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Performance improvement of fully ambient air fabricated perovskite solar cells in an anti-solvent process using TiO2 hollow spheres
Journal Pre-proofs Performance improvement of fully ambient air fabricated perovskite solar cells in an anti-solvent process using TiO2 hollow spheres Seyedeh Mozhgan Seyed-Talebi, Iraj Kazeminezhad PII: DOI: Reference:
S0021-9797(19)31461-4 https://doi.org/10.1016/j.jcis.2019.12.004 YJCIS 25744
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
15 August 2019 27 November 2019 2 December 2019
Please cite this article as: S. Mozhgan Seyed-Talebi, I. Kazeminezhad, Performance improvement of fully ambient air fabricated perovskite solar cells in an anti-solvent process using TiO2 hollow spheres, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.004
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Performance improvement of fully ambient air fabricated perovskite solar cells in an anti-solvent process using TiO2 hollow spheres Seyedeh Mozhgan Seyed-Talebi,1* Iraj Kazeminezhad1 1Department
of Physics, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran E-mail: [email protected] , [email protected]
Tel: 00989123094967; Fax: 00986133331040
Abstract: The porosity optimization of electron transporting layer (ETL) in the perovskite solar cells (PSCs) can make the effective pathways for transporting electron and blocking the holes. In the present study, the porosity modification effect of TiO2 paste as most efficient ETL using TiO2 hollow spheres (TiO2 -HSs) on the air-processed formation of perovskite films is studied. In this procedure, the TiO2 -HSs were synthesized using removable carbonaceous sphere templates. Our characterization results demonstrated that prepared TiO2 - HSs showed an external diameters less than 200 nm with shell thickness about 20-30 nm. Due to high porosity of the TiO2 - HSs, CH3NH3PbI3 sufficiently infiltrate into the modified ETL. Thus a high- quality perovskite film with large grain size and smooth surface fabricated on the modified ETL. Further time resolved photoluminescence (TRPL) investigation reveals that an increase in the electron injection and recombination resistance leading to the performance improvement of the PSCs. The best fully ambient processed device with modified electron transporting layer exhibited an efficiency of 19.62%, which is 16.37% higher than the efficiency of the standard PSC. Application of TiO2-HSs in the ETL can help to the development of air-processed perovskite solar cells for commercialization in the future. Keywords: Anti-solvent; Carbon sphere; Mesoporous TiO2; Perovskite Solar cell; TiO2 hollow sphere.
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1. Introduction In general, perovskite solar cells can be assembled as either planar heterojunction architectures [1-8] or mesoscopic structures [9–13]. It has been reported that mesoscopic structure with an inorganic metal oxide semiconductor as the electron transporting layer (ETL) has attracted much attention. Due to the transportation of electrons through both of perovskite layer and mesoporous ETL, design of the ETL plays a crucial role in the efficiency of mesoscopic perovskite solar cells not only as a scaffold of perovskite absorber but also as the pathway for transporting electrons and blocking the holes [14- 18]. In the present work, advanced interface engineering has been applied by porosity modification of commonly used TiO2 layer as a more efficient ETL in the normal perovskite solar cells, which helps more CH3NH3PbI3 penetration into the TiO2 lattice and excellent pore filling through the highly interconnected mesoporous structure. The resultant ETL which is not exactly mesoporous helps to obtain a high efficient and stable PSCs based on the combination of planar and mesoporous structures. In comparison with a standard solar cell, more easy mass migration results in the growth of perovskite deep inside the modified TiO2 layer and obtaining a perovskite layer with larger crystal size and more electron injection [19, 20]. The combination effect of PbI2 and MAI in the crystallization and morphology of mesoscopic CH3NH3PbI3 based PSCs discussed in previous reports [21-26]. In the conventional solution processes, the porous structure of TiO2 hindered the diffusion of CH3NH3PbI3 into the pore channel of TiO2 filled by PbI2, resulting in relatively small amounts of unreacted PbI2 remaining in the finally obtained perovskite solar cell [27]. Therefore, crystallization process maybe results in some large crystals on the TiO2 surface but poor coverage due to the localized nucleation of perovskite layer which tends to obstruct the continual diffusion of MAI to react with PbI2 [28]. The fast combination of PbI2 and MAI at the outside capping layer, leading to a small contact area between perovskite and TiO2 layer. In addition, it seems most of PbI2 is converted to the perovskite, but some amounts of unreacted PbI2 may be retained in the interface of TiO2/perovskite layer. The residual PbI2 has a detrimental effect
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on carrier separation and blocks the transformation of electrons from CH3NH3PbI3 to TiO2 inducing a decrease in PCE of PSCs [28, 29]. In comparison with solution methods, in the anti-solvent treatment technique can largely improve the coverage of a compact perovskite layer with uniform grain structure up to micrometers on the surface of ETL [30]. The antisolvent deposition in the air pressure is preferred here due to all of the fabrication process performed using a simple lab heater. Thus, the results are more applicable for industrial production of solar panels by a higher degree of reproducibility and easily generating of an extremely uniform and flat film with no any pinhole and uncovered area on the surface. This work is organized as follows: It first summarizes the synthesis and characterization of carbon sphere templates and TiO2 - HSs that we used. It then outlines the effect of TiO2 HSs usage on the properties of perovskite layers deposited on different ETLs. Finally, the characterization results of modified solar cell are compared with the standard perovskite solar cell.
2. Results and Discussion All the chemical materials, material characterization instruments, and preparation methods are presented in supplementary informations. 2. 1. Sample preparation In this section, the characterization of carbon spheres, and TiO2 - HSs were firstly explained. The preparation of TiO2 -HSs and effects of their usage on the morphology and grain size of the perovskite layer were then investigated. In the end, the TiO2 - HSs paste was used to build perovskite solar cell according to the procedure described in the experimental section. 2. 2. 1. Characterization of TiO2 hollow sphere Preparation of carbon sphere from sucrose as precursor using hydrothermal method and TiO2 - HSs using carbon sphere templates were described in detail elsewhere [31, 32]. Figure 1 demonstrates typical field emission scanning electron microscopy (FESEM) images of
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synthesized carbon spheres and TiO2 - HSs. The obtained HSs have a regular shape with an average diameter of about 160 ± 30 nm.
Figure 1. FESEM, and particle distribution of (a) carbon spheres, and (b) TiO2 hollow spheres.
The experimental analysis of elemental mapping (MAP) pattern, and energy dispersive Xray spectroscopy (EDX) of the samples shown in Figure 2.
Figure 2. MAP pattern, and Corresponding EDX spectrum of (a) carbon spheres and (b) TiO2 hollow spheres on top of the Aluminum substrate. The peak of Al comes from the Aluminum substrate used in the measurements.
It can be seen that carbon spheres are composed of carbon and oxygen which amount of carbon is higher than that of oxygen. Oxygen peak can be appeared due to the incomplete decomposition of sucrose, adsorption of water on the surface of carbon spheres, or existence
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of carboxyl and hydroxyl functional groups on the carbon surfaces which is in good agreement with FT-IR results (Figure S1, Supplementary Informations). The absence of carbon in TiO2 – HS’s spectrum indicated that all the carbon sphere templates were burned. 2. 1. 2. Preparation of TiO2 hollow sphere paste The TiO2 hollow sphere paste was synthesized from α-terpineol as a solvent, and ethyl cellulose as a binder. The preparation details can be found in the Supporting Informations. The pores with uniform size distribution in the TiO2 hollow sphere paste as effective light scattering centers extend the light traveling distance, thereby improve the optical absorption of the ETL. 2. 2. PSC devices fabrication and measurement techniques As shown in the schematic illustration of Figures 3(a), 3(b), the layered structure of present perovskite solar cells consist of a compact layer of TiO2 deposited on top of F-doped SnO2 and a pure or modified TiO2 layer. The mesoporous TiO2 layer filled with a perovskite layer and Spiro as a hole-transport layer. More details about the fabrication process can be found in the Supporting Informations. 2. 2. 1. Fabrication of PSCs In this study, we fabricated a compact and pin-hole free perovskite film with full coverage on the TiO2 layer via one step method using nonpolar diethyl ether as anti-solvent solution while spinning in the atmospheric pressure [30]. Images of perovskite deposition process using this method is shown in Figure 3(c-e).
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Figure 3. (a) FESEM cross section image of standard and modified perovskite solar cell, (b) Schematic illustration of modified mp-TiO2 using TiO2 HSs, Size of particles and thickness of layers are not in scale. Deposited CH3NH3PbI3 layer on ETL (c) right after washing the spin-coated layer by ether, (d) after annealing at 65 °C for 1 min (e) after annealing at 100 °C for 2 min;
The FESEM images of deposited CH3NH3PbI3 films on both mp-TiO2 and TiO2 - HS films (Figure 4) show rather homogeneous surfaces with no indications of pinhole defects. Fine tuning porosity of TiO2 films using TiO2 – HSs results in very well penetration of MAI into the pores of TiO2 and creating a uniform, continuous, and homogeneous compact CH3NH3PbI3 film with a complete surface coverage by larger grain sizes. Perovskite film with larger grain size causes to mitigate the possibility of trap states or carrier blocking at grain boundaries. Perovskite devices fabricated on such highly porous layers with better pore-filling of perovskite show higher PCE and diminished hysteresis. As shown in Figure 4, all of the deposited CH3NH3PbI3 layers have compact structure, but the size of the CH3NH3PbI3 grains are obviously different. For the CH3NH3PbI3 layers deposited on the TiO2, the average size of CH3NH3PbI3 grains is approximately 800 nm
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(Figure 4(a)). Using TiO2 HSs, the average size of CH3NH3PbI3 grains increased to 1300 nm.
Figure 4. FESEM image, and grain size distribution of deposited perovskite films on to (a) standard TiO2, (b) TiO2 -HS, and (c) XRD pattern, and (d) UV-visible optical absorption spectra of CH3NH3PbI3 layer deposited on different ETLs. The inset of (d) is the (αhυ)2 ~hυ Tauc plots for estimating the direct band gap energies of the prepared perovskite layers.
Relatively lower defect density observed in fabricated perovskite on the modified ETL as compared to that on the standard ETL could also be attributed to the slow crystallization rate inside the scaffold. During the perovskite formation process, the color of the coated precursor on the modified ETL undergoes slower color change as compared to the one fabricated on the standard mp-TiO2 substrate, indicating the slow crystallization of perovskite film. A slow crystallization rate allows for a high-quality perovskite crystal with lower defect density. The XRD pattern of perovskite films is shown in figure 4(c). The appearance of main diffraction peaks at 2θ: 14.18, 28.45, 31.81, and 43.08 can be assigned as the (110), (220), (310) and (330) plans of tetragonal CH3NH3PbI3 crystals, respectively. The absence of PbI2 peak at 12.6 ˚ shows the complete conversion of PbI2 to CH3NH3PbI3. The observed XRD peak positions of CH3NH3PbI3 crystals match well with the previously reported data [33-35]. The intensities of main tetragonal phase peaks are significantly enhanced for CH3NH3PbI3 layer deposited on TiO2–HS film (Figure 4(c)). This implies the favourable crystallographic orientations for carrier transport perpendicular to the substrate increase upon the TiO2 optimization which is important to obtain a uniform, compact, and smooth perovskite layer
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with lesser voids to avoid unnecessary leakage of current [36]. Interestingly, the (110) peak shift of ∼0.04 ° toward a lower angle is observed for fabricated perovskite on modified ETL, can be attributed to the volume extension without formation of additional recombination centres [37]. This volume expansion might be ascribed to the tensile stress developed by the attachment of the growing perovskite crystal to the TiO2 -HSs while constraining the shrinkage during crystallization. The slightly enlarged estimated band gap of modified perovskite (about 0.01 eV) shown in insert of figure 4 (d) can be attributed to the crystal volume expansion of fabricated perovskite on the TiO2-HSs. Owing the higher absorbance of deposited perovskite on the HS-TiO2 ETL, it is expected for the PSK on the HS-TiO2 film to generate higher photocurrent in the modified perovskite solar cell device. 2. 2. 2. PL and TRPL of CH3NH3PbI3 layer on different ETLs The steady-state photoluminescence (PL) measurements performed to confirm the better charges separation, charges extraction, and recombination resistance between ETL and perovskite absorber by application of TiO2 – HS as an efficient ETL. Although the position of PL peaks for all of the CH3NH3PbI3 films deposited on the different substrates is approximately centred at a constant wavelength of ∼770 nm (Figure 5 (a)), when the CH3NH3PbI3 layer was in contact with and without TiO2 layers showing different PL intensities with order of PSK> PSK- TiO2 > PSK - TiO2 -HS, indicating the electron extracting ability in the reverse order. The strongest PL quenching has been observed in CH3NH3PbI3 layer deposited on TiO2 –HS film due to the existence of more recombination resistance and electron extracting ability than other samples. The great PL quenching caused by the improved interconnection on the interface of TiO2 –HS / CH3NH3PbI3 due to more effective electron-injection from perovskite to ETL.
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Figure 5. Normalized a) Photoluminescence (PL) spectra and b) Transient PL decay profile of CH3NH3PbI3, and CH3NH3PbI3 deposited on TiO2 Film with and without TiO2-HSs. The transient PL decays were obtained using the Time-Correlated Single Photon Counting (TCSPC) technique with excitation at 635 nm and probe at 770 nm.
The corresponding band gap (Eg=1.61 eV) determined from the PL spectra of modified perovskite (Figure 5(a)) is in good agreement with the previous researcher reports [38-40]. This observation also is supported by the determined band gap from UV-vis absorption spectra using Tauc plot (Figure 4 (d)). Time-resolved photoluminescence (TRPL) decay profiles were studied to investigate the effect of TiO2 –HSs on the lifetime of charge carriers. In Figure 5 (b), the PL decay of the perovskite films on glass without and with various ETLs was compared. The observance of higher PL quenching in the case of deposited CH3NH3PbI3 on the TiO2-HS surface than the standard TiO2 layer, indicates more electron-extracting ability of the TiO2-HS surface. The fitted lifetimes with bi-exponential decay equations and two components under different substrates are listed in Table 1. The fast decay components (τ1) are attributed to the charge carrier recombination and trap-assisted recombination related to the interface (or surface) defects. The slow components (τ2) are ascribed to the radiative recombination related to the bulk defects. Coated perovskite on glass substrate exhibited the longest decay components for both fast and slow recombination (τ1= 39.3 ns and τ2 =127.3 ns), indicating its highest non-radiative recombination centers. The fitted bi-exponential decay equation for
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CH3NH3PbI3 film on standard mp-TiO2 layer resulted in an effective lifetime with a fast component of
1
= 3.5 ns and a slow component of
2
= 16.3 ns. Assuming that all PL
quenches are due to the effect of electron extraction, the time of interfacial electron extraction (τe) at the interface between perovskite layer and ETL can be calculated as: 1 1 1 e PSK /ETL PSK
(1)
in which ,𝜏psk and PSK/ETL are the PL lifetimes for perovskite layer deposited on glass substrate, and those with perovskite deposited on standard and modified ETLs, respectively. The average PL lifetime of perovskite layer (τPSK) was determined using the statistical definition PSK
A A
2 i i
to be 97.8 ns, in which τi and Ai are lifetimes and corresponding
i i
amplitudes of each component, respectively. Table 1. Time coefficients (relative amplitudes) of corresponding perovskite films obtained from fitting PL transients (Figure 5b) (excitation at 635 nm, probe at 770 nm). Sample
τ1(ns)
A1
τ2 (ns)
A2
τPSK (ns)
τe (ns)
PSK
39.3
0.62
127.3
0.38
97.8
TiO2/PSK
3.5
0.73
16.3
0.27
13.2
HS-TiO2/PSK
2.7
0.57
8.6
0.43
7.4
As one can see in Table 1, the τe in the interface of perovskite and ETL decreases from 13.2 to 7.4 ns when perovskite layer was in contact with the standard and TiO2 -HSs based ETL, respectively. This implies that the trend of electron extraction (1/τe) in the CH3NH3PbI3/ETL interface of TiO2 -HSs based solar cell is higher than that of the standard one, indicating that TiO2 –HSs based ETL as a more efficient ETL is responsible for the high-performance CH3NH3PbI3 based solar cells. The revealed change in the slow component of modified cell can be indicative of the suppression of trap-assisted recombination and improvement of the quality of the bulk perovskite. These results are in agreement with the FESEM observations
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of figure 4(b) in which standard perovskite layer has a defect density and grain boundaries higher than that of the modified perovskite layer. The enhanced charge carrier extraction with reduced trap density can explain the improved PCE along with less hysteresis for the HS-TiO2 based perovskite solar cells. 2. 3. Photovoltaic study results The experimental results of current density-voltage characterization of TiO2 and HS- TiO2 based perovskite solar cells are listed in Table 2. The statistical analysis of data demonstrated good reproducibility of devices. Figure 6 compares the distributions of Jsc, Voc, fill factor, and PCE of best devices of each type under the same experimental conditions with various ETLs.
Figure 6. Boxplots of photovoltaic parameters a) JSC, b) VOC, c) FF, and d) PCE for devices made of HS-TiO2, and standard mp- TiO2 paste as ETLs. Each data set is carried out from at least 25 devices. The devices were measured under simulated AM1.5, 100 mWcm-2 sun light. e) Current Density-voltage curves of best efficiency devices, and f) IPCE and integrated current density of the best device in comparison with standard device.
The improved FF and JSC of samples made of modified TiO2 films by TiO2 –HSs are attributed mainly to the larger CH3NH3PbI3 grains and uniform covering the entire substrate,
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which improve light-harvesting properties. The larger fill factor shows that the modification on the surface reduces the loss of photo-excited carriers being extracted from the device due to the suppression of surface electron recombination. The increase in short circuit current density is also evidence for better light absorption and light scattering of modified ETL by TiO2 -HSs. Higher charge concentration in the interface of TiO2 and perovskite absorber for a certain number of injected electrons leads to retarding the recombination in PSC and getting high Voc and PCE. Our results indicate that photovoltaic performance of the TiO2 – HS based device shows the greater VOC, JSC, FF, and PCE values than those of standard device which reasonably explains the superior shorter electron-extraction time and higher photocurrent response of the perovskite deposited onto the TiO2 -HS layer than that of on the standard ETL. Table 2. Photovoltaic Parameters of Perovskite Solar Cells Fabricated with Varied TiO2 layer. Devices are measured under simulated AM-1.5G illumination with active area of 0.09cm2. aETL
bAverage
TiO2 nanoparticle cBest bAverage
TiO2 – HS cBest
JSC(mAcm-2)
VOC (V)
FF
PCE (%)
21.55 ± 𝟎.𝟎𝟔
1.03 ± 0.02
71.16 ± 𝟏.𝟓𝟕
15.94 ± 𝟎.𝟒𝟓
21.50
1.04
75.40
16.86
23.33 ± 𝟎.𝟒𝟕
1.05 ± 𝟎.𝟎𝟐
75.64 ± 𝟏.𝟐𝟐
18.67 ± 𝟎.𝟒𝟕
23.6
1.08
76.96
19.62
[a] The devices were fabricated using the CH3NH3PbI3 layers prepared via one step methods using anti solvent treatment on to the different electrontransporting layers (ETLs) detailed in the Experimental Methods in supporting information (SI). [b] The average values shown with the uncertainties representing one standard deviation were obtained from 25 devices fabricated under the same experimental conditions analyzed by the analysis of variance (ANOVA) detailed in Tables S1 and S2 in supporting information (SI). [c] The best photovoltaic performances are reported for each type of TiO2 based devices.
The best performing devices with modified TiO2 layer show the PCE up to 19.62% with very small hysteresis in J-V characteristics. Enlarging the interfacial contact between perovskite and TiO2 by effective pore-filling suppresses the charge accumulation caused by ion migration and diminishes the possibility of hysteresis. The J-V characteristics of the
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CH3NH3PbI3 based solar cells prepared with varying standard ETL by TiO2 –HSs in forward and reverse scans are shown in Figure 7. Although the hysteresis effect in regular planar structures is greater than that of PSCs with mesoporous ETL [14]. In this structure, the hysteresis effects have been significantly relieved compared with the standard device with mesoporous TiO2 layer. Reducing the hysteresis effect (Figure 7 (b)), is related to trapping and detrapping mechanism of electrons at the interface of perovskite and modified ETL.
Figure 7. J-V characteristics of the devices based on devices in contact to (a) TiO2 and (b) TiO2 – HS layers, under illumination condition in reverse and forward sweep directions. This devices were measured under simulated AM1.5 and 100 mW.cm-2 sun light.
The majority of located electron and hole traps near the surface and grain boundaries of the perovskite layer could be a reason for the reduction of ion migration at grain boundaries and J-V hysteresis. The observance of hysteresis in the perovskite solar cells is in accordance with the modification of the interfacial surfaces and time-dependent photocurrent density (Jsc) behavior shown in Figure 8. As observed in the hysteresis curves, at certain voltages for both standard and modified devises, the J−V curves at forward and backward stepwise scan separate from each other. This effect is clearly visible at ∼0.7−0.9 open circuit voltage. The discrepancy in the J–V characteristic of the hysteresis curves varies trends, consequently, results in the scanning direction-dependent of photocurrent density behavior of devices. For the time-dependent
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photocurrent density measurements, the devices were kept under one sun illumination (AM 1.5, 100 mW/cm2) at 0.8 Voc. In the reverse scan (RS) curves, the detected current increases sharply as a large photocurrent spike while sudden increase in the photo induced separation of the electron hole pairs occurs. As charge carriers migrated from the interior of perovskite layer to the interfaces (electrons to the ETL and holes to the HTL), the photocurrent spike followed by a slow exponentially decay with time to the equilibrium state. When the recombination rate and charge generation reach to equilibrium, the steady state current is achieved. The subsequent decrease in photocurrent indicates that more recombination is occurring within the perovskite layer. The smaller difference in observed photo current density of modified devise (∼1.34 eV) in spite of Jsc difference monitored in the time dependent Jsc of standard devise (∼1.63 eV) is due to the less hysteresis behavior of HS-TiO2 based solar cell.
Figure 8. Time-dependent photocurrent response of standard and modified cells at the applied voltage of 0.8 V under (a) reverse (RS), and (b) forward (FS) scans.
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Under forward stepwise scan (FD) with possessing the applied bias, detected current shows a sharp decrease in the time-dependent photocurrent density at first, followed by a continuous recovery and reaches to the saturation point. The TiO2 -HSs as the electron transporting layer provided high photocurrent due to better connection between ETL/perovskite layer, and growing the smooth perovskite layer on its surface than the standard ETL substrate. The impedance spectroscopy (IS) measurement of standard and modified solar cells with different ETLs is performed to monitor the interfacial changes such as charge transfer, recombination, and charge accumulation in the interfaces of perovskite layer. The Nyquist plots of the standard and modified solar cells, which are measured under one-sun illumination and open-circuit conditions in the frequency range from 1 MHz to 1 Hz shown in Figure 9. In the corresponding equivalent circuit of Figure 9, Rs, Rcon and Rrec are related to the series resistance, the contact resistance at perovskite’s interfaces, and recombination resistance, respectively.
Figure 9. The fitted equivalent circuit, and impedance response of typical Nyquist plots of standard and modified perovskite solar cells.
The contact resistance (Rcon) derived from high-frequency arc in the Nyquist plots literally means the charge transfer resistance at the ETM/perovskite/HTM interfaces. It can be related
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to the electron transporting ability of TiO2 layer resulting from the geometrical factors such as roughness and the structure of the mesoporous TiO2 layer. Thus, the contact resistance value significantly changed when porosity modification of ETL is involved because of better contact and easier electron transfer at the interface of perovskite and modified ETL. The Rcon value is decreased for the modified solar cell compared to the standard solar cell (from 290 Ω to 189.5 Ω) which is indicative of better contact between HS-TiO2 and perovskite in comparison to that between mp-TiO2 and perovskite. Moreover, the Rrec is significantly higher for modified solar cell (78.73 Ω compared to 62 Ω) which pointed to the more blocked charge recombination and more electrons extraction at the interface of perovskite/ETL of modified solar cell, which leads to the improvement of photovoltaic characteristics.
3. Conclusions Our research focused on the importance of morphology control of ambient air processed perovskite layer through substrate surface modification to obtain highly efficient PSC device, which has an impact on developing PSC’s commercialization in the future. We have fabricated a TiO2-HS based ETL with similar thickness of mp-TiO2 in order to demonstrate the morphology and porosity of ETL play a crucial role to attain a satisfactory device performance and decreased hysteresis for PSC. The TiO2 based ETL designed with TiO2HSs leading to formation of a dense, uniform and homogeneous perovskite layer, which ensures smooth surface with low defect density and maximum contact areas at the interfaces with the PSK/ETL; it thus enhances charge separation at the interface of PSK/ETL. The characterization of deposited perovskite on the standard mp-TiO2 layer and modified layer with TiO2 – HSs was investigated with FESEM, EDX-mapping, and UV-vis analysis. Moreover, the enhancement in photocurrent of HS-TiO2 based PSC is relatively higher than response of solar cell with mp-TiO2 layer due to the enormous contact surface area between CH3NH3PbI3 and HS-TiO2. This is supported by the smooth perovskite layer seen in FESEM results. The electron-transfer properties were investigated via steady state PL and TRPL (via TCSPC) spectra. We found a correlation between the rate of electron transfer and the
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morphology of mp-TiO2 layer. When the perovskite layer was coated on the mp-TiO2 layer, ETL exhibited a PL lifetime longer than that of the HS-TiO2 based substrate, representing much slower charge transport in a standard ETL. The best device performance attained PCE 19.62 % for TiO2-HS based PSK, at which larger perovskite grain size and rapid electron transfer occurred. The TiO2–HS based devices also showed a minor effect of hysteresis with excellent reproducibility during a fully air processed fabrication. As our ETL structure is something between mesoscopic and planar, we believe the idea is valuable for readers to test it in other devices and check the efficiency enhancement of their standard cells in air atmosphere and pressure for commercial application.
Experimental Section Experimental Details could be found in the Supporting Informations.
References [1] Y. Yang, J. Wu, T. Wu, Z. Xu, X. Liu, Q. Guo, and X. He, An efficient solvent additive for the preparation of anion-cation-mixed hybrid and the high performance perovskite solar cells, J. Colloid. Interf. Sci. 2018, 531, 602-608 [2] D. Luo, W. Yang, Z. Wang, A. Sadhanala, Q. Hu, R. Su, R. Shivanna, G. F. Trindade, J. F. Watts, Z. Xu, T. Liu, K. Chen, F. Ye, P. Wu, L. Zhao, J. Wu, Y. Tu, Y. Zhang, X. Yang, W. Zhang, R. H. Friend, Q. Gong, H. J. Snaith, R. Zhu, Enhanced photovoltage for inverted planar heterojunction perovskite solar cells, Science. 2018, 360, 1442-1446. [3] Y. Saygili, S. ‐H. Turren‐Cruz, S. Olthof, B. W. H. Saes, I. B. Pehlivan, M. Saliba, K. Meerholz, T. Edvinsson, Sh. M. Zakeeruddin, M. Grätzel, J. ‐P. Correa‐Baena, A. Hagfeldt, M. Freitag, W. Tress, Planar Perovskite Solar Cells with High Open‐Circuit Voltage Containing a Supramolecular Iron Complex as Hole Transport Material Dopant, ChemPhysChem. 2018, 19, 1363-1370. [4] G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely, H. J. Snaith, Morphological control for high performance, solution‐processed planar heterojunction perovskite solar cells. Adv. Funct. Mater. 2014, 24, 151–7. [5] N. Marinova, S. Valero, and J. L. Delgado, Organic and perovskite solar cells: Working principles, materials and interfaces, J. Colloid. Interf. Sci. 2017, 488, 373-389.
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[6] F. Huang, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S. Rubanov, Y. Zhu, X. Lin, L. Jiang, Y. Zhou, A. Gray-Weale, J. Etheridge, C. R. McNeill, R. A. Caruso, U. Bach, L. Spiccia, Y. -B. Cheng, Gas-assisted preparation of lead iodide perovskite films consisting of a monolayer of single crystalline grains for high efficiency planar solar cells, Nano Energy. 2014, 10, 10-18. [7] Michael Grätzel, The light and shade of perovskite solar cells, Nat. Mater. 2014, 13, 838– 842. [8] X. Gan, O. Wang, K. Liu, X. Du, L. Guo, H. Liu, 2D homologous organic-inorganic hybrids as light-absorbers for planer and nanorodbased perovskite solar cells, Sol. Energy Mater. Sol. Cells. 2017, 162, 93–102. [9] B. Zhang, Z. Song, J. Jin, W. Bi, H. Li, C. Chen, Q. Dai, L. Xu, and H. Song, Efficient rare earth co-doped TiO2 electron transport layer for high-performance perovskite solar cells, J. Colloid. Interf. Sci. 553 (2019) 14–21. [10] L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, and M. Grätzel, Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells, J. Am. Chem. Soc. 2012, 134, 17396–17399. [11] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science. 2012, 338, 643–647. [12] N. –G. Park, Organometal Perovskite Light Absorbers toward a 20% Efficiency LowCost Solid-State Mesoscopic Solar Cell, J. Phys. Chem. Lett. 2013, 4, 2423–2429. [13] Y. Zhao, A. M. Nardes, and K. Zhu, Solid-State Mesostructured Perovskite CH3NH3PbI3 Solar Cells: Charge Transport, Recombination, and Diffusion Length, J. Phys. Chem. Lett. 2014, 5, 490−494. [14] M. F. Mohamad Noh, C. H. Teh, R. Daik, E. L. Lim, C. C. Yap, M. A. Ibrahim, N. A. Ludin, A. R. Mohd Yusoff, J. Jang, and M. A. M. Teridi, The architecture of the electron transport layer for a perovskite solar cell, J. Mater. Chem. C. 2018, 6, 682-712. [15] G. Yang, H. Tao, P. Qin, W. Ke, and G. Fang, Recent progress in electron transport layers for efficient perovskite solar cells, J. Mater. Chem. A. 2016, 4, 3970-3990. [16] N. –G. Park, M. Grätzel, T. Miyasaka, K. Zhu and K. Emery, Towards stable and commercially available perovskite solar cells. Nat. Energy. 2016, 1, 16152.
Journal of Colloid and Interface Science
19 of 22
[17] L. Yang, X. Wang, X. Mai, T. Wang, C. Wang, X. Li, V. Murugadoss, Q. Shao, S. Angaiah, and Z. Guo, Constructing efficient mixed-ion perovskite solar cells based on TiO2 nanorod array, J. Colloid. Interf. Sci. 2019, 534, 459-468. [18] F. Gao, H. Dai, H. Pan, Y. Chen, J. Wang, Z. Chen, Performance Enhancement of Perovskite Solar Cells by Employing TiO2 Nanorod Arrays Decorated with CuInS2 Quantum Dots, J. Colloid. Interf. Sci, 2018, 513, 693-699. [19] Z. Y. Wu, B. –L. Jian, and H. –C. Hsu, Photoluminescence characterizations of highly ambient-air-stable CH3NH3PbI3/PbI2 hetero structure, Opt. Mater. Express. 2019, 9, 18821892. [20] S. –Y. Kim, H. J. Jo, S. –J. Sung, and D. –H. Kim, Perspective: Understanding of ripening growth model for minimum residual PbI2 and its limitation in the planar perovskite solar cells, APL Mater. 2016, 4, 100901. [21] T. Du1, C. H. Burges, J. Kim, J. Zhang, J. R. Durrant, and M. A. McLachlan, Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells, Sustain. Energ. Fuels. 2017, 1, 119-126. [22] X. Sun, J. Xu, L. Xiao, J. Chen, B. Zhang, J. Yao, and S. Dai, Influence of the Porosity of the TiO2 Film on the Performance of the Perovskite Solar Cell, Int. J. Photoenergy. 2017, 2017, 4935265. [23] H. Chen, Z. Wei, H. He, X. Zheng, K. S. Wong, S. Yang, Solvent Engineering Boosts the Efficiency of Paintable Carbon based Perovskite Solar Cells to Beyond 14 %, Adv. Energy Mater. 2016, 6, 1502087. [24] B. –E. Cohen, L. Etgar, Parameters that control and influence the organo-metal halide perovskite crystallization and morphology, Optoelectron. 2016, 9, 44–52. [25] Y. Li, B. Ding, G. –J. Yang, C. –J. Li, C. –X. Li, Achieving the high phase purity of CH3NH3PbI3 film by two-step solution processable crystal engineering, J. Mater. Sci. Technol. 2018, 34, 1405-1411. [26] Q. Chen, H. Zhou, T. –B. Song, S. Luo, Z. Hong, H. –S. Duan, L. Dou, Y. Liu, and Y. Yang, Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells, Nano. Lett. 2014, 14, 4158−4163. [27] H. K. Adli, T. Harada, W. Septina, S. Hozan, S. Ito, and S. Ikeda, Effects of Porosity and Amount of Surface Hydroxyl Groups of a Porous TiO2 Layer on the Performance of a CH3NH3PbI3 Perovskite Photovoltaic Cell, J. Phys. Chem. C. 2015, 19, 22304−22309.
Journal of Colloid and Interface Science
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[28] M. Jung, S. – G. Ji, G. Kim and S. I. Seok, Perovskite precursor solution chemistry: from fundamentals to photovoltaic applications, Chem. Soc. Rev. 2019, 48, 2011-2038. [29] H. Wei, Y. Tang, B. Feng, and H. You, Importance of PbI2 morphology in two-step deposition of CH3NH3PbI3 for high-performance perovskite solar cells, Chin. Phys. B. 2017, 26, 128801. [30] N. Ahn, D.- Y. Son, I.- H. Jang, S. M. Kang, M. Choi, and N. –G. Park, Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead (II) Iodid, J. Am. Chem. Soc. 2015, 137, 8696−8699. [31] S. M. Seyed-Talebi, I. Kazeminezhad, M. Nematpour, Synthesis and characterization of Ag@Carbon core-shell spheres as a novel catalyst for room temperature N-arylation reaction, J. Catal. 2018, 361, 339-346. [32] S. M. Seyed-Talebi, I. Kazeminezhad, H. Motamedi, TiO2 hollow spheres as a novel antibiotic carrier for the direct delivery of gentamicin, Ceram. Int. 2018, 44, 13457-13462. [33] Y. Li, X. L. He, B. Ding, L. L. Gao, G. J. Yang, C. X. Li, C. J. Li, Realizing full coverage of perovskite film on substrate surface during solution processing: Characterization and elimination of uncovered surface, J. Power Sources. 2016, 320, 204–211. [34] J. Kim, J. S. Yun, X. Wen, A. Mahboubi Soufiani, C. F. Jonathan Lau, B. Wilkinson, J. Seidel, M. A. Green, S. Huang, and A. W. Y. Ho-Baillie, Nucleation and Growth Control of HC(NH2)2PbI3 for Planar Perovskite Solar Cell, J. Phys. Chem. C. 2016, 120, 11262–11267. [35] H. Gao, C. Bao, F. Li, T. Yu, J. Yang, W. Zhu, X. Zhou, G. Fu and Z. Zou, Nucleation and Crystal Growth of Organic-Inorganic Lead Halide Perovskites under Different Relative Humidity, ACS Appl. Mater. Interfaces. 2015, 7, 9110–9117. [36] S. M. Seyed-Talebi, I. Kazeminezhad, S. Shahbazi, E. W. Diau, Efficiency and Stability Enhancement of Fully Ambient Air Processed Perovskite Solar cells using TiO2 Paste with Tunable Pore Structure, Adv. Mat. Interfaces. Accepted 18 Nov 2019. [37] J. N. Sherwood, R. I. Ristic, The influence of mechanical stress on the growth and dissolution of crystals, Chem. Eng. Sci. 2001, 56, 2267-2280. [38] H. Pourradi, K. Ghani, and M. Mahdavi, Fully Ambient Air Processed Perovskite Solar Cell Based on Co (Co, Cr)2O4/ TiO2 P-N Heterojunction Array in Photoanode, J. Phys. Chem. C. 2019, 123, 4044–4055. [39] R. Sedighi, F. Tajabadi, N. Taghavinia, Vapor assisted deposition of alkaline doped perovskites: Pure phase formation of CsxMA1−xPbI3, Electrochim. Acta. 2018, 485-491.
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[40] T. Du1, C. H. Burges, J. Kim, J. Zhang, J. R. Durrant, and M. A. McLachlan, Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells, Sustain. Energ. Fuels. 2017, 1, 119-126.
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S0021-9797(19)31461-4 https://doi.org/10.1016/j.jcis.2019.12.004 YJCIS 25744
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
15 August 2019 27 November 2019 2 December 2019
Please cite this article as: S. Mozhgan Seyed-Talebi, I. Kazeminezhad, Performance improvement of fully ambient air fabricated perovskite solar cells in an anti-solvent process using TiO2 hollow spheres, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.004
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© 2019 Published by Elsevier Inc.
Performance improvement of fully ambient air fabricated perovskite solar cells in an anti-solvent process using TiO2 hollow spheres Seyedeh Mozhgan Seyed-Talebi,1* Iraj Kazeminezhad1 1Department
of Physics, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran E-mail: [email protected] , [email protected]
Tel: 00989123094967; Fax: 00986133331040
Abstract: The porosity optimization of electron transporting layer (ETL) in the perovskite solar cells (PSCs) can make the effective pathways for transporting electron and blocking the holes. In the present study, the porosity modification effect of TiO2 paste as most efficient ETL using TiO2 hollow spheres (TiO2 -HSs) on the air-processed formation of perovskite films is studied. In this procedure, the TiO2 -HSs were synthesized using removable carbonaceous sphere templates. Our characterization results demonstrated that prepared TiO2 - HSs showed an external diameters less than 200 nm with shell thickness about 20-30 nm. Due to high porosity of the TiO2 - HSs, CH3NH3PbI3 sufficiently infiltrate into the modified ETL. Thus a high- quality perovskite film with large grain size and smooth surface fabricated on the modified ETL. Further time resolved photoluminescence (TRPL) investigation reveals that an increase in the electron injection and recombination resistance leading to the performance improvement of the PSCs. The best fully ambient processed device with modified electron transporting layer exhibited an efficiency of 19.62%, which is 16.37% higher than the efficiency of the standard PSC. Application of TiO2-HSs in the ETL can help to the development of air-processed perovskite solar cells for commercialization in the future. Keywords: Anti-solvent; Carbon sphere; Mesoporous TiO2; Perovskite Solar cell; TiO2 hollow sphere.
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1. Introduction In general, perovskite solar cells can be assembled as either planar heterojunction architectures [1-8] or mesoscopic structures [9–13]. It has been reported that mesoscopic structure with an inorganic metal oxide semiconductor as the electron transporting layer (ETL) has attracted much attention. Due to the transportation of electrons through both of perovskite layer and mesoporous ETL, design of the ETL plays a crucial role in the efficiency of mesoscopic perovskite solar cells not only as a scaffold of perovskite absorber but also as the pathway for transporting electrons and blocking the holes [14- 18]. In the present work, advanced interface engineering has been applied by porosity modification of commonly used TiO2 layer as a more efficient ETL in the normal perovskite solar cells, which helps more CH3NH3PbI3 penetration into the TiO2 lattice and excellent pore filling through the highly interconnected mesoporous structure. The resultant ETL which is not exactly mesoporous helps to obtain a high efficient and stable PSCs based on the combination of planar and mesoporous structures. In comparison with a standard solar cell, more easy mass migration results in the growth of perovskite deep inside the modified TiO2 layer and obtaining a perovskite layer with larger crystal size and more electron injection [19, 20]. The combination effect of PbI2 and MAI in the crystallization and morphology of mesoscopic CH3NH3PbI3 based PSCs discussed in previous reports [21-26]. In the conventional solution processes, the porous structure of TiO2 hindered the diffusion of CH3NH3PbI3 into the pore channel of TiO2 filled by PbI2, resulting in relatively small amounts of unreacted PbI2 remaining in the finally obtained perovskite solar cell [27]. Therefore, crystallization process maybe results in some large crystals on the TiO2 surface but poor coverage due to the localized nucleation of perovskite layer which tends to obstruct the continual diffusion of MAI to react with PbI2 [28]. The fast combination of PbI2 and MAI at the outside capping layer, leading to a small contact area between perovskite and TiO2 layer. In addition, it seems most of PbI2 is converted to the perovskite, but some amounts of unreacted PbI2 may be retained in the interface of TiO2/perovskite layer. The residual PbI2 has a detrimental effect
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on carrier separation and blocks the transformation of electrons from CH3NH3PbI3 to TiO2 inducing a decrease in PCE of PSCs [28, 29]. In comparison with solution methods, in the anti-solvent treatment technique can largely improve the coverage of a compact perovskite layer with uniform grain structure up to micrometers on the surface of ETL [30]. The antisolvent deposition in the air pressure is preferred here due to all of the fabrication process performed using a simple lab heater. Thus, the results are more applicable for industrial production of solar panels by a higher degree of reproducibility and easily generating of an extremely uniform and flat film with no any pinhole and uncovered area on the surface. This work is organized as follows: It first summarizes the synthesis and characterization of carbon sphere templates and TiO2 - HSs that we used. It then outlines the effect of TiO2 HSs usage on the properties of perovskite layers deposited on different ETLs. Finally, the characterization results of modified solar cell are compared with the standard perovskite solar cell.
2. Results and Discussion All the chemical materials, material characterization instruments, and preparation methods are presented in supplementary informations. 2. 1. Sample preparation In this section, the characterization of carbon spheres, and TiO2 - HSs were firstly explained. The preparation of TiO2 -HSs and effects of their usage on the morphology and grain size of the perovskite layer were then investigated. In the end, the TiO2 - HSs paste was used to build perovskite solar cell according to the procedure described in the experimental section. 2. 2. 1. Characterization of TiO2 hollow sphere Preparation of carbon sphere from sucrose as precursor using hydrothermal method and TiO2 - HSs using carbon sphere templates were described in detail elsewhere [31, 32]. Figure 1 demonstrates typical field emission scanning electron microscopy (FESEM) images of
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synthesized carbon spheres and TiO2 - HSs. The obtained HSs have a regular shape with an average diameter of about 160 ± 30 nm.
Figure 1. FESEM, and particle distribution of (a) carbon spheres, and (b) TiO2 hollow spheres.
The experimental analysis of elemental mapping (MAP) pattern, and energy dispersive Xray spectroscopy (EDX) of the samples shown in Figure 2.
Figure 2. MAP pattern, and Corresponding EDX spectrum of (a) carbon spheres and (b) TiO2 hollow spheres on top of the Aluminum substrate. The peak of Al comes from the Aluminum substrate used in the measurements.
It can be seen that carbon spheres are composed of carbon and oxygen which amount of carbon is higher than that of oxygen. Oxygen peak can be appeared due to the incomplete decomposition of sucrose, adsorption of water on the surface of carbon spheres, or existence
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of carboxyl and hydroxyl functional groups on the carbon surfaces which is in good agreement with FT-IR results (Figure S1, Supplementary Informations). The absence of carbon in TiO2 – HS’s spectrum indicated that all the carbon sphere templates were burned. 2. 1. 2. Preparation of TiO2 hollow sphere paste The TiO2 hollow sphere paste was synthesized from α-terpineol as a solvent, and ethyl cellulose as a binder. The preparation details can be found in the Supporting Informations. The pores with uniform size distribution in the TiO2 hollow sphere paste as effective light scattering centers extend the light traveling distance, thereby improve the optical absorption of the ETL. 2. 2. PSC devices fabrication and measurement techniques As shown in the schematic illustration of Figures 3(a), 3(b), the layered structure of present perovskite solar cells consist of a compact layer of TiO2 deposited on top of F-doped SnO2 and a pure or modified TiO2 layer. The mesoporous TiO2 layer filled with a perovskite layer and Spiro as a hole-transport layer. More details about the fabrication process can be found in the Supporting Informations. 2. 2. 1. Fabrication of PSCs In this study, we fabricated a compact and pin-hole free perovskite film with full coverage on the TiO2 layer via one step method using nonpolar diethyl ether as anti-solvent solution while spinning in the atmospheric pressure [30]. Images of perovskite deposition process using this method is shown in Figure 3(c-e).
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Figure 3. (a) FESEM cross section image of standard and modified perovskite solar cell, (b) Schematic illustration of modified mp-TiO2 using TiO2 HSs, Size of particles and thickness of layers are not in scale. Deposited CH3NH3PbI3 layer on ETL (c) right after washing the spin-coated layer by ether, (d) after annealing at 65 °C for 1 min (e) after annealing at 100 °C for 2 min;
The FESEM images of deposited CH3NH3PbI3 films on both mp-TiO2 and TiO2 - HS films (Figure 4) show rather homogeneous surfaces with no indications of pinhole defects. Fine tuning porosity of TiO2 films using TiO2 – HSs results in very well penetration of MAI into the pores of TiO2 and creating a uniform, continuous, and homogeneous compact CH3NH3PbI3 film with a complete surface coverage by larger grain sizes. Perovskite film with larger grain size causes to mitigate the possibility of trap states or carrier blocking at grain boundaries. Perovskite devices fabricated on such highly porous layers with better pore-filling of perovskite show higher PCE and diminished hysteresis. As shown in Figure 4, all of the deposited CH3NH3PbI3 layers have compact structure, but the size of the CH3NH3PbI3 grains are obviously different. For the CH3NH3PbI3 layers deposited on the TiO2, the average size of CH3NH3PbI3 grains is approximately 800 nm
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(Figure 4(a)). Using TiO2 HSs, the average size of CH3NH3PbI3 grains increased to 1300 nm.
Figure 4. FESEM image, and grain size distribution of deposited perovskite films on to (a) standard TiO2, (b) TiO2 -HS, and (c) XRD pattern, and (d) UV-visible optical absorption spectra of CH3NH3PbI3 layer deposited on different ETLs. The inset of (d) is the (αhυ)2 ~hυ Tauc plots for estimating the direct band gap energies of the prepared perovskite layers.
Relatively lower defect density observed in fabricated perovskite on the modified ETL as compared to that on the standard ETL could also be attributed to the slow crystallization rate inside the scaffold. During the perovskite formation process, the color of the coated precursor on the modified ETL undergoes slower color change as compared to the one fabricated on the standard mp-TiO2 substrate, indicating the slow crystallization of perovskite film. A slow crystallization rate allows for a high-quality perovskite crystal with lower defect density. The XRD pattern of perovskite films is shown in figure 4(c). The appearance of main diffraction peaks at 2θ: 14.18, 28.45, 31.81, and 43.08 can be assigned as the (110), (220), (310) and (330) plans of tetragonal CH3NH3PbI3 crystals, respectively. The absence of PbI2 peak at 12.6 ˚ shows the complete conversion of PbI2 to CH3NH3PbI3. The observed XRD peak positions of CH3NH3PbI3 crystals match well with the previously reported data [33-35]. The intensities of main tetragonal phase peaks are significantly enhanced for CH3NH3PbI3 layer deposited on TiO2–HS film (Figure 4(c)). This implies the favourable crystallographic orientations for carrier transport perpendicular to the substrate increase upon the TiO2 optimization which is important to obtain a uniform, compact, and smooth perovskite layer
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with lesser voids to avoid unnecessary leakage of current [36]. Interestingly, the (110) peak shift of ∼0.04 ° toward a lower angle is observed for fabricated perovskite on modified ETL, can be attributed to the volume extension without formation of additional recombination centres [37]. This volume expansion might be ascribed to the tensile stress developed by the attachment of the growing perovskite crystal to the TiO2 -HSs while constraining the shrinkage during crystallization. The slightly enlarged estimated band gap of modified perovskite (about 0.01 eV) shown in insert of figure 4 (d) can be attributed to the crystal volume expansion of fabricated perovskite on the TiO2-HSs. Owing the higher absorbance of deposited perovskite on the HS-TiO2 ETL, it is expected for the PSK on the HS-TiO2 film to generate higher photocurrent in the modified perovskite solar cell device. 2. 2. 2. PL and TRPL of CH3NH3PbI3 layer on different ETLs The steady-state photoluminescence (PL) measurements performed to confirm the better charges separation, charges extraction, and recombination resistance between ETL and perovskite absorber by application of TiO2 – HS as an efficient ETL. Although the position of PL peaks for all of the CH3NH3PbI3 films deposited on the different substrates is approximately centred at a constant wavelength of ∼770 nm (Figure 5 (a)), when the CH3NH3PbI3 layer was in contact with and without TiO2 layers showing different PL intensities with order of PSK> PSK- TiO2 > PSK - TiO2 -HS, indicating the electron extracting ability in the reverse order. The strongest PL quenching has been observed in CH3NH3PbI3 layer deposited on TiO2 –HS film due to the existence of more recombination resistance and electron extracting ability than other samples. The great PL quenching caused by the improved interconnection on the interface of TiO2 –HS / CH3NH3PbI3 due to more effective electron-injection from perovskite to ETL.
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Figure 5. Normalized a) Photoluminescence (PL) spectra and b) Transient PL decay profile of CH3NH3PbI3, and CH3NH3PbI3 deposited on TiO2 Film with and without TiO2-HSs. The transient PL decays were obtained using the Time-Correlated Single Photon Counting (TCSPC) technique with excitation at 635 nm and probe at 770 nm.
The corresponding band gap (Eg=1.61 eV) determined from the PL spectra of modified perovskite (Figure 5(a)) is in good agreement with the previous researcher reports [38-40]. This observation also is supported by the determined band gap from UV-vis absorption spectra using Tauc plot (Figure 4 (d)). Time-resolved photoluminescence (TRPL) decay profiles were studied to investigate the effect of TiO2 –HSs on the lifetime of charge carriers. In Figure 5 (b), the PL decay of the perovskite films on glass without and with various ETLs was compared. The observance of higher PL quenching in the case of deposited CH3NH3PbI3 on the TiO2-HS surface than the standard TiO2 layer, indicates more electron-extracting ability of the TiO2-HS surface. The fitted lifetimes with bi-exponential decay equations and two components under different substrates are listed in Table 1. The fast decay components (τ1) are attributed to the charge carrier recombination and trap-assisted recombination related to the interface (or surface) defects. The slow components (τ2) are ascribed to the radiative recombination related to the bulk defects. Coated perovskite on glass substrate exhibited the longest decay components for both fast and slow recombination (τ1= 39.3 ns and τ2 =127.3 ns), indicating its highest non-radiative recombination centers. The fitted bi-exponential decay equation for
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CH3NH3PbI3 film on standard mp-TiO2 layer resulted in an effective lifetime with a fast component of
1
= 3.5 ns and a slow component of
2
= 16.3 ns. Assuming that all PL
quenches are due to the effect of electron extraction, the time of interfacial electron extraction (τe) at the interface between perovskite layer and ETL can be calculated as: 1 1 1 e PSK /ETL PSK
(1)
in which ,𝜏psk and PSK/ETL are the PL lifetimes for perovskite layer deposited on glass substrate, and those with perovskite deposited on standard and modified ETLs, respectively. The average PL lifetime of perovskite layer (τPSK) was determined using the statistical definition PSK
A A
2 i i
to be 97.8 ns, in which τi and Ai are lifetimes and corresponding
i i
amplitudes of each component, respectively. Table 1. Time coefficients (relative amplitudes) of corresponding perovskite films obtained from fitting PL transients (Figure 5b) (excitation at 635 nm, probe at 770 nm). Sample
τ1(ns)
A1
τ2 (ns)
A2
τPSK (ns)
τe (ns)
PSK
39.3
0.62
127.3
0.38
97.8
TiO2/PSK
3.5
0.73
16.3
0.27
13.2
HS-TiO2/PSK
2.7
0.57
8.6
0.43
7.4
As one can see in Table 1, the τe in the interface of perovskite and ETL decreases from 13.2 to 7.4 ns when perovskite layer was in contact with the standard and TiO2 -HSs based ETL, respectively. This implies that the trend of electron extraction (1/τe) in the CH3NH3PbI3/ETL interface of TiO2 -HSs based solar cell is higher than that of the standard one, indicating that TiO2 –HSs based ETL as a more efficient ETL is responsible for the high-performance CH3NH3PbI3 based solar cells. The revealed change in the slow component of modified cell can be indicative of the suppression of trap-assisted recombination and improvement of the quality of the bulk perovskite. These results are in agreement with the FESEM observations
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of figure 4(b) in which standard perovskite layer has a defect density and grain boundaries higher than that of the modified perovskite layer. The enhanced charge carrier extraction with reduced trap density can explain the improved PCE along with less hysteresis for the HS-TiO2 based perovskite solar cells. 2. 3. Photovoltaic study results The experimental results of current density-voltage characterization of TiO2 and HS- TiO2 based perovskite solar cells are listed in Table 2. The statistical analysis of data demonstrated good reproducibility of devices. Figure 6 compares the distributions of Jsc, Voc, fill factor, and PCE of best devices of each type under the same experimental conditions with various ETLs.
Figure 6. Boxplots of photovoltaic parameters a) JSC, b) VOC, c) FF, and d) PCE for devices made of HS-TiO2, and standard mp- TiO2 paste as ETLs. Each data set is carried out from at least 25 devices. The devices were measured under simulated AM1.5, 100 mWcm-2 sun light. e) Current Density-voltage curves of best efficiency devices, and f) IPCE and integrated current density of the best device in comparison with standard device.
The improved FF and JSC of samples made of modified TiO2 films by TiO2 –HSs are attributed mainly to the larger CH3NH3PbI3 grains and uniform covering the entire substrate,
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which improve light-harvesting properties. The larger fill factor shows that the modification on the surface reduces the loss of photo-excited carriers being extracted from the device due to the suppression of surface electron recombination. The increase in short circuit current density is also evidence for better light absorption and light scattering of modified ETL by TiO2 -HSs. Higher charge concentration in the interface of TiO2 and perovskite absorber for a certain number of injected electrons leads to retarding the recombination in PSC and getting high Voc and PCE. Our results indicate that photovoltaic performance of the TiO2 – HS based device shows the greater VOC, JSC, FF, and PCE values than those of standard device which reasonably explains the superior shorter electron-extraction time and higher photocurrent response of the perovskite deposited onto the TiO2 -HS layer than that of on the standard ETL. Table 2. Photovoltaic Parameters of Perovskite Solar Cells Fabricated with Varied TiO2 layer. Devices are measured under simulated AM-1.5G illumination with active area of 0.09cm2. aETL
bAverage
TiO2 nanoparticle cBest bAverage
TiO2 – HS cBest
JSC(mAcm-2)
VOC (V)
FF
PCE (%)
21.55 ± 𝟎.𝟎𝟔
1.03 ± 0.02
71.16 ± 𝟏.𝟓𝟕
15.94 ± 𝟎.𝟒𝟓
21.50
1.04
75.40
16.86
23.33 ± 𝟎.𝟒𝟕
1.05 ± 𝟎.𝟎𝟐
75.64 ± 𝟏.𝟐𝟐
18.67 ± 𝟎.𝟒𝟕
23.6
1.08
76.96
19.62
[a] The devices were fabricated using the CH3NH3PbI3 layers prepared via one step methods using anti solvent treatment on to the different electrontransporting layers (ETLs) detailed in the Experimental Methods in supporting information (SI). [b] The average values shown with the uncertainties representing one standard deviation were obtained from 25 devices fabricated under the same experimental conditions analyzed by the analysis of variance (ANOVA) detailed in Tables S1 and S2 in supporting information (SI). [c] The best photovoltaic performances are reported for each type of TiO2 based devices.
The best performing devices with modified TiO2 layer show the PCE up to 19.62% with very small hysteresis in J-V characteristics. Enlarging the interfacial contact between perovskite and TiO2 by effective pore-filling suppresses the charge accumulation caused by ion migration and diminishes the possibility of hysteresis. The J-V characteristics of the
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CH3NH3PbI3 based solar cells prepared with varying standard ETL by TiO2 –HSs in forward and reverse scans are shown in Figure 7. Although the hysteresis effect in regular planar structures is greater than that of PSCs with mesoporous ETL [14]. In this structure, the hysteresis effects have been significantly relieved compared with the standard device with mesoporous TiO2 layer. Reducing the hysteresis effect (Figure 7 (b)), is related to trapping and detrapping mechanism of electrons at the interface of perovskite and modified ETL.
Figure 7. J-V characteristics of the devices based on devices in contact to (a) TiO2 and (b) TiO2 – HS layers, under illumination condition in reverse and forward sweep directions. This devices were measured under simulated AM1.5 and 100 mW.cm-2 sun light.
The majority of located electron and hole traps near the surface and grain boundaries of the perovskite layer could be a reason for the reduction of ion migration at grain boundaries and J-V hysteresis. The observance of hysteresis in the perovskite solar cells is in accordance with the modification of the interfacial surfaces and time-dependent photocurrent density (Jsc) behavior shown in Figure 8. As observed in the hysteresis curves, at certain voltages for both standard and modified devises, the J−V curves at forward and backward stepwise scan separate from each other. This effect is clearly visible at ∼0.7−0.9 open circuit voltage. The discrepancy in the J–V characteristic of the hysteresis curves varies trends, consequently, results in the scanning direction-dependent of photocurrent density behavior of devices. For the time-dependent
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photocurrent density measurements, the devices were kept under one sun illumination (AM 1.5, 100 mW/cm2) at 0.8 Voc. In the reverse scan (RS) curves, the detected current increases sharply as a large photocurrent spike while sudden increase in the photo induced separation of the electron hole pairs occurs. As charge carriers migrated from the interior of perovskite layer to the interfaces (electrons to the ETL and holes to the HTL), the photocurrent spike followed by a slow exponentially decay with time to the equilibrium state. When the recombination rate and charge generation reach to equilibrium, the steady state current is achieved. The subsequent decrease in photocurrent indicates that more recombination is occurring within the perovskite layer. The smaller difference in observed photo current density of modified devise (∼1.34 eV) in spite of Jsc difference monitored in the time dependent Jsc of standard devise (∼1.63 eV) is due to the less hysteresis behavior of HS-TiO2 based solar cell.
Figure 8. Time-dependent photocurrent response of standard and modified cells at the applied voltage of 0.8 V under (a) reverse (RS), and (b) forward (FS) scans.
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Under forward stepwise scan (FD) with possessing the applied bias, detected current shows a sharp decrease in the time-dependent photocurrent density at first, followed by a continuous recovery and reaches to the saturation point. The TiO2 -HSs as the electron transporting layer provided high photocurrent due to better connection between ETL/perovskite layer, and growing the smooth perovskite layer on its surface than the standard ETL substrate. The impedance spectroscopy (IS) measurement of standard and modified solar cells with different ETLs is performed to monitor the interfacial changes such as charge transfer, recombination, and charge accumulation in the interfaces of perovskite layer. The Nyquist plots of the standard and modified solar cells, which are measured under one-sun illumination and open-circuit conditions in the frequency range from 1 MHz to 1 Hz shown in Figure 9. In the corresponding equivalent circuit of Figure 9, Rs, Rcon and Rrec are related to the series resistance, the contact resistance at perovskite’s interfaces, and recombination resistance, respectively.
Figure 9. The fitted equivalent circuit, and impedance response of typical Nyquist plots of standard and modified perovskite solar cells.
The contact resistance (Rcon) derived from high-frequency arc in the Nyquist plots literally means the charge transfer resistance at the ETM/perovskite/HTM interfaces. It can be related
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to the electron transporting ability of TiO2 layer resulting from the geometrical factors such as roughness and the structure of the mesoporous TiO2 layer. Thus, the contact resistance value significantly changed when porosity modification of ETL is involved because of better contact and easier electron transfer at the interface of perovskite and modified ETL. The Rcon value is decreased for the modified solar cell compared to the standard solar cell (from 290 Ω to 189.5 Ω) which is indicative of better contact between HS-TiO2 and perovskite in comparison to that between mp-TiO2 and perovskite. Moreover, the Rrec is significantly higher for modified solar cell (78.73 Ω compared to 62 Ω) which pointed to the more blocked charge recombination and more electrons extraction at the interface of perovskite/ETL of modified solar cell, which leads to the improvement of photovoltaic characteristics.
3. Conclusions Our research focused on the importance of morphology control of ambient air processed perovskite layer through substrate surface modification to obtain highly efficient PSC device, which has an impact on developing PSC’s commercialization in the future. We have fabricated a TiO2-HS based ETL with similar thickness of mp-TiO2 in order to demonstrate the morphology and porosity of ETL play a crucial role to attain a satisfactory device performance and decreased hysteresis for PSC. The TiO2 based ETL designed with TiO2HSs leading to formation of a dense, uniform and homogeneous perovskite layer, which ensures smooth surface with low defect density and maximum contact areas at the interfaces with the PSK/ETL; it thus enhances charge separation at the interface of PSK/ETL. The characterization of deposited perovskite on the standard mp-TiO2 layer and modified layer with TiO2 – HSs was investigated with FESEM, EDX-mapping, and UV-vis analysis. Moreover, the enhancement in photocurrent of HS-TiO2 based PSC is relatively higher than response of solar cell with mp-TiO2 layer due to the enormous contact surface area between CH3NH3PbI3 and HS-TiO2. This is supported by the smooth perovskite layer seen in FESEM results. The electron-transfer properties were investigated via steady state PL and TRPL (via TCSPC) spectra. We found a correlation between the rate of electron transfer and the
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morphology of mp-TiO2 layer. When the perovskite layer was coated on the mp-TiO2 layer, ETL exhibited a PL lifetime longer than that of the HS-TiO2 based substrate, representing much slower charge transport in a standard ETL. The best device performance attained PCE 19.62 % for TiO2-HS based PSK, at which larger perovskite grain size and rapid electron transfer occurred. The TiO2–HS based devices also showed a minor effect of hysteresis with excellent reproducibility during a fully air processed fabrication. As our ETL structure is something between mesoscopic and planar, we believe the idea is valuable for readers to test it in other devices and check the efficiency enhancement of their standard cells in air atmosphere and pressure for commercial application.
Experimental Section Experimental Details could be found in the Supporting Informations.
References [1] Y. Yang, J. Wu, T. Wu, Z. Xu, X. Liu, Q. Guo, and X. He, An efficient solvent additive for the preparation of anion-cation-mixed hybrid and the high performance perovskite solar cells, J. Colloid. Interf. Sci. 2018, 531, 602-608 [2] D. Luo, W. Yang, Z. Wang, A. Sadhanala, Q. Hu, R. Su, R. Shivanna, G. F. Trindade, J. F. Watts, Z. Xu, T. Liu, K. Chen, F. Ye, P. Wu, L. Zhao, J. Wu, Y. Tu, Y. Zhang, X. Yang, W. Zhang, R. H. Friend, Q. Gong, H. J. Snaith, R. Zhu, Enhanced photovoltage for inverted planar heterojunction perovskite solar cells, Science. 2018, 360, 1442-1446. [3] Y. Saygili, S. ‐H. Turren‐Cruz, S. Olthof, B. W. H. Saes, I. B. Pehlivan, M. Saliba, K. Meerholz, T. Edvinsson, Sh. M. Zakeeruddin, M. Grätzel, J. ‐P. Correa‐Baena, A. Hagfeldt, M. Freitag, W. Tress, Planar Perovskite Solar Cells with High Open‐Circuit Voltage Containing a Supramolecular Iron Complex as Hole Transport Material Dopant, ChemPhysChem. 2018, 19, 1363-1370. [4] G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely, H. J. Snaith, Morphological control for high performance, solution‐processed planar heterojunction perovskite solar cells. Adv. Funct. Mater. 2014, 24, 151–7. [5] N. Marinova, S. Valero, and J. L. Delgado, Organic and perovskite solar cells: Working principles, materials and interfaces, J. Colloid. Interf. Sci. 2017, 488, 373-389.
Journal of Colloid and Interface Science
18 of 22
[6] F. Huang, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S. Rubanov, Y. Zhu, X. Lin, L. Jiang, Y. Zhou, A. Gray-Weale, J. Etheridge, C. R. McNeill, R. A. Caruso, U. Bach, L. Spiccia, Y. -B. Cheng, Gas-assisted preparation of lead iodide perovskite films consisting of a monolayer of single crystalline grains for high efficiency planar solar cells, Nano Energy. 2014, 10, 10-18. [7] Michael Grätzel, The light and shade of perovskite solar cells, Nat. Mater. 2014, 13, 838– 842. [8] X. Gan, O. Wang, K. Liu, X. Du, L. Guo, H. Liu, 2D homologous organic-inorganic hybrids as light-absorbers for planer and nanorodbased perovskite solar cells, Sol. Energy Mater. Sol. Cells. 2017, 162, 93–102. [9] B. Zhang, Z. Song, J. Jin, W. Bi, H. Li, C. Chen, Q. Dai, L. Xu, and H. Song, Efficient rare earth co-doped TiO2 electron transport layer for high-performance perovskite solar cells, J. Colloid. Interf. Sci. 553 (2019) 14–21. [10] L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, and M. Grätzel, Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells, J. Am. Chem. Soc. 2012, 134, 17396–17399. [11] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science. 2012, 338, 643–647. [12] N. –G. Park, Organometal Perovskite Light Absorbers toward a 20% Efficiency LowCost Solid-State Mesoscopic Solar Cell, J. Phys. Chem. Lett. 2013, 4, 2423–2429. [13] Y. Zhao, A. M. Nardes, and K. Zhu, Solid-State Mesostructured Perovskite CH3NH3PbI3 Solar Cells: Charge Transport, Recombination, and Diffusion Length, J. Phys. Chem. Lett. 2014, 5, 490−494. [14] M. F. Mohamad Noh, C. H. Teh, R. Daik, E. L. Lim, C. C. Yap, M. A. Ibrahim, N. A. Ludin, A. R. Mohd Yusoff, J. Jang, and M. A. M. Teridi, The architecture of the electron transport layer for a perovskite solar cell, J. Mater. Chem. C. 2018, 6, 682-712. [15] G. Yang, H. Tao, P. Qin, W. Ke, and G. Fang, Recent progress in electron transport layers for efficient perovskite solar cells, J. Mater. Chem. A. 2016, 4, 3970-3990. [16] N. –G. Park, M. Grätzel, T. Miyasaka, K. Zhu and K. Emery, Towards stable and commercially available perovskite solar cells. Nat. Energy. 2016, 1, 16152.
Journal of Colloid and Interface Science
19 of 22
[17] L. Yang, X. Wang, X. Mai, T. Wang, C. Wang, X. Li, V. Murugadoss, Q. Shao, S. Angaiah, and Z. Guo, Constructing efficient mixed-ion perovskite solar cells based on TiO2 nanorod array, J. Colloid. Interf. Sci. 2019, 534, 459-468. [18] F. Gao, H. Dai, H. Pan, Y. Chen, J. Wang, Z. Chen, Performance Enhancement of Perovskite Solar Cells by Employing TiO2 Nanorod Arrays Decorated with CuInS2 Quantum Dots, J. Colloid. Interf. Sci, 2018, 513, 693-699. [19] Z. Y. Wu, B. –L. Jian, and H. –C. Hsu, Photoluminescence characterizations of highly ambient-air-stable CH3NH3PbI3/PbI2 hetero structure, Opt. Mater. Express. 2019, 9, 18821892. [20] S. –Y. Kim, H. J. Jo, S. –J. Sung, and D. –H. Kim, Perspective: Understanding of ripening growth model for minimum residual PbI2 and its limitation in the planar perovskite solar cells, APL Mater. 2016, 4, 100901. [21] T. Du1, C. H. Burges, J. Kim, J. Zhang, J. R. Durrant, and M. A. McLachlan, Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells, Sustain. Energ. Fuels. 2017, 1, 119-126. [22] X. Sun, J. Xu, L. Xiao, J. Chen, B. Zhang, J. Yao, and S. Dai, Influence of the Porosity of the TiO2 Film on the Performance of the Perovskite Solar Cell, Int. J. Photoenergy. 2017, 2017, 4935265. [23] H. Chen, Z. Wei, H. He, X. Zheng, K. S. Wong, S. Yang, Solvent Engineering Boosts the Efficiency of Paintable Carbon based Perovskite Solar Cells to Beyond 14 %, Adv. Energy Mater. 2016, 6, 1502087. [24] B. –E. Cohen, L. Etgar, Parameters that control and influence the organo-metal halide perovskite crystallization and morphology, Optoelectron. 2016, 9, 44–52. [25] Y. Li, B. Ding, G. –J. Yang, C. –J. Li, C. –X. Li, Achieving the high phase purity of CH3NH3PbI3 film by two-step solution processable crystal engineering, J. Mater. Sci. Technol. 2018, 34, 1405-1411. [26] Q. Chen, H. Zhou, T. –B. Song, S. Luo, Z. Hong, H. –S. Duan, L. Dou, Y. Liu, and Y. Yang, Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells, Nano. Lett. 2014, 14, 4158−4163. [27] H. K. Adli, T. Harada, W. Septina, S. Hozan, S. Ito, and S. Ikeda, Effects of Porosity and Amount of Surface Hydroxyl Groups of a Porous TiO2 Layer on the Performance of a CH3NH3PbI3 Perovskite Photovoltaic Cell, J. Phys. Chem. C. 2015, 19, 22304−22309.
Journal of Colloid and Interface Science
20 of 22
[28] M. Jung, S. – G. Ji, G. Kim and S. I. Seok, Perovskite precursor solution chemistry: from fundamentals to photovoltaic applications, Chem. Soc. Rev. 2019, 48, 2011-2038. [29] H. Wei, Y. Tang, B. Feng, and H. You, Importance of PbI2 morphology in two-step deposition of CH3NH3PbI3 for high-performance perovskite solar cells, Chin. Phys. B. 2017, 26, 128801. [30] N. Ahn, D.- Y. Son, I.- H. Jang, S. M. Kang, M. Choi, and N. –G. Park, Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead (II) Iodid, J. Am. Chem. Soc. 2015, 137, 8696−8699. [31] S. M. Seyed-Talebi, I. Kazeminezhad, M. Nematpour, Synthesis and characterization of Ag@Carbon core-shell spheres as a novel catalyst for room temperature N-arylation reaction, J. Catal. 2018, 361, 339-346. [32] S. M. Seyed-Talebi, I. Kazeminezhad, H. Motamedi, TiO2 hollow spheres as a novel antibiotic carrier for the direct delivery of gentamicin, Ceram. Int. 2018, 44, 13457-13462. [33] Y. Li, X. L. He, B. Ding, L. L. Gao, G. J. Yang, C. X. Li, C. J. Li, Realizing full coverage of perovskite film on substrate surface during solution processing: Characterization and elimination of uncovered surface, J. Power Sources. 2016, 320, 204–211. [34] J. Kim, J. S. Yun, X. Wen, A. Mahboubi Soufiani, C. F. Jonathan Lau, B. Wilkinson, J. Seidel, M. A. Green, S. Huang, and A. W. Y. Ho-Baillie, Nucleation and Growth Control of HC(NH2)2PbI3 for Planar Perovskite Solar Cell, J. Phys. Chem. C. 2016, 120, 11262–11267. [35] H. Gao, C. Bao, F. Li, T. Yu, J. Yang, W. Zhu, X. Zhou, G. Fu and Z. Zou, Nucleation and Crystal Growth of Organic-Inorganic Lead Halide Perovskites under Different Relative Humidity, ACS Appl. Mater. Interfaces. 2015, 7, 9110–9117. [36] S. M. Seyed-Talebi, I. Kazeminezhad, S. Shahbazi, E. W. Diau, Efficiency and Stability Enhancement of Fully Ambient Air Processed Perovskite Solar cells using TiO2 Paste with Tunable Pore Structure, Adv. Mat. Interfaces. Accepted 18 Nov 2019. [37] J. N. Sherwood, R. I. Ristic, The influence of mechanical stress on the growth and dissolution of crystals, Chem. Eng. Sci. 2001, 56, 2267-2280. [38] H. Pourradi, K. Ghani, and M. Mahdavi, Fully Ambient Air Processed Perovskite Solar Cell Based on Co (Co, Cr)2O4/ TiO2 P-N Heterojunction Array in Photoanode, J. Phys. Chem. C. 2019, 123, 4044–4055. [39] R. Sedighi, F. Tajabadi, N. Taghavinia, Vapor assisted deposition of alkaline doped perovskites: Pure phase formation of CsxMA1−xPbI3, Electrochim. Acta. 2018, 485-491.
Journal of Colloid and Interface Science
21 of 22
[40] T. Du1, C. H. Burges, J. Kim, J. Zhang, J. R. Durrant, and M. A. McLachlan, Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells, Sustain. Energ. Fuels. 2017, 1, 119-126.
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