The Effect of Pressing Pressure on the Performance of Perovskite

Ho-Baillie*. The Australian Centre for Advanced Photovoltaics (ACAP), .... For the purpose of the pressing test, a film of polyolefin (PO) is sandwich...
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Letter

The Effect of Pressing Pressure on the Performance of Perovskite Solar Cells Lei Shi, Meng Zhang, Yongyoon Cho, Trevor L Young, Dian Wang, Haimang Yi, Jincheol Kim, Shujuan Huang, and Anita W. Y. Ho-Baillie ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01608 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Effect of Pressing Pressure on the Performance of Perovskite Solar Cells Lei Shi, Meng Zhang, Yongyoon Cho, Trevor L. Young, Dian Wang, Haimang Yi, Jincheol Kim, Shujuan Huang, and Anita W. Y. Ho-Baillie* The Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia Keywords: perovskite solar cell, packaging, encapsulation, pressure, pressing, stability, degradation, laminating Abstract Metal halide perovskite solar cells (PSC’s) have undergone remarkably rapid progress with their power conversion efficiency (PCE) increasing from 3.8 % to 23.7 % in merely 9 years. Although enormous research effort has been devoted to performance improvement, there has been little investigation of the effect of post-fabrication processes such as encapsulation or packaging on device PCE. In this work, the effect of pressing on the performance of mesoporous PSC’s is studied. Cells with the stateof-the-art architecture glass/FTO/c-TiO2/mp-TiO2/perovskite/spiro-OMeTAD/gold are used in this study. It is found that pressing under the condition typically used for encapsulation is beneficial for these PSC’s, improving their PCE consistently by more than 7 % relative on average. The effect of pressing was characterised by light current density-voltage measurement (LJV), cross-sectional scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrical impedance spectroscopy (EIS). The PCE enhancement is due to improved interface resulting in higher fill factor, lower

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recombination and lower hysteresis. It is found that pressing pressure of 400 to 500 mbar is appropriate, higher pressure at 1000 mbar is detrimental. Moreover, pressure must be maintained to maintain the PCE improvement. Pressing is an essential part of an encapsulation process and this work demonstrates the beneficial effect of pressing on PSC’s, inspiring future development of encapsulation and packaging.

Introduction Metal halide perovskite solar cells (PSC’s) have undergone remarkably rapid progress in power conversion efficiency (PCE) increasing from 3.8 % to 23.7 % in merely 9 years1–4. Enormous research effort has been devoted to performance improvement, focussing on optimising perovskite composition, varying material preparation and cell fabrication processes and new device architectures5–7. In contrast, there has been little investigation of the effect of post-fabrication processes such as tabbing, encapsulation or packaging processes on device PCE. In this work, we focus on the effect of applying pressure to PSC’s, which is inherent in standard encapsulation process. For Si solar modules, an encapsulation process typically involves i) sandwiching the Si cells between layers of encapsulants, a front glass window and a back sheet; ii) heating the “stack” under vacuum at elevated temperature to remove the air and soften the encapsulants and iii) pressing the stack in the vacuum laminator for a few minutes to 2 ACS Paragon Plus Environment

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promote intimate contact, good adhesion and crosslink the polymer. For thin-film solar modules, the process is similar except that in addition, the edges of the modules are usually protected by sealant8,9. Therefore, it is essential to understand the effect of the laminating process on perovskite device performance and stability. In this work, perovskite solar cells that have the state-of-the-art mesoporous structure and mixed ion composition are investigated to examine the effect of pressing at room temperature. Both pressing-induced PCE improvement and pressing-induced degradation were observed. In addition, it is found that pressure must be maintained to sustain the PCE improvement.

Experiment Cell Fabrication The perovskite solar cells used in this study had a structure of glass/FTO/c-TiO2/mpTiO2/perovskite/spiro-OMeTAD/gold. The fabrication procedure is detailed in the Supporting Information (SI). Pressing tests For the purpose of the pressing test, a film of polyolefin (PO) is sandwiched between the completed PSC and a 3 mm thick cover glass (see Figure 1). The PSC/EVA/cover glass stack was loaded into a vacuum laminator (Spire 240, see Figure S1) at room temperature and pressed. Because the test is conducted at room temperature, the PO does not soften or adhere to any surfaces during pressing. The PO and cover glass were easily removed from the PSC immediately after the pressing test. The pressing process

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consists of two steps that are similar to the lamination process performed on commercial modules by a laminator except that the pressing tests were conducted at room temperature. In step 1, the stack is evacuated for 5 min to a background pressure below 50 Pa. Subsequently, in step 2, the stack is pressed by the laminator while the vacuum is maintained.

Figure 1 Cross-sectional schematics of the PSC/PO/glass stack for the pressing experiment (not to scale).

The conditions used for pressing tests performed on various PSC’s with the laminator (pressing tests #LL, #LH) are listed in Table 1 where LL stands for laminator pressing at low pressing pressure (400 mbar) and LH standards for laminator pressing at high pressing pressure (1000 mbar). In addition, a pressing test was conducted using spring-loaded clamps. The purpose of this test is to show the effect of maintaining the pressing pressure on device performance. In this test, no evacuation process is involved; the pressure on the PSC

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glass stack is maintained by spring force. The conditions for pressing test #CL and #CH, are also included in Table 1 where CL stands for clamp pressing at low pressing pressure (400 mbar - 500 mbar) and CH standards for clamp pressing at high pressing pressure (1000 mbar). The pressure of 400 mbar was selected due to its effectiveness for encapsulating perovskite solar cells previously repoted10 The pressure of 1000 mbar was also selected as it is typically used for encapsulating commercial silicon solar modules.

Table 1 The conditions for pressing tests at room temperature. By Laminator

Pressing Pressure

Pressing Duration

Background Pressure

Test #LL

400 mbar

5 min

50 Pa

Test #LH

1000 mbar

5 min

50 Pa

Test #CL

400-500 mbar

continuous

1 atm.

Test #CH

1000-1100 mbar

continuous

1 atm.

By Spring Clamps

Pressing tests were repeated on some cells a few days after the initial tests to investigate the longevity, reversibility and repeatability of their effects on device performance. Cell Characterisation Light J-V (LJV), electrical impedance spectroscopy (EIS), cross-sectional scanning electron microscopy (SEM), X-ray Diffraction (XRD) and photoluminescence (PL) were used to characterize the PSC’s before and after the pressing tests. See SI for details. 5 ACS Paragon Plus Environment

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Results and Discussion Pressure-induced improvement Figure 2 (a) shows the PCE’s of cells measured before, a few minutes after, 1 day after and 3 days after the first application of pressing test #LL (0.4 bar for 5 min). After that, the PSC’s underwent another pressing test #LL and were measured a few minutes, 3 days, 6 days and 10 days after that test. The corresponding short circuit current densities (JSC), open circuit voltages (VOC), fill factors (FF), and hysteresis indices (HI) are reported in Figure 2 (b), (c), (d) and (e). HI is defined by Equation S1. A cell without hysteresis has HI = 0.00. As shown in Figure 2 (a), the PCE of these PSC’s increased relatively by 7 % on average after the first pressing test. All of the cells’ electrical characteristics improved to some extent with FF improving the most. The increase in FF originates from a substantial decrease in RS (Figure S2a). Hysteresis indices for the cells also decreased after the pressing due to a larger improvement in the forward scanned PCE than the reverse scanned PCE (Figure S3). Interestingly, after storing the PSC’s in N2 for 3 days, their PCE’s, JSC, VOC and Hysteresis Indices returned to close to their initial values. A second pressing Test #LL produced another round of improvement (even better than before ~ 10% relative on average compared to initial). When the pressure was removed, once again, the electrical parameters, in particular, FF and RS, returned to the initial values during storage for a period of time (~ 10 days).

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20

PCE (%)

19

20

25%~75% Mean ± 1.5 SD Median Line Mean

19

18

18

17

17

16

16

15

15

14

Initial

1st Test #LL

1st day

3rd day

2nd Test #LL

3rd day

6th day

10th day

14

JSC (mA/cm2)

(a)

22

22

21

21

20

Initial

1st Test #LL

1st day

3rd day

2nd Test #LL

3rd day

6th day

10th day

20

(b)

VOC (mV)

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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1150

1150

1100

1100

1050

1050

1000

1000

950

950 Initial

1st Test #LL

1st day

3rd day

2nd Test #LL

3rd day

6th day

10th day

(c)

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78

78

76

76

74

74

72

72

70

70

68

68

66

Initial

1st Test #LL

1st day

3rd day

2nd Test #LL

3rd day

6th day

10th day

66

(d)

0.2

0.2

0.1

0.1

0.0

0.0

HI (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

FF (%)

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-0.1

Initial

1st Test #LL

1st day

3rd day

2nd Test #LL

3rd day

6th day

10th day

-0.1

(e) Figure 2 (a) PCE, (b) VOC, (c) JSC, (d) FF, and (e) Hysteresis Index (HI) for before (“Initial”); immediately after (“1st Test #LL”), 1 day after (“1st day”) and 3 days after (“3rd day”) the 1st pressing test #LL (0.4 bar for 5 min). The same pressing test was applied again, and the PSC’s were measured immediately after (“2nd Test #LL”), 3 days after (“3rd day”) and 6 days after (“6th day”) and10 days (“10th day”) after the 2nd #LL test. It seems that the inability to maintain the performance improvement was due to the absence of constant applied pressure. To prove this, another group of samples underwent a continuous pressing Test #CL (details in Table 1). PCE’s were measured after 1 day of continuously applied pressure (Test #CL 1st day). The cells went through

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another 6 days of continuously pressing and were measured immediately after (Test #CL 7th day). A final round of continuously pressing for 8 days was then performed. The cells were measured immediately after (Test #CL 15th day) and the pressing was removed. Cells were measured again on the 5th and 7th day of the “relaxation” period. Results are shown in Figures 3 and S4. Similar to results before, all electrical parameters improved after pressing while FF contributed most towards PCE gain due to reduced RS (Figure S4). Their hysteresis indices improved (i.e., decreased) after the pressing due to a larger improvement in the forward scanned PCE than the reverse scanned PCE (Figure S5a). PCE continued to improve as pressing continued but eventually levelled off. Moreover, the longer pressing helped to narrow the PCE gaps between the samples. In other words, the initially poorer cells improved and therefore improve the overall yield for the batch of PSC’s. Again, after the removal of pressing pressure, PCE and the electrical parameters dropped in a more gradual manner (especially for FF) compared to the results of test #LL.

PCE (%)

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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19

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17

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15

15 Initial

Test #CL 1st day

Test #CL 7th day

Test #CL 15th day

5th day

7th day

(a)

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22

22

21

21

20

20 Initial

Test #CL 1st day

Test #CL 7th day

Test #CL 15th day

5th day

7th day

VOC (mV)

(b) 1150

1150

1140

1140

1130

1130

1120

1120

1110

1110

1100

1100

1090

1090

1080

Initial

Test #CL 1st day

Test #CL 7th day

Test #CL 15th day

5th day

7th day

1080

(c)

FF (%)

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

JSC (mA/cm2)

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78

78

76

76

74

74

72

72

70

70 Initial

Test #CL 1st day

Test #CL 7th day

Test #CL 15th day

5th day

7th day

(d)

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HI (a.u.)

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0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0

-0.1

-0.1

-0.2

Initial

Test #CL 1st day

Test #CL 7th day

Test #CL 15th day

5th day

7th day

-0.2

(e) Figure 3 (a) PCE, b) VOC, (c) JSC, (d) FF, and (d) Hysteresis Index (HI) for before (“Initial”); immediately after 1 day of Test #CL (Test #CL 1st day), after another 6 days of continuous (Test #CL 7th day) and after another 8 days (Test #CL 15th day). After that, pressing was removed and the cells were measured again after on the 5th day and 7th days of “relaxation”.

The effect of pressure force was further investigated by laminator pressing (Test #LH) and clamp pressing (Test #CH) at higher pressure (Table 1). The cells were measured before the tests, immediately after the laminator press, and on the 1st and 2nd day of continuous pressing. Results are shown in Figures 4 and S6. The majority of PCE improvement (7 % relative) was achieved after the first pressing test (see results of Test #LH). Continuous pressing at higher pressure is shown to be detrimental (see results of Test #CH 1st day and Test #CH 2nd day). Rather than returning to initial values, the cell performance dropped after the pressing was removed. The main driver for PCE deterioration is the drop in JSC - by 1 mA/cm2 after pressing followed by another drop of 0.5 mA/cm2.

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19

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Initial

Test #LH

Test #CH 1st day

Test #CH 2nd day

3rd day

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JSC (mA/cm2)

(a)

23

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Test #LH

Test #CH 1st day

Test #CH 2nd day

3rd day

(b)

VOC (mV)

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

PCE (%)

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1150

1100

1100

1050

1050

1000

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950

Initial

Test #LH

Test #CH 1st day

Test #CH 2nd day

3rd day

950

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78

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Initial

Test #LH

Test #CH 1st day

Test #CH 2nd day

3rd day

60

(d)

HI (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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FF (%)

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0.15

0.15

0.10

0.10

0.05

0.05

0.00

0.00

-0.05

-0.05 Initial

Test #LH

Test #CH 1st day

Test #CH 2nd day

3rd day

(e) Figure 4 (a) PCE, b) VOC, (c) JSC, (d) FF, and (d) Hysteresis Index (HI) for before (“Initial”); immediately after Test #LH (1 bar for 5 min), on the 1st (Test #CH 1st day) and 2nd (Test #CH 2nd day) day of clamp test (1bar). After that, pressing was removed and the cells were measured 1 day after.

Mechanism of pressure-induced PCE improvement

To investigate the mechanism of pressure-induced effect, XRD measurements of a PSC were conducted before and after 1 day of pressing test #CH. Figure 5 shows that the XRD patterns are identical, indicating that no damage was introduced to the perovskite 13 ACS Paragon Plus Environment

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or FTO layers immediately after pressing. Therefore, any pressure-induced effect is believed to originate from changes in TiO2/perovskite and perovskite/spiro-OMeTAD interfaces after pressing.

Electrical impedance spectroscopy (EIS) was used to provide insights into carrier recombination mechanisms11,12. Figure 6 (a) shows the Nyquist plot for a PSC before and after test #LH. A bias of 0.9 V is chosen since it is typically the voltage near the maximum power point (VMPP) for an operating PSC. The experimental data were fitted with an equivalent circuit model which consists of a series resistance, RS in conjunction with a parallel R–C component and a parallel R-CPE (constant phase element) component (see Figure S7)13,14. The fitted values are given in Table S1. As shown in Figure 6 (b), pressing resulted in an increase in the recombination resistance. During the dark impedance measurement, carriers are injected only from an external voltage source. The increase in recombination resistance especially at low injection levels can be attributed to reduced interfacial carrier trapping. As a result, the improved ETL/PSC and PSC/HTL interfaces15,16 contribute to reduced Hysteresis Index in cells immediately after the pressing tests, as shown in Figure 2 to Figure 4. Test structures: (1) FTO/c-TiO2/mp-TiO2/perovskite and (2) FTO/perovskite/spiroOMeTAD were also prepared and were subject to pressing Test #LL. Time resolved photoluminescence (PL) measurements were performed before and after the test (Figure S8). The PL decay traces were fitted using a bi-exponential function to deduce the two decay components. While the radiative lifetimes drop for structure (1) after pressing, lifetimes for structure (2) improve suggesting an improvement of the 14 ACS Paragon Plus Environment

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perovskite/spiro-OMeTAD interface. As shown in results in Figure 3, the improved interface can be maintained by sustaining pressing pressure at appropriate level, such as 400-500 mbar.

Figure 5 XRD patterns of a PSC before and after pressing Test #CH for 1 day. The top Au electrode of the PSC was peeled off by tape before XRD measurement.

(a)

(b)

Figure 6 Electrical impedance spectroscopy of a PSC measured before and after pressing test #LH (1 bar, 5 min): (a) Nyquist plot at 0.9 V bias in the dark; (b) recombination resistance as a function of applied voltage extracted from the Nyquist curves

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Further work will be carried out using cells with different structures to determine how pressing affects their device performance and to help elucidate the mechanisms of pressure induced enhancement and pressure induced degradation. For example, planar cells, “inverted” cells, cells with polymer and/or fullerene based transport layers and cells fabricated by different methods such as the blow-drying method of Zheng et al.17 that produces highly compact hole transport layers, should be investigated. In addition, the origin of the detrimental effect of high pressure pressing should be elucidated in future work building on published work such as work that has reported strained related degradation18

Conclusion

In this work, the effect of pressing, which is an inherent part of a common encapsulation process, on perovskite solar cells was investigated. The effect on PCE has been shown to be positive when pressing pressure equals 400 to 500 mbar. The main contributor to PCE improvement is improved fill factor and mainly from reduced series resistance. The lack of changes in the XRD patterns suggests that the effect of pressing on the perovskite layer is minimal. Therefore, improvement in performance by pressing was mainly due to improved interfaces as shown by increased recommendation resistance which also helps explain the reduced hysteresis. The improvement is likely to come from improved perovskite/spiro-OMeTAD interface from time resolved PL results. The applied pressure needs to be maintained to sustain the improvement. Higher pressure 16 ACS Paragon Plus Environment

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(e.g., 1000 mbar), however, was shown to be detrimental. These findings have important implications for the development of low-cost encapsulation processes for perovskite solar cells and for optimising the lamination processes.

ASSOCIATED CONTENT Supporting Information Available: ** Details of material preparation; PSC fabrication; characterisation; supporting figures: series resistance, reverse and forward JV curves; steady state curves, circuit model diagram, time resolve PL results **

AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected]; Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian

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Renewable Energy Agency (ARENA). M. Zhang acknowledges financial support from Australian Research Council through DP160102955 program.

ACKNOWLEDGMENT Authors thank the Australian Microscopy & Microanalysis Research Facility (AMMRF) for SEM imaging and Mr. Bernhard Vogl for assistance measuring the clamping pressure.

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and

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J, Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells, Science Advances

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eaao5616. DOI: 10.1126/sciadv.aao5616

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