Electron-withdrawing End Groups Research Articles

Acceptor molecules with different central and terminal units for P3HT based solar cells: Effects of the molecular structures on their properties

The design strategy of acceptor materials matched with poly(3-hexylthiophene) (P3HT) is highly desired for the development of low-cost organic solar cells (OSCs). Herein, we introduced strong electron-donating central units and weak electron-withdrawing end groups to construct the acceptors with high HOMO and LUMO, modulated the molecular symmetry and conjugation length by changing the structures of central units, and synthesized six acceptor molecules, namely, TDTC-O2F, TTDTC-O2F, TTTDTC-O2F, TDTC-ThBr, TTDTC-ThBr and TTTDTC-ThBr. The high LUMO energy level of the acceptors endowed the P3HT based OSCs with relatively high Voc values. Molecular symmetry, conjugation length of central units and the terminal structure jointly affect the molecular crystallinity and miscibility of the blends. Owing to the proper film morphology, the P3HT:TTDTC-ThBr based devices achieved the champion power conversion efficiency of 6.15 %.

Unfused-ring acceptor based on a quinoxaline unit functionalized with meta-positioned alkoxy chains for efficient organic solar cells with high open-circuit voltage

Owing to the simple synthesis and flexible chemical modifiability of unfused-ring non-fullerene acceptors (UF-NFAs), they have gained great research attraction for fabricating organic solar cells (OSCs). However, UF-NFA-based devices still underperform fused-ring non-fullerene acceptor-based devices. Herein, a series of UF-NFAs based on a fluorinated quinoxaline (Qx) building block functionalized with meta-positioned alkoxy chains, namely FQxOC8-H, FQxOC8-F, and FQxOC8-Cl, are synthesized. Density-functional theory (DFT) calculations demonstrated that all UF-NFAs exhibited good backbone coplanarity, owing to the S⋯N and H⋯F nonbonding interactions. The resulting UF-NFAs showed broad absorption ranged from 500 to 900 nm as well as deep highest occupied molecular orbital (HOMO) levels. FQxOC8-Cl exhibited more red-shifted and greater absorption coefficient owing to the stronger molecular stacking caused by chlorine atoms of the end groups. Moreover, PM6:FQxOC8-Cl-based blended film exhibited reduced charge recombination, higher carrier mobilities, and suitable phase-separation domains. Accordingly, OSCs based on FQxOC8-Cl yielded a superior power conversion efficiency (PCE) of 10.52%. In contrast, FQxOC8-H-based device afforded a lower efficiency of 6.13%. Our work indicates that combining the fluorinated Qx motif functionalized with meta-positioned alkoxy chains with electron-withdrawing end-groups showed great potential to construct efficient UF-NFAs.

Isomerization Directed Molecular Packing toward Suppressing Energy Loss in Nonfullerene Ternary Solar Cells

Organic semiconductors based upon conjugated frameworks are often isomeric and display distinct optoelectronic properties within minor structural variation. In this work, two isomeric nonfullerene acceptors (NFAs) ThMeCl-1 and ThMeCl-2, having the methyl and chlorine atoms attached on different positions of the electron-withdrawing end group, are synthesized and incorporated as the third component in ternary solar cells. Although these NFA isomers exhibit a similar bandgap, energy levels, and energy loss in their PM6 based binary devices, the efficiency enhancements in ternary devices differ significantly. Compared to ThMeCl-1, the incorporation of ThMeCl-2 in PM6:C5-16 solar cells enables less energy loss, leading to an extra 0.03 eV open-circuit voltage gain and a maximum efficiency increase from 17.8 to 18.9%. Grazing-incidence X-ray diffraction and molecular dynamics simulations reveal that this is attributed to the versatile intermolecular π–π stacking forms between ThMeCl-2 and the host NFA, which result in improved charge transport and suppressed recombination. This work provides a rational guidance for controlling the molecular packing in ternary systems to prepare high performance organic photovoltaics.

Novel Third Components with (Thio)barbituric Acid as the End Groups Improving the Efficiency of Ternary Solar Cells.

Developing novel third component is critical for the ternary organic solar cells (TOSCs). Herein, we design and synthesize two novel third components, MAZ-1 and MAZ-2, with 1,3-diethyl-2-thiobarbituric acid and 1,3-dimethylbarbituric acid as the weak electron withdrawing end groups, respectively. Both MAZ-1 and MAZ-2 could improve the photovoltaic performance of the binary OSCs based on D18:Y6 which exhibit the power conversion efficiency (PCE) of 17%, because the third components can optimize the phase separation, suppress the bimolecular recombination, and decrease the nonradiative energy loss in ternary blends. The PCE of the optimized TOSCs approaches 18% along with the simultaneous increase in open circuit voltage, short circuit current density, and fill factor by incorporating 10 wt % MAZ-1 and MAZ-2 in acceptors. This work enriches the building blocks for novel third components for achieving highly efficient TOSCs.

Designing High-Performance Nonfused Ring Electron Acceptors via Synergistically Adjusting Side Chains and Electron-Withdrawing End-Groups.

Three nonfused ring electron acceptors, Hexyl-0F, Isopropyl-0F, and Isopropyl-2F, are designed and synthesized. Unlike Hexyl-0F, Isopropyl-0F with two sterically hindered 2,4,6-triisopropyl-phenyl groups is highly soluble, which provides a good opportunity for solution processability. Compared with Isopropyl-0F, Isopropyl-2F with fluorinated end-groups exhibits red-shifted absorption. Due to these synergistic adjustment, Isopropyl-2F-based devices displayed a high power conversion efficiency of 12.55%, higher than that of Isopropyl-0F (9.49%). The result demonstrates that the introduction of large steric substituents in the π-bridge units and electron-withdrawing end-groups plays a positive role in the construction of high-efficiency nonfused ring electron acceptors.

Non-fused medium bandgap electron acceptors for efficient organic photovoltaics

The cost-effective organic semiconductors are strongly needed in organic photovoltaics (OPVs). Herein, two medium bandgap (MBG) electron acceptors, TPT4F and TPT4Cl are developed via the new design of multi-noncovalent interaction assisted unfused core, flanked with two electron withdrawing end groups. These fullly non-fused MBG acceptors adapt the planar and rigid conformation in solid, therefore exhibiting the ordered face-on stacking and strong photoluminescence in films. As results, TPT4Cl-based OPVs, upon blending with the PBDB-TF polymer donor, have achieved a power conversion efficiency of 10.16% with a low non-radiative loss of 0.27 eV, representing one of the best fullly non-fused medium bandgap acceptors with desirable cost-efficiency balance.

Molecular description of charge transport in the IDIC non-fullerene acceptor for organic solar cells

Non-fullerene acceptors (NFAs) have demonstrated remarkable performance when used in bulk heterojunction organic solar cell devices. Their exceptional properties, such as the ability to tune their electronic properties by chemical modifications, increased absorption in the visible and near infrared part of the spectrum, and general ease of synthesis, make them a key alternative for traditional fullerene-based acceptors. IDIC is a typical NFA material having a five-membered fused ring core and strong electron withdrawing end groups. In this work we use a multiscale computational approach based on a hopping model and Marcus’ charge transport theory to investigate electron and hole transport in crystal and amorphous IDIC samples. The basic transport parameters, namely reorganization energy, electronic couplings and site energies are calculated and compared with the corresponding values for fullerene-based acceptors where available in the literature. Electrostatic and induction contributions to the total energetic disorder are also examined and discussed. Using Kinetic Monte Carlo simulation, the electric field and temperature dependence of mobility is systematically evaluated for electrons and holes, and the data are fitted against the widely used Gaussian Disorder Model. We find that compared to the commonly used PCBM fullerene acceptor, IDIC has larger reorganization energy and similar energetic disorder. However, it exhibits comparable electron mobility, due to enhanced electronic couplings with neighboring molecules in the crystalline brickwork-like structure.

A highly crystalline non-fullerene acceptor enabling efficient indoor organic photovoltaics with high EQE and fill factor

<h2>Summary</h2> The growth of the internet of things (IoT) is creating a demand for convenient energy sources, like organic photovoltaics, to power various small IoT devices. Here, we report a highly crystalline small molecular acceptor (named FCC-Cl) with an optical band gap of 1.71 eV suitable for indoor applications. The important design rationale of FCC-Cl is the combination of a weak electron-donating core and a moderate electron-withdrawing end group, which leads to needed band gap and high crystallinity. The OPVs based on D18:FCC-Cl achieved a high external quantum efficiency up to 85% and a high fill factor of 80% due to the high absorption coefficient and strong crystallinity of FCC-Cl. Consequently, an impressive power conversion efficiency of 28.8% was achieved under a 2,600 K LED lamp at 500 lux. It was also demonstrated that PM6:FCC-Cl-based devices can achieve high efficiencies over a wide range of active-layer thicknesses, which is a feature necessary for large-scale roll-to-roll printing processes.

Small-molecule acceptors with long alkyl chains for high-performance as-cast nonfullerene organic solar cells

Abstract Three fused-ring small-molecule electron acceptors, IDTC16-IC, IDTC16-Th, and IDTC16-4F, were designed and synthesized by introducing indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), fluorinated IC, and a thiophene-based unit as the electron-withdrawing end group. Here, instead of the commonly used n-hexyl or n-hexylphenyl side chains, n-hexadecyl peripheral substituents were employed at the IDT core to study the influence of alkyl groups on photovoltaic performance of the nonfullerene acceptors. The introduction of flexible n-hexadecyl group endowed the three acceptors with excellent solubility in common organic solvents. All the three acceptors presented strong absorption ranging from 450 nm to 720 nm in solution with high molar extinction coefficients. As a result, the as-cast organic solar cells (OSCs) based on IDTC16-IC and the wide bandgap polymer donor PM6 exhibited a power conversion efficiency (PCE) of 5.12%. The OSCs based on PM6:IDTC16-Th and PM6:IDTC16-4F showed much better photovoltaic performance with PCEs of 8.76% and 8.55%, respectively. The PCE values were improved to 5.89%, 9.09%, and 9.42% for the PM6:IDTC16-IC, PM6:IDTC16-Th, and PM6:IDTC16-4F OSCs, respectively, with the addition of the solvent additive 1,8-diiodooctane. These findings demonstrate that the combination of alkyl chains at the fused rings and fluorination or aromatic structure change of the terminal groups leads to greatly enhanced photovoltaic performance of nonfullerene acceptors through improving the photophysical, molecular orbital, and film morphological properties.

Effects of Halogenated End Groups on the Performance of Nonfullerene Acceptors.

The end groups' halogenations among the nonfullerene acceptors (NFAs) were a very useful method to fabricate high-performance NFAs-based organic solar cells (OSCs). We report three high-performance NFAs, BTIC-4EO-4F, BTIC-4EO-4Cl, and BTIC-4EO-4Br. They all have a fused benzothiadiazole as the core unit and different dihalogenated end groups (IC-2F, IC-2Cl, and IC-2Br) as the terminal unit. Thanks to the improved intramolecular charge-transfer ability of the brominated NFAs, bromination is more effective than fluorination and chlorination in lowering the energy levels and red-shifting the absorption spectra of the resulting NFAs. When compared with the chlorinated and fluorinated counterparts, the BTIC-4EO-4Br blend films exhibit lower roughness, better phase separation size, and stronger face-on stacking. When blended with poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b']-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3-diyl]]} (PBDB-TF) as the polymer donor material, the BTIC-4EO-4Br-based OSCs exhibit the highest power conversion efficiency (12.41%), with a higher current density and a higher open-circuit voltage than the BTIC-4EO-4Cl-based OSCs (11.29%) and BTIC-4EO-4F-based OSCs (10.64%). These results show that the bromination of the NFAs' electron-withdrawing end groups can also be very effective in constructing high-performance photovoltaic materials.

Fused-Ring Electron Acceptors for Photovoltaics and Beyond.

ConspectusEmerging solar cells that convert clean and renewable solar energy to electricity, such as organic solar cells (OSCs) and perovskite solar cells (PSCs), have attracted increasing attention owing to some merits such as facile fabrication, low cost, flexibility, and short energy payback time. The power conversion efficiencies (PCEs) of OSCs and PSCs have exceeded 18% and 25%, respectively.Fullerene derivatives have high electron affinity and mobility with an isotropic transport feature. Fullerene-based OSCs yielded superior PCEs to other acceptors and have dominated electron acceptor materials from 1995 to 2015. However, some drawbacks of fullerenes, such as weak visible absorption, limited tunability of electronic properties, laborious purification, and morphological instability, restrict further development of OSCs toward higher PCEs and practical applications. The theoretical PCE of fullerene-based OSCs is limited to ∼13% due to the relatively large energy losses. Many efforts have been dedicated to developing new acceptor systems beyond fullerenes, and some successful systems such as rylene diimides have achieved PCEs up to ca. 11%.In 2015, our group pioneered a new class of electron acceptors, fused-ring electron acceptor (FREA), as represented by the star molecule ITIC. The chemical features of FREAs include: (1) a modular structure, consisting of an electron-donating core, electron-withdrawing end groups, π-bridges, and side chains, which benefits molecular tailoring; (2) facile synthesis, purification, and scalability. The physical features of FREAs include: (1) a broad modulation range of absorption and energy levels; (2) strong absorption, especially in the 700-1000 nm region; (3) high electron mobility. The device features of FREAs include: (1) low voltage loss; (2) high efficiency; (3) good stability. The FREAs boosted PCEs of the OSCs up to 18% and initiated the transformation from the fullerene to nonfullerene era of this field. FREAs can also be used in PSCs as interfacial layers, electron transport layers, or active layers, improving both efficiency and stability of the devices. Beyond photovoltaic applications, FREAs can also be used in photodetectors, field-effect transistors, two-photon absorption, photothermal therapy, solar water splitting, etc.In this Account, we review the development of the FREAs and their applications in OSCs, PSCs, and other related fields. Molecular design, device engineering, photophysics, and applications of FREAs are discussed in detail. Future research directions toward performance optimization and commercialization of FREAs are also proposed.

A new chlorinated non-fullerene acceptor based organic photovoltaic cells over 12% efficiency

The method to fluorinate the terminal group has achieved remarkable success and been widely used to fine-tune the intrinsic properties of organic acceptor materials. Referring to chlorination, however, it gets less attention and remains ambiguous effect on organic photovoltaic (OPV) cells. Herein, a new non-fullerene acceptor named Y19 was reported with benzotriazole as the electron-deficient core and 2Cl-ICs as the strong electron-withdrawing end groups. Y19 exhibits a wide film absorption band from 600 nm to 948 nm and low LUMO (the lowest unoccupied molecular orbital) energy level of −3.95 eV Photovoltaic devices based on PM6:Y19 show high-power conversion efficiency (PCE) of 12.76 % with high open-circuit voltage (Voc) of 0.84 V, short-circuit current density (Jsc) of 22.38 mA/cm2 and fill factor (FF) of 68.18 %. Broad external quantum efficiency (EQE) response of over 60 % in the range of 480–860 nm can be obtained. This study demonstrates that chlorination, as a low-cost molecular design strategy, has its own superiorities to improve device performance and promote the potential application in OPV.

Non-fullerene electron acceptors constructed by four strong electron-withdrawing end groups: Potential to improve the photoelectric performance of organic solar cells by theoretical investigations

Recent advances in non-fullerene acceptors have enabled the power conversion efficiencies of organic solar cells (OSCs) to exceed 16%. Fused-ring electron acceptors (FREAs) are some of the most widely investigated non-fullerene acceptors. Classical FREAs, containing two strong electron-withdrawing end groups, are typically constructed using linear architecture. Meanwhile, the development of 2D FREAs is rarely studied. Therefore, we proposed a series of novel 2D FREAs, namely, 2D-IC, 2D-ICT, 2D-ICTS and 2D-ICTSS, which were composed of four strong electron-withdrawing end groups linked by a 2D fused-ring core to form a rotational C2h-symmetric structure. Through theoretical calculations we found that as the degree of conjugation increased, the spectrum red shifted, the integrated absorption intensity was enhanced, and the hole-electron Coulomb attraction decreased. Compared with linear molecule ITOIC-2F, which had the same end groups, 2D-ICTSS exhibited two significant absorption peaks at red and near-infrared regions. In addition, its integrated absorption intensity was 1.8 times that of ITOIC-2F. Simultaneously, the hole-electron Coulomb attraction of 2D-ICTSS was smaller than that of ITOIC-2F. When alkyl chains were introduced, the electron mobility of 2D-ICTSS was estimated to be 1.98 times that of ITOIC-2F. Given that numerous critical factors (including absorption intensity, exciton separation, and electron mobility) of 2D-ICTSS were superior to those of ITOIC-2F, we propose that this rotational C2h-symmetric molecule is highly suitable as a high-performance electron acceptor. This molecule is expected to be an important candidate for the development of next-generation FREAs for high-performance OSCs.

Diflurobenzothiadiazole core-based noncovalently fused small molecule acceptor exhibiting over 12% efficiency and high fill factor

The versatility and flexibility of organic photoelectric materials endow organic photovoltaic cells fine function modulation and huge commercial potential. In this work, a new noncovalent fused-ring small molecule acceptor (SMA) BID-4F has been synthesized for high-efficient organic solar cells (OSCs). BID-4F consists of a diflurobenzothiadiazole (DFBT) core, ladder-like indacenodithiophene (IDT) spacers, and dicyanoindanone electron-withdrawing end groups, which are supposed to be conformationally interlocked by noncovalent interactions, leading to good molecular planarity. In addition, compact solid state stacking was revealed by UV–vis–NIR absorption spectrum. The optimized PM6:BID-4F based device delivers an eminent power conversion efficiency (PCE) of 12.30% with a high open-circuit voltage (Voc) of 0.92 V and a high fill factor (FF) of 74.3%. Most importantly, the PCE and FF are among one of the highest values reported for the OSCs based on the unfused-ring SMAs. Overall, our work demonstrates that the unfused ring central framework with high molecular planarity through noncovalent interactions provides a good strategy to construct highly efficient SMAs.

Spirobifluorene-based non-fullerene acceptors for the environmentally benign process

Spirobifluorene based non-fullerene acceptors (NFAs) having meta-ethylhexyl oxy (m-OEh) substituents (PBDB-T: spiro-bridged NFAs) were synthesized. The orthogonal molecular structure of the m-OEh-substituted spirobifluorene with different end groups provided high solubility and kept the stable morphology between donor-acceptor. Organic Photovoltaic (OPV) device performance was not significantly affected by switching the processing solvent from a chlorinated solvent to an environmentally benign solvent, because suitable morphology and crystallinity were retained. The ITO/ZnO/PBDB-T:spiro-bridged NFA/MoOx/Ag OPV devices prepared using a chlorobenzene as the processing solvent displayed maximum efficiency in the range of 6.44–5.78%, while the devices made using xylene as the processing solvent had a lower maximum efficiency ranging from 6.14–4.66%. The OPV performance of the devices made using chlorobenzene decreased in the order of sp-mOEh-ITIC > sp-mOEh-ITIC-M > sp-mOEh-ITIC-F > sp-mOEh-ITIC-Cl because an electron donating end group increased open-circuit voltage (Voc), while the electron withdrawing end group increased the short-circuit current density (Jsc). Although the OPV performance of the four spiro-bridged derivatives by xylene was showed to be similar order of that by chlorobenzene solvent processing, the efficiency drop from chlorobenzene solvent to xylene solvent showed following order sp-mOEh-ITIC < sp-mOEh-ITIC-M < sp-mOEh-ITIC-F < sp-mOEh-ITIC-Cl, which depended on the polarity effect of end groups.

Functionalizing triptycene to create 3D high-performance non-fullerene acceptors.

Non-fullerene acceptors have been widely investigated for organic solar cells (OSCs). In particular, fused-ring electron acceptors (FREAs), composed of two strongly electron-withdrawing end groups connected by a planar fused-ring core, have been successfully applied to develop high-performance OSCs (>16%). In this work, we proposed two novel 3D FREAs named BFT-3D and BFTT-3D, which can reduce the formation of crystalline domains and increase the interface with donors to promote exciton separation. These 3D FREAs consist of three strongly electron-withdrawing end groups linked by a central triptycene hub to form a three-bladed propeller nanostructure. In comparison with high-performance FREA (ITOIC-2F), these FREAs have stronger absorption intensity and smaller exciton binding energy. These findings demonstrated that these three-bladed propeller-shaped FREAs can absorb abundant energy from sunlight to generate excitons, easily separate excitons to free electrons and holes, and reduce the recombination of excitons. In addition, the electron mobility of BFT-3D (8.4 × 10−4 cm2 V−1 s−1) is higher than that of BFTT-3D (1.0 × 10−4 cm2 V−1 s−1), which indicated that the appropriate 3D core structure was conducive to the electron mobility of the three-bladed propeller-shaped FREAs. It can effectively improve the current density to enhance the performance of OSCs. These findings will provide new perspectives for experimental scientists to synthesize high-performance FREAs.

An A–D–D–A-type non-fullerene small-molecule acceptor with strong near-infrared absorption for high performance polymer solar cells

An A–D–D–A-type near-infrared non-fullerene small-molecule acceptor IDT2-DFIC with indacenodithiophene–indacenodithiophene (IDT2) as donating core and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) as electron withdrawing end groups has been synthesized.

Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells

We report two novel small-molecule non-fullerene acceptors, IF-TN and IDT-TN, with indenofluorene (IF) and indacenodithiophene (IDT) as their respective central electron-rich cores and naphthyl-fused indanone (N) as the electron-withdrawing end-groups. By pairing these non-fullerene acceptors with a widely used polymer PBDB-T, the fabricated polymer solar cells based on PBDB-T:IDT-TN obtain a power conversion efficiency up to 5.89%, associated with a relatively high open-circuit voltage of 0.97 V and a remarkably low non-radiative open-circuit voltage loss of 0.23 V. These findings provide an effective approach for the rational molecular design of small-molecule acceptors with low non-radiative open-circuit voltage loss.

Effect of electron-withdrawing terminal group on BDT-based donor materials for organic solar cells: a theoretical investigation

Rational end group modification has been found as an effective strategy to improve power conversion efficiencies (PCEs) for photovoltaic materials. However, due to different electronic processes competition, various interaction factors must be taken into account to make materials design. Through density functional theory (DFT) and time-dependent DFT (TD-DFT), the effect of electron-withdrawing substitution on benzodithiophene-based donor materials from the open circuit voltage ( $$ V_{\text{OC}} $$ ), light absorption, exciton dissociation to charge transport in bulk materials has been investigated. The results point to that strong electron-withdrawing end group remarkably (1) enhances $$ V_{\text{OC}} $$ due to lowered HOMO energy level; (2) induces photon absorption redshift due to narrow optical gap (Eg); (3) facilitates exciton dissociation because of enhanced intramolecular charge transfer character. However, there is no direct correlation between electron-withdrawing ability and charge transport properties, since steric hindrance, noncovalent interaction and electrostatic interaction altogether have large impact on intermolecular stacking and then charge mobility. Comprehensive factors should be considered to improve PCEs for photovoltaic materials. Impressively, the designed molecule SM8 with dicyanovinyl-capped reveals excellent optical-electron properties, which may be a promising donor for high performance SM-OSCs.

Asymmetric triphenylamine-phenothiazine based small molecules with varying terminal acceptors for solution processed bulk-heterojunction organic solar cells.

Three compounds consisting of the electron-donating triphenylamine-phenothiazine conjugate backbone and each of the electron-withdrawing groups 3-ethylrhodanine, malononitrile and 1,3-indandione have been synthesized and used as donors in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) for organic solar cell devices. After improvements of the active layer structure by a selected donor-to-acceptor weight ratio and a two-step solvent and thermal annealing, the organic solar cells showed power conversion efficiency (PCE) values in the range of 4.79-7.25%. The highest PCE was obtained for the bulk heterojunction device with the indandione compound, which can be attributed to its better absorption profile, higher crystallinity, more balanced electron and hole transport, higher charge collection efficiency and reduced recombination, in comparison with the photovoltaic cells from the other two compounds. DFT-calculated characteristics, absorption spectra and cyclic voltammetry of the compounds, along with X-ray diffraction patterns of the blend films, are used to validate the photovoltaic results.