Journal Pre-proof Green low-temperature-solution-processed in situ HI modified TiO2/SnO2 bilayer for efficient and stable planar perovskite solar cells build at ambient air conditions Yaxin Deng, Shuxian Li, Xuandong Li, Rui Wang, Xin Li, Y. Deng, S. Li, X. Li, R. Wang, X. Li PII:
S0013-4686(19)31795-5
DOI:
https://doi.org/10.1016/j.electacta.2019.134924
Reference:
EA 134924
To appear in:
Electrochimica Acta
Received Date: 28 June 2019 Revised Date:
18 September 2019
Accepted Date: 20 September 2019
Please cite this article as: Y. Deng, S. Li, X. Li, R. Wang, X. Li, Y. Deng, S. Li, X. Li, R. Wang, X. Li, Green low-temperature-solution-processed in situ HI modified TiO2/SnO2 bilayer for efficient and stable planar perovskite solar cells build at ambient air conditions, Electrochimica Acta (2019), doi: https:// doi.org/10.1016/j.electacta.2019.134924. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Green low-temperature-solution-processed in situ HI modified TiO2/SnO2 bilayer for efficient and stable planar perovskite solar cells build at ambient air conditions Yaxin Deng 1, Shuxian Li 1, Xuandong Li*, Rui Wang and Xin Li* Y. Deng, S. Li, X. Li, R. Wang, Prof. X. Li MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, 150090, P. R. China E-mail:
[email protected] E-mail:
[email protected] *Corresponding authors 1
These authors contributed equally to this work.
Keywords: TiO2/SnO2; electron transfer layer; ambient condition; perovskite solar cell Planar structures for halide perovskite solar cells with the high efficiencies typically use high-temperature processed TiO2 as the electron transporting layers (ETLs). Here, we demonstrate that an in-situ passivation strategy for TiO2 film through the introduction of HI during low-temperature preparation process. HI not only controls hydrolysis of TiO2 precursor but also eliminates defects and suppresses trap states in TiO2 film. Meanwhile, the double-layer architecture is constructed by coating TiO2 with SnO2 layer, the double ETLs can improve the photovoltaic performance of methylamine lead iodide (MAPbI3) perovskite solar cells. The TiO2(HI)/SnO2 ETL can effectively reduce the interfacial charge recombination and facilitate electron transfer. More importantly, the preparation of TiO2 and SnO2 are totally performed at low-temperature (150 °C) and the devices are fabricated in uncontrolled ambient conditions. Our best-performing planar perovskite cell using such a TiO2(HI)/SnO2 ETL has achieved a maximum power conversion efficiency (PCE) of 16.74%, and the devices exhibit good stability which maintaining 85% and 83% of their initial efficiency after heating at 100 °C for 22 h and under illuminating upon 1 sun irradiation for 6 1
h, respectively. Our results suggest a new approach for further improving the stability of PSCs fabricated in the air condition. 1. Introduction In the last decade, organic-inorganic hybrid perovskite solar cells (PSCs) have drawn much attention due to their superior optoelectronic performance, low-cost, and easy preparation [1-3]. To achieve efficient and stable PSCs, the exploration of charger transport materials is very important [4]. Compared with organic semiconductor materials, inorganic metal oxides show better stability and higher mobility [5]. Various metal oxides, such as Cu2O, CuGaO2, MoO3, and NiOx have been used as hole-transport layers in PSCs. Electrontransport materials in PSCs commonly include ZnO, SnO2, and TiO2 [6-10]. Particularly, the low-temperature processed TiO2 has attracted significant interest due to its compatibility with flexible and large-area PSCs [11-13]. However, it still needs to be continuously optimized to satisfy the fast growing requirement of efficient and stable PSCs. Oxygen vacancies prevalent in pristine TiO2 film can trap injected electrons and lead to carrier recombination [14, 15]. Furthermore, oxygen-related defects at the surface of TiO2 may induce perovskite decomposition and degradation of photovoltaic performance [16, 17]. Notably, the lowtemperature TiO2 as ETL for PSCs has to carry out the post-processing in previous reports, in which organic solvents, such as 2-methoxy ethanol, n-butanol, methanol, chloroform, oleic acid, n-hexane or benzyl alcohol, have been used to precipitate and redissolve as-prepared product for the final high-crystallized TiO2, leading to time-consuming and environmental safety risk [18-20]. Additionally, the lower electron mobility and extraction ability of TiO2 can result in insufficient charge separation and charge accumulation at the ETL/perovskite layer interface [21-23]. In terms of this, to implement superior performance of PSCs requires the collaboration of multiple optimization methods. The cooperation of surface passivation on TiO2 and the application of TiO2/SnO2 double layer ETL has been proved to be an effective strategy to 2
address these issues [24-28]. Generally, SnO2 has higher electron mobility, wider bandgap, less photocatalytic activity and better chemical stability than TiO2 or other traditional ETL materials, which makes it a good candidate for high-performance PSCs [21, 29-32]. Besides electron extraction, ETL is also a hole blocking layer which prevents holes transfer from perovskite layer to the transparent electrode [33]. TiO2/SnO2 double ETL can effectively reduce the leakage current and suppress the charge recombination [26, 34]. Despite these progresses, the few studies have been done on the TiO2/SnO2 double ETL for PSCs fabricated at ambient conditions. Although the surface passivation by various post-treatments has been widely used to reduce surface defects, it is different to eliminate the bulk defects, resulting in deterioration of electronic properties [35]. Moreover, the use of post-treatment would lead to the complex procedure and increased cost [36]. Thus, it is imperative to seek new strategies to modify ETL in the solution preparation process for low-temperature TiO2. For the further practical applications of PSCs, low manufacturing costs and simple production process are mandatory, in this contexts, the air-condition fabrication technology for PSCs emerged [37-43]. Our previous works also have shown that efficient and stable PSCs can be obtained at room temperature without well-controlled atmosphere [44-49]. To be competitive with the performance of PSCs made in the glove-box, however, some shortcomings, such as severe hysteresis and unsatisfactory reproductive of devices, should be overcome. In this study, we report an environmentally friendly, facile yet efficient method to passivate defects in low-temperature-processed TiO2 by introducing HI during the preparation process. As indicated by Snaith and co-workers, device performance deterioration during operation is mainly due to oxygen-induced defects in TiO2 [50, 51]. In light of this, we chose HI not only to control hydrolysis of TiO2 precursor but also to eliminate surface and bulk defects in TiO2 film. Interestingly, TiO2 can be obtained by an one-step low-temperature process, without using any organic solvent for post-treatment. Meanwhile, we constructed 3
double ETL by coating a low-temperature SnO2 upon TiO2 film, which could increase the compactness and electronic properties of ETL. PSC based on TiO2/SnO2 was used as comparison, the two PSCs were denoted as TiO2/SnO2-PSCs and TiO2(HI)/SnO2-PSCs respectively. Importantly, all fabricating processes were carried out under ambient air conditions. As anticipated, the cooperation of TiO2(HI) and SnO2 film can effectively enhance photovoltaic effect and stability of PSCs. The champion PCE is 16.74% with improving hysteresis, which is improved by 10.57% of the control device. Simultaneously, the unencapsulated TiO2(HI)/SnO2-PSCs compared to the TiO2/SnO2-PSCs demonstrate considerable ambient stability maintaining 91% of its initial efficiency after storage in the air for 1400 h at room temperature. The device also exhibited excellent thermal and illumination stability, maintaining 85% of their initial efficiency after heating at 100 °C for 22 h, and maintaining 83% after illuminating upon 1 sun irradiation for 6 h. 2. Experimental Section 2.1. Materials Titanium (IV) isopropoxide (TTIP) and hydriodic acid (HI) were purchased from Aladdin, SnO2 colloidal dispersion (tin (IV) oxide, 15% in H2O colloidal dispersion) was purchased from Alfa Aesar, chlorobenzene and N,N-dimethyl-Formamide (DMF) were purchased from Youxuan Trade, and dimethyl sulfoxide (DMSO) was purchased from Bailinwei Trade, other anhydrous solvents were obtained from Alfa Aesar. Perovskite and HTL materials were obtained from Xi´an Polymer Light Technology Corp. All chemicals and reagents were used as received without any further purification. 2.2. Preparation of TiO2 and SnO2 precursor TiO2 precursor solution was prepared by a simple method reported by Christophe J. Barbe et al [18]. TTIP (1.0 mL) was slowly added into HNO3 (6.0 mL, 0.1 mM) and HI (6.0 mL, 0.1 mM) respectively, heating and stirring at 80 °C for 8 h. The solution was further filtered through a 0.45 mm PVDF syringe filter after falling to room temperature. SnO2 4
precursor was prepared by diluting the SnO2 colloidal with deionized water (volume ratio 1:5) and stirring for 12 h, followed by filtering through a 0.45 mm PVDF syringe filter. 2.3. Device fabrication Fluorine-doped tin oxide (FTO) glasses were ultrasonically cleaned in detergent solution, deionized water, ethyl alcohol and acetone for 30 min respectively, then the glasses were transferred to the ultraviolet-ozone (UV-O3) cleaner for 15 min. For double ETL, TiO2 film was prepared by spin-coating TiO2 or TiO2(HI) precursor solution at 3000 rpm for 30 s, followed by annealing at 150 °C for 1 h in air. SnO2 film was prepared by spin-coating SnO2 precursor solution on the TiO2 film at 3000 rpm for 30 s, and then baked at 150 °C for 2 h. For the deposition of Perovskite film, a normal one-step spin-coating method similar to a previous report was used [52]. CH3NH3I (1.0 M) and PbI2 (1.0 M) were dissolved in a co-solvent of DMF/DMSO (volume ratio 4:1) at room temperature and stirred for 12 h to form a perovskite precursor solution. This precursor solution was spin-coated on ETL at 3000 rpm for 50 s and ethyl acetate was slowly dripped on the substrate 12 s after the beginning of spin-coating, then the film was annealed at 60 °C for 5 min and 100 °C for 10 min respectively. Spiro-OMeTAD was deposited after ETL cooling down, solution containing Spiro-OMeTAD (76.0 mg), 4-tertbutypyridine (28.5 µL), Li-TFSI (17.5 µL, 520 mg mL-1) and chlorobenzene (1.0 mL), deposited by spin-coating at 3000 rpm for 30 s. Finally, about 60 nm thick Au counter electrode was deposited via vacuum thermal evaporation at rate of 1.0 Å s-1. The active area of PSCs was confirmed to be 0.09 cm2 by a non-reflective metal mask. All processes including fabrication, measurement and storage of devices with the structure of FTO/TiO2/SnO2/MAPbI3/Spiro-OMeTAD/Au were carried out under ambient conditions. 2.4. Measurements and characterizations For analyzing crystal structure, X-ray diffraction (XRD) pattern was recorded on a Panalytical Empyrean X-ray diffractometer at a scan rate of 10 ° min-1 with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS, Perkin-Elmer, PHI 5400 ESCA 5
system) analysis was used to calibrate binding energy value. For the observation of morphologies of nanoparticles and films, New Generation Cold Field Emission scanning electron microscopy (SEM, Hitachi, SU8000) and atomic force microscope (AFM, Bruber, Dimension Icon) have been used. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were conducted on V2.7 fluorescence spectrometer from HORIBA. UV-visible (UV-vis) absorption spectroscopy surveys were performed on TU1901 spectrometer (Beijing Purkinje General Instrument Co., Ltd). The ultraviolet photoelectron spectroscopy (UPS, AXIS NOVA, Kratos Analytical Ltd, UK) using He I (21.2 eV) as the photon source was employed to measure the work function. Photocurrent densityphotovoltage (J-V) curves and open-circuit photovoltage decay (OCVD) of PSCs were measured with an electrochemical workstation (VersaSTAT 3, Ametek, USA) and a class ABB solar simulator (model 94021A, Newport, USA) under AM 1.5G sunlight (100 mW cm2
) illuminated with a sweep rate of 0.2 V s-1. The incident photon-to-electron conversion
efciency (IPCE) was measured using the Crowntech solar cell quantum efciency measurement system (QTest Station 500AD, USA). Electrochemical impedance spectra (EIS) were measured on VersaSTAT 3 electrochemical workstation (Ametek, USA) with a frequency ranging from 0.1 to 106 Hz. 3. Results and Discussions 3.1. Characterization of ETL For an ideal ETL, some basic criteria, such as high optical transmittance, high electron mobility, high electron extraction, and high chemical stability should be meeting [29, 33]. As shown in Fig. 1(a), the TiO2(HI) film displays much higher transmittance in the visible spectrum range compared to the control sample. The high transmittance implies that the TiO2(HI) film would ensure more light to be absorbed by the perovskite active layer and to minimize the light loss, which is helpful for higher Jsc. In addition, the surface morphology of the as-prepared TiO2 films was investigated by AFM, as shown in Fig. 1(c), (d). Compared 6
with TiO2 film, TiO2(HI) film exhibits smoother morphology, with room-mean-square (RMS) surface roughness as low as 8.43 nm, resulting in high light transmission. The anatase structure of obtained TiO2 nanoparticles was confirmed by XRD (Fig. 1(b)). All positions of diffraction peaks are in good with the standard anatase phase (PDF#21-1272), indicating pure phase and nanocrystalline nature. Generally, the oxygen-induced defects in TiO2 films play a vital role in the deterioration of electronic properties. To assess this change in as-prepared TiO2 films, XPS that is an effective surface technique for determining the elemental composition distribution and chemical state of samples was performed. Fig. 2(a), (b) show Ti2p and O1s (284.6 eV binding energy for C1s is the internal reference) peaks of TiO2 film and TiO2(HI) film. The Ti2p XPS spectra consist of Ti2p3/2 at the binding energy of 458.4 eV and Ti2p1/2 at the binding energy of 464.1 eV, which is generated by spin-orbit splitting and accords with titanium in the IV oxidation state, indicating that Ti4+ exists in TiO2 lattice [53]. The O1s XPS spectra are composed of three kinds of O with binding energy of 529.7, 530.7 and 532.7 eV (Gaussian distributions), corresponding to O–Ti–O bond (denoted as OTi–O), oxygen vacancy (denoted as OV) and hydroxyl oxygen (denoted as OO–H) respectively [14, 48]. The relative amount of each state of O in the entire O1s can be calculated by the ratio of the peak area of the different O1s (OTi–O, OV, OO–H) to the overall O1s, the detail values are summarized in Table 1. Particularly, the XPS peak area ratio of OV stands for defects in TiO2 induced by oxygen vacancy, associated with carrier transport and recombination [54]. The OV ratio of TiO2(HI) decreases from 37.03% to 29.46%, indicating that the introduction of iodine into TiO2 reduces the density of oxygen-related defects, thereby enhancing the quality of TiO2(HI) film. The electronic properties of TiO2 ETL was further characterized by UPS. The work functions, determined from the secondary electron cutoff band, decreased from 3.77 eV for TiO2 to 3.62 eV for TiO2(HI) (Fig. S2). Such work function shift can be related to a higher built-in
7
potential, which effectively promotes charge transfer and decrease the carrier recombination, and thus increase open-circuit voltage [16, 55, 56]. To scrutinize the charge transfer dynamics, the trap density and conductivity needs to be explored. Trap density (nt) was first measured using space-charge-limited current (SCLC) technique based on electron-only device structure of FTO/TiO2/Au in the dark. The nt value is calculated according to the relationship of VTFL = entL2/2εε0 [57], in which VTFL stands for the onset voltage of trap filled limit, e is electric charge, L is the thickness of TiO2, ε0 is the vacuum dielectric permittivity and ε is dielectric constant of TiO2. As illustrated in Fig.. 2(c), the VTFL of TiO2(HI) reduces to 0.204 V relative to the 0.374 V of the TiO2, and nt decreases accordingly from 7.82×1015 cm-3 to 4.42×1015 cm-3. The lower trap density in TiO2(HI) film is related to passivation effect of HI on the optoelectronic properties of ETL. The conductivity of TiO2/SnO2 and TiO2(HI)/SnO2 films was then examined by the structure of FTO/TiO2/SnO2/Au. As shown in Fig. 2(d), TiO2(HI)/SnO2 has higher conductivity than TiO2/SnO2. The high conductivity might be affected by the decrease of trap state density. The improved optoelectronic properties of ETL result in faster and balanced charge transportation, and suppress J-V hysteresis. 3.2. Characterization of Perovskite Film and PSCs Fig. 3(a) shows a schematic of FTO/ETL/Perovskite/spiro-OMeTAD/Au structure used to evaluate our present research, where the ETL is manufactured using TiO2 and SnO2. Fig. 3(b) is the cross-sectional of SEM image of device based on TiO2(HI)/SnO2. The top view of SEM images of perovskite films deposited on TiO2/SnO2 and TiO2(HI)/SnO2 are shown in Fig. 3(c), (d) and the insert images are the contact angles of two double ETLs. The contact angle of TiO2(HI)/SnO2 is 29.68 ° , which is much smaller than 48.01 ° of TiO2/SnO2. Furthermore, AFM images of TiO2/SnO2 and TiO2(HI)/SnO2 of Fig. S1 show that TiO2(HI)/SnO2 has a slightly small roughness (6.61 nm) than TiO2/SnO2 (9.83 nm). The 8
smaller contact angle and roughness of TiO2(HI)/SnO2 facilitate the spreading of the perovskite precursor solution, which promotes the formation of a more dense and flat film. Therefore, although both perovskite films have large scale crystalline grains as shown in Fig. 3(c) and (d), there are exiguous gaps on the boundary of perovskite crystal film upon TiO2/SnO2. On the contrary, perovskite film on TiO2(HI)/SnO2 is compact without gaps. The results of AFM show that TiO2(HI)/SnO2 based perovskite films have smaller roughness (5.78 nm) compared to roughness (8.13 nm) of TiO2/SnO2, as exhibited in Fig. 3(e), (f). The results of the above-mentioned tests show that smaller particle size and lower OV density of TiO2(HI) will facilitate a uniform SnO2 film preparation, further promote the formation of uniform, few-pinholes and flat perovskite films. Subsequently, the charge extraction and recombination process were discussed by recording EIS under dark as shown in Fig. 4(a). The relative parameters (Table S1) can be obtained via the equivalent circuit (inset) revised with the ZView2 software. TiO2(HI)/SnO2 based device exhibits a significant reduction in charge transfer resistance (Rct) from 250.10 to 167.51
, which facilitates charge extraction from MAPbI3 to ETL. The reduction in Rct
can be understood as improving the charge carrier transfer capacity of the ETL film. In order to comprehend the behavior of photo-genetic charge carrier transfers at ETL/MAPbI3 interface, steady-state and time-resolved PL of the MAPbI3 film on varies ETLs were measured. Fig. 4(b) shows the steady–state PL spectroscopy, TiO2(HI)/SnO2/MAPbI3 quenching is stronger than TiO2/SnO2/MAPbI3 because TiO2(HI)/SnO2 can prompt the electron
transfer.
Fig.
4(c),
(d)
show
TRPL
of
TiO2/SnO2/MAPbI3
and
TiO2(HI)/SnO2/MAPbI3 films. The data is fitted with biexponential decay yields two constants of time by a previous report [58], i.e., a fast decay lifetime τ1 and a slow decay lifetime τ2, as summarized in Table S2. TiO2(HI)/SnO2/MAPbI3 has lower τ1 and τ2, and the ratio of τ1 is far outweighing than the ratio of τ2. It suggests that a faster electron transfer 9
occurs at the interface of TiO2(HI)/SnO2/MAPbI3, and it can be inferred that TiO2(HI) has a tiny trap state density [59]. These results coincide with the enhanced Jsc and FF of TiO2(HI)/SnO2-PSCs. In addition, the charge recombination process was further investigated by the OCVD test, which is a direct observation method monitoring the decay of applied voltage over time [60, 61]. As shown in Fig. S3, the Voc decay of the TiO2(HI)/SnO2-based PSC is obviously mitigated than that of the control PSC, indicating the reduced trap-assisted recombination in TiO2(HI)/SnO2-based device [62], which is the result of the passivation effect of HI, in accordance with the earlier PL and TRPL results showing the lower trap density. To study the reproducibility of PSCs, 30 individual devices for TiO2/SnO2 and TiO2(HI)/SnO2 ETL were fabricated by the same fabrication procedure. The PCE distribution histogram with Gaussian fitting and detailed parameters are shown in Fig. S4. High average PCE of 16.42 with low standard deviation of 0.20 for TiO2(HI)/SnO2-based PSCs, demonstrated good reproducibility. Fig. 5(a) and (b) present the J−V curves for the optimized device. The TiO2/SnO2-PSC yielded a PCE of 15.14% with a Jsc of 20.32 mA cm-2, a Voc of 1.04 V, and a FF of 71.37%. Meanwhile, the best TiO2(HI)/SnO2-PSC shows an outstanding enhanced performance with a Jsc of 21.08 mA cm−2, a Voc of 1.06 V, and a FF of 75.10%, and the corresponding PCE of 16.74%. Fig. 5(c) reveals the photocurrent under a constant bias voltage of -0.81 V for TiO2/SnO2-PSC and -0.86 V for TiO2(HI)/SnO2-PSC, which corresponds to the voltage at maximum power points of its J−V curves. The champion device of TiO2(HI)/SnO2-PSC achieves steady-state photocurrent of 19.11 mA cm-2 upon 300 s of continuous illumination. The corresponding steady-state PCE is 16.50%, which is more stable than TiO2/SnO2-PSC. The incident photon-to-electron conversion efficiency (IPCE) spectra of devices are shown in Fig. 5(d). The champion TiO2(HI)/SnO2-PSC has slightly larger IPCE values than TiO2/SnO2-PSC in the absorbed wavelength range of 400-750 nm. The integrated
10
Jsc values of TiO2/SnO2-PSC and TiO2(HI)/SnO2-PSC according to the IPCE spectra are 19.40 mA cm-2 and 20.00 mA cm-2, which are closed to the results of J−V curves. 3.3. Stability of PSCs The stability of PSCs is a key issue for commercialization. In general, stability tests done under an inert gas protected condition have been adopted by the majority of reported works. In order to simulate the actual working environment, we conducted the stability tests of unencapsulated devices in ambient atmosphere. Fig. 6(a) shows the long-term ambient stability measurements. The devices were kept in air with room temperature and relative humidity of 35%. The detail performance parameters are shown in Fig. S5 and Table S3, S4. The TiO2(HI)/SnO2-PSCs still retain 91% of their initial efficiency after storage even for 1400 h in ambient air, whereas the TiO2/SnO2-PSCs can merely maintain approximately 80% of their initial efficiency. For the thermal stability test, the PSCs were heated continuously at 100 °C in ambient. Fig. 6(b) shows the change of PCE, the detail performance parameters are shown in Fig. S6 and Table S5, S6. The PCE of TiO2(HI)/SnO2-PSCs retains 85% of the initial performance after heating up to 22 h, while the PCE of TiO2/SnO2-PSCs is only 39% retention of initial performance. Correspondingly, it can be seen from Fig. 6(d) that the perovskite film on TiO2/SnO2 is almost entirely degraded, whereas the perovskite film on TiO2(HI)/SnO2 is just beginning to degenerate at the edge after annealing for 22 h. To further explore the operational stability, the test of constant light-soaking of 1 sun irradiation under ambient was conducted as shown in Fig. 6(c). The detail performance parameters are shown in Fig. S7 and Table S7, S8. Similar to the above consequences, TiO2(HI)/SnO2-PSCs lost only 17% of their initial PCE after 6 h, conversely, the lost value of TiO2/SnO2-PSCs are 43%. The significantly improved air stability of TiO2(HI)/SnO2-PSCs is attributed to the high quality of perovskite films. Fewer pinholes and grain boundaries would prevent the attack of water and thus protected the perovskite films. In addition, efficient reduction of oxygen vacancies and trap state density in TiO2 film due to HI passivation, and the shielding effect of 11
SnO2 to inhibit photocatalytic activity of TiO2 are the main reasons for the better photostability. Furthermore, the TiO2(HI)/SnO2-PSCs have balanced charge transport and are more resistant to ion migration, which results in structural stability and can prevent the thermal degradation of perovskite film. Undoubtedly, the synergistic effect that HI deactivates the TiO2 film and introduces SnO2 to produce a double layer of ETL will result in a high overall stability device. 4. Conclusion We have shown that a facile and effective bilayer TiO2(HI)/SnO2 as ETL can improve the PCE and stability of the MAPbI perovskite solar cells. The best performing device built upon a TiO2(HI)/SnO2 ETL achieved a PCE of 16.74% with small hysteresis and better stability. The results show that TiO2(HI)/SnO2-based PSCs demonstrated considerable stability maintaining over 85% of the initial efficiency after heating at 100 °C for 22 h, and 83% under continuous illumination for 6 h. The enhancement of PCE and stability is mainly attributed to the TiO2(HI)/SnO2 interface layer effectively reducing the interfacial recombination while simultaneously to inactivate the oxygen vacancy and alleviate the trap states density confirmed by both PL and EIS measurements. Our results demonstrate that optimization of the electron transport layer, such as having a bilayer structure, will be essential in pushing the efficiency and stability of perovskite solar cells higher.
12
Acknowledgements 1
These authors contributed equally to this work. We are grateful for the financial support
of this research from the National Natural Science Foundation of China (51779065, 51579057) and State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2019DX11).
Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))
13
References
[1]
S.I. Seok, M. Grätzel, N.G. Park, Methodologies toward highly efficient perovskite solar cells, Small 14 (2018) 1704177.
[2]
J. Burschka, N. Pellet, S. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Grätzel, Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (2013) 316.
[3]
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.
[4]
S.S. Shin, S.J. Lee, S.I. Seok, Metal Oxide Charge Transport layers for efficient and stable perovskite solar cells, Adv. Funct. Mater. (2019) 1900455.
[5]
X. Ren, Z.S. Wang, W.C.H. Choy, Device physics of the carrier transporting layer in planar perovskite solar cells, Adv. Opt. Mater. (2019) 1900407.
[6]
Y. Chen, Z. Yang, S. Wang, X. Zheng, Y. Wu, N. Yuan, W. Zhang, S.F. Liu, Design of an inorganic mesoporous hole-transporting layer for highly efficient and stable inverted perovskite solar cells, Adv. Mater. 30 (2018) 1805660.
[7]
S.S. Mali, H. Kim, H.H. Kim, S.E. Shim, C.K. Hong, Nanoporous p-type NiOx electrode for p-i-n inverted perovskite solar cell toward air stability, Mater. Today 21 (2018) 483-500.
[8]
C. Zuo, L. Ding, Solution-processed Cu2O and CuO as hole transport materials for efficient perovskite solar cells, Small 11 (2015) 5528-5532.
[9]
W. Yu, F. Li, H. Wang, E. Alarousu, Y. Chen, B. Lin, L. Wang, M.N. Hedhili, Y. Li, K. Wu, X. Wang, O.F. Mohammed, T. Wu, Ultrathin Cu2O as an efficient inorganic hole transporting material for perovskite solar cells, Nanoscale 8 (2016) 6173-6179.
[10]
K. Yang, J. Fu, L. Hu, Z. Xiong, M. Li, X. Wei, Z. Xiao, S. Lu, K. Sun, Impact of ZnO photoluminescence on organic photovoltaic performance, Acs Appl. Mater. Interfaces 10 (2018) 3996239969.
[11]
G.E. Eperon, V.M. Burlakov, P. Docampo, A. Goriely, H.J. Snaith, Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells, Adv. Funct. Mater. 24 (2014) 151-157.
[12]
J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T. Song, C. Chen, S. Lu, Y. Liu, H. Zhou, Lowtemperature solution-processed perovskite solar cells with high efficiency and flexibility, Acs Nano 8 (2014) 1674-1680.
14
[13]
F. Wang, Y. Cao, C. Chen, Q. Chen, X. Wu, X. Li, T. Qin, W. Huang, Materials toward the upscaling of perovskite solar cells: progress, challenges, and strategies, Adv. Funct. Mater. 28 (2018) 1803753.
[14]
K. Wang, W. Zhao, J. Liu, J. Niu, Y. Liu, X. Ren, J. Feng, Z. Liu, J. Sun, D. Wang, CO2 plasma-treated TiO2 film as an effective electron transport layer for high-performance planar perovskite solar cells, Acs Appl. Mater. Interfaces 9 (2017) 33989-33996.
[15]
T. Leijtens, G.E. Eperon, S. Pathak, A. Abate, M.M. Lee, H.J. Snaith, Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells, Nat. Commun. 4 (2013) 2885.
[16]
W. Li, W. Zhang, S. Van Reenen, R.J. Sutton, J. Fan, A.A. Haghighirad, M.B. Johnston, L. Wang, H.J. Snaith, Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification, Energy Environ. Sci. 9 (2016) 490-498.
[17]
N. Aristidou, I. Sanchez Molina, T. Chotchuangchutchaval, M. Brown, L. Martinez, T. Rath, S.A. Haque, The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactive layers, Angew. Chem. 54 (2015) 8208-8212.
[18]
C.J. Barbé, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Grätzel, Nanocrystalline titanium oxide electrodes for photovoltaic applications, J. Am. Ceram. Soc. 80 (1997) 1151-2916.
[19]
Y. Zhang, B. Li, L. Zhang, L. Yin, Efficient electron transfer layer based on Al2O3 passivated TiO2 nanorod arrays for high performance evaporation-route deposited FAPbI3 perovskite solar cells, Sol. Energy. Mat. Sol. C. 170 (2017) 187-196.
[20]
Y. Zhou, B. Wu, G. Lin, Y. Li, D. Chen, P. Zhang, M. Yu, B. Zhang, D. Yun, Enhancing performance and uniformity of perovskite solar cells via a solution-processed C70 interlayer for interface engineering, Acs Appl. Mater. Interfaces 9 (2017) 33810-33818.
[21]
Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cells, Nat. Energy 2 (2017) 16177.
[22]
Y. Hou, X. Chen, S. Yang, C. Li, H. Zhao, H.G. Yang, A band-edge potential gradient heterostructure to enhance electron extraction efficiency of the electron transport layer in high-performance perovskite solar cells, Adv. Funct. Mater. 27 (2017) 1700878.
[23]
W. Ke, G. Fang, J. Wang, P. Qin, H. Tao, H. Lei, Q. Liu, X. Dai, X. Zhao, Perovskite solar cell with an efficient TiO2 compact film, Acs Appl. Mater. Interfaces 6 (2014) 15959-15965.
[24]
Z. Zhou, S. Pang, Z. Liu, H. Xu, G. Cui, Interface engineering for high-performance perovskite hybrid
15
solar cells, J. Mater. Chem. A 3 (2015) 19205-19217. [25]
Y. Li, L. Ji, R. Liu, C. Zhang, C.H. Mak, X. Zou, H. Shen, S. Leu, H. Hsu, A review on morphology engineering for highly efficient and stable hybrid perovskite solar cells, J. Mater. Chem. A 6 (2018) 12842-12875.
[26]
Z. Liu, B. Sun, X. Liu, J. Han, H. Ye, Y. Tu, C. Chen, T. Shi, Z. Tang, G. Liao, 15% efficient carbon based planar-heterojunction perovskite solar cells using a TiO2/SnO2 bilayer as the electron transport layer, J. Mater. Chem. A 6 (2018) 7409-7419.
[27]
M.M. Tavakoli, P. Yadav, R. Tavakoli, J. Kong, Surface engineering of TiO2 ETL for highly efficient and hysteresis-less planar perovskite solar cell (21.4%) with enhanced open-circuit voltage and stability, Adv. Energy Mater. 8 (2018) 1800794.
[28]
M.M. Tavakoli, F. Giordano, S.M. Zakeeruddin, M. Grätzel, Mesoscopic oxide double layer as electron specific contact for highly efficient and UV stable perovskite photovoltaics, Nano Lett 18 (2018) 24282434.
[29]
Q. Jiang, X. Zhang, J. You, SnO2: a wonderful electron transport layer for perovskite solar cells, Small 14 (2018) 1801154.
[30]
L. Xiong, Y. Guo, J. Wen, H. Liu, G. Yang, P. Qin, G. Fang, Review on the application of SnO2 in perovskite solar cells, Adv. Funct. Mater. 28 (2018) 1802757.
[31]
M. Zhu, W. Liu, W. Ke, L. Xie, P. Dong, F. Hao, Graphene-modified tin dioxide for efficient planar perovskite solar cells with enhanced electron extraction and reduced hysteresis, Acs Appl. Mater. Interfaces 11 (2018) 666-673.
[32]
W.Q. Wu, D. Chen, Y.B. Cheng, R.A. Caruso, Thin films of tin oxide nanosheets used as the electron transporting layer for improved performance and ambient stability of perovskite photovoltaics, Solar RRL 1 (2017) 1700117.
[33]
P. Zhang, J. Wu, T. Zhang, Y. Wang, D. Liu, H. Chen, L. Ji, C. Liu, W. Ahmad, Z.D. Chen, Perovskite solar cells with ZnO electron-transporting materials, Adv. Mater. 30 (2018) 1703737.
[34]
D. Liu, Y. Wang, H. Xu, H. Zheng, T. Zhang, P. Zhang, F. Wang, J. Wu, Z. Wang, Z. Chen, SnO2based perovskite solar cells: configuration design and performance improvement, Solar RRL 3 (2019) 1800292.
[35]
Y. Zhao, Q. Li, W. Zhou, Y. Hou, Y. Zhao, R. Fu, D. Yu, X. Liu, Q. Zhao, Double-side-passivated perovskite solar cells with ultra-low potential loss, Solar RRL 1800296.
16
[36]
W. Hu, W. Zhou, X. Lei, P. Zhou, M. Zhang, T. Chen, H. Zeng, J. Zhu, S. Dai, S. Yang, Lowtemperature in situ amino functionalization of TiO2 nanoparticles sharpens electron management achieving over 21% efficient planar perovskite solar cells, Adv. Mater. (2019) 1806095.
[37]
H. Ko, J. Lee, N. Park, 15.76% efficiency perovskite solar cells prepared under high relative humidity: importance of PbI 2 morphology in two-step deposition of CH3NH3PbI3, J. Mater. Chem. A 3 (2015) 8808-8815.
[38]
Y. Jin, G. Chumanov, Solution-processed planar perovskite solar cell without a hole transport layer, Acs Appl. Mater. Interfaces 7 (2015) 12015-12021.
[39]
Q. Tai, P. You, H. Sang, Z. Liu, C. Hu, H.L.W. Chan, F. Yan, Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity, Nat. Commun. 7 (2016) 11105.
[40]
J. Troughton, K. Hooper, T.M. Watson, Humidity resistant fabrication of CH3NH3PbI3 perovskite solar cells and modules, Nano Energy 39 (2017) 60-68.
[41]
Y. Cheng, X. Xu, Y. Xie, H.W. Li, Q. Jian, C. Ma, C.S. Lee, F. So, S.W. Tsang, 18% high-efficiency air-processed perovskite solar cells made in a humid atmosphere of 70% RH, Solar RRL 1 (2017) 1700097.
[42]
Y. Wang, T. Mahmoudi, W.Y. Rho, H.Y. Yang, S. Seo, K.S. Bhat, R. Ahmad, Y.B. Hahn, Ambient-airsolution-processed efficient and highly stable perovskite solar cells based on CH3NH3PbI3-xClx-NiO composite with Al2O3/NiO interfacial engineering, Nano Energy 40 (2017) 408-417.
[43]
T. Singh, T. Miyasaka, Stabilizing the efficiency beyond 20% with a mixed cation perovskite solar cell fabricated in ambient air under controlled humidity, Adv. Energy Mater. 8 (2018) 1700677.
[44]
W. Zhang, Y. Li, X. Liu, D. Tang, X. Li, X. Yuan, Ethyl acetate green antisolvent process for highperformance planar low-temperature SnO2-based perovskite solar cells made in ambient air, Chem. Eng. J. 379 (2020) 122298.
[45]
Y. Guo, L. Kang, M. Zhu, Y. Zhang, X. Li, P. Xu, A strategy toward air-stable and high-performance ZnO-based perovskite solar cells fabricated under ambient conditions, Chem. Eng. J. 336 (2018) 732740.
[46]
Y. Guo, X. He, X. Liu, X. Li, L. Kang, One-step implementation of plasmon enhancement and solvent annealing effects for air-processed high-efficiency perovskite solar cells, J. Mater. Chem. A 6 (2018) 24036-24044.
[47]
W. Zhang, Z. Ren, Y. Guo, X. He, X. Li, Improved the long-term air stability of ZnO-based perovskite solar cells prepared under ambient conditions via surface modification of the electron transport layer
17
using an ionic liquid, Electrochim. Acta 268 (2018) 539-545. [48]
Z. Ren, J. Wu, N. Wang, X. Li, An Er-doped TiO2 phase junction as an electron transport layer for efficient perovskite solar cells fabricated in air, J. Mater. Chem. A 6 (2018) 15348-15358.
[49]
Z. Ren, M. Zhu, X. Li, C. Dong, An isopropanol-assisted fabrication strategy of pinhole-free perovskite films in air for efficient and stable planar perovskite solar cells, J. Power Sources 363 (2017) 317-326.
[50]
S.K. Pathak, A. Abate, T. Leijtens, D.J. Hollman, J. Teuscher, L. Pazos, P. Docampo, U. Steiner, H.J. Snaith, Towards long-term photostability of solid-state dye sensitized solar cells, Adv. Energy Mater. 4 (2014) 1301667.
[51]
S.K. Pathak, A. Abate, P. Ruckdeschel, B. Roose, K.C. Gödel, Y. Vaynzof, A. Santhala, S.I. Watanabe, D.J. Hollman, N. Noel, Performance and stability enhancement of dye-sensitized and perovskite solar cells by Al doping of TiO2, Adv. Funct. Mater. 24 (2014) 6046-6055.
[52]
T. Bu, L. Wu, X. Liu, X. Yang, P. Zhou, X. Yu, T. Qin, J. Shi, S. Wang, S. Li, Z. Ku, Y. Peng, F. Huang, Q. Meng, Y. Cheng, J. Zhong, Synergic interface optimization with green solvent engineering in mixed perovskite solar cells, Adv. Energy Mater. 7 (2017) 1700576.
[53]
R. Sanjinés, H. Tang, H. Berger, F. Gozzo, G. Margaritondo, F. Lévy, Electronic structure of anatase TiO2 oxide, J Appl Phys 75 (1994) 2945-2951.
[54]
N. Wanyi, T. Hsinhan, A. Reza, B. Jean-Christophe, A.J. Neukirch, G. Gautam, J.J. Crochet, C. Manish, T. Sergei, M.A. Alam, Solar cells. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains, Science 347 (2015) 522-525.
[55]
Y. Zheng, X. Li, E. Zhao, X. Lv, F. Meng, C. Peng, X. Lai, M. Huang, G. Cao, X. Tao, Hexamethylenetetramine-mediated growth of grain-boundary-passivation CH3NH3PbI3 for highly reproducible and stable perovskite solar cells, J. Power Sources 377 (2018) 103-109.
[56]
X. Gong, Q. Sun, S. Liu, P. Liao, Y. Shen, C. Grätzel, S.M. Zakeeruddin, M. Grätzel, M. Wang, Highly efficient perovskite solar cells with gradient bilayer electron transport materials, Nano Lett 18 (2018) 3969-3977.
[57]
D. Qingfeng, F. Yanjun, S. Yuchuan, M. Padhraic, Q. Jie, C. Lei, H. Jinsong, Solar cells. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967970.
[58]
N. Pellet, P. Gao, G. Gregori, T. Yang, M.K. Nazeeruddin, J. Maier, M. Grätzel, Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting, Angew. Chem. 53 (2014) 3151-3157.
18
[59]
Y. Li, Y. Zhao, Q. Chen, Y.M. Yang, Y. Liu, Z. Hong, Z. Liu, Y. Hsieh, L. Meng, Y. Li, Y. Yang, Multifunctional fullerene derivative for interface engineering in perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 15540-15547.
[60]
F. Cai, Y. Yan, J. Yao, P. Wang, H. Wang, R.S. Gurney, D. Liu, T. Wang, Ionic additive engineering toward high-efficiency perovskite solar cells with reduced grain boundaries and trap density, Adv. Funct. Mater. 28 (2018) 1801985.
[61]
Y. Zou, H.Y. Wang, Y. Qin, C. Mu, Q. Li, D. Xu, J.P. Zhang, Reduced defects of MAPbI3 thin films treated by FAI for high-performance planar perovskite solar cells, Adv. Funct. Mater. 29 (2019) 1805810.
[62]
D. Yang, R. Yang, K. Wang, C. Wu, X. Zhu, J. Feng, X. Ren, G. Fang, S. Priya, S.F. Liu, High efficiency planar-type perovskite solar cells with negligible hysteresis using EDTA-complexed SnO2, Nat. Commun. 9 (2018) 3239.
19
Figure captions Figure 1. (a) UV-vis spectra of FTO/TiO2, FTO/TiO2(HI). (b) XRD patterns of TiO2 and TiO2(HI) films. AFM images of (c) TiO2 and (d) TiO2(HI). Figure 2. XPS spectra (a) the Ti2p of TiO2 and TiO2(HI), (b) the O1s of TiO2 and TiO2(HI). (c) Dark current-voltage curve of TiO2 and TiO2(HI) films with structure of FTO/TiO2 /Au, (d) J-V curves of conductivity measurements of various dual-ETLs. Figure 3. (a) Schematic diagram of the PSCs structure based on TiO2/SnO2 double ETLs, (b) cross-sectional SEM images of PSCs based on TiO2(HI)/SnO2 and top view SEM images of MAPbI3 films based on (c) TiO2/SnO2 and (d) TiO2(HI)/SnO2, the insert images is contact angles of TiO2/SnO2 and TiO2(HI)/SnO2. AFM images of MAPbI3 films based on (e) TiO2/SnO2 and (f) TiO2(HI)/SnO2. Figure 4. (a) EIS of PSCs with various ETLs, the inset shows the equivalent circuit diagram. (b) PL spectra and TRPL decay transient spectra of MAPbI3 on (c) TiO2/SnO2 and (d) TiO2(HI)/SnO2. Figure 5. The J-V curves at forward and reverse voltages of the excellent performing PSCs using (a) TiO2/SnO2 and (b) TiO2(HI)/SnO2 ETLs at a scan rate of 0.2 V s-1 under 100 mW cm-2. (c) Steady-state efficiencies of TiO2/SnO2 and TiO2(HI)/SnO2 based PSCs surveyed at constant bias voltage of -0.81 V and -0.86 V respectively. (d) IPCE of TiO2/SnO2 and TiO2(HI)/SnO2 based PSCs. Figure 6. (a) Degradation of devices kept in air (room temperature, relative humidity 35%) up to 1400 h. (b) Heat-induced degradation of devices at 100 °C up to 22 h. (c) Degradation of devices upon 1 sun irradiation up to 6 h. (d) Photographs of perovskite at 100 °C.
1
Figure 1.
2
Figure 2.
3
Figure 3.
4
Figure 4.
5
Figure 5.
6
Figure 6.
7
Table captions Table 1. Position, area and area ratio of OTi–O, OV and OO–H for the TiO2 and TiO2(HI).
8
Table 1. TiO2
TiO2(HI)
Position (eV)
529.7
529.6
Area
15775.1
21100.3
Position (eV)
530.7
530.6
Area
15810.8
13450.1
Position (eV)
532.7
532.7
Area
11108.5
11106.9
OTi–O area ratio (%)
36.95
46.21
OV area ratio (%)
37.03
29.46
OO–H area ratio (%)
26.02
24.33
O1s OTi–O
OV
OO–H
9