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Passivation of Grain Boundaries by Phenethylammonium in Formamidinium-Methylammonium Lead Halide Perovskite Solar Cells Da Seul Lee, Jae Sung Yun, Jincheol Kim, Arman Mahboubi Soufiani, Sheng Chen, Yongyoon Cho, Xiaofan Deng, Jan Seidel, Sean Lim, Shujuan Huang, and Anita W. Y. Ho-Baillie ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00121 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Energy Letters

Passivation of Grain Boundaries by Phenethylammonium in Formamidinium-Methylammonium Lead Halide Perovskite Solar Cells Da Seul Lee1, Jae Sung Yun1, Jincheol Kim1, Arman Mahboubi Soufiani1, Sheng Chen1, Yongyoon Cho1, Xiaofan Deng1, Jan Seidel2, Sean Lim3, Shujuan Huang1, and Anita W. Y. Ho-Baillie1*

1

Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and

Renewable and Engineering, University of New South Wales, Sydney 2052, Australia 2

School of Materials Science and Engineering, University of New South Wales, Sydney 2052,

Australia 3

Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia

*Corresponding author. E-mail: [email protected]

1 ACS Paragon Plus Environment

ACS Energy Letters 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

Abstract: In this work, we report the benefits of incorporating phenethylammonium cation (PEA+) into (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15 perovskite for the first time. After adding small amounts of PEA cation (< 10%), perovskite film morphology is changed but most importantly grain boundaries are passivated. This is supported by Kelvin Probe Force Microscopy (KPFM). The passivation results in the increase in photoluminescence intensity and carrier lifetimes of test structures and open circuit voltages (VOC) of the devices as long as the addition of PEA+ is ≤4.5%. The presence of higher bandgap quasi-2D PEA incorporated perovskite is responsible for the grain boundary passivation and the quasi-2D perovskites are also found to be concentrated near the TiO2 layer revealed by PL spectroscopy. Results of moisture exposure tests show that PEA+ incorporation is effective in slowing down the degradation of un-encapsulated devices compared to the control devices without PEA+. These findings provide insights into the operation of perovskite solar cells when large cations are incorporated.


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ACS Energy Letters

Graphic Abstract:



Organic-inorganic lead halide perovskites with three dimensional (3D) structures have been considered as one of the most promising materials for thin film solar cell.1 Their outstanding photo-physical properties and versatility in terms of solution processes2 have led to outstanding improvements in power conversion efficiencies (PCE) of 22.1% in a short period of time based on mixed cations: CH3NH3(MA) & HC(NH2)2(FA) cations and mixed halides: Br/I Pb-based perovskites.3 However, some of the main challenges for commercialization include thermal-, oxygen-, illumination- and moisture- induced instability under operational conditions. In particular, the material can react with water molecules and decompose into PbI2 while MAI or FAI evaporates.4 It has been reported that moisture penetrates through the surface of the film, migrates along the grain boundaries, and subsequently degrades the entire film.5 There have been various attempts to prevent moisture-induced degradation employing functional moisture barriers.6 Hu. et al.7 and Ma. et al.8 deposited hydrophobic materials such as phenylethylammonium, n-butylammonium and cyclopropylammonium (at times in conjunction with a MAI layer) on the surface of the MAPbI3 perovskite layer to provide protection. Besides using as a moisture barrier layer, the structural combination of two dimensional (2D) and 3D perovskites have been shown to be a promising photovoltaic material possibly locking in mobile ionic elements of the otherwise softer structure of threedimensional perovskite.9-11 However, inserting the hydrophobic components often induces an excessively large tolerance space in a 3D perovskite framework, which is detrimental to solar cell performance.12-13 Moreover, incorporation of large amount of larger cations widens the band-gap, lowers the charge-carriers mobility and increases the exciton binding energies. Therefore, the optimum amount of the larger cations in 3D perovskite films is often limited and needs to be carefully tuned.

12, 14-18

Due to such complexity, incorporation of the larger

cations has been often limited to single-cation perovskite systems such as MAPbI3 and FAPbI3.12,

16-17, 19

However, state of the art high performance PSCs employ the more

complicated perovskite systems such as triple cation with Cs or Rb with better performance 4 ACS Paragon Plus Environment

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ACS Energy Letters

and phase stability.2-3 Until recently, Wang et al reported the incorporation of nbutylammonium into FA0.83Cs0.17Pb(IyBr1-y)3 perovskite and observed the formation of 2D perovskite platelets at the grain boundaries. 20 Herein, we demonstrate incorporation of larger PEA cation into a mixed-cation mixed-halide perovskite, (FAPbI3)0.85(MAPbBr3)0.15 to improve moisture stability. It was found perovskite film morphology is changed but most importantly, grain boundaries are passivated supported by results of KPFM and PL measurements. This results in increased VOC when the incorporating of PEA+ is ≤4.5%. The higher bandgap quasi-2D PEA incorporated perovskite is responsible for the grain boundary passivation. Comparison of PL intensities measured from the glass and from perovskite side of the test structures shows that the quasi-2D material resides near the TiO2 interface. It is apparent that the passivation by or existence of the quasi2D material at the grain boundaries and at the interface slows down the degradation of unencapsulated devices compared to the control devices without PEA+.

Our

perovskite

films

(FAPbI3)0.85(MAPbBr3)0.15.

are

based

We

on

the

tune

the

mixed-cation PEAPbI3

lead

mixed

halide

mixing

ratio

with

(FAPbI3)0.85(MAPbBr3)0.15 at 0, 1.5, 3.0, 4.5, and 10 mol %. Although PEA has been known to form layered perovskite phases when mixed with MA in (PEA)2(MA)n-1PbI3n+1,16 it is found to be not as straight forward when PEA is mixed with MA and FA due to the nonuniform distribution of PEA within the film as discussed later. This will likely result in the formation of multiple phases (a mixture of different ‘n’ values)20. Therefore we have not used the “n” notation system to designate the PEAI incorporated perovskite films and devices at different concentrations. For the characterization of (FAPbI3)0.85(MAPbBr3)0.15 perovskite before and after the incorporation of PEAI, test structures glass/FTO/c-TiO2/mpTiO2/perovskite are fabricated according to details given in the Experimental Section by adding different amounts of PEA+: 0 %, 1.5 %, 3.0 %, 4.5 % and 10 mol %. 5 ACS Paragon Plus Environment


To investigate the crystalline properties, X-ray diffraction (XRD) patterns were measured on the test structures. Results are displayed in Figure 1a. For perovskite films with different concentrations of PEAI, the characteristic perovskite peaks of (110), (200), (202), (220) and (222) are indexed to 14.0 °, 19.9 °, 24.44 °, 28.3 ° and ≈31.82 ° respectively. The PbI2 crystal peak of (001) was indexed at 12.6 °.21 As the concentration of PEAI increases, the intensity of the (110) perovskite peak increases and its full width at half maximum (FWHM) decreases (Figure 1b), indicating improvement in crystallinity20 and/or more preferred (110) orientation with PEAI. The intensity ratio of (110)/PbI2 is shown Figure 1c which increases with PEAI . Similar trend is observed for the (110)/(200) peak ratio indicating the incorporation of PEAI promotes preferred orientation at (110).


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Figure 1. (a) X-ray diffraction patterns of (FAPbI3)0.85(MAPbBr3)0.15 perovskite with 0%, 1.5%, 3.0%, 4.5% and 10 mol % of PEAI incorporation. # refers to FTO peaks. (b) FWHM of (110) peak of the corresponding XRD patterns. (c) Peak ratio of 14° ((110)) peak to 12.6° (PbI2) peak from the corresponding XRD patterns. To confirm the incorporation of PEA+, Fourier transform infrared spectroscopy (FTIR) was carried out on the control samples (FAPbI3)0.85(MAPbBr3)0.15 without PEA+ and with the addition of 10% PEA+. Results are shown in Figure S1. In the FTIR spectra, the feature 7 ACS Paragon Plus Environment


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peaks of the alkenyl C=C stretching vibrations from the aromatic ring of PEA+ appears in the range of 1600 to 1400 cm-1 in the 10% PEA+ sample .12 We investigate the optical properties of perovskite material without and with the incorporation of PEAI through UV-vis absorption, which was measured on test structures and results are displayed in Figure 2a. It can be seen that both absorption and the optical bandgap (1.46 eV) 21 are quite similar after PEAI incorporation, except the absorption of PEAI 10% is a bit lower. Steady-state photoluminescence22 measurements were also performed on the same test structures from both the perovskite side (Figure 2b) and the glass side (Figure 2c). Figure S2 shows the same information grouped according to the amount of PEAI incorporation. In addition, the steady-state PL with 2D perovskite (PEA2PbI4) is shown in Figure S2f. In Figure 2b, there is the negligible PL peak shift when measured from the perovskite side after PEAI incorporation.23 The increase in PL intensity however is most prominent

indicating

better

quality

material

with

PEAI.

For

the

control

(FAPbI3)0.85(MAPbBr3)0.15 sample (Figure S2a) and the 2D perovskite sample (Figure S2f), no difference is observed in the PL spectra whether it is illuminated from glass or perovskite side. Interestingly, with PEAI incorporation, when illuminated from the glass side, short wavelength tail began to emerge and became more prominent with increase of PEAI concentration (see Figure 2c and Figure S2). The differences observed in Figure S2b to S2e indicate the presence of a different material that has different emissive states which is likely to be caused by the existence of the quasi-2D PEA incorporated perovskites at or near the perovskite/mp-TiO2 interface. Grancini et al.24 has observed a similar PL spectral behavior when incorporating 3% HOOC(CH2)4NH3)2PbI4 into MAPbI3 perovskite matrix. It was explained that 2D/3D phase has been preferentially formed at the TiO2 side forming new emissive states compared to pure 3D perovskite.24 In our case, newly formed emissive state of the PL spectra with PEAI incorporation is consistent with Grancini’s finding indicating higher concentration of this larger bandgap material in the vicinity of TiO2 /perovskite interface. For 8 ACS Paragon Plus Environment

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ACS Energy Letters

the 10% PEAI sample, PL spectrum (when illuminated from the glass side) shows additional peaks at around 720 nm, 610 nm and 550 nm. These are likely to be emissive states from intermediate phases with energies falling between the pure 2D perovskite (at ~525 nm; see Figure S2f) and the control (FAPbI3)0.85(MAPbBr3)0.15 perovskite (Figure S2a). At low concentrations of PEAI, these peaks are likely to be hidden within the relatively broad shortwavelength tail of the PL spectra.24-25 A simpler test structure is also prepared which does not have any TiO2. PL is measured on this glass/FTO/perovskite sample with 10% PEAI and the result (red line) is shown in Figure S3. For comparison, PL measured on sample that has cTiO2 and mp-TiO2 is also shown (black line). It can be seen that the shoulder in the PL spectrum disappears indicating the absence of much of the quasi-2D material in the absence of TiO2. There are two possible anchoring mechanisms between TiO2 and PEAI; i) hydrogen bond due to amino group in PEA, and ii) addition-elimination mechanism (reported by Yuzawa et al.26) when a phenyl ring (in the PEA in our case) acts as an electron-donor. Via these two possible mechanisms, PEAI can be favorably positioned near the TiO2 layer. Timeresolved photoluminescence (TRPL) was also measured for the different compositions and radiative recombination lifetimes were determined. The samples were excited from the perovskite layer side and the PL traces are presented in Figure 2d. The PL decay traces were fitted using a bi-exponential function to deduce the two decay components. The values are summarized in the inset Table in Figure 2d. The improvement in the slow component τ 2 with PEAI (as long as it is below 10 mol%) is a result of improved radiative recombination lifetime of the free photo-generated charge-carriers.15, 27 The ultrafast component τ1 of the PL traces is attributed to the dominantly trap-mediated recombination regime which is the lowest when PEAI is low (1.5-3.0 mol%).28



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Figure 2. (a) Absorption of the test structure FTO/glass/c-TiO2/mesoTiO2/(FAPbI3)0.85(MAPbBr3)0.15 with 0% 1.5%, 3.0%, 4.5% and 10 mol% of PEAI. Steady state PL of the same structure measured from the (b) perovskite layer side and (c) glass side. (d) TRPL of the same structure measured from the perovskite layer side. Table in the inset shows the time constants extracted from the fitting of PL decay traces.

To further investigate the effect of PEAI incorporation on morphology and the electrical properties of variation between grain interiors and grain boundaries, KPFM was performed on the same test structures whereby the perovskite surface is directly accessible by the AFM probe. Open-circuit conditions were applied as in previous reports,23,

29-30

where the FTO

layer is grounded. Such measurement set up has been shown to be a powerful tool in quantifying local electrical properties of the thin film surface. Figure 3a-e show the contact potential difference (CPD) maps of samples with different levels of PEAI incorporation measured in the dark while Figure 3f-j show the CPD maps of the same samples under 10 ACS Paragon Plus Environment

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ACS Energy Letters

illumination. It can be seen that grain size decreases with PEAI incorporation, which is also observed in the transmission electron microscopy (TEM) images (Figure S4) and the corresponding topography images shown in Figure S5. Grain size reduction as a result of an incorporation of large cation has also been observed previously.14, 20 With regards to CPD measurement results, the averaged difference between grain boundary and grain interior (CPDGB-CPDGI) are given in the chart in Figure 3k for CPD measured in the dark (dark line, calculated from Figure 3a-e) and under light (red line, calculated from Figure 3f-j). When measured in the dark, the CPD at the grain boundaries is lower than the CPD in the grain interior (darker outline) which has been observed previously.31 In addition, the difference becomes gradually larger e. g. CPDGB-CPDGI = -13mV without PEAI compared to CPDGBCPDGI = -44mV with 4.5% of PEAI. This indicates that the electronic property of the grain boundaries has been changed by the incorporation of PEAI.32-34 This is due to the formation of quasi-2D phase at the grain boundaries. Wang et al.20 also observed that plate-like features exist at grain boundaries which was suspected as a 2D phase and form interface with grains that is 3D perovskite when n-BA is incorporated in FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite. In our perovskite film, a pure 2D phase is not expected to be present (due to the absence of the 525nm peak, see Figures S2b to S2e). Rather, it is suspected that 2D/3D phases co-exist (quasi-2D) at the grain boundaries and amount of quasi-2D phase gradually increases with the amount of PEAI incorporation. The existence of this higher bandgap quasi-2D (as depicted in Figure 3l) material has as positive effect in passivating the grain boundaries.32-33 This is supported by the results from CPD measurements under light illumination. The grain boundaries “light up” results in positive values for CPDGB-CPDGI which translates to enhanced photovoltage (CPDlight- CPDdark) at the grain boundaries as a result of reduced carrier recombinations30 Zhao et al. also reported the grain boundary passivating effect by the large cation NH3I(CH2)8NH3I when it is used to post-treat MAPbI3 film.35 However, here we report grain boundary passivation without any post-treatment by simply incorporating PEAI 11 ACS Paragon Plus Environment


into (FAPbI3)0.85(MAPbBr3)0.15 during the precursor preparation process. However, this enhancement ceases when PEAI is at 10%. At this concentration, results from both dark and light measurements are very similar to the results for the control film (0% PEAI). This could be due to the presence of an excessive amount of PEAI which did not result in the formation of quasi-2D phase at the grain boundaries, but rather result in the accumulation of PEAI which is insulating and inhibiting in terms of grain formation (see Figure S5e for the lack of grains observed). This is indicated in white arrow in Figure 3e. The surface photovoltage (SPV) is also calculated by subtracting averaged CPD measured in the dark from averaged CPD measured under light, see Figure 3k. This value is commonly used to determine minority-carrier density and lifetime under illumination in semiconductors.36-38 As shown in Figure 3k, SPV increases with PEAI before it reaches 10% showing an increasing enhancement in photovoltage over the entire area of the film suggesting an overall improvement in electrical properties. These are evidence of the positive effects when incorporating PEAI providing passivation of grain boundaries.


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ACS Energy Letters

Figure 3. Spatial CPD maps (1.5 x 1.5 µm2) of perovskites samples with PEAI incorporated at (a) 0 mol%, (b) 1.5 mol%, (c) 3.0 mol%, (d) 4.5 mol% and (e) 10 mol% measured under dark. (f) –(j) shows the Spatial CPD maps of the same samples measured under illumination. (k) CPD between grain boundary and grain interior (CPDGB-CPDGI) in dark and light. SPV is indicated with bars. (l) A schematic band diagram illustrating larger bandgap at the grain boundary compared to the grain interior due to the presence of quasi- 2D perovskites and Eg is expected to increase with increasing amount of PEAI incorporation.

(FAPbI3)0.85(MAPbBr3)0.15 solar cells with PEAI incorporated at 0%, 1.5%, 3.0%, 4.5% and 10 mol% are fabricated on glass/FTO/c-TiO2/mp-TiO2/perovskite/Spiro-OMeTAD/Au structure. Their PCEs, fill factors (FF), current densities (JSC) and open-circuit voltages (VOC) are shown in Figure 4. It can be seen that VOC increases (up to 1.1 V when PEAI = 4.5%) with PEAI incorporation. The same trend is observed in the PL, KPFM and VOC results, Figure S6a.



Figure 4. Plot of (a) PCE, (b) FF (c) JSC and (d) VOC for cells with different levels of PEAI incorporations (0, 1.5, 3.0, 4.5 and 10 mol%). The data are average values of 4 solar cells from JV curves swept from VOC to JSC under 1Sun. Error boxes represent standard deviations. In terms of device efficiencies, they decrease with the amount of PEAI incorporated due to reductions in FF (Figure 4b) and JSC (Figure 4c). This is because of the insulating nature of PEAI inhibiting carrier transport. Figure S6b shows the corresponding external quantum efficiency (EQE) of the devices showing the dramatic reduction in short wavelength responses as the concentration of PEAI increases, which is most sensitive to the charge-carrier generation, transport and collection at the perovskite/TiO2 interface where quasi-2D phase has been found to be more concentrated near it from our previous discussion of the results in Figure 2 and Figure S2. We can therefore explain that the increase in VOC, reduction in JSC and reduction in FF after the incorporation of PEAI is as a combined effect of grain boundary


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ACS Energy Letters

passivation by PEAI and the accumulation of PEAI near the perovsktie/TiO2 interface(as illustrated in Figure 5a). For preliminary stability tests, we exposed the control and the PEAI incorporated devices without encapsulation to 70% relative humidity in the dark for humidity stability test (Figure 5). Interestingly, after exposing to RH 70% for 240 hours, the PEAI incorporated device performance has significantly enhanced although as fabricated PEAI-containing perovskite solar cells has lower PCE compared to control devices. Average efficiencies, JSC, VOC and FF are shown in Figure 5c, 5d, 5e and 5f respectively. The values are listed in Table 1 and the changes for each parameter after 1200 hours humidity exposure are listed in Table 2. While the control devices under-went marked degradation after only 240 hours (10 days) of humidity exposure (Figure 5c), the PCE of PEAI incorporated devices improved during that period. In fact, the best performing device in the 1.5 % PEAI group had an efficiency of 17.2% with JSC of 21.7mA/cm2, VOC of 1.07 V and FF of 74.2% as shown in Figure 5b. The parameters are comparable to those of the control device before the humidity exposure. After 1200 hours (50 days) of humidity exposure, the average PCE of control devices dropped by almost 71% due to deteriorations in FF and JSC (Figure 5d and 5b respectively) with the JSC drop being the main contributor. JSC drop is also observed in PEAI incorporated devices when PEAI incorporation is very small (at 1.5%) with an overall PCE degradation of ~17%. As the amount of PEAI increase, the devices are more stable with only 15% of degradation mainly due to FF drop. The average PCE of 10% PEAI devices even improve. To further investigate the effect of ageing on PL spectra, both sides of the control and PEAI incorporated test structures were measured after 10 days of storage under RH 70%. The results are shown in Figures S7a and b. Comparing with Figures 2b and 2c, few observations are worth noting. First, the PL peak positions measured from the perovskite side remain the same after 10 days of moisture exposure. However, when measured from the glass side, the PL peaks were redshifted for devices with more than 3% PEAI. The positons of the PL peaks for the control 15 ACS Paragon Plus Environment


device and the 1.5% PEAI device remain the same. The short wavelength tails that were previously observed in the PEAI incorporated films also disappear after exposure to moisture (cf. Figure 2c and Figure S7b). In particular, the shoulder observed in the 10% PEAI which is a signature of the quasi-2D phase also disappears. This phenomenon will be further investigated in future work to understand the underlying mechanism so as to harness this phenomenon to further improve stability of these devices in the future. As the 1.5% PEAI incorporated device shows better stability compared to other PEAI incorporated devices, this type of device together with control device was subjected to 1 sun illumination at RH of 55% without encapsulation. Results are shown in Figure S8. Similar to what was observed in the humidity exposure test, the 1.5% PEAI non-encapsulated devices show better photostability compared to the control device in terms of efficiency due to the slower degradation in JSC and FF.


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ACS Energy Letters

Figure 5. (a) Schematic illustrating the segregation of PEA incorporated 2D perovskite at the grain boundaries and at the (FAPbI3)0.85(MAPbBr3)0.15 perovskite/ TiO2 interface. (ABX3) BX6 units are red; A atoms are green). (b) J-V curve of best device in 1.5% PEAI group before and after 240 hours exposure to RH 70 %. Average efficiencies (c), current densities (d), voltages (e) and fill factors (f) of 4 un-encapsulated solar cells in the control group and in the PEAI incorporated group during exposure to RH 70% in the dark. Error bar represents standard deviation.

Table 1. Averaged photovoltaic characteristics (from JV curve swept from VOC to JSC under 1Sun) of the control group and PEAI incorporated devices before and after 1200 hours of RH 70% exposure PEAI JSC(mA/cm2) VOC(V) FF(%) PCE(%) Before

Control

21.7

0.99

75

16.5

1.5% PEAI

19.9

0.96

74

14.3



After 1200 hrs.

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3.0% PEAI

18.7

1.04

64

12.3

4.5% PEAI

17.0

1.10

66

12.4

10% PEAI

8.3

1.04

45

3.7

Control

7.8

0.98

64

4.8

1.5% PEAI

17.4

1.03

67

11.9

3.0% PEAI

18.8

1.01

55

10.5

4.5% PEAI

15.6

1.00

51

10.6

10% PEAI

10.3

1.03

53

5.7

Table 2. Change in photovoltaic characteristics of the control group and PEAI incorporated devices after 1200 hours of RH 70% exposure PEAI JSC VOC FF PCE After 1200 hrs.

Control

-64%

-1%

-15%

-71%

1.5% PEAI

-13%

+7%

-10%

-17%

3.0% PEAI

1%

-3%

-13%

-15%

4.5% PEAI

-8%

-9%

-22%

-15%

10% PEAI

+25%

-1%

18%

52%

In summary, we demonstrate the benefits of incorporating PEAI into mixed perovskites namely (FAPbI3)0.85(MAPbBr3)0.15 which are enhanced crystallinity (confirmed by XRD results), passivation of the grain boundaries (confirmed by KPFM, PL and tr-PL measurements and VOC improvement in demonstrated solar devices) and slowing down of moisture induced degradation (confirmed by moisture stability tests). Although the accumulation of the larger bandgap incorporated quasi-2D material (as illustrated in Figure 5a) at the grain boundaries is beneficial providing passivation, its insulating nature causes the drop in FF and JSC in the associated PEAI incorporated solar devices. However, the aging test has shown remarkable benefit against moisture induced degradation when incorporating PEAI. The concentration of the quasi-2D material near the TiO2 (as illustrated in Figure 5a) has been observed to change after aging for the first time. This opens up opportunities for further studies into the mechanism of PEAI incorporation, different stages of reactions including environmentally-induced reaction within the bulk and near the interfaces. This will provide 18 ACS Paragon Plus Environment

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ACS Energy Letters

better understanding of the role of large cation in metal halide perovskites in passivation and stabilization. This work contributes to solutions to the moisture in-stability of perovskite cells that is a multi-faceted problem which is determined by interface condition, grain size, grain boundary properties and the presence of segregated materials.

ASSOCIATED CONTENT Supporting Information Experimental section, characterization method and additional data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +61 2 9385 4257 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australianbased activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). This project is also supported by ARENA via the project 2014 RND075. We thank the Electron Microscopy Unit and the BioMedical Imaging Facility at UNSW for the SEM and fluorescence imaging supports. References (1) Park, N. G. Organometal Perovskite Light Absorbers Toward a 20% Efficiency LowCost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423-2429. (2) Green, M. A.; Ho-Baillie, A. Perovskite Solar Cells: The Birth of a New Era in Photovoltaics. ACS Energy Lett. 2017, 2, 822-830. (3) Yang, W. S.; P, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin. S. S.; Seo. J.; Kim, E. K.; Noh, J. H. et al. Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells. SCIENCE. 2017, 356, 1376-1379. 19 ACS Paragon Plus Environment


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