Natively textured surface aluminum-doped zinc oxide transparent conductive layers for thin film solar cells via pulsed direct-current reactive magnetron sputtering

Natively textured surface aluminum-doped zinc oxide (ZnO:Al) layers for thin film solar cells were directly deposited without any surface treatments via pulsed direct-current reactive magnetron sputtering on glass substrates. Such an in-situ texturing method for sputtered ZnO:Al thin films has the advantages of efficiently reducing production costs and dramatically saving time in photovoltaic industrial processing. High purity metallic Zn-Al (purity: 99.999%, Al 2.0 wt.%) target and oxygen (purity: 99.999%) were used as source materials. During the reactive sputtering process, the oxygen gas flow rate was controlled using plasma emission monitoring. The performance of the textured surface ZnO:Al transparent conductive oxides (TCOs) thin films can be modified by changing the number of deposition rounds (i.e. thin-film thicknesses). The initially milky ZnO:Al TCO thin films deposited at a substrate temperature of ~ 553 K exhibit rough crater-like surface morphology with high transparencies (T ~ 80-85% in visible range) and excellent electrical properties (ρ ~ 3.4 × 10^− 4 Ω cm). Finally, the textured-surface ZnO:Al TCO thin films were preliminarily applied in pin-type silicon thin film solar cells.     

Influence of working pressure on ZnO:Al films from tube targets for silicon thin film solar cells

Mid-frequency magnetron sputtering of aluminum doped zinc oxide films (ZnO:Al) from tube ceramic targets has been investigated for silicon based thin film solar cell applications. The influence of working pressure on structural, electrical, and optical properties of sputtered ZnO:Al films was studied. ZnO:Al thin films with a minimum resistivity of 3.4 × 10⁻⁴ Ω cm, high mobility of 50 cm²/Vs, and high optical transmission close to 90% in visible spectrum region were achieved. The surface texture of ZnO:Al films after a chemical etching step was investigated. A gradual increase in feature sizes (diameter and depth) was observed with increasing sputter pressure. Silicon based thin film solar cells were prepared using the etched ZnO:Al films as front contacts. Energy conversion efficiencies of up to 10.2% were obtained for amorphous/microcrystalline silicon tandem solar cells.     

The fabrication and application of ZnO:Al thin films in low-frequency inductively coupled plasma fabricated silicon solar cell

A home-made radio frequency magnetron sputtering is used to systematically study the structural, electrical, and optical properties of aluminum doped zinc oxide (ZnO:Al) thin films. The intensity of the (002) peak exhibits a remarkable enhancement with increasing film thickness. Upon optimization, we achieved low resistivity of 4.2 × 10^− 4 Ω cm and high transmittance of ~ 88% for ZnO:Al films. Based on the present experimental data, the carrier transport mechanism is discussed. It is found that the grain boundary scattering needs to be considered because the mean free path of free carrier is comparable to the grain size. The 80 nm-ZnO:Al thin films are then deposited onto low-frequency inductively coupled plasma fabricated silicon solar cells to assess the effect of ZnO:Al thin films on the performance of the solar cells. Optimized ZnO:Al thin films are identified as transparent and conductive oxide thin film layers.     

Textured surface boron-doped ZnO transparent conductive oxides on polyethylene terephthalate substrates for Si-based thin film solar cells

Textured surface boron-doped zinc oxide (ZnO:B) thin films were directly grown via low pressure metal organic chemical vapor deposition (LP-MOCVD) on polyethylene terephthalate (PET) flexible substrates at low temperatures and high-efficiency flexible polymer silicon (Si) based thin film solar cells were obtained. High purity diethylzinc and water vapors were used as source materials, and diborane was used as an n-type dopant gas. P-i-n silicon layers were fabricated at ~ 398 K by plasma enhanced chemical vapor deposition. These textured surface ZnO:B thin films on PET substrates (PET/ZnO:B) exhibit rough pyramid-like morphology with high transparencies (T ~ 80%) and excellent electrical properties (Rs ~ 10 Ω at d ~ 1500 nm). Finally, the PET/ZnO:B thin films were applied in flexible p-i-n type silicon thin film solar cells (device structure: PET/ZnO:B/p-i-n a-Si:H/Al) with a high conversion efficiency of 6.32% (short-circuit current density J_SC = 10.62 mA/cm², open-circuit voltage V_OC = 0.93 V and fill factor = 64%).     

Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells

High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highly-efficient p-i-n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10-40 W) and pressure (20-50 Pa) used during the deposition of absorber layers in p-i-n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD.     

Contact resistivity measurements of the buried Si-ZnO:Al interface of polycrystalline silicon thin-film solar cells on ZnO:Al

An experimental method is developed for contact resistivity measurements of a buried interface in polycrystalline silicon (poly-Si) thin-film solar cell devices on aluminum doped zinc oxide (ZnO:Al) layers. The solar cell concept comprises a glass substrate covered with a temperature-stable ZnO:Al film as transparent front contact layer, a poly-Si n⁺/p⁻/p⁺ cell, as well as a metal back contact. Glass/ZnO:Al/poly-Si/metal test stripe structures are fabricated by photolithographic techniques with the ZnO:Al stripes locally bared by laser ablation. The high-temperature treatments during poly-Si fabrication, e.g. a several hours lasting high-temperature step at 600 °C, are found to have no detrimental impact on the ZnO:Al/Si interface contact resistivity. All measured ρ_C values range well below 0.4 Ω cm² corresponding to a relative power loss ΔP below 3% for a solar cell with 500 mV open circuit voltage and 30 mA/cm² short circuit current density. By inclusion of a silicon nitride (SiN_x) diffusion barrier between ZnO:Al and poly-Si the electrical material quality of the poly-Si absorber can be significantly enhanced. Even in this case, the contact resistivity remains below 0.4 Ω cm² if the diffusion barrier has a thickness smaller than 10 nm.     

Advantages of using amorphous indium zinc oxide films for window layer in Cu(In,Ga)Se2 solar cells

The advantages of using indium zinc oxide (IZO) films instead of conventional Ga-doped zinc oxide (ZnO:Ga) films for Cu(In,Ga)Se₂ (CIGS) solar cells are described. The electrical properties of IZO are independent of film thickness. IZO films have higher mobility (30-40 cm²/Vs) and lower resistivity (4-5 × 10^− 4 Ω cm) compared to ZnO:Ga films deposited without intentional heating, because the number of grain boundaries in amorphous IZO films is small. The properties of a CIGS solar cell using IZO at the window layer were better than those obtained using a conventional ZnO:Ga at the window layer; moreover, the properties tended to be independent of thickness. These results indicate that use of IZO as a transparent conducting oxide layer is expected to increase the efficiency of CIGS solar cells.     

Optical and electrical properties of electron-irradiated Cu(In,Ga)Se2 solar cells

The optical and electrical properties of electron-irradiated Cu(In,Ga)Se₂ (CIGS) solar cells and the thin films that composed the CIGS solar cell structure were investigated. The transmittance of indium tin oxide (ITO), ZnO:Al, ZnO:Ga, undoped ZnO, and CdS thin films did not change for a fluence of up to 1.5 × 10¹⁸ cm^− 2. However, the resistivity of ZnO:Al and ZnO:Ga, which are generally used as window layers for CIGS solar cells, increased with increasing irradiation fluence. For CIGS thin films, the photoluminescence peak intensity due to Cu-related point defects, which do not significantly affect solar cell performance, increased with increasing electron irradiation. In CIGS solar cells, decreasing J_SC and increasing R_s reflected the influence of irradiated ZnO:Al, and decreasing V_OC and increasing R_sh mainly tended to reflect the pn-interface properties. These results may indicate that the surface ZnO:Al thin film and several heterojunctions tend to degrade easily by electron irradiation as compared with the bulk of semiconductor-composed solar cells.     

Influences of oxygen contamination on evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The influences of the oxygen contaminations on the crystal quality and performances of the evaporated polycrystalline silicon (poly-Si) thin-film solar cells prepared by solid-phase epitaxy were investigated by applying different deposition rates and base pressures. The experimental results show that although the evaporated poly-Si thin-film solar cell obtained at high base pressures (9.33 × 10^− 5 Pa) and high deposition rate (300 nm/min) has small amount of SiO₂ precipitations, it still shows the similar good material quality and performances as the cell prepared at low base pressure (1.33 × 10^− 6 Pa) and high deposition rate (300 nm/min) with oxygen interstitials. On the other hand, the poly-Si thin-film solar cell deposited at low base pressure (1.33 × 10^− 6 Pa) and low deposition rate (50 nm/min) has large amount of SiO₂ precipitations and resulting worse material quality and hence cell performances. Therefore, the high deposition rate is desirable to maximize the solar cell performance, as well as the throughput. It is a more influential factor than the base pressure.     

Thin-film polycrystalline silicon solar cells formed by flash lamp annealing of a-Si films

We have fabricated thin-film solar cells using polycrystalline silicon (poly-Si) films formed by flash lamp annealing (FLA) of 4.5-µm-thick amorphous Si (a-Si) films deposited on Cr-coated glass substrates. High-pressure water-vapor annealing (HPWVA) is effective to improve the minority carrier lifetime of poly-Si films up to 10 µs long. Diode and solar cell characteristics can be seen only in the solar cells formed using poly-Si films after HPWVA, indicating the need for defect termination. The actual solar cell operation demonstrated indicates feasibility of using poly-Si films formed through FLA on glass substrates as a thin-film solar cell material.     

Large-sized-mismatched group-V element doped ZnO films fabricated on silicon substrates by pulsed laser deposition

Pulsed laser deposition has been utilized to synthesize impurity-doped ZnO thin films on silicon substrate. Large-sized-mismatched group-V elements (A^V) including P, As, Sb and Bi were used as dopants. Hall effect measurements show that hole concentration in the order of 10¹⁶-10¹⁸ cm⁻³, resistivity in the range of 10-100 Ω cm, Hall mobility in the range of 10-100 cm²/Vs were obtained only for ZnO:As and ZnO:Bi thin films. X-ray diffraction measurements reveal that the films possess polycrystallinity or nanocrystallinity with ZnO (002) preferred orientation. Guided by X-ray photoemission spectroscopy analyses and theoretical calculations for large-sized-mismatched group-V dopant in ZnO, the A_Zn^V-2V_Zn complexes are believed to be the most possible acceptors in the p-type A^V-doped ZnO thin films.     

As-grown textured zinc oxide films by ion beam treatment and magnetron sputtering

This work presents as-grown textured ZnO:Al films by rf magnetron sputtering initiated by pre-treatment of glass substrate with mixed argon and oxygen ions. A 650 nm thick of this film exhibits surface texture features with lateral size around 500 nm; the resistivity is below 5 × 10⁻⁴ Ω · cm and the transparency in the near-infrared spectral range is high (> 80% at 1000 nm). Microcrystalline silicon thin film solar cells grown on the textured glass exhibit excellent light trapping effect with a short circuit current density of 18.2 mA/cm².     

Discharge power dependence of structural and electrical properties of Al-doped ZnO conducting film by magnetron sputtering (for PDP)

In order to investigate the possible application of ZnO films as a transparent conducting oxide (TCO) electrode for AC PDP, ZnO:Al films were prepared by DC magnetron sputtering method. The effects of discharge power and doping concentration on the structural and electrical properties of ZnO films were mainly studied experimentally. Five-inch PDP cells using either a ZnO:Al or indium tin oxide (ITO) electrode were also fabricated separately under the same manufacturing conditions. The luminous properties of both the PDP cells were measured and compared with each other.By doping the ZnO target with 2 wt% of Al₂O₃, the film deposited at a discharge power of 40 W resulted in the minimum resistivity of 8.5 × 10⁻⁴ Ω-cm and a transmittance of 91.7%. However, a high doping concentration of 3 wt% of Al₂O₃ and excessive sputtering power over 40 W may induce high defect density and limit the growth of small grains. Although the luminance and luminous efficiency of the cell using ZnO:Al are lower than those of the cell with the ITO electrode by about 10%, these values are sufficient enough to be considered for the normal operation of AC PDP.     

Improving low pressure chemical vapor deposited zinc oxide contacts for thin film silicon solar cells by using rough glass substrates

Compared to zinc oxide grown (ZnO) on flat glass, rough etched glass substrates decrease the sheet resistance (R_sq) of zinc oxide layers grown on it. We explain this R_sq reduction from a higher thickness and an improved electron mobility for ZnO layers deposited on rough etched glass substrates. When using this etched glass substrate, we also obtain a large variety of surface texture by changing the thickness of the ZnO layer grown on it. This new combination of etched glass and ZnO layer shows improved light trapping potential compared to ZnO films grown on flat glass. With this new approach, Micromorph thin film silicon tandem solar cells with high total current densities (sum of the top and bottom cell current density) of up to 26.8 mA cm^− 2 were fabricated.     

Electrical mechanism analysis of Al2O3 doped zinc oxide thin films deposited by rotating cylindrical DC magnetron sputtering

Cost efficient and large area deposition of superior quality Al₂O₃ doped zinc oxide (AZO) films is instrumental in many of its applications, including solar cell fabrication due to its numerous advantages over indium tin oxide (ITO) films. In this study, AZO films were prepared by a highly efficient rotating cylindrical direct current (DC) magnetron sputtering system using an AZO target, which has a target material utilization above 80%, on glass substrates in argon (Ar) ambient. A detailed analysis on the electrical, optical, and structural characteristics of AZO thin films was performed for the solar cell, as well as display applications. The properties of films were found to critically depend on deposition parameters, such as sputtering power, substrate temperature, working pressure, and film thickness. A low resistivity of ~ 5.5 × 10^− 4 Ω cm was obtained for films deposited at 2 kW, keeping the pressure, substrate temperature and thickness constant at 3 mTorr, 230 °C and ~ 1000 nm respectively. This was due to an increase in carrier mobility and large grain size. Mobility is found to be controlled by ionized impurity scattering within the grains, since the mean free path of carriers is much smaller than the grain size of the films. The AZO films showed a high transparency of ~ 90% in the long wavelength region. Our results offer a cost-efficient AZO film deposition method that can fabricate films with significant low resistivity and high transmittance that can be applied in thin-film solar cells, as well as thin film transistor (TFT) and non-volatile memory (NVM).     

Aluminium doped zinc oxide sputtered from rotatable dual magnetrons for thin film silicon solar cells

This study addresses the electrical and optical properties as well as the surface structure after wet-chemical etching of mid-frequency magnetron sputtered aluminium doped zinc oxide (ZnO:Al) films on glass substrates from rotatable ceramic targets. Etching of an as-deposited ZnO:Al film in acid leads to rough surfaces with various feature sizes. The influence of working pressure and substrate temperature on the surface topography after etching was investigated. It was found that the growth model which Kluth et al. applied to films sputtered in radio frequency mode from planar ceramic target can be transferred to film growth from tube target. Furthermore, the influence of Ar gas flow and discharge power on the film properties was investigated. We achieved low resistivity of about 5.4 × 10^− 4 Ω·cm at high growth rates of 120 nm·m/min. Finally, surface textured ZnO:Al films were applied as substrates for microcrystalline silicon solar cells and high efficiencies of up to 8.49% were obtained.     

Sol-gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells

Sol-gel grown polycrystalline Al doped zinc oxide (AZO) thin films have been deposited on Si wafers, microscopy slide glass and fluorine doped tin oxide coated glass substrates using the spin coating technique. The atomic ratio of Al:Zn in the films is 0.2. From the X-ray diffraction investigations it is found that the preferential growth of (100) reflection peak has taken place in the 450, 550 and 600 °C annealed films. Scanning electron microscopic study has shown that the films contain well-defined grains arranged in a closely packed array. The resistivity of the 500 °C annealed film is measured to be 5 × 10^− 1 Ω cm. The films have exhibited excellent optical transmittance (~ 90%) in the 400-1100 nm wavelength range. Refractive indices (n = 1.9-1.95) of the films on Si wafer are independent of the annealing temperature. Thickness of the films produced at 4000 rpm is in the range of 58-62 nm. The refractive index and thickness of these films are nearly appropriate to cause destructive interference after reflection from front emitters of solar cells. These films have demonstrated a reflectivity value of about 3% at a wavelength of 700 nm. The AZO coated silicon solar cells possess V_oc and I_sc values of 573 mV and 237 mA, respectively.     

Low-temperature deposition of ZnO thin films on PET and glass substrates by DC-sputtering technique

The structural, optical and electrical properties of ZnO thin films (260 - 490 nm thick) deposited by direct-current sputtering technique, at a relatively low-substrate temperature (363 K), onto polyethylene terephthalate and glass substrates have been investigated. X-ray diffraction patterns confirm the proper phase formation of the material. Optical transmittance data show high transparency (80% to more than 98%) of the films in the visible portion of solar radiation. Slight variation in the transparency of the films is observed with a variation in the deposition time. Electrical characterizations show the room-temperature conductivity of the films deposited onto polyethylene terephthalate substrates for 4 and 5 h around 0.05 and 0.25 S cm^− 1, respectively. On the other hand, for the films deposited on glass substrates, these values are 8.5 and 9.6 S cm^− 1 for similar variation in the deposition time. Room-temperature conductivity of the ZnO films deposited on glass substrates is at least two orders of magnitude higher than that of ZnO films deposited onto polyethylene terephthalate substrates under identical conditions. Hall-measurements show the maximum carrier concentration of the films on PET and glass substrate around 2.8 × 10¹⁶ and 3.1 × 10²⁰ cm^− 3, respectively. This report will provide newer applications of ZnO thin films in flexible display technology.     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.     

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10^- 6, correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.