Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

DC‐sputtered ZnO:Al as transparent conductive oxide for silicon heterojunction solar cells with µc‐Si:H emitter

Silicon heterojunction (SHJ) solar cells are highly interesting, because of their high efficiency and low cost fabrication. So far, the most applied transparent conductive oxide (TCO) is indium tin oxide (ITO). The replacement of ITO with cheaper, more abundant and environmental friendly material with texturing capability is a promising way to reduce the production cost of the future SHJ solar cells. Here, we report on the fabrication of the SHJ solar cells with direct current‐sputtered aluminum‐doped zinc oxide (ZnO:Al) as an alternative TCO. Furthermore, we address several important differences between ITO and the ZnO:Al layers including a high Schottky barrier at the emitter/ZnO:Al interface and a high intrinsic resistivity of the ZnO:Al layers. To overcome the high Schottky barrier, we suggest employing micro‐crystalline silicon (µc‐Si:H) emitter, which also improves temperature threshold and passivation of the solar cell precursor. In addition, we report on the extensive studies of the effect of the ZnO:Al deposition parameters including layer thickness, oxygen flow, power density and temperature on the electrical properties of the fabricated SHJ solar cells. Finally, the results of our study indicate that the ZnO:Al deposition parameters significantly affect the electrical properties of the obtained solar cell. By understanding and fine‐tuning all these parameters, a high conversion efficiency of 19.2% on flat wafer (small area (5 × 5 mm²) and without any front metal grid) is achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Efficient silicon heterojunction solar cells based on p‐ and n‐type substrates processed at temperatures < 220°C

We present the optimization and characterization of heterojunction solar cells consisting of an amorphous silicon emitter, a single crystalline absorber and an amorphous silicon rear side which causes the formation of a back surface field (a‐Si:H/c‐Si/a‐Si:H). The solar cells were processed at temperatures <220°C. An optimum of the gas phase doping concentration of the a‐Si:H layers was found. For high gas phase doping concentrations, recombination via defects located at or nearby the interface leads to a decrease in solar cell efficiency. We achieved efficiencies >17% on p‐type c‐Si absorbers and >17·5% on n‐type absorbers. In contrast to the approach of Sanyo, no additional intrinsic a‐Si:H layers between the substrate and the doped a‐Si:H layers were inserted. Copyright © 2006 John Wiley & Sons, Ltd.     

Opto‐electrical modelling and optimization study of a novel IBC c‐Si solar cell

Interdigitated back contact (IBC) crystalline silicon (c‐Si) solar cells are attracting a lot of attention because of their capability to reach world record conversion efficiency. Because of the relatively complex contact pattern, their design and optimization typically require advanced numerical simulation tools. In this work, a TCAD‐based simulation platform has been developed to account accurately and in detail the optical and passivation mechanisms of front texturization. Its validation has been carried out with respect to a novel homo‐junction IBC c‐Si solar cell based on ion implantation and epitaxial growth, comparing measured and simulated reflectance, transmittance, internal quantum efficiency, external quantum efficiency spectra, and current density–voltage characteristics. As a result of the calibration process, the opto‐electrical losses of the investigated device have been identified quantitatively and qualitatively. Then, an optimization study about the optimal front surface field (FSF) doping, front‐side texturing morphology, and rear side geometry has been performed. The proposed simulation platform can be potentially deployed to model other solar cell architectures than homo‐junction IBC devices (e.g., passivated emitter rear cell, passivated emitter rear locally diffused cell, hetero‐IBC cell). Simulation results show that a not‐smoothed pyramid‐textured front interface and an optimal FSF doping are mandatory to minimize both the optical and the recombination losses in the considered IBC cell and, consequently, to maximize the conversion efficiency. Similarly, it has been showed that recombination losses are affected more by the doping profile rather than the surface smoothing. Moreover, the performed investigation reveals that the optimal FSF doping is almost independent from the front texturing morphology and FSF passivation quality. According to this result, it has been demonstrated that an IBC cell featuring an optimal FSF doping does not exhibit a significant efficiency improvement when the FSF passivation quality strongly improves, proving that IBC cell designs based on low‐doped FSF require a very outstanding passivation quality to be competitive. Deploying an optimization algorithm, the adoption of an optimized rear side geometry can potentially lead to an efficiency improvement of about 1%_abs as compared with the reference IBC solar cell. Further, by improving both emitter and c‐Si bulk quality, a 22.84% efficient solar cell for 280‐μm thick c‐Si bulk was simulated. Copyright © 2017 John Wiley & Sons, Ltd.     

Enhancing the Selectivity and Transparency of the Electron Contact in Silicon Heterojunction Solar Cells by Phosphorus Catalytic Doping

An intrinsic hydrogenated amorphous silicon (a-Si:H(i)) film and a doped silicon film are usually combined in the heterojunction contacts of silicon heterojunction (SHJ) solar cells. In this work, a post-doping process called catalytic doping (Cat-doping) on a-Si:H(i) is performed on the electron selective side of SHJ solar cells, which enables a device architecture that eliminates the additional deposition of the doped silicon layer. Thus, a single phosphorus Cat-doping layer combines the functions of two other layers by enabling excellent interface passivation and high carrier selectivity. The overall thinner layer on the window side results in higher spectral response at short wavelengths, leading to an improved short-circuit current density of 40.31 mA cm⁻² and an efficiency of 23.65% (certified). The cell efficiency is currently limited by sputter damage from the subsequent transparent conductive oxide fabrication and low carrier activation in the a-Si:H(i) with Cat-doping. Numerical device simulations show that the a-Si:H(i) with Cat-doping can provide sufficient field effect passivation even at lower active carrier concentrations compared to the as-deposited doped layer, due to the lower defect density.     

Preparation and measurement of highly efficient a‐Si:H single junction solar cells and the advantages of μc‐SiOx:H n‐layers

Reducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiO_x:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiO_x:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiO_x:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiO_x:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiO_x:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiO_x:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

Interdigitated back‐contact heterojunction solar cell concept for liquid phase crystallized thin‐film silicon on glass

We present an interdigitated back‐contact silicon heterojunction system designed for liquid‐phase crystallized thin‐film (~10 µm) silicon on glass. The preparation of the interdigitated emitter (a‐Si:H(p)) and absorber (a‐Si:H(n)) contact layers relies on the etch selectivity of doped amorphous silicon layers in alkaline solutions. The etch rates of a‐Si:H(n) and a‐Si:H(p) in 0.6% NaOH were determined and interdigitated back‐contact silicon heterojunction solar cells with two different metallizations, namely Al and ITO/Ag electrodes, were evaluated regarding electrical and optical properties. An additional random pyramid texture on the back side provides short‐circuit current density (j_SC) of up to 30.3 mA/cm² using the ITO/Ag metallization. The maximum efficiency of 10.5% is mainly limited by a low of fill factor of 57%. However, the high j_SC, as well as V_OC values of 633 mV and pseudo‐fill factors of 77%, underline the high potential of this approach. Copyright © 2015 John Wiley & Sons, Ltd.     

Back amorphous‐crystalline silicon heterojunction (BACH) photovoltaic device with facile‐grown oxide ‐ PECVD SiNx passivation

This article reports on the integration of facile native oxide‐based passivation of crystalline silicon surfaces within the back amorphous‐crystalline silicon heterojunction solar cell concept. The new passivation scheme consists of 1‐nm thick native oxide and nominally 70‐nm thick PECVD silicon nitride. The low temperature passivation scheme provides uniform high quality surface passivation and low parasitic optical absorption. The interdigitated doped hydrogenated amorphous silicon layers were deposited on the rear side of the silicon wafer using the direct current saddle field PECVD technique. A systematic analysis of a series of back amorphous‐crystalline silicon heterojunction cells is carried out in order to examine the influence of the various cell parameters (interdigital gap, n‐doped region width, ratio of widths of p, and n‐doped regions) on cell performance. A photovoltaic conversion efficiency of 16.7 % is obtained for an untextured cell illuminated under AM 1.5 global spectrum (cell parameters: V_OC of 641 mV, J_SC of 33.7 mA‐cm ^{− 2} and fill factor of 77.3 %). Copyright © 2014 John Wiley & Sons, Ltd.     

20.1%‐efficient crystalline silicon solar cell with amorphous silicon rear‐surface passivation

We have developed a crystalline silicon solar cell with amorphous silicon (a‐Si:H) rear‐surface passivation based on a simple process. The a‐Si:H layer is deposited at 225°C by plasma‐enhanced chemical vapor deposition. An aluminum grid is evaporated onto the a‐Si:H‐passivated rear. The base contacts are formed by COSIMA (contact formation to a‐Si:H passivated wafers by means of annealing) when subsequently depositing the front silicon nitride layer at 325°C. The a‐Si:H underneath the aluminum fingers dissolves completely within the aluminum and an ohmic contact to the base is formed. This contacting scheme results in a very low contact resistance of 3.5 ±0.2 mΩ cm² on low‐resistivity (0.5 Ω cm) p‐type silicon, which is below that obtained for conventional Al/Si contacts. We achieve an independently confirmed energy conversion efficiency of 20.1% under one‐sun standard testing conditions for a 4 cm² large cell. Measurements of the internal quantum efficiency show an improved rear surface passivation compared with reference cells with a silicon nitride rear passivation. Copyright © 2005 John Wiley & Sons, Ltd.     

Simulation and fabrication of heterojunction silicon solar cells from numerical computer and hot‐wire CVD

In this paper, we will present a Pc1D numerical simulation for heterojunction (HJ) silicon solar cells, and discuss their possibilities and limitations. By means of modeling and numerical computer simulation, the influence of emitter‐layer/intrinsic‐layer/crystalline‐Si heterostructures with different thickness and crystallinity on the solar cell performance is investigated and compared with hot wire chemical vapor deposition (HWCVD) experimental results. A new technique for characterization of n‐type microcrystalline silicon (n‐µc‐Si)/intrinsic amorphous silicon (i‐a‐Si)/crystalline silicon (c‐Si) heterojunction solar cells from Pc1D is developed. Results of numerical modeling as well as experimental data obtained using HWCVD on µc‐Si (n)/a‐Si (i)/c‐Si (p) heterojunction are presented. This work improves the understanding of HJ solar cells to derive arguments for design optimization. Some simulated parameters of solar cells were obtained: the best results for J_sc = 39·4 mA/cm², V_oc = 0·64 V, FF = 83%, and η = 21% have been achieved. After optimizing the deposition parameters of the n‐layer and the H₂ pretreatment of solar cell, the single‐side HJ solar cells with J_sc = 34·6 mA/cm², V_oc = 0·615 V, FF = 71%, and an efficiency of 15·2% have been achieved. The double‐side HJ solar cell with J_sc = 34·8 mA/cm², V_oc = 0·645 V, FF = 73%, and an efficiency of 16·4% has been fabricated. Copyright © 2009 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: Thin‐film free‐standing monocrystalline si solar cells with heterojunction emitter

We propose a novel approach to thin‐film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS‐HJ). High crystallographic quality mono‐Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free‐standing mono‐Si (FMS) films were processed into solar cells by creating a‐a‐Si/c‐Si heterojunction. In our preliminary experiments a thin‐film FMS‐HJ solar cell with 9.6% efficiency was realized in a 20‐μμm‐thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.     

Over 16.7% Efficiency Organic‐Silicon Heterojunction Solar Cells with Solution‐Processed Dopant‐Free Contacts for Both Polarities

Realization of synchronous improvement in optical management and electrical engineering is necessary to achieve high‐performance photovoltaic device. However, inherent challenges are faced in organic‐silicon heterojunction solar cells (HSCs) due to the poor contact property of polymer on structured silicon surface. Herein, a remarkable efficiency boost from 12.6% to over 16.7% in poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/n‐silicon (PEDOT:PSS/n‐Si) HSCs by independent optimization of hole‐/electron‐selective contacts only relying on solution‐based processes is realized. A bilayer PEDOT:PSS film with different functionalizations is utilized to synchronously realize conformal contact and effective carrier collection on textured Si surface, making the photogenerated carriers be well separated at heterojunction interface. Meanwhile, fullerene derivative is used as electron‐transporting layer at the rear n‐Si/Al interface to reduce the contact barrier. The study of carriers' transport and independent optimization on separately contacted layers may lead to an effective and simplified path to fabricate high‐performance organic‐silicon heterojunction devices.     

CuOx/a‐Si:H heterojunction thin‐film solar cell with an n‐type µc‐Si:H depletion‐assisting layer

We showed that thin n‐type CuO_x films can be deposited by radio‐frequency magnetron reactive sputtering and demonstrated the fabrication of n‐CuO_x/intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) heterojunction solar cells (HSCs) for the first time. A highly n‐doped hydrogenated microcrystalline Si (n‐µc‐Si:H) layer was introduced as a depletion‐assisting layer to further improve the performance of n‐CuO_x/i‐a‐Si:H HSCs. An analysis of the external quantum efficiency and energy‐band diagram showed that the thin depletion‐assisting layer helped establish sufficient depletion and increased the built‐in potential in the n‐CuO_x layer. The fabricated HSC exhibited a high open‐circuit voltage of 0.715 V and an efficiency of 4.79%. Copyright © 2015 John Wiley & Sons, Ltd.     

Passivation of n‐type emitter and p‐type base in solar cells via oxygen terminated silicon nanoparticles

Various measurements and experiments are performed to establish the mechanism of passivation on emitter and base of conventionally manufactured solar cell with p‐type base. The surface coatings on the emitter are removed. The bare surface is then coated with silicon (Si) nanoparticles (NPs) with oxygen termination. It shows an increase in the cell efficiency up to 14% over bare surface of solar cell. The NPs show enhancement in light scattering from the surface, but shows an increase in the recombination lifetime indicating an improved passivation. When back contact is partially removed, the coating on bare back side ( p‐type) of the solar cell also improves the cell efficiency. This is also attributable to the increased recombination lifetime from the measurements. Same NPs are seen to degrade the surface of n and p‐type Si wafers. This apparently contradictory behaviour is explained by studying and comparing the emitter (n‐type) surface of the solar cell with that of n‐type Si wafer and the back surface ( p‐type) with that of p‐type Si wafer. The emitter surface is distinctly different from the n‐type wafer because of the shallow p–n junction causing the surface depletion. Back surface has aluminium (Al) metal trace, which plays an important role in forming complexes with the oxygen‐terminated Si NPs (Si–O NPs). With these studies, it is observed that increase in the efficiency can potentially reduce the thermal budget in solar cell preparation. Copyright © 2013 John Wiley & Sons, Ltd.     

Laser‐fired contact optimization in c‐Si solar cells

In this work we study the optimization of laser‐fired contact (LFC) processing parameters, namely laser power and number of pulses, based on the electrical resistance measurement of an aluminum single LFC point. LFC process has been made through four passivation layers that are typically used in c‐Si and mc‐Si solar cell fabrication: thermally grown silicon oxide (SiO₂), deposited phosphorus‐doped amorphous silicon carbide (a‐SiC_x/H(n)), aluminum oxide (Al₂O₃) and silicon nitride (SiN_x/H) films. Values for the LFC resistance normalized by the laser spot area in the range of 0.65–3 mΩ cm² have been obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of the impact of the rear‐dielectric/silver back reflector design on the optical performance of thin‐film silicon solar cells by means of detached reflectors

Thin‐film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear‐dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (µc‐Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non‐conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the µc‐Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Realistic efficiency potential of next‐generation industrial Czochralski‐grown silicon solar cells after deactivation of the boron–oxygen‐related defect center

We measure carrier lifetimes of different Czochralski‐grown silicon (Cz‐Si) materials of various boron and oxygen concentrations and determine the maximum achievable lifetime after an optimized thermal treatment. We obtain very high and stable bulk lifetimes of several milliseconds, virtually eliminating the boron–oxygen (BO) defect complex, which previously limited the carrier lifetime in boron‐doped Cz‐Si materials after prolonged illumination. Based on these experimental results, we introduce a new parameterization of the bulk lifetime of B‐doped Cz‐Si after permanent deactivation of the BO center. Notably, we measure lifetimes up to 4 ms on 2‐Ωcm Cz‐Si wafers at an injection level of 1/10 of the doping concentration. Importantly, these high lifetime values can be reached within 10 and 20 s of BO deactivation treatment. A detailed analysis of the injection‐dependent lifetimes reveals that the lifetimes after permanent deactivation of the BO center can be well described by a single‐level recombination center characterized by an electron‐to‐hole capture cross‐section ratio of 12 and located in the middle of the silicon band gap. We implement the novel parameterization into a two‐dimensional device simulation of a passivated emitter and rear solar cell using technologically realistic cell parameters. The simulation reveals that based on current state‐of‐the‐art solar cell production technology, efficiencies reaching 22.1% are realistically achievable in the near future after complete deactivation of the BO center. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimization and characterization of amorphous/crystalline silicon heterojunction solar cells

Amorphous hydrogenated silicon/crystalline silicon (a‐Si:H/c‐Si) heterojunction solar cells are investigated and optimized with regard to efficiency and simplicity of processing. Starting with a survey of a‐Si:H/c‐Si heterojunction solar cell results from the literature, we describe the fabrication steps of our a‐Si:H/c‐Si technology and analyze the electronic device properties by quantum efficiency, current–voltage, admittance, and capacitance–voltage measurements. The open‐circuit voltage and the fill factor of the a‐Si:H/c‐Si heterojunction solar cells under investigation are limited by recombination in the neutral zone of the crystalline Si absorber. Recombination at the a‐Si:H/c‐Si‐interface is subsidiary in respect of the limitation of the open‐circuit voltage. Our best n‐type a‐Si:H/p‐type c‐Si solar cell prepared without high‐efficiency features such as back‐surface field or surface texturing has an independently confirmed efficiency of 14.1% and an open‐circuit voltage of 655 mV. Copyright © 2001 John Wiley & Sons, Ltd.     

Screen‐printed Emitter‐Wrap‐Through solar cell with single step side selective emitter with 18.8% efficiency

A fabrication process for Emitter‐Wrap‐Through solar cells on monocrystalline material with high quality gap passivation by wet thermal silicon dioxide is investigated. Masking and structuring steps are performed by screen‐printing technology. Via‐holes are created by an industrially applicable high‐speed laser drilling process. The cell structure features a selective emitter structure fabricated in a single high temperature step: a highly doped emitter at the via‐holes and the rear side, allowing for a low via‐hole resistivity as well as a low resistivity contact to screen‐printed pastes, and a moderately doped front side emitter exhibiting high quantum efficiency in the low wavelength range. Therefore a novel approach is applied depositing either doped or undoped PECVD silicon dioxide layers on the front side. It is shown that doping profiles advantageous for the EWT‐cell structure can be achieved. The screen‐printed aluminum paste is found to penetrate the underlying thermal dioxide layer at appropriate contact firing conditions leading to a zone of high recombination in the overlap region of aluminum and silicon dioxide. It is shown that conventional PECVD‐anti‐reflection silicon nitride acts as effective protection layer reducing the recombination in this region. Designated area conversion efficiencies up to 18.8% on FZ material are obtained applying the single step side selective emitter fabrication technique. Copyright © 2010 John Wiley & Sons, Ltd.     

Full‐wave optoelectrical modeling of optimized flattened light‐scattering substrate for high efficiency thin‐film silicon solar cells

The flattened light‐scattering substrate (FLiSS) is formed by a combination of two materials with a high refractive index mismatch, and it has a flat surface. A specific realization of this concept is a flattened two‐dimensional grating. When applied as a substrate for thin‐film silicon solar cells in the nip configuration, it is capable to reflect light with a high fraction of diffused component. Furthermore, the FLiSS is an ideal substrate for growing high‐quality microcrystalline silicon (µc‐Si:H), used as bottom cell absorber layer in most of multijunction solar cell architectures. FLiSS is a three‐dimensional structure; therefore, a full‐wave analysis of the electromagnetic field is necessary for its optimal implementation. Using finite element method, different shapes, materials, and geometrical parameters were investigated to obtain an optimized FLiSS. The application of the optimized FLiSS in µc‐Si:H single junction nip cell (1‐µm‐thick i‐layer) resulted in a 27.4‐mA/cm² implied photocurrent density. The absorptance of µc‐Si:H absorber exceeded the theoretical Yablonovitch limit for wavelengths larger than 750 nm. Double and triple junction nip solar cells on optimal FLiSS and with thin absorber layers were simulated. Results were in line with state‐of‐the‐art optical performance typical of solar cells with rough interfaces. After the optical optimization, a study of electrical performance was carried out by simulating current–voltage characteristics of nip solar cells on optimized FLiSS. Potential conversion efficiencies of 11.6%, 14.2%, and 16.0% for single, double, and triple junction solar cells with flat interfaces, respectively, were achieved. Copyright © 2012 John Wiley & Sons, Ltd.