Enhancing photovoltaic performance of inverted perovskite solar cells via imidazole and benzoimidazole doping of PC61BM electron transport layer

Enhancing photovoltaic performance of inverted perovskite solar cells via imidazole and benzoimidazole doping of PC61BM electron transport layer

Journal Pre-proof Enhancing photovoltaic performance of inverted perovskite solar cells via imidazole and benzoimidazole doping of PC61BM electron tra...

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Journal Pre-proof Enhancing photovoltaic performance of inverted perovskite solar cells via imidazole and benzoimidazole doping of PC61BM electron transport layer Yu Wang, Yang Yang, Filip Uhlik, Zdenek Slanina, Dongwei Han, Qifeng Yang, Quan Yuan, Ying Yang, Dong-Ying Zhou, Lai Feng PII:

S1566-1199(19)30600-7

DOI:

https://doi.org/10.1016/j.orgel.2019.105573

Reference:

ORGELE 105573

To appear in:

Organic Electronics

Received Date: 30 August 2019 Revised Date:

11 November 2019

Accepted Date: 21 November 2019

Please cite this article as: Y. Wang, Y. Yang, F. Uhlik, Z. Slanina, D. Han, Q. Yang, Q. Yuan, Y. Yang, D.-Y. Zhou, L. Feng, Enhancing photovoltaic performance of inverted perovskite solar cells via imidazole and benzoimidazole doping of PC61BM electron transport layer, Organic Electronics (2019), doi: https:// doi.org/10.1016/j.orgel.2019.105573. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical Abstract

Enhancing Photovoltaic Performance of Inverted Perovskite Solar Cells via Imidazole and Benzoimidazole Doping of PC61BM Electron Transport Layer

Yu Wang,1 Yang Yang,1 Filip Uhlik,2 Zdenek Slanina,3 Dongwei Han,1 Qifeng Yang,1 Quan Yuan,1 Ying Yang,1 Dong-Ying Zhou,1,* and Lai Feng1,*

1

College of Energy, Soochow Institute for Energy and Materials InnovationS& Key

Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, China 2

Department of Physical and Macromolecular Chemistry, Charles University, Faculty of

Science, 128 43 Praha 2, Czech Republic 3

Department of Chemistry and Biochemistry, University of Arizona, Tucson Arizona

85721−0041, United States

*Corresponding author, E-mail addresses: [email protected]; [email protected]

1

Abstract. Although [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been widely used as electron transport layer (ETL) for inverted perovskite solar cells (PeSCs), it still remains to be improved. Herein, we report that amphoteric imidazole (IZ) and benzimidazole (BIZ) can act as effective and multifunctional dopants for PC61BM-based ETL. The resultant MAPbI3-xCl3-based PeSCs outperform the devices with pristine PC61BM ETL with power conversion efficiency (PCE) significantly improved from 14.38% to 15.62/16.47%. Detailed mechanism studies demonstrate that IZ and BIZ-doping cause remarkable reduction of ETL surface roughness and significant increase in electrical conductivity of ETL, both of which facilitate the electron transport from perovskite (PVK) and Ag cathode. More importantly, IZ and BIZ as dopants may passivate both Lewis-acid and Lewis-base type defects on the PVK surface due to their amphoteric nature. DFT computations further reveal that BIZ is a better Lewis-base and Lewis-acid than IZ due to its larger π-conjugation, which thus makes it a superior dopant over IZ. In addition, the PeSCs with PC61BM:BIZ based ETL display enhanced device stability by retarding the I- anion migration from PVK to Ag cathode.

Keywords: Benzoimidazole, electron transport layer, inverted perovskite solar cell, passivation effect, iodine ions diffusion

2

1. Introduction In the last decade, organic-inorganic hybrid perovskite solar cells (PeSCs)have attracted great attention in both academia and industry due to their rapidly-boosted power conversion efficiency (PCE) (i.e., from 3.8% to over 23%) [1-5] and facilesolution processing [6], which provide accessibility for low-cost, large area and large scale preparation of thin film solar cells.It is noteworthy that the PeSCs with record PCE mainly adopted n-i-p-type structure, in which mesoporous and/or compact-TiO2 were used as electron transport layer (ETL) [1-5]. However, employing TiO2 as ETL may induce a series of problematic issues: (i) TiO2 based ETLs often require a high-temperature processing (~500 °C), which makes them processable only on rigid glass substrate. (ii) The inherent catalytic nature of TiO2 may result in fast decomposition of perovskite (PVK) layer under UV illumination, thus reducing the lifetime of PeSCs [7]. (iii) TiO2-based n-i-p PeSCs exhibit severe current−voltage (J-V) hysteresis probably due to unbalanced hole and electron transport [8]. In contrast, planar p−i−n type PeSCs with comparable PCE allows low-temperature solution processing (<150 °C) and features almost negligible (J-V) hysteresis [9,10], which make them more promising for real applications. For typical p−i−n PeSCs, PC61BM is often used as ETL due to the facile processing and suitable LUMO level matching well with the conductive band of perovskite [11]. Nevertheless, the inherent drawbacks of PC61BM cannot be ignored: (i) Low electrical conductivity of PC61BM, much lower than most of typical HTLs (i.e., PEDOT:PSS and PTAA) and thus resulting in unbalanced carrier transport [12]. 3

(ii) Weak Lewis acidity of PC61BM, which makes it have no advantage in efficiently passivating the inherent ionic defects at the PVK crystals, including the Lewis base and Lewis acid-type defects (i.e., under-coordinated halide or metal ions) [13]. Thus, PC61BM-based ETL still remains to be modified and improved. It has been demonstrated that the post treatment of PVK crystals surfaces with Lewis base molecules such as thiophene and pyridine could efficiently passivate the Pb2+ defects and significantly prolong the photoluminescence lifetimes [14,15]. Probably inspired by this result, Lewis base molecules such as pyridine and 2,6-dimethoxypyridine have been used for doping PC61BM ETL[16,17]. It was found that thus doped PC61BM features not only improved passivation capacity but also higher electrical conductivity, yielding a significant increase in device PCE. Nevertheless, Lewis base doped PC61BM has no advantage in passivating the Lewis base type defects (i.e., halide ions). Thus, simple, effective and multifunctional dopants are still desirable to enhance the performance of PC61BM ETL. Unlike pyridine and its derivatives which are typical Lewis bases, imidazole (IZ) is amphoteric. That is, IZ can function as both a base and as an acid. The basic site is the nitrogen with the lone pair (and not bound to hydrogen), whilethe acidic proton is the one bound to nitrogen. Note that IZ is a strong base, approximately sixty times more basic than pyridine, but a weak acid. Benzimidazole (BIZ) is one of IZ derivatives. As compared to IZ, BIZ features a larger π-conjugation system with richer π-electrons, which makes it better electron-donor and electron-acceptor. Inspired by the unique amphoteric features of IZ and BIZ, in this work, we 4

demonstrate that PC61BM ETL can be efficiently modifiedby doping IZ or BIZ. It is verified that IZ and BIZ indeed interact with not only Lewis acid defects (Pb2+) but also Lewis base defects (I-), thus providing accessibility for more efficient passivation. Moreover, the doped PC61BM layer exhibits higher electrical conductivity as well as reduced surface roughness, which would be beneficial to the electron transport and collection. When applying PC61BM:BIZ and PC61BM:IZ to planar inverted PeSCs, the devices yield the PCEs up to 16.47% and 15.62%, respectively, obviously outperforming the control devices with PCE of ~14.38%. The devices based on PC61BM:BIZ also exhibit superior stability as compared to the control devices. That is, they maintain nearly 95% of their initial PCEs after 400 h storage under N2 atmosphere, comparing to 88% of their initial PCEs for control devices. 2. Experimental Section 2.1 Material preparation Imidazole (>99.0%) and benzimidazole (>98.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. PTAA was obtained from Xi'an Polymer Light Technology Corp. Methylammonium iodide (MAI, 99.5%) was purchased from Shanghai Mater Win New Materials Co., Ltd. PbCl2 (99.999%), PbI2 (99.999%), pyridine and bathocuproine (BCP) were purchased from Sigma-Aldrich. Anhydrous dimethylformamide (99.9%), and chlorobenzene (99.9%) were purchased from Alfa Aesar. PC61BM was manufactured by Solarmer Materials Inc. All materials were used as received without further purification. 2.2 Physical characterizations 5

1

H NMR spectra were recorded on a Bruker AV400 spectrometer. Dimethyl

sulfoxide-d6 was used as solvent and internal standard. Raman spectra were recorded on a Horiba instrument (HR Evolution, 514 and 633 nm excitation laser). The UV−vis absorption spectra were recorded on a Shimadzu UV2600 spectrophotometer. Steady-state photoluminescence (PL) spectra were obtained using a Horiba Fluoromax-4 instrument with an excitation wavelength of 475 nm. The time-resolved PL (TRPL) spectra were measured with Lifespec II (Edinburgh Instrument, U.K.) with picosecond light pulser (Hamamatsu) using a pump light wavelength of 477 nm and probe wavelength of 776 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250 spectrometer equipped with an Al Kα excitation source with photon energy of 1486.6 eV. Atomic force microscopy (AFM) height images were obtained in tapping mode on an MFP-3D-BIO (Asylum Research) instrument. Scanning electron microscopy (SEM) characterizations were performed with a Hitachi SU8010 instrument. The ESR spectra were obtained at room temperature under nitrogen atmosphere using a JEOL JES-X320 instrument. PC61BM solution (10 mg in 500 µL chlorobenzene) with 160 µL pyridine solution containing 1.6 mg BIZ was added to an ESR test tube and then degassed with nitrogen to form films on the tube walls. The electrical conductivities of the PC61BM thin films were measured by using a four point probe setup with a source measurement unit (Keithley 4200). 2.3 Solar cells fabrication and measurements 6

The inverted planar PeSCs were fabricated utilizing previously reported methods[18,19]. Briefly, ITO-coated glass sheets were subsequently washed with deionized water, acetone, isopropanol and finally treated with UVO for 15 min. A PTAA HTL was prepared by spin-coating the PTAA toluene solution (2 mg mL-1) onto ITO at 5000 rpm for 40 s and dried at 100 oC for 10 min. The MAPbI3-xClx PVK layer was prepared by a simple one-step spin-coating method without antisolvent treatment. The PVK precursor solution composed of 1.2 M MAI, 0.36 M PbI2 and 0.22 M PbCl2 was spin-coated onto the substrate at 4000 rpm for 40 s under N2 atmosphere, followed by a thermal annealing at 80 oC for 2 h. For the deposition of ETL, 20 µL chlorobenzene solution of PC61BM (20 mg mL-1) was spin-coated onto the PVK layer at 2000 rpm for 30 s. For preparing doped PC61BM layer, IZ or BIZ additive was dissolved in a pyridine solution with a concentration of 10 mg mL-1 and 1-25 µL such solution was added into the PC61BM solution (500 µL) before spin-coating. The doping ratio of IZ or BIZ is optimized by varying the amount of their pyridine solution (see Figure S1). Then, an ethanol solution of BCP (1.5 mg mL-1) was spin-coated on the ETL layer. Finally, Ag electrodes (ca. 100 nm thick) were deposited under high vacuum (<1.0 × 10−5 Pa) through a shadow mask to define the effective active area of the device (0.04 cm2). The electron-only device was fabricated with a structure of ITO/PC61BM/perovskite/ETL/BCP/Ag. The device stability was measured for the PeSCs stored under N2 atomosphere. Alternatively, white light-emitting diode (WLED; 40 W) was continuously illuminated onto the PeSCs at open-circuit conditions with a diatance of 20 cm under 7

N2 atomosphere. The time-dependant PCE and re;ated parameters were measured under standard contionds. 3.Results and Discussion According to the literature reports [13,20,21], PVK defects mainly include under-coordinated Pb2+ ions and negatively-charged Pb–I antisite defects (PbI3-) or undercoordinated halide ions. The chemical interactions between IZ, BIZ and the PVK defects were verified by means of 1H NMR studies. Figure 1a provides the 1H NMR spectra of the sample of IZ or BIZ with PbI2 with a fixed molar ratio of 1:3. The neat IZ sample shows two major 1H NMR peaks at 7.01 and 7.64 ppm, which are attributed to the protons at 2 and 4, 5-positions (H-2 and H-4, H-5), respectively. With the addition of PbI2, these signals are downfield-shifted by 0.07 and 0.13 ppm to 7.08 and 7.77 ppm, respectively, suggesting an obvious deshielding effect induced by PbI2. We thus propose the formation of coordinate bonding between IZ and Pb2+ ion. As the lone-pair electrons on the N atom of IZ were donated to the d-orbital of Pb2+ ion, the electron density of the IZ ring which induces shielding effect to protons would be remarkably reduced. Similar trend is also observed for the sample of BIZ with PbI2. On the other hand, the signal of H-2 of BIZ is slightly shifted from 8.21 to 8.22 ppm upon PbI2 addition. The less pronounced 1H NMR shift might be attributed to the larger conjugation system of BIZ, which reduces the deshielding effect induced by the coordination between N and Pb2+ ion. Furthermore, the proposed coordination between IZ, BIZ and Pb2+ ion is confirmed by means of Raman analysis. As illustrated in Figure S2, pristine IZ and BIZ display characteristic Raman peaks at 8

1267 and 1271 cm-1, respectively, which are corresponding to their ring breathing vibrations. Upon the PbI2 addition, these characteristic peaks are shifted to lower frequencies of 1253 and 1268 cm-1, respectively, indicative of the coordinate attachment of Pb2+ ion. The chemical interaction between IZ, BIZ and halide ion was evaluated by studying the samples of IZ, BIZ with tetrabutylammonium iodide (TBAI). As shown in Figure 1b, the proton signals corresponding to the H-2 and H-4, H-5 of IZ are both upfield-shifted by ~0.01 ppm, indicative of a slightly enhanced shielding effect on IZ ring probably due to the formation of NH…I- hydrogen bond [22]. As for BIZ, the upfield shift is even less pronounced probably due to its larger π-conjugation.

Figure 1. 1H NMR spectra of the samples of IZ and BIZ with (a) PbI2 and (b) tetrabutylammonium iodide (TBAI).

Table 1. The complexation Gibbs-energy change (△ △Go) at room temperature for the associations of IZ, BIZ with Pb2+ or I- ion. Lewis acid Pb2+

Lewis base IZ BIZ

△Go (kcal/mol) -99.45 -108.89

Lewis base I9

Lewis acid IZ BIZ

△Go (kcal/mol) -11.58 -13.42

Furthermore, the DFT computations were performed at the B3LYP/Def2QZVP level including the BSSE correction (in Gaussian 09 package) [23] to estimate the Gibbs energy change (△Go) of the chemical reaction of IZ or BIZ with Pb2+ ion or halide ion (I-). As illustrated in Table 1, the △Go in the complexation of IZ, BIZ with Pb2+ or I- ion is negative, indicating the complexation process is expected to be practically irreversible. For the complexation with Pb2+ ion, the absolute value of △Go is by 9.44 kcal/mol larger in the case of BIZ, giving that BIZ is a better Lewis base with respect to Pb2+. On the other hand, the complexation of BIZ with I- ion also yields higher absolute value of △Go, thus confirming the expectation that BIZ is a better Lewis acid than IZ. Therefore, we conclude that BIZ is more reactive towards Pb2+ or I- ion than IZ, thus a better dopant for the passivation of perovskite. 2.2. Doping effects of IZ and BIZ on PC61BM film The solution processed PC61BM ETL usually exhibit somewhat rough surface due to the good crystallinity of PC61BM. To elucidate the doping effects of IZ and BIZ on the morphology of PC61BM ETL, various ETLs were deposited on the PVK layer and characterized by means of AFM. As shown in Figure 2a-d, pristine PVK layer shows a rough surface with a root-mean-square roughness (RMS) of 6.06 nm. After coating with PC61BM ETL, the RMS of PVK/ETL film is reduced to 2.14 nm. When replacing PC61BM with PC61BM:IZ or PC61BM:BIZ, the RMS is further reduced to 1.64 and 1.36 nm, respectively. A plausible reason is that the IZ or BIZ doping prohibits the aggregation of PC61BM and hence reduce the surface roughness [24,25]. Important is the reduced roughness of ETL is beneficial for the interfacial 10

contact between PVK and Ag cathode and thus facilitate the electron transport between them.

Intensity (a.u.)

e

3220

PC61BM:BIZ PC61BM:IZ PC61BM

3240 3260 3280 Magnetic Field (G)

3300

Figure 2. (a) AFM height images of pristine PVK film, (b) PVK/PC61BM, (c) PVK/PC61BM:IZ and (d) PVK/PC61BM:BIZ films. (e) ESR spectrum of PC61BM:BIZ, PC61BM:IZ and PC61BM films. The electrical conductivity of doped PC61BM film was evaluated using four-point probe method. It was found that the PC61BM:BIZ film has the highest electrical conductivity of 1.26×10-5 S cm-1, followed by PC61BM:IZ film (6.84×10-6 S cm-1) and pristine PC61BM film (3.35×10-7 S cm-1). In addition, obvious ESR signal with a g value of 2.003 can be detected for PC61BM:BIZ and PC61BM:IZ films but not for PC61BM film (see Figures 2e and S3), indicating the formation of fullerene radical anions upon the BIZ and IZ doping [26,27]. Therefore, we reasonably propose that the BIZ or IZ (in pyridine) dopant causes n-type doping of fullerene and hence results in improved electrical conductivity, which is very similar to those reported for the PC61BM films doped with CTAB and DBU[26,27]. 11

2.3. Photovoltaic Performance of PeSCs

b 25 -2

Current Density (mA cm )

20 15 10 PC61BM:BIZ/RS PC61BM:BIZ/FS PC61BM/RS PC61BM/FS

5 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

12 9

60

15

40

10

20

5

6 3 0 13

14

15

16

17

0 300

Voltage (V)

0 400

500

600

700

800

PCE (%)

80

15

22 19.6 mA cm

20

-2

20

e 22 18

18.3 mA cm

-2

-2

20 18

16

15.6%

16

14

13.7%

14

12

12

PC61BM:BIZ PC61BM

10 0

20

40

@0.80 V

60

80

10

100

Time (s)

Wavelength (nm)

Figure 3. (a) Schematic structure of inverted PeSC; (b) J−V curves and (c) histogram of PCE of PeSCs with and without BIZ doping under 1 sun illumination; (d) EQE spectra of the PeSCs along with the integrated JSC; (e) Steady-state photocurrent and PCE of PeSCs with and without BIZ doping under 1 sun illumination. To investigate the performance of PC61BM:IZ and PC61BM:BIZ ETLs, we fabricate

inverted

planar

PeSCs

with

a

configuration

of

ITO/PTAA/perovskite/ETL/BCP/Ag. (see Figure 3a). As shown in the SEM image (Figure S4), it is clearly seen that the PVK is well crystalized on the PTAA HTL. The cross sectional SEM image of PeSC (Figure S5) illustrates that the PC61BM:BIZ ETL was deposited with a total thickness of ca. 40 nm, which is very comparable to that of pristine PC61BM ETL. The typical current density−voltage (J-V) curves of the PeSCs 12

-2

25 PC61BM:BIZ PC61BM

Integrated Jsc (mA cm )

d100

PC61BM:BIZ PC61BM

18

EQE (%)

-2

Current Density (mA cm )

c 21

Current Density (mA cm )

Voltage (V)

with pristine or doped PC61BM are provided in Figures 3b and S6. For comparison, the devices with pristine PC61BM and pyridine doped PC61BM were employed as the references, as the pyridine was doped along with BIZ and IZ as well.The extracted parameters are summarized in Table 2. It is clearly seen that the only pyridine doping results in slightly improved device performance with the average PCE increasing from 13.58% (for devices with pristine PC61BM) to 14.03%. Nevertheless, IZ and BIZ doping further increases the average PCE to 14.80% and 15.74%, respectively. The champion PCE of 16.47% (with a VOC of 1.03 V, a JSC of 20.44 mA cm−2 and an FF of 78.31%) is achieved for the device with PC61BM:BIZ. It is also noteworthy that the device with PC61BM:BIZ displays almost negligible J-V hysteresis, obviously smaller than that of the device with pristine PC61BM, which might be attributed to the more efficient electron transport from PVK layer to Ag cathode. Table 2. Photovoltaic Parametersa of PeSCs with pristine PC61BM and doped PC61BM. ETL w/o w/Py w/ IZ w/ BIZ

VOC (V) 0.96 (0.98) 0.97 (1.00) 0.98 (1.02) 1.00 (1.03)

JSC (mA cm-2) 19.34 (19.49) 19.50 (19.80) 19.59 (20.14) 20.20 (20.44)

FF (%) 71.80 (73.46) 73.56 (75.58) 74.25 (76.61) 77.90 (78.32)

PCE (%) 13.58 (14.38) 14.03 (14.83) 14.80 (15.62) 15.74 (16.47)

a

Parameters averaged over 30 individual devices. The best parameter is in

bracket. Py: pyridine. To probe the device reproducibility, over 30 independent devices were prepared for each type of PeSCs. The statistical distributions of photovoltaic parameters and PCEs were plotted in Figure S7. The statistical improvements in parameters and PCEs can be observed for the devices with doped PC61BM. To

13

verify the increase in JSC, EQE spectrum of PeSCs with PC61BM:BIZ was measured and compared with that of the device with pristine PC61BM. The EQE enhancement in the spectral region of 350−750 nm is clearly seen for the device with PC61BM:BIZ (Figure 3d). The integrated JSC was calculated to be 19.68 and 18.83 mA/cm-2 for the devices with and without BIZ-doping, respectively. These values are in good agreement with those obtained from J−V tests within 5% error. Furthermore, to verify the J-V scanning results, the photocurrents of the devices with and without BIZ-doping were tracked at the bias of 0.80 V and 0.72 V, respectively (Figure 3e), which suggest stable PCEs of 15.6% and 13.7% can be achieved for at least over 100 s.

b 10

Glass/PVK/PC61BM:BIZ Glass/PVK/PC61BM Glass/PVK

Intensity (a.u.)

Intensity (a.u.)

a

700

750

800

10

10

3

Glass/PVK/PC61BM Glass/PVK/PC61BM:BIZ 2

1

0

850

Wavelength (nm)

20

40 Time (ns)

60

80

Figure 4. (a) Steady-state and (b) time-resolved PL of the PVK, PVK/PC61BM, and PVK/PC61BM:BIZ films. To further elucidate the mechanism behind the improved performance of PC61BM:BIZ ETL, the steady-state and time-resolved photoluminescence (TRPL) were recorded for pristine PVK film as well as the bilayer of PC61BM/ETL. As shown in Figure 4a, the pristine PVK film exhibits a strong PL emission around 775 nm. Meanwhile, remarkably quenched PL emissions

14

are observed for the PVK/PC61BM and PVK/PC61BM:BIZ films, indicating that the photo-induced electron can be efficiently extracted by either PC61BM or PC61BM:BIZ ETL. Furthermore, the TRPL spectra of PVK/PC61BM and PVK/PC61BM:BIZ films are provided in Figure 4b. By fitting the TRPL with a biexponential function, the lifetimes (τ1 and τ2) of the fast decay and slow decay were calculated and listed in Table 3, which may be related to the interfacial defects caused non-radiative recombination and the charge recombination in the radiative channel, respectively [28,29]. Particularly, the PVK/PC61BM:BIZ film yielded τ1/τ2 of 1.49/6.08 ns along with an averaged lifetime (τavg) of 5.42 ns, all shorter than those of PVK/PC61BM film (1.95, 7.09, and 6.47 ns). These results thus indicate more or less reduced non-radiative and radiative recombinations in PeSCs upon the BIZ doping, in good agreement with the enhanced VOC and FF of the device with PC61BM:BIZ ETL. Table 3. Lifetime data extracted from the TRPL spectra by fitting with biexponential decay function. Film τ1(ns) f1 (%) τ2(ns) f2 (%) τavg (ns)a PVK/PC61BM 1.95 PVK/PC61BM:BIZ 1.49 a

33.12 40.66

τavg is calculated from the equation τ avg = ∑

7.09 6.08 Aiτ i 2

∑ Aτ

66.88 59.34

6.47 5.42

[30]

i i

Dark J−V curves were measured for the devices with various ETLs. As illustrated in Figure 5a, PC61BM:BIZ-based device displays reduced current density at the reversed bias relative to that of PC61BM device, suggesting a reduced leakage current upon the BIZ doping. To better understand this result, the trap density(Ntrap) at

15

the PVK/ETL interface was estimated using the space charge limited current (SCLC) medel [31-33].

b 10

PC61BM:BIZ PC61BM

10

0

10

-1

10

-2

10

-3

10

10 10 10 10 10 10

-0.5

0.0

2

c 1.02

PC61BM:BIZ PC61BM

1.01

0.5

Voltage (V)

1.0

1.5

1

PC61BM:BIZ PC61BM

1.00

VTFL=0.18 V 0

Voc (V)

1

10

-1.0

3

-2

-2

Current Density (mA cm )

2

Current Density (mA cm )

a 10

-1

-2

1.138kBT/q

0.99 0.98

1.365kBT/q

0.97

VTFL=0.29 V -3

0.96

-4

0.01

0.95

0.1

Voltage (V)

1

10

100 -2

Light Intensity (mW cm )

Figure 5. (a) Dark J-V of the PeSCs with PC61BM and PC61BM:BIZ. (b) Dark J-V of the electron-only devices with and without BIZ doping, displaying the VTFL kink point behaviour. (c)VOC dependence upon different light intensities for the devices with and without BIZ doping. The dark J-V curves of the electron-only devices are provided in Figure 5b, in which the VTFL can be clearly identified.Accordingly, Ntrap is calculated using the equation of Ntrap=2εε0VTFL/ed2, where ε0, ε and d are the vacuum permittivity, the dielectric constant and thickness of the PVK layer, respectively [34], while e is the elementary charge. The PC61BM:BIZ-based device yields a Ntrap of 2.37×1015 cm-3, much lower than that (4.83×1015 cm-3) of the PC61BM device. This indicates that the trap density at the PVK/ETL interface is significantly reduced when using PC61BM:BIZ as ETL, hence resulting in mitigated interfacial charge recombination. This may well account for the smaller leakage current and higher VOC of PC61BM:BIZ-based PeSC, as compared to that with pristine PC61BM.

16

To further clarify the passivation effect of PC61BM:BIZ ETL, the dependence of VOC upon the light intensities were examined for the PeSC with and without BIZ. As shown in Figure 5c, the device with PC61BM:BIZ exhibits a weaker VOC dependence on the illumination intensity with a slope of 1.138KBT/q, while the control device with pristine PC61BM yields a stronger dependence with a slope of 1.365KBT/q. These results suggest that employing PC61BM:BIZ as ETL resulted in remarkably reduced nonradiative recombination at the PVK/ETL interface, confirming the improved passivation effect of PC61BM upon the BIZ-doping. 2.4. Device Stability The long-term stability of PeSCs was evaluated under N2 atmosphere. The PCE decay versus storage time is plotted in Figure 6a. It is clearly seen that the PCE of the PC61BM device is dropped ca. 17% after 400 h, which is mainly caused by the remarkable decrease in JSC and FF (see Figure S8). In comparison, the degradation of PC61BM:BIZ device is slower with PCE drop of 5% under the same condition. According to the previous reports [35,36], the PeSC degradation under N2 atmosphere is mainly induced by the immigration of I- anions, which causes corrosion of Ag cathode gradually. Nevertheless, it has been approved that some N-containing compounds might be able to capture the immigrated I- anions and thus protect Ag cathode from corrosion. To probe if BIZ dopants interact with immigrated I- anions, XPS was employed to analysis the surface compositions of ETLs and Ag cathodes of the PC61BM and PC61BM:BIZ devices after two-week aging. As shown in the XPS spectra 17

(Figure 6b), the I 3d signals can be detected on both the PC61BM and PC61BM:BIZ surfaces, confirming the immigration of I- anions from PVK to ETL. Particularly, the PC61BM device exhibits an I 3d peak at 619.5 eV, which can be assigned to Ag−I species. In contrast, PC61BM:BIZ device displays an I 3d peak at 618.5 eV, corresponding to the I− ions associated with BIZ. These XPS results thus suggest that the BIZ dopants might be able to capture the mitigated I− ions by associating or complexion with them, hence protecting the Ag cathode from the corrosion of I− ions and improving the long-term stability of PeSCs. In addition, the device stability was probed under continuous irradiation WLED that is often used as light source to avoid UV-induced degradation [37]. As shown in Figure S9, the decay of device PCE is mitigated with the addition of BIZ in PC61BM layer. Particularly, the PCE was dropped to 98% of initial value after 190 h for the device with PC61BM:BIZ, which is higher than that (93% of initial value) of the device without BIZ. The relative device stabilities under light irradiation are fully consistent with those measured under

thermal

aging conditions,

confirming

the

advantage of

PC61BM:BIZ as ETL in improving the device operational stability.

18

using

b

1.0 0.9

Intensity (a.u.)

Normalized PCE (%)

a

0.8 0.7

I 3d

PVK/PC 61 BM /Ag

PVK/PC 61 BM :BIZ/Ag

PC61BM PC61BM:BIZ

0.6 0

100

200

300

400

500

635

630

625

620

615

Binding Energy (eV)

Time (h)

Figure 6. (a) PCE decay of PC61BM and PC61BM:BIZ devices as a function of storage time under N2 atmosphere; (b) High-resolution XPS spectra of I 3d for PC61BM and PC61BM:BIZ devices after two-week aging. 3. Conclusion In summary, we have demonstrated that IZ and BIZ can act as effective and multifunctional dopants for PC61BM-based ETL. Detailed characterizations reveal that this simple doping approach provides reduced surface roughness and higher electrical conductivity of the PC61BM ETL, which improve the electron extraction from PVK to Ag cathode. IZ and BIZ doping also enable efficient passivations of the undercoordinated-ion surface defects (both Lewis acid and Lewis base type defects) on the PVK surface. The DFT computations further predict the superiority of BIZ over IZ as dopant. As a result, the best-performing device based on PC61BM:BIZ achieved a PCE up to 16.47%, much higher than 14.38% of the control devices with pristine PC61BM. More encourage is that PC61BM:BIZ-based devices also display reduced J-V hysteresis and improved device stability. Therefore, the low-cost and effective 19

BIZ can be widely used as dopant for realizing highly stable and efficient PeSCs. Acknowledgements This work is supported in part by the Natural Science Foundation of China (51372158, 51772195 and 61705150), the Six Talent Peaks Project in Jiangsu Province, the Natural Science Foundation of Jiangsu Province (BK20160325) and Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies.

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Highlight

• We report that amphoteric imidazole (IZ) and benzimidazole (BIZ) can act as effective and multifunctional dopants for PC61BM-based electron transport layer (ETL). • IZ and BIZ as dopants may passivate both Lewis-acid and Lewis-base type defects on the perovskite surface due to their amphoteric nature. • The device with PC61BM:BIZ based ETL display enhanced device stability by retarding the I- anion migration from perovskite layer to Ag cathode.

There is no conflict of interests.

Lai Feng 2019-10-17