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Semiconductor thin films directly from minerals—study of structural, optical, and transport characteristics of Cu2O thin films from malachite mineral and synthetic CuO
Thin Solid Films 520 (2012) 3914–3917
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Semiconductor thin films directly from minerals—study of structural, optical, and transport characteristics of Cu2O thin films from malachite mineral and synthetic CuO K.R. Balasubramaniam a,⁎, V.M. Kao a, J. Ravichandran b, P.B. Rossen c, W. Siemons d, 1, J.W. Ager III
a
a
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Applied Science and Technology, University of California, Berkeley, CA 94720, USA Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA d Department of Physics, University of California, Berkeley, CA 94720, USA b c
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 23 January 2012 Accepted 26 January 2012 Available online 31 January 2012 Keywords: Photovoltaics Copper oxide Minerals X-ray diffraction Optical properties Transport characteristics Mass spectroscopy of recoiled ions
a b s t r a c t We demonstrate the proof-of-concept of using an abundantly occurring natural ore, malachite (Cu2CO3 (OH)2) to directly yield the semiconductor Cu2O to be used as an active component of a functional thin film based device. Cu2O is an archetype hole-conducting semiconductor that possesses several interesting characteristics particularly useful for solar cell applications, including low cost, non-toxicity, good hole mobility, large minority carrier diffusion length, and a direct energy gap ideal for efficient absorption. In this article, we compare the structural, optical, and electrical transport characteristics of Cu2O thin films grown from the natural mineral malachite and synthetic CuO targets. Growth from either source material results in single-phase, fully epitaxial cuprous oxide thin films as determined by x-ray diffraction. The films grown from malachite have strong absorption coefficients ( 10 4 cm − 1), a direct allowed optical bandgap ( 2.4 eV), and majority carrier hole mobilities ( 35 cm 2 V − 1 s− 1at room temperature) that compare well with films grown from the synthetic target as well as with previously reported values. Our work demonstrates that minerals could be useful to directly yield the active components in functional devices and suggests a route for the exploration of low cost energy conversion and storage technologies. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Low cost energy conversion and storage devices will be crucial for solving the current global energy crisis [1,2]. Realization of functional devices for this purpose by using the natural mineral directly is very enticing; however, such an approach usually results in components with degraded performance due to the presence of numerous impurities in the material. One other serious impediment towards realizing low cost devices is the prohibitive cost of purification for the active layer materials [3]. Thus, the candidate low cost materials should be earthabundant and should lend themselves to facile, inexpensive purification procedures. In the particular case of solar cells, it has become clear that the eventual implementation of a specific materials platform will come down to the key factors of material extraction cost, annual electricity potential of the material, abundance, and environmental friendliness of the material [1]. In this manuscript, we demonstrate the idea of low cost energy conversion devices using Cu2O based solar cells derived directly from minerals as our model system. Cu2O is one of the cheap, abundant, non-toxic oxide material that has been the focus of several studies as the p-type absorber, Cu2O, in a
p–n heterojunction with different n-type materials [4–11] or as a Schottky barrier cell with various metals or transparent conducting oxides [12–14]. Cu2O as the active layer is widely abundant in the form of naturally occurring minerals, making it an excellent candidate for our proof-of-concept study. Cuprite (the mineral form of Cu2O), however, is not one of the most abundant copper-based minerals; this designation is reserved for copper sulfides, namely copper pyrites and chalcocite [15]. One of the more common copper ore is malachite (copper carbonate copper hydroxide, Cu2CO3(OH)2 [15]), which on heating to modest temperature of 300 °C yields Cu2O [16,17]. In this work, we demonstrate the viability of directly obtaining the active absorber layer in solar cells using naturally occurring minerals as the starting material. The similarity between films grown from a commercial CuO target and a malachite target obtained through the straightforward decomposition of malachite into p-type Cu2O during pulsed laser deposition (PLD) is demonstrated in this work. Material characteristics relevant to the use of this material as the active layer in a heterojunction solar cell, specifically the structural characteristics, optical absorption, and electrical transport are compared. 2. Experimental details
⁎ Corresponding author. Tel.: + 1 510 486 4995. E-mail address: [email protected] (K.R. Balasubramaniam). 1 The author is currently at Oakridge National Laboratory. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.01.041
Films were deposited on one-side and two-side polished single crystal substrates of (LaAlO3)0.3–(Sr2AlTaO6)0.7 [LSAT] (001) obtained
K.R. Balasubramaniam et al. / Thin Solid Films 520 (2012) 3914–3917
3. Results and discussion 3.1. Structural and chemical characterization Fig. 1a shows a typical XRD θ–2θ? scan (along the (00l) of the LSAT substrate) of Cu2O film grown from both the malachite and synthetic CuO target on the LSAT substrate. The substrate peaks are marked as “s” and correspond to LSAT (00l). The only other peak in this 2θ range at 2θ = 41.94° (d = 2.15 Å, marked as “f”) corresponds to the Cu2O (002), and proves that we have produced highly-oriented, phasepure Cu2O for films grown from both the malachite (top data, Fig. 1a) and CuO (bottom data, Fig. 1a) targets. This suggests that under the growth conditions used, along with the thermal energy supplied through laser impingement on the target enables Cu2O phase formation during PLD growth. The measured (similar) full width at half maximum (FWHM) of the rocking curve is shown as the inset in Fig. 1a of 0.26° for the (002) reflection of Cu2O. The inplane epitaxy of the Cu2O film grown from malachite was determined by registering azimuthal ϕ-scans and by comparing the locations in ϕ-space of the Cu2O{022} and the LSAT{022} reflections (similar results were obtained for the Cu2O film obtained from the CuO target but are not shown here). The two phi scans are overlaid in Fig. 1b, although it should be noted that they were acquired at different 2θ angles (2θ = 61.32° for Cu2O{022} and 2θ = 68.44° for LSAT{022}) with ψ = 45°. The observed four-fold intensity of the Cu2O{022} suggests that there is a cube-on-cube texture. The ϕ-scans indicate that the Cu2O{022} peaks are aligned with the LSAT{022} peaks in ϕ-space. Taken together, the diffraction scans for Cu2O given in Fig. 1a and b
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from Crystec GmbH, Berlin, Germany. All the LSAT substrates underwent ultrasonic cleaning in trichloroethylene, acetone, and isopropanol before an annealing step at 1050 °C for 3 hours in flowing O2 prior to loading into the pulsed laser deposition (PLD) chamber. PLD was carried out using a KrF excimer laser operating at a wavelength of 248 nm. ≈400 nm thick Cu2O films were obtained from the malachite target (Wards Natural Science Establishment and originally mined in the Kolwezi mines, Shaba, Zaire) as well as a synthetic CuO target (Praxair Specialty Ceramics, Woodinville, WA). All the Cu2O films were grown at a temperature of 700 °C, chamber pressure of 0.66 Pa O2, at an energy density of ≈1.0 J/cm 2; the films were then cooled down in the deposition atmosphere before retrieving from the chamber. The crystalline nature (phase, crystalline quality, and epitaxial relationship) of the films was characterized using a Panalytical X'Pert X-ray diffractometer (CuKα radiation, λ = 1.54054 Å). For in situ time-of flight-ion scattering and recoil spectroscopy (TOF-ISARS) 39 [18] measurements were performed using K ions with an energy of 10 keV and an Ionwerks (Houston, USA) mass spectrometry of recoiled ions reflectron detector. The angle of incidence, α, was set to 15° and the detector was fixed at 60°. Ions with masses up to 250 u can be detected. The Cu2O film was grown from malachite target under the conditions described above and in situ measurements were performed after cooling the sample to 150 °C. A Hitachi U3000 UV–vis spectrometer was used to obtain reflection and transmission data from ≈400 nm thick Cu2O films grown on double-side polished LSAT substrates. This was converted to absorbance (α) using the Beer–Lambert relation. Hall measurements were carried out in a custom setup equipped with a 1.5 Tesla electro-magnet and a sample holder whose temperature can be varied from 300 to 800 K. All measurements were carried out in the van der Pauw geometry with Au metal contacts of dimensions ≈1 mm at the corners of the sample (5 mm × 5 mm). Current–voltage sweeps were carried out to ensure ohmic contact and magnetic field–voltage sweeps were carried out to ensure there is no contribution from magnetoresistance. The carrier concentration, n was calculated from the Hall resistance, RH with the transverse voltage measured at 1 Tesla field.
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φ (degrees) Fig. 1. (a) Normal XRD? θ–2θ scans of the Cu2O on LSAT(001) substrates synthesized from a synthetic CuO (bottom red graph) and malachite mineral (top black graph) target. The LSAT(00l) peaks are marked “s” and Cu2O(002) peak is marked as “f.” The inset shows the rocking curve around the Cu2O 002 peak, FWHM = 0.26°. (b) Azimuthal offaxis φ scan of {022} Cu2O film grown from the malachite target and LSAT substrate.
show that the epitaxial relationship between the Cu2O and the LSAT(001) layers is {001}Cu2O|{001}LSAT||〈110〉Cu2O|〈110〉LSAT. These combined XRD results thus indicate the high crystalline quality of our Cu2O films obtained from both the targets. Fig. 2 shows the ionic spectra obtained from a TOF-ISARS study from a malachite film grown on LSAT (001) (red curve) and from a malachite target (black curve). A qualitative assessment of the surface chemical composition of both samples reveals the presence of Cu mainly [the strong K peak reflects incident ion beam used]. However, the other major species present in the target, namely C, and the impurities P, S, Cl do not transfer to the substrate. Even though there is negligible transfer of impurities from the malachite target to the film, it is worthwhile comparing the optical and transport properties of films grown from malachite and synthetic CuO targets, as these properties are important for the photovoltaic performance of the devices. 3.2. Optical absorption The optical absorbance (α) spectra were derived from the measured transmittance (T) and reflectance (R) spectra of the Cu2O films deposited on double-side polished LSAT (001) substrates. In order to determine the optical band gaps (Eg) of copper oxide films, (αhν) 2 is plotted as a function of the incident photon energy (hν) (Fig. 3). Linear extrapolation of the bounding absorption curves yields direct allowed transitions in the range of 2.47–2.54 eV for the films. This value of the band gap for films grown from the malachite target (all curves in black) agrees well with the absorption band gap for
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Fig. 2. Time-of-flight mass spectra obtained with an incident potassium ion beam to scatter surface species from the malachite target (red curve) and film grown from this target (black curve) at room temperature showing the transfer of elements. Note: the large peak marked K in the figure is due to the reflected potassium ion beam.
Cu2O films grown from synthetic CuO targets (red curves, Fig. 3). The values observed here, however, are different from the oft-quoted band gap of around 2.1 eV [19] and it is important to make the distinction here between the electronic bandgap and the optical band gap obtained from absorption measurements. In Cu2O the highest valence band and the lowest conduction band (Eg = 2.1 eV) have the same s-symmetry, and a transition involving photon absorption is dipole forbidden [20]. For this reason, measurable absorption happens only at higher energies, in this case, for our film thicknesses, at 2.4 eV. This implies a small UV shift in the absorbed light wavelengths, and hence a reduction in the maximum possible Jsc with Cu2O as the main absorber layer in a solar cell. 3.3. Transport characteristics More importantly, the effect of impurities, even in minor quantities is usually seen in a reduction of the majority and minority carrier mobility. The hole mobility values reported for Cu2O range from 15 to 100 cm 2 V − 1 s − 1 at room temperature; the large range is primarily due to deviations in stoichiometry, which strongly affects the carrier transport properties in Cu2O [21,22]. The highest hole mobility for epitaxial thin films reported to date is 90 cm 2 V − 1 s − 1 [21]. Minority
Fig. 4. Hall mobility of 400 nm thick Cu2O films obtained from CuO (red data points) and malachite (black data points) targets on two-side polished LSAT substrates plotted as a function of the absolute temperature. The red curve is a typical T− 4 fit of the mobility data for CuO sample 1.
carrier diffusion length is one of the important metrics for the high performance of a solar cell. Even though it is not related directly to the majority carrier mobility, the electrical transport characteristics of a material can be qualified with majority carrier mobility as a scale. In Fig. 4 we show the temperature dependent mobility for the films grown from malachite (black data points) and synthetic CuO (red data points) target. It is clear that there is no significant difference in the mobility at room temperature and all samples show a T − 4 dependence above room temperature. One such fit (red line) of the CuO sample 1 in Fig. 4 is shown for reference. Several groups have reported the high temperature mobility of Cu2O in the bulk single crystalline form with or without photoexcitation [23–25] and our current observation is in accordance with these reports. Currently, there is no consensus in the literature on a theoretical understanding of the observed temperature dependence of mobility for Cu2O at high temperatures. At high temperatures, typically the dominant scattering of holes is attributed to the longitudinal optical (LO) phonon modes in oxides but this mechanism alone cannot account for the observed temperature dependence in our case. In short, both the optical and transport properties are not significantly different for the films grown from the malachite and synthetic CuO targets. 4. Summary In conclusion, we have demonstrated realization of the idea of producing materials for active components in energy conversion devices directly from natural minerals with semiconducting Cu2O synthesized from natural mineral malachite for solar cells asour model system. Thin Cu2O films obtained from the malachite target exhibit very similar optical and transport properties in comparison to that grown from pure, synthetic CuO targets. This suggests that the possibility of solar cells produced directly from minerals as the characteristics of the absorber layer in our demonstration is limited by similar mechanisms, and hence, is not related to the impurities present in the mineral. Acknowledgement
Fig. 3. Tauc plots, (αhν)2 vs. hν where α was determined from measured transmission and reflection spectra on ≈ 400 nm thick Cu2O films obtained from CuO (red data points) and malachite (black data points) on two-side polished LSAT substrates. The black dotted lines are guides to the eye for the linear extrapolation of the bounding curves in the data shown. The intercepts of the linear extrapolation yield the direct allowed optical transitions corresponding to the bandgap.
The work at Berkeley was performed within the Helios Solar Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. The authors wish to acknowledge the help and support provided by Prof. R. Ramesh at UC Berkeley. The authors also thank Prof. Lane W Martin (currently at UIUC) for useful
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discussions. J.R. would like to thank the Link energy fellowship. W.S. thanks the Dutch Organization for Scientific Research (NWO). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C. Wadia, A.P. Alivisatos, D.M. Kammen, Environ. Sci. Technol. 43 (2009) 2072. N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. 103 (2006) 15729. J. Kantner, A. Mileva, D. Kammen, , 2009, p. 1. A. Mittiga, E. Salza, F. Sarto, M. Tucci, R. Vasanthi, Appl. Phys. Lett. 88 (2006) 163502. H. Tanaka, T. Shimakawa, T. Miyata, H. Sato, T. Minami, Appl. Surf. Sci. 244 (2005) 568. J. Katayama, K. Ito, M. Matsuoka, J. Tamaki, J. Appl. Electrochem. 34 (2004) 687. T. Minami, H. Tanaka, T. Shimakawa, T. Miyata, H. Sato, Jpn. J. Appl. Phys. 43 (2004) L917. H. Tanaka, T. Shimakawa, T. Miyata, H. Sato, T. Minami, Thin Solid Films 469–470 (2004) 80. W. Siripala, A. Ivanovskaya, T.F. Jaramillo, S.H. Baeck, E.W. McFarland, Sol. Energy Mater. Sol. Cells 77 (2003) 229. L. Papadimitriou, N. Economou, D. Trivich, Solar Cells 3 (1981) 73. J. Herion, E.A. Niekisch, G. Scharl, Sol. Energy Mater. 4 (1980) 101.
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[12] W. Sears, E. Fortin, J. Webb, Thin Solid Films 103 (1983) 303. [13] V.F. Drobny, D.L. Pulfrey, Thin Solid Films 61 (1979) 89. [14] V.F. Drobny, D.L. Pulfrey, Photovoltaic Specialists Conference, 13th, Washington, DC, June 5–8, 1978, Conference Record.(A79-40881 17-44), Institute of Electrical and Electronics Engineers, Inc, New York, 1978, p. 180. [15] W.G.I. Davenport, M.J. King, M.E. Schlesinger, A.K. Biswas, Extractive Metallurgy of Copper, Pergamon, , 2002. [16] J.F. Scaife, Can. J. Chem. 35 (1957) 1332. [17] E. Holland, G. Gilligan, J. Phys. Chem. 31 (1927) 728. [18] W.J. Rabalais, Principles and Applications of Ion Scattering Spectrometry: Surface and Chemical and Structural Analysis, Wiley Interscience, 2002. [19] A. Chen, H. Long, X. Li, Y. Li, G. Yang, P. Lu, Vaccum 83 (2009) 927. [20] V.T. Agekyan, Phys. Status Solidi A 43 (1977) 11. [21] K. Matsuzaki, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Appl. Phys. Lett. 93 (2008) 202107. [22] E. Fortin, F. Weichman, Can. J. Phys. 44 (1966) 1551-&. [23] H. Shimada, T. Masumi, J. Phys. Soc. Jpn. 58 (1989) 1717. [24] B. Tazenkov, F. Gruzdev, Sov. Phys. Solid State 16 (1974) 460. [25] M. Zouaghi, M. Tapiero, J.P. Zielinger, R. Burgraf, Solid State Commun. 8 (1970) 1823.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Semiconductor thin films directly from minerals—study of structural, optical, and transport characteristics of Cu2O thin films from malachite mineral and synthetic CuO K.R. Balasubramaniam a,⁎, V.M. Kao a, J. Ravichandran b, P.B. Rossen c, W. Siemons d, 1, J.W. Ager III
a
a
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Applied Science and Technology, University of California, Berkeley, CA 94720, USA Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA d Department of Physics, University of California, Berkeley, CA 94720, USA b c
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 23 January 2012 Accepted 26 January 2012 Available online 31 January 2012 Keywords: Photovoltaics Copper oxide Minerals X-ray diffraction Optical properties Transport characteristics Mass spectroscopy of recoiled ions
a b s t r a c t We demonstrate the proof-of-concept of using an abundantly occurring natural ore, malachite (Cu2CO3 (OH)2) to directly yield the semiconductor Cu2O to be used as an active component of a functional thin film based device. Cu2O is an archetype hole-conducting semiconductor that possesses several interesting characteristics particularly useful for solar cell applications, including low cost, non-toxicity, good hole mobility, large minority carrier diffusion length, and a direct energy gap ideal for efficient absorption. In this article, we compare the structural, optical, and electrical transport characteristics of Cu2O thin films grown from the natural mineral malachite and synthetic CuO targets. Growth from either source material results in single-phase, fully epitaxial cuprous oxide thin films as determined by x-ray diffraction. The films grown from malachite have strong absorption coefficients ( 10 4 cm − 1), a direct allowed optical bandgap ( 2.4 eV), and majority carrier hole mobilities ( 35 cm 2 V − 1 s− 1at room temperature) that compare well with films grown from the synthetic target as well as with previously reported values. Our work demonstrates that minerals could be useful to directly yield the active components in functional devices and suggests a route for the exploration of low cost energy conversion and storage technologies. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Low cost energy conversion and storage devices will be crucial for solving the current global energy crisis [1,2]. Realization of functional devices for this purpose by using the natural mineral directly is very enticing; however, such an approach usually results in components with degraded performance due to the presence of numerous impurities in the material. One other serious impediment towards realizing low cost devices is the prohibitive cost of purification for the active layer materials [3]. Thus, the candidate low cost materials should be earthabundant and should lend themselves to facile, inexpensive purification procedures. In the particular case of solar cells, it has become clear that the eventual implementation of a specific materials platform will come down to the key factors of material extraction cost, annual electricity potential of the material, abundance, and environmental friendliness of the material [1]. In this manuscript, we demonstrate the idea of low cost energy conversion devices using Cu2O based solar cells derived directly from minerals as our model system. Cu2O is one of the cheap, abundant, non-toxic oxide material that has been the focus of several studies as the p-type absorber, Cu2O, in a
p–n heterojunction with different n-type materials [4–11] or as a Schottky barrier cell with various metals or transparent conducting oxides [12–14]. Cu2O as the active layer is widely abundant in the form of naturally occurring minerals, making it an excellent candidate for our proof-of-concept study. Cuprite (the mineral form of Cu2O), however, is not one of the most abundant copper-based minerals; this designation is reserved for copper sulfides, namely copper pyrites and chalcocite [15]. One of the more common copper ore is malachite (copper carbonate copper hydroxide, Cu2CO3(OH)2 [15]), which on heating to modest temperature of 300 °C yields Cu2O [16,17]. In this work, we demonstrate the viability of directly obtaining the active absorber layer in solar cells using naturally occurring minerals as the starting material. The similarity between films grown from a commercial CuO target and a malachite target obtained through the straightforward decomposition of malachite into p-type Cu2O during pulsed laser deposition (PLD) is demonstrated in this work. Material characteristics relevant to the use of this material as the active layer in a heterojunction solar cell, specifically the structural characteristics, optical absorption, and electrical transport are compared. 2. Experimental details
⁎ Corresponding author. Tel.: + 1 510 486 4995. E-mail address: [email protected] (K.R. Balasubramaniam). 1 The author is currently at Oakridge National Laboratory. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.01.041
Films were deposited on one-side and two-side polished single crystal substrates of (LaAlO3)0.3–(Sr2AlTaO6)0.7 [LSAT] (001) obtained
K.R. Balasubramaniam et al. / Thin Solid Films 520 (2012) 3914–3917
3. Results and discussion 3.1. Structural and chemical characterization Fig. 1a shows a typical XRD θ–2θ? scan (along the (00l) of the LSAT substrate) of Cu2O film grown from both the malachite and synthetic CuO target on the LSAT substrate. The substrate peaks are marked as “s” and correspond to LSAT (00l). The only other peak in this 2θ range at 2θ = 41.94° (d = 2.15 Å, marked as “f”) corresponds to the Cu2O (002), and proves that we have produced highly-oriented, phasepure Cu2O for films grown from both the malachite (top data, Fig. 1a) and CuO (bottom data, Fig. 1a) targets. This suggests that under the growth conditions used, along with the thermal energy supplied through laser impingement on the target enables Cu2O phase formation during PLD growth. The measured (similar) full width at half maximum (FWHM) of the rocking curve is shown as the inset in Fig. 1a of 0.26° for the (002) reflection of Cu2O. The inplane epitaxy of the Cu2O film grown from malachite was determined by registering azimuthal ϕ-scans and by comparing the locations in ϕ-space of the Cu2O{022} and the LSAT{022} reflections (similar results were obtained for the Cu2O film obtained from the CuO target but are not shown here). The two phi scans are overlaid in Fig. 1b, although it should be noted that they were acquired at different 2θ angles (2θ = 61.32° for Cu2O{022} and 2θ = 68.44° for LSAT{022}) with ψ = 45°. The observed four-fold intensity of the Cu2O{022} suggests that there is a cube-on-cube texture. The ϕ-scans indicate that the Cu2O{022} peaks are aligned with the LSAT{022} peaks in ϕ-space. Taken together, the diffraction scans for Cu2O given in Fig. 1a and b
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from Crystec GmbH, Berlin, Germany. All the LSAT substrates underwent ultrasonic cleaning in trichloroethylene, acetone, and isopropanol before an annealing step at 1050 °C for 3 hours in flowing O2 prior to loading into the pulsed laser deposition (PLD) chamber. PLD was carried out using a KrF excimer laser operating at a wavelength of 248 nm. ≈400 nm thick Cu2O films were obtained from the malachite target (Wards Natural Science Establishment and originally mined in the Kolwezi mines, Shaba, Zaire) as well as a synthetic CuO target (Praxair Specialty Ceramics, Woodinville, WA). All the Cu2O films were grown at a temperature of 700 °C, chamber pressure of 0.66 Pa O2, at an energy density of ≈1.0 J/cm 2; the films were then cooled down in the deposition atmosphere before retrieving from the chamber. The crystalline nature (phase, crystalline quality, and epitaxial relationship) of the films was characterized using a Panalytical X'Pert X-ray diffractometer (CuKα radiation, λ = 1.54054 Å). For in situ time-of flight-ion scattering and recoil spectroscopy (TOF-ISARS) 39 [18] measurements were performed using K ions with an energy of 10 keV and an Ionwerks (Houston, USA) mass spectrometry of recoiled ions reflectron detector. The angle of incidence, α, was set to 15° and the detector was fixed at 60°. Ions with masses up to 250 u can be detected. The Cu2O film was grown from malachite target under the conditions described above and in situ measurements were performed after cooling the sample to 150 °C. A Hitachi U3000 UV–vis spectrometer was used to obtain reflection and transmission data from ≈400 nm thick Cu2O films grown on double-side polished LSAT substrates. This was converted to absorbance (α) using the Beer–Lambert relation. Hall measurements were carried out in a custom setup equipped with a 1.5 Tesla electro-magnet and a sample holder whose temperature can be varied from 300 to 800 K. All measurements were carried out in the van der Pauw geometry with Au metal contacts of dimensions ≈1 mm at the corners of the sample (5 mm × 5 mm). Current–voltage sweeps were carried out to ensure ohmic contact and magnetic field–voltage sweeps were carried out to ensure there is no contribution from magnetoresistance. The carrier concentration, n was calculated from the Hall resistance, RH with the transverse voltage measured at 1 Tesla field.
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φ (degrees) Fig. 1. (a) Normal XRD? θ–2θ scans of the Cu2O on LSAT(001) substrates synthesized from a synthetic CuO (bottom red graph) and malachite mineral (top black graph) target. The LSAT(00l) peaks are marked “s” and Cu2O(002) peak is marked as “f.” The inset shows the rocking curve around the Cu2O 002 peak, FWHM = 0.26°. (b) Azimuthal offaxis φ scan of {022} Cu2O film grown from the malachite target and LSAT substrate.
show that the epitaxial relationship between the Cu2O and the LSAT(001) layers is {001}Cu2O|{001}LSAT||〈110〉Cu2O|〈110〉LSAT. These combined XRD results thus indicate the high crystalline quality of our Cu2O films obtained from both the targets. Fig. 2 shows the ionic spectra obtained from a TOF-ISARS study from a malachite film grown on LSAT (001) (red curve) and from a malachite target (black curve). A qualitative assessment of the surface chemical composition of both samples reveals the presence of Cu mainly [the strong K peak reflects incident ion beam used]. However, the other major species present in the target, namely C, and the impurities P, S, Cl do not transfer to the substrate. Even though there is negligible transfer of impurities from the malachite target to the film, it is worthwhile comparing the optical and transport properties of films grown from malachite and synthetic CuO targets, as these properties are important for the photovoltaic performance of the devices. 3.2. Optical absorption The optical absorbance (α) spectra were derived from the measured transmittance (T) and reflectance (R) spectra of the Cu2O films deposited on double-side polished LSAT (001) substrates. In order to determine the optical band gaps (Eg) of copper oxide films, (αhν) 2 is plotted as a function of the incident photon energy (hν) (Fig. 3). Linear extrapolation of the bounding absorption curves yields direct allowed transitions in the range of 2.47–2.54 eV for the films. This value of the band gap for films grown from the malachite target (all curves in black) agrees well with the absorption band gap for
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Fig. 2. Time-of-flight mass spectra obtained with an incident potassium ion beam to scatter surface species from the malachite target (red curve) and film grown from this target (black curve) at room temperature showing the transfer of elements. Note: the large peak marked K in the figure is due to the reflected potassium ion beam.
Cu2O films grown from synthetic CuO targets (red curves, Fig. 3). The values observed here, however, are different from the oft-quoted band gap of around 2.1 eV [19] and it is important to make the distinction here between the electronic bandgap and the optical band gap obtained from absorption measurements. In Cu2O the highest valence band and the lowest conduction band (Eg = 2.1 eV) have the same s-symmetry, and a transition involving photon absorption is dipole forbidden [20]. For this reason, measurable absorption happens only at higher energies, in this case, for our film thicknesses, at 2.4 eV. This implies a small UV shift in the absorbed light wavelengths, and hence a reduction in the maximum possible Jsc with Cu2O as the main absorber layer in a solar cell. 3.3. Transport characteristics More importantly, the effect of impurities, even in minor quantities is usually seen in a reduction of the majority and minority carrier mobility. The hole mobility values reported for Cu2O range from 15 to 100 cm 2 V − 1 s − 1 at room temperature; the large range is primarily due to deviations in stoichiometry, which strongly affects the carrier transport properties in Cu2O [21,22]. The highest hole mobility for epitaxial thin films reported to date is 90 cm 2 V − 1 s − 1 [21]. Minority
Fig. 4. Hall mobility of 400 nm thick Cu2O films obtained from CuO (red data points) and malachite (black data points) targets on two-side polished LSAT substrates plotted as a function of the absolute temperature. The red curve is a typical T− 4 fit of the mobility data for CuO sample 1.
carrier diffusion length is one of the important metrics for the high performance of a solar cell. Even though it is not related directly to the majority carrier mobility, the electrical transport characteristics of a material can be qualified with majority carrier mobility as a scale. In Fig. 4 we show the temperature dependent mobility for the films grown from malachite (black data points) and synthetic CuO (red data points) target. It is clear that there is no significant difference in the mobility at room temperature and all samples show a T − 4 dependence above room temperature. One such fit (red line) of the CuO sample 1 in Fig. 4 is shown for reference. Several groups have reported the high temperature mobility of Cu2O in the bulk single crystalline form with or without photoexcitation [23–25] and our current observation is in accordance with these reports. Currently, there is no consensus in the literature on a theoretical understanding of the observed temperature dependence of mobility for Cu2O at high temperatures. At high temperatures, typically the dominant scattering of holes is attributed to the longitudinal optical (LO) phonon modes in oxides but this mechanism alone cannot account for the observed temperature dependence in our case. In short, both the optical and transport properties are not significantly different for the films grown from the malachite and synthetic CuO targets. 4. Summary In conclusion, we have demonstrated realization of the idea of producing materials for active components in energy conversion devices directly from natural minerals with semiconducting Cu2O synthesized from natural mineral malachite for solar cells asour model system. Thin Cu2O films obtained from the malachite target exhibit very similar optical and transport properties in comparison to that grown from pure, synthetic CuO targets. This suggests that the possibility of solar cells produced directly from minerals as the characteristics of the absorber layer in our demonstration is limited by similar mechanisms, and hence, is not related to the impurities present in the mineral. Acknowledgement
Fig. 3. Tauc plots, (αhν)2 vs. hν where α was determined from measured transmission and reflection spectra on ≈ 400 nm thick Cu2O films obtained from CuO (red data points) and malachite (black data points) on two-side polished LSAT substrates. The black dotted lines are guides to the eye for the linear extrapolation of the bounding curves in the data shown. The intercepts of the linear extrapolation yield the direct allowed optical transitions corresponding to the bandgap.
The work at Berkeley was performed within the Helios Solar Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. The authors wish to acknowledge the help and support provided by Prof. R. Ramesh at UC Berkeley. The authors also thank Prof. Lane W Martin (currently at UIUC) for useful
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discussions. J.R. would like to thank the Link energy fellowship. W.S. thanks the Dutch Organization for Scientific Research (NWO). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C. Wadia, A.P. Alivisatos, D.M. Kammen, Environ. Sci. Technol. 43 (2009) 2072. N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. 103 (2006) 15729. J. Kantner, A. Mileva, D. Kammen, , 2009, p. 1. A. Mittiga, E. Salza, F. Sarto, M. Tucci, R. Vasanthi, Appl. Phys. Lett. 88 (2006) 163502. H. Tanaka, T. Shimakawa, T. Miyata, H. Sato, T. Minami, Appl. Surf. Sci. 244 (2005) 568. J. Katayama, K. Ito, M. Matsuoka, J. Tamaki, J. Appl. Electrochem. 34 (2004) 687. T. Minami, H. Tanaka, T. Shimakawa, T. Miyata, H. Sato, Jpn. J. Appl. Phys. 43 (2004) L917. H. Tanaka, T. Shimakawa, T. Miyata, H. Sato, T. Minami, Thin Solid Films 469–470 (2004) 80. W. Siripala, A. Ivanovskaya, T.F. Jaramillo, S.H. Baeck, E.W. McFarland, Sol. Energy Mater. Sol. Cells 77 (2003) 229. L. Papadimitriou, N. Economou, D. Trivich, Solar Cells 3 (1981) 73. J. Herion, E.A. Niekisch, G. Scharl, Sol. Energy Mater. 4 (1980) 101.
3917
[12] W. Sears, E. Fortin, J. Webb, Thin Solid Films 103 (1983) 303. [13] V.F. Drobny, D.L. Pulfrey, Thin Solid Films 61 (1979) 89. [14] V.F. Drobny, D.L. Pulfrey, Photovoltaic Specialists Conference, 13th, Washington, DC, June 5–8, 1978, Conference Record.(A79-40881 17-44), Institute of Electrical and Electronics Engineers, Inc, New York, 1978, p. 180. [15] W.G.I. Davenport, M.J. King, M.E. Schlesinger, A.K. Biswas, Extractive Metallurgy of Copper, Pergamon, , 2002. [16] J.F. Scaife, Can. J. Chem. 35 (1957) 1332. [17] E. Holland, G. Gilligan, J. Phys. Chem. 31 (1927) 728. [18] W.J. Rabalais, Principles and Applications of Ion Scattering Spectrometry: Surface and Chemical and Structural Analysis, Wiley Interscience, 2002. [19] A. Chen, H. Long, X. Li, Y. Li, G. Yang, P. Lu, Vaccum 83 (2009) 927. [20] V.T. Agekyan, Phys. Status Solidi A 43 (1977) 11. [21] K. Matsuzaki, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Appl. Phys. Lett. 93 (2008) 202107. [22] E. Fortin, F. Weichman, Can. J. Phys. 44 (1966) 1551-&. [23] H. Shimada, T. Masumi, J. Phys. Soc. Jpn. 58 (1989) 1717. [24] B. Tazenkov, F. Gruzdev, Sov. Phys. Solid State 16 (1974) 460. [25] M. Zouaghi, M. Tapiero, J.P. Zielinger, R. Burgraf, Solid State Commun. 8 (1970) 1823.