Front Transparent Conductive Oxide Layer Research Articles

Power Losses in the Front Transparent Conductive Oxide Layer of Silicon Heterojunction Solar Cells: Design Guide for Single-Junction and Four-Terminal Tandem Applications

In silicon heterojunction solar cells, optimization of the front transparent conductive oxide (TCO) layer is required in order to minimize both electrical and optical losses. In this article, design guidelines for this overall power loss minimization are presented-extending previous TCO optimization work that was limited to the maximization of the short-circuit current density alone-and these are used to prescribe the best TCOs for both single-junction and silicon-based four-terminal tandem applications. The employed procedure determines the loss associated with the front TCO layer as a function of the TCO carrier density, mobility, and thickness, as well as the pitch between the front electrode fingers. For a representative indium tin oxide (ITO) film with a mobility of approximately 20 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and a carrier density of 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , the loss over the 700-1200 nm infrared wavelength range-the spectrum reaching the silicon bottom cell in a typical tandem structure-is minimized by using a finger pitch of 3 mm and an ITO thickness of 100-110 nm. This compares with an optimal finger pitch of 2 mm and an optimal ITO thickness of 70 nm for the same cell operating as a single-junction device under full spectrum. The methodology presented can also readily be applied to TCO materials other than ITO, to a wide variety of specific four-terminal tandem architectures and, with minor modifications, to rear TCO layers.

Using the light scattering properties of multi-textured AZO films on inverted hemisphere textured glass surface morphologies to improve the efficiency of silicon thin film solar cells

We report the light scattering characteristics of multi-textured aluminum-doped zinc oxide (AZO) thin films with high optical transmittance, haze ratio and step coverage for amorphous silicon thin film solar cells (a-Si TFSCs). Multi-textured AZO films were deposited on inverted hemisphere textured (IHT) glass surface morphologies. Multi-textured process was used to create modulated surface morphologies with micro and nano-features useful for the light scattering in visible as well as near-infrared wavelength region. Highly transparent IHT glass surface morphologies with high haze ratio were prepared by wet chemical etching of periodic glass substrates. Total transmittance of the IHT glass substrates showed higher average values (91.38–93.40%) as compared to that of bare glass (90.94%) in the visible wavelength region due to lower reflectance. Multi-textured AZO films showed higher rms roughness 100.479 nm, optical transmittance 82.60% and haze ratio 75.09% in the visible wavelength region. X-ray photoelectron spectroscopic analysis of multi-textured AZO thin films was used to study the atomic compositions and chemical binding states. Multi-textured AZO films with high transmittance and haze ratio were used as front transparent conductive oxide layers for the fabrication of a-Si TFSCs using an absorber layer thickness of 400 nm. Multi-textured AZO films deposited on the optimal textured glass surface morphologies yield the maximum performance of a-Si TFSCs as; Voc = 871 mV, Jsc = 16.55 mA/cm2, FF = 66.6% and η = 9.61%. An enhancement of photocurrent was noticed from 15.64 to 16.55 mA/cm2 for the a-Si TFSCs deposited on as deposit AZO to optimal textured AZO films.

High-efficiency p–i–n superstrate amorphous Si solar cells on SiOx periodic arrays of three-dimensional microstructure prepared by soft imprinting

Efficient amorphous Si thin-film solar cells in a p–i–n superstrate configuration with a high initial conversion efficiency of 10.3% were successfully fabricated on periodically three-dimensional (3D) micropatterned SiOx/glass substrates prepared by soft imprinting. Conformal film deposition on a 3D microstructure was realized owing to the shape of our newly designed 3D pattern and the triode plasma-enhanced CVD technique, which enables the selective transport of favorable film precursors to the substrate surface. The nanoscale surface texture of the front transparent conductive oxide layer was found to be crucial for optical confinement, unexceptionally for amorphous Si solar cells on a 3D microstructure, which results in an improved short-circuit current density.

High mobility In2O3:H transparent conductive oxides prepared by atomic layer deposition and solid phase crystallization

The preparation of high‐quality In2O3:H, as transparent conductive oxide (TCO), is demonstrated at low temperatures. Amorphous In2O3:H films were deposited by atomic layer deposition at 100 °C, after which they underwent solid phase crystallization by a short anneal at 200 °C. TEM analysis has shown that this approach can yield films with a lateral grain size of a few hundred nm, resulting in electron mobility values as high as 138 cm2/V s at a device‐relevant carrier density of 1.8 × 1020 cm–3. Due to the extremely high electron mobility, the crystallized films simultaneously exhibit a very low resistivity (0.27 mΩ cm) and a negligible free carrier absorption. In conjunction with the low temperature processing, this renders these films ideal candidates for front TCO layers in for example silicon heterojunction solar cells and other sensitive optoelectronic applications. (© 2014 WILEY‐VCH Verlag GmbH &amp;Co. KGaA, Weinheim)

Role of double ITO/In2O3 layer for high efficiency amorphous/crystalline silicon heterojunction solar cells

The high work function transparent conductive oxide films can be use to modify the front barrier height of amorphous/crystalline silicon heterojunction solar cells. We report the implementation of double ITO/In2O3 films as a front anti-reflection electrode in amorphous/crystalline silicon heterojunction solar cells. The In2O3 and ITO films were deposited by in-situ radio frequency (RF) magnetron sputtering system. The thin In2O3 films were used to modify the front contact barrier height of amorphous/crystalline silicon heterojunction solar cell due to their high work function while the ITO films were used to improve the conductivity of front transparent conductive oxide layer. We investigated the electrical and optical properties of double ITO/In2O3 layer with the variation of film thickness. In order to satisfy the requirement of solar cell applications, the optimum combination of thickness in terms of sheet resistance, resistivity, transmittance, etc. was sought. The double ITO/In2O3 layer with the thickness of 80/20nm were applied as front anti-reflection electrode and the best performance of the device was found to be; Voc=670mV, Jsc=37.42mA/cm2, FF=71.16% and η=17.84%.

Development of textured ZnO-coated low-cost glass substrate with very high haze ratio for silicon-based thin film solar cells

Zinc oxide (ZnO) films with a very high haze ratio and low resistivity were developed on soda–lime glass substrate by using reactive ion etching (RIE) treatment with carbon tetrafluoride (CF4) to modify the substrate surface morphology before the deposition of ZnO films. We found that the surface morphology of the ZnO films deposited by metal organic chemical vapor deposition (MOCVD) technique could be modified by varying the glass treatment conditions and the gas pressure was a key parameter. With increasing glass-etching pressure, the surface morphology of the ZnO films changed from conventional pyramid-like single texture to greater cauliflower-like double texture, leading to significant increases in root mean square roughness and haze ratio of the films. By employing the developed high-haze ZnO films as a front transparent conductive oxide (TCO) layer in microcrystalline silicon solar cells, an enhancement in the quantum efficiency in the long-wavelength region has been achieved. Experimental results have verified that our unique and original glass etching treatment is a simple and effective technique to improve the light-scattering properties of the ZnO films while preserving their good transparency and electrical properties. Thus, the ZnO films deposited on etched soda–lime glass have a high potential for the use as a front TCO layer in thin-film Si solar cells.

Hydrogen-doped indium oxide/indium tin oxide bilayers for high-efficiency silicon heterojunction solar cells

The front transparent conductive oxide layer is a source of significant optical and electrical losses in silicon heterojunction solar cells because of the trade-off between free-carrier absorption and sheet resistance. We demonstrate that hydrogen-doped indium oxide (IO:H), which has an electron mobility of over 100cm2/Vs, reduces these losses compared to traditional, low-mobility transparent conductive oxides, but suffers from high contact resistance at the interface of the IO:H layer and the silver front electrode grid. This problem is avoided by inserting a thin indium tin oxide (ITO) layer at the IO:H/silver interface. Such IO:H/ITO bilayers have low contact resistance, sheet resistance, and free-carrier absorption, and outperform IO:H-only or ITO-only layers in solar cells. We report a certified efficiency of 22.1% for a 4-cm2 screen-printed silicon heterojunction solar cell employing an IO:H/ITO bilayer as the front transparent conductive oxide.

Development of Novel Al-Doped Zinc Oxide Films Fabricated on Etched Glass and Their Application to Solar Cells

We have successfully developed novel aluminum-doped zinc oxide (AZO-X) films with a high haze ratio by the combined use of an etched glass substrate and wet-etched AZO-X films. The effects of the use of an etched glass substrate and wet-chemical etching on the properties of AZO-X films were investigated. The texture size and rms roughness of these films largely increased with glass surface roughening. Post-treatment using wet chemical etching slightly increased the texture size and rms roughness. The etched glass approach has been found to be a promising method for achieving an AZO-coated glass substrate with a high haze ratio. Using high-haze ratio AZO-X films as the front transparent conductive oxide (TCO) layers in solar cells, we improved the quantum efficiency (QE) of these solar cells particularly in the long-wavelength region. Thus, the AZO-X films deposited on etched glass have a high potential for use as front TCO layers in silicon-based thin-film solar cells.

Fundamental Properties of Titanium-Doped Indium Oxide and Its Application to Thin-Film Silicon Solar Cells

We have investigated the fundamental optoelectronic properties of newly developed transparent conductive oxide (TCO) materials, e.g., titanium-doped indium oxide (InTiO). InTiO films, being deposited at 50 °C by the RF-magnetron-sputtering method followed by thermal annealing at 200 °C, show excellent optoelectronic properties for solar-cell application. We have demonstrated the improved photovoltaic performance of n–i–p microcrystalline-silicon (µc-Si:H) solar cells whose i layer is prepared at a high rate of 2.3 nm/s using a stacked structure of InTiO with aluminum-doped zinc oxide (AZO) as top (front) TCO layers.

Growth of ZnOx:Al by high-throughput CVD at atmospheric pressure

Aluminum doped zinc oxide films (ZnOx:Al) have been deposited on a moving glass substrate by a metalorganic CVD process at atmospheric pressure in an in-line industrial type reactor. Tertiary-butanol has been used as an oxidant for diethylzinc and trimethylaluminium as the dopant gas. The effect of the deposition temperature (from 380 to 540°C) on the deposition rate has been investigated by a numerical code, where a gas phase reaction among tertiary-butanol and diethylzinc is assumed to occur. The structural (crystallinity and morphology) properties of the films as a function of the deposition temperature have been analyzed by using X-ray diffraction and Scanning Electron Microscopy. A maximum growth rate of ∼11nm/s was found at a deposition temperature of 480°C, for which ZnOx:Al films show (002) preferential orientation, good crystalline quality and a naturally rough surface. ZnOx:Al films deposited at 480°C are also highly conductive (R<10Ω/□ for film thicknesses above 1050nm) and transparent (>85% in the visible range). These films have been used as front transparent conductive oxide layers in pin a-Si:H solar cells, achieving an initial efficiency approaching 8%.