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Polymeric Surface Modification of NiOx Based Inverted Planar Perovskite Solar Cells with Enhanced Performance Yawen Du, Chenguang Xin, Wei Huang, Biao Shi, Yi Ding, Changchun Wei, Ying Zhao, Yuelong Li, and Xiaodan Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04078 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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ACS Sustainable Chemistry & Engineering

Polymeric Surface Modification of NiOx Based Inverted Planar Perovskite Solar Cells with Enhanced Performance Yawen Dua,b,c,d, Chenguang Xin

a,b,c,d,

Wei Huang

a,b,c,d,

Biao Shi

a,b,c,d,

Yi Ding

a,b,c,d,

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

aInstitute

of Photoelectronic Thin Film Devices and Technology of Nankai University, #38

Tongyan Road, Jinnan district, Tianjin 300350, P. R. China bKey

Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, #38 Tongyan

Road, Jinnan district, Tianjin 300350, P. R. China cKey

Laboratory of Optical Information Science and Technology of Ministry of Education, #38

Tongyan Road, Jinnan district, Tianjin 300350, P. R. China dCollaborative

Innovation Center of Chemical Science and Engineering (Tianjin), #94 Weijin

Road, Nankai district, Tianjin 300072, P. R. China

*Corresponding author: Prof. Yuelong Li, [email protected], Tel.: +86-22-23500197; Fax: +86 22-23499304 Prof. Xiaodan Zhang, [email protected], Tel.: +86-22-23499304; Fax: +86 2223499304

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ABSTRACT Interfacial engineering has been considered to be one of the most effective methods for further enhancing the performance of perovskite solar cells. Herein a facile but effective interfacial engineering method of modifying NiOx coated substrate by PTAA was demonstrated for inverted planar perovskite solar cells. The surface modification by polymeric PTAA could effectively tailor the quality of perovskite absorber layer with larger grain size and better crystallinity by controlling wettability of NiOx surface with varied thickness of hydrophobic PTAA. The improvement of Jsc was ascribed to the improved perovskite film quality with reduced trap state densities. Based on the EIS and TRPL analysis, it was also confirmed that PTAA modification significantly facilitated interfacial charge transfer at the interface between perovskite and PTAA/NiOx due to the gradient band alignment. As a consequence, the highest power conversion efficiency of 17.1% together with negligible hysteresis effect was achieved from planar perovskite solar cells with 0.5mg/ml optimal concentration of PTAA.

KEYWORDS: planar perovskite solar cell; interfacial engineering; NiOx; PTAA; wettability


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INTRODUCTION The applications of hybrid organic/inorganic perovskite solar cells (PSCs) are in rapid progress for optoelectronic devices in recent years owing to their adorable advantages, such as long carrier diffusion length, simple solution-based processes, adjustable band structure, efficient light absorption,

etc.[1-3].The power conversion efficiency (PCE) record for

perovskite solar cell has promoted from the original 3.8% to over 23% rapidly in recent years, positioning them as a very appealing candidate for new generation photovoltaic technology.[47]

There are few different structures of PSCs have been investigated, all the main structures can be classified into two categories, which are known as planar and mesoporous devices, the former includes p-i-n and n-i-p architecture.[4, 5, 8-10] In recent years, the p-i-n devices researched by many researchers because of their virtues compared with the traditional n-i-p devices such as the negligible hysteresis effect, low processing temperature, and

potential

tandem

configuration

with

the

conventional

low-bandgap

p-type

photovoltaics.[11-14] In p-i-n type PSCs, PEDOT:PSS is normally used for the hole transport materials (HTMs), while phenyl-C61-butyric acid methyl ester (PCBM) is often used as the electron transport layer. As for the hole transport layer, PEDOT:PSS is not good enough because of its acidity, hygroscopicity, and inability to effectively block electrons. Therefore, some other HTMs which could be used in p-i-n device have been intensively studied recently



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to substitute PEDOT:PSS,[14-16] for instance metal oxides like nickel oxide, or polymer HTMs such as Poly(trial amine) (PTAA) [17-20]. It’s well known that metal oxides have higher carrier mobility and better stability than the organic materials[21-25]. In addition, metal oxides are apt to be processed from corresponding precursor solution or nanoparticles. But there is weak chemical interaction between perovskite films and nickel oxide, which limits the efficiency of the NiOx-based p-i-n PSCs. It is well known that surface modification is a facile and effective way to enhance the contact and transport of carries between perovskite and electron/hole transport layer. As for PTAA, it showed impressive great capability to increase device Voc and survive from N,N-dimethylformamide (DMF) solvent wash, however the higher resistivity of PTAA may reduce charge transport efficiency and decrease FF of these devices. In this work, we employ PTAA to modify the surface of NiOx film in order to improve the connection between NiOx and perovskite, along with the perovskite quality. On the other hand, both NiOx and PTAA have matched band alignment with perovskite to get a higher Voc and faster charge extraction. In consequence, the hole transfer efficiency and perovskite quality were considerably improved through inserting this polymeric interface modification layer (PTAA). The p-i-n PSCs with the device structure of FTO/NiOx/PTAA/Perovskite/PCBM/Au exhibits a conversion efficiency of 17.1% without hysteresis.

EXPERIMENTAL SECTION

Preparation of NiOx precursor solution and NiOx film: To prepare NiOx precursor solution, dissolve Nickel nitrate hexahydrate (Ni(NO3)2·H2O) into ethylene solution to form 0.9M nickel nitrate hexahydrate solution. Prior to using, ultrasonic cleaning the FTO-coated glass


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substrates with industrial cleaning agent, ethanol and deionized water, and then dried. And then the substrates were treated with UV ozone for 20min before spin-coating NiOx precursor solution. Then the solution was spin coating on the glass/FTO substrate at the speed of 4000r.p.m. Subsequently the substrate was post-annealed at 300oC in ambient air for 60min. To form the very thin PTAA layer on the FTO/NiOx, various concentrations of PTAA/toluene (0-1mg/ml) were deposited by spin-coating at 5000r.p.m for 60s and annealed at 100oC for 10min.

Device Fabrication and Characterization: The FA1-xMAxPb(I3-yBry) precursor solution is prepared in a glovebox. The FAPbI3 and MAPbBr3 is mixed in the solvent DMF and DMSO. And the concentration of the Pb2+ is 1.35M. The volume ratio of DMF and DMSO is 4:1 (v/v). The molar ratio of PbI2 (TCI, purity 98%)/PbBr2 (Alfa Aesar, purity 99.999%) is set in 0.85:0.15, and the molar ratio of MABr/PbBr2 is set in 1:1.The solutions was deposited by two step spincoating at 1000rpm 10s and 5000rpm for 50s. When at the last ten second of the second step, dropping the toluene, and substrates were transferred to a heating plate at 120oC and annealing for 10min. Subsequently, the PCBM solution (20mg/mL in chlorobenzene) was spin-coated at 2000rpm for 60s. At last, a 100nm thick Au electrode was deposited by thermal evaporation.

Characterization: The crystal structure was examined by X-ray diffraction (RigaKu ATX-XRD) using Cu Kα radiation (λ = 0.15406 nm) as the radiation source. The surface morphology of samples was measured using a scanning electron microscope (SEM, Jeol JSM-6700F). Film transmittance, reflectance, and absorbance were characterized with a UV-vis-NIR spectrophotometer (Cary 5000, VARIAN). The time-resolved photoluminescence (TRPL)



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spectroscopy was measured with the laser wavelength of 475nm and power of 0.2 mW, respectively (Edinburgh FS5). Device photocurrent density-voltage (J-V) curves are measured with a Keithley 2400 digital source meter under 1 sun illumination. The dark I-V measurement is also performed by this Keithley 2400 digital source meter under dark state to extract the trap state density with basic structure of FTO/HTM/PVK/HTM/Au. The external quantum efficiency (EQE) spectrum was measured with the EQE system (QEX10, PV Measurement Co, Ltd) in the DC mode without any voltage bias. RESULT AND DISCUSSIONS As the reference, the NiOx film were first prepared by a solution method, as Figure S1 shows where X-ray diffraction (XRD) exhibits strong diffraction peak at 62.2o which can be assigned to the (220) plane of NiOx, and also diffraction peaks of Ni(OH)2 residual, which is identical with the reference.[8, 18, 26, 27]


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Fig. 1 Contact angle measurements in (a), (b) and (c), surface SEM images in (d), (e) and (f), and its corresponding histograms of grain size distribution in (g), (h) and (i) of NiOx, PTAA and NiOx/PTAA based films.

Figure 1 displays the contacting angles of water on NiOx, PTAA and NiOx/PTAA surface and the surface scanning electron microscope (SEM) images of NiOx/perovskite (PVK), PTAA/PVK and NiOx/PTAA/PVK. The contact angles on the above three substrates with water in the Fig.1a-c are 42.8°, 92.5° and 85.1° for NiOx, PTAA and NiOx/PTAA, respectively. The measured contacting angles significantly increase on PTAA or PTAA modified NiOx surface compared with bare NiOx surface, representing the reduced wettability of those substrates compared with NiOx surface to water, which is also valid for perovskite solution with polar solvents such as DMF. It was reported that the difference in wettability of substrates



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could effectively tailor the nucleation and growth process of perovskite grains and therefore influence the charge transport/extraction behaviors between perovskite and HTMs.[28-30] As shown in the Fig. 1d, many small grains with size less than 100 nm are distinctly observed in the perovskite film grown on bald NiOx substrate where the average grain size according to the statistical analysis is about 281 nm for this perovskite film. Contrastively, the grain size of perovskite films on PTAA and PTAA/NiOx (as shown in Fig. 1e, 1f) is more uniform and slightly larger than that on NiOx, and the average size is about 340 and 325 nm on PTAA and PTAA/NiOx surface, respectively, although there are more grains distributed with size of 450-600 nm in the PTAA/NiOx sample. This fact is in good agreement with the wettability of surfaces where the hydrophobicity of PTAA or PTAA/NiOx surface tends to reduce the amount of nucleation sites and provides sufficient space for larger grain growth. Except impact the quality of perovskite films, surface wettability also has the possible influence on roughness or morphology of substrates. The SEM and atomic force microscopy (AFM) in Fig. S2 shows the quality of perovskite films, however, based on the measurements of SEM and AFM, the influence of substrates roughness in tailoring the grain size or morphology should be excluded due to almost identical root square roughness of 11 nm for three substrates of NiOx, NiOx/PTAA and PTAA with similar morphology obtained from SEM.


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Fig. 2 X-ray diffraction spectrum of perovskite films grown on NiOx, PTAA and NiOx/PTAA based substrate while the red dot line indicates the residual of PbI2 and the green line indicates the perovskite.

X-ray diffraction (XRD) diagram gains further insight into how the organic interfacial layer PTAA on NiOx could affect the crystallinity of perovskite layer. As shown in the Figure 2, typical characteristic peaks of FA1-xMAxPb(I3-yBry) perovskite present at 13.96 o, 20.05 o, 24.54o, etc, indicating well crystalized nature of perovskite films from three substrates coated with NiOx, PTAA and PTAA/NiOx HTM. At the same time, it is noticed that residual peak (001) of PbI2 at 12.65o only appears from pure PTAA coated substrate, which could be attributed to the fact that smooth and hydrophobic surface of PTAA shortens the retention time of perovskite precursor solution on substrate and further locally prevents the complete conversion of PbI2 to perovskite. It is also observed that the (110) peak intensity of perovskite on NiOx/PTAA substrate behaves relatively stronger than others which reflects the result of better crystallinity of the perovskite layer and more grains distributed with size of 450-600 nm in the PTAA/NiOx sample. To quantitatively evaluate the quality of perovskite formed on these substrates, the trap state



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density (ntrap) was calculated by the Space-Charge-Limited Currents (SCLC) curve which employs the dark current–voltage (I–V) characteristic based on a hole-only device with the structure FTO/HTM/Perovskite/HTM/Au. As shown in Figure 3, it’s obviously observed that the dark I–V curve has three parts. When the applied voltage is higher than the first kink-point voltage, the current shows a fast nonlinear increase (n > 3), which indicates that the injected carriers have filled all the trap states. The definition of the applied voltage at the kink point is trap-filled limit voltage (VTFL), and it is determined by the trap state density[31] 𝑉𝑇𝐹𝐿 =

𝑒𝑛𝑡𝑟𝑎𝑝𝑑2

(1)

2𝜀𝜀0

where ε is the relative dielectric constant perovskite films, d is the thickness of perovskite film, and ε0 is the constant of permittivity in free space. Therefore, the trap state density ntrap is calculated using Equation (1). The calculated trap state density is 1.06 × 1016 cm−3 for perovskite on NiOx, 1.18 × 1016 cm−3 for perovskite on PTAA and 6.58 × 1015 cm−3 for perovskite on NiOx/PTAA substrates, respectively. Therefore, the perovskite layer on NiOx/PTAA substrate has larger average grain size and preferable film quality and crystallinity with a reduced trap density.

Fig. 3. Trap state density measurements by dark I-V curve of a perovskite film grown on NiOx in (a), NiOx/PTAA in (b) and PTAA coated FTO/glass in (c). The basic structure for this measurement is FTO/HTM/PVK/HTM/Au

where

the

structure

is

FTO/NiOx/PVK/PTAA/Au


for

(a),

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FTO/NiOx/PTAA/PVK/PTAA/Au for (b) and FTO/PTAA/PVK/PTAA/Au for (c), respectively.

Figure 4 illustrates the absorption spectra, Nyquist plots, photoluminescence (PL) and timeresolved photoluminescence (TRPL) of films or cells. Compared with almost identical absorption of NiOx/PVK or NiOx/PTAA/PVK films, PTAA/PVK substrate has lower light absorbed in range of 560 to 750 nm which is not desirable for light harvesting and might damnify the short-circuit current density (Jsc). This might also indicate better crystallization of perovskite layer from NiOx or NiOx/PTAA based substrates because the thicknesses of the perovskite layer on the above three substrates are adjacent. As shown in Fig. S3, the reflection in (a) and transmittance spectrum in (b) of perovskite layers on the NiOx/PVK and NiOx/PTAA/PVK substrate are similar while PTAA/PVK substrate presents relatively higher reflectance in the whole spectra range and higher transmittance in the range of 550 to 750 nm, which all indicates that pure PTAA as hole transporting layer might lead to optical loss and further deteriorate the Jsc of cells in the end.

Fig. 4 Absorption spectra (a), Nyquist plots (b), steady-state photoluminescence (c) and time-resolved



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photoluminescence (d) of NiOx, PTAA and NiOx/PTAA based perovskite films or cells.

The Nyquist plots of these device devices present a single semicircle with relatively high characteristic frequencies (>10kHz). The high-frequency semicircle can be associated with the total series resistance (Rs) of the solar cell, and it can influence the interfacial charge transfer resistance (Rct) at the interfaces of perovskite and hole transport layer[23, 32, 33]. By fitting the Nyquist plots equivalent circuit in the inset of Fig. 4b, the Rct for NiOx/PTAA based device is 7.71Ω, which is much smaller than that of NiOx-based device (9.63Ω) and PTAAbased device (11.41Ω). This illustrates that the NiOx/PTAA based device has a more efficient charge transfer process. Meanwhile, the series resistance Rs is almost identical for all the devices (19.23Ω, 19.54Ω, and 19.01Ω for NiOx, PTAA and NiOx/PTAA based devices, respectively, since all the devices have same planar structures, electron transport layers and conducting substrates. Fig. 4c shows the steady-state PL of FTO/PVK, FTO/NiOx/PVK, FTO/PTAA/PVK and FTO/NiOx/PTAA/PVK. It is generally recognized that the lower PL intensity (i.e., higher PL quenching) represent more efficient charge separation (or injection) from the photoactive material to the charge transport layer. PTAA modification on the NiOx/PVK interface remarkably reduces the PL intensity, indicating an enhanced hole extraction capability after inserting the PTAA interface layer. The TRPL measurement was further performed to investigate the interfacial charge transport properties. Fig. 4d indicated PL decay curves of the perovskite on NiOx, PTAA, and NiOx/PTAA substrate, which were further analyzed by a bi-exponential rate law [26, 34, 35]: Y= A1exp (-t/τ1)+A2exp (-t/τ2)+y0


(2)

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Where A1 and A2 are the relative amplitudes and τ1 and τ2 are the lifetimes for the fast and slow recombination, respectively. The long lasting component is due to the recombination occurring in the bulk of the perovskite structures, whereas a fast relaxation component could be attributed to the surface recombination.[5, 30, 36, 37] The decay is mainly on account of trapassisted recombination at defects. According to the fitting results listed in Table S2, PVK/PTAA/NiOx film has the smallest τ1 indicating faster charge extraction at the interface of PVK/PTAA/NiOx than other substrates. PVK/PTAA/NiOx also has the larger τ2 indicating fewer defects, which is consistent to the result of steady-state PL spectra, EIS and the trap state density measurements. This result confirms that the PTAA modification significantly enhanced the hole injection properties. The device architecture is illustrated in Figure S4a with sequential components of glass/FTO/NiOx/PTAA/perovskite/PCBM/BCP/Au, and Fig. S4b shows the corresponding cross-sectional scanning electron microscopy (SEM) images and its band graph in Fig. S4c. The edge level of valence band (5.71ev) in perovskite exhibit a better alignment with the lowest unoccupied

Fig. 5 J-V curves (a) and its corresponding external quantum efficiency spectra with integrated Jsc (b) of NiOx, PTAA and NiOx/PTAA based perovskite cells.



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molecular orbit (LUMO) level of PTAA (5.22ev) than NiOx (5.05ev), potentially facilitating the hole extraction from perovskite to NiOx. Figure 5 demonstrates J-V curves of NiOx-based, PTAA-based and NiOx/PTAA-based cells. It can be obviously seen that NiOx/PTAA-based cells achieve a considerably higher Jsc and open-circuit voltage (Voc), and thus higher PCE. As discussed before, the surface properties of substrates could effectively influence the formation of perovskite layer and further tailor its intrinsic properties for instance morphology and crystallinity. The better crystal quality, smoother morphology, and larger grain size is likely leading to higher Jsc and higher Voc of PSCs. Dark J-V in Fig. S5 also confirmed less recombination in NiOx/PTAA based PSCs and thus higher Voc. As shown in Table 1, devices made with NiOx and PTAA as HTM exhibited generally inferior performance with lower Jsc, Voc, and FF because of unsatisfactory interface characteristics between HTM and perovskite layer. The NiOx/PTAA hole transport layers is prepared by spincoating a diluted PTAA solution with different concentrations (from 0.3mg/ml to 1.0mg/ml) onto NiOx surface. In consequence, the perovskite solar cell based on NiOx/PTAA (with the optimal concentration of 0.5mg/mL) hole transport layer exhibits the highest PCE of 16.7%, which has been improved significantly

Table 1 Summary of photovoltaic properties of NiOx, PTAA and NiOx/PTAA based perovskite solar cells.


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Integrated Jsc (mA/cm2)

Voc (mV)

F.F. (%)

PCE (%)

Rsh

Rs

Sample

Jsc (mA/cm2)

（ Ω（

（ Ω（

NiOx

19.7

940

71.2

13.2

581

4.99

19.7

PTAA

18.8

980

61.3

11.3

577

6.90

18.5

0.3mg/ml PTAA/NiOx

19.8

980

74.4

14.5

988

4.07

-

0.5mg/ml PTAA/NiOx

20.8

1020

78.3

16.7

2267

3.54

20.0

0.7mg/ml PTAA/NiOx

19.8

1020

76.8

15.5

3460

4.01

-

1.0 mg/ml PTAA/NiOx

19.7

980

76.3

14.8

1352

4.24

-

compared with PCE of 13.2% on pure NiOx and PCE of 11.3% on pure PTAA hole transport layer, respectively. The improvement in Jsc and Voc by PTAA modification could be attributed to better perovskite crystallinity, larger grain size and reduced trap state density, which have been confirmed in the abovementioned results. At the same time, better interfacial contact accelerates hole extraction and transport through the interface as well, leading to obvious improvement of Voc and FF. And the FF of the PSCs with NiOx/PTAA has been improved because of higher Rsh and smaller Rs. Figure 6 presents the J-V curves and the external quantum efficiency (EQE) of champion device with NiOx/PTAA HTM with both scanning directions. The device achieves a PCE of 17.1% (Jsc = 21.54mA/cm2, Voc = 1.06V, FF = 0.748) from forward sweep and 16.9% (Jsc = 21.49mA/cm2, Voc = 1.02V, FF = 0.769) from the reverse scan, which demonstrates almost no hysteresis phenomenon as shown in Fig. 6a, indicating efficient charge extraction at the interface of perovskite/HTM. The integrated Jsc value (20.59mA/cm2) obtained from EQE curve in Fig. 6b is consistent with that of J-V measurement.



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CONCLUSIONS To conclude, we demonstrated a facile but effective surface modification method of modifying NiOx

Fig. 6 J-V curves (a) and its corresponding external quantum efficiency spectra with integrated Jsc (b) of perovskite solar cell based on NiOx/PTAA with champion efficiency.

coated substrate by PTAA for inverted planar perovskite solar cells. The surface modification by polymeric PTAA could effectively tailor the grain size as well as the crystallinity of perovskite absorber layer due to controlled wettability of NiOx surface by varying coated amount of hydrophobic PTAA. The improvement in Jsc was ascribed to the improved perovskite crystallinity, larger grain size and enhanced film quality with reduced trap state densities. It was also confirmed by EIS and TRPL that PTAA modification significantly facilitated interfacial charge transfer at the interface of perovskite/PTAA/NiOx. As a result, planar PSCs with 0.5 mg/ml PTAA modified NiOx HTM exhibited the highest PCE of 17.1% together with negligible hysteresis effect.

ASSOCIATED CONTENT


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Supporting Information: Summary of peak position of steady-state PL, fitted result of TRPL and EIS of varied films and devices; XRD pattern of NiOx powder synthesized in this experiment; Reflectance and absorption of NiOx/PVK, PTAA/PVK and NiOx/PTAA/PVK films on FTO/glass; The device architecture with its corresponding cross-sectional SEM images and band diagram.

AUTHOR INFORMATION Corresponding Author

Prof. Yuelong Li, [email protected], Tel.: +86-22-23500197; Fax: +86 22-23499304

Prof. Xiaodan Zhang, [email protected], Tel.: +86-22-23499304; Fax: +86 22-23499304

Author Contributions Y. Li and X. Zhang conceived the idea and supervised the project; Y. Du performed the experiment; C. Xin and W. Huang helped in SEM measurement; B. Shi and Y. Ding helped in XRD analysis; C. Wei and Y. Zhao discussed the project and data; Y. Li and Y. Du wrote the manuscript and discussed with all authors; All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 61474065, 61674084 and 61874167), Fundamental Research Funds for Central Universities of China, the Natural Science Foundation of Tianjin (17JCYBJC41400), Tianjin Research Key Program of Application Foundation and Advanced



Page 18 of 23

Technology (15JCZDJC31300), the Open Fund of the Key Laboratory of Optical Information Science & Technology of Ministry of Education of China (2017KFKT014); and the 111 Project (B16027). REFERENCES

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For Table of Contents Use Only. Synopsis: Interfacial modification by thin PTAA layer on NiOx improves the charge extraction and reduces the defect density of perovskite layer grown on top.


Polymeric Surface Modification of NiOx-Based ... - ACS Publications

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