Silicon Wafer Solar Cells Research Articles

Excellent c-Si surface passivation by low-temperature atomic layer deposited titanium oxide

In this work, we demonstrate that thermal atomic layer deposited (ALD) titanium oxide (TiOx) films are able to provide a—up to now unprecedented—level of surface passivation on undiffused low-resistivity crystalline silicon (c-Si). The surface passivation provided by the ALD TiOx films is activated by a post-deposition anneal and subsequent light soaking treatment. Ultralow effective surface recombination velocities down to 2.8 cm/s and 8.3 cm/s, respectively, are achieved on n-type and p-type float-zone c-Si wafers. Detailed analysis confirms that the TiOx films are nearly stoichiometric, have no significant level of contaminants, and are of amorphous nature. The passivation is found to be stable after storage in the dark for eight months. These results demonstrate that TiOx films are also capable of providing excellent passivation of undiffused c-Si surfaces on a comparable level to thermal silicon oxide, silicon nitride, and aluminum oxide. In addition, it is well known that TiOx has an optimal refractive index of 2.4 in the visible range for glass encapsulated solar cells, as well as a low extinction coefficient. Thus, the results presented in this work could facilitate the re-emergence of TiOx in the field of high-efficiency silicon wafer solar cells.

Impact of Auger recombination parameterisations on predicting silicon wafer solar cell performance

For high-efficiency silicon wafer solar cells, Auger recombination is becoming one of the most important efficiency limiting factors. For this purpose it is desirable to be able to use different Auger recombination parameterisations in advanced computer simulations. In this paper we present a method to implement arbitrary Auger parameterisations in the software package Sentaurus TCAD, enabling two- and three-dimensional simulation of solar cells using different Auger parameterisations. As examples, we implemented and investigated three different Auger parameterisations (proposed by Altermatt et al., by Kerr and Cuevas, and by Richter et al.) from the literature. For verification, we simulate Auger lifetimes for different doping densities and injection levels in crystalline silicon. The simulated Auger lifetimes are found to agree well with analytical solutions (differences less than 0.001 %). We then employ the three different Auger parameterisations for fitting measured effective lifetime curves of both $$n$$ n -type and $$p$$ p -type float-zone silicon lifetime samples and show which models are applicable under which conditions. We further compare the difference between the three Auger parameterisations by simulating characteristics of a screen-printed aluminium local back surface field silicon wafer solar cell. The simulation results agree well with the characterisation results. We find that the choice of Auger parameterisation can lead to significant differences in the predicted solar cell behaviour under one-Sun illumination. We demonstrate that different Auger parameterisations may result in significant differences in the blue response, by simulating a heavily doped emitter of an aluminium local back surface field silicon wafer solar cell.

Towards ultra-thin plasmonic silicon wafer solar cells with minimized efficiency loss.

The cost-effectiveness of market-dominating silicon wafer solar cells plays a key role in determining the competiveness of solar energy with other exhaustible energy sources. Reducing the silicon wafer thickness at a minimized efficiency loss represents a mainstream trend in increasing the cost-effectiveness of wafer-based solar cells. In this paper we demonstrate that, using the advanced light trapping strategy with a properly designed nanoparticle architecture, the wafer thickness can be dramatically reduced to only around 1/10 of the current thickness (180 μm) without any solar cell efficiency loss at 18.2%. Nanoparticle integrated ultra-thin solar cells with only 3% of the current wafer thickness can potentially achieve 15.3% efficiency combining the absorption enhancement with the benefit of thinner wafer induced open circuit voltage increase. This represents a 97% material saving with only 15% relative efficiency loss. These results demonstrate the feasibility and prospect of achieving high-efficiency ultra-thin silicon wafer cells with plasmonic light trapping.

Novel non-metallic non-acidic approach to generate sub-wavelength surface structures for inline-diffused multicrystalline silicon wafer solar cells

Abstract One of the main restrictions on further enhancement of the multicrystalline silicon (multi-Si) wafer solar cell efficiency is the high reflectance loss from the front surface. Double-texturing, including a micro-texturing by conventional isotropic acidic-texturing followed by nano-texturing using sub-wavelength structures (SWS), provides a possibility to remove this limitation. However, presently available double-texturing processes involve multiple and high-cost nano-texturing process steps and thus have not become industrially viable. This study presents an improved double-texturing process and reports its successful implementation using low-cost inline-diffusion and non-acidic ‘SERIS etch’ etch-back technology. The process is based on reactive ion etching (RIE) as a metal-free nano-texturing step, along with the conventional acidic iso-texturing process. The process is optimised based on effective minority carrier lifetime and weighted average reflectance (WAR) measurements, on 156 × 156 mm2, industrial-grade, p-type multi-Si wafers. This optimised double-textured SWS surface has a WAR of 3.2% after silicon nitride deposition, which is significantly lower than that of conventional acidic iso-textured wafers (WAR of 6–7%). However, this optimised surface maintains the same values of the effective minority carrier lifetime as for the reference conventional acidic-textured multi-Si wafers. The optimised nano-texturing recipe is also successfully applied to alkaline-textured monocrystalline silicon wafers.

Detailed Current Loss Analysis for a PV Module Made With Textured Multicrystalline Silicon Wafer Solar Cells

We present a top-down method to quantify optical losses due to encapsulation of textured multicrystalline silicon wafer solar cells in a photovoltaic module. The approach is based on a combination of measurements and mathematical procedures. Seven different loss mechanisms are considered: 1) reflection at the glass front surface, 2) reflection at the metal fingers, 3) reflection at the textured solar cell surface, 4) absorption in the antireflection coating, 5) absorption in the glass pane and the encapsulation layer, 6) front surface escape, and 7) losses due to a non-perfect solar cell internal quantum efficiency. Losses for each of these mechanisms are obtained as a function of wavelength, and the corresponding current loss for each loss mechanism is calculated. Comparing simulated and measured results, the method predicts the module quantum efficiency with an error of less than 2% and the collected current with an error of less than 1%. In the presented example, the biggest loss (7.4 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) is due to the nonperfect quantum efficiency, followed by reflection losses at the glass front (2.2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and absorption in the glass and encapsulation layer (1.1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).

Hybrid silver nanoparticle and transparent conductive oxide structure for silicon solar cell applications

AbstractTransparent conductive oxides (TCOs) have been widely used as electrodes for various solar cell structures. For heterojunction silicon wafer solar cells, the front TCO layer not only serves as a top electrode (by enhancing the lateral conductance of the underlying amorphous silicon film), but also as an antireflection coating. These requirements make it difficult to simultaneously achieve excellent conductance and transparency, and thus, only high‐quality indium tin oxide (ITO) has as yet found its way into industrial heterojunction silicon wafer solar cells. In this Letter, we present a cost‐effective hybrid structure consisting of a TCO layer and a silver nano‐particle mesh. This structure enables the separate optimization of the electrical and optical requirements. The silver nanoparticle mesh provides high electrical conductance, while the TCO material is optimized as an antireflection coating. Therefore, this structure allows the use of cost‐effective (and less conductive) TCO materials, such as aluminium‐doped zinc oxide. The performance of the hybrid structure is demonstrated to achieve a similar visible transmission (∼86% in the 380–780 nm range) as an 80 nm thick ITO layer, but with 10 times better lateral conductance. The presented hybrid structure thus seems well suited for a variety of photovoltaic devices. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Detection limits for glow discharge mass spectrometry (GDMS) analyses of impurities in solar cell silicon

The measurement of both doping elements and trace elements in solar cell silicon plays a key role for achieving high conversion efficiency of the solar cell device. Doping element concentrations in the range of few hundreds part per billions (ppb) and trace elements in the ppb or sub-ppb concentration range are typically present in multicrystalline silicon wafers for solar cells. Accurate and reliable measurements of these small amounts are not straightforward. The present work describes a fast-flow direct-current high resolution glow discharge mass spectrometer (GDMS). Detection limits for a number of impurities (B, Al, P, Ca, Ti, V, Cr, Mn, Fe, Ni, Co, Cu, Mo, Sn, W and Pb) of interest for solar cell applications have been investigated by GDMS. These detection limits are approximately 1ppba or below, except for B, Al, P, Ca and Pb. All concentrations reported are quantitative since calculated relative sensitivity factors (RSF‘s) for Si matrix have been used. The detection limits have been achieved with minimum sample preparation and short analysis time.

Excellent Silicon Surface Passivation Achieved by Industrial Inductively Coupled Plasma Deposited Hydrogenated Intrinsic Amorphous Silicon Suboxide

We present an alternative method of depositing a high-quality passivation film for heterojunction silicon wafer solar cells, in this paper. The deposition of hydrogenated intrinsic amorphous silicon suboxide is accomplished by decomposing hydrogen, silane, and carbon dioxide in an industrial remote inductively coupled plasma platform. Through the investigation on CO2partial pressure and process temperature, excellent surface passivation quality and optical properties are achieved. It is found that the hydrogen content in the film is much higher than what is commonly reported in intrinsic amorphous silicon due to oxygen incorporation. The observed slow depletion of hydrogen with increasing temperature greatly enhances its process window as well. The effective lifetime of symmetrically passivated samples under the optimal condition exceeds 4.7 ms on planarn-type Czochralski silicon wafers with a resistivity of 1 Ωcm, which is equivalent to an effective surface recombination velocity of less than 1.7 cms−1and an implied open-circuit voltage (Voc) of 741 mV. A comparison with several high quality passivation schemes for solar cells reveals that the developed inductively coupled plasma deposited films show excellent passivation quality. The excellent optical property and resistance to degradation make it an excellent substitute for industrial heterojunction silicon solar cell production.

Novel Hybrid Electrode Using Transparent Conductive Oxide and Silver Nanoparticle Mesh for Silicon Solar Cell Applications

Transparent conductive oxides (TCOs) have been widely used as the front electrodes for various solar cell structures, including heterojunction silicon wafer solar cells and the vast majority of thin-film solar cells. For heterojunction silicon wafer solar cells, the front TCO layer not only serves as a top electrode (by enhancing the lateral conductance of the underlying amorphous silicon film), but also as an antireflection coating. These requirements make it difficult to simultaneously achieve excellent conductivity and transparency, and thus only high-quality indium tin oxide (ITO) has as yet found its way into industrial heterojunction silicon wafer solar cells. For thin-film solar cells, in order to provide efficient lateral conductance of the charge carriers, normally a TCO layer of a few hundred nanometers thickness is used which impedes the optical transparency due to the enhanced free carrier absorption. To reduce the conflict between conductivity and transparency, and to separately engineer the electrical and optical properties, a hybrid electrode is proposed and fabricated by us which consists of a TCO layer (optical layer) and a silver nanoparticle mesh (electrical layer). This hybrid electrode is demonstrated to have a 10 times higher lateral conductance compared to a single TCO layer, while maintaining high light transmission in a wide wavelength range. Due to the excellent performance of the hybrid electrode, it is demonstrated that such an electrode is suitable for various solar cell structures.

Micro Raman Spectroscopy Analysis of Doped Amorphous and Microcrystalline Silicon Thin Film Layers and its Application in Heterojunction Silicon Wafer Solar Cells

Micro Raman Spectroscopy Analysis of Doped Amorphous and Microcrystalline Silicon Thin Film Layers and its Application in Heterojunction Silicon Wafer Solar Cells

Novel selective emitter process using non-acidic etch-back for inline-diffused silicon wafer solar cells

In this work, a novel etch-back approach for the fabrication of a selective emitter (SE) structure is reported for inline-diffused p-type monocrystalline silicon wafer solar cells. The complete SE process, named the ‘SERIS SE’ process, involves screen printing of an etch mask, use of a HF-free etch-back solution and screen-printed metallisation. Both the emitter etch-back and etch-mask dissolution are performed simultaneously in a single processing step, thereby reducing the number of processing steps as compared to other etch-back based SE technologies. For inline-diffused emitter (ILDE) solar cells, the ‘SERIS SE’ process actually requires only one additional processing step, as an emitter etch-back is typically applied to remove the detrimental top layer. An average cell efficiency gain of 0.4% (absolute) is reported for the SE cells fabricated using the single-step SERIS SE process, as compared to etch-back homogeneous-emitter (HE) solar cells. An average batch efficiency of 18.5% is achieved for screen-printed p-type 156 mm pseudo-square Cz mono-Si full-area aluminium back surface field (Al-BSF) SE solar cells. The minimal increase in the number of processing steps, reliance on the robust screen printing process, HF-free non-acidic chemical etch-back and applicability to both tube and inline-diffused emitters make the ‘SERIS SE’ process suitable for industrial application on both mono- and multicrystalline silicon wafers.

Excellent surface passivation of heavily doped p+ silicon by low-temperature plasma-deposited SiOx/SiNy dielectric stacks with optimised antireflective performance for solar cell application

The passivation of p+ Si surfaces is challenging due to the fact that most passivation films have an intrinsically high positive fixed charge. In this work we show experimentally that low-temperature plasma-enhanced chemical vapor deposited SiOx/SiNy stacks with a low positive fixed charge density (+1011cm−2) and very low interface defect density (~3×1010eV−1cm−2) as measured by contactless corona-voltage measurements can effectively passivate p+ surfaces resulting in emitter saturation current density (J0e) values of 25 and 45fA/cm2 on planar and textured 75Ω/sq p+ silicon after industrial firing with a set-temperature of ~800°C, respectively. Based on contactless corona-voltage measurements and advanced device simulations, we explain the mechanism of surface passivation by PECVD SiOx/SiNy dielectric stack to be completely dominated by chemical passivation rather than field-effect passivation. Furthermore, from advanced device simulations we illustrate the role of fixed charge in surface passivation and in the extraction of fundamental surface recombination velocity parameter for p+ silicon surfaces. The fundamental surface recombination velocity parameter for electrons is determined to be about 400cm/s at these c-Si/SiOx interfaces. With excellent optical and passivation properties, SiOx/SiNy dielectric stacks are suitable for high-efficiency and cost-effective industrial n-type silicon wafer solar cells.

Efficiency and stability enhancement of laser-crystallized polycrystalline silicon thin-film solar cells by laser firing of the absorber contacts

Polycrystalline silicon thin-film solar cells produced by continuous-wave diode-laser crystallization at the University of New South Wales were recently reported to have reached a conversion efficiency above 10%. One drawback of these cells, however, was that they exhibited efficiency degradation within several hours after the cell fabrication was completed. In this work we show that by applying laser firing to the rear point contacts of the solar cells, it is possible to stabilize and even to enhance the performance of these devices. Our investigation indicates that it is the poor quality of the contact between the aluminum and the silicon absorber that causes the cell degradation and offers an elegant and industrial-compatible process to improve the cell performance. This is the first time that the laser firing process, initially developed for alloying an aluminum layer through a dielectric layer on crystalline silicon wafer solar cells, is being applied to polycrystalline silicon thin-film solar cells.

A Fill Factor Loss Analysis Method for Silicon Wafer Solar Cells

The fill factor of silicon wafer solar cells is strongly influenced by recombination currents and ohmic resistances. A practical upper limit for the fill factor of crystalline silicon solar cells operating under low-level injection is set by recombination in the quasi-neutral bulk and at the two cell surfaces. Series resistance, shunt resistance, and additional recombination currents further lower the fill factor. For process optimization or loss analysis of solar cells, it is important to determine the influence of both ohmic and recombination loss mechanisms on the fill factor. In this paper, a method is described to quantify the loss in fill factor due to series resistance, shunt resistance, and additional recombination currents. Only the 1-Sun J-V curve, series resistance at the maximum power point, and shunt resistance need to be determined to apply the method. Application of the method is demonstrated on an 18.4% efficient inline-diffused p-type silicon wafer solar cell and a 21.1% efficient heterojunction n-type silicon wafer solar cell. Our analysis does not require J-V curve fitting to extract diode saturation current densities or ideality factor; however, the results are shown to be consistent with curve fitting results if the cell's two-diode model parameters can be unambiguously determined by curve fitting.

Effects of indium concentration on the properties of In-doped ZnO films: Applications to silicon wafer solar cells

In the present paper, high-quality In-doped ZnO (ZnO:In) thin films have been prepared by rf-magnetron sputtering on glass and p-type monocrystalline silicon substrates from an aerogel nanopowder target material. The nanoparticles with the [In]/[Zn] ratio varying between 0.01 and 0.05 were synthesized by the sol–gel method and the structural properties have been analyzed. The effect of different dopant concentrations on the electrical, optical, structural and morphological properties of the films has been investigated. The obtained ZnO:In films at room temperature are polycrystalline with a hexagonal structure and a highly preferred orientation with the c-axis perpendicular to the substrate. Scanning electron microscopy and atomic force microscopy have been applied for a morphology characterization of the films' cross-section and surface. The results revealed a typical columnar structure and very smooth surface. Films with good optical transmittance, around 85%, within the visible wavelength region, and low resistivity in the range of 10−3Ω·cm and high mobility of 4cm2/Vs, were produced at low substrate temperature. On the other hand, we have studied the position of the p–n junction involved in an Au/In2O3:SnO2/ZnO:In(n)/c-Si(p)/Al structure by electron beam induced current. Current density–voltage characterizations in the dark and under illumination were also performed. The cell exhibits an efficiency of 6%.

18.7% Efficient inline-diffused screen-printed silicon wafer solar cells with deep homogeneous emitter etch-back

One of the critical fabrication processes of silicon wafer solar cells is the emitter formation. Tube based diffusion, using phosphorus oxychloride as the dopant source, is the standard in the photovoltaic industry. Inline-diffusion, using phosphoric acid as a dopant source, is a potentially low-cost alternative; however it typically results in lower solar cell efficiency. This paper presents an improved etch-back process, ‘SERIS etch’, for inline-diffused emitters, which achieves batch average efficiency of 18.6% using 156mm pseudo-square industrial-grade p-type Cz mono-silicon wafers with conventional screen-printed metallisation and aluminium back surface field (Al-BSF). The best cell is 18.7% efficient, with an open-circuit voltage of 631mV and a fill factor of 80.6%. These high efficiencies were achieved by co-optimisation of the emitter dopant profile and the emitter etch-back process. A variety of characterisation techniques, such as scanning electron microscopy, effective lifetime measurement, dopant profile analyser, photo- and electroluminescence images, and current–voltage (dark and illuminated) measurements were employed in this work.

Analysis of intrinsic hydrogenated amorphous silicon passivation layer growth for use in heterojunction silicon wafer solar cells by optical emission spectroscopy

The structure and quality of intrinsic hydrogenated amorphous silicon thin films are studied with intended use as passivation layer in heterojunction silicon wafer solar cells. The thin film layers are formed by radio-frequency parallel-plate plasma-enhanced chemical vapor deposition. While the passivation quality of the films is found to improve steadily with increasing deposition temperature, a very narrow process window in terms of pressure variation is observed. The best effective lifetime is obtained at a hydrogen to silane dilution ratio of 1 and a pressure of 66.7 Pa for the used tool configuration. Raman crystallinity and Urbach energy obtained from fitting ellipsometry data confirm that the degradation of the passivation quality outside the process window is due to a phase change into microcrystalline silicon with different growth mechanisms and an increase in bonding related defects. Film growth mechanisms are proposed to account for the observed narrow process window, which are verified by optical emission spectroscopy measurements.

Large-size, high-uniformity, random silver nanowire networks as transparent electrodes for crystalline silicon wafer solar cells

Metal nanowire networks are emerging as next generation transparent electrodes for photovoltaic devices. We demonstrate the application of random silver nanowire networks as the top electrode on crystalline silicon wafer solar cells. The dependence of transmittance and sheet resistance on the surface coverage is measured. Superior optical and electrical properties are observed due to the large-size, highly-uniform nature of these networks. When applying the nanowire networks on the solar cells with an optimized two-step annealing process, we achieved as large as 19% enhancement on the energy conversion efficiency. The detailed analysis reveals that the enhancement is mainly caused by the improved electrical properties of the solar cells due to the silver nanowire networks. Our result reveals that this technology is a promising alternative transparent electrode technology for crystalline silicon wafer solar cells.

Optimizing the Front Electrode of Silicon-Wafer-Based Solar Cells and Modules

Front electrode optimization is one of the important design considerations that affects the efficiency of a silicon wafer solar cell. The optimization of the front electrode is usually done to maximize cell efficiency at standard test conditions (STC). However, with increasing prices in silver, optimization of the front electrode should be done by taking into account the cost of the silver paste. In this study, optimization of the front electrode is done at the cell level at STC (dollars per watt peak), module level at STC (dollars per watt peak), and under real-world module conditions (dollars per kilowatthour), taking into account the cost of the silver paste. For commercial screen-printed multicrystalline silicon wafer solar cells, it is found that to achieve the most cost-effective cell design at the outdoor module level (dollars per kilowatthour), the number of front metal fingers can be strongly reduced (by more than 20) compared with a conventional cell design, which is maximized for STC cell efficiency. For silver price of $1286/kg, optimization at the cell and module level for lowest cost will yield up to 1% cost savings compared with optimization for maximum efficiency. Optimization for the lowest levelized cost of electricity (LCOE) will yield on average 0.6% lower LCOE compared with optimization for maximum annual energy output.

Investigation of Screen-Printed Rear Contacts for Aluminum Local Back Surface Field Silicon Wafer Solar Cells

Silicon wafer solar cells with an aluminum local back surface field (Al-LBSF) are currently intensively investigated for industrial application. One of the main challenges for the Al-LBSF solar cell is the formation of the local Al rear contacts. In our previous work, we have introduced the relative photoluminescence (PL) intensity method to study the Al-Si local contact formation. In this study, we apply this method to experimentally investigate the impact of the geometry (lines or points) of the rear contacts and compare the experimental results with theoretical results that are obtained using Fischer's model. We find that the PL intensity strongly correlates with the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> layer thickness and inversely correlates with the void density at the rear surface. Al-LBSF solar cells with different rear contact geometries are fabricated. High <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rs</i> was found, especially for those cells with narrower line widths and a large number of voids.