Templating Synthesis of SnO2 Nanotubes Loaded with Ag2O Nanoparticles and Their Enhanced Gas Sensing Properties

A new kind of SnO₂ nanotubes loaded with Ag₂O nanoparticles can be synthesized by using Ag@C coaxial nanocables as sacrificial templates. The composition of silver in SnO₂ nanotubes can be controlled by tuning the compositions of metallic Ag in Ag@C sacrificial templates, and the morphology of tubular structures can be changed by use of nanocables with different thicknesses of carbonaceous layer. This simple strategy is expected to be extended for the fabrication of similar metal‐oxide doped nanotubes using different nanocable templates. In contrast to SnO₂@Ag@C nanocables as well as to other types of SnO₂ reported previously, the Ag₂O‐doped SnO₂ nanotubes exhibit excellent gas sensing behaviors. The dynamic transients of the sensors demonstrated both their ultra‐fast response (1–2 s) and ultra‐fast recovery (2–4 s) towards ethanol, and response (1–4 s) and recovery (4–5 s) towards butanone. The combination of SnO₂ tubular structure and catalytic activity of Ag₂O dopants gives a very attractive sensing behavior for applications as real‐time monitoring gas sensors with ultra‐fast responding and recovering speed.     

The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2‐Based Gas Sensors: Synthesis Strategies,and Phenomenological and Spectroscopic Studies

The humidity dependence of the gas‐sensing characteristics in SnO₂‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO₂ nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO₂ nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO₂ but by the NiO, and thus the electrochemical interaction between the humidity and the SnO₂ sensor surface is totally blocked. NiO‐doped hierarchical SnO₂ sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (R_a/R_g = 2.8 at 50 ppm CO (R_a: resistance in air, R_g: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO₂ nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.     

One‐Step Synthesis of Highly Ordered Mesoporous Silica Monoliths with Metal Oxide Nanocrystals in their Channels

A simple, one‐step synthetic route to prepare ordered mesoporous silica monoliths with controllable quantities of metal oxide nanocrystals in their channels is presented. The method is based on the assisted assembly effect for mesostructure‐directing of the metal complexes formed by the interaction of metal ions with the –O– groups of copolymers. Highly ordered hexagonal silica monoliths, loaded with various metal oxide nanocrystals, including those of Cr₂O₃, MnO, Fe₂O₃, Co₃O₄, NiO, CuO, ZnO, CdO, SnO₂, and In₂O₃, can be obtained by this one‐step pathway. In the NiO/SiO₂ nanocomposite, nickel oxide nanorods with face‐centered cubic lattices are formed at low doping ratios, and they can be transformed into nanowires by increasing the quantities of the precursors. In the Fe₂O₃/SiO₂ nanocomposites, a one‐dimensional assembly of iron oxide nanoparticles is observed. In the In₂O₃/SiO₂ nanocomposites, single crystal nanowires with high aspect ratios are obtained. For the other metal oxide nanocomposites, including Cr₂O₃, MnO, Co₃O₄, CuO, ZnO, CdO, and SnO, only crystalline nanorods are obtained. N₂ sorption results of the metal oxide/SiO₂ mesostructured nanocomposites reveal that nanocrystals inside the pores do not severely decrease the pore volume or the Brunauer–Emmett–Teller (BET) surface area of the mesoporous silica host. The bandgaps of SnO₂ and In₂O₃ nanocrystals, calculated from UV‐vis spectra, are much larger than the corresponding bulk materials, implying the quantum confinement effect in the small particles. Co₃O₄/SiO₂ mesostructured nanocomposites catalyze the complete combustion of CH₄. These studies provide a new and simple method for templating synthesis of metal oxide nanostructures.     

Effects of Sb,Zn doping on structural,electrical and optical properties of SnO2 thin films

Highly transparent, low resistive pure and Sb, Zn doped nanostructured SnO₂ thin films have been successfully prepared on glass substrates at 400° C by spray pyrolysis method. Structural, electrical and optical properties of pure and Sb, Zn doped SnO₂ thin films are studied in detail. Powder X-ray diffraction confirms the phase purity, increase in crystallinity, size of the grains (90–45 nm), polycrystalline nature and tetragonal rutile structure of thin films. The scanning electron microscopy reveals the continuous change in surface morphology of thin films and size of the grains decrease due to Sb, Zn doping in to SnO₂. The optical transmission spectra of SnO₂ films as a function of wavelength confirm that the optical transmission increases with Sb, Zn doping remarkably. The optical band gap of undoped film is found to be 4.27 eV and decreases with Sb, Zn doping to 4.19 eV, 4.07 eV respectively. The results of electrical measurements indicate that the sheet resistance of the deposited films improves with Sb, Zn doping. The Hall measurements confirm that the films are degenerate n-type semiconductors.     

Low-temperature carbon monoxide gas sensors based gold/tin dioxide

Tin dioxide nanocrystals were synthesized by a precipitation process and then used as the support for 2 wt.% gold/tin dioxide preparation via a deposition–precipitation method, followed by calcination at 200 °C. Thick films were fabricated from gold/tin dioxide powders, and the sensing behavior for carbon monoxide gas was investigated. The gold/tin dioxide was found to be efficient carbon monoxide gas-sensing materials under low operating temperature (83–210 °C). The Au/SnO₂ sensor with SnO₂ calcined at 300 °C exhibited better CO gas-sensing behavior than the SnO₂ calcined at other temperatures. The experimental results indicated the potential use of Au doped SnO₂ for CO gas sensing.     

Gas Sensors: Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading (Adv. Funct. Mater. 24/2010)

This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO₂ fibers. Unloaded and Pd‐loaded SnO₂ nanofiber mats are synthesized by electrospinning followed by hot‐pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd‐loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO₂ crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO₂ crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd‐loaded specimen, indicating that Pd‐loading could effectively suppress the SnO₂ grain growth during the calcination step. The Pd‐loaded SnO₂ sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H₂ and lower sensitivity to NO₂ compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO‐decorated SnO₂ crystallites and catalytic effect of PdO in promoting the oxidation of H₂ into H₂O. These phenomena appear to have a much larger effect on the sensitivity of the Pd‐loaded sensors than the reduction in grain size.     

Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading

This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO₂ fibers. Unloaded and Pd‐loaded SnO₂ nanofiber mats are synthesized by electrospinning followed by hot‐pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd‐loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO₂ crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO₂ crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd‐loaded specimen, indicating that Pd‐loading could effectively suppress the SnO₂ grain growth during the calcination step. The Pd‐loaded SnO₂ sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H₂ and lower sensitivity to NO₂ compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO‐decorated SnO₂ crystallites and catalytic effect of PdO in promoting the oxidation of H₂ into H₂O. These phenomena appear to have a much larger effect on the sensitivity of the Pd‐loaded sensors than the reduction in grain size.     

Bottom-contact passivation for high-performance perovskite solar cells using TaCl5-doped SnO2 as electron-transporting layer

Organic-inorganic hybrid perovskite solar cells (PSCs) possess the promising potential to substitute photovoltaic technologies in the traditional model. The modified SnO₂ as an electron-transporting layer (ETL) has been studied extensively because of its excellent properties. Herein, we implemented the TaCl₅ doped SnO₂ ETL in the n-i-p structure perovskite solar cells. The doped SnO₂ solution was demonstrated the characterization of neutral power of value and hydrophobicity. The conduction band of changed ETLs shifted downward by 0.26 eV resulting in the efficient electron transfer. Furthermore, the doped SnO₂ films affect the perovskite crystallite size with passivated traps and reduced nonradiative recombination loss. After employing TaCl₅-doped SnO₂ ETL, the open-circuit voltage enhances from 0.97 to 1.08 V and a united power conversion efficiency increases from 16.38% to 18.23% achieved when adopted 1.0 wt% doped TaCl₅–SnO₂ TEL. The developed doping method provides an effective method to passivate SnO₂ for fabricating high-performance perovskite solar cells.     

Preparation and gas sensitivity of WO3 hollow microspheres and SnO2 doped heterojunction sensors

Dispersed tungsten trioxide (WO₃) microsphere aggregates were prepared by chemical reduction with hydrazine hydrate in a glycol–water system, and the composites of WO₃/tin oxide (SnO₂) with different SnO₂ weight fractions were prepared by microwave refluxing. The products were characterized by x-ray diffraction, field emission scanning electron microscopy, thermogravimetric-differential thermal analysis, Fourier transform infrared spectroscopy, and the Brunauer–Emmett–Teller method. The gas-sensing characteristics based on the composites were investigated by a stationary-state gas distribution method. The results show that the noncompact WO₃ microspheres with hollow structure were obtained. The phase composition and the morphology of WO₃ were changed by SnO₂ doping. The heterojunction structure was formed between WO₃ and SnO₂, and the heterojunction sensors have high sensitivity to H₂S, NO_X, and xylene at relatively lower operating temperature, especially the sensor doped by 3% SnO₂ operating just at 90 °C for H₂S gas.     

Low‐Temperature Growth of SnO2 Nanorod Arrays and Tunable n–p–n Sensing Response of a ZnO/SnO2 Heterojunction for Exclusive Hydrogen Sensors

Uniform SnO₂ nanorod arrays have been deposited at low temperature by plasma‐enhanced chemical vapor deposition (PECVD). ZnO surface modification is used to improve the selectivity of the SnO₂ nanorod sensor to H₂ gas. The ZnO‐modified SnO₂ nanorod sensor shows a normal n‐type response to 100 ppm CO, NH₃, and CH₄ reducing gas whereas it exhibits concentration‐dependent n–p–n transitions for its sensing response to H₂ gas. This abnormal sensing behavior can be explained by the formation of n‐ZnO/p‐Zn‐O‐Sn/n‐SnO₂ heterojunction structures. The gas sensors can be used in highly selective H₂ sensing and this study also opens up a general approach for tailoring the selectivity of gas sensors by surface modification.     

Hydrogen Gas Sensing Based on SnO<Subscript>2</Subscript> Nanostructure Prepared by Sol–Gel Spin Coating Method

A novel H₂ gas sensor based on a SnO₂ nanostructure was operated at room temperature (RT) (25°C). The SnO₂ nanostructure was grown on Al₂O₃ substrates by a sol–gel spin coating method. The structural characteristics, surface morphology, and gas sensing properties of the SnO₂ nanostructure were investigated. Thin film annealing at 500°C produced a high-quality SnO₂ nanostructure with a crystallite size of 33.98 nm. A metal–semiconductor–metal gas sensor was fabricated using the SnO₂ nanostructure and palladium metal. The gas sensor exhibited a sensitivity of 2570% to 1000 ppm H₂ gas at RT. The sensing measurements for H₂ gas at different temperatures (RT to 125°C) were repeatable?for 50 min. Sensor sensitivity was tested under different H₂ concentrations (150 ppm, 250 ppm, 375 ppm, 500 ppm, and 1000 ppm) at different operating temperatures. Adding glycerin to the sol solution increased the porosity of the SnO₂ nanostructure surface, which increased the adsorption/desorption of gas molecules which leads to the high sensitivity of the sensor. Therefore, this H₂ gas sensor is a suitable?portable?RT gas sensor.     

Watermelon‐Like Structured SiOx–TiO2@C Nanocomposite as a High‐Performance Lithium‐Ion Battery Anode

A unique watermelon‐like structured SiO_x–TiO₂@C nanocomposite is synthesized by a scalable sol–gel method combined with carbon coating process. Ultrafine TiO₂ nanocrystals are uniformly embedded inside SiO_x particles, forming SiO_x–TiO₂ dual‐phase cores, which are coated with outer carbon shells. The incorporation of TiO₂ component can effectively enhance the electronic and lithium ionic conductivities inside the SiO_x particles, release the structure stress caused by alloying/dealloying of Si component and maximize the capacity utilization by modifying the Si–O bond feature and decreasing the O/Si ratio (x‐value). The synergetic combination of these advantages enables the synthesized SiO_x–TiO₂@C nanocomposite to have excellent electrochemical performances, including high specific capacity, excellent rate capability, and stable long‐term cycleability. A stable specific capacity of ≈910 mAh g^?1 is achieved after 200 cycles at the current density of 0.1 A g^?1 and ≈700 mAh g^?1 at 1 A g^?1 for over 600 cycles. These results suggest a great promise of the proposed particle architecture, which may have potential applications in the improvement of various energy storage materials.     

Engineering of Facets,Band Structure,and Gas‐Sensing Properties of Hierarchical Sn2+‐Doped SnO2 Nanostructures

Hierarchical SnO₂ nanoflowers, assembled from single‐crystalline SnO₂ nanosheets with high‐index (11$ \bar 3 $ ) and (10$ \bar 2 $ ) facets exposed, are prepared via a hydrothermal method using sodium fluoride as the morphology controlling agent. Formation of the 3D hierarchical architecture comprising of SnO₂ nanosheets takes place via Ostwald ripening mechanism, with the growth orientation regulated by the adsorbate fluorine species. The use of Sn(II) precursor results in simultaneous Sn²⁺ self‐doping of SnO₂ nanoflowers with tunable oxygen vacancy bandgap states. The latter further results in the shifting of semiconductor Fermi levels and extended absorption in the visible spectral range. With increased density of states of Sn²⁺‐doped SnO₂ selective facets, this gives rise to enhanced interfacial charge transfer, that is, high sensing response, and selectivity towards oxidizing NO₂ gas. The better gas sensing performance over (10$ \bar 2 $ ) compared to (11$ \bar 3 $ ) faceted SnO₂ nanostructures is elucidated by surface energetic calculations and Bader analyses. This work highlights the possibility of simultaneous engineering of surface energetics and electronic properties of SnO₂ based materials.     

Structural,morphological, electrical and optical studies of Cr doped SnO2 thin films deposited by the spray pyrolysis technique

Tin oxide (SnO₂) and chromium (Cr) doped tin oxide (Cr:SnO₂) thin films were deposited on the preheated glass substrates at 673 K by spray pyrolysis. Concentration of Cr was varied in the solution by adding chromium (III) chloride hexahydrate from 0 to 3 at%. The effect of Cr doping on the structural, electrical and optical properties of tin oxide films is reported. X-ray diffraction pattern confirms the tetragonal crystal structure for undoped and Cr doped tin oxide films. Scanning electron microscopic photographs show the modification of surface morphology of tin oxide film due to varying concentration of Cr. X-ray photoelectron spectra of Cr:SnO₂ (3 at%) thin film revealed the presence of carbon, tin, oxygen, and chromium. Carrier concentration and mobility of the SnO₂ films decrease with increasing concentration of Cr and 0.5 at% Cr doped tin oxide film acquires a mobility of 70 cm²/V s. Average optical transmittance in the 550–850 nm range varies from 38% to 47% with varying Cr concentration in the solution.     

Dopant‐Free,Amorphous–Crystalline Heterophase SnO2 Electron Transport Bilayer Enables >20% Efficiency in Triple‐Cation Perovskite Solar Cells

Improving the ohmic contact and interfacial morphology between an electron transport layer (ETL) and perovskite film is the key to boost the efficiency of planar perovskite solar cells (PSCs). In the current work, an amorphous–crystalline heterophase tin oxide bilayer (Bi‐SnO₂) ETL is prepared via a low‐temperature solution process. Compared with the amorphous SnO₂ sol–gel film (SG‐SnO₂) or the crystalline SnO₂ nanoparticle (NP‐SnO₂) counterparts, the heterophase Bi‐SnO₂ ETL exhibits improved surface morphology, considerably fewer oxygen defects, and better energy band alignment with the perovskite without sacrificing the optical transmittance. The best PSC device (active area ≈ 0.09 cm²) based on a Bi‐SnO₂ ETL is hysteresis‐less and achieves an outstanding power conversion efficiency of ≈20.39%, which is one of the highest efficiencies reported for SnO₂‐triple cation perovskite system based on green antisolvent. More fascinatingly, large‐area PSCs (active areas of ≈3.55 cm²) based on the Bi‐SnO₂ ETL also achieves an extraordinarily high efficiency of ≈14.93% with negligible hysteresis. The improved device performance of the Bi‐SnO₂‐based PSC arises predominantly from the improved ohmic contact and suppressed bimolecular recombination at the ETL/perovskite interface. The tailored morphology and energy band structure of the Bi‐SnO₂ has enabled the scalable fabrication of highly efficient, hysteresis‐less PSCs.     

A General Synthesis of Crumpled Metal Oxide Nanosheets as Superior Chemiresistive Sensing Layers

Metal oxide nanosheets having high mesoporosity, grain size distribution of 5–10 nm, and ultrathin thickness have attracted much attention due to their intriguing properties such as high surface‐to‐volume ratio and superior chemical activities. However, 2D nanostructures tend to restack, inducing a decrease in accessible surface area and a number of pores. To solve this problem, herein, a unique synthetic method of crumpled metal oxide nanosheets using spray pyrolysis of metal ion–coated graphene oxide, followed by heat treatment, is reported. This method is applicable not only to single‐component metal oxides but also to heterogeneous multicomponent metal oxides in which composition can be controlled. Crumpled SnO₂, ZnO, and Co₃O₄ as well as SnO₂/ZnO and SnO₂/Co₃O₄ nanosheets with heterogeneous interfaces are successfully synthesized and used as superior gas sensing layers. Because of the abundant reaction sites, well‐developed porosity for high gas accessibility, the formation of heterojunctions, the crumpled SnO₂/ZnO and SnO₂/Co₃O₄ nanosheets exhibit outstanding sensing performance (R_air/R_gas = 20.25 toward 5 ppm formaldehyde, and R_air/R_gas = 14.13 toward 5 ppm acetone, respectively). This study can contribute to the realization of a family of heterogeneous crumpled metal oxide nanosheets that can be applied to various research fields.     

Thin‐Wall Assembled SnO2 Fibers Functionalized by Catalytic Pt Nanoparticles and their Superior Exhaled‐Breath‐Sensing Properties for the Diagnosis of Diabetes

Hierarchical SnO₂ fibers assembled from wrinkled thin tubes are synthesized by controlling the microphase separation between tin precursors and polymers, by varying flow rates during electrospinning and a subsequent heat treatment. The inner and outer SnO₂ tubes have a number of elongated open pores ranging from 10 nm to 500 nm in length along the fiber direction, enabling fast transport of gas molecules to the entire thin‐walled sensing layers. These features admit exhaled gases such as acetone and toluene, which are markers used for the diagnosis of diabetes and lung cancer. The open tubular structures facilitated the uniform coating of catalytic Pt nanoparticles onto the inner SnO₂ layers. Highly porous SnO₂ fibers synthesized at a high flow rate show five‐fold higher acetone responses than densely packed SnO₂ fibers synthesized at a low flow rate. Interestingly, thin‐wall assembled SnO₂ fibers functionalized by Pt particles exhibit a dramatically shortened gas response time compared to that of un‐doped SnO₂ fibers, even at low acetone concentrations. Moreover, Pt‐decorated SnO₂ fibers significantly enhance toluene response. These results demonstrate the novel and practical feasibility of thin‐wall assembled metal oxide based breath sensors for the accurate diagnosis of diabetes and potential detection of lung cancer.     

Effect of fluorine doping on the structural,optical and electrical properties of spray deposited cadmium stannate thin films

Cadmium stannate (Cd₂SnO₄) thin films were coated on Corning 1737 glass substrates at 540 °C by spray pyrolysis technique, from the aqueous solution of cadmium acetate and tin (II) chloride precursors. Fluorine doped Cd₂SnO₄ (F: Cd₂SnO₄) thin films were prepared by adding ammonium fluoride in the range of 0–5 wt% of the total weight of cadmium acetate and tin (II) chloride in the spray solution. Thickness of the prepared films is about 300 nm. X-ray diffraction analysis of the Cd₂SnO₄ and 3 wt% F: Cd₂SnO₄ films shows the signature for the growth along (222) direction. Scanning electron micrographs showed that fluorine doping effectively modifies the surface morphology of Cd₂SnO₄ films. Average optical transmittance in the visible region (500–850 nm) for Cd₂SnO₄ is ~79% and it is increased to ~83% for 1 wt% doping concentration of the NH₄F in the solution. Fluorescence spectra of F: Cd₂SnO₄ (1 wt% and 3 wt%) exhibit peak at 601 nm. F: Cd₂SnO₄ film (1 wt%) shows mobility of ~42 cm²/V s, carrier concentration of ~9.5×10¹⁹ cm^?3 and resistivity of ~1.5×10^?3 Ω cm.     

Effect of Bi doping on structural,morphological, optical and ethanol vapor response properties of SnO2 nanoparticles

The gas sensing behavior of thick films of Bi doped SnO₂ has been investigated towards ethanol vapor. The screen printing technique was used to prepare the thick films. The films were sintered at 650 °C for 2 h. The structural, surface morphological, optical and gas sensing properties of undoped and Bi doped SnO₂ thick films have been studied. X-ray diffraction and Raman spectroscopy confirmed that the films consisted exclusively of tetragonal tin oxide, without any impurity phases. FE-SEM studies revealed the formation of highly porous microstructure with grain size in few tens of nanometers. From the optical studies, the band gap was found to be decreased with bismuth doping (3.96 eV for undoped, 3.83 eV, 3.71 eV and 3.6 eV for 1 mol%, 2 mol% and 3 mol% Bi, respectively). The 3 mol % Bi doped SnO₂ thick films exhibited the highest sensitivity to 100 ppm of ethanol vapor at 300 °C. The effect of microstructure on sensitivity, response time and recovery time of the sensor was studied and discussed.     

Electrical properties of CIGS/Mo junctions as a function of MoSe2 orientation and Na doping

The electrical properties of Cu(In,Ga)Se₂/Mo junctions were characterized with respect of MoSe₂ orientation and Na doping level using an inverse transmission line method, in which the Cu(In,Ga)Se₂ (CIGS)/Mo contact resistance could be measured separately from the CIGS film sheet resistance. The MoSe₂ orientation was controlled by varying the Mo surface density, with the c‐axis parallel and normal orientations favored on Mo surfaces of lower and higher density, respectively. The effect of Na doping was compared by using samples with and without a SiO_x film on sodalime glass. The conversion of the MoSe₂ orientation from c‐axis normal to parallel produced a twofold reduction in CIGS/Mo contact resistance. Measurements of the contact resistances as a function of temperature showed that the difference in CIGS/Mo contact resistance between the samples with different MoSe₂ orientations was due to different barrier heights at the back contact. Comparison between Na‐doped and Na‐reduced samples revealed that the contact resistance for the Na‐reduced system was four times of that of the doped sample, which showed more pronounced Schottky‐junction behavior at lower temperature, indicating that Na doping effectively reduced the barrier height at the back contact. Copyright © 2013 John Wiley & Sons, Ltd.