Exploring the electrochemical properties of hole transport materials with spiro-cores for efficient perovskite solar cells from first-principles

Effect of heterocyclic and non-heterocyclic units on FDT-based hole transport materials for efficient perovskite solar cells: a DFT study

The development of a new structure is one of the important approaches for the advancement of efficient hole-transporting materials (HTMs). In this work, novel and efficient HTMs are designed based on the experimentally reported fluorene-dithiophene (FDT) system which shows the effect of four different units phenyl, pyridine, thiane, and oxane in the FDT unit. The structural, optoelectronic, and charge transport properties of the newly developed HTMs are probed using density functional theory (DFT) and time-dependent DFT (TD-DFT) methodologies. The calculated highest occupied molecular orbital (HOMO) energies for all HTMs are higher compared to the valence band energy level of the perovskite which exhibits outstanding hole extraction ability of all HTMs at the charge buffer interface. In addition, the designed HTMs have red-shifted absorption spectra compared to FDT. The computed hole mobilities of newly designed HTMs are faster compared to that of FDT. Moreover, newly tailored HTMs demonstrate improved solubility. The results indicate that a one thiane and one phenyl unit-based system among all materials is the most suitable for HTM design.

Removal of heavy metal ions from wastewater using two-dimensional transition metal carbides

Water is an indispensable material for human life. Unfortunately, the development of industrial activities has reduced the quality of water resources in the world. Meantime, heavy metals are an important factor in water pollution due to their toxicity. This study highlights the method for the capture of heavy metal ions from wastewater using the procedure of adsorption. The adsorption of toxic heavy metal ions (Pb2+, Hg2+, and Cd2+) on Ca2C as well as Cr2C carbide-nitride MXene monolayers is investigated using the density functional theory. We have carried out the optimization of the considered MXenes by nine DFT functionals: PBE, TPSS, BP86, B3LYP, TPSSh, PBE0, CAM-B3LYP, M11, and LC-WPBE. Our results have shown a good agreement with previously measured electronic properties of the Ca2C and Cr2C MXene layers and the PBE DFT method. The calculated cohesive energy for the Ca2C and Cr2C MXene monolayers are -4.12 eV and -4.20 eV, respectively, which are in agreement with the previous studies. The results reveal that the adsorbed heavy metal ions have a substantial effect on the electronic properties of the considered MXene monolayers. Besides, our calculations show that the metal/MXene structures with higher electron transport rates display higher binding energy as well as charge transfers between the metal and Ca2C and Cr2C layers. Time-dependent density functional analysis also displayed "ligand to metal charge transfer" excitations for the metal/MXene systems. The larger Ebin for the Pb@Ca2C as well as Pb@Cr2C are according to larger redshifts which are expected (Δλ = 45 nm and 71 nm, respectively). Our results might be helpful for future research toward the application of carbide-nitride MXene materials for removing wastewater pollutants.

Donor engineering of a benzothiadiazole-based D-A-D-type molecular semiconductor for perovskite solar cells: a theoretical study

Due to the easy formation of compact molecular packing arrangements and the favorable photophysical and electrochemical properties, donor-acceptor-donor (D-A-D)-type small molecule hole-transporting materials (HTMs) have been widely synthesized and researched to improve the efficiency and stability of perovskite solar cells (PSCs). The main approach in recent experiments has been to seek good acceptors, whereas the influence of the electron-donating units has been less reported. In this work, six new benzothiadiazole-based D-A-D-type HTMs are tailored by employing the ethyl-substituted phenoxazine (POZ), phenothiazine (PTZ) and carbazole (CZ) as the donors. To obtain an elementary understanding of new HTMs, the electronic, optical, hole-transporting and interfacial properties are simulated with quantum chemistry methods. The results indicate that all tailored HTMs exhibit suitable energy alignment compared with the band structures of the perovskite, and the continuous highest occupied molecular orbital (HOMO) levels will be helpful for interfacial energy regulation. In comparison with the YN1, the maximum absorption wavelengths of the newly designed HTMs are red-shifted due to the decreased excitation energies from the ground-state to the first singlet excited-state. Importantly, the hole mobilities of all designed HTMs are distinctly higher than the referenced YN1, which is contributed by the better planarity of the molecular skeleton and the easier orbital overlapping between adjacent molecules. The interfacial simulations manifest that the FAPbI3/SM37 system displays a more stable adsorption configuration and greater charge redistributions at the interface compared to YN1, which further promotes the separation of photogenerated electron-hole pairs. Moreover, larger Stokes shifts and better solubility are also acquired for the new HTMs. In summary, our calculations not only propose several potential highly efficient HTMs, but also provide useful insights at the atomic level for the experimental synthesis of new D-A-D-type HTMs.

Designing Thieno[3,4-c]pyrrole-4,6-dione Core-Based, A2-D-A1-D-A2-Type Acceptor Molecules for Promising Photovoltaic Parameters in Organic Photovoltaic Cells

Nonfullerene-based organic solar cells can be utilized as favorable photovoltaic and optoelectronic devices due to their enhanced life span and efficiency. In this research, seven new molecules were designed to improve the working efficiency of organic solar cells by utilizing a terminal acceptor modification approach. The perceived A2-D-A1-D-A2 configuration-based molecules possess a lower band gap ranging from 1.95 to 2.21 eV compared to the pre-existing reference molecule (RW), which has a band gap of 2.23 eV. The modified molecules also exhibit higher λmax values ranging from 672 to 768 nm in the gaseous and 715-839 nm in solvent phases, respectively, as compared to the (RW) molecule, which has λmax values at 673 and 719 nm in gas and chloroform medium, respectively. The ground state geometries, molecular planarity parameter, and span of deviation from the plane were analyzed to study the planarity of all of the molecules. The natural transition orbitals, the density of state, molecular electrostatic potential, noncovalent interactions, frontier molecular orbitals, and transition density matrix analysis of all studied molecules were executed to validate the optoelectronic properties of these molecules. Improved charge mobilities and dipole moments were observed, as newly designed molecules possessed lower internal reorganization energies. The open circuit voltage (Voc) of W4, W5, W6, and W7 among newly designed molecules was improved as compared to the reference molecule. These results elaborate on the superiority of these novel-designed molecules over the pre-existing (RW) molecule as potential blocks for better organic solar cell applications.

Improving the Conductivity of Amide-Based Small Molecules through Enhanced Molecular Packing and Their Application as Hole Transport Mediators in Perovskite Solar Cells

Organic-inorganic hybrid halide perovskite solar cells (PSCs) have attracted substantial attention from the photovoltaic research community, with the power conversion efficiency (PCE) already exceeding 26%. Current state-of-the-art devices rely on Spiro-OMeTAD as the hole-transporting material (HTM); however, Spiro-OMeTAD is costly due to its complicated synthesis and expensive product purification, while its low conductivity ultimately limits the achievable device efficiency. In this work, we build upon our recently introduced family of low-cost amide-based small molecules and introduce a molecule (termed TPABT) that results in high conductivity values (∼10-5 S cm-1 upon addition of standard ionic additives), outperforming our previous amide-based material (EDOT-Amide-TPA, ∼10-6 S cm-1) while only costing an estimated $5/g. We ascribe the increased optoelectronic properties to favorable molecular packing, as shown by single-crystal X-ray diffraction, which results in close spacing between the triphenylamine blocks. This, in turn, results in a short hole-hopping distance between molecules and therefore good mobility and conductivity. In addition, TPABT exhibits a higher bandgap and is as a result more transparent in the visible range of the solar spectrum, leading to lower parasitic absorption losses than Spiro-OMeTAD, and has increased moisture stability. We applied the molecule in perovskite solar cells and obtained good efficiency values in the ∼15% range. Our approach shows that engineering better molecular packing may be the key to developing high-efficiency, low-cost HTMs for perovskite solar cells.

Diphenylamine-based hole-transporting materials for excessive-overall performance perovskite solar cells: Insights from DFT calculations

Density functional theory calculations were employed to identify the ability of some diphenylamine-based hole-transporting materials (HTMs) for use in top-performance perovskite solar cells. The effects of donor/acceptor electron groups and the new π-bridge section in the three-part of structures were investigated thoroughly. The results indicated that adding electron-withdrawing functional groups such as CN in the phenylazo-indol moiety and substituting electron donor groups such as CH3 in the NH2 hydrogen atoms of the diphenylamine section can cause higher power conversion light-harvesting efficiency in new HTMs. Also, the replacement of thieno [3,2-b] benzothiophene as a part of the π bridge with the phenyl group according to the optical and electronic structure properties improves the efficiency of the new phenylazoindole derivatives.

Theoretical calculations and experimental investigation toward the π-conjugated modulation in arylamine derivative-based hole transporting materials for perovskite solar cells

Excellent hole transporting materials (HTMs) are beneficial to promote the performance of perovskite solar cells (PSCs). Herein, starting from the modulation of the π-conjugated groups of carbazole-diphenylamine derivatives, HTMs CY1 and CY2 were designed and investigated using density functional theory and Marcus theory. Theoretical simulations show that CY1 and CY2 exhibit appropriate HOMO/LUMO energy levels, small recombination energy, good optical properties and molecular stability. Compared with CY1, CY2 with a larger π-conjugated group on its side chain can yield a higher hole mobility and better charge separation. The experimental results confirm that CY2 in PSCs exhibits superior properties such as good hole transporting ability, good film morphology, and efficient charge extraction and dissociation at perovskite/HTM inerfaces. Therefore, a PSC device with CY2 yields a higher efficiency than those of CY1- and Spiro-OMeTAD-based devices. Hence, the results demonstrate that the strategy of the extended π-π conjugation on a side chain is a practicable approach to design potential HTMs for application in PSCs.

Graphene quantum dots (GQD) and edge-functionalized GQDs as hole transport materials in perovskite solar cells for producing renewable energy: a DFT and TD-DFT study

This study investigated the potential suitability of graphene quantum dots (GQD) and certain edge-functionalized GQDs (GQD-3Xs) as hole transport materials (HTMs) in perovskite solar cells (PSCs). The criteria for appropriate HTMs were evaluated, including solubility, hole mobility, light harvesting efficiency (LHE), exciton binding energy (Eb), hole reorganization energy (λh), hole mobility, and HTM performance. It was found that several of the compounds had higher hole mobility than Spiro-OMeTAD, a commonly used HTM in PSCs. The open circuit voltage and fill factor of the suitable GQD and GQD-3Xs were found to be within appropriate ranges for HTM performance in MAPbI3 PSCs. GQD-COOH and GQD-COOCH3 were identified as the most suitable HTMs due to their high solubility, small λh, and appropriate performance.

Molecular Engineering of Anthracene Core-Based Hole-Transporting Materials for Organic and Perovskite Photovoltaics

Anthracene core-based hole-transporting material containing TIPs (triisopropylsilylacetylene) has been spotlighted as potential donors for perovskite solar cells (SCs) due to their appropriate energy levels, efficient hole transport capacity, high stability, and high power conversion efficiency. Herein, we have efficiently designed seven new highly conjugated A-B-D-C-D molecules (AS1-AS7) containing an anthracene core. We used end-capped modifications of donor units with acceptor units on one side and then theoretically characterized them for their appropriate use for SC applications. Modern quantum chemistry techniques have theoretically described the R (reference molecule) and developed (AS1-AS7) molecules. Moreover, the proposed (AS1-AS7) molecules are explored with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) employing B3LYP/6-31G(d,p), and numerous parameters like photovoltaic, optical and electronic characteristics, frontier molecular orbital, excitation, binding and reorganization (λe and λh) energies, open circuit voltage, light harvesting efficiency, transition density matrix, fill factor, and the density of states have been studied. End-capped modification causes a smaller band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), higher UV-vis absorption maxima, tuned energy levels, lower binding and reorganizational (λe and λh) energies, and larger Voc values in proposed (AS1-AS7) molecules than R. AS5 has a remarkable absorption maximum of 495.94 nm and a narrow optimal energy gap (Eg) of 1.46 eV. Furthermore, a complex study of AS5:PC61BM has revealed extraordinary charge shifting at the HOMO (AS5)-LUMO (PC61BM) interface. Our results suggested that newly developed anthracene core-based compounds (AS1-AS7) would be effective candidates with excellent photovoltaic and optoelectronic properties and could be employed in future organic and perovskite SC applications.

Designing and Theoretical Study of Dibenzocarbazole Derivatives Based Hole Transport Materials: Application for Perovskite Solar Cells

Hole-transporting materials (HTMs) are essentials in producing the efficient and stable perovskite solar cells (PSCs). In this article, we provided the investigation results of electronic structures and photophysical characteristics of eight designed derivatives (HTM1a-HTM4a and HTM1b-HTM4b) of a dibenzocarbazole-based compound HTMR. HTMR was modified by substituting the terminal groups located on the diphenylamine moieties with two and four electron donor groups (ED1-ED4) of different character. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been used to optimize the geometry of the ground state and for excited state calculations, respectively. The nature and number of electron donor substitutions on the frontier molecular orbitals (FMOs), ionization potential (IP), electronic affinity (AE), maximum absorption wavelengths ([Formula: see text], solubility ([Formula: see text], stability (η), exciton binding energy ([Formula: see text], reorganization energies ([Formula: see text] and charge mobility (k) are examined and discussed in detail. On this basis, the features such as proper HOMO levels (-5.464 and -4.745 eV), comparable hole mobilities ([Formula: see text] (4.632 × 10¹³ and 1.177 × 10¹⁴ s^-1), a significant [Formula: see text] (367.13 and 398.27 nm), and high η (1.440 and 1.667 eV) have made these structures suitable hole transport materials (HTMs) to provide perovskite solar cells with a high efficiency.

An optimal molecule-matching co-sensitization system for the improvement of photovoltaic performances of DSSCs

Three biphenyl co-sensitizers (4OBA, 8OBA and 12OBA) with different terminal oxyalkyl chains were synthesized and co-sensitized respectively with the main dye (NP-1) in co-sensitized solar cells (co-DSSCs). The effects of the terminal oxyalkyl chains on the photophysical, electrochemical and photovoltaic properties of the co-DSSCs were systematically investigated. The optimal molecular matching relationship between the co-sensitizers and the main dye was obtained through density functional theory (DFT) calculations. Consequently, 4OBA has the most appropriate three-dimensional (3D) molecular structure, which could not only fill the gap between the large-size dyes but also plays a partial shielding role, inhibiting dye aggregation and electron recombination, therefore yielding the highest power conversion efficiency (PCE) for the co-DSSCs with NP-1@4OBA. This study suggests that adjusting the terminal oxyalkyl chains of the co-sensitizers can be used to enhance the intramolecular charge transfer efficiency and inhibit electron recombination, ultimately improving the photovoltaic performances of the co-DSSCs.

Efficient hole transport materials based on naphthyridine core designed for application in perovskite solar photovoltaics

Naphthyridine-based compounds with a donor-acceptor-donor (D-A-D) skeleton were considered as hole transport materials (HTMs) for perovskite solar cells (PSCs). The optical characteristics, stability, solubility, Hirshfeld surface analysis, crystal structure, and hole transport properties of the HTMs were studied systematically. The HOMO energies of all HTMs were higher than valence band of CH₃NH₃PbI₃ (MAPbI₃) perovskite signifying naphthyridine-based HTMs had appropriate energy alignments for usage in PSCs. The LUMO level of designed HTMs were higher than MAPbI₃ conduction band ensuring prevention of backward electronic movement from MAPbI₃ to the cathode. The λ_abs^max amounts of all HTMs were close 400 nm, which showed their competition with perovskite was impossible. The 18NP and 26NP HTMs had higher hole mobilities compared to that of the Spiro-OMeTAD. Considering aligned HOMO energies, suitable hole mobilities, satisfactory stability and solubility, 18NP (1,8-Naphthyridine) and 26NP (2,6-Naphthyridine) were introduced as the best HTM materials for PSCs which could replace Spiro-OMeTAD.

Molecular engineering of several butterfly-shaped hole transport materials containing dibenzo[b,d]thiophene core for perovskite photovoltaics

Several butterfly-shaped materials composed of dibenzo[b,d]thiophene (DBT) and dibenzo-dithiophene (DBT5) cores were designed as hole transporting materials (HTMs) and their properties were studied by density functional theory (DFT) computations for usage in mesoscopic n-i-p perovskite solar cells (PSCs). To choose suitable HTMs, it was displayed that both of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies of molecules were located higher than those of CH₃NH₃PbI₃ (MAPbI₃) perovskite as they were able to transfer holes from the MAPbI₃ toward Ag cathode. Negative solvation energy (ΔE_solvation) values for all HTMs (within the range of - 5.185 to - 18.140 kcal/mol) revealed their high solubility and stability within CH₂Cl₂ solvent. The DBT5-COMe demonstrated the lowest values of band gap (E_g = 3.544) and hardness (η = 1.772 eV) (the greatest chemical activity) and DBT5-CF₃ displayed the biggest η = 1.953 eV (maximum stability) that were predominantly valuable for effective HTMs. All HTMs presented appropriately high LHEs from 0.8793 to 0.9406. In addition, the DBT5 and DBT5-SH depicted the lowest exciton binding energy (E_b) values of 0.881 and 0.880 eV which confirmed they could produce satisfactory results for the PSCs assembled using these materials. The DBT5-SH and DBT5-H had maximum hole mobility (μ_h) values of 6.031 × 10^-2 and 1.140 × 10^-2 which were greater than those measured for the reference DBT5 molecule (μ_h = 3.984 × 10^-4 cm²/V/s) and about 10 and 100 times superior to the calculated and experimental μ_h values for well-known Spiro-OMeTAD. The DBT5-COOH illustrated the biggest open circuit voltage (V_OC), fill factor (FF) and power conversion efficiency (PCE) values of 1.166 eV, 0.896 and 23.707%, respectively, establishing it could be as the best HTM candidate for high performance PSCs.

Rational design of small molecule hole-transporting materials with a linear π-bridge for highly efficient perovskite solar cells

Developing highly efficient small molecule hole-transporting materials (HTMs) to improve the performance of devices is one of the hot topics in the progress of perovskite solar cells (PSCs). In this work, a series of molecules are designed by utilizing benzocyclobutadiene (C1), bicyclo[6.2.0]decapentaene (C2), naphthalene (C3), biphenylene (C4), fluorene (C5), and azulene (C6) as the π-cores, and p-methoxydiphenylamine (R1), p-methoxytriphenylamine (R2) and p-methoxydiphenylamine-substituted carbazole (R3) as the peripheral groups. For isolated molecules, frontier molecule orbitals, absorption and emission spectra, Stokes shift, stability, solubility, and hole mobility are assessed by density functional theory calculations along with the Marcus theory of electron transfer. The molecules adsorbed on the surface of CH₃NH₃PbI₃ are used to simulate the interfacial properties between HTMs and perovskites. Our results indicate that varying the central bridge and the terminal groups has a remarkable influence on the properties. The designed R2-Cn (n = 3, 4, 5) have deeper HOMO levels, stronger absorption in the UV region, and larger Stokes shift than Spiro-OMeTAD. They also possess good solubility, stability, and hole mobility. The proper alignment of interfacial energy levels ensures the transfer of holes from CH₃NH₃PbI₃ to the studied molecules and blocks the backflow of electrons simultaneously. Significant charge redistributions around the interfacial region benefit the separation and transfer photogenerated electron-hole pairs. The results of the present study can be further employed in the process of synthesizing new HTMs with promising features.

Revealing the Sole Impact of Acceptor's Molecular Conformation to Energy Loss and Device Performance of Organic Solar Cells through Positional Isomers

Two new fused-ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m-BAIDIC and p-BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p-BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p-BAIDIC-based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open-circuit voltage (VOC ) and short-current current density (JSC ) of the devices. Therefore, the linear FREA, p-BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m-BAIDIC. When p-BAIDIC is incorporated into the binary PM6/BO-4Cl system to form a ternary system, synergistic enhancements in VOC , JSC , fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.

A π-extended triphenylamine based dopant-free hole-transporting material for perovskite solar cells via heteroatom substitution

The triphenylamine (TPA) group is an important molecular fragment that has been widely used to design efficient hole-transporting materials (HTMs). However, the applicability of triphenylamine derived HTMs that exhibit low hole mobility and conductivity in commercial perovskite solar cells (PSCs) has been limited. To aid in the development of highly desirable TPA-based HTMs, we utilized a combination of density functional theory (DFT) and Marcus electron transfer theory to investigate the effect of heteroatoms, including boron, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, germanium, arsenic, and selenium atoms, on the energy levels, optical properties, hole mobility, and interfacial charge transfer behaviors of a series of HTMs. Our computational results revealed that compared with the commonly referenced OMeTPA-TPA molecule, most heteroatoms lead to deeper energy levels. Furthermore, these heteroatom-based HTMs exhibit improved hole mobility due to their more rigid molecular structures. More significantly, these heteroatoms also enhance the interface interaction in perovskite/HTM systems, resulting in a larger internal electric field. Our work represents a new approach that aids in the understanding and designing of more efficient and better performing HTMs, which we hope can be used as a platform to propel the developmental commercialization of these highly desirable PSCs.

Electronic structure and interfacial features of triphenylamine- and phenothiazine-based hole transport materials for methylammonium lead iodide perovskite solar cells

Recently, great research efforts have been devoted to perovskite solar cells (PSCs) leading to sunlight-to-power conversion efficiencies above 25%. However, several barriers still hinder the full deployment of these devices....

Exploration of conjugated π-bridge units in N,N-bis(4-methoxyphenyl)naphthalen-2-amine derivative-based hole transporting materials for perovskite solar cell applications: a DFT and experimental investigation

Organic small molecules as hole-transporting materials (HTMs) are an important part of perovskite solar cells (PSCs). On basis of the arylamine-based HTM (e.g. H101), two N,N-bis(4-methoxyphenyl)naphthalen-2-amine derivative-based HTMs (CP1 and CP2) with different conjugated π-bridge cores of fused aromatic ring are designed. The CP1 and CP2 were investigated by DFT and TD-DFT in combination with Marcus theory. The calculated results indicate that the designed CP1 and CP2 have better properties with good stability and high hole mobility compared with the parent H101. To validate the computational model for the screening of N,N-bis(4-methoxyphenyl)naphthalen-2-amine derivative-based HTMs, the promising CP1 and CP2 were synthesized and applied to PSC devices. The results show that the experimental data used in this paper can reproduce the theoretical results, such as frontier molecular orbital energies, optical properties and hole mobility, very well. Among them, the results show that the power conversion efficiency (PCE) of the H101-based PSC device is 14.78%, while the CP1-based PSC shows a better PCE of 15.91%, due to its high hole mobility and uniform smooth film morphology, which ultimately promoted a higher fill factor. Finally, this work shows that the computational model is a feasible way to obtain potential N,N-bis(4-methoxyphenyl)naphthalen-2-amine derivative-based HTMs.

Enhancing the Photoelectric Properties of Zinc Porphyrin Dyes by Introducing Five-Membered Heterocyclic Rings into the Electron Donor: A Density Functional Theory and Time-Dependent Density Functional Theory Study

To fabricate highly efficient dye sensitizers for dye-sensitized solar cells, new zinc porphyrin dye sensitizers were designed based on one of the most efficient dyes, YD2-o-C8, by introducing electron-rich heterocyclic rings into the electron donor. Five potentially efficient dyes, Dye1-5, were obtained by replacing the phenyl group of the donor in YD2-o-C8 with pyrrolyl, furyl, and thienyl groups. The electronic structures, absorption spectra, intramolecular charge-transfer characteristics, and excited-state lifetimes of the designed dyes were investigated using the density functional theory and time-dependent density functional theory methods. All the designed dyes exhibit better photoelectric properties than those of YD2-o-C8. Compared with YD2-o-C8, the designed new dyes have smaller frontier molecular orbital energy gaps and obvious red-shifting absorption spectra in the Q band. The analyses of charge density difference plots and intramolecular charge-transfer characteristics indicated that the designed dyes can better promote intramolecular charge transfer and electron-hole separation. Among the five designed dyes, Dye1 with a pyrrolyl group exhibits the best performance. Dye3 and Dye5 with methyl-furyl and methyl-thienyl groups, respectively, exhibit the next best performance. Dye2 and Dye4 with furyl and thienyl groups, respectively, are the worst performers. The introduced methyl group can further improve the electron-donating ability of heterocyclic rings and promote the red shift of the Q bands and intramolecular charge transfer of dyes. The excited-state lifetimes of the new dyes were in the following order: YD2-o-C8 < Dye4 < Dye2 < Dye5 < Dye3 < Dye1, which shows their stronger abilities to inject electrons into semiconductor films.

Designing Organic Electron Transport Materials for Stable and Efficient Performance of Perovskite Solar Cells: A Theoretical Study

In this article, electron transporting layer (ETL) materials are designed to enhance the performance and stability of methyl ammonium lead iodide (MAPbI₃) perovskite solar cells (PSCs). The optical and electronic properties of the designed ETLs are investigated using density functional theory. The designed ETLs show better charge mobility compared to nickel phthalocyanines (NiPcs). The NiPc, a hole transporting layer material, shows ETL-like behavior for PSCs with the substitution of different electron withdrawing groups (X = F, Cl, Br, and I). The stability and electron injection behavior of the designed ETLs are improved. The Br₁₆NiPc shows the highest charge mobility. Further, the stability of the designed ETLs is relatively better compared to NiPc. Due to the hydrophobic nature, the designed ETLs act as a passivation layer for perovskites and prevent the absorber materials from degradation in the presence of moisture and provide extra stability to the PSCs. The effect of designed ETLs on the performance of MAPbI₃ solar cells is also investigated. The PSCs designed with Br₁₆NiPc as an ETL shows a relatively better (23.23%) power conversion efficiency (PCE) compared to a TiO₂-based device (21.55%).

Boron-nitrogen substituted planar cores: designing dopant-free hole-transporting materials for efficient perovskite solar cells

Developing dopant-free hole-transporting materials (HTMs) is very important for improving the stability and increasing the power conversion efficiency of perovskite solar cells (PSCs). Herein, nine boron-nitrogen substituted tetrathienonaphthalene (BN-TTN) derivatives as hole-transporting materials (HTMs) were investigated using theoretical calculations combined with the Marcus theory and the Einstein relation. The results showed that the introduction of a boron-nitrogen group in tetrathienonaphthalene leads to a deep HOMO level, good thermal stability, and enhanced hydrophobicity. Importantly, most BN-TTN molecules possess larger hole mobility due to a broader distribution of the frontier molecular orbitals of the dimer. The BN-TTN core that matches with the size of the perovskite interface also increases the interfacial interaction and hole transfer from the perovskite layer to the HTM layer. The present findings can highlight the potential of BN-TTN core-based HTMs for efficient PSCs.

Designing Hole Transport Materials with High Hole Mobility and Outstanding Interface Properties for Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have attracted much attention due to their rapid increase in power conversion efficiencies (PCEs), and many efforts are devoted to further improving the PCEs. Designing highly efficient hole transport materials (HTMs) for PSCs may be one of the effective ways. Herein we theoretically designed three new HTMs (FDT-N, FDT-O, and FDT-S) by introducing a nitrogen-phenyl group, an oxygen atom, and a sulfur atom into the spiro core of an experimentally synthesized HTM (FDT), respectively. And then we performed quantum chemical calculation to study their application potential. The results show that the devices with FDT-O and FDT-S instead of FDT may have higher open circuit voltages owing to their lower highest occupied molecular orbital (HOMO) energy levels. Moreover, FDT-S exhibits the best hole transport performance among the studied HTMs, which may be due to the significant HOMO-HOMO overlap in the hole hopping path with the largest transfer integral. Furthermore, the results on interface properties indicate that introducing oxygen and sulfur atoms can enhance the MAPbI₃ /HTM interface interaction. The present work not only offers two promising HTMs (FDT-O and FDT-S) for PSCs but also provides theoretical help for subsequent research on HTMs.

Novel star-shaped D-π-D-π-D and (D-π)2-D-(π-D)2 anthracene-based hole transporting materials for perovskite solar cells

Three types of novel star-shaped molecular architectures, D-π-D-π-D and (D-π)₂-D-(π-D)₂ anthracene (ANTTPA, AOME, AOHE) based hole transporting materials, are designed for hybrid perovskite solar cells using the Gaussian 09 computation program with the B3LYP/6-31g (d, p) basis set level. The HOMO energy level of the designed materials has a higher HOMO energy level compared to the perovskite HOMO energy level, which is more facile for hole transport from the hole transporting layer to the oxidized perovskite layer. Thereafter, anthracene-based derivatives were synthesized from Buchwald-Hartwig and Mizoroki-Heck cross coupling reactions. The behaviors of the transporting charges were determined by both UV-visible absorbance and emission spectroscopy through solvatochromism experiments. Furthermore, the electrochemical properties also proved that the synthesized compounds had an optimal HOMO energy level in the TiO₂/perovskite/HTM interface. Our hole transport materials (HTMs) have a good film formation compared to the spiro-OMeTAD, which was confirmed from scanning electron microscopy images. The obtained theoretical and experimental data show the suitability of designing anthracene-based derivatives with the potential to be used as hole transporting materials in organic-inorganic hybrid perovskite solar cells.

Asymmetric Benzotrithiophene-Based Hole Transporting Materials Provide High-Efficiency Perovskite Solar Cells

In this study, we synthesized a series of small-molecule benzotrithiophenes (BTTs) and used them as hole transporting materials (HTMs) in perovskite solar cells (PSCs). The asymmetric benzo[2,1-b:-3,4-b':5,6-b″]trithiophene unit was used as the central core to which were appended various donor groups, namely, carbazole (BTT-CB), thieno thiophene (BTT-FT), triphenylamine (BTT-TPA), and bithiophene (BTT-TT). The extended aromatic core in the asymmetric BTT provided full planarity, thereby favoring intermolecular π-stacking and charge transport. The physical, optical, and electrical properties of these small-molecule HTMs are reported herein. BTT-TT displayed good crystallinity and superior hole mobility, when compared with those of the other three HTMs, and formed smooth and uniform surfaces when covering the perovskite active layer. Accordingly, among the devices prepared in this study, a PSC incorporating BTT-TT as the HTM achieved the highest power conversion efficiency (18.58%). Moreover, this BTT-TT-containing device exhibited good stability after storage for more than 700 h. Thus, asymmetric BTTs are promising candidate materials for use as small-molecule HTMs in PSCs.

Positional Effect of the Triphenylamine Group on the Optical and Charge-Transfer Properties of Thiophene-Based Hole-Transporting Materials

Hybrid organic-inorganic perovskite solar cells (PSCs) have shown significant potential for use in the energy field. Typically, hole-transporting materials (HTMs) play an important role in affecting the power conversion efficiency (PCE) of PSCs. A deep understanding of the structure-property relationship plays a vital role in developing efficient HTMs. Herein, the relationship between the structure and properties of two small organic HTMs H2,5 and H3,4 were systematically investigated in terms of the electronic and optical properties, the hole-transporting behavior by using density functional theory (DFT) and Marcus electron transfer theory. The results demonstrated that the high power conversion efficiency of the H2,5-based PSC was caused by strong interactions with the perovskite material on the interface and an enhanced hole mobility in H2,5 compared with H3,4. The strong interaction derives from the short bond length of O atom of HTM and Pb atom of perovskite material, and the highly hole mobility derives from the quasi-planar conjugated conformation and tight packing model of neighboring molecules in H2,5. In addition, we found that the planar structure enhances the intermolecular interaction between HTM and perovskite materials compared with the 'V'-shaped molecule. Importantly, we also note that the HOMO level of the isolated molecule is not always proportional to the open-circuit voltages of PSCs since the HOMO level might move toward a higher level when the interaction between HTM and interface of perovskite was included. The work gives essential information for rational designing efficient HTMs.

Boosting the performance of D–A–D type hole-transporting materials for perovskite solar cells via tuning the acceptor group

Phenanthrothiadiazole (PT) and triphenylenobisthiadiazole (TBT) are proposed as the acceptor groups of D–A–D-type HTMs, and compared with the benzothiadiazole (BT) unit, three small molecule HTMs are investigated theoretically.

Tailoring of the core structure towards promising small molecule hole-transporting materials for perovskite solar cells: a theoretical study

The influence of the core on the performance of small molecule HTMs is investigated by using quantum chemistry methods, and potential HTMs are proposed.

On the relations between backbone thiophene functionalization and charge carrier mobility of A–D–A type small molecules

Backbone thiophene functionalization: an efficient way to improve the charge carrier mobility of A–D–A type small molecules.

Rational design of D-π-A organic dyes to prevent "trade off" effect in dye-sensitized solar cells

It is an easy task to simulate the spectrum properties for the organic dyes applied in dye-sensitized solar cells (DSSCs) if the suitable method is chosen. However, it is still difficult to quantitatively determine the overall performance for them. In this work, the short-circuit photocurrent density (J_SC) and open circuit photovoltage (V_OC) are quantitatively calculated by combination of the density functional theory and first principle for DSSCs based on four different organic dyes, 2-((4'-((4-(bis(4-methoxyphenyl)amino)phenyl)diazenyl)biphenyl-4-yl)methylene)but-3-ynoic acid (1), 2-((5-(4-((4-(bis(4-methoxyphenyl)amino)phenyl)diazenyl)phenyl)thiophen-2-yl)methylene)but-3-ynoic acid (2), 3-(7-(4-((4-(bis(4-methoxyphenyl)amino)phenyl)diazenyl)-4H-cyclopenta[2,1-b:3,4-b']-dithiophene)-2-cyanoacrylic acid (3), and 3-(7-(4-((4-(bis(4-methoxyphenyl)amino)phenyl)diazenyl)phenyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-cyanoacrylic acid (4), in which the triarylamine is donor and the cyanoacrylic acid is acceptor along with variable π group. The 3 and 4 are new theoretically designed organic dyes on the basis of 1 and 2 with different electron-rich group as π group. Both J_SC and V_OC of 3 and 4 are improved as compared with those of 1 and 2, which breaks the normal "trade-off" rule. As a result, the power conversion efficiency (PCE) of 3 and 4 is improved, especially for 3. The aggregation effect is also considered to evaluate the overall performance, which is favorable to further enhance the reliability of theoretical design.

Optimizing electron-rich arylamine derivatives in thiophene-fused derivatives as π bridge-based hole transporting materials for perovskite solar cells

Based on the observations of thienothiophene derivatives as π-bridged small molecule hole transporting materials (HTMs), adjusting their electron-rich arylamine derivatives is an effective approach to obtain the alternative HTMs for perovskite solar cells (PSCs). In this work, starting from a new electron-rich arylamine derivative and different π-bridged units of thienothiophene derivatives, a series of arylamine derivative-based HTMs were designed, and their properties were investigated using density functional theory combined with the Marcus charge transfer theory. Compared with the parental Z26 material, the designed H01-H04 exhibit appropriate frontier molecular orbitals, good optical properties, better solubility, good stability and higher hole mobilities. H01-H04 materials with high hole mobility (∼× 10-2) can serve as promising HTMs for improving the efficiency of PSCs. The results confirm that the design strategy of adjusting the electron-rich arylamine derivatives in thienothiophene derivatives as π-bridged HTMs is a reliable approach to obtain the promising HTMs for PSC applications.

Probing effects of molecular conformation on the electronic and charge transport properties in two- and three-dimensional small molecule hole-transporting materials: a theoretical investigation

Thiophene/benzene-fused π-conjugated systems are normally employed as the core units of two- and three-dimensionally expanded small molecule hole-transporting materials (HTMs) to improve their electronic and charge transport properties, whereas comparison studies between two-dimensional and three-dimensional core conformations are less reported. To further find useful clues for the design of highly-efficient small molecule HTMs and to find new core units, in this work, four HTM molecules are designed by employing triphenylene, benzotrithiophene, triptycene, and thiophenetriptycene as the core units, and simulated with density functional theory combined with the Marcus hopping model. Our results show that all the considered HTMs display appropriate molecular energy levels, less optical absorption in the visible light region and large Stokes shifts, and high hole mobilities (9.80 × 10-2 cm2 V-1 s-1). Compared with the two-dimensional core structures, the three-dimensional cores exhibit evident superiorities with the same chemical components. Meanwhile, we also find that the quasi-degenerate HOMO energy levels will be helpful to enlarge the transfer integrals between adjacent molecules, and further to promote the hole transport in HTMs. By considering the various elements simultaneously, these investigated HTMs (S-1-S-4) with thiophene- and benzene-fused cores can be expected as potential promising candidates to help create more efficient solar cells.

Exploring the electrochemical properties of hole transporting materials from first-principles calculations: an efficient strategy to improve the performance of perovskite solar cells

Perovskite solar cells (PSCs) have been achieved with impressively dynamic improvement in power conversion efficiency (PCE), becoming the hottest topic in photovoltaics. One of the hot topics is to develop inexpensive and efficient hole transporting materials (HTMs). In the present work, we systematically investigated the impact of different atoms in the heteromerous structure on the performance of perovskite solar cells. In addition, the influence of the structural modification of the HTM molecular building blocks was also revealed. To further understand the relationship between the charge-transport properties and the structural modification, the electronic properties, reorganization energy, and hole transporting properties of a series of organic hole transporting materials were investigated using first-principles calculations combined with Marcus theory. Moreover, the orientation function μ_Φ (V, λ, r, θ, γ; Φ) was applied to quantitatively evaluate the overall carrier mobility of HTMs in PSCs. It is revealed that this model predicts the hole mobility of HTMs correctly. The calculated results indicate that hole transporting materials with heteroatoms and larger dimensional structures show higher hole mobility, which may significantly improve the photovoltaic performance of PSCs.

Improving the electron transport performance by changing side chains in sulfur-containing azaacenes: a combined theoretical investigation on free molecules and an adsorption system

The properties of designed sulfur-containing azaacene electron transport materials by changing side chains are theoretically investigated.

How to design more efficient hole-transporting materials for perovskite solar cells? Rational tailoring of the triphenylamine-based electron donor

Designed with a symmetrical naphthatetrathiophene (NTT) core and triphenylamine (TPA)-based side arms, a series of novel organic small molecule hole-transporting materials are simulated for perovskite solar cells (PSCs) using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. As a fundamental understanding, the energy level alignments and the charge transport behavior are explored for their potential applications. Our results show that, adding an oxygen-bridge between the neighboring phenyl groups of TPA side arms makes the highest occupied molecular orbital (HOMO) levels up-shift, whereas the carbon-carbon single bond stabilizes the HOMOs by about 0.3-0.4 eV. By structural tailoring of the TPA side arms, the HOMO levels of newly designed molecules range from -5.08 eV to -5.61 eV, which provides more possibilities for the interfacial energy regulation. Meanwhile, our results also indicate that the quasi-planar molecular architecture and the delocalized frontier molecular orbitals can effectively enhance the electronic coupling between adjacent molecules. In addition, the reorganization energies are distinctly lowered in the cases of the mixed carbon-carbon bond and oxygen-bridge, and the double oxygen-bridge models. As a result, these molecules with the additional carbon-carbon bond and oxygen-bridge exhibit high hole mobilities. Several promising candidates are proposed toward more efficient PSCs, and more importantly, this work offers some new insights for the design of organic small molecule materials.