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Cu2ZnSnS4 solar cells with over 10% power conversion efficiency enabled by heterojunction heat treatment Chang Yan1,6, Jialiang Huang 1,6, Kaiwen Sun 1,6, Steve Johnston2, Yuanfang Zhang1, Heng Sun1, Aobo Pu1, Mingrui He1, Fangyang Liu1*, Katja Eder3, Limei Yang3, Julie M. Cairney3, N. J. Ekins-Daukes1, Ziv Hameiri1, John A. Stride4, Shiyou Chen 5, Martin A. Green1 and Xiaojing Hao 1* 1 Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales, Australia. 2National Renewable Energy Laboratory, Golden, CO, USA. 3Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, New South Wales, Australia. 4School of Chemistry, University of New South Wales, Sydney, New South Wales, Australia. 5School of Information Science and Technology, East China Normal University, Shanghai, China. 6These authors contributed equally: Chang Yan, Jialiang Huang, Kaiwen Sun. *e-mail:
[email protected];
[email protected]
Nature Energy | www.nature.com/natureenergy
Supplementary Information Cu2ZnSnS4 solar cell power conversion efficiency breaking the 10% barrier through heterojunction heat treatment Chang Yan1+, Jialiang Huang1+, Kaiwen Sun1+, Steve Johnston2, Yuanfang Zhang1, Heng Sun1, Aobo Pu1, Mingrui He1, Fangyang Liu1*, Katja Eder3, Limei Yang3, Julie M. Cairney3, N.J. Ekins-Daukes1, Ziv Hameiri1, John A. Stride4, Shiyou Chen5, Martin A. Green1, Xiaojing Hao1* 1
Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable
Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia. 2
3
National Renewable Energy Laboratory, Golden, Colorado 80401, United States Australian Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006,
Australia. 4
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
5
School of Information Science & Technology, East China Normal University, Shanghai,
200241, China. +These authors contributed equally to this work Corresponding Author:
[email protected];
[email protected]
1
Supplementary Figure 1 Statistical performance parameters of the CZTS devices without and with HT at 270 °C, 300 °C and 330 °C, respectively. The box plot denotes median (centre line), mean value (dots), 25th (bottom edge of the box), 75th (top edge of the box), 95th (upper whisker) and 5% (lower whisker) percentiles. The sample size in each column are 10 devices. The red star denotes the efficiency of champion cell.
2
Supplementary Figure 2 a) External quantum efficiency of CZTS devices with and without heterojunction heat treatment and b) estimated bandgap from EQE curves. The solid lines are the linear extrapolation lines for [hv Ln (1-EQE)]2.
Supplementary Figure 3 The certificate of over 10% efficiency CZTS devices with area of 0.2339 and 1.113 cm2 by NREL.
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Supplementary Figure 4 Photos of our champion CZTS devices with an area of around 0.24 cm2 (left, where 14 cells are shown) and around 1.07 cm2 (right, where 3 cells are shown). All the CZTS devices are fabricated on 2.5 cm×2.5 cm glass substrates. The dimensions of small and standard (large) devices are 3 mm×8 mm and 6.7 mm×16 mm, respectively.
Supplementary Table 1 PV performance parameters for CZTS at different storage time Day after device VOC (mV) JSC (mA/cm2)
FF (%)
fabrication
Efficiency
Storage condition
(%)
1 Day
724.7
22.4
66.6
10.81
In the open air
225 Day
721.2
22.1
61.8
9.85
In the open air
4
Supplementary Figure 5 J-V characteristic of high efficiency CZTS cell measured under AM 1.5 (100 mW/cm2) after different storage times.
Supplementary Figure 6 Light absorption coefficient of typical CZTS thin films and the penetration depth asumming 90% light absorption at different wavelength. The blue dashline and red dashline indicate the wavelength of 470 nm and 640 nm, respectively. For the 470 nm 5
laser, only 256 nm thick CZTS was able to absorb 90% of the incident radiation. As the PL characterization was conducted on the finished CZTS devices, the CdS should have a significant absorption as the laser energy of 470 nm (corresponding to 2.64 eV) is higher than bandgap of CdS (2.4 eV). As the CdS layer in CZTS or CIGS solar cell is very thin, with a thickness of only 50~60 nm. According to the light absorption equation (Beer-Lamber-Law), 50~60 nm CdS with absorption coefficient of (~8×10^4 cm-1) is able to absorb 33%~39% of incident laser (470 nm), allowing most of the incident laser light to transmit and be absorbed by the CZTS absorber. Consequently, the effective laser signal arriving at CZTS should be shallower than 256 nm. In terms of the 640 nm laser, it requires 517 nm thick CZTS to absorb 90% of incident laser power. Therefore, the photoluminence information from the 470 nm laser reflects more of the recombination at the p-n interface and the very top of the CZTS, whereas at 640 nm, more reflects the deep bulk properties.
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Supplementary Figure 7 Device performance parameters extracted from Suns-VOC measurement by fitting with two-diode models. The sample size are 3 devices with highest efficiencies for each column.
7
Supplementary Note 1. Detailed discussion on over 300°C high temperature treatment: As shown in Supplementary Figure 7, it is noted that applying further higher temperatures for HT process (300-330 °C) led to a detrimental impact on the p-n junction diode quality by increasing the non-radiative recombination within the SCR, evident from the increased J 02 and nmpp. In addition, the higher temperature treatment introduced a shunting problem as indicated by a reduced RSH. However, after implementing an experiment using a higher temperature treatment and observing the XPS elemental profile for the heterojunction after treatment, we found the decrease in efficiency might also be related to over-diffusion of Zn into CdS. (See Supplementary Table 2 and Supplementary Figure 15) In order to gain more insight on the efficiency degradation at high temperature heat treatment, we used an even higher temperature 400 °C for heterojunction heat treatment and then did another CdS buffer layer. Unexpectedly, this second CdS buffer cannot compensate efficiency. Even worse, the efficiency dropped a lot after depositing a second buffer (See Supplementary Table 2 for device performance parameters). After a double CdS deposition, VOC, JSC and FF experienced a significant decrease. After the heterojunction heat treatment at high temperature, too much Zn diffused into the CdS (See Supplementary Figure 15, which shows large amount of Zn diffusion into CdS buffer at HT 330 °C). Therefore we think the efficiency drop is due to both over-diffused Zn into CdS forming an unfavorable band alignment and the degradation of the quality of CdS itself.
Supplementary Table 2 Device performance of CZTS solar cell after HT at 400 °C with single and double CdS buffer. Condition
VOC (mV)
JSC (mA/cm2)
FF (%)
Efficiency (%)
Single buffer
542
18.2
53.3
5.3
Double buffer
381
4.6
32.3
0.56
8
Supplementary Note 2 Direct evidence of Cd existed in the CZTS lattice: The very top region of CZTS (close to the CZTS/CdS heterointerface) was examined by atomic resolution high-angle annular dark-field imaging (HAADF) measurements to investigate the effect of Cd diffusion on the structural properties of CZTS. When sufficiently high-angle scattering is used for imaging, the contrast of atomic columns in HAADF–STEM images (see Figure 3c) is approximately proportional to the mean square atomic number (Z2) of the constituent atoms from the surface to the bottom of the specimen1. More rigorous calculation in later study using the Bloch wave model, indicated a Z-dependence of the contrast to be I ~ Z1.8–1.9 2. Supplementary Figure 8 illustrates the tetragonal crystal structure of CZTS and its projection along the [100] zone axis, where spheres of different colour and diameter represent different cations. Along the [100] or [010] zone axes, the columns of cation sites are crystallographically independent and the atomic ratio of cations is the same as in the unit cell, which means that each column is occupied by a single type of cation of the same number of atoms. This offers a perfect condition for examining localized defects in high resolution HAADF-STEM images by using the Z2 contrast. As is demonstrated in Supplementary Figure 6, arrangement of the cations can be categorized into 3 different sites based on the characteristic of symmetry with Sn, Cu/Zn (Cu or Sn) and S occupying the sites M1, M2, M3 respectively. Therefore, assuming Z2 dependence, the contrast ratios of atomic columns should be M1:M2:M3=502:292(302):162≈10:3:1. With such large discrepancy between cation site columns, the distribution of different elements or the localized precipitate cation should be reasonably easy illustrated and distinguished using the atomic resolution HAADF images. The original HAADF image is noisy due to amorphous material formed on the surface of the crystal by radiation damage (Supplementary Figure 9). To remove such noisy components, the HAADF–STEM images need to be processed with various techniques3
4 5
. In the present study, we applied the Wiener filter to process the
images (Supplementary Figure 9b)6 7. Each cation column then became clearly resolved. Next, a band-pass filter was applied to further improve the image quality using FFT images (Supplementary Figure 9b c)8. 9
Figure 3c presents a filtered atomic resolution HAADF image taken at the region near the CdS/CZTS interface. From the enlarged image on the right, a clear contrast difference can be observed for different cation columns. Figure 3d shows the intensity profile in a row of cations marked by the red rectangular in Figure 3c. The Sn columns on M1 sites can be clearly distinguished as its intensity is at least twice as strong as most of the M2 sites in between. This shows good consistency with the Z2 dependence between the Sn and Cu/Zn atomic columns. Meanwhile, it is very obvious that two columns on the M2 site are much stronger than other M2 sites, having intensity higher than half of the Sn on M1 site. This is a strong indication of localized precipitation of elements of larger Z than either Cu or Zn, such as Cd; this is consistent with the XPS results, where Cd diffusion was observed at the interface region. Moreover, this finding provides more direct evidence of the Cd diffusion into the CZTS lattice after the heat treatment. We cannot rule out the possibility of Sn atoms being on Cu or Zn sites solely based on the observation in HAADF images. However, based on first principle calculations9,10, the formation energy of SnCu/SnZn is found to be much higher than CdCu/CdZn. Thermodynamically, it is therefore much more feasible that CdCu/CdZn occurs instead of SnCu/SnZn. After comparing the filtered atomic resolution HAADF image taken near the CZTS/CdS interface with the image taken in the CZTS deep bulk (See Supplementary Figure 10), we think the intensity variation in HAADF should be mostly likely due to the Cd on Cu or Zn positions rather than Sn on Cu or Zn positions.
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Supplementary Figure 8 Crystal structure of Cu2ZnSnS4.
Supplementary Figure 9 Atomic resolution HAADF image with different filters.
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Supplementary Figure 10 (a) Filtered atomic resolution HAADF image taken at the region near the CdS/CZTS interface. (b) Filtered atomic resolution HAADF image taken at the region at deep CZTS bulk. (c) The intensity profile in a raw of cations marked by the red rectangular near CdS/CZTS interface in showing the cation exchange (Cd occupying Cu/Zn site). (d) The intensity profile in a raw of cations marked by the red rectangular showing no significant intensity variations on M2 site. By comparing the HAADF of CZTS between near the p/n interface and deep in the bulk, the Cd diffusion phenomenon is much more obvious near the interface region as no clear replacement of the Cu/Zn in the lattice can be observed in the deep CZTS bulk region. This, combining the XPS (Figure 3) and SIMS (Figure 4) result where obvious Cd diffusion signal was detected, we think the intensity variation in HAADF should be mostly likely due to the Cd on Cu or Zn position rather than Sn on Cu or Zn position.
12
Supplementary Figure 11 Statistical box plot of Raman peak position for CZTS surface before HT and after HT. The box plot denotes median (centre line), mean value (dots), 25th (bottom edge of the box), 75th (top edge of the box), 95th (upper whisker) and 5% (lower whisker) percentiles. The sample size in each column are around 10 dots. Two CdS/CZTS samples (CdS deposited on CZTS) with and without heat treatment were prepared for this purpose. HCl was used to remove the CdS buffer layer and a fresh CZTS surface is obtained for the Raman measurement.
13
Supplementary Figure 12 Typical Raman spectrum (514nm excitation) for CZTS before and after Cd introduction.
Supplementary Figure 13 Typical Raman spectra (514nm exitation) for bare CZTS before and after HT process.
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Supplementary Figure 14 Statistical peak position of Raman spectra for CZTS before and after HT process by using a 325 nm Laser. The box plot denotes median (centre line), mean value (dots), 25th (bottom edge of the box), 75th (top edge of the box), 95th (upper whisker) and 5% (lower whisker) percentiles. The sample size in each column are around 10 dots.
Supplementary Note 3 Elemenal diffusion at different HT temperature: We have added XPS elemental depth profiles (See Supplementary Figure 15) of heterojunction treated at different temperatures. It is clear that, driven by the temperature, Cd from CdS and Zn from CZTS diffuse into absorber and buffer, respectively. Cu and Sn did not show any significant diffusion. For Cd diffusion into CZTS, higher temperature induces longer Cd tails within the CZTS (see Cd profile at etching time > 600 s). We also estimated the apparent diffusion depths which are summarized in Supplementary Table 3. Note that the Cd diffusion length may be overestimated due to the plasma etching process (Ar 1 keV) during the XPS depth profile data collection, although general trends are immune to this effect. The diffused Cd in CZTS forms a Cu2CdxZn1-xSnS4 (CZCTS) layer within the top part of CZTS (please see the evidence in Figure 3 HR-HADDF and Raman results). In order to evaluate the quality and properties of this thin Cu2CdxZn1-xSnS4 layer, we have measured the effective minority carrier lifetime by steady-state photoluminescence and Time-Resolved 15
Photoluminescence (TRPL) (640 nm excitation), as well as the Urbach energy by photo thermal deflection spectroscopy (PDS) measurements, all on Cu2CdxZn1-xSnS4 with different Cd concentrations (See summary in Supplementary Table 4). The Urbach energy shows a negative relationship with increasing Cd concentration. This indicates that Cd incorporation in the CZTS contributes to a cleaner band edge with lower band tailing. Moreover, minor Cd incorporation into the CZTS improves the effective minority carrier lifetime and significantly increases PL yield, both indicating dramatically reduced non-radiative recombination. For Zn diffusion into CdS, the Zn appears able to elevate the conduction band minimum (CBM) of CdS. Based on our previous work11, minor Zn diffusion is beneficial to the band alignment of CdS/CZTS, however, too much Zn (Zn/(Zn+Cd)>33%) at the p-n interface will form a “high-spike” CBO thus blocking carrier collection, deleterious to the device performance11. Along with these benefits brought by elemental inter-diffusion, the Supplementary Figure 16 demonstrates the photovoltaic performance for devices with different diffusion profiles.
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Supplementary Figure 15 XPS elemental profiles for CdS/CZTS stack under different heat treatment temperature. The red arrows highlight the diffusion of Cd into CZTS.
Supplementary Table 3 Diffusion length assume >1% atom percent threshold Distance (nm)
Without HT
210 °C
270 °C
330 °C
Cd into CZTS
73 nm
129 nm
182 nm
Over 500nm
Supplementary Table 4 Summary of PL yield, minority lifetime and Urbach energy of CZCTS with different Cd atomtic percent. Cd concentration in
Normalized PL
Minority
CZCTS
yield
lifetime
0% atom
1
2.7 ns
65 meV
2.5% atom
4.2
4.9 ns
48 meV
5% atom
4.4
5.2 ns
42 meV
17
Urbach Energy
Supplementary Figure 16 Statistical performance parameters of the CZTS devices without and with HT at 210 °C, 270 °C and 330 °C. The box plot denotes median (centre line), mean value (dots), 25th (bottom edge of the box), 75th (top edge of the box), 95th (upper whisker) and 5% (lower whisker) percentiles. The sample size in each column are around 10 devices. The Voc and efficiency show steady increase with increasing HT temperature (≤ 270 °C). The performance benefits from the elemental inter-diffusion (especially Cd), which effectively reduces the band tailing problem and hence non-radiative recombination. However, device performance drops at high HT temperature (330 °C). Despite the benefit from more Cd diffusion into CZTS, the CdS buffer layer may have been over-consumed, which may deteriorate the properties and function of CdS. Besides, too much Zn diffusion into CdS buffer might form ZnxCd1-xS with high Zn content (Zn/(Zn+Cd)> 40%), which could result in the unfavorable high-spike conduction band offset previously mentioned.
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Supplementary Table 5 PV performance parameters for CZTS at different cooling rate of heterojunction annealing process, showing that the annealing temperature has more impact on PV performance rather than the ramping and cooling.
Cooling rate
VOC(mV)
JSC(mA/cm2)
FF(%)
Efficiency (%)
40 °C/min
710
22.0
62.7
9.8
10 °C/min
720
20.7
66.5
9.9
Supplementary Figure 17.Carrier concentration depth profiles of CZTS devices without HT and with HT at 270 °C, 300 °C and 330 °C, respectively, obtained by capacitance-voltage measurements.
Supplementary Table 6 Carrier concentration of CZTS at different annealing conditions Condition
Carrier concentration (cm-3)
Without HT
2.32×1016
270 °C HT on bare CZTS
2.71×1016
270 °C HT on CZTS heterojunction
3.36×1016
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Supplementary Table 7 average composition of the clusters and composition of the bulk outside the clusters Composition cluster
Composition bulk outside the cluster
Ion type
Count (at %)
Ion type
Count (at %)
S
53.25±0.153
S
54.36±0.015
Cu
22.86±0.08
Cu
23.63±0.008
Sn
13.04±0.071
Sn
12.14±0.006
Zn
6.51±0.098
Zn
6.39±0.011
Cd
3.41±0.045
Cd
3.39±0.004
Na
0.70±0.016
Na
0.045±0.001
Supplementary Note 4 Band bending calculation: In order to gain more information on the band bending, we analysed both the core level of each element at the interface as well as bulk CdS and CZTS. By comparing the energy of the core-levels between the interface and their bulk, the band bending Vbb can be estimated12 according to the following equation13. (3) Where
and
are the core level energies of two selected elements in the bulk
materials of CZTS and CdS respectively. And
and
are the core level energies
of the same corresponding elements at the p-n interface. In this case, the elements in CZTS are selected using Cu, Zn, Sn whilst element in CdS is using Cd. Supplementary Figure 18 demonstrates the core-levels of Cu (2p3), Zn (2p3), Sn (3d5) and Cd (3d5) at different depth. The band bending value is an average of Cd/Cu, Cd/Sn and Cd/Sn values. By using the equation (3) the band bending value without HT and with HT are 0.09 eV and 0.19 eV
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respectively. The valence band offset (VBO) can be estimated by using the Vbb and VBM position of CZTS and CdS12. Without HT process, the VBO is calculated to be -1.03 eV while with HT process, the VBO is -0.94 eV.
Supplementary Figure 18 XPS Core-level depth peak position profile of different elements. The bulk CdS are is defined with etching time 65-95 s and the bulk CZTS is defined with etching time 1500~1600 s, which is 500~600 nm deep into the CZTS bulk assuming the Ar etching time for CdS and CZTS are similar. The interface area is defined to be 185-335 s for 21
sample without HT and 185-395 s for sample with HT, respectively. The core-levels of all the cation elements in CZTS shift upwards from p-n interface to CZTS bulk with HT process. This suggests after HT process, the band bends downwards more severely comparing to that without HT process.
Supplementary Figure 19 CZTS film morphologies under different ramping-up rate. (Left 180 °C/min, middle 60 °C/min and right 10 °C/min), showing the film can stay flat and compact only at the low ramping-up rate.
Supplementary Table 8 PV device performance parameters (without antireflection coating) for CZTS absorber sulfurized at different holding durations at 560 °C. 3 min holding time yielded the highest efficiency CZTS cells. The PV performance for longer holding time highly depends on the sulfur pressure remaining in the sulfurisation chamber. For our case, there is insufficient sulfur pressure within the chamber after long holding duration (>10 min). However, if the sulfurization pressure can be well controlled, this optimized sulfurization condition might change.
Holding time
VOC(mV)
JSC(mA/cm2)
FF(%)
Efficiency (%)
1 min
673
16.6
55.8
6.2
3 min
668
18.3
61.8
7.6
10 min
547
17.0
50.0
4.6
22
Supplementary Table 9 PV device performance parameters for CZTS absorber at different cooling rates of sulfurization process. The slow cooling yields higher Voc but lower Jsc, whilst faster cooling gives higher Jsc but lower Voc. Overall, the cooling rate for the sulfurization process does not have a significant impact on the efficiency. Cooling rate
VOC(mV)
JSC(mA/cm2)
FF(%)
Efficiency (%)
30 °C/min
648
18.5
60.0
7.2
10 °C/min
601
19.8
58.5
7.0
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2
3
4
5
6 7 8 9
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