4 (x ≈ 0.83) Thin Film for Solar Energy Conversion - ACS Publications

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Earth-abundant Orthorhombic BaCu2Sn(SexS1-x)4 (x#0.83) Thin-Film for Solar Energy Conversion Jie Ge, Yue Yu, and Yanfa Yan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00324 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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Earth−Abundant Orthorhombic BaCu2Sn(SexS1−x)4 (x≈0.83) Thin−Film for Solar Energy Conversion Jie Ge,* Yue Yu, and Yanfa Yan* Department of Physics and Astronomy & Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States Corresponding Author E−mail: Dr Jie Ge, [email protected]; Prof Yanfa Yan, [email protected]

ABSTRACT

Exploiting renewable solar energy in terms of solar−driven water splitting and photovoltaic devices provides the clean and efficient paths to overcome the diminishing fossil fuel storage

and

greenhouse

effect.

Here

a

state−of−the−art

earth−abundant

BaCu2Sn(Se0.83S0.17)4 (BCTSSe) thin film has been presented as a promising top−cell absorber in the tandem photoelectrochemical water splitting and photovoltaic conversion devices. Our BCTSSe thin film exhibits a direct bandgap of 1.85 eV with strong optical absorption coefficients (α > 104 cm−1). Without extensive interface and electrode optimization, our best BCTS PEC cell showed a photocurrent of 5 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE). Moreover, our best−performing BCTSSe prototype

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photovoltaic

cell

with

a

configuration

of

fluorine

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doped

SnO2

(FTO,

back

contact)/BCTSSe/CdS/ZnO/aluminium doped ZnO (AZO, front contact) has achieved an 1.57% efficiency.

Maintext Abundant solar energy emerges as candidate commensurate with our growing energy requirements to displace the diminishing fossil fuel resources and reduce CO2 emissions. Solar−driven water splitting and photovoltaic (PV) devices pave the way for efficient solar energy scavenging, since they can directly convert solar energy into storable & clean chemical fuel of hydrogen and electricity power. CdTe1 and CuInxGa1−xSe2 (CIGS)2 based PV and PEC technologies have demonstrated reasonable solar to electricity (>20%) and solar to hydrogen (STH, >10%) conversion efficiencies, but at high cost due to the scarcity of Cd/Te/In related raw materials. Materials utilization is becoming increasingly critical to determine the manufacture cost. Thus, it requires the manufacture of solar devices to use low−cost earth−abundant materials as much as possible. Kesterite Cu2ZnSn(SexS1−x)4 (CZTSSe) compound made of earth−abundant Cu/Zn/Sn metal elements is one promising candidate for low−cost PEC and PV solar devices.3-4 But, the current power conversion efficiencies (PCE) remain considerably lower than those of CdTe and CIGS. This issue primarily stems from the defect disparity in kesterite materials: first, CZTSSe exhibits more deep cation anti−site defects due to the close ionic sizes among Cu+ (0.91

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Å) and Zn2+ (0.88 Å) and Sn4+ (0.83 Å), as a result, more non−radiative recombination centers are produced; second, since kesterite structure contains three cation elements with the same coordination, a large number of defects and defect clusters would easily form and cause severe band tailing and potential fluctuation.5 One trial approach to mitigate this problem is to replace Cu or Zn with larger sized atoms, for instance, Ag for Cu or Cd for Zn.6-7 But not as expected, this approach does not alter the coordination among the cation elements in lattice and it may cause additional issues in electronic structures.8 Besides, Ag and Cd are not earth−abundant elements, the use of which also increases the cost for device fabrication. Thus, it motivates us to explore new earth−abundant materials that can meet these requirements for low−cost and efficient solar energy conversion. It has been recently suggested that replacing Zn2+ (0.88 Å) by cations with much larger ionic sizes such as Ba2+ (1.49 Å) and Sr2+ (1.32 Å) may lead to more favourable defect properties.5 BaCu2SnS4 (BCTS) and SrCu2SnS4 (SCTS) adopt the trigonal strucuture.9-12 A prototype solar cell based on trigonal BCTS has been reported.13 In the trigonal lattice, each Cu and Sn is tetrahedrally surrounded by S ions, wherein only [CuSe4]7− tetrahedron is quite regular. Two [CuS4]7− and one [SnS4]4− tetrahedrons share a corner to form a three−dimensional framework, and thus each sulfur ion is three−fold coordinated with two Cu and one Sn. Each Ba/Sr is in eight−fold coordination, forming a distorted cubic antiprism polyhedron. In band structures, the antibonding states of Cu 3d and S 3p consist of the valence band edges of (Ba, Sr)Cu2SnS4, while their conduction band edges are primarily composed of Sn 5s. The orbitals of Ba/Sr contribute little to the band edges, therefore, BCTS and SCTS have the same band gap value.5 The experimental results suggest this type of compounds with a red color exhibits a 2.05 eV optical gap.13-15 But, this

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band gap value is too large to sufficiently absorb the solar spectrum in the visible region. One legitimate approach to minimize the band gap is to replace S with Se. The selenide compounds, BaCu2SnSe4 (BCTSe) and

SrCu2SnSe4 (SCTSe), crystallize in the

orthorhombic system with Ama2 symmetry.16-17 Similar to trigonal, orthorhombic structure also comprises eight−fold coordinated Ba/Sr with Se, forming a square antiprism polyhedron. Still, Cu and Sn are tetrahedrally surrounded by Se ions, wherein only [SnSe4]4− tetrahedron is quite regular. Besides, the [CuSe4]7− polyhedron are pairwise interconnected, and isolated [SnSe4]4− tetrahedron connect them via corner−sharing to a three−dimensional network.18-19 Given the similarities in structure and constituent elements, orthorhombic selenide is also expected to exhibit the favorable defect properties analogous to the trigonal sulfide counterpart. For band structures, Sn 5s states are still the primary component of the conduction band edge of BCTSe, whereas the antibonding states of Cu 3d and Se 4p consist of its valence band edge.13, 18 Since the Se 4p orbitals are higher than S 3p in energy, BCTSe will have a much higher valence band edge than BCTS. Thus, BCTSe with a black color shows a smaller band gap (~1.75 eV) than its sulfide.13 Overall, the bandgap of BCTSe has been considered suitable as a top cell in tandem PEC and PV solar cell applications,20-21 but the PEC and PV properties of orthorhombic BCTSe thin films have not been studied. In this work, we use scalable co−sputtering of Cu, SnS, and Ba targets to make precursor films on fluorine doped SnO2 (FTO) substrate at 100 °C (Figure S1). The recrystallization and grain growth was carried out at 550 °C for 30 min with selenium vapour under argon atmosphere, under which condition FTO substrate is thermodynamically stable.22-23 Multiple circles of prerequisite experiments were carried out to ensure a Cu poor composition of Cu/(Sn+Ba)=0.8−0.9. This composition facilitates the formation of shallow

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acceptor Cu vacancy and enables the p−type conductivity. Followed by a set of characterizing

and

testing

experiments,

a

Se

rich

orthorhombic

sulfoselenide

BaCu2Sn(SexS1−x)4 (BCTSSe, x≈0.83) thin film was obtained for the first time in experimental, which exhibits a band gap of 1.85 eV and the ability as a promising light−harvesting material in PEC and PV solar energy conversion. Figure 1a and 1b show the cross−sectional and top view SEM images of the selenized film on FTO substrate, respectively, which indicate an equiaxed large−grain (>2 µm) microstructure along with micro−voids being observed at the film rear side and on the film top. The composition analysis by Energy Dispersive X−ray Spectroscopy (EDX) of the area boxed in Figure 1b shows a Cu poor & Se rich composition for the annealed BCTSSe film with Cu/(Sn+Ba)≈0.86 and Se/(S+Se)≈0.83. The electron beam for the EDX analysis was excited by 15 kV. Thus, the detecting depth is estmiated to be 1 µm,24 much less than the film thickness. This can be evidenced by the absence of O signal at ~0.52 keV from FTO substrate. Thus, FTO contributed little to the Sn EDS signals due to limited detecting depth of 15 kV excited electron beam.

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Figure 1 SEM cross−sectional (a) and top−view (b) images of the BCTSSe film on FTO substrate; Energy Dispersive X−ray Spectroscopy (EDX) profile from the boxed area in Figure 1b, with the electron beam being excited by 15 kV. The scale bars on Figure 1a and Figure 1b represent 1 µm and 10 µm, respectively.

Figure 2a shows the Theta−2Theta X–ray diffraction (XRD) pattern of the selenized BCTSSe film on FTO. XRD pattern shows high crystallinity of the selenized film without impurity phases segregation (Figure S2) and the entire XRD peaks are assignable to the reflex (red markers) from orthorhombic BaCu2SnSe4 (PDF 97−017−0857). An apparent peak−shift to higher angles observed from the magnified XRD peaks suggests the smaller lattice constants than BCTSe, which is due to the smaller atomic size of sulfur than selenium. The calculated lattice constants, a = 11.0551 Å, b = 11.1712 Å and c = 6.7181 Å, are smaller than those of pure selenide BaCu2SnSe4 (a = 11.1215 Å, b = 11.2373 Å and c = 6.7531 Å, PDF 97−005−2685). Irreducible representation of orthorhombic BCTSe with Ama2 symmetry [point group C2v (mm2)] exhibits 45 optical modes (Γoptic = 12 A1 + 10 A2+

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10 B1 +13 B2) regardless TO/LO splitting. Among the optical modes, infrared active modes include 12A1 + 10B1 + 13B2 and Raman active modes include 12A1 + 10A2 + 10B1 + 13B2.25 Figure 2b shows the Raman scattering spectrum of the BCTSSe film on FTO substrate, where BCTSSe exhibits five visible Raman active modes with the dominant peak at 190 cm−1. No secondary phases was resoluble by Raman, for instance, CuSe (260 cm-1), SnSe (107 cm-1 & 181 cm-1), and Cu2SnSe3 (179 cm-1).26 Since the XRD pattern in Figure S2 shows a single phase film, these observed Raman peaks can be entirely assignable to orthorhombic BCTSSe. For the quaternary chalcogenides, the strongest vibrational line should come from the totally symmetric vibrations corresponding to the motion of chalcogen atoms alone.27 Thus, the dominant Raman band at 190 cm-1 should correspond to the motion of selenium atoms while the rest atoms remain rest. A weak wideband at the higher frequency of 333 cm−1 can be assignable to the vibrational mode of sulfur atoms with respect to the metal atoms, since the atomic mass of sulfur is much smaller than that of selenium.27-29

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Figure 2 X–ray diffraction (XRD) pattern (a), Raman spectrum (b), optical transmission spectrum (c), and absorption coefficient [α] and band gap plot (d) of the BCTSSe film grown on FTO substrate. Note: in Figure 2a, the intense peaks @ at 2-Theta = 26.500°, 33.738°, 37.800°, and 51.440° correspond to the 110, 101, 200, and 211 reflex of FTO substrate (SnO2, PDF 97– 026–2768), respectively; the rest peaks are indexed to orthorhombic BaCu2SnSe4 (PDF 97−017−0857, red bars); absorption coefficient was derived from transmission spectrum as per the Beer−Lambert’s law.

Figure 2c shows the optical transmission spectrum of the BCTSSe film grown on FTO substrate. As seen, the film demonstrates the optical uniformity with interference extrema. The low IR transmittance indicates FTO substrate remains well conductive as the intact one after selenization. As shown in Figure 2d, this BCTSSe film exhibits an absorption coefficient α ≈ 0.8−5×104 cm−1 at the photon energy region E > 1.8 eV. The fundamental bandgap can be estimated to be 1.84 eV from the Tauc plot of  ∙   . 

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for a direct semiconductor. This bandgap value can ensure the thermodynamic voltage to split water while absorb the solar spectrum more efficiently than trigonal BCTS sulfide. This bandgap value additionally suggests BCTSSe film is suitable as a top−cell absorber in the tandem PEC and PV solar cells.20-21

Figure 3 Photoelectrochemical (PEC) testing of BCTSSe photocathode (glass/FTO/BCTSSe/Pt): linear sweep voltammetric (LSV) curves under manually−chopped/unchopped light and in dark (a); Incident Photon to Current Efficiency (IPCE) spectrum of FTO/BCTSSe/Pt photocathode, with the IPCE/  ∝  (wavelength) curve for band gap estimation, and integrated current densities from IPCE data (b); Note: the PEC and IPCE measurements were carried out using 3−electrode configuration in a neutral electrolyte (pH=6.4); IPCE was measured at −0.15 V vs RHE; RHE: reversible to hydrogen electrode.

Figure 3a shows the cathodic photocurrent generated by bare BCTSSe film measured in a neutral solution using a 300 W Xe light source (~one sun intensity), which indicates a p−type conductivity for BCTSSe. A photocurrent of 5 mA cm−2 is reached at 0 V vs RHE for a FTO/BCTS/electrolyte PEC cell. The onset of photocurrent at ~0.4 V is much less than the attainable photovoltage for a 1.85 eV bandgap (~1.5 V) based on the Shockley−Queisser limit, possibly due to the presence of over−potential, the non−optimized band alignment with electrolyte, and the less-than-ideal material quality. The Incident Photon to Current Efficiency (IPCE) measurement was carried out at −0.15 V vs RHE in a

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neutral solution to show the spectral distribution of generated photocurrent (Figure 3b). IPCE data suggests that BCTSSe converts photons into currents and contributes to the hydrogen evolution efficiency. The photocurrent integrated from the IPCE data, 7.65 mA cm−2, is consistent with that from the light LSV scan at −0.15 V vs RHE. The 1st derivative curve of IPCE shows the inflection point at 1.85 eV, quite in line with the band gap edge of BCTSSe film from the optical measurement in Figure 2d.

Figure 4 light and dark current−voltage (J−V) characteristics (a); unbiased external quantum efficiency (EQE) curve, with the EQE/  ∝  (wavelength) curve for band gap estimation, and integrated current densities from EQE data (b) of BCTSSe PV solar cell (glass/FTO/BCTSSe/CdS/ZnO/AZO).

Using the selenized BCTSSe films, solar cells with a total area of 0.08 cm2 were fabricated based on the structure of FTO back contact/BCTSSe/CdS/ZnO/AZO front contact, where AZO is aluminium−doped ZnO. Figure 4a shows the J–V characteristics of our best PV device under one sun illumination from AZO side, with the statistic distribution of device parameters being given in Figure S3. Our best device shows a PCE of 1.57%, an open circuit voltage (VOC) of 0.613 V, a Fill Factor (FF) of 37.7%, and a short circuit current

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density (JSC) of 6.78 mA cm−2. However, PCE is still much less than the ideal value for a 1.85 eV band gap (~27%) based on the Shockley−Queisser limit, due to the low VOC, FF, and JSC. These issues could stem from non−optimized buffer layers, window layers, and material quality of BCTSSe. Nonetheless, this device result still presents a milestone for developing an initial BCTSe solar device prototype, but calls for further optimization to improve the less−than−ideal device performance. The external quantum efficiency (EQE) spectrum shown in Figure 4b suggests a band gap of 1.85 eV for BCTSSe, which is consistent with IPCE and optical transmission measurements. The EQE values are reduced in the wavelength range from 350 to 550 nm, due to the absorption losses of ZnO and CdS window layers. The integrated JSC from the unbiased EQE, 6.86 mA cm−2, is consistent with the J−V measurement. In this work, we prepared a polycrystalline BaCu2Sn(Se0.83S0.17)4 (BCTSSe) thin film on a FTO glass substrate by scalable co−sputtering of a sulfide precursor followed by selenization annealing. This new compound crystallizes in the orthorhombic system with Ama2 symmetry as its selenide counterpart BaCu2SnSe4 (BCTSe) does. This sulfoselenide alloy exhibits a fundamental band gap of ~1.85 eV and a high absorption coefficient of ~104 cm−1 and a p−type conductivity. The bare BCTSSe photocathode without any interface modification exhibits a photocurrent of 5 mA cm−2 at 0 V vs RHE measured in a neutral electrolyte. Also, we establish a PV device prototype with the configuration of FTO/BCTSSe/CdS/ZnO/AZO, which yields an 1.57% conversion efficiency under AM 1.5G illumination. These trial results suggest that orthorhombic BCTSe and its sulfoselenide alloy are promising top−cell absorbers for efficient and low−cost tandem PEC water−splitting and PV devices and deserve further investigation in the future.

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Acknowledgements The work was supported by the National Science Foundation under contract no. CHE−1230246 and

DMR−1534686.

This

paper

presents

results

from

an

NSF

project

(award

number CBET−1433401) competitively−selected under the solicitation “NSF 14−15: NSF/DOE Partnership on Advanced Frontiers in Renewable Hydrogen Fuel Production via Solar Water Splitting Technologies”, which was co−sponsored by the National Science Foundation, Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET), and the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Supporting Information Available: experimental details; SEM cross-sectional image and digital photo of a precursor film; statistics of PV solar device results.

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(22). Simchi, H.; Larsen, J. K.; Kim, K.; Shafarman, W. N. Improved Performance of Ultrathin Cu(InGa)Se Solar Cells With a Backwall Superstrate Configuration. IEEE J. Photovoltaics 2014, 4, 1630-1635. (23). Simchi, H.; McCandless, B. E.; Meng, T.; Shafarman, W. N. Structure and Interface Chemistry of MoO3 Back Contacts in Cu(In,Ga)Se2 Thin Film Solar Cells. J. Appl. Phys. 2014, 115, 033514. (24). Ishizuka, S.; Yamada, A.; Fons, P. J.; Shibata, H.; Niki, S. Structural Tuning of WideGap Chalcopyrite CuGaSe2 Thin Films and Highly Efficient Solar Cells: Differences from Narrow-Gap Cu(In,Ga)Se2. Prog. Photovoltaics 2014, 22, 821-829.25. (25). Kroumova, E.; Aroyo, M. I.; Perez-Mato, J. M.; Kirov, A.; Capillas, C.; Ivantchev, S.; Wondratschek, H. Bilbao Crystallographic Server : Useful Databases and Tools for PhaseTransition Studies. Phase Transitions 2003, 76 (1-2), 155-170. (26). Fontane, X.; Izquierdo-Roca, V.; Fairbrother, A.; Espindola-Rodriguez, M.; LopezMarino, S.; Placidi, M.; Jawhari, T.; Saucedo, E.; Perez-Rodriguez, A. In Selective Detection of Secondary Phases in Cu2ZnSn(S, Se)4 based Absorbers by Pre-resonant Raman Spectroscopy, Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 16-21 June 2013; 2013; pp 25812584. (27). Himmrich, M.; Haeuseler, H. Far Infrared Studies on Stannite and Wurtzstannite Type Compounds. Spectrochim. Acta, Part A 1991, 47, 933-942. (28). Ge, J.; Chu, J.; Jiang, J.; Yan, Y.; Yang, P. The Interfacial Reaction at ITO Back Contact in Kesterite CZTSSe Bifacial Solar Cells. ACS Sustainable Chem. Eng. 2015, 3, 3043-3052. (29). Ge, J.; Yu, Y.; Ke, W.; Li, J.; Tan, X.; Wang, Z.; Chu, J.; Yan, Y. Improved Performance of Electroplated CZTS Thin-Film Solar Cells with Bifacial Configuration. ChemSusChem 2016. DOI: 10.1002/cssc.201600440

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