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Article

MDACl2-Modified SnO2 Film for Efficient Planar Perovskite Solar Cells

by
Yaodong Xiao
,
Xiangqian Cui
,
Boyuan Xiang
,
Yanping Chen
,
Chaoyue Zhao
,
Lihong Wang
,
Chuqun Yang
,
Guangye Zhang
,
Chen Xie
,
Yulai Han
,
Mingxia Qiu
*,
Shunpu Li
* and
Peng You
*
College of New Materials and New Energies, Shenzhen Technology University, Shenzhen 518118, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(6), 2668; https://doi.org/10.3390/molecules28062668
Submission received: 26 February 2023 / Revised: 13 March 2023 / Accepted: 14 March 2023 / Published: 15 March 2023
(This article belongs to the Section Materials Chemistry)
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Abstract

:
The electron transport layer (ETL) with excellent charge extraction and transport ability is one of the key components of high-performance perovskite solar cells (PSCs). SnO2 has been considered as a more promising ETL for the future commercialization of PSCs due to its excellent photoelectric properties and easy processing. Herein, we propose a facile and effective ETL modification strategy based on the incorporation of methylenediammonium dichloride (MDACl2) into the SnO2 precursor colloidal solution. The effects of MDACl2 incorporation on charge transport, defect passivation, perovskite crystallization, and PSC performance are systematically investigated. First, the surface defects of the SnO2 film are effectively passivated, resulting in the increased conductivity of the SnO2 film, which is conducive to electron extraction and transport. Second, the MDACl2 modification contributes to the formation of high-quality perovskite films with improved crystallinity and reduced defect density. Furthermore, a more suitable energy level alignment is achieved at the ETL/perovskite interface, which facilitates the charge transport due to the lower energy barrier. Consequently, the MDACl2-modified PSCs exhibit a champion efficiency of 22.30% compared with 19.62% of the control device, and the device stability is also significantly improved.

1. Introduction

In recent years, organic metal halide perovskite solar cells (PSCs) have shown great commercial prospects because of their high efficiency and low-cost processing [1,2,3,4,5,6]. Perovskite materials have unique optoelectronic properties, such as long carrier diffusion length and mobility, high light absorption coefficient, small exciton binding energy, and adjustable bandgap [7,8,9,10,11]. Therefore, perovskite materials have significant advantages as an active layer material of solar cells, compared with other traditional light-absorbing materials, such as silicon and GaN [12,13,14,15,16]. Since 2009, the power conversion efficiencies (PCEs) of PSCs have increased greatly from 3.8% to 25.7% [17,18,19,20,21]. The fast development of PSCs is inseparable from the device structure improvement, especially the optimization of the electron transport layer (ETL) [22,23,24]. ETLs in PSCs not only work as electron transport and hole-blocking layers but also their morphologies, structures, and surface properties will also affect the crystal growth conditions of the adjacent perovskite layer [25,26]. Therefore, ETL modification has become an effective way to further improve the performance of PSCs [27,28].
Titanium dioxide (TiO2) and Tin oxide (SnO2) films are widely used ETLs in high-efficiency PSCs. Both TiO2 and SnO2 have their distinctive advantages as ETLs, and device efficiencies have exceeded 20% in recent studies, thus it is worth considering which material is more appropriate [29,30,31,32]. Compared with the traditional TiO2 ETL, SnO2 has outstanding advantages, such as high electron mobility, suitable energy level alignment, good chemical stability, and a simple preparation process [33,34]. SnO2 films can be prepared at low temperatures, which is compatible with the preparation processes of flexible optoelectronic devices [35,36]. Therefore, the SnO2 ETL is considered more suitable for the future commercialization of PSCs. However, the SnO2 films tend to have surface or bulk defects caused by oxygen vacancies [37], which is not conducive to carrier transport and collection. Recent research has indicated that SnO2 modification by physical or chemical methods is an effective way to achieve simultaneous improvements in SnO2 ETL, perovskite layer, and the ETL/perovskite interface, which could significantly enhance the device performance [38,39]. Generally, the SnO2 modification strategies can be divided into two categories, surface modification, and bulk blending. The surface modification involves directly coating the modified material on the SnO2 surface as an interfacial buffer layer between the SnO2 and the perovskite film, while bulk blending indicates that the modified material is incorporated into the SnO2 precursor solution and is blended with the SnO2 to form a hybridized film [40,41,42]. Until now, various metal cations such as K+, Mg2+, Al3+, and Ga3+ have been utilized for the SnO2 bulk blend modification, and the incorporated external elements can modulate the SnO2 self-doping defects, resulting in enhanced n-type conduction of the films [35,43,44]. In addition, the halide ions (F−, Cl−, and I−) have also been widely used for the bulk blending modification of the SnO2 films. This strategy based on halide incorporation improves the SnO2 film absorbance and reduces the defect concentration in the SnO2/perovskite interface region [37,45,46,47]. For example, in 2019, Liu et al. dispersed NH4Cl in SnO2 colloidal solution for the first time [48]. The introduced NH4+ and Cl− ions help enhance the charge transport capability of the ETL and inhibit the non-radiative recombination at the ETL/perovskite interface. As a result, the efficiency of the planar PSCs based on NH4Cl-doped SnO2 ETL was improved to 21.38%, compared to the 18.71% efficiency of the control device. In 2021, Lin et al. reported a simple and effective method for electron transport layer modification by introducing methylamine hydrochloride (MACl) and formamidine hydrochloride (FACl) into the SnO2 colloidal solution [49]. These dopants promote the nucleation and growth of the perovskite crystals on the ETL, leading to the improved quality of the perovskite films. Meanwhile, the energy level arrangement between the ETL and perovskite layers is adjusted, and the carrier trap density is reduced. Compared with the undoped devices, the efficiency of MACl/FACl-modified devices increased to 21.87% and 21.72%, respectively, which proves that the ETL precursor solution engineering strategy is an effective way to obtain high-performance PSCs. The bulk blending modification strategy modulated SnO2 properties while passivating the SnO2/perovskite interfacial defects, therefore we attempted to find a novel material for SnO2 modification. Methylenediammonium dichloride (MDACl2) was reported to have a stabilizing effect on the α-phase FAPbI3, and the corresponding devices were prepared with PCEs reaching 23.7% and 25.5% [50,51]. Although MDACl2 has been demonstrated to be significantly effective in optimizing the performance of perovskite layers, their applications in SnO2 ETL have never been reported previous in the literature.
In this study, we introduced a type of small molecule material MDAC12 into the SnO2 precursor solution for the first time and systematically explored the effect of MDAC12 incorporation on the SnO2 film and PSCs. The SnO2 film modification strategy not only passivates surface defects on the film surface but also reduces the energy barrier at the SnO2/perovskite interface. Meanwhile, the improved SnO2 surface property is also beneficial for the nucleation and growth of the perovskite grains, resulting in enhanced crystallinity of the perovskite films, which reduces the carrier defect density and suppresses the non-radiative recombination. Consequently, the PCE of the MDAC12-modified device was increased to 22.30% compared with the PCE of 19.62% for the control device. This study demonstrates the effectiveness of the ETL precursor solution modification strategy for performance enhancement of the PSCs.

2. Results and Discussion

2.1. Device Performance Distribution

A dense electron transport layer was deposited on the FTO substrate based on the SnO2 colloidal precursor solution containing different concentrations of MDACl2. The corresponding PSCs with the device structure shown in Figure 1a were prepared. The distributions of photovoltaic parameters including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and PCE of the PSCs prepared under conditions of different MDACl2 concentrations are shown in Figure 1b–e. The statistic photovoltaic parameters are listed in Table 1. The PSCs with undoped SnO2 as ETL (control) show an average PCE of 18.92%, VOC of 1.075 V, JSC of 23.35 mA/cm2, and FF of 75.39%. In contrast, the PSCs with MDACl2-modified SnO2 as ETL show enhanced device performance than that of the control devices. The best device performance was achieved when 4 mM MDACl2 was added to the SnO2 precursor solution, with an average PCE of 21.53%, VOC of 1.105 V, JSC of 24.45 mA/cm2, and FF of 79.65%. The underlying working mechanism of the MDACl2 additive is discussed in the following part.
We have compared the performances of PSCs prepared by using MDACl2-incorporated SnO2 precursor solution and direct spin-coating MDACl2 solution (4 mM) on top of the SnO2 film, as shown in Table S1. Notably, the introduction of MDACl2 molecules in both conditions all contribute to improved device performances. However, the average PCE of PSCs prepared based on MDACl2-incorporated SnO2 precursor solution (average PCE: 20.41%) is much lower than that of the devices prepared by direct spin-coating MDACl2 buffer layer on top of the SnO2 film (average PCE: 21.53%). Therefore, we believe that the incorporated MDACl2 molecules not only locate on the SnO2 film surface but also modify the interconnection parts of the SnO2 nanoparticles in the SnO2 thin film [52]. Furthermore, the concentration of MDACl2 we introduced into the SnO2 solution here was very low (4 mM), and it was insufficient to form a continuous MDACl2 film on top of the SnO2 film, which can be confirmed by the AFM images shown in Figure S1. To better understand the reasons for the efficiency enhancement of the MDACl2-modified PSCs, we further investigated the ETLs for more details.

2.2. ETL Characterizations

Based on the device efficiency distributions, we further compared the difference between the unmodified SnO2 and MDACl2-modified SnO2 films (4 mM). For convenience, we define them as SnO2 and SnO2-MDACl2 films, respectively. The surface morphologies of the ETL films were characterized by an atomic force microscope (AFM). As shown in Figure S1, the roughness of the two films is quite similar, and the average roughness of the SnO2-MDACl2 film (8.1 nm) is slightly lower than that of the control SnO2 film (8.8 nm). The transmittance spectra of the ETL films (see Figure S2a) show that MDACl2 modification has little effect on the optical transmittance of the SnO2 film. The effect of MDACl2 modification on the energy band arrangement of the ETLs was evaluated by ultraviolet photoelectron spectroscopy (UPS) and UV-visible absorption spectra (UV-vis). As shown in Figure 2a,b, the Fermi levels (EF) of the SnO2 and SnO2-MDACl2 films are −5.50 eV and −5.66 eV, respectively, and the valence band maximum (VBM) are −8.29 eV and −8.13 eV, respectively [49]. According to the absorption spectra of the ETL films (see Figure S2b,c), the band gaps (Eg) of the SnO2 and SnO2-MDACl2 films are both 3.90 eV. Therefore, the conduction band minimum (CBM) of the SnO2 and SnO2-MDACl2 films can be calculated to be −4.39 eV and −4.23 eV (Table S2), respectively [53,54]. The energy level arrangement of the two ETL films and the perovskite film is shown in Figure 2c. Compared with the SnO2 film, the CBM of the SnO2-MDACl2 film is closer to that of the perovskite film, which is more conducive to the transport and extraction of photogenerated charges at the ETL/perovskite interface.
The incorporation effects of MDACl2 in the ETLs were further evaluated by X-ray photoelectron spectroscopy (XPS). The XPS full spectra shown in Figure S3 confirm the existence of Sn and O elements in the SnO2 and SnO2-MDACl2 films. As shown in the high-resolution Cl 2p spectra (Figure 2d), a characteristic peak (198.3 eV) was detected in the SnO2-MDACl2 film, while the peak was not found in the SnO2 control film, indicating that the MDACl2 material was successfully incorporated in the SnO2-MDACl2 film. As shown in Figure 2e, the Sn 3d characteristic peak (486.6 eV) of the SnO2-MDACl2 film is shifted by 0.2 eV to the high binding energy direction compared to the peak (486.4 eV) of the control SnO2 film [55], indicating that the chemical environment of the Sn element has changed. Figure S4a demonstrates the C 1s peaks, and the C−C/C−H components are positioned at 284.8 eV. In addition, the C−O and O−C=O bonds could be attributed to external adventitious carbon contamination and SnO2 surface defects [56,57]. From Figure S4b, the O 1s characteristic peak splits into two parts corresponding to O2− and OH−, and the proportion of OH− is 34.88% for the SnO2 film and 31.35% for the SnO2-MDACl2 film [58,59], respectively. The slightly reduced OH− proportion can be attributed to the incorporation of MDACl2 molecules in the SnO2 film. In addition, the K 2p peak is observed at 292.7 eV (Figure S4c), which is attributed to the presence of K+ (as a stabilizer) in the commercial SnO2 colloidal solution. Therefore, the incorporation of MDACl2 as an additive in the SnO2 precursor solution was demonstrated to be an effective strategy for enhancing film conductivity and passivating surface defects of the SnO2 films. The modification of the surface, as well as the inner part of the SnO2 film by MDACl2 molecules, contribute together to the improved film properties of the SnO2 film.
As shown in Figure S5, the contact angles of water droplets on the SnO2 and SnO2-MDACl2 substrates were measured to be 41.25° and 46.90°, respectively. The higher contact angle of the SnO2-MDACl2 film is consistent with the decreased concentration of the hydroxyl groups on the SnO2-MDACl2 film surface. The wettability of the substrate after MDACl2 treatment is lower, which is beneficial to inhibit the heterogeneous nucleation during the following perovskite film deposition process and reduce the number of nucleation centers [61,62,63,64]. Meanwhile, the increase in contact angle indicates a decrease in surface energy, which reduces the energy barrier for crystal growth and promotes the vertical growth of the perovskite grains. Therefore, the MDACl2 modification of the ETLs will contribute to the formation of high-quality perovskite films, which will be further discussed in the following part.
The electrical conductivity changes of the ETL films can be obtained by measuring the current-voltage (I–V) characteristics of the FTO/SnO2/Ag devices, as shown in Figure 2f. The conductivity can be calculated according to Formula (1) [65,66]:
I = σ 0 A d − 1 V
where I is current, σ0 is the ETL film conductivity, A is the active area, d is the thickness of the ETL film, and V is the voltage. Regarding the SnO2 film thickness, we obtained the cross-sectional SEM image of the FTO/SnO2-MDACl2/Perovskite sample (Figure S6) and estimated the SnO2 film thickness to be around 40 nm. The calculated conductivity results are shown in Table S3. Incorporating MDACl2 into the SnO2 ETL can enhance the film conductivity at different levels. When the MDAC12 concentration is 4 mM, the maximum electrical conductivity of the ETL is calculated to be 5.27 × 10−3 mS/cm, which is 1.93 times that of the control film (2.73 × 10−3 mS/cm). The increased ETL conductivity can accelerate electron extraction and transportation.

2.3. Perovskite Film Characterizations

The surface morphologies of the perovskite films prepared on the SnO2 and SnO2-MDACl2 films were characterized by scanning electron microscopy (SEM), as shown in Figure 3a,b. Notably, the average grain sizes of the perovskite films grown on SnO2 and SnO2-MDACl2 ETLs are around 700 nm and 1000 nm, respectively. The larger perovskite grain sizes represent a decrease in grain boundary concentration, thereby reducing the non-radiative carrier recombination caused by grain boundary defects [67], which is beneficial to carrier transport. Then we further carried out XRD characterizations of the perovskite films prepared on the SnO2 and SnO2-MDACl2 films, as shown in Figure 3c. Both perovskite films have two strong diffraction peaks, corresponding to the (110) and (220) lattice planes, respectively. The full width at half maximum (FWHM) values of the perovskite (110) XRD peaks based on the SnO2 and SnO2-MDACl2 films are 0.086 and 0.077 (degree), respectively. The smaller FWHM value of the SnO2-MDACl2/perovskite film indicates the increased average perovskite grain size, which is consistent with the SEM results. Notably, the diffraction peaks of the perovskite film on SnO2-MDACl2 ETL are stronger, which indicates that the perovskite crystallinity is improved [68]. Furthermore, the UV-Visible absorption spectra of the perovskite films prepared on the SnO2 and SnO2-MDACl2 films were obtained (see Figure 3d). In the wavelength range of 500–750 nm, the absorption intensity of the perovskite film prepared on the SnO2-MDACl2 ETL is significantly higher than that of the control film, indicating the enhanced light absorption properties of the SnO2-MDACl2/perovskite films. Moreover, the energy band edge of the perovskite film calculated from the Tauc plots (inset of Figure 3d) were both 1.56 eV, which means that the perovskite film prepared on the SnO2-MDACl2 ETL has a comparable band gap with that of the control film [69]. Therefore, the MDACl2 modification contributes to the formation of high-quality perovskite films with increased crystallinity and light absorption properties.
Steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) measurements were utilized to further study the charge dynamics at the ETL/perovskite interface. As shown in Figure 4a, the luminescence intensity of the SnO2-MDACl2/perovskite film is significantly reduced compared to the undoped SnO2 ETL, indicating that MDACl2 incorporating is beneficial to charge extraction and transport at the SnO2/perovskite interface [37,70,71]. This should be ascribed to the increased conductivity and the passivated surface defects of the SnO2 film. This conclusion is further verified by the TRPL characterization results of the ETL/perovskite films (Figure 4b). Notably, the PL decay rate of the SnO2-MDACl2/perovskite film is faster than that of the control sample. The decay time constants were calculated by using the double exponential function fitting, and the results are summarized in Table S4. The average decay time constants (τave) of the SnO2/perovskite and SnO2-MDACl2/perovskite are 91.27 ns and 55.64 ns, respectively, indicating that the SnO2-MDACl2 ETL has higher electron extraction ability [52,72]. The stronger PL quenching implies enhanced ETL charge extraction, whereas the simultaneously enhanced non-radiative recombination and lower quasi-Fermi energy level splitting are detrimental to the device VOC [73,74,75,76]. Therefore, further analysis is required for the device charge transfer performance. Based on the space-charge-limited current (SCLC) technique, we fabricated the electronic-only devices and evaluated the defect state density of the perovskite films prepared on the SnO2 and SnO2-MDACl2 ETLs. Figure 4c,d illustrates the dark I–V curves of the electron-only devices with a device structure of FTO/ETL/Perovskite/PCBM/Ag. The trap-filling limit voltage (VTFL) can be obtained by fitting the dark I–V curve. The VTFL value of the electron-only device based on SnO2-MDACl2 ETL (0.308 V) is lower than that of the SnO2 ETL (0.438 V). The trap densities of the perovskite films can be calculated according to Formula (2) [44,77,78]:
n t r a p = 2 ε ε 0 V T F L e L 2  
where ε represents the dielectric constant of perovskite (ε = 31.18), ε0 is the vacuum permittivity, e is the elementary charge, and L is the thickness of the perovskite film (L = 550 nm). After calculation, the trap density of the perovskite film decreased from 4.99 × 1015 cm−3 to 3.51 × 1015 cm−3 after MDACl2 modification, which is attributed to the improved perovskite crystallinity [58,63].

2.4. Device Characterizations

Figure 5a shows the J–V curves and photovoltaic parameters of the best-performing PSCs prepared based on the unmodified SnO2 (control) and SnO2-MDACl2 ETLs. The best control device shows a champion efficiency of 19.62%, with VOC of 1.077 V, FF of 77.06%, and JSC of 23.64 mA/cm2, while the PSCs prepared based on the SnO2-MDACl2 ETL demonstrate a champion efficiency of 22.30%, with VOC of 1.111 V, FF of 81.40% and JSC of 24.65 mA/cm2. The significant improvement of the device efficiency is mainly attributed to the enhancement of FF and VOC, which is mainly attributed to the film quality improvement of SnO2 and perovskite. Figure 5b shows the external quantum efficiency (EQE) and the integrated current density of the PSCs. The SnO2-MDACl2 device exhibits higher EQE values with an integrated current density of 24.06 mA/cm2, compared with that of the control device (23.11 mA/cm2), which is related to the enhanced charge extraction and transport ability of the MDACl2-based SnO2 film [79]. In addition, MDACl2 incorporation also reduces the J-V hysteresis of the PSCs (see Figure S7). The PSC prepared on the SnO2-MDACl2 ETL demonstrates high PCEs of 22.30% (reverse scan) and 22.28% (forward scan), respectively, showing almost no hysteresis, while PCEs of 19.62% (reverse scan) and 19.36% (forward scan) were obtained for the control device.
The stable power output measurements of the PSCs were performed at the maximum power points, as shown in Figure 5c. The PSC based on the SnO2-MDACl2 ETL achieves a stable PCE of 22.07%, while the control device only gets a 19.37% efficiency. Notably, the time to reach the stable PCE is much shorter for the target PSCs than that of the control device, meaning that the MDACl2-modified PSCs have smaller capacitive currents, which is consistent with the reduced J–V hysteresis of the PSCs [80,81]. To further evaluate the effects of MDACl2 modification on the long-term stability of the device, we compared the performance of the unencapsulated devices stored in a 35% humidity environment, as shown in Figure 5d. After aging for 600 h, the MDACl2-modified device maintains 93.1% of its initial efficiency, whereas the PCE of the control device decays to 81.3% of its initial efficiency. These results indicate that MDACl2 incorporation into the SnO2 ETL can greatly improve long-time device stability.
In addition, to better understand the reasons for the improved device performance after MDACl2 modification, we carried out light intensity dependence characterizations of the PSCs (see Figure 6a,b), to estimate the charge recombination kinetics inside the devices. Figure 6a shows the relationship between JSC and light intensity. The PSCs based on the SnO2 and SnO2-MDACl2 ETLs exhibit very similar fitting slopes (α values), indicating that the devices have similar bimolecular recombination conditions [82,83]. Figure 6b shows the dependence of VOC on light intensity. Notably, the derived ideal factor of the MDACl2-modified PSC (1.36) is smaller than that of the control device (1.53), which indicates the reduction of SRH monomolecular recombination in the MDACl2-modified PSC [48,84].
The dark-state J–V curves of the PSCs are shown in Figure 6c. The decreased leakage current of the MDACl2-modified PSC indicates the reduced defect density and defect-assisted non-radiative recombination after MDACl2 modification [85]. We further evaluated the charge transport performance by electrochemical impedance spectroscopy (EIS) measurements in dark conditions at a bias voltage of 0.8 V. Figure 6d shows the Nyquist plots, from which the series resistance (Rs) and recombination resistance (Rrec) are derived, as listed in Table S5. The Rs value is decreased from 37.16 Ω (control device) to 30.78 Ω (MDACl2-modified PSC), confirming the improved charge transport in the optimized PSC. Moreover, the MDACl2-modified PSC demonstrates an increased Rrec (337.3 Ω) compared to that of the control device (217.3 Ω), which indicates the suppressed charge recombination inside the MDACl2-modified PSC [24,86].

3. Materials and Methods

3.1. Materials

Formamidinium iodide (FAI), methylammonium bromide (MABr), methylammonium chloride (MACl), and Butylammonium Iodide (BAI) were ordered from Greatcell Solar Ltd. PbI2 (99.999%) and SnO2 (15 wt % colloidal dispersion tin (IV) oxide) was purchased from Alfa Aesar. Cesium iodide (CsI) (99.999%), dimethylformamide (DMF), dimethylsulfoxide (DMSO), isopropanol, chlorobenzene (CB), ethyl acetate, 4-tert-Butylpyridine (tBP), bis(trifluoromethylsulphonyl)imide lithium salt (Li-TFSI), acetonitrile, and methylenediamine dihydrochloride (MDACl2) were all bought from Sigma-Aldrich Ltd. (Saint Louis, MO, USA) PbBr2 (99.999%) was purchased from Xi’an Polymer Light Technology Corp. (Xi’an, China) Spiro-OMeTAD was purchased from Lumtec Ltd. (Taiwan, China) All the chemicals and reagents were used as received without further purification.

3.2. Device Fabrication

PSCs were fabricated through a one-step solution process with a formal structure of glass/FTO/SnO2/perovskite/Spiro-OMeTAD/Au. The FTO substrates were scrubbed with soap water and ultrasonically cleaned sequentially in deionized water, acetone, and isopropanol, before drying with nitrogen flow. Prior to use, the FTO substrates were treated with UV-ozone for 15 min. The commercial SnO2 colloid solution was diluted to 2.5 w% with deionized water and mixed with different concentrations of MDACl2. Subsequently, 80 μL mixed SnO2 solution was dropped on the FTO substrates and spin-coated at 4000 rpm for 30 s. After sintering at 100 °C for 30 min, the film was treated under UV-ozone for 15 min and transferred to the glove box immediately. The Cs0.05FA0.89MA0.06Pb(I0.94Br0.06)3 perovskite precursor solution was prepared by dissolving 726 mg PbI2, 258 mg FAI, 11.2 mg MABr, 36.7 mg PbBr2, 19.5 mg CsI and 35 mg MACl in 1 mL mixed solvent (DMF: DMSO = 8:1). The solution was stirred overnight before use. The 60 μL Cs0.05FA0.89MA0.06Pb(I0.94Br0.06)3 perovskite precursor solution was dropped on the SnO2 layer at 1000 rpm for 5 s and 5000 rpm for 25 s. At the 20th second after the start of the spin coating process, 300 μL ethyl acetate as antisolvent was dripped onto the perovskite film. Then the perovskite film was annealed at 150 °C for 10 min on a hot plate immediately. After cooling down to room temperature, 50 μL of BAI solution (1 mg mL−1 in IPA) was spin-coated onto the perovskite film followed by 100 °C annealing on the hotplate for 10 min. Subsequently, 75 mg of Spiro-OMeTAD was dissolved in 1 mL CB. Then the solution was doped with 28.5 μL tBP and 17.5 μL Li-TFSI (520 mg/mL in acetonitrile) to prepare the hole transporting layer (HTL) solution. Afterwards, 80 μL HTL solution was spin-coated on the perovskite films at 4000 rpm for 30 s. Finally, an 80 nm Au electrode was deposited on the HTL by thermally evaporating under the condition of 3 × 10−5 Pa, and the active area of all devices is 0.06 cm2 defined by a metal mask.

3.3. Characterizations

A field emission scanning electron microscope (GeminiSEM 300, Carl Zeiss Microscopy Ltd., Jena, Germany) was used to obtain the morphology of the perovskite films based on SnO2 and SnO2-MDACl2 ETLs. The surface morphology of SnO2 and SnO2-MDACl2 films was measured with an atomic force microscope (AFM) (Cypher S, Oxford Instruments Asylum Research, Inc., Oxford, UK). X-ray diffraction system (SmartLab XRD, Rigaku, Tokyo, Japan) was performed to characterize the crystallinity and crystal structure of Cs0.05FA0.89MA0.06Pb(I0.94Br0.06)3 perovskite films. The UV−Vis absorption spectra and transmission spectra were measured with a UV−Vis spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan). Both the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) were performed on a steady-state transient fluorescence lifetime test system (FluoTime 300, PicoQuant, Berlin, Germany). The perovskite films for all PL measurements were prepared on FTO/SnO2 substrates and used a 405 nm laser as the excitation light source (illuminated from the perovskite film side). The Ultraviolet photoemission spectroscopy (UPS) and the X-ray photoemission spectroscopy (XPS) analysis of SnO2 and SnO2-MDACl2 films on glass substrates using an XPS Escalab Xi+ (Thermo Fisher Scientific (China) Co., Ltd., Shanghai, China). Curve fitting was performed using Thermo Avantage software. The XPS curve was calibrated for the C 1s peak at 284.8 eV. The contact angles of the samples were measured by the contact angle measuring instrument (Fed-A, Dongguan Furunde Intelligent Equipment Co., Ltd., Dongguan, China).

3.4. Device Measurements

The current density–voltage (J–V) curves of the PSCs were measured under standard 1-sun light illumination of 100 mW/cm2 (Enlitech solar simulator, equipped with an AM 1.5 filter). The light intensity was calibrated before the J–V test by using a standard reference silicon cell (Enlitech, Taiwan, China). The J–V measurements were conducted by forward scan (from 0 to 1.2 V) and reverse scan (from 1.2 to 0 V), with a scan rate of 20 mV/s. During the J–V measurement of devices, we used a metal aperture mask to ensure the reliability of the measurement. EQE was measured using an Enlitech QE-S EQE system (Taiwan, China) equipped with a standard Si detector and monochromatic light (Enlitech 300 W lamp source). The measurements were performed in AC mode at room temperature. The EQE response from a wavelength of 300 nm to 900 nm was recorded by a computer. The stabilized power output (SPO) was measured at the maximum power point of the PSCs under simulated AM 1.5G, 100 mW/cm2 solar irradiation by Enlitech solar simulator with an AM 1.5G filter. The Electrochemical Impedance Spectroscopy (EIS) measurements were performed by Modulab XM electrochemical workstation (Advanced Measurement Technology Inc., Oak Ridge, TN, USA).

4. Conclusions

In summary, we have proposed an effective and convenient precursor solution doping strategy for ETL modification. After MDACl2 modification, the surface defects of the SnO2 film are effectively passivated and the film conductivity is also improved, which is beneficial to the charge extraction and transport at the ETL/perovskite interface. Meanwhile, the MDACl2 modification contributes to the formation of high-quality perovskite films with high crystallinity and low defect density. Moreover, better energy level alignment is achieved at the ETL/perovskite interface, which can facilitate the charge transport due to the lower energy barrier. As a result, the champion PSC based on the MDACl2-modified SnO2 film demonstrates a PCE of 22.30%, with negligible hysteresis and enhanced device stability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28062668/s1, Figure S1. (a,b) AFM images of SnO2 and SnO2-MDACl2 films grown on glass substrates. Figure S2. (a) Transmission spectra (b) absorption spectra and (c) Tauc plots for the SnO2 and SnO2-MDACl2 films. Figure S3. Full XPS spectra of the SnO2, SnO2-MDACl2 films. Figure S4. High-resolution (a) C 1s, (b) O 1s, and (c) K 2p XPS spectra of the SnO2 and SnO2-MDACl2 films. Figure S5. The contact angle of water droplets on the (a) SnO2 and (b) SnO2-MDACl2 films. Figure S6. Cross-sectional SEM image of FTO/ SnO2-MDACl2/perovskite sample. Figure S7. J–V curves of the PSCs based on SnO2 and SnO2-MDACl2 ETLs under reverse scan and forward scan. Table S1. Photovoltaic parameters of PSCs prepared by using MDACl2-incorporated SnO2 precursor solution or directly spin-coating MDACl2 solution (4mM) on top of the SnO2 film (statistical data of 20 devices for each condition). Table S2. Band gaps (Eg), secondary−electron cutoff (Ecutoff), fermi level (EF), valence band (EVB), and conduction band (ECB) for SnO2 film and SnO2-MDACl2 film. Table S3. Conductivities of FTO/SnO2-MDACl2/Ag with different doping concentrations. Table S4. Fitting parameters of the TRPL spectra for Glass/FTO/SnO2/perovskite films. The average time constant τave was calculated according to the equation: τave = (A1τ12 + A2τ22) /(A1τ1 + A2τ2). Table S5. Fitting values of Rs and Rrec of the corresponding devices in a dark state.

Author Contributions

Conceptualization, P.Y. and Y.X.; methodology, Y.X., P.Y., X.C., B.X., Y.C., C.Z., L.W., C.Y. and Y.H.; software, Y.X.; validation, G.Z. and C.X.; investigation, P.Y. and Y.X.; data curation, Y.X.; writing—original draft preparation, Y.X.; writing—review and editing, P.Y., S.L. and M.Q.; visualization, Y.X.; supervision, P.Y. and S.L.; project administration, P.Y.; funding acquisition, P.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 62104159, 12274303), the Natural Science Foundation of Top Talent of SZTU (Grant No. GDRC202104), Education Department of Guangdong Province (Grant No. 2021KCXTD045), Guangdong Basic and Applied Basic Research Foundation (Grant No. 2019A1515011673).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are grateful for the testing of XRD, XPS, UPS, SEM and AFM by the Analysis and Testing Center of Shenzhen Technology University.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the compounds are not available from the authors.

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Figure 1. (a) Schematic diagram of the glass/FTO/SnO2/Perovskite/Spiro/Au PSC structure. (b) VOC, (c) JSC, (d) FF, and (e) PCE distributions of PSCs prepared under conditions of different MDACl2 concentrations. The data were collected from 20 devices for each condition.
Figure 1. (a) Schematic diagram of the glass/FTO/SnO2/Perovskite/Spiro/Au PSC structure. (b) VOC, (c) JSC, (d) FF, and (e) PCE distributions of PSCs prepared under conditions of different MDACl2 concentrations. The data were collected from 20 devices for each condition.
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Figure 2. (a,b) UPS spectra of the SnO2, SnO2-MDACl2 films. (c) Energy band diagram of the SnO2, SnO2-MDACl2, and perovskite films. The ETL energy level alignments are based on the UPS and UV−vis measurements, and the perovskite energy level alignments are based on the literature listed [60]. (d,e) High-resolution Cl 2p and Sn 3d XPS spectra of the SnO2 and SnO2-MDACl2 films. (f) Conductivity measurement curves of the FTO/SnO2/Au sample with different MDACl2 doping concentrations.
Figure 2. (a,b) UPS spectra of the SnO2, SnO2-MDACl2 films. (c) Energy band diagram of the SnO2, SnO2-MDACl2, and perovskite films. The ETL energy level alignments are based on the UPS and UV−vis measurements, and the perovskite energy level alignments are based on the literature listed [60]. (d,e) High-resolution Cl 2p and Sn 3d XPS spectra of the SnO2 and SnO2-MDACl2 films. (f) Conductivity measurement curves of the FTO/SnO2/Au sample with different MDACl2 doping concentrations.
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Figure 3. (a,b) SEM images of perovskite films grown on SnO2 and SnO2-MDACl2 ETLs. Insets show the statistical diagrams of the grain size distributions based on the SEM images. (c) XRD patterns of perovskite films grown on FTO/SnO2 and FTO/SnO2-MDACl2 substrates. (d) UV-visible absorption spectra and Tauc plots of the perovskite films grown on SnO2 and SnO2-MDACl2 ETLs.
Figure 3. (a,b) SEM images of perovskite films grown on SnO2 and SnO2-MDACl2 ETLs. Insets show the statistical diagrams of the grain size distributions based on the SEM images. (c) XRD patterns of perovskite films grown on FTO/SnO2 and FTO/SnO2-MDACl2 substrates. (d) UV-visible absorption spectra and Tauc plots of the perovskite films grown on SnO2 and SnO2-MDACl2 ETLs.
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Figure 4. (a,b) Steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) spectra of perovskite films deposited on the SnO2 and SnO2-MDACl2 ETLs. (c,d) Dark I–V curves of the electron-only devices. Insets show the schematic device structures.
Figure 4. (a,b) Steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) spectra of perovskite films deposited on the SnO2 and SnO2-MDACl2 ETLs. (c,d) Dark I–V curves of the electron-only devices. Insets show the schematic device structures.
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Figure 5. (a) J–V curves of the champion devices based on SnO2 and SnO2-MDACl2 substrates under reverse scan. (b) EQE spectra and the integrated current density of the PSCs. (c) Stable power output curves of the PSCs measured at the maximum power point under AM 1.5G illumination in glovebox. (d) Long-term stability performance of the unencapsulated PSCs under conditions of RH: 35%, T: 25 °C.
Figure 5. (a) J–V curves of the champion devices based on SnO2 and SnO2-MDACl2 substrates under reverse scan. (b) EQE spectra and the integrated current density of the PSCs. (c) Stable power output curves of the PSCs measured at the maximum power point under AM 1.5G illumination in glovebox. (d) Long-term stability performance of the unencapsulated PSCs under conditions of RH: 35%, T: 25 °C.
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Figure 6. (a,b) Light intensity dependence characterizations of JSC and VOC for PSCs based on the SnO2 and SnO2-MDACl2 ETLs. (c) Dark-state J–V curves of the PSCs. (d) Nyquist plots for the PSCs (measured in dark conditions).
Figure 6. (a,b) Light intensity dependence characterizations of JSC and VOC for PSCs based on the SnO2 and SnO2-MDACl2 ETLs. (c) Dark-state J–V curves of the PSCs. (d) Nyquist plots for the PSCs (measured in dark conditions).
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Table
Table 1. Statistic photovoltaic parameters of PSCs prepared under conditions of different MDACl2 concentrations.
Table 1. Statistic photovoltaic parameters of PSCs prepared under conditions of different MDACl2 concentrations.
ETLVOC (V)JSC (mA/cm2)FF (%)PCE (%)
SnO21.075 ± 0.01023.35 ± 0.3975.39 ± 1.4218.92 ± 0.47
SnO2-MDACl2-2 mM1.096 ± 0.00824.14 ± 0.2076.71 ± 0.8820.29 ± 0.27
SnO2-MDACl2-4 mM1.105 ± 0.00724.45 ± 0.2279.65 ± 1.2421.53 ± 0.38
SnO2-MDACl2-8 mM1.078 ± 0.00923.78 ± 0.2176.48 ± 0.7219.61 ± 0.26
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Xiao, Y.; Cui, X.; Xiang, B.; Chen, Y.; Zhao, C.; Wang, L.; Yang, C.; Zhang, G.; Xie, C.; Han, Y.; et al. MDACl2-Modified SnO2 Film for Efficient Planar Perovskite Solar Cells. Molecules 2023, 28, 2668. https://doi.org/10.3390/molecules28062668

AMA Style

Xiao Y, Cui X, Xiang B, Chen Y, Zhao C, Wang L, Yang C, Zhang G, Xie C, Han Y, et al. MDACl2-Modified SnO2 Film for Efficient Planar Perovskite Solar Cells. Molecules. 2023; 28(6):2668. https://doi.org/10.3390/molecules28062668

Chicago/Turabian Style

Xiao, Yaodong, Xiangqian Cui, Boyuan Xiang, Yanping Chen, Chaoyue Zhao, Lihong Wang, Chuqun Yang, Guangye Zhang, Chen Xie, Yulai Han, and et al. 2023. "MDACl2-Modified SnO2 Film for Efficient Planar Perovskite Solar Cells" Molecules 28, no. 6: 2668. https://doi.org/10.3390/molecules28062668

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