Non-fullerene Small Molecule Acceptors Research Articles

Medium Bandgap Nonfullerene Acceptor for Efficient Ternary Polymer Solar Cells with High Open-Circuit Voltage.

We have designed a new medium bandgap non-fullerene small-molecule acceptor consisting of an IDT donor core flanked with 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]-thiophene-4-ylidene) malononitrile (TC) acceptor terminal groups (IDT-TC) and compared its optical and electrochemical properties with the IDT-IC acceptor. IDT-TC showed an absorption profile from 300 to 760 nm, and it has an optical bandgap of 1.65 eV and HOMO and LUMO energy levels of -5.55 and -3.83 eV, respectively. In contrast to IDT-IC, IDT-TC has an upshifted LUMO energy level, which is advantageous for achieving high open-circuit voltage. Moreover, IDT-TC showed higher crystallinity and high electron mobility than IDT-IC. Using a wide bandgap D-A copolymer P as the donor, we compared the photovoltaic performance of IDT-TC, IDT-IC, and IDT-IC-Cl nonfullerene acceptors (NFAs). Polymer solar cells (PSCs) using P: IDT-TC, P: IDT-IC, and P:IDT-IC-Cl active layers achieved a power conversion efficiency (PCE) of 14.26, 11.56, and 13.34%, respectively. As the absorption profiles of IDT-IC-Cl and IDT-TC are complementary to each other, we have incorporated IDT-TC as the guest acceptor in the P: IDT-IC-Cl active layer to fabricate the ternary (P:IDT-TC: IDT-IC-Cl) PSC, demonstrating a PCE of 16.44%, which is significantly higher than that of the binary BHJ devices. The improvement in PCE for ternary PSCs is attributed to the efficient exploitation of excitons via energy transfer from IDT-TC to IDT-IC-Cl, suitable nanoscale phase separation, compact stacking distance, and more evenly distributed charge transport.

Challenging PM6-like donor polymers for pairing with a Y-type state-of-the-art acceptor in binary blends for bulk heterojunction solar cells.

Being fluorine-free and a high performance material as a small organic acceptor molecule, BTP-eC9 has been well mixed with BDT-based PM6 donor polymers for providing satisfactory photovoltaic properties, especially towards future large scale/large area solar cell production. However, as one of the key electrical outputs from such binary active layer materials, the open circuit voltage (VOC) was limited to ca. 0.84 V, which needs to be further improved for BTP-eC9 to have a bright future. This paper focuses on the molecular design of alkylthio- and alkoxy-phenyl flanked benzo[1,2-b:4,5-b']dithiophene-based conjugated polymers (PBDT-PS-ttTPD or P10 for short and PBDT-PO-ttTPD or P11), which were successfully synthesized and applied as donor materials for pairing with BTP-eC9 in organic photovoltaic (OPV) devices. By fine-tuning the side chains of the benzodithiophene (BDT) moiety, such non-fullerene OPV devices with normal configuration demonstrate an attractively high open circuit voltage (VOC) of 0.89 and 0.87 V in P10/BTP-eC9 and P11/BTP-eC9 based binary single bulk heterojunction OPV devices, while still maintaining an excellent JSC of 22.7 and 20.0 mA cm-2 with a final power conversion efficiency (PCE) of 12.93% and 9.37%, respectively. The alkylthio-phenyl chain substituted BDT polymer exhibits better photovoltaic performance in all aspects than the alternative with alkoxy chains due to the synergistic effect of the alkylthio-phenyl flanked BDT, TPD, and π-bridge (thieno[3,2-b]thiophene).

Organic photovoltaic performance and structural relationship of non-fullerene small molecule acceptors based on a tetraarylphenazine core and perylene diimide

Two small molecular acceptors based on perylene diimide tetramer with a phenazine core and benzene or thiophene linkers are developed.

Design and screening of B–N functionalized non-fullerene acceptors for organic solar cells via multiscale computation

Multiscale computation driven molecular engineering of small molecule non-fullerene acceptors for organic solar cells.

Side-chain engineering of nonfullerene small-molecule acceptors for organic solar cells

This review summarizes the side-chain engineering of nonfullerene small-molecule acceptors (SMAs) in the field of organic solar cells, discusses the key structure–property relationships in depth and the future directions of side-chain engineering.

Third component with a high LUMO energy level enables 17.69% efficiency in ternary organic solar cells

The ternary strategy is thought to be a practical method for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). To create ternary OSCs, a PM6:BTP-BO-4F host blend was combined with the non-fullerene small molecule acceptor BTA3, which has a high lowest unoccupied molecular orbital (LUMO). BTA3 demonstrated complementary absorption spectra and excellent compatibility with BTP-BO-4F, which made it easier to modulate the ternary film's morphology and catch more photons. As a result, the short circuit current density increased. Additionally, BTA3's LUMO level was marginally higher than BTP- BO-4F's, which led to a higher open-circuit voltage of the TOSCs. The addition of BTA3 improved the OSCs' energy transfer and phase separation, which in turn sped up charge transfer and reduced exciton recombination. The PCE of the TOSCs included with 10 wt% BTA3 rose to 17.69% in comparison to a PCE of 16.46% based on host binary OSCs. This research demonstrated that the incorporation of BTA3 with a high LUMO level enabled the fabrication of ternary OSCs with good performance.

Unraveling the Stretch-Induced Microstructural Evolution and Morphology-Stretchability Relationships of High-Performance Ternary Organic Photovoltaic Blends.

The stretchability and stretch-induced structural evolution of organic solar cells (OSCs) are pivotal for their collapsible, portable, and wearable applications, and they are mainly affected by the complex morphology of active layers. Herein, a highly ductile conjugated polymer P(NDI2OD-T2) is incorporated into the active layers of high-efficiency OSCs based on nonfullerene small molecule acceptors to simultaneously investigate the morphological, mechanical, and photovoltaic properties and structural evolution under stretching of ternary blend films with various acceptor contents. The structural robustness of the blend films is indicated by their stretch-induced structural evolution, which is monitored in real-time by a combination of in situ wide/small angle X-ray scattering. It is found that adding the soft P(NDI2OD-T2) can enhance the stretchability and structural robustness of ternary blend films by more entangled chains and tie chains to dissipate strain. Furthermore, the stretchability of the ternary blends can be superbly predicted by a 3D equivalent box model. This work provides instructive insight and guidance for designing stretchable electronics and predicting the stretchability of multicomponent blends.

Statistical analysis and visualization of data of non-fullerene small molecule acceptors from Harvard organic photovoltaic database. Structural similarity analysis with famous non-fullerene small molecule acceptors to search new building blocks

Data-driven material design has gained the position of “fourth paradigm” with the first three being experiments, theory, and simulation. The statistical analysis and visualization of data can provide many information about hidden trends of data. It also can help to plan future material design in efficient way. Traditional methods need long experimental time and much money. Data driven research has become new hope to revolutionize materials discovery. A detail statistical and visualization analysis of Harvard organic photovoltaic database is performed. Various new information is found. Similarity analysis is also performed. Famous experimentally reported non-fullerene small acceptors are selected as reference molecules. Very less similarity is found that is indicating that acceptors in Harvard organic photovoltaic database are much different from acceptors that are reported in literature. The molecules in database are not suitable acceptors, however, their suitable combination can result efficient acceptors of organic solar cells. New acceptors are designed using some building block from database and their electronic properties are studied using density functional theory. Our data mining assisted material design framework is based on unique approach that is rare in photovoltaics research. This framework will be helpful to find versatile building blocks.

Small-Molecule Acceptors with Asymmetric Thieno[3,2-c]isochromene Bridged Units for Boosting the Efficiency and Stability of Fullerene-Based Organic Solar Cells

Ternary strategy is an effective approach to boost the photovoltaic performance of organic solar cells (OSCs). However, the ternary OSCs using a nonfullerene small-molecule acceptor (SMA) as the third component into the binary fullerene–donor system have not received satisfactory progress. Here, we developed a simple A–D–D′–D–A-type SMA of BDT–2TiC16–2CN, employing benzodithiophene (BDT) as an electron-donating (D′) centra, asymmetric bis(hexadecyl) thieno[3,2-c]isochromene (TiC) as an electron-donating (D) bridged unit, and cyanoindanone (CN) as a strong electron-withdrawing (A) terminal. Its thermal, optical, electrochemical, and photovoltaic properties were systemically studied. It is demonstrated that BDT–2TiC16–2CN exhibits absorption maxima at 703 nm as well as the cascaded HOMO and LUMO energy levels (−5.61/–3.96 eV) with polymers PM6 and PC71BM. Encouragingly, incorporating the BDT–2TiC16–2CN guest into the PM6:PC71BM blend, the ternary OSCs exhibit better photovoltaic properties and higher device stability than the binary OSCs. An improved PCE of 9.39% is obtained in the ternary OSCs, which is increased by 12.2% in comparison with that of the binary OSCs (8.37%). More importantly, 73.71% of the initial PCE is still maintained after 1440 h at room temperature and 67.22% after 1440 min at 65 °C. Our work indicates that BDT–2TiC16–2CN with an asymmetric TiC bridged unit is a potential SMA used as a third component to improve the device efficiency and stability for the ternary fullerene OSCs.

A Polyfluoroalkyl-Containing Non-fullerene Acceptor Enables Self-Stratification in Organic Solar Cells.

The elaborate control of the vertical phase distribution within an active layer is critical to ensuring the high performance of organic solar cells (OSCs), but is challenging. Herein, a self-stratification active layer is realised by adding a novel polyfluoroalkyl-containing non-fullerene small-molecule acceptor (NFSMA), EH-C8 F17 , as the guest into PM6:BTP-eC9 blend. A favourable vertical morphology was obtained with an upper acceptor-enriched thin layer and a lower undisturbed bulk heterojunction layer. Consequently, a power conversion efficiency of 18.03 % was achieved, higher than the efficiency of 17.40 % for the device without EH-C8 F17 . Additionally, benefiting from the improved charge transport and collection realised by this self-stratification strategy, the OSC with a thickness of 350 nm had an impressive PCE of 16.89 %. The results of the study indicate that polyfluoroalkyl-containing NFSMA-assisted self-stratification within the active layer is effective for realising an ideal morphology for high-performance OSCs.

A Polyfluoroalkyl‐Containing Non‐fullerene Acceptor Enables Self‐Stratification in Organic Solar Cells

AbstractThe elaborate control of the vertical phase distribution within an active layer is critical to ensuring the high performance of organic solar cells (OSCs), but is challenging. Herein, a self‐stratification active layer is realised by adding a novel polyfluoroalkyl‐containing non‐fullerene small‐molecule acceptor (NFSMA), EH‐C8F17, as the guest into PM6:BTP‐eC9 blend. A favourable vertical morphology was obtained with an upper acceptor‐enriched thin layer and a lower undisturbed bulk heterojunction layer. Consequently, a power conversion efficiency of 18.03 % was achieved, higher than the efficiency of 17.40 % for the device without EH‐C8F17. Additionally, benefiting from the improved charge transport and collection realised by this self‐stratification strategy, the OSC with a thickness of 350 nm had an impressive PCE of 16.89 %. The results of the study indicate that polyfluoroalkyl‐containing NFSMA‐assisted self‐stratification within the active layer is effective for realising an ideal morphology for high‐performance OSCs.

Two asymmetric small molecule acceptors with aromatic and non-aromatic ring side chains were developed for organic solar cells

Asymmetric side-chain engineering has been demonstrated to be an effective approach for modifying the solubility and morphology of non-fullerene small molecule acceptors (NF-SMAs). However, prior research has mostly concentrated on optimizing the length and branching locations of alkyl chains. Phenyl and cyclohexyl groups suspended on the side chains have been shown to be effective for tuning the crystallinity and self-assembly of symmetric organic molecules for efficient organic solar cells. To investigate the impact of the aromatic and non-aromatic ring side chains on the performance of asymmetric NF-SMAs, two asymmetric acceptors, Y-Ph4F and Y–CH4F, are designed with phenyl and cyclohexyl as two distinct side chains, respectively, in this work. Due to their common backbone, the two NF-SMAs exhibit similar absorption spectra and energy levels as determined by UV–vis absorption, cyclic voltammetry, and density functional theory calculations. Differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) demonstrate that Y–CH4F with 4-cyclohexylbutyl side chains has stronger molecular packing and crystallinity in the solid state compared to Y-Ph4F with 4-phenylbutyl side chains. By pairing these NFAs with a well-known polymer donor PM6, both PM6:Y-Ph4F and PM6:Y-Ph4F-based organic solar cells (OSCs) achieve power conversion efficiencies (PCEs) of more than 15%. When fabricated with a 1 cm2 active area, the Y-Ph4F and Y–CH4F-based devices can also obtain exceptional PCEs of 15.05% and 14.10%, respectively. These findings suggest that the aromatic and non-aromatic ring side chains can effectively tune the molecular packing and crystallinity for efficient OSCs.

Picosecond Charge-Transfer-State Dynamics in Wide Band Gap Polymer-Non-Fullerene Small-Molecule Blend Films Investigated via Transient Infrared Spectroscopy.

Organic solar cells based on wide band gap polymers and nonfullerene small-molecule acceptors have demonstrated remarkably good device performances. Nevertheless, a thorough understanding of the charge-transfer process in these materials has not been achieved yet. In this study, we use Fano resonance signals caused by the interaction of broad electronic charge carrier absorption and the molecular vibrations of the electron acceptor molecule to monitor the charge-transfer state dynamics. In our time-resolved infrared spectroscopy experiments, we find that in the small-molecule acceptor, they have additional dynamics on the order of a few picoseconds. A change in the solvent used in thin film deposition, leading to different morphologies, influences this time further. We interpret our findings as the dynamics of the charge-transfer state at the interface of the electron donor and the electron- acceptor. The additional mid-infrared transient signal is generated in this state, as both electron and hole polarons can interact with small-molecule acceptor vibrational modes.

Recent Advances in Organic and Inorganic Hole and Electron Transport Layers for Organic Solar Cells: Basic Concept and Device Performance

The emergence of nonfullerene small-molecule acceptors (NFSMA) with the advantages of synthetic versatility, high absorption coefficient in wide wavelength range, and high thermal stability has attained the power conversion efficiency (PCE) exceeding 19% for resulted organic solar cells (OSCs) with the optimization of interface engineering and active layer morphology. Interfacial layers including both hole transporting layer (HTL) and electron transporting layer (ETL) are equally important in the OSCs for facilitating electron and hole extraction from the bulk heterojunction (BHJ) photoactive layer by the respective electrodes. In this Review, we summarize the recent progress in the materials used as HTL and ETL in conventional and inverted OSCs on the basis of their effect on the PCE. Finally, the prospects of HTL and ETL materials for NFSMA-OSCs will be provided.

Precise Control of Selenium Functionalization in Non‐Fullerene Acceptors Enabling High‐Efficiency Organic Solar Cells

AbstractCentral π‐core engineering of non‐fullerene small molecule acceptors (NF‐SMAs) is effective in boosting the performance of organic solar cells (OSCs). Especially, selenium (Se) functionalization of NF‐SMAs is considered a promising strategy but the structure‐performance relationship remains unclear. Here, we synthesize two isomeric alkylphenyl‐substituted selenopheno[3,2‐b]thiophene‐based NF‐SMAs named mPh4F‐TS and mPh4F‐ST with different substitution positions, and contrast them with the thieno[3,2‐b]thiophene‐based analogue, mPh4F‐TT. When placing Se atoms at the outer positions of the π‐core, mPh4F‐TS shows the most red‐shifted absorption and compact molecular stacking. The PM6 : mPh4F‐TS devices exhibit excellent absorption, high charge carrier mobility, and reduced energy loss. Consequently, PM6 : mPh4F‐TS achieves more balanced photovoltaic parameters and yields an efficiency of 18.05 %, which highlights that precisely manipulating selenium functionalization is a practicable way toward high‐efficiency OSCs.

Precise Control of Selenium Functionalization in Non-Fullerene Acceptors Enabling High-Efficiency Organic Solar Cells.

Central π-core engineering of non-fullerene small molecule acceptors (NF-SMAs) is effective in boosting the performance of organic solar cells (OSCs). Especially, selenium (Se) functionalization of NF-SMAs is considered a promising strategy but the structure-performance relationship remains unclear. Here, we synthesize two isomeric alkylphenyl-substituted selenopheno[3,2-b]thiophene-based NF-SMAs named mPh4F-TS and mPh4F-ST with different substitution positions, and contrast them with the thieno[3,2-b]thiophene-based analogue, mPh4F-TT. When placing Se atoms at the outer positions of the π-core, mPh4F-TS shows the most red-shifted absorption and compact molecular stacking. The PM6 : mPh4F-TS devices exhibit excellent absorption, high charge carrier mobility, and reduced energy loss. Consequently, PM6 : mPh4F-TS achieves more balanced photovoltaic parameters and yields an efficiency of 18.05 %, which highlights that precisely manipulating selenium functionalization is a practicable way toward high-efficiency OSCs.

Improving the performance of PM6 donor polymer by random ternary copolymerization of BDD and DTBT segments

Developing high-performance wide bandgap polymer donors to match the rapidly growing non-fullerene small molecule acceptors (NF-SMAs) is of great importance to further improve the photovoltaic performances of organic solar cells (OSCs). PM6 and D18 are two leading wide bandgap donor polymers, but their performances are highly sensitive to the molecular weights, which have distinct impact on the morphology and thereby the charge generation and recombination processes. In this work, we reported the random ternary copolymerization of PM6 and D18 segments to develop random terpolymers with finely tuned photoelectronic properties and optimized blend morphology for high-performance OSCs. The resulting random terpolymers (PMD-5, PMD-10, PMD-15 and PMD-20) exhibited finely tuned absorptions, energy levels and blend morphology. The optimal morphology of PMD-15:L8-BO contributed to the fast exciton diffusion and dissociation, efficient charge extraction with suppressed recombination. Thus, the power conversion efficiency was improved from 17.22% of the control PM6 devices and 17.47% of the control D18 devices to 18.12% of PMD-15 based devices, which is the top performance of random terpolymers. Moreover, the PMD-15 exhibited much higher batch-to-batch reproducibility at a broad molecular weight range than that of PM6 and D18, showing promising for practical applications.

Asymmetric Non-Fullerene Small Molecule Acceptor with Unidirectional Non-Fused π-Bridge and Extended Terminal Group for High-Efficiency Organic Solar Cells.

We designed and synthesized an asymmetric non-fullerene small molecule acceptor (NF-SMA) IDT-TNIC with an A–D–π–A structure, based on an indacenodithiophene (IDT) central core, with a unidirectional non-fused alkylthio-thiophene (T) π-bridge, and 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NIC) extended terminal groups. IDT-TNIC molecules still maintain a good coplanar structure, which benefits from the non-covalent conformational locks (NCL) between O···S and S···S. The asymmetric structure increases the molecular dipole moment, and the extended terminal group broadens the absorption of the material, resulting in an excellent photovoltaic performance of IDT-TNIC. The photovoltaic device, based on PBDB-T:IDT-TNIC, exhibits an energetic PCE of 11.32% with a high Voc of 0.87 V, high Jsc of 19.85 mA cm−2, and a low energy loss of 0.57 eV. More importantly, IDT-TNICs with asymmetric structures show a superior property compared to symmetric IDT-Ns. The results demonstrate that it is an effectual strategy to enhance the properties of asymmetric A–D–π–A-based NF-SMAs with non-fused NCL π-bridges and extended terminal groups.

Synthesizing Near‐Ultraviolet‐Absorbed Donor for Transparent Polymer Solar Cells

Wavelength‐selective transparent polymer solar cells (T‐PSCs) have huge developing potential in power‐window applications for their excellent visible transparency and high color‐rendering index. However, the main research interest of devices until now is focused on near‐infrared absorbers for both polymer donor and nonfullerene small‐molecule acceptor (NF‐SMA) due to the lack of appropriate near‐ultraviolet absorbers. Herein, the successful implementation of the T‐PSC devices based on novel kinds of near‐ultraviolet polymer donors combined with a near‐infrared NF‐SMA Y6 is reported. The reported polymer donors are “D1–D2”‐type copolymers employing fluorene and triphenylamine as the two electron‐donating units. The best‐performed one not only shows ultrawide optical bandgap (2.87 eV), leading to high average visible transmittance (AVT) of 96.8% in the thin film, but also has efficient hole mobility (about 5.0 × 10−6 cm2 V−1 s−1), well‐matched frontier molecular orbital energy levels, and good miscibility with Y6. As a result, the T‐PSC device is obtained with an AVT of 51.4% and PCE of 3.15%, along with favorable light utilization efficiency (LUE) of 1.62% and CRI of 91.6. This is the first example that uses near‐ultraviolet‐absorbed polymer donor to achieve high‐efficiency T‐PSC devices.

Impact of end-capped modification of MO-IDT based non-fullerene small molecule acceptors to improve the photovoltaic properties of organic solar cells

Density functional theory, along with its time dependent computational approach were employed in order to fine tune the photovoltaic attributes along with the efficiency of the MO-IDIC-2F molecule. Thus, five new molecules were designed by substitution of the different notable acceptor fragments in the MO-IDIC-2F molecule, along with the addition of the “[1, 2, 5] thiadiazolo[3,4-d] pyridazine” spacer moieties between donor core and newly substituted acceptor groups. In this research work, various photovoltaic properties, which could affect the efficiency of an organic chromophores, such as bandgap, oscillator strength, dipole moment, binding energy, light-harvesting efficiency, etc. were studied. All the newly proposed molecules demonstrated significantly improved outcomes in comparison to that of the reference molecule, in their absorption spectrum, excitation, as well as binding energy values, etc. In order to confirm the results of optoelectronic properties, density of states, transition density matrix, and electrostatic potential analyses of molecules were also performed, which supported our computational findings. All of the results confirmed the high potential of all the newly proposed molecules for the development of improved OSCs.