Low HOMO Level Research Articles

Engineering A Boron-Rich Interphase with Nonflammable Electrolyte toward Stable Li||NCM811 Cells Under Elevated Temperature.

Despite the high energy of LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathode, it still suffers serious decay due to the continuous solvents decomposition and unstable cathode electrolyte interphase (CEI) layers, especially under high temperatures. The intense exothermic reaction between delithiated NCM811 and flammable electrolyte, on the other hand, pushes the batteries to their safety limit. Herein, these two issues are tackled via engineering the electrolytes, that is, utilizing salts with higher HOMO levels and nonflammable solvents with lower HOMO levels, to reduce the massive decomposition of solvents and improve battery safety under elevated temperatures. Consequently, a thin and boron-rich CEI is generated, which effectively inhibited the side reactions, thus improving the cycling stability and safety. Deviated from the highly concentrated electrolytes which heavily relies on the usage of massive salts, the electrolyte recipe can introduce a robust inorganic-rich CEI but use much less salt (i.e., dilute electrolyte), and thus, offer an encouraging alternative toward practical applications. As such, the NCM811 cathode exhibits a high-capacity retention of 81.2% after 950 cycles at 25°C and 75% after 300 cycles at 55°C. This work provides a universal electrolyte design strategy for designing stable and safe high-temperature electrolytes for the NCM811 cathode.

Thieno[3,2 -b]thiophene-based linear dopant-free hole transport materials for efficient perovskite solar cells

Organic hole transport materials (HTMs) are extensively studied in perovskite solar cells (PSCs). However, due to the inherently low charge carrier mobility, chemical dopants are often required to increase mobility and achieve high power conversion efficiency. Unfortunately, the use of dopants not only brings high costs, but also accelerates the degradation of perovskite, severely reducing the stability of perovskite solar cells. Therefore, developing efficient dopant-free HTMs has become a big challenge. Here, two linear molecules DTTTP-DPA and DTTTP-TPA were designed and synthesized, with diethyl 2,5-bis(thieno [3,2-b]thiophen-2-yl)terephthalate (DTTTP) as the core structure, methoxydiphenylamine/methoxytriphenylamine as the end group, respectively. Compared to diphenylamine, the triphenylamine end group endowed DTTTP-TPA with a longer conjugation length and a lower HOMO level. The longer conjugation is more conducive to charge transporting. While the lower HOMO level due to the weaker electron donating ability of triphenylamine, is potential to achieve a higher open-circuit voltage when used in PSCs. In addition, the carbonyl group in molecular structure could passivate the defect at perovskite/hole transport layer (HTL) interface. As a result, the dopant-free DTTTP-TPA device shows a high photovoltaic performance of 21.62%. At room temperature (30% relative humidity), the initial PCE of the unencapsulated device remains above 93% after 1000 h, showing excellent stability. Our work demonstrates that constructing linear type molecular structure with extended conjugation is an efficient way to enhance hole mobility and is promising to obtain dopant-free HTMs.

Molecular engineering of novel dithiophene-fused hole-transporting materials for highly efficient perovskite solar cells

Understanding the relationship between structure and property of hole-transporting materials (HTMs) plays an important role in developing highly efficient perovskite solar cells (PSCs). Herein, we report a density functional theory study (DFT) combined with Marcus theory on the structural, electronic, optical, electrochemical and hole-transporting properties of designed molecules. The calculated results demonstrate that extending π-conjugation of the center by replacing CDT with IDT can lower the HOMO level, increase Stokes shift, enhance the solubility, strengthen the defect passivation ability and improve the hole mobility of molecules. The HOMO and LUMO energy alignments of the designed molecules properly match with the VBM and CBM of conventional perovskite CH3NH3PbI3 or mixed perovskite (FAPbI3)0.85(CH3NH3PbBr3)0.15, respectively. The introduction of electron-accepting groups in the π-linker results in the blue-shift of absorption edge into the UV region, which has no competition with the absorption of perovskites. Besides possessing lower HOMO levels, better solubilities and spectral properties, three designed molecules para-IDT-CN (1.76 × 10−1 cm2 V−1 s−1), meta-IDT-O (2.09 × 10−2 cm2 V−1 s−1) and meta-IDT-CN (7.53 × 10−3 cm2 V−1 s−1) exhibit higher hole mobility than the reported HTMs CDT (4.15 × 10−3 cm2 V−1 s−1) and CDT-CN (1.15 × 10−3 cm2 V−1 s−1), which ensure that these novel molecules have potentials to be efficient HTMs in the application of PSCs.

Effect of fluorine substitution on properties of hole-transporting materials for perovskite solar cells

In photovoltaic technologies, hole-transporting materials (HTMs) play a critical role in realizing highly efficient performance of perovskite solar cells (PSCs). Herein, a series of fluorine substituted small molecules based on triphenylamine backbone are designed by controlling fluorine atoms number and position. A comprehensive study on the structural modification is carried out to reveal the effect of fluorine substitution of HTMs. The energy level alignment, reorganization energy and the charge-transport properties are explored based on Quantum-chemical calculations. It is found that all difluorinated molecules have lower HOMO levels, which indicates that perovskite solar cells with two fluorine substituted hole-transporting materials may have higher open-circuit voltages. Moreover, the UV–vis absorption spectra of the studied molecules are slightly red-shifted with the increase of fluorine atoms. To further understand the effect of fluorine substitution on the hole transport properties, the hole mobilities of all HTMs are evaluated quantitatively using the Marcus theory and Einstein relation. The calculated results show that difluorinated molecules TPB-m-2F exhibit relatively higher hole mobility owing to efficient intermolecular interactions. As for monofluorinated molecules TPB-1F-a and TPB-1F-b, TPB-1F-a with fluorine atoms substituted on the inside position exhibit higher hole-transporting performance. Overall, we conclude that suitable modification of fluorine substitution is expected to improve the electrochemical and hole transport property of HTMs. We hope our theoretical investigation provide useful information for further improving the photovoltaic performance of PSCs.

New Donor–Acceptor Random Terpolymers with Wide Absorption Spectra of 300–1000 nm for Photovoltaic Applications

The development of conjugated polymers with wide absorption spectra is imperative to achieve high efficiency in polymer solar cells (PSCs), since most of these polymers usually absorb only a limited range of the solar spectrum. Random terpolymers consisting of three blocks (one electron-donor and two electron-acceptor blocks) are promising p-type polymers for PSCs, because the inclusion of a third block in the polymer macromolecules provides a synergistic effect of physical properties, such as absorption capacity, charge transfer, HOMO/LUMO energy levels, and photovoltaic characteristics. In this regard, we have developed and synthesized random terpolymers consisting of two different chromophores (DPP and BFCTP) with complementary absorption spectra as co-acceptor blocks in conjugated donor–acceptor (D–A) copolymers. Random copolymers exhibit both broad absorption and low HOMO levels favoring short-circuit current and idle voltage in PSCs. It is expected that new terpolymers consisting of one electron-donor unit and two electron-acceptor moieties will make a significant contribution to the development of highly efficient PSCs. It is expected that new ternary copolymers consisting of one electron-donor unit and two electron-acceptor fragments will make a significant contribution to the development of high-performance PSСs.

Anion Solvation Reconfiguration Enables High‐Voltage Carbonate Electrolytes for Stable Zn/Graphite Cells

AbstractConventional carbonate solvents with low HOMO levels are theoretically compatible with the low‐cost, high‐voltage chemistry of Zn/graphite batteries. However, the nucleophilic attack of the anion on carbonates induces an oxidative breakdown at high potentials. Here, we restore the inherent anodic stability of carbonate electrolytes by designing a micro‐heterogeneous anion solvation network. Based on the addition of a strongly electron‐donating solvent, trimethyl phosphate (TMP), the oxidation‐vulnerable anion‐carbonate affinities are decoupled because of the preferential sequestration of anions into solvating TMP domains around the metal cations. The hybridized electrolytes elevate the electrochemical window of carbonate electrolytes by 0.45 V and enable the operation of Zn/graphite dual‐ion cells at 2.80 V with a long cycle life (92 % capacity retention after 1000 cycles). By inheriting the non‐flammability from TMP and the high ion‐transport kinetics from the carbonate systems, this facile strategy provides cells with the additional benefits of fire retardancy and high‐power capability.

Anion Solvation Reconfiguration Enables High-Voltage Carbonate Electrolytes for Stable Zn/Graphite Cells.

Conventional carbonate solvents with low HOMO levels are theoretically compatible with the low-cost, high-voltage chemistry of Zn/graphite batteries. However, the nucleophilic attack of the anion on carbonates induces an oxidative breakdown at high potentials. Here, we restore the inherent anodic stability of carbonate electrolytes by designing a micro-heterogeneous anion solvation network. Based on the addition of a strongly electron-donating solvent, trimethyl phosphate (TMP), the oxidation-vulnerable anion-carbonate affinities are decoupled because of the preferential sequestration of anions into solvating TMP domains around the metal cations. The hybridized electrolytes elevate the electrochemical window of carbonate electrolytes by 0.45 V and enable the operation of Zn/graphite dual-ion cells at 2.80 V with a long cycle life (92 % capacity retention after 1000 cycles). By inheriting the non-flammability from TMP and the high ion-transport kinetics from the carbonate systems, this facile strategy provides cells with the additional benefits of fire retardancy and high-power capability.

Chlorination and thiophene strategies to adjust molecular energy levels for high-performance organic polymer solar cells

Three novel polymers with D-A (π)-BT backbone, PBDBT-0, PBDBT-1, and PBDBT-2, were built through effective side-chain chlorination strategies and thiophene-induced approaches. Meanwhile, the effect of different thiophene numbers was studied. As expected, all of the chlorinated polymers showed low HOMO levels. The introduction of the thiophene unit improved the solubility of the polymer and slightly adjusted the energy level of the molecule. Furthermore, polymers as donors and PC71BM as acceptors are used to construct organic solar cell devices. The devices based on polymer PBDBT-1 with one thiophene unit delivered the best efficiency of 5.73 % with an open-circuit voltage of 0.85 V, a short-circuit current density of 13.07 mA cm−2 and a fill factor of 51.6 %, which is superior to that of PBDBT-0 and PBDBT-2.

The Potentials in Solar Cells for MEH‐PPV Derivatives: Molecular Design and Performance Prediction

In this contribution, the polymer poly(2‐methoxy‐5(2′‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene) (MEH‐PPV) is researched at the aspect of electronic and structural properties to locate the relationships between structures and the experimental properties by employing DFT and TD‐DFT method. The studied properties refer to the geometry features, absorption spectra, frontier molecular orbital properties, hole transport properties, and exciton dissociation ability at polymer/PC61BM interface. In addition, four MEH‐PPV derivatives, MEH‐PPV substituted with F atoms, are designed and studied through comparing with the properties of MEH‐PPV. The results indicate that MEH‐PPV has low HOMO levels and large hole transport property, which result in its preferable performance. Besides, the designed polymers are promising candidates as PSC donors due to better exciton dissociation ability and larger theoretical VOC owing to lower highest occupied molecule levels, in special, MEH‐PPV‐Fc has better hole transport properties than MEH‐PPV due to smaller m*.

Molecular Engineering of Simple Carbazole‐Triphenylamine Hole Transporting Materials by Replacing Benzene with Pyridine Unit for Perovskite Solar Cells

Four carbazole‐triphenylamine hole transporting materials (HTMs) with a pyridine or benzene on the N of carbazole have been designed and developed. The molecular structures on their photophysical, electrochemical, thermal, as well as photovoltaic properties in perovskite solar cells (PSCs) are comprehensively analyzed. It is noted that the pyridine‐contained compounds show bathochromic spectra response, slightly lower HOMO level, and higher thermal stability than their benzene‐contained counterparts. Photoluminescence study indicates that the molecule with a pyridine unit possess better hole extraction properties on the perovskite/HTM interface. When used in PSCs, pyridine‐contained HTMs exhibit higher efficiency and better stability than benzene‐contained HTMs. Moreover, the PSCs based on the pyridine‐contained HTM exhibit a promising power conversion efficiency of 18.45% with good light‐soaking and long‐term stability, which is even better than that of conventional spiro‐OMeTAD under the same conditions. Therefore, the results provide a promising strategy to design and develop novel HTM molecules for efficient and stable PSCs.

Phenothiazine-based small molecules for bulk heterojunction organic solar cells; variation of side-chain polarity and length of conjugated system

Three small molecules denoted SM1, SM2 and SM3, with the phenothiazine donor moiety connected to benzothiadiazole and 3-ethylrhodanine acceptor units through thiophene π-linkers have been synthesized for use in organic solar cells with PC71BM as electron acceptor. SM1 has a 2-ethylhexyl group at the phenothiazine nitrogen, while SM2 and SM3 have a 2-(2-methoxyethoxy)ethyl group at that N. Opto-electronic and dielectric properties, charge carrier mobilities, morphology of active layers, and photovoltaic properties were investigated in detail. The three molecules have wide absorption bands with high molar absorption coefficients and relatively low HOMO levels (−5.21 to −5.27 eV). Compared with the N-alkylated SM1, both SM2 and SM3 exhibit red-shifts of the long-wavelength absorption band in thin films, and show enhanced crystallinity in thin films with smaller stacking distances, higher hole mobility, and higher dielectric constant. After solvent vapour annealing, the power conversion efficiencies (PCEs) were significantly improved for the solar cell devices, from 1.69 to 3.95% for SM1, 2.78–6.62% for SM2 and 3.22–7.16% for SM3. This increase in PCEs was due to the enhancement in Jsc and FF attributed to the formation of nanoscale domains of donor and acceptor resulting in efficient charge separation, balanced charge transport and suppressed charge recombination. These results demonstrate that use of an oxygen-containing side chain, in conjunction with alkyl side chains for solubility during solar cell processing, is an alternative and effective strategy for achieving high-performance small molecule donor materials.

Novel D-A type dyes based on BODIPY for solution processed organic polymer solar cells

Boron dipyrromethene (BODIPY) derivatives have good optoelectronic properties and thermal stability. However, it is rarely for such materials to use in bulk heterojunction (BHJ) organic solar cell. In this study, a variety of donor-acceptor (D-A) type boron dipyrromethene (BODIPY) based dyes were designed and synthesized for polymer solar cells (PSCs), namely BDH1-4. Thermal stability, photochemical properties, charge transport and BHJ morphology were investigated to elucidate factors affecting the photovoltaic performance in this class of polymers. These BDHs exhibit fine solubility in conventional solvents, good thermal stability, excellent panchromatic absorption in solution and in film. Furthermore, the absorption wavelength of BDH1-4 covers full color absorption in the range of 300–800 nm and has a relatively low HOMO level from −5.01 to −5.66 eV. Among them, BDH4/PC71BM based PSCs exhibit highest power conversion efficiency (PCE) of 3.69% with a Jsc of 9.92 mA cm−2, a Voc of 0.63 V and a FF of 59.1%.

Indigo-Based Acceptor Type Small Molecules: Synthesis, Electrochemical and Optoelectronic Characterizations.

In this paper, we report design and synthesis of novel low bandgap small molecules, indigo-benzimidazole (Tyr-3) and indigo-schiff base (Tyr-4) type acceptors. In these structures, tert-butoxycarbonyl (t-BOC) group has been attached to indigo nitrogen atom in order to increase the solubility. UV-vis absorption spectra of Tyr-3 and Tyr-4 dyes exhibit wide absorption bands ranging from 350 to 600nm, indicating the relatively low bandgap giving around 2.07eV for each. Excitation of both Tyr-3 and Tyr-4 dyes at 485nm displays characteristic emission features of indigo moiety and also intramolecular charge transfer complex (ICT) related with their subunits. Besides increasing fluorescence quantum yields as compared to model compound, biexponential decay times for fluorescence life times were also obtained for Tyr-3 and Tyr-4 dyes. Their appropriate energy levels along with low HOMO levels are desired for light harvesting acceptors for organic solar cells when blended with poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole] (PCDTBT) as donor polymer. Photovoltaic behavior of the synthesized dyes were examined in bulk heterojunction concept and achieved photovoltaic conversion efficiencies were discussed.

Regioregular polymers containing benzodithiophene and thienothiophene segments with different electron donating side chains for high-performance polymer solar cells

This study describes a systematic study on side chain effect in the poly(thieno[3,4-b]thiophene)benzo[1,2-b:4,5-b′]dithiophene (PTB) polymer series based regioregular polymers with D1-A-D2-A configuration, denoted as PTT-BDT(R1)-TT-BDT(R2). The synthesized three regioregular polymers employ the same conjugated backbone but with different side chains (R1, R2), P1; hexyldecyl thiophene (HD, R1) and ethylhexyl thiophene (EH, R2), P2; hexyldecyl thiophene (HD, R1) and ethylhexylthio thiophene (S-EH, R2), P3; hexyldecylthio thiophene (S-HD, R1) and ethylhexylthio thiophene (S-EH, R2). Introduction of different side chains influenced optical, electrochemical, molecular packing properties, and device performance. The P3 polymer with alkylthio thiophene groups as R1 and R2 showed much broader light absorption and lower HOMO level than other polymers with relatively less alkylthio thiophene groups. Furthermore, the P3 polymer exhibited favorable polymer orientation and short π-π stacking distance for efficient charge transport in the BHJ PSCs. The P3 based bulk-heterojunction (BHJ) polymer solar cell (PSC) showed an improved PCE of 7.20% while the P1 and P2 based BHJ PSC devices showed PCEs of 5.27% and 6.45%, respectively. This result, within our knowledge, is the highest record among BHJ PSCs based PTB polymer series composed of benzo[1,2-b:4,5-b’]dithiophene (BDT) segment and non-fluorinated alkyl ester (COOR) side chain substituted thieno[3,4-b]thiophene (TT) segment.

How the Connectivity of Methoxy Substituents Influences the Photovoltaic Properties of Dissymmetric Core Materials: A Theoretical Study on FDT

There are a few reports that the optoelectronic properties of the methoxyaniline-based hole-transporting materials are intimately correlated with the positions of −OMe substituents. To dig into this phenomenon deeply, we theoretically design five new hole-transporting materials (HTMs) based on 2′,7′-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene] (FDT) through altering the positions of −OMe substituents. Then, the electronic structures, optical properties, and hole-transporting properties are investigated at the molecular level via density functional theory and Marcus theory coupled to Einstein relation. The calculated results reveal that the derivatives with o-OMe or m-OMe substituent exhibit lower HOMO levels, favoring higher open-circuit voltages. Most importantly, benefitting from greater order and compact intermolecular stacking, the derivatives with o-OMe substituents (F1, F3) as HTMs exhibit relatively decent hole mobilities (F1: 6.29 × 10–2 cm2 V–1 s–1; F3: 2...

Tuning the Optoelectronic Properties of Naphtho‐Dithiophene‐Based A‐D‐A Type Small Donor Molecules for Bulk Hetero‐Junction Organic Solar Cells

AbstractFour acceptor‐donor‐acceptor (A−D‐A) type small molecules using naphtho‐dithiophene as central building block, trithiophene as π‐bridges, malononitrile and 2‐Thioxo‐thiazolidin‐4‐one as end acceptor groups were designed by using density functional theory (DFT) and investigated as donor materials for organic solar cells (OSCs). The effects of end acceptor groups on absorption, charge transport, morphology, energy level and photovoltaic properties of the molecules were investigated and compared with reference molecule (R1= NDTP‐CNCOO). These designed molecules showed relatively low HOMO levels of −5.46 to −5.56 eV, strong absorption between 440–650 nm by using chloroform as solvent. The designed donor molecules have an excellent electron mobility from 0.0161 to 0.0201 V, hole mobility from 0.0275 to 0.0307 V and open circuit voltage (Voc) from 1.76 to 1.86 V. This study revealed that the designed donor materials are suitable and recommended for high performance organic solar cell devices.

Effect of the Alkyl Chain Length Incorporated into Donor Part on the Optoelectronic Properties of the Carbazole Based Dyes: Theoretical Study

In this paper, we report a theoretical study using density functional theory (DFT) and time-dependent (TD-DFT) for R-D-π-A systems with various alkyl chains (R). Results show that the LUMO of the dye lies above the semiconductor conduction band, promoting the injection of electrons; the lower HOMO level promotes dye regeneration. The incorporation of methyl chain (CH 3 ) has a significant reduction in the gap energy, improved red-shift absorption spectrum and increase the molar extinction coefficient at the maximum absorption wavelength compared to D. While, the increase in alkyl chain length from C 2 H 5 to C 6 H 13 present a relatively reduce of gap energies, low effect on the wavelength (438 nm) and converged excitation energies. DOI: http://dx.doi.org/10.17807/orbital.v9i5.1003

Improving the performance of arylamine-based hole transporting materials in perovskite solar cells: Extending π-conjugation length or increasing the number of side groups?

In this work, we prepared three simple arylamine-based hole transporting materials from commercially available starting materials. The effect of extending π-conjugation length or increasing the number of side groups compared with reference compound on the photophysical, electrochemical, hole mobility properties and performance in perovskite solar cells were further studied. It is noted that these two kinds of molecular modifications can significantly lower the HOMO level and improve the hole mobility, thus improving the hole injection from valence band of perovskite. On the other hand, the compound with more side groups showed higher hole injection efficiency due to lower HOMO level and higher hole mobility compared with the compound with extending π-conjugation length. The perovskite solar cells with the modified molecules as hole transporting materials showed a higher efficiency of 15.40% and 16.95%, respectively, which is better than that of the reference compound (13.18%). Moreover, the compound with increasing number of side groups based devices showed comparable photovoltaic performance with that of conventional spiro-OMeTAD (16.87%).

Supported gold clusters as effective and reusable photocatalysts for the abatement of endocrine-disrupting chemicals under visible light

Photocatalysts based on glutathione-protected Au clusters (1.5±0.4nm) were synthesized and assessed for the visible-light degradation of ten different endocrine-disrupting chemicals (EDCs) and related compounds. In its as-prepared form, the Au clusters undergo rapid photocharge recombination resulting in no appreciable photocatalytic activity. Coupling with non-activated TiO2 acceptor was necessary to achieve efficient interfacial charge separation that allowed the photocharges on the Au clusters to be effectively harvested for surface redox reactions. Under visible light excitation, the Au cluster undergoes ligand-to-metal charge transfer and resulting in the accumulation of photohole on the glutathione ligand (i.e., the HOMO state). The transfer of photoholes to the hydrogen-bonded EDCs is identified as the main oxidation pathway. The hole transfer oxidation, which was probably complemented by superoxide radicals attack, was necessary since the low HOMO level of the Au clusters (+1.21V vs. RHE, pH 0) disallowed the production of hydroxyl radicals (+2.68V vs. RHE, pH 0). As such, the photocatalytic degradation rates of EDCs are in accordance with the trend of Langmuir adsorption capacities. Lastly, we demonstrated the high reusability of the Au cluster/TiO2 photocatalyst, retaining 96% of the initial activity even after 5 repeated cycles or equivalent to 25h of continuous irradiation.

Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells

High performance, hysteresis-free, low temperature n-i-p perovskite solar cells are successfully fabricated by solution processing using fullerene electron transport layer (ETL). PC71BM fullerene, with broader absorption spectrum and lower HOMO level, when incorporated in the perovskite solar cell yielded average power conversion efficiency (PCE) of 13.9%. This is the highest reported PCE in n-i-p perovskite solar cells with PC71BM ETL. The devices exhibited negligible hysteresis and high open-circuit voltage (Voc). On the contrary, devices with PC61BM, a common fullerene ETL in perovskite solar cell, exhibited large hysteresis and lower Voc. The underlying mechanisms of superior performance of devices with PC71BM ETL were found to be correlated with fullerene surface wettability and perovskite grain size. The influence of fullerene ETL on the perovskite grain growth and subsequent photovoltaic performance was investigated by contact angle measurement, morphological characterization of the surface topography and electrochemical impedance analysis.