The relationship between intermolecular interactions and charge transport properties of trifluoromethylated polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) with the electron-withdrawing groups such as halogen atom, cyanide, perfluoroalkyl (PFA), or perfluoroary, etc. exhibit good air stability and better solid-state charge carrier mobility. To obtain a better understanding of structure property relationships of this kind of compounds, a series PAH(CF₃)_n derivatives a1, a2, b1, b2, c1, and c2, which contain different numbers of trifluoromethyls and benzene rings, were chosen and studied by both band-like model and hopping model. Their crystals contain different intermolecular interactions. It turns out that intermolecular hydrogen bonding interactions are mainly responsible for electron transport, while π-stacking interactions dominate hole transport. When the π-stacking and intermolecular hydrogen bonding interactions coexist in the same direction, a competitive relationship occurs between hole and electron transport, which tend to cause enhancement of electron transport, and restrain hole transport.     

Effect of dihydropyrazine on structures and charge transport properties of N-heteropentacenes matters: A theoretical investigation

A family of N-heteropentacenes acted as promising candidates for organic semiconductor materials is of immense interest. It should be attributed to the following reasons that (1) the positions, numbers and valence-states of N atom in N-heteropentacenes can effectively tune their electronic structure, stability, solubility, and molecular stacking; (2) diverse intermolecular interaction and π-stacking motifs appear in their crystals. The effect of the position and number of the 6-π-pyrazine on their structures and charge-transport properties has been systematically investigated in our previous work (J. Phys. Chem. C 115 (2011) 21416). Therefore, in this work, the study on the role of 8-π-dihydropyrazine with another valence-state N atoms is our focus. Density functional theory, Marcus electron transfer theory and Brownian diffusion assumption coupled with kinetic Monte-Carlo simulation are applied into this investigation. Our theoretical results indicate that in contrast with pyrazine, dihydropyrazine introduced is more helpful for promoting p-type organic semiconductor materials. For molecule 4, hole mobility of its single crystal theoretically reach 0.71 cm² V⁻¹ s⁻¹, and coupled with its fine hole-injection ability, it should be a promising candidate for p-type organic semiconductor materials. Although the lowest triplet-state energies of the molecules studied are very small, introduction of dihydropyrazine is very helpful for increasing the energies.     

Understanding electron-withdrawing substituent effect on structural, electronic and charge transport properties of perylene bisimide derivatives

A series of n-type perylene bisimide (PBI) derivatives with electron-withdrawing substituents at both bay and imide nitrogen positions were investigated. The effects of these substituents on internal energy relaxation, molecular orbitals, air stability, electronic properties and charge transport behaviors were systematically discussed with density functional theory (DFT) which has been demonstrated reliable for organic semiconductor study. The investigated derivatives with electron-withdrawing substituents show favorable performances in terms of these properties. The LUMO levels are greatly stabilized by at least 0.3 eV and these derivatives show the strong absorption from 400 to 700 nm which match with the solar spectra very well. The charge transport mainly happens between molecules in the same organic molecular layer and electronic couplings between layer-to-layer molecules are very weak, thus the mobility along layer-to-layer direction is less efficient. The variation of molecular packings and intermolecular interactions produce the highly anisotropic mobilities. The derivative with two fluorine atoms at bay positions and –CH₂C₃F₇ at imide group has broad and strong absorption in the UV-Visible region and the electron mobility could get to 0.514 cm² V⁻¹ s⁻¹ which is greatly encouraging for molecular and material design in organic solar-cell devices. These calculated results are in good agreement with the experimental data. However, not all the functionalization with halogen substituents would induce the increase of the electronic coupling and electron mobility. The derivatives with four halogen substituents at the bay positions could not show advantages in terms of electron mobility which is induced by the distorted conjugated structures. The theoretical understanding of these n-type organic semiconductors figures out the importance of tuning the molecular geometry to get high performance semiconductor materials.     

Charge transport and optical properties of cross-conjugated organic molecules: A theoretical study

The structure, charge transport and optical properties of two-dimensional cruciform molecules 1,4-distyryl-2,5-bis(arylethynyl)benzenes, 1,4-distyryl-2,5-bis(arylethynyl)pyridines and 1,4-bis(ethylenediyl)dipyridine-2,5-bis(benzene) have been studied using density functional theory methods. The effective charge transfer integral and site energy corresponding to hole and electron transports in the above molecules were calculated directly from the matrix elements of Kohn–Sham Hamiltonian. The charge carrier mobility on the cruciform molecules was calculated using Monte Carlo simulation based on the Marcus rate theory and the results show that these cruciform molecules are the p-type organic semiconductors. The excited state calculations were performed using the time-dependent density functional theory method in gas phase, hexane and dichloromethane mediums. The trifluoromethyl substituted 1,4-distyryl-2,5-bis(arylethynyl)benzenes molecule possesses the maximum absorption and emission wavelength of 443 and 504 nm, respectively in dichloromethane medium.     

Effects of disorder on charge transport in semico++ nducting polymers

Polaron transport process in semiconducting polymers with disordered structure and morphology is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include diagonal disorder, off-diagonal disorder and an external electric field. It is found that the polaron transport mechanism is determined by the competition between the disorder and the electric field. For the case of pure diagonal disorder, the polaron transport undergoes a crossover from adiabatic drift to nonadiabatic hopping under a large scope of electric field (E < 2.0 mV/Å), whereas the field-assisted tunneling dominates charge transport for a higher electric field. For the combined disorder, it is demonstrated that the competition effects and the synergetic effects between the diagonal and off-diagonal disorder are equally important. The effects of diagonal disorder are negative in most cases, and the energetically easier pathways for charge transport are mainly opened by the off-diagonal disorder.     

A comprehensive theoretical study of halide perovskites ABX3

Solar cells based on halide perovskites have recently been attractive due to their excellent power conversion efficiency (PCE), lower cost and simple manufacture. Here, a series of halide perovskites (ABX₃: A = CH₃NH₃, CH(NH₂)₂, Cs, Rb; B = Pb, Sn, Ge; X = I, Br, Cl, F) were investigated by Density Functional Theory (DFT) calculations, together with Shockley-Queisser Maximum Solar Cell Efficiency (S-Q) and Spectroscopic Limited Maximum Efficiency (SLME) mathematical models. The results indicate that: the electronic structure of germanium perovskites bears a close similarity to that of lead perovskites with a small energy difference between the nonbonding orbital and antibonding orbitals, but with a large energy difference comparing with that of tin perovskites (0.6–1.7 eV for CsGeI₃ at Z point of the Brillouin zone, 0.7–1.4 eV for CH₃NH₃PbI₃ and 1.4–2.2 eV for CH₃NH₃SnI₃ at R point of the Brillouin zone), which is attributable to the atomic level, where the 4s orbital energy of Ge (−11.5 eV) is close to the 6s orbital energy of Pb (−11.6 eV), but the 5s orbital energy of Sn (−10.1 eV) is significantly high. Therefore, germanium perovskites possess as high absorption coefficient around solar spectrum as lead perovskites, while tin perovskites only have low absorption coefficient, which makes the short-circuit current of CsGeI₃ and CH₃NH₃PbI₃ (0.017 Acm⁻² and 0.016 Acm⁻², simulated by SLME with a 200 nm absorber under AM1.5G) are higher than that of CH₃NH₃SnI₃ (0.015 Acm⁻²) even if the bandgap of CsGeI₃ and CH₃NH₃PbI₃ (1.51 eV and 1.55 eV) are larger than that of CH₃NH₃SnI₃ (1.21 eV). Meanwhile, the effective mass of electrons and holes are approximate for germanium perovskites and lead perovskites (0.14:0.19 for CsGeI₃ and 0.12:0.12 for CH₃NH₃PbI₃), indicating a balanced electrons and holes transport, whereas the electrons transport is much slower than the holes transport for tin perovskites due to the effective mass of electron is much larger than that of hole (0.17:0.04 for CH₃NH₃SnI₃). As a result, the PCE of CsGeI₃ (27.9%) and CH₃NH₃PbI₃ (26.7%) is higher than that of CH₃NH₃SnI₃ (19.9%).     

Modeling of charge transport across disordered organic heterojunctions

Organic electronic devices often consist of a sandwich structure containing several layers of disordered organic semiconductors. In the modeling of such devices it is essential that the charge transport across the organic heterojunctions is properly described. The presence of energetic disorder and of strong gradients in both the charge density and the electric field at the heterojunction complicates the use of continuum drift-diffusion approaches to calculate the electrical current, because of the discrete positions of the sites involved in the hopping transport of charges. We use the results of three-dimensional Monte Carlo simulations to construct boundary conditions in a one-dimensional continuum drift-diffusion approach that accurately describe the charge transport across the junction. The important effects of both short- and long-range Coulomb interactions at the junction are fully accounted for. The developed approach is expected to have a general validity.     

The role of local potential minima on charge transport in thin organic semiconductor layers

We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time.     

Charge transport in poly(p-phenylene vinylene) at low temperature and high electric field

Charge transport in poly(2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)-based hole-only diodes is investigated at high electric fields and low temperatures using a novel diode architecture. Charge carrier densities that are in the range of those in a field-effect transistor are achieved, bridging the gap in the mobility versus charge carrier density plot between polymer-based light-emitting diodes and field-effect transistors. The extended field range that is accessed allows us to discuss the applicability of current theoretical models of charge transport, using numerical simulations. Finally, within a simple approximation, we extract the hopping length for holes in MEH-PPV directly from the experimental data at high fields, and we derive a value of 1.0 ± 0.1 nm.     

Improving charge transport and suppressing charge recombination in small molecule ternary solar cells via incorporating Bis-PC71BM as a cascade material

The development of small molecule organic solar cells (SMOSCs) has attracted considerable attention and achieved comparable power conversion efficiency (PCE) with polymer solar cells. Here, we demonstrate a bulk heterojunction (BHJ) small molecular solar cell with PCE of 5.31% by incorporating Bisadduct of phenyl-C₇₁-butyric acid methyl ester (Bis-PC₇₁BM) as an additional acceptor material into the host binary blend of 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP): [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM). The short circuit current (J_SC) and the fill factor (FF) of ternary SMOSCs are improved by decreasing the carrier recombination loss, increasing exciton dissociation and enhancing the carrier transport. The transient photovoltage (TPV) measurement indicates that the gradient HOMO energy alignment suppresses the charge recombination and leads to the increased charge carrier lifetime in ternary SMOSCs. As a result, the PCE of ternary devices with 5 wt% Bis-PC₇₁BM is about 20% greater than that of SQ-BP: PC₇₁BM based binary SMOSCs.     

The substituent effect on charge transport property of triisopropylsilylethynyl anthracene derivatives

The charge transport property of two triisopropylsilylethynyl anthracene (TIPSAnt) derivatives TIPSAntBt and TIPSAntNa (bithiophene and naphthalene are introduced at the 2, 6-positions of the TIPSAnt core) were explored through quantum chemical method. To gain a better understanding of the substituent effect on the charge transport property, the results of the parent molecule TIPSAnt was also provided here for comparison. The substituent effect on the molecular geometry, reorganization energy, frontier orbitals, ionization potential (IP) and electronic affinity (EA), crystal property, transfer integrals and charge mobility, band structure and effective mass of the two compounds were investigated to establish the relationship between structures and properties. The introduced bulky TIPS groups made the two compounds adopt two-dimensional, face-to-face, π-stacking structures. The efficient overlaps of π-orbital and smaller π-stacking distance are proved to be the main reason for the high charge mobility of TIPSAntBt and TIPSAntNa. The hole mobilities of TIPSAntBt and TIPSAntNa are 0.88 and 3.60 cm² V⁻¹ s⁻¹, respectively, which is well consistent with experiment values (0.2 and 3.7 cm² V⁻¹ s⁻¹, respectively). For TIPSAntBt, the electron mobility (1.29 cm² V⁻¹ s⁻¹) is a little higher than that of hole due to the more effective transfer integrals of electron. On the contrary, the hole mobility of TIPSAntNa is 20 times larger than that of electron because of the smaller reorganization energy and larger transfer integral of hole, indicating that TIPSAntNa could be used as p-type semiconductor. For TIPSAntBt, the transfer integral is smaller than the reorganization energy, so the hopping mechanism plays a key role in the charge transport property. While the bandwidths and effective mass of TIPSAntNa agreed well with the calculated transfer integrals and charge mobility results. The introduced small substituents to TIPSAnt core contributed to the dramatically different charge transport property from an n-type semiconductor of TIPSAntBt to p-type semiconductor of TIPSAntNa, which shed light on molecular design for an n-type semiconductor through simple chemical structural modification.     

Insight into the charge transport and degradation mechanisms in organic transistors operating at elevated temperatures in air

Operational stability of organic devices at above-room-temperatures in ambient environment is of imminent practical importance. In this report, we have investigated the charge transport and degradation mechanisms in pentacene based organic field effect transistors (OFETs) operating in the temperatures ranging from 25 °C to 150 °C under ambient conditions. The thin film characterizations techniques (X-ray photoelectron spectroscopy, X-ray diffraction and atomic force microscopy) were used to establish the structural and chemical stability of pentacene thin films at temperatures up to 150 °C in ambient conditions. The electrical behavior of OFETs varies differently in different temperature bracket. Mobility, at temperatures below 110 °C, is found to be thermally activated in presence of traps and temperature independent in absence of traps. At temperatures above 110 °C mobility degrades due to polymorphism in pentacene or interfacial properties. The degradation of mobility is compensated with the decrease in threshold voltage at high temperatures and OFETs are operational at temperatures as high as 190 °C. 70 °C has been identified as the optimum temperature of operation for our OFETs where both device behavior and material properties are stable enough to ensure sustainable performance.     

Modulation of ambipolar charge transport characteristics in side-chain polystyrenes as host materials for solution processed OLEDs

A series of side-chain polystyrenes was developed as ambipolar hosts for solution processed organic light emitting diodes (OLEDs). The series was derived from the hole-only transport host molecule 1,3-Bis(N-carbazolyl)benzene (mCP). Electron transport ability was incorporated into the host polymers by the introduction of electron-poor heterocycles (pyridine or triazine) and extending delocalization of the lowest unoccupied molecular orbital (LUMO). The materials were tested in Ir-based green OLED devices with all organic layers processed from solution. Devices with the polymer combining triazine and carbazole on its side-chain exhibited a low luminance on-set voltage of 3.0 V and a current efficacy of 28.9 cd/A, which was more than 10 times higher than for devices with the mCP-based polymer (1.6 cd/A). The increase in performance is most likely due to an improvement of charge balance in the emissive layer, showing that our ambipolar polymers are good candidates for further wet-process optoelectronic applications.     

Solution p-doped fluorescent polymers for enhanced charge transport of hybrid organic-silicon nanowire photovoltaics

The solution p-type doping of various conjugated polymers and small molecules via blending with tetrafluorotetracyano-quinodimethane (F4-TCNQ) is investigated to facilitate hole transport for hybrid organic-silicon-nanowire solar cells. Among all, the highly-fluorescent conjugated polymers such as poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) demonstrate superior transport characteristics, where both the device fill-factor and power conversion efficiency (PCE) are positively correlated with the doping concentration. The best PCE achieves 13.66% from the device with 30% p-doped PFO, compared to the reference at 12.50%. The enhancement is ascribed to the efficient interfacial charge-carrier recombination with minimal energy losses. A vertically graded doping profile is further revealed with F4-TCNQ molecules preferentially accumulated near the silicon surface.     

Transverse charge transport in inkjet printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

Transverse (z) alignment of PEDOT grains was demonstrated in inkjet printed PEDOT:PSS. This explained the superior transverse charge conduction mode in inkjet printed PEDOT:PSS films, best fitted by the Efros-Shklovskii 1D-VRH (variable range hopping) model in this study compared with spin coated PEDOT:PSS films, which have demonstrated layers of generally in-plane aligned PEDOT:PSS grains. The findings of this study, regarding the microstructure of inkjet printed PEDOT:PSS films and their transverse charge transport model, justify measurements of the transverse conductivity of inkjet printed films in this study being 600 times higher than that of spin coated films. In addition, it was found that the addition of 5 wt% DMSO in the printing PEDOT:PSS ink lowers the workfunction by 3% approximately.     

Influence of equilibrium charge reservoir formation on photo generated charge transport in TiO2/organic devices

Charge transport measurements have been performed using the photo induced charge extraction by linearly increasing voltage (photo-CELIV) technique on indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO₂/P3HT:PCBM/Cu) devices. By adjusting the offset voltage such that holes are accumulated at the ITO/TiO₂ contact we obtain space charge limited current (SCLC) extraction in the dark. Using photo-generation the current response is limited by SCLC extraction at low carrier concentrations but becomes purely recombination limited at high photo-generated carrier concentration. A 1-D drift diffusion model has been developed to simulate our results and we show that the hole blocking ITO/TiO₂ contact is responsible for the SCLC behavior. The highly reduced recombination of charges seen in P3HT:PCBM devices is necessary to obtain the large extraction current transients that are seen in the experimental measurements. By comparing the simulated dark CELIV and photo-CELIV we show that photo-generated extraction is more sensitive towards changes in the surface recombination velocity.     

Theoretical study on charge transport properties of cyanovinyl-substituted oligothiophenes

When the oligothiophene is substituted by dicyanovinyl (DCV) or tricyanovinyl (TCV) group, how does its transport property change? Here, we will mainly focus on exploring the influence on charge transport properties of introducing a strong electron-withdrawing DCV/TCV group to the thiophene units within Marcus–Levich–Jortner formalism at the level of density functional theory. The results show that the introduction of cyanovinyl-substituents improves the molecular π-stacking, decreases the frontier molecular orbital energy levels and reorganization energies, and increases the transfer integrals and mobilities, comparing with their parent thiophene molecules. It is interesting to find the phenomenon that enriching intermolecular interactions can be favorable for controlling the transport channel and thus get high mobility, which would be shown by the angular resolution anisotropic mobilities analysis. Besides, the simulated packing motifs of dimers for 3a and 3b without crystal structures reported indicate that their packing may form the slip π–π stacking, and that 3b may be a good ambipolar material. In a word, compared with corresponding thiophene analogues and tetracyanoquinodimethane, these compounds may become the candidates for the n-type or ambipolar organic semiconductor materials.     

Chemical synthesis and charge transport mechanism in solution processed flexible polypyrrole films

Polypyrrole (PPy) has been synthesized by a chemical oxidation method using ammonium persulfate to obtain a solution processable PPy powder. The resultant PPy powder was then solution processed to deposit flexible thin films of PPy over flexible substrates. PPy film samples were then characterized using UV–vis spectroscopy, FTIR spectroscopy and X-ray diffraction. It was found that conductivity of PPy (σ=2.4×10⁻² S/cm) reduces by an order of magnitude after solution processing in the form of films. The temperature dependent conductivity of PPy pellet and flexible films of PPy were measured in the temperature range of 80–300 K. It was observed that PPy films show stronger temperature dependence than pelletized samples. Charge transport in PPy samples has been investigated using Kivelson׳s and Mott׳s variable range hopping models. Mott׳s parameters such as density of states at the Fermi level N(E_F), average hopping distance (R) and average hopping energy (W) have been estimated for PPy samples. The results showed that at room temperature average hopping distance for PPy film was about 22.3 Å and average hopping energy was 128.6 meV.     

Charge transport properties of bulk-heterojunction organic solar cells investigated by displacement current measurement technique

We have investigated charge transport properties of bulk-heterojunction (BH) solar cells in which P3HT (Poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) are used as the active layer, by using the displacement current measurement (DCM) method. In order to investigate the charge transport properties of the BH solar cells, we fabricated a dedicated device that consists of P3HT and PCBM, and used the DCM method to measure the charge distribution of the devices with different composition ratios of P3HT and PCBM. DCM data suggested that a BH film with 50 wt% of PCBM exhibits a preferable charge transport property suited for BH solar cells. We confirmed that the DCM results are consistent with the measured performance of the BH solar cells, indicating that the DCM method is a simple and effective method for optimizing the structure of BH solar cells as well as other electronic devices composed of binary materials.