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Received 00th January 2012, Accepted 00th January 2012
Meng Zhang, Hua Yu, Jung-Ho Yun, Miaoqiang Lyu, Qiong Wang, Lianzhou Wang*
DOI: 10.1039/x0xx00000x www.rsc.org/
Smooth organolead halide perovskite films for meso/planar hybrid structured perovskite solar cells were prepared by a simple compressed air blow-drying method under ambient condition. The resultant perovskite films show high surface coverage, leading to a device power conversion efficiency of over 10% with an open circuit voltage up to 1.003 V merely using pristine poly(3-hexylthiophene) (P3HT) as hole transporter. As a new class of low-cost photovoltaic devices, organolead halide perovskite solar cells (PSCs) have attracted increasing attention in recent years1-5. A number of PSCs with different structures/configurations have been developed to date. Among various PSC structures, meso/planar hybrid structure consisting of a perovskite infiltrated mesoporous TiO2 layer and a pure perovskite capping layer has been proven to be one of the most efficient designs6, 7. With this configuration, the devices can achieve a high power conversion efficiency (PCE) without exhibiting severe IV hysteresis5. To date, the certified PCE record of PSCs is achieved by the meso/planar hybrid architecture8. The rapid improvement in PCE of PSCs stems from optimization in deposition process of the perovskite film. With the conventional spin coating method, the organolead halide perovskite film tends to form micro-sized branch-like crystals and left over large uncovered area9, 10. The direct contact between TiO2 and hole transporting material (HTM) is considered to be responsible for the undesirable recombination and hence reduces the photovoltaic performance of the devices11. To date, several approaches have been developed to deposit a uniform and smooth perovskite film5, 9-15. Most of them can be categorized as a two-step method. Firstly a PbI2 layer was deposited by spin coating and then reacts with methylammonium iodide either in solution phase9, 12 or vapour phase13, 14 to form a uniform and smooth perovskite film. Such a high quality film can be alternatively obtained by the solvent
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engineering method5 and the fast deposition-crystallization method10, . However, these methods usually require expensive facility or complicated operation procedures, and some of them are not applicable to the meso/planar hybrid architecture. Note that the perovskite film deposition is normally to be processed in an inert atmosphere (such as glove box) due to its high sensitivity to moisture14-16. A more simple fabrication process of the devices without restricted humidity control is highly desirable. 15
In this communication, we report a very simple compressed air blow-drying method to prepare a smooth perovskite film for meso/planar hybrid structured PSCs. This method does not require any complicated equipment or atmospheric protection. More importantly, it does not rely on the non-scalable spin-coating and therefore has great potential in large-scale roll-to-roll fabrication of perovskite light absorbing film. In order to minimize the influence of moisture for ambient air fabrication, mixed halide perovskite CH3NH3PbI2.4Br0.6 is chosen as the perovskite light absorber5, 17-19. A power conversion efficiency of over 10% was achieved by simply using less expensive pristine poly(3-hexylthiophene) (P3HT) instead of modified spiro-MeOTAD as the hole transporter. Fig. 1a shows the scheme of perovskite film deposition process. Firstly, the perovskite precursor solution was dispensed on the mesoporous TiO2 substrate. Then, the free flowing part of the precursor solution was removed by absorbing the solution using a tissue paper. The precursor filled mesoporous substrate was then blow-dried by compressed air. During blow-drying the perovskite film presented a dark brown color immediately indicating the fast crystallization. The film was annealed at 100⁰C for 10 min on a hotplate in order to further crystalize the film and remove the excess methylammonium halide. The XRD pattern (Fig. 1b) demonstrates that the perovskite film with good crystallinity has been successfully deposited on the substrate. All peaks of the perovskite film are consistent with the previous study17. The I : Br atomic ratio of 4 : 1
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Facile preparation of smooth perovskite films for efficient meso/planar hybrid structured perovskite solar cells
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was further confirmed by X-ray energy-dispersive spectroscopy (EDS; Supporting Information, Fig. S1).
Fig. 1 (a) Scheme of the blow-drying deposition process of a perovskite film. (b) XRD pattern of the blow-dried CH3NH3PbI2.4Br0.6 film deposited on mesoporousTiO2 coated FTO glass substrates. (c) SEM surface morphology of a blow-dried perovskite film.
Surface morphology of the resultant films after annealing for 10 min was characterized by scanning electron microscope (SEM) as shown in Fig. 1c. It is clear that the capping layer of the perovskite film is composed of submicron-sized grains. No mesoporous substrate can be observed, indicating a good coverage of the capping perovskite layer. With this facile method, the obtained perovskite films are able to achieve a high quality similar to those using other methods as previously reported5, 13. In contrast, the surface coverage of the capping layer prepared by spin-coating using the same precursor solution is very poor (Fig. S2). Compared to the spincoated perovskite film, the blow-dried perovskite film also exhibited very different appearance. To be specific, the colour of the blowdried perovskite film was much darker and consequently exhibited a much higher absorbance than the spin-coated perovskite film (Fig. S3). That is probably resulted from the full coverage of the capping layer which can absorb light evenly and efficiently across the entire films. In comparison, the spin-coated perovskite film exhibit insufficient absorption due to the poor surface coverage of the film. Another difference of the two methods is the surface roughness. It can be seen that the blow-dried perovskite film presents better smoothness with glassy surface compared to the spin-coated sample (Fig. S3). As known, a smooth and dense capping layer is very important in eliminating shunting path and achieving good photovoltaic performance5. In this regard, obviously, the blow-dried perovskite film is superior to the spin-coated film. As pointed out, the undesirable branch-like morphology can be attributed to the slow nucleation rate during the evaporation of solvent15. The function of compressed air flow is to create supersaturation of the perovskite components in the wet film and facilitate fast nucleation and crystal growth10, 15. Therefore, a certain time window (e.g. 2s at 6500 rpm during spin-coating) for gas treatment was required in the gas-assisted spin-coating method10. If the gas treatment is not accurately applied during this time window,
2 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C5CC02534F the branch-like morphology might not be avoided. However, this blow-drying method does not require such specific time window, because the wet condition of the precursor is not affected by the spin-coating procedure. Moreover, the exclusion of nonscalable spin-coating process brings great application potential in large-scale fabrication of perovskite film. In this blow-drying method, the main factor that affects the uniformity of films is the air flow speed applied on the film surface. Fig. 2 shows the SEM images of the blow-dried perovskite films prepared under different air flow speed. The air flow speed was determined by the distance between the compressed air outlet and the precursor filled mesoporous substrate and measured by a wind speed meter. The surface morphology of those films varied drastically. At a relatively low air flow speed of 10±2 m/s, the obtained perovskite film still showed large branch-like morphology which is similar to the spin-coated sample. As the air flow speed increases, the grain size of the branch-like crystals has been gradually reduced. Eventually, at a high air flow speed of 25±2 m/s, the resulted film presented a very smooth surface. These SEM images suggest that a high speed air flow applied on the wet film is essential in obtaining a smooth perovskite films. In fact, the morphology of the perovskite film is determined by the evaporation rate of the solvent. Under natural condition or low speed air flow, the low evaporation rate facilitates migration of ions in solution and form overgrown crystals which lead to branch-like morphology20. With high evaporation rate produced by high speed air flow, the crystals nucleate immediately before ions migrate and hence a smooth perovskite film consisting of small crystals is formed.
Fig. 2 SEM surface morphology of blow-dried perovskite films under varied air flow speed: a) 10±2 m/s; b) 15±2 m/s; c) 20±2 m/s; d) 25±2 m/s. Films are annealed at 100 ⁰C for 10 min.
In order to investigate the effect of the subsequent annealing on the blow-dried perovskite film, SEM images of the perovskite film samples subject to different annealing time are shown in Fig. 3. The as-prepared film without annealing already exhibits obvious grains (Fig. 3a). XRD result has further confirmed the good crystallinity of the film without annealing process (Fig. S4). It indicates that the initial crystal growth of pin-hole free perovskite film has already occurred during the blow-drying process at room temperature rather
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than in the subsequent annealing treatment. Interestingly, the grains of the film without annealing are considerably larger than that obtained using the gas-assisted spin-coating method10, while the grain size does not grow significantly with the extension of annealing time. After annealing at 100 ⁰C for 15 min (Fig. 3d), a number of sheet-like crystals were generated on the grain boundaries, which obviously reduce the smoothness of the perovskite film. Therefore, the 10 min annealing are considered to be the optimum in obtaining a smooth film with good crystallinity. And this annealing condition has been applied to all the devices discussed subsequently.
Fig. 3 SEM surface morphology of blow-dried perovskite films annealed at 100 ⁰C for different time: a) 0 min; b) 5 min; c) 10 min; d) 15 min.
The final solar cells were constructed using the low cost pristine P3HT as the HTM. Although the additives such as bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) may enhance the hole transporting property of HTM21, 22, a negative effect of accelerating the degradation of the perovskite was demonstrated in recent literature23-25. Considering the simplification of device fabrication procedure as well as the long-term stability of the devices, hole conducting P3HT are used in its pristine form without any additives. As shown in the cross-sectional SEM image of the PSC using blow-dried perovskite film (Fig. 4a), a 50 nm compact TiO2 layer is deposited on a 500 nm FTO substrate working as a hole blocking layer. The hybrid structured perovskite layer can be clearly observed, which consists of a ~300 nm perovskite infiltrated mesoporous TiO2 layer and a ~200 nm perovskite capping layer. The grain boundaries of the capping layer can be visible as well. P3HT HTM layer of less than 50 nm thick lies next to the perovskite capping layer, and Au metal is finally deposited as the photocathode. Typical J-V curves of the devices fabricated employing blowdried perovskite film and spin-coated perovskite film are shown in Fig. 4b. Under simulated air-mass (AM) 1.5 sun light, the device fabricated with blow-dried perovskite film exhibited a PCE of 10.2% with a short circuit current density (Jsc) of 15.45 mA/cm2, an open circuit voltage (Voc) of 0.952 V and a fill factor (FF) of 0.693,
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COMMUNICATION DOI: 10.1039/C5CC02534F apparently exceeding performance of the solar cells employing spincoated perovskite film, which only showed a PCE of 4.8%. With an even shorter scan delay, the measured PCE was improved to 11.3% due to an increased FF of 0.741. However, the hysteresis effect became more obvious when the delay time of the J-V scan was shortened with a dropped PCE of 7.7 %, which can be mainly attributed to the deteriorated FF in the forward scan. Similar trend can also be found in previously reported devices5, 26. But under the normal scan delay (10 ms) the hysteretic behaviour in forward scan is much less significant. In order to estimate the steady-state PCE, the photocurrent density at the maximum power output voltage was recorded27. With a forward scan from 0.760 V to 0.761 V, the PCE quickly stabilized at around 9.7 %, which is closer to the result from the normal scan condition (reverse scan with 100 ms delay). Notably, the difference between the initial and the stabilized current density is only 1 mA/cm2, much smaller than the planar structured PSC reported previously10, 15, 27. It is considered to be resulted from the suppressed hysteretic effect of the planar/meso hybrid architecture. Note that for some of the devices fabricated using the blow-drying method, their Voc can reach up to 1.003 V (Fig. S5), which is considerably high among the pristine P3HT based PSCs21, 22, 28-30 . The high Voc of the device benefits from two important configurations. Firstly, the Br substitution for I leads to a wider band gap of the perovskite light absorber17. And under illumination, perovskite with wider band gap contributes to a larger difference between quasi-Fermi level of electron and quasi-Fermi level of hole, where Voc is originated31. Secondly, the smooth perovskite capping layer provides an effective barrier between TiO2 electron extraction layer and HTM thus minimizing the recombination loss.
Fig. 4 (a) Cross-sectional SEM image of the meso/planar hybrid structured device using blow-dried perovskite film. (b) J−V curves of devices prepared with blowdryied perovskite film and spin-coated perovskite film measured under AM 1.5 simulated sunlight (100 mW/cm2 irradiance) (c) J−V curves of the device under different scan condition, reverse scan (RS) and forward scan (FS) are carried out both with 10 mv scan step. (d) Photo-current density and PCE as a function of time for the same device held at forward bias of 0.760 V to 0.761 V.
In summary, a very simple blow-drying method for perovskite film forming has been developed to fabricate meso/planar hybrid structured PSCs with any atmospheric protection. This method does
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not require a sensitive time window for compressed air blowing. Detailed experimental investigation revealed that the perovskite films were crystallized with the fast solvent evaporation under intensive air flow and grains were formed immediately after the blowing process. Stronger air flow applied on the wet film is beneficial to form a smooth and well-crystalline perovskite film. The blow-dried perovskite film has significantly improved the performance of the solar cells in comparison with the conventional spin-coated films. The corresponding devices fabricated in ambient air employing pristine P3HT achieved efficiency over 10% with an open circuit voltage up to 1.003 V. The new method reported herein not only provides a low cost and facile approach for fabricating efficient solid state perovskite solar cells but also freed the film fabrication process from the limitation of non-scalable spin-coating, which may find great potential in large-scale fabrication, e.g. roll-toroll printing, of perovskite light absorbing films.
Acknowledgements Financial support from CRC for Polymers program and ARC DP and FT programs is acknowledged. This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF-Q). M.Z. acknowledges the support from Chinese Scholarship Council (CSC).
Journal Name DOI: 10.1039/C5CC02534F 17. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano Lett. , 2013, 13, 1764-1769. 18. M. Zhang, M. Lyu, H. Yu, J. H. Yun, Q. Wang and L. Wang, Chem. Eur. J., 2015, 21, 434-439. 19. E. T. Hoke, D. J. Slotcavage, E. R. Dohner, A. R. Bowring, H. I. Karunadasa and M. D. McGehee, Chem. Sci. , 2015, 6, 613-617. 20. K. Hwang, Y.-S. Jung, Y.-J. Heo, F. H. Scholes, S. E. Watkins, J. Subbiah, D. J. Jones, D.-Y. Kim and D. Vak, Adv. Mater. , 2015, 27, 1241-1247. 21. Y. Guo, C. Liu, K. Inoue, K. Harano, H. Tanaka and E. Nakamura, J. Mater. Chem. A, 2014, 2, 13827-13830. 22. J. H. Heo and S. H. Im, Phys. Status Solidi RRL, 2014, 8, 816-821. 23. S. N. Habisreutinger, T. Leijtens, G. E. Eperon, S. D. Stranks, R. J. Nicholas and H. J. Snaith, Nano Lett. , 2014, 14, 5561-5568. 24. S. N. Habisreutinger, T. Leijtens, G. E. Eperon, S. D. Stranks, R. J. Nicholas and H. J. Snaith, J. Phys. Chem. Lett., 2014, 5, 4207-4212. 25. J. Yang, B. D. Siempelkamp, D. Liu and T. L. Kelly, ACS Nano, 2015, 9, 1955-1963. 26. H. S. Kim and N. G. Park, J. Phys. Chem. Lett., 2014, 5, 2927-2934. 27. H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. K. Noel, S. D. Stranks, J. T. W. Wang, K. Wojciechowski and W. Zhang, J. Phys. Chem. Lett. , 2014, 5, 1511-1515. 28. J. Xiao, J. Shi, H. Liu, Y. Xu, S. Lv, Y. Luo, D. Li, Q. Meng and Y. Li, Adv. Energy Mater., 2015, doi: 10.1002/aenm.201401943. 29. B. Conings, L. Baeten, C. De Dobbelaere, J. D'Haen, J. Manca and H. G. Boyen, Adv. Mater. , 2014, 26, 2041-2046. 30. M. L. Cai, V. T. Tiong, T. Hreid, J. Bell and H. X. Wang, J. Mater. Chem. A, 2015, 3, 2784-2793. 31. S. Ryu, J. H. Noh, N. J. Jeon, Y. Chan Kim, W. S. Yang, J. Seo and S. I. Seok, Energy Environ. Sci. , 2014, 7, 2614-2618.
Notes and references Nanomaterials Centre, School of Chemical Engineering, the University of Queensland, QLD 4072, Australia. E-mail: [email protected]; Fax: +61 7 3365218 † Electronic Supplementary Information (ESI) available: See DOI: 10.1039/c000000x/ 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15.
16.
M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338, 643-647. P. Gao, M. Gratzel and M. K. Nazeeruddin, Energy Environ. Sci. , 2014, 7, 2448-2463. M. A. Green, A. Ho-Baillie and H. J. Snaith, Nat. Photonics 2014, 8, 506-514. N. G. Park, J. Phys. Chem. Lett. , 2013, 4, 2423-2429. N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. I. Seok, Nat. Mater. , 2014, 13, 897-903. M. Gratzel, Nat. Mater. , 2014, 13, 838-842. Y. Zhao and K. Zhu, J. Phys. Chem. Lett., 2014, 5, 4175-4186. N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo and S. I. Seok, Nature, 2015, 517, 476-480. J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316-319. 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 and Y.-B. Cheng, Nano Energy, 2014, 10, 10-18. M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395-398. Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao and J. Huang, Energy Environ. Sci. , 2014, 7, 2619-2623. Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu, G. Li and Y. Yang, J. Am. Chem. Soc. , 2013, 136, 622-625. F. Hao, C. C. Stoumpos, Z. Liu, R. P. H. Chang and M. G. Kanatzidis, J. Am. Chem. Soc. , 2014, 136, 16411-16419. M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y. B. Cheng and L. Spiccia, Angew. Chem. Int. Ed. , 2014, 53, 9898-9903. J.-Y. Jeng, Y.-F. Chiang, M.-H. Lee, S.-R. Peng, T.-F. Guo, P. Chen and T.-C. Wen, Adv. Mater. , 2013, 25, 3727-3732.
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Smooth organolead halide perovskite films were prepared by a facile blow-drying method in ambient air for achieving efficient and low cost meso/planar hybrid structured perovskite solar cells.
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ChemComm Accepted Manuscript
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ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Wang, M. Zhang, H. Yu, M. Lyu and J. H. Yun, Chem. Commun., 2015, DOI: 10.1039/C5CC02534F.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 2012, Accepted 00th January 2012
Meng Zhang, Hua Yu, Jung-Ho Yun, Miaoqiang Lyu, Qiong Wang, Lianzhou Wang*
DOI: 10.1039/x0xx00000x www.rsc.org/
Smooth organolead halide perovskite films for meso/planar hybrid structured perovskite solar cells were prepared by a simple compressed air blow-drying method under ambient condition. The resultant perovskite films show high surface coverage, leading to a device power conversion efficiency of over 10% with an open circuit voltage up to 1.003 V merely using pristine poly(3-hexylthiophene) (P3HT) as hole transporter. As a new class of low-cost photovoltaic devices, organolead halide perovskite solar cells (PSCs) have attracted increasing attention in recent years1-5. A number of PSCs with different structures/configurations have been developed to date. Among various PSC structures, meso/planar hybrid structure consisting of a perovskite infiltrated mesoporous TiO2 layer and a pure perovskite capping layer has been proven to be one of the most efficient designs6, 7. With this configuration, the devices can achieve a high power conversion efficiency (PCE) without exhibiting severe IV hysteresis5. To date, the certified PCE record of PSCs is achieved by the meso/planar hybrid architecture8. The rapid improvement in PCE of PSCs stems from optimization in deposition process of the perovskite film. With the conventional spin coating method, the organolead halide perovskite film tends to form micro-sized branch-like crystals and left over large uncovered area9, 10. The direct contact between TiO2 and hole transporting material (HTM) is considered to be responsible for the undesirable recombination and hence reduces the photovoltaic performance of the devices11. To date, several approaches have been developed to deposit a uniform and smooth perovskite film5, 9-15. Most of them can be categorized as a two-step method. Firstly a PbI2 layer was deposited by spin coating and then reacts with methylammonium iodide either in solution phase9, 12 or vapour phase13, 14 to form a uniform and smooth perovskite film. Such a high quality film can be alternatively obtained by the solvent
This journal is © The Royal Society of Chemistry 2012
engineering method5 and the fast deposition-crystallization method10, . However, these methods usually require expensive facility or complicated operation procedures, and some of them are not applicable to the meso/planar hybrid architecture. Note that the perovskite film deposition is normally to be processed in an inert atmosphere (such as glove box) due to its high sensitivity to moisture14-16. A more simple fabrication process of the devices without restricted humidity control is highly desirable. 15
In this communication, we report a very simple compressed air blow-drying method to prepare a smooth perovskite film for meso/planar hybrid structured PSCs. This method does not require any complicated equipment or atmospheric protection. More importantly, it does not rely on the non-scalable spin-coating and therefore has great potential in large-scale roll-to-roll fabrication of perovskite light absorbing film. In order to minimize the influence of moisture for ambient air fabrication, mixed halide perovskite CH3NH3PbI2.4Br0.6 is chosen as the perovskite light absorber5, 17-19. A power conversion efficiency of over 10% was achieved by simply using less expensive pristine poly(3-hexylthiophene) (P3HT) instead of modified spiro-MeOTAD as the hole transporter. Fig. 1a shows the scheme of perovskite film deposition process. Firstly, the perovskite precursor solution was dispensed on the mesoporous TiO2 substrate. Then, the free flowing part of the precursor solution was removed by absorbing the solution using a tissue paper. The precursor filled mesoporous substrate was then blow-dried by compressed air. During blow-drying the perovskite film presented a dark brown color immediately indicating the fast crystallization. The film was annealed at 100⁰C for 10 min on a hotplate in order to further crystalize the film and remove the excess methylammonium halide. The XRD pattern (Fig. 1b) demonstrates that the perovskite film with good crystallinity has been successfully deposited on the substrate. All peaks of the perovskite film are consistent with the previous study17. The I : Br atomic ratio of 4 : 1
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Facile preparation of smooth perovskite films for efficient meso/planar hybrid structured perovskite solar cells
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was further confirmed by X-ray energy-dispersive spectroscopy (EDS; Supporting Information, Fig. S1).
Fig. 1 (a) Scheme of the blow-drying deposition process of a perovskite film. (b) XRD pattern of the blow-dried CH3NH3PbI2.4Br0.6 film deposited on mesoporousTiO2 coated FTO glass substrates. (c) SEM surface morphology of a blow-dried perovskite film.
Surface morphology of the resultant films after annealing for 10 min was characterized by scanning electron microscope (SEM) as shown in Fig. 1c. It is clear that the capping layer of the perovskite film is composed of submicron-sized grains. No mesoporous substrate can be observed, indicating a good coverage of the capping perovskite layer. With this facile method, the obtained perovskite films are able to achieve a high quality similar to those using other methods as previously reported5, 13. In contrast, the surface coverage of the capping layer prepared by spin-coating using the same precursor solution is very poor (Fig. S2). Compared to the spincoated perovskite film, the blow-dried perovskite film also exhibited very different appearance. To be specific, the colour of the blowdried perovskite film was much darker and consequently exhibited a much higher absorbance than the spin-coated perovskite film (Fig. S3). That is probably resulted from the full coverage of the capping layer which can absorb light evenly and efficiently across the entire films. In comparison, the spin-coated perovskite film exhibit insufficient absorption due to the poor surface coverage of the film. Another difference of the two methods is the surface roughness. It can be seen that the blow-dried perovskite film presents better smoothness with glassy surface compared to the spin-coated sample (Fig. S3). As known, a smooth and dense capping layer is very important in eliminating shunting path and achieving good photovoltaic performance5. In this regard, obviously, the blow-dried perovskite film is superior to the spin-coated film. As pointed out, the undesirable branch-like morphology can be attributed to the slow nucleation rate during the evaporation of solvent15. The function of compressed air flow is to create supersaturation of the perovskite components in the wet film and facilitate fast nucleation and crystal growth10, 15. Therefore, a certain time window (e.g. 2s at 6500 rpm during spin-coating) for gas treatment was required in the gas-assisted spin-coating method10. If the gas treatment is not accurately applied during this time window,
2 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C5CC02534F the branch-like morphology might not be avoided. However, this blow-drying method does not require such specific time window, because the wet condition of the precursor is not affected by the spin-coating procedure. Moreover, the exclusion of nonscalable spin-coating process brings great application potential in large-scale fabrication of perovskite film. In this blow-drying method, the main factor that affects the uniformity of films is the air flow speed applied on the film surface. Fig. 2 shows the SEM images of the blow-dried perovskite films prepared under different air flow speed. The air flow speed was determined by the distance between the compressed air outlet and the precursor filled mesoporous substrate and measured by a wind speed meter. The surface morphology of those films varied drastically. At a relatively low air flow speed of 10±2 m/s, the obtained perovskite film still showed large branch-like morphology which is similar to the spin-coated sample. As the air flow speed increases, the grain size of the branch-like crystals has been gradually reduced. Eventually, at a high air flow speed of 25±2 m/s, the resulted film presented a very smooth surface. These SEM images suggest that a high speed air flow applied on the wet film is essential in obtaining a smooth perovskite films. In fact, the morphology of the perovskite film is determined by the evaporation rate of the solvent. Under natural condition or low speed air flow, the low evaporation rate facilitates migration of ions in solution and form overgrown crystals which lead to branch-like morphology20. With high evaporation rate produced by high speed air flow, the crystals nucleate immediately before ions migrate and hence a smooth perovskite film consisting of small crystals is formed.
Fig. 2 SEM surface morphology of blow-dried perovskite films under varied air flow speed: a) 10±2 m/s; b) 15±2 m/s; c) 20±2 m/s; d) 25±2 m/s. Films are annealed at 100 ⁰C for 10 min.
In order to investigate the effect of the subsequent annealing on the blow-dried perovskite film, SEM images of the perovskite film samples subject to different annealing time are shown in Fig. 3. The as-prepared film without annealing already exhibits obvious grains (Fig. 3a). XRD result has further confirmed the good crystallinity of the film without annealing process (Fig. S4). It indicates that the initial crystal growth of pin-hole free perovskite film has already occurred during the blow-drying process at room temperature rather
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than in the subsequent annealing treatment. Interestingly, the grains of the film without annealing are considerably larger than that obtained using the gas-assisted spin-coating method10, while the grain size does not grow significantly with the extension of annealing time. After annealing at 100 ⁰C for 15 min (Fig. 3d), a number of sheet-like crystals were generated on the grain boundaries, which obviously reduce the smoothness of the perovskite film. Therefore, the 10 min annealing are considered to be the optimum in obtaining a smooth film with good crystallinity. And this annealing condition has been applied to all the devices discussed subsequently.
Fig. 3 SEM surface morphology of blow-dried perovskite films annealed at 100 ⁰C for different time: a) 0 min; b) 5 min; c) 10 min; d) 15 min.
The final solar cells were constructed using the low cost pristine P3HT as the HTM. Although the additives such as bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) may enhance the hole transporting property of HTM21, 22, a negative effect of accelerating the degradation of the perovskite was demonstrated in recent literature23-25. Considering the simplification of device fabrication procedure as well as the long-term stability of the devices, hole conducting P3HT are used in its pristine form without any additives. As shown in the cross-sectional SEM image of the PSC using blow-dried perovskite film (Fig. 4a), a 50 nm compact TiO2 layer is deposited on a 500 nm FTO substrate working as a hole blocking layer. The hybrid structured perovskite layer can be clearly observed, which consists of a ~300 nm perovskite infiltrated mesoporous TiO2 layer and a ~200 nm perovskite capping layer. The grain boundaries of the capping layer can be visible as well. P3HT HTM layer of less than 50 nm thick lies next to the perovskite capping layer, and Au metal is finally deposited as the photocathode. Typical J-V curves of the devices fabricated employing blowdried perovskite film and spin-coated perovskite film are shown in Fig. 4b. Under simulated air-mass (AM) 1.5 sun light, the device fabricated with blow-dried perovskite film exhibited a PCE of 10.2% with a short circuit current density (Jsc) of 15.45 mA/cm2, an open circuit voltage (Voc) of 0.952 V and a fill factor (FF) of 0.693,
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COMMUNICATION DOI: 10.1039/C5CC02534F apparently exceeding performance of the solar cells employing spincoated perovskite film, which only showed a PCE of 4.8%. With an even shorter scan delay, the measured PCE was improved to 11.3% due to an increased FF of 0.741. However, the hysteresis effect became more obvious when the delay time of the J-V scan was shortened with a dropped PCE of 7.7 %, which can be mainly attributed to the deteriorated FF in the forward scan. Similar trend can also be found in previously reported devices5, 26. But under the normal scan delay (10 ms) the hysteretic behaviour in forward scan is much less significant. In order to estimate the steady-state PCE, the photocurrent density at the maximum power output voltage was recorded27. With a forward scan from 0.760 V to 0.761 V, the PCE quickly stabilized at around 9.7 %, which is closer to the result from the normal scan condition (reverse scan with 100 ms delay). Notably, the difference between the initial and the stabilized current density is only 1 mA/cm2, much smaller than the planar structured PSC reported previously10, 15, 27. It is considered to be resulted from the suppressed hysteretic effect of the planar/meso hybrid architecture. Note that for some of the devices fabricated using the blow-drying method, their Voc can reach up to 1.003 V (Fig. S5), which is considerably high among the pristine P3HT based PSCs21, 22, 28-30 . The high Voc of the device benefits from two important configurations. Firstly, the Br substitution for I leads to a wider band gap of the perovskite light absorber17. And under illumination, perovskite with wider band gap contributes to a larger difference between quasi-Fermi level of electron and quasi-Fermi level of hole, where Voc is originated31. Secondly, the smooth perovskite capping layer provides an effective barrier between TiO2 electron extraction layer and HTM thus minimizing the recombination loss.
Fig. 4 (a) Cross-sectional SEM image of the meso/planar hybrid structured device using blow-dried perovskite film. (b) J−V curves of devices prepared with blowdryied perovskite film and spin-coated perovskite film measured under AM 1.5 simulated sunlight (100 mW/cm2 irradiance) (c) J−V curves of the device under different scan condition, reverse scan (RS) and forward scan (FS) are carried out both with 10 mv scan step. (d) Photo-current density and PCE as a function of time for the same device held at forward bias of 0.760 V to 0.761 V.
In summary, a very simple blow-drying method for perovskite film forming has been developed to fabricate meso/planar hybrid structured PSCs with any atmospheric protection. This method does
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not require a sensitive time window for compressed air blowing. Detailed experimental investigation revealed that the perovskite films were crystallized with the fast solvent evaporation under intensive air flow and grains were formed immediately after the blowing process. Stronger air flow applied on the wet film is beneficial to form a smooth and well-crystalline perovskite film. The blow-dried perovskite film has significantly improved the performance of the solar cells in comparison with the conventional spin-coated films. The corresponding devices fabricated in ambient air employing pristine P3HT achieved efficiency over 10% with an open circuit voltage up to 1.003 V. The new method reported herein not only provides a low cost and facile approach for fabricating efficient solid state perovskite solar cells but also freed the film fabrication process from the limitation of non-scalable spin-coating, which may find great potential in large-scale fabrication, e.g. roll-toroll printing, of perovskite light absorbing films.
Acknowledgements Financial support from CRC for Polymers program and ARC DP and FT programs is acknowledged. This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF-Q). M.Z. acknowledges the support from Chinese Scholarship Council (CSC).
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Smooth organolead halide perovskite films were prepared by a facile blow-drying method in ambient air for achieving efficient and low cost meso/planar hybrid structured perovskite solar cells.
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