- Nano Express
- Open Access
Hybrid solar cell on a carbon fiber
© Grynko et al. 2016
- Received: 11 March 2016
- Accepted: 5 May 2016
- Published: 23 May 2016
In this work, a method to assemble nanoscale hybrid solar cells in the form of a brush of radially oriented CdS nanowire crystals around a single carbon fiber is demonstrated for the first time. A solar cell was assembled on a carbon fiber with a diameter of ~5–10 μm which served as a core electrode; inorganic CdS nanowire crystals and organic dye or polymer layers were successively deposited on the carbon fiber as active components resulting in a core-shell photovoltaic structure. Polymer, dye-sensitized, and inverted solar cells have been prepared and compared with their analogues made on the flat indium-tin oxide electrode.
- Carbon fiber
- CdS nanowire
- Flexible nanobrush
- Hybrid solar cells
The advantages of the nanowire (NW) or nanorod morphology compared to the textured or flat one have been proved in our previous works [2–5]. In particular, application of semiconductor NW arrays in solar cells leads to better light absorption due to the reduced reflection and stronger light trapping (so-called shadow effect) and improvement of charge collection from the active layer, since charge carriers move straight to the respective electrode through a NW crystal .
Among typically used inorganic NW components of the hybrid PV cells, CdS nanocrystals and their aligned arrays have attracted much attention due to their relative cheapness, comparatively easy preparation, and adhesion to different surfaces as well [7, 8]. Over the past few years, tremendous efforts have been made to decrease the size and to control the shape of CdS nanocrystals, and a number of new methods were reported for the synthesis of CdS nanocrystals integrated into low-dimensional nano- and microstructures [9, 10]. Specifically, a significant extension of surface area of a NW CdS array can be obtained by using the branched morphology. This approach has resulted in effective application of branched and hyper-branched semiconductor nanocrystals in energy conversion devices , sensors , electronic logic gates , etc. The advantage of the branched morphologies stems from an increased surface of such nanocrystals and therefore from higher contribution of processes at the interface of the nanocrystal and the environment. Therefore, the modern trends in photovoltaics reveal the gradual changes from the flat morphology of hybrid heterojunctions to the textured one and to the branched NW configuration (Fig. 1).
Overview of some recent results in the world for PV cells based on CdS.
Composition of hybrid heterojunction
Morphology of heterojunction (HJ)
Nanorod core—polymer shell HJ
Y.Guo et al., J. Phys. Chem. Lett.1 (2010) 327
S.A. Yuksel et al., Thin Solid Films 540 (2013) 242.
S.Ren et al., Nano Letters, 11 (2011) 3998
Y.Kang, D.Kim, Sol. Energ. Mat. Sol. Cells 90 (2006) 166
NW array—polymer BHJ
X.Jiang et al., Sol. Energ. Mat. Sol. Cells 94 (2010) 338.
NW array—polymer BHJ
L. Wang et al., J. Phys. Chem. C 111(2007) 9538
NW array—polymer BHJ
J.-C. Lee et al., Electrochem. Commun. 11 (2009) 231.
Dye-sensitized polycrystalline film
M. Zhong et al., Sol. Energ. Mat. Sol. Cells 96 (2012) 160.
NW array loaded with dye in DSSC
0.12 × 10−5
B. Sankapal et al., J. Alloy. Compd. 651 (2015) 399.
Nanorod core-shell HJ
J.Tang et al., Nature Nanotechnol. 6 (2011) 568.
Nanorod core-shell HJ
Z. Fang et al., Nature Mater. 8 (2009) 648
Nanorod core-shell HJ
W.-C. Kwak et al., Cryst. Growth Des. 10 (2010) 5297.
In this work, we discuss the original method of design of nanoscale hybrid solar cells of four different types based on a single carbon fiber (CF) as a core electrode supporting active layers of the developed solar cell in the form of subsequent shells. It should be noted that fiber-shaped solar cells have attracted great attention recently in view of their potential integration into large-scale and low-cost textile and wearable electronic devices. It has been shown that carbon-based materials can be used both as a core electrode [17, 18] and as a counter-electrode [19–22] in respective solar cells. In this work, we use a novel approach to construct a shell in the form of a brush of radially oriented CdS NWs around a single CF. In such a cell, the flexible hybrid core-shell CF-CdS nanobrush serves as the inorganic acceptor component, whereas organic shell of Zn phthalocyanine (ZnPc) or poly(3-hexylthiophene) (P3HT) was used as a donor light-absorbing overlayer.
Single carbon fibers (CFs, diameter was about 5–10 μm and the length 15–30 mm) were taken out of the commercial carbon cloth LU-3 (Ukraine) produced by carbonization of polyacrylonitrile cloth and characterized with Young’s modulus E~250 GPa and tensile strength 2.5–3.0 GPa. The CF separation was handled using an optical microscope with the help of a homemade microinstrument. Resistivity of CF was ca. 3.8 · 10−3 Ohm · cm. Studies on fiber flexibility showed that a single CF could be bent at 180° without fiber cracking when the bent radius is as small as 500 μm. CdS powder of the chemical grade (purity 99.998 %) was used for the growth of NW crystals on the surface of a single CF. Zinc 2,9,16,23-tetra-tert-butyl-29H, 31H-phthalocyanine (ZnPc-4R) (Sigma-Aldrich) or P3HT (Rieke Metals) served as organic donor counterparts. To prepare the top electrode poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (Aldrich) has been drop-cast from the 1.3 wt.% water dispersion.
Preparation of the CdS/CF Nanobrush Structure
Solid-State Dye-Sensitized Solar Cell (SSDSSC)
A layer of ZnPc-4R in 1 wt% ethanol was drop cast onto the CdS nanobrush surface to prepare the core-shell CF/CdS/ZnPc-4R active heterojunction layer followed by deposition of the PEDOT:PSS top electrode for the SSDSSC formation. Indium pads with corresponding leads were clamped directly to a metallic holder attached to CF and to PEDOT:PSS as counter-electrode, respectively, for the electrical measurements.
Electrochemical Dye-Sensitized Solar Cell (DSSC)
A drop of Na2S-S electrolyte (10 μl) was drop-cast onto the same CF/CdS/ZnPc-4R nanobrush structure and covered by glassy carbon electrode via 15 μm PTFE spacer.
Solid-State Polymer-Sensitized Solar Cell (SSPSSC)
P3HT from 1 wt% chlorobenzene solution was drop-cast on the CF/CdS nanobrush array followed by annealing during 10 min (110 °C) under argon atmosphere. It should be noted that the polymer deposition can somewhat affect the fragile CdS NWs (Fig. 2). The analogous PV cells have been prepared on the flat indium-tin-oxide (ITO) surface by a similar successive deposition of CdS NW array, organic overlayer, and PEDOT:PSS electrode to compare PV performance of the cells of the different geometry and similar CdS NW arrays.
Inverted Solar Cell (ISC)
P3HT and fullerene derivative [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) mixture (1:2 molar ratio) from 1 wt% chlorobenzene solution was drop-cast on the CF/CdS nanobrush array followed by annealing during 10 min (110 °C) under argon atmosphere. The analogous PV cells have been prepared on the flat ITO surface by a similar successive deposition of CdS NW array, organic overlayer, and PEDOT:PSS electrode to compare PV performance of the cells of the different geometry and similar CdS NW arrays.
where k is the Boltzmann constant, e the electron charge, and T the absolute temperature.
The major charge carrier type in CF was determined by the hot-probe experiment. In this method, a bundle of CFs was fixed by two crocodile clamps and then heated from the one side while measuring the potential difference between the clamps. Depending on the sign of the potential difference, the main type of charge carriers was determined.
Morphologies of the samples were studied by scanning electron microscopy (SEM) using JEOL JSM35, JXA-8200 instruments and by optical microscope ULAB XY-B2.
The needle-like crystals of CdS were obtained at the surface of the CF (Fig. 2). Their growth mechanism was discussed in detail elsewhere . Particularly, it was shown that the formation of CdS NWs on the CF proceeds through vapor-solid (VS) mechanism due to adsorption of the reactive gas phase directly on dangling bonds, polar groups, defects, etc., along the CF surface. Based on this approach, a rather robust contact of the CdS nanocrystals to CF can be formed, resulting in a quasi-ohmic behavior of the CF/CdS NW heterostructure. From the SEM images of the heterostructures made (Fig. 2), it was estimated an average diameter and length of the needle-like CdS crystals to be 300–700 nm and up to 10 μm, respectively; in some cases, the crystal length extended up to 50 μm. The estimated surface density was several CdS NW crystals per square micron.
Comparison of PV performance of the different types of CdS NW array solar cells prepared on CF core and flat ITO electrode, respectively.
Type of solar cell
Organic material used
V oc, V CF core-shell/flat geometry
I sc, μA/cm2 CF core-shell/flat geometry
FF CF core-shell/flat geometry
PCE, % CF core-shell/ flat geometry
3.8 · 10−4/1.0 · 10−6
1.4 · 10−4/1.1 · 10−2
1.0 · 10−5/1.0 · 10−2
1.5 · 10−2/3.3 · 10−2
Analysis of I-V characteristics was performed for the related SSDSSC and DSSC structures (which have the same charge generating CF/CdS/ZnPc-4R heterostructure), which is illustrated in Figs. 4 and 5. In most cases, I-V curves can be described by power dependence behavior, excepting the currents under illumination in the range from 0 to 0.2 V, where the I-V curve can be described by exponent with γ(V) = 1 and α(V) = 12 V (Fig. 4c, Eqs. (3) and (5)).
Formally, we can describe the I-V curve also by exponent with γ(V) = 1 and α (V) = 3.95 V in the range between 0.2 and 0.5 V (Fig. 5b) with negative potential on CF and between 0.5 and 0.7 V with positive potential on CF under illumination and in the dark (Fig. 4c). The large part of α curves follows the power dependence with α = 2 (Fig. 4b, e) that corresponds to monomolecular recombination with p >> n, i.e., in the structure the concentration of injected charge carriers is not enough for bimolecular recombination with p ≈ n which corresponds to α = 1.5. The regime of bimolecular recombination is absolutely necessary for solar cells, where both types of charge carriers contribute equally to the photocurrent. Such a behavior can be seen in a very small range of voltages from 0.1 to 0.2 V (Fig. 4b, e). Here, the formal approximation by exponent with α = 3.95 (Fig. 4b) or even with α = 12 gives ideality factor (according to Eq. (6)) η = 9.77 and η = 3.21, respectively, that is far from ideal diode characteristic. So, for better PV performance of this type of structure, it would be necessary, first, to improve the injection of both types of charge carriers from the contacts and, second, to decrease bulk resistance that will improve the ideality factor of the structure.
In the case of the DSSC structure (Fig. 5), the current behavior with negative potential on CF can be described by exponential function in the range from 0 to 0.1 V. Here the I-V curve can be described by exponent with γ(V) = 1 and α(V) = 35 V in the dark and α(V) = 32 V under illumination. It gives the ideality factor (according to Eq. (3)) η = 1.10 and η = 1.2, respectively, which is quite close to the ideal diode I-V characteristics described by Eq. (2). Then, the current behavior follows power function with α = 3, that corresponds to high injection into the dielectric medium when concentration of injected charge carriers is much more than the bulk one [25, 29]. Thus, injection behavior in this case is better than in SSDSSC (Fig. 4). With positive potential on CF (Fig. 5d), there are saturation regions with α = 0.5. Therefore, in this case, there is almost an ideal diode behavior of I-V curves. In order to improve PV performance of this structure, therefore, a substantial decrease of bulk resistance is necessary.
The above analysis of the I-V curves suggests that in the both nanobrush structures (SSDSSC and DSSC), the interface of CdS NWs with ZnPc-4R plays the major role in generation of photoexcited charge carriers. This suggestion agrees well with the fact that at high voltages, the difference between dark and light currents is very small (Figs. 4 and 5). At the same time, the notable photosensitivity is observed only at small voltages (up to ~0.08 V). Therefore, despite better performance of the SSDSSC found in this study, the DSSC structure displays better PV behavior than the SSDSSC from the viewpoint of charge injection and therefore it could be more attractive for further improvement in PV application.
It should be noted that here we demonstrate only a proof of concept of new nanoscale solar cells using a commercial carbon textile (i.e., carbon cloth in our case). Naturally, the cells should be optimized because there are several factors which limit their performance. First, application of CF itself implies that its work function should be consistent with the energy levels of other materials of the solar cell assembly. Particularly, the difference in the work function of the anode and the cathode is the driving force (the built-in potential) in BHJ solar cells which moves the electrons and holes in the opposite directions. The exact work function of CF is not known but it should be close to the other carbon-related materials. It is known, for example, that the work function of carbon nanotubes is about 5.0 eV, while that of highly oriented pyrolytic graphite is 4.8–4.9 eV [30, 31]. Therefore, one can suggest that the CF material has work function around 5 eV as well. But this value is close to that of PEDOT:PSS (5.0 eV) which is the counter electrode in the above system. Therefore, there is a very small driving force which separates electrons and holes in the PV cell of that type, which can be the reason of the observed small open-circuit voltages, respectively. Moreover, the hot-probe experiment has revealed that the major charge carriers in the used CF are holes that can worsen collection of electrons from CdS and therefore, in general, leads to disadvantages in charge collection since the major charge carriers in the PEDOT:PSS counter electrode are holes as well. On the other hand, this situation suggests a possibility to solve the above problem and to improve the PV performance through replacement of CdS by p-type semiconductor, for example CdTe, along with the change of PEDOT:PSS counter electrode by that possessing a low work function and electrons as the major charge carriers.
The other drawback of the above system is the loosely distributed CdS array on the CF surface, possessing large pores, which allows for polymer to penetrate deeply into the CdS nanobrush structure (see insert in Fig. 3) and to contact with the CF electrode and thus to contribute to undesirable current leakage. This drawback is clearly seen upon comparison of performance of solar cells prepared on the CF and on the flat ITO electrode. The latter geometry provides tighter CdS layer and better solution of the shortcutting problem which is particularly important in cells where the polymer layer (SSPSSC and ISC) or a liquid contact (DSSC) is used. As a result, a significant increase in the open-circuit voltage can be achieved (Table 2). Therefore, deposition of a tighter CdS shell layer around the CF core electrode is necessary to solve the above problem. Finally, there is a problem with the reliable contact between the organic layer and the top PEDOT:PSS electrode. We have revealed that casting of the PEDOT:PSS electrode from a solution leads to shortcutting problem, but the mechanically pressed PEDOT:PSS film onto the top of the assembly does not provide a tight contact. The above contact problems affect reproducibility of the device substantially, with variation of photocurrent within one order of magnitude depending on the contact quality (cast or pressed, etc.).
We hope that the future solution of the above problems will result in the flexible fiber-based PV cell possessing a better performance.
In this work, we have demonstrated the textile-based hybrid solar cells using inorganic CdS nanocrystals and organic dye or polymer as photoactive components. As a textile component, the conductive CF taken from the carbon cloth was used. We have showed that a single CF can serve as an aligned core electrode for the growth of CdS NW array followed by deposition of organic donor layer (ZnPc-4R, P3HT or P3HT:PCBM) resulting in active BHJ layers in new micron-sized core-shell PV structures.
It was found that behavior of charge carriers in the SSDSSC structure obeys mainly the power-law dependence with α = 2 that corresponds to the first-order (monomolecular) recombination with p >> n, that means that concentration of the injected minority charge carriers is not enough in the structure. In the case of the DSSC structure, the charge carriers behavior follows the cubic dependence with α = 3. In both structures based on hyperbranched CdS nanobrushes, the interface of CdS nanowires with ZnPc-4R plays the main role in formation of photo-generated charge carriers.
Analysis of the I-V curves allowed us to suggest the ways of optimization of the above PV structures, namely, to substantially decrease bulk resistance in SSDSSC and DSSC and to improve injection of both types of charge carriers from the contacts in case of SSDSSC. In SSPSSC and ISP, the use of the polymer layer requires a tighter CdS layer around the core CF electrode to escape shortcutting problems. The replacement of CdS for the p-type semiconductor would be useful as well for the future experiments.
Although a great work in order to get better performance of the respective PV cells should be undertaken, in principle, the developed technology can be considered as a major step towards “photovoltaics on curtains” .
This publication is based on work supported by Award No. UKE2-7035-KV-11 of the U.S. Civilian Research & Development Foundation (CRDF).
DAG and ANF performed the synthesis of CdS nanowires on CF, NAO performed the organic layer deposition, OPD carried out the photoelectrical measurements, PSS carried out the treatment and interpretation of current-voltage characteristics, and OPD and AAP prepared the manuscript. All authors took part in the discussion of the results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Nozik AJ, Beard MC, Luther JM, Law M, Ellingson RJ, Johnson JC (2010) Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to third-generation photovoltaic solar cells. Chem Rev 110:6873–6890View ArticleGoogle Scholar
- Grynko DO, Fedoryak OM, Smertenko PS, Ogurtsov NA, Pud AA, Noskov YV, Dimitriev OP (2013) Application of CdS nanostructured layer in inverted solar cells. J Phys D Appl Phys 46:495114View ArticleGoogle Scholar
- Grynko DO, Fedoryak OM, Smertenko PS, Ogurtsov NA, Pud AA, Noskov YV, Dimitriev OP (2014) Hybrid solar cells based on CdS nanowire arrays. Adv Mat Res 854:75–82View ArticleGoogle Scholar
- Grynko DO, Fedoryak OM, Smertenko PS, Ogurtsov NA, Pud AA, Noskov YV, Dimitriev OP (2015) Multifunctional role of nanostructured CdS interfacial layers in hybrid solar cells. J Nanosci Nanotechnol 15:752–758View ArticleGoogle Scholar
- Kislyuk VV, Dimitriev OP (2008) Nanorods and nanotubes for solar cells. J Nanosci Nanotechnol 8:131–148View ArticleGoogle Scholar
- Garnett EC, Brongersma ML, Cui Y, McGehee MD (2011) Nanowire solar cells. Annu Rev Mater Res 41:269–295View ArticleGoogle Scholar
- Buonsanti R, Carlino E, Giannini C, Altamura D, De Marco L, Giannuzzi R, Manca M, Gigli G, Cozzoli PD (2011) Hyperbranched anatase TiO2 nanocrystals: nonaqueous synthesis, growth mechanism, and exploitation in dye-sensitized solar cells. J Am Chem Soc 133:19216–19239View ArticleGoogle Scholar
- Wang K, Qian XM, Zhang L, Li YG, Liu HB (2013) Inorganic-organic p-n heterojunction nanotree arrays for a high-sensitivity diode humidity sensor. ACS Appl Mater Interfaces 5:5825–5831View ArticleGoogle Scholar
- Chen N, Chen S, Ouyang C, Yu Y, Liu T, Li Y, Liu H, Li Y (2013) Electronic logic gates from three-segment nanowires featuring two p–n heterojunctions. NPG Asia Mater 5:e59View ArticleGoogle Scholar
- Grynko DA, Fedoryak AN, Dimitriev OP, Lin A, Laghumavarapu RB, Huffaker DL (2013) Growth of CdS nanowire crystals: vapor–liquid–solid versus vapor–solid mechanisms. Surf Coat Techn 230:234–238View ArticleGoogle Scholar
- Grynko DA, Fedoryak AN, Dimitriev OP, Lin A, Laghumavarapu RB, Huffaker DL, Kratzer M, Piryatinski YP (2015) Template-assisted synthesis of CdS nanocrystal arrays in chemically inhomogeneous pores by vapor-solid mechanism. RSC Adv 5:27496–27501View ArticleGoogle Scholar
- Jie J, Zhang W, Bello I, Lee CS, Lee ST (2010) One-dimensional II–VI nanostructures: Synthesis, properties and optoelectronic applications. Nano Today 5:313–336View ArticleGoogle Scholar
- Li H, Wang X, Xu J, Zhang Q, Bando Y, Golberg D, Ma Y, Zhai T (2013) One-dimensional CdS nanostructures: a promising candidate for optoelectronics. Adv Mater 25:3017–3037View ArticleGoogle Scholar
- Smertenko PS, Kostylev VP, Kislyuk VV, Syngaevsky AF, Zynio SA, Dimitriev OP (2008) Photovoltaic cells based on cadmium sulphide–phthalocyanine heterojunction. Sol Energ Mater Sol Cells 92:976–979View ArticleGoogle Scholar
- Grynko DO, Kislyuk VV, Smertenko PS, Dimitriev OP (2009) Bulk heterojunction photovoltaic cells based on vacuum evaporated cadmium sulfide–phthalocyanine hybrid structures. J Phys D Appl Phys 42:195104View ArticleGoogle Scholar
- Kislyuk VV, Fedorchenko MI, Smertenko PS, Dimitriev OP, Pud AA (2010) Interfacial properties and formation of a Schottky barrier at the CdS/PEDOT:PSS hybrid junction. J Phys D Appl Phys 43:185301View ArticleGoogle Scholar
- Xu W, Choi S, Allen MG. Hairlike carbon-fiber-based solar cell. Proc. IEEE Int. Conf. MEMS. 2010; 1187-90Google Scholar
- Unalan HE, Wei D, Suzuki K, Dalal S, Hiralal P, Matsumoto H, Imaizumi S, Minagawa M, Tanioka A, Flewitt AJ, Milne WI, Amaratunga GAJ (2008) Photoelectrochemical cell using dye sensitized zinc oxide nanowires grown on carbon fibers. Appl Phys Lett 93:133116View ArticleGoogle Scholar
- Pan S, Yang Z, Li H, Qiu L, Sun H, Peng H (2013) Efficient dye-sensitized photovoltaic wires based on an organic redox electrolyte. J Am Chem Soc 135:10622–10625View ArticleGoogle Scholar
- Zhang Z, Chen X, Chen P, Guan G, Qiu L, Lin H, Yang Z, Bai W, Luo Y, Peng H (2014) Integrated polymer solar cell and electrochemical supercapacitor in a flexible and stable fiber format. Adv Mater 26:466–470View ArticleGoogle Scholar
- Chen T, Wang S, Yang Z, Feng Q, Sun X, Li L, Wang ZS, Peng H (2011) Flexible, light-weight, ultrastrong, and semiconductive carbon nanotube fibers for a highly efficient solar cell. Angew Chem Int Ed 50:1815–1819View ArticleGoogle Scholar
- Zhang Z, Li X, Guang G, Pan S, Zhu Z, Ren D, Peng H (2014) A Lightweight polymer solar cell textile that functions when illuminated from either side. Angew Chem Int Ed 53:11571–11574View ArticleGoogle Scholar
- Smertenko PS, Grynko DA, Osipyonok NM, Dimitriev OP, Pud AA (2013) Carbon fiber as a flexible quasi-ohmic contact to cadmium sulfide micro- and nanocrystals. Phys Stat Solidi A210:1851–1855Google Scholar
- Smertenko P, Fenenko L, Brehmer L, Schrader S (2005) Differential approach to the study of integral characteristics in polymer films. Adv Colloid Interface Sci 116:255–261View ArticleGoogle Scholar
- Ciach R, Dotsenko YP, Naumov VV, Shmyryeva AN, Smertenko PS (2003) Injection technique for study of solar cells test structures. Sol Energ Mater Sol Cells 76:613–624View ArticleGoogle Scholar
- Luka G, Kopalko K, Lusakowska E, Nittler L, Lisowski W, Sobczak JW, Jablonski A, Smertenko PS (2015) Charge injection in metal/organic/metal structures with ZnO:Al/organic interface modified by Zn1−x Mg x O:Al layer. Org Electron 25:135–142View ArticleGoogle Scholar
- Wilcoxon JP (2000) Catalytic photooxidation of pentachlorophenol using semiconductor nanoclusters. J Phys Chem B 104:7334–7343View ArticleGoogle Scholar
- Chae WS, Ko JH, Choi KH, Jung JS, Kim YR (2010) Photocatalytic efficiency analysis of CdS nanoparticles with modified electronic states. J Anal Sci Technol 1:25–29View ArticleGoogle Scholar
- Baron R, Mayer JW. Double injection in semiconductors and semimetals. In: Willardson RK, Beer RC, editors. V 6. New York & London: Academic Press; 1970. p. 201-313Google Scholar
- Shiraishi M , Ata M. Work function of carbon nanotubes. Carbon. 2001; 31913-7.Google Scholar
- Tipler PA, Lewellyn RA (2008) Modern physics, 5th edn. W.H. Freeman, New YorkGoogle Scholar
- Fan Z, Javey A (2008) Photovoltaics: solar cells on curtains. Nature Mater 7:835–836View ArticleGoogle Scholar