Density functional theory study on organic semiconductor for field effect transistors: Symmetrical and unsymmetrical porphyrazine derivatives with annulated 1,2,5-thiadiazole and 1,4-diamyloxybenzene moieties

Density functional theory (DFT) calculations were carried out to investigate the organic field effect transistor (OFET) performance of the symmetrical metal-free tetrakis (1,2,5-thiadiazole) porphyrazine (S₄)PzH₂ and tetrakis (1,4-diamyloxybenzene) (A₄)PzH₂ as well as the low-symmetry metal-free porphyrazine with annulated 1,2,5-thiadiazole and 1,4-diamyloxybenzene groups in the ratio 2:2 (cis) and 1:3, that is, (cis-S₂A₂)PzH2 and (SA₃)PzH₂, (S = 1,2,5-thiadiazole ring, A = annulated 1,4-diamyloxy-benzene ring, Pz = porphyrazine) in terms of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy, ionization energy (IE), electron affinity (EA), and their reorganization energy (λ) during the charge-transport process. On the basis of Marcus electron transfer theory, electronic couplings (V) and field effect transistor (FET) properties for the four compounds with known crystal structure have been calculated. The electron transfer mobility (μ₋) is revealed to be 0.056 cm²·V⁻¹·s⁻¹ for (S₄)PzH₂. The hole transfer mobility (μ₊) is 0.075, 0.098, and 8.20 cm²·V⁻¹·s⁻¹ for (cis-S₂A₂)PzH₂, (SA₃)PzH₂, and (A₄)PzH₂, respectively. The present work represents the theoretical effort towards understanding the OFET properties of symmetrical and unsymmetrical porphyrazine derivatives with annulated 1,2,5-thiadiazole and 1,4-diamyloxybenzene. Supported by the National Natural Science Foundation of China (Grant No. 50673051) and Beijing Municipal Commission of Edueation     

Effects of modifications of the heteroatoms on fine-tuning the photophysical proprieties of A-π-D-π-A small molecules for photovoltaic applications

An effective way to enhance the photovoltaic properties of a small molecule is to modify the side groups into donor units. Herein three news small molecules A-π-D-π-A, denoted Dye1-3, have been designed, from experimentally reported one noted (R), by insertion of various heteroatoms (S, O, Se) on the electron-donating benzodithiophene (Donor (D = BDT)) part. From the calculated results, the dihedral angle between BDT and side-chain affects the distribution of density on the ground state, gap energy, and intramolecular charge transfer of Dyes. In particular, the Dye3 compound, with the smaller dihedral angle, shows that the furan groups in the side chain of D, participates in the distribution of density on the ground state and consequently the charge transfer is improved. Additionally, Dye3 has the lower reorganization energy revealing that this material exhibits better charge mobility. Using the Scharber diagram, Dye3-PCBM heterojunction shows a power conversion efficiency of around 7%. Overall, this work suggests that the photovoltaic properties can be affected by the modification of heteroatoms on side groups of donor parts in small molecules.     

对称和不对称的1，2，5-噻二唑-1，4-戊氧苯基取代的四氮杂卟啉化合物有机半导体场效应性质的密度泛函理论研究

在密度泛函理论基础上研究了一系列对称和不对称的1,2,5-噻二唑-1,4-戊氧苯基取代的四氮杂卟啉化合物（s4）PzH2,（A4）PzH2,（cis-S2A2）PzH2和（SA3）PzH2（S=1,2,5-噻二唑-环,A=1,4-戊氧苯基,Pz=四氮杂卟啉）有机半导体场效应性质．分别研究了这一系列化合物的最高占有和最低未占有轨道能量,离子化能,电子亲合能和电荷传导过程中的重组能．在Marcus电子传导理论基础上计算了具有晶体结构的这四种化合物的电子耦合和迁移率．计算结果表明：化合物（S4）PzH2的电子迁移率为0．056cm^2·V^-1·S^-1,其他三种化合物（cis—S2A2）PzH2,（SA3）PzH2和（A4）PzH2的空穴迁移率分别为0．075,0．098和8．20cm^2·V^-1·S^-1．目前的工作是对这一系列1,2,5-噻二唑--1,4-戊氧苯基取代的四氮杂卟啉化合物有机半导体场效应性质的理论研究．     

Toward the rational design of functionalized pentacenes: reduction of the impact of functionalization on the reorganization energy.

For the widely studied high-carrier-mobility species, pentacene, it is found that perfluorination turned it into an n-type organic field-effect transistor (OFET), but doubled the internal reorganization energy (lambda) of the electron/hole hopping. Here, Cl- and N-functionalized pentacenes are designed to test two strategies that may reduce the impact of functionalization on lambda. Calculation results show that the strategies are feasible. Moreover, combined with crystal-structure data of Cl- and N-functionalized aromatic compounds, it is concluded that compared to fluorination, the presence of Cl and N atoms in pentacene not only reduces the influence of functionalization on lambda and affords a larger window for tuning HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies, but also provides the opportunity to promote pi-stacked structures through ClCl and C--HN interactions.     

A theoretical study of carbazole dimers: Does carbazole form an excimer that undermines the performance of organic light emitting diodes?

Carbazole (Cz) dimers in various cofacial conformations, including staggered (Stg), anti, and syn, were explored by means of ab initio calculations at scaled opposite-spin (SOS)-MP2, SOS-CIS(D₀), and additional coupled cluster calculation levels. Similar to other π-conjugated molecules, strong Cz excimers form in the syn conformation in both S₁ and T₁ states, leading to significantly reduced optical excitation energies. Upon excitation, the dimers in the Stg and anti-conformations remain simple excited dimers, exhibiting similar optical energy gaps to those of the monomer. Being far more stable in the ground state, however, the Stg dimer is nearly isoenergetic to the syn dimer in the S₁ state and even more stable in the T₁ state. Given that the intermolecular interactions in the ground state are expected to govern the dimer conformations of Cz-based materials in the solid-state films of organic electronics, our results strongly demonstrate that the electronic excitation of Cz dimers do not necessarily lead to the strong excimer formation unless Cz molecules are forced to be arranged in the syn conformation.     

Charge carrier transport in high purity perylene single crystal studied by time-of-flight measurements and through field effect transistor characteristics

Electronic transport has been studied by measuring the characteristics of field effect transistors using high purity perylene and the results have been compared with those from time-of-flight measurements. The purity of the material has been monitored by carrier trapping time and delayed fluorescence lifetime. Three types of field effect transistors have been studied: (1) thin film transistor, (2) transistor prepared by placing a single crystal flake on a substrate and (3) transistor fabricated on a single crystal by depositing electrodes and insulating layer onto it. Compared to thin film transistors prepared by evaporating perylene onto a SiO₂/Si substrate, higher mobility values were obtained with transistors using single crystals, but the electrical characteristics of the transistors were far from ideal: large threshold gate voltage observed in the second class of FETs indicated that a high density of traps are present at the interface between the organics and the insulator. A transistor of the third class showed that it functioned indeed as a FET with a reasonably high mobility, but the operation was not stable enough to allow reliable measurements. Much remains to be improved in the design and construction of a perylene FET before the potentiality of the material is fully developed. Also, it remains to be explored to what extent the bulk purity and the molecular order at the organics/insulator interface influence the transport of the charge carriers in an organic FET.     

Non-exponentiality in electron transfer kinetics: Static versus dynamic disorder models

Non-exponential electron transfer kinetics in complex systems are often analyzed in terms of a quenched, static disorder model. In this work we present an alternative analysis in terms of a simple dynamic disorder model where the solvent is characterized by highly non-exponential dynamics. We consider both low and high barrier reactions. For the former, the main result is a simple analytical expression for the survival probability of the reactant. In this case, electron transfer, in the long time, is controlled by the solvent polarization relaxation—in agreement with the analyses of Rips and Jortner and of Nadler and Marcus. The short time dynamics is also non-exponential, but for different reasons. The high barrier reactions, on the other hand, show an interesting dynamic dependence on the electronic coupling element,V _el.