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Stable high-performance perovskite solar cells based on inorganic electron transporting bi-layers To cite this article: Hao Gu et al 2018 Nanotechnology 29 385401
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Nanotechnology Nanotechnology 29 (2018) 385401 (9pp)
https://doi.org/10.1088/1361-6528/aacf7c
Stable high-performance perovskite solar cells based on inorganic electron transporting bi-layers Hao Gu, Chen Zhao, Yiqiang Zhang
and Guosheng Shao
School of Materials Science and Engineering, State Centre for International Cooperation on Designer LowCarbon and Environmental Material (SCICDLCEM), Zhengzhou University, Zhengzhou 450001, Henan, People’s Republic of China E-mail:
[email protected] Received 12 April 2018, revised 10 June 2018 Accepted for publication 27 June 2018 Published 11 July 2018 Abstract
As one of the significant electron transporting materials (ETMs) in efficient planar heterojunction perovskite solar cells (PSCs), SnO2 can collect/transfer photo-generated carriers produced in perovskite active absorbers and suppress the carrier recombination at interfaces. In this study, we demonstrate that a mild solution-processed SnO2 compact layer can be an eminent ETM for planar heterojunction PSCs. Here, the device based on chemical-bath-deposited SnO2 electron transporting layer (ETL) exhibits a power conversion efficiency (PCE) of 16.10% and with obvious hysteresis effect (hysteresis index=19.5%), owing to the accumulation and recombination of charge carriers at the SnO2/perovskite interface. In order to improve the carrier dissociation and transport process, an ultrathin TiO2 film was deposited on the top of the SnO2 ETL passivating nonradiative recombination center. The corresponding device based on the TiO2@SnO2 electron transporting bi-layer (ETBL) exhibited a high PCE (17.45%) and a negligible hysteresis effect (hysteresis index=1.5%). These findings indicate that this facile solution-processed TiO2@SnO2 ETBL paves a scalable and inexpensive way for fabricating hysteresis-less and high-performance PSCs. Supplementary material for this article is available online Keywords: tin dioxide, hysteresis effect, electron transporting bi-layer, perovskite solar cells (Some figures may appear in colour only in the online journal) Among the five layers, the ETL is extremely significant for extracting and transporting photo-generated charge, as well as blocking holes [12–18]. Considering SnO2 possesses high electron mobility (about 100–200 cm2 V−1 s−1), suitable band structure and similar crystal structure as transparent conductive layer, an increasing number of researchers employ it as ETL in planar heterojunction PSCs [19, 20]. To date, the Grätzel group have shown planar devices using atomic-layer-deposited (ALD) SnO2 as ETL yielding efficiencies of 19.5% [21, 22]. Similarly, Yan et al also demonstrated planar organometal halide PSCs can be obtained by plasma-enhanced ALD SnO2 ETL [23]. However, the ALD is expensive and complicated for scalable manufacturing, thus it may hinder low-cost industrialized
1. Introduction Organic–inorganic lead halide perovskite solar cells (PSCs) have made impressive progress in the past few years with champion power conversion efficiencies (PCEs) leaping from 3.8% in 2009 to a certified 22.7% in 2017 [1, 2]. The unprecedented progress achieved to date can been ascribed to exceptional and ideal material properties such as high charge carrier mobility, long exciton diffusion distance, strong light absorption over the solar spectrum, large absorption coefficient, low exciton binding energy and low fabrication cost [3–11]. The typical PSCs consist of five parts: transparent conductive layer, electron transporting layer (ETL), light absorption layer, hole transporting layer, and metal electrode. 0957-4484/18/385401+09$33.00
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AlfaAesar) and hydrochloric acid (HCl, 36%–37%) were used as received without further purification. 0.5 ml HCl was first added to ice DI water and then anhydrous tin tetrachloride was added to the dilute HCl solution under stirring. Finally, an aqueous solution of SnCl4 was diluted to 40 mM at 0 °C and the as-prepared solutions were stored before use. The FTO substrates were then put into the aqueous SnCl4 solution and placed into an oven at 75 °C for 1 h. After 1 h, the FTO substrates were rinsed with DI water and dried at 100 °C for 1 h. To further obtain TiO2 thin layer onto the SnO2film, an aqueous stock solution of TiCl4 (99.9%) was diluted to precursor solutions with 200 mM TiCl4 at 0 °C. The substrates were then immersed into the precursor solution and kept in an oven at 70 °C for 1 h. Afterwards the FTO substrates were washed with DI water and dried at 100 °C for 1 h [26]. The PbI2 and CH3NH3I were vigorous stirred in a mixture of dimethyl sulfoxide: γ-butyrolactone (GBL) (3:7, v/v) at 60 °C overnight under inert argon environment. The raw material precursor solution was coated onto FTO/SnO2 substrate by a two-step successive spin-coating procedure at 1500 rpm and at 4000 rpm for 15 and 25 s, respectively, and the appropriate amount of toluene in second spin-stage was dropped onto the precursor coated substrate in the last 5 s. The perovskite-raw film was annealed on a hot plate at 100 °C for 10 min. The HTL was then spin-coated at 2000 rpm for 30 s. The spin-coating formulation was prepared by dissolving 144.6 mg (2,2,7,7-tetrakis(N,N-di-pmethoxyphenylamine)-9,9spirobifluorene) (Spiro-MeOTAD), 57.6 μl 4-tert-butylpyridine, and 35 μl of a stock solution of 520 mg ml−1 lithium bis (trifluoromethylsulphonyl)imide (LiTFSI) in acetonitrile in 1 ml chlorobenzene. Finally, Au cathode was evaporated by Trovato 300 C thermal evaporator via a shadow mask at 2×10−8 Torr. The device active area was fixed at 0.09 cm2.
fabrication of PSCs. Subsequently, Fang et al reported that a planar PSC using a low-temperature solution-processed SnO2 as ETL achieved a high PCE of 17.21% [24]. And Jen et al demonstrated that a high-performance inverted PSCs using highly crystallized SnO2 nanocrystals as a robust ETL yield a high PCE of 18.8% [25]. However, these studies are based on a spin-coating process, so they may not achieve large-scale ETL for commercialized PSCs. Previously, we reported that a mild solution-processed can prepare high-quality TiO2 film, which was employed to planar PSCs yielding PCE of 12.62% [26]. Therefore, the similar solution processing technique is expected to fabricate robust SnO2 thin film for high-performance planar heterojunction PSCs. Herein, we developed a simple low-temperature solutionprocessed method to prepare high-quality SnO2 films. The corresponding PSCs exhibit a PCE of 16.10% with obvious hysteresis effect (19.5%), owing to the accumulation and recombination of charge carriers at ETL/perovskite interface. In order to eliminate this serious hysteresis effect and further boost the device efficiency, a layer-by-layer chemical-bathdeposited method was designed to construct TiO2@SnO2 electron transporting bi-layer (ETBL). As a result, the device based on this ETBL shows a high-performance (short circuit current density (Jsc)=22.53 mA cm−2, open-circuit voltage (Voc)=1.088 V, fill factor (FF)=71.21% and PCE= 17.45%) and a negligible hysteresis effect (1.5%) for CH3NH3PbI3 (MAPbI3) planar heterojunction PSCs. Furthermore, the fabrication process does not exceed 100 °C, which can be potentially used to prepare flexible solar cells. These findings pave a scalable and inexpensive way for fabricating hysteresis-less and high-performance PSCs.
2. Experimental section 2.3. Instruments 2.1. Materials
The phase structure information was characterized on a Rigaku (RINT-2500) x-ray diffractometer (Cu Ka radiation, λ=1.5418 Å). SEM images were obtained via a focused ion beam SEM (ZEISS Gemini Sigma 500). UV–vis–NIR fluorescence spectrophotometer absorption spectra were recorded on a Shimadzu UV 3600 spectrophotometer at room temperature. The J–V curves’ characteristics were recorded with a Keithley2400 source meter and 425 W collimated Xenon lamp (Newport) calibrated with the light intensity to 100 mW cm−2 under AM 1.5 G solar light condition by the certified silicon solar cell. The J–V curves were obtained by reverse (forward bias (1.2 V)→short circuit (0 V)) or forward (short circuit (0 V)→forward bias (1.2 V)) scan with different scan rate. IPCE data was obtained on a computer-controlled IPCE system (Newport 74125 system). PL (photoluminescence excitation at 480 nm) was measured with Edinburgh Instruments (FLSP920). The chemical states of O, Sn and Ti in TiO2@SnO2 were studied by Multilab 2000 XPS system, and all the binding energies were an internal referenced to the C 1 s peak at 284.6 eV. Electrochemical impedance spectroscopy was measured via ZAHNER electrochemical work station in
A majority of solvent materials were purchased from AlfaAesar and used without purification. And the PbI2 (99.99%) and Spiro-OMeTAD (99.5%) were purchased from Xi’an Polymer Light Technology Corp. The CH3NH3I (MAI) was preliminarily synthesized by the reaction of 24 ml CH3NH2 (33 wt% in absolute ethanol, Alfa) and 30 ml of hydroiodic acid (57 wt% in water, Alfa) in a 250 ml flask at 0 °C for 2 h. The precipitate was then dried at 65 °C for 1 h. MAI was dissolved in anhydrous ethanol, then recrystallized from diethyl ether, and ultimately dried at 60 °C in vacuum oven for 24 h. 2.2. Fabrication of PSCs
F-doped SnO2 (FTO) substrates with a sheet resistance of 7 ohm−2 and an optical transmittance over 85% in the visible range were used. The substrates were sequentially ultrasonicated with acetone, 2-isopropanol and de-ionized (DI) water, and the cleaned FTO substrates were then treated under UV-ozone for 20 min. Tin tetrachloride (SnCl4, 99.99%,
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Figure 1. (a) The schematic illustration of all steps in low-temperature solution-processed methods of SnO2 and TiO2 thin film deposition. (b) XRD pattern of SnO2 on glass by chemical-bath deposited. (c) Absorption spectra of SnO2 films on FTO.
Figure 2. Top-view SEM images for (a) bare FTO substrate, (b)–(f) varied SnO2 film deposited by chemical-bath method with different reaction times of 15, 30, 60, 90 and 120 min, correspondingly.
is shown in figure 1(a). Figures 1(b) and (c) indicate the deposited SnO2 thin film was in amorphous feature [27] with high transmittance in the visible range, which is crucial for active perovskite layer to absorb sufficient light. The morphologies of various SnO2 layers were characterized by scanning electron microscope (SEM) (figures 2(a)–(f)). The bare FTO exhibits a much rougher surface with clearly sharp grain boundaries than the SnO2 thin films deposited.
the dark and with 0.8 V. The EIS was measured in the frequency ranging from 100 kHz to 1 Hz.
3. Results and discussion The schematic illustration of the layer-by-layer chemicalbath-deposited method for SnO2 or TiO2 thin film deposition 3
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Figure 3. (a) The XRD pattern and (b) absorption spectra of CH3NH3PbI3 based on SnO2 ETL. (c) The J–V curves of PSCs using different SnO2 film as ETL. (d) EQE of corresponding PSCs.
devices for each condition (20 devices fabricated under same conditions) were recorded and shown in figure 4. The PSCs with SnO2ETL showed excellent statistic of PCE (15.6%± 0.9%) with high reproducibility. This phenomenon can be attributed to two factors. On the one hand, complete coverage of the FTO surface by SnO2 layer leads to reduced current leakage and low series resistance [27, 29, 30]. On the other hand, SnO2 layer can extract and transport of electrons due to its appropriate Fermi level and conductivity [31, 32]. The external quantum efficiency (EQE) of PSCs depends on the efficiency of light-harvesting and the charge transportation in the device. The EQE of various SnO2-based devices is presented in figure 3(d). We can see that EQE values of 60 min-SnO2 based device are higher than that of others in the whole visible range from 350 to 780 nm. The integrated current densities for different SnO2 ETLs based devices were 18.73, 20.81, 21.39, 21.27, and 21.18 mA cm−2, respectively, in good agreement with the similar measured Jsc. To elucidate charge carrier recombination process in different SnO2 ETL/perovskite interface, several samples based on various ETL are investigated. The steady-state PL spectra were shown in figure 5(a), with excitation at 480 nm. The fluorescence is quenched obviously as inserted SnO2 ETL, and the lowest intensity of PL is the 60 min-SnO2 based sample. This phenomenon indicates that 60 min-SnO2 layer is the most effective for the collection and transporting of electrons than that of other ETLs [33, 34]. Furthermore, timeresolved PL spectrum presents that the electron–hole lifetime is shorter in 60 min-SnO2 ETL-based sample than that of bare FTO one (figure 5(b)). This fast PL decay in
Obviously, the dense pinhole-free SnO2 layer could be obtained, as the deposition time is increased to 60 min. However, when deposition time was further increased, the asprepared SnO2 film showed evident cracks and segregation of large particles, which might hinder the transportation of photon-generated electron and result in the accumulation of carriers at perovskite/ETL interface. The thickness of SnO2 films were measured employing a stylus profiler (figure S1 is available online at stacks.iop.org/NANO/29/ 385401/mmedia). It can be seen that the thickness increased linearly with the deposition time, ranging from 10 to 40 nm. In this work, the MAPbI3 layers are obtained by a modified solvent engineering method [28]. The top-view SEM shows a uniform perovskite film with a grain size of ∼500 nm (figure S2). The crystal structure of perovskite film on the amorphous SnO2 was characterized by x-ray diffraction (XRD), which exhibited high crystallinity corresponding to the tetragonal perovskite structure (figure 3(a)). And light absorbance at range of visible with a sharp onset at 780 nm as seen in figure 3(b). The various SnO2 layers with different reaction time were used as ETL in the planar heterojunction PSCs (FTO/ETL/MAPbI3/Spiro-OMeTAD/Au). The current density–voltage (J–V ) characteristics of the PSCs with different ETLs are shown in figure 3(c). It is found that the device based 60 min SnCl4 possesses a high PCE (16.10%), with a Jsc of 21.37 mA cm−2, Voc of 1.08 V, and FF of 69.29%, under the reverse scan with a scan rate of 50 mV s−1. While the ETL-free device demonstrates a low PCE of 6.63% with 16.05 mA cm−2, 0.99 V, and 41.77% for Jsc, Voc, and FF, correspondingly (table S1). Deviations of PCEs within 20 4
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Figure 4. The histograms of devices fabricated by SnO2 ETL, including PCE, FF, Jsc and Voc for 20 samples each set.
Figure 5. (a) Photoluminescence (PL) spectra of the CH3NH3PbI3 film. (b) Time-resolved PL (excitation at 480 nm) of the bare perovskite
film and on top of 60 min SnO2 ETL.
60 min-SnO2/MAPbI3 could be attributed to accelerated charge carrier transfer from the photo-excited perovskite to the ETL [34, 35]. In order to eliminate the serious hysteresis effect in SnO2-based PSCs, an ultrathin TiO2 layer (ca. 10 nm) was deposited onto SnO2/FTO substrate as ETBL. Figure S6 shows the XRD patterns of as-deposited TiO2 film. Without impurity peaks were observed, confirming the high purity of TiO2 and FTO. The broad peaks at 36.1°, 41.2°, and 44.05° were well assigned to the (101), (111), and (210) crystal planes of the rutile TiO2 phase (JCPDS#21-1276), correspondingly. To analysis the surface composition of the TiO2@SnO2 films prepared by CBD method, x-ray photoelectron spectroscopy (XPS) measurement was conducted. The full XPS spectrum given in figure 6(a) certificates the presence of O, Ti and Sn. The binding energies of 487.0 and
495.4 eV correspond to the Sn 3d5/2 and Sn 3d3/2 peaks, respectively (figure 6(b)). O 1 s peaks can be separated into two obvious peaks in figure 6(c). Obviously, the binding energy of 530.5 eV is tend to Ti–O bonding of TiO2, and the binding energy of 532.1 eV is ascribed to Sn–O bonding in SnO2 [36]. While the two distinct peaks of Ti 2p1/2 (465.0 eV) and Ti 2p3/2 (459.3 eV) be attributed to Ti4+ in TiO2, respectively (figure 6(d)). To further reveal the relationship between TiO2 and SnO2 thin film, the cross-sectional SEM image and energy dispersive spectroscopy (EDS) mappings of TiO2@SnO2 ETBL layer on the FTO is shown in figure 7. It can be obviously seen that there is a pinhole-free uniform TiO2@SnO2 ETBL layer with smooth surface on the top of FTO [37]. The distribution of Ti and Sn in the EDS mapping strongly demonstrates that the TiO2 and SnO2 upper layer is formed on the top of FTO glass. 5
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Figure 6. XPS spectra of (a) survey, (b) Sn 3d3/2, 3d5/2 peaks (c) O 1s peaks, and (d) Ti 2p1/2, Ti 2p3/2 peaks for a low-temperature solutionprocessed TiO2@SnO2 ETBL coated on a FTO substrate.
Figure 7. SEM image and EDS mapping of the cross section of FTO/TiO2@SnO2 ETBL (≈50 nm).
Figure 8. (a) Cross-sectional SEM image and (b) energy level of the device with a structure of FTO/TiO2@SnO2 ETBL/CH3NH3PbI3/Spiro-OMeTAD/Au, with TiO2@SnO2 ETBL, CH3NH3PbI3, and Spiro-OMeTAD thicknesses of ≈50, 480, and 200 nm.
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Figure 9. (a) J–V curves of devices based on the TiO2@SnO2 and pristine SnO2 from FB-SC and from SC-FB. (b) PL spectra of the CH3NH3PbI3 film on the top of SnO2 and TiO2@SnO2 ETL.
Figure 10. (a) Nyquist plots and (b) enlarged view plots of high frequency area applied 0.8 V extracted from Nyquist plots of PSCs based on TiO2@SnO2 ETBL and SnO2 ETL in the dark (Inset: equivalent circuit). Table 1. Photovoltaic parameters of Jsc, Voc, FF and PCE for the PSCs based on SnO2 and TiO2@SnO2 ETL from forward bias (FB) to short circuit (SC) and from short circuit to forward bias (SC-FB) with a scan rate of 50 mV s−1.
Sample
Scan direction
Jsc (mA cm−2)
Voc (V)
FF (%)
PCE (%)
SnO2
Forward scan Reverse scan Forward scan Reverse scan
21.23 21.37 22.42 22.53
0.99 1.08 1.088 1.088
67.39 69.29 70.04 71.21
14.16 16.10 17.08 17.45
TiO2@SnO2
We further utilized the TiO2@SnO2 ETBL layer as ETL in PSCs, the cross-sectional SEM image and energy level of the device with a structure of FTO/TiO2@SnO2/CH3NH3PbI3/ spiro-OMeTAD/Au was shown in figure 8. It is noted that the ETBL-based device demonstrates a Jsc of 22.53 mA cm−2, a Voc of 1.088 V, a FF of 71.27% and a PCE of 17.45% under one sun illumination (AM 1.5 G 100 mW cm−2), under the reverse scan with a scan rate of 50 mV s−1 (figure 9(a) and table 1). Surprisingly, the device based on this ETBL exhibits a negligible hysteresis effect (hysteresis index=1.5%), which is much lower than SnO2-based device (hysteresis index=19.5%). The improved hysteresis effect for PSCs based on TiO2@SnO2 ETBL may be attributed to the more efficient charge dissociation and reduced charge accumulation at the ETL/perovskite interface, compared to the SnO2-based PSCs. The reduced
charge recombination and improved electron extraction in TiO2@SnO2-based PSCs, which leads to the reduced hysteresis, can be further corroborated by the characterization of PL and electrochemical impedance spectroscopy. To investigate the reproducible capability, the photovoltaic parameter statistics of 20 devices for each condition (20 devices fabricated under same conditions) based on TiO2@SnO2 film as ETBL with different scan directions, measured under a simulated AM 1.5 G light irradiation shown in table S4. In order to study the influence of TiO2@SnO2 ETL on charge transfer, PL measurements were executed to further assess the quenching ability of ETBL, shown in figure 9(b). Compared to the 60 min-SnO2 ETL, the TiO2@SnO2 ETBL (more obvious quench) resulted in increased charge carrier extraction, reduced charge carrier recombination and negligible J–V hysteresis. Figure 10 is the Nyquist plots of 7
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Department of Henan Province (Grant no. 14A510001). And we thank Mr Chao Liang for discussion of the manuscript.
PSCs based on SnO2 ETL and TiO2@SnO2 ETBL in the dark. Typically, the semicircle at low frequency is corresponding to the recombination resistance (Rrec) [38, 39]. The Rrec of TiO2@SnO2 ETBL-based device is larger than that of SnO2 ETL-based PSC, indicating the recombination of carriers could be restrained efficiently. Reduced recombination of carriers leads to increased Voc and Jsc in PSCs. This result is consistent with the PL measurement. Finally, this ETBL will lead to significant improvements in performance and stability of the PSCs. The PCE at maximum output power point is 17.20% for the TiO2@SnO2-based device. This value also is higher than the SnO2 based PSCs (15.7%) (figure S7(a)). The long-term stability of 20 devices with TiO2@SnO2 ETBL and SnO2ETL for each set is shown in figure S7(b). The PSCs based on TiO2@SnO2 ETBL showed improved stability than the devices based on SnO2ETL, which are extracted from measuring J–V curves for PSCs under one sun illumination (100 mW cm−2, AM 1.5 G) in ambient environment with a relative humidity of 40%±5%. The TiO2@SnO2 ETBL-based devices retained over 70% of the initial PCE after 100 h. In comparison, the PCE of SnO2 devices decayed rather fast, about 55% of their original efficiency lost. The enhanced stability of PSCs is closely related to the optimized ETL/perovskite interface and interfacial charge accumulation during operation. In addition, a suitable ETL may lead to increased quality of the perovskite crystal, thereby increasing the stability of the devices.
ORCID iDs Yiqiang Zhang
https://orcid.org/0000-0002-2437-925X
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4. Conclusion In conclusion, efficient and hysteresis-less CH3NH3PbI3 PSCs based on TiO2@SnO2 ETBL are demonstrated. A maximum PCE of 17.45% (Jsc=22.53 mA cm−2, Voc= 1.088 V, and FF=71.21%) is achieved under one sun illumination (AM 1.5 G 100 mW cm−2). Compared to the SnO2 ETL, the TiO2@SnO2 ETBL resulted in increased charge carrier extraction, reduced charge carrier recombination. Furthermore, the PSCs based on potential low-temperature ETBL are of highly reproducibility and stability with negligible hysteresis. In addition, the PSCs based on TiO2@SnO2 ETBL show improved stability compared to the devices based on SnO2 ETBL. These ETBL are greatly promising for optimizing carrier transport behaviors in varied optoelectronic devices.
Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (Grant Nos. 21401167), the Fundamental and Advanced Technology Research Program from the Science and Technology Department of Henan Province (Grant no. 142300410031.0), the China Postdoctoral Science Foundation (Grant No. 2013M540573) and the Science and Technology Key Project from the Education 8
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