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Improving the open circuit voltage through surface oxygen plasma treatment and 11.7% efficient CuZnSnSe solar cell 2

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Hitoshi Tampo, Shinho Kim, Takehiko Nagai, Hajime Shibata, and Shigeru Niki ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01756 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Materials & Interfaces

Improving the open circuit voltage through surface oxygen plasma treatment and 11.7% efficient Cu2ZnSnSe4 solar cell Hitoshi Tampo†*, Shinho Kim†, Takehiko Nagai†, Hajime Shibata†, and Shigeru Niki‡ †

Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science

and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. ‡

Department of Energy and Environment (E&E), National Institute of Advanced Industrial

Science and Technology (AIST), Central 1, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.

KEYWORDS: kesterite, CZTSe, solar cell surface treatment

ACS Paragon Plus Environment

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ABSTRACT

The photovoltaic performance of Cu2ZnSnSe4 (CZTSe) solar cells subject to surface oxygen plasma treatments is investigated. The observed improvements are related to an enhancement of the open circuit voltage VOC, that is, the suppression of the VOC deficit. The VOC monotonically increases with treatment time up to 0.460 V. The origin of this improvement is discussed, and it is concluded that the effectiveness of the surface treatment is not due to oxygen-related alloying, but instead to the homogeneous oxidization and removal of the oxidized CZTSe surface layer. The surface oxygen content increases with surface treatment time, although surface oxides are fully removed after ammonia treatment, which is conducted in a similar manner to CdS buffer deposition. The reduction of surface recombination is confirmed by time-resolved photoluminescence measurements and the minority carrier lifetime deduced using the fast decay component increases with increasing treatment time. The relationship between photovoltaic properties and lifetime is clearly demonstrated. The best-performing CZTSe solar cell obtained using surface oxygen treatment demonstrates a conversion efficiency of 11.7%, which is higher than that of previous reports on CZTSe cells.


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INTRODUCTION In order to achieve the goals of the Paris Agreement and Sustainable Development Goals, terawatt-scale mass production of solar cells is anticipated by 2030.1 Terawatt-scale photovoltaic (PV) production corresponds to tens of times the current production, and is a very challenging target. Therefore, the use of solar cells that employ earth-abundant materials is crucial in order to meet the demands of terawatt-scale production, in terms of both cost and resources. Cu2ZnSn(S,Se)4 (CZTSSe)-based solar cells are good candidates to meet this demand because CZTSSe is comprised of earth-abundant materials, and the commercialized technologies used for Cu(In,Ga)Se2 (CIGS) and CdTe solar cells can be employed. CZTSSe is a I2-II-IV-VI4 compound semiconductor and a derivative of I-III-VI2 CIGS. There has been a great deal of technological development related to CIGS solar cells that may be applied to CZTSSe solar cells. However, the PV performance of CZTSSe-based cells is still low compared to that of CIGS solar cells; their certified highest conversion efficiency of 12.6%2 is much lower than that of 23.35% for CIGS solar cells,3 and many researchers are trying to bridge this gap. This is mainly because CZTSSe is a quaternary compound that contain various defects which are difficult to control, and the technologies related to defect control are immature compared to those for the commercialized chalcogenide CIGS and CdTe solar cells.4 The most serious issue related to CZTSSe-based solar cells is their large open circuit voltage (VOC deficit), compared to the bandgap energy.45 The VOC deficits of GaAs Si, CIGS, and CdTe solar cells are reported to be ~0.3, ~0.4, ~0.4 and ~0.6 V, respectively.6 The VOC deficit of CZTSe-based solar cell must be reduced to a level comparable to that of CIGS cells in order to replace them. In general, the fundamental mechanism that determines VOC is a recombination process; the lower the recombination rate in the solar cell, the higher the VOC.7 The structure of the CZTSSe-


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based solar cell is similar to that of CIGS solar cells except for the absorber. Therefore, the VOC is considered to be limited by the recombination in the bulk and/or at the interface of the CZTSSe.5 There are several papers related to improvements of kesterite-based cells from the perspective of the quality of the bulk or/and the surface. In terms of improving bulk quality, alloying and doping are typical techniques. For example, Ge alloying8, Ge doping,910 Li alloying,11 compositional grading,121314 and also alkaline element doping; 1516171819202122 various types of surface treatment and corresponding improvements were also reported. 23242526 However, the relationships between photovoltaic properties and basic limiting properties such as minority carrier lifetime, diffusion length, and mobility are not clearly understood. This is a large barrier to improving the conversion efficiency of kesterite-based cells and should be investigated further. Time-resolved photoluminescence (TRPL) is a powerful tool for evaluating the quality of materials and devices in terms of the carrier lifetime and decay mechanism. These have been reported and discussed in both theoretical and experimental terms,2728 also in the case of kesterite materials and devices.1518293031323334 In our previous paper,18 we reported improvements of CZTSe bulk properties that were obtained using Na doping. The minority carrier lifetime estimated using TRPL measurements, which is related to CZTSe bulk recombination and a slow decay component, increases from 2 to 15 ns. However, the Na doping did not induce any improvement in the VOC in spite of the improvement of the bulk lifetime. Moreover, one paper reported no correlation between the measured PL decay times and photovoltaic properties over a wide range of efficiency (1−12%) and VOC.34 These results imply that the recombination process in kesterite is not only limited by the bulk region but also by other recombination mechanisms. In this study, we use a surface treatment in order to increase the VOC of CZTSe cells and discuss the origins of the resulting improvements and limitations.


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EXPERIMENTAL SECTION CZTSe thin films were grown by the coevaporation method using molecular beam epitaxy equipment, and the films were thermally treated in a tube furnace under Se and SnSe atmospheres. Surface treatment was carried out using oxygen plasma ashing equipment before CdS deposition. The equipment is originally designed for ashing residuals of photoresist. The oxygen plasma treatments were conducted under an oxygen flow of 200 sccm maintaining a pressure of 100 Pa. The stage is 160 mm in diameter, and RF power of 200 W was applied. KCN treatment was also used, then CdS deposition was continuously conducted by chemical bath deposition. The CZTSe solar cells were fabricated based on a CIGS solar cell except for the absorbers, ZnO:Al (350 nm)/iZnO (50 nm) /CdS (50 nm)/CZTSe (1.8μm)/Mo/soda lime glass. ZnO:Al and i-ZnO layers were deposited using the sputtering method. The other structure was used for obtaining the bestpermorming cell, namely ZnO:Al (350 nm)/i-ZnO (30 nm) /CdS (30 nm)/CZTSe (1.8μm)/Mo/soda lime glass. The thickness of the CdS layer was 30 nm, and the i-ZnO layer was deposited using atomic layer deposition (Oxford FlexAL). The current-voltage measurements were conducted under simulated AM 1.5G (100 mW/cm2) illumination. A scribed designated area (not active area) of 0.522 cm2 was used in this study. Anti-reflection coating was not used except for the bestperforming cell. Time-resolved photoluminescence (TRPL ) measurements were conducted on CZTSe device structures using a Hamamatsu Photonics C12132 NIR TRPL system with the second harmonic of an yttrium-aluminum-garnet laser (532 nm). The repetition frequency, pulse length and power of the laser were 15 kHz, 540 nm). This implies that the recombination loss near the top surface of CZTSe was reduced by the CZTSe surface treatment. Moreover, EQE was also improved in the CdS absorption region (400 − 540 nm), which might be due to suppression of CdS/CZTSe interface recombination as demonstrated for the CdS/CIGS interface.42 The poor EQE of CZTSe cells with a rapid degradation in the longer wavelength region (λ > 600 nm) is due to the short collection length Lc of CZTSe, that was estimated using the e-ARC method.43 Figure 6 shows a simulation of the EQE spectrum for a sample after a 30-min treatment, and the collection length was estimated to be 470 nm. This simulation result implies that surface recombination still occurs because there is a relatively large deviation between the calculated and experimental EQE spectra in the shortwavelength region of CZTSe absorption, while the case without treatment showed a greater


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deviation. This deviation might indicate the extent of surface and near surface recombination, although more detailed investigations into this relationship are necessary. It is of note that the band edges were almost identical from 1.07 to 1.08 eV according to the EQE spectra as shown in Figure 5. Therefore, it might be concluded that the oxygen-incorporated CZTSe is not formed after aqueous ammonia treatment, which is consistent with the removal of the homogeneous oxidized layer as shown in Figure 2. The effect of alloying with oxygen is negligible according to the EPMA and EQE results; however, the top surface of CZTSe might be slightly oxidized, which might improve the VOC.

Best cell (a) Simulated spectrum of (a) 30 min treatment in Fig. 5 (b) Simulated spectrum of (b)

1.0

0.8

EQE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Lc=2.0 µm 2

µ=110 cm /Vs

0.6 Lc=0.47 µm

0.4 0.2

0.0 400

600

800

1000

1200

1400

Wavelength (nm) Figure 6. EQE simulation using the e-ARC method for the surface treated and best-performing CZTSe cell. The J-V curve for the best-performing CZTSe cell with surface treatment is shown in Figure 7 with a conversion efficiency of 11.7%. The VOC, JSC, and FF were 0.423 V, 41.7 mA/cm2, and 0.666, respectively. In this optimal cell, 10 min surface treatment was used, and a thinner CdS


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buffer of 30 nm was used to improve the JSC, although a lower VOC was obtained for the thinner CdS. In order to protect the CdS/CZTSe heterointerface from sputtering damage to the thinner CdS layer, intrinsic ZnO was applied using atomic layer deposition (ALD). Moreover, 10 nm NaF was deposited before thermal treatment to improve the CZTSe bulk properties through Na incorporation, and MgF2 was also used for an anti-reflection layer. The slightly lower VOC of the best-performing cell compared to that indicated in Figure 4 might be due to the thinner CdS and the NaF layers, although this must be investigated further. We observed lowering of the VOC for the thinner CdS, and found that it was possible to suppress the degradation of VOC using i-ZnO deposited via ALD. However, the relationship on PV properties between CdS layer thicknesses, the usage of ALD of ZnO, and the alkaline incorporation are not sufficiently investigated and require extensive investigations. The device parameters of the cell are compared with those reported for the CZTSe champion cell in Table 2.44 The parameters of these two cells were almost identical, for example, the efficiency of the current cell (champion cell) was 11.7% (11.6%), the VOC values were both 0.423V, and the ideality factor was almost same at 1.56 (1.57). It is notable that the JSC values calculated from EQE of the best-performing cell and the cell with the 30 min treatment shown in figure 6 were 39.6 and 30.5 mA/cm2, respectively, which are slightly smaller compared to those obtained via J-V measurements. The difference is considered to be mainly because of DC measurements in EQE, and the measured JSC is acceptable because the JSC for the best-performing cell was certified to be 40.91 mA/cm2 by Japan Electrical Safety & Environment Technology Laboratories (JET). The certified cell performance of efficiency, VOC, and FF were 11.2%, 0.4218 V, and 0.647, respectively (see the Supporting Information). Degradation in the efficiency was also confirmed via in-house measurement, and the efficiency measured in-house


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just before the JET measurement was 11.1%. Degradation is sometimes observed after cell fabrication.

2

Current Density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

30

eff.: 11.7% VOC: 0.423 V JSC: 41.7 mA/cm2 FF: 0.666 D. A.: 0.522 cm2

20 10 0 0.00

0.10

0.20

0.30

0.40

0.50

Voltage (V)

Figure 7. J-V curve for the best-performing CZTSe cell with surface treatment and Na incorporation. Table 2. Comparison of device parameters of the best-performing CZTSe cell and the reported champion CZTSe cell. eff (%)

VOC (V)

JSC (mA/cm2)

FF

n

j0 (Acm-2)

Rs (Ω cm2)

Rsh (Ω cm2)

Champion CZTSSe*

12.6

0.513

35.2

0.698

1.45

7.0 × 10-8

0.72

621

Champion CZTSe**

11.6

0.423

40.6

0.673

1.57

1.38 × 10-6

0.32

602

CZTSe (this work)

11.7

0.423

41.7

0.666

1.56

1.15 × 10-6

0.38

1000

* Ref. 2


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** Ref. 44

Figure 6 also shows the EQE spectrum for the best-performing cell with simulated spectrum, and the bandgap energy was estimated to be 1.00 eV at the inflection point of the EQE spectrum. Furthermore, it was found that a much larger collection length Lc of 2.0 μm was obtained compared to a length of 0.47 μm obtained without the surface treatment. The relatively lower VOC compared to that of the samples presented in Table 1 might be because of the lower bandgap energy, or the different estimated bandgaps might be because of the different collection lengths, resulting in different EQE spectra near the band edge, which is observed via e-ARC simulations.43 This aspect requires further investigations. The large Lc is due to the incorporation of Na as previously reported,18 which leads to improvements in bulk quality and a lowering of the carrier concentration. The diffusion length Ld is calculated to be 1.9 μm using the equation: Lc = Ld + W, where the depletion length W of 0.1 μm is used, which was estimated by capacitance-voltage measurement. Moreover, the electron mobility of CZTSe can be evaluated to be 110 cm2/Vs using the equation: Ld=√τμkT/q, where the lifetime is 13 ns and the diffusion length is 1.9 μm. These values of the mobility and diffusion length are similar to those previously reported in the high performance CZTSSe cells.45

CONCLUSIONS The VOC deficit of a CZTSe solar cell has been decreased up to 0.540 V through surface oxygen plasma treatment. A clean surface with low recombination was obtained by homogeneous oxidization and removal of the oxidized layer. A relationship between the PV properties and PL lifetime was clearly observed, and VOC was found to linearly increase with increasing lifetime, an effect which is related to surface recombination. A conversion efficiency of 11.7% was obtained


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for the CZTSe solar cell using this surface treatment and Na incorporation, which provides the highest reported efficiency in the case of CZTSe cells. The increased VOC of the CZTSe cells described in this study implies that these kesterite-based cells could be a viable option to replace CIGS cells in the future. Corresponding Author *E-mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ORCID Hitoshi Tampo: 0000-0002-6666-0285 Shinho Kim: 0000-0002-8895-0232 Takehiko Nagai: 0000-0002-1873-5764 Hajime Shibata: 0000-0002-2405-2329 Shigeru Niki: 0000-0002-3877-2028 ACKNOWLEDGMENT This work is partly supported by the New Energy and Industrial Technology Development Organization (NEDO) “Feasibility Study Program”. The thermal treatment in this study was conducted by Mr. Sekizen Takaesu, and H. T. would like to thank him for his precise and reproducible control of the treatment.


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Shin, B.; Zhu, Y.; Bojarczuk, N. A.; Jay Chey, S.; Guha, S. Control of an Interfacial MoSe 2 Layer in Cu 2 ZnSnSe 4 Thin Film Solar Cells: 8.9% Power Conversion Efficiency with a TiN Diffusion Barrier. Appl. Phys. Lett. 2012, 101 (5), 053903. https://doi.org/10.1063/1.4740276.


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Hages, C. J.; Koeper, M. J.; Agrawal, R. Optoelectronic and Material Properties of Nanocrystal-Based CZTSe Absorbers with Ag-Alloying. Sol. Energy Mater. Sol. Cells 2016, 145, 342–348. https://doi.org/10.1016/j.solmat.2015.10.039.

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DecayFit. www.fluortools.com. The measured decay curves inclue effect of finite instrument reponse, which limit the time resolution in the meaurement, and it is called as instruments response function (IRF). The IRF is actually measuremed by a reference materilal with very fact decay comared to the instrument response. The real decay curve can be evaluated by decomvolution of the


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measured curve using IRF. The decomvolution and following fitting were conducted by the DecayFit software. (36)

Shobu, K. CaTCalc: New Thermodynamic Equilibrium Calculation Software. Calphad 2009, 33 (2), 279–287. https://doi.org/10.1016/j.calphad.2008.09.015. The standard Gibbs free energy of formation, for exapmle, Cu2Se and Cu2O are calculated using the chemical reaction below, and the difference can be calculated the two standard Gibbs energy of formation. Cu + 1/2Se2 ⇄ Cu2Se, Cu + 1/2O2 ⇄ Cu2O

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Kim, K. M.; Tampo, H.; Kim, S.; Shibata, H.; Niki, S.; Han, H. Improved Performance in Cu2ZnSnSe4 Solar Cells Using a Sandwich-Structured ZnSe/Cu2SnSe3/ZnSe Precursor. Curr. Appl. Phys. 2017, 17 (3), 366–369. https://doi.org/10.1016/j.cap.2016.12.006.


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Lee, Y. S.; Gershon, T.; Gunawan, O.; Todorov, T. K.; Gokmen, T.; Virgus, Y.; Guha, S. Cu 2 ZnSnSe 4 Thin-Film Solar Cells by Thermal Co-Evaporation with 11.6% Efficiency and Improved Minority Carrier Diffusion Length. Adv. Energy Mater. 2015, 5 (7), 1401372. https://doi.org/10.1002/aenm.201401372.

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Gokmen, T.; Gunawan, O.; Mitzi, D. B. Minority Carrier Diffusion Length Extraction in Cu2ZnSn(Se,S)4solar Cells. J. Appl. Phys. 2013, 114 (11). https://doi.org/10.1063/1.4821841.


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TOC GRAPHIC

Longer Lifetime, Larger VOC

10

10

e-ARC simulation

4

VOC=0.460 V VOC=0.458 V VOC=0.451 V VOC=0.424 V VOC=0.416 V

3

surface treatment + alkaline doping

0.8

VOC up

2

τ up

PCE (CZTSe)=11.7% Lc=2.0 µm

1.0

EQE

10

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4

10

10

surface treatment

1

Lc=0.47 µm

0.2 0.0

0

0

2

4

6

8

10

12

14

16

18

20

400

Time (ns)

600

800

1000

1200

1400

Wavelength (nm)


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