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

Authors: Xiaodan Sun; Jia Xu; Li Xiao; Jing Chen; Bing Zhang; Jianxi Yao; Songyuan Dai
Journal: International Journal of Photoenergy
 (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.


---

## Body

## 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 Jsc 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 Jsc 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.Figure 1
Schematic illustration of the distribution of PbI2 and TiO2 in mp-TiO2 film (a) without and (b) with PS spheres.
(a) (b)


## 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, 10.5μA). 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.


## 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, 10.5μA). 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.Figure 2
Plane-view SEM images of (a) PS-0, (b) PS-0.5, (c) PS-1.0, and (d) PS-1.5. Cross-sectional SEM images under high magnification of (e) PS-0 and (f) PS-1.0.
(a) (b) (c) (d) (e) (f)


### 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.Figure 3
SEM and AFM images (5μm × 5 μm) of the perovskite films based on the mesoporous TiO2: (a), (b) PS-0; (c), (d) PS-0.5; (e), (f) PS-1.0; and (g), (h) PS-1.5.
(a) (b) (c) (d) (e) (f) (g) (h)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.Figure 4
SEM-EDX mapping along the mp-TiO2 film thickness to show the change in the distribution of Ti and Pb in CH3NH3PbI3 loaded on (a) PS-0 and (b) PS-1.0.
(a) (b)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.Figure 5
X-ray diffraction patterns of CH3NH3PbI3 films based on TiO2 mesoporous layers with different mass fractions of PS spheres.


### 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 (Voc) of 0.91 V, a short-circuit current (Jsc) 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, Jsc, Voc, 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 (Voc of 0.93, Jsc 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 Jsc of 19.54 mA/cm2, Voc 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 Jsc. The improvements in the performance for the PSCs are mainly due to the increase of FF. Generally, FF depends largely on the series resistance (Rs) and shunt resistance (Rsh).Table 1
Photovoltaic performance of CH3NH3PbI3 based devices as a function of different wt% of PS.
wt% of PS R s h (Ω) R s (Ω·cm2) J s c (mA/cm2) V o c (V) Fill factor (%) Efficiency (%) 0 1220 1.57 19.07 0.91 57.99 10.07 0.5 3860 0.43 19.64 0.94 64.91 11.92 1.0 5320 0.56 19.44 0.93 69.91 12.62 1.5 3700 0.71 19.54 0.90 63.47 11.14Figure 6
(a) Current density-voltage curves and the best-performing solar cells based on mp-TiO2 film with different wt% of PS. (b) Plots ofdV/dJ versus 1/(J+Jsc) and the linear fitting curves.
(a) (b)The equivalent circuit of the perovskite solar cell is shown in Figure7. The output current density J can be expressed by the following equation:(1)J=J0eqV-JRs/AkT-1+V-JRsRsh-Jsc,where J0 represent the reverse saturation current density, A is ideality factor, k is Boltzmann’s constant, T represent the temperature, and q represent electron charge. When Rs≪Rsh, (1) can be expressed as(2)dVdJ=Rs+AkTq1J+Jsc.It is found that, from (2),dV/dJ has a linear relation with (Jsc-J)-1. The intercept of the linear fitting curve gives the value of series resistance. Figure 6(b) shows the plot ofdV/dJ versus 1/(J+Jsc) 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 Rs 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 smallerRs 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 increasedRs 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 Rsh (Rsh 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 Rsh can improve FF and electron mobility [33], which is consistent with the results of theJ-V test.Figure 7
Equivalent circuit of the perovskite solar cell.R s h is closely related to the charge recombination at interfaces inside solar cells. A lower charge recombination contributes to a higher Rsh [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 τ1 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.Figure 8
Normalized transient PL decay profiles of the perovskites based on TiO2 with different wt% PS.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 Jsc values of 15.537 mA/cm2, 16.994 mA/cm2, 16.825 mA/cm2, and 16.397 mA/cm2. The calculated Jsc values from the IPCE spectrum are well matched with the Jsc values obtained from theJ-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%.Figure 9
IPCE spectra of the best-performing solar cells based on different mass fractions of PS spheres.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 Jsc, Voc, FF, and PCE have been shown in Table S2. As observed in Figure 10, Jsc 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.Figure 10
Statistic results of the cells based on the mp-TiO2 film with different mass fractions of PS spheres.
(a) (b) (c) (d)


## 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.Figure 2
Plane-view SEM images of (a) PS-0, (b) PS-0.5, (c) PS-1.0, and (d) PS-1.5. Cross-sectional SEM images under high magnification of (e) PS-0 and (f) PS-1.0.
(a) (b) (c) (d) (e) (f)


## 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.Figure 3
SEM and AFM images (5μm × 5 μm) of the perovskite films based on the mesoporous TiO2: (a), (b) PS-0; (c), (d) PS-0.5; (e), (f) PS-1.0; and (g), (h) PS-1.5.
(a) (b) (c) (d) (e) (f) (g) (h)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.Figure 4
SEM-EDX mapping along the mp-TiO2 film thickness to show the change in the distribution of Ti and Pb in CH3NH3PbI3 loaded on (a) PS-0 and (b) PS-1.0.
(a) (b)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.Figure 5
X-ray diffraction patterns of CH3NH3PbI3 films based on TiO2 mesoporous layers with different mass fractions of PS spheres.


## 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 (Voc) of 0.91 V, a short-circuit current (Jsc) 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, Jsc, Voc, 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 (Voc of 0.93, Jsc 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 Jsc of 19.54 mA/cm2, Voc 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 Jsc. The improvements in the performance for the PSCs are mainly due to the increase of FF. Generally, FF depends largely on the series resistance (Rs) and shunt resistance (Rsh).Table 1
Photovoltaic performance of CH3NH3PbI3 based devices as a function of different wt% of PS.
wt% of PS R s h (Ω) R s (Ω·cm2) J s c (mA/cm2) V o c (V) Fill factor (%) Efficiency (%) 0 1220 1.57 19.07 0.91 57.99 10.07 0.5 3860 0.43 19.64 0.94 64.91 11.92 1.0 5320 0.56 19.44 0.93 69.91 12.62 1.5 3700 0.71 19.54 0.90 63.47 11.14Figure 6
(a) Current density-voltage curves and the best-performing solar cells based on mp-TiO2 film with different wt% of PS. (b) Plots ofdV/dJ versus 1/(J+Jsc) and the linear fitting curves.
(a) (b)The equivalent circuit of the perovskite solar cell is shown in Figure7. The output current density J can be expressed by the following equation:(1)J=J0eqV-JRs/AkT-1+V-JRsRsh-Jsc,where J0 represent the reverse saturation current density, A is ideality factor, k is Boltzmann’s constant, T represent the temperature, and q represent electron charge. When Rs≪Rsh, (1) can be expressed as(2)dVdJ=Rs+AkTq1J+Jsc.It is found that, from (2),dV/dJ has a linear relation with (Jsc-J)-1. The intercept of the linear fitting curve gives the value of series resistance. Figure 6(b) shows the plot ofdV/dJ versus 1/(J+Jsc) 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 Rs 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 smallerRs 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 increasedRs 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 Rsh (Rsh 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 Rsh can improve FF and electron mobility [33], which is consistent with the results of theJ-V test.Figure 7
Equivalent circuit of the perovskite solar cell.R s h is closely related to the charge recombination at interfaces inside solar cells. A lower charge recombination contributes to a higher Rsh [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 τ1 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.Figure 8
Normalized transient PL decay profiles of the perovskites based on TiO2 with different wt% PS.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 Jsc values of 15.537 mA/cm2, 16.994 mA/cm2, 16.825 mA/cm2, and 16.397 mA/cm2. The calculated Jsc values from the IPCE spectrum are well matched with the Jsc values obtained from theJ-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%.Figure 9
IPCE spectra of the best-performing solar cells based on different mass fractions of PS spheres.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 Jsc, Voc, FF, and PCE have been shown in Table S2. As observed in Figure 10, Jsc 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.Figure 10
Statistic results of the cells based on the mp-TiO2 film with different mass fractions of PS spheres.
(a) (b) (c) (d)


## 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.



---

*Source: 4935265-2017-02-01.xml*