Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes

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Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes for High-Efficient Perovskite Solar Cells Shan Cong, Guifu Zou, Yanhui Lou, Hao Yang, Ying Su, Jie Zhao, Cheng Zhang, Peipei Ma, Zheng Lu, Hongyou Fan, and Zhifeng Huang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00760 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes for High-Efficient Perovskite Solar Cells Shan Cong1, Guifu Zou1,*, Yanhui Lou1, Hao Yang1, Ying Su1, Jie Zhao1,2,*, Cheng Zhang3, Peipei Ma1, Zheng Lu1, Hongyou Fan4,5,6,*, and Zhifeng Huang2,7,* 1College

of Energy, Soochow Institute for Energy and Materials InnovationS,

and Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou, 215000, China; 2Department

of Physics, Institute of Advanced Materials, State Key Laboratory

of Environmental and Biological Analysis, Hong Kong Baptist University (HKBU), Kowloon Tong, Kowloon, Hong Kong SAR, China; 3School

of Optoelectronic Science and Engineering, Soochow University,

Suzhou, 215000, China; 4Center

for Integrated Nanotechnologies, Sandia National Laboratories,

Albuquerque, NM 87185, United States; 5Chemical

and Biological Engineering, Center for Micro-Engineered Materials,

University of New Mexico, Albuquerque, NM 87122, United States; 6Advanced

Materials Laboratories, Sandia National Laboratories, Albuquerque,

NM 87185, United States 7HKBU

Institute of Research and Continuing Education, Industrialization

Complex Building, Shenzhen Virtual Univ. Park, No. 2 Yuexing 3rd Road, Shenzhen, Guangdong 518000, China *Correspondence and requests for materials should be addressed to G.Z. (email: [email protected]); Z.H. (email: [email protected]); J.Z. (email: [email protected]); H.F. (email: [email protected])

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ABSTRACT

Semiconductor nanomaterials with controlled morphology and architectures are of critical importance for high performance optoelectronic devices. However, fabrication of such nanomaterials on polymer-based flexible electrodes is particularly challenging due to degradation of the flexible electrodes at high temperature. Here we report fabrication of nickel oxide nanopillar arrays (NiOx NaPAs) on a flexible electrode by vapor deposition, which enables high efficient perovskite solar cells (PSCs). The NiOx NaPAs exhibit enhanced light transmittance for light harvesting, prohibit exciton recombination, promote irradiation-generated hole transport and collection, and facilitate the formation of large perovskite grains. These advantageous features result in high efficiency of 20% and 17% for the rigid and flexible PSCs, respectively. Additionally, the NaPAs show no cracking after 500 times bending, consistent with the mechanic simulation results. This robust fabrication opens a new opportunity for fabrication of large area of high performance flexible optoelectronic devices.

KEYWORDS: nickel oxide, nanopillar arrays, hole transporting layer, flexible perovskite solar cells, glancing angle deposition.

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Semiconductor nanomaterials with controlled morphology and architectures are desirable candidates for hole transporting layers (HTL) for high performance perovskite solar cells due to chemical stability, high hole mobility, and facile fabrication1-9. Among these materials, NiOx have been demonstrated for promising HTL because of appropriate energy band related to that of perovskite and high hole mobility10,11. Recently, it is of increasing interest to produce flexible PSCs hybridized with NiOx8,

12-16.

For example, NiOx nanostructured films were coated on flexible

electrodes via room-temperature solution process13, and Najafi et al. optimized the triple cation PSCs hybridized with solution-derived NiOx nanoparticles to obtain high power conversion efficiency (PCE) of 16.6% on flexible substrates8, 15, 17, 18. Despite of the previous efforts, fabrication of nanostructured NiOx array architectures have been desired in optoelectronics as they can effectively reduce reflection loss, suppress recombination dynamics7-9,

19-21,

and release stress and strain upon mechanical

bending to improve mechanical stability of optoelectronic flexible devices17,

22, 23.

However, fabrication of the nanostructured NiOx arrays for hybrid flexible PSCs has been particularly challenging because of no compatibility of polymer-based flexible electrodes to high-temperature (>300 °C) thermal synthesis conditions of NiOx 24-27. Here we report on fabrication of NiOx nanopillars HTL vertically aligned on flexible substrates by using glancing angle deposition (GLAD) at near room temperature for flexible hybrid PSCs. The as-grown NiOx NaPAs effectively reduce light reflection loss, enhance light harvesting by the perovskite layers, and improve hole collection efficiency. Such hybridization of the NiOx HTL leads to high PCE of the flexible PSCs. Finally, the NiOx NaPAs are advantageous for relaxing stress and

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strain upon mechanic bending, to suppress crack nucleation in the flexible PSCs and markedly enhance photovoltaic stability.

Figure 1. Fabrication and characterization of NiOx HTL. (a) Schematic of GLAD of a NiOx compact layer (CL, at a deposition angle α of 0° and angular velocity of substrate rotation φ of 2° s−1) and a nanopillar array (NaPA, at α of 85° and φ of 2° s−1). SEM images: (b, d) top-down view; (c, e) cross-sectional view; (f) tilted view. (b, c) A NiOx CL with a thickness of 50 nm (i.e., 50-CL), deposited on an ITO-coated glass; (d-f) an HTL composed of a NaPA (with a height of 100 nm) deposited on the “50-CL” (i.e., “50-CL+100-NaPA”). (g) (Left) High-resolution TEM image of a NiOx nanopillar with an inset of fast Fourier transform pattern of selected area diffraction; (Right) TEM images of two individual NiOx nanopillars (up), and EDX elemental maps of Ni (middle) and O (down) of the two NiOx nanopillars. Scale bar, 100 nm. (h) XRD spectra of a “50-CL+100-NaPA” NiOx HTL deposited on a glass: the as-deposited HTL (black line), and the HTL annealed at 300 °C for 30 minutes (red line). (i) XPS spectrum: Ni 2p3/2 of a NiOx 50-CL. NiOx HTL was deposited by GLAD on a supporting electrode, including rigid (ITO-coated glasses) and flexible (ITO-coated polyethylene terephthalate, or ITO-coated polyethylene terephthalate, PET) electrodes. During GLAD, electrode temperature (Te) was controlled roughly at room temperature. At φ (the angular velocity of substrate rotation) of 2° s-1, a NiOx compact layer (CL) was deposited first at a deposition angle (α) of 0° with respect to the direction of substrate normal, and subsequently the NiOx NaPA was deposited on the CL at α of 85° (Figure 1a). The

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CL (Figure 1b, c) exhibits nanometer scale roughness at the surface (Figure S1), and the NaPA is composed of individual nanopillars vertically protruding on the CL (Figure 1d-f). The NiOx nanopillars appear to gradually widen in pillar diameter and some neighboring nanopillars contact each other in the upper fractions with their growth (Figure 1g and Figure S2), i.e., the broadening effect due to competitive growth during GLAD induced by the ballistic shadowing effect27. The NiOx nanopillars have crystalline structure with (200) and (111) planes of cubic nickel oxides, and the (200) planes are highly preferential (Figure 1g, h)28. Annealing at 300 °C has negligible effect on the NiOx crystallinity, indicating that the as-deposited NiOx HTL exhibits high crystalline quality favored to produce hybrid PSCs with high PCE. Chemical analysis reveals that the elements Ni and O distribute uniformly in the nanopillars (Figure 1g), and the nanopillar surfaces contain Ni2+ ions (the characteristic of the standard Ni-O octahedral bonding configuration of cubic nickel oxides) and vacancy-induced Ni3+ ions imposing the HTL functionality on the NiOx NaPAs (Figure 1i and Figure S3)29, 30. Elongating deposition duration causes the CL thickness to increase from 10 nm (Figure S1a, e) to 50 nm (Figure 1b, c; Figure S1b, f), 80 nm (Figure S1c, g) and 100 nm (Figure S1d, h), and makes the NaPAs’ height rise from 50 nm (Figure S2a, e) to 100 nm (Figure 1d-f; Figure S2b, f), 150 nm (Figure S2c, g) and 200 nm (Figure S2d, h). It is of fundamental significance to study the optical and electric properties of the NiOx HTL. When a NiOx NaPA with a height of 100 nm was deposited on a 50 nm-thick CL (i.e., “50-CL+100-NaPA”; unless otherwise specified, the NiOx HTL is described in such abbreviation to represent its nanostructure in this work), optical transmittance was enhanced and diffuse reflectance was suppressed in a wavelength range of 400-800 nm (Figure 2a) compared to those of the 50-CL HTL. The optical

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results are in an excellent agreement with finite element simulation performed with COMSOL Multiphysics (Figures S4a-d). For example, at the incident wavelength of 500 nm (Figure 2b) and 570 nm (Figure S4e), the 100-NaPA contributes to confining

Figure 2. Optical and electronic characterizations of the NiOx HTL composed of “50-CL” (a, e, f: in black) and “50-CL+100-NaPA” (a, e, f: in red) deposited on an ITO-coated glass. (a) Ultraviolet (UV)-visible spectra of transmittance and diffuse reflectance. (b) Finite element simulation of electrical field localization in the NiOx “50-CL+100-NaPA” (left) and “50-CL” (right), excited by 500 nm-wavelength light incident into the glasses (marked by the blue arrows). In the simulation the spatial distance between the two NiOx nanopillars is 50 nm, the nanopillar height is 100 nm, and the nanopillar diameter is 50 nm. (c) Space-charge-limited current (SCLC) plot of an Au/NiOx 200-CL/ITO thin film. Deposition of perovskite (FAyMA1-yPbI3-xClx) on the NiOx “50-CL” and “50-CL+100-NaPA”, characterized by (d) SEM top-down images (top: on the “50-CL”; down: on the “50-CL+100-NaPA”), (e) histogram of perovskite grain diameters, and (f) PL spectra excited at a wavelength of 525 nm (blue line: the perovskite deposited on an ITO-coated glass). the light in the nanopillar’s vicinity according to Mie’s theory31,32, resulting in high light transmittance and low diffuse reflectivity that is favored for light absorption by perovskite. Space-charge-limited current (SCLC) of an ITO/NiOx 200-CL/Au diode is measured to evaluate the hole transporting ability of GLAD-fabricated nanocrystalline NiOx (Figure 2c). The work functions of ITO, Au and NiOx are 4.7, 5.1 and 4.6-5.2 eV, respectively (Figure S5). Therefore, such hole-dominated device was evaluated 6


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by SCLC to exhibit the hole mobility of 0.9 cm2 V-1 s-1 in the NiOx film, which is comparable to the previous result33. Furthermore, average grain diameter of the perovskite (FAyMA1-yPbI3-xClx) films increases from 670 nm on the “50-CL” to 864 nm on the “50-CL+100-NaPA” (Figures 2d, e), illuminating that the NaPA is favored for growing the perovskite film composed of large grains. Compared to the NiOx CL, although the 100-NaPA has no evident effect on the perovskite crystallinity (Figure S6), the NaPA makes the PbX2 film more porous with fewer nucleation sites34,35 (Figure S7). The porous PbX2 film can provide many growing spaces and lead to high-quality transformation of perovskite with large grains through interfacial chemical reactions with MAI/FAI sequentially36. Therefore, the NaPAs contribute to enlarging the perovskite’s grain size. Notably, the steady-state photoluminescence (PL) of the perovskite films is markedly quenched by the “50-CL”, and further quenched by adding the “100-NaPA” in the “50-CL” (Figure 2f). Time-resolved PL, typically employed to study the charge recombination process16, was monitored to reveal that the carrier lifetime in the perovskite film on ITO glass is roughly 176 ns, which is markedly reduced to 73 ns by the NiOx “50 CL” and to 56 ns by the NiOx “50-CL+100-NaPA” (Figure S8). It is illustrated that holes optically generated in the perovskite layers can be effectively extracted through the NiOx HTL, especially through the NaPAs with lager interfacial contact surfaces embedded in the perovskite layer37. All these results clearly illuminate that the NiOx HTLs composed of CL and NaPA with high hole mobility are favored for light harvesting in the perovskite films, the formation of the crystalline perovskite films with large grains, and charge carrier collection38. The

rigid

PSCs,

composed

of

glass/ITO/NiOx

HTL/perovskite

(FAyMA1-yPbI3-xClx)/PC61BM/BCP/Ag (Figure 3a), were assembled with high PCE.

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Under light irradiation, the generated electrons and holes in the perovskite layer are subsequently collected by PC61BM and the NiOx HTL, respectively. When the NiOx HTL is only composed of CL with the thickness in a range of 10-100 nm, the rigid PSCs exhibit the

Figure 3. Photovoltaic evaluation of the hybrid PSCs assembled on rigid electrodes made of ITO-coated glasses. (a) Schematic (left) and SEM cross-sectional image (right) of a hybrid PSC composed of glass/ITO/NiOx “50-CL+100-NaPA”/Perovskite (FAyMA1-yPbI3-xClx)/PC61BM/BCP/Ag. (b) Photovoltaic plots of current density versus voltage of the PSCs hybridized with diverse HTLs: “50-CL” (black line), “50-CL+50-NaPA” (red line), “50-CL+100-NaPA” (orange line), “50-CL+150-NaPA” (green line), and “50-CL+200-NaPA” (blue line). (c) Schematic of photogenerated hole transport in the PSCs hybridized with diverse HTLs of different heights of NaPA: “50-CL” (left), “50-CL+100-NaPA” (middle), and “50-CL+200-NaPA” (right). Photovoltaic plots of the champion PSCs hybridized with “50-CL” (in black) and “50-CL+100-NaPA” (in red); And their typical SEM images of NiOx nanostructures are shown into the upper layer). (d) current density versus voltage, and (e) IPCE and integrated JSC versus incident wavelength. Inset in (d): table of summary of the photovoltaic performance of the champion PSCs hybridized with “50-CL” (in black) and “50-CL+100-NaPA” (in red). (f) Plot of VOC versus light intensity of the PSCs hybridized with “50-CL” (in black) and “50-CL+100-NaPA” (in red). (g) PCE histograms of the PSCs hybridized with “50-CL” (in black) and “50-CL+100-NaPA” (in red). 8


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best PCE of 14.58% at the 50 nm thickness (Figure S9 and Table S1). Although the CL thickening has no evident effect on the optical transmittance in the whole spectrum with the wavelength of 350-800 nm (Figure S10a), the PCE maximization at the 50 nm thickness results from that thin CL will not completely cover the rigid electrode to reduce the VOC and FF, while thick CL will increase the series resistance of the PSCs and thus decrease the JSC39. Accordingly, it was on the “50-CL” that the NaPA was deposited to generate the “50-CL+NaPA” HTL, and the NaPA height was controlled in a range of 50-200 nm. It was found that the nanopillar height of 100 nm led to the optimization of the photovoltaic performance with VOC of 1.06 V, JSC of 22.79 mA cm-2, FF of 0.79 and PCE of 19.13% (Figure 3b; Table S1). On the “50-CL”, the growth of the NaPA with increasing the height has a negligible effect on the VOC, makes the JSC larger than that of the “50-CL” at the height 100 nm, and causes the FF and PCE to be superior to those of the “50-CL”. Compared to the “50-CL”, the NaPAs have markedly larger surface area for the perovskite infiltration, which is favored to reduce the exciton recombination rate and increase the JSC19. However, the NaPA elongation above 100 nm causes the obvious self-shadowing-induced broadening effect to make neighboring nanopillars contact each other in the upper fractions (Figure 3c), leading to the suppression of optical transmittance through the NiOx HTL in a wavelength range of 400-550 nm (Figure S10b), the reduction of surface areas for the perovskite infiltration, and consequently the JSC decrease. Furthermore, the steady-state PL was monitored to reveal that the deposition of the NaPA on the CL evidently suppresses the perovskite’s PL, and the NaPA with the 100 nm height gives rise to maximizing the PL quenching (Figure S11). Our finding is in well agreement with the previous report

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that the JSC sensitively depends on the array ordering and pore filling fraction40. The maximized photovoltaic performance of the rigid PSCs hybridized with the “50-CL” HTL has a PCE of 16.63% (VOC of 1.08 V, JSC of 21.94 mA cm-2, and FF = 0.70), and that of the “50-CL+100-NaPA”-hybridized rigid PSCs exhibits a maximized PCE of 20.05% (VOC of 1.08 V, JSC of 23.23 mA cm-2, and FF = 0.80) (Figure 3d). The growth of the “100-NaPA” on the “50-CL” HTL causes the JSC and FF to evidently increase, leading to the enhancement of the photovoltaic conversion. Given that the JSC is proportional to the incident photon-to-current efficiency (IPCE), it is coincident to find that the addition of the “100-NaPA” on the “50-CL” makes the IPCE increase in a wavelength region of 300-850 nm (Figure 3e). IPCE can be calculated by

IPCE  lh  e inj  h inj  cc

(1)

where ηlh is the light harvesting efficiency, ηe-inj and ηh-inj are the electron and hole injection efficiency, respectively, and ηcc is the charge collection efficiency39. Compared to the “50-CL”, the addition of the “100-NaPA” is favored to enhance the light transmittance and optical absorption by the perovskite layer (Figure 2a), quench the exciton recombination (Figure 2e), and guide the hole transport effectively to the collection electrode (Figure 3c), leading to an increase of ηlh, ηh-inj and ηcc, respectively. Hence, the “100-NaPA” comprehensively leads to increasing the IPCE and JSC of the hybrid PSCs. The integrated current density of the hybrid rigid PSCs was evaluated to be 20.66 and 22.05 mA cm-2 for the “50-CL” and “50-CL+100-NaPA” HTLs, respectively (Figure 3e), consistent with the measured JSC values (21.94 mA cm-2 for the “50-CL” and 23.23 mA cm-2 for the “50-CL+100-NaPA”). At 0.94 V where the rigid PSCs hybridized with the “50-CL+100-NaPA” have the maximum power, the steady-state photocurrent and 10


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PCE were measured to be 21.20 mA cm-2 and 19.94%, respectively (Figure S12). These two photovoltaic parameters are roughly equal to those measured by the J-V scanning (Table S1), indicating that the photovoltaic performance evaluated by the J-V scanning is reliable31. When the light ideality factor n is ≈ 2, the Shockley-Read-Hall recombination mechanism describes the carrier recombination process given by

VOC  nkT ln( I ) / q  constant

(2)

where k is Boltzmann constant, T is absolute temperature, I is light intensity, q is electron charge (1.6×10-19 C)16,

41.

The rigid PSCs with the “50-CL” and

“50-CL+100-NaPA” exhibit n of 1.85 and 1.41, respectively, derived from the plot of VOC versus lnI (Figure 3f). The “100-NaPA”-induced decrease in n illustrates the suppression of exciton recombination, accounting for the increase of FF. 70 rigid PSCs containing the “50-CL+100-NaPA” were statistically evaluated to have a PCE of 18.51% ± 0.89% (i.e., average PCE ± standard deviation), superior to those with the “50-CL” having a PCE of 14.39% ± 1.01% (Figure 3g). It is statistically verified that the addition of the “100-NaPA” leads to high PCE. Control of Te at roughly room temperature enables GLAD of NiOx HTLs on an ITO-coated flexible PET uniformly over an area of 10×10 cm2 (Figure 4a), and the NiOx-coated PET is free of cracks at the surface (Figure 4b, c). After manually bent for 500 times at a curvature radius of roughly 10 mm, the flexible electrodes deposited with the “50-CL” appear to exhibit mm-scale cracks at the surfaces (Figure 4d), while those deposited on the “50-CL+100-NaPA” appear to be free of cracks at the surfaces (Figure 4e). During bending process, the spatial distance between two NiOx nanorods is at least 50 nm, which is large enough for better bending cycles. finite element simulation, using AutoCAD, shown in Figure 4f, was performed to

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study the mechanic bending of a PET/ITO/NiOx HTL with a geometric surface area of 10×3 μm2. In the simulation model, the right side was fixed and the left side was bent downwards by 3 μm. The highest stress in the “50-CL” is 3.51 KPa, and that in the “50-CL+100-NaPA” is only 2.88 KPa. The deposition of the “100-NaPA” on the “50-CL” allows to reduce mechanical stress during bending, and gives rise to the homogeneous stress distribution along the bending to effectively prevent the HTL/perovskite interfacial delamination22, which supports the experimental results well.

Figure 4. Mechanic and photovoltaic performance of flexible PSCs hybridized with the NiOx “50-CL” (a-left, b, d, f-top, h-in black, and i-in black) and 12


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“50-CL+100-NaPA” (a-right, c, e, f-down, g-in yellow, h-in red, and i-in red), deposited on ITO-coated PET. (a) Photographs and (b, c) SEM top-down images of the NiOx HTLs deposited on ITO-coated PET without mechanic bending. (d, e) SEM top-down images of the NiOx HTLs deposited on ITO-coated PET after 500 subsequent bending, with bending curvature radius of 10 mm. (f) Finite element simulation of stress distribution in the NiOx HTLs, whose left edge is fixed and right edge is bent downward by 3 μm vertically. (g) Schematic of PET/ITO/NiOx “50-CL+100-NaPA”/Perovskite (FAyMA1-yPbI3-xClx)/PC61BM/BCP/Ag. (h) Photovoltaic plots of current density versus voltage of the champion hybrid flexible PSCs. Inset: table of summary of the photovoltaic performance of the champion flexible PSCs hybridized with “50-CL” (in black) and “50-CL+100-NaPA” (in red). (i) Distribution diagram of multiple measurement results of normalized PCE versus bending cycles of the hybrid flexible PSCs. Given such mechanic stability, we generated the flexible PSCs composed of PET/ITO/“50-CL+100-NaPA”/perovskite/PC61BM/BCP/Ag (Figure 4g), exhibiting the champion PCE of 17.23%, with a VOC of 1.06 V, JSC of 22.23 mA cm-2 and FF of 0.73 (Figure 4h). Without the “100-NaPA”, the flexible hybrid PCEs have the maximum PCE of 14.64%, with a VOC of 1.03 V, JSC of 20.72 mA cm-2 and FF of 0.68. The mechanic bending at a curvature radius of roughly 10 mm makes the flexible PSCs with the “50-CL” exhibit a gradual photovoltaic degradation and retain 56% of their initial PCE value after 500 bending, while that causes the flexible PSCs with the “50-CL+100-NaPA” to degrade much slowly and remain more than 80% of their initial PCE value after 500 bending (Figure 4i). Finite element simulation reveals that the highest stress in the flexible PSCs hybridized with the “50-CL” is 5.24 KPa, which is larger than that of 3.82 KPa in the flexible PSCs with the “50-CL+100-NaPA” (Figure S13). It is illuminated that the addition of the 100-NaPA on the 50-CL release stress during the mechanic bending, accounting for the weak bending-caused photovoltaic degradation. In conclusion, we demonstrate the in-situ growth of NiOx HTLs composed of a CL and NaPA on the flexible electrodes to generate the flexible hybrid PSCs. The NiOx NaPAs reduce light reflection loss, enhance light harvesting in perovskite, 13


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suppress exciton recombination, promote hole transport and collection, and facilitate the formation of large perovskite grains. Repeatedly mechanical bending results in no crack nucleation in the hybrid PSCs. This is likely due to the gaps or porosity between the NaPAs to release stress and strain, leading to good mechanic stability of the photovoltaic performance. In the case without NaPAs, CL has no such gap or porosity, which results in more rigid thin film structure and less mechanical stability. All these features result in high efficiency of 17.23%, which is retained more than 80% after 500 bending. The photovoltaic performances of the flexible PSCs hybridized with diverse NiOx HTLs are summarized in Table S2. The comparison reveals that no usage of the NiOx NaPAs tends to deteriorate the photovoltaic performances, and the results reported in this work show the best photovoltaic performance for the flexible PSCs hybridized with the undoped NiOx HTLs. The doping of NiOx with Cu gives rise to a VOC of 1.10 V, JSC of 21.45 mA cm-2, FF of 0.738 and PCE of 17.41%42, slightly superior to our results. It is indicated that doping the NiOx HTLs is favored for high photovoltaic efficiency. We devise an advanced technique of low-substrate-temperature GLAD generally adapted to in-situ deposit charge carrier transporting layers made of semiconductor NaPAs on flexible electrodes, to significantly enhance optoelectronic performance of flexible devices.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors G.Z. (email: [email protected]); J.Z. (email: [email protected]);

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Z.H. (email:[email protected]); H.F. (email: [email protected]) Author contributions Authors Z. H. and G. Z. conceived the idea. S. C., Y. L., H. Y., and Y. S. characterized the structures of NaPAs, S. C. fabricated and characterized the devices. C. Z. carried out the finite element simulation of the thin films. P. M. and Z. L. performed the AFM and X-ray diffraction measurements. S. C., J. Z., Z. H., and G. Z. discussed the results, analyzed the data, and co-wrote this manuscript. H.F. provided user service/support in materials assembly and chemistry through CINT user project and co-wrote the manuscript. The work was directed by Z. H. and G. Z.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the support from the National Natural Science Foundation of China (21671141, 21504061, 21473149), "973 Program---the National Basic Research Program of China" Special Funds for the Chief Young Scientist (2015CB358600), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions for Optical Engineering in Soochow University, RC-ICRS/15-16/02 (HKBU), Hong Kong Scholars Program (HKSP 2017-065), and HKBU8/CRF/11E. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. We thank support from the Center for Integrated Nanotechnologies (CINT), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-NA-0003525). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, 15


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LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

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TOC Figure

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Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes

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