Efficiency Enhancement of Kesterite Cu2ZnSnS4

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Article Cite This: ACS Appl. Energy Mater. 2018, 1, 154−160

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Efficiency Enhancement of Kesterite Cu2ZnSnS4 Solar Cells via Solution-Processed Ultrathin Tin Oxide Intermediate Layer at Absorber/Buffer Interface

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Heng Sun,† Kaiwen Sun,† Jialiang Huang,† Chang Yan,† Fangyang Liu,*,† Jongsung Park,† Aobo Pu,† John A. Stride,‡ Martin A. Green,† and Xiaojing Hao*,† †

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia ‡ School of Chemistry, University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: The ultrathin SnO2 intermediate layer deposited by a successive ionic layer adsorption and reaction (SILAR) method was introduced into the heterointerface between p-type Cu2ZnSnS4 (CZTS) absorber and n-type CdS buffer for interface defect passivation in kesterite thin film solar cells. CZTS solar cells with SnO2 intermediate layers show higher open circuit voltage (Voc) of 657 mV and fill factor (FF) of 62.8%, compared to its counterpart cells without the SnO2 intermediate layer, which have Voc of 638 mV and FF of 52.4%, resulting in improvement in the overall efficiency from 6.82% to 8.47%. The mitigation of the Voc deficit and the improvement of FF are believed to result from the integrated effects of CZTS/CdS heterointerface passivation, shunt blocking, and band alignments. The passivation effect is further affirmed by the improved carrier lifetime. Furthermore, external quantum efficiency profiles show a strengthened blue optical response which is contributed by the decrease of CdS thickness. This work provides a new insight into the CdS/CZTS interface passivation, shunt blocking, and optimization of band alignments, based on solution-processed SnO2. KEYWORDS: kesterite CZTS solar cells, SnO2, interface recombination, passivation, shunt blocking, electron-selective, band alignment, parasitic absorption

1. INTRODUCTION

However, severe Voc deficit (∼790 mV for the champion cell2), defined by Eg/q − Voc, is one of the major causes of much lower efficiency than that of its counterpart CIGSe solar cells. The Voc loss in sulfide CZTS solar cells is, to a large extent, induced by interface recombination of photogenerated carriers.4,5 The electrical and structural discrepancies between CZTS absorber and CdS buffer give rise to lattice mismatches, defects, interface states, and band discontinuities, which promote the interface recombination.4,6−9 Previous studies have reported efforts to tackle these problems on kesterite solar cells for diminishing the Voc deficit. For example, the

Thin film photovoltaics have the advantage of high specific power and have drawn great attention toward meeting the terawatts-scale global energy consumption.1 Among the thin film solar cells with excellent long-term stability, chalcogenide Cu(In,Ga)Se2 (CIGSe) and CdTe are the forerunners of thin film photovoltaics, with both having champion efficiencies of over 22%.2 However, the scarcity of In and Te restricts the production capacity of CIGS and CdTe solar cells to less than 100 gigawatt-peak (GWp),3 respectively. Kesterite Cu2ZnSnS4 solar cell is one of the promising derivatives of chalcogenide CIGSe devices, substituting scarce In/Ga with abundant Zn and Sn and toxic Se with S, thereby exploiting earth-abundant, cost-effective and environmentally benign materials. © 2017 American Chemical Society

Received: October 18, 2017 Accepted: December 20, 2017 Published: December 20, 2017 154

DOI: 10.1021/acsaem.7b00044 ACS Appl. Energy Mater. 2018, 1, 154−160

Article

ACS Applied Energy Materials discrepancy of lattice constants could be reduced by employing a lattice-matched CeO2 between CZTS and CdS, resulting in the alleviation of interface recombination.10 Additionally, CdS has been replaced by Zn1−xCdxS as a buffer layer to obtain a favorable “spike”-like CBO preventing the recombination from the trap states of a “cliff”-like CdS/CZTS interface.11 Widebandgap (WBG) insulating materials, such as atomic layer deposited (ALD) Al2O3 and ALD-TiO2, have been applied as an intermediate passivation layer between Cu2ZnSn(S,Se)4 (CZTSSe) or Cu2ZnSnSe4 (CZTSe) and CdS buffer to passivate defect states at the interface and thereby boost carrier lifetime and performance.12−16 SnO2 is also one of the potential materials for interface passivation in kesterite solar cells. However, rather limited reports have ever presented the use of SnO2 as an interface passivation layer for chalcogenide and kesterite solar cells. With regard to binary absorber materials, ALD-SnO2 have been implemented for interface passivation for SnS17 solar cells. Besides, the formation of SnOx-rich grain boundaries18 by air annealing at a temperature of 300−400 °C was found to yield outstanding passivation effects in CZTSSe solar cells. In contrast to the widely used ALD method,19 solution-based methods are cost-effective and vacuum-free, thereby possessing great potential for mass production. In this work, ultrathin SnO2 intermediate layers were deposited by an environmentally benign, cost-effective, simple, and reproducible successive ionic layer adsorption and reaction (SILAR) method using NH4OH as the complexing agent. This method has been previously introduced for the fabrication of SnO2 thin film.20 The use of SILAR-SnO2 is capable of passivating the interface and optimizing the band alignments of both conduction and valence bands. Remarkable improvements in FF and Voc were consequently achieved compared to our reference devices without any intermediate layer between CZTS and CdS, prompting the overall efficiency from 6.8% to 8.5%.

Figure 1. Schema of device structures with details of CdS/SnO2/ CZTS heterojunctions (not to scale). composition of Cu-poor and Zn-rich (Cu/Sn = 1.8; Zn/Sn = 1.2), followed by sulfurization at 560 °C in a combined sulfur and SnS atmosphere. Ultrathin SnO2 layers were synthesized on these sulfurized CZTS surfaces by the SILAR method, followed by airannealing at 250 °C for 3 min. 50 nm CdS buffer layers were then prepared by chemical bath deposition (CBD) method. A stack of 60 nm i-ZnO and 200 nm indium tin oxide (ITO) as window layers was coated sequentially using RF magnetron sputtering. Aluminum fingers were fabricated on the top of the ITO by thermal evaporation to form the front contact. The total area of the unit cell was defined by mechanical scribing to approach 0.24 cm2. The CZTS devices were finalized with a thermally evaporated MgF2 antireflection coating. Films and Devices Characterizations. X-ray photoelectron spectroscopy (XPS) was employed to confirm the formation and chemical states of the ultrathin SnO2 layer using an ESCALAB250Xi (Thermo Scientific, U.K.) under ultrahigh vacuum (lower than 2 × 10−9 mbar). Optical transmittance of the fabricated SnO2 layer on glass substrates was measured via UV−vis−near-IR spectrometer (PerkinElmer−Lambda 950). For device characterization, the current density−voltage (J−V) curves were tested using a simulator (Newport) with AM 1.5 illumination and 1000 W/m2 intensity calibrated with a standard silicon reference cell. External quantum efficiency (EQE) measurements were performed by a QEX10 spectral response system (PV Measurements, Inc.), using a monochromatic light chopped at a frequency of 120 Hz. For the capacitance−voltage (C−V) measurement, an impedance analyzer was used with a frequency at 100 Hz and a DC bias voltage sweeping from −1.5 V to 0.5 V. Time-resolved photoluminescence (TRPL) characterizations were carried out by applying the time-correlated single photon counting technique (MicroTime 200, PicoQuant) exploiting a laser source of 470 nm wavelength and 20 MHz pulse with tunable repetition for excitation.

2. EXPERIMENTAL SECTION Precursor Solution Preparation. The precursor solution for the ultrathin SnO2 layer was prepared by dissolving SnCl4·5H2O (1.25 mM, AR) in deionized (DI) water. NH4OH solution (3 M, AR) was added in the SnCl4 solution with constant stirring to adjust the pH of the reaction solution to a value of 10. Ammonia solution was used as the complexing agent to form aqueous tin−ammonia complex ions ([Sn(NH3)4]4+), which were chosen for the cation precursor. Ultrathin SnO2 Film Deposition. The sulfurized CZTS absorber layers were dipped in the precursor solution at room temperature for 20 s. After air drying, the [Sn(NH3)4]4+-adsorbed CZTS layers were immersed in DI water at 90 °C for 5 s. The air-dried samples were ultrasonicated for 20 s, and one SILAR cycle was counted. Several SILAR cycles were required to achieve the desired thicknesses. After the deposition, all SnO2-coated samples were air-annealed at 270 °C for 3 min. The growth rate (0.4 nm/cycle) was determined by the thickness measurement on hundreds of cycles of SILAR deposition via Veeco Dektak 8 Advanced Development Profiler. One to three cycles, equivalent to the thickness of 0.4−1.2 nm, were desired in this research to match the required thickness of around 1 nm and compare the performance of completed devices. CZTS Solar Cell Devices Fabrication. The schema of device structure is shown in Figure 1 with the emphasized position of SnO2. The typical methods and processes for the device fabrication employed in this study have been described in our previous publication.11 CZTS absorbers were fabricated by cosputtering Cu, ZnS, and SnS precursors on Mo-coated soda lime glass via magnetron sputtering (AJA International, Inc., model ATC-2200) with average

3. RESULTS AND DISCUSSION Figure 2 exhibits the XPS spectra of three different samples: a SnO2 film (∼4 nm) on a CZTS absorber, a SnO2 film (∼40 nm) on a quartz substrate, and a CZTS absorber only. Since the detection depth for XPS measurement can reach up to 10 nm, two distinctive binding energy peaks for Sn 3d5 were observed, shown in Figure 2a by fitting the measured XPS spectra for SnO2 film on a CZTS absorber sample. The peaks at 486.4 eV21−23 and 487.2 eV24,25 represent the binding energies of Sn−S and Sn−O bonds, respectively, which are further confirmed by XPS measurements for SnO2 film on quartz and for CZTS absorber as shown in Figure 2b and 2c, respectively. The formation of SnO2 on the surface of CZTS absorber using the SILAR method is conclusively demonstrated from the presence of Sn−O bonds. To investigate the electrical properties of SnO2 at the CdS/ SnO2/CZTS heterointerface, the overall heterojunction band 155


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ACS Applied Energy Materials

Figure 3. (a) Schematic band diagrams of the CZTS/SnO2/CdS heterojunctions; (b) J−V characteristics of reference CZTS device and CZTS devices with SnO2 intermediate layer in different thickness.

recombination stemming from trap states. In consideration of −2 to −2.2 eV valence band offset (VBO), the tin oxide film can also act as an electron-selective contact for CZTS to passivate the interface by blocking holes away from the buffer layer. Figure 3b illustrates the current density−voltage (J−V) characteristics of the CZTS devices with SnO2 intermediate layers in different thickness between CZTS absorber and CdS buffer layer. Electric parameters are collected and displayed in Table 1, showing the rise of the device efficiency from 6.82% to 8.47%. The improved performance is mainly contributed by the significant boost of fill factor (FF) from 52.4% to 62.8% and the improvement of Voc from 638 to 657 mV, both stemming from the doubling of shunt resistance (Rsh) and the alleviation of interface recombination. The rise of Rsh can be reasonably explained as the existence of a resistive SnO2 intermediate layer, to some extent, that quenches the undesired current drains via shunt pathways, which are possibly induced by a series of factors, including surface pin holes,35 voids,36 and cracks with high aspect ratios,37 local small grain size,38 CZTS roughness, and secondary phases, etc. Pin holes, in particular, are generally found in the CZTS absorber surfaces, likely induced by the decomposition of CZTS, without the application of Al2O3 rear surface intermediate layer.39 Moreover, the direct evidence for the

Figure 2. (a) XPS spectra of SILAR-SnO2 (10 cycles) on CZTS absorber surface; (b) XPS spectra of SILAR-SnO2 for 100 cycles on quartz; (c) XPS spectra of as grown CZTS after sulfurization.

diagram is estimated from the reported CZTS/CdS conduction band offset (CBO),26−28 the CBO of CdS/SnO2,18,29 electron affinities of CZTS and SnO230−33 and the measured band gap of SnO2. The derivation and calculation of the SnO2 bandgap can be seen in the Supporting Information (see Figure S1). The CBO between CZTS and SnO2 can be readily calculated as 0−0.2 eV by employing a band gap value of CZTS of 1.5 eV, revealing a “spike”-like CBO of the heterojunction interface. The schematic diagram is depicted in Figure 3a. A cliff-like conduction band offset at the CZTS/ CdS interface is known to deteriorate Voc by the trap stateassisted recombination.34 Therefore, the optimized conduction band alignment contributes to the reduced interface 156


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Table 1. Overview of CZTS Device Performance of Reference CZTS Device and Devices with SnO2 Intermediate Layer in Different Thickness SnO2 thickness(nm)

η (%)

Voc (mV)

Jsc (mA/cm2)

FF (%)

Rs (Ω cm2)

Rsh (Ω cm2)

0 0.4 0.8 1.2

6.82 7.01 8.16 8.47

638 637 648 657

20.4 19.3 20.4 20.5

52.4 57.0 61.7 62.8

1.13 1.13 0.89 0.70

163 312 315 319

diminution of interface recombination was furnished from the time-resolved photoluminescence (TRPL) measurements on finished CZTS devices with 470 nm laser excitation, which is generally used for measuring the lifetime at the heterointerface and CZTS surface region. Using the biexponential function model fitting described by Ohnesorge et al.,40 the values of carrier lifetime in the vicinity of the heterointerface for devices with and without SnO2 were obtained from the decay profiles showing an improvement from 5.0 to 7.4 ns upon introducing the SnO2 intermediate layer, as shown in Figure 4a. Apart from the lifetime promotion, the dramatic decrease of reverse saturation current density, J02 and ideality factor, A2 for diode 2 while J01 and A1 for diode 1 remain relatively stable (Figure 4b and 4c) further affirm the effectiveness of heterojunction passivation. These parameters were acquired from J−V curves fitting by using the double-diode model.41,42 The term J02 compensates for the recombination loss in the depletion region, and n02 indicates the differential mechanisms for moving carriers across the heterojunction. For junction recombination dominant devices, using the double-diode model shows a more credible match with the experimental results by setting n01 ≥ 1 and n02 ≥ 2.43 The optimization of the heterointerface performance could be reasonably attributed to the passivation of surface defect states and optimization of band alignments, thereby, reducing the overall interface recombination. Furthermore, our experiments revealed that the device with thicker SnO2 demonstrates inferior device performances: although Voc continues increasing with higher SnO2 thickness to some extent, the performance suffers from remarkably lower Jsc which stems from the thicker SnO2 with greater resistivity. Since Voc improves with higher Rsh and slightly lower J02 and A2, the reduced shunting problem and the lower recombination rate could be the reasons for Voc improvement. Since a thicker SnO2 is deposited at the heterojunction by repeating more SILAR deposition cycles, the rapidly increased Rs dominates the FF decrease, resulting in a poorer efficiency. Moreover, it is reasonably anticipated that Voc would reach a limit when passivation effect would not improve with the thickness of the SnO2 intermediate layer. However, the determination of the optimal SnO2 thickness requires extra study for SnO2 passivation layer optimization. The SnO2 layer with around 1 nm thickness would supply sufficient passivation effect for this study since the aim of this study is placed on the passivation effects and other roles of SnO2 between CdS and CZTS rather than peak efficiency. Figure 5a shows the EQE curves of CZTS devices with SnO2 intermediate layers in different thicknesses between CZTS absorber and CdS buffer layer. It is observed that the shortwavelength optical response is improved. Such EQE enhancement in the blue region is believed to result from a slightly thinner CdS buffer layer in the case of CZTS devices with the SnO2 intermediate layer. As CdS buffer layers in all devices were deposited in the same batch, it is supposed that the growth rate of CdS on the SnO2 matrix is lower than that on

Figure 4. (a) TRPL transient of reference CZTS device and the device with a 1.2 nm SnO2 at the CZTS/CdS heterointerface, with 470 nm excitation wavelength. Double-diode model obtained via J−V curves fitting: (b) reverse saturation current densities; (c) ideality factors.

the CZTS absorber, the latter likely with epitaxial growth of CdS due to their similar cubic lattice structure.44 The variation of CdS growth rate is confirmed by TEM (see Figure S2) 157


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layer deposited on SnO2 intermediate layers. Moreover, the additional resistive intermediate layer would increase the series resistance as another factor to limit Jsc, although it improves the charge separation by hole blocking and passivates interface defects.

4. CONCLUSION We have employed ultrathin SnO2 film as an intermediate layer between the CdS buffer and CZTS absorber layers to enhance the overall device performance. Our experimental results clearly demonstrate that the boost of FF and the rise of Voc are mainly attributed to the passivation of CdS/CZTS heterointerface. The SnO2 layer can also optimize the conduction band alignment and block holes away from the heterojunction. Overall, the implementation of ultrathin SnO2 film effectively promotes the efficiency without sacrificing other parameters. With the aid of this shunt-blocking layer, the CdS buffer thickness can be further minimized to reduce the parasitic absorption and thereupon obtain higher Jsc for future research. Furthermore, the SnO2 layer inserted between CZTS and CdS would also be used as a protective layer against the mechanical damage that may occur during the buffer deposition via some vacuum methods. The application of an intrinsic stannic oxide film provides a roadmap of, interface passivation, both conduction and valence bands alignments optimization, shunt blocking, and wider range of accessible buffer deposition methods. Postannealing after CdS deposition is under investigation for addressing the issue of depletion region narrowing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site at DOI: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00044. Cross-sectional CZTS/CdS interface TEM images with or without 1.2 nm SnO2 layer and the bandgap value derivation including Tauc plot (PDF)

Figure 5. (a) EQE of the same devices shown in the JV characteristics. (b) Doping profiles for the reference CZTS device and the device with 1.2 nm SnO2 from C−V measurement at 100 Hz frequency.

where the thickness of CdS decreases from 52 nm for a device without a SnO2 intermediate layer to 44 nm for a device with a 1.2 nm tin oxide intermediate layer. The introduction of the SnO2 intermediate layer seems to allow the reduction in parasitic absorption by reducing the CdS thickness without sacrificing shunt resistance. Figure 5b shows the doping profiles which are determined from the capacitance−voltage (C−V) measurement for CZTS devices with SnO2 in different thicknesses between CZTS absorber and CdS buffer layer, tested on the finished devices. The doping profiles display a decrease in depletion width from 241 to 196 nm by employing a SnO2 film. During the CBD process for reference CZTS devices, Cd tends to occupy Cu vacancy sites for the Cu-depleted CZTS surface to form CdCu+ donor sites.44,45 Hence a convincing assumption could be made that the ultrathin stannic oxide layer forms an impediment to the interaction between CZTS and CdS, such as Cd diffusion, giving rise to a shrunken depletion width. SnO2 has been reported for this similar behavior as a diffusion barrier to prevent Zn diffusion from Zn(O,S):N buffer into SnS absorber in SnS solar cells.17 A narrower depletion width results in a weaker near-infrared (near-IR) response (refer to Figure 5a), which could explain why Jsc remains unchanged at ∼20 mA/cm2, albeit the short-wavelength optical response enhances significantly, contributed by a thinner CdS buffer

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AUTHOR INFORMATION

Corresponding Authors

*(X.H.) E-mail: [email protected]. *(F.L.) E-mail: [email protected]. ORCID

Heng Sun: 0000-0002-0340-5754 Jongsung Park: 0000-0002-5423-1176 Martin A. Green: 0000-0002-8860-396X Xiaojing Hao: 0000-0001-5903-4481 Author Contributions

H.S and K.S contributed equally to this work. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Baosteel, the Australian Government through the Australian Renewable Energy Agency (ARENA), Australian Research Council (ARC), and the Australian Government Research Training Program Scholarship. Responsibility for the views, information, or advice 158


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Distribution in High-Performance Kesterite Solar Cells. Adv. Energy Mater. 2015, 5, 1402180. (19) Chambers, B. A.; MacDonald, B. I.; Ionescu, M.; Deslandes, A.; Quinton, J. S.; Jasieniak, J. J.; Andersson, G. G. Examining the role of ultra-thin atomic layer deposited metal oxide barrier layers on CdTe/ ITO interface stability during the fabrication of solution processed nanocrystalline solar cells. Sol. Energy Mater. Sol. Cells 2014, 125, 164−169. (20) Yıldırım, M. A.; Akaltun, Y.; Ateş, A. Characteristics of SnO 2 thin films prepared by SILAR. Solid State Sci. 2012, 14, 1282−1288. (21) Hurtado, M.; Cruz, S. D.; Becerra, R. A.; Calderón, C.; BartoloPérez, P.; Gordillo, G. XPS analysis and structural characterization of CZTS thin films prepared using solution and vacuum based deposition techniques. Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th; IEEE: New York, 2014; pp 0368−0372. (22) Hurtado Morales, M. F. XPS study for CZTS thin films growth by PVD co-evaporation method. Elementos (Bogota, Colomb.) 2014, 4, 87−95. (23) Dang, W.; Ren, X.; Zi, W.; Jia, L.; Liu, S. F. Composition controlled preparation of Cu−Zn−Sn precursor films for Cu 2 ZnSnS 4 solar cells using pulsed electrodeposition. J. Alloys Compd. 2015, 650, 1−7. (24) Süzer, Ş .; Voscoboinikov, T.; Hallam, K. R.; Allen, G. C. Electron spectroscopic investigation of Sn coatings on glasses. Anal. Bioanal. Chem. 1996, 355, 654−656. (25) Di Giulio, M.; Micocci, G.; Serra, A.; Tepore, A.; Rella, R.; Siciliano, P. SnO2 thin films for gas sensor prepared by rf reactive sputtering. Sens. Actuators, B 1995, 25, 465−468. (26) Bär, M.; Schubert, B.-A.; Marsen, B.; Wilks, R. G.; Pookpanratana, S.; Blum, M.; Krause, S.; Unold, T.; Yang, W.; Weinhardt, L.; Heske, C.; Schock, H.-W. Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction. Appl. Phys. Lett. 2011, 99, 222105. (27) Santoni, A.; Biccari, F.; Malerba, C.; Valentini, M.; Chierchia, R.; Mittiga, A. Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy. J. Phys. D: Appl. Phys. 2013, 46, 175101. (28) Ericson, T.; Scragg, J. J.; Hultqvist, A.; Watjen, J. T.; Szaniawski, P.; Torndahl, T.; Platzer-Bjorkman, C. Zn (O, S) Buffer Layers and Thickness Variations of CdS Buffer for Cu $ _ {2} $ ZnSnS $ _ {4} $ Solar Cells. IEEE Journal of Photovoltaics 2014, 4, 465−469. (29) Fritsche, J.; Kraft, D.; Thißen, A.; Mayer, T.; Klein, A.; Jaegermann, W. Band energy diagram of CdTe thin film solar cells. Thin Solid Films 2002, 403-404, 252−257. (30) Repins, I.; Vora, N.; Beall, C.; Wei, S.-H.; Yan, Y.; Romero, M.; Teeter, G.; Du, H.; To, B.; Young, M.; Noufi, R. Kesterites and chalcopyrites: a comparison of close cousins. MRS Online Proc. Libr. 2011, 1324, 10.1557/opl.2011.844 (31) Islam, M.; Hakim, M. Electron affinity and work function of polycrystalline SnO 2 thin film. J. Mater. Sci. Lett. 1986, 5, 63−65. (32) Ge, J.; Chu, J.; Jiang, J.; Yan, Y.; Yang, P. Characteristics of Insubstituted CZTS thin film and bifacial solar cell. ACS Appl. Mater. Interfaces 2014, 6, 21118−21130. (33) Olopade, M.; Oyebola, O.; Adeleke, B. Investigation of some materials as buffer layer in copper zinc tin sulphide (Cu2ZnSnS4) solar cells by SCAPS-1D. Adv. Appl. Sci. Res. 2012, 3, 3396−3400. (34) Minemoto, T.; Matsui, T.; Takakura, H.; Hamakawa, Y.; Negami, T.; Hashimoto, Y.; Uenoyama, T.; Kitagawa, M. Theoretical analysis of the effect of conduction band offset of window/CIS layers on performance of CIS solar cells using device simulation. Sol. Energy Mater. Sol. Cells 2001, 67, 83−88. (35) Sun, K.; Liu, F.; Yan, C.; Zhou, F.; Huang, J.; Shen, Y.; Liu, R.; Hao, X. Influence of sodium incorporation on kesterite Cu 2 ZnSnS 4 solar cells fabricated on stainless steel substrates. Sol. Energy Mater. Sol. Cells 2016, 157, 565−571. (36) Cui, H.; Lee, C.-Y.; Li, W.; Liu, X.; Wen, X.; Hao, X. Improving efficiency of evaporated Cu2ZnSnS4 thin film solar cells by a thin Ag

expressed herein is not accepted by the Australian Government. We also acknowledge the facilities and the scientific and the technical assistance of the Mark Wainwright Analytical Centre, University of New South Wales (UNSW). We acknowledge use of the facilities in the Electron Microscopy Centre at University of Wollongong and the assistance of David Mitchell and Gilberto Casillas Garcia.

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REFERENCES

(1) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Green, M. A.; Hishikawa, Y.; Warta, W.; Dunlop, E. D.; Levi, D. H.; Hohl - Ebinger, J.; Ho - Baillie, A. W. Solar cell efficiency tables (version 50). Prog. Photovoltaics 2017, 25, 668−676. (3) Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Materials availability expands the opportunity for large-scale photovoltaics deployment. Environ. Sci. Technol. 2009, 43, 2072−2077. (4) Yin, L.; Cheng, G.; Feng, Y.; Li, Z.; Yang, C.; Xiao, X. Limitation factors for the performance of kesterite Cu 2 ZnSnS 4 thin film solar cells studied by defect characterization. RSC Adv. 2015, 5, 40369− 40374. (5) Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Loss mechanisms in hydrazine-processed Cu 2 ZnSn (Se, S) 4 solar cells. Appl. Phys. Lett. 2010, 97, 233506. (6) Li, J.; Mitzi, D. B.; Shenoy, V. B. Structure and electronic properties of grain boundaries in earth-abundant photovoltaic absorber Cu2ZnSnSe4. ACS Nano 2011, 5, 8613−8619. (7) Meril, M. Engineering the properties of indium sulfide for thin film solar cells by doping. Ph.D. Thesis, Cochin University of Science & Technology, Cochin, India, 2009. (8) Courel, M.; Andrade-Arvizu, J.; Vigil-Galán, O. Loss mechanisms influence on Cu 2 ZnSnS 4/CdS-based thin film solar cell performance. Solid-State Electron. 2015, 111, 243−250. (9) Yan, C.; Liu, F.; Song, N.; Ng, B. K.; Stride, J. A.; Tadich, A.; Hao, X. Band alignments of different buffer layers (CdS, Zn (O, S), and In2S3) on Cu2ZnSnS4. Appl. Phys. Lett. 2014, 104, 173901. (10) Crovetto, A.; Yan, C.; Iandolo, B.; Zhou, F.; Stride, J.; Schou, J.; Hao, X.; Hansen, O. Lattice-matched Cu2ZnSnS4/CeO2 solar cell with open circuit voltage boost. Appl. Phys. Lett. 2016, 109, 233904. (11) Sun, K.; Yan, C.; Liu, F.; Huang, J.; Zhou, F.; Stride, J. A.; Green, M.; Hao, X. Over 9% efficient kesterite Cu2ZnSnS4 solar cell fabricated by using Zn1−xCdxS buffer layer. Adv. Energy Mater. 2016, 6, 1600046. (12) Herz, K.; Eicke, A.; Kessler, F.; Wächter, R.; Powalla, M. Diffusion barriers for CIGS solar cells on metallic substrates. Thin Solid Films 2003, 431-432, 392−397. (13) Hsu, W.-W.; Chen, J.; Cheng, T.-H.; Lu, S.; Ho, W.-S.; Chen, Y.-Y.; Chien, Y.-J.; Liu, C. Surface passivation of Cu (In, Ga) Se2 using atomic layer deposited Al2O3. Appl. Phys. Lett. 2012, 100, 023508. (14) Lee, Y. S.; Gershon, T.; Todorov, T. K.; Wang, W.; Winkler, M. T.; Hopstaken, M.; Gunawan, O.; Kim, J. Atomic Layer Deposited Aluminum Oxide for Interface Passivation of Cu2ZnSn (S,Se)4 ThinFilm Solar Cells. Adv. Energy Mater. 2016, 6, 1600198. (15) Erkan, M. E.; Chawla, V.; Scarpulla, M. A. Reduced defect density at the CZTSSe/CdS interface by atomic layer deposition of Al2O3. J. Appl. Phys. 2016, 119, 194504. (16) Wu, W.; Cao, Y.; Caspar, J. V.; Guo, Q.; Johnson, L. K.; Mclean, R. S.; Malajovich, I.; Choudhury, K. R. Characterization of CZTSSe photovoltaic device with an atomic layer-deposited passivation layer. Appl. Phys. Lett. 2014, 105, 042108. (17) Sinsermsuksakul, P.; Sun, L.; Lee, S. W.; Park, H. H.; Kim, S. B.; Yang, C.; Gordon, R. G. Overcoming Efficiency Limitations of SnS - Based Solar Cells. Adv. Energy Mater. 2014, 4, 1400496. (18) Sardashti, K.; Haight, R.; Gokmen, T.; Wang, W.; Chang, L.-Y.; Mitzi, D. B.; Kummel, A. C. Impact of Nanoscale Elemental 159


Article

ACS Applied Energy Materials intermediate layer between absorber and back contact. Int. J. Photoenergy 2015, 2015, 170507. (37) Steinmann, V.; Chakraborty, R.; Rekemeyer, P. H.; Hartman, K.; Brandt, R. E.; Polizzotti, A.; Yang, C.; Moriarty, T.; Gradečak, S.; Gordon, R. G.; Buonassisi, T. A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System. ACS Appl. Mater. Interfaces 2016, 8, 22664−22670. (38) Tao, J.; Zhang, K.; Zhang, C.; Chen, L.; Cao, H.; Liu, J.; Jiang, J.; Sun, L.; Yang, P.; Chu, J. A sputtered CdS buffer layer for coelectrodeposited Cu 2 ZnSnS 4 solar cells with 6.6% efficiency. Chem. Commun. 2015, 51, 10337−10340. (39) Liu, F.; Huang, J.; Sun, K.; Yan, C.; Shen, Y.; Park, J.; Pu, A.; Zhou, F.; Liu, X.; Stride, J. A.; Green, M. A.; Hao, X. Beyond 8% ultrathin kesterite Cu2ZnSnS4 solar cells by interface reaction route controlling and self-organized nanopattern at the back contact. NPG Asia Mater. 2017, 9, e401. (40) Ohnesorge, B.; Weigand, R.; Bacher, G.; Forchel, A.; Riedl, W.; Karg, F. Minority-carrier lifetime and efficiency of Cu (In, Ga) Se 2 solar cells. Appl. Phys. Lett. 1998, 73, 1224−1226. (41) Chan, D. S.; Phang, J. C. Analytical methods for the extraction of solar-cell single-and double-diode model parameters from IV characteristics. IEEE Trans. Electron Devices 1987, 34, 286−293. (42) Ishaque, K.; Salam, Z.; Taheri, H. Simple, fast and accurate two-diode model for photovoltaic modules. Sol. Energy Mater. Sol. Cells 2011, 95, 586−594. (43) Humada, A. M.; Hojabri, M.; Mekhilef, S.; Hamada, H. M. Solar cell parameters extraction based on single and double-diode models: A review. Renewable Sustainable Energy Rev. 2016, 56, 494− 509. (44) Liu, F.; Yan, C.; Huang, J.; Sun, K.; Zhou, F.; Stride, J. A.; Green, M. A.; Hao, X. Nanoscale Microstructure and Chemistry of Cu2ZnSnS4/CdS Interface in Kesterite Cu2ZnSnS4 Solar Cells. Adv. Energy Mater. 2016, 6, 1600706. (45) Maeda, T.; Nakamura, S.; Wada, T. First-principles study on Cd doping in Cu2ZnSnS4 and Cu2ZnSnSe4. Jpn. J. Appl. Phys. 2012, 51, 10NC11.

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