Facile Solution Spin-Coating SnO2 Thin Film Covering Cracks of TiO2 Hole Blocking Layer for Perovskite Solar Cells

The hole blocking layer plays an important role in suppressing recombination of holes and electrons between the perovskite layer and fluorine-doped tin oxide (FTO). Morphological defects, such as cracks, at the compact TiO2 hole blocking layer due to rough FTO surface seriously affect performance of perovskite solar cells (PSCs). Herein, we employ a simple spin-coating SnO2 thin film solution to cover cracks of TiO2 hole blocking layer for PSCs. The experiment results indicate that the TiO2/SnO2 complementary composite hole blocking layer could eliminate the serious electrical current leakage existing inside the device, extremely reducing interface defects and hysteresis. Furthermore, a high efficiency of 13.52% was achieved for the device, which is the highest efficiency ever recorded in PSCs with spongy carbon film deposited on a separated FTO-substrate as composite counter electrode under one sun illumination.


Introduction
Currently, perovskite solar cells (PSCs) have been recognized as the most promising alternatives to conventional silicon solar cells due to high conversion efficiency, low manufacturing cost, and simple process [1][2][3][4][5][6][7].As for typical planar heterojunction PSCs composed of working electrode (cathode)/hole blocking layer/perovskite layer/electron blocking layer/counter electrode (anode), the hole blocking layer plays an important role in extracting electrons from the perovskite layer and, as well as, suppressing recombination of holes and electrons between the perovskite layer and FTO [8][9][10][11].A uniform, pinhole-free and well electrically conductive hole blocking layer is highly demanded for well performed planar heterojunction PSCs [8,12].Liao et al. found that the pinholes on the surface of the single SnO 2 hole blocking layer coating on the rough FTO substrate were still hard to eliminate.Therefore, radio-frequency magnetron sputtering TiO 2 fully covered the surface of the FTO electrode, thus passivating the FTO surface defects and reducing the recombination, finally utilizing TiO 2 /SnO 2 bilayer as hole blocking layer to improve device performance [8,13].Furthermore, TiO 2 compact layer has commonly been used as the hole blocking layer in the planar heterojunction PSCs.Since the spin-coating process formed a highly irregular thick TiO 2 layer on top of the rough FTO layer, the pinholes appeared on the surface of the TiO 2 hole blocking layer, so that the perovskite light-absorber layer directly contacted the FTO layer, resulting in recombination of holes and electrons between the perovskite light-absorber layer and FTO layer.Though uniform and pinhole-free compact TiO 2 layers were prepared using atomic layer deposition to improve device performance, the atomic layer deposition method had the disadvantage of including a low crystallinity of thin film due to the low deposition temperature [14].However, Choi et al. mitigated the influence of the morphological defects (such as an irregular film thickness and poor physical contact between the TiO 2 and the FTO layers) at the TiO 2 interface due to a rough FTO by implementing a TiO 2 hole blocking layer based on the anodization method [15].Though the research by Choi et al. mitigated the influence of the morphological defects, the preparation technology was very complicated.Up to now, several research groups have developed carbon counter electrodes to reduce the costs of perovskite devices [16][17][18].In our research group, previously, the PSC based on sponge carbon/FTO composite counter electrode obtained the power conversion efficiency of 10.7% [19].
In this work, we employ TiO 2 /SnO 2 complementary composite layer prepared by facile solution spin-coating SnO 2 on compact TiO 2 -coated rough FTO as the hole blocking layer for planar heterojunction PSCs to solve morphological defects, such as cracks, at the compact TiO 2 hole blocking layer due to rough FTO surface.However, due to TiO 2 hole blocking layer has relative lower electron mobility, leading to unbalanced carrier transport in the device, thus regular planar structure PSC usually has a strong hysteresis behavior [20].Meanwhile, SnO 2 is currently recognized as the promising hole blocking layer substitution for the conventional TiO 2 due to high electron mobility, low conduction band minimum, and low-temperature preparation [20][21][22][23][24][25].Grätzel et al. and co-workers reported planar PSCs with efficiencies close to 21% using a simple, solution-processed technological approach for depositing SnO 2 layers [26].Thereby, we obtained the modified hole blocking layer thin film without cracks through solution spin-coating SnO 2 on compact TiO 2 -coated rough FTO at 3000 rpm.Correspondingly, this TiO 2 /SnO 2 thin film and component optimization of perovskite precursor solution contributed to the formation of high-quality perovskite thin films (such as high crystallinity, large grain size, low defect density, and good flatness).Notably, the largest size of individual perovskite grain reached 2.67 µm.Therefore, PSC based on the TiO 2 /SnO 2 complementary composite hole blocking layer achieved high power conversion efficiencies (PCE) of 13.52% in size of 0.2 cm 2 under absolute ambient condition, which is the highest efficiency ever recorded in PSCs with such spongy carbon/FTO composite counter electrode.These improvements indicate that the TiO 2 /SnO 2 complementary composite hole blocking layer could eliminate the serious electrical current leakage existing inside the device and reduce interface defects.

Device Fabrication
The surface treatment of the FTO for complete coverage before deposition of any film which affect severely the film deposition and properties.Malviya et al. demonstrated that rigorous cleaning process yielded clean FTO surface, and rigorous cleaning of the substrates prior to the hematite deposition was crucial for achieving highly reproducible results [27].In order to ensure the quality of the subsequent film deposition, we thoroughly cleaned the FTO glass substrate before proceeding to the next step.FTO substrate was firstly wiped using a mixed solution of detergent and deionized water.The resultant FTO substrates were ultrasonically cleaned with glass water (deionized water: acetone: 2-propanol = 1:1:1) and ethanol for 20 min, separately.The FTO glasses were dried with dryer for 30 min and then cleaned with ultraviolet ozone for 10 min.Compact TiO 2 layer was deposited on the cleaned FTO substrate by spin-coating an acidic titanium dioxide solution at 2000 rpm for 60 s.The substrate was annealed on a hotplate at 100 • C for 10 min, and then sintered in muffle furnace at 500 • C for 30 min.For TiO 2 /SnO 2 complementary composite hole blocking layer based devices, due to the hydrophobicity of the TiO 2 thin film layer and the complete aqueous solution of the SnO 2 precursor solution, the FTO/TiO 2 substrates were firstly cleaned by ultraviolet ozone for 10 min in order to form a hydrophilic group on the surface of the film.Then the SnO 2 functional layer was prepared by spin-coating 3 wt % SnO 2 precursor solution on the compact TiO 2 -coated FTO substrate for 30 s at 2000, 3000, and 4000 rpm, respectively [23].Subsequently, the FTO/TiO 2 /SnO 2 based substrates were dried on a hotplate at 150 • C for 30 min, and then cleaned by ultraviolet ozone for 10 min again to improve its hydrophilicity.
The perovskite thin film was deposited by a two-step spin-coating method [19].Firstly, the prepared FTO/TiO 2 /SnO 2 substrates were preheated on a hotplate at 70 • C. The precursor solution of PbI 2 was prepared by dissolving 0.5993 g PbI 2 powder in 1 mL mixture of DMF/DMSO (0.95:0.05 of volume ratio), and then spin-coated on the preheated FTO/TiO 2 /SnO 2 substrates at 1500 rpm for 30 s.After that, the doped FAI precursor solution was prepared by mixing 60 mg FAI and 6 mg MABr and 6 mg MACl in 1 mL isopropanol, and then spin-coated on the PbI 2 layer at 1300 rpm for 30 s.In order to obtain a high-quality perovskite light-absorber layer, the spin-coated substrates were annealed on a hotplate at 150 • C for 15 min under ambient condition.The electron blocking layer was deposited on top of the perovskite layer by 3000 rpm for 30 s using Spiro-OMeTAD solution.
Finally, some cleaned FTO glasses were used as substrates to collect soot of a burning candle as spongy carbon counter electrodes.The spongy carbon/FTO composite counter electrode was then pressed on the Spiro-OMeTAD layers of uncompleted devices.The whole process is carried out in air condition with a relative humidity of 10%-20% at room temperature.The entire preparation process was described in Scheme 1.

Device Fabrication
The surface treatment of the FTO for complete coverage before deposition of any film which affect severely the film deposition and properties.Malviya et al. demonstrated that rigorous cleaning process yielded clean FTO surface, and rigorous cleaning of the substrates prior to the hematite deposition was crucial for achieving highly reproducible results [27].In order to ensure the quality of the subsequent film deposition, we thoroughly cleaned the FTO glass substrate before proceeding to the next step.FTO substrate was firstly wiped using a mixed solution of detergent and deionized water.The resultant FTO substrates were ultrasonically cleaned with glass water (deionized water: acetone: 2-propanol = 1:1:1) and ethanol for 20 min, separately.The FTO glasses were dried with dryer for 30 min and then cleaned with ultraviolet ozone for 10 min.Compact TiO2 layer was deposited on the cleaned FTO substrate by spin-coating an acidic titanium dioxide solution at 2000 rpm for 60 s.The substrate was annealed on a hotplate at 100 °C for 10 min, and then sintered in muffle furnace at 500 °C for 30 min.For TiO2/SnO2 complementary composite hole blocking layer based devices, due to the hydrophobicity of the TiO2 thin film layer and the complete aqueous solution of the SnO2 precursor solution, the FTO/TiO2 substrates were firstly cleaned by ultraviolet ozone for 10 min in order to form a hydrophilic group on the surface of the film.Then the SnO2 functional layer was prepared by spin-coating 3 wt % SnO2 precursor solution on the compact TiO2coated FTO substrate for 30 s at 2000, 3000, and 4000 rpm, respectively [23].Subsequently, the FTO/TiO2/SnO2 based substrates were dried on a hotplate at 150 °C for 30 min, and then cleaned by ultraviolet ozone for 10 min again to improve its hydrophilicity.
The perovskite thin film was deposited by a two-step spin-coating method [19].Firstly, the prepared FTO/TiO2/SnO2 substrates were preheated on a hotplate at 70 °C.The precursor solution of PbI2 was prepared by dissolving 0.5993 g PbI2 powder in 1 mL mixture of DMF/DMSO (0.95:0.05 of volume ratio), and then spin-coated on the preheated FTO/TiO2/SnO2 substrates at 1500 rpm for 30 s.After that, the doped FAI precursor solution was prepared by mixing 60 mg FAI and 6 mg MABr and 6 mg MACl in 1 mL isopropanol, and then spin-coated on the PbI2 layer at 1300 rpm for 30 s.In order to obtain a high-quality perovskite light-absorber layer, the spin-coated substrates were annealed on a hotplate at 150 °C for 15 min under ambient condition.The electron blocking layer was deposited on top of the perovskite layer by 3000 rpm for 30 s using Spiro-OMeTAD solution.
Finally, some cleaned FTO glasses were used as substrates to collect soot of a burning candle as spongy carbon counter electrodes.The spongy carbon/FTO composite counter electrode was then pressed on the Spiro-OMeTAD layers of uncompleted devices.The whole process is carried out in air condition with a relative humidity of 10%-20% at room temperature.The entire preparation process was described in Scheme 1.

Characterization
X-ray diffraction (XRD) data from samples of perovskite films deposited on FTO/TiO 2 /SnO 2 substrates were collected using an X-ray diffractometer (D8 Focus, Bruker, Dresden, Germany).The morphology of the perovskite films were measured using a scanning electron microscope (SEM) (SIGMA, Zeiss, Jena, Germany).The photo-current density-voltage (J-V) characteristics were measured under simulated standard air-mass AM 1.5 sunlight with using a solar simulator (Sol 3A, Oriel, Newport, RI, USA).All the measurements of the PSCs were performed under ambient atmosphere at room temperature without encapsulation.

Results and Discussion
Figure 1 shows the top-view SEM images of the bare FTO glass substrate (Figure 1a) and 3D roughness reconstruction of bare FTO glass (Figure 1f).As we can see, the surface of the bare FTO is very rough which corresponds to Sa (arithmetic-mean-height) of 74 nm.Many cracks (marked with a red square in Figure 1b) are observed on the surface of TiO 2 thin film.Figure 1c is the enlarged SEM surface image of the red square (in Figure 1b), however, the bottom FTO can be seen through the cracks, which will cause a large number of photocarriers to be recombination and seriously affect the performance of the PSC device.As shown in the cross-sectional SEM image (Figure 1d), the poor uniformity of TiO 2 thin film and the obvious fracture (marked with a red circle in Figure 1d) which was due to surface roughness of FTO, reveals that the bare FTO surface was exposed and could form direct contact with the perovskite layer, resulting in serious recombination of holes and electrons between the perovskite layer and FTO, in agreement with the results obtained with top-view SEM images (Figure 1b,c).The cross-sectional SEM image of bare FTO layer is shown in Figure 1e, we can find that the surface of bare FTO layer was very rough due to the presence of sharp peaks.Even some peaks and valleys reached a height of ~200 nm.However, as can be seen from Figure 1d, the spin-coating process formed a highly irregular thick TiO 2 layer due to overfilling the valleys on the rough FTO surface with TiO 2 .The average thickness of single-layer TiO 2 compact layer was ~100 nm, which indicated some FTO peaks higher than 100 nm could not be covered, leading to the appearance of cracks in single-layer TiO 2 thin films.This is consistent with the observation from SEM images (Figure 1b,c) of the hole blocking layer thin films prepared with single-layer TiO 2 compact layer deposited on rough FTO substrates.The PSC devices based on single-layer TiO 2 hole blocking layer and double-layer TiO 2 hole blocking layer are named as device R1 and device R2, respectively.Table 1 shows the summary of J-V characteristics of device R1 and device R2 from reverse scan.As we can see, device R1 achieved low power conversion efficiency of 4.63% due to the presence of cracks.Compared with the device R1, the performance of the device R2 was improved.Huang et al. reported a facile dip-coating route to sequentially prepare a dense sol-gel-derived titanium dioxide (TiO 2 ) hole blocking layer and a smooth organolead halide perovskite film for pseudo-planar heterojunction perovskite solar cells [28].Masood et al. used a scalable and low-cost dip coating method to prepare uniform and ultra-thin (5−50 nm) compact TiO 2 films on FTO glass substrates [29].Though the dip-coating process does not require any complicated equipment and greatly minimizes the waste of source materials, enabling fast and easy production of uniform thin films, the method is not the same as our spin coating process.Figure 2 presents the top-view SEM images of SnO2 thin films deposited on compact TiO2-coated rough FTO for 30 s at 2000 rpm (Figure 2a,b), 3000 rpm (Figure 2c,d), and 4000 rpm (Figure 2e,f), respectively.As shown in Figure 2a,b, the cracks on the surface of the TiO2/SnO2 thin film were obviously filled with SnO2 crystal grains.Due to the rotational speed being too low, a thick SnO2 thin film formed.Thus, many cracks appeared on the surface of the SnO2 thin film.Jiang et al. demonstrated a dense, pinhole-free film formed by spin coating SnO2 nanoparticles solution onto glass/Indium-Tin Oxide (ITO) substrate at 3000 rpm [23].However, in our experiment, as shown in Figure 2c,d, modified TiO2/SnO2 thin film without visible cracks through solution spin-coating SnO2 on compact TiO2-coated rough FTO at 3000 rpm formed, and is flat, uniform, and dense, which indicates the TiO2/SnO2 complementary composite hole blocking layer is able to fully cover the rough surface of the FTO electrode.Choi et al. demonstrated the TiO2 film was so too thin that pinholes were observed in the TiO2 layer, and electrons in the FTO layer easily recombined with holes due to the lack of hole blocking [15].However, in our experiment, as the rotating speed increased to 4000 rpm, the bare FTO was exposed from a few large cracks in Figure 2e,f, which indicated that the thickness of the functional layer of SnO2 was thinner due to the excessive spin-coating speed, and coverage of FTO/TiO2 based substrate is incomplete.This result suggests that the direct contact between bare FTO and the perovskite light-absorber layer can cause the recombination of holes and electrons, leading to the degradation of device performance.It is consistent with the results obtained with summary of photovoltaic parameters with the J-V characteristics from reverse scan of the PSC devices based on TiO2/SnO2 complementary composite hole blocking layer.
However, the reasons for formation of cracks are different under high speed (4000 rpm) and low speed (2000 rpm), presented in detail in Figure 3. Figure 3 presents schematics for formation process of SnO2 thin films at different rotational speed.Compared with situations of other rotational speed, the SnO2 thin film from 2000 rpm was thicker, and the total amount of solution in the film was more.The solvent volatilization rate is lower during the spin coating process of 30 s, so that the solution film of SnO2 was formed.During the annealing process of the solution film at 150 °C for 30 min, the solvent evaporated and the solution film contracted in the thickness direction, resulting in the exposure of the sharp peak of substrate, thus forming the cracks, as shown as Figure 3, corresponding to the SEM images (Figure 2a,b).
For the SnO2 film from 3000 rpm, the solvent volatilized rate was more than the situation of SnO2 film from 2000 rpm during the spin coating of 30 s, and the film itself was relatively thinner, thus forming a semi-solid/semi-liquid film after finishing the spin coating.In particular, the SnO2 film at the location of the sharp peak of substrate almost became solid, thus preserving the shape at the location of the sharp peak, as shown as Figure 3, corresponding to the SEM images (Figure 2c,d).[15].However, in our experiment, as the rotating speed increased to 4000 rpm, the bare FTO was exposed from a few large cracks in Figure 2e,f, which indicated that the thickness of the functional layer of SnO 2 was thinner due to the excessive spin-coating speed, and coverage of FTO/TiO 2 based substrate is incomplete.This result suggests that the direct contact between bare FTO and the perovskite light-absorber layer can cause the recombination of holes and electrons, leading to the degradation of device performance.It is consistent with the results obtained with summary of photovoltaic parameters with the J-V characteristics from reverse scan of the PSC devices based on TiO 2 /SnO 2 complementary composite hole blocking layer.
However, the reasons for formation of cracks are different under high speed (4000 rpm) and low speed (2000 rpm), presented in detail in Figure 3. Figure 3 presents schematics for formation process of SnO 2 thin films at different rotational speed.Compared with situations of other rotational speed, the SnO 2 thin film from 2000 rpm was thicker, and the total amount of solution in the film was more.The solvent volatilization rate is lower during the spin coating process of 30 s, so that the solution film of SnO 2 was formed.During the annealing process of the solution film at 150 • C for 30 min, the solvent evaporated and the solution film contracted in the thickness direction, resulting in the exposure of the sharp peak of substrate, thus forming the cracks, as shown as Figure 3, corresponding to the SEM images (Figure 2a,b).In the process of spin-coating at 4000 rpm, the solvent volatilization was very fast, and the whole SnO2 film was very thin and the total amount of solvent was very little, so the solid SnO2 film had been formed after 30 s spin coating.It is worth noting that at the position of the sharp peak of substrate, the SnO2 film was so thin that the ability to withstand strain of the film was very weak, resulting in a crack after annealing at 150 °C, as shown as Figure 3, corresponding to the SEM images (Figure 2e,f).
In addition, the film from spin coating at high speed was thinner than the one at low speed, and the film from spin coating at high speed was rougher than the one at low speed.Light trapping is a technique to improve optical absorption in the active layer of a solar cell.The rough surface can act as a light trapping structure to enhance the absorption perovskite solar cell [30][31][32][33].However, the feature size here (<74 nm) is less than the effective trapping sizes mentioned by previous research groups (~300 nm [30], ~6 μm [31] and ~800 nm [32]), so that the TiO2/SnO2 thin film with lower roughness is not the main reason for the efficiency drop.In the process of spin-coating at 4000 rpm, the solvent volatilization was very fast, and the whole SnO2 film was very thin and the total amount of solvent was very little, so the solid SnO2 film had been formed after 30 s spin coating.It is worth noting that at the position of the sharp peak of substrate, the SnO2 film was so thin that the ability to withstand strain of the film was very weak, resulting in a crack after annealing at 150 °C, as shown as Figure 3, corresponding to the SEM images (Figure 2e,f).
In addition, the film from spin coating at high speed was thinner than the one at low speed, and the film from spin coating at high speed was rougher than the one at low speed.Light trapping is a technique to improve optical absorption in the active layer of a solar cell.The rough surface can act as a light trapping structure to enhance the absorption perovskite solar cell [30][31][32][33].However, the feature size here (<74 nm) is far less than the effective trapping sizes mentioned by previous research groups (~300 nm [30], ~6 μm [31] and ~800 nm [32]), so that the TiO2/SnO2 thin film with lower roughness is not the main reason for the efficiency drop.For the SnO 2 film from 3000 rpm, the solvent volatilized rate was more than the situation of SnO 2 film from 2000 rpm during the spin coating of 30 s, and the film itself was relatively thinner, thus forming a semi-solid/semi-liquid film after finishing the spin coating.In particular, the SnO 2 film at the location of the sharp peak of substrate almost became solid, thus preserving the shape at the location of the sharp peak, as shown as Figure 3, corresponding to the SEM images (Figure 2c,d).
In the process of spin-coating at 4000 rpm, the solvent volatilization was very fast, and the whole SnO 2 film was very thin and the total amount of solvent was very little, so the solid SnO 2 film had been formed after 30 s spin coating.It is worth noting that at the position of the sharp peak of substrate, the SnO 2 film was so thin that the ability to withstand strain of the film was very weak, resulting in a crack after annealing at 150 • C, as shown as Figure 3, corresponding to the SEM images (Figure 2e,f).
Coatings 2018, 8, 314 7 of 13 In addition, the film from spin coating at high speed was thinner than the one at low speed, and the film from spin coating at high speed was rougher than the one at low speed.Light trapping is a technique to improve optical absorption in the active layer of a solar cell.The rough surface can act as a light trapping structure to enhance the absorption perovskite solar cell [30][31][32][33].However, the feature size here (<74 nm) is far less than the effective trapping sizes mentioned by previous research groups (~300 nm [30], ~6 µm [31] and ~800 nm [32]), so that the TiO 2 /SnO 2 thin film with lower roughness is not the main reason for the efficiency drop.
Figure 4 presents the cross-sectional SEM images of the TiO 2 /SnO 2 complementary composite hole blocking layer with SnO 2 spin-coated on compact TiO 2 -coated FTO at 3000 rpm, which indicated the formation of modified TiO 2 /SnO 2 film without visible breakages.As shown in Figure 4a, the entire film of TiO 2 /SnO 2 complementary composite hole blocking layer was uniform in thickness and completely covered.Compared with single-layer TiO 2 hole blocking layer thin film, the SnO 2 thin film spin-coated on compact TiO 2 -coated rough FTO exhibited a flatter surface.Notably, as shown in Figure 4b, the TiO 2 /SnO 2 complementary composite hole blocking layer also has good coverage at the protrusions of the underlying substrate and no breakage or large bulging in the protrusions could be observed.This is consistent with the observation from top-view SEM images of SnO 2 films deposited on compact TiO 2 -coated rough FTO at 3000 rpm (Figure 2c,d).
Coatings 2018, 8, x FOR PEER REVIEW 7 of 12 Figure 4 presents the cross-sectional SEM images of the TiO2/SnO2 complementary composite hole blocking layer with SnO2 spin-coated on compact TiO2-coated FTO at 3000 rpm, which indicated the formation of modified TiO2/SnO2 film without visible breakages.As shown in Figure 4a, the entire film of TiO2/SnO2 complementary composite hole blocking layer was uniform in thickness and completely covered.Compared with single-layer TiO2 hole blocking layer thin film, the SnO2 thin film spin-coated on compact TiO2-coated rough FTO exhibited a flatter surface.Notably, as shown in Figure 4b, the TiO2/SnO2 complementary composite hole blocking layer also has good coverage at the protrusions of the underlying substrate and no breakage or large bulging in the protrusions could be observed.This is consistent with the observation from top-view SEM images of SnO2 films deposited on compact TiO2-coated rough FTO at 3000 rpm (Figure 2c,d).Figure 5 shows schematic illustration for the solution spin-coating process of TiO2/SnO2 complementary composite hole blocking layer.As we can see, the surface of the bare FTO was very rough due to the presence of sharp peaks (Figure 5a), corresponding to the SEM image (Figure 1e).As shown in Figure 5b, the spin-coating process formed a highly irregular thick TiO2 layer due to overfilling the valleys on the rough FTO surface with TiO2, resulting in many cracks appearing on the surface of TiO2, and the bare FTO surface was exposed, corresponding to the SEM images (Figure 1b-d).However, as can be seen from Figure 5c, the cracks were absent from the surface of SnO2 thin film deposited on compact TiO2-coated rough FTO at 3000 rpm and the SnO2 thin film fully covered the surface of the FTO electrode, corresponding to the SEM images (Figure 2c,d).Meanwhile, the SnO2 thin film is flat, uniform, and dense, corresponding to the SEM images (Figure 4).This result suggests that the modified TiO2/SnO2 complementary composite hole blocking layer is beneficial to suppressing recombination of holes and electrons between the perovskite layer and FTO and enhances the performance of devices.Figure 5 shows schematic illustration for the solution spin-coating process of TiO 2 /SnO 2 complementary composite hole blocking layer.As we can see, the surface of the bare FTO was very rough due to the presence of sharp peaks (Figure 5a), corresponding to the SEM image (Figure 1e).As shown in Figure 5b, the spin-coating process formed a highly irregular thick TiO 2 layer due to overfilling the valleys on the rough FTO surface with TiO 2 , resulting in many cracks appearing on the surface of TiO 2 , and the bare FTO surface was exposed, corresponding to the SEM images (Figure 1b-d).However, as can be seen from Figure 5c, the cracks were absent from the surface of SnO 2 thin film deposited on compact TiO 2 -coated rough FTO at 3000 rpm and the SnO 2 thin film fully covered the surface of the FTO electrode, corresponding to the SEM images (Figure 2c,d).Meanwhile, the SnO 2 thin film is flat, uniform, and dense, corresponding to the SEM images (Figure 4).This result suggests that the modified TiO 2 /SnO 2 complementary composite hole blocking layer is beneficial to suppressing recombination of holes and electrons between the perovskite layer and FTO and enhances the performance of devices.
XRD data of the SnO 2 thin film deposited on FTO/TiO 2 substrate were collected in Figure 6.Each of the marked peak positions in the figure (Figure 6) corresponded to the (110), (101), ( 200), (211), (310), (301) crystal planes of the SnO 2 crystal, which was in consistent with the results of the previous report [23] about XRD pattern of the SnO 2 material, indicating SnO 2 thin film composite counter electrodes had been successfully deposited on the FTO/TiO 2 substrate.
Coatings 2018, 8, 314 8 of 13 film deposited on compact TiO2-coated rough FTO at 3000 rpm and the SnO2 thin film fully covered the surface of the FTO electrode, corresponding to the SEM images (Figure 2c,d).Meanwhile, the SnO2 thin film is flat, uniform, and dense, corresponding to the SEM images (Figure 4).This result suggests that the modified TiO2/SnO2 complementary composite hole blocking layer is beneficial to suppressing recombination of holes and electrons between the perovskite layer and FTO and enhances the performance of devices.XRD data of the SnO2 thin film deposited on FTO/TiO2 substrate were collected in Figure 6.Each of the marked peak positions in the figure (Figure 6) corresponded to the (110), ( 101), ( 200), (211), (310), (301) crystal planes of the SnO2 crystal, which was in consistent with the results of the previous report [23] about XRD pattern of the SnO2 material, indicating SnO2 thin film composite counter electrodes had been successfully deposited on the FTO/TiO2 substrate.Figure 7 presents top-view SEM images (Figure 7a,b) and cross-sectional SEM image (Figure 7c) of perovskite thin films deposited on TiO2/SnO2 of solution spin-coating SnO2 on compact TiO2coated rough FTO at 3000 rpm, respectively.From Figure 7a,b, it can be seen that the entire perovskite film was high-quality and quite uniform free of cracks.However, high-quality perovskite film is essential for the performance of planar PSCs, as it can avoid the direct contact of hole blocking layer and electron blocking layer [20].It is worth noting that the individual grain size of perovskite crystals was especially large, and the largest grain size reached a rare 2.67 μm.Furthermore, due to the passivation of the doping, the grain boundary was too smooth, which reduced defects of grain boundaries in the perovskite film to a great extent.From the cross-sectional SEM image (Figure 7c), it can be found that the entire perovskite film was composed of single layer of large grains and uniformly dense, which is coincident with the morphology of the film on the top-view SEM images (Figure 7a,b).7c) of perovskite thin films deposited on TiO 2 /SnO 2 of solution spin-coating SnO 2 on compact TiO 2 -coated rough FTO at 3000 rpm, respectively.From Figure 7a,b, it can be seen that the entire perovskite film was high-quality and quite uniform free of cracks.However, high-quality perovskite film is essential for the performance of planar PSCs, as it can avoid the direct contact of hole blocking layer and electron blocking layer [20].It is worth noting that the individual grain size of perovskite crystals was especially large, and the largest grain size reached a rare 2.67 µm.Furthermore, due to the passivation of the doping, the grain boundary was too smooth, which reduced defects of grain boundaries in the perovskite film to a great extent.From the cross-sectional SEM image (Figure 7c), it can be found that the entire perovskite film was composed of single layer of large grains and uniformly dense, which is coincident with the morphology of the film on the top-view SEM images (Figure 7a,b).
Figure 8 displays the XRD pattern of the perovskite thin film deposited on FTO/TiO 2 /SnO 2 substrate in the case of spin-coating SnO 2 at 3000 rpm.Obviously, the XRD pattern presents strong and sharp (101) and ( 200) diffraction peaks at 14.02 • and 28.32 • , demonstrating the good crystallinity of perovskite film, which is consistent with the SEM results (Figure 7) of the large-grain-size perovskite film.The perovskite grains of good crystallinity effectively enhance the efficiency of injection of photo-carriers of the perovskite light-absorber layer into the hole blocking layer and electron blocking layer, thereby further improving the overall performance of the PSC device.
Coatings 2018, 8, 314 9 of 13 was especially large, and the largest grain size reached a rare 2.67 μm.Furthermore, due to the passivation of the doping, the grain boundary was too smooth, which reduced defects of grain boundaries in the perovskite film to a great extent.From the cross-sectional SEM image (Figure 7c), it can be found that the entire perovskite film was composed of single layer of large grains and uniformly dense, which is coincident with the morphology of the film on the top-view SEM images (Figure 7a,b).200) diffraction peaks at 14.02° and 28.32°, demonstrating the good crystallinity of perovskite film, which is consistent with the SEM results (Figure 7) of the large-grain-size perovskite film.The perovskite grains of good crystallinity effectively enhance the efficiency of injection of photo-carriers of the perovskite light-absorber layer into the hole blocking layer and electron blocking layer, thereby further improving the overall performance of the PSC device.Figure 10 shows the J-V curves from reverse scan (RS) and forward scan (FS) for the PSC devices based on TiO2/SnO2 complementary composite hole blocking layer under different SnO2 spin-coating speeds, measured under simulated sunlight with intensity of 100 mW cm −2 (AM 1.5 G).Obviously, when the SnO2 spin-coating speed was 3000 rpm, the RS and FS curves of the device basically   200) diffraction peaks at 14.02° and 28.32°, demonstrating the good crystallinity of perovskite film, which is consistent with the SEM results (Figure 7) of the large-grain-size perovskite film.The perovskite grains of good crystallinity effectively enhance the efficiency of injection of photo-carriers of the perovskite light-absorber layer into the hole blocking layer and electron blocking layer, thereby further improving the overall performance of the PSC device.Figure 10 shows the J-V curves from reverse scan (RS) and forward scan (FS) for the PSC devices based on TiO2/SnO2 complementary composite hole blocking layer under different SnO2 spin-coating speeds, measured under simulated sunlight with intensity of 100 mW cm −2 (AM 1.5 G).Obviously, when the SnO2 spin-coating speed was 3000 rpm, the RS and FS curves of the device basically Figure 10 shows the J-V curves from reverse scan (RS) and forward scan (FS) for the PSC devices based on TiO 2 /SnO 2 complementary composite hole blocking layer under different SnO 2 spin-coating speeds, measured under simulated sunlight with intensity of 100 mW cm −2 (AM 1.5 G).Obviously, when the SnO 2 spin-coating speed was 3000 rpm, the RS and FS curves of the device basically coincided and the hysteresis phenomenon basically disappeared, which matches the thin film morphology seen from the top-view SEM images (Figure 7a,b).When the SnO 2 spin-coating speeds were 2000 and 4000 rpm, a SnO 2 functional layer that was wither too thick or too thin could not match well with the TiO 2 compact layer to form modified TiO 2 /SnO 2 complementary composite hole blocking layer.This led directly to a drop in the optoelectronic performance of the device.The presence of cracks also caused serious electrical current leakage existing inside the device, resulting the PSC devices under these two processes having different degrees of hysteresis.Table 2 shows summary of J-V characteristics from RS of the PSC devices based on TiO2/SnO2 complementary composite hole blocking layer under different SnO2 spin-coating speeds.The best cell based on TiO2/SnO2 complementary composite hole blocking layer under the SnO2 spin-coating speed of 3000 rpm obtained a PEC of 13.52% corresponding to a Jsc of 22.48 mA cm −2 , a Voc of 1.01 V and a FF of 59.51.Although this efficiency is not the highest in PSCs, it is the highest efficiency ever recorded in PSCs with such spongy carbon/FTO composite counter electrode, and furthermore, the preparation techniques of high-quality perovskite thin films and the cheap spongy carbon/FTO composite counter electrode under absolute ambient condition are beneficial for large-scale applications and commercialization.At the same time, since the problem of the defect at the interface between the hole blocking layer and the perovskite light-absorber layer was solved, the hysteresis phenomenon of the entire device was well solved.

Conclusions
In summary, we optimized a rotational speed of spin-coating SnO2 at 3000 rpm to cover the cracks of compact TiO2-coated on rough FTO.Using composite layered TiO2/SnO2 can reduce Table 2 shows summary of J-V characteristics from RS of the PSC devices based on TiO 2 /SnO 2 complementary composite hole blocking layer under different SnO 2 spin-coating speeds.The best cell based on TiO 2 /SnO 2 complementary composite hole blocking layer under the SnO 2 spin-coating speed of 3000 rpm obtained a PEC of 13.52% corresponding to a J sc of 22.48 mA cm −2 , a V oc of 1.01 V and a FF of 59.51.Although this efficiency is not the highest in PSCs, it is the highest efficiency ever recorded in PSCs with such spongy carbon/FTO composite counter electrode, and furthermore, the preparation techniques of high-quality perovskite thin films and the cheap spongy carbon/FTO composite counter electrode under absolute ambient condition are beneficial for large-scale applications and commercialization.At the same time, since the problem of the defect at the interface between the hole blocking layer and the perovskite light-absorber layer was solved, the hysteresis phenomenon of the entire device was well solved.

Scheme 1 .
Scheme 1.The entire process of the device fabrication.Scheme 1.The entire process of the device fabrication.

Figure 1 .
Figure 1.SEM images: (a) Top-view SEM image of bare FTO glass; (b,c) Top-view SEM image of TiO2 film; (d) Cross-sectional SEM image of TiO2 film; (e) Cross-sectional SEM image of bare FTO glass; (f) Top-view SEM image of 3D roughness reconstruction of bare FTO glass.

Figure 1 .
Figure 1.SEM images: (a) Top-view SEM image of bare FTO glass; (b,c) Top-view SEM image of TiO 2 film; (d) Cross-sectional SEM image of TiO 2 film; (e) Cross-sectional SEM image of bare FTO glass; (f) Top-view SEM image of 3D roughness reconstruction of bare FTO glass.

Figure 2
Figure2presents the top-view SEM images of SnO 2 thin films deposited on compact TiO 2 -coated rough FTO for 30 s at 2000 rpm (Figure2a,b), 3000 rpm (Figure2c,d), and 4000 rpm (Figure2e,f), respectively.As shown in Figure2a,b, the cracks on the surface of the TiO 2 /SnO 2 thin film were obviously filled with SnO 2 crystal grains.Due to the rotational speed being too low, a thick SnO 2 thin film formed.Thus, many cracks appeared on the surface of the SnO 2 thin film.Jiang et al. demonstrated a dense, pinhole-free film formed by spin coating SnO 2 nanoparticles solution onto glass/Indium-Tin Oxide (ITO) substrate at 3000 rpm[23].However, in our experiment, as shown in Figure2c,d, modified TiO 2 /SnO 2 thin film without visible cracks through solution spin-coating SnO 2 on compact TiO 2 -coated rough FTO at 3000 rpm formed, and is flat, uniform, and dense, which indicates the TiO 2 /SnO 2 complementary composite hole blocking layer is able to fully cover the rough surface of the FTO electrode.Choi et al. demonstrated the TiO 2 film was so too thin that pinholes were observed in the TiO 2 layer, and electrons in the FTO layer easily recombined with holes due to the lack of hole blocking[15].However, in our experiment, as the rotating speed increased to 4000 rpm, the bare FTO was exposed from a few large cracks in Figure2e,f, which indicated that the thickness of the functional layer of SnO 2 was thinner due to the excessive spin-coating speed, and coverage of FTO/TiO 2 based substrate is incomplete.This result suggests that the direct contact between bare FTO and the perovskite light-absorber layer can cause the recombination of holes and electrons, leading to the degradation of device performance.It is consistent with the results obtained with summary of photovoltaic parameters with the J-V characteristics from reverse scan of the PSC devices based on TiO 2 /SnO 2 complementary composite hole blocking layer.However, the reasons for formation of cracks are different under high speed (4000 rpm) and low speed (2000 rpm), presented in detail in Figure3.Figure3presents schematics for formation process of SnO 2 thin films at different rotational speed.Compared with situations of other rotational speed, the SnO 2 thin film from 2000 rpm was thicker, and the total amount of solution in the film was more.The solvent volatilization rate is lower during the spin coating process of 30 s, so that the solution film of SnO 2 was formed.During the annealing process of the solution film at 150 • C for 30 min, the solvent evaporated and the solution film contracted in the thickness direction, resulting in the exposure of the sharp peak of substrate, thus forming the cracks, as shown as Figure3, corresponding to the SEM images (Figure2a,b).

Figure 3 .
Figure 3. Schematics for formation of SnO2 thin films at different rotational speed.

Figure 3 .
Figure 3. Schematics for formation of SnO2 thin films at different rotational speed.Figure 3. Schematics for formation of SnO 2 thin films at different rotational speed.

Figure 3 .
Figure 3. Schematics for formation of SnO2 thin films at different rotational speed.Figure 3. Schematics for formation of SnO 2 thin films at different rotational speed.

Figure 5 .
Figure 5. Schematic illustration for the solution spin-coating process of TiO2/SnO2 complementary composite hole blocking layer: (a) The bare FTO substrate; (b) TiO2 thin film deposited on rough FTO

Figure 5 .
Figure 5. Schematic illustration for the solution spin-coating process of TiO2/SnO2 complementary composite hole blocking layer: (a) The bare FTO substrate; (b) TiO2 thin film deposited on rough FTO substrate; (c) SnO2 thin film deposited on compact TiO2-coated rough FTO at 3000 rpm.

Figure 5 .
Figure 5. Schematic illustration for the solution spin-coating process of TiO 2 /SnO 2 complementary composite hole blocking layer: (a) The bare FTO substrate; (b) TiO 2 thin film deposited on rough FTO substrate; (c) SnO 2 thin film deposited on compact TiO 2 -coated rough FTO at 3000 rpm.

Figure 7
Figure 7 presents top-view SEM images (Figure 7a,b) and cross-sectional SEM image (Figure7c) of perovskite thin films deposited on TiO 2 /SnO 2 of solution spin-coating SnO 2 on compact TiO 2 -coated rough FTO at 3000 rpm, respectively.From Figure7a,b, it can be seen that the entire perovskite film was high-quality and quite uniform free of cracks.However, high-quality perovskite film is essential for the performance of planar PSCs, as it can avoid the direct contact of hole blocking layer and electron blocking layer[20].It is worth noting that the individual grain size of perovskite crystals was especially large, and the largest grain size reached a rare 2.67 µm.Furthermore, due to the passivation of the doping, the grain boundary was too smooth, which reduced defects of grain boundaries in the perovskite film to a great extent.From the cross-sectional SEM image (Figure7c), it can be found that the entire perovskite film was composed of single layer of large grains and uniformly dense, which is coincident with the morphology of the film on the top-view SEM images (Figure7a,b).Figure8displays the XRD pattern of the perovskite thin film deposited on FTO/TiO 2 /SnO 2 substrate in the case of spin-coating SnO 2 at 3000 rpm.Obviously, the XRD pattern presents strong and sharp (101) and (200) diffraction peaks at 14.02 • and 28.32 • , demonstrating the good crystallinity of perovskite film, which is consistent with the SEM results (Figure7) of the large-grain-size perovskite film.The perovskite grains of good crystallinity effectively enhance the efficiency of injection of photo-carriers of the perovskite light-absorber layer into the hole blocking layer and electron blocking layer, thereby further improving the overall performance of the PSC device.

Figure 8 .
Figure 8. XRD pattern of the perovskite thin film deposited on FTO/TiO2/SnO2 substrate in the case of spin-coating SnO2 at 3000 rpm.

Figure 9
Figure9shows energy bandgap diagram for TiO2/SnO2 based PSC, the shift of conduction band is not the main reason for the efficiency drop[34].

Figure 8 .
Figure 8. XRD pattern of the perovskite thin film deposited on FTO/TiO 2 /SnO 2 substrate in the case of spin-coating SnO 2 at 3000 rpm.

Figure 9 12 Figure 8
Figure9shows energy bandgap diagram for TiO 2 /SnO 2 based PSC, the shift of conduction band is not the main reason for the efficiency drop[34].

Figure 8 .
Figure 8. XRD pattern of the perovskite thin film deposited on FTO/TiO2/SnO2 substrate in the case of spin-coating SnO2 at 3000 rpm.

Figure 9
Figure9shows energy bandgap diagram for TiO2/SnO2 based PSC, the shift of conduction band is not the main reason for the efficiency drop[34].

Coatings 2018, 8 , 12 Figure 10 .
Figure 10.The J-V curves from reverse scan (RS) and forward scan (FS) for the PSC devices based on TiO2/SnO2 complementary composite hole blocking layer under different SnO2 spin-coating speeds, measured under simulated sunlight with intensity of 100 mW cm −2 (AM 1.5 G).

Figure 10 .
Figure 10.The J-V curves from reverse scan (RS) and forward scan (FS) for the PSC devices based on TiO 2 /SnO 2 complementary composite hole blocking layer under different SnO 2 spin-coating speeds, measured under simulated sunlight with intensity of 100 mW cm −2 (AM 1.5 G).

Table 1 .
Summary of J-V characteristics of device R1 and device R2 from reverse scan.
Notes: a Number of TiO 2 spin-coating layers; b Short-circuit photocurrent density; c Open-circuit voltage; d Fill factor; e Power conversion efficiency.Coatings 2018, 8, 314 5 of 13 Coatings 2018, 8, x FOR PEER REVIEW 5 of 12

Table 2 .
Summary of J-V characteristics from reverse scan of the PSC devices based on TiO2/SnO2 complementary composite hole blocking layer under different SnO2 spin-coating speeds.
Notes: a Rotating speed of spin-coating SnO2; b Short-circuit photocurrent density; c Open-circuit voltage; d Fill factor; e Power conversion efficiency.

Table 2 .
Summary of J-V characteristics from reverse scan of the PSC devices based on TiO 2 /SnO 2 complementary composite hole blocking layer under different SnO 2 spin-coating speeds.
Notes: a Rotating speed of spin-coating SnO 2 ; b Short-circuit photocurrent density; c Open-circuit voltage; d Fill factor; e Power conversion efficiency.