Surface Doping Level Research Articles

Anti-site defect-induced disorder in compensated topological magnet MnBi2-xSbxTe4

The gapped Dirac-like surface states of compensated magnetic topological insulator MnBi2-xSbxTe4 (MBST) are a promising host for exotic quantum phenomena such as the quantum anomalous Hall effect and axion insulating state. However, it has become clear that atomic defects undermine the stabilization of such quantum phases as they lead to spatial variations in the surface state gap and doping levels. The large number of possible defect configurations in MBST make studying the influence of individual defects virtually impossible. Here, we present a statistical analysis of the nanoscale effect of defects in MBST with x=0.64, by scanning tunnelling microscopy/spectroscopy. We identify (Bi,Sb)Mn anti-site defects to be the main source of the observed doping fluctuations, leading towards the formation of nanoscale charge puddles and effectively closing the transport gap. Our findings will guide further optimization of this material system via defect engineering, to enable exploitation of its promising properties.

Flexible and easy-handling pristine polypyrrole membranes with bayberry-like vesicle structure for enhanced Cr(VI) removal from aqueous solution

Polypyrrole has been extensively explored for Cr(VI) removal from wastewater towing to the advantages of superior performance, low cost, facile synthesis, and high environmental stability. However, the unsatisfactory adsorption capacity and complicated process of adsorbent separation from aqueous solutions remain a huge challenge, limiting its practical application. Herein, a flexible PPy membrane with bayberry-like vesicle structures (PPy-B) was prepared via template-assisted interfacial polymerization. It was found that sodium sulfosalicylate not only improved the flexibility and strength of the PPy-B membrane for easy-handling but also participated in the polymerization of PPy as a dopant to improve the specific surface area and doping level for increasing adsorption sites. Benefiting from these, the easy-handling PPy-B membrane exhibited a high adsorption capacity (586.90–682.50 mg/g at 298–318 K), a high reusability (five adsorption-desorption cycles), and a high ultimate adsorption capacity after adsorption-desorption cycles until membrane failure (1174.86 mg/g at 298 K). The proposed mechanisms of the enhanced Cr(VI) removal involve electrostatic adsorption, reduction, and ion exchange. This flexible PPy membrane therefore shows attractive advantages in wastewater treatment.

Carrier Transmission Mechanism-Based Analysis of Front Surface Field Effects on Simplified Industrially Feasible Interdigitated Back Contact Solar Cells

Interdigitated back contact (IBC) n-type silicon solar cells with a different front surface layer doping concentration were fabricated and studied and the influence of the front surface doping level was analyzed via simulation (PC1D). The IBC cells were processed by industrially feasible technologies including laser ablation and screen printing; photolithography was not used. A maximum efficiency of up to 20.88% was achieved at an optimal front surface field (FSF) peak doping concentration of 4.8 × 1019 cm−3 with a sheet resistance of approximately 95 Ω/square, corresponding to Jsc = 40.05 mA/cm2, Voc = 671 mV and a fill factor of 77.70%. The effects of the front surface doping level were studied in detail by analyzing parameters related to carrier transmission mechanisms such as minority carrier concentration, minority carrier lifetime and the saturation current density of the FSF (J0e). The influence of the front surface recombination velocity (FSRV) on the performance of IBC solar cells with different FSF layer doping concentrations was also investigated and was verified by examining the variation in the minority carrier density as a function of the distance from the front surface. In particular, the impact of the FSF doping concentration on the Jsc of the IBC cells was clarified by considering carrier transmission mechanisms and the charge-collection probability. The trends revealed in the simulations agreed with the corresponding experimental data obtained from the fabricated IBC solar cells. This study not only verifies that the presented simulation is a reasonable and reliable guide for choosing the optimal front surface doping concentration in industrial IBC solar cells but also provides a deeper physical understanding of the impact that front surface layer doping has on the IBC solar cell performance considering carrier transmission mechanisms and the charge-collection probability.

Numerical model for the chemical adsorption of oxygen and reducing gas molecules in presence of humidity on the surface of semiconductor metal oxide for gas sensors applications

Charge transfer between the interacting gas molecules and the surface of the sensing layer is the main mechanism for the detection of gases in semiconductor metal oxides. In the presented work, the Wolkenstein's theory of adsorption is used instead of the conventional Langmuir isotherm for the numerical modelling of adsorption of oxygen, reducing gas (CO) molecules and water vapours. A numerical model of chemical adsorption of these gas species at the surface of tin oxide (SnO2) semiconductor gas sensor is presented in this paper. Using this model, quantitative calculation of the combined effect of environmental oxygen, reducing gas(CO) and water vapor adsorption on various electronic properties like electrical conductivity, surface potential and the work function of the metal oxide surface has been carried out. The surface coverage of the chemically adsorbed oxygen gas molecules is simulated as a function of oxygen gas pressure, the temperature of the sensor surface and bulk doping level of the n-type SnO2 semiconductor. Along with oxygen, the adsorption of CO gas is simulated as a function of CO gas pressure at constant atmospheric pressure of Oxygen gas. Furthermore, the sensor response is simulated and compared both in presence of dry Carbon Monoxide (CO) gas as well in the humid environment. It is shown that in the presence of water vapor there is an increase in the conductivity due to the decrease in the surface potential barrier at the semiconductor surface.

(Invited) Materials Properties and Their Impact on Beta Gallium Oxide for High Power Electronics Applications

Beta gallium oxide represents a promising material for high power device applications. However, the incorporation of this material into device structures is not as well understood as materials which have been used previously for high power device applications. Those materials (for example silicon, SiC, and GaN) possess crystal structures with higher symmetry. The monoclinic structure of beta gallium oxide leads to non-uniform thermal stresses in bulk and epitaxial layers and significant differences in reactivity due to surface orientations and doping levels. The low thermal conductivity of beta gallium oxide can be compensated by integrating it with other high conductivity materials. We address the crystalline perfection in state-of-the-art beta gallium oxide using high resolution x-ray scattering and topography techniques, the role of chemical mechanical polishing on sub-surface damage, the impact of the differences in thermal expansion coefficients along different crystallographic directions on gallium oxide structures in epitaxial systems, and initial studies into large scale exfoliation techniques for materials integration of gallium oxide with high thermal conductivity materials.

Optimized n-type amorphous silicon window layers via hydrogen dilution for silicon heterojunction solar cells by catalytic chemical vapor deposition

A comprehensive study of the microstructures and properties of n-type hydrogenated amorphous silicon (n-a-Si:H) films, deposited by catalytic chemical vapor deposition, for the window layers of silicon heterojunction (SHJ) solar cells is presented. With increasing hydrogen-to-silane dilution ratio (RH), the deposited films first become dense, after which they loosen. With further increases in RH, the films tend to crystallize with native post-oxidization. The doping efficiencies of phosphorus in the various n-a-Si:H films are similar, but the upper surface doping levels of the films are affected by RH. The post-oxidized n-a-Si:H film is more transparent at short wavelengths than a dense film deposited at low RH, exhibiting an external quantum efficiency gain of 20% at 300 nm. Finally, a higher efficiency and short-circuit current density (Jsc) are obtained with the post-oxidized n-type a-Si:H window layer; a Jsc gain of 0.25 mA/cm2 and an efficiency increase of 0.36% were achieved for the optimized SHJ solar cell. At the device level, a dense intrinsic a-Si-H passivated layer is beneficial for suppressing fill-factor (FF) deterioration. The natively post-oxidized n-a-Si:H window layer is a potential choice for improving Jsc by apparently enhancing light absorption in crystalline silicon at short wavelengths.

The Electrochemical Performance of Nitrogen-Containing Carbon Nanotubes As the Electrode Material of Supercapacitors

Electrochemical electric double layer capacitors (EDLC) known as supercapacitors are used for high power density, long cycle life and good reversibility of charge/discharge processes[1]. Recent years, many Schiff base transition metal composites acting as an electroactive electrode material deposited on carbon nanotubes(CNTs) have been researched[2]. However, the CNTs tend to be random orientation, twining round and cross linked. So it urgently need to be addressed. In this paper, a given mass of NH4BF4 was added to CNTs and the electrode was prepared by the homodispersion of the raw support material in N-methyl- 2-pyrrolidone (NMP) through sonicating. The slurry was coated on a Ti sheet with the coating mass of 0.3 mg. The electropolymerization of Ni(salen) on the resulting electrode was conducted in a closed three-electrode compartment cell adopted chronoamperometry method. The introduction of NH4BF4 to the CNTs with a specific percentage (5wt. %, 10wt. %, 15 wt. %), and the modified samples were named as NBF5, NBF10, NBF15 respectively and the CNTs without NH4BF4 were taken as a control group and named as NBF0. As the cyclic voltammetric data summarized in Table 1, the values of apparent surface coverage(Γ) and doping level(n) of NBF5, NBF10 and NBF15 didn't appear to be much different from those of NBF0. However, the specific capacitance varied a lot. It can be seen that Cm of NBF15 is the most among the samples. Figure 1 depicts the GCD plots of the composite and CNTs at the current density of 1.0 mA cm-2. CNTs have symmetric change in the charge/discharge process which is the characteristic of high Coulombic efficiency. It is clear that the composites possess longer charge/discharge time than CNTs which indicating that poly[Ni(salen)] contributes the pseudo-capacitance derived from reversible redox switching between neutral and oxidative states on the basis of electric double-layer capacitance of MWCNTs. Because of the introduction of NBF, it became difficult for CNTs to twint together disorderly. And the morphylogy of the electrode support got homogeneous. NBF-containing carbon nanotube could provide basic sites, and consequently improved the deposition of polymer and enhance the performance of the electrode material. Acknowledgement This work is financially supported by the National Natural Science Foundation of China (No. 51372021).

(Invited) Microplasmas Technologies for Engineering of Silicon Based Quantum Dot Solar Cells

Silicon nanocrystals (Si-ncs) are fascinating candidates for numerous applications such as photovoltaic cells. Solar cell technologies are highly dependent on silicon materials and Si-ncs with optimal energy band gap present potential for further development high efficiency solar cells. Especially, the tunability of the Si energy bandgap by precisely controlling the size of Si-ncs due to quantum confinement effects is expected to increase the efficiency of third generation all silicon tandem solar cells. Among the different semiconductor nanocrystals, Si-ncs have a special value as they are generally non-toxic and have been widely used on an industrial scale. A crucial aspect for Si-ncs integration into PV devices is represented by the control of the surface characteristics. Since quantum confinement effects are observed for small nanocrystal sizes (<10 nm in diameter) and at these dimensions the Si-nc surface chemistry plays a critical role for the overall optoelectronic properties and device performance. While the use of bulky and/or long molecular passivation is frequent and it is effective in stabilizing the Si-nc properties in colloids, it is highly detrimental for transport and device-related properties. In particular, photon absorption and subsequently exciton dissociation and carrier transport suffer from the separation between Si-ncs due to the bulky surfactants and considerably decrease the performance of devices. Recently, plasma-based approaches have been successfully applied to induce non-equilibrium chemical reactions on the surface of Si-ncs directly in colloids and to achieve surface-engineered Si-ncs without any bulky surfactants. Such approaches led to the enhancement of the Si-nc transport properties and its electronic interaction with a polymer matrix [1] as well as Si-nc long-term stability in water based solutions [2]. Another important issue is the doping of the Si-ncs. A number of issues especially regarding the large impurity formation energy by doping and transport are still in controversial debate. Doping of quantum confined Si-ncs offers unique opportunities to control the bandgap and the Fermi energy level. Particularly, boron-doped and phosphorus-doped quantum confined silicon nanocrystals (Si-ncs) are surface-engineered in colloidal solution by an atmospheric pressure radio frequency microplasma a will be underlined in this talk. In particular, we have performed the surface engineering of electrochemically etched p- and n-type Si-ncs where the no-change position of the dopant in Si-ncs is assured by room temperature processing [3]. We report on how the microplasma induced surface engineering can lead to drastically different outcomes due to different doping and how this impacts the corresponding optical and transport properties [3]. Next we report on Fermi level for doped Si-ncs. Our measurements demonstrate that surface engineering can tune the band energy levels and in particular the Fermi level. Indeed, we fabricate solar cells to test p-/n-type Si-ncs and induced surface chemistry to demonstrate the resulting effects. In particular it is noted that band alignment, dissociation efficiency and transport are all directly affected by the interplay of varying surface chemistry and doping levels. We demonstrate that surface chemistries induced on the Si-ncs strongly depend on the type of dopants and implications for third generation solar cells are discussed in details [4, 5]. As a final point, surfactant free microplasma surface-engineered Si-ncs can be integrated into the device architecture to be only optically active and provide a means of effective down-conversion of blue photons into red photons leading to 24% enhancement of the photocurrent under concentrated sunlight. We also show that the down-conversion effect under 1-sun is enhanced in the case of hybrid solar cells where microplasma engineered Si-ncs are also beneficial in the active absorption layer of the solar cells.

Engineering Palladium Surfaces to Enhance the Electrochemical Storage of Hydrogen

Materials can be engineered to have enhanced hydrogen storage capabilities during electrolysis by modifying the composition of the first few surface layers at the electrolytic interface. The proper choice of surface layer can radically affect the electrochemical insertion of hydrogen. To this end, between 1 μg cm−2 and 23 μg cm−2 of Pb was deposited onto Pd cathodes during galvanostatic experiments in the 0.1 M LiOH electrolyte. The optimum surface doping level 2.9 μg cm−2 (~1.4 mass equivalent monolayers) of Pb was found to achieve the highest quantity of inserted hydrogen at a potential of approximately −0.5 V vs RHE. Additionally, the hydrogen content increased from PdH0.75 to PdH0.86 with increasing Pb amounts up to 2.9 μg cm−2 at a constant current of −14.5 mA cm−2. For comparison, the same change in hydrogen content from pressurized gas loading experiments would require an increase in hydrogen fugacity from about 6 to 1420 atm. Furthermore, the addition of a small amount of Pt co-deposited with the Pb resulted in a further increase in hydrogen content to PdH0.89 at −14.5 mA cm−2. Preliminary analysis suggests the Pb and Pt are changing the balance between the Volmer, Heyrovsky, and Tafel reaction rates, which changes the chemical potential of the adsorbed hydrogen, and ultimately controls the hydrogen insertion. This work provides a fundamental basis for the future design of surface alloys yielding enhanced electrochemical hydrogen insertion in Pd and other hydrogen absorbing materials.

Optimal Surface Doping of Lead for Increased Electrochemical Insertion of Hydrogen into Palladium

Increasing the amount of hydrogen that is electrochemically inserted into materials is important for studying superconductivity and hydrogen embrittlement, and improving hydrogen storage capabilities. Surfaces can be engineered to accomplish this task with better insight into how the composition of a material's first few atomic layers affects the electrochemical insertion of hydrogen. To this end, different amounts of Pb were added to the 0.1M LiOH electrolyte to be deposited onto Pd cathodes during galvanostatic experiments. The investigated amount of added Pb was between 1μgcm−2 and 23μgcm−2 with respect to the geometric area of the Pd cathode. The optimum surface doping level 2.9μgcm−2 of Pb (∼1.4 mass equivalent monolayers) was found to achieve the highest quantity of inserted hydrogen at approximately −0.5V vs RHE. Additionally, the hydrogen content increased from PdH0.75 to PdH0.86 with increasing Pb amounts up to 2.9μgcm−2 at a constant current of −14.5mAcm−2. For comparison, the same change in hydrogen content from pressurized gas loading experiments would require an increase in hydrogen fugacity from about 6 to 1420 atm. Preliminary analysis concerning the adsorbed hydrogen chemical potential suggests the Pb is affecting the balance between the Volmer, Heyrovsky, and Tafel reaction rates, which changes the hydrogen surface chemical potential, and ultimately controls the hydrogen insertion. Furthermore, the addition of Pb was found to decrease the rate of hydrogen insertion. This work provides a fundamental basis for the future design of metal surfaces yielding enhanced electrochemical hydrogen insertion in Pd and other hydrogen absorbing materials.

MBE growth and doping of AlGaP

In this work, we investigate the impact of growth parameters on surface morphology, doping levels and dopant activation in AlGaP epilayers grown on GaP substrate by solid source molecular beam epitaxy. Atomic Force Microscopy analysis reveals that a smooth surface can be obtained only in the [580–680]°C growth temperature range for a sufficiently large V/III ratio. From C(V), Hall measurements and SIMS analysis, it is shown that a reasonable activated p-doping (Be) value (typically 1018cm−3) can be reached if the growth temperature remains larger than 580°C. For the n-doping (Si), the situation is much more critical, as a growth temperature below 650°C leads to a strongly insulating layer. This dopant activation issue is related to the appearance of deep traps that are generated when growth temperature is not high enough, as evidenced by deep level transient spectroscopy and isothermal deep level transient spectroscopy. It is therefore suggested that electrically-driven devices using AlGaP epilayers have to be carefully designed in order to match both roughness and dopants activation constraints on growth temperatures and growth rates.

Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal.

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.

The Electrostatics of the Semiconductor-Electrolyte Interface

The semiconductor-electrolyte interface presents a great interest both form fundamental and applied perspective. Both phases have free mobile charges – ions in the electrolyte and electrons and/or holes in the semiconductor. Hence there will be distributions of charge density and potential in the electrolyte and in the semiconductor, which will be dependent on the interfacial charge as well as each other. This means that a perturbation of the charge or potential in one of the phases will elicit a response in the other one and vice versa. An important and interesting feature of the semiconductor-electrolyte interface is that the charge distribution may have to account for the fermionic nature of the charges present in the semiconductor. Exploring the high doped semiconductor limit allows us to also draw conclusion about the properties of metal-electrolyte interfaces. Understanding the charge and potential interactions between that semiconductor and electrolyte has many practical implications. These include controlled assembly of semiconductor nanocolloids1, electronics and optoelectronics2-5, catalysis6, corrosion, sending and detection.7-11 Recently we have performed a theoretical analysis of the electrostatics of semiconductor-electrolyte interfaces.12-13 We have found that charge distributions in semiconductor and the electrolyte affect each other and this effect is particularly strong if the interfacial charge is accounted for by examining the surface chemistry. An illustration of this effect is shown in the Figure below which examined the interaction between two semiconductor nanocolloids suspended in electrolyte solution. We have found that the doping level of the particles has a strong effect on their stability against flocculation and coalescence. The effect of parameters such as electrolyte concentration, surface chemistry thermodynamics and doping levels were studied in details. The effect of the fermionic nature of the charge carriers in the semiconductor phase was also established. 1. F. Meseguer, R. Fenollosa, I. Rodriguez, E. Xifré-Pérez, F. Ramiro-Manzano, M. Garín, and M. Tymczenko, J. Appl. Phys., 2011. 109: p. 102424. 2. M.V. Kovalenko, M.I. Bodnarchuk, J. Zaumseil, J.-S. Lee, and D.V. Talapin, J. Am. Chem. Soc., 2010. 132: p. 10085–10092. 3. M.V. Kovalenko, M. Scheele, and D.V. Talapin, Colloidal nanocrystals with molecular metal chalcogenide surface ligands. Science, 2009. 324: p. 1417-1420. 4. A. Nag, M.V. Kovalenko, J.-S. Lee, W. Liu, B. Spokoyny, and D.V. Talapin, J. Am. Chem. Soc., 2011. 133: p. 10612–10620. 5. D.V. Talapin, J.-S. Lee, M.V. Kovalenko, and E.V. Shevchenko, Chem. Rev., 2010. 110: p. 389-458. 6. T. Blasco, A. Corma, M.T. Navarro, and J.P. Pariente, J. Catalysis., 1995. 156: p. 65-74. 7. Y. Cui, Q. Wei, H. Park, and C.M. Lieber, Science, 2001. 293: p. 1289-1292. 8. J.-L. Hahm and C.M. Lieber, Nano Lett., 2004. 4: p. 51-54. 9. F. Patolsky and C.M. Lieber, Materials Today, 2005. 8: p. 20-28. 10. F. Patolsky, G. Zheng, and C.M. Lieber, Anal. Chem., 2006. 78: p. 4261-4269. 11. A.K. Wanekaya, M.A. Bangar, M. Yun, W. Chen, N.V. Myung, and A. Mulchandani, J.Phys. Chem. C., 2007. 111: p. 5218-5221. 12. M.E. Fleharty, F. Van Swol, and D.N. Petsev, Phys. Rev. Lett., 2014. 113: p. 158302. 13. M.E. Fleharty, F. Van Swol, and D.N. Petsev, J. Colloid Interface Sci., 2015. 449: p. 409-415. Figure 1

(Invited) Microplasmas Technologies for Third Generation Solar Cells Based on Colloidal Nanocrystals with Quantum Confinement Effects

Solar cell technologies are highly dependent on silicon materials and silicon nanocrystals (Si-ncs) with optimal energy band gap present potential for further development high efficiency solar cells at low cost and harmless environmental impact. Especially, the tunability of the Si energy bandgap by precisely controlling the size of Si-ncs due tio quantum confinement effects is expected to increase the efficiency of third generation all silicon tandem solar cells. A crucial aspect for Si-ncs integration into PV devices is represented by the control of the surface characteristics. Since quantum confinement effects are observed for small nanocrystal sizes (<10 nm in diameter) and at these dimensions the Si-nc surface chemistry plays a critical role for the overall optoelectronic properties and device performance. While the use of bulky and/or long molecular passivation is frequent and it is effective in stabilizing the Si-nc properties in colloids, it is highly detrimental for transport and device-related properties. In particular, photon absorption and subsequently exciton dissociation and carrier transport suffer from the separation between Si-ncs due to the bulky surfactants and considerably decrease the performance of devices. Recently, plasma-based approaches have been successfully applied to induce non-equilibrium chemical reactions on the surface of Si-ncs directly in colloids and to achieve surface-engineered Si-ncs without any bulky surfactants. Such approaches led to the enhancement of the Si-nc transport properties and its electronic interaction with a polymer matrix [1] as well as Si-nc long-term stability in water based solutions [2]. Another important issue is the doping of the Si-ncs. A number of issues especially regarding the large impurity formation energy by doping and transport are still in controversial debate. Doping of quantum confined Si-ncs offers unique opportunities to control the bandgap and the Fermi energy level. Particularly, boron-doped and phosphorus-doped quantum confined silicon nanocrystals (Si-ncs) are surface-engineered in colloidal solution by an atmospheric pressure radio frequency microplasma a will be underlined in this talk. In particular, we have performed the surface engineering of electrochemically etched p- and n-type Si-ncs where the no-change position of the dopant in Si-ncs is assured by room temperature processing [3]. We report on how the microplasma induced surface engineering can lead to drastically different outcomes due to different doping and how this impacts the corresponding optical and transport properties [3]. Next we report on Fermi level for doped Si-ncs. Our measurements demonstrate that surface engineering can tune the band energy levels and in particular the Fermi level. Indeed, we fabricate solar cells to test p-/n-type Si-ncs and induced surface chemistry to demonstrate the resulting effects. In particular it is noted that band alignment, dissociation efficiency and transport are all directly affected by the interplay of varying surface chemistry and doping levels. We demonstrate that surface chemistries induced on the Si-ncs strongly depend on the type of dopants and implications for third generation solar cells are discussed in details [4, 5]. As a final point, surfactant free microplasma surface-engineered Si-ncs can be integrated into the device architecture to be only optically active and provide a means of effective down-conversion of blue photons into red photons leading to 24% enhancement of the photocurrent under concentrated sunlight. We also show that the down-conversion effect under 1-sun is enhanced in the case of hybrid solar cells where microplasma engineered Si-ncs are also beneficial in the active absorption layer of the solar cells. References V. Svrcek, T. Yamanari, D. Mariotti, K. Matsubara, M. Kondo, Enhancement of hybrid solar cell performance by polythieno [3,4-b]thiophenebenzodithiophene and microplasma-induced surface engineering of silicon nanocrystals, Appl. Phys. Lett. 2012, 100, 223904. S. Mitra, V. Svrcek, D. Mariotti, K. Matsubara, M. Kondo, Microplasma-induced liquid chemistry as a tool for stabilization of silicon nanocrystals optical properties in water, Plasma Processes and Polym., 2014, 11 158. T. Velusamy, S. Mitra, M.l Macias-Montero, V. Svrcek, D. Mariotti, Varying Surface Chemistries for p-Doped and n-Doped Silicon Nanocrystals and Impact on Photovoltaic Devices. ACS Appl Mater Interfaces, 2015, 7(51), 28207-14. V. Svrcek, T. Yamanari, D. Mariotti, S. Mitra, T. Velusamy, K. Matsubara, A silicon nanocrystal/polymer nanocomposite as a down-conversion layer in organic and hybrid solar cells, Nanoscale, 2015, 7, 11566. D. Mariotti, T. Belmonte, J. Benedikt, T. Velusamy, G. Jain, V. Svrcek, Low-Temperature Atmospheric Pressure Plasma Processes for ‘‘Green’’ Third Generation Photovoltaics, Plasma Process. Polym. 2016, 13, 70.

Influence of surface near doping concentration on contact formation of silver thick film contacts

We investigate the properties of silver thick film contacts on phosphorus doped alkaline textured silicon surfaces with varying surface doping levels. Using scanning electron microscopy, we find that the surface near doping concentration influences the formation of silver crystallites especially those that are in direct contact with both the silicon surface and the contact finger. The observed correlation between direct silver crystallite imprint coverage and specific contact resistance allows for extracting the specific contact resistance of individual crystallites to be approximately 3.3µΩcm2, which agrees well with theoretical predictions. Furthermore we present a new approach that allows for a quantitative separation of direct and indirect current conduction channels. For our experimental conditions, the assumption of current conduction exclusively by transport through silver crystallites adequately describes the experimental data. Finally we fabricate bifacial n-type solar cells with peak efficiencies of 19.9%.

The Fabrication and Optimization of Thin-Film Transistors Based on Poly(3-Hexylthiophene) Films for Nitrogen Dioxide Detection

Poly(3-hexylthiophene) (P3HT) films were used as active layers in bottom contact organic thin-film transistor (OTFT) gas sensors to detect nitrogen dioxide (NO2). The films with different thicknesses were deposited on the devices to measure current–voltage characteristics and to test the gas sensing properties of the OTFT devices. All prepared OTFTs exhibited p-type semiconductor behaviors. The effect of film thickness on the electronic characteristics of OTFTs was investigated, and the OTFT gas sensor with the thinnest film presented best electronic characteristics. The OTFT gas sensors with thinner P3HT film showed smaller baseline drift, higher response to NO2 gas of a certain concentration, and larger sensitivity enhancement. The sensitivity increased from 0.038 to 0.11 ppm $^{{-1}}$ (up to 190%) with the decrease in film thickness from 600 to 200 nm. The absorption of NO2 changed the surface doping level, and thereby perturbed the charge transport properties of the devices, which made OTFT gas sensors with thinner film more sensitive. Compared with the response curves and repeatability curves of all devices, the response of the OTFT gas sensor with the thickest film was not precise owing to the baseline drift. The selectivity was also investigated. The results demonstrated that the response to NO2 was improved due to the film thickness optimization. Moreover, the sensing mechanism was analyzed as well.

Mechanism of direct current electrical charge conduction in p-toluenesulfonate doped polypyrrole/carbon composites

Polypyrrole/carbon (PPy/C) composites have been synthesized using varying concentration of p-toluenesulfonate (pTS) dopant by surface initiated in-situ chemical oxidative polymerization. The synthesis and influence of pTS on the structure of the PPy/C composites are confirmed by Fourier transform infrared studies and the morphological features have been examined by scanning electron microscopy. X-ray photoelectron spectroscopy, employed to examine the surface composition and doping level of these composites, confirms the anionic doping into the polymer backbone. Electron spin resonance measurement has been carried out on these samples to identify the nature of the charge carriers and their concentration at different doping levels. The dc electrical conductivity of these composites has been measured in the temperature range ∼10–305 K. The observed results have been analyzed in the framework of existing theoretical models. Different Mott's parameters, such as characteristic temperature (T0), density of states at the Fermi level {N(EF)}, average hopping distance (R), and average hopping energy (W), evaluated from dc conductivity data supports the applicability of Mott's three dimensional variable range hopping mechanism in this system.

Specific Contact Resistance Measurement of Screen-Printed Ag Metal Contacts Formed on Heavily Doped Emitter Region in Multicrystalline Silicon Solar Cells

Multicrystalline silicon (mc-Si) wafers are widely used to develop low-cost high-efficiency screen-printed solar cells. In this study, the electrical properties of screen-printed Ag metal contacts formed on heavily doped emitter region in mc-Si solar cells have been investigated. Sintering of the screen-printed metal contacts was performed by a co-firing step at 725°C in air ambient followed by low-temperature annealing at 450°C for 15 min. Measurement of the specific contact resistance (ρc) of the Ag contacts was performed by the three-point probe method, showing a best value of ρc = 1.02 × 10−4 Ω cm2 obtained for the Ag contacts. This value is considered as a good figure of merit for screen-printed Ag electrodes formed on a doped mc-Si surface. The plot of ρc versus the inverse of the square root of the surface doping level (Ns−½) follows a linear relationship for impurity doping levels Ns ≥ 1019 atoms/cm3. The power losses due to current traveling through various resistive components of finished solar cells were calculated by using standard expressions. Cross-sectional scanning electron microscopy (SEM) views of the Ag metal and doped mc-Si region show that the Ag metal is firmly coalesced with the doped mc-Si surface upon sintering at an optimum firing temperature of 725°C.

Characterization of heavily doped SOI wafers under pseudo-MOSFET configuration

The pseudo-MOSFET (Ψ-MOSFET) method is extended for the electrical characterization of heavily doped (1019–1020cm−3) SOI wafers with 10–40nm film thickness. The field-effect modulation is small and does not enable the formation of an inversion channel. Only accumulation and depletion-controlled volume conduction modes are activated by increasing the back-gate voltage to 40V. An updated model describing the conduction regimes for heavily doped SOI wafers is derived. This model provides a simple method for parameters extraction such as surface and volume mobility and doping level. Four-point probe and Hall effect measurements fully validate our Ψ-MOSFET results. It is found that high-dose implantation results in good redistribution and electrical activation of impurities, without affecting the quality of the buried oxide and Si–SiO2 interface.

Properties and fabrication status of capsules for ignition targets in inertial confinement fusion experiments

The inertial confinement fusion program has proposed a laser capable of producing ignition and gain as the next step. Several choices exist in the design and production of capsules. In this paper the important features of each ablator material and the status of production are summarized. The design consists of ablators made of germanium-doped carbon hydrogen (CH), beryllium doped copper, polyimide, B4C and diamond. The CH and beryllium capsules are two of the most important choices. Compared with the beryllium shell, the CH shell has no microstructure and has a transparent wall that allows optical characterization of the fuel ice layer. The CH shell has the advantage that the specification can be easy to satisfy the ignition acquirements. The current ignition point has been designed in USA since 2010. The ignition target design has a series of demands for the capsule, such as capsule dimensions, coating density, void defects and volume, surface roughness, uniformity, doping and impurity levels. Now, the CH capsule can meet ignition requirements in USA, while the relevant work has just started in China.