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Journal of Nanomaterials
Volume 2015, Article ID 921903, 9 pages
http://dx.doi.org/10.1155/2015/921903
Research Article

Efficiency Enhanced Colloidal Mn-Doped Type II Core/Shell ZnSe/CdS Quantum Dot Sensitized Hybrid Solar Cells

1Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology (KTH), 106 91 Stockholm, Sweden
2Department of Applied Physics/Bimolecular Physics, School of Science, Royal Institute of Technology (KTH), 106 91 Stockholm, Sweden
3Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden

Received 15 June 2015; Revised 16 July 2015; Accepted 2 August 2015

Academic Editor: Jun Chen

Copyright © 2015 A. Jamshidi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Colloidal Mn-doped ZnSe/CdS core/shell quantum dots (QDs) are synthesized for the first time and employed as a strategy to boost the power conversion efficiency of quantum dot sensitized solar cells. By using Mn-doping as a band gap engineering tool for core/shell QDs an effective improvement of absorption spectra could be obtained. The mid-states generated by a proper Mn content alleviate carrier separation and enhance the electron injection rate, thus facilitating electron transport to the TiO2 substrate. It is demonstrated that a device constructed with 0.25% Mn-doped ZnSe/CdS leads to an enhancement of the electron injection rate and power conversion efficiency by 4 times and 1.3, respectively.

1. Introduction

Nanotechnology has led to huge progress in the use of semiconductor nanocrystals for applications in diverse areas like organic light emitting diodes [1], biosensing, biolabeling [2, 3], solar cells [46], and imaging and detection [7], to mention a few examples. The world energy demands for renewable and cheap resources of solar energy have generated a large interest in sensitized solar cell technology due to high power conversion efficiency with low cost of production [8, 9]. Among various kinds of sensitizers employed in sensitized solar cells, quantum dots (QDs) are regarded as promising candidates by virtue of their size-dependent optical and electronic properties, high light-absorption ability, photostability, and multiple exciton generation [10]. In particular, type II core/shell quantum dots are promising for efficient sensitization due to their long-time charge separation and possibility for electron confinement in the conduction band of the shell when their band structure is carefully designed [7, 11]. These nanoscale crystals are capable of integrating multistructures with different functionalization into a single nanoscale particle with controllable electronic structure for development of photovoltaic cells. In this heterostructure, the core and shell are made up of two different semiconductors, with a higher conduction (valence) band of the core than the conduction (valence) band of the shell (Scheme 1).

Scheme 1: Schematic illustration of charge transfer mechanism in Mn-doped ZnSe/CdS.

This offers charge carrier localization in two separate materials so that electrons and holes are confined in the shell and core, respectively. Moreover, the use of type II nanocrystals in solar cell applications leads to better power conversion efficiency compared to the corresponding nanocrystals made up entirely from the core or shell materials [12, 13]. Apart from the wide photon absorption range for type II QDs, the improvement also refers to an effective charge separation of electron-hole pairs in the type II nanostructures that facilitates electron abstraction from QDs, suppresses recombination, and therefore leads to better electron transportation [1416]. It follows that much recent research efforts have been devoted to the synthesis of different type II core/shell structured QDs, like TiO2/CdS [17], CdSe/ZnSe [18], CdTe/CdSe [1922], CdTe/CdS [23], CdSe/ZnTe [24], CdSe/ZnTe [25], ZnTe/CdSe [26, 27], CdS/CdSe [28], CdS/ZnSe [16, 18], and ZnSe/CdS [29], as well as to the use of such QDs in emerging technology for solar cell applications. As an example, theoretical calculations from density-functional and many-body perturbation theory show the conduction and valence band offsets of 0.66 and 0.32 eV for ZnSe/CdS, respectively [30]. Since the effective mass of the electron is lower than the hole in ZnSe [31, 32], one can expect efficient conductivity in the ZnSe/CdS interface particularly where the electron accumulation is made by doping a paramagnetic material like Mn with semifilled d orbital.

It has been shown that use of CdTe/CdSe core/shell nanocrystals prepared by the one-pot synthesis method without core seed purification could make structural and optical properties of nanocrystals comparable to the nanocrystals synthesized using purified core seed, which can give higher absorption and better crystallinity [33]. Neo et al. have investigated the effect of shell thickness and surface passivation as another strategy to improve the efficiency of type II PbS/CdS-based solar cells [34]. Metal ion doping as a band gap engineering tool has also been employed for improvement of type II-based solar cell performance because metal ions could make changes in the Fermi level, band gap, and conductance [35]. Particularly, Mn-doping was recently used in CdS/CdSe sensitized cells as a strategy to boost solar cell efficiency due to very long lifetime of Mn d−d transitions (-) [36]. Mn-doping in semiconductors like ZnS, ZnSe, CdS, and CdSe shows dramatic increase in lifetime due to the spin forbidden - transition of the Mn [3742]. In case of Mn-doped CdS, through doping with Mn2+, a pair of d bands ( and ) is inserted between the conduction and valence bands of CdS QDs, which alters the electron-hole separation and recombination dynamics, allowing the generation of long-lived charge carriers with microsecond-scale lifetime more slowly than in the case of undoped QDs [37]. Utilization and transfer (transfer of electron and hole toward TiO2 substrate and electrolyte, resp.) of these long-lived and special-separated carriers by band alignment engineering in core/shell structures should be a superior opportunity to design a QDSSCs device with higher power conversion efficiency. To the best of our knowledge, Mn-doped ZnSe/CdS has not been synthesized and investigated in quantum dots sensitized solar cells (QDSSCs). Since the lattice constant of the CdS shell is larger than that of the ZnSe core (5.82 Å and 5.67 Å for shell and core, resp.), CdS will become strained if it is carefully grown on ZnSe with a smaller lattice constant. In case of strain, both the direct and indirect band gaps (- and L-band, resp.) in the CdS shell are affected by the strain amount and type (compressive or tensile). As the direct band gap shrinks faster than the indirect one in a strained material, -type doping like Mn2+-doping could make -band transitions suitable for detecting the light [4345].

Here we present a colloidal synthesis of a novel type II Mn-doped ZnSe/CdS core/shell QD system as sensitizer with different Mn concentration (0–3%) and test its utilization in QD sensitized solar cells. This is the first time that Mn-doped ZnSe/CdS core/shell QDs are successfully synthesized by the method of hot injection and used as a strategy to boost solar cell efficiency. As explained in what is to follow it was found that a proper balance of Mn concentration could tailor the band gap and core/shell conduction band edge, causing a better electron transfer from QDs to the TiO2 photoelectrode, a broader absorption, and, consequently, higher solar cell efficiency.

2. Results and Discussions

Novel Mn-doped ZnSe/CdS QDs with different Mn concentration were synthesized by hot injection method. To investigate the effect of doping, all QDs were synthesized at the same conditions and with the same amount of ZnSe cores. To get precise amount of ZnSe core, they were synthesized in one batch and after washing by methanol and acetone, they were divided into several parts to be utilized as ZnSe core in ZnSe/CdS and Mn-doped ZnSe/CdS. To grow shell material onto ZnSe core, the mixture of S and (Mn-doped) Cd precursors was slowly injected into core solution at 240°C, followed by keeping 240°C for 20 min. Afterwards the reaction was cooled down to room temperature for purification and characterization. Transmission electron microscopy, operating in 100 KV, was employed to evaluate quality and size distribution of the nanocrystals. TEM images of ZnSe/CdS core/shell (Figure 1) demonstrate the size of core/shell to be 6-7 nm.

Figure 1: TEM image of colloidal ZnSe/CdS core/shell.

In order to measure the actual Mn concentration incorporated into the nanocrystals, inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Scientific ICAP 6500) was employed. Nanocrystals were digested completely with nitric acid (67%, 0.1 mM) and diluted with DI water to obtain 10 mL of clear solution for ICP-AES measurement [4648]. The results with details are listed in Table 1 where the real Mn/Cd molar ratio is confirmed.

Table 1: Summary of Cd and Mn concentration of nanocrystals.

After being successful in synthesizing the QDs, we prepared different photoanodes sensitized with these QDs: (A) ZnSe, (B) pure ZnSe/CdS and doped ZnSe/CdS with different concentration of Mn: (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%. Figure 2 shows the absorption of QDs-deposited photoelectrodes and fluorescence of QDs dispersed in chloroform. It was seen that pure ZnSe/CdS give broader absorption with a band edge up to 488 nm compared to ZnSe with 437 nm. It shows that both the absorption and emission peak wavelengths in type II ZnSe/CdS core/shell QDs are much longer than those of the pure core or shell materials, resulting from the fact that the energy states (conduction and valence band) for the photogenerated electrons and holes are located in CdS shell and ZnSe core, respectively.

Figure 2: Absorption of QDs-deposited photoelectrodes and photoluminescence of QDs: (A) ZnSe core, (B) pure ZnSe/CdS core/shell and Mn-doped ZnSe/CdS with concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.

In order to calculate the band gap of the nanocrystals, the absorption coefficient () was applied for near-edge optical absorption using the following equation [49, 50]:where is a constant and is the optical band gap. According to the graph in Figure 3 plotted for in terms of (), the band gap values of the nanocrystals were obtained from extrapolating the straight portion of the curve to zero absorption coefficients and they are listed in Table 2.

Table 2: Band gap values of nanocrystals derived from Figure 3.
Figure 3: Plot of versus for different QDs.

Therefore, the heterostructure band gap corresponds to the energy separation between the conduction band (CB) edge of the shell and the valence band (VB) edge of the core [11, 16], which results in a smaller band gap and a broader absorption spectrum compared to the ZnSe or CdS nanocrystals. Although there is a red shift for low concentration of Mn (0.25–1%), a blue shift is obtained for high concentration (2-3%). The red and blue shifts are attributed to the decrease and increase of the band gap (Table 2), respectively. The band gap decreased for these samples of (C)–(E) and increased for these samples of (F)-(G) due to the contribution of Mn as a metal dopant in CdS and the low solid solubility of Mn (<1%) in the CdS nanocrystals [5154]. It means that, below 1%, introduction of Mn could lead to decrease of the band gap but, after 1%, it leads to increase of the band gap because of the MnS phase.

To examine the performance of the devices, they were tested under AM 1.5 G simulated solar irradiation with intensity of 100 mW·cm−2. Figure 4 shows the - characteristics of solar cell devices sensitized with various QDs. The device parameters including the short-circuit current (), open-circuit voltage (), fill factor (), and power conversion efficiency () of all samples are summarized in Table 3.

Table 3: Summary of device parameters.
Figure 4: Current-voltage characteristics of the QDSSCs device sensitized with various nanocrystals: (A) ZnSe core, (B) pure ZnSe/CdS core/shell and Mn-doped ZnSe/CdS with concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.

The ZnSe/CdS core/shell shows much higher current than the ZnSe core which may be attributed to special carrier extraction and extension of the light-absorption range in type II heterostructures. Compared to previous work in which the CdS shell of the investigated ZnSe/CdS QDs was synthesized by deposition of Cd and S separately [11], in this study the colloidal ZnSe/CdS core/shell, of which the CdS shell was synthesized by direct deposition of mixture of Cd and S solution, shows higher current and voltage and more than 2 times higher efficiency of 0.47 V and 2.23 mA and 0.56%, respectively, indicating the better crystallization quality of CdS shell synthesized by direct deposition. The strain is another issue in heterostructures that can increase electron mobility [55, 56]. Since the crystal size of CdS bulk (5.82) is bigger than the ZnSe bulk (5.67 Å) [57], one can expect strain in the CdS crystals. Since the strain can lead to enhanced electron transport as reported, for instance, in [58], it can be expected that the electrons in the strained CdS shell are transported faster than in unstrained CdS.

A device sensitized with low Mn-doped ZnSe/CdS (0.25%) shows a dramatic increase in all device parameters with about 10 times and 3 times higher efficiency compared to ZnSe and ZnSe/CdS, respectively. Since all samples are coated with 2 cycles of a ZnS SILAR layer, which blocks the electron recombination with the electrolyte [59] and passivates defect states that can trap electron-hole pairs [60], we expect a stabilizing effect by this coating. In case of the Mn-doped ZnSe/CdS sensitized device, the open-circuit voltage, short-circuit current, fill factor, and consequently device efficiency decrease with increase of Mn concentration due to the decrease of absorption range and the increase in nanocrystal band gap (Figures 2 and 3). New energy states in the band gap of the nanocrystal can make the carriers better separated and decrease recombination [1214].

The best values of device parameters are found for the lowest Mn concentration, possibly due to lower carrier recombination induced by new states generated by proper concentrations of Mn. It should also be mentioned that, like Mn-doped GaAs [61], Mn atoms can occupy three types of sites in the lattice matrix. They can occupy the substitutional or interstitial Cd lattice sites to form S with the wurtzite lattice structure. Mn atoms can also precipitate out to form different phases, for example, MnS nanocrystals. Since the solid solubility of Mn in CdS is too low [5153] (<1%), less than 1% induces substitutional or interstitial sites and more than 1% induces other phases like MnS that can act as impurity and therefore decrease the device performance.

The Incident Photon to Current Efficiency (IPCE) of all devices was measured to evaluate the photocurrent response to incident light (shown in Figure 5). IPCE, sometimes referred to as external quantum efficiency (EQE), is expressed by the following equation [62]:where is the quantum yield, is the electron collection efficiency, and is the light harvesting efficiency at the wavelength of the incident light and is derived via

Figure 5: The IPCE spectra of devices based on (A) ZnSe, (B) pure ZnSe/CdS and Mn-doped ZnSe/CdS with Mn concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.

The power conversion efficiency is comparatively low for devices sensitized by ZnSe due to limited light absorption but increases in ZnSe/CdS. This is related to the broader absorption and the special carrier separation in this type II heterostructure. The observed rather dramatic increase in IPCE spectra of Mn-doped ZnSe/CdS compared to undoped ZnSe/CdS may be due to a more efficient electron collection and electron injection efficiency. It is worthwhile to stress that band structure and CB manipulation via band gap engineering (Figures 2 and 3) by proper Mn concentration doped into the nanocrystals not only give superior ability to enhance light absorption but also can facilitate carrier transfer, which may be responsible for the improvement of the device efficiency. However, higher doping ratios may generate impurities or defects like MnS, which can increase charge recombination and inhibit electron injection into the TiO2 substrate.

The excited states dynamics of QDs were further investigated by time-resolved fluorescence lifetime measurements (experimental details in the supporting information in Supplementary Material available online at http://dx.doi.org/10.1155/2015/921903). Figures 6(a) and 6(b) show the fluorescence emission decay of different QDs deposited on insulator glass and conductive glass/TiO2, respectively. The fluorescence intensity decay curves recorded from QDs were successfully fitted to a three-exponential decay model. The fitting parameters are summarized in Table S1 (supporting information). The average lifetime of QDs was estimated based on [63]where is the th exponential component. In our procedure, a triexponential decay model () was found to be necessary to properly fit the experimental fluorescence decay curves [64, 65], and no substantial improvement in the goodness-of-fit was obtained for higher . (See fitting residuals in Figures S1 and S2 in the supporting information.)

Figure 6: Time-resolved fluorescence intensity decay curves of QDs onto (a) insulator and (b) TiO2 film; QDs: (A) ZnSe, (B) pure ZnSe/CdS and Mn-doped ZnSe/CdS QDs with Mn concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.

It is found that the average fluorescence lifetime of ZnSe/CdS QDs (49 ns) is much longer than that of ZnSe (2 ns) when they are attached to the insulator. This is due to special charge separation in type II nanostructures that can decrease their wave function overlap and delay radiative recombination, resulting in a long fluorescence lifetime [11]. It was mentioned that photogenerated holes can be moved to the valence band of ZnSe and that photogenerated electrons will be localized in CdS. When the QDs are attached to the insulator, there is no electron injection, but if deposited to the TiO2, another electron deactivation route is created that causes electrons to be injected from the QDs into the TiO2 with lower conduction band energy, which results in a decrease of the lifetime. The corresponding electron injection rate constant can be estimated from [64, 65]where and are the fluorescence lifetimes of the QDs attached to the TiO2 and insulator glass, respectively. As it is shown in Table S1, a higher injection rate constant of the QDs is ascribed to the lower Mn-doped type II ZnSe/CdS QDs that are responsible for the efficiency of the device.

To get more understanding of the charge transport in type II ZnSe/CdS QDs with different Mn concentration, a physical model like Scheme 1 is considered. The ZnSe as core is covered by the CdS shell and the conduction bands and valence bands edge of TiO2, ZnSe core, and CdS shell are formed while considering the quantum confinement effect in the type II heterostructure. Coupling of the ZnSe and CdS moieties leads to special carrier separation and interfacial wave function interaction increased by increasing the Mn concentration. The results indicate that by increasing Mn content until 1% not only is the band gap decreased, which results in special indirect band gap with some red shift of the absorption band edge, but also the device parameters are increased because of lower recombination and enhancement of the electron injection rate. It is notable that the Mn mid-state between CdS CB and TiO2 CB can facilitate electron transfer from QDs into TiO2. Further amount of doping can reverse the results (not shown here).

3. Conclusions

Novel colloidal Mn-doped ZnSe/CdS core/shell QDs with various Mn concentrations were successfully synthesized and applied to sensitized solar cells. It was demonstrated that QDs with proper Mn-doping could cause an increase in the absorption spectra and red shift in the absorption band edge and in the photoluminescence emission peak. The mid-states generated by Mn can facilitate electron transfer from the QDs to the TiO2 substrate. With superior light absorption, better carrier separation, and efficient electron injection rate a power conversion efficiency of 1.27% is presented, which is about 2 and 3 times larger than those of core and undoped QDs sensitized solar cells, respectively. The present work suggests that band structure manipulation in type II core/shell nanostructure offers an effective way to improve light harvesting and control of charge transfer via efficient charge separation in sensitized solar cells and that Mn-doping opens a new window to increase device efficiency. There is a multitude of lines of research to embark upon for future improvement of QDs sensitized solar cell of the kind proposed here. One such line of research concerns surface passivation. In fact, the organic ligands used can act as dangling bonds on the QDs surface and trap carriers, which calls for surface passivation of the QDs as a way to improve device performance, for instance, by using hydrophilic materials or ions. These and other measures will be pursued in our future studies.

4. Experimental

4.1. Quantum Dots Synthesis

Zinc stearate, gray selenium, cadmium oxide, sulfide, and manganese nitrate tetrahydrate were bought from Aldrich Company as precursors for Zn, Se, Cd, S, and Mn sources, respectively. The targeted heteronanocrystals are fabricated by a two-step synthesis composed of the fabrication of ZnSe core nanoparticle followed by a deposition of pure or Mn-doped CdS shell. Synthesis of both ZnSe core and pure or Mn-doped ZnSe/CdS core/shell under different Mn concentrations is based on previously published procedures [16, 29] with some modifications briefly explained as follows.

Firstly, in order to synthesize ZnSe core nanocrystals from organic solution, specific amounts of selenium 0.0118 g and 0.4 mL of trioctylphosphine (TOP) were placed in a one-necked flask while stirring to make selenium dissolved in TOP. The reaction was conducted under nitrogen atmosphere. When the mixture became clear, the solution was kept in a clean syringe to be used in the next step. 0.0950 g zinc stearate, 0.1874 g stearic acid, and 2 mL of octadecane (ODA) were mixed together in a 25 mL three-necked flask. The mixture was slowly heated to 120°C (about 2°C/min) while stirring and pumping to remove additional elements from solution. The mixture was then heated to 240°C under nitrogen flow to make zinc dissolved in ODE where the solution appeared colorless and clear. Then a selenium stock solution prepared in the last step was swiftly injected into the reaction flask. The solution temperature was controlled and monitored to be kept at about 280°C. After 20 min, the reaction solution was cooled down to 60°C and 5 mL of chloroform was added to the solution to allow the quantum dots to be dissolved and suspended. The product of this step was ZnSe core, which contains byproducts and free ligands. To purify, it was washed 4 times by acetone and methanol. Typically, 5 mL of chloroform, 10 mL of acetone, and 2 mL of methanol were slowly added to the QDs solution followed by centrifugation for 3 min at 12400 rpm. The upper colorless layer was removed and QDs precipitated on the bottom were dissolved in chloroform for the next washing. Monodisperse ZnSe core produced in this step was sealed and kept in a cool and dark area to be used for shell deposition.

For preparing the CdS shell, three steps were considered. First, totally 0.0050 g of sulfur was dissolved in 3 mL of ODE while pumping and heating slowly to 80°C and then cooled to room temperature. Second, totally 0.0190 g of cadmium oxide (in case of Mn-doping, a proper amount of Mn was added to Cd precursor) was mixed in 0.4 mL of oleic acid (OA) and 3.5 mL of ODE in a 25 mL flask while stirring and heating slowly to 100°C. After degassing process, the temperature was increased to 280°C to make Cd dissolved completely in the solution. When the solution became clear, it was cooled down to 60°C. Third, mix these two precursor solutions (S and Cd) together as a source for shell growth. The final mixture was slowly injected (3 mL/h) into the reaction vessel, which contained 375 g ODA and ZnSe core dispersed in 2 mL of ODE at 240°C. After 20 min, the reaction was terminated and cooled down to room temperature for purification by chloroform and acetone.

4.2. Photoanode Preparation

FTO (fluorine tin oxide) conductive glasses (Sigma Aldrich, sheet resistance of 7 Ω/sq) were used as photoelectrode substrates. FTO glasses were cut to targeted size (1 × 2 cm) and sequentially washed by soap, KOH dissolved in 2-propanol, acetone, ethanol, and DI water via sonication for 30 min for each washing step, followed by immersion in 40 mM TiCl4 solution at 80°C for 40 minutes to give a TiO2 blocking layer. Mesoporous TiO2 layers were prepared by deposition of three transparent layers and one scattering layer with commercial TiO2 pastes (Ti-Nanoxide T/SP with 20 nm and Ti-Nanoxide R/SP with > 100 nm particle size for transparent and scattering layer, resp.) by screen-printing technique concluding by heating at 70°C for each layer deposition. The samples were postannealed at 500°C for 30 min to make the layers porous by removing the organic part at high temperature.

QDs dissolved in chloroform were deposited drop by drop on TiO2 films and left to be dried in the air. Excess QDs not adsorbed on TiO2 were washed by chloroform. To passivate QDs, ZnS block layers were deposited by two cycles of SILAR method. Each cycle contains dipping the samples into sulfur solution (Na2S dissolved in DI) for 1 min followed by washing with DI water and zinc solution (zinc nitrate hydrate dissolved in DI) for 1 min followed by washing with DI water.

4.3. Counter Electrode Preparation

Cu2S counter electrode was prepared by dipping brass sheet (Sigma Aldrich, resistivity of 1.673 μΩ·cm) in hydrochloric acid at 80°C for 10 minutes and then washing with DI water and methanol.

4.4. Device Assembly

Both photoelectrode and Cu2S counter electrode prepared in the last steps were stocked together by cell spacer while the electrodes were heating at 80°C. The electrolyte (2 M S, 2 M Na2S, and 0.2 M KCl in a methanol-water (v/v, 3/7) solution) was finally injected into the cell through the hole preconstructed on top of the brass sheet. After electrolyte injection, the hole was sealed by sealing tape for the next electrical characterization.

4.5. Fluorescence Lifetime Measurements by Time-Correlated Single-Photon Counting

Time-correlated single-photon (TCSPC) measurements were performed on a spectrofluorometer with a TCSPC option (FluoroMax3, Horiba Jobin Yvon). A NanoLED (Horiba Jobin Yvon) emitting at 488 nm with a repetition rate of 1 MHz and pulse width of 1,4 ns was used as an excitation. Measurements were stopped when 3000 photon counts were collected in the peak channel using 2048 channels with 0,4 ns/channel. The instrument response function was recorded using a 2% Ludox (Sigma Aldrich) solution. To avoid reabsorption and reemission effects and also not to saturate the detectors, the sample concentration was kept strictly below 5 μM. Recorded curves were fitted with a three-exponential decay using the software Decay Analysis Software v.6 (IHB).

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors acknowledge Jing Huang for helpful advice. This work was supported by a grant from the Swedish Science Research Council (Contract G21-2012-3347).

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