Texturization of ZnO:Al surface by reactive ion etching in SF<Subscript>6</Subscript>/Ar,CHF<Subscript>3</Subscript>/Ar plasma for application in thin film silicon solar cells

As-deposited sputtered ZnO:Al (AZO) thin films having high transparency (T?≥?85% at 550 nm of wavelength) and good electrical properties (ρ?=?2.59?×?10^?04 Ω cm) are etched to get suitable light trapping in thin film solar cells, using reactive ion etching method in sulfur hexafluoride–argon (SF₆/Ar) plasma and trifluoromethane–argon (CHF₃/Ar) plasma to texture their surface. Though the electrical properties of the films are not affected much by the etching process but significant increment in the average haze values in the wave length range of 350–1100 nm in the etched AZO films (19.21% for SF₆/Ar and 22.07% for CHF₃/Ar plasma etched) are found compared to as-deposited AZO films (5.61%). Increment in haze value is due to more scattering of light from the textured surface. These textured substrates are used as front transparent conducting oxide electrode for the fabrication of amorphous silicon solar cells. Solar cells fabricated on etched AZO substrates show 7.76% increase in conversion efficiency compared to as-deposited AZO substrates.     

Study of optoelectronics and microstructures on the AZO/nano-layer metals/AZO sandwich structures

In this study, growth nano-layer metals (Al, Cu, Ag) and Al-doped ZnO (AZO) thin films are deposited on glass substrates as the transparent conducting oxides (TCOs) to form AZO/nano-layer metals/AZO sandwich structures. The conductivity properties of thin films are enhanced when the average transmittance over the wavelengths 400–800 nm is maintained at higher than 80 %. A radio frequency magnetron sputtering system is used to deposit the metal layers and AZO thin films of different thickness, to form AZO/Al/AZO (ALA), AZO/Cu/AZO (ACA) and AZO/Ag/AZO (AGA) structures. X-ray diffraction and field emission scanning electron microscopy are used to analyze the crystal orientation and structural characteristic. The optical transmission and resistivity are measured by UV–VIS–NIR spectroscopy and Hall effect measurement system, respectively. The results show that when the Ag thickness is maintained at approximately 9 nm, the TCOs thin film has the lowest resistivity of 8.9 × 10^?5 Ω-cm and the highest average transmittance of 81 % over the wavelengths 400–800 nm. The crystalline Ag nano-crystal structures are observed by high-resolution transmission electron microscopy. In addition, the best figure of merit for the AZO/Ag/AZO tri-layer film is 2.7 × 10^?2 (Ω^?1), which is much larger than that for other structures.     

Effect of ZnO buffer layer on AZO film properties and photovoltaic applications

Aluminum-doped ZnO (AZO) transparent conducting films were deposited on glass substrates with and without intrinsic ZnO (i-ZnO) buffer layers by a home made and low cost radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient and under a low vacuum level. The films were examined and characterized for electrical, optical, and structural properties for the application of CIGS solar cells. The influence of sputter power, deposition pressure, film thickness and residual pressure on electrical and optical properties of layered films of AZO, i-ZnO and AZO/i-ZnO was investigated. The optimization of coating process parameters (RF power, sputtering pressure, thickness) was carried out. The effects of i-ZnO buffer layer on AZO films were investigated. By inserting thin i-ZnO layers with a thickness not greater than 125 nm under the AZO layers, both the carrier concentration and Hall mobility were increased. The resistivity of these layered films was lower than that of single layered AZO films. The related mechanisms and plasma physics were discussed. Copper indium gallium selenide (CIGS) thin film solar cells were fabricated by incorporating bi-layer ZnO films on CdS/CIGS/Mo/glass substrates. Efficiencies of the order of 7–8% were achieved for the manufactured CIGS solar cells (4–5 cm² in size) without antireflective films. The results demonstrated that RF sputtered layered AZO/i-ZnO films are suitable for application in low cost CIGS solar cells as transparent conductive electrodes.     

Improved short-circuit current density of a-Si:H thin film solar cells with n-type silicon carbide layer

In this work, the performance of p–i–n hydrogenated amorphous silicon thin film solar cells by adopting n-type silicon carbide (n-SiC_x:H) layer was investigated. By varying CH₄/SiH₄ gas flow ratio, refractive index and electrical conductivity of n-SiC_x:H thin films were changed in the range of 3.4 to 3.8 and 1.48E?5 to 1.24 S/cm, respectively. Compared with solar cells with n-Si:H/Ag configuration, short-circuit current density (J _sc ) of solar cells with n-SiC_x:H/Ag configuration was improved up to 8.4%, which was comparable with that of solar cells with n-Si:H/ZnO/Ag configuration. Improved J _sc was related with enhanced spectral response at long wavelength of 500–800 nm. It was supposed that the decreased refractive index of n-SiC_x:H layer resulted in the increased back reflectance, which contributed to the improved J _sc. Our experiments demonstrated that n-SiC_x:H thin films were attractive choice because they functioned both as n-layer and interlayer in back reflector, and their deposition method was compatible with preparation process of solar cells.     

Optical enhancement by back reflector with ZnO:Al2O3(AZO) or NiCr diffusion barrier for amorphous silicon germanium thin film solar cells

Different composite films, including Al, Ag/NiCr/Al and Al₂O₃-doped ZnO (AZO)/Ag/NiCr (AZO)/Al, were utilized as the back reflectors for p-i-n hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells. The experimental results indicated that the AZO leyer between silicon layers and Ag/NiCr/Al back reflector was effect in improving solar cell performance, mainly owing to an increase in short-circuit current density (J_sc) of the solar cells. In addition, the thickness of AZO film could strongly affect the J_sc. The highest solar cell performance was achieved at the AZO thickness of about 90 nm. A nickel-chromium (NiCr) or AZO film was inserted between Ag and Al as a diffusion barrier against mutual diffusion of them, the similar performances of solar cells were achieved. So AZO/Ag/NiCr (AZO)/Al could be utilized as an advanced AZO/metal back reflector for p-i-n a-SiGe:H solar cells.     

Influence of RF power on structural optical and electrical properties of hydrogenated nano-crystalline silicon (<Emphasis Type="Italic">nc</Emphasis>-Si:H) thin films deposited by PE-CVD

We report synthesis of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by using conventional plasma enhanced chemical vapor deposition (PE-CVD) system from gas mixture of pure silane (SiH₄) and hydrogen (H₂). We investigated the effect of RF power on structural, optical and electrical properties using various characterization techniques including Raman spectroscopy, FTIR spectroscopy, UV–visible spectroscopy etc. Low angle XRD and Raman spectroscopy analysis revealed that the RF power in PE-CVD is a critical process parameter to induce nanocrystallization in Si:H films. The FTIR spectroscopy analysis results indicate that with increase in RF power the predominant hydrogen bonding in films shifts from Si–H to Si–H₂ and (Si–H₂)_n bonded species bonded species. However, the bonded hydrogen content didn’t show particular trend with change in RF power. The UV–visible spectroscopy analysis shows that the band tail width (E₀₄–E_Tauc) with increase in RF power. The defect density and Urbach energy also increases with increase in RF power. The highest dark conductivity (and lowest charge carrier activation energy) was obtained for the film deposited at RF power of 125 W indicating that 125 W is optimized RF power of our PE-CVD unit. At this optimized RF power nc-Si:H films with crystallite size ~3.7 nm having good degree of crystallinity (~86.7 %) and high band gap (E_Tauc ~ 2.01 eV and E₀₄ ~ 2.58 eV) were obtained with a low hydrogen content (6.2 at.%) at moderately high deposition rate (0.24 nm/s).     

Low-temperature preparation of transparent conductive Al-doped ZnO thin films by a novel sol–gel method

We developed a novel sol–gel method to prepare transparent conductive Al-doped ZnO (AZO) thin film at low temperature. The AZO nanocrystals were prepared by a solvothermal method and then they were dispersed in the monoethanolamine and methanol to form AZO colloids. A (002)-oriented ZnO thin film was used as a nucleation layer to induce the (002)-oriented growth of AZO thin films. The AZO thin films were prepared on Si(100) and fused quartz glass substrates with the (002)-oriented ZnO nucleation layer and annealed at 400 °C for 60 min. All AZO thin films showed (002) orientation. For electrical and optical measurements, the films deposited on glass substrates were post-annealed at 400 °C for 30 min in forming gas (100 % H₂) to improve their conductivity. These samples had high transparency in the visible wavelength range, and also showed good conductivity. A 0.2 mol L^?1 AZO solution with 3 at.% Al content was heated in a Teflon autoclave at 160 °C for 30 min to form AZO nanocrystals, and then the AZO nanocrystals were suspended in the MEA and methanol to obtain the stable AZO colloid. The Al content in the AZO nanocrystals was 2.7 at.%, and the high Al doping coefficient was mainly attributed to the formation of AZO nanocrystals in the autoclave. The AZO thin film using this colloid had the lowest resistivity of 3.89 × 10^?3 Ω cm due to its high carrier concentration of 3.29 × 10²⁰ cm^?3.     

Transparent conductive and infrared reflective AZO/Cu/AZO multilayer film prepared by RF magnetron sputtering

AZO/Cu/AZO multilayer films were prepared on glass substrate by radio frequency magnetron sputtering technology. The prepared films were investigated by a four-point probe system, X-ray diffraction, optical transmittance spectra, scanning electron microscope, atomic force microscopy and Fourier transform infrared spectroscopy. The results showed that Cu inner layer started forming a continuous film at the thickness around 11 nm. The prepared AZO/Cu/AZO samples exhibited the visible transmittance of 60–80 % and sample with 15 nm Cu inner layer showed the highest infrared reflection rate of 67 % in FIR region and the lowest sheet resistance of 16.6 Ω/sq. The proper visible transmittance and infrared reflection property of the AZO/Cu/AZO multilayer film make it a promising candidate for future energy conservation materials.     

The optimization of interfacial properties of nc-Si:H/c-Si solar cells in hot-wire chemical vapor deposition process

Passivation of c-Si surface was processed by hot-wire chemical vapor deposition through the pretreatment of atomic hydrogen. The interfacial properties were examined by the admittance spectroscopy. The interfacial property is sensitive to the atomic hydrogen treatment time t _H of c-Si substrates rather than the crystallization of the buffer layer. It was found that the t _H of 30–60 s is essential for the low defects states in the emitter. The interfacial morphology between nano-crystalline Si (nc-Si:H) layer and c-Si substrates was investigated by high resolution transmission electron microscopy in cross section geometry. The amorphous, mixed epitaxial/amorphous and epitaxy phases were observed as the buffer layer growth at hydrogen dilution ratios of 0, 0.5, 0.86, 0.97, respectively. By optimizing t _H and using an epitaxial silicon buffer layer, the efficiency of 17.36% of solar cell with the structure of n epi-Si/i epi-Si/p c-Si was fabricated at 250 °C.     

Physical properties of hydrogenated Al-doped ZnO thin layer treated by atmospheric plasma with oxygen gas

Hydrogenated Al-doped ZnO (H:AZO) thin films were deposited on glass substrates at room temperature by radio-frequency magnetron sputtering at various hydrogen flow rates. The addition of hydrogen improved the resistivity of the H:AZO films significantly. A thin insulating layer was produced on H:AZO films by atmospheric pressure plasma with Ar/O₂ reactive gas. The resistivity degenerated and the optical band gap of the oxygen plasma-treated H:AZO films decreased from 3.7 eV to 3.4 eV. This was attributed to a decrease in the hydrogen concentration at the film surface according to elemental depth analysis.     

Quality enhancement of AZO thin films at various thicknesses by introducing ITO buffer layer

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d₍₀₀₂₎, strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (E_g) of AZO film are increased from 4.62?×?10¹⁹ to 8.21?×?10¹⁹ cm^?3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6?×?10^?4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19?×?10⁴ (Ω cm)^?1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.     

Surface passivation of silicon by rf magnetron-sputtered silicon nitride films

Si-rich silicon nitride (SiN_x(:H)) films are deposited on single crystalline p-type silicon to investigate their properties as a passivation and antireflection coating for solar cells. The SiN_x(:H) films were reactively sputtered from an intrinsic Si-target in an Ar/N₂/H₂ rf (13.56 MHz) magnetron plasma at substrate temperatures from 150°C to 350°C. The optical band gap of Si-rich SiN_x(:H) becomes lower than 3 eV which was determined from spectral data of the complex refractive index. Infrared spectra show a strong Si–H band in Si-rich films. The effective surface recombination velocity (SRV) was calculated from the effective life time in SiN_x(:H) covered p-Si wafers by the microwave detected photoconductivity decay (MW PCD) technique. Very low values for the effective SRV of about 60 cm/s were determined. The low values of the effective SRV are due to field-effect passivation. The field-effect passivation of the SiN_x(:H)/Si contact is explained with the model of a hetero junction.     

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

The effect of ZnO film depositions using various film deposition methods such as magnetron sputtering (MSP), pulsed laser deposition (PLD) and vacuum arc plasma evaporation (VAPE) on the photovoltaic properties of ZnO–Cu₂O heterojunction solar cells is described in this report. In addition, the relationship between the resulting photovoltaic properties and the film deposition conditions such as supply power and substrate arrangement was investigated in Al-doped ZnO (AZO)–Cu₂O heterojunction devices fabricated using AZO thin films prepared by d.c. magnetron sputtering (d.c.MSP) or r.f. magnetron sputtering (r.f.MSP). The results showed that the measured photovoltaic properties of devices fabricated with films deposited on substrates oriented perpendicular to the target were better than those of devices fabricated with films deposited on substrates oriented parallel to the target. It was also found that ZnO film depositions under conditions where a relatively weaker oxidizing atmosphere was used yield better properties than films derived from MSP, which utilizes a high-density and high-energy plasma. Using VAPE and PLD, for example, high efficiencies of 1.52 and 1.42%, respectively, were obtained under AM2 solar illumination in devices fabricated at a substrate temperature around 200 °C.     

Novel properties of AZO film sputtered in Ar+H2 ambient at high temperature

AZO (ZnO:Al) transparent conductive thin film was prepared by RF magnetron sputtering with a AZO (98 wt% ZnO 2 wt% Al₂O₃) ceramic target in the same Ar+H₂ ambient at different substrate temperatures ranging from 100 to 300 °C. The minimum resistivity of AZO films was 7.9×10⁻⁴ Ω cm at the substrate temperature of 200 °C. The average transmission in the visible rang was more than 90%. Scanning electron microscopy and XRD analyses showed that the surface morphology of the AZO samples altered with the increasing of the substrate temperature. AZO film prepared at 200 °C in the pure Ar ambient was also made as comparison about the resistivity, carrier concentration and the average crystallite size. The resistivity became about 3 times higher. The carrier concentration became lower and the average crystallite size was smaller.     

Damp heat stability of AZO transparent electrode and influence of thin metal film for enhancing the stability

The electrical properties of Aluminum doped ZnO (AZO) thin films prepared by sol–gel method were investigated as a function of annealing atmosphere (vacuum, argon +5 % hydrogen and pure hydrogen) and doping concentration (1, 2, 3 and 4 wt%). An optimal annealing atmosphere (pure hydrogen) and doping concentration (2 wt%) was obtained with a minimum resistivity of 1.6 × 10^?3 Ω cm. The structural, optical and electrical stability has been investigated by a damp-heat test in an environment with 85 % relative humidity at 85 °C. The degradation of the electrical film properties was due to the decrease of carrier concentration and mobility, whereas, no significant change was observed for structural and optical properties. The thin metallic layer (Ti or Cr) was deposited on AZO by sputtering to prevent the penetration of oxygen and water into film thus increasing the electrical stability. Oxide layer of metal was formed on surface when it comes to air at room temperature which was confirmed by X-ray photoelectron spectroscopy and thus a bi-layer of metal/metal oxide layer on AZO film enhances the electrical stability.     

Characterization of CuInGeSe<Subscript>4</Subscript> thin films and Al/n–Si/p–CuInGeSe<Subscript>4</Subscript>/Au heterojunction device

CuInGeSe₄ thin films of various thicknesses were prepared on a glass substrate by thermal evaporation followed by selenization at 700 K. Energy dispersive X-ray analysis shows that the CuInGeSe₄ thin films are near stoichiometric. The X-ray diffraction patterns indicate that the as-deposited CuInGeSe₄ thin films are amorphous, while the CuInGeSe₄ thin films annealed at 700 K are polycrystalline with the chalcopyrite phase. The structure of the films was further investigated by transmission electron microscopy and diffraction, with the results verifying the X-ray diffraction data. High-resolution scanning electron microscopy images show well-defined grains that are nearly similar in size. The surface roughness increases with film thickness, as confirmed by atomic force microscopy. The optical transmission and reflection spectra of the CuInGeSe₄ thin films were recorded over the wavelength range of 400–2500 nm. The variation of the optical parameters of the CuInGeSe₄ thin films, such as the refractive index n and the optical band gap E_g, as a function of the film thickness was determined. The value of E_g decreases with increasing film thickness. For the studied films, n were estimated from the Swanoepl’s method and were found to increase with increasing film thickness as well as follow the two-term Cauchy dispersion relation. A heterojunction with the configuration Al/n–Si/p–CuInGeSe₄/Au was fabricated. The built-in voltage and the carrier concentration of the heterojunction was determined from the capacitance–voltage measurements at 1 MHz and were found to be 0.61 V and 3.72?×?10¹⁷ cm^?3, respectively. Under 1000 W/m² solar simulator illumination, the heterojunction achieved a conversion efficiency of 2.83%.     

Aluminum-doped zinc oxide electrode for robust (Pb,La)(Zr,Ti)O3 capacitors: effect of oxide insulator encapsulation and oxide buffer layer

Conductive aluminum-doped zinc oxide (AZO) was deposited by pulsed laser deposition as top electrodes for chemical solution deposition derived lanthanum modified lead zirconate titanate (PLZT) capacitors. Compared with PLZT capacitors with Pt top electrodes (Pt/PLZT/Pt), PLZT capacitors with AZO as top electrodes (AZO/PLZT/Pt) showed improved fatigue endurance after cycles of 200 kV/cm (10 V) were applied with a 100 μs pulse width at 1 ms intervals. By using a combination of AZO as the top electrode and as a thin buffer layer (10 nm) under PLZT thin films (AZO/PLZT/AZO/Pt), fatigue behavior was also improved compared with the case without an AZO buffer layer. The addition of either an Al₂O₃ or HfO₂ encapsulation layer increased the remnant polarization ratio of PLZT capacitors (after annealing at 200 °C, 1 Torr, 3 % hydrogen atmosphere) to 0.80 and 0.57, respectively, comparing with 0.52 without an encapsulation layer.     

Fabrication of Cu<Subscript>2</Subscript>SnS<Subscript>3</Subscript> thin-film solar cells with oxide precursor by pulsed laser deposition

In this paper, Cu₂SnS₃ (CTS) thin film is fabricated through sulfurization of oxide precursor which is deposited by pulsed laser deposition with a mixed CuO/SnO₂ target. XRD and Raman analyses indicate a pure monoclinic Cu₂SnS₃ phase has been obtained by sulfurization at temperature from 500 to 600 °C. A compact and smooth film with polycrystalline structure is observed through SEM result. In addition, the CTS films show excellent absorbance with the band gap around 0.91 eV estimated by UV–Vis, which is suitable for the absorption layer of solar cells. Final devices were fabricated with a SLG/Mo/CTS/CdS/i-ZnO/AZO/Al structure. Device performance is improved with the temperature increasing. The best efficiency of CTS-based solar cells is 0.69% with an open-circuit voltage of 144 mV and a short-circuit current density of 18.30 mA/cm^?2.     

Influence of H2 introduction on properties in Al-doped ZnO thin films prepared by RF magnetron sputtering at room temperature

Al-doped ZnO (AZO) thin films were prepared by RF magnetron sputtering on quartz substrates at room temperature in different Ar + H₂ ambient. The influence of H₂ flow ratio on the structure and optoelectronic properties in AZO films was investigated. The prepared films are hexagonal wurtzite structure with c-axis preferred orientation, and the intensity of (002) peak decreases with the increase of H₂ flow ratio. The resistivity significantly decreases with increasing the H₂ flow ratio to 1.0 % by almost four orders of magnitude. X-Ray photoelectron spectroscopy and X-ray diffraction measurements exhibit that the effectiveness of Al doping in the substitutional positions is not influenced by H₂ addition. We suggest that there exist a large number of acceptors in the films, the introduced H₂ will passivate the acceptors, which raises both carrier concentration and Hall mobility. The increase of carrier concentration consequently induces the blue shift of optical absorption edge according to the Burstein-Moss effect.     

Improving the characteristics of CdS and CIAS films and the performances of CIAS solar cells by electrodeposition Cu–Se/CIAS binary structure precursors on FTO substrate

Cu-poor electrodeposited CuIn_1?xAl_xSe₂ (CIAS) precursor films were prepared to investigate the alteration in surface morphology of post-annealed CIAS films through post-annealing temperature adjustment. Scanning electron microscopy (SEM) and atomic force microscope (AFM) analyses demonstrated that surface morphology and root–mean–square (RMS) roughness of post-annealed CIAS films exhibited uneven and rough triangular structures. The crystal size of post-annealed CIAS films can be increased by increasing post-annealing temperature. The precursor film structure was modified by substituting Cu–Se/CIAS binary structure with CIAS single structure to proceed with the investigation. The apparent variation in surface morphology of post-annealed CIAS films changed from rough triangular structures to smooth round structures, and the RMS roughness of post-annealed CIAS films was reduced to <100 nm. The reduction was attributed to the formation of Cu–Se liquid phases during the post-annealing process, which enhanced elemental migration, recombination, and promotion of large grains and smooth surface formation. X-ray diffraction patterns showed three preferred growth orientations along the (112), (204/220), and (116/312) planes with chalcopyrite structures for all species. In addition, the characteristics of surface morphology, RMS roughness, and current measurement of subsequently deposited cadmium sulfide (CdS) film were studied and examined via SEM and AFM analyses. The surface morphology of CdS films deposited on binary structure post-annealed CIAS films exhibited smoothness, compactness, small RMS roughness, and large crystals with round and film-like structure. The AFM current images indicated that the distribution of leakage current paths was greatly diminished by changing the precursor film structure from CIAS single structure to Cu–Se/CIAS binary structure. The dark current–voltage characteristics of the CdS/CIAS heterojunctions showed that the reverse dark current density was decreased by approximately one order of magnitude from 4.02 × 10^?4 (single structure) to 4.26 × 10^?5 A/cm² (binary structure). Furthermore, the conversion efficiency of CIAS solar cells was enhanced from 0.52 (single structure) to 1.44 % (binary structure) with increase in V_oc and J_sc.