Boron-doped Zinc Oxide Films Research Articles

Growth of low resistivity and high transparency boron-doped zinc oxide film by pulse laser deposition

0.5 wt% boron-doped zinc oxide (BZO) films were fabricated by utilizing pulse laser deposition under different growth temperature ranged from 250 to 450 °C. The effect of the growth temperature on the structural, optical, and electrical properties was investigated and discussed. The crystal structure and orientation of BZO thin films were examined by X-ray diffraction. All of the BZO films under various growth temperatures had strong c-axis (002)-preferred orientation. Optical transparency was high (>80%) over a wide spectral range from 400 nm to 700 nm, and the optical bandgap value of BZO are found to be in the range from 3.18 to 3.47eV. According to the experimental data, the resistivity of the BZO film was optimized at ~1.13 × 10−3 Ω-cm and significantly influenced by the growth temperature.

Transparent boron-doped zinc oxide films for antibacterial and magnetic applications

B-doped ZnO thin films have been synthesized by a sol–gel dip-coating deposition technique. Deposition conditions were optimized to achieve highly ordered thin films. XRD patterns confirm that B ions are inserted into the ZnO; lattice and the crystallite size decrease from 25.83 to 20.94 nm as the amount of B increases to optimal value (9 at.wt%). Synthesized B-doped ZnO thin films have ordered hexagonal wurtzite structures and granular morphology with high specific surface area. Optical transmittance spectra display transparency in the visible region. The band gap of the films decreases from 3.89 to 3.04 eV with the increase in B concentration. Band gap lowering is due to increase of defect level with the increase in B dopant percentage. The origin of the ferromagnetism is explained in terms of oxygen vacancies and Zn interstitials. Dielectric constant increases while DC conductivity and AC conductivity decrease with the increase in B doping. Antibacterial activity increases with the increase in B doping percentage but lower than undoped ZnO due to large crystallite size of B-doped ZnO nanostructures.

FESEM, XRD and DRS studies of electrochemically deposited boron doped ZnO films

AbstractIn this study, the effect of boron (B) incorporation into zinc oxide (ZnO) has been investigated. The undoped, 2 at.%. and 4 at.% B doped ZnO films were deposited on p-type silicon (Si) substrates by electrodeposition method using chronoamperometry technique. Electrochemical depositions were performed by applying a constant potentiostatic voltage of 1.1 V for 180 min at 90 °C bath temperature. To analyze the surface morphology, field emission scanning electron microscopy (FESEM) was used and the results revealed that while a small amount of boron resulted in smoother surface, a little more incorporation of boron changed the surface morphology to dandelion-like shaped rods on the whole surface. By using X-ray diffraction (XRD) analysis, the crystal structures of the films were detected and the preferred orientation of the ZnO, which exhibited polycrystalline and hexagonal wurtzite structure, changed with B doping. For the estimation of the optical band gap of obtained films, UV-Vis diffuse reflectance spectra (DRS) of the films were taken at room temperature and these data were applied to the Kubelka-Munk function. The optical band gap of ZnO narrowed due to incorporation of B, which was confirmed by red-shift.

On the origin of the changes in the opto-electrical properties of boron-doped zinc oxide films after plasma surface treatment for thin-film silicon solar cell applications

The modification of the steep and sharp valleys on the surface of the boron-doped zinc oxide (BZO) front electrodes by plasma surface treatment is a critical process for avoiding a significant reduction in the electrical performance of thin-film silicon solar cells. In this work, we report the origin of the changes in the electrical and optical properties of the BZO films that occur after this process. On the basis of an analysis of the chemical states, we found an improvement of the carrier concentration along with the treatment time that was mainly due to an increase of the oxygen vacancy. This indicated a deficiency of the oxygen in the BZO films under argon-ion bombardment. The red-shift of the A1 longitudinal optical mode frequency in the Raman spectra that was attributed to the existence of vacancy point defects within the films also strengthened this argument. The significant reduction of the haze ratio as well as the appearance of interference peaks on the transmittance spectra as the treatment time was increased were mainly due to the smoothing of the film surface, which indicated a degradation of the light-scattering capability of the BZO films. We also observed a gain of the visible-region transmittance that was attributed to the decrease of the thickness of the BZO films after the plasma surface treatment, instead of the crystallinity improvement. On the basis of our findings, we have proposed a further design rule of the BZO front electrodes for thin-film silicon solar cell applications.

Structural, optical and electrical properties of ZnO: B thin films with different thickness for bifacial a-Si:H/c-Si heterojunction solar cells

Textured surface boron-doped zinc oxide (BZO) thin films were fabricated by metal organic chemical vapor deposition as transparent conductive oxide (TCO) for solar cells. The surface microstructure was characterized by X-ray diffraction spectrum and scanning electron microscope. The optical transmittance was shown by optical transmittance microscope and the electrical properties were tested by Hall measurements. The thickness of the BZO film has crucial impact on the surface morphology, optical transmittance, and resistivity. The electrical and optical properties as well as surface microstructure varied inconsistently with the increase of the film thickness. The grain size and the surface roughness increased with the increase of the film thickness. The conductivity increased from 0.96×103 to 6.94×103 S/cm while the optical transmittance decreased from above 85% to nearly 80% with the increase of film thickness from 195 to 1021 nm. The BZO films deposited as both front and back transparent electrodes were applied to the bifacial ptype a-Si:H/i-type a-Si:H/n-type c-Si/i-type a-Si:H/n+-type a-Si:H heterojunction solar cells to obtain the optimized parameter of thickness. The highest efficiency of all the samples was 17.8% obtained with the BZO film thickness of 829 nm. Meanwhile, the fill factor was 0.676, the opencircuit voltage was 0.63 Vand the short-circuit density was 41.79 mA/cm2. The properties of the solar cells changing with the thickness were also investigated.

Boron-doped zinc oxide thin films grown by metal organic chemical vapor deposition for bifacial a-Si:H/c-Si heterojunction solar cells

Boron-doped zinc oxide (BZO) films were grown by metal organic chemical vapor deposition. The influence of B2H6 flow rate and substrate temperature on the microstructure, optical, and electrical properties of BZO films was investigated by X-ray diffraction spectrum, scanning electron microscope, optical transmittance spectrum, and Hall measurements. The BZO films with optical transmittance above 85% in the visible and infrared light range, resistivity of 0.9–1.0×10−3 Ωcm, mobility of 16.5–25.5cm2/Vs, and carrier concentration of 2.2–2.7×1020cm−3 were deposited under optimized conditions. The optimum BZO films were applied on the bifacial BZO/p-type a-Si:H/i-type a-Si:H/n-type c-Si/i-type a-Si:H/n+-type a-Si:H/BZO heterojunction solar cell as both front and back transparent electrodes. Meanwhile, the bifacial heterojunction solar cell with indium tin oxide (ITO) as both front and back transparent electrodes was fabricated. The efficiencies of 17.788% (open-circuit voltage: 0.628V, short-circuit current density: 41.756mA/cm2 and fill factor: 0.678) and 16.443% (open-circuit voltage: 0.590V, short-circuit current density: 36.515mA/cm2 and fill factor: 0.762) were obtained on the a-Si/c-Si heterojunction solar cell with BZO and ITO transparent electrodes, respectively.

Resistive Switching Characteristics in Boron Doped Zinc Oxide Films

In this work, metal/oxide/metal capacitors were fabricated and investigated using transparent boron-doped zinc oxide (ZnO:B) films for nonvolatile memory applications. Both top and bottom electrodes are tungsten. The average value of transmittance of ZnO:B films grown on silicon substrates is found to be about 91% in the visible light region. According to the relationship between transmittance and wavelength, the optical band gap of ZnO:B films is determined to be about 3.26 eV. The temperature dependent current-voltage curves show that the current density increases with increasing temperature in low-resistance state (LRS), meanwhile, the current density decreases with increasing temperature in high-resistance state (HRS). From the resistive switching behavior of the W/ZnO:B/W memory devices, the reset voltage which triggers the memory devices from an LRS to an HRS is independent of temperature. On the other hand, the set voltage which triggers the memory devices from an HRS to an LRS is increased with temperature.

Trade-offs of the opto-electrical properties of a-Si:H solar cells based on MOCVD BZO films.

Boron-doped zinc oxide (BZO) films, deposited by metal-organic chemical vapor deposition (MOCVD), have been widely used as front electrodes in thin-film solar cells due to their native pyramidal surface structure, which results in efficient light trapping. This light trapping effect can enhance the short-circuit current density (Jsc) of solar cells. However, nanocracks or voids in the silicon active layer may form when the surface morphology of the BZO is too sharp; this usually leads to degraded electrical properties of the cells, such as open-circuit voltage (Voc) and the fill factor (FF), which in turn decreases efficiency (Eff) [Bailat et al., Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on. IEEE, 2006, vol. 2, pp. 1533-1536]. In this paper, an etching and coating method was proposed to modify the sharp "pyramids" on the surface of the BZO films. As a result, an evident enhancement was achieved for these modified, BZO-based cells' Voc, FF, and Eff, although the Jsc exhibited a small decrease. In order to increase the Jsc and maintain the improved electrical properties (Voc, FF) of the cell, a thin BZO coating, deposited by MOCVD, was introduced to coat the sputtering-treated BZO film. Finally, we optimized the trade-off among the Voc, FF, and Jsc, that is, we identified a regime with an increase of the Jsc as well as a further improvement of the other electrical properties.

Conduction Mechanisms in Resistance Switching Memory Devices Using Transparent Boron Doped Zinc Oxide Films.

In this work, metal/oxide/metal capacitors were fabricated and investigated using transparent boron doped zinc oxide (ZnO:B) films for resistance switching memory applications. The optical band gap of ZnO:B films was determined to be about 3.26 eV and the average value of transmittance of ZnO:B films was about 91% in the visible light region. Experimental results indicated that the resistance switching in the W/ZnO:B/W structure is nonpolar. The resistance ratio of high resistance state (HRS) to low resistance state (LRS) is about of the order of 105 at room temperature. According to the temperature dependence of current-voltage characteristics, the conduction mechanism in ZnO:B films is dominated by hopping conduction and Ohmic conduction in HRS and LRS, respectively. Therefore, trap spacing (1.2 nm) and trap energy levels in ZnO:B films could be obtained.

Improved properties of boron-doped zinc oxide films with In2O3：Sn interlayers for solar cells

Boron-doped zinc oxide (BZO) films with a natural pyramid-textured surface grown by metal organic chemical vapor deposition (MOCVD) have large light trapping effect in thin film silicon solar cells when used as front contact electrodes. However, the surface topography of traditional BZO films is so sharp as to damage the quality of the subsequent silicon thin film materials and to reduce the photovoltaic conversion efficiency of the solar cells. In this work, an ultra-thin In2O3:Sn(ITO)film (～ 4 nm) is used as the interlayer in the sandwiched structure of the multilayer films, i.e. glass/bottom BZO layer /ITO interlayer/top BZO layer. The surface properties can be improved through modulating the thickness of the top BZO layer. Appropriate thickness of top BZO layer and ITO interlayer are helpful for obtaining the cauliflower-like surface morphology and thus the sharp structure becomes relatively gentle, but the surface morphology still keeps a pyramid feature when depositing thicker top BZO layer. The relatively gentle surface morphology could promote crystallization quality ofc-Si:H thin film materials and reduce cracks in intrinsic layer and TCO/P-Si interface defects. Finally, this new sandwiched structure of multilayer ZnO films is applied in c-Si:H p-i-n thin film solar cells. Compared with traditional BZO films, the quantum efficiency (QE) of solar cells with a sandwiched structure of ZnO increases by about 10%, and both the open-circuit voltage (Voc) and short-circuit current density (Jsc) may increase and thus improve the solar cell efficiency.

Study of boron-doped zinc oxide film serving as front contact with high haze used in amorphous silicon thin film solar cells

Boron-doped zinc oxide (BZO) deposited by metal organic chemical vapor deposition (MOCVD) method is used as front contact in amorphous silicon thin film solar cells. Asahi-U type SnO2:F is used as the reference front contact for comparison. When the a-Si:H intrinsic layer thickness is changing changed, the performance of a-Si:H solar cells shows different evolution trends. These different results can be understood from the shadowing effect during the growth of intrinsic silicon material, which is caused by the as-grown pyramid texture in the surface of BZO substrate. In order to reduce this negative effect on the performance of solar cells, the deposition temperature of the a-Si:H intrinsic layer is optimized, to thereby improving improve the open circuit voltage and fill factor. The conversion efficiency of a-Si:H solar cells can reach up to 7.34%, with the thickness of absorber layer being only around 200 nm. and only Al back reflector is being used.

Influences of deposition temperature on characteristics of B-doped ZnO films deposited by metal–organic chemical vapor deposition

Boron-doped zinc oxide films were fabricated by metal–organic chemical vapor deposition at deposition temperatures (Td) from 150 to 210°C. The deposition rate increases abruptly and monotonically with increasing Td. The resistivity also varies drastically, and a minimum resistivity of 1.6×10−3Ωcm is obtained at Td=175°C. The crystal orientation and surface texture show Td dependence. These characteristics correlate with each other. The dependence of these characteristics on Td is caused by the reactivity of the source materials.

Highly transparent and conducting boron doped zinc oxide films for window of Dye Sensitized Solar Cell applications

Highly transparent and conducting boron doped zinc oxide (ZnO:B) films grown by sol–gel method are reported. The annealing temperature is varied from 350 to 550°C and doping concentration of boron is kept fixed for 0.6at.% for all the films. At low temperature the stress in the films is compressive, which becomes tensile for the films annealed at higher temperature. A minimum resistivity of 7.9×10−4Ωcm and maximum transmittance of ∼91% are observed for the film annealed at 450°C. This could be attributed to minimum stress of films, which is further evident by the evolution of A1 and defect related Raman modes without any shifting in its position. Such kind of highly transparent and conducting ZnO:B thin film could be used as window material in Dye Sensitized Solar Cell (DSSC).

Effect of boron doping on optical properties of sol–gel based nanostructured zinc oxide films on glass

Boron doped zinc oxide thin films (∼80 nm) were deposited onto pure silica glass by sol–gel dip coating technique from the precursor sol/solution of 4.0 wt.% equivalent oxide content. The boron concentration was varied from 0 to 2 at.% w.r.t. Zn using crystalline boric acid. The nanostructured feature of the films was visualized by FESEM images and the largest cluster size of ZnO was found in 1 at.% boron doped film (B1ZO). The presence of mixed crystal phases with hexagonal as major phase was identified from XRD reflections of the films. Particle size, optical band gap, visible specular reflection, room temperature photoluminescence (PL) emissions (3.24–2.28 eV), infra-red (IR) and Raman active longitudinal optical (LO) phonon vibration were found to be dependent on dopant concentration. For the first time, we report the room temperature fine structured PL emissions as phonon replicas originated from the LO phonon (both IR and Raman active) in 1 at.% boron doped zinc oxide film.

ChemInform Abstract: Deposition of Boron Doped Zinc Oxide Films and Their Electrical and Optical Properties.

Boron doped zinc oxide films were deposited by atmospheric pressure chemical vapor deposition in a laminar flow reactor from diethyl zinc, tert-butanol, and diborane in the temperature range from 300 to 420 o C. When the deposition temperature was above 320 o C, both doped and undoped films have highly oriented crystallites with their c-axes perpendicular to the substrate plane

Polycrystalline ZnO: B grown by LPCVD as TCO for thin film silicon solar cells

Conductive zinc oxide (ZnO) grown by low pressure chemical vapor deposition (LPCVD) technique possesses a rough surface that induces an efficient light scattering in thin film silicon (TF Si) solar cells, which makes this TCO an ideal candidate for contacting such devices. IMT-EPFL has developed an in-house LPCVD process for the deposition of nanotextured boron doped ZnO films used as rough TCO for TF Si solar cells. This paper is a general review and synthesis of the study of the electrical, optical and structural properties of the ZnO:B that has been performed at IMT-EPFL. The influence of the free carrier absorption and the grain size on the electrical and optical properties of LPCVD ZnO:B is discussed. Transport mechanisms at grain boundaries are studied. It is seen that high doping of the ZnO grains facilitates the tunnelling of the electrons through potential barriers that are located at the grain boundaries. Therefore, even if these potential barriers increase after an exposition of the film to a humid atmosphere, the heavily doped LPCVD ZnO:B layers show a remarkable stable conductivity. However, the introduction of diborane in the CVD reaction induces also a degradation of the intra-grain mobility and increases over-proportionally the optical absorption of the ZnO:B films. Hence, the necessity to finely tune the doping level of LPCVD ZnO:B films is highlighted. Finally, the next challenges to push further the optimization of LPCVD ZnO:B films for thin film silicon solar cells are discussed, as well as some remarkable record cell results achieved with LPCVD ZnO:B as front electrode.

Temperature-dependent growth of zinc oxide thin films grown by metal organic chemical vapor deposition

Transparent conductive un-doped zinc oxide (ZnO) thin films are deposited on glass substrate by metal organic chemical vapor deposition and the effect of temperature in the range 393–443 K on the structural, electrical, and optical properties of these films are investigated. X-ray diffraction spectra and scanning electron microscope images indicate that substrate temperature plays a great role on the microstructure of ZnO films and the morphological transition takes place obviously at around 418 K. The morphology of the ZnO films shows sphere-like structure at low temperature (<418 K) and then changes from pyramid-like to rock-like structure at higher temperature (>418 K). The grain of ZnO films grows up with an increase of substrate temperature. Hall measurements indicate that decreased resistivity and increased mobility of ZnO films result from the improvement of grain size and crystal quality. Under the optimal growth condition, the boron-doped ZnO films at 423 K exhibit the lowest resistivity of 1.2×10 −3 Ω cm (its thickness=1000 nm) with a high mobility of 30.4 cm 2/V s and average optical transmittance above 85% in the range 400–900 nm, specially suitable for thin film Si solar cells.

Boron-doped zinc oxide thin films prepared by sol-gel technique

Multilayer transparent conducting boron-doped zinc oxide films have been prepared on glass substrates by the sol gel dip coating process. Zinc acetate solutions of 0.4 M in isopropanol stabilized by diethanolamine and doped with boron tri-i-propoxide were used. Each layer was fired at 400–650∘C in a conventional furnace for 30 min. Selected samples were vacuum annealed at 400–450∘C for 1 h to improve their electrical properties. The electrical resistivity curve with doping shows a minimum around 0.8 at.%. Excess boron caused a drop of the carrier mobility without acting as donors. Post-deposition annealing sequence was crucial for dopant partial regeneration. Films with an average optical transmittance exceeding 90% can be achieved reproducibly.

Studies on structural, optical and electrical properties of boron doped zinc oxide films prepared by spray pyrolysis technique

Boron doped and undoped zinc oxide (ZnO) thin films were prepared by pyrolytic decomposition of methanolic solution of zinc acetate onto glass substrates. The structural properties of the films were studied by using X-ray diffraction. The results show that both the boron doped and undoped films exhibit hexagonal wurtzite structure with strong c-axis orientation as evidenced by X-ray diffraction patterns. The crystallite size wanes with increasing doping concentration. The electrical resistivity studies were carried out using the four-probe method. It is found that the films with 0.8 wt% boron doping concentration attains the lowest resistivity (10 −4 Ω m), whereas for higher and lower doping concentrations resistivity increases. The thermoelectric power studies showed that the doped and undoped films exhibit n-type conductivity, optical studies revealed that with increase in doping concentration the transmittance of the film increases in the wavelength range 400–600 nm. The optical absorption data were used to determine band gap energy and it is found to be 3.22 eV for undoped ZnO films and 3.32 for boron doped films.

Transparent Conducting Boron-Doped Zinc Oxide Films Deposited by DC-Magnetron Sputtering in B2H6-Ar Mixtures

Transparent conducting ZnO:B films for solar cells were produced by DC magnetron sputtering in B2H6-Ar mixtures. Low-resistivity textured films could be obtained at relatively low substrate temperatures of 150-200° C. The resistivity, electron mobility and carrier concentration were 4×10-4 Ω cm, 60 cm2/ Vs and 2×1020 cm-3, respectively. The 2-µ m-thick films showed a sheet resistance of 2 Ω/ sq and optical transmission of greater than 85% at wavelengths of 0.6-1.0 µ m. These films exhibited higher optical transmission in the near infrared region than ZnO:Al films with the same resistivity because of high electron mobility. No degradation of resistivity and optical transmission was observed after exposure at 85° C and 95%RH for 5 h.