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Low-temperature preparation of HTM-free SnO2-based planar heterojunction perovskite solar cells with commercial carbon as counter electrode
Journal of Alloys and Compounds 809 (2019) 151817
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Low-temperature preparation of HTM-free SnO2-based planar heterojunction perovskite solar cells with commercial carbon as counter electrode Yue Qiang, Jian Cheng, Ying Qi, Haokun Shi, Haichao Liu, Cong Geng, Yahong Xie* Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education and Xinjiang Uyghur Autonomous Region, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, 830046, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2019 Received in revised form 5 August 2019 Accepted 10 August 2019 Available online 11 August 2019
In order to meet the demand for fabricating flexible and wearable solar cell products, low-temperature preparation technology of perovskite solar cells (PSCs) has become critical. As a highly promising electron transport layer (ETL) material, SnO2 has many advantages, such as high charge mobility, wide band gap, and could be prepared at relatively low temperatures. However, the application of SnO2-based PSCs is limited by the expensive hole transport material (HTM) and electrode material (e.g. spiro-OMeTAD and gold) in the common device structure. In this paper, with SnO2 as the ETL, cheap carbon as the counter electrode, HTMefree PSCs structured as FTO/SnO2/CH3NH3PbI3/Carbon was fabricated by the full lowtemperature technology. The thickness of the SnO2 ETL was optimized by adjusting the concentration of the SnO2 precursor. The highest PCE of 8.32% was obtained and after 32 days in an air environment without packaging, the PCE still remained approximately 92% of its original performance. The factors affecting the stability of the cell were analyzed according to the structure of the PSCs. © 2019 Elsevier B.V. All rights reserved.
Keywords: Low temperature SnO2 ETL Carbon electrode PSCs HTM-Free
1. Introduction Over the past few years, Perovskite Solar Cells (PSCs) have attracted a great deal of attention due to their easy fabrication, high absorption coefficient, high charge carrier mobility and high power conversion efficiency (PCE). Their PCEs have increased dramatically from 3.8% [1] in 2009 to a certified 23.7% [2] in 2018. In the PSCs, the electron transport layer (ETL) plays a crucial role on the extraction and transportation of charge and stability. A lot of inorganic n-type metal oxides have been investigated as the ETL, such as TiO2 [3,4], SnO2 [5], ZnO [6,7], Zn2SO4 [8], WO3 [9], In2O3 [10], SrTiO3 [11], Nb2O5 [12], and BaSnO3 [13]. Among them, TiO2 is the most commonly used ETL material, and its preparation temperature is usually higher than 500 C. High-temperature condition (>450 C) tends to cause nano-scale pinholes and crack between TiO2 and transparent conductive oxides due to crystallization and thermal stress [14], which deteriorates the cell performance and limits the use of many flexible substrates. Mriganka Singh et al. reported a simple ball milling method to obtain TiO2 and SnO2 at
* Corresponding author. E-mail address: [email protected] (Y. Xie). https://doi.org/10.1016/j.jallcom.2019.151817 0925-8388/© 2019 Elsevier B.V. All rights reserved.
low temperature processing, which achieved champion PCEs of 17.43% and 17.92% based Ag and Au counter electrodes (CEs), respectively [15,16]. To reduce the toxicity of lead, lead-free singlelayer (C4H9NH3)2Sn(BH4)4 hybrid halide perovskite is expected to manufacture high performance PSCs [17]. Furthermore, as the demands for flexible and wearable solar cell products increase, lowtemperature preparation technology of PSCs is becoming increasingly important. Other wide band gap n-type metal oxides, such as SnO2 and ZnO, have higher transparency and electron mobility (SnO2: 100e200 cm V1 S1; ZnO: 200e300 cm V1 S1) and are more suitable for low temperature preparation [18]. Nevertheless, the chemical instability of ZnO and the thermally induced degradation of the ZnO/perovskite interface may hinder its application as an ETL in PSCs. Comparatively, n-type SnO2 is a highly promising photovoltaic material with deep conduction band (CB), valence band (VB), excellent stability and good band alignment to perovskites [19]. In 2018, Yang et al. used EDTA-complexed SnO2 (E-SnO2) as the ETL in the planar-type PSCs and achieved a PCE of 21.60% (certified PCE was 21.52%), which was the highest value reported for the planar-type PSCs [20]. Some optimization strategies are used to improve the properties of PSCs. CdSe quantum dots/PCBM composites as an ETL can reduce the roughness of perovskites and
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form high quality films with compact morphology [21]. Furthermore, the application of superalkali perovskite achieves stable and efficient kinetic performance [22]. The band gap tuning of perovskite by iodide ion concentration gradient can also improve the photovoltaic performance [23]. In order to make solar cells widely used as important energy source, environmentally friendly, low cost, high stability, high efficiency are all factors that should be considered in commercialization. At present, the CEs of most PSCs still uses precious metal gold or silver. However, due to the synergistic effect of perovskite decomposition and metal migration, both gold and silver can react with halide (iodide) ions in perovskites, which results in poor longterm stability and increases cost of the device. Carbon can be used as an alternative material for CE because its work function (5.0 eV) is close to Au (5.1 eV), and the carbon material is inexpensive [24]. To reduce the cost of the device, it's a wonderful choice to replace hole-transfer materials (HTMs) and noble metal CEs, such as spiroOMeTAD and gold, with commercial cheap conductive-carbon materials. However, the usual carbon electrodes preparation process also requires a high temperature above 400 C to volatilize organic solvents, which limits the application of flexible devices fabricated on the plastic substrates [25]. Therefore, further development of more low-temperature processes for the preparation of carbon electrodes is also needed. In this paper, SnO2 ETL, the perovskite CH3NH3PbI3 light absorber and carbon electrode were all prepared at low temperatures, and HTM-free planar heterojunction PSCs structured as FTO/ SnO2/CH3NH3PbI3/Carbon were fabricated. The thickness of the SnO2 ETL was optimized, and the effect of diverse SnO2 ETL films thickness on the device performance, as well as stability, was investigated. 2. Experimental 2.1. Materials PbI2 (99.9%) was obtained from Xi'an Polymer Light Technology Corp. CH3NH3I (MAI) was obtained from Dyesol. SnCl2$2H2O (99.99%), ethylacetate (99.5%), N, N-dimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.9%), and chlorobenzene (99%) were purchased from Aladdin. Besides, commercial conductive carbon paste was received from Shenzhen Dongdalai. All the Chemical reagents directly used without any processing.
2.3. Device fabrication Fig. 1(A) illustrates the preparation process of the SnO2-based planar heterojunction PSCs, including a SnO2 ETL, a perovskite light absorbing layer and a carbon electrode layer: At first, a Fluorinedoped tin oxide (FTO) glass substrate with a size of 2 cm 2 cm was ultrasonically cleaned by detergent, deionized water, isopropanol, and ethanol successively, each step for 20 min. SnO2 precursor solution was spin-coated onto the FTO-glass substrate at 3000 rpm for 30 s, followed by annealing for at 200 C for 1 h to obtain a SnO2 ETL. The perovskite layer was fabricated by the antisolvent one-step spin-coating method [27,28]. Briefly, MAI (159 mg) and PbI2 (461 mg) as well as DMSO (78 mg) were mixed in DMF (600 mg), magnetic stirring at 60 C for 2 h, forming a CH3NH3PbI3 precursor solution. 80 mL of CH3NH3PbI3 precursor solution was spin-coated onto the SnO2 ETL at 4000 rpm for 30 s, followed by adding 350 mL of ethylacetate in the last 20 s, and then heated at 100 C for 10 min on the hot plate. Ultimately, the asprepared carbon paste was deposited onto the perovskite layer through the doctor blade method, and dried in an oven at 60 C for 10 min. The entire preparation process was carried out under 200 C and the perovskite layer is completed in a nitrogen atmosphere glove box. 2.4. Characterisations The structure of SnO2 ETL was investigated using an X-ray diffractometer (XRD, D8 Advance, BRUKER) with Cu Ka radiation (l ¼ 1.54056 Å) from 10 to 80 . Transmission electron microscopy (TEM) and Selective area electron diffraction (SAED) images were taken with a field emission transmission electron microscope (FETEM, JEM2100) operating at 200 kV accelerating voltage to analyze in-depth structural and morphological analysis. The morphology of the SnO2-based planar heterojunction PSCs was observed by field emission scanning electron microscopy (FEGeSEM, S-4800, HITACHI, Japan). The J-V curves of PSCs were recorded on electrochemical workstation system (CHI660D, Chenhua and Shanghai) under simulated AM 1.5 G, 100 mW cm2 with an active area fixed mask of 0.08 cm2. The intensity (100 mW cm2) was calibrated via a standard monocrystalline silicon solar cell (CEL-RCCN, Beijing China Education Au-light). Electrochemical impendence spectroscopy (EIS) was tested at 0.7 V bias voltage with a frequency range between 0.1 MHz and 0.01 Hz under AM 1.5 G illumination. 3. Results and discussion
2.2. Preparation of SnO2 precursor solution and carbon paste SnO2 precursor solution was obtained via a low-temperature processed sol-gel method: SnCl2$2H2O was dissolved in absolute ethanol to form a 0.1 M solution, and then the solution was refluxed at 78 C for 3 h, aged at 40 C for another 3 h, and then rested at room temperature in the dark for 24 h. In this process, the colour of the SnO2 precursor solution was pale-yellow at the end of reflux and then gradually deepens during aging until it does not change over time. The obtained SnO2 precursor solution was diluted to some extent with ethanol (dilution ratios of SnO2:ethanol are 1:0, 1:1, 1:2, 1:3, 1:4, 1:5 vol ratios). A solvent-exchange method [26] was used to treat the commercial conductive carbon paste. About 10 g of the commercially conductive carbon paste was dispersed in 200 mL ethanol, and magnetically stirred at room temperature for 2 h. The resulting mixture was suction filtered while washing with ethanol, and then dried at 60 C for 2 h. After dispersing 0.5 g of the dried carbon powder in 2 mL of chlorobenzene, 2 g of zirconium beads were added and ground for 4 h.
The structure and corresponding energy gap of the SnO2 based planar PSCs are demonstrated in Fig. 2(A) and (B). The VB and CB values of perovskite are 5.4 eV and 3.9 eV, respectively [29]. The ambipolar properties of perovskite material is beneficial to manufacture a series of structure types of flexible devices because perovskite layer can replace the electron transport layer or the hole transport layer [30]. In addition, SnO2 is a stable n-type wide bandgap semiconductor material with high bulk electron mobility, outstanding chemical stability wide optical bandgap, which can be processed by a simple low-temperature method. As shown in Fig. 2(B), it is a fantastic choice that SnO2 works as ETL in our device structure, for that the favorable CB and deep VB of SnO2 can provide efficient electron transfer. Fig. 3(A) is the XRD patterns of the SnO2 ETL film, which shows that six sharp and strong peaks at different 2q values, corresponding to (110), (101), (200), (211), (220), (310) and (301) crystal faces of SnO2, respectively. No characteristic peaks of other impurities were observed, confirming that the obtained product was pure SnO2. It is reported that the mechanism for synthesizing SnO2
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Fig. 1. Manufacturing process diagram of SnO2-based planar perovskite solar cells.
Fig. 2. (A) The PSCs structure in this work and (B) corresponding energy level.
ETL by sol-gel method can be expressed as follows [31]: SnCl2$2H2O þ 2C2H5OH / Sn(OH)2 þ 2C2H5Cl
(1)
Sn(OH)2 / SnO þ H2O
(2)
2SnO þ O2 / SnO2
(3)
This process is spontaneous and has great potential for the preparation of high quality SnO2 ETL films [32]. It can also be proved that the low temperature sol-gel method is a very effective method for preparing SnO2 ETL. TEM and high resolution (HR) TEM micrographs (Fig. 3(B) and (C)) shows a slightly irregular elliptical shape with a size of about 7e10 nm. The SAED image (illustration of Fig. 3(C)) confirmed the high crystallinity of the SnO2 nanocrystalline particles. Fig. 4 (A) shows the FESEM image of the SnO2 ETL surface after sintering at 200 C for 1 h. The SnO2 nanoparticles are extremely small in size and evenly cover the FTO substrate. Fig. 4 (B) shows that the dense grains of the CH3NH3PbI3 film with sizes of 100e500 nm have been formed, and the entire film consists of a uniform, well-crystallized perovskite layer. The carbon film treated by solvent-exchange method exhibits a macroporous structure
with bulges of uneven size, ravines and gullies criss-cross on the surface displayed in Fig. 4(C). The thickness of the SnO2 layer can also be varied according to the preparation conditions. Highmagnification SEM images reveal that different SnO2 ETL film thicknesses can be obtained by diluting SnO2 precursor solution with some extent with ethanol, and dilution ratios of SnO2: ethanol are 1:0 (Fig. 4(D)), 1:3 (Fig. 4(E)), 1:5 (Fig. 4(F)) volume ratios corresponding SnO2 ETL thicknesses of 460, 400 and 360 nm, respectively. From the SEM images, it is detected that carbon penetrated into the perovskite layer to varying degrees, which is conductive to the formation of CH3NH3PbI3/Carbon heterojuction. Since the thickness of the SnO2 ETL is controlled by adjusting the dilution ratios of SnO2 to ethanol, Fig. 5(A) actually shows the linear sweep voltammetry curves of conductivity test for the SnO2 ETL films with different thicknesses. Fig. 5(B) shows the test sample structure and the corresponding band structure of the SnO2 ETL film sandwiched between the FTO and carbon electrode, respectively. The slope of the linear sweep voltammetry curves directly reflects the conductivity of the film. As can be seen from Fig. 5(A), the curve has the largest slope when the dilution ratio is 1:3, which means the optimum conductivity and the thickness of SnO2 ETL directly affects the conductivity. The open-circuit voltage decay (OCVD) curves are collected to evaluate the charge recombination in PSCs based different SnO2 ETL thickness, as shown in Fig. 5(C). A lower recombination rate was observed for the 400 nm thickness SnO2 (SnO2:ethanol ¼ 1:3). The accelerated recombination of too thin or too thick SnO2 ETL films have verified the existence of electron trap site, which consequently increase the recombination rate and reduce the performance of the PSCs. In OCVD measurement, the devices were illuminated by a solar simulator at an opencircuit voltage, and light was blocked after building a stable opencircuit voltage. The decay of open-circuit voltage was recorded as a function of time, and electron lifetime can be calculated by formula as follows [33]:
tn ¼ (kB T /e) (dVoc/dt)1
Fig. 3. (A) XRD diffraction patterns of SnO2 ETL; TEM image of SnO2 (B), HR-TEM image of the SnO2 sintered at 200 C for 1 h(C), and insert of Fig. 3(C): corresponding diffraction pattern.
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Fig. 4. Top-view SEM images of SnO2 ETL (A); illustration (a) and (b) are element Maping of Sn and O, respectively. CH3NH3PbI3 deposited on SnO2 ETL (B), and carbon surface onto the CH3NH3PbI3 (C). Cross-sectional SEM images of the device at ethanol diluted SnO2 precursor solution with different dilution ratio (SnO2 precursor: ethanol ¼ 1 : 1 (D), 1 : 3 (E), and 1 : 5 (F)).
Fig. 5. (A) Linear sweep voltammetry curves of different dilution ratios of SnO2 precursor solution and ethanol; (B) are sample structure diagram and corresponding band structure respectively; OCVD curves (C) and Electron lifetime curves (D) of different thickness SnO2 ETL.
Y. Qiang et al. / Journal of Alloys and Compounds 809 (2019) 151817
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Fig. 6. (A) The current density (J)-voltage (V) curves of different SnO2 dilution ratios. (B) The steady-state output current density and calculated PCE of champion PSCs at 0.583 V bias. (C) Nyquist plots of different SnO2 thickness planar perovskite solar cells, and (D) corresponding simplified equivalent fitting circuit diagram.
where kB T is the thermodynamics constant, e is the electron charge, and dVoc/dt is the derivative of open-circuit voltage with respect to time. The calculated electron lifetime curves (Fig. 5(D)) of 400 nm thickness SnO2 ETL film (SnO2:ethanol ¼ 1:3) is higher than other thickness SnO2. This result is confirmed that suitable thickness of SnO2 film is beneficial to enhance the charge transport and inhibit recombination in PSCs. Fig. 6(A) is the J-V curves of the planar heterojunction PSCs with different thicknesses of SnO2 ETL, which shows the effect of the thickness of SnO2 ETL on photovoltaic performance, and the corresponding photovoltaic parameters are shown in Table 1. From these data, it can be seen that when the dilution ratio is 1:3, the optimum voltage (Voc) of 0.93 V, the short-circuit current (Jsc) of 22.50 mA/cm2, the fill factor (FF) of 0.40 and the PCE of 8.32% were obtained. The width of the depletion region formed by SnO2/CH3NH3PbI3 heterojunction is very important for the performance of the solar cell. When the concentration of the perovskite precursor is the same, the width of the depletion region of the SnO2/CH3NH3PbI3 heterojunction is mainly affected by
Table 1 Photovoltaic parameters of the devices at different ethanol diluted SnO2 precursor solution with different dilution ratios. Dilution ratio SnO2: SnO2: SnO2: SnO2: SnO2: SnO2:
ethanol ¼ 1:0 ethanol ¼ 1:1 ethanol ¼ 1:2 ethanol ¼ 1:3 ethanol ¼ 1:4 ethanol ¼ 1:5
Voc
Jsc
FF
PCE
0.86 0.89 0.92 0.93 0.87 0.68
20.84 21.00 21.33 22.50 21.79 19.95
0.38 0.41 0.39 0.40 0.38 0.32
6.78 7.67 7.59 8.32 7.18 4.35
the thickness of the SnO2 ETL film. It is known that an important function of ETL is to fill the pores of the perovskite layer and provide an effective barrier to avoid direct contact of the perovskite layer with the FTO substrate and minimize charge shunt paths, so the increase in the thickness of the SnO2 ETL can ensure this blocking effect [34]. On the other hand, in order to improve electron extraction and reduce charge recombination, a thin and dense SnO2 ETL film is required. Therefore, in order to obtain the best photoelectric efficiency, the optimum ETL thickness is necessary and is obtained using a dilution ratio of SnO2 to ethanol of 1:3 (400 nm thickness of SnO2 ETL) in our work. The steadystate efficiency of the champion PCE device is measured under AM 1.5 G 100 mW/cm2 illumination and the results are shown in Fig. 6(B). A steady-state current density of 10.36 mA/cm2 is obtained at a constant bias of 0.583 V, which produces a stable maximum power output of 6.04% for 400 s. In order to fully understand the photovoltaic principle, electrochemical impedance spectroscopy (EIS) of planar PSCs with different SnO2 thicknesses are measured to explore the interface charge transport properties, and their Nyquist plots are shown in Fig. 6(C). The EIS data are fitted with an equivalent circuit and each component in the circuit is assigned to a specific charge transfer process and shown in Fig. 6(D). The devices with FTO/SnO2/ CH3NH3PbI3/Carbon structure show two arcs in the Nyquist plots. Generally, the radius of the first high frequency circle corresponds to the internal resistance (Rct) through FTO/SnO2/CH3NH3PbI3, carbon electrode, and the radius of the second low frequency circle corresponds to the SnO2/CH3NH3PbI3 interface resistance (Rrec). The fitted Rrec of SnO2 (dilution ratio is 1:5) based PSC is obviously lower than that of other devices, indicating that more electron-hole
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Fig. 7. (A) Photoelectric property parameter of PCE distribution histogram. (B) The long-term stability test of SnO2-based planar heterojunction PSCs preserved in atmosphere without encapsulation measured under one sun illumination.
recombination occurs at the SnO2/CH3NH3PbI3 interface, and too thin SnO2 film is detrimental to the rapid transmission of electrons. The SnO2 (dilution ratio is 1:3) based PSC owned a lower Rct and a higher Rrec, indicating that holes have better transport ability from CH3NH3PbI3 to carbon electrode, and electrons have faster transmission rate from CH3NH3PbI3 to SnO2 ETL. This result is consistent with the J-V curves. To test the performance reproducibility of the SnO2-based planar device, the PCE of 20 samples were tested under reverse scanning in Fig. 7(A). It is manifest that the device has good repeatability and reliable results in the histogram distribution of photoelectric performance parameters. Fig. 7(B) shows the longterm stability test of the cell, which keep in a dark air condition without encapsulation (temperature: 25 C; humidity: 20%). The result illustrates that the cell performance decreases from the initial Voc of 0.8 V, Jsc of 17.19 mA/cm2, FF of 38% and PCE of 5.23% to the final 0.79 V, 14.84 mA/cm2, 41% and 4.81%, respectively. According to the initial and final PCE results, the PCE maintain approximately 92% of its original performance after preserving cells in atomosphere for 32 days, reflecting a good repeatability and reliable results. The stability of PSCs is closely related to environmental conditions, such as atmosphere and contact layer of the perovskite absorber. The admirable stability in our devices can be attributed to the protection of the carbon electrode worked as waterproof, preventing damage to the perovskite layer by water. In addition, the SnO2 with a relatively wide bandgap absorbs less UV light, has lower hygroscopicity and acid resistance also contribute to device stability [35,36]. The use of carbon as a CE instead of Ag or Au can greatly improve the stability of the device. Throughout the testing process, the value of PCE increased but eventually declined. The growth of PCE values may be due to combined effects of light or field-induced ion movement with associated structural rearrangement. Light-induced trap formation or interface charge accumulation alter device behaviour. After a long period of illumination, the size of the perovskite crystals will gradually increase, and the small crystals will merge into each other to become large crystals [37], which is conducive to the transmission of electrons and holes.
4. Conclusions In summary, the SnO2 ETLs were successfully prepared by a simple sol-gel process and the carbon CE was obtained from a commercially available conductive carbon paste treated by a
solvent-exchange method. The PCEs with the structure of FTO/ SnO2/CH3NH3PbI3/Carbon were fabricated at a low temperature and the thickness of the SnO2 ETL was optimized by adjusting the concentration of the SnO2 precursor. The highest PCE of 8.32% was obtained when the thickness of SnO2 ETL was 400 nm. And after 32 days in an air environment without packaging, the PCE of the cell still remained approximately 92% of its original performance with an excellent stability. Acknowledgements This research was financially supported by Xinjiang Uygur Autonomous Region Natural Science Foundation of China (No.2017D01C023). The authors also acknowledge the facilities and staffs at the Physical and Chemical Testing Center of Xinjiang University. 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) 6050e6051. [2] Z. Wang, Q. Lin, B. Wenger, M.G. Christoforo, Y.-H. Lin, M.T. Klug, M.B. Johnston, L.M. Herz, H.J. Snaith, High irradiance performance of metal halide perovskites for concentrator photovoltaics, Nat. Energy 3 (2018) 855e861. [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, Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells, Science 356 (2017) 1376e1379. [4] A. Khorasani, M. Marandi, N. Taghavinia, Application of combinative TiO2 nanorods and nanoparticles layer as the electron transport film in highly efficient mixed halides perovskite solar cells, Electrochim. Acta 297 (2019) 1071e1078. [5] W. Ke, G. Fang, Q. Liu, L. Xiong, P. Qin, H. Tao, J. Wang, H. Lei, B. Li, J. Wan, Lowtemperature solution-processed tin oxide as an alternative electron transporting layer for efficient perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 6730e6733. [6] D.-Y. Son, J.-H. Im, H.-S. Kim, N.-G. Park, 11% efficient perovskite solar cell based on ZnO nanorods: an effective charge collection system, J. Phys. Chem. C 118 (2014) 16567e16573. [7] 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 using an ionic liquid, Electrochim. Acta 268 (2018) 539e545. [8] S.S. Shin, W.S. Yang, J.H. Noh, J.H. Suk, N.J. Jeon, J.H. Park, J.S. Kim, W.M. Seong, S.I. Seok, High-performance flexible perovskite solar cells exploiting Zn2SnO4 prepared in solution below 100 C, Nat. Commun. 6 (2015) 7410. [9] K. Mahmood, B.S. Swain, A.R. Kirmani, A. Amassian, Highly efficient perovskite solar cells based on a nanostructured WO3-TiO2 core-shell electron transporting material, J. Mater. Chem. A 3 (2015) 9051e9057. [10] M. Qin, J. Ma, W. Ke, P. Qin, H. Lei, H. Tao, X. Zheng, L. Xiong, Q. Liu, Z. Chen, Perovskite solar cells based on low-temperature processed indium oxide electron selective layers, ACS Appl. Mater. Interfaces 8 (2016) 8460e8466.
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[25] Z. Liu, T. Shi, Z. Tang, B. Sun, G. Liao, Using a low-temperature carbon electrode for preparing hole-conductor-free perovskite heterojunction solar cells under high relative humidity, Nanoscale 8 (2016) 7017e7023. [26] H. Zhang, J. Xiao, J. Shi, H. Su, Y. Luo, D. Li, H. Wu, Y.B. Cheng, Q. Meng, Selfadhesive macroporous carbon electrodes for efficient and stable perovskite solar cells, Adv. Funct. Mater. 28 (2018) 1802985. [27] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476. [28] F. Han, J. Luo, Z. Wan, X. Liu, C. Jia, Dissolution-recrystallization method for high efficiency perovskite solar cells, Appl. Surf. Sci. 408 (2017) 34e37. [29] L. Huang, X. Sun, C. Li, J. Xu, R. Xu, Y. Du, J. Ni, H. Cai, J. Li, Z. Hu, UV-sintered low-temperature solution-processed SnO2 as robust electron transport layer for efficient planar heterojunction perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 21909e21920. [30] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, A holeconductor-free, fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295e298. [31] V.-H. Tran, R.B. Ambade, S.B. Ambade, S.-H. Lee, I.-H. Lee, Low-temperature solution-processed SnO2 nanoparticles as a cathode buffer layer for inverted organic solar cells, ACS Appl. Mater. Interfaces 9 (2017) 1645e1653. [32] S. Lin, B. Yang, X. Qiu, J. Yan, J. Shi, Y. Yuan, W. Tan, X. Liu, H. Huang, Y. Gao, Efficient and stable planar hole-transport-material-free perovskite solar cells using low temperature processed SnO2 as electron transport material, Org. Electron. 53 (2018) 235e241. [33] F. Lan, M. Jiang, Q. Tao, G. Li, Revealing the working mechanisms of planar perovskite solar cells with cross-sectional surface potential profiling, IEEE J. Photovolt. 8 (2017) 125e131. €rantner, H.J. Snaith, T. Park, Well-defined nano[34] J. Choi, S. Song, M.T. Ho structured, single-crystalline TiO2 electron transport layer for efficient planar perovskite solar cells, ACS Nano 10 (2016) 6029e6036. [35] J. Song, E. Zheng, J. Bian, X.-F. Wang, W. Tian, Y. Sanehira, T. Miyasaka, Lowtemperature SnO 2-based electron selective contact for efficient and stable perovskite solar cells, J. Mater. Chem. A 3 (2015) 10837e10844. [36] G. Niu, X. Guo, L. Wang, Review of recent progress in chemical stability of perovskite solar cells, J. Mater. Chem. A 3 (2015) 8970e8980. n-Carmona, I. Zimmermann, E. Mosconi, X. Lee, [37] G. Grancini, C. Rolda D. Martineau, S. Narbey, F. Oswald, F. De Angelis, M. Graetzel, One-Year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. 8 (2017) 15684.
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Low-temperature preparation of HTM-free SnO2-based planar heterojunction perovskite solar cells with commercial carbon as counter electrode Yue Qiang, Jian Cheng, Ying Qi, Haokun Shi, Haichao Liu, Cong Geng, Yahong Xie* Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education and Xinjiang Uyghur Autonomous Region, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, 830046, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2019 Received in revised form 5 August 2019 Accepted 10 August 2019 Available online 11 August 2019
In order to meet the demand for fabricating flexible and wearable solar cell products, low-temperature preparation technology of perovskite solar cells (PSCs) has become critical. As a highly promising electron transport layer (ETL) material, SnO2 has many advantages, such as high charge mobility, wide band gap, and could be prepared at relatively low temperatures. However, the application of SnO2-based PSCs is limited by the expensive hole transport material (HTM) and electrode material (e.g. spiro-OMeTAD and gold) in the common device structure. In this paper, with SnO2 as the ETL, cheap carbon as the counter electrode, HTMefree PSCs structured as FTO/SnO2/CH3NH3PbI3/Carbon was fabricated by the full lowtemperature technology. The thickness of the SnO2 ETL was optimized by adjusting the concentration of the SnO2 precursor. The highest PCE of 8.32% was obtained and after 32 days in an air environment without packaging, the PCE still remained approximately 92% of its original performance. The factors affecting the stability of the cell were analyzed according to the structure of the PSCs. © 2019 Elsevier B.V. All rights reserved.
Keywords: Low temperature SnO2 ETL Carbon electrode PSCs HTM-Free
1. Introduction Over the past few years, Perovskite Solar Cells (PSCs) have attracted a great deal of attention due to their easy fabrication, high absorption coefficient, high charge carrier mobility and high power conversion efficiency (PCE). Their PCEs have increased dramatically from 3.8% [1] in 2009 to a certified 23.7% [2] in 2018. In the PSCs, the electron transport layer (ETL) plays a crucial role on the extraction and transportation of charge and stability. A lot of inorganic n-type metal oxides have been investigated as the ETL, such as TiO2 [3,4], SnO2 [5], ZnO [6,7], Zn2SO4 [8], WO3 [9], In2O3 [10], SrTiO3 [11], Nb2O5 [12], and BaSnO3 [13]. Among them, TiO2 is the most commonly used ETL material, and its preparation temperature is usually higher than 500 C. High-temperature condition (>450 C) tends to cause nano-scale pinholes and crack between TiO2 and transparent conductive oxides due to crystallization and thermal stress [14], which deteriorates the cell performance and limits the use of many flexible substrates. Mriganka Singh et al. reported a simple ball milling method to obtain TiO2 and SnO2 at
* Corresponding author. E-mail address: [email protected] (Y. Xie). https://doi.org/10.1016/j.jallcom.2019.151817 0925-8388/© 2019 Elsevier B.V. All rights reserved.
low temperature processing, which achieved champion PCEs of 17.43% and 17.92% based Ag and Au counter electrodes (CEs), respectively [15,16]. To reduce the toxicity of lead, lead-free singlelayer (C4H9NH3)2Sn(BH4)4 hybrid halide perovskite is expected to manufacture high performance PSCs [17]. Furthermore, as the demands for flexible and wearable solar cell products increase, lowtemperature preparation technology of PSCs is becoming increasingly important. Other wide band gap n-type metal oxides, such as SnO2 and ZnO, have higher transparency and electron mobility (SnO2: 100e200 cm V1 S1; ZnO: 200e300 cm V1 S1) and are more suitable for low temperature preparation [18]. Nevertheless, the chemical instability of ZnO and the thermally induced degradation of the ZnO/perovskite interface may hinder its application as an ETL in PSCs. Comparatively, n-type SnO2 is a highly promising photovoltaic material with deep conduction band (CB), valence band (VB), excellent stability and good band alignment to perovskites [19]. In 2018, Yang et al. used EDTA-complexed SnO2 (E-SnO2) as the ETL in the planar-type PSCs and achieved a PCE of 21.60% (certified PCE was 21.52%), which was the highest value reported for the planar-type PSCs [20]. Some optimization strategies are used to improve the properties of PSCs. CdSe quantum dots/PCBM composites as an ETL can reduce the roughness of perovskites and
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form high quality films with compact morphology [21]. Furthermore, the application of superalkali perovskite achieves stable and efficient kinetic performance [22]. The band gap tuning of perovskite by iodide ion concentration gradient can also improve the photovoltaic performance [23]. In order to make solar cells widely used as important energy source, environmentally friendly, low cost, high stability, high efficiency are all factors that should be considered in commercialization. At present, the CEs of most PSCs still uses precious metal gold or silver. However, due to the synergistic effect of perovskite decomposition and metal migration, both gold and silver can react with halide (iodide) ions in perovskites, which results in poor longterm stability and increases cost of the device. Carbon can be used as an alternative material for CE because its work function (5.0 eV) is close to Au (5.1 eV), and the carbon material is inexpensive [24]. To reduce the cost of the device, it's a wonderful choice to replace hole-transfer materials (HTMs) and noble metal CEs, such as spiroOMeTAD and gold, with commercial cheap conductive-carbon materials. However, the usual carbon electrodes preparation process also requires a high temperature above 400 C to volatilize organic solvents, which limits the application of flexible devices fabricated on the plastic substrates [25]. Therefore, further development of more low-temperature processes for the preparation of carbon electrodes is also needed. In this paper, SnO2 ETL, the perovskite CH3NH3PbI3 light absorber and carbon electrode were all prepared at low temperatures, and HTM-free planar heterojunction PSCs structured as FTO/ SnO2/CH3NH3PbI3/Carbon were fabricated. The thickness of the SnO2 ETL was optimized, and the effect of diverse SnO2 ETL films thickness on the device performance, as well as stability, was investigated. 2. Experimental 2.1. Materials PbI2 (99.9%) was obtained from Xi'an Polymer Light Technology Corp. CH3NH3I (MAI) was obtained from Dyesol. SnCl2$2H2O (99.99%), ethylacetate (99.5%), N, N-dimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.9%), and chlorobenzene (99%) were purchased from Aladdin. Besides, commercial conductive carbon paste was received from Shenzhen Dongdalai. All the Chemical reagents directly used without any processing.
2.3. Device fabrication Fig. 1(A) illustrates the preparation process of the SnO2-based planar heterojunction PSCs, including a SnO2 ETL, a perovskite light absorbing layer and a carbon electrode layer: At first, a Fluorinedoped tin oxide (FTO) glass substrate with a size of 2 cm 2 cm was ultrasonically cleaned by detergent, deionized water, isopropanol, and ethanol successively, each step for 20 min. SnO2 precursor solution was spin-coated onto the FTO-glass substrate at 3000 rpm for 30 s, followed by annealing for at 200 C for 1 h to obtain a SnO2 ETL. The perovskite layer was fabricated by the antisolvent one-step spin-coating method [27,28]. Briefly, MAI (159 mg) and PbI2 (461 mg) as well as DMSO (78 mg) were mixed in DMF (600 mg), magnetic stirring at 60 C for 2 h, forming a CH3NH3PbI3 precursor solution. 80 mL of CH3NH3PbI3 precursor solution was spin-coated onto the SnO2 ETL at 4000 rpm for 30 s, followed by adding 350 mL of ethylacetate in the last 20 s, and then heated at 100 C for 10 min on the hot plate. Ultimately, the asprepared carbon paste was deposited onto the perovskite layer through the doctor blade method, and dried in an oven at 60 C for 10 min. The entire preparation process was carried out under 200 C and the perovskite layer is completed in a nitrogen atmosphere glove box. 2.4. Characterisations The structure of SnO2 ETL was investigated using an X-ray diffractometer (XRD, D8 Advance, BRUKER) with Cu Ka radiation (l ¼ 1.54056 Å) from 10 to 80 . Transmission electron microscopy (TEM) and Selective area electron diffraction (SAED) images were taken with a field emission transmission electron microscope (FETEM, JEM2100) operating at 200 kV accelerating voltage to analyze in-depth structural and morphological analysis. The morphology of the SnO2-based planar heterojunction PSCs was observed by field emission scanning electron microscopy (FEGeSEM, S-4800, HITACHI, Japan). The J-V curves of PSCs were recorded on electrochemical workstation system (CHI660D, Chenhua and Shanghai) under simulated AM 1.5 G, 100 mW cm2 with an active area fixed mask of 0.08 cm2. The intensity (100 mW cm2) was calibrated via a standard monocrystalline silicon solar cell (CEL-RCCN, Beijing China Education Au-light). Electrochemical impendence spectroscopy (EIS) was tested at 0.7 V bias voltage with a frequency range between 0.1 MHz and 0.01 Hz under AM 1.5 G illumination. 3. Results and discussion
2.2. Preparation of SnO2 precursor solution and carbon paste SnO2 precursor solution was obtained via a low-temperature processed sol-gel method: SnCl2$2H2O was dissolved in absolute ethanol to form a 0.1 M solution, and then the solution was refluxed at 78 C for 3 h, aged at 40 C for another 3 h, and then rested at room temperature in the dark for 24 h. In this process, the colour of the SnO2 precursor solution was pale-yellow at the end of reflux and then gradually deepens during aging until it does not change over time. The obtained SnO2 precursor solution was diluted to some extent with ethanol (dilution ratios of SnO2:ethanol are 1:0, 1:1, 1:2, 1:3, 1:4, 1:5 vol ratios). A solvent-exchange method [26] was used to treat the commercial conductive carbon paste. About 10 g of the commercially conductive carbon paste was dispersed in 200 mL ethanol, and magnetically stirred at room temperature for 2 h. The resulting mixture was suction filtered while washing with ethanol, and then dried at 60 C for 2 h. After dispersing 0.5 g of the dried carbon powder in 2 mL of chlorobenzene, 2 g of zirconium beads were added and ground for 4 h.
The structure and corresponding energy gap of the SnO2 based planar PSCs are demonstrated in Fig. 2(A) and (B). The VB and CB values of perovskite are 5.4 eV and 3.9 eV, respectively [29]. The ambipolar properties of perovskite material is beneficial to manufacture a series of structure types of flexible devices because perovskite layer can replace the electron transport layer or the hole transport layer [30]. In addition, SnO2 is a stable n-type wide bandgap semiconductor material with high bulk electron mobility, outstanding chemical stability wide optical bandgap, which can be processed by a simple low-temperature method. As shown in Fig. 2(B), it is a fantastic choice that SnO2 works as ETL in our device structure, for that the favorable CB and deep VB of SnO2 can provide efficient electron transfer. Fig. 3(A) is the XRD patterns of the SnO2 ETL film, which shows that six sharp and strong peaks at different 2q values, corresponding to (110), (101), (200), (211), (220), (310) and (301) crystal faces of SnO2, respectively. No characteristic peaks of other impurities were observed, confirming that the obtained product was pure SnO2. It is reported that the mechanism for synthesizing SnO2
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Fig. 1. Manufacturing process diagram of SnO2-based planar perovskite solar cells.
Fig. 2. (A) The PSCs structure in this work and (B) corresponding energy level.
ETL by sol-gel method can be expressed as follows [31]: SnCl2$2H2O þ 2C2H5OH / Sn(OH)2 þ 2C2H5Cl
(1)
Sn(OH)2 / SnO þ H2O
(2)
2SnO þ O2 / SnO2
(3)
This process is spontaneous and has great potential for the preparation of high quality SnO2 ETL films [32]. It can also be proved that the low temperature sol-gel method is a very effective method for preparing SnO2 ETL. TEM and high resolution (HR) TEM micrographs (Fig. 3(B) and (C)) shows a slightly irregular elliptical shape with a size of about 7e10 nm. The SAED image (illustration of Fig. 3(C)) confirmed the high crystallinity of the SnO2 nanocrystalline particles. Fig. 4 (A) shows the FESEM image of the SnO2 ETL surface after sintering at 200 C for 1 h. The SnO2 nanoparticles are extremely small in size and evenly cover the FTO substrate. Fig. 4 (B) shows that the dense grains of the CH3NH3PbI3 film with sizes of 100e500 nm have been formed, and the entire film consists of a uniform, well-crystallized perovskite layer. The carbon film treated by solvent-exchange method exhibits a macroporous structure
with bulges of uneven size, ravines and gullies criss-cross on the surface displayed in Fig. 4(C). The thickness of the SnO2 layer can also be varied according to the preparation conditions. Highmagnification SEM images reveal that different SnO2 ETL film thicknesses can be obtained by diluting SnO2 precursor solution with some extent with ethanol, and dilution ratios of SnO2: ethanol are 1:0 (Fig. 4(D)), 1:3 (Fig. 4(E)), 1:5 (Fig. 4(F)) volume ratios corresponding SnO2 ETL thicknesses of 460, 400 and 360 nm, respectively. From the SEM images, it is detected that carbon penetrated into the perovskite layer to varying degrees, which is conductive to the formation of CH3NH3PbI3/Carbon heterojuction. Since the thickness of the SnO2 ETL is controlled by adjusting the dilution ratios of SnO2 to ethanol, Fig. 5(A) actually shows the linear sweep voltammetry curves of conductivity test for the SnO2 ETL films with different thicknesses. Fig. 5(B) shows the test sample structure and the corresponding band structure of the SnO2 ETL film sandwiched between the FTO and carbon electrode, respectively. The slope of the linear sweep voltammetry curves directly reflects the conductivity of the film. As can be seen from Fig. 5(A), the curve has the largest slope when the dilution ratio is 1:3, which means the optimum conductivity and the thickness of SnO2 ETL directly affects the conductivity. The open-circuit voltage decay (OCVD) curves are collected to evaluate the charge recombination in PSCs based different SnO2 ETL thickness, as shown in Fig. 5(C). A lower recombination rate was observed for the 400 nm thickness SnO2 (SnO2:ethanol ¼ 1:3). The accelerated recombination of too thin or too thick SnO2 ETL films have verified the existence of electron trap site, which consequently increase the recombination rate and reduce the performance of the PSCs. In OCVD measurement, the devices were illuminated by a solar simulator at an opencircuit voltage, and light was blocked after building a stable opencircuit voltage. The decay of open-circuit voltage was recorded as a function of time, and electron lifetime can be calculated by formula as follows [33]:
tn ¼ (kB T /e) (dVoc/dt)1
Fig. 3. (A) XRD diffraction patterns of SnO2 ETL; TEM image of SnO2 (B), HR-TEM image of the SnO2 sintered at 200 C for 1 h(C), and insert of Fig. 3(C): corresponding diffraction pattern.
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Fig. 4. Top-view SEM images of SnO2 ETL (A); illustration (a) and (b) are element Maping of Sn and O, respectively. CH3NH3PbI3 deposited on SnO2 ETL (B), and carbon surface onto the CH3NH3PbI3 (C). Cross-sectional SEM images of the device at ethanol diluted SnO2 precursor solution with different dilution ratio (SnO2 precursor: ethanol ¼ 1 : 1 (D), 1 : 3 (E), and 1 : 5 (F)).
Fig. 5. (A) Linear sweep voltammetry curves of different dilution ratios of SnO2 precursor solution and ethanol; (B) are sample structure diagram and corresponding band structure respectively; OCVD curves (C) and Electron lifetime curves (D) of different thickness SnO2 ETL.
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Fig. 6. (A) The current density (J)-voltage (V) curves of different SnO2 dilution ratios. (B) The steady-state output current density and calculated PCE of champion PSCs at 0.583 V bias. (C) Nyquist plots of different SnO2 thickness planar perovskite solar cells, and (D) corresponding simplified equivalent fitting circuit diagram.
where kB T is the thermodynamics constant, e is the electron charge, and dVoc/dt is the derivative of open-circuit voltage with respect to time. The calculated electron lifetime curves (Fig. 5(D)) of 400 nm thickness SnO2 ETL film (SnO2:ethanol ¼ 1:3) is higher than other thickness SnO2. This result is confirmed that suitable thickness of SnO2 film is beneficial to enhance the charge transport and inhibit recombination in PSCs. Fig. 6(A) is the J-V curves of the planar heterojunction PSCs with different thicknesses of SnO2 ETL, which shows the effect of the thickness of SnO2 ETL on photovoltaic performance, and the corresponding photovoltaic parameters are shown in Table 1. From these data, it can be seen that when the dilution ratio is 1:3, the optimum voltage (Voc) of 0.93 V, the short-circuit current (Jsc) of 22.50 mA/cm2, the fill factor (FF) of 0.40 and the PCE of 8.32% were obtained. The width of the depletion region formed by SnO2/CH3NH3PbI3 heterojunction is very important for the performance of the solar cell. When the concentration of the perovskite precursor is the same, the width of the depletion region of the SnO2/CH3NH3PbI3 heterojunction is mainly affected by
Table 1 Photovoltaic parameters of the devices at different ethanol diluted SnO2 precursor solution with different dilution ratios. Dilution ratio SnO2: SnO2: SnO2: SnO2: SnO2: SnO2:
ethanol ¼ 1:0 ethanol ¼ 1:1 ethanol ¼ 1:2 ethanol ¼ 1:3 ethanol ¼ 1:4 ethanol ¼ 1:5
Voc
Jsc
FF
PCE
0.86 0.89 0.92 0.93 0.87 0.68
20.84 21.00 21.33 22.50 21.79 19.95
0.38 0.41 0.39 0.40 0.38 0.32
6.78 7.67 7.59 8.32 7.18 4.35
the thickness of the SnO2 ETL film. It is known that an important function of ETL is to fill the pores of the perovskite layer and provide an effective barrier to avoid direct contact of the perovskite layer with the FTO substrate and minimize charge shunt paths, so the increase in the thickness of the SnO2 ETL can ensure this blocking effect [34]. On the other hand, in order to improve electron extraction and reduce charge recombination, a thin and dense SnO2 ETL film is required. Therefore, in order to obtain the best photoelectric efficiency, the optimum ETL thickness is necessary and is obtained using a dilution ratio of SnO2 to ethanol of 1:3 (400 nm thickness of SnO2 ETL) in our work. The steadystate efficiency of the champion PCE device is measured under AM 1.5 G 100 mW/cm2 illumination and the results are shown in Fig. 6(B). A steady-state current density of 10.36 mA/cm2 is obtained at a constant bias of 0.583 V, which produces a stable maximum power output of 6.04% for 400 s. In order to fully understand the photovoltaic principle, electrochemical impedance spectroscopy (EIS) of planar PSCs with different SnO2 thicknesses are measured to explore the interface charge transport properties, and their Nyquist plots are shown in Fig. 6(C). The EIS data are fitted with an equivalent circuit and each component in the circuit is assigned to a specific charge transfer process and shown in Fig. 6(D). The devices with FTO/SnO2/ CH3NH3PbI3/Carbon structure show two arcs in the Nyquist plots. Generally, the radius of the first high frequency circle corresponds to the internal resistance (Rct) through FTO/SnO2/CH3NH3PbI3, carbon electrode, and the radius of the second low frequency circle corresponds to the SnO2/CH3NH3PbI3 interface resistance (Rrec). The fitted Rrec of SnO2 (dilution ratio is 1:5) based PSC is obviously lower than that of other devices, indicating that more electron-hole
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Fig. 7. (A) Photoelectric property parameter of PCE distribution histogram. (B) The long-term stability test of SnO2-based planar heterojunction PSCs preserved in atmosphere without encapsulation measured under one sun illumination.
recombination occurs at the SnO2/CH3NH3PbI3 interface, and too thin SnO2 film is detrimental to the rapid transmission of electrons. The SnO2 (dilution ratio is 1:3) based PSC owned a lower Rct and a higher Rrec, indicating that holes have better transport ability from CH3NH3PbI3 to carbon electrode, and electrons have faster transmission rate from CH3NH3PbI3 to SnO2 ETL. This result is consistent with the J-V curves. To test the performance reproducibility of the SnO2-based planar device, the PCE of 20 samples were tested under reverse scanning in Fig. 7(A). It is manifest that the device has good repeatability and reliable results in the histogram distribution of photoelectric performance parameters. Fig. 7(B) shows the longterm stability test of the cell, which keep in a dark air condition without encapsulation (temperature: 25 C; humidity: 20%). The result illustrates that the cell performance decreases from the initial Voc of 0.8 V, Jsc of 17.19 mA/cm2, FF of 38% and PCE of 5.23% to the final 0.79 V, 14.84 mA/cm2, 41% and 4.81%, respectively. According to the initial and final PCE results, the PCE maintain approximately 92% of its original performance after preserving cells in atomosphere for 32 days, reflecting a good repeatability and reliable results. The stability of PSCs is closely related to environmental conditions, such as atmosphere and contact layer of the perovskite absorber. The admirable stability in our devices can be attributed to the protection of the carbon electrode worked as waterproof, preventing damage to the perovskite layer by water. In addition, the SnO2 with a relatively wide bandgap absorbs less UV light, has lower hygroscopicity and acid resistance also contribute to device stability [35,36]. The use of carbon as a CE instead of Ag or Au can greatly improve the stability of the device. Throughout the testing process, the value of PCE increased but eventually declined. The growth of PCE values may be due to combined effects of light or field-induced ion movement with associated structural rearrangement. Light-induced trap formation or interface charge accumulation alter device behaviour. After a long period of illumination, the size of the perovskite crystals will gradually increase, and the small crystals will merge into each other to become large crystals [37], which is conducive to the transmission of electrons and holes.
4. Conclusions In summary, the SnO2 ETLs were successfully prepared by a simple sol-gel process and the carbon CE was obtained from a commercially available conductive carbon paste treated by a
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