A Carbon Nanotube Bridging Method for Hole Transport Layer-Free

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A Carbon Nanotube Bridging Method for Hole Transport Layer-Free Paintable Carbon-Based Perovskite Solar Cells Yue Wang, He Zhao, Yeming Mei, Hongli Liu, Shirong Wang, and Xianggao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18530 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Applied Materials & Interfaces

A Carbon Nanotube Bridging Method for Hole Transport Layer-Free Paintable Carbon-Based Perovskite Solar Cells Yue Wang a,b, He Zhao a,b, Yeming Mei a,b, Hongli Liu a,b,*, Shirong Wang a,b, Xianggao Li a,b,* a School

of Chemical Engineering and Technology, Tianjin University, 300350 Tianjin, China

b Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

300072, China. Corresponding Author *Email: [email protected]; *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. The incredible stability of carbon-based perovskite solar cells (C-PSCs) has aroused enormous concern. However, for the paintable C-PSCs, the fill factor (FF) and power conversion efficiency (PCE) remain low, which is because the insufficient contact at the interface between the perovskite and the electrode and the low conductivity of the electrode. In this work, a carbon nanotube (CNT) bridging method is introduced into the devices by adding single-walled carbon nanotubes (SWCNTs) in both perovskite and carbon layers to form a high-quality perovskite/carbon interface. The CNT bridges penetrating into both the CH3NH3PbI3 layer and the carbon cathode not only facilitate charge extraction and transport between the two layers but also promote the electrical conductivity of the carbon electrode. The hole transport layer (HTM)-free C-PSC with a structure of FTO / compact TiO2 / mesoporous TiO2 / CH3NH3PbI3•SWCNT / SWCNT•C gained a remarkable PCE of 15.73% with an FF of 0.72, accompanied by an outstanding stability of 90 days in dark under high humidity (65±5% relative humidity (RH), 25 ±5℃) and high temperature (75±5℃, 25±5% RH) conditions. The low-cost fabrication process makes highly stable and efficient C-PSCs promising candidates for future applications.

KEYWORDS: carbon nanotubes; efficient; stability; carbon electrode; perovskite solar cell


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Introduction Perovskite solar cells (PSCs) have aroused widespread concern because of their unique photovoltaic properties and increased power conversion efficiency (PCE).1 Since Miyasaka et al. applied perovskite as a light harvesting layer and reported a power efficiency of 3.8% in 2009,2 several studies have been made on various aspects of PSCs.3-7 To date, the reported maximum efficiency has promptly surged to more than 23%.8 However, the commercialization of these stateof-the-art PSCs has been hindered by their costly hole transport materials (HTMs) and noble metal counter electrodes.9 Fortunately, an ambipolar property has been demonstrated in perovskite, which indicates the potential of manufacturing HTM-free PSCs. Hence, carbon, a low-cost chemically stabilized material, emerges as an alternative to metal electrodes due to its suitable work function, moisture resistance and high conductivity.10-13 Moreover, apart from eliminating high-consumption processes, carbon-based perovskite solar cells (C-PSCs) demonstrate promising stability even under extreme conditions.14-16 Therefore, due to such merits, considerable investigations have been conducted in C-PSCs.17 Han et al. first applied mesoscopic carbon as an electrode in PSCs in 2013.18 The resulting PSC gave an efficiency of 6.64%. Several efforts was made to elevate the performance of HTM-free CPSCs in recent years. By applying SrCl2 to the precursor solution, eventually, Alex et al. acquired a PCE of 15.9%,19 which is the highest PCE of HTM-free C-PSCs ever reported. However, without a hole-transporting layer, low hole extraction and transport remain a key problem in C-PSCs. Considering their work function, charge transport feature, electrical and mechanical properties, carbon nanotubes (CNTs) are considered superior electrode materials and outstanding hole extraction materials.20-22 Recently, CNTs were applied to PSCs to enhance the


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carrier mobility.23 For example, by applying a mesoscopic TiO2/Al2O3/NiO/ single-walled carbon nanotubes (SWCNTs) as a scaffold into PSCs and introducing CH3NH3PbI3 perovskite by a twostep sequential method, a PCE of 12.7% was obtained by Wang et al.24 Yang et al. prepared a fluorine-doped tin oxide (FTO)/TiO2/PbI2/multiwalled carbon nanotube (MWCNT) substrate and dropped CH3NH3I solution on top of the MWCNT layer to cause an in situ chemical reaction. As a result, an interpenetrating interface between CH3NH3PbI3 and carbon electrodes was formed, and a PCE of 12.67% was achieved.25 The preformed mesoscopic structure mentioned above ensure seamless interfaces between CH3NH3PbI3 and carbon electrodes and efficient charge extraction and transfer. However, the preformed structure may affect the crystallization of perovskite and damage the photovoltaic performance. In comparison, crystallization was ensured in paintable C-PSCs; however, the fill factor (FF) of these devices was usually lower than 0.70 because of insufficient contact as well as deficient charge extraction and transfer at the perovskite/carbon interfaces arising from the inferior interface quality. Thus, it is significant to modify the perovskite/electrode interface and enhance exciton separation and charge transfer in HTM-free paintable C-PSCs. Ryu et al. introduced a solvent dripping method by dripping the chlorobenzene solution dispersed with MWCNTs onto the precursor solution to facilitate fast charge transport between perovskite and carbon layer and achieved a PCE of 13.57%.26 Ryu’s remarkable work provides a possibility of modifying the perovskite/carbon interface by introducing functional carbon nanomaterials into perovskite layer through the anti-solvent. Although numerous studies have been carried out by adjusting only the carbon layer or only the perovskite layer,27-28 there is no study of fabricating intimately contacted perovskite/electrode interfaces in paintable C-PSCs by simultaneously modifying both the carbon and perovskite layers in paintable C-PSCs. Thus,


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inspired by Ryu’s and others’ work, we decided to modify the perovskite/carbon interface and enhance the charge transport by adding SWCNT into both perovskite layer and carbon material to combine the two merits and further improve the performance of the devices. Compared with MWCNT, SWCNT demonstrate a higher charge carrier transport ability, a highly uniformed network format, which make the exciton can be effectively separated by the CNTs in perovskite film and the charge be efficiently transferred by the CNTs in the carbon layer.29-30 Besides, unlike the mixed perovskite Cs0.06(MA0.17FA0.83)Pb(I0.84Br0.16)3 that Ryu et al. used, a more common pure perovskite methylammonium lead iodide (MAPbI3) was used as light harvester in this work and the principle why this perovskite•CNT composite would facilitate hole transport was further studied. Herein, we report a CNT bridging method for the fabrication of paintable C-PSCs. By introducing SWCNTs into both anti-solvent chlorobenzene and carbon electrodes, CNTs are embedded into both the MAPbI3 layer and SWCNT/carbon composite electrode, acting as a charge extraction and transport highway between them. Meanwhile, by adding SWCNTs into the champion carbon counter electrode selected from pastes containing three types of graphite (flaky graphite of 12500 mesh, 8 μm spheroidal graphite and flaky graphite of 15000 mesh), the electrical conductivity of the carbon electrode is enormously enhanced. The CNT bridging method contributes to the formation of intimate perovskite/carbon interfaces, resulting in a promising PCE of 15.73% with an FF of 0.72. The unsealed devices exhibited remarkable long-term stability under severe hydrothermal conditions (65±5% relative humidity (RH), 25±5℃ and 75±5℃, 25±5% RH) for 90 days when they were kept in dark.


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Experimental section Materials Patterned FTO conducting glasses were obtained from Nippon Sheet Glass Corporation. Dimethyl sulfoxide (DMSO), N, N ′ -dimethylformamide (DMF), acetyl acetone and isopropyl alcohol were purchased from J&K Chemical Corporation. Titanium diisopropoxide bis(acetylacetonate) was acquired from Alfa Aesar. Methylammonium iodide and lead iodide (PbI2) were products of Youxuan Technology Corporation. The SWCNTs (diameter: 1-2 nm, length: 5-30 μm) were purchased from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. Preparation of the carbon paste Three different types of graphite powders (flaky graphite of 12500 mesh, 8 μm spheroidal graphite and flaky graphite of 15000 mesh) were mixed with carbon black powders at a mass ratio of 3:1 in a complex solvent of terpineol, ethanol and 2-acetoxy-1-methoxypropane (PGMEA).31 ZrO2 nanoparticles and ethyl cellulose were added as an auxiliary and binder. The blended powders were dispersed in the composite solution via ultrasonic dispersion for 4 h and ball milling for 12 h. CNTs (1 wt.%) were composited to the carbon electrode by grinding with a mortar and pestle to prepare the carbon composite. Device fabrication FTO glass substrates were successively cleaned by ultrasonic treatment in methylbenzene, acetone, deionized water and anhydrous ethanol. Then, a layer of compact TiO2 was deposited on the ascleaned FTO substrate by spray pyrolysis at 450℃. Subsequently, a 400 nm thick mesoporous TiO2 layer was spin-coated onto the compact layer with TiO2 paste (Dyesol 30 NR-Dyesol), which


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was diluted in ethanol at a mass ratio of 1:3.5. After drying at 70℃ for 5 min, the substrate was sintered at 500℃ for 30 min. The CH3NH3PbI3 light absorption layer was fabricated by a one-step anti-solvent method. The perovskite precursor solution containing 1.35 M Pb2+ in a mixed solvent of DMF and DMSO (volume ratio of DMF and DMSO is 9:1) was dripped onto the mesoporous TiO2 layer and then spin-coated at 1000 rpm for 10 s and 6000 rpm for 30 s. At 25 s before the last spin-coating step, 180 μL of chlorobenzene containing 1 mg/mL of SWCNT was pipetted onto the substrate. Subsequently, the substrate was sintered at 100℃ for 60 min. Eventually, a 10 μm carbon composite was coated on top of the perovskite film by the doctor-blading method and sintered at 100℃ for 60 min. Device characterization The morphology of the PSCs was analyzed by a field-emission scanning electron microscope (FESEM, ZEISS SUPRA 55 VP, Germany) with an accelerating voltage of 5 kV. Structural examinations of the perovskite films were carried out by X-ray diffraction (XRD, Mini Flex 600) measurements from 2θ=10° to 2θ= 60° with CuKα1 radiation. Steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were obtained with Edinburgh Instruments (FLS980, UK) recorded at 775 nm using excitation with a 485 nm light source. Ultraviolet-visible (UV-vis) absorbance spectra were collected with a Thermo Evolution 300 UV/Visible-spectrometer. Energy band spectra were measured by Ultraviolet Photoelectron Spectroscopy (UPS, Thermo ESCALAB 250XI). Using an Oriel solar simulator 91160A (Newport, USA), the current density-voltage (J-V) characteristic curves were recorded by a digital source meter (Keithley 2400) under simulated air mass 1.5 (AM 1.5) illumination at 100 mW/cm2. A mask of 0.09 cm2 was applied to ensure the effective area of the PSCs. An electrochemical


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workstation (IVIUM) was employed for electrochemical impedance spectroscopy (EIS). The frequency range of the EIS spectra is from 1 Hz to 100 KHz. Incident photon-to-current conversion efficiency (IPCE) was investigated by an IPCE testing system (E0201b) fabricated by the Institute of Physics, Chinese Academy of Sciences. Results and discussion Exciton separation and charge transport are two significant processes in PSCs. A sufficient connection between the perovskite and carbon layer will result in effective charge extraction and transport, leading to superior photovoltaic properties. In paintable C-PSCs, carbon paste is simply painted on the perovskite, causing a balky gap and crack at the perovskite/C interface. With the CNT bridging method, CNTs could insert into the perovskite layer and capture the carbon layer at the same time, forming a seamless interface. CNT bridges connecting the perovskite layer and carbon layer form a exciton separation highway in the perovskite layer and a carrier transfer highway in the carbon layer, resulting in a high PCE.


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Figure 1. (a) Fabrication process of CNT bridge-based paintable PSCs. (b) Cross-sectional SEM image of the MAPbI3 with embedded CNTs. (c) Schematic structure of three types of carbon counter electrodes, and schematic representation of hole extraction and transport at the MAPbI3/electrode interface with/without CNT bridging method. (d) Energy band diagram of the as-fabricated devices.

An illustration presenting the device structure and fabrication process of a CNT bridge-based paintable perovskite solar cell is depicted in Figure 1a. On the FTO/ compact TiO2/ mesoporous TiO2 substrate, a 400 nm thick CH3NH3PbI3•SWCNT layer was formed by a one-step anti-solvent method. Chlorobenzene containing SWCNTs was dripped onto the substrate of the MAPbI3 precursor solution, resulting in a sheer MAPbI3 bottom layer and a MAPbI3•SWCNT composite upper layer (Figure 1b).26 When the device was finished by coating with the SWCNT•C composite electrode paste, SWCNTs were partly embedded into the perovskite layer (Figure S1) and partly inserted into the carbon layer, enabling rapid exciton separation and carrier transport. The corresponding energy band diagram is depicted in Figure 1d according to the UPS measurement results (Figure S2). Compared with pure MAPbI3 (-5.45 eV to -3.88 eV), the valance band and conduction band of the MAPbI3•SWCNT composite decreased to -5.19 eV and -3.64 eV, which may cause an inhibition of the recombination and enhancement of the carrier conduction.32 After the generation of exciton, the extraction of holes from the valance band of MAPbI3 perovskite to the carbon counter electrode is implemented efficiently due to the suitable energy level of the MAPbI3-SWCNT composite, increased number of conduction highways and extraordinary hole extraction ability of the SWCNTs.33 Meanwhile, the extraction of electrons from MAPbI3 to titanium dioxide is also improved because of the efficient hole extraction, which can reduce charge accumulation and enhance hole-electron separation.34


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Although CNTs demonstrated superior conductivity, the carbon paste made of pure CNT could not form a uniform and continuous film in paintable C-PSCs due to the agglomeration of pure CNTs. Thus, preparing a graphite/carbon black (CB)/CNT composite electrode seemed to be a reasonable way to advance the electrical conductivity of carbon electrodes (CEs) and device performance. As a contrastive sample, carbon composites containing three different types of graphite were first prepared. Then, the CNTs were blended with the champion carbon paste to form the composite electrode. Three types of graphite (flaky graphite of 12500 mesh, 8 μm spheroidal graphite and flaky graphite of 15000 mesh) were mixed with carbon black at a mass ratio of 3:1. Their corresponding CEs, namely, counter electrode CE A, CE B and CE C, were characterized by four-point probe sheet resistance tests and SEM measurements. In Table 1 list the detailed results of the square resistance (Rsq) of the carbon composite. The Rsq values of CE A, CE B and CE C are 12.76 ± 0.13, 12.59 ± 0.13 and 11.28 ± 0.12 Ω/sq, respectively. CE C exhibits the lowest square resistance. Figure 1c shows a graphic of the carbon composite containing three types of graphite. According to the corresponding SEM images shown in Figure 2a-f, identical 10 μm thick carbon layers were prepared, and obvious cracks in CE A and CE B could be seen in their top-view SEM images. These cracks could be harmful to charge transport. In contrast, no palpable crack was observed in CE C. The lower resistivity of CE C could be attributed to its higher continuity and higher contents of graphite in an equivalent volume of carbon paste.35 Subsequently, CE A, CE B and CE C were further blended with the SWCNTs (Table 2). With the addition of SWCNTs, the Rsq of CE C+SWCNTs was dramatically reduced to 9.84 ± 0.10 Ω/sq, indicating an enhanced charge transfer ability.36 This result may thanks to a better connection between graphite, CB and SWCNT, as


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demonstrated in Figure 3a.37 Preliminarily, CE C was selected to be blended with the CNTs and further fabricate devices with the CNT bridging method.

Table 1. Square resistance of different counter electrodes Sample

CE A

CE B

CE C

Rsq(Ω/sq)

12.76 ± 0.13

12.59 ± 0.13

11.28 ± 0.12

Table 2. Square resistance of different carbon composite electrodes Sample

CE A + SWCNT

CE B + SWCNT

CE C + SWCNT

Rsq(Ω/sq)

11.80 ± 0.12

10.93 ± 0.11

9.84 ± 0.10

Figure 2. (a), (b) and (c) Top-view and (d), (e) and (f) cross-sectional SEM images of CE A, CE B and CE C.

Figure 3b illustrates the top-view SEM images of the MAPbI3•SWCNT layer prepared by the CNT bridging method. SWCNTs are inserted into the perovskite layer, and as a result, a CNT


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network is shaped on the MAPbI3 layer. The pristine perovskite layer demonstrates a smooth surface (Figure 3c), which would result in insufficient contact at the MAPbI3/CE interface (Figure 3e), resulting in deficient hole extraction. By comparison, using the CNT bridging method, upward SWCNTs facilitate hole extraction and transport, while transverse SWCNTs ensure an intense connection between perovskite and the carbon electrode (Figure 3d ， 3f), enabling improved charge carrier mobility.24 Other details of the cross-sectional SEM results are displayed in Figure S3.

Figure 3. (a) SEM image of the graphite/CB/SWCNT composite. (b) Top-view SEM image of the perovskite embedded with CNTs using the CNT bridging method. Cross-sectional SEM images of the MAPbI3 layer formed without (c) and with (d) CNT bridges and those of the interfaces of devices fabricated without (e) and with (f) the CNT bridging method.


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Figure 4. (a) UV-Vis absorption spectra of CH3NH3PbI3 films with/without CNTs. Currentvoltage (I-V) characteristics of ITO/perovskite/Au devices (b) with CNTs and (c) without CNTs. (d) PL and (e) TRPL spectra of CH3NH3PbI3 films with/without CNTs bridges. (f) XRD spectra of the MAPbI3 films with/without CNTs.

In Figure 4a shows the UV-vis spectra of CH3NH3PbI3 film and CH3NH3PbI3•SWCNT composite layer, revealing the influence of SWCNTs on optical properties of CH3NH3PbI3 layer. The CH3NH3PbI3•SWCNT composite presents a slightly higher absorbance and a slight redshift in the region from 400 to 800 nm in the UV-vis absorption spectra, indicating that SWCNT addition might slightly reduce the defect density of the perovskite layer.38 Space-charge-limited current (SCLC) measurements (Figure 4b-c) were implemented to further quantitatively measure the trap-state densities (ntrap) of the perovskite films.39 An ohmic response is indicated in lower voltage region because the current demonstrate a linear increase with the growth of voltage in this range. Afterwards, the curves demonstrate a nonlinear growth in the higher voltage, exhibiting the trap-filled limit (TFL). The TFL voltage (VTFL), determined by the


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trap state, is the intersection voltage between the ohmic response region and trap-filled region. Thus, the ntrap could be calculated with the equation:

𝑁𝑡𝑟𝑎𝑝𝑠 =

2𝜀0𝜀𝑉𝑇𝐹𝐿 𝑒𝐿2

(1)

where e is the elementary charge, L is the thickness of the MAPbI3 film, ε0 is vacuum permittivity and ε is the relative dielectric constant of MAPbI3. The VTFL values of MAPbI3 films with and without CNTs are 0.36 V and 0.45 V, respectively, and the corresponding ntrap are 1.19 × 1016 cm-3 and 1.49 × 1016 cm-3. Compared with the pristine MAPbI3 film, the film with CNTs demonstrate a slightly lower ntrap. This result is consistent with the UV-vis measurements. PL characterization was performed to probe the charge carrier extraction and transfer behavior at the surface of the CH3NH3PbI3•SWCNTs. As demonstrated in Figure 4d, a strong quenching effect can be observed in the CH3NH3PbI3•SWCNT films, indicating that the introduction of a CNT bridge remarkably enhances the charge carrier extraction in the perovskite layer.40 TRPL decay measurements (Figure 4e) were actualized to investigate the charge-recombination lifetime of the as-prepared samples. The perovskite samples without and with SWCNTs give fast and slow phase lifetimes of τ1=28.0 ns, τ2=750.3 ns and τ1=10.5 ns, τ2=405.2 ns, respectively. The decreased radiative lifetime of the CH3NH3PbI3•SWCNT film might be ascribed to an enhanced charge extraction and transfer ability in the film with CNTs, which is probably responsible for the increase in the short-circuit current density (Jsc), open-circuit voltage (Voc) and FF.41 XRD measurements were actualized to analyze the crystallinities of perovskite films. Three main diffraction peaks (14.48°, 28.39° and 32.03°) indicate the existence of a MAPbI3 perovskite crystal structure.42 According to Figure 4f, the same XRD patterns and similar intensities of the two samples demonstrate that the addition of SWCNTs does not harm to the formation of perovskite crystals.


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Using the CNT bridging method, PSCs with FTO substrate/compact TiO2/mesoporous TiO2/ CH3NH3PbI3•SWCNT/ SWCNT•CE C structures were fabricated. The cross-sectional SEM image is shown in Figure S4 and clearly depicts a 450 nm TiO2/CH3NH3PbI3 layer and a 400 nm CH3NH3PbI3•SWCNT top layer. Moreover, an intense connection between the perovskite and the CE can be observed. Figure 5a-c demonstrates the performance of PSCs based on CE A, CE B, CE C and CNT bridging methods. Figure 5a displays the current-voltage (J-V) characteristics of the four types of as-fabricated C-PSCs. In Table 3 are the corresponding photovoltaic parameters. Obviously, the PSC based on CE A provides the lowest performance with a Jsc of 19.44 mA/cm2, a Voc of 935.82 mV, an FF of 0.6444 and a PCE of 11.72%. In contrast, devices based on CE B and CE C present a noticeable improvement in FF. The CE B-based perovskite solar cell exhibits better performance with a Jsc of 19.76 mA/cm2, a Voc of 941.08 mV, an FF of 0.6790 and a PCE of 12.63%, while much better performance is achieved by the CE C-based PSC with a Jsc of 20.92 mA/cm2, a Voc of 950.41 mV, an FF of 0.6971 and a PCE of 13.86%. To gain farther improved the device performance, the CNT bridging method was invited into the paintable C-PSCs to form interpenetrated perovskite/carbon interfaces and enhance hole extraction and transfer. Impressively, the champion device with the CNT bridging method displays a Jsc of 22.36 mA/cm2, a Voc of 974.37 mV, an FF of 0.7222 and a PCE of 15.73%. The substantial improvement was ascribed to enhanced exciton separation and carrier transport as well as the superior conductivity caused by the CNT bridge.43


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Figure 5. (a) J-V curves of various devices. (b) Corresponding IPCE spectra. (c) Nyquist plots of devices based on different CEs. (d) J-V curves measured in the forward and reverse scanning directions. (e) Stabilized power output under simulated AM 1.5G sunlight of 100 mW/cm2. (f) Dark J-V curves of devices with/without a CNT bridge.

The IPCE spectra (Figure 5b) exhibit photocurrent responses from 310 nm to 800 nm. For the devices based on the CE A, CE B, CE C and CNT bridging methods, these spectra provide integrated photocurrent densities of 18.86 mA/cm2, 19.38 mA/cm2, 19.93 mA/cm2 and 21.31 mA/cm2, respectively. The results and the measured Jsc values gained from the J-V curves show a consistency. Table 3. Photovoltaic performance parameters of the C-PSCs Sample Jsc (mA/cm2) Voc (mV) FF CE A 19.44±0.39 935.82±18.72 0.6444±0.0129 CE B 19.76±0.40 941.08±18.82 0.6790±0.0136 CE C 20.92±0.42 950.41±19.01 0.6971±0.0140 With 22.36±0.45 974.37±19.49 0.7222±0.0144 CNT

PCE (%) 11.72±0.23 12.63±0.25 13.86±0.28 15.73±0.31


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The interfacial charge carrier transport processes of these four types of as-fabricated C-PSCs was investigated by EIS characterization. The measurement was performed under illumination at -0.9 V in the frequency region between 100 kHz and 1 Hz. The Nyquist plots for all devices and corresponding Bode phase plots are demonstrated in Figure 5c and Figure S5, respectively. Only one semicircle is obtained for each device in the Nyquist plots, and correspondingly, in the Bode phase plots, there is only a single peak detected at the high frequency area (100 kHz - 10 kHz). An equivalent circuit which fits the EIS plots is displayed in the inset of Figure 5c. Because the photocurrent is extremely higher in this situation than in the dark, EIS provides information regarding the charge transfer resistance (Rct) and series resistance (Rs),24 showing no obvious interface of charge recombination.23 The intercept on the real axis at high frequency is Rs, which consists of the Rs of the FTO glass, as well as the intrinsic contact resistance of TiO2, MAPbI3 and the carbon electrode.36 The device with the CNT bridging method exhibits the lowest Rs value due to the superior conductivity of the SWCNTs. Because all four devices contain similar TiO2/MAPbI3 interfaces, the difference in Rct is attributed to the different charge transfer dynamics at perovskite/CE interfaces. The CNT bridge-based PSC presents the lowest Rct, revealing an outstanding charge extraction and transfer properties. The extraordinary electrochemical property and high FF in the champion cell resulted from the enhanced interface contact and superior charge transfer caused by the CNT bridge. Hysteresis effects have always been a serious problem in PSCs.43-45 To further investigate the hysteresis effect of PSCs with and without CNT bridges, J-V curves with forward (-50 mV to 1200 mV) and reverse (1200 mV to -50 mV) scan directions at 0.1 V/s were measured (Figure 5d). For the control device, an obvious hysteresis effect is illustrated. In contrast, the device with a CNT bridge presents superior performance with barely no hysteresis effect. This finding may be


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ascribed to enhanced charge extraction by the CNT bridge reducing the charge accumulation since charge accumulation is suggested to have a strong correlation with J-V hysteresis.34 In Table S1 list the corresponding photovoltaic parameters. Measurements of the steady-state power output at the maximum power voltage (790 mV) were conducted (Figure 5e), and the results indicate a rapid photoresponse and a 300 seconds stable output. Dark J-V curves could provide the leakage current of fabricated devices.25 The dark current of the device with a CNT bridge under a reverse bias is lower than that of the control sample (Figure 5f), which suggests that the leakage current is reduced in the PSCs with the CNT bridge.46 The effect of SWCNT concentration in anti-solvent was also examined (Figure S6). The perovskite film treated by 1 mg/ml of SWCNT anti-solvent demonstrated the best performance and too much SWCNT would cause a decline in cells’ performance. This is because too much SWCNT would cause aggregation and inhibit the growth of perovskite grain.26 The performance of devices with and without CNT in the carbon layer was also studied and the optimum concentration was 1 wt.%. (Figure S7) The J-V characterization of devices with interfaces of MAPbI3•CNT/CE C, MAPbI3/CNT•CE C and MAPbI3•CNT/CNT•CE C was conducted to better reveal the superiority of the CNT bridges (Figure S8a). The device based on the CNT bridging method exhibits the highest PCE, largely ascribed to the increased number of conduction carriers. The CNT bridges shape fast charge collection and transport pathways in both perovskite and carbon layers. However, the exciton can only be effectively separated but not efficiently transferred in devices with interfaces of MAPbI3•CNT/CE C, whereas exciton can only be efficiently transferred but not effectively separated in devices with interfaces of MAPbI3/CNT•CE C (Figure S8b-d).


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To further evaluate the reproducibility, the photovoltaic performances of sixteen cells of a batch are characterized. As shown in Figure 6a-d, the devices present excellent reproducibility and an outstanding statistical distribution. Long-term stability is vitally essential to PSCs as well. The stability of the as-fabricated C-PSCs was examined under high humidity (65±5% RH, 25±5℃ in ambient air) and high temperature (75±5℃, 25±5% RH in ambient air) conditions without encapsulation for 90 days. All samples were kept in dark condition. As presented in Figure 7a-b, all of the normalized photovoltaic metrics maintain approximately 100% of initial parameters, showing no obvious decrease after 90 days testing. The excellent long-term stability can be mainly ascribed to the hydrophobicity of the SWCNT/carbon layer, which prevents moisture from contacting the perovskite layer.47

Figure 6. Statistical deviation of the photovoltaic parameter for 16 solar cells using CE A, B, C and the CNT bridging method: (a) Voc, (b) Jsc, (c) FF, and (d) PCE.


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Figure 7. Device performance parameters (normalized) of the C-PSCs with the CNT bridging method under (a) 65±5% RH, 25±5℃ and (b) 75±5℃, 25±5% RH after aging for 90 days.

Conclusion To conclude, we report a CNT bridging method for the fabrication of paintable C-PSCs. The CNT bridge penetrated into both the perovskite and carbon electrode resulting in superior charge extraction and transfer ability, forming an intimate perovskite/CE interface, and enhancing the electrical conductivity of the carbon composite electrode. Due to such merits, a PCE of 15.73% with an FF of 0.72 was obtained. Furthermore, PSCs based on the CNT bridging method presented superior long-term stability under severe hydrothermal conditions (65±5% RH, 25±5℃ and 75 ± 5 ℃ , 25 ± 5% RH) over 90 days. This investigation may be beneficial for future practical applications and the commercialization of C-PSCs.

ASSOCIATED CONTENT Supporting Information.


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Detailed cross-sectional SEM of perovskite layer and devices fabricated with CNT bridging method. Bode phase plots of four deices. J−V curves measured at different scanning directions. UPS spectra of perovskite layers with and without SWCNT.

AUTHOR INFORMATION Corresponding Author *Prof. Hong Li Liu, E-mail: [email protected]; *Prof. Xiang Gao Li, E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21676188); the National Key R&D Program of China (2016YFB0401303); and the Key Projects in National Science Foundation of Tianjin (16JCZDJC37100).

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Table of Contents Graphic


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A Carbon Nanotube Bridging Method for Hole Transport Layer-Free

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