ZnO Transparent Conductive Oxide Research Articles

Enhancing performance of heterojunction silicon solar cells through atomic-layer-deposited MoOx hole contact and atomic-layer-deposited AlZnO layer

We present the performance enhancement of a heterojunction Si solar cell facilitated by incorporating an atomic-layer-deposited (ALD) MoOx hole contact layer and an ALD Al-doped ZnO (AZO) transparent conductive oxide (TCO). ALD MoOx thin films as a hole-selective contact layer instead of P+-doped amorphous Si in Si heterojunction solar cells, with subsequent growth of ALD AZO films grown on the MoOx layer preceding the indium-tin-oxide sputtering deposition. Integrating ALD MoOx hole-selective contact and ALD AZO TCO manifests an augmentation in the short-circuit current in heterojunction Si solar cells due to the heightened carrier transport and concurrent reduction in sputtering-induced damage. Consequently, the heterojunction Si solar cell employing the ALD MoOx film and ALD AZO layer yielded a 1.1 % increase in efficiency relative to the reference heterojunction solar cell.

Annealing of ZnO and SnO2 transparent conductive oxides

Transparent Conductive Oxide (TCO) are used in different field [1], especially as antireflective layers on the surface of solar cells [2,3].But in the solar cells process, annealing steps are often used , i.e ; the front metallic contacts of solar cells are obtained by serigraphy at high temperature (800 – 830°C). In this case what happen in TCOs used as antireflective layers ?. In this work we present the comparison of changes in physical properties of two TCOs: tin oxide SnO2 and zinc oxide ZnO when they are annealed at low and high temperatures. These films are deposited by the Atmospheric Pressure Chemical Vapour Deposition APCVD technique. Tin oxide is deposited from tin dichloride (SnCl2, 2H2O) precursor and zinc oxide is obtained by the use of zinc acetylacetonate Zn(C5H7O2)2. The electrical and optical properties of tin oxide and zinc oxide are determined by the four points probes method and spectrophotometry measurement. The values of the resistivity of tin oxide and zinc oxide are 10-4 Ω.cm and 10-3 Ω.cm respectively. These films present an optical transmission higher than 80%. The scanning electronic microscopy images show that the films have a polycrystalline aspect. A post annealing of these TCOs at 450°C improves their electrical properties, while an annealing at higher temperature damage them.

Ab initio study of photoelectric properties in ZnO transparent conductive oxide

Zinc oxide is regarded as a third generation transparent conductive film which can replace indium doped tin oxide. It can be widely used in the electrodes of solar cells, the display of electronic instruments and other devices. In this paper, the photoelectric properties of ZnO thin films and their different growth directions were studied, and the best growth direction was obtained. The ZnO system was calculated by density functional theory. The band structure, density of states and other electronic properties of the stable structure are calculated by using Quantum Espresso program. It is found that the Valence Band Maximum and Conduction Band Minimum of ZnO (002) crystal plane both pass through the Fermi level, and the system exhibits semi-metallic properties, which has an important effect on the electrical properties. The band structure is interpolated by wannier90 program, and the precise maximum localized Wannier functions are obtained. Phonons properties are studied by density functional perturbation theory, and the accurate phonon dynamic matrix and the electron phonon coupling matrix elements are obtained. Based on that, the Perturbo program is used to solve the Boltzmann Transport Equation by using relaxation time approximation and electron phonon coupling theory. The scattering rate and relaxation time under different temperature are obtained and the reason why the conductivity and mobility decrease with the increase of temperature is analyzed. In the optical properties of the calculation, the time-dependent density functional theory and epsilon.x program are used to solve the Kohn–Sham equation, and then find out the refractive index, absorption coefficient, visible light transmittance and other important optical properties by obtaining the dielectric function. Combined with the analysis of electrical and optical properties, the (002) crystal plane is determined to be the best growth direction, which is well consistent with the preferred orientation obtained from the experiment. Therefore, it provides a theoretical basis and support for the preparation of better ZnO thin films. The work in this paper fills the blank of theoretical research on ZnO.

Electrical and Optical Properties of Al-Doped ZnO Transparent Conductive Oxide Films Prepared via Radio Frequency Magnetron Co-Sputtering System.

In this study, the optical and electrical properties of a transparent conductive oxide (TCO) film synthesized via the radio frequency (RF) magnetron co-sputtering of Al-doped ZnO (AZO) and ZnO targets on a glass substrate were investigated. In the visible region, the resistivity, transmittance, and carrier concentration of the TCO film are influenced by the ratio of Al doping. The samples were prepared using two targets with the same deposition condition, except several different power levels on an AZO target to obtain different Al compositions in the film. The power range was 100-160 W in 20 W steps on the AZO target with a constant 50 W power level on the ZnO target. The electrical and optical characteristics of the film were measured using several apparatuses. The cross-section of the films was measured with via field emission scanning electron microscopy (FESEM) to determine the thickness of the film. The electrical and optical properties of the AZO films were measured via Hall measurement and UV-visible spectroscopy. The structural characteristics of the AZO films were confirmed by Raman spectroscopy.

Comparison study of ZnO-based quaternary TCO materials for photovoltaic application

ZnO is of great interest for large-area optoelectronic devices, e.g., flat panel displays, light-emitting diodes, transparent semiconductors, and transparent conductive oxides (TCOs) etc., owing to its good optical properties. In the present study, quaternary ZnO thin films are deposited on a soda lime glass substrate using Mg and group III elements, such as Al, Ga, and In, to enhance the optical and electrical properties. The structural, optical, and electrical properties of quaternary ZnO TCO materials are investigated for improved thin film solar cell performance. All of the films show a uniform microstructure without any void and crack and have a transmittance over 75% in the visible region. They possess comparable band gap differences. Especially, as for the optical properties, a Mg and Ga co-doped ZnO, MgGaZnO (MGZO) thin film shows a high optical band gap of 3.87 eV with a transmittance of about 90% in the visible region. In addition, the MGZO thin film shows improved electrical properties with the lowest resistivity of 3.97 × 10−4 Ωcm, higher carrier concentration of 1.11 × 1021cm−3, high mobility of 14.07 cm2V−1s−1 and lower sheet resistance of 7.39Ω/sq. Moreover, a wide band gap of 3.87 eV is obtained for MGZO thin films, and the performance of a Cu2ZnSn(S, Se)4 (CZTSSe) photovoltaic device fabricated with a MGZO TCO layer is improved owing to higher band gap and outstanding electronic properties. The improved short circuit current results in a device with a MGZO thin film with a power conversion efficiency of 8.79%.

Influence of the Thickness of a Layer of Potassium Fluoride Incorporated in the CIGS/CdS Interface on the Macroscopic Electrical Parameters of the Solar Cell

In this work, the heterojunction composed of a n-type ZnO transparent conductive oxide (OTC) layer, a n-type CdS buffer layer and a absorber layer based Cu (In, Ga)Se2 p doped is studied under the influence of a KF layer placed in the CIGS/CdS interface. This study was done by varying the thickness of KF using thin-film simulation software named SCAPS-1D. The presence of KF for a doping of the CIGS absorber of 1016cm-3 improves strongly the electrical parameters that are the Vco, the Jcc the FF, the maximum power and the conversion efficiency of the solar cell Ƞ. However, a decrease of the FF and the Jcc is noticed when the thickness of the KF is greater than 30nm causing a deterioration of the performances of the cell.

Critical design criteria for silicon nanocrystals downshifting layers enhancement in CIGS solar cells

Conventional Cu ( In , Ga ) Se2 (CIGS) solar cells suffer poor response in the short-wavelength region (280 to 520 nm) due to the parasitic absorption in the ZnO transparent conductive oxide and inefficient collection of generated electron–hole pairs in the CdS layer. The short-wavelength response can be enhanced using a downshifting (DS) layer mounted on the top surface of the solar cell. The performance effects of coupling silicon nanocrystals DS layer to a CIGS solar cell are analyzed using numerical simulations. Measured photoluminescence of fabricated Si-nC using plasma-enhanced chemical vapor deposition is used to calibrate the model. A 13.9% relative enhancement of the conversion efficiency is observed when a DS layer with a photoluminescence quantum yield (PLQY) of 20% is added. The relative enhancement increases to 17.8% when the DS layer has a PLQY of 100%. The results are further analyzed to decouple and quantify the surface reflectance effect and the DS effect using a passive cell. The surface reflectance effect is dominant for DS layers with PLQY of 20% and accounts for 13.6% relative enhancement. Whereas the DS effect dominates when the PLQY is >70 % and reaches 17.1% relative enhancement for a DS layer with PLQY of 100%. Overall, cells with poor UV spectral responses are found to benefit the most from the coupling of a DS layer, albeit only for a very high PLQY.

Muscovite mica as a flexible substrate for transparent conductive AZO thin films deposited by spray pyrolysis

In this study, Aluminum doped and undoped ZnO transparent conductive oxide thin films on a specific substrate, i.e., the muscovite mica sheet, were prepared by the spray pyrolysis technique. The substrate was used in this study for its unique properties, namely simultaneous flexibility and high thermal stability. The effects of doping on structural, morphological, optical and electrical characteristics of the samples were investigated and discussed. The obtained thin films were mostly polycrystalline with a hexagonal structure. UV–Visible spectroscopy analysis showed high average transparency (more than 80%) in visible and near-infrared region. Field emission scanning electron microscopy analysis proved the homogenous morphology of the films and showed their resistivity to be in the range of 10−3 Ω cm.

Si‐Doped Cu(In,Ga)Se2 Photovoltaic Devices with Energy Conversion Efficiencies Exceeding 16.5% without a Buffer Layer

AbstractIn this communication, novel and simplified structure Cu(In,Ga)Se2 (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si‐doped CIGS films grown by the three‐stage coevaporation method, B‐doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si‐doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer‐free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer‐based CIGS device performance, are found to be also effective in enhancing buffer‐free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer‐free CIGS photovoltaic devices.

Structural, electrical and optical properties of Al–Sn codoped ZnO transparent conducting layer deposited by spray pyrolysis technique

Low cost Al-Sn codoped ZnO (ATZO) Transparent Conductive Oxide films were deposited by spray pyrolysis on glass substrate. The influence of Al-Sn codoping on the structural, optical and electrical properties of ZnO thin films was studied by comparing the same properties obtained in undoped ZnO, Al doped ZnO (AZO) and Sn doped ZnO (TZO) thin films. The so-obtained films crystallized in hexagonal wurtzite structure. The morphology and structural defects have been investigated by both High resolution Field Effect Scanning Electron Microscopy (FE-SEM) and Raman spectroscopy at 532 nm excitation source. In the visible region, the undoped and doped films show an average transmittance of the order of 85%, while for ATZO thin film, it is of the order of 72%, which points out a degradation of the optical properties due to the co-doping. The optical band gap of ATZO thin film achieves 3.31eV and this shift, compared to the referred samples is attributed to the Burstein–Moss (BM) and band gap narrowing (BGN) opposite effects which is due to the increase of the carrier concentration in degenerate semiconductors. Within all the samples, the ATZO thin film exhibits the lowest electrical resistivity of 4.56 × 10−3 Ωcm with a Hall mobility equal to 2.13 cm2 V−1s−1, and the highest carrier concentration of 6.41 × 1020 cm−3. The performance of ATZO transparent conductive oxide film are determined by its figure of merit (φTC), found equal to 1.69 10−4 Ω−1, which is a suitable value for potentially high-performance solar cell applications.

Codoping and Interstitial Deactivation in the Control of Amphoteric Li Dopant in ZnO for the Realization of p-Type TCOs.

We report on first principle investigations about the electrical character of Li-X codoped ZnO transparent conductive oxides (TCOs). We studied a set of possible X codopants including either unintentional dopants typically present in the system (e.g., H, O) or monovalent acceptor groups, based on nitrogen and halogens (F, Cl, I). The interplay between dopants and structural point defects in the host (such as vacancies) is also taken explicitly into account, demonstrating the crucial effect that zinc and oxygen vacancies have on the final properties of TCOs. Our results show that Li-ZnO has a p-type character, when Li is included as Zn substitutional dopant, but it turns into an n-type when Li is in interstitial sites. The inclusion of X-codopants is considered to deactivate the n-type character of interstitial Li atoms: the total Li-X compensation effect and the corresponding electrical character of the doped compounds selectively depend on the presence of vacancies in the host. We prove that LiF-doped ZnO is the only codoped system that exhibits a p-type character in the presence of Zn vacancies.

A method for optimizing the electrical conductivity of Al:ZnO TCO films

In this paper, we propose a new strategy for optimizing the conductivity of AZO (ZnO:Al) thin films as transparent conductive oxide (TCOs) based on adapting the atomic percentage of the dopant to the precursor sol concentration. A comparison between the electrical current of pure ZnO and AZO thin films indicated that, the increase in molar concentration of ZnO solution and percentage of Aluminum, respectively up to 1M and 2at.%, increased the charge carrier concentration continuously. But a little more increase in both of them actually decreased the conduction. Optical behavior of thin films showed blue shifts in the optical band gap that could be explained by the Burstein-Moss effect and Brus equation. The lattice stress and crystal size of thin films showed that Al+3 ions were properly localized in the ZnO structure prepared with 1M concentration at 2at.% Al.

Improvement of parameters in a-Si(p)/c-Si(n)/a-Si(n) solar cells

We analyzed and discussed the influence of thickness and doping concentration of the different layers in a-Si(p)/c-Si(n)/a-Si(n) photovoltaic (PV) cells with the aim of increasing its efficiency while decreasing its global cost. Compared to the efficiency of a standard marketed PV cell, elaborated with a ZnO transparent conductive oxide (TCO) layer but without Back Surface Field (BSF) layer, an optimization of the thickness and dopant concentration of both the emitter a-Si(p) and absorber c-Si(n) layers will gain about 3% in the global efficiency of the cell. The results also reveal that with introduction of the third layer, i.e. the BSF layer, the efficiency always achieves values above 20% and all other parameters of the cell, such as the open-circuit voltage, the short-circuit current and the fill-factor, are strongly affected by the thickness and dopant concentration of the layers. The values of all parameters are given and discussed in the paper. Thereby, the simulation results give for an optimized a-Si(p)/c-Si(n)/a-Si(n) PV cells the possibility to decrease the thickness of the absorber layer down to 50 μm which is lower than in the state-of-the-art. This structure of the cell achieves suitable properties for high efficiency, cost-effectiveness and reliable heterojunction (HJ) solar cell applications.

Growth and characterization of nonpolar (10-10) ZnO transparent conductive oxide on semipolar (11–22) GaN-based light-emitting diodes

We have grown thin films of nonpolar m-plane (10-10) ZnO on a semipolar (11–22) GaN template by atomic layer deposition (ALD) at low growth temperatures (<200 °C). The surface morphology of the ZnO film is found to be an arrowhead-like structure, which is a typical surface structure of the semipolar (11–22) GaN films. On increasing the growth temperature of the ZnO films, the concentration and mobility of the charge carriers in the ZnO film are increased. However, the optical transmittance decreases with an increase in the growth temperature. Based on these results, we have fabricated semipolar (11–22) GaN-based light-emitting diodes (LEDs) with nonpolar m-plane ZnO film as a transparent conductive oxide (TCO) to improve the light extraction efficiency. In spite of a decrease in the optical transmittance, the operation voltage of semipolar (11–22) GaN-based LEDs is found to decrease with an increase in the growth temperature, which might be due to the improvements in the electrical properties and current spreading effect, resulting in an increase in the optical output power.

Investigation of V doped ZnO transparent conductive oxide films

The performance of the ZnO film that is an indispensable part of pin-type Si-based thin-film solar cells, plays a crucial role in high-efficiency thin-film solar cells and also forms a significant part in photovoltaic research and development. In this paper, low resistivity and wide broadband spectrum transmittance vanadium (V) doped ZnO (VZO) films are successfully fabricated on Corning XG substrates at various substrate temperatures (STs). The properties of VZO films are investigated by the radio-frequency magnetron sputtering technique and plane wave pseudo-potential method based on the density-functional theory. The experimental results demonstrate that all the VZO flms have (002) preferred orientation with the c-axis perpendicular to the substrate, and the crystalline quality is found to increase with the substrate temperature (ST) rising up to 280 ℃ and decrease when the ST increases further. The optimal VZO film is achieved at 280 ℃ with a resistivity of 3.810-3 cm and an average transmittance of more than 85% in a range of 500-2000 nm. The theoretical result shows that after incorporation of V the Fermi level goes through the conduction band, showing a typical n-type metallic characteristic. The carriers originate from the orbits of V 3d and O 2p. The calculated lattice constants and mobility for VZO film are in agreement well with the experimental results. The consistency of the theoretical results with the experimental results shows that the VZO thin film has a great potential application as a front contact in high-efficiency thin film solar cells.

High-haze and wide-spectrum hydrogenated MGZO TCO films on micro-textured glass substrates for thin-film solar cells

High-haze and wide-spectrum hydrogenated Mg and Ga co-doped ZnO (HMGZO) transparent conductive oxide films on ‘U-shaped’ micro-textured glass are developed using magnetron sputtering. The wet-chemically etched glass substrates with large crater-like features obtained via a frosting method exhibit strong light-scattering capability in a wide wavelength range (~350–1100nm). The morphological, structural, optical and electrical properties of HMGZO films on textured and flat glass are investigated in detail. The HMGZO thin films deposited on textured glasses show conformal growth and have a polycrystalline hexagonal wurtzite phase with a c-axis preferred orientation, which is similar to that of films deposited on flat glass. The electrical resistivity of HMGZO films decreases from 9.74×10−4 to 7.67×10−4Ωcm and the optical band gap Eg is in the range from 3.76 to 3.72eV as the deposition time is increased from 25 to 40min. The HMGZO films with textured glass substrate exhibit stronger light-scattering capability compared with films with flat glass substrate. Finally, an a-Si solar cell deposited on textured glass shows a higher conversion efficiency and short-circuit current density than that deposited on flat glass, as a result of effectively improved light scattering. ZnO-TCO films on low-cost chemically etched glass substrates are promising candidates as front electrodes for thin-film solar cells.

Optoelectronic properties and color chemistry of native point defects in Al:ZnO transparent conductive oxide

The effects of native defects (e.g. VO, VZn, H) on the TCO properties and color of an Al:ZnO (AZO) material are investigated using first principles calculations.

Wide-spectrum Mg and Ga co-doped ZnO TCO thin films for solar cells grown via magnetron sputtering with H2 introduction

Wide-spectrum Mg and Ga co-doped ZnO transparent conductive oxide (TCO) thin films are deposited via magnetron sputtering at various H2 flow rates on glass substrates. The structural, electrical, and optical properties of MGZO thin films are investigated with different H2 flow rates. The experiment results show that the MGZO thin films are polycrystalline with a hexagonal wurtzite structure exhibiting a preferred (002) crystal plane orientation. The carrier concentration remarkably increases from 5.15×1019cm−3 to 2.12×1020cm−3 with increasing the H2 flow rate from 0sccm to 4.0sccm and then decreases when further increasing the H2 flow rate. The glass/MGZO thin film deposited at the H2 flow rate of 4.0sccm exhibits the lowest resistivity of 1.96×10−3Ωcm (film thickness d∼548nm) and an average transmittance (Ta) of 80.5% in the wavelength range from 340nm to 1100nm. Optical measurements indicate that the optical band gap (Eg) of MGZO thin films varies from 3.45eV to 3.78eV with adjusting H2 flow rate from 0sccm to 12.0sccm. The obtained MGZO thin films with an appropriate thickness are preliminarily applied in p–i–n type hydrogenated amorphous silicon (a-Si:H) thin film solar cells. The a-Si:H solar cell with MGZO layer presents higher quantum efficiency in the short wavelength region than that with GZO layer, resulting from widened optical band gap.

Nanocolumnar zinc oxide as a transparent conductive oxide film for a blue InGaN-based light emitting diode

A comprehensive study of ZnO nanocolumnar structure has been conducted through x-ray diffraction spectroscopy, atomic force microscopy, field emission scanning electron microscopy and UV-visible spectroscopy measurements. This study involves the development of an efficient ZnO transparent conductive oxide film fabricated by RF magnetron sputtering for use in a blue indium gallium nitride based light emitting diode. FESEM measurements revealed that a maximum growth temperature of 500°C induced the formation of a nanocolumnar structure. The XRD measurements revealed that the stress reduction in the film also contributed to a superior ZnO nanocolumnar structure. In addition, the reduction of the oxygen percentage during deposition resulted in significant improvement of the structure. The reduced atomic peening effect yielded lower stress in the film. Thus, a dense, uniform-thickness, fine nanocolumnar ZnO of lateral size measuring 30 to 60nm was successfully fabricated at the lowest oxygen percentage of 7%. This film also exhibited good transparency at the blue region with 78% optical transmission.

Influence of external magnetic field on properties of aluminum-doped zinc oxide films prepared by RF magnetron sputtering

Al-doped ZnO (AZO) transparent conductive oxide films were prepared by RF magnetic sputtering. An external magnetic field was applied to the traditional magnetron sputtering system. The influence of the external magnetic field on the crystalline structure, surface topography and photoelectric properties of the AZO transparent conductive film have been studied. XRD diffraction patterns show that under the same processing condition, the intensity of (002) diffraction peak is significantly increased with the external magnetic field, suggesting a higher degree of c-axis preferred orientation. Scanning electron microscope shows that the external magnetic field can enlarge the grain size and density of films; the surface topography of the AZO films deposited without an external magnetic field is wormlike. Deposition rate and square resistance test results show that in an external magnetic field, the deposition rate will increase from 13.04 nm/min to 19.93 nm/min, and the sheet resistance reduce to 12.88 Ω /□ from 30.74 Ω /□ at a sputtering time of 90 min. Optical transmittance spectra shows that the average transmittance of all the films in visible light spectrum is over 85% when the sputtering time is not more than 60 min, while the external magnetic field has little effect on the transmittance of the films, but making a larger blue shift of the absorption edge. Ansys software is used to simulate the two-dimensional magnetic field distribution above the target. Results show that the intensity of the horizontal magnetic field and the uniformity of it are improved by the external magnetic field, the secondary electrons near the target are tightly bound, leading to a much larger target current intensity. So the deposition rate, surface topography and photoelectric properties of the AZO films are improved.