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(CHCHNH)CuBr: A Lead-Free, Highly Stable TwoDimensional Perovskite for Solar Cell Applications Xiaolei Li, Bochao Li, Jianhui Chang, Bin Ding, Shuaizhi Zheng, Yiwei Wu, Junliang Yang, Guanjun Yang, Xiangli Zhong, and Jinbin Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00372 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Energy Materials

(C6H5CH2NH3)2CuBr4: A Lead-Free, Highly Stable TwoDimensional Perovskite for Solar cell Applications Xiaolei Li,†, ‡ Bochao Li,† Jianhui Chang,† Bin Ding,‡ Shuaizhi Zheng,† Yiwei Wu,† Junliang Yang,§ Guanjun Yang,*, ‡ Xiangli Zhong,*,† and Jinbin Wang*, †

†

Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of

Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, P. R. China ‡

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and

Engineering, Xi'an Jiaotong University, Xi'an 710049, P. R. China §

Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of

Physics and Electronics, Central South University, Changsha 410083, P. R. China

Corresponding Author *Xiangli Zhong, E-mail: [email protected]. *Jinbin Wang, E-mail: [email protected]. *Guanjun Yang, E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

The toxicity and the instability of lead-based perovskites might eventually hamper the commercialization of perovskite solar cells. Here, we present the optoelectronic properties and stability of a two-dimensional layered (C6H5CH2NH3)2CuBr4 perovskite. This material has a low Eg of 1.81 eV and high absorption coefficient of ~1×105 cm‒1 at the most intensive absorption at 539 nm, implying that it is suitable for light-harvesting in thin film solar cells, especially in tandem solar cells. Furthermore, X-ray diffraction (XRD), ultraviolet‒visible (UV‒Vis) absorption spectra and thermo gravimetric analysis (TGA) confirm the high stability toward humidity, heat and ultraviolet light. Initial studies produce a mesoscopic solar cell with a power conversion efficiency of 0.2%. Our work may offer some useful inspiration for the further investigation of environment-friendly and stable organic–inorganic perovskite photovoltaic materials.

KEYWORDS lead-free, highly stable, (C6H5CH2NH3)2CuBr4, two-dimensional, copper-based, perovskite solar cells


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Hybrid organic–inorganic lead (Pb) halide perovskite materials (MAPbX3, where MA is CH3NH3+; X is halides) have recently attracted unprecedented attention due to their impressive photovoltaic performance and facile fabrication procedure.1-7 During the past several years, the power conversion efficiency (PCE) of perovskite solar cells (PVSCs) has soared to certified 22.7%.8-14 However, the carcinogenicity of hazardous Pb15 and the instability of Pb-based halide perovskite materials16-17 may eventually hamper the large-scale industrial application of PVSCs. Consequently, it is highly desirable to further explore environment-friendly and stable lightharvesting materials.18 An important way to solve the toxicity issue is to replace Pb in perovskites with environment-friendly elements.19-21 The metal elements used to replace Pb mainly contain IVA group elements,22-27 e.g., Sn(II) and Ge(II), VA group elements,28-33 e.g., Bi(III) and Sb(III), and transition Cu(II).34-38 As similar elements to Pb in IVA group, Sn(II) and Ge(II) are the most obvious candidates to replace Pb. The similar electronic configurations and ionic radius (Pb 1.49 Å, Sn 1.35 Å) might encourage researchers to replace Pb by Sn with no significant sacrifice in crystal structure and photovoltaic behavior. For instance, photovoltaic device based on threedimensional (3D) MASnI3 perovskites achieved a PCE of approximate 7%.24 Liao et al. reported a highly oriented two-dimensional (2D) (PEA)2(FA)8Sn9I28 perovskite with a PCE of 5.94% and enhanced stability.26 Recently, highly reproducible 2D/3D Sn-based PVSCs with 9% efficiency and high stability toward light and ambient have been achieved.27 However, Sn-based perovskites still suffer from extremely inferior air-stability owing to the oxidation of Sn2+ to Sn4+; as a result, Sn-based perovskites tend to decompose very rapidly upon exposure to ambient air, therefore its solar cells preparation requires processing in N2-filled glove-boxes.24 Meanwhile, VA group elements (Bi and Sb) with 6s26p3 electronic configuration have been


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applied in Pb-free perovskites, e.g., Bi-based perovskite-type compounds have been studied as light absorbers, including AgBi2I7,31 MA3Bi2I9,32 and AgBiI4.33 Recently, Zhang et al. presented a high-quality (MA)3Bi2I9 film-based solar cell with a record PCE of 1.64% by a novel two-step vacuum deposition procedure.30 Nevertheless, Bi- and Sb-based perovskite-type compounds usually exhibit large band gaps (Eg > 2.1 eV), which are not very suitable for light-harvesting.19 Regarding another alternative of Cu, it is an environment-friendly, cost-effective and abundant transition element used in our daily life. Moreover, Cu is also essential to all living organisms as part of enzymes and the adult body contains between 1.4 and 2.1 mg of Cu per kg of body weight.39 In 2015, Cui et al.37 reported two Cu-based hybrid perovskites, (p–F– C6H5C2H4NH3)2CuBr4 and (CH3(CH2)3NH3)2CuBr4, with PCEs of 0.51% and 0.63%, respectively. Later, 2D layered copper halide perovskite (MA)2CuCl4-xBrx has been investigated as light-harvesting material in solar cells with a low PCE of 0.017%, demonstrating the potential photovoltaic performance of this Cu-based perovskite; however, the absorption coefficient and stability of this Cu-based perovskite need to be improved.34 In 2017, Li et al.35 reported a highly stable C6H4NH2CuBr2I-based solar cell with a PCE of 0.46%. To enhance the moisture stability of perovskite materials, one useful strategy is to replace hygroscopic MA with hydrophobic cation, such as butylammonium (BA)40,

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and

phenylethylammonium (PEA).42 For example, 2D Ruddlesden–Popper layered perovskite (BA)2(MA)3Pb4I13 (n = 4) with outstanding moisture stability has been used in solar cells with a PCE of 12.52%.40 Based on former research, it's rational that a hydrophobic moiety containing Cu-based perovskite material would well resolve the problems of humidity instability and toxicity in the mean time. In these regards, (PMA)2CuBr4 (PMA = C6H5CH2NH3+) turns out to be a suitable


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candidate, while the optoelectronic properties of layered perovskite (PMA)2CuBr4 have not yet been studied.43 With hydrophobic PMA molecules, this material might be more stable compared to (MA)2CuCl4‒xBrx perovskite. Therefore, it is meaningful to apply the layered perovskite (PMA)2CuBr4 as a novel light absorber alternative to Pb-based perovskites. Here, for the first time, we report the optoelectronic properties and stability of (PMA)2CuBr4 perovskite. This material exhibits a low Eg of 1.81 eV and high absorption coefficient of ~1×105 cm‒1 at the most intensive absorption at 539 nm, implying that it is suitable for light-harvesting in thin film solar cells, especially in tandem solar cells. Stability measurements indicate that (PMA)2CuBr4 perovskite is humidity-stable, heat-stable, and ultraviolet (UV) light-stable. Finally, we applied this lead-free and stable (PMA)2CuBr4 perovskite in solar cells for the preliminary trials and obtained a PCE of ~0.2%. The crystal structure of 2D perovskite (PMA)2CuBr4 consists of inorganic layers of planar CuBr42‒ anions separated by double organic layers of PMA+ cations. The structure scheme of the (PMA)2CuBr4 was shown in Figure 1a, which is similar to that of (C2H5NH3)2CuC14.44 The crystal structure of (PMA)2CuBr4 is hexagonal with a = 10.558 Å, b = 10.486 Å, c = 63.473 Å.43 As shown in Figure 1b, (PMA)2CuBr4 crystals form plates. The layered structure of (PMA)2CuBr4 crystals can be observed from SEM images. Figure 1c and Figure S1 show the edge of a thick (PMA)2CuBr4 crystal, which consists of stacks of sheets. The obtained Cu/Br atomic ratio is approximately 1:4, which is consistent with the stoichiometry in the molecule formula (Figure S2). Furthermore, the XRD patterns of (PMA)2CuBr4 in comparison to those of CuBr2 and C6H5CH2NH3Br (Phenmethylammonium bromide, PMABr) were also given in Figure 1d, indicating that no remnants of raw materials could be detected within the detection limit of XRD.


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The (PMA)2CuBr4 thin film was prepared by spin-coating its ethanol solution onto FTO glasses in ambient air without the use of a glovebox. UV‒Vis absorption spectra were performed to test the absorption properties of the (PMA)2CuBr4 thin film. As shown in Figure 2a, the (PMA)2CuBr4 thin film exhibits a relatively wide absorption range. From the corresponding Tauc plot of (PMA)2CuBr4 thin film in the inset of Figure 2a, we obtained a Eg of 1.81 eV for (PMA)2CuBr4 assuming a direct Eg, which is suitable for wide-band gap top cell application in multi-junction solar cells.45 To further assess the optical properties of the (PMA)2CuBr4 perovskite, the steady-state photoluminescence (PL) was performed. The PL peak (1.82 eV, Figure S3) is in good agreement with the Eg of 1.81 eV. High optical absorption coefficient is one of key properties for high-performance solar cells, which is relevant to the collection of photogenerated carriers.46 As shown in Figure 2b, the optical absorption coefficient of (PMA)2CuBr4 thin film is estimated to ~1×105 cm‒1 at the most intensive absorption at 539 nm, which is suitable for light-harvesting in thin film solar cells and comparable to that of the standard MAPbI3.47 To estimate the recombination time of the photo-excited carriers, we use a 450 nm wavelength laser to excite the sample, and the transient PL delay curve was recorded by a time-correlated single-photon counting setup. As shown in Figure S4, the (PMA)2CuBr4 thin film has a short-lifetime of 0.17 ns (37.85%) and a long-lifetime of 3.15 ns (62.15%). The shortlived process might arise from charge-carrier trapping, while the long-lived process might be attributed to the exciton recombination.48 The charge carrier lifetime (2.02 ns = 0.17 ns × 37.85% + 3.15 ns × 62.15%) is approximate to that of other Pb-free perovskites (The detailed values were summarized in Table S1) and even much longer than that of the Pb-based layered perovskite (PMA)2PbI449 (73.51 ps). This relatively short carrier lifetime might be attributed to the 2D layered structure of the PMA2CuBr4 perovskite, which is not beneficial for carriers


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transport.40,

49, 51

At the early stages of development for this new solar absorber, the carrier

lifetime (2.02 ns) is longer than the first-pass threshold value (1 ns) required for solar cells.50 Therefore, (PMA)2CuBr4 is worth to be investigated as a novel light-harvesting material and might be improved for solar cell applications in the future. Ultraviolet photoelectron spectrum (UPS) was performed to evaluate the Fermi energy (EF) and the valence band energy (Ev) level of (PMA)2CuBr4 perovskite (Figure 2c). The EF and Ev were estimated to be ‒ 5.47 eV and ‒ 5.67 eV, respectively.35 The conduction band energy (Ec) was calculated by adding the Eg of (PMA)2CuBr4 perovskite to Ev, which is estimated to be ‒ 3.86 eV. The band diagram of (PMA)2CuBr4 perovskite is shown in Figure 2d. The lack of stability toward moisture, heat, and light is an important barrier that perovskite materials faced. Therefore, we evaluated the moisture and thermal stability of perovskite (PMA)2CuBr4 as well as its tolerance toward UV light. To test the humidity stability, we exposed (PMA)2CuBr4 film spin-coated on FTO substrate continuously for 7 days to ambient air at ~60% relative humidity (see Figure S5), and monitored the process by recording XRD patterns before and after storage in ambient air for 7 days. Upon humidity exposure, the XRD patterns of (PMA)2CuBr4 does not show additional reflections after 7 days (Figure 3a). To further confirm the humidity stability of the (PMA)2CuBr4 perovskite thin film, enlarged areas of XRD patterns around the baseline were shown in Figure S6. No obvious changes were observed, demonstrating that the (PMA)2CuBr4 thin film is humidity-stable. Furthermore, the UV‒Vis spectra measurements of the (PMA)2CuBr4 thin films were performed before and after storage in ambient air for 7 days (Figure 3b); no obvious changes were observed, demonstrating that the (PMA)2CuBr4 thin film is humidity-stable. The satisfactory humidity stability of the (PMA)2CuBr4 thin film can be attributed to the use of hydrophobic PMA groups.40-42 The


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hydrophobic chain of PMA groups might reduce direct contact of water with (PMA)2CuBr4 and thus may enhance the humidity stability. Apart from moisture stability, heat stability of perovskite materials is also vital for commercial development of PVSCs. As shown in Figure 3c, thermo gravimetric analysis (TGA) exhibits the beginning of weight loss at ~160 °C, which is higher than that of MA2CuClxBr4‒x (120 °C for MA2CuCl0.5Br3.5 and 140 °C for MA2CuCl2Br2).34 The reason for the enhanced heat stability might be the replacement of MA groups with larger PMA groups. To further confirm the heat stability of perovskite (PMA)2CuBr4, the XRD patterns of the perovskite (PMA)2CuBr4 thin film were performed before and after heating at 100 °C for 6 h; no additional reflections were observed (Figure 3d and Figure S7). The above results demonstrate that (PMA)2CuBr4 perovskite is a thermally stable material for photovoltaic application and confirm that the introduction of hydrophobic PMA groups is an effective way to substantially improve the heat- and humidity-stability of perovskite materials. Furthermore, we exposed (PMA)2CuBr4 thin film for 14 h under UV light to investigate the UV aging effect (Figure S8). No obvious changes with XRD patterns (Figure 4a and Figure S9) and UV–Vis spectra (Figure 4b) over this period indicate that the material is UV light stable. To assess the photovoltaic performance of the (PMA)2CuBr4 perovskite, we fabricated photovoltaic device with structure of fluorine-doped tin oxide (FTO)/compact (c) TiO2/mesoporous (m) -TiO2 + (PMA)2CuBr4 perovskite/P3HT/gold electrode (Figure 5a). Figure 5b shows a cross section SEM image of the (PMA)2CuBr4-based device, consisting of a ~600 nm thick mesoporous TiO2 layer loaded with (PMA)2CuBr4 perovskite, a ~500 nm thick (PMA)2CuBr4 perovskite capping layer, a thin P3HT layer and a ~160 nm thick gold film. Figure 5c shows the current density-voltage (J-V) curves tested under simulated AM1.5 100 mW/cm2 illumination. A preliminary PCE of 0.2% was achieved with a short-current density (Jsc)


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of 0.73 mA/cm2, an open-circuit voltage (Voc) of 0.68 V and a fill factor (FF) of 0.41. The J–V curves for the device are almost identical when the voltage scanned both in reverse and forward directions. The Jsc was not high, so it is necessary to confirm the contribution from the P3HT. Therefore, we further fabricated devices with structure of FTO/c-TiO2/m-TiO2/P3HT/Au and measured their J–V curves (Figure S10 and Table S2). The P3HT indeed contributes to the total PCE of the solar cell. An average PCE of 0.0117% was obtained based on the structure of FTO/c-TiO2/m-TiO2/P3HT/Au, whereas the highest PCE of the solar cell based on (PMA)2CuBr4 film is 0.2%. The average Jsc (0.2861 mA/cm2, Table S2) contributed from the P3HT is ~39.7% of the total Jsc (0.73 mA/cm2) of the best-performing device, which is consistent with the results in Bi-based devices with P3HT layer.52 The measured external quantum efficiency (EQE) spectra of the (PMA)2CuBr4-based solar cell are illustrated in Figure 5d. The Eg (1.82 eV) derived from EQE spectrum is in good agreement with the value (1.81 eV) determined from the Tauc plot (Figure 2a). To compare with other Cu-based PVSCs, the detailed values are summarized in Table S2. To the best of our knowledge, this is first time to prove the photovoltaic effect of this copper-based layered perovskite with high stability. The low photovoltaic efficiency achieved in this copper-based photovoltaic material can be attributed to the intrinsic layered structure of (PMA)2CuBr4 perovskites and the relatively short carrier lifetime (2.02 ns, Figure S4). For layered perovskite structure, the poor out-of-plane charge transport is not beneficial for the transport and extraction of charge carrier in solar cells,40, 41, 49

which might be the main reason for low Jsc. Especially, the vertical charge carrier transport

might be seriously hampered in planar devices, due to the preferential orientation of the (PMA)2CuBr4 toward the (00l) direction. This was supported by the very low Jsc (~0.07 mA/cm2) achieved by a planar device with structure of FTO/c-TiO2/PMA2CuBr4/P3HT/Au (Figure S11).


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So, the use of m-TiO2 layer likely allows the disruption of the continuous 2D planes, which might be useful for the charge extraction and vertical charge transport in 2D Cu-based mesoscopic PVSCs.34 Controlled crystal growth to obtain an orthogonal alignment of the 2D planes on the substrate should be an effective way to overcome the intrinsic disadvantages of layered perovskite, which might be useful for the improvement of Jsc. By hot casting method, the PCE of the 2D (BA)2(MA)3Pb4I13 device is improved from 4.73% to 12.5% due to the wellcontrolled crystal orientation of the 2D (BA)2(MA)3Pb4I13 perovskite films.40 So, it is meaningful to explore the methods to obtain the crystal orientation in this 2D Cu-based perovskites, which may be useful for the enhancement of the photovoltaic performance. In conclusion, this 2D perovskite (PMA)2CuBr4 exhibits a low Eg of 1.81 eV and high absorption coefficient of ~1×105 cm‒1 at the most intensive absorption at 539 nm, implying that it is suitable for light-harvesting in thin film solar cells, especially in tandem solar cells. We found that (PMA)2CuBr4 has a carrier lifetime exceeding 2 ns, which suggests that it is worth further investigation as a new light-harvesting material and could be improved for solar cell application in the future. Stability measurements indicate that the material is very stable to moisture, heat, and UV light. Finally, we applied this lead-free and stable light perovskite material in solar cells and obtained a PCE of ~0.2%. Additional understanding of the working principle of this material and investigations of photovoltaic device preparation is needed for the future development of the (PMA)2CuBr4 perovskite. Importantly, our study can offer some useful inspiration for the further development of environment-friendly and stable organic– inorganic perovskite photovoltaic materials.


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Figure 1. (a) Schematic picture of the (PMA)2CuBr4 structure. (b) Digital photograph of (PMA)2CuBr4 plates. (c) SEM image of (PMA)2CuBr4 showing the layered structure. (d) X-ray diffraction patterns of (PMA)2CuBr4, CuBr2 and C6H5CH2NH3Br compound.


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Figure 2. (a) UV‒Vis spectrum of (PMA)2CuBr4 thin film and Tauc plot of (PMA)2CuBr4 from UV‒Vis spectrum. (b) Absorption coefficients of (PMA)2CuBr4 thin film (The film thickness is about 200 nm). (c) UPS of (PMA)2CuBr4 material. (d) Band level diagram of the (PMA)2CuBr4 material.


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Figure 3. XRD patterns (a) and UV‒Vis spectrum (b) of (PMA)2CuBr4 thin films before and after storage in ambient air (~60% relative humidity) for 7 days. (c) TGA curve of (PMA)2CuBr4 material. (d) XRD patterns of (PMA)2CuBr4 thin film before and after heat at 100 °C for 6 h.


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Figure 4. XRD patterns (a) and UV‒Vis spectrum (b) of (PMA)2CuBr4 thin film before and after UV light radiation for 14 h.

Figure 5. Scheme (a) and SEM image (b) of (PMA)2CuBr4-based device, J‒V curves (c) and EQE spectrum (d) of the best performing device.


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Scheme 1. Schematic illustration of C6H5CH2NH3Br (a) and (C6H5CH2NH3)2CuBr4 crystals (b) synthesis process.

EXPERIMENTAL METHODS Materials. All the chemicals were used as received, including CuBr2 (Copper bromide, 99.9%, Aladdin), C6H5CH2NH2 (Benzylamine solution, 99% Aladdin), HBr (Hydrobromic acid, 40wt%, 99.99%, Aladdin), absolute ethanol (99.5%, Aladdin), isopropanol (99.5% Aladdin), diethyl ether (99.5%, Sinopharm Group Co. Ltd), poly (3-hexylthiophene) (P3HT, Xi'an Polymer Light Technology Corp). Abbreviations used: PMA = benzylamine, EtOH = absolute ethanol, IPA = isopropanol. The mesoporous TiO2 (Dyesol, particle size ≈ 18 nm) paste was diluted with EtOH (1:4 weight ratio). Synthesis of PMABr salts. HBr solution (6.67 mL, 0.0455 mmol) was cautiously added in a cold (0 °C) mixture solution of 5 mL PMA (0.0445 mmol) and 10 mL EtOH and then stirred for 2 h. The white precipitates were filtered and washed three times with diethyl ether. After recrystallization from IPA and then dried at 60 °C for 12 h in vacuum drying cabinet, white


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PMABr crystals were obtained (see Scheme 1a). Synthesis of (PMA)2CuBr4 perovskite crystals. Stoichiometric C6H5CH2NH3Br (376.3 mg) and CuBr2 (223.35 mg) were dissolved in 2 mL HBr and then the solution was stirred at 60 °C for 2 h. The (PMA)2CuBr4 perovskite crystals were recrystallized after 12 h at 0 °C. The dark violet flake-like crystals were collected by filtration, and then dried at 60 °C for 12 h in vacuum drying cabinet (see Scheme 1b). Preparation of (PMA)2CuBr4 film. (PMA)2CuBr4 crystals (100 mg) were dissolved in EtOH (100 mL) and then the solution (Figure S12a) was stirred at 60 °C for 2 h. The dark violet (PMA)2CuBr4 thin films (Figure S12b) used for material characterization were prepared by spincoating method at 2000 rpm for 30 s, and then heated at 70 °C for 10 min on a hot plate. Material characterization. XRD measurements were completed by a Ultima IV X-ray Diffractometer with Cu Kα from 10 to 90° (2θ). UV‒Vis spectra and transmission spectrum were measured with the (PMA)2CuBr4 precursor solutions spin-coated on FTO using a PerkinElmer Lambda 950 spectrophotometer. The transient fluorescence spectrometer (FLS980) was applied for testing the PL decay. UPS measurement was performed with Thermo Scientific ESCALAB 250Xi instrument, using monochromatized Al Kα radiation (hν = 1486.7 eV). SEM tests were performed on a field-emission SEM (MIRA3 TESCAN). Instrument SDT Q600 V20.5 Build 15 was performed for the TGA. The measurement was carried out under N2 (flow rate 100 mL/min) from 42 to 600 °C (ramp rate 5 °C/min). The UV stability tests were carried using a 6 W UV lamp (lmax = 365 nm, 8 mW/cm2). The humidity and UV stability tests were carried out under ambient conditions (Summer, Xiangtan, P. R. China) with a temperature of ~30 °C and humidity of ~60%, respectively.


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Device fabrication and characterization. FTO substrates were cleaned by sonicating sequentially in acetone, IPA, and deionized water, each for 20 min. The c-TiO2 was spin-coated at 2000 rpm for 30 s onto the FTO substrates, and then the resulting substrates were annealed at 135 °C for 10 min. After cooling to room temperature, the TiO2 paste was spin-coated at 4000 rpm for 30 s on top of c-TiO2-coated FTO substrates, followed by sintering at 500 °C for 30 min. Then the (PMA)2CuBr4 films were spin-coated at 2000 rpm for 30 s, followed by heating at 80 °C for 10 minutes. After this the P3HT in chlorobenzene (20 mg/mL) was spin-coated at 4000

rpm for 30 s and annealed at 100 °C for 10 min. Finally, 160 nm thick gold layer was thermally evaporated on the top of P3HT layer. J–V curves of the solar cells with an active area of 0.08 cm2 were tested by using a source meter (Keithley, 2400) under 100 mW cm‒2 illumination using a solar simulator (Peccell Technologies, PEC-L01). The devices were measured by reverse (from 1.2 to ‒ 0.2 V) and forward (from ‒ 0.2 to 1.2 V) voltage scanning with a scan rate of 0.05 V/s. The EQE spectrum was tested in air at room temperature with action spectrum measurement system (Peccell Technologies, PEC-S20).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images and EDS of (C6H5CH2NH3)2CuBr4 samples; time-resolved PL decay; photographs of (C6H5CH2NH3)2CuBr4 solution and film; planar device J‒V curve; table.

AUTHOR INFORMATION


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Corresponding Authors *Email address: [email protected] *Email address: [email protected] *Email address: [email protected] Author Contributions X. L. Li and B. C. Li contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51572233 and 61574121) and the National Key Research and Development Program of China (2016YFB0501303). We also thank Instrument Analysis Center of Xi'an Jiaotong University for performing various characterization and measurements.

REFERENCES (1) Brenner, T. M.; Egger, D. A.; Kronik, L.; Hodes, G.; Cahen, D. Hybrid Organic‒Inorganic Perovskites: Low-Cost Semiconductors with Intriguing Charge-Transport Properties. Nat. Rev. Mater. 2016, 1, 15007. (2) Jung, H. S.; Park, N. G. Perovskite Solar Cells: From Materials to Devices. Small 2015, 11, 10‒25.


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