Article pubs.acs.org/cm
Structural, Electronic, and Optical Properties of Cu2NiSnS4: A Combined Experimental and Theoretical Study toward Photovoltaic Applications Sachin Rondiya,†,⊥ Nitin Wadnerkar,‡,⊥ Yogesh Jadhav,§ Sandesh Jadkar,† Santosh Haram,§ and Mukul Kabir*,‡,∥ †
Department of Physics, Savitribai Phule Pune University, Pune 411007, India Department of Physics, Indian Institute of Science Education and Research, Pune 411008, India § Department of Chemistry, Savitribai Phule Pune University, Pune 411007, India ∥ Centre for Energy Science, Indian Institute of Science Education and Research, Pune 411008, India ‡
S Supporting Information *
ABSTRACT: Earth-abundant quaternary chalcogenides are promising candidate materials for thin-film solar cells. Here we have synthesized Cu2NiSnS4 nanocrystals and thin films in a novel zincblende type cubic phase using a facile hot-injection method. The structural, electronic, and optical properties are studied using various experimental techniques, and the results are further corroborated within first-principles density functional theory based calculations. The estimated direct band gap ∼ 1.57 eV and high optical absorption coefficient ∼ 106 cm−1 indicate potential application in a low-cost thin-film solar cell. Further, the alignments for both conduction and valence bands are directly measured through cyclic voltametry. The 1.47 eV electrochemical gap and very small conduction band offset of −0.12 eV measured at the CNTS/CdS heterojunction are encouraging factors for the device. These results enable us to model carrier transport across the heterostructure interface. Finally, we have fabricated a CNTS solar cell device for the first time, with high open circuit voltage and fill factor. The results presented here should attract further studies.
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vacuum based approaches.13−16 An analogous material, Cu2NiSnS4 (CNTS), has been synthesized very recently;24−31 however, a detailed experimental understanding of electronic and optical properties and band alignments leading to solar cell device fabrication remain elusive. Moreover, to the best of our knowledge, to date there has been no first-principles investigation to complement experimental efforts. Here, we synthesize stoichiometric CNTS nanocrystals and thin films in zincblende structure through cost-effective and robust hot-injection method, for the first time. The zincblende crystal structure is achieved over the metastable wurtzite phase by altering the initial reaction condition. The structure, size distribution, surface morphology, and composition of the asprepared CNTS nanocrystals and thin films are characterized by X-ray diffraction (XRD) pattern, Raman spectrum, (highresolution) transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and other complementary experimental
INTRODUCTION
Solar photovoltaic (PV) materials generate electron−hole pairs through light absorption and, in a device architecture, generate electricity without any carbon footprint.1−4 Much effort has been invested in designing PV materials such as amorphous silicon,5,6 CdTe,7,8 and Cu(In,Ga)Se2 (CIGS)9−11 thin films due to their excellent optical properties and concurrent high PV efficiency. However, most of these materials contain rare-earth and/or toxic metals limiting sustainable large-scale applications. In contrast, earth-abundant quaternary chalcogenides Cu2MSnX4 (M = Zn, Fe, Co, Ni and X = Se, S) have attracted attention in recent years as potential alternatives.12−30 A direct band gap commensurate with optical absorption in the visible range and high absorption coefficient (>104 cm−1) make these materials promising. Indeed, as high as 12.6% energy conversion efficiency has been achieved recently for Cu2ZnSn(S,Se)4 (CZTSSe) based solar cells.19 The optical properties of these materials crucially depend on the particle size, chemical composition, crystal structure, and surface morphology, which could be controlled by preparation techniques. Further, lowcost, robust, and scalable solution based techniques17−27 are being developed to replace more expensive and low-throughput © 2017 American Chemical Society
Received: January 15, 2017 Revised: March 7, 2017 Published: March 14, 2017 3133
DOI: 10.1021/acs.chemmater.7b00149 Chem. Mater. 2017, 29, 3133−3142
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microscopy (FE-SEM, Hitachi S-4800) with 5 kV operating voltage. Surface topology of the films was further investigated using noncontact atomic force microscopy (NC-AFM; JEOL, JSPM-5200). The XPS spectra were investigated (VSW ESCA spectrometer with 105 cm−1. The experimental band gap for the CNTS nanoparticle is estimated to be 1.57 eV (Figure 7b). The experimental band gap is in close agreement with the HSE06 calculated gap (1.22 eV) and comparable to Cu2ZnSnS4 (1.4−1.6 eV), which has already shown enormous promise in PV applcation.32,33 Thus, we
propose CNTS to be a promising candidate for thin-film solar cell applications. Further, the valence and conduction band edges are measured through cyclic voltametry (CV) and estimate ionization potential Ip, electron affinity Ea, and electrochemical band gap Eelg of semiconducting CNTS nanocrystals. In the previous reports, only the anodic peak was measured due to poor electrochemical signals.28,29 In contrast, here we have successfully detected both the peaks (Figure 8a) and observed a prominent anodic peak at 0.32 V (marked as A1) and a cathodic peak at −1.15 (marked as C1) over repeated cycles. The potential difference of 1.47 V between these peaks is the measure of Eelg , which is in good agreement with the optical band gap of 1.57 eV (Figure 7b), that was measured from the optical absorption spectrum. The exact positions of conduction and valence bands with respect to the vacuum level are calculated using EVBM/eV = −Ip = −(Eoxidation + Eref) and ECBM/ peak eV = −Ea = −(Ereduction + Eref). Here, ferrocene was used as peak internal reference Eref (4.5 eV vs NHE), and Eoxidation and peak Ereduction are energies corresponding to A1 and C1 peaks, peak respectively. Thus, the EVBM and ECBM are measured to be 3138
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tecture (Figure 9a) with an effective device area of 0.25 cm2. The back-contact Mo layer was deposited on glass substrate using direct current sputtering technique, which was spincoated with a CNTS absorber layer. A thin n-type CdS layer was deposited on the CNTS layer, and a Al-ZnO layer was deposited as transparent conducting layer through radio frequency sputtering. To improve the current collection, Al has been used as a front contact, which was deposited using a thermal evaporation method. The device XRD pattern indicate the individual Mo, CdS, and Al-ZnO layers are in their pure and stable phase (Supporting Information, Figure S7). The J−V characteristics were measured under the 1.5 AM illumination with 100 mW/cm2 intensity (Figure 9b). The measured fill factor is found to be 0.43. Despite the fact that CNTS nanocrystals have encouraging semiconducting properties with optical band gap of 1.57 eV and high absorption coefficient > 105 cm−1 in the visible range, the measured energy conversion efficiency η for the present PV device is found to be 0.09%. This is lower than the efficiencies achieved in the optimized CZTS (9.2%)33 and CZTSSe (12.6%)19 devices. Although the measured η for the first ever CNTS PV device is small, it is comparable to the very first pure sulfide CZTS device with 0.23% efficiency,32 which attracted enormous attention in optimizing the material synthesis and device fabrication processes further. Indeed, these resulted in enormous success in increasing η to 9.2% at present.33 Similarly, with encouraging optical properties, we predict that the optimization of materials synthesis and device fabrication will increase efficiency of thinfilm CNTS solar cells. Particularly, the Mo/CNTS/CdS/AlZnO/Al device architecture, operation of the metal contacts in the ohmic regime, optimization of the differential layer thickness, and CNTS/CdS heterojunction to reduce interfacial defect states should be addressed in the future. A clear understanding about the band alignment and the concurrent carrier transport mechanism at and across the CNTS/CdS heterojunction is necessary for such device optimization. The electrochemical band gap and electron affinity for the p-type CNTS are measured to be 1.47 and 3.35 eV, respectively, and the same for the n-type CdS are 2.42 and 3.47 eV, respectively.56 These data have been used to model the transport across the CNTS/CdS heterojunction (Figure 9c).
Figure 8. (a) Cyclic voltammograms for CNTS nanocrystal dispersion drop-casted on gold electrode (scan rate, 100 mV s−1). The anodic and cathodic peaks are indicated by A1 and C1, respectively. This indicates an electrochemical band gap of 1.47 eV, in close agreement with the optical measurements (1.57 eV), discussed earlier. (b) Conduction and valence bands with respect to the vacuum level EVBM and ECBM. Normal hydrogen electrode (NHE) is calculated using these data and compared with recent experimental measurements.
−4.82 and −3.35 eV, respectively (Figure 8b), which are compared with the previous reports. Moreover, the catalytic effect of CXTS (X = Zn, Ni, and Co) nanocrystals on the photocatalytic hydrogen evolution reaction (HER) has been investigated recently. Under visible light irradiation, the CNTS nanocrystals were found to have good HER activity,28−30 and the present results also corroborate these earlier findings. Next, to investigate the photoelectrical properties, we have fabricated Cu2NiSnS4 thin films, coated on Mo layer, which was deposited on a glass substrate. Finally, an Al layer was deposited on the CNTS film. The corresponding I−V characteristics (Supporting Information, Figure S6) have been measured using solar simulator AM 1.5, 100 mW/cm2 for white light illumination. A current enhancement of 27.74 μA at voltage 1 V was observed under light illumination due to electron−hole pair generation and concurrent separation. These photogenerated electrons and holes move in the opposite direction and toward the metal contacts (Al, Mo), and thus increase the net current. Such photoactive nature of CNTS thin films indicates high potential use in solar cell applications. Finally, for the first time, we have fabricated a CNTS thinfilm PV device with glass/Mo/CNTS/CdS/Al-ZnO/Al archi-
Figure 9. (a) Schematic photovoltaic device fabricated with CNTS thin films in the glass/Mo/CNTS/CdS/Al-ZnO/Al architecture. While Mo acts as the back-contact and CdS as the n-type layer, the Al-doped ZnO acts as transparent conducting layer. Present optical measurements and accompanied theoretical calculations indicate enormous promise. (b) Current−voltage characteristics of the fabricated CNTS PV device, measured under light illumination with 100 mW/cm2 intensity. Although the fabricated device has a high open circuit voltage, VOC, and fill factor, FF, the poor efficiency, η = 0.09%, for the current PV device is due to nonohmic contacts. Thus, further device optimizations are needed. (c) Schematic energy band diagram of p-Cu2NiSnS4 and n-CdS after heterojunction formation in the PV cell, and the carrier collection mechanism at the back- and frontcontact. Band offsets of −0.12 eV (CBO) and 1.07 eV (VBO) are observed after the CNTS/CdS heterojunction formation. 3139
DOI: 10.1021/acs.chemmater.7b00149 Chem. Mater. 2017, 29, 3133−3142
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Chemistry of Materials Further, the location of the Fermi level has been estimated using a work function, which was measured from the shift in retarding the potential relative to the reference sample as a function of applied current (Supporting Information, Figure S8).57 The polycrystalline gold was used as a reference sample, with 4.90 eV work function, and the measured work function of CNTS thin film is found to be 4.51 eV. Thus, there is a mismatch in their respective ECBM and EVBM for CNTS and CdS, which leads to conduction and valence band offsets, ΔEc, and ΔEv, respectively (Figure 9c). After the heterojunction is formed, ΔEc and ΔEv offsets are found to be −0.12 and 1.07 eV, respectively. It is interesting to note that the conduction band offset (CBO) at the heterojunction is related to the open circuit voltage VOC and concurrent fill factor. Thus, low CBO of −0.12 eV at the CNTS/CdS heterostructure results in higher VOC and fill factor (Figure 9c), compared to the first ever CZTS and CFTS device.32,58 In contrast, the lower JSC for the present device is predicted to originate from the nonohmic contacts resulting in inefficient carrier collection at the electrodes. Therefore, one could optimize the metal contacts to increase device efficiency. Further, replacing the n-type material could also increase the device performance. Indeed, the Cu2FeSnS4 thin-film solar cells were reported to have higher efficiency while Bi2S3 was used as the n-type material instead of CdS.58 These illustrate opportunities in optimizing the band alignment at the CNTS/CdS interface for better carrier transport, and we hope further tweaking of the synthesis, and device fabrication will lead to an increase in PV efficiency.
the excellent optical properties along with small conduction band offset, we predict that there are enormous opportunities to increase efficiency through optimizing material synthesis and device fabrication/architecture. Especially, optimizing the differential thickness of individual layers, reducing the interfacial defect states at the CNTS/CdS heterojunction, and designing ohmic metal contacts will play key roles. While these factors are optimized properly for the CNTS device, we anticipate at least a comparable efficiency as those reported for CZTS and CZTSSe devices.19,33 We hope that the present study will attract further investigations to address these issues.
CONCLUSIONS We have synthesized zincblende Cu2NiSnS4 stoichiometric nanocrystals and thin films using cost-effective solution based hot-injection and spin-coating methods, which have been characterized using various complementary experimental techniques. The structural, electronic, and optical properties are experimentally studied, and the results are further corroborated within the first-principles density functional theory. The calculations are done using various exchange-correlation functionals including on-site Coulomb interaction U and the more accurate hybrid HSE06 functional. Although, the experimental data indicate zincblende crystal structure, it is difficult to predict the correct cation disorder. In this regard, complementary theoretical calculation predicts the P4̅2c polytype to be thermodynamically most stable. Moreover, the underlying magnetic ordering strongly influences the electronic structure, and for all polytypes, the G-AFM magnetic ordering is found to be energetically more favorable. The calculated optical properties are in excellent agreement with the experimental measurements. The CNTS is measured to be a direct-gap semiconductor with 1.57 eV gap and has excellent optical absorption coefficient > 105 cm−1 in the entire visible range. These experimental results are in excellent agreement with the theoretical calculations (Figure 7). These properties make CNTS an exciting candidate material for cost-effective thin-film solar cell application. Moreover, we have proposed a carrier transport mechanism across the CNTS/CdS heterostructure, through successful determination of both valence and conduction band edges for CNTS thin film through CV measurements (Figure 8) Further, a CNTS photovoltaic device was fabricated for the first time with glass/Mo/CNTS/CdS/AlZnO/Al architecture, with impressive VOC and fill factor compared to the first CZTS and CFTS devices.32,58 Owing to
ORCID
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00149. Structural data, FESEM and AFM images, EDS spectrum, calculated lattice parameters and band gaps, phonon dispersion, density of states, I−V measurement, device XRD, and potential shift of CNTS with respect to gold reference (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +91 (20) 2590 8112. Fax: +91 (20) 2025 1566.
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Santosh Haram: 0000-0002-0618-1215 Mukul Kabir: 0000-0002-3230-280X Author Contributions ⊥
S.R. and N.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.R. and S.J. are thankful for partial support by the Indian Nanoelectronics Users Program, Indian Institute of Technology, Bombay project sponsored by the Ministry of Electronics and Information Technology, Government of India. S.R. is also grateful to the Dr. Babasaheb Ambedkar Research and Training Institute, Pune for the research fellowship and financial assistance. Y.J. thanks the DST INSPIRE Ph.D. program (Grant 2013/606) for financial support. N.W. and M.K. acknowledge the supercomputing facilities at the Centre for Development of Advanced Computing, Pune; at Inter University Accelerator Centre, Delhi; and at the Center for Computational Materials Science, Institute of Materials Research, Tohoku University. M.K. acknowledges the funding from the Department of Science and Technology, Government of India under the Ramanujan Fellowship and the Nano Mission Project Grant SR/NM/TP-13/2016(G).
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