Designing of Thiophene [3, 2-b] Pyrrole Ring-Based NFAs for High-Performance Electron Transport Materials: A DFT Study

Among the blended components of a photoactive layer in organic photovoltaic (OPV) cells, the acceptor is of high importance. This importance is attributed to its increased ability to withdraw electrons toward itself for their effective transport toward the respective electrode. In this research work, seven new non-fullerene acceptors were designed for their possible utilization in the OPVs. These molecules were designed through side-chain engineering of the PTBTP-4F molecule, with its fused pyrrole ring-based donor core and different strongly electron-withdrawing acceptors. To elucidate their effectiveness, the band gaps, absorption characteristics, chemical reactivity indices, and photovoltaic parameters of all of the architecture molecules were compared with the reference. Through various computational software, transition density matrices, graphs of absorption, and density of states were also plotted for these molecules. From some chemical reactivity indices and electron mobility values, it was proposed that our newly designed molecules could be better electron-transporting materials than the reference. Among all, TP1, due to its most stabilized frontier molecular orbitals, lowest band gap and excitation energies, highest absorption maxima in both the solvent and gas medium, least hardness, highest ionization potential, superior electron affinity, lowest electron reorganization energy, as well as highest rate constant of charge hopping, seemed to be the best molecule in terms of its electron-withdrawing abilities in the photoactive layer blend. In addition, in terms of all of the photovoltaic parameters, TP4-TP7 was perceived to be better suited in comparison to TPR. Thus, all our suggested molecules could act as superior acceptors to TPR.

Novel A-π-D-π-A type non-fullerene acceptors of dithienyl diketopyrropopyrrole derivatives to enhance organic photovoltaic applications: a DFT study

To boost the photovoltaic attributes of organic photovoltaic cells, seven dithienyl diketopyrropopyrrole (TDPP) donor-based A-π-D-π-A (acceptor-bridge-donor-bridge-acceptor) type molecules (TM1-TM7) were formulated by modifying the electron accepting ends of the reference molecule (TMR). Optical and quantum chemical parameters of seven synthesized molecules were investigated using density functional theory with the MPW1PW91/6-31G(d,p) functional. Several parameters that can be used to measure and improve the efficiency of solar cells have been analyzed and summed up. These parameters include binding energy of exciton, excitation energy of electron, reorganization energies, dipole moment, molecular electrostatic potential, charge mobility, wavelength of maximum absorption, open circuit voltage, short circuit current, fill factor, density of states, transition density matrices, as well as iso-surface and non-covalent interactions. Thus, all of our proposed structures are perceived to be superior to the reference in terms of the maximum possible solar energy yield in solar cells with bulk heterojunctions, as determined by analyses of our designed molecules for the aforementioned parameters.

Computational Design of Crescent Shaped Promising Nonfullerene Acceptors with 1,4-Dihydro-2,3-quinoxalinedione Core and Different Electron-withdrawing Terminal Units for Photovoltaic Applications

This study aims to design a series of nonfullerene acceptors (NFAs) for photovoltaic applications having 1,4-dihydro-2,3-quinoxalinedione fused thiophene derivative as the core unit and 1,1-dicyanomethylene-3-indanone (IC) derivatives and different π-conjugated molecules other than IC as terminal acceptor units. All the investigated NFAs are found air-stable as the computed highest occupied molecular orbitals (HOMOs) are below the air oxidation threshold (ca. -5.27 eV vs saturated calomel electrode). The studied NFAs can act as potential nonfullerene acceptor candidates as they are found to have sufficient open-circuit voltage (V_oc) and fill factor (FF) ranging from 0.62 to 1.41 V and 83%-91%, respectively. From the anisotropic mobility analysis, it is noticed that the studied NFAs except dicyano-rhodanine terminal unit containing NFA, exhibit better electron mobility than the hole mobility, and therefore, they can be more promising electron transporting acceptor materials in the active layer of an organic photovoltaic cell. From the optical absorption analysis, it is noted that all the designed NFAs have the maximum absorption spectra ranging from 597 nm-730 nm, which lies in the visible region and near-infrared (IR) region of the solar spectrum. The computed light-harvesting efficiencies for the PM6 (thiophene derivative donor selected in our study):NFA blends are found to lie in the range of 0.96-0.99, which indicates efficient light-harvesting by the PM6:NFA blends during photovoltaic device operation.

Impact of end capped modification on BT-CIC molecule for high-performance photovoltaic attributes: a DFT approach

With the aim of utilizing structural modeling techniques to design efficient organic solar cells, a quantum chemical density functional theory (DFT) and its time-dependent DFT (TD-DFT) study have been carried out for the examination of the photovoltaic properties of four BT-ClC-based novel non-fullerene acceptor (NFA) molecules. The designed entities (BT1-BT4) have an A-π-D-π-A configuration with seven fused ring-based BDT central core and newly substituted peripheral acceptor moieties. The optical parameters (absorption maxima, light-harvesting efficiency, first excitation energies, and dipole moments), electronic properties (frontier molecular orbitals, density of states, and molecular electrostatic potential), and charge transfer characteristics (open-circuit voltage, transition density matrix, and fill factor) of the investigated molecules were evaluated using the selected B3LYP/6-31G (d,p) level of theory. The systematic computational analysis reveals that under the influence of terminal acceptor groups, there is an augmentation in the absorption range, and reduction in the band gap values. The electron withdrawing effect of acceptor moieties is evident from the electronic density distribution on the HOMO-LUMO orbitals, along with the density of state (DOS) graphs. Transition density matrix (TDM) analyses reveal consistent charge transfer in the newly devised entities. Reorganization energies computed for electron and hole are significantly lower than the reference, making the transfer of charge carriers efficient. Open-circuit voltage (V_oc) of reported acceptor entities, theoretically computed with PTB7-Th donor, revealed maximum output. Furthermore, the estimated fill factor (FF) of the investigated molecules predicted an increase in power conversion efficiencies. Consequently, all the computed parameters favor the applicability of our designed molecules in the field of organic photovoltaics by virtue of their excellent charge mobilities, increased absorption maximum values, and reduced band gaps.

Impact of various heterocyclic π-linkers and their substitution position on the opto-electronic attributes of the A–π–D–π–A type IECIO-4F molecule: a comparative analysis

To investigate the consequence of different substitution positions of various π-linkers on the photovoltaic properties of an organic solar cell molecule, we have introduced two series of six three-donor molecules, by the substitution of some effective π-linkers on the A–π–D–π–A type reference molecule IECIO-4F (taken as IOR). In series “a” the thienyl or furyl bridge is directly linked between the donor and acceptor moieties, while in series “b” the phenyl ring of the same bridge is working as the direct point of attachment. The frontier molecular orbitals, density of states, transition density matrix, molecular electrostatic potential surfaces, exciton binding energy, excitation energy, wavelength of maximum absorption, open-circuit voltage, fill factor, and some other photovoltaic attributes of the proposed molecules were analyzed through density functional theory (DFT) and its time-dependent (TD) approach; the TD-DFT method. Though both series of newly derived molecules were a step up from the reference molecule in almost all of the studied characteristics, the “a” series (IO1_a to IO3_a) seemed to be better due to their desirable properties such as the highest maximum absorption wavelength (λ_max), open-circuit voltage, and fill factor, along with the lowest excitation and exciton dissociation energy, etc. of its molecules. Also, the studied morphology, optical characteristics, and electronic attributes of this series of proposed molecules signified the fact that the molecules with thienyl or furyl ring working as the direct link between the acceptor and donor molecules showed enhanced charge transfer abilities, and could provide a maximum quantum yield of the solar energy supplied.

We have introduced two series of six three-donor molecules, by the substitution of some effective π-linkers on the A–π–D–π–A type reference molecule IECIO-4F (taken as IOR) for efficient organic solar cells.

Assessing Effects of Different π bridges on Properties of Random Benzodithiophene-thienothiophene Donor and Non-fullerene Acceptor Based Active Layer

This report presents the effect of insertion of four different π bridges, furan, thienothiophene, thiophene, and thiazole, into a random benzodithiophene (BDT)-fluorinated-thienothiophene (TT-F) based donor. Starting from a structure of synthesized donor (D)-acceptor (A) random copolymer with 3:1 ratio, we have designed four D-π-A systems with four different π bridges. Structural, optoelectronic, and charge transport/transfer properties of these donors and donor/NDI (NDI = poly[N,N'-bis(2-hexyldecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) blends are investigated using DFT and TD-DFT methodologies. Our results show that the thiazole based TzP1 oligomer has the deepest HOMO value resulting in the highest open circuit voltage among all systems. The maximum absorption wavelengths of π-linked systems are red-shifted compared to the parent molecule. Rates of charge transfer and charge recombination are the highest and smallest in case of the thiazole/NDI blend system. In addition, hole mobilities in thiophene, thienothiophene, and thiazole based systems are larger than in the parent system. The results indicate that the thiazole unit among the four π bridge units is the most suitable for active layer construction.

Theoretical Design of Dithienopicenocarbazole-Based Molecules by Molecular Engineering of Terminal Units Toward Promising Non-fullerene Acceptors

Dithienopicenocarbazole (DTPC), as the kernel module in A-D-A non-fullerene acceptors (NFA), has been reported for its ultra-narrow bandgap, high power conversion efficiency, and extremely low energy loss. To further improve the photovoltaic performance of DTPC-based acceptors, molecular engineering of end-capped groups could be an effective method according to previous research. In this article, a class of acceptors were designed via bringing terminal units with an enhanced electron-withdrawing ability to the DTPC central core. Their geometrical structures, frontier molecular orbitals, absorption spectrum, and intramolecular charge transfer and energy loss have been systematically investigated on the basis of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Surprisingly, NFA 4 highlights the dominance for its increased open circuit voltages while NFA 2, 7, and 8 exhibit great potential for their enhanced charge transfer and lower energy loss, corresponding to a higher short-circuit current density. Our results also manifest that proper modifications of the terminal acceptor with extensions of π-conjugation might bring improved outcomes for overall properties. Such a measure could become a feasible strategy for the synthesis of new acceptors, thereby facilitating the advancement of organic solar cells.

Insight into the optoelectronic characteristics of diimide-based acceptors in organic solar cells by performing DFT calculation and molecular dynamics simulation

In order to compare the main difference of two diimide derivatives on the modulation of electronic and optical properties of P3HT-based organic solar cell, the density functional theory and molecular dynamics simulations were implemented to achieve elementary data on geometrical structure, molecular orbital, open-circuit voltage, absorption spectra, energetic driving force, and interface parameter of P3HT/D1 and P3HT/D2 systems. According to the investigation, P3HT/D1 system not only exhibits higher open circuit voltage and enough energetic driving force than P3HT/D2 system, but also possesses low-lying LUMO +1 orbital which can favor the exciton separation efficiency. Moreover, on the basis of some typical interface models choose from MD simulation, the estimation of the interface rate manifests that the P3HT/D1 interface possesses the smaller charge recombination rates and larger charge separation rates than those of the P3HT/D2 interface. It is expect that this work can provide certain guidelines for the further develop the performance of organic solar cell. We hope this work can further study on non-fullerene acceptor materials as a certain guides.

Modification of NFA-Conjugated Bridges with Symmetric Structures for High-Efficiency Non-Fullerene PSCs

As electron acceptors, non-fullerene molecules can overcome the shortcomings of fullerenes and their derivatives (such as high cost, poor co-solubility, and weak light absorption). The photoelectric properties of two potential non-fullerene polymer solar cells (PSCs) PBDB-T:IF-TN (PB:IF) and PBDB-T:IDT-TN (PB:IDT) are studied by density functional theory (DFT) and time-dependent DFT (TD-DFT). Based on the optimized structure of the ground state, the effects of the electron donor (D) and electron acceptor (A) (D/A) interfaces PBDB-T/IF-TN (PB/IF) and PBDB-T/IDT-TN (PB/IDT) are studied by a quantum-chemical method (QM) and Marcus theory. Firstly, for two non-fullerene acceptors (NFAs) IF-TN and IDT-TN, the NFA IDT-TN has better optical absorption ability and better electron transport ability than IF-TN. Secondly, for the D/A interfaces PB/IF and PB/IDT, they both have high optical absorption and electron transfer abilities, and PB/IDT has better optical absorption and lower exciton binding energy. Finally, some important parameters (open-circuit voltage, voltage loss, fill factor, and power conversion efficiency) are calculated and simulated by establishing the theoretical model. From the above analysis, the results show that the non-fullerene PSC PB:IDT has better photoelectric characteristics than PB:IF.

Non-Fullerene Acceptor-Based Solar Cells: From Structural Design to Interface Charge Separation and Charge Transport

The development of non-fullerene small molecule as electron acceptors is critical for overcoming the shortcomings of fullerene and its derivatives (such as limited absorption of light, poor morphological stability and high cost). We investigated the electronic and optical properties of the two selected promising non-fullerene acceptors (NFAs), IDIC and IDTBR, and five conjugated donor polymers using quantum-chemical method (QM). Based on the optimized structures of the studied NFAs and the polymers, the ten donor/acceptor (D/A) interfaces were constructed and investigated using QM and Marcus semi-classical model. Firstly, for the two NFAs, IDTBR displays better electron transport capability, better optical absorption ability, and much greater electron mobility than IDIC. Secondly, the configurations of D/A yield the more bathochromic-shifted and broader sunlight absorption spectra than the single moiety. Surprisingly, although IDTBR has better optical properties than IDIC, the IDIC-based interfaces possess better electron injection abilities, optical absorption properties, smaller exciton binding energies and more effective electronic separation than the IDTBR-based interfaces. Finally, all the polymer/IDIC interfaces exhibit large charge separation rate (KCS) (up to 1012⁻1014 s-1) and low charge recombination rate (KCR) (<10⁶ s-1), which are more likely to result in high power conversion efficiencies (PCEs). From above analysis, it was found that the polymer/IDIC interfaces should display better performance in the utility of bulk-heterojunction solar cells (BHJ OSC) than polymer/IDTBR interfaces.

A theoretical exploration of the effect of fluorine and cyano substitutions in diketopyrrolopyrrole-based polymer donor for organic solar cells

A series of polymer donor materials 1-5 based on diketopyrrolopyrrole and thiophene unit which have been widely used in organic solar cells (OSCs) were investigated based on quantum chemical calculations. The effect of fluorine and cyano substitutions in polymer donor materials was focused on. Based on the investigation on electronic structures and optical properties of the reported molecules 1 and 2 and the analysis on some parameters relevant to charge dissociation ability at donor/acceptor interface constituted by 1 and 2 with PC₆₁BM such as intermolecular charge transfer and recombination, driving force and Coulombic bound energy, we explained why fluorine substitution can improve OPV efficiency through strengthening eletron-withdrawing ability from a theoretical perspective. Then we designed cyano-substituted polymers 3-5 with the aim of obtaining better photovoltaic donor materials. The results reveal that our attempt to design donor materials which can balance large open-circuit voltage (V_oc) and high short-circuit current (J_sc) in OSCs has worked out. It is worth noting that the substitutions of fluorine and cyano groups synergistically reduce energy gap and HOMO energy level of polymers 3 and 4. Moreover, 3/PC₆₁BM and 4/PC₆₁BM heterojunctions show over 10⁷ and 10⁴ times higher than 1/PC₆₁BM on the ratios of intermolecular charge transfer and recombination rates (k_inter-CT/k_inter-CR). Thus, our work here may provide an efficient strategy to design promising donor materials in OPVs and we hope it could be useful in the future experimental synthesis.

Theoretical characterization on photoelectric properties of benzothiadiazole- and fluorene-based small molecule acceptor materials for the organic photovoltaics

The upper efficiency of heterojunction organic photovoltaics depends on the increased open-circuit voltage (V _oc) and short-circuit current (J _sc). So, a higher lowest unoccupied molecular orbital (LUMO) level is necessary for organic acceptor material to possess higher V _oc and more photons absorbsorption in the solar spectrum is needed for larger J _sc. In this article, we theoretically designed some small molecule acceptors (2∼5) based on fluorene (F), benzothiadiazole, and cyano group (CN) referring to the reported acceptor material 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (1), the crucial parameters affecting photoelectrical properties of compounds 2∼5 were evaluated by the density functional theory (DFT) and time dependent density functional theory (TDDFT) methods. The results reveal that compared with 1, 3 and 4 could have the better complementary absorption spectra with P3HT, the increased LUMO level, the improved V _oc, and the decreased electronic organization energy (λ _e). From the simulation of transition density matrix, it is very clear that the excitons of molecules 3 and 4 are easier to separate in the material surface. Therefore, 3 and 4 may become potential acceptor candidates for organic photovoltaic cells. In addition, with the increased number of CN, the optoelectronic properties of the molecules show a regular change, mainly improve the LUMO level, energy gap, V _oc, and absorption intensity. In summary, reasonably adjusting CN can effectively improve the photovoltaic properties of small molecule acceptors. Graphical Abstract Structure-property relationship of small molecule acceptors could be rationally evaluated in the article. The changes of conjugate length and CN are important strategies to alter the photovoltaic properties of small molecule acceptors. Therefore, taking the K12/1 as a reference, we have theoretically designed a series of small molecule acceptors (2-4). The calculated results by means of DFT and TDDFT manifest that molecules 3 and 4 have the better complementary absorption spectra with P3HT, the increased LUMO level, the improved V _oc, the decreased electronic organization energy and the easier separation in the material surface than 1. In summary, reasonably increasing conjugate length and decreasing CN can effectively improve the PCE, which will provide a theoretical guideline for the design and synthesis of new small molecule acceptors.

Theoretical design of three-dimensional non-fullerene acceptor materials based on an arylenediimide unit towards high efficiency organic solar cells

DFT and TDDFT calculations were performed to search for high-performance non-fullerene organic acceptor materials in organic solar cells.

The conversion of donor to acceptor and rational design for diketopyrrolopyrrole-containing small molecule acceptors by introducing nitrogen-atoms for organic solar cells

Following the introduction of nitrogen, a donor could convert into an acceptor, the conversion mechanism of which, acquired from theoretical calculations, will promote the development of acceptors.

From blue to full color – theoretical design and characterization of a series of Ir(iii) complexes containing azoline ligand with potential application in OLEDs

A series of full-color Ir(iii) phosphorescence materials with high quantum efficiency was designed with both the radiative rate and the nonradiative ability taken into consideration.

Exploring what prompts ITIC to become a superior acceptor in organic solar cell by combining molecular dynamics simulation with quantum chemistry calculation

A comparison on charge transfer process in OSC between non-fullerene electron acceptor ITIC and PC71BM was taken by microscopic analysis based on the molecular dynamics and quantum chemistry method.