Ultrasound-spray deposition of multi-walled carbon nanotubes on NiO nanoparticles-embedded perovskite layers for high-performance carbon-based perovskite solar cells

Author’s Accepted Manuscript Ultrasound-spray Deposition of Multi-Walled Carbon Nanotubes on NiO Nanoparticlesembedded Perovskite Layers for High-performance Carbon-based Perovskite Solar Cells Yinglong Yang, Haining Chen, Xiaoli Zheng, Xiangyue Meng, Teng Zhang, Chen Hu, Yang Bai, Shuang Xiao, Shihe Yang

PII: DOI: Reference:

www.elsevier.com/locate/nanoenergy

S2211-2855(17)30680-8 https://doi.org/10.1016/j.nanoen.2017.11.003 NANOEN2304

To appear in: Nano Energy Received date: 20 June 2017 Revised date: 14 September 2017 Accepted date: 2 November 2017 Cite this article as: Yinglong Yang, Haining Chen, Xiaoli Zheng, Xiangyue Meng, Teng Zhang, Chen Hu, Yang Bai, Shuang Xiao and Shihe Yang, Ultrasound-spray Deposition of Multi-Walled Carbon Nanotubes on NiO Nanoparticles-embedded Perovskite Layers for High-performance Carbon-based Perovskite Solar Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.11.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Ultrasound-spray Deposition of Multi-Walled Carbon Nanotubes on NiO Nanoparticles-embedded Perovskite Layers for High-performance Carbon-based Perovskite Solar Cells

Yinglong Yang,a Haining Chen,a, Xiaoli Zheng,a, Xiangyue Meng,a Teng Zhang,a Chen Hu,a Yang Bai,a Shuang Xiao,a Shihe Yang*a a

Department of Chemistry, The Hong Kong University of Science and Technology, Clear

Water Bay, Kowloon, Hong Kong. *E-mail: [email protected]

Abstract Carbon-based perovskite solar cells (C-PSCs) are prized for their simple device structure, high cost-efficiency and super stability. However, the absolute efficiency of C-PSCs is still low due to inappropriate C/perovskite interfaces. Herein, we report a simple ultrasound spray method to deposit pure multi-walled carbon nanotubes (MWCNT) films. When directly deposited on perovskite films, MWCNT could form a seamless contact at the perovskite/carbon interface, thus boosting the power-conversion efficiency (PCE) of C-PSCs to 14.07% as compared to drop cast MWCNT electrodes. In addition, when the perovskite surface was embedded with a sub-monolayer of nickel oxide nanoparticles (NiO NPs) prior to the MWCNT deposition, we achieved a champion efficiency as high

as 15.80%, which is among the highest C-PSCs efficiencies reported to date. Detailed characterizations revealed that by planting NiO NPs into the surface region of CH3NH3PbI3 crystals, the hole extraction efficiency was effectively enhanced and interfacial recombination was reduced. With silicon dioxide (SiO2 NPs) as a control, the NiO NPs layer was found to favorably bend the energy levels at the interface for selective hole extraction, thereby enhancing the overall photovoltaic performance. The combination of ultrasound spray and nanoparticle embedment technologies opens the way to cost-efficient and scalable production of C-PSCs.

Graphical Abstract

The ultrasound spray deposition technique was adopted firstly to deposit the pure multi-walled carbon nanotubes (MWCNT) electrode on pure PbI2 or PbI2 with NiO nanoparticles sub-monolayer substrates. After the conversion reaction from PbI2 to perovskite dipping in the CH3NH3I solution, two different structures of carbon based perovskite solar cells (C-PSCs) were fabricated, with both NiO NPs and/or MWCNT implanted into the surface region of perovskite film. The integrated perovskite/carbon interfaces guaranteed the fast hole extraction and transporting, and reduced the interface nonradiative recombination process, thereby enhancing the overall photovoltaic performance..

Keywords: Carbon; Perovskite solar cells; Dual-embedment; band bending; NiO

1. Introduction The past few years have witnessed an upsurge of perovskite solar cells (PSCs),[1-8] but to bring PSCs to commercialization calls for addressing the issues of efficiency and stability hand in hand.[9-11] Researchers are attempting from different aspects to improve PSCs’ lifetime at high temperature, high humidity and continuous light illumination. For instance, cesium and rubidium were introduced into the constituents of ABX3 to partly replace the organic methylammonium (MA+ or CH3NH3+) or formamidinium (FA+ or H2NCHNH2+), which exhibited much more stable against high humidity or heat.[12, 13] On another front, new and stable hole transporting materials (HTM, such as NiO,[3, 14-17] CuSCN[18]) were

explored

to

replace

unstable

organic

HTM

2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMETAD)[19]. Etgar et al. found that the typical lead organohalide perovskite (CH3NH3PbI3) could simultaneously work as both light harvester and hole conductor.[20] Therefore, PSCs could still work without using HTM, promising to considerably improve the stability of PSCs. And at this point, carbon electrode came on the scene, which has exhibited an

obvious promise in HTM-free PSCs as a hole extraction electrode, because carbon materials are abundant, low-cost, and stable.[21-24] Ku et al. firstly reported a fully printable mesoscopic carbon-based PSCs (C-PSCs) by employing multi-layer porous structure of TiO2/ZrO2/carbon which demonstrated a decent PCE with excellent long-term stability. By dint of much effort, this kind of C-PSCs have achieved the PCE of 13-14%.[25, 26] Recently, Xu et al. enhanced the efficiency to 14.90% by combining the NiO NPs as one layer of the multi-layer porous structure to work as the HTM, to suppress charge recombination and facilitate the hole extraction.[17] Meanwhile, Wei et al. have introduced a new device structure with an associated embedment process, in which the highly porous carbon electrode was first deposited on lead iodide (PbI2) layer at low temperature, followed by chemical conversion in methylammonium iodide (MAI) solution to form C-PSCs.[27-29] However, the carbon electrode in embedment C-PSCs was commonly deposited by the drop casting method, which, if not carefully processed, could result in loose and poor interface contacts with the perovskite layer. Consequently, the efficiency was severely restrained by recombination at the CH3NH3PbI3/MWCNT interface. In order to solve the above issues, we developed a simple, low-cost ultrasound spray method to deposit carbon electrode. For proof of principle, we chose MWCNTs, which have proved to be a promising carbon electrode material, was used as electrode materials because of their suitable work function, good filming ability, high conductivity, and beneficial tubular morphology for securing interface contact.[23, 30] Significantly,

uniform and smooth MWCNT electrodes were formed by the spray method, affording a high power-conversion efficiency (PCE) of 14.07%, which was obviously higher than those by the drop casting method. We further developed a dual embedment technique by embedding a sub-monolayer of NiO NPs to the perovskite surface region prior to the embedding deposition of MWCNTs, achieving a champion efficiency as high as 15.80%, with superior stability. NiO as a wide band gap p-type semiconductor is believed to enhance hole extraction, block electron leakage and reduce the nonradiative recombination.[14] This was indeed verified through our detailed characterizations. 2. Results and discussion 2.1 Carbon cathode fabrication

Figure 1. Schematic illustrating the fabrication of carbon cathode for C-PSCs by the dual-embedment method. 1) NiO NPs embedment by drop casting; 2) Ultrasound spray deposition of MWCNTs; 3) Formation of perovskite in CH3NH3I bath.

Figure 1 schematically depicts the fabrication process of C-PSCs based on the ultrasound spray deposition technique. Firstly, a compact TiO2 (c-TiO2) layer and a mesoporous TiO2 (m-TiO2) layer were sequentially deposited on a fluorine-doped tin oxide (FTO) glass, followed by the deposition of PbI2 layer by spin coating. The ultrasound spray method was then used to deposit MWCNT electrode at 70 oC without using any binders. (For NiO NPs embedded interlayer, the NiO NPs were deposited prior to the MWCNT) Home-made steel masks were used to confine the area size and the shape of MWCNT films. To convert the PbI2 into CH3NH3PbI3, the samples were immersed into CH3NH3I solution in mixed solvents (isopropanol (IPA)/cyclohexane (CYHEX)) with the concentration of 1 mg/ml),[29] completing the whole fabrication process of C-PSCs. In the as-fabricated structure of pure MWCNT based HTM-free C-PSCs (CNT-PSCs) shown in Figure 1, both TiO2 and MWCNT are embedded into the perovskite crystal layer and form a well-contacted sandwich layers structure, guaranteeing the high carrier separating and transporting efficiencies in C-PSCs. The dual-embedment structure of C-PSCs with NiO sub-monolayer (NiO-CNT-PSCs) was formed by first depositing NiO NPs and then the MWCNTs, two-step conversion reaction in CH3NH3I bath making both of them partially implanted into perovskite surface region. This will take advantages of both MWCNT and NiO, without compromising the interface quality. This method is also advantageous for making large-area high-quality MWCNT films in high yields, making it promising for scaling-up and mass production of C-PSCs. As any polymer binder or surfactant would undermine the conductivity and mobility

of the MWCNTs,[31, 32] MWCNTs in this work were simply dispersed in various pure solvents to form inks for the spray process. Various physical parameters such as boiling point, evaporation rate, surface tension and viscosity critically impact on the compactness, completeness and homogeneity of the MWCNT films.[33] Naturally, the MWCNTs tend to bundle together because of their strong attractive interaction.[34] Without the help of surfactants or polymers having similar functions, only solvents with a large surface tension could balance the strong attractive interaction force to open the bundles.[35] On the other hand, solvents that are compatible with CH3NH3PbI3 must have a small dipole moment and thus could not dissolve CH3NH3PbI3.[11]

Table 1. Information of solvents compatible with perovskite. Data from Sigma Aldrich. Vapor

Surface

Density o

b.p. ( C)

Pressure

Viscosity

Dipole

(mPa•s)

moment (D)

tension

(g/ml) (mmHg)

(mN/m)

4-Methyl-2117-118

0.801

19.9

23.6

0.586

2.69

chlorobenzene

132

1.106

12.0

33.5

0.806

1.54

ethyl acetate

76.5-77.5

0.902

93.2

24

0.423

1.78

pentanone

We have tested three solvents in our experiment, which appear to satisfy the above requirements, including ethyl acetate, 4-Methyl-2-pentanone and chlorobenzene (Table 1). After initial screening of the MWCNT concentration, we found that only solutions with

concentrations below 1.5 mg/ml could work well for depositing a smooth MWCNT film with the ultrasound spray method. Therefore, the MWCNT concentrations in these solvents were all fixed at 1 mg/ml. All these solutions could form a pure uniform ink without any precipitation after preparation for at least seven days (Figure S1). However, the SEM images in Figure 2a-d show that very different morphologies arise from the different MWCNT inks. For ethyl acetate (Figure 2a), a quite unideal MWCNT film was obtained with numerous holes and protuberances. Plausibly, ethyl acetate has a quite low boiling point (~77 oC), high vapor pressure (93.2 mmHg) and thus a much higher evaporation rate than other solvents, which may easily result in many gas bubbles in the film during the film formation process. Additionally, the low surface tension (24 mN/m) of ethyl acetate also resulted in ineffective de-bundling of the MWCNTs, as seen from the numerous protuberances. To justify our conjectures, 4-Methyl-2 pentanone with higher boiling point (~117 oC) was introduced. As can be seen from Figure 2b, the spray of MWCNTs dispersed in 4-Methyl-2-pentanone yielded much smoother and more compact films. On the other hand, protuberances could still be seen from the image, which could be ascribed to the solvent’s low surface tension (23.6 mN/m) and viscosity (0.586 mPa•s) with a low de-bundling efficiency. Chlorobenzene is a commonly used solvent for spiro-OMETAD, which is friendly to the perovskite. Besides, both the much higher boiling point of 132 oC and the higher surface tension of 33.5 mN/m are desirable for our system under investigation. So we sprayed the MWCNT ink in chlorobenzene with the concentration of 1 mg/ml on the substrate. As expected, a rather smooth, compact and

homogeneous MWCNT film was formed, without any gas bubbles or aggregations (Figure 2c). We further decreased the concentration to 0.5 mg/ml (Figure 2d), no discernable variation was found on the film, confirming that surface tension indeed has a great impact on de-bundling the MWCNTs, and a concentration of 1 mg/ml is low enough for tearing up the MWCNT bundles in solution.

Figure 2. Top-view SEM images of MWCNT films deposited with the ink solvent of a) ethyl acetate; b) 4-Methyl-2 pentanone; c) chlorobenzene at a concentration of 1 mg/ml; d) chlorobenzene at a concentration of 0.5 mg/ml. Cross sectional SEM images of e) MWCNT film and f) CNT-PSCs. The cross-sectional SEM images of the spray deposited MWCNT film and solar cells with structure of FTO/c-TiO2/m-TiO2/CH3NH3PbI3/MWCNT are shown in Figure 2e and 2f, respectively. From Figure 2e, it can be seen that the MWCNT film is rather compact, uniform and complete. The rough selvedge coming from the sample cutting indicates that the MWCNT film was intertwined and entangled, making them difficult to be sliced

through cleanly. After the MWCNT deposition, the PbI2 could be converted into CH3NH3PbI3 with the two-step chemical reaction embedment method as we reported earlier.[29] (Figure 1). Figure 2f shows that the CH3NH3PbI3/MWCNT interface was rather appressed, with the nanotubes partly embedded into the perovskite film. This good interface is much sought after to ensure efficient interfacial charge extraction. Figure 3a shows the photocurrent density-voltage (J-V) characteristics of MWCNT C-PSCs with the MWCNT electrodes fabricated from different inks measured by reversely scanning from the high to low voltage at a scan rate of 100 mV/s. As indicated, the different photovoltaic performances are also well in register with the film quality of the corresponding electrodes. MWCNT from ethyl acetate gave the poorest performance with a PCE of 1.7% (Jsc of 5.12 mA/cm2, Voc of 0.678 V and FF of 0.48). MWCNT electrode from 4-Methyl-2 pentanone gave a little better photovoltaic performance, with a Jsc of 14.05 mA/cm2, Voc of 0.870 V, FF of 0.43 and PCE of 5.24%. That from chlorobenzene presented the best photovoltaic performance with a PCE of 9.9% (Jsc of 17.02 mA/cm2, Voc of 0.900 V and FF of 64.7%). These performance results convincingly demonstrate that the MWCNT electrode film quality has a large impact on the carrier extraction at the perovskite/MWCNT interface and consequently the final photovoltaic efficiency. To further improve the solar cell performance, the MWCNT film thickness was then optimized. The sheet resistance of the MWCNT films as a function of the thickness is presented in Figure 3b. Clearly, the sheet resistance decreases with film thickness,

indicating that when the MWCNT film was very thin, the conductivity was rather poor. The conductivity was enhanced as film thickness increased, which will favor carrier transport in the electrode. The C-PSCs based on the MWCNT electrodes with different thicknesses were constructed for performance evaluation and optimization. As shown in Figure S2, at the thickness of 5 µm, all the photovoltaic parameters were very low, with a Voc of 0.750 V, Jsc of 15.50 mA/cm2, FF of 60% and a low PCE of 8%, with a rather wide distribution. When the thickness was increased to 10 µm, all the photovoltaic parameters were improved synchronously, but the PCE distribution was still wide. And after thickness was further increased to 15 µm, a champion PCE as high as 14.07% was achieved, with the Voc of 0.899 V, Jsc of 21.03 mA/cm2 and FF of 74.4%. (Figure 3d) The distribution was also quite slim. Further increasing the thickness to 20 µm, both Voc and FF decreased slightly. This indicates that the performance was influenced by the MWCNT film thickness in a large extent. Too thin MWCNT film generated poorer photovoltaic performance because of the low conductivity. The larger MWCNT film thickness would enhance the conductivity of the electrodes and make the electrode more efficient for carrier transporting,[36] thus promoting the device performance. However, too thick MWCNT electrodes would lead to a far distance between the electrode surface and the perovskite layer. The carrier depletion and the electrode conductivity enhancement reached to a balance.[37, 38] As a result, the far-end MWCNT layer showed small contribution to the cell performances. We also notice that FF distribution didn’t change seriously with the varying electrode thickness, which certified the strength of the perfect

perovskite/MWCNT interface as indicated in Figure 2f.

Figure 3. a) J-V curves of C-PSCs from different MWCNT ink solvent of ethyl acetate (black), 4-methyl-2-pentanone (red), chlorobenzene (blue), scan rate 100 mV/s; b) sheet resistance (Ω/sq) of the MWCNT films as a function of thickness (µm); c) schematic diagram and optical photographs of drop-casting (above) and spray (below) deposition processes; d) J-V curves of champion solar cell fabricated by spray method or drop cast method at forward and reverse scanning directions, scan rate 100 mV/s. To check the uniqueness of spray process, drop-casting method was also used for MWCNT deposition as control samples, with the method we reported in our previous work.[28] As could be seen, drop-casted MWCNT films only show good quality in small

area, but numerous cracks, holes and protuberances could be seen in large scale. (Figure 3c, above) What is worse, parts of the films had already peeled off, making the films useless for working as the back electrodes in C-PSCs. The reason for such a difference in film quality with spray process is suggested here. The MWCNT ink in drop casting process actually has a rather high concentration (10 mg/ml) for the efficient deposition, numerous MWCNT bundles could not be well opened in the paste (Figure 3c), which could introduce protuberances and holes. For large area, bundles made the adhesion force with the substrate rather weak and leaded to the exfoliation during the solvent evaporation. For spray deposition (Figure 3c, below), because of the high solvent surface tension balanced the attractive interaction force between single MWCNTs, bundles in MWCNT ink dispersed in chlorobenzene with the much lower concentration were opened completely. Single nanotubes were sprayed from the nozzle and stacked on the substrate one by one. The nanotube with the diameter around 10 nm would catch every balustrade of the PbI2 substrate (Figure S3) one by one to get the rather strong adhesion force between the substrate and the electrode. As a result, the spray deposited MWCNT films were almost in the same quality at large scale, which was rather smooth and compact, no cracks, sags or crests. More importantly, spray deposited MWCNT films have strong adhesion force to prevent from peeling off the substrate in the perovskite conversion process, guaranteeing the interface quality and interfacial charge transport efficiency. J-V curves of the champion C-PSCs fabricated by spray method or drop cast method were measured at forward and reverse scanning directions, respectively. (Figure 3d) In

reverse scanning, spray MWCNT electrodes had the PCE up to 14.07%, resulting from the Jsc of 21.03 mA/cm2, Voc of 0.899 V and FF as high as 74.4%. Well the drop-casting MWCNT electrode only achieved a PCE of 12.08%, with a Voc of 0.88 V, Jsc of 19.31 mA/cm2 and FF of 71.2%. Therefore, the spray MWCNT electrode has well improved the PCE by 16.47% compared with drop cast MWCNT electrode. For the forward scanning, both cells exhibited rather small hysteresis, which should be attributed to the good carrier transportation in the CH3NH3PbI3/MWCNT interface by the embedment technology.[28] In addition, thick TiO2 mesoporous layer could also contribute to the electron separation on the other direction, which also help to reduce the hysteresis.[39, 40] The small hysteresis had already been confirmed in our previous work.[21, 28] We also prepared larger scale cells with the active area of 1 cm2 and the performance is shown in Figure S4. As could be seen, the sprayed MWCNT C-PSCs also showed its great superiority over the drop cast device in large scale. The PCE of 10.33%, resulting from the Voc of 0.930 V, Jsc of 19.25 mA/cm2 and FF of 57.7%, was achieved by spray MWCNT. While drop cast MWCNT C-PSCs only gave the PCE of 5.13% in 1 cm2 active area, with a Voc of 0.870 V, Jsc of 14 mA/cm2 and FF of 42.1%. 2.2 Effects of NiO NPs embedment in the spray fabricated C-PSCs devices To further improve the efficiency, NiO NPs synthesized by the hydrothermal method was deposited prior to MWCNT on the PbI2 substrate. With the two-step conversion reaction in CH3NH3I bath, we developed the embedment technology in our previous work[28] into this dual-embedment method, forming the C-PSCs devices with the

structure

of

TiO2/perovskite/NiO-MWCNT.

Indicated

from

Figure

1,

in

the

NiO-CNT-PSC structure, sub-monolayer NiO NPs and MWCNT were both implanted into the perovskite crystals, with the MWCNT sticking in a pin wherever there's room of NiO NPs sub-monolayer, forming the integrated hole transport highway! The NiO NPs sub-monolayer embedded CH3NH3PbI3 film is shown in Figure 4a, indicating the incomplete coverage of the implanted NiO NPs. The corresponding cross-sectional SEM image of the cell is shown in Figure 4c, showing both MWCNT and NiO NPs were implanted into perovskite crystals, the same as shown in the schematic diagram of Figure 1e. Finally, with the help of this embedded NiO sub-monolayer, the PCE of C-PSCs with ultrasound sprayed MWCNT electrode was further promoted to exceeding 15%!

Figure 4. Top view SEM image of a) NiO NPs b) SiO2 NPs embedded in CH3NH3PbI3; c) cross-sectional SEM image of NiO-CNT-PSCs; d) J-V curves of C-PSCs with different NiO NPs sub-monolayer coverage, scan rate 100 mV/s. The coverage percentage of NiO NPs sub-monolayer had a great effect on the photovoltaic performance. As shown in Figure 4d, deposition times of the as prepared NiO NPs solution on PbI2 substrate were applied here to control the coverage percentage. One-time deposition resulted in rather a low coverage NiO NPs film, two times for the medium and three times for the high coverage. J-V curves also showed very different parameters. With it increased from low coverage to the medium one, both of the Voc and Jsc were enhanced, and the best PCE was 15.38%, resulting from a Voc of 0.903 V, Jsc of 22.38 mA/cm2, FF of 0.76. However, when the NiO coverage was too high, even though the Voc increased continuously, the Jsc decreased a lot. Considering all of the factors in the experiment, when too thick NiO NPs was deposited prior on PbI2, there were several drawbacks. Firstly, too thick a NiO NPs layer could influence the interaction of MAI and PbI2. Color changing from yellow to dark brown was much slower when too thick NiO was loaded. Secondly, too thick a NiO NPs layer could also reduce the contact between perovskite and MWCNT film, resulting in much poorer charge transport and reduced Jsc. Thirdly, too thick a NiO NPs layer could weaken the adhesive force between MWCNT and PbI2 substrate, since too thick NiO NPs are actually in a multi-layer and loose structure. Finally, too thick a NiO NPs layer could also increase the cell’s series resistance, again leading to a reduced Jsc. Nevertheless, the sub-monolayer of NiO NPs with

MWCNT implanted into perovskite crystals synchronously would not only remove all the drawbacks but also promote synergistic effect to boost the cell efficiency. To uncover the key functions of NiO NPs, SiO2 nanoparticles was used as a control (Figure 4b). SiO2 was totally insulated, without semiconductor property like NiO. Figure 5a

shows

the

photovoltaic

TiO2/MAPbI3/MWCNT,

parameters

of

devices

TiO2/MAPbI3/NiO-MWCNT,

with

different

structure:

TiO2/MAPbI3/SiO2-MWCNT,

named device A, B, C, respectively. Compared with device A of pure MWCNT electrode, the addition of NiO NPs sub-monolayer mainly promoted the current density from 21.03 mA/cm2 to 22.38 mA/cm2, and the final efficiency was increased from 14.07% to 15.38%, the Voc and FF also increased slightly. However, the addition of SiO2 interlayer resulted in all the photovoltaic parameters decreasing, showing the Voc of 0.885 V, Jsc of 19.60 mA/cm2, FF of 0.70 and the reduced PCE of 12.14%. (Table 2) Table 2. Photovoltaic parameters obtained from the MWCNT -based PSCs.

a)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Device A

0.899

21.03

74.4

14.07

Device B

0.903

22.38

76.1

15.38

Device C

0.885

19.60

70.0

12.14

Device A: without interlayer;

b)

Device B: with NiO NPs interlayer;

c)

Device C: with

SiO2 NPs interlayer. The incident photon-to-electron conversion efficiency (IPCE) spectrum (Figure 5b) plotted as a function of the wavelength of the incident light, between 365 nm to 850 nm,

was here to certify the above J-V test results. The IPCE value of device B is higher than that of device A in almost the whole wavelength region, especially in a longer wavelength from 650 nm to 700 nm. The integrated Jsc are calculated to be 22.10 mA/cm2 and 20.01 mA/cm2, respectively. On the contrary, the addition of SiO2 NPs interlayer leads to the decrease of IPCE value of device C, especially in the shorter wavelength region and the lower integrated Jsc is 19.37 mA/cm2. Thus, photocurrents obtained from the IPCE data are consistent with the J-V curves.

Figure 5. a) J-V curves of C-PSCs with different devices, scan rate 100 mV/s; b) IPCE spectra; c) Nyquist plots of different devices in light with bias at 0.2 V over the frequency range of 500 mHz to 2 MHz; and (d) Steady state photoluminescence (PL) spectra of different CH3NH3PbI3 films: Pure CH3NH3PbI3; CH3NH3PbI3 with MWCNT electrode;

CH3NH3PbI3 with embedded NiO NPs sub-monolayer and MWCNT. We further conducted the electrochemical impedance spectroscopy (EIS) to examine the electrical characteristics in TiO2/MAPbI3/MWCNT, TiO2/MAPbI3/NiO-MWCNT and TiO2/MAPbI3/SiO2-MWCNT devices. Figure S6 shows the typical Bode plots for these three types of devices under illumination at a bias of 0.2 V over the frequency of 500 mHz to 2 MHz. Generally, three peaks appearing at high, medium and low frequency can be observed in the EIS spectrum of a perovskite solar cell. The higher frequency features of the impedance spectra in this kind of devices are commonly attributed to the charge transfer resistance (Rct), whereas the medium features contain information about the charge recombination resistance (Rrec) and the lower frequency feature is related to slow dynamics and hysteresis processes in the perovskite.[41, 42] For devices A and B, in the low applied voltage, results of analysis show three peaks in the Bode plots (Figure S6) and three separated arcs in the Nyquist diagram (Figure 5c). The calculated results of Bode plots show that the Rct for device A is 167.6 Ω, with the Rrec of 350.6 Ω. For the device B, Rct is 146.3 Ω and Rrec is 666.8 Ω. Notably, the introduced NiO NPs sub-monolayer didn’t affect the Rct, much and in fact, it is even lower in device B, suggesting an even faster charge transfer process in device B. On the other hand, the Rrec of device B is much higher than that of device A. This result is important, as it illustrates that the electron leakage was blocked efficiently and recombination process was reduced effectively in device B by the NiO NPs sub-monolayer. The energy alignment for the different devices are shown in Figure 8 and Figure S5. We will discuss that in detail later.

For device C, which adopted the SiO2 as the interlayer, we just observed one arc at high frequency and a linear curve from the medium to lower frequency regions. The Rct from higher frequency is 1619 Ω, which is much higher than other devices, proving that charge transfer process in device C is much slower, which is consistent with the decreased Jsc in Figure 5a. The insulating SiO2 layer influenced the charge transfer process heavily in this kind of C-PSCs. For the linear curve, that usually could be seen for perovskite solar cell with the insulating scaffold.[43] We believe it belongs to dielectric properties of SiO2. A device D (NiO-PSCs) with the structure of TiO2/MAPbI3/NiO was fabricated as a control

for

comparison

with

devices

A

(TiO2/MAPbI3/MWCNT)

and

B

(TiO2/MAPbI3/NiO-MWCNT). The structure layout is shown in Figure S7a, with another FTO glass serving as the back electrode for the measurements. It is shown in the J-V curve (Figure S7b) that without the MWCNT as the electrode, the photovoltaic performance is rather poor. Compared with devices A and B (Figure 5a), all of the performance parameters decreased sharply, with the Jsc of less than 0.08 mA/cm2, Voc of 0.179 V and FF of 0.31. To find out the reasons behind it, EIS measurement was conducted under the same conditions as for devices A and B, under illumination at a bias of 0.04 V (also 2/9 of the open-circuit voltage) over the frequency range of 500 mHz to 2 MHz. (Figure S7c-d) Only one arc was observed in the high frequency region in the Bode plots for device D. As we discussed above, this arc should be associated with the charge transfer process. The Rct is calculated to be 1769 Ω, and it is much larger than those of device A (167.6 Ω) and B (146.3 Ω). This certified that the MWCNT electrode is

indispensable to C-PSCs for the efficient charge transfer. In contrast, when using only the embedded NiO sub-monolayer without the implanted MWCNT electrode, the photocarriers could not be collected efficiently. In order to investigate the charge extraction efficiency of the dual embedded NiO NPs with MWCNT in the surface region of perovskite, the steady state photoluminescence (PL) spectra (excited at 514 nm) were measured and are shown in Figure 5d. For this experiment, we deposited the thin CH3NH3PbI3 film on quartz glass substrates, followed by implanting the NiO NPs and/or pure MWCNT layers. The excitation laser beam was directed to the perovskite/MWCNT interface region to induce light emission. As shown in Figure 5d, the quenching degree of the perovskite emission with both the implanted NiO NPs sub-monolayer and the MWCNT layer is greater than that with the MWCNT layer alone, proving that the embedded NiO NPs sub-monolayer could enhance the hole extraction efficiency. We further explored the impact of annealing temperature of NiO NPs on the cell performance. The temperature rising rate was controlled to be 1 oC/min to avoid the sintering process. Figure S8 shows the typical X-ray diffraction (XRD) patterns of the as-prepared products at different annealing temperatures from 300 oC to 600 oC, respectively. All the diffraction peaks in this pattern can be indexed to the pure phase of face-centered cubic NiO, which matches well with JCPDS card no. 47-1049. Higher annealing temperature helps the better crystallization of NiO. The slightly narrowing of half peak width indicates the slightly high-temperature induced aggregation because of the

nature of NiO. Figure 6b depicts the ultraviolet photoelectron spectra (UPS) (He I, 21.22 eV) of the NiO NPs treated at different temperatures. With the valance band onset (Ei) and secondary electron cut-off (Ecut-off) energy analysis, the valence band maximum (VBM) energies of the NiO NPs, defined as VBM =21.22-  (Ecut-off − Ei), were calculated. After annealing at different temperatures of 300 oC, 400 oC, 500 oC, 600 oC, the VBM values were calculated to be 5.18 eV, 5.35 eV, 5.37 eV and 5.50 eV relative to the vacuum energy level, respectively. The fermi level down shifted from 4.52 eV at 300 oC to 4.77 eV at 600 oC, indicating the enhanced p-type characteristics when the NiO NPs were annealed at higher temperature. Actually, NiO reacted easily with oxygen in air and generated Ni2O3 when the heating temperature was below 400 oC.[44] The excess oxygen in Ni2O3 offers more electrons and the fermi level shifts up. With the annealing temperature increasing, Ni3+ in the form of Ni2O3 was reduced to NiO, and the reaction completed when the temperature reached 600 oC. It was the reduced oxygen in NiOx that caused the downward shift of the fermi level and VBM, and this is tied in with the UPS results discussed above. As the pure CH3NH3PbI3 has the VBM value at around 5.54 eV (Figure 7d-e), we obtained a more matched energy alignment between the VBM of CH3NH3PbI3 and NiO as the VBM of NiO was down-shifted, thus minimizing the Voc loss. After systematic comparison, the champion solar cells with NiO NPs annealed at 550 o

C was achieved, with the Voc of 0.912 V, Jsc of 22.84 mA/cm2, FF of 0.76 and PCE of

15.80% in the reverse scanning direction at a scan rate of 100 mV/s. (Figure 6a) To test

the hysteresis effect, the forward scanning was also tested and we got the Voc of 0.900 V, Jsc of 22.64 mA/cm2, FF of 0.74 and PCE of 15.07%. Comparing with the photovoltaic parameters in the reverse scanning, the Voc showed a small decreasing, but the hysteresis effect was quite small.

Figure 6. a) J-V curves of champion solar cell with NiO NPs interlayer at forward and reverse scanning directions, scan rate 100 mV/s; b) UPS spectra of the VBM onset (Ei) and photoemission cut-off (Ecut-off) energy boundary of NiO NPs annealed at various temperatures; c) Steady-state photocurrent measurement and the corresponding PCE output at the maximum power point (0.73 V) of the device B champion solar cell; d) Variation in the PCE of champion solar cells as a function of stored time at the humidity

of 15% in air. We also measured the steady-state photocurrent output at the voltage close to the maximum power point (0.73 V) to check the cell’s output stability. (Figure 6c) The photocurrent could stabilize over the whole time of the measurement (around 20.8 mA/cm2) and the PCE is calculated to be 15.18%, which matched well with the small hysteresis and J-V curves. The C-PSCs were stored in a dry box with the humidity of 15% in air. For a 3000 h test, the PCE showed almost no changes (Figure 6d), or even a slight increase. We measured the XRD spectra of freshly prepared CH3NH3PbI3 (red) and that of CH3NH3PbI3 collected from cells after two months of storage (blue). (Figure S9) It can be seen clearly that the freshly obtained CH3NH3PbI3 from the two-step conversion reaction in CH3NH3I solution showed pure phases without PbI2 peaks. However, after two months of storage, a small peak of PbI2 appears. This suggests that PbI2 was slowly released from CH3NH3PbI3 during the storage. This small excess of PbI2 could exist in perovskite grain boundaries, suppressing defect trapping and reducing nonradiative recombination process, and thus improving the overall performance of the PSCs.[45-49] The perfect stability of the completed solar cells demonstrated that the quite compact and completed MWCNT films deposited by spray method with the nature of hydrophobicity could prevent the water molecular from penetrating into perovskite layer, which suppresses the decomposing process of the perovskite crystal efficiently. 2.3 Mechanistic investigation of the dual-embedment fabricated solar cells Based on the above analysis, we further investigated the effect of embedded NiO

NPs sub-monolayer on the interface energy levels, and studied the function of NiO NPs more deeply and systematically. Currently, most reported works simply assume flat band conditions, without considering the interface band bending effect for the band alignment of perovskite with functional layers. Actually, the flat band assumption could not hold true for the real interface states.[50, 51] Here, XPS and UPS spectra were measured to examine the electronic structure and energy level alignments of CH3NH3PbI3/NiO interface and CH3NH3PbI3/MWCNT interface.

Figure 7. X-ray photoelectron spectra (XPS) measurements of perovskite related elements (a) Pb 4f; (b) I 3d; (c) Ni 2p3/2 of NiO; UPS spectra of three type interfaces: CH3NH3PbI3, NiO NPs embedded CH3NH3PbI3 and NiO. d) VBM onset; e) photoemission cut-off. f) UPS spectrum of as prepared MWCNT film. It would be great if in situ XPS and UPS measurements could be used to monitor the slow deposition at the interface in a nanometer scale region,[52] but that is very difficult

for the embedded NiO NPs on solution processed perovskite films. Instead, we prepared three types of films to address the problem with gradually changing interfaces: pure CH3NH3PbI3 film, the NiO NPs embedded CH3NH3PbI3 film and thick NiO film. Firstly, we used the XPS spectra for the examination of elements’ core level shifts. Figure 7a and 7b show the XPS spectra for Pb 4f and I 3d in the pure CH3NH3PbI3 film, the NiO NPs embedded CH3NH3PbI3 film. Pb 4f and I 3d peaks could only be seen in CH3NH3PbI3. No apparent binding energy shift was observed at Pb 4f peak of 138.66 eV and I 3d peak of 619.50 eV, which proves the absence of any band bending of the CH3NH3PbI3 after the NiO NP embedment. However, the binding energy of Ni 2p3/2 peaks shifted from 853.57 eV in thick NiO film to 854.19 eV in NiO NPs embedded CH3NH3PbI3 film. So the core level of NiO shows 0.62 eV upward shift when contacted with CH3NH3PbI3. (Figure 8) The valance band changes and work function shift were measured by UPS spectra. The VBM is determined by linear extrapolation of valance band onset subtracted to the background around Fermi level. From Figure 7d, the VBM of pure CH3NH3PbI3 is 1.35 eV below the Fermi level, in agreement with the reported data. After NiO NPs embedment, the VBM upshifts to only 1.18 eV and 0.64 eV below the Fermi level in increasingly thick NiO films, again manifesting the band bending in NiO interfaced with the perovskite. The VBM is believed to have a 0.71 eV upward shift. The work function also shows the shift in the same direction, from 4.19 eV in pure CH3NH3PbI3, to 4.43 eV with an upward shift in NiO NPs embedded CH3NH3PbI3 and 4.58 eV of thick NiO film. However, the upward shift value of the work function (0.39 eV)

is much lower than that of core level shift (0.62 eV) from XPS spectrum and the VBM bending of 0.71 eV, but they all show the same band bending trend near the CH3NH3PbI3/NiO interface. Since our method is based the gradually changing interfaces of the three types of films, the accurate band bending value obtained may be limited. Vacuum evaporation of NiO or CH3NH3PbI3 forming the gradually changing interface thickness may be needed combining with the in-situ XPS and UPS measurements for more accurate measurements of the interfacial band bending. Finally, from the UV-vis absorption spectra of CH3NH3PbI3 and NiO in Figure S10, we found that NiO is a wide band gap semiconductor, with a bandgap of 3.70 eV. Combining with the bandgaps of CH3NH3PbI3 (1.60 eV), we obtain the energy band alignment of the fabricated C-PSCs, which is shown in Figure 8. For the CNT-PSC structure without the NiO interlayer, the UPS result shows that the VBM relative to the Fermi level of MWCNT is zero, and the work function is calculated to be 4.54 eV (Figure 7f). This indicates that the MWCNT used in our experiment is metallic as was also certified in our previous work.[43, 53-56] The larger difference (1.04 eV) between the VBM of CH3NH3PbI3 and the Fermi level of MWCNT results in a bigger Voc loss. Also the higher Fermi level induces electrons leakage from the perovskite to MWCNT, and this would increase the possibility of nonradiative recombination process at the interface and reduce the overall performance of C-PSCs. However, for C-PSCs with the NiO NPs sub-monolayer embedment, the upward band bending of the NiO at NiO/perovskite interface energetically favors fast hole

extraction and transportation from CH3NH3PbI3 to NiO. Besides, from the band gap and VBM values, the conduction band minimum (CBM) of NiO and CH3NH3PbI3 could be calculated as shown in Figure 8. One can see that the CBM of NiO is much higher than that of perovskite, so the separated electrons could not go through from perovskite to NiO because of the energy barrier, and this blocks the electron leakage and reduces the nonradiative recombination effectively. The reduced recombination process has already been discussed above on the basis of the EIS results. All of these contribute to the improved overall photovoltaic performance, with Voc, Jsc and FF all increased of the C-PSCs after embedding the NiO NPs sub-monolayer.

Figure

8.

Energy level

diagram

of

Perovskite/NiO

interface

(middle)

and

perovskite/MWCNT interface (right). 3. Conclusions We have demonstrated the effectiveness of the ultrasound spray method to deposit

MWCNT film electrodes for C-PSCs. Through systematic screening and investigation, chlorobenzene was proved to be an ideal solvent for dispersing MWCNTs to form a compact and homogeneous MWCNT film deposited on PbI2 substrate with strong adhesion force. After optimizing the MWCNT film thickness, the PCE as high as 14.07% was achieved, which is obviously higher than that of the C-PSCs based on drop casting MWCNT. Moreover, we have shown that a sub-monolayer of NiO NPs prior to the MWCNT deposition enhanced the hole extraction and reduced the recombination process due to the band bending effect and wide band gap of NiO at the CH3NH3PbI3/NiO interface. This has further boosted the PCE up to 15.80%, which is among the highest efficiency for C-PSCs. Therefore, the simple and reproducible spray method has paved the way for the scaling-up and mass production of C-PSCs in the future.

4. Material and methods 4.1 Material Preparation Titanium diisopropoxide bis(acetylacetonate) (Tiaca, 75 wt% in isopropanol), N, N-dimethylformamide (DMF, 99%), lead (II) nitrate (Pb(NO3)2), potassium iodide (KI), methylamine (MA, 40% in methanol), hydroiodic acid (HI, 57 wt% in water), cyclohexane (99.7%), 2-isopropanol (IPA, 99.9%), boric acid (H3BO3) and chlorobenzene (99.8%) were purchased from Sigma-Aldrich and directly used without any treatment or purification. TiO2 paste was purchased from Dyesol Australia Pty Ltd. Multi-walled carbon nanotube (MWCNT, ≥98% carbon basis) was purchased from Sigma-Aldrich and

treated with H3BO3 for a boron (B) doping with our previous reported method.[43] 4.2 Boron (B) doping of Multi-walled carbon nanotubes 0.2 g of MWCNT and 1 g of boric acid (H3BO3, Aldrich) were mixed and heated at 1000 o

C for 4 h (heating rate 5 oC/min) in a quartz tube reactor with flowing pure argon as the

carrier gas. The product was then washed and filtered three times using hot deionized water (100 oC) to remove residual H3BO3 and/or B2O3. After washing, the samples were dried at 60 oC for 48 h in the vacuum oven. 4.3 Preparation of PbI2 The PbI2 was synthesized by dissolving 6.62 g Pb(NO3)2 and 6.86 g KI in 50 and 200 ml deionized water. Under vigorous stirring, the KI solution was added into Pb(NO3)2 solution drop by drop. After 30 min, the light-yellow product was collected with a filtration and washed by plenty of deionized water. Finally, the product was dried at 70 oC in vacuum oven overnight. 4.4 Preparation of CH3NH3I CH3NH3I was prepared by reacting 24 ml of methylamine and 12 ml hydroiodic acid in ice bath for 2 h with stirring. The precipitate was recovered by rotary evaporation at 50 °C and carefully removing the solvents with the help of low-temperature circulator. To reduce I2 content, the product was re-dissolved in ethanol and precipitated with diethyl ether, which was repeated twice. Finally, the white solid was collected by filtration and dried at 60 °C in vacuum oven overnight. 4.5 Preparation of NiOx nanoparticles

The NiOx nanoparticles used in the cells was synthesized according to a modified reported method. Firstly, 2.3 g NiCl2•6H2O and 1.5 g NaHCO3 were dissolved in 10 ml and 20 ml deionized water, respectively. Then the solutions were stirred by magnetic stirrer for 15 min to make sure them dissolved well. After that, the NaHCO3 solution was added to the NiCl2 solution drop by drop with vigorous stirring. The products were collected after 15 min by centrifugation in 5000 rpm and washed with deionized water for three times. Secondly, the collected products were dried as 70 oC in a vacuum oven for 3 h. Then they were heated in air at different temperature for 2 h, respectively. 4.6 Solar device fabrication Firstly, the FTO glass was cleaned using the detergent, deionized water, acetone, IPA and ethanol in ultrasonic bath for 15 min, respectively. Then, the 50 nm dense TiO 2 (c-TiO2) layer was deposited on the cleaned FTO glass by ultrasonic spray pyrolysis using the 0.05 M titanium diisoprop-oxide bis(acetylacetonate) in ethanol in 500 oC. After that, the mesoporous TiO2 scaffolds (m-TiO2) with the thickness about 400 nm was spin-coated on the substrate at 5000 rpm for 30 s using the commercial TiO2 paste diluted in ethanol for 2.5 times in weight. After preheating at 100 oC for 10 min, the mesoporous TiO2 scaffolds were calcined at 550 oC for 2 h and cooled down to room temperature. The CH3NH3PbI3 layer and carbon nanotubes deposition were completed by our reported modified two-step chemical reaction embedment method and the novel spray coating method, respectively. Firstly, 599 mg PbI2 was dissolved in 1 ml DMF to form the 1.3 M solution at 100 oC. The FTO/c-TiO2/m-TiO2 substrate was preheating at 100 oC for

15 min. 50 μl PbI2 solution was deposited on the TiO2 scaffolds film by spin-coating at 4000 rpm for 20 s and spin-coated twice. After that, the PbI2 film was annealed at 100 oC for 10 min. The NiO nanoparticles was dispersed into chlorobenzene with the concentration about 1 mg/ml and ultrasonic dispersed for 1 h, then it was filter with the PTFE filter of 0.4 um. The NiOx was deposited on the PbI2 substrate for different times by drop-casting and heating at 70 oC to remove the solvent. Then, the H3BO3 treated multiwall carbon nanotubes were dispersed in chlorobenzene with the concentration of 1 mg/ml in ultrasonic bath for 15 min to form homogeneous MWCNT ink. Before the MWCNT deposition, the homed steel mask was put on the PbI2 to prevent the short circuit on the cell edge as well as confine the electrode active area size. Then the MWCNT layer with the thickness from 5 μm to 20 μm was deposited on the substrate using the homed gun-spraying system powered by the compressed air. The deposition time was measured to control the thickness of the MWCNT films. For the drop cast method of the MWCNT electrode, the details could be found in our previous work.[28] The MWCNT was well dispersed in chlorobenzene at a concentration of 10 mg/mL with a probe ultrasonic processor. The carbon material solution was then drop casted on the top of the TiO2/PbI2 substrates. After coating, the substrates were heated at 50 ℃ for another 5 min and then cooled down to room temperature. In the end, after removing the mask, the FTO/c-TiO2/m-TiO2/PbI2/MWCNT substrates were immersed into the CH3NH3I solution for 12 h to complete the conversion from PbI2 to CH3NH3PbI3.The CH3NH3I solution was obtained by adding enough

CH3NH3I powders into 50 ml IPA/cyclohexene (9: 1, v: v) mixing solution to form the clear and saturated CH3NH3I solution. After that, the substrate was annealed at 100 oC for 10 min in dry air. The devices were measured under simulated AM 1.5 G, 100 mW/cm2 sunlight with an active area of 0.08 cm2 or 1 cm2. 4.7 Material Characterization Morphology was evaluated on a JEOL6700F SEM at an accelerating voltage of 10 kV. X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron spectra (UPS) were carried out by a Kratos Axis Ultra DLD multitechnique surface analysis system using monochromatized Al Kα X–ray photons for XPS and a He I discharge lamp (21.22 eV) for UPS. Sheet resistance of MWCNT films were measured using a four-point probe method (HL5500PC (Bio-Rad)). UV-vis absorption was measured using the Cary 500 UV-vis-NIR Spectrophotometer. For steady-state PL, a 514.5 nm ultrafast laser was used as the excitation light source (Laser sources: Ar ion laser, 50mW). The solar light simulator (Newport solar simulator, model number 6255, 150 W Xe lamp, AM 1.5 global filter) was calibrated to 1 sun (100 mW/cm2) using a silicon reference solar cell equipped with KG-5 filter. The active cell area was all fixed at about 0.08 cm2 or 1 cm2 and J-V curves were recorded on an IM6x electrochemical work station (ZAHNER-Elektrik GmbH & Co. KG, Germany). IPCE spectra were recorded using IPCE kit developed by ZAHNER-Elektrik in AC mode with frequency of 1 Hz. The EIS plots were also recorded on the IM6x electrochemical workstation (ZAHNER-Elektrik GmbH & Co., KG, Germany).

Acknowledgment This work was supported by the HK-RGC General Research Funds (GRF grant no. 16312216) and the HK Innovation and Technology Funds (grant no. ITS/219/16).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at .

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Vitae

Yinglong Yang received his B.S. degree in 2013 from University of Science and Technology of China. He is currently a Ph.D. candidate in Prof. Shihe Yang's group in Department of Chemistry of the Hong Kong University of Science and Technology. His current research focuses on carbon materials based perovskite solar cells and synthesis of nanomaterials.

Haining Chen is currently an associate professor at School of Materials Science and Engineering, Beihang University. He obtained his PhD degree from Beihang University in 2013 followed by working as a postdoctoral fellow at The Hong Kong University of

Science and Technology (Prof. Shihe Yang's group). His recent research interests include organic-inorganic hybrid perovskite solar cells, quantum dot-sensitized solar cells, photoelectrochemical water splitting.

Xiaoli Zheng obtained her M.S. degree in 2012 from Zhengzhou University, and Ph.D. degree from The Hong Kong University of Science and Technology in 2016 (Prof Shihe Yang’s group). Currently, she is working at Zhengzhou University. Her research mainly focuses on synthesis of nanomaterials and their applications in perovskite solar cells.

Xiangyue Meng received his PhD degree from Institute of Chemistry Chinese Academy of Sciences in 2014. Now, he is visiting scholar in the Hong Kong University of Science and Technology (with Prof. Shihe Yang). Since 2014, he has been associate professor at Beijing University of Chemical Technology. His main research activities focus on the development of organic functional materials for various optoelectronic applications.

Teng Zhang received his Ph.D. degree (2017) in School of Science from Hong Kong University of Science and Technology. He is currently a Post-doctoral fellow in Professor Shihe Yang’s group in Department of Chemistry from The Hong Kong University of Science and Technology. His current research focuses on solar conversion particularly perovskite solar cell and photoelectrochemical water splitting.

Chen Hu obtained his bachelor degree from a “2+2″ program in Chemistry with The Hong Kong Polytechnic University associated with Sun Yat-Sen University. He is now the Ph.D. candidate in Professor Shihe Yang’s research group at Department of Chemistry in the Hong Kong University of Science and Technology with project on solution-processed perovskite solar cells.

Yang Bai received his B.S. degree (2013) in School of Chemistry and Environment from Beihang University and his Ph.D degree (2017) in Department of Chemistry from The Hong Kong University of Science and Technology. He is currently a postdoctoral fellow at The Hong Kong University of Science and Technology. (Prof. Shihe Yang’s group). His research focuses on perovskite solar cell, interface design and 2D material.

Shuang Xiao received his B.S. degree (2012) in School of Physics from Huazhong University of Science and Technology in China and his Ph.D degree (2017) in Department of Chemistry from The Hong Kong University of Science and Technology. He is currently a postdoctoral fellow in Professor Shihe Yang’s group in Department of Chemistry from The Hong Kong University of Science and Technology. His current research focuses on perovskite solar cells, photoelectrochemical cells and catalysts.

Shihe Yang is now Professor at The Hong Kong University of Science and Technology. His research has spanned material size range of clusters, fullerenes and metallofullerenes and nanomaterials. His current research interests are focused on the understanding, manipulation and applications of zero-, one-, and two-dimensional nanosystems for applications in solar energy conversion.

Highlights 1. Simple ultrasound spray method was firstly established to fabricate CNT electrodes for carbon-based perovskite solar cells (C-PSCs). 2. A dual-embedment technology was developed to form integrated CH3NH3PbI3/NiO/C interfaces in C-PSCs. 3. The power conversion efficiency as high as 15.80% was achieved in C-PSCs! 4. The upward band bending phenomenon at NiO/CH3NH3PbI3 was first measured and reported.