Article pubs.acs.org/JPCC
A Nonvacuum Approach for Fabrication of Cu2ZnSnSe4/In2S3 Thin Film Solar Cell and Optoelectronic Characterization Dhruba B. Khadka,† SeongYeon Kim,† and JunHo Kim* Department of Physics, Incheon National University, 12-1 Songdo-dong Yeonsu-gu, 406-772 Incheon, South Korea S Supporting Information *
ABSTRACT: Cd-free kesterite-based Cu2ZnSnSe4 (CZTSe)/In2S3 champion solar cell of 5.74% efficiency has been fabricated by chemical spray pyrolysis. In this fabrication route, CZTSe absorber layer was sprayed by using a precursor solution, where metallic salts were dissolved in water-based solvent and subsequently selenized with Se powder at high temperature. In2S3 buffer as an alternative to CdS buffer was also deposited by chemical spray pyrolysis. The device characteristics were studied by measuring dark/light illuminated J−V−T, external quantum efficiency, temperature dependence of open circuit voltage (VOC) and series resistance (Rs), and admittance spectroscopy. The performance of sprayed CZTSe/ In2S3 solar cell was found to be limited by high back-contact barrier potential, poor carrier collection, and detrimental intrinsic defect states in device.
1. INTRODUCTION Chalcogen-based thin film photovoltaic cells have drawn considerable interest as an alternative energy source for safe and sustainable solution of energy crisis. Despite the photovoltaic (PV) technology with Cu(In,Ga)Se2 (CIGSe) and CdTe thin films which have achieved highest efficiencies of 21.7% and 21.0%, respectively, the scarcity of elements such as In, Ga, Te, and toxic Cd has raised high cost and environmental issues for large-scale and long-term production.1−3 To address these issues, quaternary chalcogenide Cu2ZnSnS4 (Se4) (CZTS (Se)) has gained much attention due to earth abundant, nontoxic, and low-cost constituents along with its high absorption coefficient α > 104 cm−1 and tunable band gap 1.0−1.5 eV, which is suitable for efficient light harvesting.3,4 Despite these promising properties of CZTS (Se), the device performance is still low as reference to CIGSe solar cell. Therefore, to make kesterite-based CZTS (Se) thin film solar cell as mature as CIGSe, extensive research has been carried out on material fabrication, secondary phase control,2 defect formation,5 band-gap tuning,4,6−8 back-contact barrier,9−11 and band alignment.12,13 The kesterite CZTSSe solar cell processed from a hydrazine-based precursor solution has recently achieved a record efficiency of 12.6% with CdS buffer layer and 12.4% with double In2S3/CdS emitter,14,15 which implies further improvement to compete to CIGSe technology. Different fabrication approaches have been successfully employed to fabricate kesterite thin film solar cells. The CZTSSe solar cells adopting the vacuum-based deposition showed power conversion efficiencies (PCE) of 11.6% via coevaporation16 and 9.7% via sputtering.17 Since vacuum-based technologies need expensive high-vacuum facilities, the potential of chalcogenide compound can be efficiently exploited when combined with a scalable, nonvacuum fabrication © 2015 American Chemical Society
technique. A number of solution process techniques have been applied for chalcopyrite and kesterite solar cell fabrication.18 Although the performance of CZTSSe solar cell adopting hydrazine based hybrid slurry approach has been reported to be progressed to best performance, the hazards associated with highly toxic and explosive hydrazine may inhibit the adaptation of this technique.14,18 The high efficiency of CZTSSe solar cell has achieved PCE of 9.6% by using binary and ternary nanoparticle solution approach.19 The nanocrystalbased fabrication techniques have also provided competitive CZTSSe solar cell of efficiency as high as 9.0%, whereas devices with Ge alloyed CZTSSe (CZGTSSe) nanocrystal have achieved improved PCE as high as 9.4% with band-gap tuning.7,20 Furthermore, CZTSSe solar cell of 8.3% efficiency has been reported using molecular ink prepared by dissolving metal salt with dimethyl sulfoxide (DMSO) solvent.21 The organic solvent used for precursor preparation can have safety issue and carbon residue may normally remain which is detrimental for solar cell performance. Thus, as an alternative to hydrazine or organic solvent, nontoxic and low-cost solvents like water and ethanol are preferable for eco-friendly synthesis. The CZTSSe solar cell fabricated with precursor ink prepared using water and ethanol-based solvent has been reported with high efficiency of 8.6% via spray deposition and 6.2% via spin coating.22,23 The solution-based nonvacuum processes such as spin coating, doctor blade, electrodeposition, chemicals bath deposition (CBD), and spray pyrolysis have been successfully adopted for fabrication of absorber layer in kesterite thin film solar cell.14,18−26 Among these, the aqueous precursor solution Received: April 2, 2015 Revised: May 6, 2015 Published: May 8, 2015 12226
DOI: 10.1021/acs.jpcc.5b03193 J. Phys. Chem. C 2015, 119, 12226−12235
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The Journal of Physical Chemistry C
precursor solution prepared in mixed solvent (DIW + ethanol) was found to be stable for longer time compared to that of prepared only in DIW. The excess amount of sulfur source, i.e., thiourea (SC(NH2)2), was used to minimize oxidation as a consequence of sulfur loss during spray deposition. The prepared precursor solution was deposited onto Mo-SLG at substrate temperature of 450 °C using homemade electrostatic field assisted ultrasonic spray system34 in which high electrostatic field was applied between outlet of aerosol flow pipe and substrate to accelerate the molecular CZTS mist flow. It is to be noted that our spray system is nozzle-free spray, which is good for deposition of ultrafine aerosol by minimizing the droplets coalescence before reaching to the substrate. The CZTS film was prepared by spraying the precursor mist onto the substrate under a rate of 1.2 mL/min by means of carrier gas of nitrogen at pressure of 0.1 MPa. The as-sprayed films deposited on MoSLG were selenized by rapid thermal annealing 50 °C/min under selenium vapor ambient and dwelled at ∼500−520 °C for 30 min, keeping the sample inside of rectangular graphite box filled with selenium powder (99.99%). The schematic diagrams of annealing furnace and temperature profile for selenization are as our earlier reports.8,26,36 2.2. Device Fabrication. To complete thin film device structure, indium sulfide (In2S3) was used as Cd-free buffer which was also deposited by the CSP method. For the deposition of indium sulfide buffer, the precursor solution, which was prepared by dissolving InCl3 and SC(NH2)2 with the molar ratio of 1:7 in alcohol−aqueous (DIW (50%) + ethanol (50%)) solvent, was sprayed onto CZTSe absorber layer at substrate temperature of 360 °C with a flow rate of 0.8 mL/min by N2 as carrier gas. Prior to buffer deposition, the surface of postselenized CZTSe absorber layer was etched for 3 min in 10% of potassium cyanide DIW solution and immediately washed with DIW and then dried under gentle flow of nitrogen gas. Finally, the CZTSe device was completed by subsequent deposition of intrinsic zinc oxide (i-ZnO) layer and indium tin oxide (ITO) layer via radio-frequency (RF) magnetron sputtering. No metal grid and antireflection coating were further deposited on the solar cell device. 2.3. Film and Device Characterization. The stoichiometry, surface morphology, and cross-sectional image of samples were studied by field emission scanning electron microscopy (FESEM, JEOL, JSM-7001F) equipped with energy dispersive X-ray spectroscopy (EDS, Oxford, INCA). The crystal structure and phase were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns were obtained in 2θ scan at a scanning rate of 3°/min by XRD system (Rigaku, Smart Lab) equipped with a Cu Kα source of λ = 1.5412 Å operated at 45 kV and 200 mA. The Raman spectroscopy measurements were conducted using a Spectro Raman system equipped with spectrometer (Mmac 750) and laser of excitation wavelength λ = 532 nm (irradiation power Et(VCu)) deteriorates the solar cell performance.5,56,58 Furthermore, the defect distribution in device can be calculated by adopting the theory proposed by Walter et al.55
Figure 5. Admittance spectra of champion CZTSe solar cell device (a). Arrhenius plot of the inflection frequencies extracted from differentiation of the admittance spectra (b). Defect density profile corresponding to respective defect energy level derived from admittance spectra (c). Here, the dashed lines represent corresponding defect levels.
solar cell is quite complex where the absorber and interface junctions contain plenty of electronics states, i.e., defects states which cause recombination of electrons and holes and hence definitely influence the device performance.7,51 Since AS accounts for the response of the device to small ac bias voltage modulation in accordance with frequencies and temperature, those responses are considered to occur as a consequence of the emission and capture of electron in electrically active defects.55 Figure 5a shows C−f scans taken in the dark from 20 Hz to 2 MHz in the temperature range of 90−300 K. A similar C−f−T trend has been reported for the CZTSSe device.45,51 It is noticed that the C−f spectra decay from a low-frequency capacitance (Clf) to high-frequency capacitance (Chf) where the converging trend of C−f data is observed beyond 1 MHz, 12232
DOI: 10.1021/acs.jpcc.5b03193 J. Phys. Chem. C 2015, 119, 12226−12235
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The Journal of Physical Chemistry C Author Contributions
It allows estimation of the defect distribution in the absorber layer from the equation52,55 Nt(Eω) =
†
These authors contributed equally to this work.
Notes
2Vbi 3/2
dC(ω , T ) ω dω kT w q qVbi − (Eg − Eω)
The authors declare no competing financial interest.
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(4)
ACKNOWLEDGMENTS The financial support for this research was provided by the National Research Foundation of Korea (NRF) funded by the Korean government (NRF-2012R1A1A2006936, NRF2014R1A2A1A11053109) and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government and Ministry of Trade, Industry and Energy (No. 20123010010130).
where Vbi is the built-in potential, q is the fundamental charge, Eg is the band-gap energy, Eω is the energetic distance from the valence band corresponding to ω, and w is the width of the p side of the junction region. The defect density profile calculated from eq 4 is depicted in Figure 5c where the frequency axis is converted into energy axis following eq 3. The required values of parameters, Vbi, w, and Eg, were obtained from preceding C− V measurement and reported literature.12 The superimposed defect spectra in Figure 5c displays three distinct defect density spectra resulting integrated defect densities, Nt1 = 3.26 × 1015 cm−3, Nt2 = 2.21 × 1015 cm−3, and Nt3 = 1.99 × 1015 cm−3, corresponding to respective defect activation energies Et1, Et2, and Et3 estimated from Arrhenius plot (Figure 5b). These trap densities estimated above are found to be relatively higher than earlier reports.43,51 The defect density corresponding to Et3 (VCu) is found to be comparatively lower than that of detrimental defect states, Et1(CuZn) and Et3,5,58 which also impose limitations to the device performance of sprayed CZTSe/In2S3 solar cell.
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(1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (version 44). Prog. Photovoltaics 2015, 23, 1−9. (2) Siebentritt, S.; Schorr, S. Kesterites - a Challenging Material for Solar Cells. Prog. Photovoltaics 2012, 20, 512−519. (3) Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. The Path Towards a High Performance Solution-Processed Kesterite Solar Cell. Sol. Energy Mater. Sol. Cells 2011, 95, 1421−1436. (4) Yang, W.; Duan, H. S.; Bob, B.; Zhou, H.; Lei, B.; Chung, C.; Li, S.; Hou, W. W.; Yang, Y. Novel Solution Processing of High Efficiency Earth-Abundant Cu2ZnSn(S,Se)4 Solar Cells. Adv. Mater. 2012, 24, 6323−6329. (5) Chen, S.; Walsh, A.; Gong, X. G.; Wei, S. H. Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 EarthAbundant Solar Cell Absorbers. Adv. Mater. 2013, 25, 1522−1539. (6) Khadka, D. B.; Kim, J. H. Band Gap Engineering of Alloyed Cu2ZnGexSn1‑xQ4 (Q = S,Se) Films for Solar Cell. J. Phys. Chem. C 2015, 119, 1706−17013. (7) Hages, C. J.; Levcenco, S.; Miskin, C. K.; Alsmeier, J. H.; AbouRas, D.; Wilks, R. G.; Bar, M.; Unold, T.; Agrawal, R. Improved Performance of Ge-Alloyed CZTGeSSe Thin Film Solar Cells through Control of Elemental Losses. Prog. Photovoltaics 2015, 23, 376−384. (8) Khadka, D. B.; Kim, J. H. Structural Transition and Band Gap Tuning of Cu2(Zn,Fe)SnS4 Chalcogenide for Photovoltaic Application. J. Phys. Chem. C 2014, 118, 14227−14237. (9) Shin, B.; Zhu, Y.; Bojarczuk, N. A.; Chey, S. J.; Guha, S. Control of an Interfacial MoSe2 Layer in Cu2ZnSnSe4 Thin Film Solar Cells; 8.9% Power Conversion Efficiency with a TiN Diffusion Barrier. Appl. Phys. Lett. 2012, 101, 0539031−0539034. (10) Altamura, G.; Grenet, L.; Roger, C.; Roux, F.; Reita, V.; Fillon, R.; Fournier, H.; Perraud, S.; Mariette, H. Alternative Back Contacts in Kesterite Cu2ZnSn(S1‑xSex)4 Thin Film Solar Cells. J. Renewable Sustainable Energy 2014, 6, 0114011−0114017. (11) Yang, K. J.; Sim, J. H.; Jeon, B.; Son, D. H.; Kim, D. H.; Sung, S. J.; Hwang, D. K.; Song, S.; Khadka, D. B.; Kim, J. H.; et al. Effects of Na and MoS2 on Cu2ZnSnS4 Thin-Film Solar Cell. Prog. Photovoltaics 2014, DOI: 10.1002/pip.2500. (12) Haight, R.; Barkhouse, A.; Gunawan, O.; Shin, B.; Copel, M.; Hopstaken, M.; Mitzi, D. B. Band Alignment at the Cu2ZnSn(SxSe1−x)4/CdS Interface. Appl. Phys. Lett. 2011, 98, 2535021− 2535024. (13) Barkhouse, D. A. R.; Haight, R.; Sakai, N.; Hiroi, H.; Sugimoto, H.; Mitzi, D. B. Cd-Free Buffer Materials on Cu2ZnSn(SxSe1‑x)4: Band Alignment with ZnO, ZnS and In2S3. Appl. Phys. Lett. 2012, 100, 1939041−1939045. (14) Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe Thin film Solar Cells With 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 13014651−13014655. (15) Kim, J.; Hiroi, H.; Todorov, T. K.; Gunawan, O.; Kuwahara, M.; Gokmen, T.; Nair, D.; Hopstaken, M.; Shin, B.; Lee, Y. S.; et al. High
4. CONCLUSIONS We have achieved a 5.74% efficient Cd-free CZTSe/In2S3 thin film solar cell in which both CZTSe absorber and In2S3 buffer layer were fabricated by water-based solution process, spray pyrolysis route. The increased efficiency compared to our previous report is due to improved quality of absorber layer and buffer/absorber interface. The analysis of J−V−T results gives back-contact barrier height of 127.6 meV near to the room temperature regime. AS reveals defects at 19 (VCu), 58, and 118 meV (CuZn) with integrated defect densities approximately 1.99 × 1015, 2.21 × 1015, and 3.26 × 1015 cm−3, respectively. These results lead to that fabricated solar cell is limited by high backcontact barrier potential, poor spectral response, and high densities of deep defects in the absorber. The efficiency of sprayed CZTSSe/In2S3 device can be further improved by minimizing the back-contact barrier effect and passivating the detrimental defect states in device. Therefore, we believe that CSP using cheap and eco-friendly water-based precursor solution is a promising cost-effective fabrication method for the high-efficiency kesterite solar cell.
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ASSOCIATED CONTENT
S Supporting Information *
Raman spectra and band-gap estimation of indium sulfide (In2S3) buffer, J−V curves of some of solar cells and device parameters, J−V−T characteristics of champion device under white light illumination, dark condition, different filter illuminations, capacitance voltage (C−V) measurement, and Mott−Schottky plot. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03193.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*(J.K.) E-mail
[email protected]; Ph +82-32-835-8221. 12233
DOI: 10.1021/acs.jpcc.5b03193 J. Phys. Chem. C 2015, 119, 12226−12235
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