Lower HOMO Energy Levels Research Articles

Synergistic Design of Imidazole-Based Polymer Donors for Enhanced Organic Solar Cell Efficiency.

Within the realm of organic solar cells (OSCs), designing new high-efficiency polymer donors remains a significant challenge. Achieving the right balance in polymer backbone planarity is crucial: excessive planarity can lead to undesirable aggregation, while insufficient planarity can hinder the charge transport efficiency. In this study, we designed and synthesized an imidazole-based acceptor (A) unit for the first time and then investigated the impact of backbone planarity on charge transport capacity and power conversion efficiency (PCE). Backbone planarity was precisely tuned by incorporating isomeric alkyl chains on the thiophene π-bridge, resulting in four distinct polymer donors: MZC8-F, MZC8-Cl, MZEH-F, and MZEH-Cl. The results showed that the steric hindrance from the EH-branched alkyl chain induced backbone distortion and caused a blue-shift in the absorption spectrum. MZEH-Cl, with its poor planarity and excessively low HOMO energy level, achieved a PCE of just 7.6%. Through careful modulation, MZC8-Cl emerged as the most efficient, with a remarkable PCE of 17.3%, setting a new benchmark for imidazole-based polymer donors. This study not only deepens the understanding of the role of polymer backbone planarity in photovoltaic performance but also lays the groundwork for developing high-efficiency polymer donors.

Substituent Regulating Porphyrin Derivatives for Efficient Photocatalytic Hydrogen Evolution

AbstractPorphyrin and its derivatives have been seen as a type of potential organic photocatalyst for photocatalytic hydrogen evolution. Substituent regulation as a simple method can effectively regulate the photoelectric characteristic of organic semiconductors. Herein, we designed three porphyrin derivatives attached with different substituents (PP‐NH2, PP‐OH and PP‐COOH). Through a facile substituent optimization strategy, the photoelectric properties of these porphyrin derivatives show an obvious difference, which leads an optimized photocatalytic hydrogen evolution efficiency. The porphyrin derivative with carboxyl presents a reduced impedance, and a better charge separation. Compared to the amino group, PP‐COOH with carboxyl, achieved a high hydrogen evolution rate of 2.20 mmol g−1 h−1 (over 20 times than that of PP‐NH2). Moreover, enhanced stability was observed in PP‐COOH rather than PP‐OH, because of the lower HOMO energy level. And the ionization of carboxyl in water negatively charges the porphyrin derivatives and forms a relatively stable sol. This result further increases the operation stability of PP‐COOH, during photocatalytic hydrogen evolution. This study presents a promising design strategy for long‐lifetime photocatalysts for hydrogen evolution.

Thieno[3,2-b]thiophene flanked para-azaquinodimethane π-extended aromatic-quinoidal polymers

A series of thieno[3,2-b]thiophene (TT) flanked para-azaquinodimethane (p-AQM) based quinoidal conjugated polymers PAQM2TT-T, PAQM2TT-BT and PAQM3TT were designed in a strategy to extend the aromatic-quinoidal π-system. The three polymers were synthesized by Stille polycondensation, and their opto-electronic properties and device performance in field-effect transistors have been explored. From absorption spectroscopy, the polymers show low bandgaps (1.54–1.57 eV) and high HOMO energy levels. They exhibit typical p-type behavior according to the results of the characterization of the field-effect transistors with average charge carrier mobilities of 0.08 cm2 V−1 s−1 for PAQM2TT-T, 0.12 cm2 V−1 s−1 for PAQM2TT-BT and 0.05 cm2 V−1 s−1 for PAM3TT. This report presents an alternative π-extended quinoidal-donor strategy to control the molecular design and properties of new p-AQM-based conjugated polymers.

Disulfide bonded polyimide cathode of rechargeable magnesium batteries: Improved capacity via reversible break/formation of disulfide and sulfur-oxygen bonds

Rechargeable magnesium batteries are a potential selection for large-scale energy storage technologies, but development of cathode materials is the major difficulty at present. Organic polyimides are promising magnesium battery cathodes with the open and amorphous frameworks as well as enhanced charge delocalization. However, only two carbonyls are redox-active out of four for each imide unit, which largely limits the capacity. Herein, a new polyimide containing disulfide bond is fabricated and studied as cathode for rechargeable magnesium batteries. This polyimide shows a high capacity beyond the traditional two-electron principle. A mechanism study demonstrates that carbonyl enolization and disulfide bond break/formation occur in the redox process, leading to formation/break of sulfur-oxygen bond. This sulfur-oxygen bond enhances enolization and electron delocalization, resulting in an improved capacity. Further theoretical computation indicates a charge transfer within this sulfur-oxygen bond. The two-electron reduction state exhibits quite low HOMO and LUMO energy levels, which makes the disulfide bonded polyimide have a higher tendency to accept more electrons and a higher oxidation stability after reduction.

Enhancing the performance of pyridyl carboxamide-N,N-dimethyl amino chalcone (PCC) as a dye sensitizer for dye-sensitized solar cells (DSSCs) through the incorporation of electron donor moieties

This study has focused on enhancing the performance of pyridyl carboxamide-N,N-dimethyl amino chalcone (PCC) as a sensitizer for dye-sensitized solar cells (DSSCs) by strategically incorporating electron donor moieties identified in a literature review. Our reviewing the previous reports highlights the pivotal role of electron donors in optimizing light harvesting, electron injection, and overall power conversion efficiency in organic dye sensitizers, particularly those following a donor-bridge-acceptor (D-π-A) architecture. The selected donor moieties, including dithieno[3,2-b:2′,3′-d]thiophene (DTT), benzodithiophene (BDT), triphenylamine (TPA), carbazole (CBZ), and triazatruxene (TAZ), are systematically introduced into the molecular structure of PCC (abbreviated as PCC-DTT, PCC-BDT, PCC-TPA, PCC-CBZ, and PCC-TAZ, respectively). The performance of these modified compounds is rigorously evaluated using density functional theory (DFT) methods. Validation results revealed that the most reliable method for PCC derivative characterization involved the combination of the functional B3PW91 with a 6-31G(d) basis set. The results indicate that the incorporation of certain donor moieties, particularly BDT, significantly enhances the electronic and optical properties of PCC, including lower HOMO energy levels, a narrow energy gap (Egap), enhanced absorption intensity, and a redshift in absorption peaks. Evaluation of photovoltaic properties including electron injection (ΔGinj), and open circuit voltage (Voc), indicated facilitating spontaneous electron injection from the dyes to the TiO2 conduction band. Notably, PCC-TAZ demonstrated the lowest electron regeneration (ΔGreg), suggesting a notable potential for efficient electron regeneration.

Synthesis, photophysical, electrochemical, theoretical and self-assembly investigation of tetraphenylethene tethered arylimidazoles

To understand the comparative structure–property relationships, we have designed and synthesized 4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole, coded as JR-1 and 4,4′-(2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole-4,5-diyl)bis(N,Ndimethylalanine)coded as JR-2. Here, we conducted various analyses, including, structural, photophysical, thermal, electrochemical, and computational studies, to gain a deeper understanding of introduction electron releasing N,N-dimethyleamine groups on later one. The molecules exhibit high thermal stability (200 − 300 °C) low band gap (2.75 – 3.26 eV) and comparable HOMO (−4.81 to − 5.35 eV) and LUMO (−2.06 to − 2.09 eV) energy levels with those of calculated and reported ambipolar materials. The UV–vis absorption was observed in the range of 300–400 nm for JR-1 and 300 − 440 nm for JR-2. These compounds exhibit distinct mechanochromism, solvatochromism, aggregation-induced emission, and aggregation-caused quenching and selective sensing. Interestingly, JR-1 exhibits mechanochromism without solvatochromism, while JR-2 is solvatochromic without mechanochromism. Due to incorporation of N,N-Dimethylphenylamine donating groups on imidazole leads to solvatochromicity and aggregation-caused quenching. Moreover, their optical properties differ significantly in dilute solutions compared to its solid state due to distinct molecular arrangements. Consequently, morphological studies reveal the formation of 3D rod and spear-shaped s supramolecular self-assembly structures in different solvent mixtures. This research work offers a fundamental strategy for the design of fluorophores for optoelectronic applications.

Dopant-Free Pyrene-Based Hole Transporting Material Enables Efficient and Stable Perovskite Solar Cells.

Dopant-free hole transporting materials (HTMs) is significant to the stability of perovskite solar cells (PSCs). Here, we developed a novel star-shape arylamine HTM, termed Py-DB, with a pyrene core and carbon-carbon double bonds as the bridge units. Compared to the reference HTM (termed Py-C), the extension of the planar conjugation backbone endows Py-DB with typical intermolecular π-π stacking interactions and excellent solubility, resulting in improved hole mobility and film morphology. In addition, the lower HOMO energy level of the Py-DB HTM provides efficient hole extraction with reduced energy loss at the perovskite/HTM interface. Consequently, an impressive power conversion efficiency (PCE) of 24.33 % was achieved for dopant-free Py-DB-based PSCs, which is the highest PCE for dopant-free small molecular HTMs in n-i-p configured PSCs. The dopant-free Py-DB-based device also exhibits improved long-term stability, retaining over 90 % of its initial efficiency after 1000 h exposure to 25 % humidity at 60 °C. These findings provide valuable insights and approaches for the further development of dopant-free HTMs for efficient and reliable PSCs.

Dopant‐Free Pyrene‐Based Hole Transporting Material Enables Efficient and Stable Perovskite Solar Cells

AbstractDopant‐free hole transporting materials (HTMs) is significant to the stability of perovskite solar cells (PSCs). Here, we developed a novel star‐shape arylamine HTM, termed Py‐DB, with a pyrene core and carbon‐carbon double bonds as the bridge units. Compared to the reference HTM (termed Py‐C), the extension of the planar conjugation backbone endows Py‐DB with typical intermolecular π–π stacking interactions and excellent solubility, resulting in improved hole mobility and film morphology. In addition, the lower HOMO energy level of the Py‐DB HTM provides efficient hole extraction with reduced energy loss at the perovskite/HTM interface. Consequently, an impressive power conversion efficiency (PCE) of 24.33 % was achieved for dopant‐free Py‐DB‐based PSCs, which is the highest PCE for dopant‐free small molecular HTMs in n‐i‐p configured PSCs. The dopant‐free Py‐DB‐based device also exhibits improved long‐term stability, retaining over 90 % of its initial efficiency after 1000 h exposure to 25 % humidity at 60 °C. These findings provide valuable insights and approaches for the further development of dopant‐free HTMs for efficient and reliable PSCs.

Effect of substituting donors on the hole mobility of hole transporting materials in perovskite solar cells: a DFT study.

Several hole-transporting materials (HTMs) have been designed by incorporating different types of π-conjugation group such as long chain aliphatic alkenes and condensed aromatic rings of benzene and thiophene and their derivatives on both sides between the planar core and donor of a reference HTM. Various electronic, optical, and dynamic properties have been calculated by using DFT, TDDFT, and Marcus theory. In this study, all the designed HTMs show a lower HOMO energy level and match well with the perovskite absorbers. Inserting condensed rings results in better hole mobility compared to aliphatic double bonds. It is found that the charge transfer integral is the dominant factor which mainly influences the hole mobility in our studied HTMs. Other factors such as hole reorganization energy, hole hopping rate, and centroid distance have a minor effect on hole mobility. Thus, this study is expected to provide guidance for the design and synthesis of new HTMs with increased hole mobility.

Di-chlorinated benzothiadiazole unit: a new strong electron-withdrawing ability acceptor toward fast-switching green low band gap D-A electrochromic polymers

Di-halogenated benzothiadiazole (BT) is one of the potential acceptor units in constructing low band gap d-A polymers, but it has not attracted widespread attention, especially for the di-chlorinated BT unit. Herein, in this work, two kinds of d-π-A-π-D electrochromic conjugated precursors with di-halogenated BT (5,6-dichlorobenzo[c][1,2,5]thiadiazole and 5‑chloro-6-fluorobenzo[c][1,2,5]thiadiazole) as the acceptor unit, 3-dodecyl thiophene as the π bridge, and 3,4-ethylenedioxythiophene (EDOT) as the donor unit, were designed and synthesized, and their corresponding d-A polymers were successfully obtained by electrochemical deposition method. The photophysical study proves that introducing more chlorine atoms among the precursors will lead to stronger steric hindrance, lower HOMO energy level, blue-shifted absorption spectra, lower quantum yield, but larger Stokes shift. Both of them show very low oxidation potential, which is beneficial for them to obtain high-quality d-A polymers. The introduction of di-chlorinated BT unit obviously improved the polymers’ electroactivity and redox stability, and the remained activity of P(ClCl) was above 74.79 after 1000 cycles. The optical band gap of P(ClCl) is further reduced to 1.28 eV, and the reversible transition from the neutral dark green to the oxidized dark blue can be realized in the redox process. As a result, various favorable electrochromic parameters for P(ClCl) were achieved, such as the optical contrast in the near-infrared region is 27.23 %, the response time is as low as 0.4 s, and the coloration efficiency is as high as 151.3 cm2 C−1, as well as the good optical stability and satisfied memory effect. Overall, the subtle change of halogen atoms in BT units is a very efficient method to modulate the photophysical properties of d-π-A-π-D precursors, also providing good guidelines in designing highly efficient d-A electrochromic polymers.

Charge transport in individual short base stacked single-stranded RNA molecules

Charge transport in biomolecules is crucial for many biological and technological applications, including biomolecular electronics devices and biosensors. RNA has become the focus of research because of its importance in biomedicine, but its charge transport properties are not well understood. Here, we use the Scanning Tunneling Microscopy-assisted molecular break junction method to measure the electrical conductance of particular 5-base and 10-base single-stranded (ss) RNA sequences capable of base stacking. These ssRNA sequences show single-molecule conductance values around 10^{-3}G_0 (G_0= 2e^2/h), while equivalent-length ssDNAs result in featureless conductance histograms. Circular dichroism (CD) spectra and MD simulations reveal the existence of extended ssRNA conformations versus folded ssDNA conformations, consistent with their different electrical behaviors. Computational molecular modeling and Machine Learning-assisted interpretation of CD data helped us to disentangle the structural and electronic factors underlying CT, thus explaining the observed electrical behavior differences. RNA with a measurable conductance corresponds to sequences with overall extended base-stacking stabilized conformations characterized by lower HOMO energy levels delocalized over a base-stacking mediating CT pathway. In contrast, DNA and a control RNA sequence without significant base-stacking tend to form closed structures and thus are incapable of efficient CT.

Optimizing molecular conformation and photovoltaic property of simple asymmetrical SMAs by tuning electron-donating central subunit

To overcome complex synthetic routes of multiple-fused-ring small-molecule acceptors (SMAs), it is crucial to develop simple fused-ring and non-fused-ring SMAs and explore structure-property relationship for high-efficiency and low-cost organic solar cells (OSCs). For this purpose, a novel SMA named BIO-4Cl was designed and synthesized, which consist of an asymmetric dual electron-donating D-D′ central block of alkoxy- indenothiophene (INT) and benzodithiophene (BDT), as well as two electron-accepting (A) terminal group of 2-(5,6-dichloro-3-oxo-2,3-dihydro-1 H-inden-1-ylidene)malononitrile. The effect of the electron-donating D-D′ central block on molecular conformation, absorp- tion, energy level, mobility, morphology and photovoltaic performance of the SMAs was primarily studied by integrating its analogue IOEH-4Cl with the cyclopentadithiophene (DTC) and INT central block. By altering the electron-donating D′ subunit in the D-D′ central block, it is found that BIO-4Cl and IOEH-4Cl behave different molecular conformation with the S-type and C-type, respectively. As a result, BIO-4Cl exhibits a blue-shifted absorption with a decreasing molar extinction coefficient, as well as a low HOMO and LUMO energy level in comparison with IOEH-4Cl. The IOEH-4Cl blend film displays better crystallinity and film morphology than the BIO-4Cl blend film using polymer PM6 as donor material. The IOEH-4Cl-based OSCs display better photovoltaic property than the BIO-4Cl-based OSCs, in which the power convention efficiency (PCE) was increased 4.75 times to 13.7%. This work indicates that fine-tuning the electron-donating subunit in an asymmetric dual electron-donating D-D′ central block is crucial to optimize molecular conformation and PCE for simple-structure asymmetric SMAs.

Fluorenylidene-Cyclopentadithiophene-Based Asymmetric Bistricyclic Aromatic Ene Compounds: Synthesis and Substituents Effects.

Low band gap materials have always been a focus of attention due to their potential applications in various fields. In this work, a series of asymmetric bistricyclic aromatic ene (BAE) compounds with fluorenylidene-cyclopentadithiophene (FYT) skeleton were facially synthesized, which were modified with different substituents (-OMe, -SMe). The FYT core exhibit twisted C=C bond with dihedral angels around 30°, and the introduction of -SMe group can provide additional S···S interaction between molecules, which is conducive to the charge transporting. The UV-Vis spectra, electrochemistry and photoelectron spectroscopy revealed that these compounds have relatively narrow band gaps, particularly, the -SMe modified compounds have slightly lower HOMO and Fermi energy levels than that of the -OMe modified compounds. Furthermore, PSCs devices were fabricated with the three compounds as HTMs, and FYT-DSDPA exhibit the best performance among them, revealing the fine-tuning band structure could influence properties of HTMs.

Small molecules based on 6,7-difluoroquinoxaline and thieno[3,2-b]thiophene for solution-processed solar cells

Fluorinated quinoxaline-based small molecules, SM1, SM2, and SM3, were developed as the donor materials for photovoltaic cells. The small molecules containing a 2,3-didodecyl-6,7-difluoroquinoxaline as the electron-deficient unit and thienothiophene as the electron rich units were synthesized by Stille coupling. The small molecules exhibit satisfactory thermal stability and a broad absorption band from 400 to 700 nm. The HOMO/LUMO energy levels of SM1, SM2, and SM3 were –5.70/–3.48, –5.57/–3.42, and –5.66/–3.59 eV, respectively. SM3 with the alkyl group at 4-position in terminal thiophene units has lower HOMO energy levels to increase the V OC value.

Side substitution on benzothiadiazole-based hole transporting materials with a D–A–D molecular configuration for efficient perovskite solar cells

A series of benzothiadiazole-based Donor-Acceptor-Donor (D-A-D) typed hole-transporting materials (HTMs) are designed through introducing electron-withdrawing fluorine group and electron-donating alkoxy group into the core of benzothiadiazole (BT). The energy level alignment, electronic properties, absorption, and hole transport properties of these materials are explored comprehensively using First-principle calculations. As compared with the reference molecule BT, three other substituted molecules have lower HOMO energy levels and lower reorganization energy. Especially, two fluorine substituted hole-transporting materials BT-1F and BT-2F possess planar molecular configuration, suitable energy level, indicating that fluorine substitution is beneficial to the increase of open circuit voltage of device. Moreover, the hole transporting mobility, solubility and stability of all designed molecules are also estimated, which are important items to determine the cost and performance in real application of solar cells. Calculated results show that hole mobility of monofluorinated molecule BT-1F displays relatively higher hole mobility owing to efficient intermolecular interactions. Therefore, molecule BT-1F is likely to be highly efficient HTMs because of its excellent hole mobility and solubility. The present work demonstrates that fluorination engineering on core acceptors in D-A-D typed HTMs is an effective strategy to tune the photovoltaic performance of perovskite solar cells.

Blue-red emitting materials based on a pyrido[2,3-b]pyrazine backbone: design and tuning of the photophysical, aggregation-induced emission, electrochemical and theoretical properties.

Pyrido[2,3-b]pyrazine-based donor–acceptor–donor (D–A–D) molecules were designed by altering donor amines and synthesized using the Buchwald–Hartwig C–N coupling reaction. Further, the tunable opto-electrochemical properties of the dyes were studied in detail. The dye possesses intramolecular charge transfer (ICT) transition (412–485 nm), which marked the D–A architecture and induces a broad range of emissions from blue to red (486–624 nm) in the solution and solid state. Some of the dyes show aggregation-induced emission (AIE) features and formation of nanoparticles in the THF/H2O mixture, as confirmed by DLS and FEG-SEM (of 7) analysis. The AIE characteristics indicate its solid/aggregate-state application in organic electronics. The molecules exhibit high thermal stability, low band gap (1.67–2.36 eV) and comparable HOMO (−5.34 to −5.97 eV) and LUMO (−3.61 to −3.70 eV) energy levels with those of reported ambipolar materials. The relationship between the geometrical structure and optoelectronic properties of the dyes, as well as their twisted molecular conformation and small singlet and triplet excitation energy difference (ΔEST = 0.01–0.23 eV) were analyzed using the DFT/TDDFT method. Thus, potential applications of the dyes are proposed for optoelectronic devices.

Near-infrared small molecule acceptors based on 4H-cyclopenta[1,2-b:5,4-b']dithiophene units for organic solar cells

Two small-molecule non-fullerene acceptors (NFAs) CPDT-Me-2Eh and CPDT-4Cl-2Eh based on 4H-cyclopenta[1,2-b:5,4-b′] dithiophene as the electron-donating unit 2-(5/6-methyl-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the end group respectively were developed. Compared with CPDT-Me-2Eh, CPDT-4Cl-2Eh possesses red-shift absorption spectrum, lower HOMO and LUMO energy level, and higher electron mobility. Non-fullerene polymer solar cells based on PBDB-T: CPDT-4Cl-2Eh gave a power conversion efficiency (PCE) of 7.52%, with a fill factor (FF) of 64.46%, and a short-circuit current (Jsc) of 16.90 mA/cm2. As contrast, the cell based on PBDB-T: CPDT-Me-2Eh got a PCE value of 4.39%. Our results indicate that the small molecule acceptor containing the CPDT core can effectively extend the absorption spectrum to the near-infrared (NIR) region by chlorination of the capping group. Our work provides an easy routine to broaden the field of new NIR NFAs with high PCE based on non-fullerene OSCs.

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC 71 BM acceptor, the PBDTTS-DFIQ:PC 71 BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility ( μ h / μ e ), and nice morphology in comparison with PBDTTS-IQ:PC 71 BM blend film. Encouragingly, the PBDTTS-DFIQ:PC 71 BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC 71 BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage ( V oc ) of 0.84 V, short current density ( J sc ) of 13.26 mA cm −2 and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs. Two novel NBG D-A copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric indo [2,3-b]quinoxaline (IQ) unit as acceptor unit through tuning the side chains with halogen atom fluorine (F) engineering, aiming to achieve novel copolymer donors with good photovoltaic performance. • Both narrow bandgap copolymers were developed based on asymmetric planar aromatic backbone indo [2,3-b]quinoxaline as an acceptor unit. • The fluorinated PBDTTS-DFIQ exhibits a lower HOMO energy level, better planarity and higher dipole moment in comparison with PBDTTS-IQ. • The PBDTTS-DFIQ:PC 71 BM-based device presents a maximum PCE of 7.40%. • Fluorinated asymmetric indo [2,3-b]quinoxaline framework is a wonderful acceptor moiety to build light-harvesting copolymers for efficient PSCs.

Synthesis, molecular structure and photovoltaic performance for polythiophenes with β-carboxylate side chains

To lower the HOMO energy level of polythiophenes, carboxylate groups were introduced to the β-position of the thiophene unit, by which two polythiophenes with tetrathiophene (poly[5,5′′-(bis-3,3′′-((2-butyloctyl)-carboxylate)-2,2′:2′,2′′-terthiophene)-alt-5-thiophene], P-4T-2COOR) or pentathiophene (poly[5,5′′-(bis-3,3′′-((2-butyloctyl)-carboxylate)-2,2′:2′,2′′-terthiophene)-alt-5,5′-(2,2′-bithiophene)], P-5T-2COOR) repeating unit were synthesized. Absorption spectroscopy and cyclic voltammetry measurements revealed that the β-carboxylate substitution red-shifts the maximum absorption wavelength (λmaxabs) in solution owing to the electron accepting nature of the carboxylate group. In addition, the introduction of β-carboxylate reduces the HOMO level from -5.09 eV for P3HT to -5.34 eV and -5.18 eV for P-4T-2COOR and P-5T-2COOR, respectively, which is in good agreement with quantum chemisty calculation results. However, the β-carboxylate side chain showed different orientation to that of P3HT, which leads to weaker intermolecular π-π interaction as confirmed by less red-shited absorption in thin solid film and the quantum calculation results. Polymer solar cells using P-4T-2COOR and P-5T-2COOR as the electron donor, 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno‐[1,2‐b:5,6-b′]di‐thiophene (ITIC) as the electron acceptor were fabricated and tested. The P-4T-2COOR and P-5T-2COOR based cells showed high open circuit (VOC) of 0.73–0.99 V, significantly higher than that of P3HT based cell (VOC of 0.52 V), which can be ascribed to the lower HOMO energy levels and less condensed molecular packing of these two polymers.

Influence of the Location of Electron-Donating 3,4-Ethylenedioxythiophene (EDOT) Moiety in the A–π–D–π–A Type Conjugated Molecules on the Optoelectronic Properties and Photovoltaic Performances

A–π–D–π–A type conjugated small molecules play an indispensable role in organic photovoltaics. Understanding the relationship between the molecular structure and performance is a fundamental question for the further rational design of high-performance organic materials. To red-shift the absorption spectrum of benzo[1,2-b:4,5-b']dithiophene (BDT) based A–π–D–π–A type compounds, an electron-donating 3,4-ethylenedioxythiophene (EDOT) moiety was introduced into the π-conjugation bridge unit. Two new compounds with EDOT next to the central BDT core (COOP-2HT-EDOT-BDT) or next to the terminal electron acceptor unit (COOP-EDOT-2HT-BDT) were synthesized and characterized. The compound COOP-2HT-EDOT-BDT showed higher molar extinction coefficient (εabs max = 1.06 × 105 L mol−1 cm−1), lower optical band gap (E g = 1.56 eV) and high HOMO energy level (E HOMO = −5.08 eV) than COOP-EDOT-2HT-BDT (εabs max = 0.96 × 105 L mol−1 cm−1, E g = 1.71 eV, E HOMO = −5.26 eV), which is attributed to the intensive interaction between the EDOT unit and the HOMO orbital, as confirmed by the theoretical calculation results. However, the higher power conversion efficiency of 3.58% was achieved for the COOP-EDOT-2HT-BDT:PC61BM-based solar cells, demonstrating that the electron-donating EDOT unit adjacent to the electron-withdrawing end-capped group (COOP) is a better way to achieve high-performance photovoltaic materials.