Accepted Manuscript TiO2/SnOxCly double layer for highly efficient planar perovskite solar cells Cuncun Wu, Ziru Huang, Yihao He, Wei Luo, Hungkit Ting, Tieyi Li, Weihai Sun, Qiaohui Zhang, Zhijian Chen, Lixin Xiao PII:
S1566-1199(17)30381-6
DOI:
10.1016/j.orgel.2017.07.050
Reference:
ORGELE 4242
To appear in:
Organic Electronics
Received Date: 26 June 2017 Revised Date:
17 July 2017
Accepted Date: 30 July 2017
Please cite this article as: C. Wu, Z. Huang, Y. He, W. Luo, H. Ting, T. Li, W. Sun, Q. Zhang, Z. Chen, L. Xiao, TiO2/SnOxCly double layer for highly efficient planar perovskite solar cells, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.07.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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TiO2/SnOxCly double layer was employed as the electron transport layer for planar perovskite solar cell. Compared with bare TiO2, perovskite solar cell based on TiO2/SnOxCly shows drastically improved power conversion efficiency and reduced hysteresis. These improvements are attributed to TiO2/SnOxCly which could enhance electron extraction and reduce surface trap-state.
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TiO2/SnOxCly Double Layer for Highly Efficient Planar Perovskite Solar Cells Cuncun Wu,a# Ziru Huang,a# Yihao He,d Wei Luo,a Hungkit Ting,ae Tieyi Li,a Weihai Sun,a Qiaohui
a.
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Zhang,a Zhijian Chen,*abc Lixin Xiao*abc State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China. E-mail:
[email protected];
[email protected].
b. Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices, Chongqing
c.
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University of Arts and Sciences, Yongchuan Chongqing 402160, China
New Display Device and System Integration Collaborative Innovation Center of the West
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Coast of the Taiwan Strait, Fuzhou 350002, China.
d. The High School Attached to Hunan Normal University. e.
Technology Innovation Center, Dongguan Institute of Opto-Electronics Peking University, Dongguan, Guangdong 523808 P.R.China
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These authors contributed equally to this work.
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ACCEPTED MANUSCRIPT Abstract Recently, perovskite solar cells have attracted tremendous research interest due to their amazing light to electric power conversion efficiency (PCE). However, most high performance devices usually use mesoporous TiO2 as the electron transport layer
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(ETL), which increases cost for practical application. Here, TiO2/SnOxCly double layer was employed as the ETL for planar perovskite solar cells. Compared with bare TiO2, perovskite solar cell based on TiO2/SnOxCly shows drastically improved power conversion efficiency and reduced hysteresis. These improvements are attributed to
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TiO2/SnOxCly which could enhance electron extraction and reduce surface trap-state.
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Keywords : planar perovskite solar cells, TiO2/SnOxCly, electron extraction, trap-state
1. Introduction
Since the breakthrough work reported in 2012 [1], perovskite solar cells have been investigated extensively during the past four years as promising
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alternatives to conventional silicon solar cells [2-7]. Today, the highest efficiency of perovskite solar cells has reached 22.1% [8]. Such high performance solar cells usually use TiO2 as the electron transport layer (ETL) 2,20,7,70-tetrakis(N,N-di-pmethoxyphenylamine)-9,9'-spirobifluorene
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(spiro-OMeTAD) as the hole transport layer (HTL).[9,10] However, the electron extraction ability of planar TiO2 is not high enough. Thus, a mesoporous TiO2
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layer is often added to help extract electrons by increasing the surface area. In spite of this, mesoporous structure has more complicated production
process and higher cost, encouraging people to improve the efficiency of planar perovskite solar cells [11-13]. Generally, the cell structure of planar perovskite solar cell can be classified into two types: regular (n-i-p)[14] and inverted (p-i-n)[15] architectures. For inverted structure, the power conversion efficiency (PCE) over 18% has been reported [15,16]. In the case of TiO2 based regular planar structure perovskite solar cell usually has a PCE less than 15% and a strong hysteresis behavior [17,18]. This is because in the regular planar
ACCEPTED MANUSCRIPT perovskite solar cell, TiO2 as ETL has two disadvantages: (i) It has relative lower electron mobility, leading to unbalanced carrier transport in the device. Thus, the J-V curve exhibits significant hysteresis phenomenon. (ii) Oxygen vacancies in TiO2 nanoparticles are deep electron trap sites, which can bind
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with photogenerated electrons from perovskite and cause the degradation of cell performance [19].
Recent studies have shown the potential of SnO2 as a promising ETL in PSC [20-24]. Compared with TiO2, SnO2 has a lower conduction band
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minimum(CBM) and higher electron mobility [25], which can facilitate the extraction of photogenerated electrons from perovskite to ETL more efficiently.
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Very recent study shows that the interfacial Cl on TiO2 surface suppresses deep trap states at perovskite interface and improves interface binding in planar perovskite solar cell [26]. Therefore, taking the advantages of SnO2 and interfacial Cl into consideration, in this work, we propose TiO2/SnOxCly double layer as the ETL in planar perovskite solar cells. Moreover, the TiO2/SnOxCly
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based devices exhibit reduced hysteresis and an improved PCE from 14.6% to 16.6% with higher Voc, short-circuit current (Jsc), and fill factor (FF) compared with bare TiO2 based one, which is due to the enhanced electron extraction and fewer trap-states
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induced recombination at the interface of the ETL and the photoactive layer.
2. Experimental
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2.1 Materials and device fabrication Materials. CH3NH3I was prepared according to the procedure reported
previously. PbI2(99.99%) and spiro-OMeTAD(99.5%) were purchased from Xi’an Polymer Light Technology Corp. (PLT). Diisopropoxide bis(acetylacetonate) and SnCl2·2H2O were purchased from Sigma-Aldrich and Aladdin, respectively. All these commercially available materials were used as received without any further purification. Device fabrication. Fluorine doped tin oxide (FTO) glass was cleaned sequentially via deionized water, acetone, and ethanol under ultra sonication 15 min
ACCEPTED MANUSCRIPT for each, and then treated with O2 plasma for 15 min. For pure TiO2 devices, a compact TiO2 layer on the FTO glass was prepared by spin-coating of titanium diisopropoxide bis(acetylacetonate) solution (0.15 M, in 1-butanol) at 4000 rpm for 30 s, dried at 130 ℃ for 5 min, then repeated once with 0.3 M of titanium
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diisopropoxide bis(acetylacetonate) solution, finally baked at 500 ℃ for 15 min. For TiO2/SnOxCly device, the TiO2 layer was prepared by spin-coating 0.15 M of titanium diisopropoxide bis(acetylacetonate) solution at 4000 r.p.m. for 30 s, then baked at 500 ℃ for 15 min. The SnOxCly layer was prepared by spin-coating 0.1 M of
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SnCl2·2H2O on the TiO2 layer at 4000 r.p.m. for 30 s and annealed at 165 ℃ for 60 min.
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The perovskite film was deposited by a modified one-step spin coating method. 1.25 mmol CH3NH3I and PbI2, 89 µL dimethyl sulfoxide (DMSO) was mixed in 1 mL N,N-dimethylformamide (DMF) at room temperature and to form a precursor solution after 12 hours. This precursor solution was spin-coated on TiO2 or TiO2/SnOxCly electron transport layer at 6000 rpm for 30 s and 0.7 mL diethyl ether was slowly
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dripped on the substrate 6 s after the beginning of spin-coating. After spin-coating, the film was annealed at 70 ℃ for 1 min and 100 ℃ for 3 min. 60 µL 2,20,7,70-tetrakis(N,N-di-pmethoxyphenylamine)-9,9'-spirobifluorene
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(spiro-OMeTAD) solution, which is made by mixing 72.3 mg spiro-OMeTAD, 30 µL of 4-tert-butyl pyridine (TBP) and 30 µL of lithium bis(trifluoromethane) sulfonimide (Li-TFSI) solution (520 mg Li-TFSI, Sigma–Aldrich, 99.8 %, in 1 mL acetonitrile) in
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1mL chlorobenzene, was spin-coated on the perovskite layer at 3000 rpm for 30 s. Finally, Ag electrode was deposited by using thermal evaporator at a constant evaporation rate of 0.3 nm/s. Except for the thermal evaporation, the whole process is carried out in humid air with a relative humidity (RH) of 50% to 60% at room temperature.
2.2 Characterization The absorption spectrum was recorded with a UV-visible spectrophotometer (Agilent 8453). The morphology was measured using a scanning electron microscope
ACCEPTED MANUSCRIPT (SEM) (Hitachi S-4800) and an atomic force microscope (AFM) (Agilent Series 5500). The X-ray diffraction (XRD) patterns were measured using X-ray diffraction system (PANalytical Inc.) with monochromatic Cu Kα irradiation (λ = 1.5418 ). Photovoltaic performances were measured by using a Keithley 2611 source meter
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under simulated sunlight from a Newport solar simulator. The system was calibrated against a certified reference solar cell. Photoluminescence (PL) (excitation at 485 nm) was measured with NaonLog infrared fluorescence spectrometer (Nanolog FL3-2Ihr). Transient PL measurement was measured using UltraFast lifetime Spectrometer
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(Delta flex). All the measurements of the solar cells were performed under ambient
3. Results and discussions
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atmosphere at room temperature without encapsulation.
TiO2/SnOxCly ETLs were obtained by spin-coating SnCl2·2H2O precursor on TiO2 and annealing at 165
. The thickness of TiO2 and SnOxCly are about 10 nm
and 20 nm, respectively, which were measured by step profiler. Then X-ray
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photoelectron spectroscopy (XPS) characterization was carried out to further investigate the chemical composition of this ETL. The XPS spectrum shows the presence of Sn, Ti, O and Cl as shown in Fig. 1a. The Sn 3d peaks are observed (Fig.
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1a), which accords with the previous report [22]. The peaks at 458.6 eV and 464.3 eV correspond to the Ti 2p3/2 and Ti 2p1/2, respectively, in Fig. 1b. Fig. 1c shows the O 1s peaks, which is the O2- state in SnO2 and TiO2. In addition, a relatively small number
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of Cl was observed in our sample (Fig. 1d), the molar ratio of Cl and O is nearly 1:10, indicating the formation of TiO2/SnOxCly ETL. We investigated the morphology of TiO2 and TiO2/SnOxCly films using AFM as
shown in Fig. 2. The root mean square roughness of the TiO2/SnOxCly film is 5.1 nm, while that of TiO2 film is 4.9 nm. This shows that the TiO2/SnOxCly film is nearly as smooth as the TiO2 film, which should not affect the formation of perovskite film. The perovskite films were prepared using a modified method pioneered by Park and co-workers [27]. Fig. 3 presents the scanning electron microscope (SEM) images of perovskite films on TiO2 and TiO2/SnOxCly, which indicates the formation of dense
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perovskite films without visible pinholes. AFM measurement was used to further
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Fig. 1. XPS spectra of Ti 2p(a), Sn 3d(b), O 1s(c), and Cl 2p(d) for TiO2/SnOxCly ETL
Fig. 2.
AFM images of TiO2 (a, b) and TiO2/SnOxCly (c, d) films deposited on FTO substrate
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Fig. 3. SEM images of TiO2/perovskite (a) and TiO2/SnOxCly/perovskite (b)
investigate the morphology of perovskite film (Fig. S1). The root mean square roughness of perovskite film on TiO2/SnOxCly is 4.6 nm in 10×10 µm2 of scanning area. This quite smooth surface is beneficial to form a uniform spiro-OMeTAD HTL, which is crucial to the performance of perovskite solar cell. XRD patterns (Fig. S2)
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conform to the mixture of CH3NH3PbI3 perovskite on ETL. A high quality perovskite film is essential for the performance of planar PSCs, as it can avoid the direct contact of ETL and HTL. The UV-visible absorbance spectra of TiO2/perovskite and
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TiO2/SnOxCly/perovskite were measured as shown in Fig. S3, presenting a strong absorption for visible light tailing to 770 nm. No obvious difference can be observed
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from these two absorbance spectra. Moreover, the whole fabrication can be carried out in air with a relative humidity (RH) of 50% to 60% at room temperature, which is more convenient and inexpensive than other methods. This result indicates that high crystallinity perovskite film can be prepared with a relative high humidity, which agrees with the literature result [28]. In this study, the device structure and the energy level diagram of perovskite solar cell are given in Fig. 4. The energy levels of TiO2 and SnOxCly were obtained by ultraviolet photoelectron spectroscopy (UPS) and UV-visible absorption spectroscopy (Fig. S4) The CBM of SnO2 is 0.24 eV more negative than that of TiO2 and a higher
ACCEPTED MANUSCRIPT mobility of SnO2 indicates the electron injection and transportation from perovskite to
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the CBM of ETL could be more efficient. In addition, SnOxCly functions as a hole
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Fig. 4. (a) Schematic of the device structure. (b) Energy-level diagram of the device components (relative to the vacuum level)
blocking layer (HBL) to suppress the electrons back transfer from TiO2 to perovskite. For comparison, identical perovskite solar cells were fabricated using TiO2 or
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TiO2/SnOxCly as ETL with the same thickness. The photocurrent density-voltage (J–V) characteristics of two kinds of solar cells are presented in Fig. 5 and Table. 1. Obviously, all parameters are improved: The JSC increases from 20.3 to 20.9 mA/cm2,
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the VOC increases from 1.05 to 1.09 V, the FF increases from 0.68 to 0.73, and the PCE increases from 14.6% to 16.6%. These improvements indicate TiO2/SnO1-xClx ETL is beneficial to electron extraction and suppression of recombination. To explore
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the effect of the presence of Cl on the device performance, according literature result, SnO2 can be obtained by annealing SnCl2·2H2O at 180
to the
. We made
perovskite solar cell based on TiO2/SnO2 ETL, and a PCE only 12.6% was obtained (Fig.S5).
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Fig. 5. J-V curves of the perovskite solar cell based on TiO2 and TiO2/SnOxCly ETL.
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Table 1. Comparison of photovoltaic parameters of the perovskite solar cell based on TiO2 and TiO2/SnOxCly ETL
JSC(mA/cm2) 20.3 20.9
VOC (V) 1.05 1.09
FF 0.68 0.73
PCE(%) 14.6 16.6
Rs(Ω ) 147.7 80.4
Rsh(Ω ) 1.18×104 1.68×105
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Sample TiO2 TiO2/SnOxCly
To elucidate the FF improvement of TiO2/SnOxCly ETL based PSCs, the series
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resistance (Rs) and the shunt resistance (Rsh) are calculated from J-V curves. As shown in Table 1, compared with bare TiO2 based device, TiO2/SnOxCly based device shows obviously decrease of Rs from 147.7 Ω to 80.4 Ω, and the Rsh increases by one order of magnitude.
Moreover, the TiO2/SnOxCly based PSC exhibits lower hysteresis than bare TiO2 based device (Fig. S6, Table S1). The PCE of forward scan is 86% of reverse scan for PSC based on TiO2/SnOxCly ETL whereas only 80% for bare TiO2 based device. Hysteresis is possibly caused by ion migration [29], high trap-state density for carriers at the perovskite surface [30], and unbalanced charge transportation [31]. Thus, the
ACCEPTED MANUSCRIPT lower hysteresis result can be attributed to enhanced electron extraction and reduced recombination by using TiO2/SnOxCly ETL, which is further confirmed by the transient photoluminescence (TPL) measurements. To thoroughly investigate the effects of TiO2/SnOxCly ETL on the device
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performance, the recombination process of PSCs was analyzed by steady-state Photoluminescence (PL) and TPL measurements. Fig. 6a shows a steady PL spectra of TiO2/perovskite and TiO2/SnOxCly/perovskite on quartz, respectively. The PL intensity of TiO2/SnOxCly/perovskite was reduced compared with that of
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TiO2/perovskite. This indicates that TiO2/SnOxCly has enhanced electron injection and transportation which reduces Rs and increases FF of device. Moreover, the TPL was
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measured by monitoring the peak emission at 770 nm, as shown in Fig. 6b. Generally, there are two typical recombination processes, one is free carrier recombination in perovskite bulk films in a relatively short time, and the other one comes from the delayed recombination of trapped charges [32]. Therefore, we fitted the data with biexponential decay, yielding two PL lifetime constants (τ1 and τ2), as summarized in
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Table S2. For TiO2/perovskite, τ1 is 49.5 ns with a ratio of 24.39% and τ2 is 110.0 ns with a ratio of 75.61%, yielding an average PL lifetime of 95 ns. However, for TiO2/SnOxCly/perovskite, τ1 is 66.3 ns with a ratio of 74.62% and τ2 is 125.8 ns with a
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ratio of 25.38%, yielding an average PL lifetime of 80 ns. The average PL lifetimes are in accordance with PL quenching. Notably, the part of long PL lifetime (τ2) in TiO2/SnOxCly/perovskite is significantly reduced compared with TiO2/perovskite.
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This suggests reduction of the trap states by modification of SnOxCly, which contributes to higher Voc and improved device performances. In addition, the PL lifetimes (τ1 and τ2) of perovskite film on TiO2 were shorter than perovskite film on TiO2/SnOxCly,
this
indicates
that
charge
has
a
slow
recombination
at
TiO2/SnOxCly/perovskite interface, which could be attributed to a stronger binding at the interface of TiO2/SnOxCly and perovskite due to the presence of Cl.
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Fig. 6. (a) Steady-state photoluminescence (PL) and (b) transient photoluminescence of TiO2/perovskite and TiO2/SnOxCly/perovskite deposited on quartz glass.
ACCEPTED MANUSCRIPT Conclusions In summary, using double layered TiO2/SnOxCly to replace pure TiO2 can improve the JSC, VOC, FF and PCE, and reduce hysteresis behavior due to the enhanced electron transport and reduced recombination between the ETL and the
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photoactive layer. As a result, we achieved 1.09 V of Voc and high PCE of 16.6% in humid air. Our findings suggest that double layered TiO2/SnOxCly can be used as ETL to enhance PCE of planar perovskite solar cell. In addition, interfacial Cl atoms can be introduced by SnOxCly ETL to reduce electron trap-states between ETL and
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perovskite interface.
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Acknowledgements
This work was supported by the National Key R&D Program of China (No. 2016YFB0401003), the National Natural Science Foundation of China (U1605244
61575005, 11574009) and the National Fund for Fostering Talents
of Basic Science (NFFTBS) (J1030310). The authors are thankful to Mrs. Yan
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Guan in the College of Chemistry and Molecular Engineering of Peking
References
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University for her kind help in the transient PL measurements.
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ACCEPTED MANUSCRIPT Highlights 1. TiO2/SnOxCly double layer was firstly employed as the electron transport layer (ETL) for planar perovskite solar cells.
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2. Compared with bare TiO2 based solar cell, TiO2/SnOxCly based devices exhibit reduced hysteresis and remarkable improvement in device efficiency from 14.6% to 16.6%.
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3. TiO2/SnOxCly ETL could enhance electron extraction and reduce surface trap-state.
