OrganicâInorganic Copper(II)-Based Material: A Low-Toxic, Highly

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Organic-inorganic Copper(II)-based Material: a Low-Toxic, Highly Stable Light Absorber for Photovoltaic Application Xiaolei Li, Xiangli Zhong, Yue Hu, Bochao Li, Yusong Sheng, Yang Zhang, Chao Weng, Ming Feng, Hongwei Han, and Jinbin Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00086 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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The Journal of Physical Chemistry Letters

Organic-inorganic Copper(II)-Based Material: a Low-Toxic, Highly Stable Light Absorber for Photovoltaic Application Xiaolei Li,†,‡ Xiangli Zhong,† Yue Hu,‡ Bochao Li,† Yusong Sheng,‡ Yang Zhang,† Chao Weng,§ Ming Feng,# Hongwei Han,*,‡ 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 ‡

Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for

Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China §

College of Chemistry, Xiangtan University, Xiangtan 411105, P. R. China

#

Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education,

Jilin Normal University, Changchun 130103, P. R. China Corresponding Author *Jinbin Wang, E-mail: [email protected]. Tel: 0731-58292280. *Hongwei Han, E-mail: [email protected]. Tel: 027-87793027.

ACS Paragon Plus Environment

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ABSTRACT

Lead halide perovskite solar cells have recently emerged as a very promising photovoltaic technology due to their excellent power conversion efficiencies; however, the toxicity of lead and the poor stability of perovskite materials remain two main challenges that need to be addressed. Here, for the first time, we report a lead-free, highly stable C6H4NH2CuBr2I compound. The C6H4NH2CuBr2I films exhibit extraordinary hydrophobic behavior with a contact angle of ~90°, and their X-ray diffraction patterns remain unchanged even after four hours of water immersion. UV/Vis absorption spectrum shows that C6H4NH2CuBr2I compound has an excellent optical absorption over the entire visible spectrum. We applied this copperbased light absorber in printable mesoscopic solar cell for the initial trial and achieved a power conversion efficiency of ~0.5%. Our study represents an alternative pathway to develop lowtoxic and highly stable organic-inorganic hybrid materials for photovoltaic application.

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Methylammonium lead halide perovskite semiconductors (CH3NH3PbX3, where X is halide) have recently aroused significant attention in the photovoltaic community due to their high absorption coefficient, long electron-hole diffusion length, high carrier mobility, tunable direct band gap (Eg) and excellent ambipolar charge mobility.1-6 The power conversion efficiency (PCE) of perovskite solar cells (PVSCs) has dramatically improved from 3.8% to a value exceeding 21% over the past several years.7-14 Meanwhile, PVSCs with an aperture area exceeding one square centimeter with a certified PCE of 19.6% have been achieved via a vacuum flash-assisted solution process.15 Recently, perovskite-perovskite tandem solar cells have been obtained, with a highest efficiency of 20.3%.16 Unfortunately, PVSCs still face serious challenges to their market acceptance in future commercialization: the use of environmentally hazardous lead17 and the instability of perovskite materials to atmospheric moisture or heat.18,19 To solve the toxicity issue, many efforts have been devoted to research on lead-free organicinorganic hybrid perovskite materials.20-24 To date, tin (Sn),25, 26 bismuth (Bi),27-31 copper (Cu),32, 33

antimony (Sb),34, 35 and germanium (Ge)36 have been used to replace lead (Pb). A Sn-based

device has been reported with a PCE over 6%.26 However, these Sn-based perovskites are not as stable as the lead perovskites due to the oxidation of Sn2+ to Sn4+, and all device assembly and characterization processes have to be carried out under a nitrogen atmosphere.25 More recently, double-perovskites based on bismuth- and silver-halides27 (for example, Cs2AgBiBr6 and Cs2AgBiCl6) have been reported. Bismuth (Bi3+) has been used to replace lead (Pb2+) to form low-toxic Cs3Bi2I9 perovskite, and a CH3NH3Bi2I9-based solar cell with an efficiency of 0.19% has been reported.28 Solution-processed AgBi2I7 thin films29 have been considered as light absorbers in solar cells, with a PCE above 1%. Cortecchia et al. reported (CH3NH3)2CuCl4-xBrx perovskites with a very low efficiency of only 0.017%.32


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In addition to the toxicity issue, many strategies have been proposed to improve the stability of PVSCs. Some efforts have been made on the using of protective layers, such as the use of thick carbon back-contact,37 encapsulating solar cells by hydrophobic polymers,38 photocurable fluorinated coatings39 and polymer scaffolds.40 In addition, alkylphosphonic acid ωammonium,41 surface functionalization of perovskite thin films,42 cross-linked silane-modified fullerene layers43 and solution-processed metal oxide transport layers44 have been used to enhance the stability of such devices. Moreover, layered [C6H5(CH2)2NH3]2(CH3NH3)2Pb3I10 perovskites have been used in photovoltaic devices, with a PCE of 4.73%, and have shown the excellent stability after being stored under 52% relative humidity for 46 days.45 However, the toxicity of lead and the instability of lead-based perovskite materials remain two key challenges in this research field.46 Herein, for the first time, we report a lead-free, highly stable C6H4NH2CuBr2I compound. We synthesized the C6H4NH2CuBr2I compound by equimolar reaction of hydrophobic C6H4NH2I (2-iodoaniline) and low-toxic CuBr2. This work is inspired by an investigation of metal-organic frameworks (enhancing the moisture stability of metal-organic frameworks through incorporation of hydrophobic groups) 47 and the reports on layered hybrid perovskites [C 6 H5 (CH2 )2 NH3 ] 2 (CH3 NH3 ) 2 Pb 3 I10 with enhanced moisture stability. 45,48 Although the preliminary photovoltaic performance of the devices is low (PCE≈0.5%), the C6H4NH2CuBr2I shows its advantages of low toxicity, high stability and excellent optical absorption over the entire visible spectrum, which may address both the toxicity of lead and the instability of perovskites. Powder X-ray diffraction (XRD) refinement (Figure 1a) suggests that the crystal


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structure of C6H4NH2CuBr2I is hexagonal, space group R3, with a = b = 3.963(3) Å, c =

Figure 1. a) Pawley fit of XRD of C6H4NH2CuBr2I: observed (black) and calculated (red) diffraction pattern. The green line is the difference between the observed and calculated patterns. b) UV/Vis spectroscopy of C6H4NH2CuBr2I thin film. Inset in b) shows a picture of the thin film. c) Tauc plot of C6H4NH2CuBr2I from UV/Vis spectroscopy to determine Eg under the assumption of direct band gap and photoluminescence spectra. 9.698(5) Å. Crystallographic parameters are reviewed in Table S1 (Supporting Information). A high-resolution transmission electron microscope (TEM) image of the C6H4NH2CuBr2I crystal is shown in Figure S1 (Supporting Information). Moreover, the XRD patterns of C6H4NH2CuBr2I in comparison to those of CuBr2 and C6H4NH2I are also given in Figure S2 (Supporting Information), suggesting that a new phase is generated without any apparent residual starting materials. Photo of C6H4NH2CuBr2I powder is shown in Figure S3 (Supporting Information) to illustrate its dark appearance. Furthermore, the energy-dispersive X-ray spectroscopy (EDS) suggests that the ratio of elemental Cu and halogen was approximately 1:3, which is consistent with the ratio in the precursor solution (Figure S4 and Table S2, Supporting Information). As depicted in Figure 1b, the C6H4NH2CuBr2I thin film shows excellent optical absorption over the entire visible solar emission spectrum. As shown in Figure 1c, the direct Eg of C6H4NH2CuBr2I


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is estimated to be ~1.64 eV, and the photoluminescence (PL) peaked at ~1.50 eV. The discrepancy in Eg derived from PL and absorption measurements can be explained as follows. The PL is caused by the radiative recombination between the electrons in the bottom of the conduction band and holes in the top of the valence band. So the Eg derived from PL refers to the energy difference between the top of the valence band and the bottom of the conduction band in semiconductors. From the Hall effect measurements (Table S3, Supporting Information), one can see that the electron carrier concentration is as high as 1021 cm-3 which corresponds to a degenerate electron distribution found in some degenerate semiconductors. In such a degenerate semiconductor, the absorption edge will shift to higher energies because of the Burstein-Moss effect.49 Therefore, the Eg derived from absorption measurement is wider than that from PL. From the transmission spectrum in Figure S5 (Supporting Information), the absorption coefficient of C6H4NH2CuBr2I is estimated to ~0.6×105 cm-1 at 450 nm, which is high enough for efficient light absorption.50 Ultraviolet photoelectron spectroscopy (UPS) was used to determine the Fermi energy (EF) and the valence band energy (Ev) levels of the C6H4NH2CuBr2I compound. The band diagram of C6H4NH2CuBr2I is shown in Figure 2a. The EF was found to be -4.90 eV, obtained using the cutoff energy (Ecutoff) presented in Figure 2b from the equation EF = 21.22 eV (He I) - Ecutoff. The linear extrapolation in the low binding energy region indicates the value of (Ev - EF), leading to an Ev of -6.4 eV. The conduction band energy (Ec) can be calculated by adding the Eg of C6H4NH2CuBr2I to Ev and is thereby determined to be -4.76 eV. Hall effect measurements were performed at room temperature on a 4-probe sample holder placed between the plates of an electromagnet. The samples were measured under an excitation of 40 nA with a constant


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magnetic field of 0.55 T and the results are shown in Table S3 (Supporting Information). The

Figure 2. a) Band level diagram of the C6H4NH2CuBr2I compound. b) UPS of C6H4NH2CuBr2I absorber materials. The binding energy is calibrated with respect to He I photon energy (21.21 eV). c) The Seebeck coefficient at different temperatures for the C6H4NH2CuBr2I thin film in the parallel direction. (T is the average temperature between the hot side and cold side at the sample). d) The scheme of device for Seebeck coefficient test. electron mobility is relatively high (2.216×101cm2 V-1 S-1 for Sample #1; 1.984×101cm2 V-1 S-1 for Sample #2). The negative Hall coefficient indicates the n-type nature of the carriers. Meanwhile, the negative Seebeck coefficient shows that C6H4NH2CuBr2I compound is an n-type


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semiconductor (Figure 2c). The scheme of device for Seebeck coefficient test is shown in Figure 2d. It is worth mentioning that the C6H4NH2CuBr2I compound exhibits a high Seebeck coefficient (~56 µV/K) at room temperature, indicating that this material might be used for thermoelectric conversion. From the X-ray photoelectron spectroscopy (XPS) analysis in Figure S6a and Figure S7 (Supporting Information), the absence of a satellite peak clearly revealed that the surface of C6H4NH2CuBr2I compound changes to Cu+, which is consistent with the conclusion presented by Cortecchia et al.32 Besides, the C6H4NH2CuBr1.6Cl1.4 compound with chlorine (Cl) shows satellite peaks, indicating the presence of Cu2+ ions (Figure S6b, Supporting Information).


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Figure 3. a) Digital photograph of thin film showing the contact angle. Inset in a) shows a photograph of a water drop on the thin film surface after five minutes. b) UV/Vis absorption spectra of C6H4NH2CuBr2I thin film before and after washing by water for five minutes. c) XRD patterns of a C6H4NH2CuBr2I thin film before and after immersion in water for four hours. d) TGA curve of C6H4NH2CuBr2I material. Generally, a material is defined as hydrophobic when the contact angle (C.A.) is between 90 and 150°. As shown in Figure 3a, this copper-based hybrid material exhibits extraordinary nearhydrophobic behavior with a C.A. of ~90°, whereas the perovskite CH3NH3PbI3 will decompose immediately upon contact with water (Supporting Information, Figure S8). The inset in Figure 3a shows the C6H4NH2CuBr2I thin film after dropping water on the film surface. The film remained black even when in direct contact with water. The UV/vis absorption spectrum measurements of the C6H4NH2CuBr2I thin film were carried out before and after dropping water on the film surface for several minutes; no obvious changes were observed (Figure 3b). Most strikingly, the XRD patterns of the C6H4NH2CuBr2I thin film remained nearly the same even after four hours of immersion in water, successfully demonstrating that the C6H4NH2CuBr2I film is extremely water-stable (Figure 3c). The digital photograph of the C6H4NH2CuBr2I thin film spin-coated on FTO glass after immersion in water for four hours is shown in Figure S9 (Supporting Information). The exceptional stability of the C6H4NH2CuBr2I thin film in water might be caused by the incorporation of hydrophobic groups (C6H4NH2I is a type of hydrophobic organic molecule and slightly soluble in water). Moreover, thermo gravimetric analysis (TGA, Figure 3d) shows the beginning of weight loss at ~133 °C, which means that C6H4NH2CuBr2I-based solar cells may be widely used in actual daytime even if the device temperature exceeds 85 °C.18


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Figure 4. The scheme a) and cross section SEM image b) of the Cu-based solar cell. c) Top view SEM image of C6H4NH2CuBr2I thin film. d) J-V curves of the best performing device (reverse scan). The copper-based light absorber was then applied in printable mescoscopic solar cell based on carbon back-contact. A schematic cross section of the triple-layer, fully printable mesoscopic solar cell (Figure 4a) shows that the mesoporous layers of TiO2 and ZrO2 have thicknesses of ~1 and 2 µm, respectively. The mesoporous layers were infiltrated with C6H4NH2CuBr2I by dripping the C6H4NH2CuBr2I precursor solution through a 15-µm-thick carbon layer printed on the top of ZrO2. Figure 4b shows the scanning electron microscopy (SEM) image of the cross


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section of the copper-based solar cell. A dense, smooth, and pinhole-free C6H4NH2CuBr2I thin film is obtained from the precursor solutions through a gas pump method (Figure 4c).51 Figure 4d shows the current density-voltage (J-V) curves of the best performing device measured under simulated AM1.5 100 mW cm-2 illumination, giving a PCE of 0.46% with a photocurrent density (Jsc) of 6.20 mA cm-2, an open-circuit voltage (Voc) of 0.20 V and a fill factor (FF) of 0.46. To the best of our knowledge, this is first time to show the photovoltaic effect in this novel light absorber. As shown in Figure S10 (Supporting Information), the hysteresis effect was evaluated by performing a reverse scan (from Voc to Jsc) and a forward scan (from Jsc to Voc), and a large hysteresis has been found. Moreover, the device showed a PCE value as high as 2.0% when scanning from 10 V to -0.2 V (see Figure 4d). This large hysteresis effect is somewhat similar to the results for previously reported MAPbI3-xClx-based solar cells, and this large hysteretic behavior may be explained by different initial states of the ferroelectric domain due to the different starting points.52 In addition, this might be associated with the investigation of ferroelectric semiconductors in photovoltaic applications, which may help us to improve the PCE of PVSCs via an external electric field.53 It is noteworthy that Cu might also not be entirely nontoxic.54 For future research, it is apparent that some attention should be paid to the biological toxicity of this novel light absorber, just as the reports on the toxicity of Sn.55 There are many possible causes for the low PCE achieved in this preliminary study of this new photovoltaic material. Here, we note two key factors that may affect the photovoltaic performance. First, the lack of hole transport material (HTM) in our devices might be an important factor that could affect PCE. The excellent ambipolar charge mobility of CH3NH3PbI3 perovskites means it can function simultaneously as a light harvester and as HTM.56 However, the n-type C6H4NH2CuBr2I compound cannot transport holes as efficient as CH3NH3PbI3.


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Therefore, the use of HTM (such as p-type NiO and PEDOT:PSS; the device architectures could be

printable

mesoscopic

TiO2/Al2O3/NiO/carbon57

or

compact

TiO2/mesoporous

TiO2/C6H4NH2CuBr2I/PEDOT:PSS/Au) in mesoscopic device might be helpful for carrier separation and transport. Second, the higher trap density may be attributed to the formation of Cu+ during the film processing (as confirmed by XPS measurements in Figure S6), which causes an additional pathway for carrier recombination. As shown in Figure S6b, the Cu2+ reduction process could be suppressed by the presence of chlorine (Cl).32 In summary, a lead-free, highly stable C6H4NH2CuBr2I compound was developed as a new type of photovoltaic material. Interestingly, the C6H4NH2CuBr2I thin film exhibits hydrophobic behavior with a C.A. of approximately 90°. The XRD patterns of the C6H4NH2CuBr2I thin film remain nearly the same even after four hours of immersion in water, successfully demonstrating that the C6H4NH2CuBr2I film is extremely water-stable. Moreover, the C6H4NH2CuBr2I thin film exhibits a suitable Eg of ~1.64 eV, which can cover much of the visible spectrum. The combined features of exceptional stability, low toxicity, wide optical absorption range and high absorption coefficient (~0.6×105 cm-1 at 450 nm) highlight the potential of C6H4NH2CuBr2I solar cell as a possible low-toxic alternative for solar cells. By using HTM (such as p-type NiO and PEDOT:PSS) in mesoscopic devices as well as performing deeper studies of carrier recombination in the active materials, much higher PCE values might be possible.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11272274, 11372266, 61574121 and 51572233). The author H. W. Han appreciates the financial support of


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this work from the National Natural Science Foundation of China (91433203 and 61474049), the Ministry of Science and Technology of China (2015AA034601), and the Science and Technology Department of Hubei Province (2013BAA090). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology for performing various characterizations and measurements. We thank Dr. Yuhua Wang (School of Materials Science and Engineering, Shijiazhuang Tiedao University) for assistance in UV/Vis absorption spectra measurement and analysis.

ASSOCIATED CONTENT Supporting Information Experimental Section; table of crystallographic parameters; XRD patterns; TEM image, EDS results; Hall effect measurement results; Tauc plot and transmission spectra of samples; X-ray photoelectron spectroscopy; contact angle test of CH3NH3PbI3 thin film; photographs of C6H4NH2CuBr2I samples. AUTHOR INFORMATION Author Contributions X. L. Li, X. L. Zhong and Y. Hu contributed equally to this work. Notes The authors declare no competing financial interests. REFERENCES


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Efficient

Screen

Printed

Perovskite

Solar

Cells

Based

on

Mesoscopic

TiO2/Al2O3/NiO/carbon Architecture. Nano Energy 2015, 17, 171-179.


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