Polycrystalline Pentacene Research Articles

Strong light-matter coupling in pentacene thin films on plasmonic arrays.

Utilizing strong light-matter coupling is an elegant and powerful way to modify the energy landscapes of excited states of organic semiconductors. Consequently, the chemical and photophysical properties of these organic semiconductors can be influenced without the need of chemical modification but simply by implementing them in optical microcavities. This has so far mostly been shown in Fabry-Pérot cavities and with organic single crystals or diluted molecules in a host matrix. Here, we demonstrate strong, simultaneous coupling of the two Davydov transitions in polycrystalline pentacene thin films to surface lattice resonances supported by open cavities made of silver nanoparticle arrays. Such thin films are more easily fabricated and, together with the open architecture, more suitable for device applications.

Trap-Filling Magnetoconductance as an Initialization and Readout Mechanism of Triplet Exciton Spins.

Photoexcited triplet states are promising candidates for hybrid qubit systems, as they can be used as a controlling gate for nuclear spins. But microwave readout schemes do not generally offer the sensitivity needed to approach the single-molecule limit or the scope to integrate such systems into devices. Here, we demonstrate the possibility of electrical readout of triplet spins at room temperature through a specific mechanism of magnetoconductance (MC) in polycrystalline pentacene. We show that hole-only pentacene devices exhibit a positive photoinduced MC response that is consistent with a trap-filling mechanism. Spin and magnetic-field-dependent quenching of photogenerated triplets by holes quantitatively explains the MC response we observe. These results are distinct in both sign and proposed mechanism compared to previous reports on polyacene materials and provide clear design rules for future spintronic devices based on this spin-sensing mechanism.

Directly probing the charge transport in initial molecular layers of organic polycrystalline field effect transistors

We report the charge transport in pentacene polycrystalline organic thin film transistors (OTFTs) with different active layer thicknesses, ranging from a sub-monolayer, bilayer, to tens of nanometers by employing a novel electrode-contact architecture.

A study on the effect of film crystallinity and morphology on charge carrier concentration-dependent hole mobility in pentacene thin-film transistors: advantages of high deposition rate

The combined effect of deposition rate and substrate temperature on the film crystallinity, morphology, and electronic properties of pentacene is studied. It is shown that the channel mobility in polycrystalline pentacene thin-film transistors is relatively immune to substrate temperature, and the films offer good hole mobility when grown at a high rate. This is advantageous when high throughput with low deviation in electrical parameters over devices are required. The surface morphology is characterized by atomic force microscopy measurements and the crystallinity is studied using x-ray diffraction. The effect of growth parameters on the crystalline phases of pentacene is correlated to the charge carrier transport. It is found that the field-effect mobility is primarily affected by the crystalline phases of the film rather than the grain size. The charge carrier dependence of the hole mobility is used to parameterize the dispersion (width) in the density of states (DOS) of the highest occupied molecular orbital of the films in the transistor channel region. It is found that the presence of multiple phases in the path of the charge carrier flow increases the dispersion of the DOS.

Ultrafast Dynamics of Long-Lived Bound Triplet Pair Generated by Singlet Fission in 6,13-Bis(triisopropylsilylethynyl) Pentacene

Singlet fission (SF) converts a singlet exciton into two triplet excitons and holds great potential in the photovoltaic field. The triplet pair state is proposed to play an important role in SF. However, the existence and character of the triplet pair state are still ambiguous to date. In this study, we directly observed the ultrafast formation of the long-lived bound triplet pair state in 6,13-bis(triisopropylsilylethynyl) pentacene polycrystalline film using sub-10-fs broadband transient absorption spectroscopy. The triplet pair state has strong absorption in the near-infrared region and shows distinct dynamic characteristics. It shows faster formation and relaxation compared to free triplet states. Diffusion-limited triplet–triplet annihilation was proved to be the major relaxation channel for both free triplets and bound triplet pairs. Moreover, the larger constraint coefficient obtained from triplet–triplet annihilation of the bound triplet pair state reveals the inherent coupling nature between its two constituent triplets.

Slow charge transfer from pentacene triplet states at the Marcus optimum.

Singlet fission promises to surpass the Shockley-Queisser limit for single-junction solar cell efficiency through the production of two electron-hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~107 s-1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.

Two temperature regimes of triplet transfer in the dissociation of the correlated triplet pair after singlet fission

The ability to undergo spin-allowed exciton multiplication makes singlet fission materials promising for photovoltaic applications. Here, we examine the separation of correlated triplet pairs, 1(T…T), in polycrystalline pentacene films via temperature-dependent transient absorption spectroscopy. Single wavelength analysis reveals a profound delay in 1(T…T) dynamics. Moreover, the dynamics of 1(T…T) exhibit temperature dependence, whereas other features show no discernable temperature dependence. Previous literatures have suggested that correlated triplet separation is mediated by a thermally activated hopping process. Surprisingly, we found that the time constants governing triplet pair separation display two distinct temperature-dependent regimes of triplet transport. The high temperature regime follows a thermally activated hopping mechanism. The experimentally derived reorganization energy and electronic coupling is verified by density matrix renormalization group quantum chemical calculations. In addition, we evaluated the low temperature regime and show that the trend can be modelled by a Miller–Abrahams-type model that incorporates the effects of energetic disorder. We conclude that the correlated triplet pair separation is mediated by thermally activated hopping or a disorder driven Miller–Abrahams-type mechanism at high and low temperature, respectively. We observe that crossover between two regimes occurs ∼226 K. We find the time constant for triplet–triplet energy transfer to be 1.8 ps at ambient temperature and 21 ps at 77 K.

SuPR-NaP Technique for Printing Ultrafine Silver Electrodes and its Use for Low-Voltage Operation of Organic Thin-Film Transistors

ABSTRACTRecently, an epoch-making printing technology called “SuPR-NaP (Surface Photo-Reactive Nanometal Printing)” that allows easy, high-speed, and large-area manufacturing of ultrafine silver wiring patterns has been developed. Here we demonstrate low-voltage operation of organic thin-film transistors (OTFTs) composed of printed source/drain electrodes that are produced by the SuPR-NaP technique. We utilize an ultrathin layer of perfluoropolymer, Cytop, that functions not only as a base layer for producing patterned reactive surface in the SuPR-NaP technique but also as an ultrathin gate dielectric layer of OTFTs. By the use of 22 nm-thick Cytop gate dielectric layer, we successfully operate polycrystalline pentacene OTFTs below 2 V with negligible hysteresis. We also observe the improvement of carrier injection by the surface modification of printed silver electrodes. We discuss that the SuPR-NaP technique allows the production of high-capacitance gate dielectric layers as well as high-resolution printed silver electrodes, which provides promising bases for producing practical active-matrix OTFT backplanes.

Microscopic gate-modulation imaging of charge and field distribution in polycrystalline organic transistors

In this work, a high-resolution microscopic gate-modulation imaging (μ-GMI) technique is successfully developed to visualize inhomogeneous charge and electric field distributions in operating organic thin-film transistors (TFTs). We conduct highly sensitive and diffraction-limit gate-modulation sensing for acquiring difference images of semiconducting channels between at gate-on and gate-off states that are biased at an alternate frequency of 15 Hz. As a result, we observe unexpectedly inhomogeneous distribution of positive and negative local gate-modulation (GM) signals at a probe photon energy of 1.85 eV in polycrystalline pentacene TFTs. Spectroscopic analyses based on a series of μ-GMI at various photon energies reveal that two distinct effects appear, simultaneously, within the polycrystalline pentacene channel layers: Negative GM signals at 1.85 eV originate from the second-derivative-like GM spectrum which is caused by the effect of charge accumulation, whereas positive GM signals originate from the first-derivative-like GM spectrum caused by the effect of leaked gate fields. Comparisons with polycrystalline morphologies indicate that grain centers are predominated by areas with high leaked gate fields due to the low charge density, whereas grain edges are predominantly high-charge-density areas with a certain spatial extension as associated with the concentrated carrier traps. Consequently, it is reasonably understood that larger grains lead to higher device mobility, but with greater inhomogeneity in charge distribution. These findings provide a clue to understand and improve device characteristics of polycrystalline TFTs.

Low-voltage operation of organic thin-film transistors based on ultrafine printed silver electrodes

Abstract We report the low-voltage operation of organic thin-film transistors (OTFTs) based on high-resolution printed source/drain electrodes that are produced by a surface photoreactive nanometal printing (SuPR-NaP) technique. We utilized an ultrathin layer of perfluoropolymer, Cytop, that functions not only as a gate dielectric layer in the OTFTs but also as a base layer for producing a patterned reactive surface for silver nanoparticle chemisorption in the SuPR-NaP technique. We successfully demonstrate 2 V operation with negligible hysteresis in the polycrystalline pentacene OTFT with a gate dielectric thickness of 22 nm, and we achieved current amplification by the printed electrodes modified with pentafluorobenzenethiol. The SuPR-NaP technique enables the production of high-resolution printed silver electrodes required for high-performance OTFTs, which have potential practical electronic device applications.

Angular-Dependent EDMR Linewidth for Spin-Dependent Space-Charge-Limited Conduction in a Polycrystalline Pentacene

Spin-dependent space charge limited carrier conduction in a Schottky barrier diode using polycrystalline \emph{p}-type $\pi$-conjugated molecular pentacene is explored using multiple-frequency electrically detected magnetic resonance (EDMR) spectroscopy with a variable-angle configuration. The measured EDMR spectra are decomposed into two components derived respectively from mobile and trapped positive polarons. The linewidth of the EDMR signal for the trapped polarons increases with increasing resonance magnetic field for an in-plane configuration where the normal vector of the device substrate is perpendicular to the resonance magnetic field, while it is independent of the field for an out-of-plane configuration. This difference is consistent with the pentacene arrangement on the device substrate, where pentacene molecules exhibit a uniaxial orientation on the out-of-substrate plane. By contrast, the mobile polarons do not show anisotropic behavior with respect to the resonance magnetic field, indicating that the anisotropic effect is averaged out owing to carrier motion. These results suggest that the orientational arrangements of polycrystalline pentacene molecules in a nano thin film play a crucial role in spin-dependent electrical conduction.

Crystallinity of the epitaxial heterojunction of C60 on single crystal pentacene

The structure of pn heterojunctions is an important subject in the field of organic semiconductor devices. In this work, the crystallinity of an epitaxial pn heterojunction of C60 on single crystal pentacene is investigated by non-contact mode atomic force microscopy and high-resolution grazing incidence x-ray diffraction. Analysis shows that the C60 molecules assemble into grains consisting of single crystallites on the pentacene single crystal surface. The in-plane mean crystallite size exceeds 0.1µm, which is at least five time larger than the size of crystallites deposited onto polycrystalline pentacene thin films grown on SiO2. The results indicate that improvement in the crystal quality of the underlying molecular substrate leads to drastic promotion of the crystallinity at the organic semiconductor heterojunction.

Improved Electrical Properties of Pentacene MIS Capacitor With OTS Modified Ta2O5Dielectric

Ta2O5 films of 200 nm thick with and without octadecyltrichlorosilane (OTS) surface modification were prepared and used as gate dielectrics to fabricate pentacene metal–insulator–semiconductor (MIS) capacitors. The dielectric constants of Ta2O5 film and OTS-treated Ta2O5 film were determined to be 20.6 and 19.3 at 1 MHz, and both the films exhibit good permittivity stability and low leakage characteristics with the bias voltage sweeping from −10 to 10 V. Atomic force microscopy and Hall measurements show that a pentacene polycrystalline film deposited on OTS-treated Ta2O5 has a high grain packing density with an improved grain contact, and thus a high hole mobility of 3.5 cm2/Vs. Capacitance–voltage and conductance–voltage analysis of pentacene MIS capacitors demonstrate that OTS modification induces a positive shift of flatband voltage from 3.1 to 3.8 V, and effectively minimizes the interface state density of pentacene/dielectric from $5.1\times 10^{12}$ cm $^{-2}$ eV $^{-1}$ to $9.6\times 10^{10}$ cm $^{-2}$ eV $^{-1}$ . The results indicate that high- ${k}$ Ta2O5 film with OTS modification would be a promising gate dielectric for pentacene thin-film transistors in low-end electronics.

CdS Thin Film Transistor for Inverter and Operational Amplifier Circuit Applications

Organic and inorganic thin film transistors (TFTs) are fabricated, simulated, and tested for circuit applications. The devices are based on polycrystalline cadmium sulfide (CdS) and pentacene thin films. Both devices are simulated and analyzed using two-dimensional finite element simulation methodology. For CdS, grain boundary and uniform trap distribution approaches are implemented. It is assumed that traps due to grain boundaries are uniformly distributed throughout the film. For pentacene a Poole–Frenkel mobility model is employed. After matching device simulation with experimental data, the SPICE model parameters are extracted for circuit simulation. The devices are tested for both analog and digital circuits. operational amplifier (OPAMP) and inverter circuits are tested. Two OPAMP topologies are compared and the results are discussed. One of the topologies is based on all n-type transistors using CdS TFT and the other topology is the two-stage Miller compensated CMOS design based on CdS and pentacene TFTs. The operational amplifiers have an open-loop voltage gain of 25.9dB and 29.7dB respectively. The performance of the CMOS design is found to be limited by performance of pentacene transistor.

FABRICATION OF LATERAL ORGANIC SPIN VALVES BASED ON La0.7Sr0.3MnO3 ELECTRODES

We report the successful fabrication of lateral organic spin valves (OSVs) using polycrystalline pentacene as spacer and half-metallic La 0.7 Sr 0.3 MnO 3 (LSMO) as two electrodes. The distance between two electrodes ranges from 30 nm to 100 nm. The current–voltage characteristics follow the power law relation, which are attributed to the space charge limited current behavior. The devices with a spacing of 30 nm exhibits clear spin-valve characteristics with a magnetoresistance (MR) ratio of ~ 2% at 9 K. The MR effects disappear for electrode spacing about 100 nm, suggesting that the spin diffusion length is less than 100 nm.

Triplet Dynamics in Pentacene Crystals: Applications to Fission‐Sensitized Photovoltaics

The decay and transport of triplet excitons photogenerated via singlet exciton fission in polycrystalline and single-crystalline pentacene is reported. Using transient absorption spectroscopy, we find evidence for diffusion-mediated triplet-triplet annihilation. We estimate monomolecular lifetimes, bimolecular annihilation rate constants, and triplet exciton diffusion lengths. We discuss these results in the context of current solar cell device architectures.

Influence of grain size at first monolayer on bias-stress effect in pentacene-based thin film transistors

Threshold voltage shift under applied gate voltage is a key factor characterizing stability of organic thin-film transistors (OTFTs), while the physical mechanism is still controversial. In this study, we systematically examined the initial growth of pentacene polycrystalline films under different growth rates. Bias stress performance of the fabricated pentacene-based OTFTs was found to be highly related to the initial gain size of the pentacene films. Larger grain size at the first deposition layer led to smaller threshold voltage shift. The quantitative correlation can be described by a two-dimensional microscopic mobility model relating to the grains and grain boundaries.

Triplet diffusion in singlet exciton fission sensitized pentacene solar cells

Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.

Singlet Exciton Fission in Polycrystalline Pentacene: From Photophysics toward Devices

Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit.

The effect of structural order on solar cell parameters, as illustrated in a SiC-organic junction model

To understand the title topic a model system of single crystal SiC, modified with an interfacial molecular monolayer of alkyl siloxane molecules, with polycrystalline pentacene deposited on it, was fabricated. In this way a change in the length of the alkyl chain could change the structural order of the pentacene film by changing the surface's hydrophobicity, while no significant variation was found in the surface potential, and, thus, in the surface dipole. The pentacene film grown on top of the monolayers showed, with increasing alkyl chain length, increased lateral order and decreased band gap state density, as observed by X-ray diffraction and surface photovoltage spectroscopy. The Voc, Jsc and fill factor of solar cells, made with these material combinations, improved with increasing alkyl chain length. We explain this as a result of increased 2D film growth with increasing alkyl chain length of the monolayer, as the surface becomes more hydrophobic, which increases ordering of the pentacene film. Thus, this model system illustrates the role of ordering in charge separation and recombination.