CsPbI3 Interface under UV Illumination - ACS Publications

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C: Energy Conversion and Storage; Energy and Charge Transport 2

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Understanding Effects of Cesium in CH(NH)PbI for Stabilizing CH(NH)PbI/CsPbI Interface under UV Illumination 2

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Premjit Limpamanoch, Nopporn Rujisamphan, Pisist Kumnorkaew, Vittaya Amornkitbamrung, I-Ming Tang, Qiao Zhang, and Thidarat Supasai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01581 • Publication Date (Web): 20 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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The Journal of Physical Chemistry

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Understanding Effects of Cesium in CH(NH2)2PbI3 for Stabilizing CH(NH2)2PbI3/CsPbI3 Interface under UV Illumination

Premjit Limpamanoch,† Nopporn Rujisamphan,‡,§ Pisist Kumnorkaew,± Vittaya Amornkitbamrung,£ I-Ming Tang,‡ Qiao Zhang,* and Thidarat Supasai†,*

† Department

of Materials Science, Faculty of Science, Kasetsart University, Bangkok 10900,

Thailand ‡

Nanoscience and Nanotechnology Graduate Program, Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand

§

Theoretical and Computational Science Center (TaCS), Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand

± National

Nanotechnology Center (NANOTEC), National Science and Technology

Development Agency, 111 Thailand Science Park, Phahonyothin Rd., Khlong Nueng, Khlong Luang, Pathumthani 12120, Thailand £ Integrated

Nanotechnology Research Center, Department of Physics, Faculty of Science, Khon

Kaen University, Khon Kaen 40002, Thailand 

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China

Corresponding Authors *E-mail: [email protected] (T.S). *E-mail: [email protected] (Q.Z).

ACS Paragon Plus Environment

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Abstract In solution-processed perovskite semiconductors ( SPS’ s) , ultraviolet ( UV) radiation can induce structural degradation in the light-absorbing layer, for instance, methylammonium lead iodide ( MAPbI3 ) . In this paper, we use modulated surface photovoltage ( SPV) spectroscopy to track the mechanisms of photo-generated charge generation and separation as well as the formation of defects in the formamidinium lead triiodide (FAPbI3 ) ( FAx Cs1-x PbI3 )

and cesium-containing (Cs)

FAPbI3

perovskite when exposed to different durations of UV-light treatment.

The

measured SPV signals ( in-phase and out-phase ( shifted by 90o ) ) were found to be strongly dependent on the addition of Cs and on the UV exposure times. Upon the partial incorporation of Cs, the improved stability in the structural and optical properties was observed. The formation of the -CsPbI3 phase in the FAx Cs1-x PbI3 perovskite is attributed to the stabilization of the FAPbI3 / CsPbI3 interface, which would efficiently inhibit the phase segregation, and provide for a stable medium for the modulated charge separation under UV illumination.


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1. Introduction Organic-inorganic lead halide perovskites are now being used in solution based fabrication process and have gained much research attention over the past few years due to their great optoelectronic properties,1,2 i. e. , the long charge carrier diffusion length3,4 and low-exciton binding energy.

5,6

These properties make them potential candidates for use in light emitting

diodes, photodetectors, laser and photovoltaic applications.

7

One breakthrough, for example, is

an outstanding power conversion efficiency ( PCE) of over 20%

achieved over a short time. 8,9

However, when using single cation perovskite CH3 NH3 PbI3 ( MAPbI3 ) , intrinsic and extrinsic factors such as exposure to heat, UV light, oxygen, and moisture have challenged the development of the technology and of related research fields. moisture,

transformation

of

the

hydrated

10-12

perovskite

As MAPbI3 is exposed to

species,

i.e.,

monohydrate

(CH3 NH3 PbI3 H2 O) and dehydrate (( CH3 NH3 ) 4 PbI6 2H2 O) occurs and have been observed when the perovskites contain excess methylammonium iodide ( MAI).

13,14

The decomposition of

MAPbI3 first produces the by-product PbI2 , followed by the release of NH3 and CH3 I gases. 15 It was found that the MAPbI3 was susceptible to decomposition at 85 C even under inert gas atmosphere.

16

Also a structural phase transition occurs, from cubic to tetragonal phase at a

relatively low temperature of 57°C, which is in the working temperature range of solar devices. 17 Therefore, the use of the longer cations in CH( NH2 ) 2 PbI3 ( FAPbI3 ) should be of great interest. The utilization of FAPbI3 rather than MAPbI3 is preferable due to a reduced band gap in FAPbI3 , allowing the resulting energy gap to be close to the single junction optimum value needed for achieving the Shockley-Queisser limit.

18,19

The best solar cells based on formamidinium ( FA)

compounds used in a tandem solar device exhibit an impressive PCE of up to 23. 6% . 20 Also, the perovskite compounds in these solar cells do not undergo a phase transition over a wide range of temperature (25 to 120°C).21,22 However, an unstable yellow phase of FAPbI3 ( -phase) with a hexagonal structure was reported to have formed at room temperature. 23,24 This undesired -phase is non-photoactive with an indirect bandgap. 22 The partial incorporation of cesium ( Cs+ ) or rubidium ( Rb+ ) in FAPbI3 enables it to stabilize as a photoactive black phase ( -phase) . 25,26 In literature, -FAPbI3 is reported to have a direct gap transition with a band gap value of 1. 48 eV. 27 These mixed-cation



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perovskite compounds have been shown to be structural and thermal stable, and to be tolerant to light exposure and to ambient environments.

28,29

Defect states, which lie within a material’ s band gap, could theoretically capture charge carriers.

This would be an important loss mechanism for the device. Detailed information about

these defects would be very important. As is well known, defects can create new energy levels into which the excited electrons can go. Perturbations to the host crystal can also cause the energy levels to change.

For example, under environmental stress, the energy levels of the

defects could be shifted to lie inside the band gap of the perovskite semiconductors or/ and in the interfaces. carriers.

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This could greatly affect the charge transportation and cause the losses of charge The consequence of the latter effect is that the perovskite would undergo an

unintentional transition to either a n- or p-type behavior. This would of course cause a significant drop in the voltage of a solar cell.

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Information about the effects of ultraviolet ( UV) radiation on the stability of perovskite layers becomes of the paramount interest. It has been previously shown that the UV could accelerate the phase segregation in the perovskite absorber layer,10,32 causing deep traps for charge recombination at the interface between the titanium dioxide and perovskite.

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A better

understanding of the effects of UV radiation on stabilizing the photo-generated charge separation in such perovskite layers would, therefore, be one of the paramount information needed for advancement in this field. In this work, we report on the correlations between the structural properties, the optical properties and the photo-generated charge separation of the FAPbI3 and Cs-containing FAPbI3 under UV radiation. The formation of the CsPbI3 phase was found to be the key to the stabilizations of the perovskite structure and the behaviors of photo-generated charge separation under UV illumination. The in-situ surface photovoltage ( SPV) measurements were performed at 30C inside a homebuilt N2 -filled chamber, kept at a constant pressure of 300 mbar to prevent any effects due to moisture. The samples were illuminated by an UVA ( 390 nm) for different exposure times, e. g. , 2 4 , 48, 72, 96 and 120 hours. The power of the UV lamp in this study was about 398 mW/cm2 , which is about 3. 98 times higher than that of AM1. 5G.

Correlations between all the changes in

the formation of defect states during the UV exposure along with changes of the perovskite phase and the absorbance were analyzed and presented.


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2.

Experimental 2.1 Materials The chemicals and materials used in this work are: transparent fluorine-doped tin oxide

conductive glass (FTO, R = 15 /cm2 , Bangkok Solar), N,N-Dimethylformamide (DMF, anhydrous, 99.8%, Sigma Aldrich), dimethyl sulfoxide (DMSO, anhydrous, 99.9 %, SigmaAldrich), toluene (TL, 99.8%, Sigma Aldrich), lead (II) iodide (PbI2 , 99.9%, Sigma-Aldrich), methylammonium iodide (MAI, Great Cell Solar), formamidinium iodide (FAI, Great Cell Solar) and cesium iodide (99.9%, Sigma-Aldrich). All chemicals were used as received. 2.2 Sample preparation Figure 1(a)

shows the sample preparation for FAPbI3 and FAx Cs1-x PbI3 perovskite

compounds deposited onto the FTO substrates alongside their investigation. The top-view SEM images of the perovskites are shown. The perovskites were prepared inside a nitrogen-filled glovebox by spin-casting precursors at a rotation speed of 5000 rpm for 30 s with toluene as the solvent ( 5 0 0 uL) was dripped onto the spinning sample at 20 sec before the end of steps. The sample was then annealed at 170 C for 10 min inside the N2 filled glovebox. We note here that the precursors were prepared as follows: a 1. 24 M FAPbI3 solution was made by adding 1 M FAI and a 1.09 M PbI2 in a solvent consisting of DMF and DMSO (4:1 volume ratio). To prepare the FAx Cs1-x PbI3 solutions, a separate 1. 5 M CsI solution was dissolved in a DMSO solution prepared in a separated vial. This mixture was added directly to the previously prepared FAPbI3 solution having the desired Cs doping content. As a side note, before use, the mixed solutions were stirred continuously overnight at 70 C to ensure a complete reaction of the perovskite complexes.

2.3 SPV measurements and characterizations The SPV measurements were done in a fixed capacitive arrangement ( Figure 1 ( b) ) in this study. More details of the setup are described elsewhere. 14,34 The measurements were carried out in the home-built chamber, which allows for the study of the effects of different conditions such as the duration of the UV radiation and the moisture. In principle, changes in SPV signals are due to the changes in surface work function with respect to the reference electrode when the samples are illuminated.

The SPV measurements in a fixed planar capacitor provide modulated signals



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generated by the photo-generated charge separation and relaxation processes arising from the on/off illumination. The parts of the in-phase ( X) and the phase-shifted by 90

o

( Y) signals which

are due to the response times can provide information on the fast and slow response processes with respect to the modulation period. 35

For example, some changes of the SPV signals are

shown in Figure 1( c) . The positive sign of the X-signals ( Figure 1( c) , top) provides evidence for the preferential separation of the photo-generated electrons toward the internal interface ( towards a FTO substrate) whereas the negative sign of the X-signals ( Figure 1( c) , bottom) suggests that the photo-generated electrons preferentially transport toward the external surface ( towards a mica sheet) . The opposite sign ( of the X- and Y-signal) over the whole SPV spectrum implies that one dominating mechanism for the modulated charge separation occurs and that the relaxation is caused by the recombination process only. 36 Since this surface sensitive technique measures the photo-induced voltage and not the photocurrents, the SPV signals for low charge densities and/ or for even low light intensity will be able to be detected. 37 In our observation, a typical SPV spectrum will have three distinct regions: Region ( i) is the SPV signals for photon energies below the band gap. The signals of the photo-generated charge separation in this region may be attributed to excitation from the near-band-edge region of the emission spectrum.

These

are related to the Urbach tail energy ( Eu) of the investigated sample. Region ( ii) is where the SPV signals strongly increases ( evidenced by the increase in the signals from the onset energy to the maxima) . This is the dominant part of the signals. These signals are due to the photo-generated charge separation during a direct band gap transition in the perovskite compounds. As a remark, the SPV measurements allow for a very sensitive characterization of charge separation, for instance, in an extremely thin layer of the photoactive materials or monolayer of molecules.38,39 Region ( iii) is the SPV signals after reaching their maxima.

Slow decreases in both the X-and

Y-signals are caused by a reduction of the modulated light intensities ( photon flux) .

In this

work, a 100 W halogen lamp with quartz prism monochromator ( Bausch& Lomb) and optical chopper for a modulation frequency of 15 Hz were used for illumination. The SPV signals first passed through a high impedance buffer ( 20 G with the RC time constant 2 sec) , and then collected by a double-phase lock-in amplifier ( made from Elektron–Manufaktur Mahlsdorf, Germany).


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Figure 1. (a) Preparation of the perovskite layer for SPV, XRD, and UV-vis measurements; SEM images of the FAPbI3 and FAx Cs1-x PbI3 perovskites. (b) Scheme of the SPV measurement configuration in a fixed capacitor arrangement measured with modulated light. (c) Possible changes of the SPV signals under illumination, demonstrated by the X- and Y-signals with the typical three distinct features--region i, ii and iii.

The phase structure and optical properties including the optical band gap ( Eg) of the pristine and of the UV exposed samples were characterized by X-ray diffraction ( XRD, Bruker D8 using a CuK α radiation source) and a UV–Vis spectrometer ( Perkin Elmer Lambda model 650) equipped with an integrating sphere component.

3. Results and discussion 3.1 Characterization of structural and optical properties In order to clarify how UV radiation can induce instability in the structural and optical properties in FAPbI3 and FAx Cs1-x PbI3 , XRD and UV-vis measurements were performed. Figure 2 shows the XRD patterns of the FAPbI3 and FAx Cs1-x PbI3 before, and after being exposed to the



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UV illumination for periods up to 120 hr. As seen, the as-prepared FAPbI3 has characteristic peaks at around 14.05°, 19. 90°, 24.50°, 28.29°, 31.57° and 34.63° (marked by “” in the Figure 2) which are the reflections from the preferential orientations of ( -111) , ( -120) ( 012) , ( 021) ( 003) , ( -222) , ( -231) ( -123) and ( 030) ( -132) ( -114) planes of the -FAPbI3 , respectively. All the resulting peaks here are the trigonal phase ( space group P3m1) 21 and are in a perfect agreement with those reported in the literature. 40 The reflections at 11. 5° and 12. 7° indicate the presence of the non-perovskite hexagonal structure of -FAPbI3 ( a yellow phase) with the reflection being from the ( 010) plane, and from the PbI2 hexagonal polytype, which are denoted as “ ” and “ * ” , 41,42



after UV illumination

o 

FTO

FAxCs1-xPbI3



o 

*





 o  

FTO

respectively.

Log Intensity (arb.un.)

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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

FAxCs1-xPbI3 before UV illumination FAPbI3 after UV illumination FAPbI3 before UV illumination

5

10

15

20

25

30

35

2 (degrees) Figure 2. -2 X-ray patterns of the FAPbI3 and FAx Cs1-x PbI3 before and after UV illumination for 120 hr.


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It is well established that FAPbI3 has a thermodynamic preference to form into the unstable yellow phase at room temperature. 23 We note here that the PbI2 peak for FAPbI3 sample is due to an excess of PbI2 used in the preparation of the perovskite solution. After being illuminated for 120 hr, the FAPbI3 exhibits an increased number of reflection from the yellow phase, suggesting an increase in the preferred orientations of the crystal of the -phase. Additional peaks belonging to the -phase were found at around 16. 58°, 20. 81°, 22. 71° and 25. 68° whose positions are in a good agreement with literature. 40 Moreover, the prominent peak of -phase at around 11. 5° becomes more pronounced with a narrower full width at haft maximum ( FWHM) . The XRD pattern for the FAx Cs1-x PbI3 before UV illumination is presented as a blue line in Figure 2. As seen, there is no appearance of any reflection from the planes belonging to the -FAPbI3 phase. A small peak which is assigned to the orthorhombic structure of -CsPbI3 phase ( as marked by “ o” ) is observed at around 22. 50°. 43 This shows that the Cs inclusion plays a role in stabilizing the trigonal black FAPbI3 phase. It is well recognized that the Goldschmidt tolerance factor ( t) is a measure of the structural stability of these perovskites. 23 Upon addition of a small amount of Cs, the tolerance factor tends towards that of a cubic lattice structure. This is due to a slight lattice shrinkage which causes an enhancement of the bound interaction between the alkali cations and iodide anions.28 The observed XRD data here show a significant shift towards higher diffraction angles after the Cs addition. The estimated lattice constants of the FAx Cs1-x PbI3 are a= b= 8. 821Å and c= 10.865 Å whereas those for the FAPbI3 are found to be a= b= 8.912 Å and c= 10.891 Å, which closely agrees with previous reports. 28 The reduction of the size of the unit cell points to the substitution of a smaller cation of Cs+ ( 1. 81 Å) into the FA+ ( 2. 79 Å) sites. Upon UV illumination, the XRD peaks belonging to the -FAPbI3 phase are small. Additional peaks of small intensities are seen at 2 of 9. 98° and 13.20°, which are due to the -CsPbI3 phase. 43 To correlate the phase transformation, the ratio of intensities between the - and the -phase was calculated by taking the intensity of ( 010) plan divided by those from the main ( hkl) planes (indexed by (-111), (-222) and (-231)(-123)). Figure 3 (a) shows the plot of the ratio I( 010) /I( hkl) for FAPbI3 and FAx Cs1-x PbI3 . It was found that the ratio for the FAPbI3 increased by a factor of 1. 3-2 after 120-hr of UV exposure, meaning that there was an increase in the lattice spacing of the crystal in the -phase. The increase in the ratio was also observed in FAx Cs1-x PbI3 sample



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after UV illumination. Figure 3( b) shows the opposite trend for the corresponding FWHM ratio. This ratio for the FAPbI3 decreased by a factor of 0. 5-1 while that for FAx Cs1-x PbI3 was in the range of 0. 2-1. 2, and therefore suggests a decrease in the crystallinity of the -phase after UV illumination.

I(010)I(hkl)

1.2

(a)

0.8

0.4 not found for (010)

0.0

FWHM(010)FWHM(hkl)

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(b)

FAPbI3 before UV illumination FAPbI3after UV illumination FAxCs1-xPbI3 before UV illumination

1.2

FAxCs1-xPbI3after UV illumination

0.8 0.4

(-111)

(-222)

(-231)(-123)

(h k l) Figure 3. The ratio of the intensities (a) and of the full width at half maximum (FWHM) (b) of the XRD peaks indexed to the (010) -FAPbI3 to the three main (hkl) planes for -FAPbI3 for the FAPbI3 (black dots), FAx Cs1-x PbI3 (blue dots) before UV illumination, and those samples after the 120 hr-UV illumination (red and green dots for FAPbI3 and FAx Cs1-x PbI3, respectively).

Figure 4( a) shows the absorbance spectra of FAPbI3 and FAx Cs1-x PbI3 before and after UV illumination. A blue shift of about 20 nm was observed in FAx Cs1-x PbI3 sample. This agrees with the previous report. 44 The inset of Figure 4( a) shows the analysis of the optical band gap ( Eg) using Tauc’ s equation. The Eg was found to be 1. 521 eV for the fresh FAPbI3 , which slightly shifted to 1. 515 eV after UV illumination. A larger band gap of 1. 563 eV was found after adding


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the Cs and Eg changed to 1. 557 eV after the UV illumination. The increase in Eg after the Cs addition can be attributed to the stronger interaction between lead cations and iodide anions, caused by the smaller lattice constants. As presented, the Eg of the samples was larger than that for the bulk material ( 1. 43 eV) . 18 This difference could be attributed to the size confinement effect appearing in the thin films having the nanostructured grains.

45

A slight red shift of the

band gap was seen in for FAx Cs1-x PbI3 after UV illumination, providing evidence for a partial reduction of the amount of Cs migrating toward the surface/ grain boundary. The deficiency of the Cs+ in the lattice leads to a structural distortion upon relaxation. This will cause a change in the symmetry of the PbI6 octrahedron. 46 A thoretical study of the electronic structure of MAPbI3 showed that the valence band maximum ( VBM) is due mainly to the p orbitals of iodine ( I) and the conduction band minimum ( CBM) is mainly due to the p orbitals of the lead ( Pb) . 47 The change in Pb-I-Pb angles can alter the electronic structure.

In order to investigate the

decomposition rate due to the UV irradiation, the normalized absorbances at 630 nm for the FAPbI3 and FAx Cs1-x PbI3 were measured as a function of exposure time and are shown in Figure 4( b) (see Figure S1 for the whole absorption spectra, Supporting Information). An onset of the degradation of the FAPbI3 is observed after 48 hr of UV exposure. The absorbance dramatically decreases by 40% after 120 hr of illumination. The strong reduction of the absorbance indicates the degragation occurred throughout the bulk of the FAPbI3 . The opposite is seen in the FAx Cs1x PbI3

where a constant amount of absorbance, over 95%

of its original value is seen even after

120 hr of illumination. This suggests that the bulk-absorption property of the FAx Cs1-x PbI3 is not greatly influenced by the UV irradiation.

This evidence underlines that the partial Cs

incorporation into FAPbI3 effectively retards the decomposition rate due to UV illumination.



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Figure 4. UV-Vis absorption spectra of the fresh FAPbI3 (black line), of the FAx Cs1-x PbI3 (blue line) and those samples after UV exposure up to 120 hr (red and green line for FAPbI3 and FAx Cs1-x PbI3, respectively) (a). The inset displays the linear fit of the data at the absorption onset regime. Normalized absorbance collected at 630 nm for the FAPbI3 (black symbols) and FAx Cs1x PbI3

(blue symbols) under UV light at different illumination periods of time (b).

3.2 Behaviors of photo-generated charge migration In order to investigate the changes in electronic states induced by the UV irradiation and to correlate those changes with behaviors of photo-generated charge separation, in-situ surface photovoltage spectroscopy was carried out. The overview of the SPV spectra, the in-phase ( Xsignal) and phase-shifted by 90o ( Y-signal) , of FAPbI3 collected after the UV light exposure is presented in Figure 5( a) . As observed, the FAPbI3 shows a strong onset energy in the SPV signals at about 1. 47 eV, which is due to the direct band gap ( Eg) of the black -FAPbI3 phase. 48 It should be noted that the optical band gap of the black perovskite phase varies from 1. 43 eV for bulk to 1. 48 eV in the thin film specimens. 27,49 The 50 meV difference depends strongly on the preparation conditions being used. 28,50 As reported in the literature, a relatively larger Eg ( in a range of 1.50-1.57 eV) has also been reported for the black perovskite phase. 18,51 The other phase ( yellow non-perovskite -FAPbI3 ) has a relatively larger band gap of about 2. 43 eV. 22

As

observed, a pristine FAPbI3 ( Figure 5( a) -1) exhibits negative X- and positive Y- signals over the whole spectrum.

This is indicative of there being a sole mechanism for the photo-generated


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charge separation in the space charge region, i. e. , the photo-generated electrons move toward the external surface ( towards a mica sheet) and the photo-generated holes transport toward the internal interface (towards a FTO substrate). Upon increasing the UV exposure to 24 hr ( Figure 5( a) -2) , the sign of the X-signals remains unchanged while that for the Y-signals changes, i. e. , it becomes negative. This suggests that most of the photo-generated electrons and some of the holes separated and move towards the external surface. 36

Interestingly, signs of the X- and Y-signals both become positive after the

UV exposure is increased to 48 hr ( Figure 5( a) -3) . These results imply that a change of charge separation direction occurred; that is, most of the electron carriers now prefer to move towards the internal interface.

Figure 5. Modulated SPV spectra of the in-phase (black lines) and the phase-shifted by 90o (red lines) signals of the FAPbI3 (a) and FAx Cs1-x PbI3 film (b) under UV radiation, collected at various exposure periods.



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Increasing the UV exposure cycles to 72 hr ( see Figure 5( a) -4) , the change in the sign of the X-signals occurs at photon energy between 2. 42 to 3. 5 eV. These signals are attributed to the formation of the perovskite -FAPbI3 phase. 22 After 72-hr UV exposure, the SPV signals below the band gap are seen to be considerably more intense with a maximal amplitude of about 3 V. This observation is more pronounced ( a maximum of 6. 5 V) as the sample was illuminated by the UV radiation for 120 hr (Figure 5(a)-6)). The increased SPV signals at photon energies below the bandgap are experimental evidence of an increased charge separation length induced by the defect formation after the illumination. The presence of the defect states alongside the structural instability occurs concurrently with the formation of -FAPbI3 as seen by the SPV signals at about 2. 4 eV. We conclude that the UV radiation greatly affects the phase segregation and the transformation of FAPbI3 from the -phase to the -phase. As a result of this, the defect states near the surface and/ or grain boundaries are formed. In recent studies, Liu et al. showed that the formation of intrinsic defects in FAPbI3 originated from the creation of iodine vacancies ( VI) , the Pb substitution into FA sites ( PbFA) and the creation of FA interstitial ( FAi) defects. 52 These defect states are formed near the surface and are caused by iodide vacancies due to their relatively lower formation energies. Figure 5( b) shows the modulated SPV spectra of FAx Cs1-x PbI3 collected for various UV exposures times. Contrary to the FAPbI3 , the pristine FAx Cs1-x PbI3 shows a positive X-signals with a maximal intensity of about 0. 12 mV whereas the Y-signals are close to zero ( see Figure 5( b) -1) . This shows that there is only ( i) ultra-fast response of photo-generated charge separation and that there is ( ii) only photo-induced electrons being separated, and that they are moving toward the internal interface.

The strong positive X-signals in the FAx Cs1-x PbI3 are directly

related to their absorbed-light property and to their charge separation being occured in space. The positive sign of the X-signals also indicates that the FAx Cs1-x PbI3 behaves as an n-type semiconductor with a depletion region at the surface. perovskites are very susceptible conditions.

53,54

Naikeaw et al.

As mentioned, the electronic states in

to the chemical compositions and to the fabrication reported that the MAPbI3 changed from a p- to n-type

semiconductor upon moderate heating.

31

In MAPbI3 , the p-type doping originates from the

existence of Pb vacancies ( VPb) or of methylammonium vacancies ( VMA) , whereas the presence of iodide vacancies ( VI) leads to the n-type doping. 55 To sum up, an onset energy of the SPV signals for the FAx Cs1-x PbI3 appears to slightly shift towards higher energy of about 1. 52 eV, and


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15

by adding a small amount of dopant into a perovskite host, the optical properties, e. g. , the band gap and absorption coefficient, and the electronic properties ( i. e. , charge transport/ mobility) could be tuned.51,56

Interestingly, the signs of the SPV signals produced by the Cs-containing FAPbI3 did not change. This demonstrates the essential roles of Cs on the preferential charge separation and on the phase stabilization.

The variation of the Cs contents also had significant effects on the

structural stabilization, by causing the stabilization of the -FAPbI3 perovskite. As demonstrated by the SPV spectra, the partial incorporation of Cs potentially governs the direction of photoinduced charge separations and of the charge transportations. A small hump located at around 2. 85 eV seems to more pronounced after being exposed to the UV illumination for 72 hr ( Figure 5( b) -4) . A considerable increase in the X-signals was observed when increasing the UV exposure to 120 hr, ( see Figure 5( b) -6) , the results of which could be due possibly to the small segregation of the CsPbI3 at the FAPbI3 / CsPbI3 interfaces (as supported by the grazing incident X-ray diffraction (GIXRD) results, see Figure S2(a)-(e) in Supporting Imformation). This would lead to an increase in the charge separation length.

It is worth emphasizing that the formation of CsPbI3

at the surface could assist in the transfer of hole carriers towards the external surface and blocking that of the photo-induced electrons. A similar observation was previously reported for the PbI2 /CH3 NH3 PbI3 interfaces.57

3.3 Estimation of the characteristic energies of defect states and onset energies To further elucidate the UV-induced defect formation, the SPV measurements with a 10 meV photon energy step were carried out. The SPV signals were measured near the material’ s band gap and a quantitative analysis of the characteristic energy on defect states below the band gap upon the UV illumination was done.

The characteristic energy of the defect states locating

near the band edges, namely the tail states ( Et ) , can be determined from the slope of the exponential tail states of the SPV signals. 58 Figure 6( a) shows the spectra of the PV amplitude exhibited on a logarithmic scale for FAPbI3 and FAx Cs1-x PbI3 before and after the UV illumination for 120 hr. We note, the PV amplitude is defined as the square root of the sum of the squared X-and Y-signals. As seen, an increased magnitude ( by a factor of four) of the PV



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amplitude at a photon energy of about 1. 4 eV was only observed for the FAPbI3 after the UV illumination whereas this feature disappeared for the FAx Cs1-x PbI3 after the exposure.

These

results reveal that the density of states below the band gap of the FAPbI3 is strongly influenced by the UV radiation. Dependencies of the values of Et on the UV illuminations for the FAPbI3 and the FAx Cs1-x PbI3 are displayed in Figure 6( c) and ( d) , respectively. As shown, the analyzed Et for the FAPbI3 trends to increase ( from 18 to 29 meV) with continuing UV illumination. However, those values for the FAx Cs1-x PbI3 , show a slight variation ( a 18. 5 meV for the pristine) which gradually increases to about 22 meV after 48 hr of UV illumination, followed by a slightly decrease to 19 meV after 120 hr. Figure 6( b) shows the plots of the normalized square of the PV amplitude divided by photon flux as a function of photon energies for FAPbI3 and FAx Cs1-x PbI3 before and after UV illumination for 120 hr. The Eonset can be obtained by extrapolating a linear portion of the plot to the zero SPV signal. The FAx Cs1-x PbI3 exhibits a slight blue shift of the onset energy compared with that of the FAPbI3 . This has been previously reported. 42 Lee et al. reported a Eg of 1. 53 eV for the FAPbI3 whereas the Eg for FA0. 9 Cs0. 1 PbI3 is slightly shifted towards higher energy ( 1. 545 eV) . 28 A large blue shift of about 100-200 meV of the onset of the optical spectra for FAPbI3 is observed when increasing the Cs contents from 8 to 50% .25 We found that the values of Eonset for the fresh FAPbI3 and for the FAx Cs1-x PbI3 are 1. 47 and 1. 52 eV, respectively. The observed Eonset as determined from the SPV spectrum is the mobility band gap while the ( optical) band gap energy ( obtained from optical absorption measurement)

in semiconductors is the

energy to excite an electron from a localized occupied state in the valance band to a localized unoccupied state in the conduction band. In the absence of any strong interaction between the hole and the electron, the two are the same. The values of Eonset for the FAPbI3 and FAx Cs1-x PbI3 as function of increasing illumination times are shown in Figure 6(c) and (d), respectively.


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17

Figure 6. The PV amplitude (on a logarithm scale) measured near the band gap of pristine FAPbI3 (black symbols), FAx Cs1-x PbI3 (blue symbols) and those samples after UV illumination (red and green symbols for FAPbI3 and FAx Cs1-x PbI3 , respectively) (a). Dashed lines present the photon flux spectrum for the SPV measurement. The solid lines in (a) present the slopes for determination of the characteristic energy of distributed tail states (Et ). The amplitude divided by the photon flux of the corresponding samples (b). The solid lines in (b) manifest the definitions of the onset energies (Eonset ). Dependence of the characteristic energy of the tail states (left axis) and the onset energies (right axis) on the UV exposures for the FAPbI3 (c) and for the FAx Cs1-x PbI3 (d).

A mechanism for stabilization of the Cs-containing FAPbI3 is proposed in Figure 7.

The

formation of CsPbI3 phase is formed under UV illumination of the FAx Cs1-x PbI3 , by the creation of the CsPbI3 / FAPbI3 interface. This interface could be considered to be an incomplete diffusion barrier which would act as blocking barrier for any ionic species (i.e., FA+) movement.



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Figure 7. Schematic band diagram at the FAPbI3 /CsPbI3 interface, illustrating a mechanism for blocking FA+ diffusion towards the surface, CB and VB denoting the conduction and valence band edges.

4. Conclusions We have seen that SPV measurements can clarify the photo-generated and charge migrations in the Cs/ FAPbI3 systems when exposed to different durations of UV illumination. Upon UV illumination, variations in the SPV signals of the FAPbI3 were observed, showing that the UV exposure can significantly affect the formation of energy tail states and can lead to phase transitions. A prolonged UV exposure increases the Et by a factor of 2 for FAPbI3 sample. Analysis of the phase aggregation ( as analyzed by the XRD, UV-Vis data) supports a mixed and the -phase, being caused by the UV illumination. A reduction of defect state formation and stabilization of charge separation was clearly seen in the Cs-containing FAPbI3 system. Upon further UV illumination, a tiny peak located at 2. 85 eV in the SPV spectrum, which is ascribed to -CsPbI3 phase is found.

Along with the formation of this new phase, the enhancement and

stabilization of the SPV amplitude signals were observed.

The latter observation was

interpretated as being due to an improvement in the charge separation capability and the creation of an incomplete diffusion barrier for mobility of the FA+ at the FAPbI3 /CsPbI3 interfaces.


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Associated Content Supporting Information The absorption spectra of FAPbI3 and FAx Cs1-x PbI3 perovskites

before and after UV

illumination, measured in a wavelength range of 450-900 nm, grazing incidence X-ray diffraction patterns after UV illumination for 120 hr, measured at grazing angle of 0. 3, 0. 5, 1. 5 and 2.0 degrees. Acknowledgments This work was supported by the Thailand Research Fund ( MRG6280111) . P. L would like to acknowledge the Graduate School of the Kasetsart University ( Graduate Study Research Scholarship for International Publications Year 2016) . T. S is grateful to Kasetsart University Research and Development Institute ( grant no. P-3. 1 ( D) 228. 61 phase#2) for financial support. I-M. T. and N. R. would like to acknowledge the support of King Mongkut’ s University of Technology, Thonburi through the “ KMUTT 55th Anniversary Commemorative Fund” .

We

acknowledge Dr. Navaphun Kayunkid of College of Nanotechnology, King Mongkut's Institute of Technology Ladkrabang for providing grazing incidence XRD measurements and discussions.



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References 1.

Chen, Q.; Marco, N. D.; Yang, Y.; Song, T. B.; Chen, C. C.; Zhao, H.; Hong, Z.; Zhou, H. ; Yang, Y. Under the Spotlight:

The Organic–Inorganic Hybrid Halide Perovskite for

Optoelectronic Applications. Nano Today 2015, 10, 355-396. 2.

Park, N. G. Organometal Perovskite Light Absorbers Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423-2429.

3.

Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Yang, W. S.; Seo, J.; Yun, J. S.; Baillie, A. H.; Huang, S.; Green, M. A. ; Seidel, J. ; Ahn, T. K. ; Seok, S. I. Beneficial Effects of PbI2 Incorporated in Organo-Lead Halide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1502104/ 11502104/8.

4.

Shi, D.; Adinolfi, V. ; Comin, R.; Yuan, M. ; Alarousu, E.; Buin, A. ; Chen, Y.; Hoogland, S. ; Rothenberger, A.; Katsiev, K. ; Losovyj, Y. ; Zhang, X. ; Dowben, P. A. ; Mohammed, O. F. ; Sargent, E. H. ; Bakr, O. M. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522.

5.

Lin, Q. ; Armin, A. ; Burn, P. L. ; Meredith, P. Organohalide Perovskites for Solar Energy Conversion. Acc. Chem. Res. 2016, 49, 545-553.

6.

Galkowski, K. ; Mitioglu, A.; Miyata, A.; Plochocka, P.; Portugall, O. ; Eperon, G. E. ; Wang, J. T. W.; Stergiopoulos, T. ; Stranks, S. D. ; Snaith, H. J.; Nicholas, R. J. Determination of the Exciton Binding Energy and Effective Masses for Methylammonium and Formamidinium Lead Tri-Halide Perovskite Semiconductors. Energy Environ. Sci. 2016, 9, 962-970.

7.

Hassan, Y. ; Ashton, O. J. ; Park, J. H. ; Li, G. ; Sakai, N. ; Wenger, B. ; Haghighirad, A. A. ; Noel, N. K. ; Song, M. H. ; Lee, B. R. ; Friend, R. H. ; Snaith, H. J. Facile Synthesis of Stable and Highly Luminescent Methylammonium Lead Halide Nanocrystals for Efficient Light Emitting Devices. J. Am. Chem. Soc. 2019, 141, 1269-1279.

8.

Hou, Y.; Du, X.; Scheiner, S.; McMeekin, D. P.; Wang, Z. ; Li, N.; Killian, M. S.; Chen, H. ; Richter, M. ; Levchuk, I. ; Schrenker, N. ; Spiecker, E. ; Stubhan, T. ; Luechinger, N. A. ; Hirsch, A.; Schmuki, P.; Steinrück, H. P.; Fink, R. H.; Halik, M.; Snaith, H. J.; Brabec, C. J. A


Page 21 of 28 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


21

Generic Interface to Reduce the Efficiency-Stability-Cost Gap of Perovskite Solar Cells. Science 2017, 358, 1192-1197. 9.

Yang, W.S.; Park, 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. ; Seok, S. I. Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379.

10. Lee, S. W.; Kim, S.; Bae, S.; Cho, K.; Chung, T.; Mundt, L. E.; Lee, S.; Park, S.; Park, H.; Schubert, M. C.; Glunz, S. W.; Ko, Y.; Jun, Y.; Kang, Y.; Lee, H. S.; Kim, D. UV Degradation and Recovery of Perovskite Solar Cells. Sci. Rep. 2016, 6, 38150/1-1 38150/10. 11.

You, J.; Meng, L. ; Song, T. B. ; Guo, T. F.; Yang, Y. ; Chang, W. H. ; Hong, Z. ; Chen, H. ; Zhou, H. ; Chen, Q.; Liu, Y. ; Marco, N. D. ; Yang, Y. Improved Air Stability of Perovskite Solar Cells via Solution-Processed Metal Oxide Transport Layers. Nat. Nanotechnol. 2016, 11, 75-81.

12.

Troughton, J. ; Hooper, K. ; Watson, T. M. Humidity Resistant Fabrication of CH3 NH3 PbI3 Perovskite Solar Cells and Modules. Nano Energy 2017, 39, 60-68.

13.

Halder, A. ; Choudhury, D. ; Ghosh, S. ; Subbiah, A. S. ; Sarkar, S. K. Exploring Thermochromic Behavior of Hydrated Hybrid Perovskites in Solar Cells. J. Phys. Chem. Lett. 2015, 6, 3180-3184.

14.

Henjongchom, N. ; Rujisamphan, N. ; Tang, I. M. ; Supasai, T. Surface Photovoltage Spectroscopy Study of Ultrasonically Sprayed-Aerosol CH3 NH3 PbI3 Perovskite Crystals. Phys. Status Solidi A 2018, 215, 1800133/1-1800133/8.

15.

Juarez-Perez, E. J. ; Hawash, Z. ; Raga, S. R. ; Ono, L. K. ; Qi, Y. Thermal Degradation of CH3 NH3 PbI3 Perovskite

into

NH3 and

CH3 I

Gases

Observed

by

Coupled

Thermogravimetry-Mass Spectrometry Analysis. Energy Environ. Sci. 2016, 9, 3406-3410. 16.

Conings, B. ; Drijkoningen, J. ; Gauquelin, N. ; Babayigit, A. ; D'Haen, J. ; D'Olieslaeger, L. ;



Page 22 of 28

22

Ethirajan, A. ; Verbeeck, J. ; Manca, J. ; Mosconi, E.; Angelis, F. D. ; Boyen, H. G. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5, 1500477/1- 1500477/8. 17.

Whitfield, P.S.; Herron, N.; Guise, W. E.; Page, K.; Cheng, Y. Q.; Milas, I.; Crawford, M. K. Structures, Phase Transitions and Tricritical Behavior of the Hybrid Perovskite Methyl Ammonium Lead Iodide. Sci. Rep. 2016, 6, 35685/1- 35685/16.

18.

Pang, S. ; Hu, H. ; Zhang, J. ; Lv, S. ; Yu, Y. ; Wei, F. ; Qin, T. ; Xu, H. ; Liu, Z. ; Cui, G. NH2 CH═

NH2 PbI3 ═

: An Alternative Organolead Iodide Perovskite Sensitizer for

Mesoscopic Solar Cells. Chem. Mater. 2014, 26, 1485-1491. 19.

Hanusch, F. C. ; Wiesenmayer, E. ; Mankel, E. ; Binek, A. ; Angloher, P. ; Fraunhofer, C. ; Giesbrecht, N. ; Feckl, J. M. ; Jaegermann, W. ; Johrendt, D. ; Bein, T.; Docampo, P. Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium lead Bromide. J. Phys. Chem. Lett. 2014, 5, 2791-2795.

20.

Bush, K. A. ; Palmstrom, A. F. ; Yu, Z. J. ; Boccard, M. ; Cheacharoen, R. ; Mailoa, J. P. ; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T.; Peters, I. M.; Minichetti, M. C. ; Rolston, N. ; Prasanna, R. ; Sofia, S.; Harwood, D. ; Ma, W. ; Moghadam, F. ; Snaith, H. J. ; Buonassisi, T. ; Holman, Z. C. ; Bent, S. F. ; McGehee, M. D. 23. 6% -Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009 /1- 17009/7.

21.

Binek, A. ; Hanusch, F. C. ; Docampo, P. ; Bein, T. Stabilization of the Trigonal HighTemperature Phase of Formamidinium Lead Iodide. J. Phys. Chem. Lett. 2015, 6, 12491253.

22.

Ma, F. ; Li, J. ; Li, W. ; Lin, N. ; Wang, L. ; Qiao, J. Stable α/ δ Phase Junction of Formamidinium Lead Iodide Perovskites for Enhanced Near-Infrared Emission. Chem. Sci. 2017, 8, 800-805.


Page 23 of 28 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


23

23.

Li, Z. ; Yang, M. ; Park, J. S. ; Wei, S. H. ; Berry, J. J. ; Zhu, K. Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys. Chem. Mater. 2016, 28, 284-292.

24.

Zhou, Z. ; Pang, S. ; Ji, F. ; Zhang, B. ; Cui, G. The Fabrication of Formamidinium Lead Iodide Perovskite Thin Films via Organic Cation Exchange. Chem. Commun. 2016, 52, 3828-3831.

25.

Syzgantseva, O. A. ; Saliba, M. ; Grätzel, M. ; Rothlisberger, U. Stabilization of the Perovskite Phase of Formamidinium Lead Triiodide by Methylammonium, Cs, and/ or Rb Doping. J. Phys. Chem. Lett. 2017, 8, 1191-1196.

26.

Guo, Y. ; Li, C. ; Xue, Y. ; Geng, C. ; Tian, D. First-Principles Study of Cs/ Rb Co-Doped FAPbI3 Stability and Degradation in the Presence of Water and Oxygen. Mater. Res. Express 2018, 5, 026203.

27.

Eperon, G. E. ; Stranks, S. D. ; Menelaou, C. ; Johnston, M. B. ; Herz, L. M. ; Snaith, H. J. Formamidinium Lead Trihalide:

A Broadly Tunable Perovskite for Efficient Planar

Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 982-988. 28.

Lee, J. W.; Kim, D. H.; Kim, H. S.; Seo, S. W.; Cho, S. M.; Park, N. G. Formamidinium and Cesium Hybridization for Photo- and Moisture-Stable Perovskite Solar Cell. Adv. Energy Mater. 2015, 5, 1501310/1-1501310/9.

29.

Niu, G. ; Li, W. ; Li, J. ; Liang, X. ; Wang, L. Enhancement of Thermal Stability for Perovskite Solar Cells through Cesium Doping. RSC Adv. 2017, 7, 17473-17479.

30.

Sherkar, T. S.; Momblona, C.; Gil-Escrig, L.; Ávila, J.; Sessolo, M.; Bolink, H. J.; Koster, L. J. A. Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions. ACS Energy Lett. 2017, 2, 1214-1222.

31.

Naikaew, A. ; Prajongtat, P. ; Lux-Steiner, M. Ch. ; Arunchaiya, M. ; Dittrich, T. Role of Phase Composition for Electronic States in CH3 NH3 PbI3 Prepared from CH3 NH3 I/ PbCl2



Page 24 of 28

24

Solution. Appl. Phys. Lett. 2015, 106, 232104/1-232104/4. 32.

Li, W.; Zhang, W.; Reenen, S. V.; Sutton, R. J.; Fan, J.; Haghighirad, A. A.; Johnston, M. B.; Wang, L. ; Snaith, H. J. Enhanced UV-Light Stability of Planar Heterojunction Perovskite Solar Cells with Caesium Bromide Interface Modification. Energy Environ. Sci. 2016, 9, 490-498.

33.

Leijtens, T. ; Eperon, G. E. ; Pathak, S. ; Abate, A. ; Lee, M. M. ; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized TiO 2 with Meso-Superstructured Organometal Tri-Halide Perovskite Solar Cells. Nat. Commun. 2013, 4, 2885/1-2885/8.

34.

Rujisamphan, N. ; Supasai, T. ; Dittrich, T.

Photoinduced Charge Dissociation and

Transport at P3HT/ ITO Interfaces: Studied by Modulated Surface Spectroscopy. Appl. Phys. A Mater. Sci. Process. 2016, 122:77, 1-6. 35.

Fengler, S. ; Dittrich, T. Algorithm for Random Walk Simulation of Modulated Surface Photovoltage Signals in Nanostructured Systems with Localized States. J. Phys. Chem. C 2016, 120, 17777-17783.

36.

Prajongtat, P. ; Dittrich, T. Precipitation of CH3 NH3 PbCl3 in CH3 NH3 PbI3 and Its Impact on Modulated Charge Separation. J. Phys. Chem. C 2015, 119, 9926-9933.

37.

Osterloh, F. E.; Holmes, M. A.; Zhao, J.; Chang, L.; Kawula, S.; Roehling, J. D.; Moulé, A. J. P3HT: PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy. J. Phys. Chem. C 2014, 118, 14723-14731.

38.

Chen, R. ; Fan, F.; Dittrich, T. ; Li, C. Imaging Photogenerated Charge Carriers on Surfaces and Interfaces of Photocatalysts with Surface Photovoltage Microscopy. Chem. Soc. Rev. 2018, 47, 8238-8262.

39.

Gross, D. ; Susha, A. S. ; Klar, T. A. ; Como, E. D. ; Rogach, A. L. ; Feldmann, J. Charge Separation in Type II Tunneling Structures of Close-packed CdTe and CdSe Nanocrystals. Nano Lett. 2008, 8, 1482-1485.


Page 25 of 28 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


25

40.

Han, Q.; Bae, S. H.; Sun, P.; Hsieh, Y. T.; Yang, Y(M).; Rim, Y. S.; Zhao, H.; Chen, Q.; Shi, W.; Li, G. ; Yang, Y. Single Crystal Formamidinium Lead Iodide ( FAPbI3 ) : Insight into the Structural, Optical, and Electrical Properties. Adv. Mater. 2016, 28, 2253-2258.

41.

Zhang, M.; Yun, J. S.; Ma, Q.; Zheng, J.; Lau, C. F. J., Deng, X.; Kim, J.; Kim, D.; Seidel, J.; Green, M. A. ; Huang, S. ; Ho-Baillie, A. W. Y. High-Efficiency Rubidium-Incorporated Perovskite Solar Cells by Gas Quenching. ACS Energy Lett. 2017, 2, 438-444.

42.

Yi, C.; Luo, J. ; Meloni, S. ; Boziki, A.; Ashari-Astani, N. ; Grätzel, C. ; Zakeeruddin, S. M. ; Röthlisberger, U. ; Grätzel, M. Entropic Stabilization of Mixed A-Cation ABX3 Metal Halide Perovskites for High Performance Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 656-662.

43.

Choi, H.; Jeong, J.; Kim, H. B.; Kim, S.;Walker, B.; Kim, G. H. ; Kim, J. Y. Cesium-Doped Methylammonium Lead Iodide Perovskite Light Absorber for Hybrid Solar Cells. Nano Energy 2014, 7, 80-85.

44.

Bhunia, H. ; Chatterjee, S. ; Pal, A. J. Band Edges of Hybrid Halide Perovskites under the Influence of Mixed-Cation Approach: A Scanning Tunneling Spectroscopic Insight. ACS Appl. Energy Mater. 2018, 1, 4351-4358.

45.

Polavarapu, L. ; Nickel, B. ; Feldmann, J. ; Urban, A. S. Advances in Quantum-Confined Perovskite Nanocrystals for Optoelectronics. Adv Energy Mater. 2017, 7, 1700267/ 11700267/9.

46. Young, J.; Rondinelli, J. M. Octahedral Rotation Preferences in Perovskite Iodides and Bromides. J. Phys. Chem. Lett. 2016, 7, 918═922. 47.

Walsh, A. Principles of Chemical Bonding and Band Gap Engineering in Hybrid OrganicInorganic Halide Perovskites. J. Phys. Chem. C 2015, 119, 5755-5760.

48.

Zhumekenov, A. A. ; Saidaminov, M. I. ; Haque, M. A.; Alarousu, E.; Sarmah, S. P. ; Murali, B. ; Dursun, I. ; Miao, X. H. ; Abdelhady, A. L. ; Wu, T. ; Mohammed, O. F. ; Bakr, O. M.



Page 26 of 28

26

Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length. ACS Energy Lett. 2016, 1, 32-37. 49.

Xie, L. Q.; Chen, L.; Nan, Z. A.; Lin, H. X.; Wang, T.; Zhan, D. P.; Yan, J. W.; Mao, B.W.; Tian, Z. Q. Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals. J. Am. Chem. Soc. 2017, 139, 33203323.

50.

Aharon, S. ; Dymshits, A. ; Rotem, A. ; Etgar, L. Temperature Dependence of

Hole

Conductor Free Formamidinium Lead Iodide Perovskite Based Solar Cells. J. Mater. Chem. A 2015, 3, 9171-9178. 51.

Tang, Z. K. ; Xu, Z. F.; Zhang, D. Y.; Hu, S. X. ; Lau, W. M. ; Liu, L. M. Enhanced Optical Absorption via Cation Doping Hybrid Lead Iodine Perovskites. Sci. Rep. 2017, 7, 7843/ 17843/7.

52.

Liu, N. ; Yam, C. Y. First-Principles Study of Intrinsic Defects in Formamidinium Lead Triiodide Perovskite Solar Cell Absorbers. Phys. Chem. Chem. Phys. 2018, 20, 6800-6804.

53.

Yin, W. J. ; Shi, T.; Yan, Y. Unusual Defect Physics in CH3 NH3 PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903/1-063903/4.

54.

Kim, J. ; Lee, S. H. ; Lee, J. H. ; Hong, K. H. The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2014, 5, 1312-1317.

55.

Frolova, L. A. ; Dremova, N. N. ; Troshin, P. A. The Chemical Origin of the P-Type and NType Doping Effects in the Hybrid Methylammonium-Lead Iodide ( MAPbI3 ) Perovskite Solar Cells. Chem. Commun. 2015, 51, 14917-14920.

56.

Herz, L M.

Charge-Carrier Mobilities in Metal Halide Perovskites:

Fundamental

Mechanisms and Limits. ACS Energy Lett. 2017, 2, 1539-1548. 57.

Somsongkul, V. ; Lang, F. ; Jeong, A. R. ; Rusu, M. ; Arunchaiya, M. ; Dittrich, T. Hole Blocking PbI2 /CH3 NH3 PbI3 Interface. Phys. Status Solidi RRL 2014, 8, 763-766.


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58.

Juma, A. O. ; Azarpira, A.; Steigert, A. Pomaska, M.; Fischer, C. H. ; Lauermann, I.; Dittrich, T. Role of Chlorine in In2 S3 for Band Alignment at Nanoporous-TiO2 / In2 S3 Interfaces. J. Appl. Phys. 2013, 114, 053711/1-053711/5.



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CsPbI3 Interface under UV Illumination - ACS Publications

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