High-quality Transparent Conductive Oxide Research Articles

Low-damage hydrogen-doped transparent electrodes towards semitransparent perovskite photovoltaics

As one kind of transparent conductive electrodes (TCEs), transparent conductive oxides (TCOs) have been widely used in perovskite solar cells (PSCs). However, the conventional magnetron sputtering usually causes high-energy particles, thus damaging the underlying functional films and deteriorating the performance of devices. Therefore, developing a low-damage method for high-quality TCO film is crucial. Here, the hydrogen and cerium co-doped indium oxide (ICO:H) films were prepared by the reactive plasma deposition (RPD) technology at room temperature. Benefiting from the hydrogen doping, the bandgap of the ICO:H film was tuned to 3.97 eV, the average transmittances reached 91.25% (from 800 nm to 1200 nm) and 81.35% (from 300 nm to 800 nm), and the carrier mobility exceeded 30 cm2/V/s. Combined with these advantages, semitransparent perovskite solar cells (ST-PSCs) without buffer layers were fabricated using ICO:H films as transparent conductive window layers, and the power conversion efficiency (PCE) was up to 17.61% (from the glass side). Furthermore, the ICO:H films were also applied in two-terminal perovskite/silicon tandem solar cells, and the PCE exceeded 25%. This low-damage method opens a new avenue for fabricating film-based optoelectronics.

Cerium-Doped Indium Oxide as a Top Electrode of Semitransparent Perovskite Solar Cells

Semitransparent perovskite solar cells (ST-PSCs) play a very important role in high-efficiency tandem solar cells and building integrated photovoltaics (BIPV). One of the main challenges for high-performance ST-PSCs is to obtain suitable top-transparent electrodes by appropriate methods. Transparent conductive oxide (TCO) films, as the most widely used transparent electrodes, are also adopted in ST-PSCs. However, the possible ion bombardment damage during the TCO deposition and the relatively high postannealing temperature usually required for high-quality TCO films is not conducive to improving the performance of the perovskite solar cells with low ion bombardment and temperature tolerances. Herein, cerium-doped indium oxide (ICO) thin films are prepared by reactive plasma deposition (RPD) at substrate temperatures below 60 °C. A high carrier mobility of 50.26 cm2 V-1 s-1, a low resistivity of 7.18 × 10-4 Ω·cm, and an average transmittance of 86.53% in the wavelength range of 400-800 nm and 87.37% in the wavelength range of 800-1200 nm are achieved. The RPD-prepared ICO film is used as a transparent electrode on top of the ST-PSCs (band gap ∼1.68 eV), and photovoltaic conversion efficiency (PCE) of 18.96% is achieved on the champion device.

Lightwave irradiation-assisted low-temperature solution synthesis of indium-tin-oxide transparent conductive films

Transparent conductive oxide (TCO) films have important applications in many areas. Unfortunately, TCOs are usually fabricated using vacuum and high-temperature methods, preventing them from applications in low-cost flexible devices. In this paper, facile low-temperature sol-gel method is described that can be used to fabricate high-quality TCO films. This study uses lightwave (LW) irradiation (at ∼280 °C) with indium-tin-oxide (ITO) as a typical example. Both structure and key properties of ITO films are investigated for different LW irradiation conditions. ITO can be formed via LW irradiation after a period as short as 5 min. Furthermore, it is found that LW irradiation can promote the formation of M − O framework, effectively remove Cl impurities, and facilitate the elimination of hydroxyl oxygen defects - even at temperatures as low as ∼280 °C. The optimal ITO films show excellent electronic properties, including low sheet-resistance (14.5 Ω·sq−1) and high conductivity (1.7 × 103 S cm−1). Moreover, ITO films also show high transmittance (above 87%). Overall, our ITO films have a figure of merit (FOM) of 1.72 × 10−2 Ω−1, which is comparable to (or higher than) those of previous ITO films that were produced using conventional vacuum and high-temperature methods. Our LW irradiation method provides facile and effective approach to produce high-performance TCO films at remarkably low cost. This means these films could be used in affordable flexible large-area devices.

Front and Back TCO Research Review of a-Si/c-Si Heterojunction with Intrinsic Thin Layer (HIT) Solar Cell

In this paper, we report a technical approach regarding an amorphous silicon (a-Si)/crystalline silicon (c-Si) heterojunction solar cell to solve the previous issues, and we investigate the applications of front and back transparent conductive oxides (TCOs) on this high-efficiency solar cell. The presentation of front and rear-emitter structure solar cells is included, and we investigate the TCO-material candidates for the Si heterojunction (SHJ) solar cell according to the electrical and optical properties. A high-quality TCO film is very important because it is linked to the efficiency of the c-Si-based silicon solar cell. The intention here is the applying of a high-efficiency SHJ solar cell by fabricating the high-quality TCO materials of the investigation of this study.

Ga-doped ZnO films by magnetron sputtering at ultralow discharge voltages: Effects of defect annihilation

Preparation of high quality transparent conductive oxide (TCO) films by sputter deposition involves an intricate balance of defect generation by the highly energetic negative oxygen ions (depending on the discharge voltage) and the concomitant annihilation of these defects during film growth. Ga-doped ZnO films with a low Ga content (1.7at.%) were deposited to investigate the effects of defect annihilation on the microstructure evolutions as well as the optical and electrical properties. To achieve this aim, we prepared the GZO films by magnetron sputtering at ultralow discharge voltages (~70V) to minimize the defect generation, and varied the substrate temperature (from 130°C to 380°C) to adjust the annihilation rates. The microstructure was systematically characterized by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Raman Spectroscopy. The electrical and optical properties were obtained by a Hall-effect measurement system and Spectroscopic Ellipsometry (SE), respectively. It was found that (i) even under the condition of highly controlled defect generation from the bombardment of negative oxygen ions, a sufficient annihilation of the defects cannot be realized without externally heating the substrate; and (ii) there existed a threshold temperature, above which both the structural quality and the electrical properties were improved with the increased temperature over the temperature range we examined. These results reveal that the growth temperature during the GZO film deposition has played an important role in effective annihilation of the irradiation-induced structural defects, which may be due to the higher diffusion barrier of Ga atoms in our GZO films.

Design of a plasmonic back reflector using Ag nanoparticles with a mirror support for an a-Si:H solar cell

Plasmonic nanoparticles have unique optical properties and these properties are affected by any surrounding structures, or lack thereof. Nanoparticles are often added to a device without fully assessing the effect that each interface will have on the nanoparticle’s response. In this work, we simulate and fabricate devices utilizing hemispherical nanoparticles integrated into the back reflector of an amorphous silicon solar cell. 3D finite difference time domain simulations were used to calculate the optical absorption of a 300nm amorphous silicon layer as a function of the size of the nanoparticles, the distance between the nanoparticles and the active layer, and the distance between the nanoparticles and the mirror. Two transparent conducting oxides, aluminum doped zinc oxide and indium tin oxide, are investigated to determine the importance of the material properties between the nanoparticles and mirror. Silver hemispherical nanoparticles with a diameter of 150nm placed directly on the a-Si:H and a 60nm aluminum doped zinc oxide layer between the nanoparticles and the mirror lead to a maximum absorption increase of 7.2% in the 500nm to 800nm wavelength range. Experimental devices confirmed the trends predicted by theory but did not achieve enhancement, likely due to fabrication challenges. Fabricating a solar cell with the simulated design requires a high quality transparent conductive oxide and high control over the nanoparticle size distribution.

Indium oxide-based transparent conductive films deposited by reactive sputtering using alloy targets

High-quality transparent conductive oxide (TCO) films, Sn-doped In2O3 (ITO) and In2O3–ZnO (IZO), were successfully deposited on either synthetic silica or polyethylene terephthalate (PET) substrates in the “transition region” by reactive dc magnetron sputtering using In–Zn and In–Sn alloy targets, respectively, with a specially designed plasma emission feedback system. The composition, crystallinity, surface morphology, and electrical and optical properties of the films were analyzed. All of the IZO films were amorphous, whereas the ITO films were polycrystalline over a wide range of deposition conditions. The minimum resistivities of the IZO and ITO films deposited on the heated PET substrates at 150 °C were 3.3 × 10−4 and 5.4 × 10−4 Ω·cm, respectively. By applying rf bias to unheated PET substrates, ITO films with a resistivity of 4.4 × 10−4 Ω·cm were deposited at a dc self-bias voltage of −60 V.

Enhanced electrical and optical properties of room temperature deposited Aluminium doped Zinc Oxide (AZO) thin films by excimer laser annealing

High quality transparent conductive oxides (TCOs) often require a high thermal budget fabrication process. In this study, Excimer Laser Annealing (ELA) at a wavelength of 248nm has been explored as a processing mechanism to facilitate low thermal budget fabrication of high quality aluminium doped zinc oxide (AZO) thin films. 180nm thick AZO films were prepared by radio frequency magnetron sputtering at room temperature on fused silica substrates. The effects of the applied RF power and the sputtering pressure on the outcome of ELA at different laser energy densities and number of pulses have been investigated. AZO films deposited with no intentional heating at 180W, and at 2mTorr of 0.2% oxygen in argon were selected as the optimum as-deposited films in this work, with a resistivity of 1×10−3Ω.cm, and an average visible transmission of 85%. ELA was found to result in noticeably reduced resistivity of 5×10−4 Ω.cm, and enhancing the average visible transmission to 90% when AZO is processed with 5 pulses at 125mJ/cm2. Therefore, the combination of RF magnetron sputtering and ELA, both low thermal budget and scalable techniques, can provide a viable fabrication route of high quality AZO films for use as transparent electrodes.

Double-Pattern Textured ZnO:Ga Thin Films Fabricated by an APPJ and an DC Sputtering

Transparent conductive oxides (TCO) are essential for superstrate solar cells due to its outstanding optical and electrical properties. 1 In ordertoimprovetheefficiencyofsolarcells,itisimportanttofabricate high-quality TCO substrates. Recently, zinc oxide (ZnO) based TCO have attracted significant attention because of high abundance, low cost and non-toxicity. Moreover, ZnO films doped with group III elements are more stable than tin-doped indium oxides (ITO) and fluorine-doped tin oxides (FTO) in the hydrogen and silan (SiH4) plasma discharge. 2,3 Hence, they are very suitable for the process of microcrystalline silicon (μc-Si) and amorphous silicon (a-Si) solar cells. For silicon based thin film solar cells, textured ZnO electrodes are necessary to increase light absorption in μc-Si absorbers and to reduce the degradation in a-Si absorbers. This is because that textured ZnO films can scatter the incident light efficiently and enhance the light trapping effect. To date, many efforts have been made to achieve superior textured ZnO films. Post wet etching by diluted acids for aluminum-doped zinc oxides (AZO) and gallium-doped zinc oxides (GZO) is the well-known method. 4,5 However, this process creates a lot of waste in ZnO and is not fit for large-scale production. Recently, pyramid-like textured zinc oxides doped with boron (BZO) have been successfully fabricated by a low pressure chemical vapor deposition (LPCVD) or a metal organic chemical vapor deposition (MOCVD) without any post treatments. 6,7 Nevertheless, disadvantages of these techniques are expensive equipments and the usage of toxic precursors. Here, we developed a novel, safe, and low-cost technique to produce textured GZO films. A bi-layer structure of organosilicon/ GZO was conducted to fabricate double-pattern textured GZO films on glass substrates. This bi-layer structure was achieved by an atmospheric pressure plasma jet (APPJ) and a dc magnetron sputtering. The surface morphology, optical characteristics and conductivity of double-pattern textured GZO films can be effectively controlled by the haze of organosilicon underlayers. After a post-annealing treatment in high vacuum, double-pattern textured GZO films can reveal the excellent optical transmittance and electrical conductivity.

Development of natively textured surface hydrogenated Ga-doped ZnO-TCO thin films for solar cells via magnetron sputtering

The main purposes are to obtain high quality transparent conductive oxide (TCO) based on zinc oxide (ZnO) thin films with high optical transparency in the visible and near infrared spectral range, high electrical conductivity and good light-scattering capability to enhance the path of the light inside the Si-based thin film solar cells. Natively textured surface hydrogenated gallium-doped ZnO (HGZO) thin films have been deposited via pulsed direct current (DC) magnetron sputtering on glass substrates at a substrate temperature of 553K. These natively textured HGZO thin films exhibit high optical transmittance (over 80%) in the visible and near infrared region (λ=380–1100nm) and excellent electrical properties. The optimized HGZO thin film with crater-type textured surface obtained at the hydrogen flow rate of ∼2.0sccm exhibits a high electron mobility of 41.3cm2/Vs and a relatively low sheet resistance of ∼7.0Ω. The influences of hydrogen flow rates on the surface morphology, electrical and optical properties of HGZO thin films were investigated in detail. In addition, we put forward a method of gradient H2 growth technique for fabricating HGZO thin films so as to obtain rough surface structure with good light-scattering capability and high electrical conductivity. “Crater-like” surface feature size and optical transmittance can be improved through gradient H2 growth technique.

Textured surface ZnO:B/(hydrogenated gallium-doped ZnO) and (hydrogenated gallium-doped ZnO)/ZnO:B transparent conductive oxide layers for Si-based thin film solar cells

Textured surface un-doped zinc oxide (ZnO) and boron-doped ZnO (ZnO:B) thin films with pyramid-like grains were directly deposited by metal organic chemical vapor deposition (MOCVD) technique and hydrogenated gallium-doped ZnO (HGZO) thin films with good electrical properties were grown by pulsed direct-current magnetron sputtering. In order to obtain high quality transparent conductive oxides (TCOs) for Si-based thin film solar cells, through combining the advantages of MOCVD–ZnO and sputter-HGZO thin films, bi-layer textured surface glass/ZnO:B/HGZO and glass/HGZO/ZnO:B TCO thin films with milky surface were fabricated and investigated in detail. The main research purposes are to explore appropriate surface morphology and improve the electrical properties of TCO layers. Compared with textured surface un-doped ZnO and ZnO:B thin films, both the bi-layer ZnO/HGZO and ZnO:B/HGZO thin films exhibit high electron mobility. The ZnO:B/HGZO thin films present modified pyramid-like surface morphology with high transparencies (T>85%) and excellent electrical properties (resistivity of ~9.28×10−4Ωcm and mobility of ~28.8cm2/Vs). Meanwhile, the HGZO/ZnO thin films exhibit large grain size (~500nm–1000nm) with preferred growth orientations along (100) and (002) crystallographic directions. Bi-layer HGZO/ZnO and HGZO/ZnO:B thin films exhibit relatively higher diffuse transmittance than that of corresponding ZnO and ZnO:B thin films.

The properties of tin-doped indium oxide films prepared by pulsed magnetron sputtering from powder targets

Mixed indium oxide and tin oxide powder targets have been used to prepare transparent semi-conductive films on glass by pulsed magnetron sputtering. The influences of dopant concentration in the target and oxygen concentration in the plasma were investigated. Also, the properties of the coatings were compared before and after annealing. The morphological structure, crystallinity, optical properties and electrical properties of the films were investigated using a range of techniques, including SEM, XRD, spectrophotometry, four-point probe and van der Pauw method. The main preferred orientations of the coatings are (222) and (400). The transparent lines of coatings deposited with additional oxygen in the plasma showed blue shifts after annealing, with the average transmittance within the visible range being in the range 80–85%. Also, coating resistivities of the order of 10 −4 Ω cm were obtained. The results to date demonstrate that the pulsed magnetron sputtering of Sn-doped In 2O 3 films from powder targets is a versatile, novel technique for the production of high quality transparent conductive oxide (TCO) materials. Due to the flexibility of this technique, preliminary experiments of multi-component tin-doped indium oxide (ITO) (5 at.% Sn)/ZnO (3 at.% Al) coatings are also included here. The incorporation of Al-doped ZnO resulted in an increase in transmission, but also a reduction in the conductivity of these coatings.

The characteristics of aluminium-doped zinc oxide films prepared by pulsed magnetron sputtering from powder targets

Mixed zinc oxide and alumina powder targets have been used to prepare transparent semi-conductive films on glass by pulsed magnetron sputtering. The structure, crystallinity, optical properties, electrical properties and adhesion of the films were investigated using a range of techniques, including SEM, XRD, spectrophotometry, four-point probe and scratch adhesion testing. Also, the properties of the coatings before and after annealing under various conditions are compared. The crystallinity of the films increased and the preferred orientation in some of the samples changed from (1 0 0) to (0 0 2) after annealing. The transparent lines of the coatings showed blue shifts with the average transmittance within the visible range being up to 90%. Resistivities of the order of 10 −3 Ω cm were obtained after annealing, without deterioration of the coating-to-substrate adhesion. The results to date demonstrate that the pulsed magnetron sputtering of Al-doped ZnO films from powder targets is a versatile, novel technique for the deposition of high quality transparent conductive oxide materials. As a demonstration of the flexibility of this technique, preliminary results from an investigation into indium-doped zinc oxide coatings are also included here.