Diffusive Carrier Transport Research Articles

Layer‐by‐Layer Organic Solar Cells Enabled by 1,3,4‐Selenadiazole‐Containing Crystalline Small Molecule with Double‐Fibril Network Morphology

AbstractA double‐fibril network of the photoactive layer morphology is recognized as an ideal structure facilitating exciton diffusion and charge carrier transport for high‐performance organic solar cells (OSCs). However, in the layer‐by‐layer processed OSCs (LbL‐OSCs), polymer donors and small molecule acceptors (SMAs) are separately deposited, and it is challenging to realize a fibril network of pure SMAs with the absence of tight interchain entanglement as polymers. In this work, crystalline small molecule donors (SMDs), named TDZ‐3TR and SeDZ‐3TR, were designed and introduced into the L8‐BO acceptor solution, forcing the phase separation and molecular fibrilization. SeDZ‐3TR showed higher crystallinity and lower miscibility with L8‐BO acceptor than TDZ‐3TR, enabling more driving force to favor the phase separation and better molecular fibrilization of L8‐BO. On the other hand, two donor polymers of PM6 and D18 with different fibril widths and lengths were put together to optimize the fibril network of the donor layer. The simultaneously optimization of the acceptor and donor layers resulted in a more ideal double‐fibril network of the photoactive layer and an impressive power conversion efficiency (PCE) of 19.38 % in LbL‐OSCs.

Layer-by-Layer Organic Solar Cells Enabled by 1,3,4-Selenadiazole-Containing Crystalline Small Molecule with Double-Fibril Network Morphology.

A double-fibril network of the photoactive layer morphology is recognized as an ideal structure facilitating exciton diffusion and charge carrier transport for high-performance organic solar cells (OSCs). However, in the layer-by-layer processed OSCs (LbL-OSCs), polymer donors and small molecule acceptors (SMAs) are separately deposited, and it is challenging to realize a fibril network of pure SMAs with the absence of tight interchain entanglement as polymers. In this work, crystalline small molecule donors (SMDs), named TDZ-3TR and SeDZ-3TR, were designed and introduced into the L8-BO acceptor solution, forcing the phase separation and molecular fibrilization. SeDZ-3TR showed higher crystallinity and lower miscibility with L8-BO acceptor than TDZ-3TR, enabling more driving force to favor the phase separation and better molecular fibrilization of L8-BO. On the other hand, two donor polymers of PM6 and D18 with different fibril widths and lengths were put together to optimize the fibril network of the donor layer. The simultaneously optimization of the acceptor and donor layers resulted in a more ideal double-fibril network of the photoactive layer and an impressive power conversion efficiency (PCE) of 19.38 % in LbL-OSCs.

Investigations of carrier transport mechanism and junction formation in Si/CZTS dual absorber solar cell technology

We report on dual absorber n-silicon/p-Cu2ZnSnS4 (Si/CZTS) solar cell model with CZTS as top absorber, silicon as bottom absorber and ITO acting as transparent conducting oxide layer on top of CZTS material in order to exploit the advantages of well-established silicon technology and low-cost earth abundant CZTS materials. The valence band edge develops a triangular potential at the Si/CZTS interface, and hence, the drift–diffusion and thermionic carrier transport mechanisms are investigated using Silvaco ATLAS 2D simulator. The thermionic emission current component is suggested to improve the fill factor at the cost of short circuit current and open circuit voltage. The band discontinuities, built-in potential developed across the heterojunction, generation–recombination profile, electron and hole current densities are carefully examined to understand the transport phenomena of Si/CZTS structure for varying thickness, acceptor concentration, electron affinity and optical band gap of top CZTS absorber materials. Synthesis and deposition of CZTS thin films were carried out via sol-gel spin coating process with different preheating temperature range between 250 and 350 °C for 10 min, and absorption data obtained from experiment is used to explore the benefits of tunable band gap properties of CZTS absorbers in the Si/CZTS heterojunction solar cell device. The increased band gap for 350 °C preheated sample has minimized the electrical recombination losses and improved the infrared light absorption resulting in efficiency enhancement. Then the CZTS physical parameters are optimized for the Si/CZTS solar cell model and the conversion efficiency is improved from 8.97 to 14.74% by minimizing the electrical and optical losses of the materials.

Tiny Au nanoparticles mediation strategy for preparation of NIR CuInS2 QDs based 1D TiO2 hybrid photoelectrode with enhanced photocatalytic activity

Copper indium sulfide (CuInS2) is attracting growing attention as a green, photo-absorption material; yet CuInS2 alone displays poor photocatalytic activity owing to the severe charge carrier recombination resulting from its high defect tolerability — especially for quantum dots (QDs) with the high volume-to-surface ratio. In an attempt to mitigate this issue, a ternary type II heterostructure hybrid photoelectrode was successfully prepared through in situ photoreduction deposition of tiny Au nanoparticles (NPs) onto free-standing TiO2/Ti nanowire arrays (NWAs) loaded with near-infrared (NIR) CuInS2 QDs. This configuration permits CuInS2 QDs to act as a photosensitizer with a light harvest region extending 800 nm, while tiny Au NPs function as photocatalytic reaction initiators to enhance organic molecule decomposition. Most importantly, using TiO2/Ti NWAs as the supporting substrate not only provides considerably more sites for highly loaded and dispersed catalysts, but also reserves one dimension space for ions, mass diffusion, and photo-generation charge carrier transport. As expected, the results showed that incorporation of Au NPs (CuInS2/Au/TiO2 NWAs) increased photocurrent intensity by 65.2% compared with CuInS2/TiO2 NWAs. What's more, the phenol degradation efficiency was 94.3% after 180 min of light irradiation, which is significantly higher than that of other control photoelectrodes. Finally, the superior performance also benefits from the intimate contact interface between different components created through our proposed strategy. A photoluminescence (PL) technique, using terephthalic acid (TA) as the probe molecule, confirmed that a highly reactive hydroxyl radical based photocatalytic mechanism is also responsible for such significant photocatalytic efficiency.

On the ESD Behavior of Large-Area CVD Graphene Transistors: Physical Insights and Technology Implications

In this paper, for the first time, the record high-performance top-gated graphene technology platform is used for electrostatic discharge (ESD) physics exploration while investigating the implications of various design and technology options. Impact of diffusive versus ballistic carrier transport on the failure mechanism in top-gate as well as back-gate graphene FET (GFET) is investigated. A unique contact limited failure in graphene transistors is reported. Physical insights on current saturation in GFET and unique step-by-step failure in dielectric capped transistors are presented for the first time. Moreover, device degradation under ESD timescales and its implications on current saturation are revealed. Finally, the influence of various top-gate designs on the ESD performance is reported. New physical insights and matured GFET technology has eventually enabled record high ESD robustness.

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films.

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices.

Electrothermal Effects on Hot-Carrier Reliability in SOI MOSFETs—AC Versus Circuit-Speed Random Stress

Computational study of electrothermal effects on hot-carrier injection (HCI) in 100-nm silicon-on-insulator (SOI) MOSFET for digital integrated circuit is performed using in-house developed time-domain finite element algorithm. The simulated $I$ – $V$ curve is obtained by solving diffusive carrier transport equations, and it agrees well with our measured results. The time-dependent thermal conduction equation is solved to get the transient temperature response of the device. Furthermore, according to the transient temperature responses to different signal stresses, including step pulse, ac signal, and pseudorandom binary sequence (PRBS), HCI-induced threshold voltage shift (TVS) is captured by accounting for the temperature dependence of HCI. It is shown that the TVS of device under PRBS stress, which is used to mimic the circuit speed operation, is slightly larger than that of the device under ac signal with the same frequency. With the frequency of signal stress decreasing from gigahertz to tens of megahertz, TVS becomes more severe, because the temperature during ON-state is higher for the SOI MOSFET under low-frequency stress. Thick buried oxide leads to high temperature in channel and deteriorates the HCI. The method presented in this paper should also be applicable to the other MOSFET counterparts.

Finite element simulations of graphene based three-terminal nanojunction rectifiers

Electrical rectification in graphene-based three-terminal nanojunctions is simulated using the finite element method. The model is based on diffusive charge carrier transport in a field-effect transistor configuration. The influence of device geometry, temperature, and electric potential disorder on the rectification efficiency is calculated. For a typical realistic device on a Si/SiO2 substrate, the model yields a room temperature efficiency of about 1% at a bias of 100 mV. The calculations are compared to previously published experimental results.

Analysis of Internal Quantum Efficiency and Current Injection Efficiency in III-Nitride Light-Emitting Diodes

Current injection efficiency and internal quantum efficiency (IQE) in InGaN quantum well (QW) based light emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier systems. A self-consistent 6-band k ·p method is used to calculate the band structure for InGaN QW structure. Carrier-photon rate equations are utilized to describe radiative and non-radiative recombination in the QW and the barrier regions, carrier transport and capture time, and thermionic emission leading to carrier leakage out of the QW. Our model indicates that the IQE in the conventional 24-A In0.28Ga0.72 N -GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large thermionic carrier leakage. The efficiency droop phenomenon at high current density in III-nitride LEDs is thus consistent with the high-driving-current induced quenching in current injection efficiency predicted by our model. The effects of the monomolecular recombination coefficient, Auger recombination coefficient and GaN hole mobility on the current injection efficiency and IQE are studied. Structures combining InGaN QW with thin larger energy bandgap barriers such as AlxGa1-xN, lattice-matched AlxIn1-xN, and lattice-matched AlxInyGa1-x-y N have been analyzed to improve current injection efficiency and thus minimize droop at high current injection in III-nitride LEDs. Effect of the thickness of the larger energy bandgap barriers (AlGaN, AlInN and AlInGaN) on injection efficiency and IQE are investigated. The use of thin AlGaN barriers shows slight reduction of quenching of the injection efficiency as the current density increases. The use of thin lattice-matched AlInN or AlInGaN barriers shows significant suppression of efficiency-droop in nitride LEDs.

Simple Analytic Description of Collection Efficiency in Organic Photovoltaics.

The collection of charge carriers is a fundamental step in the photovoltaic conversion process. In disordered organic films, low mobility and disorder can make collection the performance-limiting step in energy conversion. We derive two analytic relationships for carrier collection efficiency in organic photovoltaics that account for the presence or absence of carrier-selective electrodes. These equations directly include drift and diffusive carrier transport in the device active layers and account for possible losses from Langevin and Shockley-Read-Hall recombination mechanisms. General relationships among carrier mobility, contact selectivity, recombination processes, and organic photovoltaic figures of merit are established. Our results suggest that device collection efficiency remains mobility-limited for many materials systems, and a renewed emphasis should be placed on materials' purity.

Charge carrier dynamics and interactions in electric force microscopy

In electric force microscopy, a charged atomic force microscope tip in vacuum senses a fluctuating electrical force generated by the sample. Such measurements can in principle probe electrical noise generated by moving charge carriers in an organic semiconductor. We present a theory of cantilever frequency fluctuations in electric force microscopy, driven by coupled charge carrier dynamics and dielectric fluctuations. The connection between observable frequency fluctuations in electric force microscopy and the Casimir-Lifshitz force is described. This classical electrodynamic calculation is based on Maxwell's equations coupled to diffusive carrier transport. The effects of carrier transport and inter-carrier interactions on the spectrum of cantilever frequency noise are elucidated. We find that a simplified model of freely diffusing carriers can overestimate cantilever frequency noise by several orders of magnitude because of the neglect of interactions. Electric force microscopy measurements on an organic field effect transistor are reported and qualitatively interpreted in terms of the suppression of electrical noise from charge carriers by Coulomb interactions.

Temperature dependence of the diffusive conductivity of bilayer graphene

Assuming diffusive carrier transport and employing an effective medium theory, we calculate the temperature dependence of bilayer graphene conductivity due to Fermi-surface broadening as a function of carrier density. We find that the temperature dependence of the conductivity depends strongly on the amount of disorder. In the regime relevant to most experiments, the conductivity is a function of T/T*, where T* is the characteristic temperature set by disorder. We demonstrate that experimental data taken from various groups collapse onto a theoretically predicted scaling function.

Current injection efficiency induced efficiency-droop in InGaN quantum well light-emitting diodes

Current injection efficiency and its impact on efficiency-droop in InGaN single quantum well (QW) based light-emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier system. A self-consistent 6-band k · p method is used to calculate the band structure for InGaN QW. The analysis indicates that the internal quantum efficiency in the conventional 24-Å In 0.28Ga 0.72N–GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large carrier thermionic emission. Structures combining 24-Å In 0.28Ga 0.72N QW with 15-Å Al 0.1Ga 0.9N barriers show slight reduction in quenching of the injection efficiency as current density increases. The use of 15-Å Al 0.83In 0.17N barriers shows significant reduction in efficiency-droop (10% reduction of the internal quantum efficiency at current density of 620 A/cm 2). Thus, InGaN QWs employing thin layers of larger bandgap AlInN barriers suppress the efficiency-droop phenomenon significantly.

Noise from metallic surfaces: Effects of charge diffusion

We study electromagnetic response functions for metallic surfaces with diffusive carrier transport. The electric field noise at a distance ${z}_{0}$ from the surface is evaluated and compared with data from ion chips that show anomalous heating with a noise power decaying as ${z}_{0}^{\ensuremath{-}4}$. We find that a layer of surface charge with strong diffusion can account for the latter result.

Current saturation in zero-bandgap, top-gated graphene field-effect transistors

The novel electronic properties of graphene, including a linear energy dispersion relation and purely two-dimensional structure, have led to intense research into possible applications of this material in nanoscale devices. Here we report the first observation of saturating transistor characteristics in a graphene field-effect transistor. The saturation velocity depends on the charge-carrier concentration and we attribute this to scattering by interfacial phonons in the SiO2 layer supporting the graphene channels. Unusual features in the current-voltage characteristic are explained by a field-effect model and diffusive carrier transport in the presence of a singular point in the density of states. The electrostatic modulation of the channel through an efficiently coupled top gate yields transconductances as high as 150 microS microm-1 despite low on-off current ratios. These results demonstrate the feasibility of two-dimensional graphene devices for analogue and radio-frequency circuit applications without the need for bandgap engineering.

In-Crystal and Surface Charge Transport of Electric-Field-Induced Carriers in Organic Single-Crystal Semiconductors

Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity sigma(square) and Hall coefficient R(H). The Hall mobility mu(H) (identical with sigma(square)R(H)) reaches nearly 10 cm(2)/V s when relatively low-density carriers (<10(11) cm(-2)) distribute into the crystal. mu(H) rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.

Unified description of ballistic and diffusive carrier transport in semiconductor structures

A unified theoretical description of ballistic and diffusive carrier transport in parallel-plane semiconductor structures is developed within the semiclassical model. The approach is based on the introduction of a thermoballistic current consisting of carriers which move ballistically in the electric field provided by the band edge potential, and are thermalized at certain randomly distributed equilibration points by coupling to the background of impurity atoms and carriers in equilibrium. The sum of the thermoballistic and background currents is conserved and identified with the physical current. The current-voltage characteristic for nondegenerate systems and the zero-bias conductance for degenerate systems are expressed in terms of a reduced resistance. For arbitrary mean free path and arbitrary shape of the band edge potential profile, this quantity is determined from the solution of an integral equation, which also provides the quasi-Fermi level and the thermoballistic current. To illustrate the formalism, a number of simple examples are considered explicitly. The present work is compared with previous attempts towards a unified description of ballistic and diffusive transport.

Electrical properties of narrow-gap HgMnTe Schottky diodes

The electrical properties of Al/Hg1−x MnxTe (x=0.08–0.1) Schottky barriers have been studied. Specific features related to a narrow band gap and to a strong difference between the effective masses of carriers have been revealed. The principal parameters defining the characteristics of a diode structure, as well as the tunneling and above-barrier (diffusion) carrier transport, are determined. The obtained experimental and theoretical data demonstrate the high detectivity of the diodes under study.

Unified model of ballistic and diffusive carrier transport: Application to photovoltaic materials

A unified model of one-dimensional ballistic and diffusive carrier transport in semiconductors, valid for arbitrary mean free path and arbitrary shape of the band edge profiles, is applied in a study of the effect of grain boundaries on the transport properties of photovoltaic materials. Adopting the trapping model to describe the grain boundaries, dark conductivities and photoconductivities are calculated as a function of donor density for samples consisting of chains of grains with length comparable to the mean free path. The results of the unified model are found to deviate substantially from those of the purely ballistic and purely diffusive limits.

Recombination in deep etched CdZnSe/ZnSe quantum wires

Picosecond photoluminescence spectroscopy was used to investigate the recombination dynamics of excitons in deep etched CdZnSe/ZnSe quantum wires with lateral extensions down to 20 nm. In the low-temperature regime (T≤10 K), no significant reduction of the exciton lifetime was found down to a wire width of 20 nm, indicating a negligible influence of carrier loss at the wire sidewalls. At higher temperatures, the lifetime decreases for decreasing wire width,e.g., from 330 ps in the mesa structure to 21 ps in the 28 nm wide wires at room temperature. Simple model calculations indicate that this drop of the lifetime is due to diffusive carrier transport to the wire sidewalls and subsequent non-radiative surface recombination.