Charge Carrier Transport Processes Research Articles

Insight into the interface effect and PTC effect on the temperature-adaptive electrical conductivity of epoxy composite

Doping functional fillers into the polymeric matrix is an effective strategy to improve the electrical, thermal, and other performance of insulating materials. It is imperative to understand the influence of fillers on the charge carrier behavior to achieve better regulation effectiveness. In this work, micrometer-sized and nano-sized ceramic particles with positive temperature coefficient (PTC) electrical resistivity are employed to prepare the epoxy composites, whose electrical conductivity under different temperature and electric field, space charge characteristics, permittivity, and electric field distribution are studied. It is found that the doping of a PTC filler shifts the electrical conduction from bulk-controlled to electrode-limited, determining the quantity of charge carriers within epoxy composites. While the interface effect mainly affects the transport process of charge carriers, it would fail to dominate the electrical conduction since the abundant charge carrier introduced by the semiconductive functional filler. Combined with the reinforced interface effect, the electrical conductivity–temperature characteristic of the epoxy nanocomposite is optimized, leading to the reduction in the maximum electric field within electrical equipment insulation by 55%. These findings emphasize the synergistic regulation of charge carrier amount and transport, which contributes to the precision design of polymeric composites doped with functional fillers.

Computer Simulation of Charge Carrier Transport Processes in the Conducting Layers of Colloidal Quantum Dots

Computer Simulation of Charge Carrier Transport Processes in the Conducting Layers of Colloidal Quantum Dots

Preparation and characterization of large-sized CdZnTe epitaxial single crystal

CdZnTe (CZT) epitaxial single crystal with a diameter of 2 inches and a thickness exceeding 1 mm was prepared by the closed spaced sublimation. In order to test the uniformity of the large-sized CZT epitaxial single crystal, the composition, crystalline quality, X-ray response performance and carrier transport property of the central and edge parts of the CZT crystal were characterized. The morphology images show that different areas have flat surfaces with dislocation densities between 3.5 × 104 cm−2 and 4.5 × 104 cm−2, respectively. The crystal quality shows a trend of better in the central region than in the edge. The bandgap is between 1.56 eV and 1.57 eV, and the resistivity is between 5 × 109Ω cm and 6 × 109Ω cm. The sensitivities of the detectors prepared with the edge and central areas to X-ray is 276.22 μCGy−1 cm−2 and 283.43 μCGy−1 cm−2, respectively. The carrier lifetime products of the edge and central parts were tested to be 3.932 × 10−4 cm 2 V−1 and 4.511 × 10−4 cm2 V−1, by fitting the Hecht equation. The lower (μτ)e values of the edge part may be higher dislocation density, which has a great impact on the transport process of charge carriers, as demonstrated by the photoluminescence (PL) results. The fabricated detectors have a stable response to 241Am@59.5 KeV gamma-ray, which shows great potential for future applications in the field of photon counting imaging.

Time-Dependent Charge Carrier Transport with Hall Effect in Organic Semiconductors for Langevin and Non-Langevin Systems.

The time-dependent charge carrier transport and recombination processes in low-mobility organic semiconductor diodes are obtained through numerical simulations using the finite element method (FEM). The application of a Lorentz force across the diode alters the charge transport process leading to the Hall effect. In this contribution, the Hall effect parameters, such as the Hall voltage and charge carrier concentration with varying magnetic fields, are computed for both Langevin and non-Langevin type recombination processes. The results indicate the charge carrier concentration within the diode for the Langevin system is about seven and fourteen times less while the maximum amount of extracted charge is nearly five and ten times less than that in the non-Langevin system of 0.01 and 0.001, respectively. The Hall voltage values obtained for the steady-state case are similar to the non-Langevin system of ββL=0.01. However, the values obtained for the Langevin and non-Langevin systems of ββL=1 and 0.001 exhibit anomalies. The implications of these findings advance the understanding of the charge transport and Hall effect measurements in organic semiconductors that underpins the device's performance.

Contact property depending on radiation intensity between the perovskite semiconductor layer and electrode film

Hybrid organic–inorganic perovskites have been widely studied in high-performance optoelectronic devices. Long-term stability is a key limitation to date in restricting their further development and commercial application. In general, interface properties between a semiconductor and an electrode highly affect device performance and stability. Herein, contact characteristics between the perovskite and electrode varying with illumination status have been first investigated. The results suggest that device's contact resistance (RC), obtained from a transfer length method, decreases as the incident light intensity increases. The RC under a higher irradiation intensity (1.26 mW/cm2) is about one-tenth of that at a lower density (0.08 mW/cm2). Simulation has been performed and shows a similar trend with experimental results. The interface physical model has been discussed based on an energy band theory. The band bending and barrier modulation at the interface under light illumination are originated from the surface states and the localized charges. This work explains the interface in determining device's fundamental properties, including stability, RC variation, and charge carrier transport process. It is significant in understanding the device working mechanism and providing a potential way for perovskite optoelectronic devices with enhanced stability and performance.

(Invited) Probing Charge Carrier Dynamics in Porphyrin-Based Bulk Heterojunction Thin Films with Time-Resolved Terahertz Spectroscopy

Organic photovoltaic (OPV) solar cells have attracted much attention in recent years because of the low production cost and ease of device fabrication. However, the power conversion efficiency is still low compared to the silicon and perovskite solar cells so that further research is necessary to improve their performance. In particular, understanding of the detailed mechanisms of charge carrier generation, recombination and carrier transport process is very important for developing design principles of new compounds for OPVs. Until now, polymer-based compounds have been widely used for p-type semiconductors. However, it is not easy to obtain information on how the microscopic structure of bulk heterojunction (BHJ) blend of donor and acceptor materials affect the charge carrier dynamics because one cannot easily control the molecular weight and conformational distributions of polymer compounds. On the other hand, small molecule-based semiconductors have several unique advantages such well-defined chemical structures, ease of purification and small batch-to-batch variations.Tetrabenzoporphyrin (BP) has been frequently used as a small-molecule organic semiconductor. Recently, Yamada and coworkers synthesized diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) conjugates that have linear alkyl groups on the DPP moieties whose chain length is defined as n, namely, Cn-DPP-BP (n=4, 6, 8, or 10). For DPP-BP based solar cells, they demonstrated that alkyl chain length affects the power conversion efficiency. Therefore, it is very interesting to see how the local BHJ structures of DPP-BP based thin films affect the initial step of the charge carrier dynamics because such studies are difficult to perform for polymer-based systems. We can take advantage of small molecule-based systems to study the relationship between the local structure and the charge carrier dynamics.In this work, we used time-resolved terahertz (THz) spectroscopy to investigate the charge carrier dynamics of DPP-BP BHJ thin films blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Time-resolved THz spectroscopy is a unique method to quantify charge carrier mobility with very fast time resolution (less than picosecond). This spectroscopy reflects the charge carrier dynamics in the local spatial region because of the high-frequency nature of the THz electric field. We chose C4-DPP-BP:PC61BM and C10-DPP-BP:PC61BM thin films for the THz measurements. In the previous study, it was found that C4-DPP-BP molecule prefers to take a face-on geometry on the substrate. A grain size of the aggregates was around 100 nm or smaller which were measured by atomic force microscopy. In such a structure, one can expect that charge transport takes place very efficiently to the electrode. Actually, power conversion efficiency is 5.2 % which is highest among a series of DPP-BP chromophores. On the other hand, C10-DPP-BP adopts an edge-on geometry on the substrate and forms micrometer-sized aggregates. In this structure, molecular stacking is oriented perpendicular to the electrode so that power conversion efficiency decreases to 0.19%.In contrast to the other measurements, the THz spectroscopy has sensitivity to in-plane motion of the charge carriers with respect to the substrate, we expect to observe higher in-plane mobility of the charge carriers in the C10-DPP-BP:PC61BM BHJ films compared to C4-DPP-BP based films. In contrast to our intuitive expectation, we found that the product of the carrier mobility and the yield of the photo-generated charge carriers is similar to each other. We consider that the similarity of the charge carrier dynamics results from the high-frequency nature of the local mobility of the polarons because most of the polarons are confined in less than a 10-nm length scale. Furthermore, the lowest limit of the carrier mobility obtained from our results is about 1.0 cm2V-1s-1 when the quantum yield of the charge carrier generation is assumed to be unity. This value is much higher than those obtained by space-charge-limited current method. We expect that charge separation occurs very efficiently when the local mobility of the charge carrier is high.In this contribution, we will present importance of the local mobility of the charge carriers and difference of charge transport between short and long length scales in DPP-BP based solar cells.

Charge Generation and Transfer Mechanism of Bilayer Organic Photovoltaics with Unconventional Energy Alignment

The generation and transport processes of charge carriers in bilayer organic photovoltaics (OPVs), which rely on a suitable interfacial energy alignment between the donor and acceptor molecules, we...

Effect of TADF Assistance on Performance Enhancement in Solution Processed Green Phosphorescent OLEDs.

Many methods have been proposed to increase the efficiency of organic electroluminescent materials applied as an emissive layer in organic light emitting diodes (OLEDs). Herein, we demonstrate enhancement of electroluminescence efficiency and operational stability solution processed OLEDs by employing thermally activated delayed fluorescence (TADF) molecules as assistant dopants in host-guest systems. The TADF assistant dopant (SpiroAC–TRZ) is used to facilitate efficient energy transfer from host material poly(N–vinylcarbazole) (PVK) to a phosphorescent Ir(III) emitter. We present the analysis of energy transfer and charge trapping—two main processes playing a crucial role in light generation in host–guest structure OLEDs. The investigation of photo-, electro- and thermoluminescence for the double-dopant layer revealed that assistant dopant does not only harvest and transfer the electrically generated excitons to phosphorescent emitter molecules but also creates exciplexes. The triplet states of formed PVK:SpiroAC–TRZ exciplexes are involved in the transport process of charge carriers and promote long–range exciton energy transfer to the emitter, improving the efficiency of electroluminescence in a single emissive layer OLED, resulting in devices with luminance exceeding 18 000 cd/m2 with a luminous efficiency of 23 cd/A and external quantum efficiency (EQE) of 7.4%.

Enhanced optoelectronic performance of 3C–SiC/ZnO heterostructure photodetector based on Piezo-phototronic effect

Abstract Piezo-phototronic effect is widely utilized to improve the performance of optoelectronic devices through the coupling of semiconducting, optical, and piezoelectric properties in the ability to adjust charge carrier transport, separation and recombination process in piezoelectric semiconductors. In this paper, we demonstrate the photoresponse enhancement of a 3C–SiC/ZnO heterostructure photodetector based on piezo-phototronic effect. By introducing the strain-induced piezo-potential in ZnO nanorods array radially grown on an individual 3C–SiC nanowire core, photodetector performance is significantly enhanced due to the carrier-dynamics modulation at the local interface. The high photoresponsivity of 1.01 × 106 A/W and fast photoresponse time of 24.1 ms are obtained by applying an external compressive strain of −0.381%, corresponding to the enhancement of 117% in responsivity and shortening to 75.9% in photoresponse time, respectively, showing its outstanding ultraviolet (UV) light detection ability. The possible mechanism of strain-induced piezo-phototronic effect involved in 3C–SiC/ZnO heterostructure photodetector is proposed and discussed. These results present a new avenue for better utilizing piezo-potential in coupling with light, and are valuable for designing and fabricating strain modulated heterostructure optoelectronic devices.

Temperature dependent electrical properties of thermoplastic polypropylene nanocomposites for HVDC cable insulation

Polypropylene has been regarded as a potential recyclable insulation material for HVDC cable, which is beneficial to improve the working temperature and transmission capacity of HVDC cables. However, the electrical properties of polymeric insulation materials exhibit apparent temperature dependent characteristics. Temperature influences the generation and migration of charge carriers, thereby affecting the electrical properties. This paper focuses on the electrical properties of polypropylene nanocomposites at different temperatures, including volume resistivity, space charge and DC breakdown strength. The temperature dependent electrical properties of polypropylene are regulated by adding MgO nanoparticles. Based on the characterization of microstructure and trap characteristics, the mechanism of regulating the charge carrier transport process and the electrical properties of the nanocomposites by introducing MgO nanoparticles is explained. The polypropylene nanocomposites show decreased electrical conduction, suppressed space charge accumulation and increased DC breakdown strength, which are desirable for recyclable HVDC cable insulation application. This work may pave a way for tailoring the high temperature electrical properties of polymeric insulation material.

Electrochemical Research in the Energy Industry & the Role of Interfaces

As the energy transition unfolds, energy generation, distribution, storage, and use are evolving rapidly. Traditionally, carbon-based fuels such as oil, coal and natural gas are readily distributed and have served as energy storage media for the power and transportation sectors. With the growth of lower-carbon energy technologies, particularly wind and solar power, there is a need for high-performing, cost-effective solutions that provide analogous storage and distribution services to traditional fuels and enable effective use of these renewable electrons. The talk will include a brief introduction to key challenges whose solutions are central to successful, expanded deployment of electrochemical energy storage technologies. This will be followed by selected examples of ongoing electrochemical research that illustrate one energy company’s approach to R&D, in order to answer questions about fundamental phenomena that have the potential to redefine present understanding of electron storage and charge carrier transport processes and enhance the performance, energy density, and lifetime of electrical energy storage systems. Finally, elements of my professional experience will be highlighted that led me to electrochemical sciences and have been central to the global energy storage research program which I lead. These include the role and navigation of interfaces within and among technical disciplines and business functions.

Structure, magnetic behavior and OER activity of CoFe2O4 powders obtained using agar-agar from red seaweed (Rhodophyta)

This work is the first report on the synthesis of cobalt ferrite (CoFe2O4) powders by a proteic sol-gel green method that uses agar-agar from red seaweed (Rhodophyta). CoFe2O4 is also prepared using flavorless gelatin for the sake of comparison. The as-prepared powders are calcined at 1073 K and characterized by XRD, FESEM, Mössbauer spectroscopy (MS), and DC magnetometry. Electrodes made of CoFe2O4 powders were also investigated as electrocatalysts for the oxygen evolution reaction (OER), a reaction of high importance to produce pure H2 through water splitting. The material obtained with agar-agar showed a particle size of 176 nm and effective magnetocrystalline anisotropy of 5.8× 107 erg/cm3 (against 74 nm and 5.7 × 107 erg/cm3 for the gelatin-based sample). Magnetic assessment at 5 K showed a ferrimagnetic behavior for both samples. As an OER electrocatalyst, CoFe2O4 obtained with agar-agar attains an overpotential of 360 mV vs. RHE to generate 10 mA cm−2, a Tafel slope of 69.2 mV dec−1 and a turnover frequency (TOF) of 8.8 × 10−2 s−1 (against 435 mV vs. RHE, 126.3 mV dec−1 and 1.9 × 10−3 s−1 for the gelatin-based sample). The electrocatalytic performance towards OER can be explained by differences in mass and charge carrier transport processes influenced by microstructural features. The performance assessment of the cobalt ferrite (CoFe2O4) powders herein synthesized by sol-gel green methods shows their potential for using in energy conversion applications.

Charge Carrier Dynamics in Metal-Oxide/Nanocarbon Hybrid Photoelectrodes

One of the most critical challenges of the 21st century is the shift in energy use from fossil fuels to renewable sources. Photoelectrochemical solar energy conversion, as a potentially attractive candidate, has gained an increased momentum during the recent years. Although both water-splitting (hydrogen evolution) and carbon dioxide reduction are promising avenues, they both have substantial shortcomings to be addressed. A common virtue of these two processes is that a good photoelectrode has to concurrently fulfill many attributes to drive these reactions efficiently. One of the most important is the efficient charge carrier transport, which ensures that most of the photogenerated charge carriers can be extracted both toward the electrode/electrolyte junction and toward the back contact. Numerous promising photoelectrode material showed poor performance due to extensive charge carrier recombination both in the bulk and at the surface. A well-known example is α-Fe2O3 (hematite), in which the hole-diffusion length is only 2-4 nm.1 One possible way to improve charge carrier transport properties is to deposit the metal-oxide on an interconnected nanocarbon network (e.g., carbon nanotubes or graphene). These highly conductive nanoscaffolds can improve the transport of the photogenerated charge carriers, which in turn, results in an enhanced photoelectrochemical performance.2 In my presentation, I am going to discuss how charge carrier dynamics of various hematite and hematite/nanocarbon composites is affected by the composition of the nanohybrids and by the applied bias. Hematite/nanocarbon photoelectrodes were synthesized by a two-step method: first the nanocarbon layer was spray-coated on the surface of the substrate electrode, and the nanostructured metal-oxide component was electrodeposited in a subsequent step. The metal-oxide/nanocarbon ratio was precisely controlled by the deposition charge density. To investigate charge carrier dynamics on a timescale ranging from seconds to femtoseconds three different techniques were employed: (i) transient absorption spectroscopy, employing a fs pump-probe setup, (ii) intensity-modulated photocurrent spectroscopy, and finally (iii) fast chronoamperometry. By covering this broad timescale, it was possible to study charge carrier transport, trapping and recombination processes, which were greatly affected by both the applied potential and the presence of the nanocarbons. 1 Energy Environ. Sci., 2016, 9, 2744—2775. 2 J. Am. Chem. Soc. 2017, 139, 6682−6692.

Mapping Strain/Pressure with ZnO Nanowire Arrays By Piezotronic and Piezo-Phototronic Effect

Emulation of human senses via electronic means has long been a grand challenge in research of artificial intelligence as well as prosthetics, and is of pivotal importance for developing intelligently accessible and natural interfaces between human/environment and machine. Unlike other senses (seeing, hearing, smelling and tasting), capability of skin for touch sensing remains stubbornly difficult to be mimicked, which necessitates the development of large-scale pressure sensor arrays with high spatial-resolution, high-sensitivity and fast response. In this talk, we present a novel design of ZnO nanowire arrays, which can be used to directly record the strain distribution by piezotronic and piezo-phototronic effect. First, we have reported large-array three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as active taxel-addressable pressure/force-sensor matrix for tactile imaging with a high resolution of 100 μm1. Using the piezoelectric polarization charges created at metal-semiconductor interface under strain to gate/modulate transport process of local charge carriers, piezotronic effect has been applied to design independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied on the devices into local electronic controlling signals. The device matrix has been demonstrated for achieving shape-adaptive high-resolution tactile imaging and self-powered, multi-dimensional active sensing. However, the signal of the piezotronic transistors array is the change of the resistance of each NWs, which can only be measured in a series way. That means such piezotronic transistors array can only mapping a static strain. Different with the electrical signal, the optical signal can be measured in a parallel way. And in our previous work, we have demonstrated how the piezo-phototronic effect can be effectively utilized to enhance the emission intensity of an n-ZnO/p-GaN NW LED. The emission light intensity and injection current at a fixed applied voltage has been enhanced by a factor of 17 and 4 after applying a 0.093% compressive strain, respectively. Here, we extend the single NW device to NW LEDs array, for pressure/force sensor arrays for mapping strain with a resolution as high as 2.7 μm. Such sensors are capable of recording spatial profiles of pressure distribution, and the tactile pixel area density of our device array is 6250000/cm2, which is much higher than the number of tactile sensors in recent reports (~ 6-27/cm2) and mechanoreceptors embedded in the human fingertip skins (~ 240/cm2). When the device is under pressure, the images unambiguously show that the change in LED intensity occurred apparently at the pixels that were being compressed by the molded pattern, while those were off the molded characters showed almost no change in LED intensity. Instead of using the cross-bar electrodes for sequential data output, the pressure image is read out in parallel for all of the pixels at a response and recovery time-resolution of 90 ms. This may be a major step toward digital imaging of mechanical signals by optical means, with potential applications in touch pad technology, personalized signatures, bio-imaging and optical MEMS. Furthermore, our recent studies achieve such piezo-phototronic effect induced strain mapping in a flexible n-ZnO NWs/p-polymer LEDs array system composed of PEDOT:PSS and patterned ZnO NWs with a spatial resolution of 7 μm for mapping of spatial pressure distributions. The emission intensity of the LED array sensor matrix is dominated by locally applied strains as indicated by piezo-phototronic effect. Therefore, spatial pressure distributions are immediately obtained by parallel-reading the illumination intensities of LED arrays based on electroluminescence working mechanism. A wide range of pressure measurements from 40 MPa to 100 MPa was achieved through controlling the growth conditions of ZnO nanowire array. Lastly, the piezo-phototronic effect was achieved on Si wafer based on a n-ZnO nanofilm/p-Si micropillar heterostructure (ZSH) LEDs array. White light emissions at room temperature, featuring with emission peaks in both visible and near-infrared regions were obtained. By applying a strain onto the top of the ZSH LEDs, the light emission intensity of ZSH LEDs array was enhanced as well by 120% under -0.05% compressive strains. A pressure map can be created by reading out in parallel the change of the electroluminescent intensities from all the pixels in the near future. This research not only introduce a novel approach to fabricate Si-based light-emitting components with high performances, but also may be a great step toward digital imaging of mechanical signals using optical means, having potential applications in artificial skin, touch pad technology, personalized signatures, bio-imaging and optical MEMS, and even and smart skin.

Dielectric relaxation and carrier transport in epoxy resin and its microcomposite

Epoxy resin and its microcomposite are widely used as electrical insulation in electrical equipment. Surface flashover and breakdown properties of these materials can be influenced by dielectric relaxation process and charge carrier behavior. The charge transport characteristics and dielectric relaxation process for neat EP and its microcomposites were studied by means of dielectric relaxation spectroscopy. The glass transition temperatures Tg of the two kinds of samples were investigated with differential scanning calorimetry measurement, which were approximately 105 °C and 120 °C, respectively. Both the two kinds of samples were sputtered with gold electrodes on two sides, and electrodes' diameter was 30 mm. The properties of dielectric relaxation was measured by Concept 80 Novocontrol broadband dielectric spectrometer at various temperatures from 100 °C to 180 °C. There is a relaxation peak which labeled α in the range of high frequencies on account of the molecular chain movement or motion of segmental chains when the temperature is above Tg. At the same time, the dc conductivity attributed by the charge carrier transport occurs in the range of low frequencies. Besides, there are different relaxation times for molecular chains with different length scales. Moreover, there is a broad distribution of relaxation time for both neat EP and its microcomposites and the relaxation time distribution at different temperatures was calculated. In addition, it was found that the relation between peak frequency of relaxation process and temperature follow the Vogel-Fulcher-Tammann (VFT) law, and temperature dependences of dc conductivity also obey the VFT equation. The Vogel temperatures of dielectric relaxation process and charge carrier transport were calculated by fitting the results of permittivity and conductivity. The glass transition temperatures for neat EP and its microcomposite were estimated to be 102 °C and 117 °C through Vogel temperatures, which are in consistence with DSC measurement results. It represents that free volume in the two kinds of samples increases with the increase of temperature, thus enhancing molecular chain movement and charge carrier transport process.

A row hammer pattern analysis of DDR2 SDRAM

A DDR2 SDRAM test setup implemented on the Griffin III ATE test system from HILEVEL Technologies is used to analyse the row hammer bug. Row hammer pattern experiments are compared to standard retention tests. The analysis confirms that the row hammer effect is caused by a charge excitation process depending on the number of stress activation cycles. The stress has to occur in the local neighborhood of the cells under test. Shallow impurity levels support the responsible charge carrier transport process in the used DDR2 SDRAM technology.

Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors

Hydration of oxygen-deficient metal oxides causes filling of oxygen vacancies and formation of hydroxyl groups with interstitial structural protons, rotating around the oxygen in localized motion. Thermal activation from 500 to 800 K triggers delocalization of the protons by jumping to adjacent oxygen ions, constituting proton conductivity. We report quantitative analyses of proton and lattice dynamics by neutron-scattering data, which reveal the interaction of protons with the crystal lattice and proton–phonon coupling. The motion for the proton trapped in the elastic crystal field yields Eigen frequencies and coupling constants, which satisfy Holstein’s polaron model for electrons and thus constitutes first experimental evidence for a proton polaron at high temperature. Proton jump rates follow a polaron model for cerium-oxygen and hydroxyl stretching modes, which are thus vehicles for proton conductivity. This confirms that the polaron mechanism is not restricted to electrons, but a universal charge carrier transport process.

Enhancing the Efficiency of Silicon-Based Solar Cells by the Piezo-Phototronic Effect.

Although there are numerous approaches for fabricating solar cells, the silicon-based photovoltaics are still the most widely used in industry and around the world. A small increase in the efficiency of silicon-based solar cells has a huge economic impact and practical importance. We fabricate a silicon-based nanoheterostructure (p+-Si/p-Si/n+-Si (and n-Si)/n-ZnO nanowire (NW) array) photovoltaic device and demonstrate the enhanced device performance through significantly enhanced light absorption by NW array and effective charge carrier separation by the piezo-phototronic effect. The strain-induced piezoelectric polarization charges created at n-doped Si-ZnO interfaces can effectively modulate the corresponding band structure and electron gas trapped in the n+-Si/n-ZnO NW nanoheterostructure and thus enhance the transport process of local charge carriers. The efficiency of the solar cell was improved from 8.97% to 9.51% by simply applying a static compress strain. This study indicates that the piezo-phototronic effect can enhance the performance of a large-scale silicon-based solar cell, with great potential for industrial applications.

Improvement of photovoltaic performance of inverted hybrid solar cells by adding single-wall carbon nanotubes in poly(3-hexylthiophene)

Solution prepared hybrid solar cells show promising low cost technology for electricity generation from sun light, although their power conversion efficiency has to be improved. One of the approaches is to increase the absorbance or charge carrier mobility of organic semiconductors. In this work, pristine single walled carbon nanotubes (SWCNT) were added into poly(3-hexylthiophene) (P3HT) solution to form P3HT:SWCNT composite films with different weight percent (wt%) of SWCNT. It is observed that optical absorbance spectra as well as the morphology of the composite films were modified by the addition of SWCNTs. This phenomenon could be explained by the π-π interaction between the conjugated polymer and carbon nanotubes. Most importantly, the electrical conductivities of the composite films increased with the SWCNT wt%. When these films were used as hole conductor layers in inverted planar hybrid solar cell, with CdS thin films as electron acceptor layers, the fill factor (FF) and open-circuit voltage (Voc) of the corresponding cells were decreased with the increase of the wt% of SWCNT. However, the short-circuit current density (Jsc) and the power conversion efficiency (PCE) showed a maximum value at about 0.4wt% of SWCNT in P3HT. The transient photovoltage measurements (TPV) revealed that the presence of SWNCT promoted the charge recombination process at P3HT/CdS interface, and as a result, reduced the Voc. The photovoltaic performance of the hybrid solar cells could be optimized by choosing an adequate weight percentage of SWCNT in P3HT to balance the charge carrier transport and charge recombination processes at the donor-acceptor interface.

Observation of Space Charge Dynamics Inside an All Oxide Based Solar Cell.

The charge transfer dynamics at interfaces are fundamental to know the mechanism of photovoltaic processes. The internal potential in solar cell devices depends on the basic processes of photovoltaic effect such as charge carrier generation, separation, transport, recombination, etc. Here we report the direct observation of the surface potential depth profile over the cross-section of the ZnO nanorods/Cu2O based solar cell for two different layer thicknesses at different wavelengths of light using Kelvin probe force microscopy. The topography and phase images across the cross-section of the solar cell are also observed, where the interfaces are well-defined on the nanoscale. The potential profiling results demonstrate that under white light illumination, the photoinduced electrons in Cu2O inject into ZnO due to the interfacial electric field, which results in the large difference in surface potential between two active layers. However, under a single wavelength illumination, the charge carrier generation, separation, and transport processes between two active layers are limited, which affect the surface potential images and corresponding potential depth profile. Because of changes in the active layer thicknesses, small variations have been observed in the charge carrier transport mechanism inside the device. These results provide the clear idea about the charge carrier distribution inside the solar cell in different conditions and show the perfect illumination condition for large carrier transport in a high performance solar cell.