Front Transparent Conductive Oxide Research Articles

Surface Modification of High Haze Front Transparent Conductive Oxide for Silicon Thin Film Solar Cell

Surface Modification of High Haze Front Transparent Conductive Oxide for Silicon Thin Film Solar Cell

Effective light management of three-dimensionally patterned transparent conductive oxide layers

For effective light harvesting, a design weighting should be implemented in a front geometry, in which the incident light transmits from a surface into a light-active layer. We designed a three-dimensionally patterned transparent conductor layer for effective light management. A transparent conductive oxide (TCO) film was formed as three-dimensional structures. This efficiently drives the incident light at the front surface into a Si absorber to yield a reduction in reflection and an enhancement of current. This indicates that an optimum architecture for a front TCO surface will provide an effective way for light management in solar cells.

NUV/VIS sensitive multicolor thin film detector based on a-SiC:H/a-Si:H/μc-SiGeC:H alloys with an in-situ structured transparent conductive oxide front contact without etching

An innovative family of hydrogenated amorphous silicon (a-Si:H) multicolor p–i–n photo sensors, sensitive in the VIS and the near UV spectrum, is presented. Typical values of the quantum efficiency at 350nm and 580nm are 5.4% and 54.7%, respectively, with −0.4V and −12V bias. Electro-optical studies were performed to explore the effect of combining linearly graded a-SiGe:H/μc-SiGeC:H layers with linearly graded a‐SiC:H-layers. The devices presented additionally contain a buried a-Si:H region. Low-reflective aluminum doped zinc oxide (ZnO:Al) back contacts improve the spectral color separation. μτ-products and absorption coefficients of graded absorbers were determined. Discrete absorbers were substituted by a linear graded a-SiC:H absorption zone in the top structure, an interior a-Si:H region and a graded a-SiGe:H/a-SiC:H alloy combination. In this paper we demonstrate a reduction of interference fringes and operation at low bias voltages, combined with a highly precise adjustment of the spectral sensitivity, even in the near UV-spectrum. The device dynamic range exceeds 50dB at 1000lx white-light illumination. As the deposited upper layers adopt the roughness of μc-SiGeC:H clusters in the rear absorber, we present an in-situ structured front contact without etching ZnO:Al.

Nanometer- and Micrometer-Scale Texturing for High-Efficiency Micromorph Thin-Film Silicon Solar Cells

Optimized transparent conductive oxide front electrodes are vital to further increase the efficiency of thin-film silicon solar devices. We report details on the fabrication of multiscale textured zinc oxide substrates and their implementation in amorphous silicon/microcrystalline silicon tandem (micromorph) devices. Such substrates allow separate optimization of light trapping in the top and bottom cells, and efficient decoupling of transparency and conduction. We show in particular the need for sharp, nanoscale texturing for antireflection and light trapping in the top cell. We also show that smooth, micrometer-scale texturing can efficiently improve large-wavelength light management without degrading the quality of the silicon material grown on the substrate. By combining the appropriate morphologies, high currents can be reached in both the top and bottom subcells, while conserving the optimal electrical properties of the solar cells.

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%.

Development of the Transparent Conductive Oxide Layer for Nanocrystalline Cubic Silicon Carbide/Silicon Heterojunction Solar Cells with Aluminum Oxide Passivation Layers

We developed an In2O3:H/indium–tin oxide (ITO) stack as the front transparent conductive oxide (TCO) layer of nanocrystalline cubic silicon carbide/crystalline silicon heterojunction solar cells with Al2O3 passivation layers. We investigated the solar cell performance and optical and electrical properties of this layer with various annealing temperatures. The solar cells with In2O3:H and In2O3:H/ITO layers show a higher short circuit current density (Jsc) than that with an ITO layer owing to their lower surface reflection and lower free carrier absorption. The solar cell with the In2O3:H/ITO stack shows a higher fill factor (FF) than that with the In2O3:H layer. The solar cell with the In2O3:H/ITO stack shows an aperture area efficiency of 16.8% (Voc = 0.638 V, Jsc = 34.5 mA/cm2, and FF= 0.762). These results indicate that the In2O3:H/ITO stack has good optical and electrical properties after annealing.

Window layer with p doped silicon oxide for high Voc thin-film silicon n-i-p solar cells

We investigate the influence of the oxygen content in boron-doped nanocrystalline silicon oxide films (p-nc-SiOx) and introduce this material as window layer in n-i-p solar cells. The dependence of both, optical and electrical properties on the oxygen content is consistent with a bi-phase model which describes the p-nc-SiOx material as a mixture of an oxygen-rich (O-rich) phase and a silicon-rich (Si-rich) phase. We observe that increasing the oxygen content enhances the optical gap E04 while deteriorating the activation energy and the planar conductivity. These trends are ascribed to a higher volume fraction of the O-rich phase. Incorporated into n-i-p a-Si:H cells, p-nc-SiOx layers with moderate oxygen content yield open circuit voltage (Voc) up to 945 mV, which corresponds to a relative gain of 11% compared to an oxygen-free p-layer. As a similar gain is obtained on planar and on textured substrates, we attribute the increase in Voc to the higher work function of the p-nc-SiOx layer made possible by its wider band gap. These results are attained without changing the dilution ratio of the 250 nm thick intrinsic layer. We also observe an enhancement of 0.6 mA cm−2 in short circuit current density in the short wavelengths due to the higher transparency of the p-nc-SiOx layer. Finally, an initial efficiency of 9.9% for a single junction 250 nm a-Si:H n-i-p solar cell on plastic foil is achieved with the optimization of the p layer thickness, the doping ratio of the front transparent conductive oxide, and the optical properties of the back reflector.

High Haze and Low Resistive MgO/AZO Bi-layer Transparent Conducting Oxide for Thin Film Solar Cells

ABSTRACTWe have propsed MgO/AZO bi-layer transparent conducting oxide (TCO) for thin film solar cells. From XRD analysis, it was observed that the full width at half maximum of AZO decreased when it was grown on MgO precursor. The Hall mobility of MgO/AZO bi-layer was 17.5cm2/Vs, whereas that of AZO was 20.8cm2/Vs. These indicated that the crystallinity of AZO decreased by employing MgO precursor. However, the haze (=total diffusive transmittance/total transmittance) characteristics of highly crystalline AZO was significantly improved by MgO precursor. The average haze in the visible region increased from 14.3 to 48.2%, and that in the NIR region increased from 6.3 to 18.9%. The reflectance of microcrystalline silicon solar cell was decreased and external quantum efficiency was significantly improved by applying MgO/AZO bi-layer TCO. The efficiency of microcrystalline silicon solar cell with MgO/AZO bi-layer front TCO was 6.66%, whereas the efficiency of one with AZO single TCO was 5.19%.

High efficiency CIGS based solar cells with electrodeposited ZnO:Cl as transparent conducting oxide front contact

AbstractIn this paper high efficiency (up to 15.8%) Mo/Cu(In,Ga)Se2/CdS/i‐ZnO solar cells terminated with an electrodeposited ZnO:Cl window layer are reported. The optoelectronic properties of these cells are comparable to reference cells terminated with standard sputtered ZnO:Al. The use of an electrochemical step, a low cost and easily up‐scalable method, in the fabrication process of this type of solar cell, opens the possibility to lower their production price and make them more competitive. The optimization involves a soft annealing treatment (T ≤ 150°C) under atmospheric conditions after the electrodeposition which improves the transparency of the ZnO:Cl layers in the infrared region, the short circuit current and the fill factor of the I–V curve. These changes are related to the increase of one order of magnitude (up to 5 × 102 Ω−1·cm−1) of the lateral conductivity of the electrochemical layer due to an improvement of the free carrier mobility. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Atomic scale observation and characterization of redox-induced interfacial layers in commercial Si thin film photovoltaics

Thermodynamics considerations and experimental evidence suggest that redox reactions occur at the interfaces between transparent conductive oxides (TCOs) and the active silicon layers in photovoltaic stacks, with potentially nefarious effects to device efficiency. The presence of interfacial layers of oxidized silicon and reduced metal is confirmed here with analytical depth profiling techniques in industrially produced Si thin film solar cells. Atomic-resolution scanning transmission electron microscopy and energy loss spectroscopy are used to show that the specific chemistry of the interface, the front TCO being Sn-rich while the back TCO is Zn-rich, has a strong influence on the size of the resulting interfacial layer. Furthermore, the morphology of the interface and the impact of annealing treatments are also studied, leading to suggestions for possible improvements of commercial device efficiency.

Effects of Growth Parameters on Surface-morphological, Structural, Electrical and Optical Properties of AZO Films by RF Magnetron Sputtering

Abstract500 nm-thick aluminum-doped zinc oxide (ZnO:Al) thin film is usually used as a front transparent conductive oxide (TCO) contact on photovoltaic devices, and for this application is often deposited by a reactive radio-frequency (r.f.) magnetron sputtering system from a ceramic target. This work reports on the preparation and characterization of AZO thin films on Corning 1737 glass substrates grown by reactive r.f.-magnetron sputtering from a ZnO ceramic target with 2 wt% Al content. It was found that the growth parameters, such as chamber pressure, working power, and deposition temperature, have significant influences on the properties of AZO films. According to the experimental results: (1) Films were polycrystalline showing a strong preferred c-axis orientation. (2) With increasing working power, the resistivity decreased, and mobility and the carrier concentration increased. (3) Lower deposition temperature leads to a decrease in resistivity, with 2.5×10-4 Ω-cm representing the lowest resistivity reached.

Simulation of losses in thin‐film silicon modules for different configurations and front contacts

AbstractA simulation tool for the quantification of electrical losses in thin‐film modules using a one‐ and two‐dimensional electrical PSpice model is presented. Two main sources of electrical losses are examined: monolithic contacts (MC) and front contacts made of a transparent conductive oxide (TCO) layer with or without a metal finger grid. Our study was focussed on amorphous and micromorph silicon modules in substrate or superstrate configuration. Results show that front contact losses (TCO losses and finger losses) prevail. While, under assumption that their subcell performances are the same, performance of amorphous silicon (a‐Si) modules do not depend on the configuration, the superstrate micromorph silicon module has a relatively slight (below 2%) advantage over the substrate counterpart due to lower electrical losses in the MC. Losses of the front contact made of a thick TCO layer or of thin TCO layer and metal finger grid on top were studied for both modules in substrate configuration and optimisation results are presented. Use of thin TCO layer and optimised finger grid and solar cell geometry is competitive and these modules can even outperform the optimised amorphous or micromorph silicon module with thick TCO front contact. In all optimised cases under standard test conditions, total relative losses can be minimised to around 10%. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The potential of textured front ZnO and flat TCO/metal back contact to improve optical absorption in thin Cu(In,Ga)Se 2 solar cells

The role of additionally textured front transparent conductive oxide − TCO (ZnO:Al) and flat TCO/metal contact on optical improvements in thin Cu(In,Ga)Se 2 (CIGS) solar cells are investigated by means of numerical simulations. A de-coupled analysis of two effects related to additional texturing of front surface of ZnO:Al TCO − (i) enhancement of light scattering and (ii) decreased total reflectance (antireflective effect) − reveals that the improvements in quantum efficiency, QE, and short-circuit current, J SC, of the solar cell originate from an antireflective effect only. In order to improve optical properties of the back contact the introduction of a TCO layer (undoped ZnO) between CIGS and back metal contact is investigated from the optical point of view. In addition to ZnO/Mo, a highly reflective ZnO/Ag contact (ZnO is also assumed to work as a protection layer for Ag) is also included in simulations. Results show significant increase in reflectance related to introduced ZnO in front of Mo. Drastically increased reflectance is obtained if ZnO/Mo is substituted with ZnO/Ag. The improvements in QE and J SC of a thin CIGS solar cell, related to ZnO/metal contacts are presented.

Rough ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells

Doped ZnO layers deposited by low-pressure chemical vapour deposition technique have been studied for their use as transparent contact layers for thin-film silicon solar cells. Surface roughness of these ZnO layers is related to their light-scattering capability; this is shown to be of prime importance to enhance the current generation in thin-film silicon solar cells. Surface roughness has been tuned over a large range of values, by varying thickness and/or doping concentration of the ZnO layers. A method is proposed to optimize the light-scattering capacity of ZnO layers, and the incorporation of these layers as front transparent conductive oxides for p–i–n thin-film microcrystalline silicon solar cells is studied.

Challenges in microcrystalline silicon based solar cell technology

This contribution discusses recent scientific and technological challenges for the development of highly efficient microcrystalline silicon (μc-Si:H) based thin film solar cells. Aluminium doped ZnO films prepared by sputtering and post deposition etching serve as transparent conductive oxide (TCO) material, which provide excellent light trapping properties. Challenges are the transfer of this approach to cost-effective reactive sputtering from metallic targets and the reduction of optical absorption losses in the front TCO films. We developed μc-Si:H i-layers by plasma-enhanced chemical vapour deposition (PECVD) using 13.56 and 40.68 MHz excitation frequency, high deposition pressures and high RF-powers. These conditions provide sufficiently “soft” deposition for the growth of high quality μc-Si:H material and yield high deposition rates. Stable aperture area module efficiencies of 10.1% and 8.1% were obtained for a-Si:H/μc-Si:H and μc-Si:H modules, respectively, on 10 × 10 cm 2 substrate size. Subsequent up-scaling to ∼1 m 2 coating area was performed and a-Si:H/μc-Si:H modules exceeding 10% initial aperture area efficiency were obtained. We discuss the questions of process stability and process reproducibility with respect to large area PECVD production systems.

P–i–n flexible imaging devices with optical readout

Abstract Large area two terminal p–i–n image sensors deposited on plastic substrates were produced at low temperatures (110 °C) by PE-CVD and compared with similar sensors deposited on glass substrates. The same sensing element structure ZnO:Al/p(SiC:H)/i(Si:H)/n(SiC:H)/Al was used in all devices although with different resistivities of the front contact. In this work the efforts are focused mainly on the optimization of the output characteristics of the sensor when fabricated on plastic substrates. The role of the sensor configuration, front contact resistivity and readout parameters on the image acquisition process is analyzed. The optical-to-electrical transfer characteristics show reasonable quantum efficiency under a red light pattern, broad spectral response, and reciprocity between light and image signal. First results show that the sensors deposited on flexible substrate present smaller light to dark sensitivity than those deposited on glass. In both, the non ohmic behavior of the transparent conductive oxide front contact blocks the carrier collection and leads to a surprising linear dependence of the image signal on the applied voltage.

Potential of amorphous and microcrystalline silicon solar cells

Low pressure chemical vapour deposition (LP-CVD) ZnO as front transparent conductive oxide (TCO), developed at IMT, has excellent light-trapping properties for a-Si:H p-i-n single-junction and ‘micromorph’ (amorphous/microcrystalline silicon) tandem solar cells. A stabilized record efficiency of 9.47% has independently been confirmed by NREL for an amorphous silicon single-junction p-i-n cell (∼1 cm 2) deposited on LP-CVD ZnO coated glass. Micromorph tandem cells with an initial efficiency of 12.3% show after light-soaking a stable performance of 10.8%. The monolithic series connection by laser-scribing for module fabrication has been developed at IMT as well, for both amorphous single-junction and micromorph tandem cells in combination with the LP-CVD ZnO technique. Mini-modules (areas between 22 and 24 cm 2) with an aperture efficiency of 8.7% in the case of amorphous single-junction p-i-n cells (independently confirmed by NREL), and of 9.8% in the case of micromorph tandem cells, have been obtained. Micromorph tandem cells with an intermediate TCO reflector between the amorphous top and the microcrystalline bottom cell show an almost stable performance (η=10.7%) with respect to light-soaking.

Potential of amorphous and microcrystalline silicon solar cells

Low pressure chemical vapour deposition (LP-CVD) ZnO as front transparent conductive oxide (TCO), developed at IMT, has excellent light-trapping properties for a-Si:H p-i-n single-junction and ‘micromorph’ (amorphous/microcrystalline silicon) tandem solar cells. A stabilized record efficiency of 9.47% has independently been confirmed by NREL for an amorphous silicon single-junction p-i-n cell (∼1 cm2) deposited on LP-CVD ZnO coated glass. Micromorph tandem cells with an initial efficiency of 12.3% show after light-soaking a stable performance of 10.8%. The monolithic series connection by laser-scribing for module fabrication has been developed at IMT as well, for both amorphous single-junction and micromorph tandem cells in combination with the LP-CVD ZnO technique. Mini-modules (areas between 22 and 24 cm2) with an aperture efficiency of 8.7% in the case of amorphous single-junction p-i-n cells (independently confirmed by NREL), and of 9.8% in the case of micromorph tandem cells, have been obtained. Micromorph tandem cells with an intermediate TCO reflector between the amorphous top and the microcrystalline bottom cell show an almost stable performance (η=10.7%) with respect to light-soaking.

Large area p-i-n flexible image sensors

AbstractLarge area p-i-n image sensors deposited on plastic substrates were produced at low temperatures (110 °C) by PE-CVD and compared with similar sensors deposited on glass substrates. The same sensing element structure ZnO:Al/p(SiC:H)/i(Si:H)/n(SiC:H)/Al was used for both devices. In this work the efforts are focused mainly on the optimization of the output characteristics of the sensor when fabricated on plastic substrates. The role of the sensor configuration and readout parameters in the image acquisition process is analyzed. The optical- to-electrical transfer characteristics show a reasonable quantum efficiency under a red light pattern, broad spectral response, and reciprocity between light and image signal.First results show that the sensors deposited on flexible substrate present smaller light to dark sensitivity than those deposited on glass. In both, the non ohmic behavior of the transparent conductive oxide front contact blocks the carrier collection and leads to a surprising linear dependence of the image signal with the applied voltage.

Role of front contact work function on amorphous silicon/crystalline silicon heterojunction solar cell performance

We investigated with numerical simulations the effects that a transparent conductive oxide front contact (TCO) can have on amorphous silicon/crystalline silicon heterojunction solar cells. We found that, due to the extremely thin emitter layer used in this kind of device, the built-in potential available cannot be merely defined by the difference between the work function of the emitter and the base, but it depends on TCO work function as well. As a consequence, because of the correlation between built-in potential and open circuit voltage, the TCO work function strongly affects the solar cell performance. Simulation results show that a higher work function leads to the best device efficiency.