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Hg(1−x)CdxTe nanowire heterostructures on conductive glass using templated electrodeposition
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Fabrication of CdS/Hg(1 x)CdxTe nanowire heterostructures on conductive glass using templated electrodeposition Brian A. Ashenfelter, Terry P. Bigioni n Department of Chemistry and Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA
a r t i c l e i n f o
abstract
Keywords: Cadmium sulfide Mercury cadmium telluride Nanowire Heterojunction Multiple exciton generation Anodic aluminum oxide Template Electrodeposition Galvanic contact deposition
Single-material and heterojunction nanowires were fabricated from CdS and Hg(1 x)CdxTe (MCT) in anodic aluminum oxide templates grown on transparent conductive oxide coated glass substrates. Structural and compositional analyses were carried out by electron microscopy, elemental analysis and x-ray diffraction. CdS was deposited using potentiostatic electrodeposition, forming CdS nanowires with a 1:1 stoichiometry and wurtzite structure. MCT was also deposited using potentiostatic electrodeposition, forming MCT nanowires with a stoichiometry of Hg0.24Cd0.76Te and a zinc blende structure. Annealing of the electrodeposited MCT nanowires increased crystallite size from 9 nm to 23 nm in the (1 1 1) direction. Heterojunction nanowires were prepared by sequential electrodeposition of CdS and MCT with control over the length and diameter. These materials have a desirable band gap and geometry for multipleexciton generation studies for nanostructured photovoltaics applications. & 2013 Elsevier Ltd. All rights reserved.
1. Introduction Increasing the efficiency and decreasing the cost of solar cells is a priority in the development of photovoltaics (PV). Third generation PV has the general promise of unique and tunable physical properties and inexpensive synthetic routes [1]. This will require the development of novel structures, materials, processing methods, and strategies, however, if it is to have a positive impact on device performance. Nanostructures inherently have the disadvantage of possessing more numerous interfaces compared to bulk or thin film materials, so it is imperative that their use enables an advantageous new strategy. The generation of multiple excitons from the absorption of a single photon is one such example [1–11]. Typically, when the energy of an absorbed photon exceeds the band gap energy (Eg) of the semiconductor, the excess energy is dissipated as heat as the electron thermalizes through phonon scattering. With multipleexciton generation (MEG), however, if there is sufficient
n
Corresponding author. Tel.: þ1 419 530 4095; fax: þ1 419 530 4033. E-mail address: [email protected] (T.P. Bigioni).
excess energy (hν42Eg), then a second electron has the potential to be excited by the process of impact ionization. In this case, the high-energy electron transfers its extra energy through a collision with a less energetic electron, exciting it into the conduction band [4]. If the incident photon energy hν4nEg, then in principle, as many as n electrons may be excited. Detailed balance calculations have shown that single-junction MEG devices could attain conversion efficiencies as high as 44%, with an optimal band gap between 0.80 eV and 1.1 eV [4]. This efficiency limit is close to that of a multijunction device [12] but with the simpler construction and engineering of a single-junction device. Although MEG has been observed in bulk materials, it is thought to be more efficient in nanostructures since their discrete energy levels suppress phonon scattering [4]. While MEG has been measured in PbS, PbSe, PbTe, and CdSe nanocrystals [2], device performance typically suffers due to poor charge transport across the many interfaces of the nanocrystalline thin films [13]. Also, the MEG threshold energy for nanocrystals is, in practice, nearly 3Eg, not 2Eg as predicted by theory [4]. One-dimensional (1D) nanowires might offer solutions to both of these problems, however. Recent reports state that a MEG threshold of just 2.2Eg was
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found in nanowires [4]. Further, their axial connectivity ought to greatly improve charge transport [4,13]. Semiconductor nanowire arrays are therefore attractive for the construction of solid-state PV devices that exploit MEG. Here, we establish electrochemical methods of fabricating aligned arrays of Hg(1 x)CdxTe (MCT) nanowires. The arrays are embedded in an optical quality Al2O3 matrix, enabling spectroscopic study of MEG in 1D nanostructures. With this architecture, contacts could also be made to either end of the nanowires such that transport measurements could be made of MEG-based PV devices. MCT was chosen because its band gap can be tuned between that of CdTe (1.5 eV) [14] and HgTe ( 0.03 eV) [14] by varying the stoichiometry (x) of the Hg(1 x)CdxTe alloy. With electrodeposition, adjusting the deposition potential can control the stoichiometry. In this way, the optimal band gap for MEG can be achieved, where x ¼0.65–0.80 [15–17]. Electrochemical synthesis of MCT thin films has been reported [15,18–21]. In fact, electrodeposited CdS/MCT thin film PV devices with an efficiency of 10.6% have been demonstrated [16]. Nanoporous anodic aluminum oxide (AAO) was chosen for the template as electrodeposited semiconductor nanowire arrays have been previously reported for related materials [22–26]. Heterostructures, such as p–n junctions, can also be fabricated by sequential deposition of different semiconductors into the same template. In this case, nanowire arrays have been electrodeposited into AAO that was grown on transparent conductive oxide (TCO) coated glass [22–31]. The strategy for fabricating MCT nanowires reported here uses this approach, as depicted in Fig. 1.
ultrasonically in a warm 10% microsoap solution for 30 min. This was followed by two ultrasonic rinses in deionized water, and an ultrasonic rinse in ethanol. Substrates were then dried with Ar gas and baked at 80 1C overnight. E-beam evaporation was used to deposit a 10 nm adhesion layer of Ti followed by a 4 μm film of Al onto the cleaned TCO surface. Al films were anodized at room temperature in 0.3 M oxalic acid using a MPJA model 9305 PS DC power supply and a two-electrode cell with constant stirring. The Al-coated TCO was used as the working electrode and graphite was used as the counter electrode. Substrates were anodized at a constant potential in a two-step process. The first anodization was conducted at 40 V for 12 min. The resulting disordered alumina film was etched away in a 0.2 M phosphoric acid and 0.4 M chromic acid solution at 60 1C for 15 min. After the disordered oxide film was completely removed, the patterned Al film was anodized for a second time at a constant potential of 15–40 V for 20 min. The volume expansion during the conversion of Al to AAO is 1:1.8, generating about 3 μm of AAO after both anodization steps. The resulting film was optically transparent and completely anodized; no Al metal remained. The barrier layer was thinned during the final stages of anodization by stepping the potential down by 1 V/min until 0 V was achieved. Following anodization, the barrier layer was completely opened by a pore-widening step, wherein the substrate was immersed in 10% phosphoric acid for 5–45 min. This enabled DC electrodeposition of semiconductor materials directly into the pores of the template and onto the underlying TCO electrode.
2.2. Electrodeposition 2. Experimental 2.1. Anodic aluminum oxide (AAO) templates Commercially available TCO-coated glass was obtained from Pilkington (Tec-15). The glass substrates were cleaned
Electrodeposition was carried out potentiostatically in a three-electrode cell using a Princeton Applied Research Model 263A Galvanostat/Potentiostat. The AAO/TCO template was used as the working electrode and graphite was used as the auxiliary electrode. Single-material and heterojunction nanowire arrays were achieved on TCO/glass by electrodepositing
Fig. 1. Fabrication strategy for electrodepositing semiconductor nanowires into an AAO template on TCO-coated glass.
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CdS and MCT into the pores of the AAO template using a method similar to that reported elsewhere [22,24–26]. 2.2.1. CdS electrodeposition Cadmium sulfide was potentiostatically electrodeposited at a constant voltage of 1.0 V, using Cd wire as the reference electrode. The electrolytic bath consisted of 0.055 M CdCl2 and 0.19 M elemental sulfur dissolved in dimethyl sulfoxide at 110 1C. 2.2.2. MCT electrodeposition Mercury cadmium telluride was grown by potentiostatic electrodeposition. All MCT was deposited from an
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electrolytic bath consisting of an aqueous solution of 1.0 M CdSO4, 1.5 mM TeO2, and 0.15 mM HgCl2. The pH was adjusted to 1.5 using concentrated H2SO4 and the deposition temperature was held at 91 1C. Electrodepositions were carried out at 0.700 V, using an Ag/AgCl reference electrode with constant stirring.
2.3. Annealing treatment CdS nanowires were annealed at 400 1C for 10 min in air. MCT nanowires were annealed in a tube furnace with a flowing mixture of 95% Ar and 5% H2 at 280 1C for 10 min.
Fig. 2. (a) Photograph of an AAO template fabricated on TCO-coated glass. ((b)–(f)) SEM images showing (b) cross section of a typical AAO template on TCOcoated glass, (c) AAO template with irregular pore structure after one anodization step, (d) AAO template with more regular pore structure after a second anodization step, (e) AAO template on TCO-coated glass that has not had the barrier layer thinned or removed, and (f) AAO on TCO-coated glass after the resistive barrier layer was completely removed.
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Powder x-ray diffraction data were collected using a PANalytical X′pert Pro MPD diffractometer equipped with a Cu Kα radiation source (λ¼ 1.54056 Å). Nanowires were measured while oriented in the AAO template. Peaks were fit using the PANalytical X′pert High Score software and crystallite sizes were calculated using the Scherrer formula.
5 min to 45 min, producing pores ranging in size from 20 nm to 100 nm in diameter. To facilitate electrodeposition into the AAO pores, the resistive barrier layer at the bottom of the pores (Fig. 2e) was removed. Ramping down the applied potential at the end of anodization and subsequently widening the pores in 10% phosphoric acid successfully opened the pores to expose the underlying TCO electrode, as shown in Fig. 2f [29].
2.5. Electron microscopy
3.2. CdS nanowire arrays
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were acquired using a JEOL JSM-7500F microscope. Nanowires were liberated from the templates for imaging by dipping the substrate in 1 M NaOH for at least 30 min to dissolve the AAO. Wires were imaged on the TCO substrate and dispersed onto Cu grids. High-resolution TEM (HRTEM) images were acquired using a Hitachi HD-2300A scanning transmission electron microscope (STEM) with 200 keV accelerating voltage. Samples were prepared on 200 mesh lacey carbon or SiO coated Cu grids.
CdS nanowires were electrodeposited into the pores of an AAO film on TCO/glass, liberated from the AAO matrix, and then imaged by SEM, as shown in Fig. 3a. The nanowires were compact and straight with an average nanowire diameter of 75 nm, in agreement with the pore structure of the AAO template. The UV–vis absorption spectrum of the wires shown in Fig. 3b indicated a band edge near the bulk band gap of 2.42 eV. Powder x-ray diffraction of the CdS nanowires embedded in the AAO matrix (Fig. 3c) showed that the CdS was polycrystalline with a wurtzite structure. Annealing the nanowires in air at 400 1C for 10 min led to recrystallization and an increase in grain size, as shown in Fig. 3c. The as-deposited CdS crystallite size was 10.4 nm, which increased to 12.4 nm upon annealing. Crystallite sizes are tabulated in Table 1.
2.4. Powder x-ray diffraction (pXRD)
2.6. Elemental analysis Energy dispersive x-ray spectroscopy (EDS) spectra were collected on a JEOL JSM-7500F microscope using a Bruker XFlash 6|60 detector with 129 eV resolution. 3. Results 3.1. AAO on TCO/glass substrates Growing AAO directly on bare TCO can be challenging due to vigorous gas evolution at the alumina–TCO interface and subsequent delamination of the film during anodization. Electrochemical degradation of the TCO has also been reported after direct exposure to anodization conditions [29], which can significantly reduce the conductivity of the TCO film making it unsuitable for electrodeposition. Depositing a thin ( 10 nm) adhesion layer of Ti between the TCO and Al films significantly improves adhesion and makes AAO film growth possible [29], since anodization of Al films with Ti adhesion layers proceeded without gas evolution or delamination. After complete anodization, the resulting films were smooth and optically transparent over their entire areas and the TCO retained its conductivity. A typical AAO template fabricated on TCO-coated glass is shown in Fig. 2a. A cross section of the resulting AAO template is shown in Fig. 2b. A two-step anodization process was essential to creating an ordered through-pore nanoarray [30]. When the substrate was completely anodized in a single step, the resulting pore structure in the oxide film was irregular, as shown in Fig. 2c. By adding a pre-anodization step, pores with uniform diameters and more regular packing were achieved, as shown in Fig. 2d. Control over pore diameter has been previously demonstrated by adjusting the applied potential and the pore widening time [32]. Here, substrates were anodized at potentials between 15 V and 40 V and pore widened for
3.3. MCT nanowire arrays MCT nanowires were grown inside the pores of an AAO film on TCO/glass using potentiostatic electrodeposition. Fig. 4a shows an SEM cross-section image of the electrodeposited MCT nanowires, still embedded in the AAO template. Although the nanowires do not appear to be compact and monolithic, their broken structure is due to the fracturing process. This is better seen in the unbroken nanowires shown in Fig. 4b, which were liberated from the AAO pores by dissolution of the template. Although monolithic, the MCT nanowires were not as compact as the CdS nanowires. Atomic resolution HRTEM imaging of as-deposited nanowires (see Fig. 4c) shows a lattice spacing of 3.7 Å, which indicates growth in the (1 1 1) direction for this particular wire. Analysis by x-ray diffraction found similar crystallite sizes in all directions, however, revealing that there was no preferred growth direction (see Table 1). The pXRD pattern showed that the MCT nanowires were polycrystalline with a zinc blend structure, as shown in Fig. 4d. Improvement of the nanowire crystallinity by annealing was not trivial, however. Annealing of the MCT nanowires at 400 1C for 10 min resulted in the complete loss of material due to volatilization of mercury from the lattice. To overcome this instability, the MCT nanowires were instead annealed in a reducing environment of 95% Arþ 5% H2 and at a decreased temperature of 280 1C. This suppressed the loss of MCT from the array and promoted grain growth in the (1 1 1) direction by more than a factor of 2.5. In order to use MCT nanowires to study MEG, the band gap and therefore the stoichiometry needs to be controlled. Changing the applied deposition potential was successfully
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Fig. 3. (a) Side view SEM image of CdS nanowires on TCO/glass. (b) UV–vis absorption spectrum of CdS nanowires embedded in an AAO template on TCO/ glass. (c) pXRD pattern for CdS nanowires embedded in an AAO template before and after annealing. TCO peak positions are indicated by n. Table 1 Electrodeposited nanowire crystallite sizes as determined by powder x-ray diffraction. hkl CdS nanowires As deposited Annealed
(1 0 0) 103.65 Å 123.97 Å
(1 0 1) 83.86 Å 113.31 Å
(1 1 0) 43.03 Å 61.79 Å
Potentiostatic MCT nanowires As deposited Annealed
(1 1 1) 88.11 Å 208.24 Å
(2 2 0) 71.89 Å 190.79 Å
(3 1 1) 61.54 Å 200.23 Å
used to vary the stoichiometry of the nanowires. Compositions suitable for obtaining band gaps in the MEG window were achieved at 700 mV vs. Ag/AgCl. This corresponded to a stoichiometry near x¼0.76, as measured by EDS. 3.4. CdS/MCT heterojunction nanowires Heterostructures were fabricated by sequential electrodeposition of CdS and MCT into the AAO template. Fig. 5a shows a top-view SEM image of an oriented array of CdS/MCT nanowires after the AAO matrix was dissolved. The heterojunctions were imaged using TEM and low-angle backscattered electron (LABe) imaging, both of which show a clear Z-contrast between the CdS and MCT regions of the nanowires (see Fig. 5b and c, respectively). The nanowires were well formed and straight. Diffraction measurements verified that the heterojunction nanowires contained polycrystalline
(1 0 3) 99.08 Å 88.72 Å
CdS and MCT, as shown in Fig. 5d, with grain sizes similar to those of the single-material nanowires. The stoichiometry of each segment of the heterostructures was analyzed using EDS, as shown in Fig. 6. The nanowires were confirmed to be composed of MCT, with the composition Hg0.24Cd0.76Te, as well as stoichiometric CdS. The MCT composition corresponds to a band gap of about 1.0 eV, which lies within the MEG window. 4. Discussion It was possible to control the stoichiometry of the MCT nanowires with potentiostatic electrodeposition. The composition of MCT could be adjusted from Hg-rich to Cd-rich by applying more negative potentials. The potentials for which MCT deposition was stoichiometric ranged from 450 mV to 750 mV vs. Ag/AgCl, as judged by the current-limited
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Fig. 4. (a) SEM cross-section image of potentiostatically deposited MCT nanowires embedded in the AAO template grown on TCO-coated glass. (b) MCT nanowires liberated from the AAO pores by dissolution of the template. (c) Atomic resolution HRTEM image of MCT nanowire indicating (1 1 1) orientation along the wire axis. (d) pXRD pattern of potentiostatically electrodeposited MCT nanowires. Wires showed crystallite size improvement of 2.5 times upon annealing at 270 1C for 10 min in 95% Arþ 5% H2.
plateau in a linear sweep voltammetry scan. Within this window, stoichiometries were determined to range from x¼0.25 for 450 mV and x¼0.76 for 700 mV. For the present study, MCT was most often deposited at 700 mV
vs. Ag/AgCl, which produced material with a composition of x¼0.76. This corresponds to a band gap of 1.0 eV, which is within the MEG window. In contrast, growth of CdS was not sensitive to the deposition potential. Deposition of CdS was
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Fig. 5. (a) Top view secondary electron SEM image of CdS/MCT heterojunction nanowires. (b) TEM image of CdS/MCT heterojunction nanowires grown by potentiostatic electrodeposition. (c) Low-angle backscattered electron image of CdS/MCT heterojunction nanowires grown by potentiostatic electrodeposition. (d) pXRD pattern following the progression of forming a CdS/MCT heterojunction on the same array of nanowires.
generally done at an over-potential and always resulted in a 1:1 stoichiometry. MCT thin film analogs electrodeposited on TCO-coated glass tended to have larger grains than the MCT nanowires. This is not surprising since the constraints imposed on the growth by the template ought to limit grain growth. It is also possible that the pore walls could serve as nucleation sites and promote the growth of many small grains, although the insulating nature of the oxide ought to suppress this. Annealing the MCT nanowires promoted grain growth up to 20 nm in size, a dimension that matches the pore diameter of the template (see Table 1). Electrodeposition often leads to oriented crystal growth as the substrate can act as a template during the initial stages of growth. The fact that the grains are essentially uniform in size, based on the x-ray diffraction analysis, and not larger than the template pore diameter supports the conclusion that these nanowires do not exhibit oriented growth. No oriented growth along the pore axis was observed. In contrast, we found that MCT thin films tended to grow preferentially in the (1 1 1) direction perpendicular to the substrate. This generated large columnar grains that span the thickness of the film, which may facilitate charge transport across the material. This continuity across the film may be one of the reasons that an electrodeposited CdS/MCT thin film solar cell was able to achieve an
efficiency of 10.6% [16]. Oriented growth of MCT nanowires could potentially lead to single-domain nanowires that span the template, which would be ideal materials for PV applications. This remains a technical challenge for these materials. Galvanic contact deposition was also investigated in order to contrast the results with potentiostatic electrodeposition. For galvanic contact deposition, the working electrode (template or TCO/glass) was immersed in the MCT electrodeposition bath and connected to a strip of Al foil, which set the potential in place of the potentiostat. This galvanic cell seemed to deposit a more uniform MCT with a consistent stoichiometry, perhaps because of the ability of the system to readjust the potential based on the local conditions at the working electrode surface. The internal galvanic potential was about 705 mV, which was close to that applied during potentiostatic deposition and therefore produced the desired stoichiometry. Like potentiostatic deposition, galvanic contact deposition was not found to produce oriented growth of MCT nanowires. Nevertheless, galvanic contact deposition was a simple and reliable deposition technique that did not require an external power source and consistently produced uniform material with a suitable stoichiometry. The mechanical stability of CdS/CdTe thin film heterojunctions is not affected by the large ( 10%) lattice
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Fig. 6. (a) Low-angle backscattered electron image of a CdS/MCT heterojunction nanowire. (b) EDS spectrum of the CdS segment, with a 1:1 stoichiometry. (c) EDS spectrum of the MCT segment, with a stoichiometry of x¼ 0.76. Cu, O, and Si peaks are from the SiO coated copper TEM grid.
mismatch between the two phases [33], due to their large contact area. It is somewhat surprising, however, that CdS/ CdTe or CdS/MCT heterojunction nanowires share this mechanical stability. It is clear from electron microscopy images, such as those in Figs. 5 and 6, that the heterojunction nanowires are mechanically robust since sample handling, including liberation from the template, did not result in separation of the two phases. While mechanical stability does not guarantee good charge transport across the interface, poor mechanical stability implies poor interfacial charge transport. Control over the diameter of the nanowires was achieved by tuning the diameter of the pores with the anodization voltage [27–30]. Post-anodization pore widening was also used to increase and fine tune the size of the nanowires, such that diameters between 20 nm and 100 nm were synthesized. The 20 nm pores were prepared by applying 15 V and pore widening for just 5 min. Applying 40 V and pore widening for 45 min resulted in the 100 nm pores. It is worth considering the effect of nanowire diameter on the band gap. The Bohr exciton radii of CdS, CdTe and HgTe are 2.8 nm [34], 7.5 nm [35], and 40 nm [36], respectively. Given the large diameters of the nanowires discussed here, properties of the CdS were not affected by quantum confinement. This was confirmed by extrapolating the band gap of the CdS nanowires from their UV–vis absorption (see Fig. 3b). For MCT nanowires, however, quantum size effects could become important below diameters of 40 nm. Based
on the Cd-rich composition, it is expected that the band gap would be closer to that of CdTe than HgTe, however. While it would seem reasonable that the nanowire diameter would not affect the MCT band gap for the diameter range considered here, absorption measurements were not able to confirm a clear band edge location. Recombination due to the presence of defects and trap states leads to a reduced performance in PV devices and has been shown to be the dominant factor limiting minority carrier diffusion lengths in single-crystal silicon nanowires [37]. In particular, surface recombination leads to a 60 mV decrease in open-circuit voltage for every order of magnitude increase in surface area [37]. Polycrystalline nanowires, such as those presented here, contain many interfaces and have large total surface areas, therefore resolving or passivating these interfaces will be key to optimizing PV performance. To that end, both annealing to eliminate defects and using common surface-passivation techniques have been shown to dramatically reduce the trapping of charges [37]. The optical density of these nanostructured thin films is also important when considering use of this nanowire architecture for PV applications. Without pore widening, approximately 25% of the cross-sectional area of the film is made up of semiconductor material, which effectively reduces the absorptivity of the films by 75% compared to a continuous thin film of the same thickness. Increasing the diameter of the nanowires by pore widening can improve light absorption for photovoltaic applications; however, a balance may be needed
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to optimize MEG efficiency. In this case, it is possible to increase the optical density of the films by increasing the length of the nanowires (note: gaps between nanowires are much smaller than the wavelength of incident solar radiation). By increasing the growth time, the lengths of the nanowires could be adjusted from a few hundred nanometers up to several microns depending on absorptivity requirements. 5. Conclusions The ability to fabricate large-area, optically-transparent AAO templates on TCO/glass has been shown. These porous oxide templates have tunable pore diameters and are ideal for optoelectronic device construction. The underlying TCO of these templates retained good conductivity after complete anodization of the Al film such that nanowire arrays could be electrodeposited across the entire electrode. CdS/MCT p–n heterojunction nanowires were fabricated by sequential deposition of CdS and MCT into the template and were found to have well-defined junctions between the materials. CdS nanowires were found to have polycrystalline wurtzite structures and MCT nanowires were found to have polycrystalline zinc blende structures, whether deposited as single-material nanowires or as heterostructures. Low temperature annealing of potentiostatically deposited MCT nanowires promoted grain growth of up to 2.5 times. The stoichiometry of the MCT was controlled by the deposition potential by potentiostatic deposition, however galvanic contact deposition was found to be a simpler deposition technique that consistently produced uniform material with a composition of x¼0.76, which is a suitable stoichiometry for MEG studies.
Acknowledgements We thank the Air Force Research Laboratory, the Wright Center for Photovoltaics Innovation and Commercialization, the School of Solar and Advanced Renewable Energy at the University of Toledo, and the National Science Foundation (Grant no. 0840474) for support. References [1] G. Conibeer, Materials Today 10 (2007) 42–50. [2] M.C. Beard, A.G. Midgett, M. Law, O.E. Semonin, R.J. Ellingson, A.J. Nozik, Nano Letters 9 (2009) 836–845. [3] D.J. Binks, Physical Chemistry Chemical Physics 13 (2011) 12693–12704. [4] P.D. Cunningham, J.E. Boercker, E.E. Foos, M.P. Lumb, A.R. Smith, J.G. Tischler, J.S. Melinger, Nano Letters 11 (2011) 3476–3481. [5] M.A. Green, Progress in Photovoltaics: Research and Applications 9 (2001) 123–135.
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Fabrication of CdS/Hg(1 x)CdxTe nanowire heterostructures on conductive glass using templated electrodeposition Brian A. Ashenfelter, Terry P. Bigioni n Department of Chemistry and Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA
a r t i c l e i n f o
abstract
Keywords: Cadmium sulfide Mercury cadmium telluride Nanowire Heterojunction Multiple exciton generation Anodic aluminum oxide Template Electrodeposition Galvanic contact deposition
Single-material and heterojunction nanowires were fabricated from CdS and Hg(1 x)CdxTe (MCT) in anodic aluminum oxide templates grown on transparent conductive oxide coated glass substrates. Structural and compositional analyses were carried out by electron microscopy, elemental analysis and x-ray diffraction. CdS was deposited using potentiostatic electrodeposition, forming CdS nanowires with a 1:1 stoichiometry and wurtzite structure. MCT was also deposited using potentiostatic electrodeposition, forming MCT nanowires with a stoichiometry of Hg0.24Cd0.76Te and a zinc blende structure. Annealing of the electrodeposited MCT nanowires increased crystallite size from 9 nm to 23 nm in the (1 1 1) direction. Heterojunction nanowires were prepared by sequential electrodeposition of CdS and MCT with control over the length and diameter. These materials have a desirable band gap and geometry for multipleexciton generation studies for nanostructured photovoltaics applications. & 2013 Elsevier Ltd. All rights reserved.
1. Introduction Increasing the efficiency and decreasing the cost of solar cells is a priority in the development of photovoltaics (PV). Third generation PV has the general promise of unique and tunable physical properties and inexpensive synthetic routes [1]. This will require the development of novel structures, materials, processing methods, and strategies, however, if it is to have a positive impact on device performance. Nanostructures inherently have the disadvantage of possessing more numerous interfaces compared to bulk or thin film materials, so it is imperative that their use enables an advantageous new strategy. The generation of multiple excitons from the absorption of a single photon is one such example [1–11]. Typically, when the energy of an absorbed photon exceeds the band gap energy (Eg) of the semiconductor, the excess energy is dissipated as heat as the electron thermalizes through phonon scattering. With multipleexciton generation (MEG), however, if there is sufficient
n
Corresponding author. Tel.: þ1 419 530 4095; fax: þ1 419 530 4033. E-mail address: [email protected] (T.P. Bigioni).
excess energy (hν42Eg), then a second electron has the potential to be excited by the process of impact ionization. In this case, the high-energy electron transfers its extra energy through a collision with a less energetic electron, exciting it into the conduction band [4]. If the incident photon energy hν4nEg, then in principle, as many as n electrons may be excited. Detailed balance calculations have shown that single-junction MEG devices could attain conversion efficiencies as high as 44%, with an optimal band gap between 0.80 eV and 1.1 eV [4]. This efficiency limit is close to that of a multijunction device [12] but with the simpler construction and engineering of a single-junction device. Although MEG has been observed in bulk materials, it is thought to be more efficient in nanostructures since their discrete energy levels suppress phonon scattering [4]. While MEG has been measured in PbS, PbSe, PbTe, and CdSe nanocrystals [2], device performance typically suffers due to poor charge transport across the many interfaces of the nanocrystalline thin films [13]. Also, the MEG threshold energy for nanocrystals is, in practice, nearly 3Eg, not 2Eg as predicted by theory [4]. One-dimensional (1D) nanowires might offer solutions to both of these problems, however. Recent reports state that a MEG threshold of just 2.2Eg was
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found in nanowires [4]. Further, their axial connectivity ought to greatly improve charge transport [4,13]. Semiconductor nanowire arrays are therefore attractive for the construction of solid-state PV devices that exploit MEG. Here, we establish electrochemical methods of fabricating aligned arrays of Hg(1 x)CdxTe (MCT) nanowires. The arrays are embedded in an optical quality Al2O3 matrix, enabling spectroscopic study of MEG in 1D nanostructures. With this architecture, contacts could also be made to either end of the nanowires such that transport measurements could be made of MEG-based PV devices. MCT was chosen because its band gap can be tuned between that of CdTe (1.5 eV) [14] and HgTe ( 0.03 eV) [14] by varying the stoichiometry (x) of the Hg(1 x)CdxTe alloy. With electrodeposition, adjusting the deposition potential can control the stoichiometry. In this way, the optimal band gap for MEG can be achieved, where x ¼0.65–0.80 [15–17]. Electrochemical synthesis of MCT thin films has been reported [15,18–21]. In fact, electrodeposited CdS/MCT thin film PV devices with an efficiency of 10.6% have been demonstrated [16]. Nanoporous anodic aluminum oxide (AAO) was chosen for the template as electrodeposited semiconductor nanowire arrays have been previously reported for related materials [22–26]. Heterostructures, such as p–n junctions, can also be fabricated by sequential deposition of different semiconductors into the same template. In this case, nanowire arrays have been electrodeposited into AAO that was grown on transparent conductive oxide (TCO) coated glass [22–31]. The strategy for fabricating MCT nanowires reported here uses this approach, as depicted in Fig. 1.
ultrasonically in a warm 10% microsoap solution for 30 min. This was followed by two ultrasonic rinses in deionized water, and an ultrasonic rinse in ethanol. Substrates were then dried with Ar gas and baked at 80 1C overnight. E-beam evaporation was used to deposit a 10 nm adhesion layer of Ti followed by a 4 μm film of Al onto the cleaned TCO surface. Al films were anodized at room temperature in 0.3 M oxalic acid using a MPJA model 9305 PS DC power supply and a two-electrode cell with constant stirring. The Al-coated TCO was used as the working electrode and graphite was used as the counter electrode. Substrates were anodized at a constant potential in a two-step process. The first anodization was conducted at 40 V for 12 min. The resulting disordered alumina film was etched away in a 0.2 M phosphoric acid and 0.4 M chromic acid solution at 60 1C for 15 min. After the disordered oxide film was completely removed, the patterned Al film was anodized for a second time at a constant potential of 15–40 V for 20 min. The volume expansion during the conversion of Al to AAO is 1:1.8, generating about 3 μm of AAO after both anodization steps. The resulting film was optically transparent and completely anodized; no Al metal remained. The barrier layer was thinned during the final stages of anodization by stepping the potential down by 1 V/min until 0 V was achieved. Following anodization, the barrier layer was completely opened by a pore-widening step, wherein the substrate was immersed in 10% phosphoric acid for 5–45 min. This enabled DC electrodeposition of semiconductor materials directly into the pores of the template and onto the underlying TCO electrode.
2.2. Electrodeposition 2. Experimental 2.1. Anodic aluminum oxide (AAO) templates Commercially available TCO-coated glass was obtained from Pilkington (Tec-15). The glass substrates were cleaned
Electrodeposition was carried out potentiostatically in a three-electrode cell using a Princeton Applied Research Model 263A Galvanostat/Potentiostat. The AAO/TCO template was used as the working electrode and graphite was used as the auxiliary electrode. Single-material and heterojunction nanowire arrays were achieved on TCO/glass by electrodepositing
Fig. 1. Fabrication strategy for electrodepositing semiconductor nanowires into an AAO template on TCO-coated glass.
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CdS and MCT into the pores of the AAO template using a method similar to that reported elsewhere [22,24–26]. 2.2.1. CdS electrodeposition Cadmium sulfide was potentiostatically electrodeposited at a constant voltage of 1.0 V, using Cd wire as the reference electrode. The electrolytic bath consisted of 0.055 M CdCl2 and 0.19 M elemental sulfur dissolved in dimethyl sulfoxide at 110 1C. 2.2.2. MCT electrodeposition Mercury cadmium telluride was grown by potentiostatic electrodeposition. All MCT was deposited from an
3
electrolytic bath consisting of an aqueous solution of 1.0 M CdSO4, 1.5 mM TeO2, and 0.15 mM HgCl2. The pH was adjusted to 1.5 using concentrated H2SO4 and the deposition temperature was held at 91 1C. Electrodepositions were carried out at 0.700 V, using an Ag/AgCl reference electrode with constant stirring.
2.3. Annealing treatment CdS nanowires were annealed at 400 1C for 10 min in air. MCT nanowires were annealed in a tube furnace with a flowing mixture of 95% Ar and 5% H2 at 280 1C for 10 min.
Fig. 2. (a) Photograph of an AAO template fabricated on TCO-coated glass. ((b)–(f)) SEM images showing (b) cross section of a typical AAO template on TCOcoated glass, (c) AAO template with irregular pore structure after one anodization step, (d) AAO template with more regular pore structure after a second anodization step, (e) AAO template on TCO-coated glass that has not had the barrier layer thinned or removed, and (f) AAO on TCO-coated glass after the resistive barrier layer was completely removed.
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Powder x-ray diffraction data were collected using a PANalytical X′pert Pro MPD diffractometer equipped with a Cu Kα radiation source (λ¼ 1.54056 Å). Nanowires were measured while oriented in the AAO template. Peaks were fit using the PANalytical X′pert High Score software and crystallite sizes were calculated using the Scherrer formula.
5 min to 45 min, producing pores ranging in size from 20 nm to 100 nm in diameter. To facilitate electrodeposition into the AAO pores, the resistive barrier layer at the bottom of the pores (Fig. 2e) was removed. Ramping down the applied potential at the end of anodization and subsequently widening the pores in 10% phosphoric acid successfully opened the pores to expose the underlying TCO electrode, as shown in Fig. 2f [29].
2.5. Electron microscopy
3.2. CdS nanowire arrays
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were acquired using a JEOL JSM-7500F microscope. Nanowires were liberated from the templates for imaging by dipping the substrate in 1 M NaOH for at least 30 min to dissolve the AAO. Wires were imaged on the TCO substrate and dispersed onto Cu grids. High-resolution TEM (HRTEM) images were acquired using a Hitachi HD-2300A scanning transmission electron microscope (STEM) with 200 keV accelerating voltage. Samples were prepared on 200 mesh lacey carbon or SiO coated Cu grids.
CdS nanowires were electrodeposited into the pores of an AAO film on TCO/glass, liberated from the AAO matrix, and then imaged by SEM, as shown in Fig. 3a. The nanowires were compact and straight with an average nanowire diameter of 75 nm, in agreement with the pore structure of the AAO template. The UV–vis absorption spectrum of the wires shown in Fig. 3b indicated a band edge near the bulk band gap of 2.42 eV. Powder x-ray diffraction of the CdS nanowires embedded in the AAO matrix (Fig. 3c) showed that the CdS was polycrystalline with a wurtzite structure. Annealing the nanowires in air at 400 1C for 10 min led to recrystallization and an increase in grain size, as shown in Fig. 3c. The as-deposited CdS crystallite size was 10.4 nm, which increased to 12.4 nm upon annealing. Crystallite sizes are tabulated in Table 1.
2.4. Powder x-ray diffraction (pXRD)
2.6. Elemental analysis Energy dispersive x-ray spectroscopy (EDS) spectra were collected on a JEOL JSM-7500F microscope using a Bruker XFlash 6|60 detector with 129 eV resolution. 3. Results 3.1. AAO on TCO/glass substrates Growing AAO directly on bare TCO can be challenging due to vigorous gas evolution at the alumina–TCO interface and subsequent delamination of the film during anodization. Electrochemical degradation of the TCO has also been reported after direct exposure to anodization conditions [29], which can significantly reduce the conductivity of the TCO film making it unsuitable for electrodeposition. Depositing a thin ( 10 nm) adhesion layer of Ti between the TCO and Al films significantly improves adhesion and makes AAO film growth possible [29], since anodization of Al films with Ti adhesion layers proceeded without gas evolution or delamination. After complete anodization, the resulting films were smooth and optically transparent over their entire areas and the TCO retained its conductivity. A typical AAO template fabricated on TCO-coated glass is shown in Fig. 2a. A cross section of the resulting AAO template is shown in Fig. 2b. A two-step anodization process was essential to creating an ordered through-pore nanoarray [30]. When the substrate was completely anodized in a single step, the resulting pore structure in the oxide film was irregular, as shown in Fig. 2c. By adding a pre-anodization step, pores with uniform diameters and more regular packing were achieved, as shown in Fig. 2d. Control over pore diameter has been previously demonstrated by adjusting the applied potential and the pore widening time [32]. Here, substrates were anodized at potentials between 15 V and 40 V and pore widened for
3.3. MCT nanowire arrays MCT nanowires were grown inside the pores of an AAO film on TCO/glass using potentiostatic electrodeposition. Fig. 4a shows an SEM cross-section image of the electrodeposited MCT nanowires, still embedded in the AAO template. Although the nanowires do not appear to be compact and monolithic, their broken structure is due to the fracturing process. This is better seen in the unbroken nanowires shown in Fig. 4b, which were liberated from the AAO pores by dissolution of the template. Although monolithic, the MCT nanowires were not as compact as the CdS nanowires. Atomic resolution HRTEM imaging of as-deposited nanowires (see Fig. 4c) shows a lattice spacing of 3.7 Å, which indicates growth in the (1 1 1) direction for this particular wire. Analysis by x-ray diffraction found similar crystallite sizes in all directions, however, revealing that there was no preferred growth direction (see Table 1). The pXRD pattern showed that the MCT nanowires were polycrystalline with a zinc blend structure, as shown in Fig. 4d. Improvement of the nanowire crystallinity by annealing was not trivial, however. Annealing of the MCT nanowires at 400 1C for 10 min resulted in the complete loss of material due to volatilization of mercury from the lattice. To overcome this instability, the MCT nanowires were instead annealed in a reducing environment of 95% Arþ 5% H2 and at a decreased temperature of 280 1C. This suppressed the loss of MCT from the array and promoted grain growth in the (1 1 1) direction by more than a factor of 2.5. In order to use MCT nanowires to study MEG, the band gap and therefore the stoichiometry needs to be controlled. Changing the applied deposition potential was successfully
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Fig. 3. (a) Side view SEM image of CdS nanowires on TCO/glass. (b) UV–vis absorption spectrum of CdS nanowires embedded in an AAO template on TCO/ glass. (c) pXRD pattern for CdS nanowires embedded in an AAO template before and after annealing. TCO peak positions are indicated by n. Table 1 Electrodeposited nanowire crystallite sizes as determined by powder x-ray diffraction. hkl CdS nanowires As deposited Annealed
(1 0 0) 103.65 Å 123.97 Å
(1 0 1) 83.86 Å 113.31 Å
(1 1 0) 43.03 Å 61.79 Å
Potentiostatic MCT nanowires As deposited Annealed
(1 1 1) 88.11 Å 208.24 Å
(2 2 0) 71.89 Å 190.79 Å
(3 1 1) 61.54 Å 200.23 Å
used to vary the stoichiometry of the nanowires. Compositions suitable for obtaining band gaps in the MEG window were achieved at 700 mV vs. Ag/AgCl. This corresponded to a stoichiometry near x¼0.76, as measured by EDS. 3.4. CdS/MCT heterojunction nanowires Heterostructures were fabricated by sequential electrodeposition of CdS and MCT into the AAO template. Fig. 5a shows a top-view SEM image of an oriented array of CdS/MCT nanowires after the AAO matrix was dissolved. The heterojunctions were imaged using TEM and low-angle backscattered electron (LABe) imaging, both of which show a clear Z-contrast between the CdS and MCT regions of the nanowires (see Fig. 5b and c, respectively). The nanowires were well formed and straight. Diffraction measurements verified that the heterojunction nanowires contained polycrystalline
(1 0 3) 99.08 Å 88.72 Å
CdS and MCT, as shown in Fig. 5d, with grain sizes similar to those of the single-material nanowires. The stoichiometry of each segment of the heterostructures was analyzed using EDS, as shown in Fig. 6. The nanowires were confirmed to be composed of MCT, with the composition Hg0.24Cd0.76Te, as well as stoichiometric CdS. The MCT composition corresponds to a band gap of about 1.0 eV, which lies within the MEG window. 4. Discussion It was possible to control the stoichiometry of the MCT nanowires with potentiostatic electrodeposition. The composition of MCT could be adjusted from Hg-rich to Cd-rich by applying more negative potentials. The potentials for which MCT deposition was stoichiometric ranged from 450 mV to 750 mV vs. Ag/AgCl, as judged by the current-limited
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Fig. 4. (a) SEM cross-section image of potentiostatically deposited MCT nanowires embedded in the AAO template grown on TCO-coated glass. (b) MCT nanowires liberated from the AAO pores by dissolution of the template. (c) Atomic resolution HRTEM image of MCT nanowire indicating (1 1 1) orientation along the wire axis. (d) pXRD pattern of potentiostatically electrodeposited MCT nanowires. Wires showed crystallite size improvement of 2.5 times upon annealing at 270 1C for 10 min in 95% Arþ 5% H2.
plateau in a linear sweep voltammetry scan. Within this window, stoichiometries were determined to range from x¼0.25 for 450 mV and x¼0.76 for 700 mV. For the present study, MCT was most often deposited at 700 mV
vs. Ag/AgCl, which produced material with a composition of x¼0.76. This corresponds to a band gap of 1.0 eV, which is within the MEG window. In contrast, growth of CdS was not sensitive to the deposition potential. Deposition of CdS was
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Fig. 5. (a) Top view secondary electron SEM image of CdS/MCT heterojunction nanowires. (b) TEM image of CdS/MCT heterojunction nanowires grown by potentiostatic electrodeposition. (c) Low-angle backscattered electron image of CdS/MCT heterojunction nanowires grown by potentiostatic electrodeposition. (d) pXRD pattern following the progression of forming a CdS/MCT heterojunction on the same array of nanowires.
generally done at an over-potential and always resulted in a 1:1 stoichiometry. MCT thin film analogs electrodeposited on TCO-coated glass tended to have larger grains than the MCT nanowires. This is not surprising since the constraints imposed on the growth by the template ought to limit grain growth. It is also possible that the pore walls could serve as nucleation sites and promote the growth of many small grains, although the insulating nature of the oxide ought to suppress this. Annealing the MCT nanowires promoted grain growth up to 20 nm in size, a dimension that matches the pore diameter of the template (see Table 1). Electrodeposition often leads to oriented crystal growth as the substrate can act as a template during the initial stages of growth. The fact that the grains are essentially uniform in size, based on the x-ray diffraction analysis, and not larger than the template pore diameter supports the conclusion that these nanowires do not exhibit oriented growth. No oriented growth along the pore axis was observed. In contrast, we found that MCT thin films tended to grow preferentially in the (1 1 1) direction perpendicular to the substrate. This generated large columnar grains that span the thickness of the film, which may facilitate charge transport across the material. This continuity across the film may be one of the reasons that an electrodeposited CdS/MCT thin film solar cell was able to achieve an
efficiency of 10.6% [16]. Oriented growth of MCT nanowires could potentially lead to single-domain nanowires that span the template, which would be ideal materials for PV applications. This remains a technical challenge for these materials. Galvanic contact deposition was also investigated in order to contrast the results with potentiostatic electrodeposition. For galvanic contact deposition, the working electrode (template or TCO/glass) was immersed in the MCT electrodeposition bath and connected to a strip of Al foil, which set the potential in place of the potentiostat. This galvanic cell seemed to deposit a more uniform MCT with a consistent stoichiometry, perhaps because of the ability of the system to readjust the potential based on the local conditions at the working electrode surface. The internal galvanic potential was about 705 mV, which was close to that applied during potentiostatic deposition and therefore produced the desired stoichiometry. Like potentiostatic deposition, galvanic contact deposition was not found to produce oriented growth of MCT nanowires. Nevertheless, galvanic contact deposition was a simple and reliable deposition technique that did not require an external power source and consistently produced uniform material with a suitable stoichiometry. The mechanical stability of CdS/CdTe thin film heterojunctions is not affected by the large ( 10%) lattice
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Energy (keV)
Fig. 6. (a) Low-angle backscattered electron image of a CdS/MCT heterojunction nanowire. (b) EDS spectrum of the CdS segment, with a 1:1 stoichiometry. (c) EDS spectrum of the MCT segment, with a stoichiometry of x¼ 0.76. Cu, O, and Si peaks are from the SiO coated copper TEM grid.
mismatch between the two phases [33], due to their large contact area. It is somewhat surprising, however, that CdS/ CdTe or CdS/MCT heterojunction nanowires share this mechanical stability. It is clear from electron microscopy images, such as those in Figs. 5 and 6, that the heterojunction nanowires are mechanically robust since sample handling, including liberation from the template, did not result in separation of the two phases. While mechanical stability does not guarantee good charge transport across the interface, poor mechanical stability implies poor interfacial charge transport. Control over the diameter of the nanowires was achieved by tuning the diameter of the pores with the anodization voltage [27–30]. Post-anodization pore widening was also used to increase and fine tune the size of the nanowires, such that diameters between 20 nm and 100 nm were synthesized. The 20 nm pores were prepared by applying 15 V and pore widening for just 5 min. Applying 40 V and pore widening for 45 min resulted in the 100 nm pores. It is worth considering the effect of nanowire diameter on the band gap. The Bohr exciton radii of CdS, CdTe and HgTe are 2.8 nm [34], 7.5 nm [35], and 40 nm [36], respectively. Given the large diameters of the nanowires discussed here, properties of the CdS were not affected by quantum confinement. This was confirmed by extrapolating the band gap of the CdS nanowires from their UV–vis absorption (see Fig. 3b). For MCT nanowires, however, quantum size effects could become important below diameters of 40 nm. Based
on the Cd-rich composition, it is expected that the band gap would be closer to that of CdTe than HgTe, however. While it would seem reasonable that the nanowire diameter would not affect the MCT band gap for the diameter range considered here, absorption measurements were not able to confirm a clear band edge location. Recombination due to the presence of defects and trap states leads to a reduced performance in PV devices and has been shown to be the dominant factor limiting minority carrier diffusion lengths in single-crystal silicon nanowires [37]. In particular, surface recombination leads to a 60 mV decrease in open-circuit voltage for every order of magnitude increase in surface area [37]. Polycrystalline nanowires, such as those presented here, contain many interfaces and have large total surface areas, therefore resolving or passivating these interfaces will be key to optimizing PV performance. To that end, both annealing to eliminate defects and using common surface-passivation techniques have been shown to dramatically reduce the trapping of charges [37]. The optical density of these nanostructured thin films is also important when considering use of this nanowire architecture for PV applications. Without pore widening, approximately 25% of the cross-sectional area of the film is made up of semiconductor material, which effectively reduces the absorptivity of the films by 75% compared to a continuous thin film of the same thickness. Increasing the diameter of the nanowires by pore widening can improve light absorption for photovoltaic applications; however, a balance may be needed
Please cite this article as: B.A. Ashenfelter, T.P. Bigioni, Materials Science in Semiconductor Processing (2013), http://dx. doi.org/10.1016/j.mssp.2013.08.015i
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to optimize MEG efficiency. In this case, it is possible to increase the optical density of the films by increasing the length of the nanowires (note: gaps between nanowires are much smaller than the wavelength of incident solar radiation). By increasing the growth time, the lengths of the nanowires could be adjusted from a few hundred nanometers up to several microns depending on absorptivity requirements. 5. Conclusions The ability to fabricate large-area, optically-transparent AAO templates on TCO/glass has been shown. These porous oxide templates have tunable pore diameters and are ideal for optoelectronic device construction. The underlying TCO of these templates retained good conductivity after complete anodization of the Al film such that nanowire arrays could be electrodeposited across the entire electrode. CdS/MCT p–n heterojunction nanowires were fabricated by sequential deposition of CdS and MCT into the template and were found to have well-defined junctions between the materials. CdS nanowires were found to have polycrystalline wurtzite structures and MCT nanowires were found to have polycrystalline zinc blende structures, whether deposited as single-material nanowires or as heterostructures. Low temperature annealing of potentiostatically deposited MCT nanowires promoted grain growth of up to 2.5 times. The stoichiometry of the MCT was controlled by the deposition potential by potentiostatic deposition, however galvanic contact deposition was found to be a simpler deposition technique that consistently produced uniform material with a composition of x¼0.76, which is a suitable stoichiometry for MEG studies.
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