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Abstract
As an important part of perovskite solar cells (PSCs), the compact electron transport layer largely determines the performance of devices. Titanium dioxide (TiO2) and tin dioxide (SnO2) are very common materials for the electron transport layer (ETL) in PSCs. However, less has been reported regarding the development of high efficiency mesoporous PSCs based on a SnO2 compact layer. Herein, we prepared Mg doped modified SnO2 film at high temperature, combined it with TiO2 to form a composite compact layer, and then applied PSCs with the mesoporous structure. Compared with the pristine SnO2 compact layer, the composite compact layer has excellent interface contact with perovskite and higher electronic extraction capacity. Moreover, the defect free contact between TiO2 and FTO provides stronger hole blocking ability. Devices based on composite compact layers have almost no hysteresis effect. With the composite compact layer, the devices achieved a champion PCE of 13.01%, which is a 9.79% increase compared to the pristine SnO2 compact layer device.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, the excessive use of limited fossil energy has led to an increasingly serious energy crisis, which made the application of renewable energy become particularly important. Solar energy is a very effective clean energy. Recently, organic-inorganic lead hybrid perovskite materials have become the research focus of researchers all over the world due to its high charge carrier mobility, strong light absorption capacity, adjustable band gap width and long carrier diffusion length, and it is an ideal light absorption material for solar cells [1–4]. In just a few years, the power conversion efficiency (PCE) of PSCs have increased from 3.8% to 25.2% [5–13].
PSCs are usually composed of fluorine-doped tin oxide (FTO) conductive substrate, compact electron transport layer (c-ETL), mesoporous electron transport layer (mp-ETL), perovskite layer, hole transport layer (HTL) and counter electrode. In addition, PSCs can be further divided into mesoporous and planar structures according to whether there is a mesoporous layer in the ETL. Hole-conductor-free PSCs based on carbon counter electrodes have been reported frequently since 2013 [8]. Although removing the hole transport layer and expensive metal electrode will have a certain impact on the efficiency of PSCs, but it will also reduce its preparation cost and simplify the preparation process, which is also of research value, which is worth studying [14–17]. So far, TiO2 is the most commonly used ETL material in mesoporous structure PSCs [18]. Preparation of mesoporous PSCs requires high-temperature sintering for TiO2 crystallization to obtain higher electrical conductivity and remove impurities such as organics in the mesoporous layer. However, as an ETL material, TiO2 has some disadvantages such as its low electron mobility (0-0.1 cm2V−1s−1) and unsatisfactory UV stability. To overcome these shortcomings, many other n-type semiconductor materials have been reported as ETL, such as metal oxides ZnO, SnO2, WO3, ZnSO4, and organic [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) et al [19–28]. As an ETL, SnO2 is a promising material with a wide band gap (∼ 3.6 eV), a deeper conduction band than TiO2 and a higher electron mobility (240 cm2V−1s−1), which has been widely favored [29–31]. And there are a lot of selection of SnO2 synthetic methods, such as: Solution Method, Atomic Layer Deposition (ALD), Chemical Bath Deposition (CBD), Spray Pyrolysis, and so on [32–40]. Unfortunately, the interface contact of SnO2 prepared at high-temperature is not satisfactory, so SnO2 ETL is mostly used in planar structure PSCs. Because the degradation of SnO2 at high temperature will lead to many surface defects, which will affect the quality of the ETL and hinder the development of SnO2 mesoporous PSCs [21,45]. To overcome this problem and improve the performance of SnO2 thin films at high temperatures, Kuang treated the compact layer of SnO2 with TiCl4 aqueous solution, effectively passivating the surface defects improving the quality of the compact layer of SnO2 and increasing the PCE of PSCs from 6.5% to 14.6% [41]. Meanwhile, doping is one of the methods to improve the interface quality. Xiong used an appropriate amount of Mg doping SnO2, which effectively inhibited the lattice distortion of SnO2 at high temperature and reduced the formation of thin film cracks, resulting in a smooth and dense SnO2 compact layer, achieving the highest energy conversion efficiency of 19.12% [22,42]. In addition, in order to reduce the film surface defects caused by poor interface contact of SnO2 and the carrier recombination on the interface, TiO2-SnO2 composite ETL formed by combining TiO2 and SnO2 has been reported recently. Liu used TiO2-SnO2 compact layer as ETL to achieve a PCE of 18.03%, and the efficiency was still over 91% after 90 days of air storage [43]. Xie prepared TiO2-SnO2 compact layer with a similar structure at low temperature and obtained a PCE of 18.85% [44]. Each part of the mesoporous structure PSCs without hole transport layer is particularly important, and a high-performance compact layer is necessary.
Herein we reported a composite compact layer prepared by Mg-doped SnO2 (denoted as “Mg-SnO2” hereafter) combined with TiO2 at high temperature for hole-conductor-free and C counter electrode mesoporous PSCs. By effectively utilizing the advantages of TiO2 and SnO2, the composite compact layer has good surface contact and carrier extraction ability. Its atomic force microscope (AFM) images and steady-state photoluminescence (PL) spectra confirm these. PSCs based on composite compact layer have a champion PCE of 13.01%, FF of 59.5%, JSC of 21.49 mA/cm2 and VOC of 1.02 V. Compared with the PSC based on Mg-SnO2 compact layer, the photovoltaic performance parameters are improved.
2. Experimental
2.1 Material
Lead iodine (PbI2, 99.99%) and TiO2 paste (solid content: 20%) were purchased from Xi’an p-OLED (China). SnCl2·2H2O (98%) were purchased from Macklin (China). MgCl2·6H2O (98%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Titanium diisopropoxide bis (acetylacetonate, 75%), dimethylsulfoxide (DMSO, 99.7%) and N, N-dimethylformamide (DMF, 99.8%) were bought from Sigma-Aldrich (US). Methylammonium iodide (MAI, 99.5%) and FTO (7Ω/sq) were bought from Yingkou YouXuan (China). ZrO2 paste (solid content: 20%) and low-temperature carbon electrode paste were acquired from Shanghai MaterWin New Material (China).
2.2 Preparation of the Mg-SnO2 and TiO2 compact precursor solution
In order to obtain the high temperature resistant SnO2 compact layer, a certain proportion of Mg was added to the solution of SnO2 precursor to ensure the quality of SnO2 film at high temperature, the SnO2 film with appropriate Mg content (7.5%) showed excellent performance [42]. Firstly, 1.04 g SnCl2·2H2O and 0.07 g MgCl2·6H2O were mixed in 50 ml anhydrous ethanol to obtain the precursor solution with 0.1 M and continuously stirred for 0.5 h for later use. Secondly, TiO2 precursor solution with 0.1 M was prepared by using the reported method. Dissolve 1 ml titanium diisopropoxide bis (75%, Sigma-Aldrich, America) in 19 ml anhydrous ethanol, seal at room temperature and stir continuously for 1 h [46]. Finally, these two solutions were used to prepare TiO2 and Mg-SnO2 compact layers.
2.3 Device fabrication
Mesoporous PSCs were prepared using our previous method [47–49]. Firstly, the FTO glass substrates were cleaned in ultrasonic cleaner with deionized water, acetone, isopropyl alcohol and alcohol successively for 30 min, and then cleaned by UV-ozone cleaning system for 20 min. For Mg-SnO2 based devices, a compact Mg-SnO2 layer were prepared on the FTO glass by spin-coating the SnO2 compact precursor solution at 4000 rpm for 30 s, heated at 180 °C for 10 min and annealed at 500°C for 30 min. For TiO2/Mg-SnO2 device, the TiO2 layer was prepared by spin-coating TiO2 precursor solution at 4000 rpm for 20 s, then heated at 150 °C for 10 min. The Mg-SnO2 layer was prepared by spin-coating Mg-SnO2 precursor solution at 4000 rpm for 20 s and annealed at 500 °C for 30 min. Next, the mesoporous TiO2 layer was deposited on the top of compact layer by spin-coating the TiO2 colloidal solution (TiO2 paste diluted in ethanol with a weight ratio of 1:5) at 3500 rpm for 20 s, heating it at 150 °C for 10 min, and annealing at 500 °C for 30 min. Then, the ZrO2 colloidal solution was obtained by the same approach described previously, the ZrO2 film was sintered at 500 °C for 30 min. Previous reports mentioned that ZrO2 could improve the stability of mp-PSCs [50]. After cooling to room temperature, the perovskite films was completed by one-step spin-coating at 1000 rpm for 10 s and 4000 rpm for 30 s using perovskite precursor solution (462 mg PbI2 and 159 mg MAI mixed in 600 mg DMF and 78 mg DMSO). During spin-coating for 15 s, 160 ul of methylbenzene was dropped to form a stable perovskite film, and the obtained perovskite films was hated at 100 °C for 10 min. Finally, the carbon electrodes were prepared by screen-printing low-temperature carbon paste and hated at 100 °C for 10 min.
2.4 Characterization
The photocurrent density-voltage characterizatics of the PSCs were measured under 1.5G illumination by a source meter (2400, Keithley, US) at 100 mW/cm2 intensity and a sunlight simulator (Oriel Sol3A, Newport Corporation, US). The surface morphology were measured by scanning electron microscope (SEM, JSM-IT300, JEOL, Japan) and atomic force microscope (AFM, Nanoscope IV, VEECO, US). The crystal structure of the sample was analyzed by X-ray diffraction (XRD, D8, Advance, AXS, Germany). Transmission electron microscope (TEM, JEM2100, Japan) were used to characterize the morphology and lattice spacing. Steady-state photoluminescence spectroscopy (PL, Shimadzu, Japan) spectra was obtained by exciting the perovskite samples deposited on the compact layer at 500 nm. The monochromatic incident photon-to-electron conversion efficiency spectrometer (IPCE, Newport Corporation, US) was used to acquire the external quantum efficiency (EQE) of the prepared devices. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, US) was used to analyze the element composition and chemical bonding of the as-prepared samples. The UV-vis spectrometer (UV-2600, Shimadzu, Japan) was used to test the absorption spectrum of the PSCs devices. The Electrochemical Impedance Spectroscopy (EIS) measurements were carried out on electrochemical workstation (Zahner Company, Kronach, Germany) from frequencies of 10 mHz to 4 MHz at a bias of 0.8 V under simulated AM 1.5G radiation.
3. Result and discussion
In order to better understand the properties of the prepared Mg-SnO2, we carried out some characteristic on their basic properties. Firstly, the Mg-SnO2 compact layer precursor solution was sintered in a muff furnace at 500°C, and the powder obtained was dispersed in alcohol for TEM test. According to high-resolution TEM (HRTEM) Fig. 1(a), it can be clearly observed that the nanoparticles are very uniform and the particle size is 3-5 nm. Figure 1(b) showed the enlarged image of Mg-SnO2 nanoparticales. Lattice fringes with lattice spacing of 0.336 nm and 0.243 nm can be clearly observed, corresponding to the crystal planes of SnO2 crystal (110) and MgO crystal (111), respectively, indicating that the addition of Mg2+ did not affect the crystallization of SnO2. In addition, Mg-doping SnO2 nanoparticles have high crystallinity. The higher crystallinity indicates that nanoparticles have fewer defects and higher charge mobility when used as ETL.
XPS was used to analyze the composition of elements and chemical bonds in Mg-SnO2 compact layer, and the images are shown in Fig. 2. Figure (a) shows the full XPS spectrum, including Sn, O, Mg, and C. The energy spectrum of Sn is shown in Fig. 2(b), the Sn 3d5/2 and Sn 3d3/2 dipoles centered at 485.2 eV and 493.6 eV correspond to the Sn4+ valence state in the Sn atom, respectively. Sn has some additional peaks in Fig. 2(a), the peaks at 735.08 eV, 757.08 eV, 884.08 eV, 1060.8 eV correspond to the Sn 3p3/2, Sn 3p1/2, Sn 3s and Sn Auger respectively. As shown in Fig. 2(c), the strong O 1s energy spectrum at the binding energy of 530 eV corresponds to the O2- valence state, indicating the presence of SnO2 in the sample. Figure 2(d) shows a strong Mg 1s energy spectrum with binding energy of 1302 eV corresponds to the Mg2+ valence state, confirming the presence of Mg in SnO2 ETL and Mg2+ was successfully incorporated into the lattice of SnO2. Since the ionic radius of Mg is similar to that of Sn, the addition of Mg does not affect the lattice of SnO2 [38]. The results are consistent with TEM test.
Fig. 2. (a) XPS spectra of Mg-SnO2 film, and XPS spectra of (b) Sn 3d core level (c) O 1s core level and (d) Mg 1s core level peak for the Mg-SnO2 film.
Figure 3 shows the XRD pattern of Mg-SnO2 and TiO2/Mg-SnO2 films on the glass. Strong diffraction peaks appeared in 26.6°, 33.9°, 37.9°, 51.8°, 54.8°, 62.6°and 65.6°, which correspond to the tetragonal rutile of SnO2 crystal (JCPDS Card: 41-1445). These characteristic diffraction peaks are respectively attributed to the (110), (101), (200), (211), (220), (221), (301) crystal planes of tetragonal rutile structure SnO2, it indicates that SnO2 has high crystallinity and no obvious impurity peak. The diffraction peak of MgO was not observed in the figure due to the low Mg content.
Figure 4(a) and (b) shows the SEM images of the surface morphologies of SnO2 films on FTO substrates with and without Mg doping. It can be clearly seen that both kinds of SnO2 films have high density at higher magnification. As the magnification ratio decreases, many defects appear on the pure SnO2 film, it may be non-uniform surface due to the degradation of SnO2, but there are almost no defects on the surface of the Mg-SnO2 film. It indicates that the addition of Mg at high temperature can effectively inhibit the surface defects of SnO2. It also indicates that a certain proportion of Mg doped in SnO2 can still obtain high quality SnO2 film at high temperature. Mg-SnO2 precursor solution was spin-coated on FTO substrate with or without TiO2 compact layer. The film surface morphology was characterized by SEM and AFM, and the images were shown in Fig. 5. It can be seen from SEM images that two different compact layers have high density and no obvious surface defects, and TiO2 is completely covered by SnO2 in the composite compact layer. In order to further study the surface morphology and roughness of ETL, the atomic force microscope (AFM) images of SnO2 layer and TiO2/Mg-SnO2 layer films are showed in Fig. 5(c) and (d). The surface root mean square (RMS) roughness of TiO2/Mg-SnO2 composite layer (20.3 nm) is smaller than that of Mg-SnO2 (23.9 nm) film. Lower roughness means better interface contact and fewer surface defects.
Fig. 5. SEM images and 3d-AFM images of the (a) and (c) Mg-SnO2 film, (b) and (d) TiO2/Mg-SnO2 composite film.
PSCs were fabricated with a mesoporous structure (glass/FTO/compact layer/mp-TiO2/mp-ZrO2/MAPbI3/C) by employing different compact layer. Figure 6(a) and (b) showed structural diagram and cross-sectional SEM image of a whole mesoporous PSCs used TiO2/Mg-SnO2 composite compact layer. The thickness of each layer of PSCs can be obtained from the cross-sectional SEM image. It is clear that the PSCs consists of a ∼500 nm FTO layer, a ∼40 nm TiO2/Mg-SnO2 composite compact layer, a ∼150 nm TiO2 mesoporous layer/perovskite and a ∼150 nm ZrO2 mesoporous layer/perovskite, respectively. In addition, the thickness of the carbon counter electrode is about 30 um, which is much thicker than the other layers. So it is not shown in the figure.
Fig. 6. (a) Schematic diagram of the whole PSC structure based on TiO2/Mg-SnO2 composite compact layer; (b) cross-sectional SEM image of the PSC.
Figure 7 shows the band alignment of each functional layer of device [22,43]. The conduction band minimum (CBM) of Mg-SnO2 film and TiO2 film are -4.8 eV and -4.2 eV, respectively. The valence band maximum (VBM) and CBM of MAPbI3 are -5.4 eV and -3.9 eV has been reported [39]. There is a larger electron injection force at the Mg-SnO2/perovskite interface due to the deeper CBM of the Mg-SnO2 film, provides a higher current density, and Mg-SnO2 could promote electron transportation to the TiO2 film. Therefore, the alignment between the CBM of the Mg-SnO2 and the VBM of perovskite is favorable, it could extract and block the holes transfer to TiO2.
Figure 8(a) shows the UV-vis absorption spectra of PSCs with different compact layers to compare the optical properties of the devices. It is found that the two devices have similar absorption characteristics. But the UV-vis absorption spectrum intensity of TiO2/Mg-SnO2 compact layer devices is higher than that of Mg-SnO2 compact layer devices, higher light absorption means that perovskite can absorb more light and produce more photoelectrons, thus improving the performance of PSCs. Meanwhile, the steady state photoluminescence (PL) spectra of glass/compact layer/perovskite were characterized to analyze the ability of ETL to extract electrons. The PL spectra measured by 500 nm excitation light were shown in Fig. 8(b). It can be seen from the figure that the glass/TiO2/Mg-SnO2 composite film/perovskite structure has a lower PL intensity than the glass /Mg-SnO2/ perovskite structure. The reason for this result is that the electron extraction rate at the TiO2/Mg-SnO2 layer interface is faster than that at the Mg-SnO2 layer interface, resulting in quenching effect.
Fig. 8. (a) UV-vis absorption spectra based on PSCs of different compact layers; (b) Steady-state PL spectra of perovskite deposited on Mg-SnO2 compact layer and TiO2/Mg-SnO2 composite compact layer.
The Current density-Voltage (J-V) curves of PSCs were measured with a reverse scan rate of 0.15 V/s under AM 1.5G irradiance (100 mW/cm2) at room temperature in air. The J-V curves of PSCs based on different compact layers are shown in Fig. 9. Other Photovoltaic parameters of PSCs can be obtained from J-V curves, as shown in Table 1. According to J-V curve in Fig. 9(a), PSCs based on Mg-SnO2 compact layer showed JSC of 20.14 mA/cm2, VOC of 1.01 V, PCE of 11.85% and FF of 57.9%. And the PCSs based on TiO2/Mg-SnO2 composite compact layer showed JSC of 21.49 mA/cm2, VOC of 1.02 V, PCE of 13.01% and FF of 59.4%. Compared with Mg-SnO2 compact layer devices, PSCs with TiO2/Mg-SnO2 composite compact layer have improved photovoltaic performance, which can be attributed to better interface contact and faster electron extraction rate of composite compact layer. The short-circuit current density of PSCs based on the composite compact layer is slightly higher than that of devices based on the Mg-SnO2 compact layer. J-V curves under forward and reverse scan are shown in Fig. 9(b), the PSCs based on Mg-SnO2 compact layer show obvious hysteresis, while composite compact layer devices have almost no hysteresis. In order to demonstrate the authenticity and repeatability of the results, photovoltaic parameters of 10 groups of PSCs based on different compact layers were taken as shown in the Fig. 10.
Fig. 9. (a) J-V characteristics of PSCs based on different compact layers, (b) The forward and reverse scan of PSC’s J-V curve based on TiO2/Mg-SnO2 composite compact layer.
Fig. 10. Statistical results of (a) PCE, (b) FF, (c) JSC and (d)VOC of PSCs employing the TiO2/Mg-SnO2 and Mg-SnO2 as compact ETL.
IPCE spectra and corresponding integral current densities of PSCs based on different compact layers are shown in Fig. 11. The IPCE of TiO2/Mg-SnO2 composite compact layer devices is significantly improved compared with that of single Mg-SnO2 compact layer devices. In addition, IPCE is above 80% in most wavelength ranges, which also indicates that TiO2/Mg-SnO2 composite compact layer devices have high light absorption and carrier mobility. Meanwhile, the short-circuit current density obtained by integrating the IPCE spectrum for Mg-SnO2 compact layer based devices and TiO2/Mg-SnO2 composite compact layer based devices is 20.57 mA/cm2 and 21.43 mA/cm2 respectively, which is consistent with the short-circuit current density characterized by the J-V curve.
Fig. 11. IPCE spectra and the corresponding integrated currents of the best Mg-SnO2 and composite based devices.
The EIS analyses were performed to illustrate charge transport and recombination in the device. As shown in Fig. 12, there were two semicircles in the Nyquist plots, which correspond to the high-frequency region and the low-frequency region, respectively. The high-frequency region illustrates the performance of the C electrode/perovskite interface, which is related to the transmission resistance (Rtr). And the low-frequency region represented charge transfer and recombination at the ETL/perovskite interface related to the recombination resistance (Rrec) [50]. The data fitted from the Nyquist plots are shown in Table 2. The series resistance (RS) is inversely proportional to the FF [47]. The lower Rtr indicates that the device based on TiO2/Mg-SnO2 compact layer has faster electron extraction capability. The perfect interface contact between TiO2 and FTO reduced carrier recombination and resulted in higher Rrec.
Fig. 12. (a) Nyquist plots of the PSCs based on different compact layer and (b) the equivalent circuit employed to fit the plots.
Table 2. Summary of EIS parameters of PSCs based on different compact layer.
4. Conclusion
In this study, we effectively improved the quality of SnO2 films prepared at high temperatures by adding a certain proportion of Mg to the of SnO2 precursor solution. Then it was stacked with TiO2 to form a composite compact layer and for high performance mesoporous PSCs. Due to the high electron mobility of SnO2 and the good surface coverage of TiO2, the TiO2/Mg-SnO2 composite compact layer has a high electron extraction capacity and good interface contact. As a result, the photovoltaic performance parameters of PSCs based on composite compact layer are higher than those of devices based on single Mg-SnO2 compact layer. And it achieved a champion PCE of 13.01%, FF of 59.4%, JSC of 21.49 mA/cm2 and VOC of 1.02 V. The hysteresis effect was hardly observed in the forward and reverse scan tests. The study also proves that the TiO2/SnO2 composite compact layer is a promising ETL and suitable for mesoporous structure PSCs.
Disclosures
The authors declare no conflict of interest.
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