Journal of Power Sources 402 (2018) 460–467
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SnO2 nanorod arrays with tailored area density as efficient electron transport layers for perovskite solar cells
T
Xiaokun Zhanga, Yichuan Ruia,∗, Yuanqiang Wanga, Jingli Xua, Hongzhi Wangb, Qinghong Zhangb,∗∗, Peter Müller-Buschbaumc a
College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai, 201620, PR China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China c Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, Jams-Franck-Strasse 1, 85748, Garching, Germany b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
approach is exploited for the • New synthesis of SnO nanorod arrays. area density of the nanorod arrays • The could be tailored. PSCs using SnO nanorod arrays as • ETL yield a high efficiency of 15.46%. charge transfer is system• Interfacial atically investigated. 2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 nanorod arrays Electron transport layer Perovskite solar cells Power conversion efficiency
Tin dioxide (SnO2) is regarded as an effective electron transport material for attaining high-performance perovskite solar cells (PSCs). Herein, vertically aligned SnO2 nanorod arrays are grown directly on fluorine-doped tin oxide (FTO) substrates in an acidic solution via hydrothermal method, where the area density of the nanorod arrays is tailored by varying the precursor concentration. Particularly, the mean diameters of the nanorods increase from 15 to 25 nm and the corresponding area densities decrease from 660 to 460 μm−2 with increasing the concentration of tin(IV) chloride pentahydrate. X-ray diffraction and X-ray photoelectron spectroscopy measurements reveal that the nanorod arrays are pure tetragonal rutile SnO2 with a high degree of crystallinity. Mixed perovskites of (FAPbI3)0.85(MAPbBr3)0.15 are infiltrated into these SnO2 nanorod arrays, and the perovskite solar cells show an enhanced photovoltaic performance as compared to the nanoparticle counterpart. Perovskite solar cells based on SnO2 nanorod arrays with the optimized area density exhibit the best power conversion efficiency of 15.46% which is attributed to an accelerated electron transport and a decreased recombination rate at SnO2/perovskite interface.
1. Introduction Organic-inorganic halide perovskite solar cells (PSCs) have gained increasing attention due to their superb characteristics featuring a high
∗
absorption coefficient, high charge carrier mobility and long charge carrier diffusion length [1,2]. The certified power conversion efficiency (PCE) has dramatically increased to over 22% to benefit from the tremendous worldwide research effort [3]. To date, state-of-the art PSCs
Corresponding author. Corresponding author. E-mail addresses:
[email protected] (Y. Rui),
[email protected] (Q. Zhang).
∗∗
https://doi.org/10.1016/j.jpowsour.2018.09.072 Received 15 May 2018; Received in revised form 21 July 2018; Accepted 20 September 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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(PCE) of 15.46%, which is attributed to an accelerated electron transport and a decreased recombination rate at SnO2/perovskite interface.
have employed either mesoporous or planar structures [4]. In either device architecture the perovskite light-absorbing layer is sandwiched between the electron transport layer (ETL) and the hole transport layer (HTL), which selectively extract electrons and holes to the conductive electrodes, respectively [5]. The electron transport layer plays an important role to effectively make contact with the perovskite light absorber and selectively extract the electrons while blocking holes at the same time, which has been considered as an indispensable part for highefficiency PSCs [6,7]. Contrarily, the photovoltaic performance of PSCs without ETL was very poor with a PCE of only 1.77% when the CH3NH3PbI3 perovskite was coated directly on the FTO substrate as reported by Grätzel and coworkers [8]. Moreover, the ETL not only affects the device efficiency but also the electron extraction capability, which is closely related to the anomalous hysteresis in J-V curves [9–11]. Therefore, it is of great importance to exploit effective electron transport materials with fast electron mobility for highly efficient and hysteresis-free PSCs. Mesoporous TiO2 nanoparticles are the most widely used electron transport materials. However, the existence of grain boundaries in the mesoporous TiO2 layer increases the number of trapping sites and consequently also the electron recombination rate [12,13]. Recently, vertically aligned one-dimensional (1D) nanorod arrays have attracted researchers’ interests, which incline to have fewer grain boundaries and exhibit advantages of unidirectional electron transport and enhanced charge collection [14–27]. Nanorod arrays could also provide wider space for effective pore-filling with the perovskites as compared to the commonly used nanoparticle based mesoscale structure due to an open porous structure. Moreover, light scattering amongst the nanorod arrays is beneficial for improving the photon absorption efficiency of the perovskite layer [16]. In the past, TiO2 [14–21] and ZnO [22–27] nanorod arrays with tailored diameters, lengths as well as area densities were exploited as electron transport materials of the PSCs. However, to the best of our knowledge, there have been few reports about PSCs using SnO2 nanorod arrays as ETLs. Only recently, SnO2 nanorod arrays have been exploited as electron transport materials as reported by Lv et al. [28] and Sun et al. [29], respectively. Actually, SnO2 has some striking advantages such as high electron mobility and superior chemical stability among the various metal oxides. PSCs based on SnO2 nanoparticles as ETLs showed very high efficiencies and were almost free of hysteresis [30–35]. Moreover, they showed a remarkable increase in the long-term stability under the full solar spectrum illumination [36,37]. In addition, the lower conduction band edge of SnO2 matches better with both perovskites, MAPbI3 and (FAPbI3)0.85(MAPbBr3)0.15, than that of TiO2, where the band misalignment in the latter causes undesirable consequences such as an accumulation of photogenerated charges [38]. Thus, it is expected that by employing the 1D SnO2 nanorod arrays as electron transport material would further improve the performance of PSCs. Oriented 1D SnO2 nanorods could be synthesized via various wetchemical processes as reported in literature [39–47]. Recently, our group also developed a unique solvothermal approach based on the aqueous–organic solvent system for the synthesis of high quality singlecrystalline SnO2 nanorods with tunable length [48]. However, all so far reported wet-chemical routes usually depend on basic solutions using alkali as mineralizing agent, which would etch the FTO glass soaked in such solution [39,45,47]. Moreover, surfactants were frequently used to direct the anisotropic growth [41,44], which is disadvantageous for electronic device applications due to seriously reducing the charge carrier transport. Thus, in-situ growth of vertically aligned SnO2 nanorod arrays on FTO glass is still a challenge. Herein, we report the in-situ synthesis of tailored SnO2 nanorod arrays on FTO glass by a facile hydrothermal method in the acidic solution, hence avoiding any FTO glass etching. SnO2 nanorod arrays with average diameters of 15 nm, 22 nm and 25 nm are obtained under different precursor concentrations. Perovskite solar cells using these SnO2 nanorod arrays as the ETLs exhibit a best power conversion efficiency
2. Experimental 2.1. Materials and reagents Unless stated otherwise, all materials were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. 4-tert-butylpyridine (t-BP), and Li-bis-(trifluoromethanesulfonyl) imide (Li-TFSI) were purchased from Sigma-Aldrich. NH2CH=NH2I, CH3NH3Br, PbI2, PbBr2 and Spiro-OMeTAD (purity ≥99.5%) were purchased from Xi'an Polymer Light Technology Corp. 2.2. Synthesis of SnO2 nanorod arrays The fluorine-doped tin oxide (FTO) conductive glass (Pilkington, TEC15) was sequentially cleaned in detergent solution, deionized water and ethanol in an ultrasonic bath. Firstly, a compact seed layer was deposited on the FTO glass substrate by spin-coating. To prepare the seed solution, 0.1 M SnCl2 2H2O isopropanol solution was refluxed at the temperature of 70 °C for one hour, and then aged for 6 h to form a transparent sol. The seed layer was deposited onto FTO by two consecutive spin-coating steps at 1000 rpm and 4000 rpm for 10 s and 30 s respectively and calcined at 500 °C for 30 min in air. The thickness of the seed layer was only about 20 nm (Fig. S1). Secondly, SnO2 nanorod arrays were grown on the seed layer via a hydrothermal method. Briefly, 25 mL of H2O, 25 mL of ethanol and 2 mL of concentrated HCl were mixed in a teflon-lined autoclave, and the mixture was stirred at ambient conditions for 5 min before the addition of SnCl4·5H2O [49]. To adjust the micro-structures of the nanorod arrays, different amount of SnCl4·5H2O were added, and the samples were named as NR-A (55 mg), NR-B (65 mg) and NR-C (75 mg), respectively. Subsequently, one piece of seeded FTO substrate was placed against the wall of the teflon-lined autoclave with the conductive side facing down. The hydrothermal reaction was performed at 200 °C for 12 h. After hydrothermal reaction, The SnO2 nanorod films were repeatedly rinsed with deionized water and ethanol to remove excess ions. Finally, the films were annealed at 500 °C for 30 min. 2.3. Solar cell fabrication Mixed perovskites of (FAPbI3)0.85(MAPbBr3)0.15 were prepared by dissolving 344 mg of NH2CH=NH2I (1.0 M), 44.8 mg CH3NH3Br (0.2 M) powders with 1014 mg PbI2 (1.1 M) and 146.8 mg PbBr2 (0.2 M) into 2 mL of anhydrous DMF/DMSO (4:1, volume ratio) at 70 °C for 60 min according to our previous recipe [50]. The perovskite was deposited on the electron transport layer by using a one-step method. Firstly, the FTO substrate with SnO2 nanorod arrays was treated with UV-O3 for 30 min to get rid of organic contaminations. The inorganic–organic lead halide perovskite solutions were then coated onto SnO2 nanorod film by spin-coating at 3000 rpm for 40 s, and 80 μL of chloroform was poured onto the substrate during the last 10 s. The FTO substrates were then annealed at 130 °C for 20 min. After the perovskites cooled to room temperature, 35 μl of spiro-OMeTAD solution (80 mg/mL in chlorobenzene) was spin-coated onto the prepared perovskite film at 2500 rpm for 30 s. Finally, the devices were deposited with a gold electrode with the thickness of about 80 nm by evaporation through an aperture mask in a vacuum chamber. The active area of this electrode was fixed at 0.16 cm2. A schematic illustration for the preparation process of SnO2 nanorod arrays as the electron transport layers for perovskite solar cells is summarized in Scheme 1. For a better comparison, PSCs using SnO2 nanoparticles (NP) as the ETLs are introduced. The SnO2 nanoparticles were synthesized via a facile solvothermal route according to our previously reported method [48]. The as-prepared SnO2 nanoparticles are pure rutile phase with the 461
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Scheme 1. Schematic illustration for the preparation process of SnO2 nanorod arrays as the electron transport layers for perovskite solar cells: (a) Preparation of SnO2 seed layer on FTO glass; (b) synthesis of SnO2 nanorod arrays via a hydrothermal method; (c) spin-coating of the precursor solution of perovskite; (d) annealing process; (e) spin-coating of the HTM layer; (f) A complete PSC device.
84% by calculating the fractions of bright areas in the SEM images using Adobe Photoshop [51]. It can be concluded that the lateral growth of the nanorods is more preferential as compared to the vertical direction in the case of an increase in precursor concentration. Additionally, the starting solution with other concentrations of SnCl4·5H2O was also studied. It was found that the nanorod arrays on FTO were very sparse when the amount of SnCl4·5H2O was lower than that of NR-A, while serious sedimentation of tin oxides glued on the backside of FTO glass when the amount of precursor was larger. TEM images in Fig. 2 show a bunch of SnO2 nanorod arrays scraped from NRB sample. The nanorods exhibit a high degree of crystallinity. The growth direction of the nanorods is along [001], and the spacing between adjacent lattice planes of 0.34 nm can be ascribed to the (110) planes. The hydrolysis of SnCl4·is largely retarded in the acidic solution in this study. By allowing a slow nucleation and growth at low interfacial tension condition, stable c-elongated anisotropic SnO2 nanocrystals exposed stable (110) faces could occur, exhibiting typical crystal habits of acicular and tabular structures [40]. Furthermore, as the chloride ions in the solution could adsorb on (110) crystal faces of SnO2 and retard the growth, the nanorods exhibit a preferential growth along [001] direction [48]. The X-ray diffraction (XRD) patterns of the SnO2 nanorod arrays are shown in Fig. 3, where the bare FTO substrate is also shown for a better comparison. As the transparent conductive layer on FTO is fluorine doped tin dioxides, the bare FTO substrate exhibits characteristic peaks consistent with the standard pattern of rutile type tetragonal structure SnO2 (JCPDS 41-1445). After the hydrothermal reaction, the intensity of (101) peaks becomes remarkably stronger, indicating the growth of SnO2 nanorod arrays on FTO. X-ray photoelectron spectroscopy (XPS) measurements further confirm that the composition of the nanorod arrays prepared from SnCl4·5H2O precursor is pure SnO2. Fig. 4a shows the typical broad-scan spectrum with the binding energy range of 0–1200 eV for NR-B sample, where the primary peaks correspond to O 1s and Sn 3d are marked. High-resolution Sn 3d spectrum in Fig. 4b shows two different peaks with the binding energy of 487.1 and 495.5 eV, corresponding to the spin-orbital splitting photoelectron lines of Sn 3d5/2 and Sn 3d3/2 peaks respectively [30]. Fig. 5a shows the transmission spectra of the bare FTO glass and those with the SnO2 nanorod arrays on it. NR-A and NR-B samples exhibit higher transmittance than the bare FTO in the wavelength range of 400–800 nm, as a result of good antireflection capability of the SnO2 nanorod arrays, which can also be distinguished easily from the optical image of the inset by its appearance. Concerning solar cells, higher transmittance demonstrates an important way to achieve stronger lightharvesting, which allows more photons to be absorbed by the perovskite layer. Fig. 5b shows the absorbance spectra of FTO/SnO2/perovskite. All samples demonstrate an excellent light absorption over the entire visible-light region.
crystallite size of about 15 nm as shown in Fig. S2 and S3 in supplementary data. The SnO2 nanoparticles were dispersed using ethanol to prepare the colloidal suspension. A monolayer of SnO2 nanoparticles was coated on the FTO glass by spin-coating at a speed of 3000 rpm for 15 s. After spin-coating, the substrates were sintered at 450 °C for 30 min. Subsequently, the perovskite and spiro-OMeTAD were sequentially spin-coated onto the nanoparticle ETLs the same as that of NR-PSCs. 2.4. Characterization The phase of SnO2 nanorod arrays was identified by X-ray diffraction (XRD) (Model D/Max 2400 X, Rigaku, Japan). The morphology of SnO2 nanorod arrays was performed by transmission electron microscopy (TEM) (JEM-2100F, JEOL Co., Japan). A field-emission scanning electron microscopy (FESEM) (SU8010, Hitachi, Japan) was employed to observe morphological properties of films and devices. X-ray photoelectron spectroscopy (XPS) measurements were performed using a XPS system (Thermo Scientific, ESCALAB 250Xi, USA). The transmittance and absorption spectra were collected on an UV–vis spectrophotometer (UV-3600, Shimadzu, Japan). Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were acquired with a FLS920 transient optical spectrometer (Edinburgh Instruments, UK). Photocurrent density-voltage (J-V) curves of the PSCs were performed using a Keithley 2400 Source Meter. A solar simulator (Model 96160 Newport Co., USA) equipped with a 300 W Xenon lamp was used as a light source, where the light intensity was adjusted to 100 mW cm−2 using a NREL-calibrated silicon solar cell. The incident photon-to-current conversion efficiency (IPCE) spectra were measured as a function of the wavelength from 300 to 900 nm using a specially designed IPCE system (Newport Co., USA). Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical analyzer (Zahner-elecktrik, Germany) and was carried out by applying a DC bias of 0.8 V and an AC voltage with an amplitude of 5 mV in a frequency range of 100 mHz to 100 kHz under dark condition. 3. Results and discussion Fig. 1 shows surface and cross-sectional SEM images of the SnO2 nanorod arrays. The nanorods are grown vertically on the FTO glass. The NR-A sample exhibits the finest nanorods among the three samples. With increasing the precursor concentration of SnCl4·5H2O the mean diameters of the nanorods increase from 15 to 25 nm, whereas the lengths of which are nearly unchanged with a similar length of about 160 nm. Statistical analysis shows that the area densities of the nanorod arrays are about 660 (NR-A), 500 (NR-B) and 460 μm−2 (NR-C), and the corresponding surface coverage rates are estimated to be 61%, 76% and 462
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Fig. 1. Surface morphology and cross-sectional SEM images of SnO2 nanorod arrays grown on FTO substrate at 200 °C for 12 h with different amounts of SnCl4·5H2O: (a, d) NR-A, 55 mg (b, e) NR-B, 65 mg and (c, f) NR-C, 75 mg.
perovskite solar cells based on different SnO2 nanorod arrays. The electron transport layer of nanorod arrays, absorber layer of perovskite (∼500 nm), hole transport layer of Spiro-OMeTAD (∼200 nm), and the gold layer can be seen clearly from the bottom to top. The grain boundaries of the perovskites are vertically oriented to the substrate which is beneficial for the charge carrier transport. Noteworthy, the interspace among the nanorod arrays is also well filled with perovskites. Fig. 7a shows the J-V curves of the best performing perovskite solar cells based on the SnO2 nanorod arrays, and the corresponding photovoltaic parameters are listed in Table 1. For a better comparison, PSCs prepared from SnO2 nanoparticles (NP) are also introduced. It can be found that PSCs from nanorod arrays show an remarkable increase in short-circuit current density (Jsc) compared with the nanoparticle counterpart, which is consistent with the previous findings of TiO2 nanorod arrays [15,16,21]. Generally, the photocurrent mainly depends on light-harvesting of the solar cells and electron transport/collection inside the semiconductor. Firstly, the good antireflection capability of the SnO2 nanorod arrays would enhance the light harvesting. Secondly, the one-dimensional nanorod arrays grow along the direction perpendicular to the FTO substrate, so they provide the shortest
Steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were measured to investigate the charge-carrier dynamics in pristine and ETL-containing perovskite films [52]. The perovskite layer on FTO shows an intense PL emission centered around 775 nm (Fig. 5c). In the presence of SnO2 nanorod arrays as the ETL, it has a much greater PL quenching. To further understand the ETL properties and recombination dynamics occurring in the ETL/ perovskite, TRPL was performed as shown in Fig. 5d. In the case of the FTO/perovskite, it shows a fast decay lifetime (τ1) of 7.20 ns and a slow decay lifetime (τ2) of 61.86 ns. The values of τ1 and τ2 of FTO/NR-A/ perovskite are 6.40 ns and 45.48 ns. The values of τ1 and τ2 of FTO/NRC/perovskite are 7.01 ns and 41.43 ns. In case of the FTO/NR-B/perovskite, the τ1 and τ2 values reduce to 4.33 ns and 26.98 ns. Thus, the PL results indicate that the lifetime can be significantly reduced when depositing perovskite on SnO2 nanorod arrays as a result of efficient charge separation at the interface between ETL and perovskite layer. Fig. 6a shows the surface morphology of the perovskite film. The perovskite of (FAPbI3)0.85(MAPbBr3)0.15 exhibits a uniform and full coverage feature, where the largest grain sizes have reached about 500 nm. Such high quality perovskite is the guarantee of the high efficiency. Fig. 6b–d shows the cross-sectional SEM images of the three
Fig. 2. (a) TEM and (b) HRTEM images of the SnO2 nanorod arrays scraped from NR-B. 463
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and such defects would produce a negative effect on the electron transport and collection and eventually result in the inferior photovoltaic performance [21]. The photovoltaic performance of PSCs using only the SnO2 seed layer as the ETL is also investigated for comparison. It shows a poor efficiency of 2.69% (Fig. S4). As the solution-processed seed layer is very thin (∼20 nm), there is a great probability of having direct contact between the FTO and the perovskite, leading to a low shunt resistance [30]. The anomalous hysteresis effect in J–V curves is a common phenomenon when using different scan directions in PSCs. Regardless of the possible reasons as ion migration and ferroelectric polarization, the charge transfer kinetics are also considered to have a significant impact on this hysteresis behavior of the devices [10]. Charge accumulation at interfaces, slow charge transport and serious charge recombination would all give rise to hysteresis. Current–voltage curves of the PSCs from different SnO2 nanorod arrays under reverse and forward bias scan conditions are shown in Fig. S5, where the hysteresis effect is almost negligible. The high electron mobility of nanocrystalline SnO2 is believed to reduce the hysteresis as demonstrated by previous reports [30,53,54]. Furthermore, the vertically aligned structures of the nanorod arrays also contribute to the elimination of the hysteresis, which could improve the interfacial contact and charge transport ability, making effectively charge extract across the interface between the perovskite and ETL layers. Fig. 7b shows the incident-photon-to-current conversion efficiency (IPCE) spectra of the best-performing perovskite solar cell based on various SnO2 nanorod arrays ETL. The IPCE values were calculated using the following equation IPCE = (1240 × J )/(λ × P ) where J is the measured photocurrent density (mA/cm2) and P is the incident light power density (mW/cm2) for each wavelength λ (nm). Obviously, the perovskite solar cells based on SnO2 nanorods show the enhanced IPCE values in the entire visible light region from 400 to 800 nm compared with the NP-PSCs. The NR-B PSC exhibits the largest values of around 90%, consistent with the highest short-circuit current density. The results of electrochemical impedance spectroscopy (EIS) could provide essential information on the charge transfer and recombination kinetics of the PSCs [55,56]. Fig. 7c shows the Nyquist plots which are probed at a bias of 0.8 V in the dark over the frequency range of 0.1 Hz–100 kHz, and the inset displays the equivalent circuit model which describes the interfaces and electronic elements. The equivalent circuit model is composed of series resistance (Rs), two resistance elements for transfer resistance (Rtr) at the ETL/perovskite and the perovskite/HTL interfaces, and recombination resistance (Rrec) forming two parallel circuits with constant phase elements (CPE1 and CPE2), respectively. Herein, the arc in the high frequency region (> 1 kHz) related to the charge transfer is overlapped with the recombination semicircle as reported in previous studies [16,56]. Since the perovskite/ HTL interface is identical for all samples, the main arcs of Rrec mainly reflect the charge recombination at the ETL/perovskite interfaces. By fitting the EIS results using the equivalent circuit, the recombination
Fig. 3. XRD patterns of the bare FTO substrate (black) and FTO/NR samples NR-A (red), NR.B (green) and NR-C (blue) with Bragg peaks indexed. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
electron diffusion path between the light absorption layer and conductive layer. The nanorod arrays also exhibit much less grain boundaries than the nanoparticle film which contribute to fast electron transport. Besides, the nanorod arrays being well inserted into the perovskite matrix is beneficial to the exciton dissociation at the ETL/ perovskite interface. The NR-PSC samples show larger open-circuit voltage (Voc) values than the NP-PSC samples because both the perfect nanocrystallinity and vertically aligned one-dimensional structure of the nanorod arrays would suppress the charge recombination, whereas the nanoparticles have many intraband gap states as electron loss centers. Additionally, the compact SnO2 seed layer between the FTO and SnO2 nanorod arrays would also play the role of blocking the holes [24]. Among the three types of PSCs prepared from nanorod arrays, the NR-B PSC samples show the best photovoltaic performance with an average Voc of 0.99 ± 0.01 V, Jsc of 22.7 ± 0.5 mA cm−2 and FF of 66.0 ± 0.4%, leading to average efficiencies of 14.90 ± 0.56% and the best efficiency of 15.46%. For the NR-A PSC samples, the electron extraction at the ETL/perovskite interface is not sufficiently efficient due to relatively sparse SnO2 nanorod arrays, leading to the poor fill factors of PSCs. On the other hand, the photovoltaic parameters all decrease for the NR-C PSC samples which have much denser nanorod arrays. Due to the increasing diameters of the SnO2 nanorod arrays, it becomes difficult to fully fill the perovskite into the interspaces of the nanorod arrays. Some voids may remain in the bottom part of the films,
Fig. 4. (a) Typical XPS survey of SnO2 nanorod array film (NR-B). (b) High-resolution XPS spectrum of Sn 3d. 464
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Fig. 5. (a) Transmittance spectra of bare FTO glass and FTO glass with SnO2 nanorod arrays (the inset shows a typical optical image of the FTO glass where SnO2 nanorod arrays grow on the lower half). (b) Absorbance spectra of FTO/NR-A/perovskite, FTO/NR-B/perovskite and FTO/NR-C/perovskite samples, respectively. (c) Steady-state PL spectra and (d) TRPL spectra of FTO/perovskite, FTO/NR-A/perovskite, FTO/NR-B/ perovskite and FTO/NR-C/perovskite samples, respectively.
resistance of NP-PSC is found to be 2670 Ω, while the Rrec values of the NR-PSCs are 2969 Ω (NR-A), 4420 Ω (NR-B), and 4077 Ω (NR-C), respectively. The NR-PSCs based on the nanorod arrays exhibit larger Rrec values than that of NP-PSCs, which supports lower recombination and enhanced charge collection at the interface by using 1D nanorod arrays as the ETLs. It also can be found that the PSCs based on NR-B ETLs exhibit the largest Rrec value, indicating the smallest charge recombination process and in turn the least potential loss at the interface, as witnessed by the largest Voc [57].
4. Conclusion In summary, we exploit a facile solution route to synthesize vertically aligned SnO2 nanorod arrays on FTO substrates. The mean diameters of the nanorods range from 15 to 25 nm and the corresponding area densities decrease from 660 to 460 μm−2 with increasing the precursor concentration. Mixed perovskites of (FAPbI3)0.85(MAPbBr3)0.15 are infiltrated into these SnO2 nanorod arrays, and the obtained perovskite solar cells show an enhanced photovoltaic performance as compared with the nanoparticle containing
Fig. 6. (a) Surface morphology of the perovskite film in top view. (b–d) Cross-sectional SEM images of the completed perovskite solar cells prepared from NR-A (b), NR-B (c) and NR-C (d), respectively. 465
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Table 1 Photovoltaic parameters of the perovskite solar cells based on different SnO2 nanorod arrays. Sample NP NR-A NR-B NR-C
Best Average Best Average Best Average Best Average
Voc (V)
Jsc (mA cm−2)
FF (%)
PCE (%)
0.94 0.92 0.95 0.93 1.00 0.99 0.98 0.97
20.9 20.1 22.4 21.6 23.1 22.7 22.8 21.9
62.5 61.9 63.2 62.6 66.4 66.0 66.0 65.3
12.38 11.54 13.56 12.60 15.46 14.90 14.97 14.21
± 0.02 ± 0.03 ± 0.01 ± 0.02
± 0.8 ± 0.8 ± 0.5 ± 0.7
± 0.6 ± 0.6 ± 0.4 ± 0.7
± 0.84 ± 0.96 ± 0.56 ± 0.76
PSCs, and the PCE may be improved to over 20% by further optimization of the PSC fabrication process. Declarations of interest None. Acknowledgments This work was supported by the National Natural Science Foundation of China (No.51172046), the Shanghai Natural Science Foundation (15ZR1401200), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. X.Z. acknowledges the foundation supported by Shanghai University of Engineering Science Innovation Fund for Graduate Students. P.M.-B. acknowledges funding from TUM.solar in the context of the Bavarian Collaborative Research Project Solar Technologies Go Hybrid (SolTech). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.09.072. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photon. 8 (2014) 506–514. [3] W.S. Yang, B.W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Iodide management in formamidinium-lead-halidebased perovskite layers for efficient solar cells, Science 356 (2017) 1376–1379. [4] M.L. Petrus, J. Schlipf, C. Li, T.P. Gujar, N. Giesbrecht, P. Müller-Buschbaum, M. Thelakkat, T. Bein, S. Hüttner, P. Docampo, Capturing the sun: a review of the challenges and perspectives of perovskite solar cells, Adv. Energy Mater. 7 (2017) 1700264. [5] G. Yang, H. Tao, P.L. Qin, W.J. Ke, G.J. Fang, Recent progress in electron transport layers for efficient perovskite solar cells, J. Mater. Chem. 4 (2016) 3970–3990. [6] Y.N. Zhang, B. Li, L.Y. Zhang, L.W. Yin, Efficient electron transfer layer based on Al2O3 passivated TiO2 nanorod arrays for high performance evaporation-route deposited FAPbI(3) perovskite solar cells, Sol. Energy Mater. Sol. Cells 170 (2017) 187–196. [7] J. Schlipf, P. Müller-Buschbaum, Structure of organometal halide perovskite films as determined with grazing-incidence x-ray scattering methods, Adv. Energy Mater. 7 (2017) 1700131. [8] A. Yella, L.P. Heiniger, P. Gao, M.K. Nazeeruddin, M. Grätzel, Nanocrystalline rutile electron extraction layer enables low-temperature solution processed perovskite photovoltaics with 13.7% efficiency, Nano Lett. 14 (2014) 2591–2596. [9] D. Yang, X. Zhou, R.X. Yang, Z. Yang, W. Yu, X.L. Wang, C. Li, S.Z. Liu, R.P.H. Chang, Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells, Energy Environ. Sci. 9 (2016) 3071–3078. [10] N.K. Elumalai, A. Uddin, Hysteresis in organic-inorganic hybrid perovskite solar cells, Sol. Energy Mater. Sol. Cells 157 (2016) 476–509. [11] J. Chen, X. Cai, D. Yang, D. Song, J. Wang, J. Jiang, A. Ma, S. Lv, M.Z. Hu, C. Ni, Recent progress in stabilizing hybrid perovskites for solar cell applications, J. Power Sources 355 (2017) 98–133. [12] D. Bi, W. Tress, M.I. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P. Correa Baena, J.-D. Decoppet, S.M. Zakeeruddin,
Fig. 7. (a) Current–voltage curves and (b) IPCE spectra of the PSCs from different SnO2 nanorod arrays and from nanoparticles for comparison. (c) Nyquist plots of PSCs based on different SnO2 ETL layers, and the inset displays the equivalent circuit.
counterparts due to the unidirectional electron transport and inhibited charge recombination. PSCs prepared from the SnO2 nanorod arrays with the optimized area density exhibit the most effective pore-filling of the perovskites, yielding average PCE values of 14.90 ± 0.56% and the best PCE of 15.46%. Thus, this study demonstrates the enormous potential of SnO2 nanorod arrays for application as an effective ETL in 466
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