Conjugated Side Chains Research Articles

Highly crystalline acceptor materials based on benzodithiophene with different amount of fluorine substitution on alkoxyphenyl conjugated side chains for organic photovoltaics

Organic solar cells based on narrow bandgap small-molecule acceptors (SMAs) with highly crystalline characteristics have attracted great attentions for their superiority in obtaining high photovoltaic efficiency. Employing highly crystalline SMAs to enhance power conversion efficiencies (PCEs) by regulating and controlling morphology and compatibility of donor and acceptor materials has turned out to be an effective approach. In this study, we synthesized three different crystalline SMAs by using fluorine substitution on alkoxyphenyl conjugated side chains to modulate the relationship of crystallinity and morphologies, namely ZY1 (zero F atoms), ZY2 (two F atoms), and ZY3 (four F atoms). The three SMAs show the broad absorption edges and similar frontier orbital energy levels, generating the analogical (over 0.9 ​V) open circuit voltage (VOC) of the polymer solar cells (PSCs). As a result, the PM6:ZY2-based PSCs yield a PCE of 10.81% with a VOC of 0.95 ​V, a short-circuit current density (JSC) of 16.154 ​mA ​cm−2, and a fill factor (FF) of 0.71, which is higher than that of 9.17% (PM6:ZY1) and 6.37% (PM6:ZY3). And the PCE (17.23%) of the PM6:Y6:ZY2 based ternary PSCs is also higher than that of 16.32% PM6:Y6 based binary device. Obviously, the results demonstrate that adding fluorine atoms on the conjugated side chains to construct high crystalline materials is a positive strategy to effectively increase the efficiencies of binary and ternary PSCs.

Efficiency Improvement of All‐Small‐Molecule Organic Solar Cells Through Fused‐Aromatic‐Ring Side‐Chained Donors

Side‐chain engineering influences the organic solar cell (OSC) devices active layer morphology fundamentally and it is an efficient strategy to improve device performances. A novel small molecular donor with conjugated fused‐aromatic‐ring side chain on a benzo[1,2‐b:4,5‐b′]dithiophene core is developed. To the best of our knowledge, this is the first example where fused‐aromatic‐ring units are being utilized as conjugated side chains to extend the conjugation system of small molecular donor materials. The material property, OSC device performance, active layer morphology, and photovoltaic mechanism are investigated. This molecular design strategy has proven to be effective in fine‐tuning and enhancing the molecular stacking/orientation and phase separation, which led to the enhanced charge separation and transportation, suppressed charge recombination, and as a result, devices with an improved short‐circuit current density of 24.9 mA cm−2, fill factor of 69.9%, and power conversion efficiency of 13.6% are achieved. The results reveal that a fused‐aromatic‐ring utilized as conjugated side chain strategy can potentially be an effective way for molecular rational design, aiming to improve the OSC performances.

Effect of Arylmethylene Substitutions on Molecular Structure, Optoelectronic Properties and Photovoltaic Performance of Dithienocyclopentafluorene‐Based Small‐Molecule Acceptors

Two dithienocyclopentafluorene-based small-molecule acceptors (SMAs) were developed that feature methylene-functionalized conjugated side chains, to study the effect of arylmethylene substitution and its number on structure, optoelectronic properties and device performance. Results showed that two SMAs have better absorption properties and planarity, lower bandgaps and higher LUMOs compared with the control SMA without conjugated side chains. The synthesized SMAs were tested in polymer solar cells for examples of their applicability. This work argues that the introduction of methylene-functionalized conjugated side chains has great potential in tuning molecular structure, optoelectronic properties, device physics and photovoltaic performance of SMAs.

Structure–Property Relationships of Precisely Chlorinated Thiophene‐Substituted Acceptors

AbstractSystematic investigation of three nonfullerene acceptors, BTIC‐4Cl‐T, BTIC‐4Cl‐TCl‐γ, and BTIC‐4Cl‐TCl‐b, with or without a chlorine substituent at the γ/b‐position of the side chain thiophene ring, reveals that molecular planarity, stacking structure, and photovoltaic performance of the compounds are dependent on the position of the chlorine substituent. Of the materials using thiophenes in conjugated side chains, BTIC‐4Cl‐T shows a relatively lower open‐circuit voltage of 0.81 V, decreased current density, leading to an efficiency of only 10.86%. BTIC‐4Cl‐TCl‐γ with chlorine at the γ‐position of the conjugated thiophene shows a 3D network structure, a greatly increased current density, and an efficiency of 14.35%. BTIC‐4Cl‐TCl‐b, with a chlorine atom in b‐position, is found to have been reformed to a quasi‐3D network, in which electron hopping can be efficiently realized in adjacently positioned, linearly arranged molecules due to S···S interactions. With this quasi‐3D network, BTIC‐4Cl‐TCl‐b promotes the open‐circuit voltage up to 0.86 V and has the highest efficiency (15.65%) among the three acceptors. These results prove that chlorination is an effective strategy to improve photovoltaic performance and highlights the decisive relationship between structural regulation and molecular arrangement. It also provides a good starting point for the exploration and design of next generation high‐performance materials.

Synergetic Effect of Side-Chain Engineering of Polymer Donors and Conformation Tuning of Small-Molecule Acceptors on Molecular Properties, Morphology, and Photovoltaic Performance

It is demonstrated that the side-chain engineering of polymer donors and molecular conformation of small-molecule acceptors (SMAs) plays a crucial role in the morphology and photovoltaic performance of polymer solar cells (PSCs). However, the synergetic effect of these two aspects has been rarely reported. Herein, two wide-band gap donor–acceptor (D–A) copolymers (PST-TTC and PSPT-TTC) featuring the same main-chain backbone but different conjugated side chains were developed and combined with three low-band gap symmetric SMAs (ITIC, DTCFO-ICCl, and Y6) with various molecular conformations (S-shape, C-shape, and U-shape) to investigate the synergetic effect of side-chain engineering of polymer donors and conformation manipulation of SMAs on molecular properties, device physics, film morphology, and photovoltaic performance. The results indicate that PST-TTC with an alkylthiothiophene side chain exhibits a broader absorption spectrum, a smaller optical band gap (1.95 vs 1.99 eV), and a deeper-lying highest occupied molecular orbital (HOMO) level (−5.39 vs −5.36 eV) as compared to PSPT-TTC with an alkylthiophenylthiophene side chain. The PSCs were constructed according to the different donor/acceptor combinations based on the two as-synthesized polymers and three selected SMAs. After optimization, PST-TTC paired with S-shaped ITIC provides a power conversion efficiency (PCE) of 10.10% and the PCE can be further improved up to 10.60% when C-shaped DTCFO-ICCl was used to replace ITIC as the acceptor. However, the device performance becomes worse, accompanied with a sharply reduced PCE of 7.36% after the substitution of ITIC for U-shaped Y6. On the contrary, when the paired donor was changed to PSPT-TTC, the PSPT-TTC:Y6-based PSC achieves a remarkably increased PCE of up to 11.46%. These delicate observations suggest that the polymer donor with different conjugated side chains should collaborate with symmetric SMAs possessing various molecular conformations to realize superior morphology and device performance.

Improving Mobility–Stretchability Properties of Polythiophene Derivatives through Ester-Substituted, Biaxially Extended Conjugated Side Chains

P3HT, as one of the most important conjugated polymers, has been reported to possess low mobility and poor stretchability. In this study, two polythiophenes are prepared by attaching ester-substitu...

Optimization of active layer morphology by small-molecule donor design enables over 15% efficiency in small-molecule organic solar cells

Two conjugated small molecules with different conjugated side chains were synthesized to study their photovoltaic performances.

Introducing methoxy or fluorine substitutions on the conjugated side chain to reduce the voltage loss of organic solar cells

Herein, three benzotriazole (BTA)-based polymers and a fused benzotriazole-based small molecule were constructed to realize a relatively high power conversion efficiency (PCE) of above 11% with a high open-circuit voltage (VOC) approaching 0.99 V.

Highly efficient fused ring electron acceptors based on a new undecacyclic core

Two FREAs, IUIC-O and IUIC-T, based on a large fused ring donor core (undecacyclic unit) were developed. IUIC-T, having 2D conjugated side chains, affords a higher PCE of 13.05%.

Design of Thienothiophene-Based Copolymers with Various Side Chain-End Groups for Efficient Polymer Solar Cells.

Three two-dimensional donor–acceptor conjugated copolymers consisting of a benzo[1,2-b:4,5-b′]dithiophene derivative and thieno[3,2-b]thiophene with a conjugated side chain were designed and synthesized for use in bulk heterojunction (BHJ) or nonfullerene polymer solar cells (PSCs). Through attaching various acceptor end groups to the conjugated side chain on the thieno[3,2-b]thiophene moiety, the electronic, photophysical, and morphological properties of these copolymers were significantly affected. It was found that the intermolecular charge transfer interactions were enhanced with the increase in the acceptor strength on the thieno[3,2-b]thiophene moiety. Moreover, a better microphase separation was obtained in the copolymer: PC71BM or ITIC blend films when a strong acceptor end group on the thieno[3,2-b]thiophene moiety was used. As a result, BHJ PSCs based on copolymer:PC71BM blend films as active layers exhibited power conversion efficiencies from 2.82% to 4.41%, while those of nonfullerene copolymer:ITIC-based inverted PSCs ranged from 6.09% to 7.25%. These results indicate the side-chain engineering on the end groups of thieno[3,2-b]thiophene unit through a vinyl bridge linkage is an effective way to adjust the photophysical properties of polymers and morphology of blend films, and also have a significant influence on devices performance.

A combined density functional theory and x-ray photoelectron spectroscopy study of the aromatic amino acids

Amino acids are essential to all life. However, our understanding of some aspects of their intrinsic structure, molecular chemistry, and electronic structure is still limited. In particular the nature of amino acids in their crystalline form, often essential to biological and medical processes, faces a lack of knowledge both from experimental and theoretical approaches. An important experimental technique that has provided a multitude of crucial insights into the chemistry and electronic structure of materials is x-ray photoelectron spectroscopy. While the interpretation of spectra of simple bulk inorganic materials is often routine, interpreting core level spectra of complex molecular systems is complicated to impossible without the help of theory. We have previously demonstrated the ability of density functional theory to calculate binding energies of simple amino acids, using ΔSCF implemented in a systematic basis set for both gas phase (multiwavelets) and solid state (plane waves) calculations. In this study, we use the same approach to successfully predict and rationalise the experimental core level spectra of phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and histidine (His) and gain an in-depth understanding of their chemistry and electronic structure within the broader context of more than 20 related molecular systems. The insights gained from this study provide significant information on the nature of the aromatic amino acids and their conjugated side chains.

Benzotriazole Based 2D-conjugated Polymer Donors for High Performance Polymer Solar Cells

To design high efficiency polymer solar cells (PSCs), it is of great importance to develop suitable polymer donors that work well with the low bandgap acceptors, providing complementary absorption, forming interpenetrating networks in the active layers and minimizing energy loss. Recently, we developed a series of two-dimension-conjugated polymers based on bithienylbenzodithiophene-alt-benzotriazole backbone bearing different conjugated side chains, generally called J-series polymers. They are medium energy bandgap (Eg) polymers (Eg of ca. 1.80 eV) with strong absorptions in the range of 400–650 nm, and exhibit ordered crystalline structures, high hole mobilities, and more interestingly, tunable energy levels depending on the structure variations. In this feature article, we highlight our recent efforts on the design and synthesis of those J-series polymer donors, including an introduction on the polymer design strategy and emphasis on the crucial function of differential conjugated side chain. Finally, the future opportunities and challenges of the J-series polymers in PSCs are discussed.

VOC enhancement of thienobenzofuran and benzotriazole backboned photovoltaic polymer by side chain sulfuration or fluoridation

In this work, four wide-bandgap (WBG) conjugated polymers based on asymmetrical building block thieno [2,3f]benzofuran (TBF) and fluorobenzotriazole (BZ), are synthesized and applied as donor materials in polymer solar cells (PSCs). The effect of conjugated side chains (alkylthiothiophenyl, alkylthiophenyl, alkylphenyl and fluoroalkoxyphenyl) on the optical absorption, electrochemical and photovoltaic properties are investigated in detail. These polymers exhibit similar properties, strong absorption from 300 to 650 nm with wide bandgaps ranging from 1.87 to 1.96 eV, excellent thermal stability, the high and balanced carrier mobility. Notably, the open-circuit voltages (VOC) of PTSDO-BZ increases by 0.16 V compared with PTDO-BZ. Similarly, larger VOC changes have also occurred in PPEH-BZ and PPFOEH-BZ comparation. The enhanced VOC can be ascribed to the low-lying highest occupied molecular orbital (HOMO) energy level by incorporating sulfur and fluorine substituents on the conjugated side chains. The as-cast devices of PTSDO-BZ:ITIC exhibit the highest power conversion efficiency (PCE) of 8.66%, with a VOC of 0.88 V, a short-circuit current density (JSC) of 15.06 mA cm−2, and a fill factor (FF) of 65.12%. The results indicate that the modification of the side chains of the TBF unit can effectively lower the HOMO energy levels and thereby obtaining high VOC.

Developing Wide Bandgap Polymers Based on Sole Benzodithiophene Units for Efficient Polymer Solar Cells.

In this work, a series of sole benzodithiophene-based wide band gap polymer donors, namely PBDTT, PBDTS, PBDTF and PBDTCl, were developed for efficient polymer solar cells (PSCs) by varying the heteroatoms into the conjugated side chains. The effects of sulfuration, fluorination and chlorination were also investigated systematically on the overall properties of these BDT-based polymers. The HOMO levels could be lowered gradually by introducing sulfur, fluorine and chlorine atoms into the side chains, which contributed to the stepwise increased Voc (from 0.78 V to 0.84 V) in the related PSCs using Y6 as the electron acceptor. This side-chain engineering strategy could promote the polymer chain interactions and fine-tune the phase separation of active blends, leading to enhanced absorption, ordered molecular packing and crystallinity. Among them, the chlorinated PBDTCl exhibited not only high level absorption and crystallinity, but also the most balanced hole/electron charge transport and the most optimized morphology, giving rise to the best PCE of 13.46 % with a Voc of 0.84 V, a Jsc of 23.16 mA cm-2 and an FF of 69.2 %. The chlorination strategy afforded PBDTCl synthetic simplicity but high efficiency, showing its promising photovoltaic applications for realizing low-cost practical PSCs in near future.

Development of Ruthenium Complex Based Sensitizers, Organic Based Sensitizers, and Co-sensitization System for Dye-sensitized Solar Cells

This paper reviews the recent development of ruthenium complex sensitizers, organic sensitizers, and co-sensitizers for dye-sensitized solar cell (DSSC) applications. The development of ruthenium sensitizers has progressed from thiocyanate (NCS) ligands containing bipyridyl, terpyridyl, quarterpyridyl, and cyclometalated ruthenium sensitizers to NCS-free analogs. The integration of organic moieties and ruthenium complex ancillary ligands into a single structure entity was also achieved in the DSSC industry. The development of organic sensitizers as an alternative to ruthenium-based sensitizers was also discussed according to their functional segment: donor, acceptor, and π-spacer. The modification of donor groups has been focused on introducing ancillary groups to increase the molar extinction coefficient and electron recombination resistance. Anchoring groups other than cyanoacrylic acids have been developed to improve the anchoring ability of organic sensitizers while maintaining their light-harvesting properties. A study was performed on π-spacers modified with different conjugation lengths and conjugated side chains in order to enhance the charge transfer within organic sensitizers. Co-sensitization systems, including metal-metal, metal-organic, and organic-organic systems with stepwise and “cocktail” co-sensitization methods will be discussed in this review.

Wide Band-gap Two-dimension Conjugated Polymer Donors with Different Amounts of Chlorine Substitution on Alkoxyphenyl Conjugated Side Chains for Non-fullerene Polymer Solar Cells

In this study, wide bandgap (WBG) two-dimensional (2D) copolymer donors (DZ1, DZ2, and DZ3) based on benzodithiophene (BDT) on alkoxyphenyl conjugated side chains without and with different amounts of chlorine atoms and difluorobenzotriazole (FBTZ) are designed and synthesized successfully for efficient non-fullerene polymer solar cells (PSCs). Three polymer donors DZ1, DZ2, and DZ3 display similar absorption spectra at 300–700 nm range with optional band-gap (Egopt) of 1.84, 1.92, and 1.97 eV, respectively. Compared with reported DZ1 without chlorine substitution, it is found that introducing chlorine atoms into the meta-position of the alkoxyphenyl group affords polymer possessing a deeper the highest occupied molecular orbital (HOMO) energy level, which can increase open circuit voltage (VOC) of PSCs, as well as improve hole mobility. Non-fullerene bulk heterojunction PSCs based on DZ2:MeIC demonstrate a relatively high power conversion efficiency (PCE) of 10.22% with a VOC of 0.88 V, a short-circuit current density (JSC) of 17.62 mA/cm2, and a fill factor (FF) of 68%, compared with PSCs based on DZ1:MeIC (a PCE of 8.26%) and DZ3:MeIC (a PCE of 6.28%). The results imply that adjusting chlorine atom amount on alkoxyphenyl side chains based on BDT polymer donors is a promising approach of synthesizing electron-rich building block for high performance of PSCs.

Dopant-free hole transporting materials with supramolecular interactions and reverse diffusion for efficient and modular p-i-n perovskite solar cells

The rational design of dopant-free organic hole-transporting layer (HTL) materials is still a challenge for realizing high-efficient and stable p-i-n planar perovskite solar cells (pero-SCs). Here, we synthesized two π-conjugated small-molecule HTL materials through tailoring the backbone and conjugated side chain to carefully control molecular conformation. The resultant BDT-TPA- s Th containing a planar fused benzo[1,2- b :4,5- b′ ]dithiophene (BDT) core and a conjugated thiophene side chain showed the planar conformation. X-ray crystallography showed a favorable stacking model in solid states under the parallel-displaced π-π and additional S-π weak-bond supramolecular interactions, thus achieving an obviously increased hole mobility without dopants. As an HTL material in p-i-n planar pero-SCs, the marginal solubility of BDT-TPA- s Th enabled inverse diffusion into the perovskite precursor solution for assisting the subsequent perovskite film growth and passivating the uncoordinated Pb2+ ion defects. As a result, the planar p-i-n pero-SCs exhibited a champion power conversion efficiency (PCE) of 20.5% and enhanced moisture stability. Importantly, the BDT-TPA- s Th HTL material also showed weak thickness-photovoltaic dependence, and the pero-SCs with blade-coated BDT-TPA- s Th as a HTL achieved a 15.30% PCE for the 1-cm2 modularized device. This HTL material design strategy is expected to pave the way toward high-performance, dopant-free and printing large-area planar p-i-n pero-SCs.

A Non‐Fullerene Acceptor with Chlorinated Thienyl Conjugated Side Chains for High‐Performance Polymer Solar Cells via Toluene Processing

Summary of main observation and conclusionSmall 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.

Structure–Mobility Relationship of Benzodithiophene-Based Conjugated Polymers with Varied Biaxially Extended Conjugated Side Chains

Semi-two-dimensional (semi-2D) benzo[1,2-b:4,5-b′]dithiophene (BDT)-based donor–acceptor copolymers have been regarded as one of the most successful organic semiconductors for photovoltaic applicat...

Highly Efficient All-Small-Molecule Organic Solar Cells with Appropriate Active Layer Morphology by Side Chain Engineering of Donor Molecules and Thermal Annealing.

It is very important to fine-tune the nanoscale morphology of donor:acceptor blend active layers for improving the photovoltaic performance of all-small-molecule organic solar cells (SM-OSCs). In this work, two new small molecule donor materials are synthesized with different substituents on their thiophene conjugated side chains, including SM1-S with alkylthio and SM1-F with fluorine and alkyl substituents, and the previously reported donor molecule SM1 with an alkyl substituent, for investigating the effect of different conjugated side chains on the molecular aggregation and the photophysical, and photovoltaic properties of the donor molecules. As a result, an SM1-F-based SM-OSC with Y6 as the acceptor, and with thermal annealing (TA) at 120 °C for 10 min, demonstrates the highest power conversion efficiency value of 14.07%, which is one of the best values for SM-OSCs reported so far. Besides, these results also reveal that different side chains of the small molecules can distinctly influence the crystallinity characteristics and aggregation features, and TA treatment can effectively fine-tune the phase separation to form suitable donor-acceptor interpenetrating networks, which is beneficial for exciton dissociation and charge transportation, leading to highly efficient photovoltaic performance.