently, the polymer solar cells (PSCs) based on the PM6 and small molecular acceptor (SMA) Y6 have attracted considerable attention in this community for their outstanding photovoltaic performance. As well known, the molecular weight of polymer has dramatic impact on the blend morphology and photovoltaic performance in the polymer solar cells, especially for those polymers with temperature‐dependent aggregation (TDA) behavior. In this work, a series of PM6 polymers with different weight‐average molecular weights (Mw = 41, 67, 74, 84 and 114 kDa) were synthesized, and the effects of the Mw on the morphology and photovoltaic performance were systematically investigated. Notably, molecular weight induces dramatic influences on the aggregation behaviors of the polymers and their corresponding blend morphology, which were explored by the temperature‐dependent UV‐vis absorptions (TD‐Abs), grazing incidence wide‐angle X‐ray scattering (GIWAXS) and resonant soft X‐ray scattering (R‐SoXS) measurements. Increasing Mw promoted the strong face‐on molecular packing and reduced the blend film domain sizes for enhanced donor‐acceptor interfacial areas, resulting in increased short‐circuit current densities (Jsc) and fill factor (FF). However, polymer donor with high Mw value processed stronger tendency of forming excessive pure donor aggregate, which hampered forming the intermixed phase for efficient charge transport, resulting in the inferior FF. Therefore, the optimized OSCs based on PM6 with moderate molecular weight (Mw = 74 kDa) and Y6 exhibited suitable size and purity of the domains, facilitating well‐balanced charge transport, reducing trap‐assisted recombination and leading to significant enhancement of photovoltaic performance with the optimal PCE of 17.1%. This work indicates that proper and precisely tuning the molecular weight of polymer donor is critical to enhance the photovoltaic performance in PSCs. [ABSTRACT FROM AUTHOR]

A non‐conjugated polymer acceptor PF1‐TS4 was firstly synthesized by embedding a thioalkyl segment in the mainchain, which shows excellent photophysical properties on par with a fully conjugated polymer, with a low optical band gap of 1.58 eV and a high absorption coefficient >105 cm−1, a high LUMO level of −3.89 eV, and suitable crystallinity. Matched with the polymer donor PM6, the PF1‐TS4‐based all‐PSC achieved a power conversion efficiency (PCE) of 8.63 %, which is ≈45 % higher than that of a device based on the small molecule acceptor counterpart IDIC16. Moreover, the PF1‐TS4‐based all‐PSC has good thermal stability with ≈70 % of its initial PCE retained after being stored at 85 °C for 180 h, while the IDIC16‐based device only retained ≈50 % of its initial PCE when stored at 85 °C for only 18 h. Our work provides a new strategy to develop efficient polymer acceptor materials by linkage of conjugated units with non‐conjugated thioalkyl segments. [ABSTRACT FROM AUTHOR]

Two wide‐bandgap (WBG) conjugated polymers (PBPD‐p and PBPD‐m) based on phenyl‐substituted benzodithiophene (BDT) with the different substitution position of the alkyl side chain and benzodithiophene‐4,8‐dione (BDD) units are designed and synthesized to investigate the influence of alkyl substitution position on the photovoltaic performance of polymers in polymer solar cells (PSCs). The thermogravimetric analysis, absorption spectroscopy, molecular energy level, X‐ray diffraction, charge transport and photovoltaic performance of the polymers are systematically studied. Compared with PBPD‐p, PBPD‐m exhibits a slight blue‐shift but a deeper highest occupied molecular orbital (HOMO) energy level, a tighter alkyl chain packing and a higher hole mobility. The PBPD‐m‐based PSCs blended with acceptor IT‐4F shows a higher power conversion efficiency (PCE) of 11.95% with a high open‐circuit voltage (Voc) of 0.88 V, a short‐circuit current density (Jsc) of 19.76 mA cm−2 and a fill factor (FF) of 68.7% when compared with the PCE of 6.97% with a Voc of 0.81 V, a Jsc of 15.97 mA cm−2 and an FF of 53.9% for PBPD‐p. These results suggest that it is a feasible and effective strategy to optimize photovoltaic properties of WBG polymers by changing the substitution position of alkyl side chain in PSCs. [ABSTRACT FROM AUTHOR]

Molecular ordering and miscibility of donor and acceptor materials play critical roles in developing high‐performance as‐cast polymer solar cells (PSCs). In this work, a highly crystalline nonfullerene small molecular acceptor, namely, C8‐IT‐4F, based on alkylated indacenodithieno[3,2‐b]thiophene as the aromatic core and 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile moieties as end groups, is selected and synthesized. The π–π stacking distance in C8‐IT‐4F film can be tuned from 3.88 Å to a more compact (3.48 Å) state by a film‐formation process and the confinement induced by the preaggregated polymer donor PM7, leading to broadened absorption and fine phase separation in the blend film. The optimal morphology with a framework of preaggregated polymer donor, J‐type face‐on π–π stacked acceptor, and appropriate donor/acceptor miscibility facilitates charge generation and transport and reduces charge recombination. As a result, the best PSC based on the as‐cast PM7:C8‐IT‐4F blend film exhibits power conversion efficiency of 14.3%, with an open‐circuit voltage of 0.82 V, a short‐circuit current density of 22.7 mA cm−2, a fill factor of 77.1%, and good photostability with a stabilized fill factor. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: Small molecular acceptors (SMAs) BTC‐2F and BTH‐2F, based on heptacyclic benzodi(cyclopentadithiophene) electron‐donating core (CBT) with chlorinated‐thienyl conjugated and thienyl conjugated side chains, respectively, are designed and synthesized. Compared with non‐chlorine acceptor BTH‐2F, BTC‐2F exhibits slightly blue‐shifted absorption spectra, similar the lowest unoccupied molecular orbital (LUMO) (–3.91 eV), deeper highest occupied molecular orbital (HOMO) energy level and higher electron mobility than that of BTH‐2F. PM6, a wide bandgap polymer, is selected as the donor material to construct bulk heterojunction polymer solar cells processed with nonhalogenated solvent toluene. The optimized PM6:BTC‐2F‐based device presents a 12.9% power conversion efficiency (PCE), while the PCE of PM6:BTH‐2F‐based device is only 11.3%. The results suggest that it is an effective strategy to optimize the photoelectric properties of SMAs by incorporating chlorine atom into the conjugated side chains. [ABSTRACT FROM AUTHOR]

Keywords: Acceptor; Molecular Conformation; Oligomer; Selenophene; Solar Cells EN Acceptor Molecular Conformation Oligomer Selenophene Solar Cells 1 1 1 10/31/23 20231106 NES 231106 B Two dimeric acceptors b , DIBP3F-Se and DIBP3F-S were synthesized by linking two Y6-derivative segments using different central linking bridges: selenophene and thiophene. Paired with the state-of-the-art polymer donor PM6, optimized solar cells employing DIBP3F-Se showcase remarkable performance with a power conversion efficiency of 18.09 %, as demonstrated by Donghong Yu, Thomas D. Anthopoulos, Ergang Wang et al. in their Research Article (e202302888). Back Cover: On the Conformation of Dimeric Acceptors and Their Polymer Solar Cells with Efficiency over 18 % (Angew. [Extracted from the article]

Keywords: Acceptor; Molecular Conformation; Oligomer; Selenophene; Solar Cells EN Acceptor Molecular Conformation Oligomer Selenophene Solar Cells 1 1 1 10/31/23 20231106 NES 231106 B Two dimeric acceptors b , DIBP3F-Se and DIBP3F-S were synthesized by linking two Y6-derivative segments using different central linking bridges: selenophene and thiophene. Acceptor, Molecular Conformation, Oligomer, Selenophene, Solar Cells Paired with the state-of-the-art polymer donor PM6, optimized solar cells employing DIBP3F-Se showcase remarkable performance with a power conversion efficiency of 18.09 %, as demonstrated by Donghong Yu, Thomas D. Anthopoulos, Ergang Wang et al. in their Research Article (e202302888). [Extracted from the article]

In this work, polymer solar cells are fabricated based on the blend of PTB7-Th: PC71BM by using a mixed solvent additive of 1,8-diiodooctane and N-methyl pyrrolidone to optimize the morphology of the blend. A high power conversion efficiency (PCE) of 10.8% has been achieved with a simple conventional device. In order to deeply investigate the influence of the mixed solvent additives on the morphology and device performance, the variations of the molecular packing and bulk morphology of the blend film cast from ortho-dichlorobenzene with single or binary solvent additives are measured. Although all the blend films exhibit similar domain size and nanoscale phase separation, the blend film processed with mixed solvent additive shows the highest domain purity, resulting in the least bimolecular recombination, relatively high Jsc and FF, and hence enhanced PCE. Therefore, the best photovoltaic performance with the Voc of 0.82 V, Jsc of 19.1 mA cm−2, FF of 69.1%, and PCE of 10.8% are obtained for the device based on the blend with binary solvent additive treatment. [ABSTRACT FROM AUTHOR]

Two poly(thiazole vinylene) derivatives, poly(4-hexylthiazole vinylene) (P4HTzV) and poly(4-nonylthiazole vinylene) (P4NTzV), were synthesized by Pd-catalyzed Stille coupling method. The polymers are soluble in common organic solvents such as o-dichlorobenzene and chloroform, and possess good thermal stability. P4HTzV and P4NTzV films exhibit broad absorption bands at 400-720 nm with an optical bandgap of 1.77 eV and 1.74 eV, respectively. The HOMO (the highest occupied molecular orbital) energy levels of P4HTzV and P4NTzV are −5.11 and −5.12 eV, respectively, measured by cyclic voltammetry. Preliminary results of the polymer solar cells based on P4HTzV : PC61BM ([6,6]-phenyl-C-61-butyric acid methyl ester) (1 : 1, w/w) show a power conversion efficiency of 0.21% with an open-circuit voltage of 0.55 V and a short circuit current density of 1.11 mA cm−2, under the illumination of AM1.5G, 100 mW cm−2. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 [ABSTRACT FROM AUTHOR]

We study the ultrafast photoexcitation dynamics in PBDTTT-C-T (P51, poly(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene-alt-alkylcarbonyl-thieno[3,4-b]thiophene)) film (~100 nm thickness) and PBDTTT-C-T:PC71BM (P51:PC71BM, phenyl-C71-butyric-acid-methyl ester) nanostructured blend (∼100 nm thickness) with/without DIO(1,8-diiodooctane) additives with sub-10 fs transient absorption (TA). It is revealed that hot-exciton dissociation and vibrational relaxation could occur in P51 with a lifetime of ~160 fs and was hardly affected by DIO. However, the introduction of DIO in P51 brings a longer lifetime of polaron pairs, which could make a contribution to photocarrier generation. In P51:PC71BM nanostructured blends, DIO could promote the Charge Transfer (CT) excitons and free charges generation with a ~5% increasement in ~100 fs. Moreover, the dissociation of CT excitons is faster with DIO, showing a ~5% growth within 1 ps. The promotion of CT excitons and free charge generation by DIO additive is closely related with active layer nanomorphology, accounting for Jsc enhancement. These results reveal the effect of DIO on carrier generation and separation, providing an effective route to improve the efficiency of nanoscale polymer solar cells. [ABSTRACT FROM AUTHOR]

Two new wide bandgap block copolymers (PL1 and PL2) with porphyrin‐incorporated side chains are designed and used as electron donors for solution‐processed bulk heterojunction polymer solar cells. The photophysical, electrochemical, and photovoltaic properties, charge transport mobility and film morphology of these two block copolymers are investigated. Detailed investigations reveal that the different alkyl groups and electron‐withdrawing substituents on the porphyrin pendant units have significant influence on the polymer solubility, absorption energy level, band gap, and charge separation in the bulk‐heterojunction thin films, and thus the overall photovoltaic performances. Organic photovoltaic devices derived from these copolymers and ([6,6]‐phenyl‐C71‐butyric acid methyl ester) (PC71BM) acceptor show the best power conversion efficiencies of 5.83% and 7.14%, respectively. These results show that the inclusion of a certain proportion of side chain porphyrin group as a pendant in the traditional donor‐acceptor (D‐A) type polymer can broaden the molecular absorption range and become a full‐color absorbing molecule. The size of the porphyrin pendant also has an obvious effect on the properties of the molecule. [ABSTRACT FROM AUTHOR]

Abstract: Fluorination of conjugated polymers is one of the effective strategies to tune the molecular energy levels and morphology for high efficient polymer solar cells (PSCs). Herein, two novel donor‐acceptor conjugated polymers, PffBT and PBT, based on bis(3,5‐bis(hexyloxy)phenyl)benzo[1,2‐ b:4,5‐b']dithiophene and benzo[c][1,2,5]thiadiazole (BT) with or without fluorination, respectively, were synthesized, and their photovoltaic properties were compared. The polymer PffBT based on fluorinated BT showed lower frontier energy levels, improved polymer ordering, and a well‐developed fibril structure in the blend with PC71BM. As a result, the PSCs based on PffBT/PC71BM exhibit a superior power conversion efficiency (PCE) of 8.6% versus 4.4% for PBT‐based devices, due to a high space charge limit current (SCLC) hole mobility, mixed orientation of polymer crystals in the active layer, and low bimolecular recombination. [ABSTRACT FROM AUTHOR]

A new conjugated polymer based on 5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione,named as PBDTBDD, was designed, synthesized, and applied in polymersolar cells (PSCs). A power conversion efficiency (PCE) of 6.67% wasobtained from the PBDTBDD/PC61BM-based PSC, which is aremarkable result for the PSCs using PC61BM as electronacceptor. The PBDTBDD/PC61BM-based device exhibits a narrowabsorption band and excellent quantum efficiency in the range from500 to 700 nm. Furthermore, PBDTBDD shows a strong aggregation effectin solution state, and the study indicates that although the temperatureused in solution preparation has little influence on molecular orientationas well as crystallinity of the D/A blend, it plays an important rolein forming proper domain size in the blend. This work provides a goodexample to reveal the correlation between the morphology of the blendfilms and the processing temperature of the solution preparation.Furthermore, the study in this work suggests an interesting and feasibleapproach to modulate domain size without changing crystallinity ofthe blend films in PSCs. [ABSTRACT FROM AUTHOR]

Interfacial engineering is crucial to improve the photovoltaic performance of polymer solar cells (PSCs). In this study, we demonstrate efficient inverted non-fullerene PSCs with ZnO modified by nontoxic glycine (Gly) as the electron transport layer (ETL). With the modification of Gly, work function of the ETL interlayer was decreased from 4.11 eV for ZnO to 4.02 eV for ZnO/Gly, which effectively increased the built-in electric field and hence improved the electron extraction. Furthermore, the surface energy of interlayer was also reduced from 69.1 mJ/m2 for ZnO to 52.5 mJ/m2, which is beneficial for the enrichment of the acceptor at the interface and hence facilities the electron transport and collection at cathode electrode. Therefore, the device based on PM6:IT-4F with ZnO/Gly as ETL exhibited a remarkably enhanced power conversion efficiency (PCE) of 14.0% relative to 12.9% for the control device with ZnO ETL, indicating a ∼9% increase of the PCE with the simple molecular modification of the ETL. Meanwhile, this strategy can also be successfully applied to other non-fullerene and fullerene based inverted PSCs. Our work provides an effective approach to modify ZnO for high performance PSCs. Nontoxic glycine modified ZnO was used as the electron transport layer (ETL), which shows fewer defects, lower work function and lower surface energy, and therefore promote the photovoltaic performance of the inverted non-fullerene polymer solar cells. Image 1 • Nontoxic glycine modified ZnO is used as the electron transport layer in inverted non-fullerene polymer solar cells (PSCs). • The glycine modified ZnO interlayer shows lower work function, fewer defects and lower surface energy. • With the glycine modified ZnO interlayer, higher efficiencies are obtained in the inverted non-fullerene PSCs. [ABSTRACT FROM AUTHOR]

Abstract Polymer solar cells (PSCs) based on narrow bandgap (NBG) photovoltaic materials have attracted significant attention due to their unique applications in semitransparent devices and organic tandem solar cells. In this work, we design and synthesize a new NBG polymer PBTT based on 2-(2-ethylhexyl)-thienyl substituted benzodithiophene (BDT-T) and n-octyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT) as donor material for the applications in non-fullerene PSCs. Compared with the widely used NBG polymer PTB7-Th, PBTT exhibited higher film extinction coefficient of 9.27 × 10 4 cm−1, higher-lying energy levels, more ordered molecule packing and higher hole mobility of 2.13 × 10 −3 cm2 V−1 s −1. The PSCs based on PBTT : IEICO obtained a power conversion efficiency (PCE) of 9.5% with a V oc of 0.86 V, a J sc of 17.9 mA cm−2 and a FF of 61.3%, which is significantly enhanced over that (7.3%) of the corresponding devices with PTB7-Th as donor. The significantly improved PCE could be attributed to the higher charge mobility and the improved morphology of the PBTT : IEICO blend film. These results demonstrate that PBTT is a very promising NBG polymer donor material for the applications in non-fullerene PSCs. Graphical abstract Image 1 Highlights • A new narrow-bandgap polymer with high mobility was developed. • The optimal PSCs devices demonstrated a PCE of 9.5%. [ABSTRACT FROM AUTHOR]

[Display omitted] • A series of 1D/2A type novel wide-bandgap terpolymer donors were synthesized. • Using a simple and low-cost electron-deficient building block DTCPz to develop terpolymers. • The hero PSC presents a particularly increased PCE of 17.8%, with ca. 11% of improvement from that of 16.0% for the parent PM6-based ones. • Our terpolymer strategy provides a positive effect on improving the thermal stability of the device. It had been commonly accepted in the organic photovoltaic (OPV) community that subtle variations in the molecular structure of active layer materials would cause profound impacts on their aggregating structure and blend morphology and therefore the performance of such polymer solar cells (PSCs). Herein, we employed an electron-deficient building block 3,6-dithiophenyl-2-carboxylate pyrazine (DTCPz) for constructing one series of promising donor terpolymers of PMZ1, PMZ2, and PMZ3, respectively, gaining their relatively lower-lying highest occupied molecular orbital (HOMO) energy levels, more closed π-π stacking and enhanced crystallinity in thin films, and lower miscibility with acceptor Y6, in comparison with their parent polymer counterpart (namely PM6). Reaching DTCPz moieties up to 20% (mol/mol%) in its terpolymer composition, the resulting polymer (PMZ2) achieved more favorable phase separation with improved exciton dissociation, and charge transport and extraction. As a result, an outstanding fill factor of 77.2% and a promising power conversion efficiency of 17.8 % was achieved. Moreover, the corresponding device shows better thermal stability over the PM6-based one. This work suggests a facile method for significantly improving the thin film morphology of the active-layer materials via fine-tuning the chemical structure of electron-deficient units on the backbone of the wide bandgap donor polymer, therefore achieving enhanced photovoltaic performance and thermal stability for practical applications. [ABSTRACT FROM AUTHOR]

In order to investigate the influence of two-dimensional(2D) conjugated structure on photovoltaic properties of benzo[1,2-b:4,5-b′]difuran (BDF)-based polymers,two low band gap photovoltaic polymers, named PBDFTT-CF-O and PBDFTT-CF-T,were designed and synthesized. These two polymers have the same backbonesand different side groups. Although these two polymers show similaroptical band gaps (ca. 1.5 eV), the polymer with alkylthienyl sidegroups, PBDFTT-CF-T, exhibits stronger absorption in long wavelengthdirection than the polymer with alkoxyl side groups, PBDFTT-CF-O.Meanwhile, PBDFTT-CF-T exhibits a HOMO level of −5.21 eV, whichis 0.23 eV lower than that of PBDFTT-CF-O due to weaker electron-donatingability of alkylthienyl side groups than that of aloxyl side groups.The hole mobility of the blend of PBDFTT-CF-T/PC71BM (1:1.5,w/w) is 0.128 cm2V–1s–1, which is 1 order of magnitude higher than that of the blend ofPBDFTT-CF-O/PC71BM. Density functional theory (DFT) modelshows thiophene pendants on dithienyl-BDF are more coplanar than iton dithienyl-BDT. These results indicate that the 2D-conjugated structureis helpful for molecular structure design of the BDF-based polymersin enhancing the intermolecular π–π stacking andimproving charge transport property. Furthermore, the photovoltaicdevices based on these two polymers show similar short circuit densityand fill factor values, while the open circuit voltage of the PBDFTT-CF-T-baseddevice is 0.78 V, which is 0.15 V higher than that of the PBDFTT-CF-O-baseddevice. Therefore, the efficiencies of the devices based PBDFTT-CF-T/PC71BM and PBDFTT-CF-O/PC71BM are 6.26% and 5.22%,respectively. The results in this work demonstrate that the weak electron-donatingability of alkylthienyl side groups can be seen as an effective strategyto improve photovoltaic properties of the BDF-based polymers and the2D-conjugated molecular structure is favorable to improve hole mobility. [ABSTRACT FROM AUTHOR]