Solar cell efficiency, self-assembly, and dipole-dipole interactions of isomorphic narrow-band-gap molecules

The effects of alkyl substitution on the aggregation of π-conjugated dyes: spectroscopic study and modelling

A family of dithienosilole-based dyes with alternating donor and acceptor conjugated groups, decorated with linear or branched alkyl chains at different positions on the backbone, have been obtained and investigated in different aggregation states. These dyes are characterized by almost panchromatic absorption and by near-IR emission, with good quantum yields in a variety of solvents with different polarity. We demonstrate that the nature and position of the alkyl substituents strongly govern the self-assembly of the dyes, whose packing is also sensitive to external stimuli, such as grinding and water addition. Thanks to computational results and theoretical modelling, we are able to interpret the results based on two possible preferential packings, characterized by distinct spectroscopic behaviour, whose abundance can be tuned according to the nature and position of the alkyl chains, as well as via external stimuli.

Tuning the optoelectronic properties of selenophene-diketopyrrolopyrrole-based non-fullerene acceptor to obtain efficient organic solar cells through end-capped modification

With the goal of developing a high-performance organic solar cell, nine molecules of A2-D-A1-D-A2 type are originated in the current investigation. The optoelectronic properties of all the proposed compounds are examined by employing the DFT approach and the B3LYP functional with a 6-31G (d, p) basis set. By substituting the terminal moieties of reference molecule with newly proposed acceptor groups, several optoelectronic and photovoltaic characteristics of OSCs have been studied, which are improved to a significant level when compared with reference molecule, i.e., absorption properties, excitation energy, exciton binding energy, band gap, oscillator strength, electrostatic potential, light-harvesting efficiency, transition density matrix, open-circuit voltage, fill factor, density of states and interaction coefficient. All the newly developed molecules (P1-P9) have improved λmax, small band gap, high oscillator strengths, and low excitation energies compared to the reference molecule. Among all the studied compounds, P9 possesses the least binding energy (0.24 eV), P8 has high interaction coefficient (0.70842), P3 has improved electron mobility due to the least electron reorganization energy (λe = 0.009182 eV), and P5 illustrates high light-harvesting efficiency (0.7180). P8 and P9 displayed better Voc results (1.32 eV and 1.33 eV, respectively) and FF (0.9049 and 0.9055, respectively). Likewise, the phenomenon of charge transfer in the PTB7-Th/P1 blend seems to be a marvelous attempt to introduce them in organic photovoltaics. Consequently, the outcomes of these parameters demonstrate that adding new acceptors to reference molecule is substantial for the breakthrough development of organic solar cells (OSCs).

Design of Isoindigo-Based Small-Molecule Donors for Bulk Heterojunction Organic Solar Cell Applications in Combination with Nonfullerene Acceptors

The development of small-molecule organic solar cells with the required efficiency depends on the information obtained from molecular-level studies. In this context, 39 small-molecule donors featuring isoindigo as an acceptor moiety have been meticulously crafted for potential applications in bulk heterojunction organic solar cells. These molecules follow the D2-A-D1-A-D2 and D2-A-π-D1-π-A-D2 framework. Similar molecules considered in the previous experimental study (molecules R1 ((3E,3″E)-6,6″-(benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione)) and R2 ((3E,3″E)-6,6″-(4,8-dimethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione))) have been chosen as reference molecules. Molecules with and without π-spacers have been considered to understand the impact of the length of the π-spacer on intramolecular charge-transfer transitions and absorption properties. A detailed investigation is carried out to establish the relationship between the structure and photovoltaic parameters using density functional theory and time-dependent density functional theory methods. The newly developed molecules exhibit better electronic, excited-state, and charge transport properties than the reference molecules. Additionally, model donor-acceptor interfaces are constructed by integrating the designed donor molecules with fullerene/nonfullerene acceptors. The electronic and excited-state properties of these interfaces are rigorously evaluated. Results elucidate that the donor comprising of isoindigo-bithiophene-pyrroloindacenodithiophene (IIG-T2-PIDT) emerges as a promising candidate for bulk heterojunction solar cells based on nonfullerene acceptors. This research provides systematic design strategies for the development of small-molecule donors for organic solar cells.

Lest We Forget-The Importance of Heteroatom Interactions in Heterocyclic Conjugated Systems, from Synthetic Metals to Organic Semiconductors

The field of synthetic metals is, and remains, highly influential for the development of organic semiconductor materials. Yet, with the passing of time and the rapid development of conjugated materials in recent years, the link between synthetic metals and organic semiconductors is at risk of being forgotten. This review reflects on one of the key concepts developed in synthetic metals - heteroatom interactions. The application of this strategy in recent organic semiconductor materials, small molecules and polymers, is highlighted, with analysis of X-ray crystal structures and comparisons with model systems used to determine the influence of these non-covalent short contacts. The case is made that the wide range of effective heteroatom interactions and the high performance that has been achieved in devices from organic solar cells to transistors is testament to the seeds sown by the synthetic metals research community.

New bithiophene-based molecules as hole transporting materials for perovskite solar cells and or as donor for organic solar cells

DFT and TDDFT approaches were used to design three (T16,17,18) molecules based on 4,4'-dimethoxy-2,2'-bithiophene core to explore the influence of substitution of triphenylamine (TPA) fragment by methoxy groups, and introduction of azomethine π-bridges on the optoelectronic properties of hole transporting materials for perovskite solar cells (PSCs) or as donor for organic solar cells (OSCs). To shed light on the efficiency, stability, and solubility several physicochemical parameters were computed in dichloromethane solvent. All designed molecules show appropriate frontier molecular orbital levels, which facilitates effective hole transfer from the perovskite materials to the HTMs in the hole-transporting layer in PSC devices. They all show good efficiency and pore-fillings and are stable and soluble in dichloromethane. Electron-hole pairs can easily dissociate into free charge carriers, especially for T16 and T17; consequently, improve short-circuit current densities and facilitate hole transport. It is also advised to use T18 which includes azomethine bridges as a donor with a non-fullerene Y6 acceptor to create effective OSCs because it exhibits high open circuit voltage, fill factor and low gap energy.

Exploration of charge transfer analysis and photovoltaics properties of A-D-A type non-fullerene phenazine based molecules to enhance the organic solar cell properties

To intensify the photovoltaic properties of organic solar cells, density functional theory (DFT) based computational techniques were implemented on six non-fullerene A-D-A type small molecules (N1-N6) modified from reference molecule (R) which consists of phenazine fused with 1,4- Dimethyl-4H-3,7-dithia-4-aza- cyclopenta [α] pentalene on both sides with one of its phenyl rings acting as the central donor unit, further attached with 2-(5,6-Difluoro-2-methylene-3-oxo-indan-1-ylidene)-malononitrile acceptor groups at terminal sites. All proposed compounds have a phenazine base modified with a variety of substituents at the terminals. Transition density matrix, density of states, frontier molecular orbitals, intramolecular charge transfer abilities and optoelectronic properties of these compounds were investigated using B3LYP/6-31G (d, p) and B3LYP/6-31G++ (d,p) level of theory. All six designed compounds exhibited a bathochromic sift in their λmax as compared to the R molecule. All designed molecules also have reduced band gap and smaller excitation energy than R. Among all, N6 exhibited highest λmax and lowest bandgap as compared to reference molecule indicating its promising photovoltaic properties. Decreased hole and electron reorganization energy in several of the suggested compounds is indicative of greater charge mobility in them. PTB7-Th donor was employed to calculate open circuit voltage of all investigated molecules. N1-N5 molecules had improved optoelectronic properties, significant probable power conversion efficiency as evident from their absorption aspects, high values of Voc, and fill factor, compared to R molecule. Designed A-D-A type NF based molecules make OSCs ideal for use in wearable devices, building-integrated photovoltaics and smart fabrics.

Increasing the Photovoltaic Power of the Organic Solar Cells by Structural Modification of the R-P2F-Based Materials

CONTEXT

The present study aims to improve the performance of optoelectronics and photovoltaics by constructing an acceptor-donor-acceptor (A-D-A) molecule with a fullerene-free acceptor moiety. The study utilizes malononitrile and selenidazole derivatives to tailor the molecule for enhanced photovoltaic abilities. The study analyzes molecular properties and parameters like charge density, charge transport, UV absorption spectra, exciton binding energies, and electron density difference maps to determine the effectiveness of the tailored derivatives.

METHODS

To optimize the geometric structures, the study used four different functionals (B3LYP, CAM-B3LYP, MPW1PW91, and ɷB97XD) along with a double zeta valence basis set 6-31G(d, p) basis set. The study compared the results of the tailored derivatives with a reference molecule (R-P2F) to determine improvements in performance. The light harvesting efficiency of the molecules was analyzed by performing simulations in the gas and solvent phases (chloroform) based on the spectral overlap between the solar irradiance and the absorption spectra of the molecules. The open-circuit voltage (V_OC) of each molecule was also analyzed, representing the maximum voltage that can be obtained from the cell under illuminated conditions. The findings indicated that the M1-P2F designed derivative is a more effective, with energy gap of 2.14 eV, and suitable candidate for non-fullerene organic solar cell application, based on various analyses such as power conversion efficiency, quantum chemical reactivity parameters, and electronic features.

Exploring the photovoltaic properties of promising non-fullerene acceptors with different degrees of asymmetry due to halogenations of terminal groups

Over the past few years, the strategy of asymmetric modification has become popular for designing new photovoltaic materials because it can effectively improve optoelectronic performance and morphology, therefore power conversion efficiency (PCE). However, how the halogenations (to further change asymmetry) of terminal groups (TGs) of an asymmetric small-molecule non-fullerene acceptor (Asy-SM-NFA) influence optoelectronic properties is still not very clear. In this work, we have selected a promising Asy-SM-NFA IDTBF (the OSC based on it has a PCE of 10.43 %), exacerbated the asymmetry through fluorinations of TGs, and finally designed six new molecules. Based on density functional theory (DFT) and time-dependent DFT calculations, we systematically examine how the changed asymmetry impacts the optoelectronic properties. We find that the halogenations of TGs may significantly affect the molecular planarity, dipole moment, electrostatic potential, exciton binding energy, energy loss, and absorption spectrum. And the results show that newly designed BR-F1 and IM-mF (m = 1,3, and 4) are potential Asy-SM-NFAs because they all have enhanced absorption spectra in the visible region. Therefore, we provide a meaningful direction for the design of asymmetric NFA.

Increasing Charge Carrier Mobility through Modifications of Terminal Groups of Y6: A Theoretical Study

The applications of non-fullerene acceptor Y6 with a new type of A1-DA2D-A1 framework and its derivatives have increased the power conversion efficiency (PCE) of organic solar cells (OSCs) up to 19%. Researchers have made various modifications of the donor unit, central/terminal acceptor unit, and side alkyl chains of Y6 to study the influences on the photovoltaic properties of OSCs based on them. However, up to now, the effect of changes of terminal acceptor parts of Y6 on the photovoltaic properties is not very clear. In the present work, we have designed four new acceptors-Y6-NO2, Y6-IN, Y6-ERHD, and Y6-CAO-with different terminal groups, which possess diverse electron-withdrawing ability. Computed results show that with the enhanced electron-withdrawing ability of the terminal group, the fundamental gaps become lower; thus, the wavelengths of the main absorption peaks of UV-Vis spectra red-shifts and total oscillator strength increase. Simultaneously, the electron mobility of Y6-NO2, Y6-IN, and Y6-CAO is about six, four, and four times faster than that of Y6, respectively. Overall, Y6-NO2 could be a potential NFA because of its longer intramolecular charge-transfer distance, stronger dipole moment, higher averaged ESP, enhanced spectrum, and faster electron mobility. This work provides a guideline for the future research on modification of Y6.

Dipole-moment-induced supramolecular assembly of a donor-acceptor-type molecule on a metal surface and in a crystal

The conformation and alignment of molecules in organic materials are important because they affect the materials' bulk physical properties. Because two-dimensional (2D) materials offer a simpler model of three-dimensional (3D) materials, the conformation and alignment of molecules in 2D assemblies have been investigated at the atomic scale by scanning tunnelling microscopy (STM). However, differences in the conformation and alignment of molecules between 2D and 3D assemblies have not been clarified. In this work, the conformation and alignment of a donor-acceptor-type molecule, 4-(3,3-dimethyl-2,3-dihydro-1H-indol-1-yl)benzonitrile (IBN), are studied in 2D and 3D assemblies. Thus, the 2D assembly of IBN on the Au(111) surface was investigated by STM and the 3D assembly of IBN in a single crystal was investigated by X-ray crystallography. Our survey revealed that the conformation of IBN is planar in both 2D and 3D assemblies because of the electron-delocalised structure resulting from the electron-donating and electron-accepting groups of IBN; thus, the values of the dipole moment of IBN in 2D and 3D assemblies are essentially the same. In both the 2D and 3D assemblies, IBN molecules align to cancel out the dipole moment even though the self-assembled structures differ. In the 2D assemblies, the orientation and self-assembled structure of IBN are changed by the surface density of IBN, and they are affected by the crystal orientation and superstructure of Au(111) because of the strong interaction between IBN and Au(111). In addition, scanning tunnelling spectroscopy revealed that the coordination structure is not included in the self-assembled structure of IBN on Au(111).

Non-covalent interactions (NCIs) in π-conjugated functional materials: advances and perspectives

The design and development of functional materials with real-life applications are highly demanding. Understanding and controlling inter- and intra-molecular interactions provide opportunities to design new materials. A judicious manipulation of the molecular structure significantly alters such interactions and can boost selected properties and functions of the material. There is burgeoning evidence of the beneficial effects of non-covalent interactions (NCIs), showing that manipulating NCIs may generate functional materials with a wide variety of physical properties leading to applications in catalysis, drug delivery, crystal engineering, etc. This prompted us to review the implications of NCIs on the molecular packing, optical properties, and applications of functional π-conjugated materials. To this end, this tutorial review will cover different types of interactions (electrostatic, π-interactions, metallophilic, etc.) and their impact on π-conjugated materials. Attempts have also been made to delineate the effects of weak interactions on opto-electronic (O-E) applications.

Ab Initio Study of Two-Dimensional Cross-Shaped Non-Fullerene Acceptors for Efficient Organic Solar Cells

In the present work, five novel non-fullerene acceptor molecules are represented to explore the significance of organic solar cells (OSCs). The electro-optical properties of the designed A-D-A-type molecules rely on the central core donor moiety associated with different halogen families such as fluorine, chlorine, and bromine atoms and acyl, nitrile, and nitro groups as acceptor moieties. Among these, M1 exhibits the maximum absorption (λ_max) at 728 nm in a chloroform solvent as M1 has nitro and nitrile groups in the terminal acceptor, which is responsible for the red shift in the absorption coefficient as compared to R (716 nm). M1 also shows the lowest value of the energy band gap (2.07 eV) with uniform binding energy in the range of 0.50 eV for all the molecules. The transition density matrix results reveal that easy dissociation of the exciton is possible in M1. The highest value of the dipole moment (4.6 D) indicates the significance of M4 and M2 in OSCs as it reduces the chance of charge recombination. The low value of λ_e is given by our designed molecules concerning reference molecules, indicating their enhanced electron mobility. Thus, these molecules can serve as the most economically efficient material. Hence, all newly designed non-fullerene acceptors provide an overview for further development in the performance of OSCs.

Unraveling the Unconventional Order of a High-Mobility Indacenodithiophene-Benzothiadiazole Copolymer

A new class of donor-acceptor (D-A) copolymers found to produce high charge carrier mobilities competitive with amorphous silicon (>1 cm² V^-1 s^-1) exhibit the puzzling microstructure of substantial local order, however lacking long-range order and crystallinity previously deemed necessary for achieving high mobility. Here, we demonstrate the application of low-dose transmission electron microscopy to image and quantify the nanoscale and mesoscale organization of an archetypal D-A copolymer across areas comparable to electronic devices (≈9 μm²). The local structure is spatially resolved by mapping the backbone (001) spacing reflection, revealing nanocrystallites of aligned polymer chains throughout nearly the entire film. Analysis of the nanoscale structure of its ordered domains suggests significant short- and medium-range order and preferential grain boundary orientations. Moreover, we provide insights into the rich, interconnected mesoscale organization of this new family of D-A copolymers by analysis of the local orientational spatial autocorrelations.

Molecular engineering of indenoindene-3-ethylrodanine acceptors with A2-A1-D-A1-A2 architecture for promising fullerene-free organic solar cells

Considering the increased demand and potential of photovoltaic devices in clean, renewable electrical and hi-tech applications, non-fullerene acceptor (NFA) chromophores have gained significant attention. Herein, six novel NFA molecules IBRD1-IBRD6 have been designed by structural modification of the terminal moieties from experimentally synthesized A2-A1-D-A1-A2 architecture IBR for better integration in organic solar cells (OSCs). To exploit the electronic, photophysical and photovoltaic behavior, density functional theory/time dependent-density functional theory (DFT/TD-DFT) computations were performed at M06/6-311G(d,p) functional. The geometry, electrical and optical properties of the designed acceptor molecules were compared with reported IBR architecture. Interestingly, a reduction in bandgap (2.528-2.126 eV), with a broader absorption spectrum, was studied in IBR derivatives (2.734 eV). Additionally, frontier molecular orbital findings revealed an excellent transfer of charge from donor to terminal acceptors and the central indenoindene-core was considered responsible for the charge transfer. Among all the chromophores, IBRD3 manifested the lowest energy gap (2.126 eV) with higher λmax at 734 and 745 nm in gaseous phase and solvent (chloroform), respectively due to the strong electron-withdrawing effect of five end-capped cyano groups present on the terminal acceptor. The transition density matrix map revealed an excellent charge transfer from donor to terminal acceptors. Further, to investigate the charge transfer and open-circuit voltage (Voc), PBDBT donor polymer was blended with acceptor chromophores, and a significant Voc (0.696-1.854 V) was observed. Intriguingly, all compounds exhibited lower reorganization and binding energy with a higher exciton dissociation in an excited state. This investigation indicates that these designed chromophores can serve as excellent electron acceptor molecules in organic solar cells (OSCs) that make them attractive candidates for the development of scalable and inexpensive optoelectronic devices.

Exploration of efficient electron acceptors for organic solar cells: rational design of indacenodithiophene based non-fullerene compounds

The global need for renewable sources of energy has compelled researchers to explore new sources and improve the efficiency of the existing technologies. Solar energy is considered to be one of the best options to resolve climate and energy crises because of its long-term stability and pollution free energy production. Herein, we have synthesized a small acceptor compound (TPDR) and have utilized for rational designing of non-fullerene chromophores (TPD1-TPD6) using end-capped manipulation in A2-A1-D-A1-A2 configuration. The quantum chemical study (DFT/TD-DFT) was used to characterize the effect of end group redistribution through frontier molecular orbital (FMO), optical absorption, reorganization energy, open circuit voltage (Voc), photovoltaic properties and intermolecular charge transfer for the designed compounds. FMO data exhibited that TPD5 had the least ΔE (1.71 eV) with highest maximum absorption (λmax) among all compounds due to the four cyano groups as the end-capped acceptor moieties. The reorganization energies of TPD1-TPD6 hinted at credible electron transportation due to the lower values of λe than λh. Furthermore, open circuit voltage (Voc) values showed similar amplitude for all compounds including parent chromophore, except TPD4 and TPD5 compounds. These designed compounds with unique end group acceptors have the potential to be used as novel fabrication materials for energy devices.

Tuning the optoelectronic properties of triphenylamine (TPA) based small molecules by modifying central core for photovoltaic applications

Small donor molecules based on fused ring acceptors exhibit encouraging photovoltaic properties and expeditious advancement in organic solar cells. Central core modification of non-fullerene acceptor materials is a favorable methodology to enhance electronic properties and efficiency for OSCs. Herein, four new donor molecules, namely, BDTM1, PYRM2, ANTM3, and NM4 are designed with a strong donor moiety triphenylamine, tetracyanobutadiene as acceptor unit, and thiophene as spacer linked to a modified central core. Geometric parameters, optical, electrical properties, effect of central core modification on tailored molecules BDTM1-NM4 are investigated and compared with reference DPPR. DFT together with TDDFT approaches using MPW1PW91 functional is used to study key parameters like absorption maximum (λ_max), frontier molecular approach, ionization potential, electron affinity, the density of states, transition density matrix along with open-circuit voltage (V_OC), dipole moment and reorganization energy. Among all these molecules, BDTM1 shows maximum calculated absorption λ_max (817 nm) and the lowest band gap (2.54 eV). This bathochromic shift in BDTM1 is due to the presence of 4,8-dimethoxy-2,6-di-2-thienylbenzodithiophene as a strong electron-withdrawing group. Computed reorganization energies (RE) shows that BDTM1 has the highest hole and electron mobility among all designed molecules. Combination of BDTM1 donor and PC₆₁BM acceptor further verifies charge transfer and their interaction. The results illustrate that designed donor molecules (BDTM1-NM4) are better in performance and are recommended for experimentation to develop efficient OSCs. Four new donor molecules, namely, BDTM1, PYRM2, ANTM3, and NM4 are designed with a strong donor moiety triphenylamine, tetracyanobutadiene as acceptor unit and thiophene as spacer linked to a modified central core. Geometric parameters, optical, electrical properties, effect of central core modification on tailored molecules BDTM1-NM4 are investigated and compared with reference DPPR.

Tuning the optoelectronic properties of oligothienyl silane derivatives and their photovoltaic properties

Four new Donor-Acceptor (D-A) type oligothiophenes based structures (C1-C4) were designed by substituting different acceptors moieties around tetrahedral silicon core to simulate their photovoltaic properties. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) quantum analysis were carried out to reconnoiter various parameters of solar cells. A comparative analysis has conducted between designed structures and reference molecule R to conclude our simulated results. Among all the structures, C2 has displayed highest absorption values (380 nm) with red shift and minimum band gap (Δ_H-L) of 4.11 eV in dichloromethane at DFT-CAM-B3LYP/6-31G (d,p) using IEFPCM model. The C2 has also shown the lowest values of electron reorganization energy (λe = 0.018eV) and hole reorganization energy (λh = 0.015eV) therefore, could be suggested for use in organic solar cells because of its most noteworthy charge carrier mobilities. Again, C2 has the different trend in TDM graph because the electron density is present in the lower right part of core unit and in the acceptor moiety due to high electron affinities of end capped acceptor having cyanide groups.

Alkyl-Side-Chain Engineering of Nonfused Nonfullerene Acceptors with Simultaneously Improved Material Solubility and Device Performance for Organic Solar Cells

Two nonfullerene small molecules, TBTT-BORH and TBTT-ORH, which have the same thiophene-benzothiadiazole-thiophene (TBTT) core flanked with butyloctyl (BO)- and octyl (O)-substituted rhodanines (RHs) at both ends, respectively, are developed as electron acceptors for organic solar cells (OSCs). The difference between the alkyl groups introduced into TBTT-BORH and TBTT-ORH strongly influence the intermolecular aggregation in the film state. Differential scanning calorimetry and UV-vis absorption studies reveal that TBTT-ORH exhibited stronger molecular aggregation behavior than TBTT-BORH. On the contrary, the material solubility is greatly improved by the introduction of a BO group in TBTT-BORH, and the inevitably low molecular interaction and packing ability of the as-cast TBTT-BORH film can be effectively increased by a solvent-vapor annealing (SVA) treatment. OSCs based on the two acceptors and PTB7-Th as a polymer donor are fabricated owing to their complementary absorption and sufficient energy-level offsets. The best power conversion efficiency of 8.33% is obtained with the SVA-treated TBTT-BORH device, where, together with a high open-circuit voltage of 1.02 V, the charge-carrier mobility and the short-circuit current density were greatly improved by the SVA treatment to levels comparable to those of the TBTT-ORH device because of the suppressed charge recombination and improved film morphology. In this work, the simultaneous improvement of both material solubility and device performance is achieved through alkyl side-chain engineering to balance the trade-offs among material solubility/crystallinity/device performance.

From Y6 to BTPT-4F: a theoretical insight into the influence of the individual change of fused-ring skeleton length or side alkyl chains on molecular arrangements and electron mobility

Organic solar cells (OSCs) based on non-fullerene acceptor (NFA) Y6 have drawn tremendous attention due to the great progress in their power conversion efficiencies (PCEs).

High-Efficiency Thermal-Annealing-Free Organic Solar Cells Based on an Asymmetric Acceptor with Improved Thermal and Air Stability

The stability of organic solar cells (OSCs) is an urgent problem for commercialization. In this work, a novel asymmetric molecule TB-4Cl was designed and synthesized. Quantum chemical computations revealed that TB-4Cl has a larger dipole moment of 1.98 Debye than that of Y6, which can induce a stronger intermolecular interaction. Without thermal annealing, devices based on PM6:TB-4Cl achieved a higher efficiency of 14.67%. Impressively, all of the devices showed a negligible difference in power conversion efficiency (PCE) before and after thermal-annealing treatment. Compared to the unencapsulated PM6:Y6-based devices, PM6:TB-4Cl-based devices exhibited improved thermal and air stability, evidenced by retaining around 75% (TB-4Cl) and 60% (Y6) after being heated at 100 °C in nitrogen for 110 h and 65% (TB-4Cl) and 50% (Y6) in air for 92 h. This work indicates that an A-D₁A'D₂-A asymmetric molecule can be a promising candidate for achieving stable OSCs with high efficiency.

Designing of 5,10-Dihydroindolo [3,2-b] Indole (DINI) Based Donor Materials for Small Molecule Organic Solar Cells

In this paper, four small molecules B1, B2, B3 and B4 based on donor–acceptor–donor–acceptor–donor (D-A-D-A-D) combination were designed by making structural modifications in R. The designed molecules contain 5,10-dihydro-indolo [3,2-b] indole central donor core and different benzo-thiadiazole and fluorine substituted benzothiadiazole (FBT) acceptor units. These molecules have different subunits introduced on 5,10-dihydroindolo [3,2-b] indole central core like benzo [1,2,5] thiadiazole in (B1), 5-Fluoro-benzo [1,2,5] thiadiazole in (B2), 5-Methyl-benzo [1,2,5] thiadiazole in (B3), 2-Fluoro-2-methyl-2-H-benzotriazole unit in (B4), flanked with [2,2’,5’,2”] terthiophene as spacer (S) and triphenyl amine as a common end-capped donor in all the molecules (B1– B4). The optoelectronic properties of these molecules were studied by performing density functional theory (DFT) at CAM-B3LYP. Among all the designed structures, B2 showed maximum absorption (457[Formula: see text]nm) due to its strong electron withdrawing 5-Fluoro-benzo [1,2,5] thiadiazole acceptor unit. Other opto-electronic properties were analyzed through reorganization energies, density of electronic states and transition density matrix (TDM) to estimate the photovoltaic potential of these newly designed molecules. Low exciton binding energies and comparable values of open circuit voltage than R indicate the worth of these candidates to be used in future solar energy driven devices.

Photoactivated Refractive Index Anisotropy in Fluorescent Thiophene Derivatives

The optical control of anisotropy in materials is highly advantageous for many technological applications, including the real-time modulation of another light signal in photonic switches and sensors. Here, we introduce three thiophene derivatives with a donor-acceptor structure, which feature different positions of an electron-acceptor nitrile group, and both photoalignment and luminescence properties. Quantum chemical calculations highlight the presence of trans-forms stable at room temperature and metastable cis-isomers. Besides photoluminescence peaked at 440-460 nm and 0.4 ns lifetime, the three nonlinear optical chromophores exhibit photoinduced anisotropy of the refractive index closely depending on the specific molecular structure, with higher values of birefringence at lower driving signal being obtained for ortho substitution of the nitrile group. All-optical modulation of an external light beam at rates of hundreds of hertz is demonstrated in the fluorescent systems. This finding opens an interesting route to multispectral photonic switches embedded in the active layers of light-emitting devices.

Noncovalently Fused-Ring Electron Acceptors with C2v Symmetry for Regulating the Morphology of Organic Solar Cells

Four noncovalently fused-ring electron acceptors p-DOC6-2F, o-DOC6-2F, o-DOC8-2F, and o-DOC2C6-2F have been designed and synthesized. p-DOC6-2F and o-DOC6-2F have the same molecular backbone but different molecular shapes and symmetries. p-DOC6-2F has an S-shaped molecular backbone and C_2h symmetry, whereas o-DOC6-2F possesses a U-shaped molecular backbone and C_2v symmetry. The molecular shape and symmetry can influence the dipole moment, solubility, optical absorption, energy level, molecular packing, and film morphology. Compared with the corresponding p-DOC6-2F, o-DOC6-2F exhibits better solubility, a wider band gap, and a larger dipole moment. When blended with the donor polymer PBDB-T, the C_2v symmetric o-DOC6-2F can form an appropriate active layer morphology, whereas the C_2h symmetric p-DOC6-2F forms oversized domains. Organic solar cells (OSCs) based on p-DOC6-2F, o-DOC6-2F, o-DOC8-2F, and o-DOC2C6-2F obtained power conversion efficiencies of 9.23, 11.87, 11.23, and 10.80%, respectively. The result reveals that the molecular symmetry can facilely regulate the performance of OSCs.

Designing of near-infrared sensitive asymmetric small molecular donors for high-efficiency organic solar cells

Herein, we have designed four small molecular donors (SMDs) with Donor–Acceptor–Acceptor (D–Á–A) backbone having different acceptor units for highly efficient organic solar cells (OSCs). The specific molecular modeling has been made by replacing the additional acceptor unit (A) of recently synthesized TPA-DAA-MDN molecule (R) by employing different highly efficient acceptor units in order to improve the photovoltaic performances of the molecules. A theoretical approach (DFT and TD-DFT) has been applied to investigate the photophysical, opto-electronic and photovoltaic parameters of the designed molecules (DAA1–DAA4) and compared with the reference molecule (R). The red-shifting absorption of SMDs is the most important factor for highly efficient OSCs. Our all formulated molecules showed a red shifted absorption spectrum and also exhibit near IR sensitivity. Acceptor unit modification of R molecule causes reduction in HOMO-LUMO energy gap; therefore, all designed molecules offer better opto-electronic properties as compared to R molecule. A variety of certain critical factors essential for efficient SMDs like frontier molecular orbitals (FMOs), absorption maxima, dipole moment, exciton binding energy along with transition density matrix, excitation energy, open circuit voltages and charge mobilities of (DAA1–DAA4) and R have also been investigated. Generally, low values of reorganizational energy (hole and electron) offer high charge mobility and our all designed molecules are enriched in this aspect. High open circuit voltage values, low excitation energies, large dipole moment values indicate that our designed SMDs are suitable candidates for high-efficiency OSCs. Furthermore, conceptualized molecules are superior and thus are suggested to experimentalist for out-looking future progresses of highly efficient OSCs devices.

Planar D-π-A Configured Dimethoxy Vinylbenzene Based Small Organic Molecule for Solution-Processed Bulk Heterojunction Organic Solar Cells

A new and effective planar D-π-A configured small organic molecule (SOM) of 2-5-(3,5-dimethoxystyryl)thiophen-2-yl)methylene)-1H-indene-1,3(2H)-dione, abbreviated as DVB-T-ID, was synthesized using 1,3-indanedione acceptor and dimethoxy vinylbenzene donor units, connected through a thiophene π-spacer. The presence of a dimethoxy vinylbenzene unit and π-spacer in DVB-T-ID significantly improved the absorption behavior by displaying maximum absorbance at ~515 nm, and the reasonable band gap was estimated as ~2.06 eV. The electronic properties revealed that DVB-T-ID SOMs exhibited promising HOMO (−5.32 eV) and LUMO (−3.26 eV). The synthesized DVB-T-ID SOM was utilized as donor material for fabricating solution-processed bulk heterojunction organic solar cells (BHJ-OSCs) and showed a reasonable power conversion efficiency (PCE) of ~3.1% with DVB-T-ID:PC61BM (1:2, w/w) active layer. The outcome of this work clearly reflects that synthesized DVB-T-ID based on 1,3-indanedione units is a promising absorber (donor) material for BHJ-OSCs.

The theoretical investigation of the opto-electronic properties of designed molecules having 2-(2-Methylene-3-oxo-indane-1-ylidene)malononitrile as end-capped acceptors

Five acceptor-donor-acceptor molecules having different core units with 2-(2-Methylene-3-oxo-indane-1-ylidene)malononitrile as end capped terminal acceptor unit were designed. The ground state geometries and electronic properties were calculated by using density functional theory (DFT) at MPW1PW91/6-31G(d,p) level of theory. The absorption spectra were computed by using time dependent DFT at MPW1PW91/6-31G(d,p) level of theory. The designed molecules have broad absorption range in visible region. M3 shows relatively lower band gap so that having high light harvesting efficiency (LHE). The molecules consider as better hole blocking materials in term of high ionization potentials. The reorganization energies calculation of M1, M2 and M4 manifests that these molecules are the optimal candidate for electron transportation. High value of Voc has been observed for molecules which would favorably contribute in power conversion efficiency. M1, M2, M4 and M5 are more stable in terms of absolute hardness and electrostatic potential surfaces. All molecules show good opto-electronic properties in the aspect of their use in photovoltaic application.

The theoretical investigation of the opto-electronic properties of designed molecules having 2-(2-Methylene-3-oxo-indane-1-ylidene)malononitrile as end-capped acceptors

Five acceptor-donor-acceptor molecules having different core units with 2-(2-Methylene-3-oxo-indane-1-ylidene)malononitrile as end capped terminal acceptor unit were designed. The ground state geometries and electronic properties were calculated by using density functional theory (DFT) at MPW1PW91/6-31G(d,p) level of theory. The absorption spectra were computed by using time dependent DFT at MPW1PW91/6-31G(d,p) level of theory. The designed molecules have broad absorption range in visible region. M3 shows relatively lower band gap so that having high light harvesting efficiency (LHE). The molecules consider as better hole blocking materials in term of high ionization potentials. The reorganization energies calculation of M1, M2 and M4 manifests that these molecules are the optimal candidate for electron transportation. High value of Voc has been observed for molecules which would favorably contribute in power conversion efficiency. M1, M2, M4 and M5 are more stable in terms of absolute hardness and electrostatic potential surfaces. All molecules show good opto-electronic properties in the aspect of their use in photovoltaic application.

Magnetic i-MXenes: a new class of multifunctional two-dimensional materials

Based on density functional theory calculations, we investigated two-dimensional in-plane ordered MXenes (i-MXenes), focusing particularly on their magnetic properties. It has been observed that robust two-dimensional magnetism can be achieved by alloying nonmagnetic MXenes with magnetic transition metal atoms. Moreover, both the magnetic ground states and the magnetocrystalline anisotropy energy of i-MXenes can be effectively manipulated by strain, indicating a strong piezomagnetic effect. Further studies on the transport properties reveal that i-MXenes provide an interesting platform to realize large thermoelectric response, antiferromagnetic topological insulators, and spin-gapless semiconductors. Thus, i-MXenes are a new class of multifunctional two-dimensional magnetic materials which are promising for future spintronic applications.

A theoretical study of the absorption spectra of electron-deficient pentacene derivatives using DFT and TDDFT

Non-fullerene acceptor based organic bulk heterojunction solar cells have been a hot topic because their power conversion efficiencies have been up to 16.35%. Functionalized 6,13‑bis (trimethylsilyl alkynyl) pentacenes with strong electron withdrawing groups, which can be easily modified to improve charge transport properties and film morphology, seem to be promising soluble non-fullerene pentacene-based organic acceptors. But how the substitutions of electron withdrawing groups influence their electronic structures, then change the absorption spectra and power conversion efficiencies, is still not clear. In this paper, we utilize density functional theory and time-dependent density functional theory to study the effects of substitutions of different electron withdrawing groups (CN, CF₃, NO₂) and different positions of these groups in 6,13‑bis (trimethylsilyl alkyl) pentacene molecule on their physical and optical properties. We find that the experimental power conversion efficiencies are positively/negatively correlated with calculated dipole moments/exciton binding energies of these functionalized molecules. The computed results indicate that the molecules substituted with CN group have much larger dipole moment than the others. For the same electron withdrawing group, the dipole moment at the R2 position is generally larger than that at the R1 position. Furthermore, we find that the calculated exciton binding energy of these molecules functionalized at the R2 position is lower than that at the R1 position. In addition, the result of absorption spectra confirm that these functionalized 6,13‑bis (trimethylsilyl alkynyl) pentacenes have stronger absorption strength than C60 in the both visible and the ultraviolet regions.

Tuning of optoelectronic properties of triphenylamines-based donor materials for organic solar cells

In the present study, four molecules have been designed by substituting various acceptor moieties around the triphenylamine donor moiety like 2-cyano acrylic acid (R), 2-methylene malonitrile (M1), 2-cyano acrylic acid methyl ester(M2), 2-(2-methylene-3-oxo-indan-1-ylidene)-malonitrile (M3), 2-(6,7-difluoro-2-methylene-3-oxo-indan-1-ylidene)-malonitrile (M4), respectively. CAM-B3LYP/6-31G (d, p) level of theory by using density functional theory (DFT) has been used for the investigation of optoelectronic properties of four new triphenylamine (TPA)-based donor materials (M1–M4) for organic solar cells. In comparison with the recently reported reference molecule, the optoelectronic properties of designed molecules were evaluated. M4 showed absorption maxima at 520[Formula: see text]nm due to extended conjugation with bridged thiophene group. Results of reorganization energy calculations also favor M4 exhibiting highest transfer rate of hole as depicted from its low reorganization energy of hole ([Formula: see text].

A novel low-bandgap pyridazine thiadiazole-based conjugated polymer with deep molecular orbital levels

A pyridazine thiadiazole acceptor (PzT) has been utilised in the synthesis of a novel low band-gap D–A copolymer PTTPz.

Synthesis, Molecular Packing, and Electrical Properties of New Regioisomeric n-type Semiconducting Molecules with Modification of Alkyl Substituents Position

We design and synthesize a series of regioisomeric n-type small molecules, which have an identical diketopyrrolopyrrole (DPP) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) terminal groups with octyl substituents at different positions. The isomeric structures are confirmed by two-dimensional NMR spectroscopy based on the heteronuclear multiple-bond coupling method. Incorporation of the electron-deficient DPP and strongly electron-withdrawing INCN groups yields deep frontier molecular orbitals with n-type charge-transport properties in solution-processed organic field-effect transistors (OFETs). Interestingly, a minor change in the substitution position of the octyl side chains significantly influences the optoelectronic and morphological properties of the thin film. The polycrystalline morphology of the as-cast films is reorganized differently with thermal annealing depending on the octyl topology, significantly affecting the OFET performance. With thermal treatment at 200 °C, the kinked DPP(EH)-INCNO1 (EH = 2-ethylhexyl) structures transform into single crystalline-like structures, exhibiting a remarkably improved electron mobility up to ∼0.6 cm²V^-1 s^-1 compared with DPP(EH)-INCNO2 isomers. The more linear DPP(EH or HD)-INCNO2 (HD = 2-hexyldecyl) molecules become more crystalline with thermal treatments, but their polycrystalline packing structures with large grain boundaries are the main reason for their lower electron mobility. When the solubilizing alkyl substituents are selected, careful molecular design is needed, with consideration of both the solubility and intermolecular packing, for optimizing the optoelectronic properties.

Selective Adsorption of C60 in the Supramolecular Nanopatterns of Donor-Acceptor Porphyrin Derivatives

The nanostructure of active layers consisting of donor and acceptor molecules is responsible for the separation and transfer processes of charge carriers, which may result in different photoelectric conversion efficiencies of organic photovoltaic cells (OPVCs). Therefore, intensive study on the relationships among nanostructures, intermolecular interactions, and molecular chemical skeletons is necessary for preparing controlled nanostructures of active layers by designing photovoltaic molecules. In this research, the self-assembled nanopatterns of three (DPP-ZnP-E)2-based molecules on highly oriented pyrolytic graphite surface were probed by scanning tunneling microscopy and analyzed by density functional theory calculations. The results indicated that different bridges, diethynylene, diethynylene-dithienyl, and diethynylene-phenylene, in (DPP-ZnP-E)2-based molecules not only made a difference to intermolecular interactions and cooperated with molecule-substrate interactions, consequently affecting the packed nanopattern, but also influenced the adsorption of fullerene acceptors in the nanopatterns of (DPP-ZnP-E)2-based molecules. C₆₀ molecules were found to be selectively adsorbed atop the dithienyl groups of (DPP-ZnP-E)2-2T donor molecules probably by S···π interactions compared with (DPP-ZnP-E)2 or (DPP-ZnP-E)2-Ph molecules. This study on the assembled nanopatterns of the three (DPP-ZnP-E)2-based molecules would be conductive to (DPP-ZnP-E)2-based optoelectronic materials design in OPVCs.

Significant Difference in Semiconducting Properties of Isomeric All-Acceptor Polymers Synthesized via Direct Arylation Polycondensation

The direct arylation polycondensation (DArP) appeared as an efficient method for producing semiconducting polymers but often requires acceptor monomers with orienting or activating groups for the reactive carbon-hydrogen (C-H) bonds, which limits the choice of acceptor units. In this study, we describe a DArP for producing high-molecular-weight all-acceptor polymers composed of the acceptor monomers without any orienting or activating groups via a modified method using Pd/Cu co-catalysts. We thus obtained two isomeric all-acceptor polymers, P1 and P2, which have the same backbone and side-chains but different positions of the nitrogen atoms in the thiazole units. This subtle change significantly influences their optoelectronic, molecular packing, and charge-transport properties. P2 with a greater backbone torsion has favorable edge-on orientations and a high electron mobility μ_e of 2.55 cm²  V^-1  s^-1 . Moreover, P2-based transistors show an excellent shelf-storage stability in air even after the storage for 1 month.

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.