Charge Modulation Spectroscopy Research Articles

Using Molecular Structure to Tune Intrachain and Interchain Charge Transport in Indacenodithiophene-Based Copolymers.

In this work, we compare two structurally near-amorphous rigid-rod polymers─poly(indacenodithiophene-co-benzothiadiazole), p(IDT-BT), and poly(indacenodithiophene-co-benzopyrollodione), p(IDT-BPD)─with orders of magnitude different mobilities to understand the effect charge carrier intrachain delocalization has on electronic transport. Quantum chemical calculations show that p(IDT-BPD) has a barrier to torsion that is significantly lower than that of p(IDT-BT) and is thus more likely to have reduced conjugation lengths. We utilize absorption and photoluminescence spectroscopy to characterize energetic disorder and show that p(IDT-BPD) has higher energetic disorder. Charge modulation spectroscopy (CMS) and model calculations are used to show that charge carriers are substantially delocalized in p(IDT-BT) and occupy near-uniform energetic environments. We find that mobility activated hopping barriers are similar in these two materials. Electronic structure calculations show that both intrachain and interchain couplings of monomer units are poor enough in p(IDT-BPD) that charge carriers collapse to single IDT units and transport via a through-space tunneling mechanism. This work highlights the remarkable charge transport properties of p(IDT-BT) by showing that high mobilities are achievable on device-relevant length scales with only 1D carrier delocalization.

Charge Transport in Sub‐Monolayer Networks of a Naphthalene‐Diimide‐Based Copolymer

AbstractSub‐monolayer networks of the electron‐transporting semiconducting polymer poly([N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)−2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)) (P(NDI2OD‐T2)) are realized through judicious choice of spin‐coating solvent and control of solution concentration. These sub‐monolayer networks provide a platform to study the effects of surface coverage, network connectivity, and orientational order on charge transport. It is found that charge transport through semiconducting polymer networks depends not only on the coverage/concentration of the material but also on the nature of the conduction paths themselves. Down to 40% surface coverage, current measured in a field‐effect architecture decreases in line with the decrease in surface coverage, but drops precipitously below a surface coverage of 40%. Below 40% surface coverage, there is a marked decrease in network connectivity (quantified through the branching point density) and a decrease in orientational order. Furthermore, an increase in the density of dangling branches (dead ends) is also seen with decreasing surface coverage. Additional supporting data in the form of grazing incidence wide‐angle X‐ray scattering (GIWAXS), optical spectroscopy, and charge modulation spectroscopy are presented, which help to establish that long‐range interconnectivity rather than local morphology dominates charge transport in P(NDI2OD‐T2) sub‐monolayers as well as thin films.

Polaron absorption in aligned conjugated polymer films: breakdown of adiabatic treatments and going beyond the conventional mid-gap state model.

This study provides the first experimental polarized intermolecular and intramolecular optical absorption components of field-induced polarons in regioregular poly(3-hexylthiophene-2,5-diyl), rr-P3HT, a polymer semiconductor. Highly aligned rr-P3HT thin films were prepared by a high temperature shear-alignment process that orients polymer backbones along the shearing direction. rr-P3HT in-plane molecular orientation was measured by electron diffraction, and out-of-plane orientation was measured through series of synchrotron X-ray scattering techniques. Then, with molecular orientation quantified, polarized charge modulation spectroscopy was used to probe mid-IR polaron absorption in the ℏω = 0.075 - 0.75 eV range and unambiguously assign intermolecular and intramolecular optical absorption components of hole polarons in rr-P3HT. This data represents the first experimental quantification of these polarized components and allowed long-standing theoretical predictions to be compared to experimental results. The experimental data is discrepant with predictions of polaron absorption based on an adiabatic framework that works under the Born-Oppenheimer approximation, but the data is entirely consistent with a more recent nonadiabatic treatment of absorption based on a modified Holstein Hamiltonian. This nonadiabatic treatment was used to show that both intermolecular and intramolecular polaron coherence break down at length scales significantly smaller than estimated structural coherence in either direction. This strongly suggests that polaron delocalization is fundamentally limited by energetic disorder in rr-P3HT.

Scattering mechanism of hole carriers in organic molecular semiconductors deduced from analyses of terahertz absorption spectra using Drude–Anderson model

To clarify the limiting factor of carrier transport in organic molecular semiconductors, we performed charge modulation spectroscopy of a field-effect transistor with a 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW) single crystal, which showed a hole-carrier mobility of 8.4 cm2 V−1 s−1 at 295 K. The terahertz absorption of electric-field-induced hole carriers increases with decreasing frequency down to 150 cm−1 (4.5 THz). However, it is not reproduced by the simple Drude model but tends to be suppressed with decreasing frequency. The spectral shape of the absorption and the mobility value were simultaneously reproduced by the Drude–Anderson model, which incorporates carrier scattering due to thermal molecular fluctuations. The frequency of the intermolecular vibration that dominates carrier scattering is estimated to be approximately 8 cm−1, which is in good agreement with the theoretically predicted value. Moreover, analyses of the absorption spectra at low temperatures reveal that the mobility increases to 14 cm2 V−1 s−1 at 240 K. These results demonstrate that thermal molecular fluctuations limit the mobility.

Charge Modulation Spectroscopy of Pristine and sp3-Functionalized Single-Walled Carbon Nanotube Networks

The controlled decoration of single-walled carbon nanotubes (SWCNTs) with luminescent sp3 defects has emerged as a powerful approach to enhance their usually low photoluminescence (PL) quantum yields and achieve tuneable emission across the near-infrared even for polymer-wrapped nanotubes [ACS Nano 2019, 13, 9259]. However, their application in optoelectronic devices requires a detailed understanding of the impact of sp3 functionalization on their electrical performance. While isolated sp3 defects on single SWCNTs increase the resistance by a few kΩ, the effect of functionalization on charge transport in networks of semiconducting SWCNTs remains unexplored. Charge modulation spectroscopy is ideally suited to probe the mobile charge carriers in SWCNT networks and has previously corroborated the notion that charge accumulation and transport in mixed SWCNT networks is highly chirality- and gate voltage-dependent. Here, we use charge-modulated PL (CMPL) spectroscopy to study the current pathways in field-effect transistors based on networks of pristine and functionalized (6,5) SWCNTs. We find that the defects are very efficiently sampled by mobile carriers and consequently, sp3-functionalized SWCNTs actively participate in the charge transport in these networks. In combination with voltage-dependent PL and electroluminescence measurements, our CMPL method provides fundamental insights into the interaction of charge carriers with sp3 defects in SWCNT thin film devices.Fig. 1: Schematic of the charge modulation PL spectroscopy setup (left) and CMPL spectra of pristine and functionalized (6,5) SWCNT network field-effect transistors (right). Figure 1

Probing Mobile Charge Carriers in Semiconducting Carbon Nanotube Networks by Charge Modulation Spectroscopy.

Solution-processed networks of semiconducting, single-walled carbon nanotubes (SWCNTs) have attracted considerable attention as materials for next-generation electronic devices and circuits. However, the impact of the SWCNT network composition on charge transport on a microscopic level remains an open and complex question. Here, we use charge-modulated absorption and photoluminescence spectroscopy to probe exclusively the mobile charge carriers in monochiral (6,5) and mixed SWCNT network field-effect transistors. Ground-state bleaching and charge-induced trion absorption features as well as exciton quenching are observed depending on applied voltage and modulation frequency. Through correlation of the modulated mobile carrier density and the optical response of the nanotubes, we find that charge transport in mixed SWCNT networks depends strongly on the diameter and thus bandgap of the individual species. Mobile charges are preferentially transported by small bandgap SWCNTs especially at low gate voltages, whereas large bandgap species only start to participate at higher carrier concentrations. Our results demonstrate the excellent suitability of modulation spectroscopy to investigate charge transport in nanotube network transistors and highlight the importance of SWCNT network composition for their performance.

Study of I-V Hysteresis of Tin Perovskite Solar Cells Using Capacitance-Voltage Measurement Coupled with Charge Modulation Spectroscopy

We investigated the I-V hysteresis of tin perovskite solar cells using capacitance-voltage (C-V) measurement coupled with charge modulation spectroscopy (CMS). The C-V characteristics and CMS of the solar cell device (ITO/PEDOT:PSS/CH3NH3SnI3 (MASnI3)/C60/Al) showed that electrons inject from a C60 layer into an MASnI3 layer at positive bias voltage, while the injected electrons return to a C60 layer at negative bias voltage. Results suggested that I-V hysteresis of tin perovskite solar cells is originated from the carrier injection into an MASnI3 layer. This study provides a method for evaluating the carrier motion in tin perovskite solar cells.

Modeling and analysis of I-V hysteresis behaviors caused by defects in tin perovskite thin films

We analyzed the I-V hysteresis behaviors of tin perovskite (MASnI3, MA: CH3NH3) thin films using impedance spectroscopy coupled with charge modulation spectroscopy (CMS). The capacitance-voltage (C-V) characteristics of the ITO/MASnI3/Al device showed hysteresis behaviors, in accordance with the trap filling process suggested by the I-V characteristics. The CMS measurement indicated the enlargement of the energy bandgap of the MASnI3. On the basis of these results, we proposed a model that trap states in the vicinity of the bottom of conduction band are filled, and concluded that the trap filling process occurring at Sn vacancies makes a significant contribution to the electrical properties of tin perovskite film and the hysteresis of the I-V and C-V characteristics of our device. The I-V hysteresis is suppressed with the decrease of defects such as Sn vacancies in MASnI3 films.

Unraveling the Effect of Conformational and Electronic Disorder in the Charge Transport Processes of Semiconducting Polymers

AbstractCharge transport in semiconducting polymers is inextricably linked to their microstructure, making the characterization of polymer morphology at all length‐scales essential for understanding the factors that limit mobility in these materials. Indeed, charge transport depends both on the ability of polarons to delocalize at the approximately nanometer length‐scale and navigate a complex energetic and morphological mesoscale landscape. While characterization of the mesoscale morphology of polymers is well‐established, studies of the local chain packing and nanoscale disorder, which affect delocalization, can be significantly more difficult to carry out. Through infrared charge modulation spectroscopy and theoretical modeling, the effect of the local chain environment on polaron delocalization is directly measured and quantified. Using a series of polymers based on the model system, poly(3‐hexylthiophene), the link between disorder and polaron localization is systematically explored. Polaron delocalization is correlated with known trends in mobility, revealing that while charge delocalization is always beneficial, the formation of tie‐chains is necessary to reach the highest mobilities in semicrystalline polymers. The results provide direct evidence for the importance of both nanoscale (charge carrier delocalization) and mesoscale (tie‐chains) orders, demonstrating the need to distinguish the key length‐scale limiting charge transport in the design of new, high mobility polymers.

In situ probing electronic dynamics at organic bulk heterojunction/aqueous electrolyte interfaces by charge modulation spectroscopy.

Interfacing an organic bulk heterojunction (OBHJ) with an aqueous electrolyte (aqE) solution has the potential for applications in biological sensing and neuronal stimulus, by taking advantage of the benefits of the high excitation efficiency and biocompatibility of the OBHJ. At the OBHJ/aqE interface, local charge transfer and transport processes, which are influenced by the polymer/fullerene interface and ion migration, are critically important for device performance but poorly understood. Here, we have introduced charge modulation spectroscopy (CMS) in aqE-gated heterojunction transistors to in situ investigate electronic dynamics at the OBHJ/aqE interface. By correlating impedance spectroscopy measurements and the gating-voltage dependence of the mobility, we show that the existence of local disordered structures, caused by an intermixed fullerene phase, can induce electrochemical doping effects with ion injections. These ions will be trapped in fullerene domains, thus limiting carrier transports via strong carrier-ion interactions with ion-induced trapping. However, carrier-ion interactions have little influence over the charge transfer process due to the existing large energy-offset between the polymer and the fullerene. Furthermore, time-resolved CMS responses reveal that carrier-ion interactions can induce obvious perturbations in polaron relaxations. Our findings provide possibilities for the design and manipulation of novel and low-cost sensing systems for future bio-recognition devices.

Large bipolaron density at organic semiconductor/electrode interfaces

Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a bipolaron sheet density >1010 cm−2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought.

Spectroscopic studies of dopant-induced conformational changes in poly (3-hexylthiophene) thin films

Abstract

Electrical and optical analyses of trapping phenomenon with temperature dependence of organic device

Abstract Carrier mechanism in actual organic devices is not simple, owing to the dielectric nature of active organic semiconductor layers, the complexity of the organic device interface, carrier trapping effects by stress biasing, and others. By coupling the conventional electrical measurement, e.g., current–voltage, capacitance–voltage and capacitance-frequency measurements, with optical charge modulation spectroscopy (CMS) measurement, we studied the hysteresis behavior and the temperature dependence of indium tin oxide/polyimide/6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS-pentacene)/Au diodes to understand the effect of carrier trapping caused by injected carriers. The coupled electrical and optical measurements were very helpful to clarify carrier injection that was followed by carrier trapping in the diode, in terms of hysteresis behavior. CMS measurement was used to observe energetic states of carriers in TIPS-pentacene double-layer diode. Finally, the carrier mechanism in organic diodes was discussed by analyzing the diodes as a Maxwell-Wagner effect element.

Fast Holes, Slow Electrons, and Medium Control of Polaron Size and Mobility in the DA Polymer F8BT

The nature of electron and hole polarons on poly(9,9-di-n-hexylfluorenyl-2,7-diyl) (pF) and a copolymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) has been studied by chemical doping, pulse radiolysis, charge modulation spectroscopy, quantum chemical calculations, and microwave conductivity. While pF exhibits very similar behavior in all respects for the electron and the hole, this paper explores the hypothesis that the donor acceptor (push–pull) nature of F8BT will tend to localize charges. Optical spectra and quantum chemical calculations point to an electron localized on the thiadiazole unit in polar liquids but becoming more delocalized as the solvent polarity decreases. Indeed, in the nonpolar solvent benzene, the electron mobility is only 2.7 times lower than that of the hole, which conversely is shown to be delocalized in all environments and has a similar mobility to polarons on the homopolymer polyfluorene. Advantageous modifications to the optoelectroni...

Study of carrier energetics in ITO/P(VDF-TrFE)/pentacene/Au diode by using electric-field-induced optical second harmonic generation measurement and charge modulation spectroscopy

By using electric-field-induced optical second harmonic generation (EFISHG) measurement and charge modulation spectroscopy (CMS), we studied carrier behavior and polarization reversal in ITO/ poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE))/pentacene/Au diodes with a ferroelectric P(VDF-TrFE) layer in terms of carrier energetics. The current-voltage (I–V) characteristics of the diodes showed three-step polarization reversal in the dark. However, the I–V was totally different under illumination and exhibited two-step behavior. EFISHG probed the internal electric field in the pentacene layer and accounted for the polarization reversal change due to charge accumulation at the pentacene/P(VDF-TrFE) interface. CMS probed the related carrier energetics and indicated that exciton dissociation in pentacene molecular states governed carrier accumulation at the pentacene/ferroelectric interface, leading to different polarization reversal processes in the dark and under light illumination. Combining EFISHG measurement and CMS provides us a way to study carrier energetics that govern polarization reversal in ferroelectric P(VDF-TrFE)/pentacene diodes.

Resistive switching in graphene-organic device: Charge transport properties of graphene-organic device through electric field induced optical second harmonic generation and charge modulation spectroscopy

Graphene-based resistive random access memory devices is a promising non-volatile memory technology that combines low operation voltage and power, extremely fast write/erase speeds, excellent reliability and storage capacity of RRAM with low-cost, large area and flexibility of carbon-based technologies. However, low-cost single-step synthesis of high-quality graphene remains a challenge. In this paper, high quality graphene synthesized directly from sustainable carbon source (M. alternifolia oil) was used as electrode and pentacene/C60 as active layers in carbon-based RRAM. I-V measurements were used to demonstrate reproducible switching (rapid increase in current) at certain voltage which was reversible. Charge transport and accumulation was visualized using electric field induced optical second harmonic generation and charge modulation spectroscopy. Hole transport from graphene layer to the organic layer was the primary cause of the observed switching behavior.

Temperature Dependence of Charge Localization in High‐Mobility, Solution‐Crystallized Small Molecule Semiconductors Studied by Charge Modulation Spectroscopy

In solution‐processable small molecule semiconductors, the extent of charge carrier wavefunction localization induced by dynamic disorder can be probed spectroscopically as a function of temperature using charge modulation spectroscopy (CMS). Here, it is shown based on combined field‐effect transistor and CMS measurements as a function of temperature that in certain molecular semiconductors, such as solution‐processible pentacene, charge carriers become trapped at low temperatures in environments in which the charges become highly localized on individual molecules, while in some other molecules the charge carrier wavefunction can retain a degree of delocalization similar to what is present at room temperature. The experimental approach sheds new insight into the nature of shallow charge traps in these materials and allows identifying molecular systems in which intrinsic transport properties could, in principle, be observed at low temperatures if other transport bottlenecks associated with grain boundaries or contacts could be removed.

In Situ Probing of the Charge Transport Process at the Polymer/Fullerene Heterojunction Interface

The polymer/fullerene interface (PFI) in polymer solar cells (PSCs) provides an energetic offset for exciton dissociation while at the same time influencing local transport of photocarriers adjacent to the interface. In this paper, we introduce a heterojunction field-effect transistor (FET) structure in charge modulation spectroscopy (CMS) to enable in situ probing of the charge transport process at PFIs. The PFIs formed by fullerene/crystalline polymer and fullerene/amorphous polymer systems are studied and compared, respectively. By correlating the steady-state and frequency-dependent CMS responses of pure polymer, polymer/fullerene bilayer, and polymer/fullerene blend FETs, we demonstrate that through different charge localization effects the interface fullerene molecules can influence the hole transport in both crystalline and amorphous polymer phases. We propose a trade-off between charge transfer and charge transport at PFIs with an aim to enhance the engineering of molecular orientation and packing...

Microscopic hole-transfer efficiency in organic thin-film transistors studied with charge-modulation spectroscopy

While the microscopic transfer properties of carriers are of primary importance for carrier transport of organic semiconductors, the mesoscopic features including the morphologies of grains and the structure of grain boundaries limit the overall carrier transport particularly in polycrystalline organic thin films. Thus the conventional evaluation methods of carrier mobility that rely on macroscopic properties such as $I\text{\ensuremath{-}}V$ curves of devices are not capable to determine carrier transfer probability at the molecular level. Here, we present a method for evaluating the relative strengths of transfer integrals using charge-modulation spectroscopy on thin-film transistors of dinaphtho[$2,3\text{\ensuremath{-}}b$:2\ensuremath{'},3\ensuremath{'}-$f$]thieno[$3,2\text{\ensuremath{-}}b$]thiophene (DNTT) and its alkylated derivatives (${\mathrm{C}}_{n}$-DNTT, $n=8$, 10, and 12). The band edges of absorption spectra of holes at around 1.9 eV show bathochromic shifts with increasing length of alkyl chains introduced at both ends of a DNTT chromophore. Applying a two-dimensional model with Holstein-type Hamiltonians to electronic transitions of holes, we have been able to simulate the features of the absorption band edges observed. The simulations indicate that the bathochromic shifts are due to an increase in the transfer integrals of holes with increasing length of alkyl chains. Thus this analysis confirmed that the subtle changes in the mutual orientations between adjacent DNTT chromophores induced by alkyl chains enhance the microscopic hole transfer rate. Although this fastener effect has been suggested by hole mobility measurements by $I\text{\ensuremath{-}}V$ curves, the spectral analysis in this study gives clear evidence of this effect at the molecular level.

Analysis of carrier transport and carrier trapping in organic diodes with polyimide-6,13-Bis(triisopropylsilylethynyl)pentacene double-layer by charge modulation spectroscopy and optical second harmonic generation measurement

We studied the carrier transport and carrier trapping in indium tin oxide/polyimide (PI)/6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)/Au diodes by using charge modulation spectroscopy (CMS) and time-resolved electric field induced optical second harmonic generation (TR-EFISHG) measurements. TR-EFISHG directly probes the spatial carrier behaviors in the diodes, and CMS is useful in explaining the carrier motion with respect to energy. The results clearly indicate that the injected carriers move across TIPS-pentacene thorough the molecular energy states of TIPS-pentacene and accumulate at the PI/TIPS-pentacene interface. However, some carriers are trapped in the PI layers. These findings take into account the capacitance-voltage and current-voltage characteristics of the diodes.