Light and Electrically Induced Phase Segregation and Its Impact on

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Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells The Duong, Hemant Kumar Mulmudi, Yiliang Wu, Xiao Fu, Heping Shen, Jun Peng, Nandi Wu, Hieu T. Nguyen, Daniel MacDonald, Mark N. Lockrey, Thomas P. White, Klaus Weber, and Kylie R. Catchpole ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06816 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Applied Materials & Interfaces

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells The Duonga,*, Hemant Kumar Mulmudia, YiLiang Wua, Xiao Fua, Heping Shena, Jun Penga, Nandi Wua, Hieu T. Nguyena, Daniel Macdonalda, Mark Lockreyb, Thomas P. Whitea, Klaus Webera, Kylie Catchpolea,** a

Centre for Sustainable Energy Systems, Research School of Engineering, Australian National University, Canberra 2601, Australia

b

Australian National Fabrication Facility, Research School of Physics and Engineering, Australian National University, Canberra 2601, Australia

*Email: [email protected] **Email: [email protected] Keywords: perovskite, high bandgap, tandem, solar cell, phase segregation, stability

Perovskite material with a bandgap of 1.7 eV – 1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase segregation phenomena is often observed in perovskite films of this kind, with implications for solar cell efficiency. Here we investigate light-induced phase segregation inside quadruple cation 1 ACS Paragon Plus Environment


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perovskite material in a complete cell structure, and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 hours operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 hours operation at the open circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photo-excitation is not required to induce phase segregation.

1. INTRODUCTION Bandgap tuning is an attractive feature in perovskite material, and provides the potential for integrating perovskite solar cells into tandem structures to achieve efficiencies up to 30% 1-2. This has been demonstrated firstly by alloying iodide and bromide anions in the form of methylammonium lead iodide/bromide MAPb(I1-xBrx)3 to tune the bandgap from 1.55 eV to 2.3 eV. However the disorder of MAPb(I1-xBrx)3, as revealed by the value of the Urbach energy and the full-width-at-half-maximum (FWHM) of the photoluminescence (PL) peak, reaches a maximum when x is between 0.2 and 0.5. This results in a large difference between the obtained open circuit voltage of the cell and the tuned bandgap of the material. Therefore the performance of MA-based perovskite solar cells is limited, which makes them less useful in tandem applications

3-5

. Additionally, light-induced segregation phenomena in mixed-

halide perovskites were reported, in which photo-excitation induces halide migration and results in lower bandgap iodide-rich domains which acts as traps and limit the performance of the solar cells 6. A recent study showed that the MAPb(I1-xBrx)3 mixture has a miscibility gap 2 ACS Paragon Plus Environment

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above room temperature when 0.3 1.1 V to less than 1.0 V while the cell performance determined by periodic J-V scans after every hour deteriorates significantly to less than 82% of the original performance after 12 hours (Figure 5a,b). The reduction in the cell performance is mainly due to the drop in Voc and FF as shown in Figure S4. The drop in the VOC might be due to the red shift of the perovskite bandgap under the effect of the light-induced phase segregation. Significantly increased recombination inside the perovskite active layer, possibly caused by iodide trap states, might also contribute to the drop in the VOC. This increased recombination is likely to be the reason for the significant reduction in the FF of the cell. Along with the drop in performance, the hysteresis in the cell increases notably (Figure 5c,d). On the other hand, the 13 ACS Paragon Plus Environment


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short circuit current of the cell increases from 19.8 mA/cm2 to 21.1 mA/cm2, which can be explained by the increase in absorption of the cell due to the red shift in the bandgap (Figure S5). The cell was kept in the dark for 12 hours and the performance was recorded again; the VOC recovers slightly however the value is significantly lower than the original value while the performance does not recover. This is consistent with the incomplete recovery as observed in the CL measurement above. Additionally, factors such as UV light, migration of additives in the hole transport layer to the perovskite and others can contribute to the degradation of the cell 26-27; those factors are currently being investigated.

Figure 5. (a) Monitoring of open circuit voltage over time. (b) Normalized performance over time determined by periodically J-V scanning after every 1.5 hours. (c) J-V curves of the cell before and after 12 hours under light in the first cycle. (d) J-V curves of cell before and after 12 hours under light in the second cycle.

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Solar cells normally operate near their maximum power point rather than under open circuit conditions. We therefore repeated the above experiments, but this time with the cell held at maximum power point. The spatial distribution of the PL peak position after 12 hours operation suggests that only localized phase segregation has occurred inside the active area while phase segregation is still consistently observed outside of the active material (Figure 6a). Inside the active area, a few locations do show signs of phase segregation, which we believe comes from defects in the perovskite film (eg. spinning defects, dust, pinholes). In term of performance, the efficiency slightly drops to 94% of the original value while the VOC monitored after every 1.5 hours shows negligible reduction (Figure 6b,c). After the cell rests in the dark for 12 hours, the performance and Voc recovers although there is a slight overall reduction. We note that this drop is less than the drop observed by Domanski et al. with FA0.83MA0.17Pb(I0.83Br0.17)3 perovskite composition (bandgap ~ 1.6 eV), where cation migration has been suggested as the mechanism for the performance drop

28

. Our results

demonstrate the link between phase segregation and the performance stability in the high bandgap perovskite, under the interplay between light and electrical bias.



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Figure 6. (a) Micro-PL scan image shows PL peak position distribution on a cell after 12 hours operating at the maximum power point. (b) Monitoring of open circuit voltage over time. (c) Normalized performance over time where the dips in the curves are caused by the response of the cell after switching from VOC to VMPP. We also observe negligible phase segregation inside the active area and pronounced phase segregation outside of the active area when the cell operates under short circuit condition for 12 hours under 1 Sun (Figure S6). The short circuit current remains stable during 12 hours of measurement. The dependence of light-induced phase segregation inside the active layer on the operating conditions of the cells can be explained by the theory of electron-phonon coupling. This theory has been proposed to explain the phase segregation phenomena on MA-based perovskite films 9. Firstly, the collection of photo-generated carriers increases the number of 16 ACS Paragon Plus Environment

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electrons on the TiO2/perovskite side and the number of holes on the perovskite/SpiroOMeTAD side. Under the open circuit condition, this accumulation of carriers lead to more electron-phonon coupling, which promotes phase segregation. In contrast, under maximum power point and short circuit condition, carriers exit the device as photo-generated current and there is no build-up of carriers and therefore phase segregation is not promoted. 3.3. Effect of Electrical Bias in the Dark on Phase Segregation To isolate the effect of light and electrical bias on the phase segregation observed in the perovskite absorber in the cell structure, we perform the micro-PL scanning measurement after the cells are biased at different voltages under dark conditions for 12 hours. Electrical bias has been reported to cause degradation in the performance of perovskite solar cells due to the accumulation of charged defects at the contact areas

29

. Under electrical bias, the

migration of MA+ cations was observed in perovskite lateral devices and proposed as the mechanism of the switchable photovoltage effect

30

. However, the question of whether

electrical bias can produce such large spectroscopic changes as observed under illumination in high bandgap perovskites remains open. This is especially important because red LEDs using MAPbBr2I have been demonstrated but the stability of the devices has not been reported 31. We show that, similar to the case when the cell operates under light at open circuit voltage, phase segregation happens inside the active area when the cell is forward biased at 1.2 V (higher than VOC of the cell) in the dark for 12 hours. This is illustrated by the shift in the PL peak to ~ 780 nm in Figure 7a. The region outside of the active area is unaffected by the electrical bias on the cell. When the cell is reverse biased at bias at -1 V (Figure 7b) or short circuited, we do not observe a significant shift in the emission peak of perovskite material either inside or outside of the active area.



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Perovskite solar cells have been shown to operate as a p-i-n device with high electronic quality –i- layer

32

. The carrier concentration within the perovskite layer depends

exponentially on the applied voltage as per the following formula:

~ ∗

Where n0 is the intrinsic carrier concentration, q is electric charge, k is the Boltzmann constant, T is temperature and V is the bias voltage. We therefore propose that the observed phase segregation under electrical bias occurs by the electron-phonon coupling theory suggested previously 9. Under forward bias higher than the built-in potential, carriers within the perovskite layer are abundant and these charges deform the surrounding lattice and generate sufficient lattice strain to promote phase segregation. Our results demonstrate that photo-excitation is not needed in this process.

Figure 7. Micro-PL scan images showing the peak position of the perovskite material after the cell was biased at 1.2 V (a) and -1 V (b) in the dark.

4. CONCLUSION In conclusion, we show light-induced phase segregation in quadruple cation 1.73 eV bandgap perovskite films after 12 hours under one Sun and incomplete recovery after 12 18 ACS Paragon Plus Environment

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hours in dark. Importantly, the phase segregation in the active layer of the cell structure depends on the operating conditions of the cell. In particular, the phenomena is negligible under maximum power point conditions and it is severe under open circuit conditions. As a result, perovskite cells with 1.73 eV bandgap retain 94% of the original efficiency after 12 hours operating at the maximum power point conditions, while the cells retain only 82% after 12 hours operating at the open circuit conditions. Additionally we show that phase segregation can be induced by electrical excitation under dark. The study gives more insight into light-induced phase segregation in the optimized 1.7 – 1.8 eV bandgap perovskite and demonstrates the potential of utilizing this material in a tandem structure.

Supporting Information Figure S1. XRD of MAPbI2Br film before and after being exposed for 12 hours under light. Figure S2. Micro-PL scan of fresh and exposed films. Figure S3. Micro-PL scan of fresh film showing spatial distribution of the intensity of PL peaks. Figure S4. Parameters of cell monitored at open circuit condition for 12 hours. Figure S5. Reflectance and absorption of fresh cell and exposed cell. Figure S6. Micro-PL scan of cell operating at short circuit condition for 12 hours.

ACKNOWLEDGEMENTS 19 ACS Paragon Plus Environment


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This work has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. Part of the experiment was performed at Australian National Fabrication Facility (ANFF) ACT Node and Centre for Advanced Microscopy (CAM) at the Australian National University. KRC acknowledges the support of a Future Fellowship from the Australian Research Council.

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Light and Electrically Induced Phase Segregation and Its Impact on

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