Highest Occupied Molecular Orbital Level Research Articles

Novel integrated reference-counter electrode for electrochemical measurements of HOMO and LUMO levels in small-molecule thin-film semiconductors for OLEDs

Organic light-emitting diodes (OLEDs) are a prominent display technology, yet the accurate characterization of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels in their constituent materials remains challenging. This study introduces a novel integrated reference-counter electrode (IRCE) assembly, leveraging a gel polymer electrolyte with an embedded silver quasi-reference electrode, facilitating the electrochemical measurement of HOMO and LUMO levels in small molecular thin-film semiconductors. Calibration of the IRCE against ferrocene enables the establishment of an absolute energy scale. Comparative stability tests against a standard Ag/AgNO3 reference electrode confirm the IRCE's reliability. Electrochemical characterization using cyclic voltammetry was performed on prototypical OLED materials, including NPB, TCTA, PO-T2T neat films, and an NPB:PO-T2T exciplex film. While NPB and PO-T2T exhibited stable voltammograms, TCTA showed signs of electropolymerization. Additionally, the HOMO level of the NPB:PO-T2T exciplex was slightly shifted compared to that of NPB, suggesting interactions within the exciplex. The results demonstrated the IRCE's capability to accurately determine frontier energy levels in thin films, paving the way for better device modeling and a better understanding of underlying electronic processes in organic semiconductors.

Non‐Electron Withdrawing Unit Copolymerization to Enhance Photo‐Stability of Thiophene‐Based Organic Solar Cells

The impact of non‐electron withdrawing unit copolymerization on photostability of fullerene‐based organic solar cells is investigated using two donor polymers namely: benzodithiophene‐4,8‐dione (BDD) unit copolymerized with α‐quaterthiophene (4 T) unit (PBDD4T) and poly‐3‐hexyl‐thiophene (P3HT). The optical and electrochemical properties of the polymers reveal a deeper highest occupied molecular orbital (HOMO) and broader absorption in PBDD4T in comparison to P3HT owing to the BDD copolymerization. The copolymer achieves a higher power conversion efficiency (PCE) compared to the P3HT‐based device due to its higher VOC and Jsc that resulted from its deeper HOMO level and broader absorption. The photostability study reveals that PBDD4T‐based devices lost 14% of its initial PCE relative to the 48% reduction in PCE for P3HT‐based devices after 7 h of irradiation. Further investigation into the reasons behind the difference in the photostability suggests that photooxidation and recombination induced by irradiation in PBDD4T‐based devcie are suppressed by BDD‐copolymerization, maintaining better stability. In contrast, P3HT:PC71BM‐based solar absorber shows bimolecular recombination due to photoaging processes, which have negatively impacted the device stability. The reduction in stability of both devices is evident by lower photogenerated current attributed to reduced charge mobility and increased surface roughness.

Functional Group-Driven Competing Mechanism in Electrochemical Reaction and Adsorption/Desorption Processes toward High-Capacity Aluminum-Porphyrin Batteries.

Nonaqueous organic aluminum batteries are considered as promising high-safety energy storage devices due to stable ionic liquid electrolytes and Al metals. However, the stability and capacity of organic positive electrodes are limited by their inherent high solubility and low active organic molecules. To address such issues, here porphyrin compounds with rigid molecular structures present stable and reversible capability in electrochemically storing AlCl2 +. Comparison between the porphyrin molecules with electron-donating groups (TPP-EDG) and with electron-withdrawing groups (TPP-EWG) suggests that EDG is responsible for increasing both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, resulting in decreased redox potentials. On the other hand, EWG is associated with decreasing both HOMO and LUMO energy levels, leading to promoted redox potentials. EDG and EWG play critical roles in regulating electron density of porphyrin π bond and electrochemical energy storage kinetics behavior. The competitive mechanism between electrochemical redox reaction and de/adsorption processes suggests that TPP-OCH3 delivers the highest specific capacity ~171.8 mAh g-1, approaching a record in the organic Al batteries.

Functional Group‐Driven Competing Mechanism in Electrochemical Reaction and Adsorption/Desorption Processes toward High‐Capacity Aluminum‐Porphyrin Batteries

AbstractNonaqueous organic aluminum batteries are considered as promising high‐safety energy storage devices due to stable ionic liquid electrolytes and Al metals. However, the stability and capacity of organic positive electrodes are limited by their inherent high solubility and low active organic molecules. To address such issues, here porphyrin compounds with rigid molecular structures present stable and reversible capability in electrochemically storing AlCl2+. Comparison between the porphyrin molecules with electron‐donating groups (TPP‐EDG) and with electron‐withdrawing groups (TPP‐EWG) suggests that EDG is responsible for increasing both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, resulting in decreased redox potentials. On the other hand, EWG is associated with decreasing both HOMO and LUMO energy levels, leading to promoted redox potentials. EDG and EWG play critical roles in regulating electron density of porphyrin π bond and electrochemical energy storage kinetics behavior. The competitive mechanism between electrochemical redox reaction and de/adsorption processes suggests that TPP‐OCH3 delivers the highest specific capacity ~171.8 mAh g−1, approaching a record in the organic Al batteries.

Enhancement of thermoelectric performance of thienoisoindigo based D‐A conjugated polymers through the incorporation of 3,4‐ethylenedioxythiophene as donor unit

AbstractWith the good planarity of conjugated backbone and high charge carrier mobilities, thienoisoindigo (TIIG)‐based polymers show great potential in organic electronic devices. In this work, two TIIG‐based D‐A conjugated polymers (PTIIG‐3T and PTIIG‐2T‐EDOT) were designed and synthesized, which exhibit high lying highest occupied molecular orbital (HOMO) energy level and can be facilely doped by FeCl3. Compared with PTIIG‐3T, one thiophene unit in the donor building block is replaced by 3,4‐ethylenedioxythiophene (EDOT) in PTIIG‐2T‐EDOT, and PTIIG‐2T‐EDOT shows a higher HOMO level with slightly higher coplanarity than PTIIG‐3T, which induce an enhancement in electrical conductivity after oxidation doping. After proper doping, doped PTIIG‐2T‐EDOT film achieved a higher TE power factor of 17.9 μW m−1 K−2 than that of PTIIG‐3T due to its higher conductivity. The results indicate that TIIG unit is a promising building block for future high‐performance conjugated polymers for thermoelectric applications, and incorporation of the EDOT unit into D‐A conjugated polymers could be an effective way to develop high performance thermoelectric polymers.

Surfactant Composition Dependent Evolution of Electronic/Structural Properties of P3HT Nanoparticle Colloids Synthesized via Mini-Emulsion Technique

: This study investigates the influence of surfactant (sodium dodecyl sulfate, SDS) concentration on the size and optoelectronic properties of poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) synthesized via the mini-emulsion technique. P3HT NPs were fabricated with varying SDS concentrations. The size dependent optoelectronic properties, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels, were analyzed through photoelectron spectroscopy in air, UV-Vis absorption spectroscopy and cyclic voltammetry. A clear trend of decreasing NP size with increasing SDS concentration was observed. Higher SDS levels resulted in smaller NPs with enhanced pi-pi stacking and shorter conjugation lengths. This structural change led to a noticeable shift in HOMO and LUMO levels, indicating a direct correlation between surfactant concentration and the electronic properties of the P3HT NPs. The absorption spectra revealed a decrease in the A0-0/A0-1 ratio with smaller NP size, suggesting a transition towards more pronounced H-aggregate characteristics. As the size of the P3HT NPs decreases, there is a notable increase in H-aggregate formation. This increase can be attributed to enhanced interchain interactions between the polymer chains. In smaller NPs, the surface area-to-volume ratio is higher, leading to more significant interactions between adjacent polymer chains. These interactions promote the formation of H-aggregates, characterized by their interchain pi-pi stacking, which differs from the intrachain coupling observed in J-aggregates.

Hyperbranched polyimide with triazole core structure for nonvolatile resistive switching memory

AbstractHyperbranched polyimides (HBPIs) possess a unique branched structure, which can make the lowest unoccupied molecular orbitals (LUMOs) degenerate and reduce the bandgap between the highest occupied molecular orbital (HOMO) and LUMO energy levels. The triazole group can facilitate the charge transfer process. Thus, a novel hyperbranched polyimide (HBPI‐TZA‐6FDA) has been prepared from new triazole derivative triamine monomer (TZA) and 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) for memory device. The memory device with the structure ITO/HBPI‐TZA‐6FDA/Al exhibits a low‐threshold voltage of −2.2 V and a high ON/OFF current ratio of about 105. Meanwhile, the memory device shows nonvolatile write once read many times (WORM) type memory behavior and excellent stability with an operation time of 104 at a continuous applied voltage of −1 V. Optical, electrochemical experiments, and molecular simulation have been carried out to illustrate the memory performance. The memory performance is governed by the charge transfer between TZA and 6FDA units. The branched structure and triazole unit can reduce the band gap between HOMO and LUMO energy levels effectively, which is favorable for improving charge transfer and reducing threshold voltage. The HBPI‐TZA‐6FDA‐based memory device is favorable for memory device applications due to its relatively low‐power consumption and high‐operation stability.

Trivalent Nickel-Catalyzing Electroconversion of Alcohols to Carboxylic Acids.

The comprehension of activity and selectivity origins of the electrooxidation of organics is a crucial knot for the development of a highly efficient energy conversion system that can produce value-added chemicals on both the anode and cathode. Here, we find that the potential-retaining trivalent nickel in NiOOH (Fermi level, -7.4 eV) is capable of selectively oxidizing various primary alcohols to carboxylic acids through a nucleophilic attack and nonredox electron transfer process. This nonredox trivalent nickel is highly efficient in oxidizing primary alcohols (methanol, ethanol, propanol, butanol, and benzyl alcohol) that are equipped with the appropriate highest occupied molecular orbital (HOMO) levels (-7.1 to -6.5 eV vs vacuum level) and the negative dual local softness values (Δsk, -0.50 to -0.19) of nucleophilic atoms in nucleophilic hydroxyl functional groups. However, the carboxylic acid products exhibit a deeper HOMO level (<-7.4 eV) or a positive Δsk, suggesting that they are highly stable and weakly nucleophilic on NiOOH. The combination (HOMO, Δsk) is useful in explaining the activity and selectivity origins of electrochemically oxidizing alcohols to carboxylic acid. Our findings are valuable in creating efficient energy conversions to generate value-added chemicals on dual electrodes.

Carbazole‐Decorated Organoboron Emitters with Low‐Lying HOMO Levels for Solution‐Processed Narrowband Blue Hyperfluorescence OLED Devices

AbstractThe hyperfluorescence has drawn great attention in achieving efficient narrowband emitting devices based on multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters. However, achieving efficient solution‐processed pure blue hyperfluorescence devices is still a challenge, due to the unbalanced charge transport and serious exciton quenching caused by that the holes are easily trapped on the high‐lying HOMO (the highest occupied molecular orbital) level of traditional diphenylamine‐decorated emitters. Here, we developed two narrowband blue organoboron emitters with low‐lying HOMO levels by decorating the MR‐TADF core with weakly electron‐donating carbazoles, which could suppress the hole trapping effect by reducing the hole traps between host and MR‐TADF emitter from deep (0.40 eV) to shallow (0.14/0.20 eV) ones for facilitating hole transport and exciton formation, as well as avoiding exciton quenching. And the large dihedral angle between the carbazole and MR‐TADF core makes the carbazole act as a steric hindrance to inhibit molecular aggregation. Accordingly, the optimized solution‐processed pure blue hyperfluorescence devices simultaneously realize record external quantum efficiency of 29.2 %, narrowband emission with a full‐width at half‐maximum of 16.6 nm, and pure blue color with CIE coordinates of (0.139, 0.189), which is the best result for the solution‐processed organic light‐emitting diodes based on MR‐TADF emitters.

Carbazole-Decorated Organoboron Emitters with Low-Lying HOMO Levels for Solution-Processed Narrowband Blue Hyperfluorescence OLED Devices.

The hyperfluorescence has drawn great attention in achieving efficient narrowband emitting devices based on multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters. However, achieving efficient solution-processed pure blue hyperfluorescence devices is still a challenge, due to the unbalanced charge transport and serious exciton quenching caused by that the holes are easily trapped on the high-lying HOMO (the highest occupied molecular orbital) level of traditional diphenylamine-decorated emitters. Here, we developed two narrowband blue organoboron emitters with low-lying HOMO levels by decorating the MR-TADF core with weakly electron-donating carbazoles, which could suppress the hole trapping effect by reducing the hole traps between host and MR-TADF emitter from deep (0.40 eV) to shallow (0.14/0.20 eV) ones for facilitating hole transport and exciton formation, as well as avoiding exciton quenching. And the large dihedral angle between the carbazole and MR-TADF core makes the carbazole act as a steric hindrance to inhibit molecular aggregation. Accordingly, the optimized solution-processed pure blue hyperfluorescence devices simultaneously realize record external quantum efficiency of 29.2 %, narrowband emission with a full-width at half-maximum of 16.6 nm, and pure blue color with CIE coordinates of (0.139, 0.189), which is the best result for the solution-processed organic light-emitting diodes based on MR-TADF emitters.

Molecular understanding of electron donor influences in dye-sensitized solar cells of novel series-based D-A’-(π-A)2

The efficiency of the designed dye-sensitized solar cells (DSSCs) containing phenothiazine phenoxazine, and 5,10-dimethylphenazine as donors and triazine, phenyl with D-A’-(π-A)2 architecture has been examined utilizing Time-dependent (TD-DFT) and Density Functional Theory (DFT) techniques. The geometrical structures, electrical characteristics, absorption, and photovoltaic characteristics were studied using these techniques. Additionally, the computed findings show that various layouts can alter the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels and reduce the energy gap between 1.63 and 2.30 eV. The absorption undergoes a redshift displacement. These bands are between 300 and 400 nm. This work aims at calculating the structural geometries and the electronic and optical properties of the designed dyes. In order to shed light on the bridging influence on geometric and TD-BHandH/6-31G(d) for optoelectronic properties, the density functional theory (DFT) and time-dependent TD-DFT at the B3LYP/6-31G(d) level were examined. Additionally, the complex [email protected] cluster's binding energies have been investigated.

The Influence of the Central Metal Ion on the Electronic and Photovoltaic Properties of Metalloporphyrins [M= Zn(II), Ni(II), Cu(II), Au(III)] Systems

Although the pioneering use of porphyrins in organic solar cells (OSCs) was disappointing as reported efficiencies were very low, the situation has changed over the last five years as Zn-porphyrins with ABAB structures linked to acceptor units through triple bonds have been applied as donors resulting in efficiencies of up to 12% in binary OSCs and more than 15% in ternary OSCs.The optical and electrochemical properties of porphyrins can be adjusted by molecular design and functionalization on the b or meso positions of the porphyrin ring as well as by introduction of different central metal ions. The choice of the centrally bounded metal cation within the porphyrin core plays an important role in the electronic properties allowing the modulation of the frontier orbital levels and thereby use the porphyrin-based molecule as donor or acceptor component of the device.Due to the high HOMO level of Zn-Porphyrin derivatives, the Voc of OSCs using this family of compounds are lower than 0.9 V limiting the PCE of devices.We have described that Ni-porphyrins derivatives allow the design of OSCs with voltage higher than 1V. These systems present slightly higher optical bandgaps but the deeper highest occupied molecular orbital (HOMO) energy level make available to reach good PCE values of the solar devices.While porphyrins and their derivatives have been extensively used as donor component in the BHJ active layer of OSCs, the investigation of porphyrin-based materials as electron acceptor is very limited. Recently, we designed a new A2-D-A1-D-A2 non fullerene small molecule acceptor (NFSMA) with broad absorption up to 920 nm and a great absorption coefficient where A1 core is Au(III) porphyrin. This molecule, when used in OSC with a polymer donor yielded a PCE=9.24%. Nevertheless, the use of other central metals such as Cu(II) in the porphyrin core has been scarcely used in OSCs, the studied devices presented low efficiencies (≤ 3%). These outcomes demonstrated that metalloporphyrins, which is different from Zn-porphyrins, also have great potential for the design of NFSMAs for efficient OSCs. Also, we have studied A-π-D-π-A small molecules based on Cu(II) Porphyrin, scarcely used in OSCs, presenting an ambipolar behaviour, as donor and as acceptor. Here, I´ll present our recent work in design, synthesis, and application of porphyrin-based small molecules with different central metal for highly efficient OSCs.References V. Cuesta, M. Vartanian, P. de la Cruz, R. Singhal, G. D. Sharma and F. Langa. J. Mater. Chem. A, 2017, 5, 1057.S. Arrechea, A. Aljarilla, P. de la Cruz, M. K. Singh, G. D. Sharma and F. Langa. J. Mater. Chem. C, 2017, 5, 4742.M. Vartanian, R. Singhal, P. de la Cruz, S. Biswas, G. D. Sharma and F. Langa. ACS Appl. Energy, Mater., 2018, 1, 1304.M. Vartanian, P. de la Cruz, F. Langa, S. Biswas and G. D. Sharma. Nanoscale, 2018, 10, 12100.M. Vartanian, R. Singhal, P. de la Cruz, G. D. Sharma and F. Langa. Chem. Commun., 2018, 54, 14144.V. Cuesta, R. Singhal, P. de la Cruz, G. D. Sharma and F. Langa. ACS Appl. Mater. Interfaces, 2019, 11, 7216.Cuesta, R. Singhal, P. de la Cruz, G. D. Sharma and F. Langa, ChemSusChem, 2021, 14, 3439.H. Dahiya, V. Cuesta, P. de la Cruz, F. Langa and G. D. Sharma ACS Appl. Energy Mater., 2021, 4, 4498. Figure 1

Unveiling the effect of side chains and fluorination on the photovoltaic performance of D-A copolymers: a comparative study of P-HBT-T, P-FBT-T and P-FBT-O

Three random D-A copolymers containing thienopyrroledione (TPD) and benzodithiophene (BDT) named P-HBT-T, P-FBT-T, and P-FBT-O were synthesized. The effects of side chains on BDT and fluorination to benzothiadiazole on the photovoltaic performances of fabricated solar cells were investigated. The highest occupied molecular orbital (HOMO) levels of the polymers were −5.57, −5.51, and −5.65 eV for P-HBT-T, P-FBT-T, and P-FBT-O, respectively, suggesting low-lying HOMO energy levels. The optimized weight ratios of the polymer to PC71BM were determined as 1:2 with 24 mg/mL blend concentration for all polymers, and the maximum power conversion efficiencies (PCEs) of the devices were 7.35%, 7.76%, and 9.21% for P-HBT-T, P-FBT-T, and P-FBT-O, respectively, after optimizations with 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN). Trap-assisted recombination and bimolecular recombination loss mechanisms, which are PCE limiting mechanisms, were examined for all devices. The morphological and topographical properties were investigated using transmission electron microscopy (TEM) and atomic force microscopy (AFM), respectively. Our findings demonstrate that P-FBT-O-bearing organic solar cells (OSCs) emerged as the best-performing device due to their deeper HOMO level, high molecular weight, lower trap-assisted and bimolecular recombination, and superior morphology.

Spray-Dried MgMn2O4 Spinel Oxide Cathode with Single Mg Ion-Conductive Polymers for Rechargeable Mg Metal Battery

To improve the cycle performance of Mg metal batteries, a spinel-type MgMn2O4 (MMO) as the active material of the cathode was coated with a poly(styrene-4-sulfonyl trifluoromethylsulfonyl)imide Mg salt (PSTFSI-Mg) on the MMO surface. Coating was performed by spray drying, freeze drying, simple mixing, and one-pot spray drying. Transmission electron microscopy measurements of the resulting polymer-coated MMO were undertaken to verify the surface and the inner morphology. The results of charge–discharge measurements suggest that the polymer coating on the MMO surface promotes electron transfer between the particles. These results are interested in terms of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the modified polymers obtained from density functional theory (DFT), confirming that the polymer reduces the gap between the valence band maximum of MMO and the HOMO level of the electrolyte solution, suppressing the degradation of the electrolyte.

Monomer-mixed hole transport layers for improving hole injection of quantum dot light-emitting diodes.

Quantum-dot light-emitting diodes (QLEDs) are promising components for next-generation displays and related applications. However, their performance is critically limited by inherent hole-injection barrier caused by deep highest-occupied molecular orbital levels of quantum dots. Herein, we present an effective method for enhancing the performance of QLEDs by incorporating a monomer (TCTA or mCP) into hole-transport layers (HTL). The impact of different monomer concentrations on the characteristics of QLEDs were investigated. The results indicate that sufficient monomer concentrations improve the current efficiency and power efficiency. The increased hole current using monomer-mixed HTL suggests that our method holds considerable potential for high-performance QLEDs.

Rhodanine-Based Non-Fullerene Acceptors for Organic Solar Cells with a High Open-Circuit Voltage of 1.07 V

To overcome the intrinsic limitations of fullerene-based organic photovoltaic (OPV) devices, research on OPV devices based on non-fullerene acceptors (NFAs) has been actively conducted in recent years. It is important to understand the relationship between the structure of the NFAs and photovoltaic properties to create high-performance OPV devices. In this study, we have designed and synthesized a series of NFAs (DFDO-RC2 and DFDE-RC2) based on electron-rich dithienosilole (D) and electron-deficient difluorobenzodiathiazole (F), benzodiathiazole-connected 3-ethylrhodanine (RC2) units, and alkyl chains of 2-ethylhexyl (E) and octyl (O) groups. The PTB7-Th:DFD-RC2 devices showed low PCEs mainly due to the highly located highest occupied molecular orbital (HOMO) energy levels of the DFD-RC2 acceptors compared to the PTB7-Th polymer donor. To lower the HOMO levels of the DFD-RC2 NFAs, the backbone structures were modified by replacing difluorobenzodiathiazole core moiety with difluorobenzene (FBz) to obtain DFBz-RC2 molecules (DFBzO-RC2 and DFBzE-RC2). PTB7-Th:DFBz-RC2 devices exhibited significantly improved PCEs compared to PTB7-Th:DFD-RC2 devices. The DFBzO-RC2 and DFBzE-RC2 molecule-based OPVs exhibited remarkably high Vocs of 1.03 and 1.07 V, respectively, which characteristic is associated with the very low energy loss (Eloss) of 0.51 eV in both PTB7-Th:DFBzO-RC2 and PTB7-Th:DFBzE-RC2 devices. Overall, our investigation of the various synthesized molecules reveals the structure-to-photovoltaic properties, which guide the design of new high-performance NFAs to advance in the field of organic solar cells.

Effects of HAT-CN Layer Thickness on Molecular Orientation and Energy-Level Alignment with ZnPc.

Efficient energy-level alignment is crucial for achieving high performance in organic electronic devices. Because the electronic structure of an organic semiconductor is significantly influenced by its molecular orientation, comprehensively understanding the molecular orientation and electronic structure of the organic layer is essential. In this study, we investigated the interface between a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) hole injection layer and a zinc-phthalocyanine (ZnPc) p-type organic semiconductor. To determine the energy-level alignment and molecular orientation, we conducted in situ ultraviolet and X-ray photoelectron spectroscopies, as well as angle-resolved X-ray absorption spectroscopy. We found that the HAT-CN molecules were oriented relatively face-on (40°) in the thin (5 nm) layer, whereas they were oriented relatively edge-on (62°) in the thick (100 nm) layer. By contrast, ZnPc orientation was not significantly altered by the underlying HAT-CN orientation. The highest occupied molecular orbital (HOMO) level of ZnPc was closer to the Fermi level on the 100 nm thick HAT-CN layer than on the 5 nm thick HAT-CN layer because of the higher work function. Consequently, a considerably low energy gap between the lowest unoccupied molecular orbital level of HAT-CN and the HOMO level of ZnPc was formed in the 100 nm thick HAT-CN case. This may improve the hole injection ability of the anode system, which can be utilized in various electronic devices.

Influence of triisopropyl(prop-1-ynyl)silane side chain on the properties of pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione–based polymeric donor for organic solar cells

A new alternating copolymer, P(BDTSi-PPD) , comprising 4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-ʹ]dithiophene (BDTSi) and 4,6-bis(thiophen-2-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (PPD) units was prepared. The photophysical, electrochemical, and photovoltaic properties of P(BDTSi-PPD) , as well as the backbone curvature, charge transport, and crystallinity, were studied and briefly compared with those of structurally similar high-energy-converting wide-bandgap copolymers, namely P(BDTO-PPD), containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-bʹ]dithiophene (BDTO) and PPD units. Polymer P(BDTSi-PPD) displayed a slightly broader absorption band, deeper highest occupied molecular orbital (HOMO), and lower π–π stacking ability compared to those of P(BDTO-PPD). Nevertheless, the poor power conversion efficiency of organic solar cells (OSCs) employing a binary blend of P(BDTSi-PPD) :PC 70 BM was inferior ( PCE ∼ 1.08%) to that of devices made from the P(BDTO-PPD):PC 70 BM blend ( PCE ∼ 5.42%). However, the inclusion of 3 vol% P(BDTSi-PPD) in the PTB7-Th:PC 70 BM blend significantly enhanced the PCE of the resulting ternary OSCs, where the PCE increased from 7.86% to 8.51%. In contrast, the PCE of the PTB7-Th:PC 70 BM blend increased to 8.34% with the addition of 10 vol% P(BDTO-PPD) to the primary blend. Overall, P(BDTSi-PPD) works as an additive material in OSCs, whereas P(BDTO-PPD) acts mainly as a co-absorber. This study demonstrates that replacing 2-ethylhexyloxy (EHO) with a triisopropyl(prop-1-ynyl)silane (TIPS) substituent on the backbone of benzo[1,2-b:4,5-bʹ]dithiophene (BDT) units of PPD-based polymers results in significant attenuation of the photovoltaic properties. • Substituent, EHO and TIPS, effects on the properties of PPD-based polymers studied. • TIPS-substituted polymer showed lower bandgap and deeper HOMO level. • EHO-substituted polymer gave higher PCE in binary blend OSCs. • TIPS-substituted polymer gave higher PCE in ternary blend OSCs. • TIPS-substituted polymer works as both additive and co-absorbing materials.

Enhancing the performance of perovskite solar cell via optimizing the solvent of hole transport layer

Poly(9-vinylcarbazole) (PVK) is an attractive hole transport layer (HTL) for high efficiency perovskite solar cells (PSCs). But there is still some room for improvement in energy level alignment between the highest occupied molecular orbital (HOMO) of PVK and the perovskite layer. In this work, the solvent of PVK-based HTL was optimized to adjust the energy level of HTLs by changing the volume ratio of chlorobenzene (CB) in dimethylformamide (DMF). Experimental results shows that 20% is the optimal ratio of CB in DMF, based on which the HOMO level of HTL was lifted from −5.62 eV to −5.44 eV. Meanwhile, the morphology of perovskite layer with larger grain size and lower trap density was obtained. As a result, the V OC and J SC of PSCs was enhanced, leading to the increase of PCE from 15.41% to 18.27%. This work paves the way to realize high-efficiency PSCs based on PVK HTL. • The solvent of PVK-based HTL was optimized to adjust the energy level of HTLs. • The HOMO level of HTL was lifted from −5.62 eV to −5.44 eV. • The V OC and J SC of PSCs was enhanced, leading to the increase of PCE from 15.41% to 18.27%.

A polymer donor based on difluoro-quinoxaline with a naphthalimide substituent unit exhibits a low-lying HOMO level for efficient non-fullerene polymer solar cells.

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (VOC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.