Long-Term Durability of Bromide-Incorporated Perovskite Solar Cells

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Long-Term Durability of Bromide Incorporated Perovskite Solar Cells via Modified Vapor-Assisted Solution Process Fatemeh Ansari, Masoud Salavati-Niasari, Pariya Nazari, Noshin Mir, Vahid Ahmadi, and Bahram Abdollahi Nejand ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01075 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Energy Materials

Long-term Durability of Bromide Incorporated Perovskite Solar Cells via Modified Vapor-Assisted Solution Process Fatemeh Ansari†, Masoud Salavati-Niasari†,*, Pariya Nazari§,ψ, Noshin Mir†, Vahid Ahmadi§, Bahram Abdollahi Nejand§,ψ,‡,* †Institute §School

of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box. 87317–51167, I. R. Iran.

of Electrical and Computer Engineering, Tarbiat Modares University, 14115-194 Tehran, Iran

ψ Institute

of Microstructure Technology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Karlsruhe,

Germany. ‡Nanomaterial

Research Group, Academic Center for Education, Culture and Research (ACECR) on TMU, 14115-343

Tehran, Iran *Email: [email protected]

Abstract Organometal halide perovskite materials as a potential light absorber attracted much attentions in the field of third-generation photovoltaics. The low stability and durability of perovskites have outstanding effects on their optoelectronic properties, which causes a big obstacle in scaling up these impressing solar cells. Here, we introduce the fabrication of efficient organometal halide perovskite devices by a simple and low-temperature vapor-assisted solution process using methyl ammonium bromide (MABr) to construct pinhole-free and uniform perovskite thin films. The results shows that using MABr as a second evaporated precursor in fabrication of perovskite layer loads the Br atoms in to the final MAPbI3-xBrx structure in which higher open circuit voltage, short circuit current density, and fill factor is achieved compared to the pure MAPbI3 perovskite structure. Perovskite solar cells based on the as prepared films show high power conversion efficiency of 15.34%, which present the interesting 50 days durability. Keywords: Perovskite solar cells, mix halide, long term durability, VASP method, MABr.

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Introduction Research in the photovoltaic technologies are being for large-scale solar energy conversion. Many materials have been used in photovoltaic (PV) devices to reach high efficiency thin film solar cells. Organometal halide perovskite has been noticed as a promising material for high efficiency PV devices. Recently, solar cells based on perovskite halide materials have been reported to achieve remarkably high efficiency after only a few years1-4. This fast development can be mainly attributed to the excellent properties of perovskite such as high absorption coefficient 5 and long charge carrier diffusion length

3, 6.

However, alternative mixed of iodide and bromide based

perovskite is interesting for application in multi junction and photo electrochemical devices

7-10.

Perovskite mixed iodide and bromide could achieve higher open circuit values compared to iodide counterpart because of a wider band gap 7, 9and they could improve durability of the devices10-11 due to lower hydrophilicity of bromide perovskite structure rather than the iodine perovskite structure. Considering the significant challenges in the way of commercialization of PV devices, perovskite-based PV with high PCE and noticeable operational stability is a good candidate for further development. Deposition of perovskite solar cells has led to substantial progress in the development of organic-inorganic perovskite solar cells. Furthermore, the main challenges for perovskite solar cells are the perovskite film coverage, pinholes, grain size, and crystal orientation12-13. Therefore, various deposition approaches have been used to reach a uniform and modified perovskite layers to decrease the carriers recombination and lengthy electron and hole life times14. Different perovskite deposition methods result in diverse morphologies and thicknesses which strongly affect the cell performance and parameters. It was declared that formation of pinhole-free perovskite layers considerably improves the cell parameters. The open


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circuit voltage and fill factor are among these parameters, which are induced by high recombination rate in the interface of electron and hole transport materials 15-16. To date, perovskite deposition methods are classified to “one step” and “two step”, chemical and physical methods. Thus, a wide variety of deposition techniques have been developed, such as fast deposition−crystallization procedure17, dual- source vacuum evaporation18, vapor assisted solution progress (VASP)

19-20,

solvent-solvent extraction21, blow-drying22 progress, and solvent free

perovskite deposition23. In this regard VASP technique could be considered as a deposition method for a uniform and pinhole-free perovskite layer and this method would be a good candidate for up scaling due to its possibility for large area deposition. Since physical deposition of perovskite layers requires high production cost, deposition of perovskite by chemical routes has been confirmed to be an easy and low cost19, 24. Herein, in order to reach higher stability with a uniform mixed halide perovskite layer, we used VASP method. A uniform and pinhole-free perovskite layer was deposited by VASP method with a high power conversion efficiency of 15.34% in a normal structure (Glass/ FTO/ TiO2/ Perovskite/ Spiro-OMeTAD/ Au) as well as long term stability of the devices. This method takes advantage of the kinetic reactivity of CH3NH3Br and thermodynamic stability of perovskite during the in situ growth process through which well-defined grain structure and good surface coverage was achieved. The bromide consistent in this approach, enhances the device durability in comparison to the previous reported VASP approaches in the fabrication of efficient perovskite solar cells.

Experimental Section Preparation of the substrate and mesostructure layer



FTO-coated glass substrates were patterned by Zn powder and 2M HCl solution etching. The patterned FTO substrates were cleaned by a soap–deionized water solution, followed by ultrasonication at 50℃ in deionized water, ethanol, and isopropanol, and then was subject to an O3/ultraviolet treatment for 20 min. A hole-blocking layer of TiO2 was deposited by spin-coating of an acidic solution of titanium isopropoxide in anhydrous ethanol (2000 rpm for 30 s) and then annealed at 500℃ for 30 min. The substrate was immersed in 40 mm TiCl4 (Sigma–Aldrich) aqueous solution for 30 min at 70℃ and washed with distilled water. It was then annealed at 500℃ for 30 min. After cooling to room temperature, the TiO2 paste containing TiO2 dyesol and ethanol with the weight ratio of 2 to 7 was spin-coated on the substrate at 4500 rpm for 30 s. By drying in 70℃ for 20 min, the film was then annealed at 500℃ for 30 min. Perovskite Film Fabrication via VASP Method Perovskite films were deposited using the vapor-assisted solution processed. Firstly, PbI2 solution in DMF with a concentration of 1.3 M was spin-coated on the mesoporous TiO2 at 6000 rpm for 20s. After annealing at 100℃ for 5 min, the film was treated by CH3NH3Br vapor at 140, 150, 160, or 170℃ (labelled as S140, S150, S160, and S170, respectively) for 12 minutes in a closed glass petri-dish (Fig. 1.a). In this method, the PbI2 substrates were placed in the closed glass petri-dishes and MABr powder was spread around the substrate. The petri-dishes were moved to a heating oven to produce the MABr vapor in the petri-dishes. After cooling down, the as-prepared substrates were washed with isopropanol, dried, and annealed on a hotplate at 100℃ for 5 minutes. For the fabrication the control cell, the CH3NH3I was evaporated in the same condition at 150 ᵒC for 12 minutes. Deposition of Spiro-OMeTAD/Au Right after cooling down the annealed perovskite layers to room temperature, the spiro-OMeTAD-based hole transporting layer (72 mg spiro-OMeTAD, 17.5 μl lithium-bis(trifluoromethanesulfonyl)imide (Li-


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TFSI) solution (520 mg Li-TFSI in 1 ml acetonitrile) and 26.6 mg 4-tert-butylpyridine all dissolved in 1 ml chlorobenzene) were deposited by spin-coating at 2000 rpm for 30 sec. By keeping the deposited spiroOMeTAD in desicator for 12 hours, the thin 100 nm gold contact was deposited on the spiro-OMeTAD by thin stainless steel shadow mask to create a 0.1 cm2 active area.

Thin Film and Device Characterization. A solar simulator (AM1.5G, 100 mW/cm2) illumination with a solar simulator (Sharif solar simulator, IRAN) equipped with a 450 W Xenon lamp (Newport 6279NS) and a Keithly 2400 source meter was used to record the I-V curves of the fabricated solar cells. An aperture mask was attached during the I-V curve measurement. The microstructure and morphology of the deposited films was studied by field emission scanning electron microscopy (FE-SEM, S416 Hitachi). The phase structure and crystal size of films were also investigated by X-ray diffraction (XRD, Philips Expert-MPD). XRD was performed in the θ−2θ mode using Cu Kα with wavelength of 1.5439 Å radiation. All the XRD experiments were performed at grazing incident angle of 2°. The optical characteristics of deposited films were analyzed by UV−Vis spectroscopy using the wavelength range 190−1000 nm.

Results and Discussion The perovskite solar cells works according to the justified perovskite absorber valence and conduction bands with the appropriate electron and hole transporting materials. The present perovskite solar cell consist of the bromide incorporated CH3NH3PbI3 structure which could increase the perovskite band gap (Fig. 1.b) regarding the lower radius of bromide compared to iodine atoms (Fig. 1.c). To study the microstructure of the perovskite layer, XRD patterns were recorded with different annealing temperature (Fig. 2.a-d). The grown perovskite layer at 140 ℃



shows three phases of MAPbI3-xBrx, MAPbBr3, and PbI2. The presence of PbI2 could be ascribed to the residual PbI2, which could not be transferred to perovskite structure due to low reaction temperature (Fig. 2.a). The grown perovskite layer at 150℃ (Fig. 2.b) shows the higher crystallinity of MAPbI3-xBrx and the residual PbI2 is not detected in the grown perovskite structure, indicating that the PbI2 phase is completely converted into MAPbI3-xBrx and MAPbBr3. As the growth temperature is increased to 160℃ (Fig. 2.c), intensities of the perovskite peaks are decreased probably due to slight decomposition of the perovskite structure to PbI2. At the growth temperatures of 170 ℃ (Fig. 2.c), there is a considerable amount of PbI2 as an impurity and undesirable phase in the perovskite structure regarding the degradation of the produced perovskite at higher temperatures. The morphology changes of perovskite films with different reaction temperatures at 140, 150, 160 and 170℃ are studied by FE-SEM as shown in Fig. 3.a-h. As already reported in previous works, the film morphology has a crucial role in achieving high performance solar cells8, 25. The average grain sizes of S140, S150, S160, and S170 were measured to be 150, 190, 231, and 760 nm (Fig. 3. j). As shown in the Fig. 3.a-f, increasing the annealing temperature up to 160℃ presents gradual increase in grain size of the perovskite microstructure. A significant grain growth of perovskite layer observes at 170℃ regarding the fast growth rate of the perovskite grains at higher temperature. Although enlarged average grain size is beneficial to improve the performance of final solar cell through decreasing the grain boundaries, the presence of numerous and big pinholes in the film and higher structural defects at high temperatures should also be into account. The observed pinholes in different samples are shown in high magnified FE-SEM imaged in Fig. 3.a, 3c, 3e, and 3g for S140, S150, S160, andS170, respectively. The detected pinholes are shown in red cycles in each image. As shown in the figure, S150 contains the lowest density of pinholes


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which reveals better morphology for fabrication of efficient perovskite solar cells. In S170, despite the large grain size of the perovskite microstructure, it contains larger pinholes which may cause a drastic problem in fabrication of efficient perovskite solar cells by considerable recombination cites in the contact of the ETL and HTLs. The cross-section FE-SEM image (Fig. 3.i) of the S150 device reveals the complete separation of the ETL and HTLs by uniform perovskite layer. As shown in the cross section image, the FTO substrate is coated with a compact electron blocking layer of TiO2 (60 nm), meso TiO2 layer (300 nm), perovskite layer (500 nm) deposited via VASP method, spiro-OMeTAD as a HTL (200 nm), and Au metal contact (100 nm). Post-treatment of perovskite film by MABr, induces Ostwald ripening which may help producing high-quality MAPbI3-xBrx films26. In this work, since the as-prepared substrates were washed with isopropanol, this also can induce an Ostwald ripening and intermediate halide exchange. Therefore, the morphology variations of the film with time was studied before washing with isopropanol (room temperature) and after washing and drying at 100 ℃ after 2, 5, and 12 min, respectively (Fig. 4.a-d). As shown in the figure, before washing with isopropanol, small grains with a number of pinholes are formed in the film. After isopropanol washing, at the first 2 minutes, the grains start growing and the number of pinholes are decreased. After 5 minutes, a compact perovskite film with larger grains is formed. By increasing the annealing time to 12 min, larger grains are formed and pinholes are appeared due to the irregular morphology. Therefore, in this work, the high quality film with the lowest pinhole was produced by using drying time and temperature of 5 min and 100 ℃, respectively. Fig. 5.a shows UV-Vis spectra of the prepared samples as a study on the change of absorption characteristics. S150 and S160 produce thicker layers compared with S140 and S170 probably because of residual PbI2 in the S140 and S170 in which higher absorbance was observed in the



S150 and S160. Besides, regarding no residual PbI2 in the thin film, higher photo absorbance is achieved in the S150 and S160. It is noteworthy to mention that by conversion of the PbI2 to perovskite structure, regarding the increment in the lattice parameters, the thin film of the PbI2 transfers to thicker layer of perovskite layer27. S150 shows higher absorbance regarding better perovskite growth and higher light scattering into the perovskite layer, which can enhance the light absorbance. As different growth temperatures could affect the MAPbI3-xBrx structure by producing different Br/I ratio in the perovskite structure, small band gap variations is observed in the different reaction temperatures (Fig 5.a). Recently, it has been demonstrated that for very thin perovskite layers with small m (layers of perovskite unit cells) i.e. 1 to 3, quantum yields decreases down to 2% and for those with m=4 and m=5, the quantum yield increased up to 30%. Therefore, from absorption study presented here, S150 and S160 samples would have higher quantum yields compared with the other samples. 28-29. The J-V analysis measurement was conducted for understanding the best performance of the prepared devices (Fig. 5.b and Table 1) at different reaction temperatures (140, 150, 160, and 170℃). Controlling deposition temperature of perovskite layer in VASP method is one of the significant challenges, which affects the growth of crystal network and morphology of perovskite layer. At 140℃, the PbI2 layer could not be completely converted to perovskite structure, while increasing the growth temperature in VASP method to 150℃ results in the complete growth of uniform and low pinhole perovskite layer. This microstructure enhancement along with noticeable absorption proposes the enhanced photovoltaic parameters. The uniform, pinhole-free, and large grained film may help to alleviate surface recombination leading to high VOC. Typical J-V plot of optimized solar cell using VASP method at 150 ℃ exhibits outstanding performance, with JSC=23.4 mA/cm2, VOC = 1.06 V, fill factor = 62%, and PCE = 15.3%. The distributions of different


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photovoltaic parameters for four groups of fabricated devices are shown in Fig. 5.c and summarized in Table 2. The average values for Voc, JSC, FF, and PCE for S150 are 1.08 eV, 20.8 mA, 0.61, and 13.6%, respectively. As shown in the figure, the fabricated devices of S150 show a good reproducibility than other devices which are produced at 140, 160, and 170 ℃. The high durability and PCE can be attributed to the high quality of the S150 absorber film. Table 1 shows the parameters extracted from the J–V curves. Increasing the reaction temperature to 160 ℃ results in reaching lower short circuit density due to phase transition in the perovskite structure. At higher reaction temperature of 170℃ despite bigger pinholes in the perovskite layer which results in lower open circuit voltage, the short circuit density of the devices increased than the reaction temperature of 160℃ due to higher crystallinity of the perovskite structure. The durability of the devices is traced by keeping the fabricated cells at 25 ℃, under ambient atmosphere with 28±2% moisture, and dark area. The VASP-assisted fabricated devices with MABr (S150) are compared to similar devices fabricated with MAI. As shown in Fig. 6.a, S150 exhibited a maximum PCE of 15.34 %, whereas the prepared device with MAI yielded a maximum PCE of 10.58% in the first hours of operation. As shown in Fig. 6.a and Fig. 6.b, the prepared device by MAI degraded rapidly, whereas S150 shows higher stability even after 50 days of working under the same condition. This is attributed to presence of bromide contained perovskite structures including MAPbI3-xBrx and MAPbBr3, simultaneously, which brings about lower hydrolyzation of the perovskite structure with moisture. “It is evident that MAPbI3 undergoes two decomposition reactions including irreversible and reversible reactions of MAPbI3 →CH3I + NH3I2 and Pb0 + X2 ⇌ PbX2, respectively. However, MAPbBr3 decompose to PbBr2 + CH3NH2 + HBr at ambient temperature resulting in a clean self-healing process ending up with enhanced stability.30”



Impedance spectroscopy (IS) is used to separate the internal electrical processes in order to obtain valuable information about carrier transport and recombination as well as chemical capacity 31-32. The IS of S140, S150, S160, and S170 were measured at 0 and 500 mV forward applied biases over the frequency range of 0.1 to 106 Hz under 100 mW/cm2 AM 1.5 illumination and the results were presented in Fig. 7.a-d, respectively. The obvious characteristics of the recorded Nyquist plots of all the samples are two main arcs at high and low frequencies. The results were fitted according to the proposed equivalent circuit shown in Fig. 7.e 31. According to the previous reports 33-35, considering electron transport and recombination behavior of solar cells, specially dye sensitized solar cells (DSCs), a classical feature of transmission line (TL) originating from the mesostructured TiO2 sublayer is observed. The TL model includes a straight line followed by the arc at lower frequency which is a result of coupling capacitance with recombination. This line cuts the semicircle at low frequencies 31. The TL patterns are clearly seen in Fig. 7.a as well as Fig. 7.b-d insets. The first observed arc at higher frequencies in all samples is related to the transport in hole transport material (HTM) which here is Spiro-OMeTAD. The as-shown circuit model in Fig. 7.e represents RHTM, as HTM resistance in parallel with CHTM, as HTM capacitance. The second semicircle at lower frequencies is originated from recombination resistance, Rrec. Obviously, at higher applied voltage, both semicircles become smaller due to upshifting the Fermi level of CH3NH3PbI3 which results in higher electron transfer from CH3NH3PbI3 to HTM and decreasing recombination resistance 36. For S140 (Fig. 7.a), it is seen that both arcs are very bigger than the other three samples (Fig. 7.b-d). Although bigger Rrec in this cell shows lower recombination compared with the other samples, high value of the semicircle at high frequencies prevents from good performance of the final cell since it shows high Rs of the sample which could be a reason


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for low FF value of this cell (see Table 1) 37. In Fig. 7.b, at low applied bias, it is seen that Rrec of S150 is bigger than those of S160 and S170. It explains higher FF of S150 compared with the other samples since the system behaves like a cell with high shunt resistance 37. The big Rrec at lower voltage shows that in S150 recombination pathway is not dominant compare with S160 and S170 samples. The similar results of IS for S160 and S170 explains their close photovoltaic values. The slightly higher VOC value of S160 could be attributed to its bigger Rrec at both voltages.

Conclusion In this work, a new perovskite layer deposition with high stability and high photovoltaic efficiency was introduced. In this approach formation of MAPbI3-xBrx/MAPbBr3 proposes high efficiency and stability compared to CH3NH3PbI3. Herein, a desirable perovskite layer was fabricated by optimization of temperature and time of VASP approach by substituting the MAI with MABr. Stabilized PCE of 15.34% was demonstrated with the MAPbI3-xBrx/ MAPbBr3 perovskite solar cell using optimized thermal reaction process. This approach is proposed as a good candidate for fabrication of large-area perovskite solar cells.



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(19). Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y., Planar Heterojunction Perovskite Solar Cells Via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2013, 136, 622-625. (20). Zhou, H.; Chen, Q.; Yang, Y., Vapor-Assisted Solution Process for Perovskite Materials and Solar Cells. MRS Bull. 2015, 40, 667-673. (21). Zhou, Y.; Yang, M.; Wu, W.; Vasiliev, A. L.; Zhu, K.; Padture, N. P., Room-Temperature Crystallization of Hybrid-Perovskite Thin Films Via Solvent–Solvent Extraction for High-Performance Solar Cells. J. Mater. Chem. A. 2015, 3, 8178-8184. (22). Zhang, M.; Yu, H.; Yun, J.-H.; Lyu, M.; Wang, Q.; Wang, L., Facile Preparation of Smooth Perovskite Films for Efficient Meso/Planar Hybrid Structured Perovskite Solar Cells. Chem. Commun. 2015, 51, 10038-10041. (23). Nejand, B. A.; Gharibzadeh, S.; Ahmadi, V.; Shahverdi, H. R., Novel Solvent-Free Perovskite Deposition in Fabrication of Normal and Inverted Architectures of Perovskite Solar Cells. Sci. Rep. 2016, 6, 33649. (24). Nejand, B. A.; Nazari, P.; Gharibzadeh, S.; Ahmadi, V.; Moshaii, A., All-Inorganic Large-Area Low-Cost and Durable Flexible Perovskite Solar Cells Using Copper Foil as a Substrate. Chem. Commun. 2017, 53, 747-750. (25). Sheng, R.; Ho-Baillie, A.; Huang, S.; Chen, S.; Wen, X.; Hao, X.; Green, M. A., Methylammonium Lead Bromide Perovskite-Based Solar Cells by Vapor-Assisted Deposition. J. Phys. Chem. C 2015, 119, 3545-3549. (26). Yang, M.; Zhang, T.; Schulz, P.; Li, Z.; Li, G.; Kim, D. H.; Guo, N.; Berry, J. J.; Zhu, K.; Zhao, Y., Facile Fabrication of Large-Grain CH3NH3PbI3−X BrX Films for High-Efficiency Solar Cells Via CH3NH3Br-Selective Ostwald Ripening. Nat. commun. 2016, 7, 12305. (27). Brenner, T. M.; Rakita, Y.; Orr, Y.; Klein, E.; Feldman, I.; Elbaum, M.; Cahen, D.; Hodes, G., Conversion of Single Crystalline PbI2 to CH3NH3PbI3: Structural Relations and Transformation Dynamics. Chem. Mater. 2016, 28, 6501-6510. (28). Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J., Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6521-6527. (29). Bouduban, M. E.; Burgos-Caminal, A.; Ossola, R.; Teuscher, J.; Moser, J.-E., Energy and Charge Transfer Cascade in Methylammonium Lead Bromide Perovskite Nanoparticle Aggregates. Chemical science 2017, 8, 4371-4380. (30). Juarez-Perez, E. J.; Ono, L. K.; Maeda, M.; Jiang, Y.; Hawash, Z.; Qi, Y., Photodecomposition and Thermal Decomposition in Methylammonium Halide Lead Perovskites and Inferred Design Principles to Increase Photovoltaic Device Stability. J. Mater. Chem. A. 2018, 6, 9604-9612. (31). Gonzalez-Pedro, V.; Juarez-Perez, E. J.; Arsyad, W.-S.; Barea, E. M.; Fabregat-Santiago, F.; MoraSero, I.; Bisquert, J., General Working Principles of CH3NH3PbI3 Perovskite Solar Cells. Nano Lett. 2014, 14, 888-893. (32). Kim, H.-S.; Mora-Sero, I.; Gonzalez-Pedro, V.; Fabregat-Santiago, F.; Juarez-Perez, E. J.; Park, N.-G.; Bisquert, J., Mechanism of Carrier Accumulation in Perovskite Thin-Absorber Solar Cells. Nat. commun 2013, 4. (33). Mora-Seró, I.; Luo, Y.; Garcia-Belmonte, G.; Bisquert, J.; Muñoz, D.; Voz, C.; Puigdollers, J.; Alcubilla, R., Recombination Rates in Heterojunction Silicon Solar Cells Analyzed by Impedance Spectroscopy at Forward Bias and under Illumination. Sol. Energy Mater. Sol. Cells 2008, 92, 505-509. (34). Bertoluzzi, L.; Boix, P. P.; Mora-Sero, I.; Bisquert, J., Theory of Impedance Spectroscopy of Ambipolar Solar Cells with Trap-Mediated Recombination. J. Phys. Chem. C 2014, 118, 16574-16580. (35). Zarazua, I.; Han, G.; Boix, P. P.; Mhaisalkar, S.; Fabregat-Santiago, F.; Mora-Seró, I.; Bisquert, J.; Garcia-Belmonte, G., Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis. J. Phys. Chem. Lett. 2016, 7, 5105-5113.



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Table 1. Photovoltaic properties of prepared devices at different reaction temperatures of 140, 150, 160, and 170 ℃ Device 140

Voc 1.09

Jsc 16.79

FF 0.23

PCE

150

1.06

23.40

0.62

15.34

160

1.09

20.91

0.54

12.26

170

1

21.98

0.56

12.43

4.18

Table 2. Statistics of device performance in S140, S150, S160, and S170 Photovoltaic parameter VOC (V)

JSC (mA)

FF

PEC (%)

Statistic values

S140

S150

S160

S170

The highest value The lowest value The average value The standard deviation The highest value The lowest value The average value The standard deviation The highest value The lowest value The average value The standard deviation The highest value The lowest value The average value The standard deviation

1.09 0.70 0.95 0.15 16.79 9.1 11.77 2.99 0.28 0.164 0.23 0.035 4.18 1.05 2.67 1.07

1.1 1.05 1.08 0.02 23.4 18.82 20.82 1.84 0.67 0.56 0.61 0.034 15.34 12.29 13.68 1.05

1.1 1.07 1.09 0.01 22.05 12.26 19.33 3.58 0.63 0.49 0.54 0.045 12.26 8.5 10.84 1.51

1.1 0.09 0.91 0.36 23.70 15.46 20.20 2.73 0.56 0.46 0.53 0.037 12.43 9.21 10.83 1.21



Fig 1. (a) Schematic procedure of MAPbI3-xBrx/ MAPbBr3 perovskite layer growth via VASP method. (b) Schematic of energy level diagrams of the FTO/TiO2/ MAPbI3-xBrx/ MAPbBr3 /SpiroOMeTAD/Au (c) Schematic illustration of the perovskite solar cell configuration, where a smooth and compact perovskite capping layer fully covers the mesoporous TiO2 layer (mp-TiO2) infiltrated with perovskite. FTO, fluorine-doped tin oxide; bl-TiO2, TiO2 compact layer.


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Fig 2. XRD pattern of samples prepared at 140, 150, 160, and 170 ℃.


500 nm


a

e

c

1 µm

b

1 µm

g

1 µm

1 µm

f

d

500 nm

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500 nm

h

500 nm

500 nm

i Au Spiro-OMeTAD Perovskite m-TiO2 FTO

500 nm

Fig 3. (a-h) Plane-view scanning electron microscopic (SEM) images of perovskite films as a function of reaction temperature at (a,b) 140, (c,d) 150, (e,f) 160 and (g,h) 170 ℃. (i) Crosssectional SEM image of a meso architecture solar cell based on evaporation of MABr with VASP technique for 12 min at 150 ℃. (j) The statistical distribution of the particle size for grown perovskite layer at different temperatures of 140, 150, 160, and 170 ℃.


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a

b

2 µm

2 µm

d

c

2 µm

2 µm

Fig. 4. Plane-view SEM images of perovskite films (a) before washing with isopropanol at room temperature and after washing with isopropanol and drying at 100 C for (b) 2 min (c) 5 min (d) 12 min.




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Fig 5. (a) Absorption spectra and inset tauc-plot of perovskite films as a function of annealing temperature for 12 minutes. The perovskite films were prepared with VASP on glass substrate and annealed at 140, 150, 160 and 170 for 12 min and. (b) Current-voltage (J-V) curves for the best performing devices using perovskite films prepared by the VASP method at different temperature ranging from 140 to170 ℃ for 12 min. (c) The distributions of different photovoltaic parameters of devices fabricated at different growth temperatures of 140, 150, 160, and 170 ℃ under 100 mW/cm2 illumination.



Fig 6. (a) and (b) Device performance stability of FTO/TiO2/ MAPbI3-xBrx/ MAPbBr3 /HTM/Au devices where the perovskite is prepared with VASP either MABr or MAI over a period of 50 days, and device performance durability in 50 days at 25℃ and 28 ± 2% moisture under dark retention and analyzing under one sun (AM1.5) illumination.


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Fig 7. The Nyquist plots at short-circuit and 500 mV bias of TiO2/ MAPbI3-xBrx/ MAPbBr3 solar cells under 100 mW/cm2 illumination prepared by VASP method at different temperatures (a) S140, (b) S150, (c) S160, and (d) S170 hole conductors (Insets: the high frequency part of the spectra. (e) The fitted equivalent circuit model for impedance analysis of the fabricated solar cells. Rs: series resistance, Rhf: high frequency resistance, Chf: high frequency capacitance, TL: transmission line element.



TOC


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Long-Term Durability of Bromide-Incorporated Perovskite Solar Cells

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