Influence of the Porosity of the TiO<sub>2</sub> Film on the Performance of the Perovskite Solar Cell

Sun, Xiaodan; Xu, Jia; Xiao, Li; Chen, Jing; Zhang, Bing; Yao, Jianxi; Dai, Songyuan

doi:https://doi.org/10.1155/2017/4935265

International Journal of Photoenergy

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Nanostructured Solar Cells

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Research Article | Open Access

Volume 2017 | Article ID 4935265 | https://doi.org/10.1155/2017/4935265

Influence of the Porosity of the TiO₂ Film on the Performance of the Perovskite Solar Cell

Xiaodan Sun,¹Jia Xu,²Li Xiao,¹Jing Chen,^1,2Bing Zhang,^1,3Jianxi Yao ,^1,2and Songyuan Dai^1,3

Academic Editor: Pushpa Pudasaini

Received08 Oct 2016

Accepted19 Dec 2016

Published01 Feb 2017

Abstract

The structure of mesoporous TiO₂ (mp-TiO₂) films is crucial to the performance of mesoporous perovskite solar cells (PSCs). In this study, we fabricated highly porous mp-TiO₂ films by doping polystyrene (PS) spheres in TiO₂ paste. The composition of the perovskite films was effectively improved by modifying the mass fraction of the PS spheres in the TiO₂ paste. Due to the high porosity of the mp-TiO₂ film, PbI₂ and CH₃NH₃I could sufficiently infiltrate into the network of the mp-TiO₂ film, which ensured a more complete transformation to CH₃NH₃PbI₃. The surface morphology of the mp-TiO₂ film and the photoelectric performance of the perovskite solar cells were investigated. The results showed that an increase in the porosity of the mp-TiO₂ film resulted in an improvement in the performance of the PSCs. The best device with the optimized mass fraction of 1.0 wt% PS in TiO₂ paste exhibited an efficiency of 12.69%, which is 25% higher than the efficiency of the PSCs without PS spheres.

1. Introduction

Perovskite solar cells based on CH₃NH₃PbI₃ have attracted much attention. Tremendous progress has been made since the seminal work of Kojima et al. in 2009 [1]. In just six years, power conversion efficiencies (PCEs) of PSCs have increased sharply from 3.8% [1] to 22.1% [2], which exceeds the PCEs of polycrystalline silicon solar cells [3–5]. Moreover, their solution processability and low cost endow them with high potential for next generation solar cells [4, 6].

Two typical PSC structures are widely used, including the planar heterojunction architectures [7] and the mesoporous structures [8–10]. Planar perovskite solar cells are advantageous because they have simple and scalable cell configurations [11]. In mesoporous structured PSCs, semiconductors, including TiO₂, ZnO, insulating Al₂O₃ and ZrO₂, are employed as the electron transporting layer (ETL) or the scaffold of the perovskite layer [12]. Due to its excellent physicochemical properties, such as large band gap, chemical stability, photostability, nontoxicity, and low cost [13], mesoporous TiO₂ is the most widely used electron transporting material in PSCs. The mp-TiO₂ film acts as not only the scaffold of the perovskite layer but also as the pathway for electron transport [14].

It has been reported that the structural properties of the mp-TiO₂ layer, such as particle size [15], thickness [16–18] and porosity, have a significant influence on the performance of PSCs [19]. Highly porous mp-TiO₂ films promote the easy infiltration of perovskite which subsequently fills the pores. A higher deposition of perovskites in mp-TiO₂ film results in increased light absorption and a higher current density [12, 14]. Moreover, the interface between the mp-TiO₂ film and perovskite film plays a key role in determining the overall conversion efficiency of PSCs [20]. Increasing the specific surface area and porosity of mp-TiO₂ film can promote a deeper infiltration of perovskites in TiO₂ films. This superior infiltration is thought as an effective way to decrease the contact barrier between the TiO₂/CH₃NH₃PbI₃ interfaces, which could improve the transport of carriers in the PSCs. Dharani et al. [21] used electrospinning to prepare TiO₂ nanofibers as the ETL for PSCs. The TiO₂ nanofiber formed a highly porous structure. The excellent porous network resulted in improved loading of PbI₂. Therefore, the CH₃NH₃I can infiltrate through the pores to completely react with PbI₂. Sarkar et al. [22] prepared well-organized mesoporous TiO₂ photoelectrodes with enlarged pores by block copolymer-induced sol−gel assembly. TiO₂ photoelectrodes with larger pores are favorable for filling of perovskite in the mp-TiO₂ film. Within a certain range, devices based on larger pores showed a higher and superior performance. Rapsomanikis et al. [19] synthesized highly meso-macroporous TiO₂ thin films as ETLs of PSCs using a sol–gel process and Pluronic P-123 block copolymer as the organic template. Their results showed that the high porosity enabled the TiO₂ thin film to act as an ideal host for perovskite. The efficient contact between mp-TiO₂ and perovskite enhanced the electron transport.

Methods of controlling the mesoporous networks of mp-TiO₂ include changing the size of TiO₂ nanoparticles, using amphiphilic block copolymers [14, 22] and using templates. Many researchers reported that polystyrene (PS) spheres can be used as a mesostructured template to fabricate macro and mesoporous TiO₂ films in dye-sensitized solar cells (DSSC) because of its size tunability [23, 24]. By using various preparation method and PS spheres with different sizes, the morphology of the films can be easily controlled. Dionigi et al. [25] used PS spheres as structure-directing agents and coated the PS spheres with titanium dihydroxide to fabricate porous TiO₂ films with ordered pore architectures. Du et al. [24] fabricated hierarchically ordered macro-mesoporous TiO₂ films as the interfacial layer of DSSC using PS spheres as a template. Because of the periodically ordered structure and large specific surface of the macro-mesoporous TiO₂ films, a higher and a PCE enhanced by 83% were obtained.

However, to our knowledge, there have been no reports on the use of PS spheres to change the mesoporous networks of mp-TiO₂ film in PSCs. In this study, we present a facile method of controlling the porosity of mp-TiO₂ by introducing PS spheres into TiO₂ paste, which can be an effective strategy for the development of mesoporous PSCs. Figure 1 shows the schematic representation of the reaction process studied in this work. As shown in Figure 1(a), TiO₂ nanoparticles are close to each other. There are only few pores between the particles. PbI₂ cannot fully infiltrate into the narrow pores of mp-TiO₂ without the introduction of PS spheres during the preparation process; instead, CH₃NH₃I would react with the superficial PbI₂, which results in a large amount of residual PbI₂ in the TiO₂ film. In Figure 1(b), PS spheres can be observed in the TiO₂ film before heat treatment. Then PS spheres were removed by heat treatment and a large amount of pores were formed in mesoporous TiO₂ film. Therefore, more PbI₂ infiltrated into the film. The subsequently deposited CH₃NH₃I reacted more completely with PbI₂, which reduced the amount of the residual PbI₂. By adjusting the mass fraction of PS spheres in TiO₂ paste, a controllable porous mp-TiO₂ film was obtained. The incorporation of the macropores in mp-TiO₂ films increased the perovskite loading in the film and improved the contact between the TiO₂ and perovskite interface, which effectively suppressed charge recombination in the interface. X-ray diffraction and fluorescence lifetime measurements confirmed that the increased porosity ensured an adequate reaction between PbI₂ and CH₃NH₃I, thus decreasing the amount of residual PbI₂ and enhancing the electron injection from the perovskite to mp-TiO₂ film. The PSCs with the porous TiO₂ films showed an enhanced short-circuit current density and higher efficiency.

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2. Materials and Methods

2.1. Materials

Titanium dioxide paste (18-RT) was purchased from Yingkou OPV Tech New Energy Co., Ltd. Ethanol (99.8%) and hydrochloric acid (36%) were purchased from Beijing Chemical Plant (Beijing, China). The 100 nm PS spheres (Mw ~ 100000) were purchased from Janus New-Materials Co., Ltd. PbI₂ (99%) was purchased from Acros. CH₃NH₃I (99.5%) and 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) (99.7%) were purchased from Borun Chemicals (Ningbo, China). Tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III)tris(bis(trifluoromethylsulfonyl)imide) (FK209-cobalt(III)-TFSI) was purchased from MaterWin Chemicals (Shanghai, China). N,N-dimethylformamide (DMF) was purchased from Alfa Aesar. Isopropanol was purchased from J&K Scientific Co., Ltd. All chemicals were used as received. Glass substrates with a transparent fluorine-doped tin oxide (FTO, sheet resistance 15 Ω/square) layer were used for the PSCs.

The solution of the compact TiO₂ is prepared by mixing titanium isopropoxide, HCl, and ethanol with a volume ratio of 7 : 22 : 100. In the solution of compact TiO₂, the concentrations of HCl and ethanol are 70.24% and 8.48%, respectively. For the preparation of the TiO₂ paste, we first diluted the TiO₂ paste in ethanol with a ratio of 1 : 3.5. Then, the mixture was added to 100 nm PS spheres with various wt% (0 wt%, 1.0 wt%, and 1.5 wt%) and stirred for 12 h. The spiro-MeOTAD solution was prepared by dissolving 72.3 mg spiro-MeOTAD in 1 mL chlorobenzene, and then 28.8 μL TBP and an 8 μL solution of LiTFSI (520 mg/mL LiTFSI in acetonitrile) were added.

2.2. Device Fabrication

Devices were fabricated on FTO glass substrates with a dimension of 1.5 cm × 1.3 cm. First, FTO was partially etched with Zn powder and HCl. Then, the etched FTO was cleaned using potassium sulfate solution, soap, deionized water, and ethanol, and, finally, it was sintered at 500°C for 30 minutes. A compact TiO₂ layer was prepared by spin coating compact TiO₂ solution, followed by annealing at 500°C for 30 min. The mesoporous TiO₂ film was prepared by spin coating 35 μL TiO₂ paste at 5000 r.p.m. for 30 s. Then, the films were heated to 450°C for 2 hours with a heating rate of 5°C/min. 462 mg of PbI₂ was dissolved in 1 mL DMF under stirring at 70°C for 12 hours. 40 μL PbI₂ solution was spin-coated on the mp-TiO₂ films at 3000 r.p.m. for 30 s. After loading was performed for 4 min, the substrates were dried at 70°C for 30 min. After the films were cooled to room temperature, 90 μL CH₃NH₃I solution in 2-propanol (8 mg/mL) was sprayed on the PbI₂ films, and the films were spun at 4000 r.p.m. for 30 s and then dried at 70°C for 30 min. The hole transporting layer (HTL) was prepared by spinning spiro-MeOTAD on the TiO₂/CH₃NH₃PbI₃ film at 3000 r.p.m. for 30 s. Finally, 70 nm gold electrodes were deposited on top of the device by thermally evaporation.

2.3. Characterization and Measurement

The surface morphology of the films was observed with a field emission scanning electron microscope (SEM, SU8010, Hitachi, 20.0 kV, 10.5 μA). The AFM images were obtained by An AC Mode III (Agilent 5500) atomic force microscope (AFM). X-ray diffraction (XRD) patterns were obtained by using a Bruker X-ray diffractometer with a Cu-Kα radiation source (40 kV, 400 mA). The 2θ diffraction angle was scanned from 10° to 80°, with a scanning speed of 1 second per step. The incident-photon-to-electron conversion efficiency (IPCE) curves were measured under ambient atmosphere using a QE-R measurement system (Enli Technology). The current-voltage characteristics (J-V curves) were obtained with a Keithley 2400 source meter and a sunlight simulator (XES-300T1, SAN-EI Electric, AM 1.5), which was calibrated using a standard silicon reference cell.

3. Results and Discussion

3.1. Morphology of the Porous TiO₂ Film

The size and mass fraction of PS spheres are crucial in determining the porosity and uniformity of the TiO₂ mesoporous layer. The surface morphology of the TiO₂ films was analyzed by scanning electron microscopy. The SEM images of sample PS-1.0 before and after heat treatment are shown in Figure S1 in Supplementary Material available online at https://doi.org/10.1155/2017/4935265. As observed in Figure 2, unlike the TiO₂ layer formed by spinning the paste without PS spheres (PS-0), pores can be observed in the mp-TiO₂ film prepared by TiO₂ paste with PS spheres. For a low mass fraction of 0.5 wt% PS spheres in the paste (PS-0.5), a few pores with an average size of 70 nm (the statistic numbers and histogram of pore size distribution are shown in Table S1 and Figure S2) are formed on the surface of the mp-TiO₂ layer. When the mass fraction of the PS spheres in TiO₂ paste increased to 1.0 wt%, (PS-1.0), lots of pores with an average size of 80 nm are formed. The average pore sizes for both PS-0.5 and PS-1.0 are almost the same, and the pore distribution is uniform. As the mass fraction of PS spheres increases to 1.5 wt% (PS-1.5), pores with relative larger pores (94 nm) were formed due to the high mass fraction of PS spheres and the agglomeration of PS spheres in the TiO₂ paste. The pore distribution is inhomogeneous compared to the PS-1.0 sample. The pore structure could be clearly observed in the cross-sectional SEM images of samples PS-0 and PS-1.0 (Figures 2(e) and 2(f)). As observed in Figures 2(e) and 2(f), the thickness of samples PS-0 and PS-1.0 is 260 nm and 360 nm, respectively. Moreover, the porous structure of sample PS-1.0 is looser than that of sample PS-0. The TiO₂ nanoparticles are densely packed and there are almost no pores among the nanoparticles in sample PS-0. However, pores can be observed from the cross-sectional SEM image in PS-1.0 (Figure 2(f)). The pores emerged not only on the surface of mp-TiO₂ film but also throughout the film. Sample PS-1.0 is thicker than sample PS-0 due to the presence of more pores in the film.

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3.2. Effect of TiO₂ Porosity on the Growth of Perovskite

The mp-TiO₂ film plays a key role in determining the structure of the perovskite layer. SEM and AFM images of the perovskite layer were obtained for mp-TiO₂ films prepared by TiO₂ paste with different mass fraction PS spheres. As shown in Figure 3, the average size of the perovskite is 300 nm regardless of if PS spheres were used in the mp-TiO₂ paste or not. The root-mean-square (RMS) roughness values of the CH₃NH₃PbI₃ films based on samples PS-0, PS-0.5, PS-1.0, and PS-1.5 obtained for a 5 μm × 5 μm area were evaluated as 53.3 nm, 55.6 nm, 53.2 nm, and 58.2 nm, respectively. This means that the amount of pores in the mp-TiO₂ films does not change the morphology and roughness of the perovskite film.

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To better understand the distribution of perovskite in the mp-TiO₂ film, energy dispersive X-ray (EDX) mapping was performed. The EDX mapping of the cross-sectional area shows the distribution of two elements, Ti and Pb, along the mp-TiO₂ film thickness. As observed in Figure 4(a), a small amount of Pb is deposited in the sample PS-0. Figure 4(b) shows that more Pb is deposited at deeper levels in the sample PS-1.0 and the distribution of Pb is more uniform along the thickness of the TiO₂ film. In addition, CH₃NH₃PbI₃ was uniformly distributed in the mp-TiO₂ film owing to the presence of more pores.

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Moreover, the perovskite crystallinity and the amount of residual PbI₂ are crucial to the performance of perovskite solar cells. It is well known that the residual PbI₂ layer exists between the interface of the mp-TiO₂ film and perovskite layer due to the incomplete reaction of PbI₂ and CH₃NH₃I [26]. Excessive amounts of residual PbI₂ will block the electron injected from the perovskite to mp-TiO₂ film, which deteriorates the cell performance of PSCs [27, 28]. However, a trace amount of residual PbI₂ acts as a passivation layer and reduces charge recombination at the interface between mp-TiO₂ film and the perovskite layer. To investigate the influence of the porosity of TiO₂ substrate on the growth of perovskite, X-ray diffraction was performed. In Figure 5, the peaks at 26.8°, 38.1°, and 51.8° correspond to the (110), (200), and (211) planes of FTO. The diffraction peaks marked by stars represent the PbI₂ (001) lattice plane, which conforms well with the literature data [29]. The Bragg peaks at 14.08°, 19.92°, 28.40°, 31.85°, 40.46°, and 43.02°, respectively, represent the reflections from the (110), (112), (220), (310), (224), and (314) crystal planes of the tetragonal perovskite structure [30, 31], which means that the change in porosity of the TiO₂ mesoporous layer has no influence on perovskite crystallinity. However, as the mass fraction of PS spheres in the TiO₂ paste increases, the intensity of the PbI₂ peaks reduces. There is only a little residual PbI₂ in the sample PS-1.0. By introducing PS spheres in TiO₂ paste, more pores were formed in the mp-TiO₂ film. The presence of more pores enables a deeper infiltration of PbI₂ and CH₃NH₃I solution, which endows complete reaction to CH₃NH₃PbI₃.

3.3. Photovoltaic Characterization of PSCs

The current density versus voltage (J-V) characteristics of PCSs based on the mp-TiO₂ layer prepared by TiO₂ paste with and without PS spheres are shown in Figure 6. The photovoltaic parameters of the devices are summarized in Table 1. PSCs based on the mp-TiO₂ film PS-0 showed a reasonable PCE of 10.07% with an open-circuit voltage () of 0.91 V, a short-circuit current () of 19.07 mA/cm², and a fill factor (FF) of 57.99%. A relatively higher performance was exhibited by the device with PS spheres. After doping 0.5 wt% PS spheres, , , FF, and PCE increased to 19.64 mA/cm², 0.94 V, 64.91% and 11.92%, respectively. For a 1.0% mass fraction of PS spheres, the best PCE ( of 0.93, of 19.44 mA/cm², FF of 69.91%, and PCE of 12.62%) was achieved. When the mass fraction of PS spheres increased to 1.5 wt%, the PCS exhibits of 19.54 mA/cm², of 0.90 V, FF of 63.47%, and PCE of 11.14%. The decrease in the residual PbI₂ contributes to rapid electron injection from the perovskite to TiO₂ and higher . The improvements in the performance for the PSCs are mainly due to the increase of FF. Generally, FF depends largely on the series resistance () and shunt resistance ().

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The equivalent circuit of the perovskite solar cell is shown in Figure 7. The output current density can be expressed by the following equation:where represent the reverse saturation current density, is ideality factor, is Boltzmann’s constant, represent the temperature, and represent electron charge. When , (1) can be expressed asIt is found that, from (2), dV/dJ has a linear relation with (. The intercept of the linear fitting curve gives the value of series resistance. Figure 6(b) shows the plot of dV/dJ versus and the linear fitting curve. As can be seen in Figure 6(b), the fitting curves are more linear by doping PS spheres in TiO₂ paste. Slightly decreased values of from 1.57 Ω·cm² to 0.43 Ω·cm², 0.56 Ω·cm², and 0.71 Ω·cm² were evaluated after doping 0.5 wt%, 1.0 wt%, and 1.5 wt% PS spheres in the mp-TiO₂ paste. The smaller R_s values are due to the reduction in both the contact resistance and bulk resistance, which means a higher photocurrent will be generated [32]. However, when the mass fraction of PS spheres is 1.5 wt%, the increased R_s is attributed to the higher resistance caused by more pores and the negative contact with TiO₂ nanoparticles. The device based on TiO₂ mesoporous layer prepared by TiO₂ paste with PS spheres showed larger ( values of the PSCs are 1220 Ω, 3860 Ω, 5320 Ω, and 3700 Ω for doping with 0 wt%, 0.5 wt%, 1.0 wt%, and 1.5 wt% PS spheres, resp.). A higher can improve FF and electron mobility [33], which is consistent with the results of the J-V test.

is closely related to the charge recombination at interfaces inside solar cells. A lower charge recombination contributes to a higher [34]. To better understand the separation of light-induced charge at the TiO₂/CH₃NH₃PbI₃ interface, we performed time-resolved photoluminescence (PL) decay measurements on the CH₃NH₃PbI₃ perovskite-filled mp-TiO₂ films prepared by TiO₂ paste with different mass fractions of PS spheres, which are presented in Figure 8. Using global biexponential fits, the PL decay of the CH₃NH₃PbI₃ perovskite in the mp-TiO₂ films without PS spheres and with 0.5 wt%, 1.0 wt% and 1.5 wt% PS spheres exhibits values of 22.53 ns, 17.57 ns, 17.71 ns, and 18.93 ns, respectively. By doping PS spheres into the TiO₂ paste, the rate of electron injection from the perovskite into TiO₂ film becomes faster, which results in lower charge recombination at the TiO₂/CH₃NH₃PbI₃ interfaces. This could be attributed to the better filling of the CH₃NH₃PbI₃ perovskite in the mp-TiO₂ film and the more complete contact between the TiO₂/perovskite as more pores emerge in the mp-TiO₂ film.

The photon-to-electron conversion efficiency (IPCE) spectra with mp-TiO₂ doping for different mass fractions of PS spheres are shown in Figure 9. The convolution of the spectral response with the photon flux of the AM 1.5G spectrum provided the estimated values of 15.537 mA/cm², 16.994 mA/cm², 16.825 mA/cm², and 16.397 mA/cm². The calculated values from the IPCE spectrum are well matched with the values obtained from the J-V curves. In addition, the PCSs from mp-TiO₂ films with PS spheres exhibited a higher and broader spectrum from 450 nm to 700 nm. Here, an IPCE of ~80% was obtained at the maximum peak, while the device based on mp-TiO₂ films without PS spheres exhibited a lower IPCE of ~70%.

To further determine the influence of the porosity of mp-TiO₂ films on the fabricated solar cells, we showed the statistic results of the cells based on the mp-TiO₂ film prepared by TiO₂ paste with different mass fractions of PS spheres in Figure 10. The deviations of , , FF, and PCE have been shown in Table S2. As observed in Figure 10, and FF increase as the mass fraction of the PS spheres increases and they attain their highest values at the mass fraction of 1.0 wt%, which contributes to the increase in the PCE.

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4. Conclusions

In conclusion, mp-TiO₂ films with tunable porosities were fabricated by doping PS spheres in TiO₂ paste and applied as the ETL of perovskite solar cells. The results indicate that the porosity of mp-TiO₂ films not only affects the infiltration and residual amounts of PbI₂ but also significantly influences the contact between the mp-TiO₂ film and perovskite layer. By adjusting the mass fraction of PS spheres, the perovskite solar cell based on mp-TiO₂ film prepared by TiO₂ paste with 1.0 wt% PS spheres exhibits the highest power conversion efficiency of 12.62% under a simulated standard AM 1.5 condition.

Competing Interests

The authors declare that they have no competing interests.

Acknowledgments

This work was supported by the National High Technology Research and Development Program of China (863 Program) (no. 2015AA050602), the National Natural Science Foundation of China (no. 51372083), Jiangsu Province Science and Technology Support Program, China (BE2014147-4), and the Fundamental Research Funds for the Central Universities (nos. 2014ZZD07 and 2015ZD11).

Supplementary Materials

As shown in Figure S1, before heat treatment, PS spheres can be observed in the TiO₂ film. After the heat treatment, the PS part was burnt out leading to the formation of a plenty of pores in the film. The statistic numbers and the histogram of pore size distribution in TiO₂ film were shown in Table S1 and Figure S2. The average pore size in TiO₂ film prepared by doping 0 wt%, 0.5 wt%, 1.0 wt% and 1.5 wt% PS spheres in TiO₂ paste are 26.45 nm, 70.09 nm, 80.01 nm and 93.90 nm, respectively. The deviation values of J_sc, V_oc, FF, and PCE of different samples were shown in Table S2. By doping PS spheres in TiO₂ paste, the deviation values of J_sc, V_oc, FF, and PCE reduced.

Supplementary Material

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Copyright © 2017 Xiaodan Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Photoenergy

Nanostructured Solar Cells

Influence of the Porosity of the TiO2 Film on the Performance of the Perovskite Solar Cell

Abstract

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Device Fabrication

2.3. Characterization and Measurement

3. Results and Discussion

3.1. Morphology of the Porous TiO2 Film

3.2. Effect of TiO2 Porosity on the Growth of Perovskite

3.3. Photovoltaic Characterization of PSCs

4. Conclusions

Competing Interests

Acknowledgments

Supplementary Materials

References

Copyright

Influence of the Porosity of the TiO₂ Film on the Performance of the Perovskite Solar Cell

3.1. Morphology of the Porous TiO₂ Film

3.2. Effect of TiO₂ Porosity on the Growth of Perovskite