Improvement of the physical properties of ZnO/CdTe core-shell nanowire arrays by CdCl2 heat treatment for solar cells
© Consonni et al.; licensee Springer. 2014
Received: 5 February 2014
Accepted: 22 April 2014
Published: 7 May 2014
CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl2 heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl2 heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl2 heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl2 heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells.
KeywordsZnO/CdTe Nanowire arrays CdCl2 Heat treatment Solar cells
Increasing interest has been devoted to core-shell semiconductor nanowires (NWs) over the past years due to their potential use in energy-harvesting devices such as nanostructured solar cells[1, 2]. Semiconductor NWs are expected to offer an efficient charge carrier transport and collection, thanks to their very high crystalline quality. The core-shell NW heterostructure can also benefit from the charge carrier separation over a small distance of the NW diameter. Furthermore, the NW arrays can act as a photonic crystal, which in turn improves significantly light absorption and trapping.
Owing to its wide bandgap energy of 3.3 eV at room temperature, high exciton binding energy of 60 meV, and high electron mobility, increasing efforts have been dedicated to the development of ZnO nanostructures[3, 4]. The ability of ZnO to grow as NWs by a wide variety of chemical deposition techniques such as metalorganic or standard chemical vapor deposition[5, 6], electrodeposition, and chemical bath deposition (CBD)[8, 9] is very attractive. ZnO NWs have therefore emerged as promising building blocks for nanostructured solar cells such as dye- and quantum dot-sensitized solar cells as well as extremely thin absorber solar cells, all of them including the type-II band alignment[10–13]. The latter offer an alternative route to the conventional p-n junction that suffers from the doping difficulty in some of the compound semiconductors belonging to the III-V or II-VI groups. The type-II band alignment occurs when one of the two semiconductors in the core-shell structure has the energy minimum of both the conduction and valence bands. The alignment is expected to induce an efficient charge carrier separation as well as an alternative absorption channel via the type-II optical transition[13, 15], which may significantly improve the light absorption and efficiency of nanostructured solar cells.
Owing to its bandgap energy of 1.5 eV at room temperature and its high optical absorption coefficient (>104 cm-1), CdTe is a very efficient absorbing layer and considered as a good candidate as the shell layer. The potential scarcity of tellurium should also be emphasized and may require the forthcoming use of CdTe in nanostructures in order to reduce the amount of raw materials consumed. In particular, solar cells made from ZnO/CdTe planar structures grown by spray pyrolysis or solution process have reached the photo-conversion efficiency of 8.8% and 12.3%, respectively, which clearly indicates their promising potential photovoltaic applications[16–18]. ZnO/CdTe nanocone tip/film structures have lead to the fabrication of solar cells with a photo-conversion efficiency as high as 3.2%. The development of ZnO/CdTe core-shell NW arrays grown by a wide variety of low-cost deposition techniques has therefore been attracting much attention[20–33]. This is supported by the systematic optical simulations of their ideal short-circuit current density, showing that the absorption capability is highly favorable in ZnO/CdTe core-shell NW arrays and even better than in Si core-shell NW arrays. Levy-Clément et al. have first deposited ZnO/CdTe core-shell NW arrays by using electrodeposition and vapor phase epitaxy, respectively. In the radial structure, the CdTe shell composed of nanograins (NGs) can be grown on ZnO NWs by vapor-phase epitaxy, MOCVD, electron beam deposition[23, 25, 28], electrodeposition[27, 33], close space sublimation or successive ion layer adsorption and reaction (SILAR). An alternative route is to deposit CdTe nanoparticles (NPs) on ZnO NWs by immersion or dip coating[24, 26, 29, 32]. In both routes, a uniform deposition of the CdTe shell has been reported from the bottom to the top of ZnO NWs. Still, the photovoltaic properties of the resulting nanostructured solar cells are fairly poor[22, 24, 25, 27, 29, 32]. One explanation may be correlated to the thermal activation of CdTe NGs and NPs. For instance, it is well-known for p-CdTe/n-CdS heterojunctions that the use of CdCl2 heat treatment can significantly enhance the photovoltaic properties of the resulting solar cells. The CdCl2 heat treatment is expected to favor recrystallization of grains[34–37] as well as passivation of grain boundaries (GBs); these are beneficial for the transport properties of the resulting solar cells. Nevertheless, very little is known concerning the effects of the CdCl2 heat treatment on the physical properties of ZnO/CdTe core-shell NW arrays. It is the aim of this paper to reveal the chemical and physical mechanisms following the CdCl2 heat treatment in ZnO/CdTe core-shell NW arrays as well as their effects on the photovoltaic performances.
Synthesis of ZnO/CdTe core-shell NW arrays on FTO thin films
The synthesis of ZnO/CdTe core-shell NW arrays was achieved on fluorine-doped tin oxide (FTO) thin films by using low-cost chemical and physical deposition techniques. Polycrystalline FTO thin films were initially deposited by ultrasonic spray pyrolysis on a Corning C1737 borosilicate glass substrate (Delta Technologies, Ltd., CO, USA) heated at a growth temperature of 420°C. The chemical precursor solution was composed of 0.16 M of SnCl4 · 5H2O and 0.04 M of NH4F in a methanolic solution and sprayed at a constant flow rate of 1.25 mL/min for a given volume of 20 mL. The thickness of the FTO thin films is about 300 nm. The growth texture of the FTO thin films was controlled along the <100 > orientation in order to favor the structural ordering of the layers grown on top of them[40, 41]. The optical transmittance and electrical resistivity of the FTO thin films are about 90% and a few 10-4 Ω · cm, respectively. A seed layer of ZnO NPs was then grown at room temperature by dip coating. The chemical precursor solution consisted of zinc acetate dihydrate (ZnAc2·2H2O) and monoethanolamine dissolved in absolute ethanol in an equimolar ratio of 0.375 M. The withdrawal speed of 3.3 mm/s was used. All of the samples were initially pre-heated on a hot plate kept at 300°C for 10 min and subsequently post-heated on another plate at 540°C for 1 h. The thickness of the seed layer is about 20 nm. The growth texture of the seed layer was induced along the c-axis in order to favor the vertical alignment of ZnO NWs grown on top of them[42, 43]. Subsequently, the ZnO NWs were grown by CBD for 3 h in a chemical precursor solution of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (C6H12N4) mixed in an equimolar ratio of 0.025 M, dissolved in de-ionized water, and heated at 90°C. CdTe NGs were eventually deposited for 6 min by close space sublimation from a source of CdTe 5 N powder heated at 480°C. The ZnO/CdTe core-shell NW arrays were dipped in a saturated CdCl2:methanol solution for 30 min and then annealed under argon atmosphere for 1 h at different annealing temperatures in the range of 300°C to 500°C.
FESEM, XRD, Raman scattering, PL, and absorption measurements
The structural properties of the ZnO/CdTe core-shell NW arrays were investigated by field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) measurements, and Raman scattering measurements. FESEM images were recorded with a ZEISS Ultra 55 microscope (Oberkochen, Germany). HRTEM specimens were prepared by dispersing ZnO/CdTe core-shell NWs kept in an ethanol solution on a copper grid. HRTEM images were recorded with a JEOL JEM-2010 microscope (Tokyo, Japan) operating at 200 kV. XRD patterns were collected with a PanAlytical diffractometer (Almelo, The Netherlands) using CuKα radiation according to the Bragg-Brentano configuration. The texture of the CdTe shell was quantitatively analyzed from the Kα1 component in the framework of the Harris method by determining both the degree of preferred orientation and texture coefficients[40, 41]. The θ-2θ XRD measurements were performed in the range of 20° to 100° (in 2θ scale). Seven CdTe diffraction peaks were taken into account for the texture analysis: (111), (220), (311), (400), (331), (422), and (531). The (511) diffraction peak was excluded from the texture analysis, as being superimposed with the (333) diffraction peak. The intensity of each CdTe diffraction peak was precisely determined by pseudo-Voigt fits, and their deconvolution with other SnO2 or ZnO diffraction peaks was carefully achieved when required. The 00-041-1445, 00-036-1451, and 00-0150770 files of the International Center for Diffraction Data (ICDD) were used for SnO2, ZnO, and CdTe, respectively. Absorption measurements were performed with a UV-visible-NIR Perkin Elmer Lambda 950 spectrophotometer (Waltham, MA, USA). An integrating sphere was used for light-harvesting efficiency measurements by determining the total optical transmittance and reflectance. The 5 K PL measurements were achieved in a helium flow cryostat by using a frequency-doubled argon laser operating at 244 nm. The 5 K PL spectra were analyzed by using a spectrometer equipped with a 600-line/mm grating and detected with a liquid-nitrogen cooled charge-coupled device (i.e., CCD detector). The excitation power was varied by using an optical attenuator. For all of the PL spectra, the spot size was about 100 μm. Raman measurements were performed with an argon laser operating at 514.5 nm, and the scattered light was analyzed using a Jobin-Yvon T64000 triple spectrometer (Palaiseau, France) equipped with a CCD detector. Raman spectra were collected in the frequency range of 80 to 700 cm-1 at room temperature in near-backscattering geometry using the subtractive configuration of the spectrometer with 100-μm slits (spectral width ≈ 2.2 cm-1). For all of the Raman spectra, the excitation power and spot size were about 2.5 mW and 1 μm, respectively. In order to investigate the homogeneity of the ZnO/CdTe core-shell NW arrays at micron and submicron scales, a Marzhauser Wetzlar motorized stage (Wetzlar, Germany) was used with a lateral step resolution of 100 nm either in steps of 200 nm or 3 μm.
Solar cell fabrication and photovoltaic performances
In order to investigate the photovoltaic properties of as-grown and annealed ZnO/CdTe core-shell NW arrays, CuSCN as a wide bandgap p-type semiconductor was deposited by impregnation. A saturated solution of CuSCN was initially prepared by dissolving 50 mg of CuSCN in 10 mL of n-propyl sulfide. The solution of 0.04 M was then spread over the ZnO/CdTe core-shell NW arrays held on a hot plate kept at 100°C. The solar cells were completed by evaporating a 40-nm-thick gold contact with an Edwards evaporator (Gennevilliers, France). Their photovoltaic properties were recorded under 100 mW/cm2 AM 1.5G simulated sunlight (model 96000, Oriel Instruments, Irvine, CA, USA). The solar simulator had previously been calibrated by using a NREL certified solar cell (Spectra Nova, Ontario, Canada). The external quantum efficiency (EQE) measurements were achieved by using a halogen lamp as the light source and a Newport monochromator (Cornestone 130, Irvine, CA, USA). The acquisition was collected via a lock-in amplifier system. A silicon calibrated diode was used for determining the absolute incident-light intensity.
where Iincident is the light intensity shining the ZnO/CdTe core-shell NW arrays from the FTO/glass substrate side.
Results and discussion
Effects on the structural ordering of ZnO/CdTe core-shell NW arrays
Importantly, the CdTe NGs uniformly cover the ZnO NWs from their bottom to their top both for as-grown and annealed ZnO/CdTe core-shell NW arrays. The CdTe shell thickness varies in the range of 50 to 100 nm and is typically larger on top of the ZnO NWs than on the vertical sidewalls. This indicates that a larger amount of CdTe is deposited on top of the ZnO NWs. The crystallite size as deduced from the Debye-Scherrer law is instead about 32 nm and thus is smaller than the range of the CdTe shell thickness, showing that several layers of CdTe NGs have been deposited. Basically, it also turns out that some CdTe NGs can cover several ZnO NWs, as depicted in Figure 1. The as-grown CdTe NGs have a zinc-blend structure and are polycrystalline, as shown by the XRD patterns in Figure 2a. No epitaxial relationships are thus expected with ZnO NWs since no strong preferential orientation is revealed. This is further shown on the local scale by HRTEM imaging and Fourier-filtered enhancements in Figure 3. No sign of the presence of a transitional layer is further revealed in Figure 3, which excludes the formation of ternary compounds, for instance, in agreement with the XRD patterns of Figure 2a. The absence of epitaxial relationship is likely due to (i) the very high lattice mismatch between ZnO and CdTe and to (ii) the high growth rate for the deposition of CdTe by CSS that typically lies in the range of 0.5 to 1 μm/h. This is also usual for the deposition of CdTe by CSS in the form of thin films. In contrast, some epitaxial relationships have been reported for ZnO/ZnSe core-shell NW arrays, despite the polycrystalline nature of the ZnSe shell; however, the growth rate for the deposition of the ZnSe shell by pulsed laser deposition is instead much lower and of the order of 0.03 μm/h, favoring the establishment of epitaxial relationships. The growth of CdTe NGs by CSS basically follows the Volmer-Weber mechanisms: 3D islands initially nucleate on the vertical sidewalls and top of the ZnO NWs, then coarsen, and eventually coalesce to form a continuous 2D shell. Interestingly, the CdTe NGs are preferentially oriented along the <531 > direction: the degree of preferred orientation as deduced from the Harris method is 0.6, corresponding to a <531 > texture coefficient of 2.4, as shown in Figure 2b. The texture magnitude is hence not pronounced, as expected for polycrystalline thin films deposited by CSS in contrast to standard physical vapor deposition or sputtering. The texture of CdTe NGs can be accounted for by thermodynamic considerations (as usually achieved for polycrystalline thin films), for which grain growth is driven by the minimization of total free energy. The total free energy is dependent upon surface, interface, and strain energy, which are strongly anisotropic in CdTe (i.e., the anisotropy factor is equal to 2.32). Here, CdTe NGs have yielded (the yield stress being fairly low), and the strain is plastically accommodated; Σ3 deformation twins, and dislocations are formed. The stored strain energy within a grain is however expected to be insufficient for further relaxation in nearby grains: accordingly, the strain energy depends on both the yield stress and elastic biaxial modulus. The <531 > texture is thus governed by strain energy minimization since the <531 > direction has one of the lowest biaxial elastic modulus. The growth of the as-grown CdTe NGs on ZnO NWs preserves the typical growth regimes for their planar growth. However, the critical film thickness separating the growth regimes driven by surface or strain energy minimization is strongly decreased.
Upon the CdCl2 heat treatment of the ZnO/CdTe core-shell NW arrays, CdTe NGs significantly grow and their crystallization is enhanced; the formation of the well-defined facets and GBs is shown in Figure 1 for high annealing temperature. Also, their crystallite size is increased up to 56 nm as annealing temperature is raised to 400°C. For higher annealing temperature, the crystallite size decreases with film thickness, owing to CdTe sublimation. The growth of CdTe NGs upon annealing is driven by diffusion-induced GB migration, which is assisted by impurity atoms[54, 55]. Interestingly, the texture of the annealed CdTe NGs along the <531 > direction is decreased, corresponding to randomization phenomena[35–37, 51, 56]. The degree of preferred orientation and <531 > texture coefficient decrease down to 0.4 and 1.9, respectively, as annealing temperature is raised to 450°C, as revealed in Figure 2b. The slight deterioration of the <531 > texture of CdTe NGs on ZnO NWs after CdCl2 heat treatment can be compared with the slight deterioration of the <111 > texture of polycrystalline CdTe thin films above a threshold annealing temperature[37, 56]. In contrast, the texture of the annealed CdTe NGs is strengthened along the <100 > direction as annealing temperature is raised to 400°C. The <100 > texture is governed by strain energy minimization[52, 53]. The underlying physical process upon CdCl2 heat treatment is still unclear, but it has recently been suggested that the formation of CdTe-CdCl2 eutectic liquid phases at GBs may favor recrystallization phenomena through the generation of compressive stresses.
Effects on the doping properties of ZnO/CdTe core-shell NW arrays
Effects on the photovoltaic properties of ZnO/CdTe core-shell NW arrays
Photovoltaic properties of the resulting solar cells
3 × 10-6
2.8 × 10-8
Annealed 300°C, 1 h
9.2 × 10-4
Annealed 450°C, 1 h
9.6 × 10-3
1.2 × 10-2
1.15 × 10-2
1.1 × 10-2
The effects of the CdCl2 heat treatment are investigated on the structural ordering, doping, and photovoltaic properties of ZnO/CdTe core-shell NW arrays grown by low-cost deposition techniques. It is found by FESEM images and XRD measurements that recrystallization phenomena are induced in CdTe NGs by the CdCl2 heat treatment. Their crystallinity is improved through the formation of well-defined facets and GBs while grain growth and texture randomization occur. The initial texture of the as-grown CdTe NGs along the <531 > direction is driven by strain energy minimization and is slightly reduced in favor of the <100 > orientation after the CdCl2 heat treatment. The occurrence of a crystalline tellurium phase is revealed by Raman scattering measurements and strongly enhanced after the CdCl2 heat treatment. The crystalline tellurium phase may decorate GBs in CdTe NGs. Furthermore, the chlorine doping of CdTe NGs is achieved after the CdCl2 heat treatment. The formation of chlorine A-centers is shown by PL measurements; after the CdCl2 heat treatment, radiative transition of excitons bound to chlorine A-centers arise at 1.589 eV, while the intensity of the related emission band involving donor acceptor pairs at 1.44 eV is increased. It is also expected that chlorine can passivate GBs. The chlorine doping and passivation are beneficial for the photovoltaic properties of ZnO/CdTe core-shell NW arrays. The absorption properties of the as-grown and annealed ZnO/CdTe core-shell NW arrays are highly efficient, and about 80% of the incident light is absorbed in the spectral range of the solar irradiance. Most of the charge carriers are photo-generated at the bottom of the ZnO/CdTe core-shell NW arrays inside the CdTe shell, as shown by the maps of the optical generation rate computed from RCWA. The photovoltaic properties of the ZnO/CdTe core-shell NW arrays when covered with the CuSCN/Au back-side contact are strongly improved after the CdCl2 heat treatment but remain low. It is expected that the main limitation originates from the poor collection of the holes generated in the CdTe shell from the CuSCN/Au back-side contact. Eventually, the CdCl2 heat treatment should systematically be achieved for the fabrication of solar cells made from ZnO/CdTe core-shell NW arrays.
The authors are grateful to B. Gayral, CEA-INAC, Grenoble, France, for his assistance in PL measurements. This work has been supported by the Nanosciences Foundation of Grenoble through the project II-VI Photovoltaic and by Grenoble INP with a Bonus Qualité Recherche grant through the project CELESTE. This work has also been partially supported by the Spanish Ministry under contract MAT2010-16116.
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