Exploring the Effects of the Pb2+ Substitution in MAPbI3 on the

Letter pubs.acs.org/JPCL

Exploring the Effects of the Pb2+ Substitution in MAPbI3 on the Photovoltaic Performance of the Hybrid Perovskite Solar Cells Lyubov A. Frolova,† Denis V. Anokhin,†,‡ Kirill L. Gerasimov,‡ Nadezhda N. Dremova,† and Pavel A. Troshin*,§,† †

IPCP RAS, Semenov Prospect 1, Chernogolovka, 142432, Russia Faculty of Fundamental Physical and Chemical Engineering, Lomonosov Moscow State University, 1-51 Leninskie Gory, Moscow 119991, Russia § Skolkovo Institute of Science and Technology, Nobel Street 3, Moscow, 143026, Russia ‡

S Supporting Information *

ABSTRACT: Here we report a systematic study of the Pb2+ substitution in the hybrid iodoplumbate MAPbI3 with a series of elements affecting optoelectronic, structural, and morphological properties of the system. It has been shown that even partial replacement of lead with Cd2+, Zn2+, Fe2+, Ni2+, Co2+, In3+, Bi3+, Sn4+, and Ti4+ results in a significant deterioration of the photovoltaic characteristics. On the contrary, Hg-containing hybrid MAPb1−xHgxI3 salts demonstrated a considerably improved solar cell performance at optimal mercury loading. This result opens up additional dimension in the compositional engineering of the complex lead halides for designing novel photoactive materials with advanced optoelectronic and photovoltaic properties.

ost recent efforts of the researchers working in the field of organo-inorganic hybrid perovskite solar cells are focused on the design of novel materials with improved optoelectronic characteristics and stability. Primary attention is paid to the compositional engineering of the conventional leadhalide perovskites such as MAPbI3 and FAPbI3 (MA = methylammonium; FA = formamidium). Thus, a combination of two organic cations and two halogen anions (I and Br) produced a “cocktail” material, which delivered an excellent photovoltaic efficiency (PCE) of >18% followed by a further improvement resulting in PCEs > 20%.1,2 More recently, a combination of FA and Cs cations produced other highperformance perovskite material, which also demonstrated advanced stability under ambient conditions.3,4 Surprisingly, much less attention has been paid to the replacement of Pb by other elements. The Goldshmidt model provides a good theoretical estimation on the stability of the ABX3 perovskites by comparing ionic radii of A, B, and X elements.5 If we stay with A = MA and X = I or Br, the M2+ metal anion can only be selected from a narrow set of elements including the heavier group 14 elements (Ge2+, Sn2+, Pb2+),6 the alkaline earths Sr2+, Ba2+,7 the rare earths Eu2+, Yb2+,8 and some others.9 Partial or complete replacement of Pb with Sn, Ge, Sr, Cd, Ca was addressed using computational and selected spectroscopic approaches.10,11 Experimental studies in photovoltaic cells were performed by now only for Pb-, Sn-, and Gebased 3D hybrid perovskites.

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© XXXX American Chemical Society

A lot of attention has been paid to Sn as the most promising replacement for Pb.12−14 MASnI3−xBrx demonstrated rather exciting optoelectronic characteristics, particularly, narrow bandgaps (1.2−1.4 eV)15 and high charge carrier mobilities in thin films.16,17 However, the lead-free Sn-based perovskite solar cells showed inferior photovoltaic performances approaching 6% only.18,19 Another drawback is a very poor stability of these devices, which is ascribed to the facile oxidation of Sn2+ to Sn4+.18,19 Enhanced stability and device efficiency was obtained for binary Sn/Pb perovskites MAPb1−xSnxI3, yielding the best power conversion efficiency of ∼10% for an optimal lead-rich material composition MASn0.15Pb0.85I3.20 Germanium is another possible replacement for Pb, although the Ge-based perovskites remain almost unexplored. It has been shown that a series of the Ge-based hybrid salts AGeI3 with different organic cations demonstrate excellent optoelectronic characteristics.21 However, the Ge2+ species are also prone to oxidation, so some severe challenges with handling of such materials are expected.6,22,23 A partial replacement of Pb with Ge also affected badly the photovoltaic performance of the lead halide perovskite solar cells.22 It has been reported recently that optoelectronic properties of the wide band gap material MAPbBr3 can be tuned by heterovalent substitution of Pb2+ with Bi3+, while In3+ and Au3+ Received: September 16, 2016 Accepted: October 19, 2016 Published: October 19, 2016 4353

DOI: 10.1021/acs.jpclett.6b02122 J. Phys. Chem. Lett. 2016, 7, 4353−4357

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The obtained results are presented in Figure 1c and Figure S1 (SI). It is seen from the diagram that only the mercury substitution enhanced the power conversion efficiency (PCE) of the solar cells. All other bivalent ions induced a significant deterioration of the photovoltaic performance, which suggests that they perform as traps for mobile charge carriers. Incorporation of univalent ions (Cu+, Ag+) did not affect the characteristics of the devices significantly. On the contrary, the aliovalent substitution of Pb2+ with Ti4+, Sn4+, Bi3+, and also In3+ ruined the photovoltaic performance of the system. In the case of bismuth, we reduced the concentration of the dopant (Bi3+) down to 5, 1, and 0.1% and still observed a dramatic decrease in the performance of the solar cells. This result suggests that the aliovalent doping of the lead halide perovskites with Bi3+ (as well as Ti4+, Sn4+ and to the less extent In3+) disturbs the fundamental mechanisms of the charge generation and transport. Most likely, this approach cannot be used for boosting the photovoltaic performance of the lead halide perovskites, thus leaving the previously expressed hopes23 unsatisfied. The observed increase in the performance of the perovskite solar cells resulting from Hg2+ doping was particularly exciting. Therefore, we explored this model system in more detail. We prepared and studied a series of the mixed MAPb1−xHgxI3 systems with x ranging from 0.03 to 1. Figure 2 shows the

were shown to be inactive. Even low loading of the dopant (∼0.01−1.0% of Bi3+) significantly improved optical properties of the material (color changed from orange to black) and its electrical conductance. Unfortunately, the effect of doping on the performance of the MAPb/BiBr3 material in solar cells was not reported.23 In the present work, we systematically explored the effect of the partial Pb substitution in MAPbI3 on the optical and photovoltaic properties of the material. Figure 1b shows the

Figure 2. Absorption spectra of MAPb1−xHgxI3 films with different concentrations of Hg.

Figure 1. (a) Schematic layout of the planar junction solar cell, (b) absorption spectra, and (c) photovoltaic performances of the MAB0.1Pb0.9I∼3 systems comprising different elements B.

evolution of the absorption spectra of the MAPb1−xHgxI3 films as a function of the mercury loading. The replacement of up to 15% of Pb with Hg does not influence the optical properties of the material. Further increase in the content of Hg up to 70% resulted in a gradual decrease in the absorbance at 350−570 nm range, while the characteristic “perovskite” band at 650−800 nm was still persistent. These findings suggested that Pb2+ can be indeed replaced easily with Hg2+ in the perovskite lattice of MAPbI3. The evolution of the crystal structure of the MAPb1−xHgxI3 films as a function of their chemical composition was studied using grazing incidence wide-angle X-ray scattering (GIWAXS) (Figure 3). The GIWAXS profile of the pristine MAPbI3 shows the formation of the crystalline beta phase (symmetry group I4cm, a = b = 8.85 Å; c = 12.64 Å16) with a weak orientation of the unit cell c-axis normal to the film surface (Figure 3). A gradual replacement of Pb2+ with Hg2+ ions does not lead to any noticeable changes in the crystal structure of the material until the mercury loading goes beyond 30%. This result proves

absorption spectra of different MAB0.1Pb0.9I∼3 films comprising a series of elements B: Hg (II), Cu (I), Ag (I), Sn(II), Cd (II), Zn (II), Co (II), Fe (II), Ni (II), Bi (III), In(III), and Ti(IV). One might notice that the aliovalent substitution of Pb2+ with Bi3+ resulted in the notable bleaching of the absorption band, which is characteristic for the MAPbI3 perovskite films. The substitution of just 10% of Pb2+ with Sn2+ in the MAPbI3 lattice produced a remarkable shift of the band edge to the longer wavelengths. The absorption spectra of all other mixed systems closely resembled the spectrum of pristine MAPbI3. We performed a screening of a series of compositions MAB0.1Pb0.9I∼3 in planar junction perovskite solar cells (Figure 1a). The device fabrication procedure is given in the Supporting Information (SI). The chemical composition of the perovskite films was confirmed using energy-dispersive X-ray spectroscopy (EDX) analysis (Table T1, SI). 4354

DOI: 10.1021/acs.jpclett.6b02122 J. Phys. Chem. Lett. 2016, 7, 4353−4357

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as it can be concluded from the SEM images shown in Figure 4b. Enhanced crystallinity and connectivity between the grains apparently result in the reduced recombination of charge carriers and improved PCE. However, further increase in the Hg loading produces inhomogeneous films with the incomplete surface coverage, which is the most plausible reason for their inferior photovoltaic performance. Thus, the mixed MAPb0.9Hg0.1I3 films delivered the best solar cell efficiencies clearly outperforming the pristine MAPbI3 material with respect to the open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF) as it follows from the J−V curves presented in Figure 5a and the photovoltaic characteristics listed in Table 1. Figure 3. GIWAXS patterns of the MAPb1−xHgxI3 films with different concentrations of Hg.

that lead and mercury species undergo cocrystallization with the formation of the common mixed perovskite phase. The GIWAXS pattern of the MAPb0.5Hg0.5I3 films still shows the main peaks corresponding to the perovskite lattice with some minor reflections attributed to the segregated nonperovskite MAHgI3 phase.24 The revealed persistence of the perovskite lattice in the mixed MAPb1−xHgxI3 films with high mercury loading is remarkable, considering the fact that the ionic radius of Hg2+ is smaller compared to that of Pb2+. The influence of the lead to mercury substitution on the performance of planar junction solar cells (Figure 1a) was investigated in more detail. Figure 4a and Table T2 (SI) show

Figure 5. (a) J−V curves of the representative perovskite solar cells measured with the forward (FS) and reverse (RS) voltage sweep directions with the rate of 0.1 V/s and (b) the external quantum efficiency (EQE) spectra of the representative devices based on the mixed MAPb0.9Hg0.1I3 and pristine MAPbI3.

The current densities of both types of devices were reconfirmed by integration of the corresponding EQE spectra (Figure 5b) against the standard AM1.5G solar irradiation spectrum.

Figure 4. (a) The power conversion efficiency of the MAPb1−xHgxI3based solar cells as a function of Hg loading and (b) SEM images of the selected perovskite films.

that increase in the Hg loading from 2.5 to 10% gradually improves the efficiency of the devices. However, a rapid deterioration of the photovoltaic performance was observed when the mercury content was further increased from 10 to 30%. The revealed PCE vs. Hg concentration behavior can be explained by the changes in the active layer morphology. Indeed, the incorporation of 10% of Hg increases the average grain size by a factor of 2−3 as compared to the pristine MAPbI3, while the films remain homogeneous and continuous,

Table 1. Mean (and the Best) Photovoltaic Parameters of the Devices Based on MAPb0.9Hg0.1I3 and Pristine MAPbI3 Extracted from the I−V Curves Measured in the Forward Direction material MAPbI3 MAPb0.9Hg0.1I3

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VOC, mV

JSC, mA/cm2

FF, %

PCE, %

780 ± 15 (800) 910 ± 16 (940)

16.2 ± 0.8 (16.9) 17.7 ± 0.5 (18.1)

67.5 ± 1.2 (69.8) 71.9 ± 1.1 (76.4)

8.6 ± 0.5 (9.4) 11.9 ± 0.9 (13.0)

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It is notable that the MAPb0.9Hg0.1I3-based solar cells provided the EQE of ∼80% at maximum, which is substantially higher than that of the reference MAPbI3 devices. Unfortunately, the MAPb0.9Hg0.1I3 devices exhibited a strong hysteresis in the current−voltage characteristics. Indeed, the solar cell efficiency drops down from 13.0% to 11.9% while going from the forward to reverse voltage sweep direction. The reference MAPbI3 solar cells demonstrated a very small difference between the J−V curves obtained with the forward and reverse scans. Importantly, the best-performing devices based on the mixed MAPb0.9Hg0.1I3 and the reference MAPbI3 films demonstrated steady-state photocurrents corresponding to the efficiencies of >12% and ∼8%, respectively (Figure S3, SI). In conclusion, we have systematically explored a partial substitution of Pb2+ in the MAPbI3 perovskite films with a series of univalent, bivalent, and trivalent ions. The effect of the lead substitution on the photovoltaic performance of the material depended strongly on the nature of the incorporated ion. Thus, using Bi3+ or Fe2+ ruined the solar cell performance even at low dopant concentrations (0.1−1%). On the contrary, incorporation of Hg2+ resulted in the considerably enhanced solar cell efficiencies peaking at the material composition corresponding to the MAPb0.9Hg0.1I3. To the best of our knowledge, this represents the first model system where a partial lead substitution boosted the photovoltaic performance of MAPbI3 and similar perovskites. It is also remarkable that up to 30% of Pb2+ in MAPbI3 can be easily replaced with the smaller Hg2+ ions without any significant changes in the crystal structure and optical properties of the material. These results prove that the compositional engineering of the lead halide perovskites by partial Pb2+ substitution can indeed be considered as a promising research direction providing wide opportunities for designing novel materials with advanced optoelectronic and photovoltaic properties.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02122. Experimental procedures, Figure S1 with J−V curves, Figure S2 with SEM images, Figure S3 with steady-state photocurrent measurement data, Table T1 with EDX data on the chemical compositions of the perovskite films, and Table T2 with the characteristics of the perovskite solar cells (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected], Tel: +7-496-522-1418, Fax: +7-496-522-3507. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in the frame of the Next Generation Skoltech-MIT collaboration program. Some part of the research performed at IPCP RAS was also supported by Russian Foundation for Basic Research (Project No. 16-03-00793). We also gratefully acknowledge a contribution of Dr. P. Barzelovich providing some starting materials. 4356

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The Journal of Physical Chemistry Letters (19) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-free Solid-state Organic−inorganic Halide Perovskite Solar Cells. Nat. Photonics 2014, 8, 489−494. (20) Zuo, F.; Williams, S. T.; Liang, P.-W.; Chueh, C.-C.; Liao, C.-Y.; Jen, A. K.-Y. Binary-Metal Perovskites Toward High-Performance Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2014, 26, 6454−6460. (21) Stoumpos, C. C.; Frazer, L.; Clark, D. J.; Kim, Y. S.; Rhim, S. H.; Freeman, A. J.; Ketterson, J. B.; Jang, J. I.; Kanatzidis, M. G. Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties. J. Am. Chem. Soc. 2015, 137, 6804− 6819. (22) Ohishi, Y.; Oku, T.; Suzuki, A. Fabrication and Characterization of Perovskite-based CH3NH3Pb1‑xGexI3, CH3NH3Pb1‑xTlxI3 and CH3NH3Pb1‑xInxI3 Photovoltaic Devices. The Irago Conference 2015 AIP Conference Proceedings, Aichi, Japan, 2015; Vol. 1709, pp 20020−1. (23) Abdelhady, A. L.; Saidaminov, M. I.; Murali, B.; Adinolfi, V.; Voznyy, O.; Katsiev, K.; Alarousu, E.; Comin, R.; Dursun, I.; Sinatra, L.; et al. Heterovalent Dopant Incorporation for Bandgap and Type Engineering of Perovskite Crystals. J. Phys. Chem. Lett. 2016, 7, 295− 301. (24) Korfer, M.; Fuess, H.; Bats, J. W.; Klebe, G. Z. Struktur und Eigenschaften von Doppelhalogeniden von substituiertem Ammonium und Quecksilber(II). V. Die Kristallstruktur von CH3NH3HgBr3 und CH3NH3HgI3. Z. Anorg. Allg. Chem. 1985, 525, 23−28.

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