Lower Highest Occupied Molecular Orbital Research Articles

2‐Ethylhexyl‐4,6‐Dibromo‐3‐Cyano‐3‐Thieno[3,4‐b]Thiophene Enables Low HOMO Energy Level Polymer Donor

AbstractThe new halogen‐free donor polymer PCN6 is constructed using 2‐ethylhexyl‐4,6‐dibromo‐3‐cyano‐thieno[3,4‐b]thiophene as acceptor (A) block, and is compared in detail with the commercially available PTB7‐Th. It is found that PCN6 has a wider film absorption (300–700 nm) and lower highest occupied molecular orbital (HOMO) energy levels (−5.52 eV) than PTB7‐Th (−5.34 eV), suggesting a great advantage of the monocyano‐functionalized modification strategy in terms of molecular absorption and energy level tuning. The performance difference between PCN6:Y6‐ and PTB7‐Th:Y6‐based organic solar cells (OSCs) is compared by a series of studies including light intensity dependence, carrier mobility, AFM, TEM, and GIWAXS. The results show that PCN6:Y6‐based OSCs have stronger crystallinity, better charge transport, higher and more balanced carrier mobility, and less exciton complex loss. Therefore, the power conversion efficiency (PCE) of PCN6:Y6‐based OSCs reaches 11.34%, while the PCE of PTB7‐Th:Y6‐based OSCs is only 9.02%. These results suggest that 2‐ethylhexyl‐4,6‐dibromo‐3‐cyano‐thieno[3,4‐b]thiophene is an excellent A block for the construction of halogen‐free donor polymers with low HOMO energy levels, and also demonstrate that the introduction of cyano in the conjugated backbone of polymers is a good strategy to achieve high‐performance OSCs.

Molecular properties of a triazole-Ce(III) complex with antioxidant activity: structure, spectroscopy, and relationships with related derivatives. Influence of the ligands in the complex.

A novel Ce(III) complex with the triazole ligand 2b, which presents four H-bonded sites with amino acids of the MMP-2 receptor, was synthesized. The experimental IR and Raman spectra of this Ce(III) complex were well-interpreted based on their comparison to the theoretical scaled spectra using the scaling equations determined by two procedures and four density functional theory (DFT) levels. Therefore, the structure predicted for the synthesized Ce(III) complex was clearly characterized and confirmed. The potential antioxidant action of this complex was compared with the analogous La(III) complex, and it was found that the coordination of ligand 2b with Ce(III) improves the ligand's ability to participate in single-electron transfer (SET), as observed in the ABTS·+ assay, and this complex seems to scavenge the stable radical much more actively compared to its La(III) counterpart. Additionally, interactions with potassium superoxide and sodium hypochlorite indicate a high pro-oxidant behavior of the complex. The effects of different ligands on the geometric parameters, atomic charges, and molecular properties of the Ce(III) complex were analyzed at four DFT levels, and several relationships were clearly established. These relationships can facilitate the selection of new ligands with improved properties in the design of novel lanthanide-triazole carboxylate complexes with promising biological activity. The ligand size increase in the complexes facilitates the electronic transfer of negative charge, and the low HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap indicates a large reactivity and low energy for their excitation.

Fluorinated and safety-reinforced copolymer solid electrolytes for high-voltage all-solid-state sodium metal batteries

Poly (ethylene oxide)-based all-solid-state electrolytes are extensively regarded as highly promising contenders for the upcoming era of sodium metal batteries. Nevertheless, the intrinsic elevated highest occupied molecular orbital (HOMO) and the strong sodium-ion adsorption energy lead to a narrow electrochemical stability window (ESW) and a sluggish ion transport kinetics, thus retarding its widespread application in high-voltage batteries. About these shortcomings, we propose a strategy of fluorine substitution through a ready UV-initiated free-radical copolymerization to regulate the orbital level sodium-ion adsorption energy of poly (ethylene oxide) electrolytes. The characterization confirmed the success in the introduction of fluorine into the polymer (referred to as P-co-F). DFT calculations demonstrated a lower HOMO (-7.061 eV) in fluorine-substituted polymer electrolyte, which enabled to expand the ESW up to 5.55 V. Besides, the sodium-ion adsorption energy on P-co-F was decreased from −2.78 eV to −3.43 eV, facilitating the rapid transport of sodium ion. In addition, Polyethylene terephthalate (PET) was adopted as a support frame, which would reinforce the mechanical properties of polymer electrolytes. The electrochemical tests suggested P-co-F exhibited a profound interfacial stability in the Na//Na symmetrical system throughout an operational duration of 4690 h at 0.1 mA cm−2. The assembled all-solid-state battery manifested remarkable stability during continuous cycling at room temperature for 200 cycles, retaining an impressive 96.59 % of its initial capacity at 0.1C. The intriguing fluorine-induced effect, coupled with a cost-effective safety substrate, will provide valuable insights for the rational molecular design of innovative electrolytes and easy production scalability for all-solid-state Na metal batteries.

Benzyl Alcohol Photo-oxidation Based on Molecular Electronic Transitions in Metal Halide Perovskites

Vacancy ordered double perovskites Cs2Te(IV)X6 have been found to exhibit molecule-like electronic behavior when X is Cl– or Br– due to the zero-dimensional (0D) nature of their octahedral units. Electronically isolated building blocks, the [TeBr6]2– ionic octahedron, serve as the fundamental electronic unit of the Cs2TeBr6 solid. Herein, a detailed understanding of the Cs2TeBr6 electronic structure and its photoexcitation is presented with consideration of individual molecular orbitals from these isolated octahedral building blocks. Two optical absorption features correspond to two unique electronic transitions, (1) a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition under 455 nm excitation and (2) mixed transitions including lower HOMO states to LUMO transition and HOMO to higher LUMO states transition under 365 nm excitation. With this in mind, we examined the excitation wavelength-dependent photo-oxidation of benzyl alcohol using Cs2TeBr6 as the photocatalyst. Significant differences in photocatalytic performance were observed, and different forms of activated alcohol radicals were detected under the two excitation wavelengths. As a case study, this work highlights the application of molecule-like halide perovskites in photocatalysis. The highly tunable energy band structures and catalytic centers in perovskites can offer a valuable platform for photocatalytic mechanistic studies and catalyst development in the foreseeable future.

Wide‐Bandgap Donor–Acceptor Copolymer Based on BDTTz Donor and TPD Acceptor for Polymer Solar Cells Using Fullerene and Nonfullerene Acceptors

A donor–acceptor copolymer based on thiazole substituted benzodithiophene (BDTTz) (donor unit) and thieno[3,4‐c]pyrrole‐4‐6‐dione (TPD) (acceptor unit) P(BDTTz‐TPD) denoted as P132 is investigated. P132 shows an absorption profile from 300 to 680 nm and the low‐lying highest occupied molecular orbital (HOMO) energy level of −5.54 eV. P132 as donor along with fullerene derivative (PC71BM) or nonfullerene denoted as BThIND‐Cl as acceptor are used for the fabrication of polymer solar cells. The P132:BThIND‐Cl‐based polymer solar cell shows high open‐circuit voltage as compared with P132:PC71BM which is attributed to the upshifted lowest unoccupied molecular orbital level of BThIND‐Cl and low energy loss. Moreover, P132:BThIND‐Cl attains higher short‐circuit current than that for P132:PC71BM which is attributed to the broader absorption spectra and low optical band gap of BThIND‐Cl, which leads to high light‐harvesting ability of the P132:BThIND‐Cl active layer. The polymer solar cells based on P132:PC71BM and P132:BThIND‐Cl attain power conversion efficiencies of 7.03% and 15.52%, respectively. The charge transport is more balanced in P132:BThIND‐Cl and results in a high value of fill factor. Small energy loss in P132:BThIND‐Cl‐based PSCs is attributed to the low HOMO offsets between P132 and BThIND‐Cl.

Harnessing the Inductive Effect To Design New Donor–Acceptor–Acceptor′-Configured Small-Molecule Donors for Vacuum-Processed Organic Photovoltaics

Three new donor–acceptor–acceptor′ (D–A–A′)-configured molecules, DTCFiBT, DTCFoBT, and DTCF2BT, with F-substituted benzothiadiazole (BT) as the A group, and two molecules, DTCPiTD and DTCPoTD, adopting pyridal[2,1,3]-thiadiazole (PTD) as the A group, were synthesized and characterized. The effects of the F-substitution number and the orientation of mono F-substituted BT and PTD relative to the D group on the physical properties and intermolecular interactions were examined, together with theoretical calculations to establish the structure–property relationship. In comparison to the parent molecule DTCPB, the inductive effects of F-substituted BTs lower both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of DTCFiBT, DTCFoBT, and DTCF2BT and, thus, similar optical energy gaps (Egopt), while DTCPiTD and DTCPoTD exhibit much lower LUMO energy levels and reduced Egopt, ascribing to the electron-deficient character of PTD. The strong dipolar features of the D–A–A′-configured structure resulting in the antiparallel dimeric packing with different intermolecular interactions in their crystal structures were observed by X-ray analyses. Small-molecule organic solar cells (SMOSCs) with a bulk heterojunction active layer comprising new D–A–A′ donors and C70 were fabricated and characterized. The deeper LUMO levels and reduced Egopt benefit the DTCPiTD- and DTCPoTD-based devices to have higher short current density (JSC), while the DTCFiBT-, DTCFoBT-, and DTCF2BT-based devices benefited from the lower HOMO energy levels that lead to the higher open circuit voltage (VOC). Transient photoluminescence, atomic force microscopy, and incident-light-intensity-dependent device characteristics were examined to reveal the recombination issue for the inferior DTCFoBT-based device. Among these new donors, the DTCFiBT-based device shows the best performance, with VOC of 0.94, JSC of 11.3 mA/cm2, fill factor (FF) of 0.65, and power conversion efficiency of 6.8%, which are attributed to the high VOC as a result of the deeper HOMO level and the superior FF as a result of good exciton separation and charge carrier transport.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

13.4 % Efficiency from All-Small-Molecule Organic Solar Cells Based on a Crystalline Donor with Chlorine and Trialkylsilyl Substitutions.

How to simultaneously achieve both high open‐circuit voltage (V oc) and high short‐circuit current density (J sc) is a big challenge for realising high power conversion efficiency (PCE) in all‐small‐molecule organic solar cells (all‐SM OSCs). Herein, a novel small molecule (SM)‐donor, namely FYSM−SiCl, with trialkylsilyl and chlorine substitutions was designed and synthesized. Compared to the original SM‐donor FYSM−H, FYSM−Si with trialkylsilyl substitution showed a decreased crystallinity and lower highest occupied molecular orbital (HOMO) level, while FYSM−SiCl had an improved crystallinity, more ordered packing arrangement, significantly lower HOMO level, and predominant “face‐on” orientation. Matched with a SM‐acceptor Y6, the FYSM−SiCl‐based all‐SM OSCs exhibited both high V oc of 0.85 V and high J sc of 23.7 mA cm−2, which is rare for all‐SM OSCs and could be attributed to the low HOMO level of FYSM−SiCl donor and the delicate balance between high crystallinity and suitable blend morphology. As a result, FYSM−SiCl achieved a high PCE of 13.4 % in all‐SM OSCs, which was much higher than those of the FYSM−H‐ (10.9 %) and FYSM−Si‐based devices (12.2 %). This work demonstrated a promising method for the design of efficient SM‐donors by a side‐chain engineering strategy via the introduction of trialkylsilyl and chlorine substitutions.

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

Ternary Polymer Solar Cells Using Two Polymers P1 and P3 with Similar Chemical Structures and Nonfullerene Acceptor Attained Power Conversion Efficiency Over 15.5% with Low Energy Loss of 0.55 eV

Herein, ternary active layer‐based polymer solar cells using two donor (D)–acceptor (A) conjugated polymers P1 and P3 having same A unit and different D units as donor along with the nonfullerene small molecule acceptor, BThIND‐Cl, are fabricated. The polymer solar cells based on P1:BThIND‐Cl and P3:BThIND‐Cl binary show power conversion efficiency of 12.13% and 13.08% with energy loss of 0.60 and 0.51 eV, respectively. The open circuit voltage of the P3 based polymer solar cells is higher than that for P1 counterpart associated with the deeper highest occupied molecular orbital (HOMO) energy level of P3 as compared to the P1. The small energy loss for P3 may be attributed to low HOMO offset between P3 and BThIND‐Cl. Moreover, the short circuit current density is higher for the P1 based polymer solar cell. Taking the advantages of high valve of short circuit current for P1 and larger value of open circuit voltage for P3 counterpart, optimized polymer solar cells using these two polymers and BThIND‐Cl as acceptor and after the optimization, the polymer solar cells based on P1:P3:BThIND‐Cl showed overall power conversion efficiency of 15.78% with energy loss of 0.55 eV.

Computational study of organic small molecules based on imidazolinone for photovoltaic applications

ABSTRACT Organic photovoltaic cells have received significant interest due to the low-cost manufacturing and compatibility to deposit on flexible substrate. However, this technology suffers from certain problems including lowest solar spectrum absorption by reason of the large band gap of donor materials. This could return to the lack of extensive studies concerning the effects of geometric structures and the energy levels associated with the synthesis of molecules. In this context, six organic π-conjugated molecules based on imidazolinone with low band gap have been studied theoretically using quantum methods namely density functional theory (DFT) and time dependent-density functional theory (TD-DFT). The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and chemicals descriptors of these molecules have been examined in this work. Spectroscopic and electronic properties have been reported in order to study the parameters which affect photovoltaic efficiency. The calculated results of these compounds show a lower HOMO energy, large absorption spectra, and good theoretical open circuit voltage. The findings of this work are helpful to enhance the performance of bulk hetero junction organic solar cells (BHJ).

Designing of benzodithiophene core-based small molecular acceptors for efficient non-fullerene organic solar cells.

Nowadays, organic solar cells (OSCs) with non-fullerene electron acceptors provide the highest efficiencies among all studied OSCs. To further improve the efficiencies of fullerene-free organic solar cells, end-capped acceptor modification is made with strong electron withdrawing groups. In this report, we have theoretically designed five new novel Benzodithiophene core-based acceptor molecules (H1-H5) with the aim to study the possible enhancement in photophysical, optoelectronic, and photovoltaic properties of newly designed molecules. The end-capped acceptor modification of famous and recently synthesized FBDIC molecule has been made with strong electron withdrawing groups. Density functional theory and time-dependent-density functional theory are extensively used to study the structural-property relationship, optical properties and various geometrical parameters like frontier molecular orbitals alignment, excitation and binding energy, transition density matrix along with open circuit voltage, density of states and dipole moment. Commonly, low reorganization energies (hole and electron) afford high charge mobility and our all designed systems are enriched in aspect (λe=0.0044-0.0104eV and λh=0.0060-0.0090eV). Moreover, H1-H5 molecules demonstrate red-shifting in absorption spectrum (λmax=741-812nm) as compare to R (λmax=728nm). Low excitation and binding energies with low HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap of H1-H5 suggested that designed molecules are better and suitable candidates for high performance organic solar cell. Results of all analysis indicate that this theoretical framework demonstrates that end-capped acceptors modification is a simple and effective alternative strategy to achieve the desirable optoelectronic properties. Therefore, H1-H5 are recommended to experimentalist for out-looking future developments of highly efficient solar cells.

Theoretical Analysis on Heteroleptic Cu(I)-Based Complexes for Dye-Sensitized Solar Cells: Effect of Anchors on Electronic Structure, Spectrum, Excitation, and Intramolecular and Interfacial Electron Transfer.

Two groups of heteroleptic Cu(I)-based dyes were designed and theoretically investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Different anchors were integrated into the dye skeleton to shed light on how the type of anchor influenced the electronic structure, absorption spectrum, electron excitation, and intramolecular and interfacial electron transfer of dyes. The results indicated that, compared with other dyes, the dyes with cyanoacrylic acid and nitric acid exhibited more appropriate electron distributions in frontier molecular orbitals (FMOs), lower HOMO (the highest occupied molecular orbital) –LUMO (the lowest unoccupied molecular orbital) energy gaps, broader absorption spectral ranges as well as improved spectral characteristics in the near-infrared region and better intramolecular electron transfer (IET) characteristics with more electrons transferred to longer distances, but smaller orbital overlap. Among all the studied Cu(I)-based dyes, B1 and P1 (with cyanoacrylic acid anchoring group) exhibited the best interface electronic structure parameters with a relatively short electron injection time (τinj) and large dipole moment (μnormal), which would have a positive effect on the open-circuit photovoltage (Voc) and short-circuit current density (Jsc), resulting in high power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). Our findings are expected to provide a new insight into the designing and screening of high-performance dyes for DSSCs.

Synergistic Use of Pyridine and Selenophene in a Diketopyrrolopyrrole‐Based Conjugated Polymer Enhances the Electron Mobility in Organic Transistors

AbstractTo achieve semiconducting materials with high electron mobility in organic field‐effect transistors (OFETs), low‐lying energy levels (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) and favorable molecular packing and ordering are two crucial factors. Here, it is reported that the incorporation of pyridine and selenophene into the backbone of a diketopyrrolopyrrole (DPP)‐based copolymer produces a high‐electron‐mobility semiconductor, PDPPy‐Se. Compared with analogous polymers based on other DPP derivatives and selenophene, PDPPy‐Se features a lower LUMO that can decrease the electron transfer barrier for more effective electron injection, and simultaneously a lower HOMO that, however, can increase the hole transfer barrier to suppress the hole injection. Combined with thermal annealing at 240 °C for thin film morphology optimization to achieve large‐scale crystallite domains with tight molecular packing for effective charge transport along the conducting channel, OFET devices fabricated with PDPPy‐Se exhibit an n‐type‐dominant performance with an electron mobility (μe) as high as 2.22 cm2 V−1 s−1 and a hole/electron mobility ratio (μh/μe) of 0.26. Overall, this study demonstrates a simple yet effective approach to boost the electron mobility in organic transistors by synergistic use of pyridine and selenophene in the backbone of a DPP‐based copolymer.

Benzo[b]selenophene/thieno[3,2-b]indole-Based N,S,Se-Heteroacenes for Hole-Transporting Layers.

Two series of new N,S,Se-heteroacenes, namely, 6H-benzo[4′,5′]selenopheno[2′,3′:4,5]thieno[3,2-b]indoles and 12H-benzo[4″,5″]selenopheno[2″,3″:4′,5′]thieno[2′,3′4,5]thieno[3,2-b]indoles, were successfully obtained using an effective strategy based on Fiesselmann thiophene and Fischer indole synthesis. The new molecules exhibit a large optical band gap (2.82 eV < Egopt < 3.23 eV) and their highest occupied molecular orbital (HOMO) energy formed by the plane π-core ranges between −5.2 and −5.6 eV, with the narrower optical band gap and lower HOMO level corresponding to selenated heteroacenes. In thin solid films of the heteroacenes, hole mobility measured using the conventional CELIV technique ranges between 10–5 and 10–4 cm2·V–1·s–1. All these make the proposed condensed-ring compounds a promising platform for the development of hole-transporting materials applicable in organic electronics.

Improving open-circuit voltage by a chlorinated polymer donor endows binary organic solar cells efficiencies over 17%

Power conversion efficiency (PCE) of single-junction polymer solar cells (PSCs) has made a remarkable breakthrough recently. Plenty of work was reported to achieve PCEs higher than 16% derived from the PM6:Y6 binary system. To further increase the PCEs of binary OSCs incorporating small molecular acceptor (SMA) Y6, we substituted PM6 with PM7 due to the deeper highest occupied molecular orbital (HOMO) of PM7. Consequently, the PM7:Y6 has achieved PCEs as high as 17.0% by the hot-cast method, due to the improved open-circuit voltage ( V OC). Compared with PM6, the lower HOMO of PM7 increases the gap between E LUMO-donor and E HOMO-acceptor, which is proportional to V OC. This research provides a high PCE for single-junction binary PSCs, which is meaningful for device fabrication related to PM7 and commercialization of PSCs.

Double Acceptor Block-Containing Copolymers with Deep HOMO Levels for Organic Solar Cells: Adjusting Carboxylate Substituent Position for Planarity.

A deep highest occupied molecular orbital (HOMO) level is a prerequisite for polymer donor material to boost the organic solar cells (OSCs) performance by achieving high open circuit voltage ( Voc). Abandoning the traditional concept of donor-acceptor (D-A) structure, two copolymers PBTZ-4TC and PBTZ-C4T based on acceptor1-π-acceptor2 (A1-π-A2) architecture, where thiophene as the bridge, the difluorinated benzotriazole (BTZ) as A1 unit alternating copolymerized with 4,4'-dicarboxylate-substituted difluorotetrathiophene (4TC) and 3,3'-dicarboxylate-substituted difluorotetrathiophene (C4T) as A2, respectively, are developed. Because of the double acceptor blocks with high electron affinity, both A1-π-A2 type copolymers possess the lower HOMO levels of 5.52-5.56 eV, which are lower than most D-A type donors. Polymer PBTZ-4TC and PBTZ-C4T have the same backbone but only differ with the position of carboxylate substituent on the A2 unit. Intriguingly, subtle optimizing the position of the carboxylate-substitute causes a significantly difference on the properties of the A1-π-A2 type copolymers. PBTZ-C4T with more planar geometry is demonstrated with better light absorption, higher crystallinity, more pronounced temperature-dependent aggregation effect, and favorable bulk heterojunction morphology but with slightly higher HOMO level and more emission energy loss relative to the PBTZ-4TC. The PBTZ-C4T device exhibits the higher power conversion efficiency (PCE) of 9.34% than the PBTZ-4TC-based one (8.75%). These results reveal that concept of A1-π-A2 type copolymers not only can afford more flexibility in tuning the energy levels to achieve the deep HOMO levels but also can provide a facial strategy to greatly enrich the types of polymer donors for high-performance OSCs.

Activity of Thioallyl Compounds From Garlic Against Giardia duodenalis Trophozoites and in Experimental Giardiasis.

Fresh aqueous extracts (AGEs) and several thioallyl compounds (TACs) from garlic have an important antimicrobial activity that likely involves their interaction with exposed thiol groups at single aminoacids or target proteins. Since these groups are present in Giardia duodenalis trophozoites, in this work we evaluated the anti-giardial activity of AGE and several garlic's TACs. In vitro susceptibility assays showed that AGE affected trophozoite viability initially by a mechanism impairing cell integrity and oxidoreductase activities while diesterase activities were abrogated at higher AGE concentrations. The giardicidal activities of seven TACs were related to the molecular descriptor HOMO (Highest Occupied Molecular Orbital) energy and with their capacity to modify the –SH groups exposed in giardial proteins. Interestingly, the activity of several cysteine proteases in trophozoite lysates was inhibited by representative TACs as well as the cytopathic effect of the virulence factor giardipain-1. Of these, allicin showed the highest anti-giardial activity, the lower HOMO value, the highest thiol-modifying activity and the greatest inhibition of cysteine proteases. Allicin had a cytolytic mechanism in trophozoites with subsequent impairment of diesterase and oxidoreductase activities in a similar way to AGE. In addition, by electron microscopy a marked destruction of plasma membrane and endomembranes was observed in allicin-treated trophozoites while cytoskeletal elements were not affected. In further flow cytometry analyses pro-apoptotic effects of allicin concomitant to partial cell cycle arrest at G2 phase with the absence of oxidative stress were observed. In experimental infections of gerbils, the intragastric administration of AGE or allicin decreased parasite numbers and eliminated trophozoites in experimentally infected animals, respectively. These data suggest a potential use of TACs from garlic against G. duodenalis and in the treatment of giardiasis along with their additional benefits in the host's health.

Bis(carboxylate) substituted benzodithiophene based wide-bandgap polymers for high performance nonfullerene polymer solar cells

To develop wide bandgap donor materials for high performance nonfullerene polymer solar cells (NF PSCs), bis(carboxylate) substituted benzo[1,2-b:4,5-b]dithiophene (BDT) weak electron acceptor (A) unit was used to polymerize with 5-alkylthiophene-2-yl-substituted BDT or 5-alkylthiothiophene-2-yl-substituted BDT donor unit to construct donor-acceptor (D-A) alternative polymers PBDT-BDTC and PBDTS-BDTC, respectively. PBDT-BDTC and PBDTS-BDTC exhibit wide bandgaps over 1.90 eV and low lying highest occupied molecular orbital (HOMO) levels below −5.50 eV. The absorption and energy levels of these two polymers match well with those of the typical low bandgap acceptor ITIC-Th, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset of about 0.15 eV. NF PSCs with an inverted device structure were fabricated and power conversion efficiencies (PCEs) higher than 7% were achieve. Furthermore, PBDT-BDTC based PSCs exhibit much better photovoltaic performance than PBDTS-BDTC based ones after the treatment with 0.5% 1,8-diiodooctane (DIO). The optimized devices gave a PCE of 8.32% with an open-circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 13.91 mA/cm2, and a fill factor (FF) of 65.21%, indicating that PBDT-BDTC is a promising polymer donor for NF PSCs. To the best of our knowledge, a PCE of 8.32% is the highest record for bis(carboxylate) substituted BDT based polymers.

Theoretical design of new small molecules with a low band-gap for organic solar cell applications: DFT and TD-DFT study

In search of novel high-performance materials for use in organic solar cells, we used density-functional theory and time-dependent density functional theory to design a series of organic small molecules derived from the recently synthesized BT(-2T-DCV-Hex)2 donor molecule. In this work, we replaced the BT unit by different acceptors in order to improve their electronic properties, optical absorption and performance in organic solar cell applications. We found that the hybrid functional B3PW91 with the 6-31G(d) basis set gave highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the BT(-2T-DCV-Hex)2 in better agreement with the experimental oxidation and reduction potentials. However, the range-separated hybrid functional WB97XD was the most appropriate functional for describing the maximum absorption wavelength. Our calculation indicate that the designed small molecules donor proposed here are expected to offer better performances compared to the BT(-2T-DCV-Hex)2, such as a lower HOMO energy, a narrower HOMO-LUMO energy gap, a larger absorption range and may lead to power conversion efficiencies reaching the (7–9)% range.