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Low-temperature easy-processed carbon nanotube contact for high-performance metal- and hole-transporting layer-free perovskite solar cells
Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 265–272
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Low-temperature easy-processed carbon nanotube contact for high-performance metal- and hole-transporting layer-free perovskite solar cells Chandu V.V.M. Gopi, Mallineni Venkata-Haritha, Kandasamy Prabakar, Hee-Je Kim* School of Electrical Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, South Korea
A R T I C L E I N F O
Article history: Received 8 May 2016 Received in revised form 29 July 2016 Accepted 7 September 2016 Available online 9 September 2016 Keywords: Low-temperature Carbon nanotubes Perovskite solar cells Stability
A B S T R A C T
Expensive and energy-consuming vacuum process of metal deposition with ambient-unstable hole transporters are incompatible with large-scale and low-cost production of perovskite solar cells (PSCs) and thus hampers their commercialization. For the first time, we demonstrate cost-effective novel carbon nanotube (CNT) paste that was applied to FTO substrate by the facile doctor blade method and processed at low temperature (100 C). Herein we report a new method of cost-efficient perovskite solar cells with the use of conventional hole transporters by directly clamping a selective hole extraction electrode made of CNT and a TiO2/perovskite photoanode. Most importantly, under optimized conditions in the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 7.83%. The PSC devices are fabricated in air without high-vacuum deposition which simplifies the processing and lowers the threshold of both scientific research and industrial production of PSCs. Electrochemical impedance spectroscopy demonstrates good charge transport characteristics of CEs on the photovoltaic performance of devices. The PSCs exhibited good stability over 50 h. The abundance, low cost, and excellent properties of the CNT material offer wide prospects for further applications in PSCs. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaics have been realized as suitable for generating electrical power by converting solar radiation into direct-current electricity with cost-effective and high-efficiency solar cells to meet the ever increasing demand for clean energy. The perovskite solar cells (PSCs) have recently attracted a great deal of attention because of their excellent photovoltaic performance, which is obtainable via facile and cheap processes [1,2]. Methylammonium lead halide (CH3NH3PbX3, X = Cl, Br, I) have invoked tremendous amounts of scientific and commercial interest for their notable characteristics of broad and intense absorption spectra [2], appropriate semiconducting properties [3], long carrier diffusion length [4,5], and facile solution processability. To date, for PSCs, extraordinary power conversion efficiencies (PCE) ranging from 3.8% [6] to more than 20.1% [7] have been obtained through optimization of preparing technology and device structure, which make it competitive in future commercialization.
* Corresponding author. E-mail address: [email protected] (H.-J. Kim). http://dx.doi.org/10.1016/j.jphotochem.2016.09.003 1010-6030/ã 2016 Elsevier B.V. All rights reserved.
To prepare CH3NH3PbI3, one of the critical challenges is to understand the influence of the ambient environment on the resultant perovskite thin films since perovskite crystals are sensitive to humidity under ambient conditions. The perovskite crystals degrade gradually when they are in contact with ambient moisture for a certain time [8,9]. Therefore, most of the highperformance perovskite solar cells are prepared in a glove box to avoid contacting moisture. However, fabricating PSCs under ambient condition is inevitable if we desire to transition from laboratory research into large-scale applications. Also, PSCs require an expensive and air-sensitive hole transporter (e.g., spiroMeOTAD) and a noble metal electrode (Au or Ag) deposited by complicated vacuum technologies [10,11]. However, noble metals such as Au or Ag as metal contacts are indispensable in these highperformance photovoltaic devices and are not conducive for the industrial development and market potential of PSCs. Therefore, the development of alternative materials and processes that show high performance and also inexpensive, abundant, environmentally friendly, easily processable, and not energy intensive is required for high-efficiency photovoltaic devices. In addition to being inexpensive, carbon nanotubes (CNTs) have rapid electron transfer kinetics, relatively large surface area, and high electrocatalytic activity. They have been applied in many other
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photovoltaic devices successfully, and impressive results have been achieved [12–14]. For most reported carbon materials, hightemperature treatments are necessary, which hinder the largescale production for solar cells [15,16]. Wei et al. reported a remarkably high PCE of 11.02% from PSCs by directly clamping a selective hole extraction electrode made of candle soot and a deliberately engineered perovskite photoanode [15]. Recently, carbon electrode layers, carbon nanotubes (CNT) and metal meshes have been applied as the top electrodes in PSCs [17–20]. Li et al. fabricated and reported a CNT network by chemical vapor deposition (CVD) method and transferred it onto a CH3NH3PbI3 substrate and achieved a solar cell efficiency of 6.87% [20]. All these evidences indicate that complicated holetransport materials and metal contacts are not necessary in fabricating a simpler perovskite-based photovoltaic device. In view of these reported results, herein, we successfully fabricated PSCs by using low-cost and robust mesoporous TiO2 and CNT layers as electron and hole selective contacts with doctor blade technique, respectively. The CNT electrode acted as a hole conductor or an electron blocking layer to suppress charge recombination and facilitate the hole extraction. The CNT-based devices TiO2/CH3NH3PbI3/CNT have much simpler architecture, in which CH3NH3PbI3 thin film is sandwiched between TiO2 (as the electron-selective layer) and a CNT electrode (as the hole-selective layer). Under a atomsperic conditions, we propose the concept of clamping solar cells by joining a perovskite photoanode and a CNT electrode, which has remarkable photovoltaic performance with a short circuit current (Jsc) of 18.54 mA/cm2, open circuit voltage (Voc) of 0.703 V, fill factor (FF) of 0.600, and power conversion efficiency (h) of 7.83% with an active area of 0.18 cm2, demonstrating the potential of applying CNTs as a charge collector, eliminating both the metal electrode and hole transporter in PSCs. The devices of TiO2/CH3NH3PbI3/CNT solar cells without encapsulation exhibited advantageous stability (over 50 h) in air. The electrochemical impedance spectroscopy (EIS) measurements demonstrated that holes in perovskite could be transported effectively to CNT counter electrode. Thus, using CNTs in mesoscopic PSCs will simultaneously cut both the cost of material and production costs, offering a more encouraging prospect for the commercialization of this photovoltaic technology. To the best of our knowledge, this is the first report for the use of low temperature processed CNT as counter electrode as well as hole-extracting electrode in holeconductor free perovskite-based solar cells. 2. Experimental section 2.1. Preparation of CNTs paste CNT thin films were deposited on well-cleaned FTO glass substrates with a sheet resistance of 7 V/cm2 (Hartford Glass) by the doctor blade method. Prior to deposition, FTO substrates were cleaned ultrasonically with acetone, ethanol, and distilled water for 10 min each. For the preparation of CNT paste, 0.05 g of MWCNTs (purchased from Carbon Nano-Material Technology Co., Ltd.; Type-Multiwall; Diameter- 20 nm; and Length- 5 mm) was placed in a pestle and mortar. Then, we added 2 mL of ethanol and grind the MWCNTs for 15 min by hand. Finally, we added 1 mL of terpineol and 0.03 mL of ethyl cellulose to the mixture and grind it for 15 min. Finally, the paste was collected in glass bottle and used for the preparation of CNT electrode in PSC. A digital photograph of the CNT paste is shown in Fig. S1y, Supporting information. 2.2. Fabrication of HTM-free perovskite solar cell Devices were fabricated on clean FTO glass substrates. A thin TiO2 compact film was coated on FTO glass. To prepare the
precursor solution, 0.15 M of titanium isopropoxide in ethanol was stirred for 1 h. A compact TiO2 film was coated by spin-coating method with a speed of 3000 rpm for 30 s, and the film was thermally annealed at 450 C for 2 h. Mesoporous TiO2 (mp-TiO2) film was deposited by spin-coating a diluted TiO2 paste (18NR-T Dyesol) in ethanol (2:7 w/w), followed by sintering at 500 C for 30 min. Lead iodide (PbI2) was then introduced into the TiO2 by spin-coating 20 mL of a 462 mg/ml solution of PbI2 in N,Ndimethylformamide (DMF) and kept at 90 C for 5 min under ambient conditions of 50–60% relative humidity. For Method 1 and Method 2 for clamping PSCs, 10 mg/ml of CH3NH3I was dissolved in isopropanol and spin-coated on top of the dried PbI2 layer at 0 rpm for 30 s (loading time), followed by 2000 rpm for 30 s at room temperature and drying at 90 C for 30 min in air. A solution of CH3NH3I on TiO2/PbI2 changes color immediately from yellow to dark brown, indicating the formation of CH3NH3PbI3. A piece of FTO/CNT film (CNT paste applied on FTO by doctor blade with an active area of 0.18 cm2 and heated at 100 C for 20 min) and the CH3NH3PbI3 film were directly clamped to make the Method 1 solar cells. For Method 2 solar cells, the CNT paste was applied on CH3NH3PbI3 film (area of 0.18 cm2) and heated at 100 C for 20 min. Then, another piece of FTO glass was clamped to the CH3NH3PbI3/CNT film. In Method 3, the CNT paste was transferred onto the PbI2 film before CH3NH3I bath treatment, and other procedures were similar to Method 2. 2.3. Characterizations The surface morphology, thickness, and elemental compositions of the electrodes were investigated using a field emission scanning electron microscope (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. High-resolution transmission electron microscopy was performed (HRTEM; Jem 2011, Jeol cop.) with a 4k x 4k CCD camera (Ultra Scan 400SP, Gatan cop.) at the Busan KBSI. X-ray diffraction (XRD) analysis was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Ka radiation operated at 40 kV and 40 mA. X-ray photon spectroscopy (XPS) was performed using a VG Scientific ESCALAB 250 with monochromatic AlKa radiation at 1486.6 eV and an electron take-off angle of 90 . The current-voltage (J-V) characteristics of PSCs were examined under one sun illumination (AM 1.5G, 100 mW cm2) using an ABET Technologies (USA) solar simulator with an irradiance uniformity of 3%. Electrochemical impedance spectroscopy (EIS) was conducted on the PSCs using a BioLogic potentiostat/ galvanostat/EIS analyzer (SP-150, France) under one sun illumination, and the frequency ranged from 100 mHz to 500 kHz. The electrical impedance was characterized using Nyquist and Bode plots. To perform the stability test, the PSCs were continuously irradiated with an AM 1.5G 100 mW cm2 illumination in working conditions, and the J-V curves were tested for different intervals of time in the course of 50 h. 3. Results and discussion Fig. S2ay shows an SEM image of CNTs deposited on FTO substrates by the doctor blade method. The top-view SEM image shows the fiber-like array morphology of the uniform CNT thin film. The thickness of the CNT film is 7.21 mm (Fig. S3y, Supporting information). Most of the fiber-like carbon was in fact tubular CNTs, which was confirmed by TEM in Fig. S2by. It is obvious from the images that all the nanotubes are hollow and tubular in shape. TEM images indicate that the nanotubes have high purity with uniform diameter distribution and contain no deformity in the structure. The crystalline structure of the CNT is highly ordered. In
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addition, uniform CNTs with lengths of up to 5 mm and diameters of around 20 nm were observed from TEM analysis, indicating no structural damage of MWCNTs. XRD was used to ascertain the quality and crystalline nature of nanotubes. Fig. S2cy shows the XRD pattern of the MWCNTs. The XRD pattern of MWCNTs exhibits a sharp peak around 2u = 26.4 and a broad peak centered at 2u = 42.4 corresponding to the graphite (002) and (100) reflections, respectively (JCPDS No: 01-0646). The other characteristic diffraction peaks of graphite at 2u of about 53 and 77 are associated with (004) and (110) diffractions of graphite, respectively. Fig. S2dy shows the XPS survey spectrum for CNTs. As expected, it contains C and O. The main C1 s peak located at 284.6 eV is a signal of adventitious elemental carbon, which originates from the graphitic sp2 carbon atoms. In addition, the energy-dispersive X-ray spectroscopy (EDX) analysis (Fig. S4y, Supporting information) also shows that the CNTs on FTO substrate contain a smaller concentration of oxygen (9.02%) than carbon
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(90.98%). Therefore, the CNTs are an excellent candidate for the charge transporting component due to their exceptional charge transport characteristics and structural and chemical stability. A viscous CNT paste was prepared by grinding MWCNT powder, ethanol, terpineol, and ethyl cellulose using a mortar and pestle. The as-prepared paste was applied in PSCs. The proposed concept of clamping PSCs is shown in Fig. 1. The CNT/perovskite interface was optimized using stepwise evolving of the clamping using three methods. In Method 1 (Fig. 1a), the PSC is prepared by directly clamping a perovskite photoanode to a CNT electrode deposited on FTO by the doctor blade method. However, poor contact between the perovskite material and CNT may result in a large internal resistance and inferior photovoltaic performance. To overcome this issue, the CNT paste was directly applied on the perovskite material by the doctor blade method, which led to Method 2 for clamping PSCs. Another type of clamping involves applying a CNT paste on PbI2 and subjecting the sample to a spin coating of
Fig 1. Preparation CNT paste and its application in PSC’s as a counter electrode. (a) Fabrication process of the Method 1 clamping solar cells by simply clamping an FTO supported CNT electrode and a CH3NH3PbI3 photoanode. (b) Fabrication of the Method 2 clamping solar cells by applying CNT paste on surface of CH3NH3PbI3. (c) Fabrication of the Method 3 clamping solar cells by CNT paste on surface of PbI2, with a CH3NH3I spin-coating for the in situ conversion of PbI2 to CH3NH3PbI3 partially embedding the CNT.
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CH3NH3I (Method 3). These methods allowed us to greatly boost the performance of a PSC with good stability. In an effort to expedite the technical optimization of the clamped solar cells, a closer examination of the CNT/perovskite interfaces is in order. Fig. 2a1 shows a schematic of Method 2 involving physical contact between CNT and CH3NH3PbI3, in which CNT is attached to the perovskite photoanode by the doctor blade method. Fig. 2a2 shows that the CH3NH3PbI3 film was composed of interconnected nanoscale domains with good film coverage. Indeed, the top surface of the CH3NH3PbI3 film is full of CNT
surfaces, which are useful for the subsequent filling of size- and morphology-matched CNTs, as shown in Fig. 2a3. This surface feature is particularly beneficial to the mechanical clamping in Method 1. Fig. 2b and c show the chemically promoted interface engineering from Method 3. The precursor PbI2 has even smaller particles (24–58 nm; Fig. 2b2), which permit more interpenetrating contacts between CNTs and PbI2 (Fig. 2b3). Fig. 2c1 shows the perovskite precursors could penetrate into the CNTs, wrap around them, and crystallize. They hold together the CNT and produce a high-quality interface for facile hole extraction. The SEM
Fig. 2. The CNT/perovskite interface engineering of the Method 2 and Method 3 clamping solar cells. (a1) schematic illustrating the interface of CNT/CH3NH3PbI3. SEM images of (a2) pure CH3NH3PbI3 and (a3) CNT/CH3NH3PbI3 film. (b and c) Interface engineering of the 3rd clamping solar cells: (b1) schematic illustrating the interface of CNT/PbI2. SEM images of the (b2) PbI2 and (b3) CNT/PbI2 film. (c1) Schematic illustrating the chemically engineered interface by in situ conversion from PbI2 to CH3NH3PbI3, partially burying CNT into the CH3NH3PbI3 film, (c2) SEM image of the PbI2/CNT/CH3NH3PbI3 film.
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in Fig. 2c2 demonstrates the high crystallinity of the perovskite, which appears to grow by filling the pores in the CNT network. Therefore, the improved interface contact from the Method 3 film by the chemically promoted interpenetration between CNTs and perovskite facilitates the hole extraction and improves the photovoltaic performance. XPS was measured to determine the composition and chemical bond configuration of the CH3NH3PbI3 thin film on the surface of TiO2. Fig. 3a shows the Pb 4f core-level spectra of CH3NH3PbI3, which exhibit chemical states of Pb 4f7/2 and Pb 4f5/2 at the binding energies of 138.1 and 143.0 eV, respectively. The lower binding energy peak located at 138.1 eV indicates the presence of metallic lead (Pb). The higher binding energy peak at 143.0 eV is related to the covalently bonded Pb-I species [21]. In the case of I 3d corelevel spectra (Fig. 3b), the doublet chemical states of I 3d5/2 and I 3d3/2 were obtained at the binding energies of 619.5 and 631.1 eV, respectively. The XPS survey spectra of CH3NH3PbI3 are shown in Fig. S5y, Supporting information. The XPS results suggest that the CH3NH3PbI3 was successfully deposited on the surface of TiO2. Fig. 4a shows the ultraviolet-visible (UV–vis) absorption spectra of PbI2 and CH3NH3PbI3 film on FTO substrate. The PbI2 film features an absorption peak at around 550 nm, which is the characteristic band-gap excitation of crystallized PbI2 semiconductors. The optical absorption of CH3NH3PbI3 film shows a broad absorption peak at approximately 800 nm. The optical bandgap determined from the absorption spectrum is about 1.55 eV for CH3NH3PbI3. Fig. 4b presents the energy band diagram of the clamping cells based previous report [15,20]. Thus, the CNT electrode has a dual role in the TiO2/CH3NH3PbI3/CNT solar cell Fig. 4. (a) UV–vis absorption spectra of PbI2 and CH3NH3PbI3 thin films. (b) Energy band diagram of the clamping solar cell.
Fig. 3. High-resolution XPS of (a) Pb 4f and (b) I 3d in CH3NH3PbI3 film.
system. First, it is used as a hole collector. Second, it works as a conductive electrode to transport holes to the external circuit. Once a working cell is illuminated by sunlight, the electrons in CH3NH3PbI3 are excited from the ground state to the excited state at 3.86 eV, injected into the CB of TiO2 (4.00 eV), and finally collected at the FTO anode (work function: 4.0 eV). At the same time, the holes in the CH3NH3PbI3 valence band could be injected into the CNT (5.0 eV) [15,20]. It follows that the key to the operation of the clamping cell is the two selective charge extraction interfaces sandwiching the ambipolar perovskite layer. The first is the TiO2/CH3NH3PbI3 interface for electron extraction, and the second is the interface of CNT/CH3NH3PbI3 for hole extraction. To understand the exact effect of CNTs in perovskite solar cells, we compared the performance of the three methods of clamping solar cells. A digital photograph of the as-prepared CNT-based clamped solar cells is shown in Fig. S6y, Supporting information. Fig. 5 presents photocurrent density–voltage characteristics of these devices under AM 1.5 illumination of 100 mW m2. The photovoltaic parameters, open circuit voltage (Voc), fill factor (FF), short circuit current density (Jsc), and power conversion efficiency (h) are summarized in Table 1. The results show that the solar cells become more and more efficient from method to method. Method 1 achieves a Voc of 0.633 V, Jsc of 6.47 mA cm2, FF of 0.377, and h of 1.55%. The poor contact between CNTs and perovskite from the simple mechanical joint in Method 1 resulting in large internal resistance and an inferior photovoltaic performance. In the Method 2, Voc, Jsc, and FF are increased to 0.653 V, 11.16 mA cm2, and 0.510, respectively, yielding an impressive h of 3.73%. The CNT electrode further lowers the electric resistance and encourages intimate contact with the perovskite, enhancing the performance for method 2. Method 3 cells showed greatly improved performance, with Jsc, Voc, and FF of 18.54 mA cm2, 0.703 V, and 0.600, respectively, with h of 7.83%. A higher Rsh contributes to a larger
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Fig. 5. Performance characteristics of the three methods clamping solar cells. J–V curves of the three methods of clamping solar cells showing an increasing h as a result from the interface engineering.
Table 1 Performance summary of the Method 1, Method 2 and Method 3 clamping solar cells. Cell
Voc (V)
Jsc (mA/cm2)
FF
h%
Rs (V)
RCE (V)
te (ms)
Method 1 Method 2 Method 3
0.633 0.653 0.703
6.47 11.16 18.54
0.377 0.510 0.600
1.55 3.73 7.83
9.43 8.26 6.61
18.1 2.84 1.46
0.0426 7.18 10.70
Voc. The Rsh of solar cells can be estimated as the inverse of the slopes of the J–V curve around 0 V. Comparing the curves in Fig. 5 reveals that the Method 3 PSC has a much higher shunt resistance (Rsh = 1851 V) than those with the Method 1 (427 V) and Method 2 (1034 V), which is responsible for the higher Voc observed in the cell with the Method 3 PSC. Method 3 presents higher Voc, FF and Jsc compared to that of Method 1 and Method 2, indicating improved interface contact between the perovskite and CNT material results the facilitates the hole extraction, eventually leading to the difference in h. In order to investigate the reproducibility of the Method 3 clamping solar cell, 5 separate devices were fabricated and tested, the cell-performance parameters are presented in Table S1. As can be seen from the results, the average h is located at about 7.83%, while the lowest efficiency is 7.18% and the highest efficiency is 8.31%. For the sake of direct comparison, an Au cathode thermal evaporated on an FTO/TiO2/CH3NH3PbI3 film exhibited a h of 6.16% (Fig. S7y). To further evaluate the influence of the CNT CE on the performance of solar cells, we carried out EIS measurements in the frequency range of 100 mHz to 500 kHz with 10 mV AC amplitude under one sun illumination. The Nyquist plots are shown in Fig. 6a. The EIS transmission-line model was investigated for the mesoscopic solar cells based on the inorganic–organic hybrid perovskite CH3NH3PbI3. The equivalent circuit is shown in inset of Fig. 6a. In this paper, we only focus on the discussion of series resistance (Rs) and charge transfer resistance between CNTs and perovskite (RCE) in consideration of relevance to the methods. The RC response in the high frequency region is assigned to the charge transfer process at the perovskite/CNT CE (RCE) interface, while the one at low frequencies represents the TiO2/perovskite interface (Rct) [22,23]. When illuminated, two semicircles can be found in the Nyquist plot. As expected, the low Rs value (6.61 V) of Method 3 solar cells led to the highest FF of 0.600 compared with the Method 1 (Rs = 9.43 V) and Method 2 (Rs = 8.26 V), which had FFs of 0.377 and 0.510, respectively. The lower Rs value of Method 3 PSC could be attributed to either the high electrical conductivity of the FTO substrate or to good bonding strength between the
Fig. 6. Electrochemical impedance spectra of three methods of clamping solar cells in the form of (a) Nyquist-plots and (b) Bode phase diagrams. Inset: the equivalent circuit employed to fit the spectra.
photoanode/MWCNT film and the FTO substrate, which in turn promotes the collection of more electrons from the external circuit. It is well known that the Rs of CEs will influence the FF parameter [24,25], which determines the shape of J–V characteristics and the maximum power output of the device. The charge transfer resistances at the perovskite/CNT interface of the three devices are not larger than 20 V, indicating that the charge transfer process at the CNT electrode is efficient. Method 3 shows lower RCE (1.46 V) than Method 1 (18.1 V) and Method 2 (4.08 V). Notably, in the Method 3 device, MAPbI3 not only acts as a light harvester but also as a hole transporter. The low RCE value of Method 3 contributes to better transport of holes from perovskite to the CNT CEs, which indicates that the CNTs can work well when used as a CE for a perovskite solar cell. The corresponding peaks at the characteristic frequencies in the Bode phase plots in Fig. 6b can be used to determine the carrier lifetime for the different methods of clamping perovskite solar cells. The Method 3 film exhibits the longest lifetime of 10.70 ms, and the lifetimes of Method 1 and Method 2 are 7.18 ms and 0.0426 ms, respectively. The chemically promoted Method 3 solar cell allows more efficient carrier extraction from perovskite to CNT. Long-term stability is another important concern for practical applications of PSCs. Herein we present a stability test of the Method 3 cell was tested at room temperature after storage for over one day without encapsulation or special protection. The detailed photovoltaic parameters are plotted in Fig. 7. The stability of perovskite solar devices is limited mainly to the ambient moisture because the alkylammonium salts are, in general, highly hygroscopic [26]. However, photovoltaic performances of representative perovskite solar cells demonstrate high stability in this
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Fig. 7. Detailed photovoltaic parameters of a Method 3 based clamping solar cell measured under the continuous illumination of AM 1.5 G (100 mW cm2) for 50 h. Voc: opencircuit voltage; Jsc: short-circuit photocurrent density; FF: fill factor; h: power conversion efficiency.
CNT-based device. During the tests, h shifted from the initial 6.91% to the final 7.07%, and an average h of 7.08% was achieved. Furthermore, an average Jsc of 17.21 mA/cm2, Voc of 0.661 V, and FF of 0.622 were achieved in the 50 h test. This remarkable stability is attributed to the CNT layer, which can work as a water-retaining layer to protect the perovskite from being destroyed [18], which indicates the potential of the CNTs as promising candidates for a highly efficient perovskite solar cell. 4. Conclusions We have established a cost-effective, simple, low-temperature processed (100 C), environmentally stable, and abundant CNT network film as an efficient CE for hole-collector-free PSCs. We have developed a concept of clamping solar cells by interfacing the CNTs with CH3NH3PbI3 films under atmospheric conditions. Three methods of clamping were evolved from direct clamping to chemically promoted clamping. Method 3 without a hole-transporting material achieved a remarkable efficiency of 7.83% and good long-term stability under atomosperic conditions. Impedance spectroscopy reveals good charge transport characteristics of the CNT CE. The efficiency might be improved by future purification and chemical doping of CNTs to increase the film conductivity and the work function. The potential to directly process low-cost CNT electrodes at low temperature on top of CH3NH3PbI3 offers numerous possibilities to choose and optimize the materials and structure of the device. The simple fabrication process and long-term stability of the devices open up new avenues for future large-scale production. Competing interest The authors declare no competing financial interest. The samples were analyzed by scanning electron microscope (SEM, Hitachi S-4200) and high-resolution transmission electron microscope (HRTEM; Jem 2011, Jeol corp.) with a 4k x 4k CCD camera (Ultra Scan 400SP, Gatan corp.) at the Busan KBSI. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korean government (No.2014005051).
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Low-temperature easy-processed carbon nanotube contact for high-performance metal- and hole-transporting layer-free perovskite solar cells Chandu V.V.M. Gopi, Mallineni Venkata-Haritha, Kandasamy Prabakar, Hee-Je Kim* School of Electrical Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, South Korea
A R T I C L E I N F O
Article history: Received 8 May 2016 Received in revised form 29 July 2016 Accepted 7 September 2016 Available online 9 September 2016 Keywords: Low-temperature Carbon nanotubes Perovskite solar cells Stability
A B S T R A C T
Expensive and energy-consuming vacuum process of metal deposition with ambient-unstable hole transporters are incompatible with large-scale and low-cost production of perovskite solar cells (PSCs) and thus hampers their commercialization. For the first time, we demonstrate cost-effective novel carbon nanotube (CNT) paste that was applied to FTO substrate by the facile doctor blade method and processed at low temperature (100 C). Herein we report a new method of cost-efficient perovskite solar cells with the use of conventional hole transporters by directly clamping a selective hole extraction electrode made of CNT and a TiO2/perovskite photoanode. Most importantly, under optimized conditions in the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 7.83%. The PSC devices are fabricated in air without high-vacuum deposition which simplifies the processing and lowers the threshold of both scientific research and industrial production of PSCs. Electrochemical impedance spectroscopy demonstrates good charge transport characteristics of CEs on the photovoltaic performance of devices. The PSCs exhibited good stability over 50 h. The abundance, low cost, and excellent properties of the CNT material offer wide prospects for further applications in PSCs. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaics have been realized as suitable for generating electrical power by converting solar radiation into direct-current electricity with cost-effective and high-efficiency solar cells to meet the ever increasing demand for clean energy. The perovskite solar cells (PSCs) have recently attracted a great deal of attention because of their excellent photovoltaic performance, which is obtainable via facile and cheap processes [1,2]. Methylammonium lead halide (CH3NH3PbX3, X = Cl, Br, I) have invoked tremendous amounts of scientific and commercial interest for their notable characteristics of broad and intense absorption spectra [2], appropriate semiconducting properties [3], long carrier diffusion length [4,5], and facile solution processability. To date, for PSCs, extraordinary power conversion efficiencies (PCE) ranging from 3.8% [6] to more than 20.1% [7] have been obtained through optimization of preparing technology and device structure, which make it competitive in future commercialization.
* Corresponding author. E-mail address: [email protected] (H.-J. Kim). http://dx.doi.org/10.1016/j.jphotochem.2016.09.003 1010-6030/ã 2016 Elsevier B.V. All rights reserved.
To prepare CH3NH3PbI3, one of the critical challenges is to understand the influence of the ambient environment on the resultant perovskite thin films since perovskite crystals are sensitive to humidity under ambient conditions. The perovskite crystals degrade gradually when they are in contact with ambient moisture for a certain time [8,9]. Therefore, most of the highperformance perovskite solar cells are prepared in a glove box to avoid contacting moisture. However, fabricating PSCs under ambient condition is inevitable if we desire to transition from laboratory research into large-scale applications. Also, PSCs require an expensive and air-sensitive hole transporter (e.g., spiroMeOTAD) and a noble metal electrode (Au or Ag) deposited by complicated vacuum technologies [10,11]. However, noble metals such as Au or Ag as metal contacts are indispensable in these highperformance photovoltaic devices and are not conducive for the industrial development and market potential of PSCs. Therefore, the development of alternative materials and processes that show high performance and also inexpensive, abundant, environmentally friendly, easily processable, and not energy intensive is required for high-efficiency photovoltaic devices. In addition to being inexpensive, carbon nanotubes (CNTs) have rapid electron transfer kinetics, relatively large surface area, and high electrocatalytic activity. They have been applied in many other
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photovoltaic devices successfully, and impressive results have been achieved [12–14]. For most reported carbon materials, hightemperature treatments are necessary, which hinder the largescale production for solar cells [15,16]. Wei et al. reported a remarkably high PCE of 11.02% from PSCs by directly clamping a selective hole extraction electrode made of candle soot and a deliberately engineered perovskite photoanode [15]. Recently, carbon electrode layers, carbon nanotubes (CNT) and metal meshes have been applied as the top electrodes in PSCs [17–20]. Li et al. fabricated and reported a CNT network by chemical vapor deposition (CVD) method and transferred it onto a CH3NH3PbI3 substrate and achieved a solar cell efficiency of 6.87% [20]. All these evidences indicate that complicated holetransport materials and metal contacts are not necessary in fabricating a simpler perovskite-based photovoltaic device. In view of these reported results, herein, we successfully fabricated PSCs by using low-cost and robust mesoporous TiO2 and CNT layers as electron and hole selective contacts with doctor blade technique, respectively. The CNT electrode acted as a hole conductor or an electron blocking layer to suppress charge recombination and facilitate the hole extraction. The CNT-based devices TiO2/CH3NH3PbI3/CNT have much simpler architecture, in which CH3NH3PbI3 thin film is sandwiched between TiO2 (as the electron-selective layer) and a CNT electrode (as the hole-selective layer). Under a atomsperic conditions, we propose the concept of clamping solar cells by joining a perovskite photoanode and a CNT electrode, which has remarkable photovoltaic performance with a short circuit current (Jsc) of 18.54 mA/cm2, open circuit voltage (Voc) of 0.703 V, fill factor (FF) of 0.600, and power conversion efficiency (h) of 7.83% with an active area of 0.18 cm2, demonstrating the potential of applying CNTs as a charge collector, eliminating both the metal electrode and hole transporter in PSCs. The devices of TiO2/CH3NH3PbI3/CNT solar cells without encapsulation exhibited advantageous stability (over 50 h) in air. The electrochemical impedance spectroscopy (EIS) measurements demonstrated that holes in perovskite could be transported effectively to CNT counter electrode. Thus, using CNTs in mesoscopic PSCs will simultaneously cut both the cost of material and production costs, offering a more encouraging prospect for the commercialization of this photovoltaic technology. To the best of our knowledge, this is the first report for the use of low temperature processed CNT as counter electrode as well as hole-extracting electrode in holeconductor free perovskite-based solar cells. 2. Experimental section 2.1. Preparation of CNTs paste CNT thin films were deposited on well-cleaned FTO glass substrates with a sheet resistance of 7 V/cm2 (Hartford Glass) by the doctor blade method. Prior to deposition, FTO substrates were cleaned ultrasonically with acetone, ethanol, and distilled water for 10 min each. For the preparation of CNT paste, 0.05 g of MWCNTs (purchased from Carbon Nano-Material Technology Co., Ltd.; Type-Multiwall; Diameter- 20 nm; and Length- 5 mm) was placed in a pestle and mortar. Then, we added 2 mL of ethanol and grind the MWCNTs for 15 min by hand. Finally, we added 1 mL of terpineol and 0.03 mL of ethyl cellulose to the mixture and grind it for 15 min. Finally, the paste was collected in glass bottle and used for the preparation of CNT electrode in PSC. A digital photograph of the CNT paste is shown in Fig. S1y, Supporting information. 2.2. Fabrication of HTM-free perovskite solar cell Devices were fabricated on clean FTO glass substrates. A thin TiO2 compact film was coated on FTO glass. To prepare the
precursor solution, 0.15 M of titanium isopropoxide in ethanol was stirred for 1 h. A compact TiO2 film was coated by spin-coating method with a speed of 3000 rpm for 30 s, and the film was thermally annealed at 450 C for 2 h. Mesoporous TiO2 (mp-TiO2) film was deposited by spin-coating a diluted TiO2 paste (18NR-T Dyesol) in ethanol (2:7 w/w), followed by sintering at 500 C for 30 min. Lead iodide (PbI2) was then introduced into the TiO2 by spin-coating 20 mL of a 462 mg/ml solution of PbI2 in N,Ndimethylformamide (DMF) and kept at 90 C for 5 min under ambient conditions of 50–60% relative humidity. For Method 1 and Method 2 for clamping PSCs, 10 mg/ml of CH3NH3I was dissolved in isopropanol and spin-coated on top of the dried PbI2 layer at 0 rpm for 30 s (loading time), followed by 2000 rpm for 30 s at room temperature and drying at 90 C for 30 min in air. A solution of CH3NH3I on TiO2/PbI2 changes color immediately from yellow to dark brown, indicating the formation of CH3NH3PbI3. A piece of FTO/CNT film (CNT paste applied on FTO by doctor blade with an active area of 0.18 cm2 and heated at 100 C for 20 min) and the CH3NH3PbI3 film were directly clamped to make the Method 1 solar cells. For Method 2 solar cells, the CNT paste was applied on CH3NH3PbI3 film (area of 0.18 cm2) and heated at 100 C for 20 min. Then, another piece of FTO glass was clamped to the CH3NH3PbI3/CNT film. In Method 3, the CNT paste was transferred onto the PbI2 film before CH3NH3I bath treatment, and other procedures were similar to Method 2. 2.3. Characterizations The surface morphology, thickness, and elemental compositions of the electrodes were investigated using a field emission scanning electron microscope (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. High-resolution transmission electron microscopy was performed (HRTEM; Jem 2011, Jeol cop.) with a 4k x 4k CCD camera (Ultra Scan 400SP, Gatan cop.) at the Busan KBSI. X-ray diffraction (XRD) analysis was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Ka radiation operated at 40 kV and 40 mA. X-ray photon spectroscopy (XPS) was performed using a VG Scientific ESCALAB 250 with monochromatic AlKa radiation at 1486.6 eV and an electron take-off angle of 90 . The current-voltage (J-V) characteristics of PSCs were examined under one sun illumination (AM 1.5G, 100 mW cm2) using an ABET Technologies (USA) solar simulator with an irradiance uniformity of 3%. Electrochemical impedance spectroscopy (EIS) was conducted on the PSCs using a BioLogic potentiostat/ galvanostat/EIS analyzer (SP-150, France) under one sun illumination, and the frequency ranged from 100 mHz to 500 kHz. The electrical impedance was characterized using Nyquist and Bode plots. To perform the stability test, the PSCs were continuously irradiated with an AM 1.5G 100 mW cm2 illumination in working conditions, and the J-V curves were tested for different intervals of time in the course of 50 h. 3. Results and discussion Fig. S2ay shows an SEM image of CNTs deposited on FTO substrates by the doctor blade method. The top-view SEM image shows the fiber-like array morphology of the uniform CNT thin film. The thickness of the CNT film is 7.21 mm (Fig. S3y, Supporting information). Most of the fiber-like carbon was in fact tubular CNTs, which was confirmed by TEM in Fig. S2by. It is obvious from the images that all the nanotubes are hollow and tubular in shape. TEM images indicate that the nanotubes have high purity with uniform diameter distribution and contain no deformity in the structure. The crystalline structure of the CNT is highly ordered. In
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addition, uniform CNTs with lengths of up to 5 mm and diameters of around 20 nm were observed from TEM analysis, indicating no structural damage of MWCNTs. XRD was used to ascertain the quality and crystalline nature of nanotubes. Fig. S2cy shows the XRD pattern of the MWCNTs. The XRD pattern of MWCNTs exhibits a sharp peak around 2u = 26.4 and a broad peak centered at 2u = 42.4 corresponding to the graphite (002) and (100) reflections, respectively (JCPDS No: 01-0646). The other characteristic diffraction peaks of graphite at 2u of about 53 and 77 are associated with (004) and (110) diffractions of graphite, respectively. Fig. S2dy shows the XPS survey spectrum for CNTs. As expected, it contains C and O. The main C1 s peak located at 284.6 eV is a signal of adventitious elemental carbon, which originates from the graphitic sp2 carbon atoms. In addition, the energy-dispersive X-ray spectroscopy (EDX) analysis (Fig. S4y, Supporting information) also shows that the CNTs on FTO substrate contain a smaller concentration of oxygen (9.02%) than carbon
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(90.98%). Therefore, the CNTs are an excellent candidate for the charge transporting component due to their exceptional charge transport characteristics and structural and chemical stability. A viscous CNT paste was prepared by grinding MWCNT powder, ethanol, terpineol, and ethyl cellulose using a mortar and pestle. The as-prepared paste was applied in PSCs. The proposed concept of clamping PSCs is shown in Fig. 1. The CNT/perovskite interface was optimized using stepwise evolving of the clamping using three methods. In Method 1 (Fig. 1a), the PSC is prepared by directly clamping a perovskite photoanode to a CNT electrode deposited on FTO by the doctor blade method. However, poor contact between the perovskite material and CNT may result in a large internal resistance and inferior photovoltaic performance. To overcome this issue, the CNT paste was directly applied on the perovskite material by the doctor blade method, which led to Method 2 for clamping PSCs. Another type of clamping involves applying a CNT paste on PbI2 and subjecting the sample to a spin coating of
Fig 1. Preparation CNT paste and its application in PSC’s as a counter electrode. (a) Fabrication process of the Method 1 clamping solar cells by simply clamping an FTO supported CNT electrode and a CH3NH3PbI3 photoanode. (b) Fabrication of the Method 2 clamping solar cells by applying CNT paste on surface of CH3NH3PbI3. (c) Fabrication of the Method 3 clamping solar cells by CNT paste on surface of PbI2, with a CH3NH3I spin-coating for the in situ conversion of PbI2 to CH3NH3PbI3 partially embedding the CNT.
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CH3NH3I (Method 3). These methods allowed us to greatly boost the performance of a PSC with good stability. In an effort to expedite the technical optimization of the clamped solar cells, a closer examination of the CNT/perovskite interfaces is in order. Fig. 2a1 shows a schematic of Method 2 involving physical contact between CNT and CH3NH3PbI3, in which CNT is attached to the perovskite photoanode by the doctor blade method. Fig. 2a2 shows that the CH3NH3PbI3 film was composed of interconnected nanoscale domains with good film coverage. Indeed, the top surface of the CH3NH3PbI3 film is full of CNT
surfaces, which are useful for the subsequent filling of size- and morphology-matched CNTs, as shown in Fig. 2a3. This surface feature is particularly beneficial to the mechanical clamping in Method 1. Fig. 2b and c show the chemically promoted interface engineering from Method 3. The precursor PbI2 has even smaller particles (24–58 nm; Fig. 2b2), which permit more interpenetrating contacts between CNTs and PbI2 (Fig. 2b3). Fig. 2c1 shows the perovskite precursors could penetrate into the CNTs, wrap around them, and crystallize. They hold together the CNT and produce a high-quality interface for facile hole extraction. The SEM
Fig. 2. The CNT/perovskite interface engineering of the Method 2 and Method 3 clamping solar cells. (a1) schematic illustrating the interface of CNT/CH3NH3PbI3. SEM images of (a2) pure CH3NH3PbI3 and (a3) CNT/CH3NH3PbI3 film. (b and c) Interface engineering of the 3rd clamping solar cells: (b1) schematic illustrating the interface of CNT/PbI2. SEM images of the (b2) PbI2 and (b3) CNT/PbI2 film. (c1) Schematic illustrating the chemically engineered interface by in situ conversion from PbI2 to CH3NH3PbI3, partially burying CNT into the CH3NH3PbI3 film, (c2) SEM image of the PbI2/CNT/CH3NH3PbI3 film.
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in Fig. 2c2 demonstrates the high crystallinity of the perovskite, which appears to grow by filling the pores in the CNT network. Therefore, the improved interface contact from the Method 3 film by the chemically promoted interpenetration between CNTs and perovskite facilitates the hole extraction and improves the photovoltaic performance. XPS was measured to determine the composition and chemical bond configuration of the CH3NH3PbI3 thin film on the surface of TiO2. Fig. 3a shows the Pb 4f core-level spectra of CH3NH3PbI3, which exhibit chemical states of Pb 4f7/2 and Pb 4f5/2 at the binding energies of 138.1 and 143.0 eV, respectively. The lower binding energy peak located at 138.1 eV indicates the presence of metallic lead (Pb). The higher binding energy peak at 143.0 eV is related to the covalently bonded Pb-I species [21]. In the case of I 3d corelevel spectra (Fig. 3b), the doublet chemical states of I 3d5/2 and I 3d3/2 were obtained at the binding energies of 619.5 and 631.1 eV, respectively. The XPS survey spectra of CH3NH3PbI3 are shown in Fig. S5y, Supporting information. The XPS results suggest that the CH3NH3PbI3 was successfully deposited on the surface of TiO2. Fig. 4a shows the ultraviolet-visible (UV–vis) absorption spectra of PbI2 and CH3NH3PbI3 film on FTO substrate. The PbI2 film features an absorption peak at around 550 nm, which is the characteristic band-gap excitation of crystallized PbI2 semiconductors. The optical absorption of CH3NH3PbI3 film shows a broad absorption peak at approximately 800 nm. The optical bandgap determined from the absorption spectrum is about 1.55 eV for CH3NH3PbI3. Fig. 4b presents the energy band diagram of the clamping cells based previous report [15,20]. Thus, the CNT electrode has a dual role in the TiO2/CH3NH3PbI3/CNT solar cell Fig. 4. (a) UV–vis absorption spectra of PbI2 and CH3NH3PbI3 thin films. (b) Energy band diagram of the clamping solar cell.
Fig. 3. High-resolution XPS of (a) Pb 4f and (b) I 3d in CH3NH3PbI3 film.
system. First, it is used as a hole collector. Second, it works as a conductive electrode to transport holes to the external circuit. Once a working cell is illuminated by sunlight, the electrons in CH3NH3PbI3 are excited from the ground state to the excited state at 3.86 eV, injected into the CB of TiO2 (4.00 eV), and finally collected at the FTO anode (work function: 4.0 eV). At the same time, the holes in the CH3NH3PbI3 valence band could be injected into the CNT (5.0 eV) [15,20]. It follows that the key to the operation of the clamping cell is the two selective charge extraction interfaces sandwiching the ambipolar perovskite layer. The first is the TiO2/CH3NH3PbI3 interface for electron extraction, and the second is the interface of CNT/CH3NH3PbI3 for hole extraction. To understand the exact effect of CNTs in perovskite solar cells, we compared the performance of the three methods of clamping solar cells. A digital photograph of the as-prepared CNT-based clamped solar cells is shown in Fig. S6y, Supporting information. Fig. 5 presents photocurrent density–voltage characteristics of these devices under AM 1.5 illumination of 100 mW m2. The photovoltaic parameters, open circuit voltage (Voc), fill factor (FF), short circuit current density (Jsc), and power conversion efficiency (h) are summarized in Table 1. The results show that the solar cells become more and more efficient from method to method. Method 1 achieves a Voc of 0.633 V, Jsc of 6.47 mA cm2, FF of 0.377, and h of 1.55%. The poor contact between CNTs and perovskite from the simple mechanical joint in Method 1 resulting in large internal resistance and an inferior photovoltaic performance. In the Method 2, Voc, Jsc, and FF are increased to 0.653 V, 11.16 mA cm2, and 0.510, respectively, yielding an impressive h of 3.73%. The CNT electrode further lowers the electric resistance and encourages intimate contact with the perovskite, enhancing the performance for method 2. Method 3 cells showed greatly improved performance, with Jsc, Voc, and FF of 18.54 mA cm2, 0.703 V, and 0.600, respectively, with h of 7.83%. A higher Rsh contributes to a larger
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Fig. 5. Performance characteristics of the three methods clamping solar cells. J–V curves of the three methods of clamping solar cells showing an increasing h as a result from the interface engineering.
Table 1 Performance summary of the Method 1, Method 2 and Method 3 clamping solar cells. Cell
Voc (V)
Jsc (mA/cm2)
FF
h%
Rs (V)
RCE (V)
te (ms)
Method 1 Method 2 Method 3
0.633 0.653 0.703
6.47 11.16 18.54
0.377 0.510 0.600
1.55 3.73 7.83
9.43 8.26 6.61
18.1 2.84 1.46
0.0426 7.18 10.70
Voc. The Rsh of solar cells can be estimated as the inverse of the slopes of the J–V curve around 0 V. Comparing the curves in Fig. 5 reveals that the Method 3 PSC has a much higher shunt resistance (Rsh = 1851 V) than those with the Method 1 (427 V) and Method 2 (1034 V), which is responsible for the higher Voc observed in the cell with the Method 3 PSC. Method 3 presents higher Voc, FF and Jsc compared to that of Method 1 and Method 2, indicating improved interface contact between the perovskite and CNT material results the facilitates the hole extraction, eventually leading to the difference in h. In order to investigate the reproducibility of the Method 3 clamping solar cell, 5 separate devices were fabricated and tested, the cell-performance parameters are presented in Table S1. As can be seen from the results, the average h is located at about 7.83%, while the lowest efficiency is 7.18% and the highest efficiency is 8.31%. For the sake of direct comparison, an Au cathode thermal evaporated on an FTO/TiO2/CH3NH3PbI3 film exhibited a h of 6.16% (Fig. S7y). To further evaluate the influence of the CNT CE on the performance of solar cells, we carried out EIS measurements in the frequency range of 100 mHz to 500 kHz with 10 mV AC amplitude under one sun illumination. The Nyquist plots are shown in Fig. 6a. The EIS transmission-line model was investigated for the mesoscopic solar cells based on the inorganic–organic hybrid perovskite CH3NH3PbI3. The equivalent circuit is shown in inset of Fig. 6a. In this paper, we only focus on the discussion of series resistance (Rs) and charge transfer resistance between CNTs and perovskite (RCE) in consideration of relevance to the methods. The RC response in the high frequency region is assigned to the charge transfer process at the perovskite/CNT CE (RCE) interface, while the one at low frequencies represents the TiO2/perovskite interface (Rct) [22,23]. When illuminated, two semicircles can be found in the Nyquist plot. As expected, the low Rs value (6.61 V) of Method 3 solar cells led to the highest FF of 0.600 compared with the Method 1 (Rs = 9.43 V) and Method 2 (Rs = 8.26 V), which had FFs of 0.377 and 0.510, respectively. The lower Rs value of Method 3 PSC could be attributed to either the high electrical conductivity of the FTO substrate or to good bonding strength between the
Fig. 6. Electrochemical impedance spectra of three methods of clamping solar cells in the form of (a) Nyquist-plots and (b) Bode phase diagrams. Inset: the equivalent circuit employed to fit the spectra.
photoanode/MWCNT film and the FTO substrate, which in turn promotes the collection of more electrons from the external circuit. It is well known that the Rs of CEs will influence the FF parameter [24,25], which determines the shape of J–V characteristics and the maximum power output of the device. The charge transfer resistances at the perovskite/CNT interface of the three devices are not larger than 20 V, indicating that the charge transfer process at the CNT electrode is efficient. Method 3 shows lower RCE (1.46 V) than Method 1 (18.1 V) and Method 2 (4.08 V). Notably, in the Method 3 device, MAPbI3 not only acts as a light harvester but also as a hole transporter. The low RCE value of Method 3 contributes to better transport of holes from perovskite to the CNT CEs, which indicates that the CNTs can work well when used as a CE for a perovskite solar cell. The corresponding peaks at the characteristic frequencies in the Bode phase plots in Fig. 6b can be used to determine the carrier lifetime for the different methods of clamping perovskite solar cells. The Method 3 film exhibits the longest lifetime of 10.70 ms, and the lifetimes of Method 1 and Method 2 are 7.18 ms and 0.0426 ms, respectively. The chemically promoted Method 3 solar cell allows more efficient carrier extraction from perovskite to CNT. Long-term stability is another important concern for practical applications of PSCs. Herein we present a stability test of the Method 3 cell was tested at room temperature after storage for over one day without encapsulation or special protection. The detailed photovoltaic parameters are plotted in Fig. 7. The stability of perovskite solar devices is limited mainly to the ambient moisture because the alkylammonium salts are, in general, highly hygroscopic [26]. However, photovoltaic performances of representative perovskite solar cells demonstrate high stability in this
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Fig. 7. Detailed photovoltaic parameters of a Method 3 based clamping solar cell measured under the continuous illumination of AM 1.5 G (100 mW cm2) for 50 h. Voc: opencircuit voltage; Jsc: short-circuit photocurrent density; FF: fill factor; h: power conversion efficiency.
CNT-based device. During the tests, h shifted from the initial 6.91% to the final 7.07%, and an average h of 7.08% was achieved. Furthermore, an average Jsc of 17.21 mA/cm2, Voc of 0.661 V, and FF of 0.622 were achieved in the 50 h test. This remarkable stability is attributed to the CNT layer, which can work as a water-retaining layer to protect the perovskite from being destroyed [18], which indicates the potential of the CNTs as promising candidates for a highly efficient perovskite solar cell. 4. Conclusions We have established a cost-effective, simple, low-temperature processed (100 C), environmentally stable, and abundant CNT network film as an efficient CE for hole-collector-free PSCs. We have developed a concept of clamping solar cells by interfacing the CNTs with CH3NH3PbI3 films under atmospheric conditions. Three methods of clamping were evolved from direct clamping to chemically promoted clamping. Method 3 without a hole-transporting material achieved a remarkable efficiency of 7.83% and good long-term stability under atomosperic conditions. Impedance spectroscopy reveals good charge transport characteristics of the CNT CE. The efficiency might be improved by future purification and chemical doping of CNTs to increase the film conductivity and the work function. The potential to directly process low-cost CNT electrodes at low temperature on top of CH3NH3PbI3 offers numerous possibilities to choose and optimize the materials and structure of the device. The simple fabrication process and long-term stability of the devices open up new avenues for future large-scale production. Competing interest The authors declare no competing financial interest. The samples were analyzed by scanning electron microscope (SEM, Hitachi S-4200) and high-resolution transmission electron microscope (HRTEM; Jem 2011, Jeol corp.) with a 4k x 4k CCD camera (Ultra Scan 400SP, Gatan corp.) at the Busan KBSI. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korean government (No.2014005051).
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