Color-Tuned Perovskite Films Prepared for Efficient Solar Cell

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Color-Tuned Perovskite Films Prepared for Efficient Solar Cell Applications Dong Cui, Zhou Yang, Dong Yang, Xiaodong Ren, Yucheng Liu, Qingbo Wei, Haibo Fan, Jinghui Zeng, and Shengzhong (Frank) Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09393 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 17, 2015

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The Journal of Physical Chemistry

Color-Tuned Perovskite Films Prepared for Efficient Solar Cell Applications Dong Cui, Zhou Yang,* Dong Yang, Xiaodong Ren, Yucheng Liu, Qingbo Wei, Haibo Fan, Jinghui Zeng, Shengzhong(Frank) Liu* Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Institute for Advanced Energy Materials, School of Materials Science & Engineering, Shaanxi Normal University, Xi’an 710062, China AUTHOR INFORMATION Corresponding Author * Prof. Shengzhong (Frank) Liu, E-mail: [email protected]; Tel: +86-029-81530785 Dr. Zhou Yang, E-mail: [email protected], Tel: +86-029-81530709

Prof. Shengzhong (Frank) Liu also works at Dalian Institute of Chemical Physics, National Laboratory for Clean Energy, Chinese Academy of Sciences, Dalian, 116023, China

ACS Paragon Plus Environment

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ABSTRACT

Color-tuned perovskite films have been recognized as a promising candidate for building integrated photovoltaics, bright-colorful display and component cell in multi-junction solar cell applications. In this paper, four representative color-tuned perovskite films with chemical formula of CH3NH3PbBrxI3-x (x=0, 1, 2 and 3) are successfully prepared by using a technique that combines advantages of direct contact lead halide film with hot methylamine halide powder and intercalcation process. The EDX results indicate that the Br/I ratio is controlled as desired. The SEM imaging shows very uniform films with good surface coverage on the substrate. The highest power conversion efficiency (PCE) of the perovskite solar cells with four different compositions are 12.76%, 6.84%, 4.12% and 3.53%, respectively.


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INTRODUCTION

The organic-inorganic hybrid solar cells based on the lead halide perovskites have attracted great interest in the past few years due to their high cell efficiency, low fabrication cost, and tunability of their optical absorption.1-4 The most widely studied perovskites are CH3NH3PbX3 (MAPbX3) (X=Cl, Br, I), among which, MAPbI3 gives a bandgap 1.55 eV, MAPbBr3 2.2 eV, and MAPbCl3 3.0 eV.5 Even more interesting, when two halide elements are mixed with controlled ratios, MAPbBr3-xClx and MAPbI3-xBrx are prepared with tunable bandgaps and optical properties to achieve superior performance. In addition to halide substitution or doping, other attempts have been demonstrated to partially replace Pb with Sn. For example, Hao et al. prepared CH3NH3SnxPb(1-x)I3 with band gap as low as 1.1eV.6, 7 However, the chemical stability of Sn doped perovskites is bad as it is easily oxidized to Sn4+ .7 Another way to tune the band gap of perovskite is achieved by replacing the organic ions. For example, replacing CH3NH3+ with formamidinium (HC(NH2)2+), a new perovskite was prepared with slightly smaller bandgap 1.47 eV, leading to extended light absorption to ~840 nm.8-13 Although the most widely studied MAPbI3 and MAPbI3-xClx solar cells have achieved high efficiency14-27, they are insufficient in the preparation of tandem solar cell with currently commercialized solar cells,28, 29 colourful display unit or the building integrated photovoltaics as its dark brown color. According to the theoretical calculation, a cell with bandgap of 1.7eV is the best choice for the top cell of an efficient tandem solar cell with c-Si.28, 29 There have been many research works reporting substitution of I with Br ions to increase the bandgap of MAPbX3.30-34 For example, Zhao et al. spin-coated perovskite precursor solution (mixed solution of PbI2/CH3NH3I (MAI) and PbBr2/CH3NH3Br (MABr)) on the mesoporous TiO2 substrate, which


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was then heated for a long time to form the perovskite films.30 Noh et al. spin-coated PbI2/PbBr2 precursor solution on the mesoporous TiO2 substrate, which subsequently was immersed CH3NH3I/CH3NH3Br isopropanol solution to produce the perovskite films.31 However, these methods are hard to control the morphology and coverage of the perovskite films. In this work, we used the direct contact and intercalation process (DCIP) 35, as we previously reported, to prepare high quality and bandgap tunable perovskite thin films. By controlling the chemical composition of lead halide film and MABr or MAI powder used during DCIP, four representative perovskite films with defined halide ratios to tune them into different colors, including MAPbI3 (dark brown), MAPbBrI2 (dark red), MAPbBr2I (red), and MAPbBr3 (orange), have been successfully prepared. The EDX results indicate the final Br/I ratios are very close to the theoretical values. The MAPbBrxI3-x (x=0, 1, 2, 3) films prepared using this technique have large grain size, small surface roughness, complete surface coverage and high reproducibility. The PCE of solar cell devices based on these films under 1 sun illumination (AM 1.5G, 100 mW/cm2) can reach 12.76% for MAPbI3, 6.84% for MAPbBrI2, 4.12% for MAPbBr2I and 3.50% for MAPbBr3, respectively.

EXPERIMENTAL METHODS

MAX preparation: Both the MAI and MABr powder were synthesized by reacting 30 mL of methylamine (40% in methanol, TCI) and 32.3 mL of hydroiodic acid (57 wt % aqueous solution, Aldrich) or 23.3 mL of hydrobromic acid (48 wt % aqueous solution, Aldrich) in a 250 mL round-bottom flask at 0 °C for 2h with active stirring. The precipitate was collected using a rotary evaporator through carefully removing the solvents at 50 °C. The light yellowish raw products of methyl ammonium iodide (CH3NH3I) or methyl ammonium bromide (CH3NH3Br)


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were dissolved in absolute ethanol by stirring for 30 min, and then precipitated with diethyl ether. The washing step was repeated three times. The final products were collected using a Buchner funnel and dried at 60 °C in a vacuum oven for 24 h. TiO2 compact layer coated FTO substrate: The FTO substrate was cleaned using standard glass cleaning agents in a base bath (cleaning agent: deionized water=1:20), followed by rinsing with deionized (DI) water and ethanol. The compact TiO2 layer was spin coated on FTO glasses using 0.15 M titanium diisopropoxide bis(acetylacetonate) (75% Aldrich) in 1-butanol (Aldrich) solution, and then heated at 125 °C for 10 min. Afterwards, the coating was cooled down to the room temperature. The spin-coating process was repeated again to finish the compact layer preparation. Then the TiO2 film was annealed at 500 oC for 30 min. After cooled to room temperature, the sample was soaked in a 0.04 M TiCl4 solution and kept at 70°C for 30 min. After taken out from the solution, the sample was rinsed with DI water and ethanol, and finally dried under N2 gas flow. The TiCl4-treated TiO2 films were annealed again at 500°C for 30 min before being used for perovskite film deposition. Color-tuned perovskite films for solar cell preparation: To deposit the perovskite layer, PbI2, PbI2/PbBr2 mixture (mole ratio 1:1) and PbBr2 DMF solutions (with concentrations of 462, 415.5, 369 mg/mL) were spin-coated on compact TiO2 coated FTO glass at 4000 rpm for 30 seconds. The samples were dried at 70 °C on a hotplate for 15 min to remove the residue solvent, and then cooled for 5 min. The PbBrxI2-x (x=0, 1 and 2) films were placed onto MAI or MABr powder that was preheated at 150 °C in an aluminium container (as shown in figure 1). To investigate the transition process from lead halide film to perovskite film, a series of samples were prepared using different length of reaction time. For solar cell devices, the MAPbBrxI3-x (x=0, 1, 2, 3) films prepared above were used. After perovskite film deposition, a hole transport layer (HTL)


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was spin coated onto perovskite film at 3000 rpm for 30s, where 1 mL spiroOMeTAD/chlorobenzene (90 mg/mL) solution was employed with addition of 27 µL LiTFSI/acetonitrile (250 mg/mL), and 36 µL tBP. Finally, a 60-nm-thick Au electrode was deposited on the HTL layer by thermal evaporation in vacuum at 5.0 × 10-4 Pa. The active area of each device was 0.09 cm2. Characterizations: X-ray diffraction (XRD) pattern was obtained using a DX-2700 diffract meter with Cu Kα (λ=1.5418 Å) radiation at scan rate of 6°/min under operation condition of V=30 kV and I=40 mA. A field-emission scanning electron microscope (FE-SEM, HITACHI SU-8020) was used to investigate the morphology of the obtained samples. The thicknesses of all samples were measured using a Veeco profiler (Dektak 150). Thermogravimetric analysis (TGA) was performed on a TA SDT-Q600 V20.9 (Build 20) from room temperature to 350 oC. The optical absorbance spectra of perovskite film were measured using a UV-vis spectrophotometer (SHIMADZU UV-3600) ranging from 400 nm to 800 nm. The J-V curve of solar cell was measured using Keithely 2400 source meter under simulated solar radiation (100 mW/cm2, AM 1.5G). The incident photon to current efficiency (IPCE) was tested using a QTest Station 2000ADI system (Crowntech Inc.) in AC mode. A tungsten–halogen lamp (150W) was used as light source.

RESULT AND DISCUSSION

Figure 1. Schematic illustration for the fabrication of perovskite films using the DCIP.


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The PbBrxX2-x (x = 0, 1, 2) film was first prepared by spin-coating of 1 M PbBrxI2-x (x = 0, 1, 2) DMF solution onto compact TiO2 coated FTO (bl-TiO2/FTO) at 4000 rpm, followed by annealing for 15 min on a 70 oC hotplate. A flat MAI or MABr powder layer was spread onto the bottom of an aluminium container and pre-heated at 150 oC, onto which the PbBrxX2-x (x = 0, 1, 2) film was directly flipped. The stabilities of MAI and MABr at the reaction temperature were investigated by TGA. As shown in figure S1, both MAI and MABr will not decomposed at 150 o

C. The intercalation reaction was performed for transition of lead halide film to perovskite film

in a closed container (Figure1). After cooled down to room temperature, the MAPbX3 perovskite film was washed with isopropyl alcohol. The thicknesses of the lead halides films and corresponding perovskite films were summarized in table 1. For all four cases, the thickness of perovskite film is around 1.8 times to the original value of lead halide film, caused by crystal expanding during transition from lead halide to perovskite.36 Table 1. The thicknesses of PbX2 film and corresponding perovskite films. PbX2 PbI2 PbI2 PbI2/PbBr2 PbBr2

Thickness (nm) 175 175 180 191

MAPbX3 MAPbI3 MAPbBrI2 MAPbBr2I MAPbBr3

Thickness (nm) 315 310 325 345

Ratio MAPbX3/PbX2 1.80 1.77 1.81 1.81

To tune the Br/I ratio of perovskites and corresponding light absorptions (band gaps), four kinds of reaction processes have been proposed, as illustrated in the flowing formulas: MAPbI3

MAI + PbI2 → MAPbI3

(1)

MAPbBrI2

MABr + PbI2 → MAPbBrI2

(2)

MAPbBr2I

MABr + PbI2/PbBr2 → MAPbBr2I

(3)

MAPbBr3

MABr + PbBr2 →MAPbBr3

(4)


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Figure 2. The XRD patterns change as a function of reaction time: (a) PbI2 to MAPbI3, (b) PbI2 to MAPbBrI2, (c) PbBrI to MAPbBr2I and (d) PbBr2 to MAPbBr3. The lead halide films were put onto hot MAI or MABr powder to form perovskite films. By tuning the composition of lead halide films Pb(BrxI2-x) (x = 0, 1, 2) and using MAI or MABr powders during DCIP, we intended to adjust the chemical composition and their light absorption properties of perovskite films. For example, for MAPbBr2I, a 1:1 mole ratio PbBr2:PbI2 mixture was first deposited on bl-TiO2/FTO. After dried on a hot plate and cooled, the PbBrI film was put onto preheated MABr powder to form perovskite MAPbBr2I, as illustrated in equation 3.


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XRD measurement was used to monitor the transition reaction process to form perovskite. As shown in figure 2c, the diffraction peaks from PbI2 and PbBr2 could be easily observed before the DCIP. As the transition reaction proceeds, the lead halides diffraction peaks became weaker and weaker, indicating that the amount of lead halides were being consumed. At the same time, new diffraction peaks were emerged and became stronger and stronger. The new diffraction peaks are from the perovskite. After 15 min of reaction, lead halides are barely found in the XRD pattern, indicating that all of the lead halides have been consumed and transformed into halide perovskite. For MAPbI3, MAPbBrI2 and MAPbBr3, they follow the similar reaction scheme except using different halide films and MAI/MABr powder. The formation process for other three perovskite films were also monitored using XRD and shown in figure 2a, b and d. Interestingly, all the transition processes take around 15 min. By carefully comparing the diffraction peaks from four different kinds of perovskites (shown in figure S2), all peaks gradually shift to larger 2θ values as more Br incorporation into MAPbI3, which is ascribed to the smaller ion radius of Br (1.96 Å) than I (2.2 Å).31 The different peak positions from the four films demonstrate four different products have been obtained. Table 2. The Br and I content in perovskite films.

MAPbI3 MAPbBrI2 MAPbBr2I MAPbBr3

Br content (%)

I Content (%)

Br content in perovskite film by EDX result (%)

0 15.5 53.1 73.0

81.8 37.7 25.9 0

0 29.1 67.2 100

Br content in perovskite film from chemical formula (%) 0 33.3 66.7 100

The XRD patterns of MAPbI3 and MAPbBr3 indicate that they have tetragonal and cubic perovskite phase at room temperature. The Br incorporation changed the crystal from low


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symmetry I4/mcm to high symmetry Pm3m. For the four kinds of perovskites, the diffraction peaks, in 28o to 31o region, gradually shift to larger angle as the Br content increasing (as shown in figure 3a). If we view tetragonal structure as pseudo-cubic structure, the (220) plane of tetragonal MAPbI3 could be recognized as (200) plane of pseudo cubic MAPbI3. The lattice parameters could be extracted and shown in figure 3b. The lattice parameters almost linearly reduce as Br contentment increases in the mixed perovskites, in good agreement with Vegard’s law, in good agreement well with previous reports.31,

34

EDX measurement was used to

investigate the Br/I element ratios in the final films. As shown in Table 2 (figure S3, Table S1), the Br content results by EDX measurement are very close to the theoretical calculation from the chemical formula. Both XRD result and EDX results indicate that our proposed method can effectively tune the chemical composition of the halide perovskites.

Figure 3. Powder X-ray diffraction patterns of mixed lead halide perovskite films MAPBrxbI3x

(x=0, 1, 2, 3) in the region of the tetragonal (220) and cubic (200) reflections (2θ = 28-31.0o);

(b) the lattice parameters of pseudocubic or cubic MAPBrxbI3-x (x=0, 1, 2, 3) as a function of the Br content (x in pencentage).


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Figure 4a provides photos of four perovskite films, including two pure halide perovskites and tow dual-halides perovskites, with controlled composition. It is clear that their colours can be tuned from dark brown for MAPbI3, to dark red for MAPbBrI2, to red for MAPbBr2I and then to orange for MAPbBr3. In other words, their band gap can be tuned by controlling the lead halide film composition and MAI/MABr powder used in DCIP. It is envisioned that these colourful perovskite materials are promising for solar panels with controlled colours, and colourful displays. Figure 4b shows optical spectra of these four materials with each presenting its distinct absorption edge. It is clear that the absorption onset systemically move from 786 nm for MAPbI3 to 542 nm for MAPbBr3 as the content of Br in perovskite increasing from 0 to 100%. The absorption onsets were used to calculate the optical band gaps. A linear dependence of the band gap with Br content is observed, as shown in figure 4c. In fact, a linear expression is established using excel fitting: Eg = 0.72x+1.58, where x is the Br content in percentage. The trend agrees well with the previous report,31, 32 demonstrating the validity of the present technique in tuning the chemical composition and energy band gap of the dual-halide perovskites.


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Figure 4. The light absorption properties and band gaps of perovskites with different Br contents: (a) the photo pictures of MAPbBrxI3-x (x=0, 1, 2, 3) films; (b) the light absorbance of MAPbBrxI3-x (x=0, 1, 2, 3) films; (c) the relationship between Br content and perovskite band gap, which shows band gaps linearly depending on the Br contents in perovskite films.


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Figure 5. The morphology of perovskite films with different Br/I ratio: (a) MAPbI3, (b) MAPbBrI2, (c) MAPbBr2I, (d) MAPbBr3 Compared with the mixed-halide perovskite films prepared using solution method30-32, the current perovskite films using the DCIP technique show better uniformity and higher coverage as shown in figure 5 (figure S4). It is clearly observed that the grain size decreases from ~700 nm for the MAPbI3 sample (figure 5a), ~600 nm for MAPbBrI2 (figure 5b), ~400 nm for MAPbBr2I (figure 5c) and ~200 nm for MAPbBr3 (figure 5d) as the Br content is increased from 0 to 100%. The surface morphology of the above perovskite films were characterized by AFM.


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Figure 6. AFM images of the perovskite film with different Br/I ratio: (a) for MAPbI3, (b) for MAPbBrI2, (c) for MAPbBr2I, (d) for MAPbBr3. Figure 6 shows the AFM images of the top surface of the as-deposited perovskite thin films. The average grain size is smaller and smaller with the increase of the content of Br. The local surface topography data reveals root-mean-square (RMS) roughnesses are 20.3 nm (figure 6a), 27.8 nm (figure 6b), 19.2 nm (figure 6c) and 22.7 nm (figure 6d) for MAPbI3, MAPbBrI2, MAPbBr2I and MAPbBr3, respectively. Those results indicate the obtained perovskite films were very smooth. Finally, all four perovskite films have been used as absorber for planar heterojunction solar cell, namely FTO/bl-TiO2/perovskite/Spiro-OMeTAD/Au.


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Figure 7. (a) Current density−voltage (J−V) characteristics of the solar cell measured under simulated sun light (AM1.5G, 100 mW/cm2). (b) IPCE spectra for four kinds of perovskites based solar cells. Table 3. The specific photovoltaic parameters of the perovskite solar cells based on MAPbI3, MAPbBrI2, MAPbBr2I and MAPbBr3 films

MAPbI3 MAPbBrI2

Voc Jsc FF PCE Integrated Jsc (V) (mA/cm2) (%) (%) (mA/cm2) 1.02 19.36 64.6 12.76 18.2 champion 0.93±0.07 19.10±0.50 65.0±3.1 11.25±1.50 average 0.96

12.12

58.8

0.97±0.06 10.47±1.65 59.2±3.8 MAPbBr2I MAPbBr3

6.84

11.6

5.83±1.01

1.06

6.37

60.8

4.12

1.04±0.08

5.80±1.58

60.1±3.3

3.46±0.66

1.22 1.20±0.06

4.12 2.94±1.11

70.3 70.9±3.0

3.53 2.48±1.05

champion average

6.18

champion average

4.05

champion average

The J-V parameters are summarized in figure 7a and table 3. The photocurrents of the champion devices decrease with increased Br content in the perovskite film, from 19.36 mA/cm2 for MAPbI3 to 4.12 mA/cm2 for MAPbBr3. While the average open circuit voltages show a different trend and increase with increased Br content from 0.93 ± 0.06 V for MAPbI3 to


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1.20±0.06 V for MAPbBr3, following the same trend as the band gaps. Larger band gap would produce a higher open circuit voltage, but reduced light absorption would render a smaller photocurrent. From the IPCE data (figure 7b), the light response region of the mixed halide perovskite solar cell moves to short wavelength as the Br content increase, which agrees well with the light absorption property of mix halide perovskite and the photocurrent data. The PCE of the champion perovskite solar cells measured under 1 sun (AM1.5G, 100 mW/cm2) illumination are 12.76%, 6.84%, 4.12% and 3.53% for MAPbI3, MAPbBrI2, MAPbBr2I and MAPbBr3, respectively.

CONCLUSION

By intentionally controlling the lead halide film composition PbBrxI2-x (x = 0, 1, 2) and using MABr or MAI powder during the DCIP process, the chemical composition and corresponding band gap or light absorption property of the perovskite films have been adjusted successfully. The EDX result indicates that our proposed method could be used to effectively control the Br/I ratio of dual-halide perovskite film, with the composition very close to the stoichiometry. The color of perovskite films can be tuned from yellow to dark brown as Br content increased in mixed halide perovskites, likely to be useful for building integrated photovoltaic application. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge (Including TGA results for MAI and MABr, XRD patterns for four perovsktis, EDX data and Perovskite film morphology at larger scale). This information is available free of charge via the Internet at http://pubs.acs.org


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AUTHOR INFORMATION Corresponding Author * Prof. Shengzhong (Frank) Liu, E-mail: [email protected]; Tel: +86-029-81530785 Dr. Zhou Yang, E-mail: [email protected], Tel: +86-029-81530709 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The

authors

acknowledge

supports

from

the

National

University

Research

Fund

(GK261001009), the Changjiang Scholar and Innovative Research Team (IRT_14R33), the 111 Project (B14041) and the Chinese National 1000-talent-plan program. REFERENCES 1. Gao, P.; Grätzel, M.; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 2248-2263. 2. Park, N.-G. Organometal Perovskite Light Absorbers Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423-2429. 3. Hodes, G. Perovskite-Based Solar Cells. Science 2013, 342, 317. 4. Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630. 5. Liu, Y.; Yang, Z.; Cui, D.; Ren, X.; Sun, J.; Liu, X.; Zhang, J.; Wei, Q.; Fan, H.; Yu, F.; et al. Two-Inch-Sized Perovskite CH3NH3PbX3 (X = Cl, Br, I) Crystals: Growth and Characterization. Adv. Mater. 2015, 27, 5176-5183. 6. Lee, B.; Stoumpos, C. C.; Zhou, N.; Hao, F.; Malliakas, C.; Yeh, C. Y.; Marks, T. J.; Kanatzidis, M. G.; Chang, R. P. Air-Stable Molecular Semiconducting Iodosalts for Solar Cell Applications: Cs2SnI6 as a Hole Conductor. J. Am. Chem. Soc. 2014, 136, 15379-15385. 7. 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. 8. Eperon, G. E.; Bryant, D.; Troughton, J.; Stranks, S. D.; Johnston, M. B.; Watson, T.; Worsley, D. A.; Snaith, H. J. Efficient, Semitransparent Neutral-Colored Solar Cells Based on Microstructured Formamidinium Lead Trihalide Perovskite. J. Phys. Chem. Lett. 2015, 6, 129138. 9. Pang, S.; Hu, H.; Zhang, J.; Lv, S.; Yu, Y.; Wei, F.; Qin, T.; Xu, H.; Liu, Z.; Cui, G.


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TOC GRAPHICS


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Color-Tuned Perovskite Films Prepared for Efficient Solar Cell

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