Field-dependent Charge Research Articles

Extracting charge carrier mobility in organic solar cells through space-charge-limited current measurements

Mobility is a critical parameter influencing the overall performance of organic solar cells (OSCs). Herein, we innovatively elucidated the intricate interrelation between the photovoltaic molecular structures and the methodologies employed for the extraction of charge carrier mobility in OSCs. We proposed a simple yet effective principle to accurately extract charge carrier mobility values using the standard space-charge-limited current (SCLC) measurement, while critically assessing theoretical and experimental deficiencies through the drift-diffusion analysis. It was found that field-dependent charge transport is necessitated to describe the prominent long-range intrachain hopping carrier behavior in polymers, while short-range intermolecular hopping results in trap-involved charge transport within small molecular acceptors. Based on the above understanding, a synergetic inter/intra-molecular hopping strategy was proposed to fabricate thick-film all-polymer OSCs, and an unprecedented power conversion efficiency (PCE) of 16.61 % was achieved in the 300 nm PM6:PY-IT OSC. This work not only presents a precise and straightforward approach for measuring mobility values, but also provides a significant reference about charge carrier transport to make optimal decisions regarding photovoltaic material design and device fabrication process of high-performance OSCs.

Field-dependent THz transport nonlinearities in semiconductor nano structures.

Charge transport nonlinearities in semiconductor quantum dots and nanorods are studied. Using a density matrix formalism, we retrieve the field-dependent nonlinear mobility and show the possibility of intra-pulse gain. We further demonstrate that the dynamics of master equations can be captured in an analytical formula for the field-dependent charge carrier mobility, e.g. for two-level systems. This equation extends the linear response theory based Kubo-Greenwood result to nonlinear processes at elevated field strength, easily reached in THz transport spectroscopy. With these tools we analyze the field strength, chirp, temperature and dephasing dependence of the charge carrier mobility in the model system of CdSe quantum dots and wires. Stark broadening and Rabi splitting result in strong alterations of the mobility spectra, pronounced at low temperatures. The mobility spectra are strongly temperature and pulse shape dependent in the nonlinear regime. The findings are of immediate interest e.g. for nonlinear THz generation, conversion and amplification in 6G technology and nano electronics. Our results further enable experimentalists to fit and understand measured charge transport nonlinearities with analytical expressions and to design nanosystems with engineered material properties.

Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells

Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in non-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5—a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states.

Elucidating How Low Energy Offset Matters to Performance of Nonfullerene Acceptor-Based Solar Cells

The energetic offset between the highest occupied molecular orbitals of the donor and acceptor components of organic photovoltaic blends is well-known to affect the device efficiency. It is well-established that a decreasing offset increases the open-circuit voltage but reduces the short-circuit current, which has been explained by insufficient exciton dissociation. However, the impact of the offset on the fill factor and underlying processes is less clear. Here, we study free charge generation and recombination in three different nonfullerene acceptors, Y6, ITIC, and o-IDBTR, blended with the same donor polymer PM6. We demonstrate that a diminishing offset results in field-dependent charge generation related to field-assisted exciton dissociation. On the other hand, reformation of excitons from free charges is identified as an additional channel for charge recombination, which goes along with a substantial rise in the bimolecular recombination coefficient. In combination of these two effects, the fill factor drops considerably with a decreasing energy offset. Using the comparison between PM6:ITIC and PM6:o-IDBTR, we show that bulk properties such as morphology and carrier mobilities can not fully explain the observed difference in performance, highlighting the importance of interfacial kinetics and thermodynamics in controlling the device efficiency, both through generation and recombination of charge carriers.

Intergranular Spin Dependent Tunneling Dominated Magnetoresistance in Helimagnetic Manganese Phosphide Thin Films.

Helical magnets are emerging as a novel class of materials for spintronics and sensor applications; however, research on their charge- and spin-transport properties in a thin film form is less explored. Herein, we report the temperature and magnetic field-dependent charge transport properties of a highly crystalline MnP nanorod thin film over a wide temperature range (2 K < T < 350 K). The MnP nanorod films of ~100 nm thickness were grown on Si substrates at 500 °C using molecular beam epitaxy. The temperature-dependent resistivity ρ(T) data exhibit a metallic behavior (dρ/dT > 0) over the entire measured temperature range. However, large negative magnetoresistance (Δρ/ρ) of up to 12% is observed below ~50 K at which the system enters a stable helical (screw) magnetic state. In this temperature regime, the Δρ(H)/ρ(0) dependence also shows a magnetic field-manipulated CONE + FAN phase coexistence. The observed magnetoresistance is dominantly governed by the intergranular spin dependent tunneling mechanism. These findings pinpoint a correlation between the transport and magnetism in this helimagnetic system.

Origin of electric field-dependent charge generation in organic photovoltaics with planar and bulk heterojunctions

The electric field dependence in the charge generation process of organic solar cells in planar heterojunction and bulk heterojunction structures is related to the energetics and molecular orientation at the donor/acceptor interfaces.

Magnetic and Electric Field Dependent Charge Transfer in Perovskite/Graphene Field Effect Transistors

AbstractStable all‐inorganic CsPbX3 perovskite nanocrystals (PNCs) with high optical yield can be used in combination with graphene as photon sensors with high responsivity (up to 106 A W−1) in the VIS‐UV range. The performance of these perovskite/graphene field effect transistors (FET) is mediated by charge transfer processes at the perovskite – graphene interface. Here, the effects of high electric (up to 3000 kV cm−1) and magnetic (up to 60 T) fields applied perpendicular to the graphene plane on the charge transfer are reported. The authors demonstrate electric‐ and magnetic‐field dependent charge transfer and a slow (&gt;100 s) charge dynamics. Magneto‐transport experiments in constant (≈0.005 T s−1) and pulsed (≈1000 T s−1) magnetic fields reveal pronounced hysteresis effects in the transfer characteristics of the FET. A magnetic time is used to explain and model differences in device behavior under fast (pulsed) and slowly (continuous) changing magnetic fields. The understanding of the dynamics of the charge transfer in perovskite/graphene heterostructures developed here is relevant for exploitation of these hybrid systems in electronics and optoelectronics, including ultrasensitive photon detectors and FETs for metrology.

External electric field-dependent photoinduced charge transfer in non-fullerene organic solar cells

Based on Marcus theory, the photoinduced electron transfer properties of D-A type non-fullerene acceptor organic solar cells (OSCs) under the dependence of external electric field (Fext) were investigated. The research results shown that the charge transfer mode under different Fext intensities changes with certain regularity. Focusing on the important parameters (ΔG, λ, and VDA) that affect the charge transfer rate, it was found that both charge separation (|ΔGCS|>λ(1.3019 vs 0.8275 eV at Fext = 0) and charge recombiation (|ΔGCR|>λ, (1.9633 vs 0.8275 eV)) processes occur in the Marcus inverted region. The ΔGCS is relatively sensitive to Fext, and the calculated ΔGCS at different Fext intensities yields an increment of 0.0073 eV, which is also the main reason for the increase in the rate of charge separation. The ΔGCR ranges between −1.9633 and −1.9637 eV, is insensitive to Fext, and ΔGCR is significantly smaller than ΔGCS, which makes the charge recombination rate significantly smaller than the charge separation rate. For VDA, it is found that VDA will transition to a new level only when the Fext intensity reaches a certain intensity, which also enables to obtain a faster charge separation rate. By studying the charge transfer parameters in different polar solvents, it is found that polar solvents can indeed increase the charge transfer rate. To a certain extent, our results also demonstrate that the addition of Fext can further improve the performance of non-fullerene acceptor OSCs.

Insights into photocatalytic mechanism for the rational design of p-n heterojunction by decorating mesoporous SnS2 over ZnFe2O4 nanocomposite for accelerated visible light photocatalysis

Binary p-n heterojunction of ZnFe2O4/SnS2 nanocomposite (NCs) was constructed by sonochemical assisted technique for enhanced visible light induced photocatalytic performance. The narrow bandgap of 2.21 eV facilitated the photocatalyst be to active under visible-light irradiation. The rate of photodegradation of methylene blue (MB) dye by optimized-NCs ratio was observed to be remarkable with multifold enhancement than pristine ZnFe2O4 and SnS2. The interfacial contact enforces an internal electric field dependent directional charge migration which hinders charge recombination that was verified by reduced photoluminescence intensity in NCs. The degradation process was found to follow pseudo-first order kinetics. The major ROS involved in the photodegradation process were found to be hydroxyl (.OH) and superoxide (.O2-) radicals which were identified through the scavengers assay. The NCs were subjected to six consecutive cycles in order to test its structural stability and reusability. This work provides guidance for rational design of heterojunction with superior activity and it reflected the potential of ZnFe2O4/SnS2 NCs as a promising photocatalyst to remove toxic pollutants from the aqueous bodies.

Coexistence of band-like and thermally activated charge transport through nuclear tunneling effect in organic semiconductors

In this study, the coexistence of band-like and thermally activated charge transport in organic semiconductors is demonstrated through the first-principles calculation of the electric field dependent charge mobilities of TIPS-pentacene, which strongly deviates from the semi-classical Marcus theory. The nuclear tunneling hopping rates simultaneously exhibit band-like and thermally activated conduction for the downward and upward hoppings, respectively, which explains the puzzling observation of the transition from thermally activated transport to band-like transport with increasing field strength. As the hopping rates are very sensitive to the site-energy fluctuations induced by defect configurations, the transition from band-like transport to thermally activated transport with increasing energetic disorder is found. The results suggest that the interplay of the quantum-mechanical effect and disorder is of particular importance in the understanding of the temperature and field dependence of mobility in organic materials.

Evidence for Field-Dependent Charge Separation Caused by Mixed Phases in Polymer-Fullerene Organic Solar Cells.

As organic photovoltaic performance approaches 20% efficiencies, causal structure-performance relationships must be established for devices to realize theoretical limits and become commercially competitive. Here, we reveal evidence of a causal relationship between mixed donor-acceptor interfaces and charge generation in polymer-fullerene solar cells. To do this, we combine a holistic loss analysis of device performance with quantitative synchrotron X-ray nanocharacterization to identify a >98% anticorrelation between field-dependent geminate recombination and nanodomain purity. Importantly, our analysis eliminates other possible explanations of the performance trends, a requirement to establish causality. The unprecedented granular level of our analysis also separates field-dependent and field-independent recombination at the interface, where we find for the first time that this system is free of field-independent recombination, a loss channel that plagues high-performance systems, including those with non-fullerene acceptors. This result broadens the case that minimizing mixed phases to promote sharp interfaces between pure aggregated domains is the ideal nanostructure for realizing theoretical efficiency limits of organic photovoltaics.

Maxwell Equations without a Polarization Field, Using a Paradigm from Biophysics.

When forces are applied to matter, the distribution of mass changes. Similarly, when an electric field is applied to matter with charge, the distribution of charge changes. The change in the distribution of charge (when a local electric field is applied) might in general be called the induced charge. When the change in charge is simply related to the applied local electric field, the polarization field P is widely used to describe the induced charge. This approach does not allow electrical measurements (in themselves) to determine the structure of the polarization fields. Many polarization fields will produce the same electrical forces because only the divergence of polarization enters Maxwell’s first equation, relating charge and electric forces and field. The curl of any function can be added to a polarization field P without changing the electric field at all. The divergence of the curl is always zero. Additional information is needed to specify the curl and thus the structure of the P field. When the structure of charge changes substantially with the local electric field, the induced charge is a nonlinear and time dependent function of the field and P is not a useful framework to describe either the electrical or structural basis-induced charge. In the nonlinear, time dependent case, models must describe the charge distribution and how it varies as the field changes. One class of models has been used widely in biophysics to describe field dependent charge, i.e., the phenomenon of nonlinear time dependent induced charge, called ‘gating current’ in the biophysical literature. The operational definition of gating current has worked well in biophysics for fifty years, where it has been found to makes neurons respond sensitively to voltage. Theoretical estimates of polarization computed with this definition fit experimental data. I propose that the operational definition of gating current be used to define voltage and time dependent induced charge, although other definitions may be needed as well, for example if the induced charge is fundamentally current dependent. Gating currents involve substantial changes in structure and so need to be computed from a combination of electrodynamics and mechanics because everything charged interacts with everything charged as well as most things mechanical. It may be useful to separate the classical polarization field as a component of the total induced charge, as it is in biophysics. When nothing is known about polarization, it is necessary to use an approximate representation of polarization with a dielectric constant that is a single real positive number. This approximation allows important results in some cases, e.g., design of integrated circuits in silicon semiconductors, but can be seriously misleading in other cases, e.g., ionic solutions.

Optically Probing Field-Dependent Charge Dynamics in Non-Fullerene Organic Photovoltaics with Small Interfacial Energy Offsets

Current non-fullerene organic photovoltaics (OPVs) can achieve high photocurrent generation yields with small donor/acceptor energy offsets. However, the effect of varying energy offset on the char...

Temperature dependent charge transport of acid-treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is an important opto-electronic material, and its room temperature electrical conductivity can be prominently enhanced by chemical treatment; it is technologically significant to investigate its temperature and magnetic field dependent charge transport characteristics. Here, we analyzed the low temperature charge transport properties of pristine and acid-treated PEDOT:PSS thin films by studying the variation in resistance as a function of temperature and magnetic field (T ∼ 300 to 4.2 K, H ∼ up to 5 T). The acid-treated sample exhibited much improved charge transport behavior at low T, with a resistivity ratio [ρ(4.2 K)/ρ(300 K)] of ∼5.7 × 102, which is three orders of magnitude smaller than that of the pristine sample. Nevertheless, both pristine and acid-treated samples followed hopping conduction, obeying R(T)∝expT−1/2andR(T)∝expT−1/3, respectively, in the low T regime. Furthermore, positive magnetoresistance of ∼16% (at 4.2 K and 5 T) was displayed by acid-treated samples that could be due to the wave function shrinkage phenomenon.

Comment on “Charge transport in disordered semiconducting polymers driven by nuclear tunneling”

In Phys. Rev. B 93, 140206(R) (2016), electron transport governed by nuclear tunneling was claimed to be a more realistic transport mechanism in semiconducting polymers as compared to Marcus and Miller-Abrahams transport mechanisms. The conclusion was based on the analysis of the field-dependent charge carrier mobility. We point out the unjustified simplification in the theoretical description of the electron transport governed by nuclear tunneling which caused such a conclusion.

Space Charge Accumulation at Material Interfaces in HVDC Cable Insulation Part II—Simulations of Charge Transport

Extruded high voltage direct current (HVDC) cable systems contain interfaces with poorly understood microscopic properties, particularly surface roughness. Modelling the effect of roughness on conduction in cable insulation is challenging, as the available results of macroscopic measurements give little information about microscopic charge distributions at material interfaces. In this work, macroscopic charge injection from interfaces is assessed by using a bipolar charge transport model, which is validated against a series of space charge measurements on cable peelings with different degrees of surface roughness. The electric field-dependent conduction and charge trapping effects stimulated by the injection current originating from rough surfaces are assessed. It is shown that by accounting for roughness enhanced charge injection with the parameters derived in part I of the paper, reasonable agreement between computed and measured results can be achieved at medium field strengths (10–40 kV/mm).

External Electric Field-Dependent Photoinduced Charge Transfer in a Donor–Acceptor System in Two-Photon Absorption

In this paper, we theoretically investigated external electric field-dependent photoinduced charge transfer (CT) in a donor–acceptor (D-A) system in two-photon absorption (TPA) where PCDTBT and PC7...

Field-dependent charge transport in organic thin-film transistors: Impact of device structure and organic semiconductor microstructure

Organic semiconductors are highly susceptible to defect formation, leading to electronic states in the gap—traps—which typically reduce the performance and stability of devices. To study these effects, we tuned the degree of charge trapping in organic thin-film transistors by modifying the film deposition procedures and device structure. The resulting charge carrier mobility varied between 10−3 and 10 cm2/V s in 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene. We analyzed the data using a Poole-Frenkel-like model and found a strong dependence of mobility on the field in low-mobility transistors and a field-independent mobility in high-performance devices. We confirmed the presence of traps in all films investigated in this study and concluded that the Poole-Frenkel model is not sufficiently sensitive to identify traps when their concentration is below the detection limit.

Field and temperature dependent charge transport mechanism in doped Poly(3-methylthiophene) devices

Thermal and electric field activated charge transport mechanism in Silver/Poly(3-methylthiophene)/Stainless Steel device with different charge carrier concentrations is studied. Temperature dependent Current-Voltage (I–V) characteristics of the devices show disorder induced non-linear behaviour Depending upon temperature range, the value of m varies suggesting that different transport mechanisms are active in the different temperature ranges. In the higher temperature range trap controlled space charge limited conduction (TCSCLC) with an exponential distribution of trap states plays a dominant role. In the low temperature range the carriers cannot perform phonon assisted hoping and the mobility varies with the applied electric field and TCSCLC mechanism changes to field dependent mobility. Field dependent charge carrier mobility is found to be more pronounced as carrier density decreases.

Electric field-dependent charge transport in a poly (dA)-(dT) DNA molecule: localization length study

Electronic transport in DNA molecules could depend on many factors, one of them is external electric field influence. In this research we analyse localization length of poly (dA)-(dT) DNA molecule to study its electronic transport properties. Tight binding Hamiltonian model is used along with electric field and temperature influence. Electric field is set to be parallel to the helix axis of DNA. Transfer-matrix method is required to calculate localization length, while gram-schmidt orthonormalization method is used to reduce errors. The results show that electric field changes localization length in all energy considered in calculation.