Properties of MAPbI3 perovskite layers grown with HCl additions

Accepted Manuscript Properties of MAPbI3 perovskite layers grown with HCl additions M. Moret, A. Tiberj, W. Desrat, O. Briot

PII:

S0749-6036(18)31036-X

DOI:

10.1016/j.spmi.2018.05.033

Reference:

YSPMI 5700

To appear in:

Superlattices and Microstructures

Received Date: 17 May 2018 Accepted Date: 17 May 2018

Please cite this article as: M. Moret, A. Tiberj, W. Desrat, O. Briot, Properties of MAPbI3 perovskite layers grown with HCl additions, Superlattices and Microstructures (2018), doi: 10.1016/ j.spmi.2018.05.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Properties of MAPbI3 perovskite layers grown with HCl additions M.Moret, A.Tiberj, W. Desrat and O.Briot

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Laboratoire Charles Coulomb (L2C), UMR5221 CNRS-Université de Montpellier, Montpellier, FR-34095, France

Abstract

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Lead halide perovskites, used to produce photovoltaic devices, have been the subject of a huge research effort these last years. This is due to the spectacular improvement of the conversion efficiency results within a short amount of time. However, some issues have been identified, which include stability, lead use and cost of some precursors. It has recently been shown that low purity, low cost PbI2 could be succesfully used in the synthesis of the MAPbI3 perovskite, provided that HCl is added during the synthesis to prevent solubility problems. Thus, it is of high interest to provide information pertaining to the material quality, in relation with the HCl additions performed during synthesis. In this work, we have grown three sets of samples with different HCl to (PbI2 + MAI) molar ratios (RHCl ), where PbI2 and methylamine iodide (MAI) are the precursors of the perovskite. We used RHCl = 0 (no HCl), 0.5 and 1.2 . We performed x-ray diffraction, transmittance and 77K photoluminescence experiments in order to assess the material structural and optoelectronic properties and found that an optimum HCl concentration must be used. HCl introduction clearly has a beneficial effect both on domain sizes and photoluminescence intensity at RHCl = 0.5, but lead to subsequent degradation of the perovskite quality at higher RHCl . Keywords: perovskite, MAPbI3 , HCl, optical properties, photoluminescence

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1. Introduction In today’s world, renewable energy sources have become the center of an intense interest. Photovoltaic (PV) devices are close to be on par with more conventionnal sources, from the economical point of view, and relatively small improvements in efficiency and costs could turn them into widely adopted solutions. In the PV material family, a newcomer has rapidly emerged as a high potential candidate toward the realization of efficient devices, namely the lead halide perovskites. The first report on these PV materials occured in 2009[1] and a tremendous research activity has resulted. Remarkable improvements in conversion efficiencies have been reported, and it would be too difficult a task

Preprint submitted to Elsevier

May 18, 2018

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to list all the steps, but some may be followed in [2, 3, 4, 5, 6], with recent overview papers in [7, 8]. Although the results have been progressing at an amazing pace, several problems have been identified : stability is an issue, the use of lead in massively produced devices raises environmental concerns, and cost must not be overlooked, as some chemical precursors (for example hole transport layer materials, such as spiro-OMETAD) are quite costly. All these issues are addressed by numerous team worldwide, and we will here focus on the particular aspect linked to the cost of lead iodide (PbI2 ), in the synthesis of CH3 NH3 PbI3 (MAPbI3 ) perovskite. As a matter of fact, very high purity PbI2 is requested to achieve high performances and reproducibility [9, 10]. In particular, it has been observed by many researchers that low purity, low cost PbI2 was quite difficult to dissolve in N,N-dimethylformamide (DMF), the organic solvent used in typical synthesis process of MAPbI3 , while high purity PbI2 dissolves correctly. Recently, Guo et al. [11] have investigated the addition of hydrochloric acid (HCl) to the PbI2 solution and observed a huge improvement in its dissolution. Thus, they realized MAPbI3 based solar cells, using both low and high purity PbI2 and demonstrated that very similar efficiencies (in the 14-15% range) could be obtained. Several questions remain open regarding the influence of the HCl on the properties of the MAPbI3 perovskite, and this paper is devoted to studying the structural and optical characteristics of MAPbI3 layers grown with various HCl additions.

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2. Experimental

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The MAPbI3 layers were realized using the so-called one step method, where a Fluorine doped Tin Oxide (FTO) coated glass substrate is spin-coated (2500 rpm) using a freshly prepared sol of lead iodide and methylammonium iodide (MAI), in order to achieve a planar layer of perovskite. In all the samples involved in this study, the sol was prepared by mixing 1,153g of PbI2 and 0.397g of MAI in 2ml of N,N-dimethylformamide (DMF) at room temperature. Even using ultrasonic process, the PbI2 can hardly dissolve fully. HCl addition results in instant and full dissolution of the PbI2 . The amount of HCl added is quantified by RHCl , the molar ratio of HCl to the sum of PbI2 and MAI. Here, the samples were prepared using either a ratio of 0 (no HCl), 0.5 or 1.2 in order to investigate increasing HCl additions. The low purity PbI2 used is commercial 98% reagent, and MAI is prepared using the following method : an HI and methylamine mixture in 1:1.1 molar ratio is stirred at 0°C during 2 hours, leading to a MAI precipitate, which is then washed with diethyl-ether and dried under vacuum at 60°C during 3 hours. Optical characterization was performed using LN2 cooled InGaAs near IR photomultiplier (Hamamatsu R5509-73). For photoluminescence, the perovskite layer was excited with a 650nm red solid state laser. X-ray diffraction experiments were done using a Bruker D8 discovery equipment fitted with a copper anticathode tube.

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3. Results and discussion

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For RHCl = 0 (no HCl addition), the precursor sol had a low PbI2 concentration, owing to the incomplete dissolution of lead iodide, and the deposited layers do not form a continuous film on the substrate as it can be seen in figure 1(a). The morphology observed in SEM clearly contains a lot of voids, while films deposited with RHCl = 0.5 or 1.2 have morphologies such as the one in 1(b), indicating a planar deposition, with full surface coverage. These last layers clearly appear more opaque to the naked eye, while the layer with RHCl = 0 is evidently more transparent.

Figure 1: Scanning electron microscopy of perovskite layers grown (a) without HCl, and (b) with a molar ratio HCl/(PbI2 +MAI) of 0.5. The magnification is x7500 in both images.

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X-ray diffraction experiments were performed on the samples. The results are reported in figure 2. All the peaks have been indexed using calculated data from reference [12], where information is available for 2θ angles below 60°, which explains why the last peaks in our figure are not attributed. Clearly, we obtain the tetragonal MAPbI3 phase in all our layers. To get a deeper insight of the crystalline properties of the sample, the linewidth of the (110) diffraction peak was analyzed using Scherrer’s formula to extract the crystalline domain sizes. The results are depicted in figure 3, where it can be seen that the largest domain size is obtained for moderate amounts of added HCl, while larger additions of HCl lead to smaller grain sizes, i.e. lower crystalline quality. In figure 2, a star (*) denotes peaks associated with PbI2 . They can only be observed in layers grown using HCl, and probably result of PbI2 incorporation due to its much higher concentration in the precursor solution.

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Figure 2: θ-2θ scan of the perovskite layers with different RHCl . All peaks can be indexed according to the tetragonal structure of MAPbI3 . The peaks labeled FTO correspond to the fluorine doped tin oxide substrate. The star indexed peak is associated to PbI2

Figure 3: Crystallite sizes for the samples grown using different RHCl .

It may be questionned whether the use of HCl can lead to Cl incorporation in the perovskite, where large Cl fraction would change the perovskite bandgap 4

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Figure 4: left : transmittance of the perovskite samples at various RHCl . The layer grown without HCl contains many holes and voids and the light transmitted through this holes prevent the observation of the bandgap edge. right : plot of the natural logarithm of the absorption coefficient for the different samples; the linear part in the 1.6 - 1.7 eV range is used to fit the Urbach energies.

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[13]. This is to be considered since the HCl concentration in the solution is of the same order of magnitude as the perovskite precursors. In order to address this question, we performed room temperature transmittance experiments on our layers. The results are shown in figure 4. Unfortunately, the transmittance profile for the sample with RHCl = 0 is flat and yields no useful information. This may be attributed to the fact that a large amount of light is transmitted through the voids in the layer, without any energy dependence. For samples with RHCl = 0.5 and 1.2, a well marked edge is visible. From these features, the absorption coefficient α can be extracted and the bandgap of the material can be calculated, in a very basic approach, using Tauc’s expression [14] : (αE)2 = A(E − Eg ) , where Eg is the material bandgap. In both cases, the bandgap obtained is the same Eg = 1.59 eV, in good agreement with the commonly reported values in the literature. This indicates that there is no massive incorporation of chlorine in our perovskites, since the bandgap increases with the chlorine fraction in the perovskite [13]. In the transmittance profiles, one can observe a smooth tailing which could be linked to different physical phenomena : either the existence of an indirect bandgap at lower energy, as proposed by Hutter et al [15], or band tailing due 5

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to any large potential fluctuations, such as defects or impurities [16]. It seems likely that the perovskites will contain large potential fluctuations, due to the simple growth techniques used. It may easily lead to non-uniformities or defects, as compared to epitaxial techniques usually employed for semiconductors. So, we applied the Urbach type analysis to our transmittance data, by plotting the logarithm of the absorption coefficient, and using : lnα = lnα0 + EEu , to extract Eu , which is the Urbach energy, quantifying the amount of potential fluctuations. Of course, this is not possible for the sample with RHCl = 0, but we obtain Eu = 90 meV for RHCl = 0.5 and Eu = 107 meV for RHCl = 1.2 . The lower Urbach energy for RHCl = 0.5 indicates that there should be an optimal HCl addition, in agreement with the x-ray diffraction results shown above. To go further, we have performed photoluminescence (PL) at liquid nitrogen temperature on the set of samples, and we report the results in figure 5, where on the left side the PL lines are depicted. A slight energy shift of the PL is noticeable upon adding HCl : the PL peaks at 1.581 eV when no HCl is used (RHCl = 0), while it shifts to 1.603 eV for RHCl = 0.5 and 1.604 eV for RHCl = 1.2 . The values given here are above those extracted from room temperature transmittance, because photoluminescence is performed at 77K. It is remarkable that the addition of HCl results in a blue shift. Several interpretation might be invoked for this : detrapping from defects, stress induced shift or Moss-Burnstein shift due to increased doping level. Recently, Colella et al. [17] have reported that the incorporation of chlorine at doping levels, without changing the bandgap, dramatically improved the charge transport, which may be linked to a reduction of the defect densities. Their calculations demonstrate a modification of the cell lattice parameters with increasing RHCl which could be the origin of inhomogeneous strains. Also, the increase in the Urbach energy that we observed above can be related to an increase in the layer doping level with increasing RHCl , the behavior of the PL intensity which drastically increases with RHCl = 0.5 does not support the hypothesis of an increasing doping level; it may rather be explained by a reduction of the defect densities. On the right part of figure 5, we have displayed the full width at half maximum of the PL lines. It is steadily increasing with increasing RHCl , which would suggest an overall degradation of the material quality. The amount of change is quite subtle, and this is not in line with the results above. X-ray data, transmittance and PL intensities all suggest an improvement of the material quality at RHCl = 0.5, followed by a latter degradation for higher RHCl .

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Figure 5: left : photoluminescence spectra at 77K for the samples with various HCl additions. right : full width at half maximum of the PL lines versus RHCl .

4. Conclusion

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We have grown three different types of samples, using increasing levels of HCl during the MAPbI3 perovskite synthesis, to improve the solubility of low purity PbI2 , which could be used for decreasing the cost of the PV material. The use of HCl lead to the deposition of planar, good quality layers. The crystalline domain sizes are larger for RHCl = 0.5, denoting an optimum of crystal quality. Transmittance experiments confirm that degradation of the material quality occur at larger RHCl , while photoluminescence intensity is dramatically improved for moderate HCl additions (RHCl = 0.5). These findings support the idea that low cost, low purity PbI2 could be used for the succesful synthesis of PV quality perovskites, by adding an optimum amount of HCl to promote PbI2 dissolution. [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells, Journal of the American Chemical Society 131 (17) (2009) 6050–6051. [2] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N.-G. Park, 6.5% efficient perovskite quantum-dot-sensitized solar cell, Nanoscale 3 (2011) 4088–4093. [3] Burschka Julian, Pellet Norman, Moon Soo-Jin, Humphry-Baker Robin, Gao Peng, Nazeeruddin Mohammad K., Grätzel Michael, Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (2013) 316. 7

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[4] Park Nam-Gyu, Organometal Perovskite Light Absorbers Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell, The Journal of Physical Chemistry Letters 4 (15) (2013) 2423–2429. [5] Y. Zhao, K. Zhu, Efficient Planar Perovskite Solar Cells Based on 1.8 eV Band Gap CH3NH3PbI2Br Nanosheets via Thermal Decomposition, Journal of the American Chemical Society 136 (35) (2014) 12241–12244.

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[6] G. Li, T. Zhang, Y. Zhao, Hydrochloric acid accelerated formation of planar CH3NH3PbI3 perovskite with high humidity tolerance, J. Mater. Chem. A 3 (2015) 19674–19678. [7] Y. Zhao, K. Zhu, Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655–689.

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[8] Snaith Henry J., Present status and future prospects of perovskite photovoltaics, Nature Materials 17 (5) (2018) 372–376. [9] T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M. Graetzel, T. J. White, Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications, J. Mater. Chem. A 1 (2013) 5628–5641.

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[11] N. Guo, T. Zhang, G. Li, F. Xu, X. Qian, Y. Zhao, A simple fabrication of CH3NH3PbI3 perovskite for solar cells using low-purity PbI2, Journal of Semiconductors 38 (1) (2017) 014004.

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[12] X. Guo, C. McCleese, C. Kolodziej, A. C. S. Samia, Y. Zhao, C. Burda, Identification and characterization of the intermediate phase in hybrid organic-inorganic MAPbI3 perovskite, Dalton Trans. 45 (2016) 3806–3813.

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[13] Noh Jun Hong, Im Sang Hyuk, Heo Jin Hyuck, Mandal Tarak N., Seok Sang Il, Chemical Management for Colorful, Efficient, and Stable Inorganic– Organic Hybrid Nanostructured Solar Cells, Nano Letters 13 (4) (2013) 1764–1769. [14] J. Tauc, Optical properties and electronic structure of amorphous Ge and Si, Materials Research Bulletin 3 (1) (1968) 37–46. [15] Hutter Eline M., Gélvez-Rueda María C., Osherov Anna, Bulović Vladimir, Grozema Ferdinand C., Stranks Samuel D., Savenije Tom J., Direct– indirect character of the bandgap in methylammonium lead iodide perovskite, Nature Materials 16 (2016) 115. 8

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[16] F. Urbach, The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids, Phys. Rev. 92 (1953) 1324–1324.

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[17] Colella Silvia, Mosconi Edoardo, Fedeli Paolo, Listorti Andrea, Gazza Francesco, Orlandi Fabio, Ferro Patrizia, Besagni Tullo, Rizzo Aurora, Calestani Gianluca, Gigli Giuseppe, De Angelis Filippo, Mosca Roberto, MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties, Chemistry of Materials 25 (22) (2013) 4613–4618.

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ACCEPTED MANUSCRIPT HIGHLIGHTS Highlights for manuscript entitled «Properties of MAPbI 3 perovskite layers grown with HCl additions» by M.Moret, A. Tiberj, W.Desrat and O.Briot

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* MAPbI3 perovskite has been grown using low purity low cost PbI2 * Thin planar films of MAPbI3 were obtained by adding Hcl during synthesis * X-ray data and optical properties of layer grown with Hcl are reported

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* an optimum molar ratio of Hcl to perovskite precursors of 0.5 is found