Unraveling the Passivation Process of PbI2 to Enhance the

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C: Energy Conversion and Storage; Energy and Charge Transport

Unraveling the Passivation Process of PbI2 to Enhance Efficiency of Planar Perovskite Solar Cells Biao Shi, Xin Yao, Fuhua Hou, Sheng Guo, Yucheng Li, Changchun Wei, Yi Ding, Yuelong Li, Ying Zhao, and Xiaodan Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08075 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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The Journal of Physical Chemistry

Fabrication process and XRD of perovskite films 136x76mm (142 x 142 DPI)

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The Journal of Physical Chemistry 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

Phase and morphology of perovskite films with remnant PbI2 146x80mm (150 x 150 DPI)


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Carrier transportation and recombination kinetics in perovskite films 118x94mm (220 x 220 DPI)



Passivation results of PbI2 in PSCs 146x77mm (220 x 220 DPI)


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The schematic of location and passivation process of PbI2 164x111mm (220 x 220 DPI)



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Unraveling the Passivation Process of PbI2 to Enhance Efficiency of Planar Perovskite Solar Cells Biao Shi† a, b, c, d, Xin Yao† a, b, c, d, Fuhua Hou a, b, c, d, Sheng Guo a, b, c, d, Yucheng Li a, b, c, d

, Changchun Wei a, b, c, d, Yi Ding a, b, c, d, Yuelong Li a, b, c, d, Ying Zhao a, b, c, d, Xiaodan Zhang * a, b, c, d

a

Institute of Photoelectronic Thin Film Devices and Technology of Nankai University, Tianjin 300071, P. R. China

b

Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Tianjin 300071, P. R. China c

Key Laboratory of Optical Information Science and Technology of Ministry of Education, Tianjin 300071, P. R. China

d

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

†

These authors contributed equally to the work.

*Corresponding author: Tel.: +86-22-23499304; fax: +86 22-23499304;

E-mail address: [email protected] 1


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ABSTRACT: There appears to be controversial whether remnant PbI2 is beneficial to the performance of perovskite solar cells (PSCs). We have shown that PSCs with residual

PbI2

deposited

by

one-step

anti-solvent

solution

and

two-step

evaporation-solution method both have presented better performance than those w/o excess PbI2. X-ray diffraction with diverse X-ray incident angles combined with scan electron microscopy and secondary ion mass spectrometry is employed to identify the position of remnant PbI2. It reveals that residual PbI2 locates at grain boundaries near perovskite/HTL interface area for the one-step anti-solvent solution method, and two-step evaporation-solution method situates the excess PbI2 at grain boundaries and ETL/perovskite interface. The cell performance implies that grain boundary passivation is beneficial for promoting short-circuit current density while interface passivation is more favorable to enhance open-circuit voltage and fill factor. The revealed passivation process indicate a deep understanding of remnant PbI2 and contribute to the development of PSCs.

2



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INTRODUCTION Perovskite solar cells (PSCs) have attracted widespread attention due to its rapid ascending photoelectric conversion efficiency from certified 15%1 to 22.1%2 in the past 4 years. Perovskite materials are promising solar cell absorbers as they have shown excellent optoelectronic properties such as large charge carrier mobility3, long diffusion length4, high absorption coefficient5 and unique tolerance to structural defects6-7. An uniform and well-crystallized perovskite absorber is indispensable to realize a good performance of PSC8-15. Since T. Supasai et al find remnant PbI2 can passivate trap states and suppress recombination 16, various methods have been utilized to prepare perovskite containing a slight PbI2 impurity, such as post-thermal decomposition of MAPbI317-21, enhancing PbI2 ratio in one-step precursor22-27 and decreasing concentration or spin rate of precursor in two-step solution28-29. On the contrary, many people hold the opinion that pure perovskite without excess PbI2 is more beneficial for efficient PSC30-31. From above, we find a controversial point view on the influence of remnant PbI2 in perovskite film. It is challengeable to find out the root effect of remnant PbI2 for PSCs. Recently, some scholars have put forward the double-edged sword effect of PbI2. Precisely, PbI2 could enhance initial efficiency of PSCs, while accelerate the degradation of devices32-33. And the positive passivation of PbI2 has been proposed17 and

evidenced

by

time-correlated

single

photon

counting22,

femtosecond

time-resolved transient absorption spectroscopy34, small-modulation transient decay35, 3


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scanning electrochemical microscopy measurements36. It is confusing that passivation effect differs with each other in previous literatures, which mainly reflects in enhancement of specific parameters, e.g. the short-circuit current density (JSC)25, 36, or open-circuit voltage (VOC) and fill factor (FF)26-27. In this work, we propose that the passivation process of remnant PbI2 is associated with its location, which is controlled through depositing methods, and identified with X-ray diffraction (XRD) with diverse X-ray incident angles, scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). Based on the enhanced parameters of devices with different PbI2 locations, we innovatively put forward the possible passivation process that can well answer previous questions. With passivation at interface more beneficial for VOC and FF, while grain boundary passivation leading to JSC enhancement, this work is helpful for settling down the confusion of PbI2 passivation process and gives guidance for manufacturing efficient PSCs with special parameters enhancement.

EXPERIMENTAL SECTION Substrate cleaning and HTL preparation. Transparent conductive FTO/Glass (13 Ω/□) films were purchased from Lattice Solar Energy Technology Co. Ltd in Wuhan. FTO/Glass substrates were cleaned firstly with semiconductor industrial cleaner (Huaxing DZ-1, Jinan Xihua technology Co. Ltd) and deionized water in ultrasonic baths for 50 minutes, respectively, and then dried using nitrogen gas. 1 mL chlorobenzene solution containing Spiro-OMeTAD (80 mg), TBP (28.5 µL) and 4



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LiTFSI (17.5 µL) solution (520 mg Li-TFSI in 1 mL acetonitrile) was prepared for the organic hole transporting layer (HTL). Cell 1 fabrication. The SnO2 colloid precursor was obtained from Alfa Aesar (tin(IV) oxide, 15% in H2O colloidal dispersion). The particles were diluted by H2O with volume ratio 1:1. After ultraviolet ozone pretreatment for 10 min, FTO/glass substrates were coated with 80 µL SnO2 diluted solution, and rotated at 5000 r.p.m for 30s, and then baked on a hot plate in ambient air at 150

for 30 min. The

perovskite precursor solution containing mixture of PbI2 (1M), FAI (0.9M), MABr (0.3M) and PbBr2 (0.3M) in anhydrous DMF: DMSO 4:1 (v/v) was spin coated with a two-step program at 1000 and 5000 rpm for 10 and 30 s, respectively. Contents of residual PbI2 were adjusted through changing the ratio of PbI2/FAI with constant FAI. During the second step, 100 µL chlorobenzene was dripped on the spinning substrate 10 s prior to the end of the program. The substrates were then annealed at 120

for

25min in a nitrogen filled glove box. And 20 µL Spiro-OMeTAD solution was spin coated on perovskite film at 6000 rpm for 30 s. Finally, a 100 nm gold layer was thermally evaporated on top of HTL to complete the device. Cell 2 fabrication. Firstly, 12 nm TiO2 film with titanium isopropoxide (TTIP) and oxygen plasma as precursor was deposited onto FTO/glass by atomic layer deposition (ALD) (Model ö SI ALDè Companyö SENTECH). Then, 50 µl of 10 mg/ml PC61BM/chlorobenzene (CB) solution was spin-coated onto TiO2/FTO/Glass substrates at 3500 rpm for 30 s. 150 nm PbI2 was subsequently deposited on 5


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substrates with evaporation. The base pressure of the system was 3×10-6 Torr and the evaporation rate was 2 Å/s. The substrates coated with 150 nm PbI2 were taken into glove box without exposure to air. MAI/ isopropyl alcohol (IPA) solution with various concentrations was loaded on PbI2 film for 1 min before rotating the substrates at 2000 rpm for 40 s, and then annealing the films at 120

for 25 min,

and 70 µl Spiro-OMeTAD solution was spin coated on the perovskite film at 4000 rpm for 60 s. Finally, a 100 nm gold layer was thermally evaporated on top of HTL to complete the device. Film and device characterization. The morphology of perovskite films were characterized using a scanning electron microscope (SEM) (JEOL JSM-6700F). X-ray diffraction (XRD, Rigaku ATX-XRD) patterns of perovskite films were examined using Cu Kα radiation as the radiation source (λ = 1.5405 Å) across a 2θ range of 5° to 70°. Impedance spectroscopy (IS) was recorded by software called NoNa 1.11 remote controlling electrochemical workstation (Metrohm PGSTAT204) at 0 V voltage bias under dark condition with frequency ranged from 1 Hz to 106 Hz. Photoluminescence (PL) and time-resolved photoluminescence (TR-PL) spectroscopy was measured with a PL spectrometer (Edinburgh Instruments, FS5), and a pulsed laser with a wavelength of 475 nm was employed as the excitation source. Secondary ion mass spectrometry (SIMS) measurements was conducted with Adept 1010 manufactured by Physical Elctronics. Photocurrent density-voltage (J-V) curves of solar cells were measured at 25 °C in N2-filled glovebox. Unless specified, bias scan 6



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from 1200 mV to -200 mV firstly (SC-FB) and return back (FB-SC) with a voltage step of 40 mV and delay time 50 ms. A metal mask with a window of 0.1 cm2 was coated on light incident side to define the active area of the cell. A solar simulator (HAL-320, ASAHI SPECTRA Co. Ltd., Japan) with compact xenon light source was used to produce the simulated AM 1.5G irradiation (100 mW/cm2), and the calibration of the light was carried out by a detector (CS-20, ASAHI SPECTRA Co. Ltd., Japan) with silicon reference cell. The external quantum efficiency (EQE) spectral response was taken by QEX10, PV Measurement. RESULTS AND DISCUSSION Location of PbI2. We deposit perovskite films on SnO2 (ETL 1) with one-step anti-solvent solution method (Film 1)22, 37-38 and on PC61BM passivated TiO2 (ETL 2) with two-step evaporation-solution technique (Film 2)15,

31, 39-43

, with fabrication

processes schematically depicted in Figure 1a and c, respectively, and the fabrication steps are detailed in the experimental section in supporting information. Perovskite films with different contents of remnant PbI2 were acquired through modifying the ratio of PbI2/FAI and the MAI concentration (MAI/IPA) in the one-step solution and two-step evaporation-solution method, respectively. With XRD spectra of perovskite films with different contents of remnant PbI2 illustrated in Figure S1a and Figure S2a, PbI2 can be hardly observed with PbI2/FAI=0.85 (Film 1B) and MAI/IPA=32.5 mg/ml (Film 2B) for one and two-step depositing method, respectively. Devices with remnant PbI2 commonly present better performance compared with those without 7


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Figure 1. Fabrication process and XRD of perovskite films. Scheme of one-step anti-solvent solution (a) and two-step evaporation-solution (c) technique for fabricating perovskite films. FIXRD of Film 1 (b) and Film 2 (d) for typical perovskite film with or without remnant PbI2. Film 1 was fabricated with anti-solvent one-step solution technique and Film 2 was fabricated with two-step evaporation-solution technique. We identified film with remnant PbI2 using character A and film without PbI2 using B. excess PbI2 for both two different deposition methods, as demonstrated in Figure S1b and S2b. Films of PbI2/FAI = 1.1 (Film 1A) and MAI/IPA = 27.5 mg/ml (Film 2A) are chosen as the typical perovskite films with excess PbI2, which present the best performance. With XRD spectra of Film 1A, Film 1B, Film 2A and Film 2B fabricated with and w/o remnant PbI2 using different depositing methods listed in Figure 1b and d. How excess PbI2 in perovskite film works is an interesting and confusing topic. In order to probe the passivation process of excess PbI2, where PbI2 situates is necessary 8



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to ascertain. In this paper, grazing incidence X-ray diffraction (GIXRD) with different X-ray incident angles and SEM is employed to investigate the position of residual PbI2. With 2θ = 12.6° and 14.1° corresponding to orientations of PbI2 (001) and perovskite (110), respectively, area ratio of peak at PbI2 (001)/perovskite (110) (R-PbI2/Perovskite) is calculated using Gaussian fitting to evaluate the content of PbI2 in perovskite film44. As depicted in Figure 2a and b, Film 1A and Film 2A present completely opposite results. Figure 2c summarizes that R-PbI2/perovskite decreases from 60.2% to 39.1% in Film 1A whereas increases from 0% to 38.5% in Film 2A with increasing X-ray incident angle, illustrating that the fraction of remnant PbI2 decreases in Film 1A while increases in Film 2A from surface down to perovskite/ETL interface. Moreover, obvious observed ‘white phase’ of PbI218, 45 in Film 1A and none ‘white phase’ of PbI2

9


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Figure 2. Phase and morphology of perovskite films with remnant PbI2. XRD of Film 1A (a) and Film 2A (b) with different X-ray incident angles. (c) The curves of relative ratio of PbI2/Perovskite with different X-ray incident angles. SEM surface morphology for Film 1A (d) and Film 2A (e). on the surface of Film 2A further verify this phenomenon, as demonstrated in Figure 2d and e. Consequently, we can deduce that excess PbI2 in Film 1A scatters at grain boundaries in film surface. While for Film 2A, remnant PbI2 situates near MAPbI3/PC61BM interface. SIMS measurements, a relatively precise elements analysis method, are employed to further verify and probe the location of remnant PbI2, with the element depth profiles shown in Figure S3a and b. Pb and I are the main elements to investigate the position of PbI2. Contents of Pb and I are observed decreased from surface down to

10



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the depth of ~50 nm and then tend to be gentle with depth deepen in Film 1A, as seen from insert of Figure 3. Carrier transportation and recombination kinetics in perovskite films. Steady-state photoluminescence (PL) spectra of Film 1 (a) and Film 2 (c). Time-resolved photoluminescence (TR-PL) spectra of Film 1 (b) and Film 2 (d). The pulsed laser excited the films from surface. Figure S3a, which is different from Film 2A äFigure S3bå, indicating that PbI2 are rich in upper surface of Film 1A. However, in Film 2A, a thin PbI2 layer situates at the interface of perovskite/ETL 2 and there are small and separated grains of PbI2 on the thin PbI2 layer. Passivation effect of PbI2. PL and TR-PL decay measurements are applied to investigate the transport and recombination dynamic of photo-induced carriers in perovskite films associated with remnant PbI2. The excess PbI2 in Film 1A situating at the top side of the film does not influence carrier extraction similar to non-radiative channel46 through ETL/perovskite interface. Compared with film 1B, the PL intensity and TR-PL decay time of film 1A are larger (Figure 3a, b), indicating depressed non-radiative recombination and longer photo-induced carrier lifetime in perovskite bulks, which results from the passivation of PbI2 at grain boundaries. As shown in Figure 3c, the PL intensity of Film 2A is lower than that of Film 2B. In fact, photo-induced carriers were generated on the upper layer of perovskite film when pulsed laser excited the films from top side25. The diminished PL intensity of film 2A 11


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referring to better carriers generated on top side transporting through perovskite bulks into ETL as amorphous organic molecular can hinder the carriers transportation25 and Film 2A contain less organic molecular. Moreover, seen from Figure 3d, the TR-PL decay time of Film 2A are longer than Film 2B though Film 2A is more beneficial for carriers transportation, indicating the PbI2 can efficiently depress the non-radiative recombination in perovskite bulks and prolong the lifetime of carrier.

Figure 4. Passivation results of PbI2 in PSCs. J-V curves of Cell 1 under one standard light density (a) or in dark (b) and Cell 2 under light (d) or in dark (e). Nyquist plots of Cell 1 (c) and Cell 2 (f) in dark with 0 V bias voltage. Cell 1A was PSC with Film 1A as photo-absorber and Cell 1B with Film 1B as photo-absorber. Cell 2A was PSC with Film 2A as photo-absorber and Cell 2B was PSC with Film 2B as photo-absorber.

12



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PSCs with excess PbI2, as shown in Figure 4a, d and Table 1, S1,S2, present increased JSC, VOC and FF, though amplitude is different for cells manufactured with different methods, which will be further discussed below. For cell 1B, the hysteresis of J-V curve is inverted, i.e., Eff. of device from FB-SC is lower than that from SC-FB, which is different from normal hysteresis in PSCs45, 47. Unbalanced photo-induced carrier extraction is one of the main reasons that caused hysteresis48-50. The inverted hysteresis in cell 1B caused by more efficient electron extraction than hole extraction because of Table 1. J-V parameters of representative perovskite solar cells with and without remnant PbI2.

Fabricating Samples technique

One-step anti-solvent solution

Two-step evaporation solution

Scanning JSC VOC direction (mA/cm2) (mV)

Cell 1 w/o PbI2 FB-SC remnant (Cell 1 SC-FB B) Cell 1 with PbI2 FB-SC remnant (Cell 1 SC-FB A) Cell 2 w/o PbI2 FB-SC remnant (Cell 2 SC-FB B) Cell 2 with PbI2 FB-SC remnant (Cell 2 SC-FB A)

Eff. FF (%)

17.8

1100

0.65

12.73

17.9

1100

0.67

13.19

19.5

1150

0.74

16.59

19.5

1150

0.73

16.37

19.7

980

0.55

10.62

19.5

980

0.49

9.36

20.3

1090

0.72

15.93

20.2

1070

0.70

15.13

“FB-SC” refers to the scan direction from JSC to VOC and “SC-FB” from VOC to JSC.

13


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deficient PbI2 contributing to hole accumulating at the perovskite/HTL interface45. The higher turn-on voltage under dark conditions of Cell 1A and 2A illustrated in Figure 4b and e indicates decreased leakage of electricity in ETL/perovskite/HTL, results from the excess PbI2 acting as a charge recombination barrier at ETL/perovskite or perovskite/HTL interface19, 28. Combined with the location of PbI2 discussed above, the remnant PbI2 in Cell 1A depresses recombination at perovskite/HTL interface and in Cell 2A depresses recombination at ETL/perovskite interface. Furthermore, electrochemical impedance spectroscopy (EIS) measurement is also carried out to investigate the recombination process in the devices. Figure 4c and f imply that Cell 1A and 2A have larger high frequency semi-arc, which indicates higher recombination resistance leading to lower recombination rate in cells19, 44, 51. Consequently, suitable excess PbI2 in perovskite film can reduce recombination in PSCs. Moreover, PbI2 in Cell 1A depresses recombination at grain boundaries and perovskite/HTL interface and contributes to transporting holes. In Cell 2A, remnant PbI2 depresses recombination at grain boundaries and ETL/perovskite. Passivation process of PbI2. It is clear that remnant PbI2 in perovskite film can effectively reduce carrier recombination and thus promote device efficiency. From the above, we try to materialize the passivation process of remnant PbI2 in PSCs. Firstly, the location of PbI2 in Film 1A and Film 2A is depicted as Figure 5a and b shown, respectively. Secondly, the corresponding energy level alignments of different passivation situations are demonstrated in Figure 5d, e and f 18, 52-53. As PbI2 is p-type 14



semiconductor27,

45

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with wide bandgap of about 2.36 eV(Figure S4), PbI2 can

contribute to transporting holes if suitable while block charge transfer if too thick25. So, as shown in Figure 5c photo-induced carriers pass into transport layers through the road without PbI2 if PbI2 is thick and hole can pass through the PbI2 if PbI2 is thin enough. For passivation I, the recombination rate is reduced as PbI2 acts as electron blocking layer at ETL/perovskite interface (Figure 5d). Similarly, for passivation II, as illustrated in Figure 5e, PbI2 at perovskite/HTL interface forms energy barriers to prevent recombination of hole in HTL with electron in perovskite and ETL. For passivation III, PbI2 at grain boundaries blocks recombination of photo-induced electron and hole transported from different perovskite grains (Figure 5f). The photo-induced holes are injected into HTL through individual perovskite grain or PbI2 at the top side of perovskite grain without recombination with electrons in another perovskite grain. And the photo-induced electrons are injected into ETL without recombination with holes in another perovskite grain. According to location of remnant PbI2, for Cell 1A, the passivation processes of PbI2 are II and III. For Cell 2A, the passivation processes are I and III. Moreover, we can deduce that the grain boundary passivation is predominant in Cell 1A while interface passivation between Film 2 and ETL dominates in Cell 2A. As detailed in Figure S5, the enhanced amplitude of JSC is larger in Cell 1A while VOC and FF increase more for Cell 2A, and the parameters are from statistical average listed in Table S1, 2. It indicates that grain boundary passivation is more beneficial to increase JSC whereas interface passivation 15


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enhances VOC and FF, most likely because grain boundaries passivation can decrease

Figure 5. The schematic of location and passivation process of PbI2. The scheme of location of remnant PbI2 in Film 1A (a) and Film 2A (b). (c) Transport path of photo-induced carriers and scheme of three possible passivation processes of PbI2 in perovskite film such as ETL/Perovskite interface I, Perovskite/HTL interface II and grain boundaries III passivation. Process scheme for PbI2 passivation I (d), II (e), III (f). Dotted line with red cross curve stands for impossible path and dotted dark blue line stands for the hole transport path if PbI2 is suitable. the loss of photo-induced current and interface passivation can decrease reverse saturation current associated with Voc and leakage of electricity. CONCLUSIONS 16



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In conclusion, we fully confirm that excess residual PbI2 acts to reduce the rate of recombination and therefore is beneficial for the performance of perovskite solar cells. Moreover, we succeed to probe the position of excess PbI2 in perovskite films, which depends on fabrication methods. We have acknowledged that residual PbI2 situates grain boundaries near perovskite/HTL interface area for film manufactured with one-step anti-solvent solution method, with the grain boundary passivation predominant to suppress the recombination process. While excess PbI2 locates at grain boundaries and ETL/perovskite (N/I) interface for film fabricated with two-step evaporation-solution method, and the N/I interface passivation dominates in restraining recombination. Besides, grain boundary passivation is more beneficial for JSC while interface passivation is more efficient for VOC and FF. Our results provide a deep understanding of the effect of remnant PbI2 in perovskite devices, which is meaningful for enhancing special device parameters by introducing special passivation.

ASSOCIATED CONTENT Supporting Information

The following files are available free of charge.

X-ray diffraction spectra and SEM of perovskite films; UV-Vis absorption spectra of PbI2; J-V curves of PSCs; Statistical parameters of PSCs. (PDF)

AUTHOR INFORMATION 17


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*Corresponding author: Tel.: +86-22-23499304; fax: +86 22-23499304;

E-mail address: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors gratefully acknowledge the supports from International Cooperation Projects of the Ministry of Science and Technology (2014DFE60170), National Natural Science Foundation of China (61474065) and (61504069), Tianjin Research Key

Program

of

Application

Foundation

and

Advanced

Technology

(15JCZDJC31300), Key Project in the Science and Technology Pillar Program of Jiangsu Province (BE2014147-3), and the 111 Project (B16027).

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Unraveling the Passivation Process of PbI2 to Enhance the

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