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Mater Renew Sustain Energy (2017) 6:18 DOI 10.1007/s40243-017-0102-8 BRIEF COMMUNICATION Heterojunction p-Cu O/ZnO-n solar cell fabricated by spark plasma sintering 1 1 1 1 • • • • Christophe Tenailleau Guillaume Salek Thi Ly Le Benjamin Duployer 1 1 1 • • Jean-Jacques Demai Pascal Dufour Sophie Guillemet-Fritsch Received: 21 April 2017 / Accepted: 27 August 2017 / Published online: 2 September 2017 The Author(s) 2017. This article is an open access publication Abstract Cuprous oxide and zinc oxide nanoparticles Introduction were prepared at room temperature by inorganic polycon- densation. X-ray diffraction (XRD) analyses show that the Solar cells are based on semi-conducting components that oxide phases formed are pure and well crystallized. The once assembled properly can convert efficiently sunlight spark plasma sintering (SPS) technique was successfully into electricity. Silicon-based solar cells are still the most used to prepare dense nanoceramics with superimposed widely commercialized. However, other families of semi- layers of Cu O and ZnO nanopowders. Sintering conditions conducting materials (GaAs, CdTe, CuInS , and deriva- 2 2 were optimized to densify the ceramics without phase tives…) have long being studied and developed to obtain transformation or diffusion. These ceramics were also thin films that can facilitate solar energy conversion, characterized by XRD and scanning electron microscopy miniaturization, and decrease costs. However, their toxicity (SEM), as well as X-ray computed tomography (XCT). and element scarcity are usually restricting factors for SEM and XCT showed that nanograins are preserved after developing large-scale solar cells. SPS throughout both oxide materials, while a smaller layer All-oxide photovoltaics constitute a promising genera- (*20 lm) of pure oxide phase with larger grains is formed tion of solar cells [1]. While n-type transparent conducting in between Cu O and ZnO during the sintering process. oxides are well reported (ZnO, TiO , In-Sn–O), p-type 2 2 The SPS technique results in high material density, with semiconductors used in solar cells are often limited to the absence of porosity and cracks, homogenous distribu- Cu O[2–5]. However, the quantum yield for Cu O is yet to 2 2 tion, and a good phase separation. This is the first time that be improved with 6% conversion efficiency reached only such as-prepared dense oxide-based heterojunction exhibits recently [6], in comparison with the *20% theoretical a photovoltaic effect under illumination opening a new value. route for preparing solar cells. Solar cells based on p-Cu O/ZnO-n heterojunctions can be made by different techniques, mainly spin-coating and Keywords Copper oxide Zinc oxide Ceramics Spark electrodeposition methods [7–9], magnetron sputtering plasma sintering X-ray tomography Photovoltaics [3, 4, 10], hydrothermal methods [11], from thermally activated metal sheets [12], or fabricated in air at low temperatures by atmospheric atomic layer deposition [13]. We have recently optimized a low-cost preparation process of crystalline oxide nanopowders that is applicable to a wide variety of structures and stoichiometries, including Cu O and ZnO materials [14]. Once dispersed in & Christophe Tenailleau tenailleau@chimie.ups-tlse.fr colloidal suspensions, the oxides can be integrated in thin- film solar cells. Centre Interuniversitaire de Recherche et d’Ingenierie des These oxide nanopowders were used to produce dense ´ ´ MATeriaux (CIRIMAT) CNRS, INPT, UPS, Universite de layered nanoceramics by the spark plasma sintering (SPS) Toulouse, 118 route de Narbonne, 31062 Toulouse Cedex 09, technique. To our knowledge, this is the first superimposed France 123 18 Page 2 of 7 Mater Renew Sustain Energy (2017) 6:18 ceramic of this type obtained in one step after sintering. Cu O layer was grown by thermal oxidation of copper foils -2 Flash sintering of ZnO is well presented in the literature and showed a 1.6 mA/cm shift of current density at 0 Volt [15–18], while it is hard to find any detailed study on the (i.e., J value) [24]. Here, this is the first all-oxide sc sintering process of Cu O by SPS. Sintering Cu O by the *300 lm-thick solar cell ever reported, to our knowledge. 2 2 conventional methods is very hard to perform without A few issues related to charge recombination and oxide phase transformation. The SPS apparatus uses a pulsed conductivities are still to be answered. High material den- current to heat very quickly a graphite die which contains sification due to SPS treatment, compactness, low activation the sample powder. This is a very interesting technique energies at small grain boundaries for both phases, and well- which allows to decrease considerably the time and tem- defined interfaces can explain these results. In addition, SPS perature of a ceramic sintering process. It can also preserve treatment under vacuum or argon can reduce the oxide the size of the raw nanopowders and nanograin size components, generating higher concentration of vacancies ceramics there obtained, so-called nanoceramics, can and improving the number of charges. exhibit unusual physical properties (see [19, 20], for instance). High sample densifications (above 95%) were obtained after SPS treatments. The new p-type Cu O and Experimental n-type ZnO assembly created by SPS was characterized by X-ray diffraction (XRD) for phase crystalline states, Our simple synthesis method of metal oxide nanopowders scanning electron microscopy (SEM) for grains distribution used at low temperature without any organic and com- essentially, and X-ray computed tomography (XCT) to plexing agent allowed the preparation of pure crystalline probe the oxides interface and visualize the 3D volume in a Cu O and ZnO nanopowders [14]. Briefly, metal salts are non-destructive manner. This original p-Cu O/ZnO-n dissolved in water in stoichiometric proportions. Solutions heterojunction was successfully tested as a solar cell. are then mixed with a large volume of lithium hydroxide Standard silicon-based solar cells are usually a few and stirred for half an hour at ambient atmosphere. The 100th of lm thick (complete Si cells are typically a few addition of dilute ascorbic acid to the copper di-hydroxide mm in thickness). Reduction of the silicon wafer thickness from its current value of about 180 lm to 10–20 lm would eliminate 90% of its effective costs. Multiple technologies exist, some of which have already demonstrated high efficiency on wafers as thin as 35 lm, including silicon grown epitaxially directly from vapor sources, silicon wafers produced directly from molten silicon without casting and wire sawing, and thinner wire saws. Thinner wafers also contribute to higher throughput processing, further reducing the costs. Specifically, the throughput of crystal growth, ingot cropping, wire sawing, and wet chemical steps are increased by having thinner wafers [21]. In wafer-based mono- and multi-crystalline Si solar cells, the absorption is routinely improved by texturing the sur- face with micrometre-sized pyramid-shaped features, which increase scattering of light into the cell, and by a silicon nitride anti-reflection coating. This approach is not practical for thin-film silicon solar cells, where the absorber layer thickness is on the order of 1–3 lm. In these cells, absorption is improved by properly engineered sub-mi- crometre surface texture, which enhances both light scat- tering into a thin absorber layer and the anti-reflection effect at the interfaces over a broad range of wavelengths and incident angles [22]. Most optimized, high-performance, bulk heterojunction solar cells have an active layer thickness of about 100 nm. However, the thin active layer of bandgap [2 eV is unfa- Fig. 1 Scanning electron microscopy images of the cuprite Cu O vorable for optical absorption and film coating. Thicker films (top) and zinc oxide ZnO (bottom) nanoparticles after synthesis by can have higher performances [23]. A 20 lm free-standing inorganic polycondensation 123 Mater Renew Sustain Energy (2017) 6:18 Page 3 of 7 18 formed after the previous stage is required to form Cu O. lution of 3.5 nm and 10 nm at 35 kV in SEI and BEI After washing with water, oxide nanoparticles of controlled modes, respectively. The microscope is equipped with an size and morphology were isolated. The aqueous solvent is Oxford INCA Energy Dispersive X-ray (EDX) Spectrom- finally changed to ethanol and colloidal suspensions, which eter for elemental and cartography analysis. are stable for a few months in an azeotrope solution, can be Two- and three-dimensional images were obtained by used for thin films preparation by the dip-coating process. X-ray computed tomography (XCT) on a few mm of For the first time to our knowledge, the co-sintering of ceramics with a Phoenix/GE Nanotom 180 using a tungsten Cu O and ZnO was possible in a short time (*30 min) by target. The VG Studio Max 2.1 software was used for data SPS using a Dr Sinter 2080 device from Sumitomo coal visualization and process. Different XCT parameters were mining (Fuji Electronic Industrial, Saitama, Japan) with tested to verify their effects on the reconstructed images both phases being preserved as such after the experiment. and to improve the characterization of the materials 300 mg of oxide powders were used. A papyex layer microstructure of similar diffusion factors. Typically, data (200 lm in thickness of C graphite) was put at both ends in measurement conditions were: Zero Mode (2.7 W power the main graphite die (8 mm in diameter) to avoid con- for an optimal resolution in between 1.8 and 60 lm), tamination of the pistons. High vacuum and argon gas were U = 90 kV, I = 160 lA, source/objet distance = 7.6 mm, used to avoid phase transformation. A pressure of 5kN source/detector distance = 250 mm, voxel size = 1.5 lm, (100 MPa) was applied to the sample from the beginning step time = 1500 ms, and five images averaged/step, while of the heating time and up to 800 C (highest temperature the first two images were skipped to avoid image remi- tested, which was reached in 8 min, and a constant dwell niscence after each rotation. 1440 images were recorded time of 6 min at high temperature). Cooling was done at after 360 sample rotation. the furnace rate with the pressure released upon cooling. I–V measurements were carried out with a BEN- The pellet density was determined by the Archimedes THAM PV 300 from ES instrument using a xenon source method using an ARJ-220-4M balance (KERN, Murnau- (150 W) and a halogen quartz source (100 W) to cover Westried, Germany). the whole sunlight spectrum for photovoltaic character- X-ray diffraction (XRD) data were recorded on a Bruker ization in standard conditions (298 K, 1000 W/m , D4-ENDEAVOR diffractometer using a CuKa wavelength AM1.5). The PV 300 system from Bentham/ES (ref. IL- radiation (40 kV, 40 mA) and collected over the DUAL-Xe/QH & 605) is specifically dedicated to effi- 10 \ 2h \ 100 range at room temperature, with a 0.02 ciency measurements in standard conditions on small step scan and 3.6 s/step. areas for different types of solar cells. A reference LP- Scanning electron microscopy (SEM) and back scatter- Si-CAL-Ex silicon photodiode was used for the system ing electron microscopy (BSEM) were performed using a calibration in between 300 and 1100 nm before effi- JEOL JSM6400 operated from 0.2 to 40 kV, with a reso- ciency measurements. Fig. 2 Room-temperature X-ray diffraction patterns of superimposed Cu O/ZnO nanoceramics prepared by SPS. Inset shows the graphite sample holder used for SPS treatment 123 18 Page 4 of 7 Mater Renew Sustain Energy (2017) 6:18 Results and discussion Recently, we have optimized a simple synthetic approach to prepare crystalline nanopowders of oxides at low costs [14, 25]. This soft-chemistry method was used to prepare Cu O and ZnO nanopowders of controlled size and mor- phology. Cu O nanoparticles are spherical (100 nm average size in diameter), while ZnO nanorods (*50 nm in width and 300 nm in length) can be prepared using a solvent mixture of 30 vol% of ethanol in water (Fig. 1). These nanopowders were favorably associated to form a dense nanoceramic (densification[ 95%) after sintering at 500–800 Cusing the SPS technique. The papyex layers were removed by polishing on both sides and the final pellet thickness was about 300 lm. X-ray diffraction patterns, recorded on the cuprite-side of the pellets at room temperature, show that Cu Oand CuO phases are present after SPS treatments and heating at 500 and 700 C in argon atmosphere (Fig. 2). ZnOisalsoseenby XRD. At 800 C, CuO is not present and the only oxide phase observed is Cu O, while a very small quantity of Cu metal starts to form due to the reducing atmosphere provided by the graphite environment (see the 8 mm in diameter matrix and pistons used for SPS in inset of Fig. 2). The pellet showing only Cu O and ZnO phases by XRD was fully characterized. SEM images show that nanopar- ticles are well preserved after SPS treatments (Fig. 3). No coexistence of the phases was determined and a net sepa- ration was observed showing a good compatibility of those materials essentially due to the fast sintering process and close oxide phase dilatation parameters. However, a smaller interface (*20 lm thick) is clearly evidenced on secondary electron microscopy and back-scattered images. This interface is composed of larger grains of the ZnO Fig. 3 Scanning secondary (top) and back-scattered (middle) electron phase as evidenced by EDX analyses (see Fig. 3). This microscopy images of the Cu O/ZnO heterojunction. Inset is a photo abrupt change in grain sizes was already observed in ZnO- of the nanoceramic obtained after SPS treatment. EDX mapping based systems sintered by flash sintering and attributed to analysis results are given at the bottom image, showing that the interlayer of larger grains consists mainly of ZnO the occurrence of electric-potential-induced abnormal grain growth [15]. This enhanced grain growth and/or coarsening would be associated with the accumulation of electrons and throughout the layer after SPS sintering. The charge formation of cation vacancies at ZnO grain boundaries and transport (electrons in the n-type ZnO semi-conducting free surfaces due to particular electrical potentials. layer) is, therefore, favorable within each nanoparticle The perpendicular orientation to the electrode support is along both longitudinal and perpendicular directions with usually more favorable for faster charge conduction. Rod- high ceramic densification. like structures facilitate penetration and effective interfa- Advances in characterization techniques are always cial area, which normally result in reduced charge carrier pushing further the comprehension of the materials’ path length and increased photovoltaic parameters. How- chemical formation/transformation and physical properties. ever, a nanocrystalline porous ZnO film constructed from Recently, X-ray computed tomography has been developed upright-standing nanosheets with the c axis parallel to the to probe and visualize inside the matter in a non-destructive substrate and incorporated into a dye sensitized solar cell manner. Indeed, with the XCT technique, it is now possible (or DSSC) indicated the highest level of efficiency due to to obtain a large set of 2D images of the inner core of a this specific morphology [26]. In our case, SEM images material and use these to reconstruct the 3D volume. show that nanorods-like particles are randomly distributed Depending on the XCT technique and beam source (X-ray, 123 Mater Renew Sustain Energy (2017) 6:18 Page 5 of 7 18 Fig. 4 2D/3D images obtained by laboratory X-ray computed tomography on Cu O/ZnO nanoceramics. The interlayer is emphasized by arrows J(nA/cm ) neutron or electron, essentially), it is now usually possible Dark to detect variable phases, cracks, defects… without any Light damages of a sample, from a few hundreds of micrometers down to a few nanometers. XCT data information can give valuable microstructural information and reach a wide community of scientists. Dense and thick materials with -0.2 -0.1 0.0 0.1 0.2 -5 U(V) high atomic numbers are hard to characterize at a high resolution with conventional XCT lab instruments. It is -10 usually necessary to minimize the sample size for valuable -15 data reconstruction. Another difficulty arises when material -20 phases contrast is lowered due to close atomic numbers and/or compositions, such as Cu O and ZnO. However, we -25 managed to record 2D and 3D images of our nanoceramics -30 and separate both phases. We could also distinguish clearly, by XCT, the intermediate phase of larger granu- Fig. 5 I–V curves in the dark and under standard solar irradiance lometry (and probably different density) at the oxide (AM1.5 and 1000 W/m at room temperature) on the oxide-based heterojunction Cu O/ZnO solar cell obtained by SPS showing a interface (Fig. 4). No cracks were seen throughout the photovoltaic effect. Scale units are 0.5 mm (left), 70 lm(top right), whole studied volumes, especially at the phase separation. and 50 lm(bottom right) Therefore, not only the high densification of the nanoce- ramics was confirmed by XCT measurements, but also the circuit voltage V * 0.07 V. These values are still very oc two oxide phases of close density and diffusion factors low, but the preparation method is very promising for could be separated over a large material volume. industrial applications. Thin films of gold (*50 nm in thickness) were depos- Sinsermsuksakul et al. recently observed some signs of a ited on both sides of the ceramic composite, with only large positive conduction band offset CBO (a small posi- *1 mm-diameter spot on the ZnO side for the light to tive CBO is desirable to reduce interface recombination illuminate a maximum of sample. I–V curves were mea- without any loss in photo-current collection) in a thicker sured in dark and under sunlight in standard conditions. SnS-based solar cell ([500 nm), including a dark/light J– The nanoceramic obtained after SPS treatment showed a V cross over, higher diode voltage (i.e., V ), small fill oc photovoltaic behavior under sunlight illumination (Fig. 5). factor (FF), and low J [27]. This CBO discrepancy may sc This is the first time, to our knowledge, that a solar cell is be because of a variation of the SnS surface condition for made by this technique. The absolute value of the short- different film thicknesses due to preferred and anisotropic -2 circuit current density J is of 10 nA cm and the open- sc crystal orientations of SnS during film preparation. 123 18 Page 6 of 7 Mater Renew Sustain Energy (2017) 6:18 Garnett and Yang demonstrated in 2010 strong light- Conclusions trapping properties of nanowire arrays, which improve the conversion efficiency of Si solar cells. Their 5 lm nano- A solar cell was produced using the spark plasma sintering wire arrays’ radial p–n junction solar cells fabricated from process. The heterojunction formed and studied in this 8 and 20 lm Si absorbing layers achieved conversion paper was composed of two oxide phases, Cu O and ZnO. efficiencies of 4.8 and 5.3%, respectively, under AM1.5 First, pure crystalline nanopowders of Cu O and ZnO were illumination. The significant light-trapping effect, above synthesized by a simple soft-chemistry method. These the theoretical limit for a randomizing system, indicates powders were then inserted on top of each other into a that there might be photonic improvement effects [28]. graphite die to prepare a junction of the two oxides. The The efficiency was found to increase as the oxide film SPS technique allows to obtain a nanoceramic with high thickness decreases in a Cu O/Cu heterojunction, up to a densification for a very short time of heat treatment under limiting thickness of 26.30 lm after which the efficiency high pressure. The oxide phases remain well fitted and decreases with decrease in the oxide film thickness, separated after SPS treatment, while a limited region of depending essentially on the oxidation conditions (with grain growth is observed on the ZnO side at the oxide T * 1000 C, time = 3 min) found for Cu O and Cu, with interface probably due to an electrical potential in the SPS I = 518 lA and V = 86.0 mV [29]. apparatus. Finally, the solar energy conversion into elec- sc oc An increasing J value was also measured as a function tricity was tested with success on this system, showing that sc of the increasing Cu O thickness for TiO /Cu O all-oxide this method can be used to prepare photovoltaic devices. 2 2 2 heterojunction solar cells entirely produced by spray ´ ´ Acknowledgements The French FERMaT Midi-Pyrenees Federation pyrolysis onto fluorine-doped tin oxide (FTO) covered FR3089 is deeply acknowledged for providing X-ray tomography glass substrates [30]. laboratory facility. Mr Geoffroy Chevallier is thanked for assistance In addition, enhanced photovoltaic effect was reported in the SPS treatments. This work was supported by the French in 2013 in the ferroelectric lanthanum-modified lead zir- Ministry of Education and Research and the Vietnamese government with the University of Sciences and Technology in Hanoi (USTH). conate titanate (PLZT) [31]. PLZT ceramics were prepared by hot-pressing calcinations at 1240 C under 40 MPa Open Access This article is distributed under the terms of the pressure. Both sides of the 1 cm in diameter resulting Creative Commons Attribution 4.0 International License (http:// pellets were then polished down to about 300 lmin creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give thickness before metal deposition (*100 nm thick, by appropriate credit to the original author(s) and the source, provide a radio-frequency magnetron sputtering). The photovoltaic link to the Creative Commons license, and indicate if changes were response of PLZT capacitor was expanded from ultraviolet made. to visible spectra and may have important impact on design and fabrication of high-performance photovoltaic devices. These preparation technique and ceramic thickness are References comparable to those described here. 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Materials for Renewable and Sustainable Energy – Springer Journals
Published: Nov 1, 2017
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