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Precursor vapor deposited perovskite solar cells with smooth absorber layer
Materials Science in Semiconductor Processing 107 (2020) 104813
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Precursor vapor deposited perovskite solar cells with smooth absorber layer �irmenci b, Muhammet Erkan Ko €se c Fatih Mehmet Cos¸kun a, *, Fatih Deg a
Department of Engineering Physics, Istanbul Medeniyet University, Istanbul, 34730, Turkey Barbaros Hayrettin Pasa Mah, 1083. Sok. No:17/6 Gaziosmanpasa, Istanbul, Turkey c Department of Chemistry, Kocaeli University, Kocaeli, 41380, Turkey b
A R T I C L E I N F O
A B S T R A C T
Keywords: Perovskite solar cell Vapor deposition Large area devices
Preparation of a smooth methylammoniumleadiodide (CH3NH3PbI3) perovskite absorber film is one of the most critical processes in fabricating a perovskite photovoltaic cell with high efficiency. In this study, we report an original coating method to prepare CH3NH3PbI3 films by using the solvent vapors of precursor methylamine (MA) and hydroiodic acid (HI) solutions on lead (II) iodide (PbI2) thin films. In this strategy, the vapors of MA and HI solutions were briefly treated to the PbI2 coated substrate and then the resultant film was annealed to obtain smooth perovskite thin layer. After the topographic analyses of the thin films, the surface roughnesses of the perovskite films were found to be limited by the surface roughnesses of the PbI2 films before precursor vapor exposure. Another important aspect of our approach is that there is no need to synthesize methylammonium iodide (CH3NH3I) salt for active layer preparation. Furthermore, this method enables large-area fabrication of high quality films by a very simple and brief process. After the characterization of preliminary devices with this method, we obtained a short circuit current density of 14.67 mA/cm2, an open circuit voltage of 0.815 V, a fillfactor of 0.59, and a power conversion efficiency of 7.08%.
1. Introduction In recent years, perovskite photovoltaic technology has become one of the most promising photovoltaic technologies due to low cost pro duction and high efficiencies of the perovskite solar cells [1–3]. CH3NH3PbI3 and CH3NH3PbI3-xClx (CH3NH3PbI3-xClx) perovskite ab sorbers are at the forefront of advancements in terms of power conver sion efficiencies (PCEs) and device optimization studies [4]. CH3NH3PbI3 films are generally deposited from blend solutions (such as CH3NH3I þ PbI2 in N,N-dimethylformamide (DMF)) by spin coating or sequential deposition of PbI2 film with spin coating and then introduc tion of CH3NH3I onto PbI2 film either in solution or in vapor form [5–9]. CH3NH3I salt is prepared by reacting equimolar amounts of CH3NH2 (MA) and hydroiodic acid (HI) precursor solutions. Sequential deposi tion method is favored by most researchers in the field due to formation of high quality perovskite layers with lesser pinholes and inhomogeneity in comparison to those prepared with blend solutions [6]. Inhomoge neous coverage of perovskite layer leads to short-circuited devices and hence decreases the photovoltaic performance of cells [10]. Although sequential deposition presents itself as an improvement over previously practiced methods, the reaction of PbI2 film with
CH3NH3I in solution could sometimes lead to rough surfaces [11]. Introduction of CH3NH3I in vapor form is a better strategy in terms of producing high quality perovskite absorber layer. There are many methods reported in the literature that exploits CH3NH3I vapor in pro ducing perovskite absorbers. For instance, Chen et al. evaporated CH3NH3I onto PbCl2 at vacuum while heating the substrate [12]. They have found that it is possible to obtain very uniform perovskite thin films with high coverage. Li et al. also demonstrated that low temperature evaporation of CH3NH3I on PbI2/PbCl2 film results in pinhole-free smooth perovskite film and the devices made out of these films exhibit a PCE of 16.8% with highly repeatable performance [13]. In another method, a large current is passed through a heater coated with CH3NH3I, causing flash evaporation of CH3NH3I in vacuum. The perovskite solar cells fabricated with this method gave PCEs around 12% [14]. Luo et al. used low-pressure chemical vapor deposition method to fabricate perovskite solar cells under ambient conditions with similar PCE figures [15]. Vapor assisted deposition has not only been used for CH3NH3PbI3 absorber but also for a large band gap CH3NH3PbBr3 absorber, in which the deposition is made on mesoporous TiO2 skeleton [16]. Although vacuum deposition processes are attractive for achieving high photovoltaic performance [8], the use of expensive and
* Corresponding author. E-mail address: [email protected] (F.M. Cos¸kun). https://doi.org/10.1016/j.mssp.2019.104813 Received 17 July 2019; Received in revised form 24 October 2019; Accepted 28 October 2019 Available online 16 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
F.M. Cos¸kun et al.
Materials Science in Semiconductor Processing 107 (2020) 104813
Scheme 1. Steps involved in HI and MA vapor deposited perovskite film.
therefore, the films were exposed to HI and MA vapor in a closed chamber under fixed vapor pressure as shown in Scheme 1. Too much exposure to MA vapor introduces excess MA into the PbI2 film, which in turn results in a rough perovskite layer. In order to decrease the exposure rate, the concentration of MA solution can be reduced with methanol addition (typically 1:5 ratio). The first HI vapor step is critical in the sense that, to an extent, it prevents the buildup of excess MA. The final HI vapor step is also an important step at which the perovskite crystal formation begins with color change to dark brown. In order to increase the extent of transformation of PbI2 into CH3NH3PbI3 perovskite absorber, additional exposure to MA and HI vapors can be applied in alternating order. Once the color change of PbI2 film is complete, the films were annealed at 110 � C for 1 h. Then, the PCBM solution (dissolved in ortho-dichlorobenzene with a concentration of 20 mg/mL) was spin coated on top of the perovskite layer at 1500 rpm for 30 s. The substrate was annealed at 70 � C for 10 min. The device was finished by evaporating C70 (20 nm), 2,9-dimethyl4,7-diphenyl-1,10phenanthroline (BCP) (10 nm) as buffer layers, and Al (60 nm) as counter electrode in a base pressure of 2 � 10 6 mbar. The device area was defined through a shadow mask from the overlap of the ITO and aluminum electrodes (10.9 mm2). The current density-voltage (J-V) measurement of the devices was conducted on a computer controlled Keithley 2602A source meter. The J-V measurement system uses a solar simulator with a Class-A match to the AM1.5 Global Reference Spectrum. Film absorption spectra of the films were compared with a Shimadzu UV-2600 UV–Vis spectropho tometer. Atomic force microscopy (AFM) images of the samples were recorded on a ParkSystems XE-100E microscope. X-ray diffraction (XRD) measurements were performed with a Bruker D8 Discover X-ray diffractometer with copper K-α target X-ray tube. Scanning electron microscopy (SEM) images were taken with JEOL 6510-LV JSM. Film thickness measurements were determined with Ambios XP-200 surface profilometer.
Fig. 1. a) Optical absorption spectra of PbI2 and CH3NH3PbI3 perovskite films on glass substrate spun at 6000 rpm.; b) The XRD spectra of CH3NH3PbI3 and PbI2 thin films are given in the lower panel.
complicated vacuum systems is required for fabrication of perovskite layer, which may present itself as a bottleneck in commercialization of perovskite technology. In this study, we report the preparation and photovoltaic performance of CH3NH3PbI3 perovskite absorber film by introduction of MA and HI vapors onto the PbI2 film. With this novel approach, it is possible to fabricate perovskite layer in a very short amount of time at ambient conditions and obtain very smooth films with high coverage of substrate surface, eliminating the need of using a vacuum system for perovskite film formation. Our approach also allows fabrication of large-area perovskite thin films, which may be beneficial in the commercialization of perovskite photovoltaic technology. 2. Experimental Perovskite solar cells were fabricated on patterned indium tin oxide (ITO) glasses with a sheet resistance of 10 Ω/sq. The ITO coated glasses were cleaned by sequential ultrasonic treatment in deionized water, acetone, and isopropanol (IPA), and then finally treated in a bench-top plasma cleaner (PE-50 bench top cleaner, The Plasma Etch, Inc., USA) for 2 min. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) solution (Heraeus Clevious pH 500) was filtered through a 0.45 μm filter and then spin coated at 4000 rpm for 60 s on the ITO coated glass substrate. Subsequently, the PEDOT:PSS layer was baked at 140 � C for 10 min in the air. The preparation of CH3NH3PbI3 layer is illustrated in Fig. 1. PbI2 is dissolved in DMF with a concentration of 250 mg/mL. The PbI2 solution was spun on PEDOT:PSS coated substrate at two different speeds for thickness comparison namely, 6000 rpm and 3000 rpm for 30 s and then the resultant film was dried on a hot plate at 110 � C for 5 min and left for cooling. The film was exposed to hydroiodic acid vapor (Merck, 67% in water) for 15 s and then MA vapor (TCI, 40% in methanol) for 5 s which is followed by another step of HI vapor exposure for 30 s. Exposing the film to uncontrolled vapor flow yields non-uniform perovskite films;
3. Results and discussion Optical absorbance of the methylamine and hydroiodic acid vapor deposited perovskite film is given in Fig. 1a. The optical absorbance profile is very similar to the published absorption spectra of CH3NH3PbI3 films [17–19]. The extrapolated absorption onset of perovskite films corresponds to a band gap of ~1.6 eV, in agreement with the literature reports [20]. The peaks located at ca. 750 nm and ca. 500 nm hints the formation of perovskite crystals in the film. For comparison, the ab sorption spectrum of PbI2 film is also given in the same figure. PbI2 film possesses much smaller extinction coefficient and has a different spectral 2
F.M. Cos¸kun et al.
Materials Science in Semiconductor Processing 107 (2020) 104813
Fig. 2. AFM images of a) PbI2, b) precursor vapor deposited CH3NH3PbI3, and c) solution deposited CH3NH3PbI3 thin films are given at top. d), e), and f) SEM images of the same samples with the same order and also g), h), and i) with higher magnification with the same order at top.
profile than perovskite film. It is clear that precursor vapor exposition causes the formation of perovskite crystals in the film. In order to study the extent of PbI2 → CH3NH3PbI3 transformation, we further investigated both films with XRD technique. The major diffraction peak of PbI2 film has been found at 12.8� along with some minor peaks at 25.6� and 38.8� , as shown in Fig. 1b. CH3NH3PbI3 absorber film, however, displays major peaks at 14.3� , 28.6� , and 43.3� ,
which can be assigned to (110), (220), and (330) planes of perovskite crystals, respectively. Nonetheless, there is also evidence that there is some unreacted PbI2 left in the film after vapor treatment since there is a small peak of PbI2 crystal present at 12.8� . AFM studies were performed to evaluate the quality of the precursor vapor deposited perovskite films. The root-mean-square surface rough ness (Rq) of the perovskite film has been mainly found limited by the
Fig. 3. a) J-V graph of the best perovskite solar cells measured in this study for the 6000 rpm (straight line) and 3000 rpm (dashed line) devices. Blue and red lines show forward and reverse scans respectively. The device structure is given in the inset. b) The photograph of precursor vapor deposited perovskite film on 4 � 4 inch2 size glass substrate is given (the scale in inches). 3
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Materials Science in Semiconductor Processing 107 (2020) 104813
CH3NH3PbI3 crystals, by exploiting other device structures, or by post-processing of perovskite layer to achieve pin-hole free layer.
Table 1 Device parameters for the films fabricated using PbI2 films spin coated at 3000 rpm and 6000 rpm. Device Parameters
Average values at 3000 rpm
Average values at 6000 rpm
Best device at 6000 rpm
Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
0.807 � 0.026 12.99 � 0.63 0.52 � 0.02 5.44 � 0.19
0.817 � 0.002 14.66 � 0.01 0.59 � 0.01 7.02 � 0.06
0.815 14.67 0.59 7.08
4. Conclusions In conclusion, we have developed a novel strategy to make perov skite layer without the need for expensive vacuum systems or time consuming processes. It is possible to synthesize homogeneous perov skite thin films in the presence of HI and MA vapor precursors at ambient conditions with controlled intercalation. Since low temperature depo sition methods are used in fabrication of perovskite solar cells, the proposed method is suitable for a wide variety of rigid and flexible substrates. Our versatile method can be extended to fabricate large area perovskite modules and can be used to decrease the cost and also simplify the fabrication process. Indeed, we have prepared a perovskite film on 4 � 4 inch2 glass substrate as shown in Fig. 3b. The measured Rq value of such films has been found as low as 8.5 nm. Such high quality perovskite films could help in decreasing the cost while securing the uniformity needed for high efficiency devices.
quality of the pre-deposited PbI2 film. The Rq value of the PbI2 film is 6.7 nm whereas Rq value of the precursor vapor deposited CH3NH3PbI3 film is 13.9 nm. Repeated experiments revealed that if one uses rougher PbI2 films, the Rq value of the resultant perovskite film becomes slightly larger than the Rq of PbI2 film. Fig. 2b shows that the grain sizes of the perovskite layer are slightly larger than those of PbI2 film. In order to show the differences in film quality prepared by our method, we also made perovskite films through solution processing. CH3NH3I solution with 10 mg/mL concentration has been introduced on PbI2 film for 5 s and then the film was spin coated at 2000 rpm for 30 s. After annealing of the substrate, AFM image was taken as illustrated in Fig. 2c. Solution processing gave a rough surface with an Rq value of 55 nm. Thus, one can propose that the method we utilized in this manuscript can yield highly smooth perovskite films in comparison to those prepared by so lution processing. SEM images (Fig. 2d, e, and 2f) also support the conclusions drawn from the analysis of AFM images. SEM images show that there are very small sized pinholes in PbI2 film, whereas the number of such pinholes is much less in the precursor vapor deposited perov skite. Solution processed perovskite film, however, has many islands present on the surface with high roughness, in accordance with AFM image. The typical thickness of perovskite films has been measured as 180 nm and 290 nm, for the samples spun at 6000 rpm and 3000 rpm, respectively. It is important to note that the PbI2 film had a thickness of 100 nm (6000 rpm) and 160 nm (3000 rpm) before precursor vapor treatment. That means, after sequential exposure to MA and HI vapors, the film swells and almost doubles its thickness as one might expect due to insertion of MA and HI into the film. The device architecture used in this study is given in the inset of Fig. 3a. The best device gave a power conversion efficiency (PCE) of 7.08% with an open circuit voltage (Voc) of 0.815 V, a short-circuit current density (Jsc) of 14.67 mA/cm2, and a fill-factor (FF) of 0.59 (Fig. 3) in forward scan. The current density-voltage (J-V) graphs in Fig. 3 belong to the best devices for the films spin coated at 6000 rpm and 3000 rpm. Table 1 summarizes the device parameters for the PbI2 films spin coated at 3000 and 6000 rpms. The device parameters were obtained from the average data on at least 6 devices. As can be seen from Table 1, an average Voc of 0.817 � 0.002 V, Jsc of 14.66 � 0.01 mA/cm2, an FF of 0.59 � 0.01, and a PCE of 7.02 � 0.06 were obtained for the devices fabricated on PbI2 thin films spun at 6000 rpm. Relatively lower photovoltaic activity was observed for the devices prepared with PbI2 thin films spun at 3000 rpm. We have observed negligible J-V hysteresis in all of our measurements [19]. The better performance and reproducibility of the thinner (6000 rpm) device can be attributed to more uniform conversion of PbI2 to perovskite crystal along the depth of the PbI2 layer during precursor vapor deposition. The presence of PbI2 crystal diffraction peaks indicate the perovskite formation is not complete and may explain the relatively poor fill factors of the devices obtained from these films compared to state of art. This is further confirmed from the SEM images of the grain boundaries in Fig. 2h. The bright spots can be speculated as unconverted PbI2, which is less conductive than CH3NH3PbI3 and leading to charge accumulation. It is also possible that small pinholes present in the film causes losses both in Voc and FF [21]. The device performances can be optimized by increasing the conversion of PbI2 crystal to the
Declaration of competing interest The author declare that he has no conflict of interest. Acknowledgements This work was supported by TUBITAK BIDEB 2211-C Fellowship Program and by the BAGEP Award of the Science Academy. References [1] N.G. Park, Organometal perovskite light absorbers toward a 20% efficiency lowcost solid-state mesoscopic solar cell, J. Phys. Chem. Lett. 4 (2013) 2423–2429. [2] 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 338 (2012) 643–647. [3] H.J. Snaith, Perovskites: the emergence of a new era for low-cost, high-efficiency solar cells, J. Phys. Chem. Lett. 4 (2013) 3623–3630. [4] J. Bisquert, The swift surge of perovskite photovoltaics, J. Phys. Chem. Lett. 4 (2013) 2597–2598. [5] H.S. Jung, N.-G. Park, Perovskite solar cells: from materials to devices, Small 11 (2015) 10–25. [6] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316–319. [7] Q. Chen, H.P. Zhou, Z.R. Hong, S. Luo, H.S. Duan, H.H. Wang, Y.S. Liu, G. Li, Y. Yang, Planar heterojunction perovskite solar cells via vapor-assisted solution process, J. Am. Chem. Soc. 136 (2014) 622–625. [8] M.Z. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395–398. [9] P. Gao, M. Graetzel, M.K. Nazeeruddin, Organohalide lead perovskites for photovoltaic applications, En. & Environ. Sci. 7 (2014) 2448–2463. [10] 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. 24 (2014) 151–157. [11] T. Du, N. Wang, H. Chen, H. Lin, H. He, Comparative study of vapor- and solutioncrystallized perovskite for planar heterojunction solar cells, Acs Appl. Mater. & Int. 7 (2015) 3382–3388. [12] C.-W. Chen, H.-W. Kang, S.-Y. Hsiao, P.-F. Yang, K.-M. Chiang, H.-W. Lin, Efficient and uniform planar-type perovskite solar cells by simple sequential vacuum deposition, Adv. Mater. 26 (2014) 6647–6652. [13] Y. Li, J.K. Cooper, R. Buonsanti, C. Giannini, Y. Liu, F.M. Toma, I.D. Sharp, Fabrication of planar heterojunction perovskite solar cells by controlled lowpressure vapor annealing, J. Phys. Chem. Lett. 6 (2015) 493–499. [14] G. Longo, L. Gil-Escrig, M.J. Degen, M. Sessolo, H.J. Bolink, Perovskite solar cells prepared by flash evaporation, Chem. Commun. (J. Chem. Soc. Sect. D) 51 (2015) 7376–7378. [15] P. Luo, Z. Liu, W. Xia, C. Yuan, J. Cheng, Y. Lu, Uniform, stable, and efficient planar-heterojunction perovskite solar cells by facile low-pressure chemical vapor deposition under fully open-air conditions, Acs Appl. Mater. & Int. 7 (2015) 2708–2714. [16] R. Sheng, A. Ho-Baillie, S. Huang, S. Chen, X. Wen, X. Hao, M.A. Green, Methylammonium lead bromide perovskite-based solar cells by vapor-assisted deposition, J. Phys. Chem. C 119 (2015) 3545–3549. [17] Y. Chen, J. Peng, D. Su, X. Chen, Z. Liang, Efficient and balanced charge transport revealed in planar perovskite solar cells, Acs Appl. Mater. & Int. 7 (2015) 4471–4475.
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[18] Y.S. Kwon, J. Lim, H.-J. Yun, Y.-H. Kim, T. Park, A diketopyrrolopyrrole-containing hole transporting conjugated polymer for use in efficient stable organic-inorganic hybrid solar cells based on a perovskite, En. & Environ. Sci. 7 (2014) 1454–1460. [19] 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, W. Zhang, Anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 5 (2014) 1511–1515.
[20] Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao, J. Huang, Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers, En. & Environ. Sci. 7 (2014) 2619–2623. [21] Q. Chen, H. Zhou, T.-B. Song, S. Luo, Z. Hong, H.-S. Duan, L. Dou, Y. Liu, Y. Yang, Controllable self-induced passivation of hybrid lead iodide perovskites toward high performance solar cells, Nano Lett. 14 (2014) 4158–4163.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Precursor vapor deposited perovskite solar cells with smooth absorber layer �irmenci b, Muhammet Erkan Ko €se c Fatih Mehmet Cos¸kun a, *, Fatih Deg a
Department of Engineering Physics, Istanbul Medeniyet University, Istanbul, 34730, Turkey Barbaros Hayrettin Pasa Mah, 1083. Sok. No:17/6 Gaziosmanpasa, Istanbul, Turkey c Department of Chemistry, Kocaeli University, Kocaeli, 41380, Turkey b
A R T I C L E I N F O
A B S T R A C T
Keywords: Perovskite solar cell Vapor deposition Large area devices
Preparation of a smooth methylammoniumleadiodide (CH3NH3PbI3) perovskite absorber film is one of the most critical processes in fabricating a perovskite photovoltaic cell with high efficiency. In this study, we report an original coating method to prepare CH3NH3PbI3 films by using the solvent vapors of precursor methylamine (MA) and hydroiodic acid (HI) solutions on lead (II) iodide (PbI2) thin films. In this strategy, the vapors of MA and HI solutions were briefly treated to the PbI2 coated substrate and then the resultant film was annealed to obtain smooth perovskite thin layer. After the topographic analyses of the thin films, the surface roughnesses of the perovskite films were found to be limited by the surface roughnesses of the PbI2 films before precursor vapor exposure. Another important aspect of our approach is that there is no need to synthesize methylammonium iodide (CH3NH3I) salt for active layer preparation. Furthermore, this method enables large-area fabrication of high quality films by a very simple and brief process. After the characterization of preliminary devices with this method, we obtained a short circuit current density of 14.67 mA/cm2, an open circuit voltage of 0.815 V, a fillfactor of 0.59, and a power conversion efficiency of 7.08%.
1. Introduction In recent years, perovskite photovoltaic technology has become one of the most promising photovoltaic technologies due to low cost pro duction and high efficiencies of the perovskite solar cells [1–3]. CH3NH3PbI3 and CH3NH3PbI3-xClx (CH3NH3PbI3-xClx) perovskite ab sorbers are at the forefront of advancements in terms of power conver sion efficiencies (PCEs) and device optimization studies [4]. CH3NH3PbI3 films are generally deposited from blend solutions (such as CH3NH3I þ PbI2 in N,N-dimethylformamide (DMF)) by spin coating or sequential deposition of PbI2 film with spin coating and then introduc tion of CH3NH3I onto PbI2 film either in solution or in vapor form [5–9]. CH3NH3I salt is prepared by reacting equimolar amounts of CH3NH2 (MA) and hydroiodic acid (HI) precursor solutions. Sequential deposi tion method is favored by most researchers in the field due to formation of high quality perovskite layers with lesser pinholes and inhomogeneity in comparison to those prepared with blend solutions [6]. Inhomoge neous coverage of perovskite layer leads to short-circuited devices and hence decreases the photovoltaic performance of cells [10]. Although sequential deposition presents itself as an improvement over previously practiced methods, the reaction of PbI2 film with
CH3NH3I in solution could sometimes lead to rough surfaces [11]. Introduction of CH3NH3I in vapor form is a better strategy in terms of producing high quality perovskite absorber layer. There are many methods reported in the literature that exploits CH3NH3I vapor in pro ducing perovskite absorbers. For instance, Chen et al. evaporated CH3NH3I onto PbCl2 at vacuum while heating the substrate [12]. They have found that it is possible to obtain very uniform perovskite thin films with high coverage. Li et al. also demonstrated that low temperature evaporation of CH3NH3I on PbI2/PbCl2 film results in pinhole-free smooth perovskite film and the devices made out of these films exhibit a PCE of 16.8% with highly repeatable performance [13]. In another method, a large current is passed through a heater coated with CH3NH3I, causing flash evaporation of CH3NH3I in vacuum. The perovskite solar cells fabricated with this method gave PCEs around 12% [14]. Luo et al. used low-pressure chemical vapor deposition method to fabricate perovskite solar cells under ambient conditions with similar PCE figures [15]. Vapor assisted deposition has not only been used for CH3NH3PbI3 absorber but also for a large band gap CH3NH3PbBr3 absorber, in which the deposition is made on mesoporous TiO2 skeleton [16]. Although vacuum deposition processes are attractive for achieving high photovoltaic performance [8], the use of expensive and
* Corresponding author. E-mail address: [email protected] (F.M. Cos¸kun). https://doi.org/10.1016/j.mssp.2019.104813 Received 17 July 2019; Received in revised form 24 October 2019; Accepted 28 October 2019 Available online 16 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
F.M. Cos¸kun et al.
Materials Science in Semiconductor Processing 107 (2020) 104813
Scheme 1. Steps involved in HI and MA vapor deposited perovskite film.
therefore, the films were exposed to HI and MA vapor in a closed chamber under fixed vapor pressure as shown in Scheme 1. Too much exposure to MA vapor introduces excess MA into the PbI2 film, which in turn results in a rough perovskite layer. In order to decrease the exposure rate, the concentration of MA solution can be reduced with methanol addition (typically 1:5 ratio). The first HI vapor step is critical in the sense that, to an extent, it prevents the buildup of excess MA. The final HI vapor step is also an important step at which the perovskite crystal formation begins with color change to dark brown. In order to increase the extent of transformation of PbI2 into CH3NH3PbI3 perovskite absorber, additional exposure to MA and HI vapors can be applied in alternating order. Once the color change of PbI2 film is complete, the films were annealed at 110 � C for 1 h. Then, the PCBM solution (dissolved in ortho-dichlorobenzene with a concentration of 20 mg/mL) was spin coated on top of the perovskite layer at 1500 rpm for 30 s. The substrate was annealed at 70 � C for 10 min. The device was finished by evaporating C70 (20 nm), 2,9-dimethyl4,7-diphenyl-1,10phenanthroline (BCP) (10 nm) as buffer layers, and Al (60 nm) as counter electrode in a base pressure of 2 � 10 6 mbar. The device area was defined through a shadow mask from the overlap of the ITO and aluminum electrodes (10.9 mm2). The current density-voltage (J-V) measurement of the devices was conducted on a computer controlled Keithley 2602A source meter. The J-V measurement system uses a solar simulator with a Class-A match to the AM1.5 Global Reference Spectrum. Film absorption spectra of the films were compared with a Shimadzu UV-2600 UV–Vis spectropho tometer. Atomic force microscopy (AFM) images of the samples were recorded on a ParkSystems XE-100E microscope. X-ray diffraction (XRD) measurements were performed with a Bruker D8 Discover X-ray diffractometer with copper K-α target X-ray tube. Scanning electron microscopy (SEM) images were taken with JEOL 6510-LV JSM. Film thickness measurements were determined with Ambios XP-200 surface profilometer.
Fig. 1. a) Optical absorption spectra of PbI2 and CH3NH3PbI3 perovskite films on glass substrate spun at 6000 rpm.; b) The XRD spectra of CH3NH3PbI3 and PbI2 thin films are given in the lower panel.
complicated vacuum systems is required for fabrication of perovskite layer, which may present itself as a bottleneck in commercialization of perovskite technology. In this study, we report the preparation and photovoltaic performance of CH3NH3PbI3 perovskite absorber film by introduction of MA and HI vapors onto the PbI2 film. With this novel approach, it is possible to fabricate perovskite layer in a very short amount of time at ambient conditions and obtain very smooth films with high coverage of substrate surface, eliminating the need of using a vacuum system for perovskite film formation. Our approach also allows fabrication of large-area perovskite thin films, which may be beneficial in the commercialization of perovskite photovoltaic technology. 2. Experimental Perovskite solar cells were fabricated on patterned indium tin oxide (ITO) glasses with a sheet resistance of 10 Ω/sq. The ITO coated glasses were cleaned by sequential ultrasonic treatment in deionized water, acetone, and isopropanol (IPA), and then finally treated in a bench-top plasma cleaner (PE-50 bench top cleaner, The Plasma Etch, Inc., USA) for 2 min. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) solution (Heraeus Clevious pH 500) was filtered through a 0.45 μm filter and then spin coated at 4000 rpm for 60 s on the ITO coated glass substrate. Subsequently, the PEDOT:PSS layer was baked at 140 � C for 10 min in the air. The preparation of CH3NH3PbI3 layer is illustrated in Fig. 1. PbI2 is dissolved in DMF with a concentration of 250 mg/mL. The PbI2 solution was spun on PEDOT:PSS coated substrate at two different speeds for thickness comparison namely, 6000 rpm and 3000 rpm for 30 s and then the resultant film was dried on a hot plate at 110 � C for 5 min and left for cooling. The film was exposed to hydroiodic acid vapor (Merck, 67% in water) for 15 s and then MA vapor (TCI, 40% in methanol) for 5 s which is followed by another step of HI vapor exposure for 30 s. Exposing the film to uncontrolled vapor flow yields non-uniform perovskite films;
3. Results and discussion Optical absorbance of the methylamine and hydroiodic acid vapor deposited perovskite film is given in Fig. 1a. The optical absorbance profile is very similar to the published absorption spectra of CH3NH3PbI3 films [17–19]. The extrapolated absorption onset of perovskite films corresponds to a band gap of ~1.6 eV, in agreement with the literature reports [20]. The peaks located at ca. 750 nm and ca. 500 nm hints the formation of perovskite crystals in the film. For comparison, the ab sorption spectrum of PbI2 film is also given in the same figure. PbI2 film possesses much smaller extinction coefficient and has a different spectral 2
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Fig. 2. AFM images of a) PbI2, b) precursor vapor deposited CH3NH3PbI3, and c) solution deposited CH3NH3PbI3 thin films are given at top. d), e), and f) SEM images of the same samples with the same order and also g), h), and i) with higher magnification with the same order at top.
profile than perovskite film. It is clear that precursor vapor exposition causes the formation of perovskite crystals in the film. In order to study the extent of PbI2 → CH3NH3PbI3 transformation, we further investigated both films with XRD technique. The major diffraction peak of PbI2 film has been found at 12.8� along with some minor peaks at 25.6� and 38.8� , as shown in Fig. 1b. CH3NH3PbI3 absorber film, however, displays major peaks at 14.3� , 28.6� , and 43.3� ,
which can be assigned to (110), (220), and (330) planes of perovskite crystals, respectively. Nonetheless, there is also evidence that there is some unreacted PbI2 left in the film after vapor treatment since there is a small peak of PbI2 crystal present at 12.8� . AFM studies were performed to evaluate the quality of the precursor vapor deposited perovskite films. The root-mean-square surface rough ness (Rq) of the perovskite film has been mainly found limited by the
Fig. 3. a) J-V graph of the best perovskite solar cells measured in this study for the 6000 rpm (straight line) and 3000 rpm (dashed line) devices. Blue and red lines show forward and reverse scans respectively. The device structure is given in the inset. b) The photograph of precursor vapor deposited perovskite film on 4 � 4 inch2 size glass substrate is given (the scale in inches). 3
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CH3NH3PbI3 crystals, by exploiting other device structures, or by post-processing of perovskite layer to achieve pin-hole free layer.
Table 1 Device parameters for the films fabricated using PbI2 films spin coated at 3000 rpm and 6000 rpm. Device Parameters
Average values at 3000 rpm
Average values at 6000 rpm
Best device at 6000 rpm
Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
0.807 � 0.026 12.99 � 0.63 0.52 � 0.02 5.44 � 0.19
0.817 � 0.002 14.66 � 0.01 0.59 � 0.01 7.02 � 0.06
0.815 14.67 0.59 7.08
4. Conclusions In conclusion, we have developed a novel strategy to make perov skite layer without the need for expensive vacuum systems or time consuming processes. It is possible to synthesize homogeneous perov skite thin films in the presence of HI and MA vapor precursors at ambient conditions with controlled intercalation. Since low temperature depo sition methods are used in fabrication of perovskite solar cells, the proposed method is suitable for a wide variety of rigid and flexible substrates. Our versatile method can be extended to fabricate large area perovskite modules and can be used to decrease the cost and also simplify the fabrication process. Indeed, we have prepared a perovskite film on 4 � 4 inch2 glass substrate as shown in Fig. 3b. The measured Rq value of such films has been found as low as 8.5 nm. Such high quality perovskite films could help in decreasing the cost while securing the uniformity needed for high efficiency devices.
quality of the pre-deposited PbI2 film. The Rq value of the PbI2 film is 6.7 nm whereas Rq value of the precursor vapor deposited CH3NH3PbI3 film is 13.9 nm. Repeated experiments revealed that if one uses rougher PbI2 films, the Rq value of the resultant perovskite film becomes slightly larger than the Rq of PbI2 film. Fig. 2b shows that the grain sizes of the perovskite layer are slightly larger than those of PbI2 film. In order to show the differences in film quality prepared by our method, we also made perovskite films through solution processing. CH3NH3I solution with 10 mg/mL concentration has been introduced on PbI2 film for 5 s and then the film was spin coated at 2000 rpm for 30 s. After annealing of the substrate, AFM image was taken as illustrated in Fig. 2c. Solution processing gave a rough surface with an Rq value of 55 nm. Thus, one can propose that the method we utilized in this manuscript can yield highly smooth perovskite films in comparison to those prepared by so lution processing. SEM images (Fig. 2d, e, and 2f) also support the conclusions drawn from the analysis of AFM images. SEM images show that there are very small sized pinholes in PbI2 film, whereas the number of such pinholes is much less in the precursor vapor deposited perov skite. Solution processed perovskite film, however, has many islands present on the surface with high roughness, in accordance with AFM image. The typical thickness of perovskite films has been measured as 180 nm and 290 nm, for the samples spun at 6000 rpm and 3000 rpm, respectively. It is important to note that the PbI2 film had a thickness of 100 nm (6000 rpm) and 160 nm (3000 rpm) before precursor vapor treatment. That means, after sequential exposure to MA and HI vapors, the film swells and almost doubles its thickness as one might expect due to insertion of MA and HI into the film. The device architecture used in this study is given in the inset of Fig. 3a. The best device gave a power conversion efficiency (PCE) of 7.08% with an open circuit voltage (Voc) of 0.815 V, a short-circuit current density (Jsc) of 14.67 mA/cm2, and a fill-factor (FF) of 0.59 (Fig. 3) in forward scan. The current density-voltage (J-V) graphs in Fig. 3 belong to the best devices for the films spin coated at 6000 rpm and 3000 rpm. Table 1 summarizes the device parameters for the PbI2 films spin coated at 3000 and 6000 rpms. The device parameters were obtained from the average data on at least 6 devices. As can be seen from Table 1, an average Voc of 0.817 � 0.002 V, Jsc of 14.66 � 0.01 mA/cm2, an FF of 0.59 � 0.01, and a PCE of 7.02 � 0.06 were obtained for the devices fabricated on PbI2 thin films spun at 6000 rpm. Relatively lower photovoltaic activity was observed for the devices prepared with PbI2 thin films spun at 3000 rpm. We have observed negligible J-V hysteresis in all of our measurements [19]. The better performance and reproducibility of the thinner (6000 rpm) device can be attributed to more uniform conversion of PbI2 to perovskite crystal along the depth of the PbI2 layer during precursor vapor deposition. The presence of PbI2 crystal diffraction peaks indicate the perovskite formation is not complete and may explain the relatively poor fill factors of the devices obtained from these films compared to state of art. This is further confirmed from the SEM images of the grain boundaries in Fig. 2h. The bright spots can be speculated as unconverted PbI2, which is less conductive than CH3NH3PbI3 and leading to charge accumulation. It is also possible that small pinholes present in the film causes losses both in Voc and FF [21]. The device performances can be optimized by increasing the conversion of PbI2 crystal to the
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