Boron Doping of Multiwalled Carbon Nanotubes Significantly

Mar 13, 2017 - In the present work, we have demonstrated that B-doping of multiwalled carbon nanotubes (B-MWNTs) significantly enhances the hole extra...
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Boron Doping of Multi-walled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-based Perovskite Solar Cells Xiaoli Zheng, Haining Chen, Qiang Li, Yinglong Yang, Zhanhua Wei, Yang Bai, Yongcai Qiu, Dan Zhou, Kam Sing Wong, and Shihe Yang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00200 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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Boron Doping of Multi-walled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-based Perovskite Solar Cells Xiaoli Zheng,1,† Haining Chen,1,†,‡ Qiang Li,§ Yinglong Yang,† Zhanhua Wei,† Yang Bai,† Yongcai Qiu,† Dan Zhou,† Kam Sing Wong,§ and Shihe Yang*,†

†

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

Kowloon, Hong Kong

‡

School of Materials Science and Engineering, Beihang University, Beijing 100191, China

§

Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay,

Kowloon, Hong Kong

1

These authors contributed equally to this work.

*Corresponding author, Email: [email protected]

ABSTRACT: Compared with the conventional perovskite solar cells (PSCs) containing hole-transport materials (HTM), carbon materials based HTM-free PSCs (C-PSCs) have often suffered from inferior power conversion efficiencies (PCEs) arising at least partially from the inefficient hole extraction at perovskite/carbon interface. Here, we show that boron (B) doping of multi-walled carbon nanotubes (B-MWNTs) electrodes are superior in enabling enhanced hole extraction and transport by increasing work function, carrier concentration and conductivity of MWNTs. The C-PSCs prepared using the ACS Paragon Plus Environment

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B-MWNTs as the counter electrodes to extract and transport hole carriers have achieved remarkably higher performances than that with the undoped MWNTs, with the resulting PCE being considerably improved from 10.70% (average 9.58%) to 14.60% (average 13.70%). Significantly, these cells show negligible hysteretic behavior. Moreover, by coating a thin layer of insulating aluminum oxide (Al2O3) on the mesoporous TiO2 film as a physical barrier to substantially reduce the charge losses, the PCE has been further pushed to 15.23% (average 14.20%). Finally, the impressive durability and stability of the prepared C-PSCs were also testified under various conditions, including long-term air exposure, heat treatment and high humidity.

KEYWORDS: carbon based perovskite solar cell, boron-doping carbon nanotubes, hole extraction, Fermi level, carrier concentration

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Over the past few years, organic-inorganic metal halide perovskite solar cells (PSCs) have attracted tremendous attention for their unique photovoltaic properties and sharp rise in power conversion efficiency (PCE).1-8 Intriguingly, it was soon found that such PSCs could still work even without the costly hole transport materials (HTM) due to the ambipolar semiconducting characteristics of the light-harvesting perovskite materials.9-11 Against this backdrop, carbon materials came on the scene due to their suitable electronic properties, moisture-resistant hydrophobicity and outstanding chemical stability. Literally, carbon-based HTM-free PSCs (C-PSCs) emerged as a commercially promising alternative because of their low cost and simplified device fabrication process, as well as the potential to overcome the environmental instability of the PSCs.12-18 Up to now, extensive efforts have been devoted to enhance the performance of C-PSCs, for instance, by modulating carbon morphologies,19-23 designing device architectures,24,

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blending organic or inorganic HTM (such as NiO),26,

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and improving

perovskite crystallization and film quality.28, 29 However, the inherently inefficient hole selectivity of carbon materials in C-PSCs makes it especially challenging to achieve an PEC parity with the conventional PSCs containing organic HTM and metal electrode. Therefore, the way forward is to develop a facile strategy to enhance the hole extraction in the C-PSCs. In terms of the electrode structural design, the carbon nano-assemblies consisting of charge transport highways with good connectivity are expected to enhance hole extraction. As such, carbon nanotubes (CNTs) have been naturally thought to be an ideal electrode material due to their excellent carrier mobility, chemical stability, mechanical flexibility and solution processability.30, 31 Recently, CNTs were incorporated into the PSCs to perform various roles.32-36 For instance, CNTs were employed as conductive fillers in organic HTM to enhance their carrier mobility and conductivity.37-41 Moreover, pure CNTs were directly configured as cathode electrode, with CNTs serving as both hole collecting and transport layer.42-45 Charge transfer dynamic studies have shown that band bending in the CNTs layers could facilitate charge extraction and slow recombination.46, 47 PSCs using CNTs as the counter electrode without companion HTM, however, often display relatively low performances.41 Here the Femi levels (EF) of pure CNTs are far above the valence band (VB) of perovskite,48 with a mismatched ACS Paragon Plus Environment

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energy level alignment, leading to inefficient hole extraction rate and low open-circuit voltage. Moreover, the inferior conductivity of the CNTs film make the devices suffer from low hole transport behavior. Fortunately, many strategies could be borrowed to manipulate the hole extraction and transport ability of CNTs and one of the most effective strategies is the element doping strategy. Theoretical and experimental studies have shown that B-doping can lead to the formation of an acceptor state by downshifting the EF and increasing the number of participating conduction channels.49-51 In fact, N- or B-doped CNTs, as highly selective electron- or hole transport enhancement materials, have been proved to remarkably enhance the performance of polymer solar cells.52 To our knowledge, however, no special research has been conducted to study interfacial charge transfer between such doped-CNTs and perovskite, not to mention the performance of the doped CNTs for hole extraction in C-PSCs. In the present work, we have demonstrated that B-doping of multi-walled carbon nanotubes (B-MWNTs) significantly enhances the hole extraction in C-PSC devices compared with the undoped ones, and the performances are improved by increasing the concentration of B-doing in B-MWNTs. Impressively, a record PEC of 15.23% has been achieved by the C-PSCs based on the B-MWNTs. B-doping of MWNTs. For the MWNTs electrode, we dope it with B atoms through thermal annealing of MWNTs and B source, i.e. H3BO3, under inert gas atmosphere. The chemical reaction for the B-MWNTs has been proposed generally (equation 1), where incomplete substitution reaction occurs, leading to the formation of B-rich islands (e.g., BC3) in the nanotube skeletons.50, 53 2x H3BO3 + (2 + 3x) C(nanotube) → 2 BxC(nanotube)+ 3x CO(g) + 3x H2O(g)

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(1)

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Figure 1. Structural, chemical and electronic characteristics of different MWNTs. (A-C) TEM images of (A) MWNTs, (B) T-MWNTs and (C) B-MWNTs. (D) XRD patterns, (E) Raman spectra, (F) deconvolution of C1s XPS spectra of (a) MWNTs, (b) T-MWNTs and (c) B-MWNTs. (G) Deconvolution of B1s XPS spectrum of B-MWNTs. (H) UPS spectra and (I) conductivity and carrier concentration of (a) MWNTs, (b) T-MWNTs and (c) B-MWNTs. To scrutinize the structure information of the MWNTs and B-MWNTs, we performed morphology and structural characterizations by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively. Thermal treated MWNTs without B-doping (T-MWNTs) was also prepared as a control. As shown in Figure 1A-C and Figure S1, the lengths and diameters of the T-MWNT and B-MWNT are similar to those of the pristine MWNTs, and the interplanar spacing of these nanotubes has no changes with approximately 0.34 nm. It reveals that the high thermal treatment and B-doping do not change the lattice constant of MWNTs. Nevertheless, a great deal of amorphous substances surrounds the outside surface of the pristine MWNTs (marked by white arrows, Figure 1A), which might be amorphous carbon and residual impurity. Conversely, the T-MWNTs (Figure 1B) and B-MWNTs (Figure 1C) exhibit relatively cleaner surfaces. XRD patterns of the MWNTs products (Figure 1D) all show a strong Bragg peak at around 2θ = 25.65o, which can be ACS Paragon Plus Environment

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assigned to (002) crystal plane of graphite. Compared with the pristine MWNTs, the peaks of T-MWNTs and B-MWNTs are almost unchanged, meaning that the thermal-annealing and B-doping process did not dramatically change the crystalline graphene framework.53 A more detailed analysis of the chemical structure and compositions of the MWNTs samples can be derived from Raman spectra and X-ray photoelectron spectroscopy (XPS) measurements. The Raman spectra of the three kinds of MWNTs are depicted in Figure 1E. We found three distinct peaks around 1350 cm-1, 1590 cm-1, and 2690 cm-1 for all MWNTs, labeled as D-band, G-band, and G’-band, respectively. The D band originates from structural defects and the ratio of D band and G band intensities (ID/IG) can be used to indicate the disorder degree and structural quality of the nanotubes.54 As shown in Figure 1E, the ID/IG decreases from 1.34 for pristine MWNTs to 1.31 for T-MWNTs, while for B-MWNTs it further decreases to 1.13. To ascertain the ID/IG changes, 4 or more spots on each sample were collected, which were summarized in Figure S2. It has been well established that high temperature annealing process can efficiently decrease defects and enhance the graphitization of the MWNTs, thus give a lower ID/IG for T-MWNTs.55 Interestingly, the B-MWNTs exhibit even lower ID/IG than those T-MWNTs, suggesting more effective reduction of MWNTs during the B-doping process.56 Moreover, the reduction and doping effects can also be monitored by the peak positions of the Raman spectra.57, 58 For the T-MWNTs, all the three bands shift to lower frequency, indicating the reduction of T-MWNTs during thermal treatment.59, 60 For the B-MWNTs, the frequencies of D band and G’ band further reduce, which might be resulted from the increased reduction degree (lowest ID/IG). Nevertheless, its G band is upshifted compared with that of T-MWNTs, suggesting carriers transfer occurs between B and the MWNTs due to the charge transfer can increase the force constant and thus enhance the lattice frequency of the B-MWNTs.56, 61 The more detailed chemical reduction and doping mechanisms were uncovered by XPS characterization. The high-resolution C1s XPS spectra of the MWNTs samples are shown in Figure 1F. Before thermal annealing, the pristine MWNTs is characterized by a distinct peak at 284.5 eV, and two peaks at 285.6 and 288.1 eV. The main peak at 284.5 eV arises from the ring C (C-C bonds), and the ACS Paragon Plus Environment

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two peak at 285.6 and 288.1 eV are typically assigned to the C-O and C=O, respectively. For the T-MWNTs and B-MWNTs, the peak at 288.1 eV disappears, indicating that oxygen functional groups such as carbonyl (C=O) and carboxyl (O=C-OH) groups are mostly removed from the MWNTs under high temperature treatment. The peak at 285.6 eV still remains but with decreased intensity and the B-MWNTs shows the lowest one (marked by light blue area). The O1s XPS spectra and the C and O atomic percentages (at%) of the three samples were also provided to further investigate the chemical reaction (Figure S3). Similarly, in the O1s XPS spectra, the peak at 531.0 eV contributed to C=O groups disappears in the T-MWNTs and B-MWNTs samples, and the peak at 532.4 eV arising from C-OH groups is still present.62 This is because most of the carbonyl and carboxyl groups in MWNTs decompose below 900 oC and partial hydroxyl groups decompose at 1000 oC.63 For quantitative characterization, the percentage of O and C decreases and increases, respectively, in the sequence of MWNT, T-MWNTs and B-MWNTs, further suggesting that the B-doping can facilitate the reduction of MWNTs, consistent with the Raman data. The B1s peaks of the B-MWNTs product are shown in Figure 1G, which is characterized by a peak at 191.0 eV with a strong shoulder near 192.6 eV. This B1s peak is different from that of H3BO3 and/or its derivative B2O3 near 193.0 eV, demonstrating that B atoms have been successfully linked to carbon atoms in the MWNT network.64 The peaks at 191.0 and 192.6 eV arise from the B atoms of BC3 and BC2O, respectively, and the total atomic percentage of B reaches up to 0.27 at%, measured by elemental analysis of XPS.65 It has previously been established that B atom will be preferentially doped into the defects of carbon materials and enhance their graphitization,53 which can well explain the increased reduction degree in B-MWNTs. Considering all the results, we conclude that a thermochemical H3BO3 reaction with MWNTs does indeed produce B substituted MWNTs. Accordingly, schematic chemical structures of the MWNTs and B-MWNTs are shown in Figure S4, where the MWNTs were successfully doped by B to B-MWNTs though thermal annealing in Ar atmosphere at 1000 oC, where B-doping and reduction occur simultaneously.63, 65 To investigate how B-doping affects the electronic properties of the MWNTs, ultraviolet photoelectron spectra (UPS) and Resistivity/Hall effect measurement of both non-doped and the doped ACS Paragon Plus Environment

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samples were conducted. Figure 1H shows typical UPS of different MWNTs and their EF was obtained by subtracting the binding energies of the secondary electron cutoffs from the excitation energy (21.2 eV) of HeI UPS spectra. To ascertain the work function data from UPS curves, each sample was measured three times by UPS and the obtained work function data was very close. The EF of the pristine MWNTs is upshifted from -4.46 to -4.43 eV for T-MWNTs, which might be resulted from the removal of some electron withdrawing groups such as carbonyl and carboxyl groups after thermal treatment. In contrast, the EF of BT-MWNT downshifts to -4.55 eV, similar to other reported works that the B doping of carbon nanotubes leads to the formation of an acceptor state in the band gap above the valence band edge.51, 66 For an efficient hole extraction at the perovskite/MWNTs interface, the MWNTs should have a EF that closely matches the valence band of perovskite. This downshift of the EF of B-MWNTs is expected to facilitate selective hole transfer at the interface.23, 52 In addition, Resistivity/Hall effect measurements were conducted to evaluate the room-temperature (RT) specific conductivity and carrier concentration of the MWNTs samples (Figure 1I). Clearly, the pristine MWNTs show low specific conductivity (22.69 S cm-1) and carrier concentration (4.49 × 1020 cm-3). After thermal treatment, the conductivity and carrier concentration of the T-MWNTs slightly increase to 26.21 S cm-1 and 5.13 × 1020 cm-3, respectively, mainly due to the reduced amorphous carbon impurities and defects. Excitingly, B-doping is proved to remarkably enhance the conductivity of B-MWNTs to 38.65 S cm-1 and also the carrier concentration to 1.97 × 1021 cm-3. This demonstrates that introducing B acceptors into the MWNTs skeleton creates free carriers in the sp2 network and renders the B-MWNTs more conductive.67 Based on the above electronic characterizations, the substitutional B can not only lower the EF of the tubes but also increase the number of participating conduction carriers, which are assumed to be beneficial to selective hole extraction and transport.49, 52 From the above results, we learn that the B-doping greatly influence the structural, chemical and electronic properties of the MWNTs. To evaluate the effects of the concentration of B heteroatoms on the properties of MWNTs, we dope the MWNTs with B under different annealing temperatures of 1000 o

C, 800 oC and 600 oC, and the characterization results are shown in Figure S5. As expected, the defects ACS Paragon Plus Environment

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and oxygen-contents decrease largely from 600 oC to 800 oC and further decrease slightly to 1000 oC (Figure S5A-D, Raman and XPS results). The B-doping concentration changes from 0.02 at% of B-MWNTs-600 to 0.20 at% of B-MWNTs-800 and 0.27 at% of B-MWNTs-1000, demonstrating the formation of more B-C bonds in the MWNTs hexagonal carbon network with increasing temperature, which has been proved to be contributed largely to the increased number of hole-type charge carriers.68 Similarly, as shown in Figure S5E, the EF of the B-MWNTs is depressed gradually when raising the annealing temperature, i.e. -4.51 eV for B-MWNTs-600, -4.54 eV for B-MWNTs-800 and -4.55 eV of B-MWNTs-1000. Consequently, the conductivity and carrier concentration of the B-MWNTs also increase with the B-doing concentration (Figure S5F). C-PSCs design based on B-doped MWNTs. As evidenced above, the B-MWNTs exhibit superior properties compared with the undoped one, i.e. reduced defects, lowered EF, and increased conductivity and numbers of conduction carriers, which are all prerequisites for high performance PSCs, especially for the HTM-free C-PSCs. Therefore, C-PSCs based on the different MWNTs samples were fabricated to further investigate the key role of B-MWNTs. We fabricated the MWNTs electrode based C-PSCs by a two-step chemical embedment method. Detailed fabrication procedures were shown in Figure S6 and Experimental section (Supporting Information). Firstly, we deposited a 50 nm TiO2 compact layer (c-TiO2) onto Fluorine-doped tin oxide (FTO) by spray pyrolysis. Then a ~350 nm of m-TiO2 was spin-coated on the compact layer and calcinated. The PbI2 layer, with a capping layer of ~380 nm, was deposited by spin-coating. This was followed by the deposition of a continuous and interlaced MWNTs network (~20 um, Figure S7), accomplished by drop-casting its chlorobenzol solution (4 mg ml-1). Finally, MAPbI3 perovskite film (with a capping layer of ~600 nm) was converted in a methylammonium iodide (MAI) mixed solvent of 2-isopropanol/cyclohexane, which can not only accelerate the conversion of PbI2 to MAPbI3 but also suppress the Ostwald ripening process, resulting in a high-quality perovskite layer.29 The morphology and crystal structure of the PbI2 and MAPbI3 layers without MWNTs were examined by means of SEM and XRD spectra (Figure S8), respectively. The

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compact and uniform capping layer exhibit pure phase and high crystallinity, which is required for high photovoltaic performance.

Figure 2. Schematic illustration of cell configuration, B doping of MWNTs and charge behavior in C-PSCs. (A) Schematic illustration of the C-PSC configuration. (B) Schematic diagram of B-doping of MWNTs to B-MWNTs. (C) Schematic illustration of charge transfer enhancement by B-MWNTs through (I) lowering the EF of MWNTs and (II) increasing the number of conduction carriers in B-MWNTs electrode. The intimate interface between perovskite and MWNTs is marked by black dotted rectangle in (II). The schematic device architecture of a typical C-PSC with uniform MWNTs counter electrode is shown in Figure 2A. Once illumination, the MAPbI3 layer absorbs light and generates electron-hole pairs. The electrons are injected into mesoporous TiO2 film (m-TiO2) and the holes are transferred to the MWNTs counter electrode, respectively. We chemically dope the MWNTs electrode by atomic B (Figure 2B), to efficiently improve the hole extraction and transport in the C-PSC device. As demonstrated in Figure 2C, we propose that the potential benefit of the B-MWNTs electrode could be threefold. First, the B-doping can induce an extra p-type band and lower the EF of B-MWNTs (I), which could be beneficial for the efficient hole extraction.50, 51 Second, the incorporated B could increase the number of conduction carriers of MWNTs electrode (II), which would improve the conductivity and hole transport.49 Third, such continuous and porous MWNTs electrode allows sufficient perovskite ACS Paragon Plus Environment

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precursor infiltration, and form intimate interface with perovskite crystals by chemical embedment approach (marked by black dotted rectangle in Figure 2C-II). Photovoltaic performance characterization. By using the two-step chemical embedment method, tens of nanometers-thick intimate cross-layers between CNTs and perovskite can be formed without destroying the overall perovskite crystals, as shown in Figure 3A (marked by the white dotted rectangle). High resolution SEM was further used to distinctly characterize the interface structure of MWNTs/perovskite (Figure S9), where the MWNTs were strongly embedded in the top-layer of perovskite crystals. And the rough surface of MWNTs clearly illustrate the strong interaction between MWNTs and perovskite crystals. The band alignments of relevant functional layers are shown in Figure 3B.20,69 Thanks to the ambipolar property of MAPbI3 perovskite, electrons can be injected into the conduction band of m-TiO2, and the holes can be effectively collected by the B-MWNTs electrode owing to the suitable EF, increased number of conduction channels and increased conductivity.70, 71

Figure 3. Photovoltaic device, energy level structures and photovoltaic performances of C-PSC devices with different MWNTs. (A) A cross-sectional SEM image of a complete C-PSC. (B) Band alignments ACS Paragon Plus Environment

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of the solar cell. (C) Photocurrent density-voltage curves and (D) IPCE spectra of the C-PSCs with different MWNTs. (E) Photocurrent density as a function of time for the cells held at the maximum output power point (0.67 V for the MWNTs-PSC, 0.65 V for the T-MWNTs-PSC and 0.73 V for the B-MWNTs-PSC). (F) Statistics on performance variations of the C-PSCs based on different MWNTs: (a) MWNTs, (b) T-MWNTs and (c) B-MWNTs (20 devices for each type). Figure 3C-F show photovoltaic performances of the C-PSCs based on the MWNTs, T-MWNTs and B-MWNTs. The photocurrent density-voltage (J-V) curves of the C-PSCs shown in Figure 3C were measured under standard global AM 1.5 light illumination (100 mW/cm2). Obviously, device with the MWNTs shows the lowest performance, with an open-circuit voltage (Voc) of 0.84 V, a short-circuit photocurrent density (Jsc) of 17.94 mA/cm2, fill factor (FF) of 0.71 and a photoelectric conversion efficiency (PCE) of 10.70%. The apparent low Jsc should be attributed to the poor quality of MWNTs with large amount of amorphous substances as well as low conductivity and high defect density (as indicated in Figure 2). While, the cell with T-MWNTs shows a higher Jsc of 20.46 mA/cm2, together with a Voc of 0.82 V, a FF of 0.72, and a PCE of 12.08%. High temperature thermal treatment can largely reduce the defects and amorphous substances, and increase the conductivity of the T-MWNTs. Thus charge transport in the T-MWNTs based device is largely improved with increased Jsc and FF. However, the slightly decreased Voc might be resulted from the upshifted EF. Impressively, the device based on B-MWNTs film delivers a significant enhancement in the photovoltaic parameters, with a Voc of 0.90 V, a Jsc of 21.35 mA/cm2, a FF 0.76, and a PCE of as high as 14.60%. Notably, the significant improvements for these parameters (Voc, Jsc, and FF) are likely to stem from two effects resulting from the improved MWNTs film quality by B-doping. First is the higher charge extraction capability with downshifted EF, reducing the charge recombination probability and shunt path, and thus enhancing the Voc and FF. Second, the improved conductivity and the increased number of conduction channels as well as carrier concentrations facilitate charge transfer and transportation, and increase the Jsc primarily. Figure 3D shows the incident-photon-to-current conversion efficiency (IPCE) spectra for the three C-PSCs based on different MWNTs. The use of the B-MWNTs gives a noticeable improvement of the ACS Paragon Plus Environment

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IPCE over the whole region. This IPCE enhancement correlates well with the higher photocurrent in the B-MWNTs based C-PSCs, mainly due to more effective charge extraction and transportation. Integrating the overlap of the IPCE spectrum yield a photocurrent density of 17.02, 18.86, and 19.88 mA/cm2 for MWNTs, T-MWNTs and B-MWNTs based C-PSCs, respectively, which are in excellent agreement with the measured Jsc (Figure 3C). Moreover, monitoring the stabilized current density at the maximum output power points is a powerful parameter to present the cell performance alongside the J-V scan derived PSCs. As shown in Figure 3E, the photocurrent densities of these devices exhibit a fast photo-response to light on and stabilize quickly within seconds and yield a stabilized PCE around 10.25%, 11.40% and 14.10% for MWNTs-PSC, T-MWNTs-PSC and B-MWNTs-PSC, respectively, measured 180 s after. To further check the reproducibility of the C-PSCs, the statistics on performance variations of the C-PSCs based on different MWNTs are shown in Figure 3F and the statistics data are summarized in Table S1. Obviously, the B-MWNTs-PSCs exhibit small standard deviations and hence improved device reproducibility. The hysteresis effect of these C-PSCs were also investigated to further examine the device performances. Interestingly, the devices based on different MWNTs all exhibit little hysteresis even under different scan rates (Figure S10 and Table S2). The superior interface between MWNTs and the perovskite prepared by chemical-embedment method together with the high quality of MWNTs layers are mainly responsible for the negligible hysteresis and the corresponding fluent hole transfer process.43 Charge-carrier dynamics characterization. To gain a deeper insight into the photovoltaic performance improvement by the B doping, we evaluated the charge carrier extraction capability by time-resolved photoluminescence (TRPL) decay measurement. Figure 4A shows the PL decays of the pure MAPbI3 films and different MAPbI3/MWNTs films deposited on glass slide. Global biexponential fits were performed to quantify the carrier dynamics and the detailed fitted PL lifetimes (τ1, τ2) are collected in Table S3.72, 73 For simplicity, the weighted average value of τ1 and τ2 is utilized to represent the PL lifetime (τ). The MAPbI3 itself exhibits a long PL lifetime (123.43 ns), demonstrating slow carrier recombination in the perovskite layer.74 When coming into contact with the MWNTs electrode, ACS Paragon Plus Environment

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the PL lifetimes are shortened to varying degrees for different MWNTs contacts. As illustrated in Figure 4A, the fitted PL lifetime follows the sequence as τ (MAPbI3/B-MWNTs) < τ (MAPbI3/T-MWNTs) < τ (MAPbI3/MWNTs) < τ (MAPbI3). We find that the τ of the MAPbI3/T-MWNTs contacts is substantially shorter than that of MAPbI3/MWNTs (23.35 versus 13.19 ns; Figure 4Aa,b), which we attribute to rapid hole extraction because the increased conductivity greatly accelerates the hole transport and prevents carrier accumulation at the interface.74,

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The lowest PL lifetime of

MAPbI3/B-MWNTs (Figure 4Ac) indicates that the photoinduced holes in perovskite can be more efficiently extracted by the B-MWNTs due to its lowest EF as well as increased conductive carrier concentration.

Figure 4. Charge-carrier dynamics of the C-PSCs based on different MWNTs. (A) TRPL spectra of (a) MAPbI3/MWNTs, (b) MAPbI3/T-MWNTs, (c) MAPbI3/B-MWNTs and (d) pure MAPbI3 on glass, and the TRPL spectra were taken at the peak emission wavelength of perovskite after excitation at 400 nm (3.8 MHz, 55 uW). (B) IS Nyquist plots of C-PSCs based on different MWNTs, obtained under short-circuit state and full-sun illumination (AM 1.5, 100 mW/cm2) condition. Inset is the equivalent circuit used for fitting the IS results, in which Rs, Rct and CPE represent the series resistance, charge transfer resistance and cell capacitance, respectively. (C) τt as a function of photon flux, obtained from ACS Paragon Plus Environment

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IMPS measurement. (D) τr as a function of photon flux, obtained from IMVS measurement. (E) Schematic diagram of hole extraction and transport capability of (III) B-MWNTs than (I) MWNTs and (II) T-MWNTs. In order to understand the charge-carrier dynamics in the devices, we resorted to impedance spectroscopy (IS) characterization under short-circuit and full-sun illumination condition.76-78 Since the photocurrent in this condition is significantly larger than dark current, the IS mainly give information on the effective charge transfer dynamics without the obvious interference of charge recombination.29, 79 As shown in Figure 4B, only one single distinct semicircle is observed in the Nyquist plots for all devices, corresponding to the single peak in Bode phase plots (Figure S11) at high frequency region (2 MHz to 10 KHz). This phenomenon suggested that all charge transfer dynamics at different interfaces in C-PSCs may be similar (at the close frequency region), which leads to the overlay of different RC features and hence only exhibits one semicircle in the Nyquist plots. As all the three C-PSCs have similar TiO2/MAPbI3 interface, we assign the predominant reason for the variation in RC semicircle in Nyquist plots to the potential difference in the charge transfer dynamic at the different MAPbI3/MWNTs interfaces.43, 80 By fitting the IS results using the equivalent circuit in the inset of Figure 4B, the charge transfer resistance (Rct) of the B-MWNTs-PSC (270 Ω) is considerably lower than those of MWNTs-PSC (710 Ω) and T-MWNTs-PSC (390 Ω), which supports the higher charge transfer rate at the perovskite/B-MWNTs interface. Another important parameter series resistance (Rs) obtained from the intersection point of IS semicircle with the real axis is purportedly affected by the charge transport in the devices. As other parts of all three PSCs are similar, the difference in Rs should reflect the different conductivity of MWNTs electrodes. The Rs follows the sequence of B-MWNTs-PSC (12.32 Ω) < T-MWNTs-PSC (17.82 Ω) < MWNTs-PSC (25.62 Ω), confirming the outstanding charge transport property in the B-MWNTs-PSCs. The photoexcited charge-carrier dynamics in these devices were further investigated by intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), which can be used to reveal the charge transport and recombination events in the PSCs.81,

82

Generally, the charge transport time

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constant (τt) and recombination time constant (τr) can be derived from the IMPS and IMVS responses, respectively, which are characterized by a semicircle with a radial frequency at its maximum (fIMPS and fIMVS).82 The τt and τr can be calculated according to the equation τt = 1/(2πfIMPS) and τr = 1/(2πfIMVS), respectively. As illustrated in Figure 4C,D, the B-MWNTs-PSCs exhibit the lowest τt and highest τr than that of MWNTs-PSCs and T-MWNTs-PSCs, confirming much faster charge transport and slower charge recombination owing to the very rapid hole carrier extraction.23 Taken together, the diverse hole extraction and transport behaviors of the different MWNTs samples are schematically illustrated in Figure 4E. The following reason might account for the superior hole extraction and transport capability of B-MWNTs in C-PSCs. The obviously more efficient hole extraction due to the lower FE, higher conductivity and charge transport property of B-MWCTs would enhance the electron-hole separation at perovskite/B-MWCTs interface and hence suppress charge recombination. The higher electron concentration resulting from more efficient separation would sequentially fill more trap states in perovskite and TiO2, which would accelerate the electron transport in perovskite and TiO2. Moreover, the influence of the concentration of B-doping on the photovoltaic performances of the B-MWNTs-PSCs was also investigated, and the results were displayed in Figure S12. As shown in Figure S12A, the PCE of C-PSCs greatly increases from 12.57% (Voc 0.85 V, Jsc 20.54 mA/cm2, FF 0.72) for B-MWNTs-600-PSC (with boron doping at 600 oC) to 14.17% (Voc 0.89 V, Jsc 21.23 mA/cm2, FF 0.75) for B-MWNTs-800-PSC (with boron doping at 800 oC). Further raising the temperature from 800 oC to 1000 oC, the PCE is slightly improved to 14.60% (Voc 0.90 V, Jsc 21.35 mA/cm2, FF 0.76). The detailed charge-carrier dynamics were also investigated by TRPL, IS and IMPS/IMVS (Figure S12C-F). These results fully confirmed that the hole extraction and transport can be remarkably improved by B-doping of MWNTs and the concentration of B-doping directly influence the performances of the devices. More works need to be conducted to investigate the effect of the high concentration of B-doing as well as polyatoms-doping of carbon electrode on the performances of C-PSCs.

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It is well known that intimate interface between perovskite and carbon electrode is a prerequisite for high performance devices. Herein, we also compared the device performances prepared by chemical embedment method adopted in this work and post-deposited method, i.e. post-depositing MWNTs on pre-formed perovskite film (post-B-MWNTs-PSC). The morphology and photovoltaic performances are shown in Figure S13. Obvious gaps can be seen at the perovskite/B-MWNTs interface while using the post-deposited method. As a result, inferior PCE of 12.67% (Voc 0.90 V, Jsc 20.40 mA/cm2, FF 0.69) with obvious J-V hysteresis was obtained, which mainly resulted from the inferior interface between the perovskite and B-MWNTs interfaces. The photovoltaic performance together with TRPL decay and IS characterizations further reveal that the intimate interface of MAPbI3/B-MWNTs is another requirement to guarantee the efficient hole extraction.79 Performance improvement by coating an ultrathin Al2O3 blocking layer. Base on the above discussion, one can conclude that there are mainly two determinants for the high efficient B-MWNTs-PSC devices: i.e., the B-doping of MWNTs and intimate perovskite/MWNTs interface, which can both contribute to the superior charge transfer behavior in the HTM-free C-PSCs. It should be noticed that the key limitation for the cell performance is the relatively low Voc, which might be affected by the recombination process in the cells. Hence, we tried to deposit a thin insulating aluminum oxide (Al2O3) layer on the m-TiO2 film (TiO2/Al2O3) to act as physical barrier for substantially avoiding contact between CNTs and meso-TiO2 as well as decreasing the back electron transfer.83-85 The homogeneously deposited Al2O3 overlayer is extremely thin with a thickness of 2~3 nm, without blocking the pores in the m-TiO2 architecture for following perovskite filling and electron injection (Figure S14-15).

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Figure 5. Performances of the devices prepared using the m-TiO2 coated with an utrathin Al2O3 layer (TiO2/Al2O3-B-MWNTs-PSC). (A) Schematic of conformably coated thin Al2O3 layer on preformed m-TiO2 film. (B) Ultrathin Al2O3 coating on m-TiO2 acts as physical barrier for reducing charge losses (process ② and ③). (C) Recombination resistance (Rrec) at a function of Vappl for both cells, which were obtained from IS characterization under dark with different Vappl. (D) J-V curves measured at forward and reverse scanning directions (100 mV/s). (E) IPCE spectrum and integrated current density. (F) Measured photocurrent output at the maximum power point (0.76 V) and calculated PCE vs. time. The

role

of

the

Al2O3

in

the

performance

of

the

B-MWNTs-PSC

devices

(TiO2/Al2O3-B-MWNTs-PSC) was investigated in Figure 5. As shown in Figure 5A,B, the conformally coated thin Al2O3 nanoparticle layer on m-TiO2 are expected to act as physical barrier to avoid possible direct-contact between m-TiO2 and B-MWNTs, as well as reduce charge losses by restraining the charge recombination processes from the m-TiO2 conduction band (CB) to the perovskite (process ②) and the B-MWNTs electrode (process ③). The effect of the Al2O3 on the restraining the interface charge recombination was investigated by IS. The IS spectra of the cells was recorded in the dark under different applied bias voltages (Vappl), where recombination behavior dominate the charge transfer processes at different interface. One obvious semicircle at intermediate frequency (10 KHz to 10 Hz) was observed for both C-PSCs with different bias voltages (IS data measured at 0.4 V were shown for ACS Paragon Plus Environment

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example, Figure S16), corresponding to the recombination reaction in C-PSCs. Since the perovskite/B-MWNTs interfaces are identical in these two devices, the difference in RC behavior should be attributed to the difference in the recombination dynamics at TiO2/perovskite and TiO2/Al2O3/perovskite interfaces. To well evaluate the recombination dynamics, recombination resistance (Rrec) are obtained by fitting the IS spectra.76, 82 Figure 5C summarized the Rrec of the two cells as a function of the Vappl. Clearly, the TiO2/Al2O3-B-MWNTs-PSC has a higher Rrec at varied Vappl, indicating that the presence of thin Al2O3 coating can efficiently reduce the carrier recombination. Therefore,

better

photovoltaic

TiO2/Al2O3-B-MWNTs-PSCs.

performances

Figure

5D

are

shows

expected the

to

photovoltaic

be

obtained

performances

by

the

of

the

TiO2/Al2O3-B-MWNTs-PSC and the photovoltaic performance statistics were summarized in Table S1. As expected, the TiO2/Al2O3-B-MWNTs-PSC gives a higher PCE of 15.23% than that of B-MWNTs-PSC (14.60%), mainly due to the increased Voc (0.92 V) and FF (0.77). To our best knowledge, this PCE is one of the top efficiencies among all of the HTM-free C-PSCs reported previously (Table S4). The apparent increased Voc and FF should be attributed to the lower recombination loss in the devices with a thin Al2O3 coating.86, 87 Forward scanning J-V curve was also recorded, which shows a small hysteresis behavior, further indicating that a thin Al2O3 coating almost has no influence on the electron transfer from perovskite to m-TiO2. Figure 5E presents the corresponding IPCE spectrum and an integrated current density of 20.08 mA/cm2, which is well consistent with the JSC at J-V curve. Moreover, the steady-state photocurrent output at the maximum power point (0.76 V) was measured as a function of time (Figure 5F). The photocurrent density stabilizes quickly within seconds and yields a PCE of 14.76% after 600 s measurement, confirming a small feature in C-PSCs. Device stability. The long-term stability of the PSCs is of vital importance for practical applications. To evaluate the stability of the C-PSCs in this work, the performances of cells (TiO2/Al2O3-B-MWNTs-PSCs), without sealing, were recorded by storing the device under dry air, high temperature as well as high humidity conditions, respectively. As shown in Figure S17, the C-PSC ACS Paragon Plus Environment

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shows a good stability and maintain 98% of its highest efficiency after 80 days storage in dry air. Moreover, the effect of the environmental thermal stressing (80 oC in an oven) and high humidity (~65 % at 25 oC) on the cell performances were further investigated. Impressively, as shown in Figure S18, the PCE decreased only by 15% and 7% of its highest value after two weeks storage at 80 oC and humidity of 65%, respectively. The good long-term stability at different conditions of the C-PSCs can be attributed chiefly to the hydrophobic property of the MWNTs and the compact interlinked MWNTs network films. In conclusion, we have demonstrated the effectiveness of B-doping carbon nanotubes in significantly enhancing the performances of carbon-based perovskite solar cells, leading to a PCE above 15%. This was achieved by addressing the issues on work function, hole extraction and transport of the carbon electrode. The high PCE is based on chemical embedding B-doped MWNTs in C-PSCs together with coating an insulating Al2O3 thin layer on m-TiO2. The B-doping of MWNT has been found to be able to lower the EF and increase the carrier concentration of MWNTs electrode. Along with the interfaces, the holes transferred toward nanotube cathodes efficiently because of the more matched EF and travelled fast due to the high conductivity of B-MWNTs. In addition, a thin layer of insulating Al2O3 was modified on the mesoporous TiO2 surface to act as physical barrier for preventing direct electrical contact between CNTs and m-TiO2 as well as decreasing the back electron transfer reaction. In particularly, the B-doped C-PSCs exhibit a considerably higher long-term stability at dry air, high temperature as well as high humidity conditions, even without sealing. Taken together, this work establishes the chemically embedded B-doped MWNTs based C-PSCs as a promising solar cells for practical applications, and provides a platform for understanding the fundamental nature and evolution of the hole extraction and transfer occurring in those C-PSCs. ASSOCIATED CONTENT Supporting Information

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Supporting Information Available: Experimental details, text, tables and figures giving detailed and additional material and devices characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the HK-RGC General Research Funds (GRF No. 16300915), the HK Innovation and Technology Fund (ITS/004/14), the NSFC/HK-RGC Joint Research Scheme (N_HKUST 610/14) and the RGC Areas of Excellence Scheme (AoE/P-02/12). REFERENCES (1) Xing, G. C.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Science 2013, 342, 344-347. (2) Dong, Q. F.; Fang, Y. J.; Shao, Y. C.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. S. Science 2015, 347, 967-970. (3) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131, 6050-6051. (4) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338, 643-647. (5) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Nature 2013, 501, 395-398. (6) Liu, D. Y.; Kelly, T. L. Nature Photon. 2014, 8, 133-138. (7) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Science 2015, 348, 1234-1237. ACS Paragon Plus Environment

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Boron Doping of Multi-walled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-based Perovskite Solar Cells

The chemically modified B-MWNTs are superior in enabling enhanced hole extraction and transport in carbon based perovskite solar cells by increasing work function, carrier concentration and conductivity of MWNTs electrode.

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