Lead-free, stable, and effective double FA4GeIISbIIICl12 perovskite for photovoltaic applications

Solar Energy Materials and Solar Cells 192 (2019) 140–146

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Lead-free, stable, and effective double FA4GeIISbIIICl12 perovskite for photovoltaic applications W.B. Dai, S. Xu, J. Zhou, J. Hu, K. Huang, M. Xu

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Engineering Research Center of Environmental Materials and Membrane Technology of Hubei Province, School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, Hubei, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Lead-free perovskite Solar Cells Semiconductor Transition metals compounds Photoelectric performance

Organic-inorganic hybrid lead halide perovskite solar cells emerge as a breakthrough photovoltaic technology with high power conversion efficiency. However, some inherent shortcomings impede their further industrialization: 1) the toxicity of Pb, 2) unstable, sensitive to moisture and/or light. To circumvent these issues, we study the lead-free and solution-processed photovoltaic devices based on the double-metals < 111 > -oriented 2D layered formamidinium germanium-antimony halide perovskites (FA4GeIISbIIICl12) in this contribution. Compared with benchmark MAPbI3, the larger formamidinium is selected to replace methylammonium to form a more stable crystal structure while the double metals germanium-antimony (GeII-SbIII) are chosen to replace Pb under the considerations that expanding the possible metal combinations in the design of new perovskite with analogous photovoltaic performance. The FA4GeIISbIIICl12 perovskite behaves as a stable and efficient semiconductor with direct bandgap of ~ 1.3 eV and its conductivity is one order of magnitude higher than that of MAPbI3. Meanwhile, FA4GeIISbIIICl12 based solar cell with power conversion efficiency up to 4.7% can be achieved without use of any additives. This approach opens up new possibilities of exploiting lead-free perovskite that incorporates metals in different valence states and offers great potential application in photoelectric field.

1. Introduction Because of its long electron-hole diffusion length, high absorption coefficient as well as low defect density, the organic-inorganic hybrid halide perovskite (AMX3, A = organic molecule, M = divalent metal, X = halide) has been attracted extensive attention over the past decades with high power conversion efficiency (PCE) for application in solar cells [1–6]. A considerable part of the previously work was focused on methylammonium lead triiodide (MAPbI3, MA = CH3NH3+) as light absorbers in perovskite solar cells (PSCs) due to the board absorption range and direct band gap, in which high PCE can be obtained via optimization of cell architectures [7–9]. Recently, Formamidinium (FA, NH2CH = NH2+)-based perovskite has also attracted attention to replace the MA due to: 1) the larger organic FA can form a more symmetric crystal structure, 2) smaller bandgap allows for near infrared absorption and 3) elevated decomposition temperature and thus potential to improve stability [10,11]. Currently, one key concern for large-scale manufacturing or commercialization of MAPbI3-based photovoltaic devices is that the toxicity of Pb2+ and the unstable device set for long-term outdoor application. Taking into account these

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drawbacks, an alternative can use of similar element to replace Pb, such as group-14 elements (Ge2+ or Sn2+), alkaline-earth metals (Ca2+, Mg2+, Sr2+…), transition metals (V2+, Mn2+, Fe2+, Co2+…) and pblock elements (Ga2+, In2+) to find more stable, non-toxic and environmentally benign perovskite materials. Unfortunately, most of the above candidates have to be excluded due to their limited ability to form perovskites and/or are not well suited for photovoltaic application because of too high band gaps, toxicity, radioactivity and/or instability in divalent (2 +) state. Actually, only limited success has been achieved for the stable and efficient lead-free perovskite [12–17]. As the pool of homovalent substitution of the 2 + metals is almost exhausted for single-metal perovskite, an alternative using double metals perovskite, where two metals (i.e., in trivalent and monovalent or two divalent) could be considered to maintain overall charge balance, is a viable method and has attracted significant attention. Until now, however, only few double perovskites have been synthesized and most of these materials have indirect bandgaps and/or unsuitable bandgap energies for photovoltaic application [18–20]. Given these limitations, in this study, we preliminary explore the synthesis and characterization of stable and efficient double perovskite FA4GeIISbIIICl12 and its

Corresponding author. E-mail address: [email protected] (M. Xu).

https://doi.org/10.1016/j.solmat.2018.12.031 Received 16 October 2018; Received in revised form 30 November 2018; Accepted 18 December 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.

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potential application in solar cells. In comparison to Pb2+, Ge2+ has the same oxidation state but exhibits a lower electronegativity, a more covalent character and an ionic radius lower than Pb2+. Notably, as Ge are not easy oxidation compared with Pb2+ and Sn2+, there is the reason for us replacing Pb2+ with the less toxic and oxidizable element Ge2+. Goldschmidt tolerance factor (t) also supports the formation of Ge-based perovskites (i.e., t = 1.005, 0.988 and 0.965 for MAGeCl3, MAGeBr3 and MAGeI3, respectively) [21,22]. In fact, these materials have been applied in solar cells, albeit with low PCEs (~ 0.2%) [3]. On the other hand, Sb3+ was chosen by considering its ability to form corner-sharing octahedral SbX6 and even halide perovskites, due to the facts of 1) the Sb3+ is isoelectronic to Pb2+ featuring the same 6 s2 lone pair, 2) shows a similar electronegativity and 3) has an ionic radius lower than Pb2+ [23,24]. However, Sb3+ cannot replace Pb2+ directly due to different valence state. Actually, Sb3+-based perovskite exhibits structural diversity in terms of connectivity and dimensionality range from 0D dimer units, 1D chain like motifs, 2D layered networks to 3D frameworks [3,5,14]. As superior photovoltaic performance is associated with properties of each moiety, identifying the role of each atom is of great importance to explore new PSCs. By adopting the first-principle calculations, the FA4GeIISbIIICl12 is investigated and an attempt is made to understand the structural and electronic influences of the double metals on the photovoltaic performance. Actually, double-metals GeII-SnII, GeII-PbII and CuI-SbIII perovskites with the formulas FAGeIISnIII3, FAGeIIPbIII3 and Cs2CuISbIIII3 have been studied and computationally examined [18], however, in our hands all these compounds have either low PCEs (GeII-SnII, GeII-PbII) or inaccessible by traditional solution processed methods (CuI-SbIII). On the other hand, although the GeII-SbIII perovskite with the formula An+1BnX3n+3 (n = 3, A = FA, B = GeII-SbIII and X = Cl) [25,26] cannot form 3D structure, it can form a < 111 > oriented 2D layered perovskite and suitable for applying in Solar cells. Notably, the layered perovskite family An+1BnX3n+3 has several known members with n = 1 and 2 [27,28], however, to the best of our knowledge, n = 3 of the FA4GeIISbIIICl12 is not reported before and it is also the first study of a mixed-metal halide layered perovskite. FA4GeIISbIIICl12-based solar cell displays high photocurrent density and short-circuit photocurrents stem from the fact that the absorption band extended to ~ 950 nm as evidenced by incident photon to current efficiency (IPCE) measurements. Furthermore, FA4GeIISbIIICl12 has high photo and thermal stability and is tolerant to humidity. Based on the optimal PSC architecture using FA4GeIISbIIICl12 as the absorber layer, the PCE up to 4.7% was obtained without using any additives. The demonstrated strategies provide guidelines and prospects for developing future double-metals high-performance lead-free perovskites.

water and ethanol, respectively, and then dried by nitrogen gas. Next, the substrate was treated by ozone plasma for half hour. Titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol) diluted in ethanol (1: 9 in v/v) was sprayed on the substrate and heated at ~ 500 °C for half hour to form a blocking layer of TiO2 (70–80 nm). Then, after cooling to RT, the substrate was treated in 40 mM TiCl4 for half hour at 80 °C, followed by annealing at 500 °C for half hour. TiO2 paste diluted in ethanol (3: 8 w/w) was spin-coated onto the blocking layer to form a mesoporous TiO2 layer. The substrate was treated in 20 mM TiCl4 solution at 80 °C for half hour and annealed at 500 °C for half hour before deposition of FA4GeIISbIIICl12 perovskite. Solution was prepared by dissolving solid-state powder FA4GeIISbIIICl12 perovskite (1 M) in dimethylformamide (DMF) and filtered using 0.2 µm filter. The filtered solution was deposition on the TiO2 substrates by spin-coating at 5000 rpm for 30 s, with annealing at 80 °C for 15 min. Au electrodes were deposited on these substrates by thermal evaporation. Spiro-OMeTAD was dissolved in chlorobenzene (75 mg/mL) and spin-coated on the substrates, forming hole transporting layer. 2.2. Characterization Phase characterization was done by Powder X-ray diffraction (XRD) measurements on a Bruker AXS (D8 Advance) X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å) with a step size of 0.02° and a time per step of 1 s. Morphologies were characterized using field-emission scanning electron microscope (JEOL JSM-7600F) with an accelerating voltage of 5 kV. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis were performed on a SDT-Q600 (TA Instruments) over a temperature range from RT to 800 °C in nitrogen with a platinum pan at a heating rate of 5 °C/min. The absorbance spectra were recorded using a microspectrometer (SD1200-LS-HA, Allied Scientific Pro (ASP), Quebec, Canada) with spectral resolution of 1.3–5.0 nm FWHM, and the detection wavelength range was 300–1000 nm. The PL spectra were recorded using a microspectrometer with a 365 nm UV light as the excitation source. The electrical characteristics were measured using a Keithley 2400 SourceMeter under simulated AM 1.5 illumination (100 mW cm−2) by a Xenon-lamp based solar simulator. The EL spectra, radiance and EQE were recorded and calculated using a spectrometer (USB2000 +, Ocean Optics, Shanghai, China) and ISM-Nit software. Photo-stability studies were performed by using a 40 W Oriel 96000 (Xe) lamp-illuminator as the light source. UV stability test were carried using a 12 W UV lamp (λmax = 365 nm). 2.3. DFT modeling

2. Experimental section

First principle calculations were preformed to understand the structure of FA4GeIISbIIICl12. The calculations were realized by using the projected augmented wave plane-wave basis, implemented in the Vienna ab initio simulation package [29–31]. An energy cutoff of 500 eV is employed and the atom positions are optimized using the conjugate gradient scheme without any symmetric restrictions until the maximum force on each of them is less than 0.02 eV Å−1. The perfect FA4GeIISbIIICl12 was modeled with 4 × 6 × 4 grid for the k-point sampling. The generalized gradient approximation exchange-correlation DFT functional Perdew-Burke-Ernzerhof (PBE) with DFT-D3, which includes the dispersion interaction, was employed for the geometric optimization. The electronic-structure calculations were performed using the PBE functional based on the optimized geometries, the XRD was calculated by Reflex module implements in Material Studio.

2.1. Materials and device fabrication All reagents and solvents were used as received without any further purification. Polycrystalline FA4GeIISbIIICl12 was precipitated by mixing of 0.886 g Formamidinium chloride (HC(NH2)2Cl, 0.5 mmol) to a solution of Sb2O3 (0.342 g, 2.5 mmol) and 0.216 g Germanium dichloride (GeCl2, 1.5 mmol) in 5.0 mL of HCl (2 M). The resultants were filtered on a glass frit and dried under reduced pressure to afford 1.308 g (90.6% yields) of product. The final products for structure study were obtained by heating a solution of FA4GeIISbIIICl12 in concentrated HCl to 110 °C in a pressure vessel. The solution was then slowly cool down to room temperature (RT) over the course of one day. Single crystalline can be also obtained by slowly cooling down a solution of FA4GeIISbIIICl12 in concentrated HCl and the structure were confirmed by using single crystal X-ray diffraction (SCXRD) studies. F-doped SnO2-coated glass substrate was etched with Zn powder and HCl (2 M) for obtaining electrode pattern. Under sonication, the substrate was washed sequentially by diluted detergent, deionized

3. Results and discussion The starting point of this study for the perovskite FA4GeIISbIIICl12 is based on the prototypical of MAPbI3. Typically, the normal route for replacing Pb2+ in MAPbI3 is via homovalent substitution of group-14 141

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investigated by adopting the first-principle using density functional theory (DFT) in the local density approximation. The DOS (density of states) and pDOS (partial density of states) were calculated (Fig. 2), which reveals that the CBM (conduction band minimum) of FA4GeIISbIIICl12 is occupied mainly by the Sb orbitals and Cl p-orbital. Meanwhile, due to the bandgap of the FA4GeIISbIIICl12 is related to the effect of Ge in the VB (valence band) and the contribution of Ge orbitals to CBM can be ignored, the small direct bandgap of ~ 1.15 eV can be obtained. This bandgap value is fairly suited for using as absorber in solar cells (due to extended light harvesting in the infrared region) with the theoretical maximum efficiency of 28.6% according to the Shockley-Queisser limit [36]. On the other hand, from the pDOS calculation results, it shows that the VBM (valence band maximum) is predominantly occupied by the Ge d-orbitals and mixed with the Cl porbitals. Actually, the small direct bandgap of FA4GeIISbIIICl12 is due to the good orbital overlap of the Ge-d with the Cl and Sb orbitals, which can broaden the VBM and result in reducing the gap. Furthermore, the presence of a d9 Sb center also generates partially occupied bands at the top of the VB, which can also potentially provide additional charge carriers. Note that the calculated bandgap with DFT is smaller than their experimental value (1.15 eV vs. 1.3 eV, vide infra). We attribute this to the inherent shortage of DFT-PBE method in bandgap calculation. More accurate bandgaps can be reproduced by higher level method such as HSE06, GW calculations, where many body effects and spin orbital coupling (SOC) should be considered together [37,38]. We note that the FA4GeIISbIIICl12 has heavy DOS close to the band edge, which may increase the probability of electronic transitions and be responsible for the higher absorption coefficient. The effective masses for holes and electrons were calculated (0.18 and 0.38 me, me is the electron mass), which shows strong anisotropy and in agreement with its 2D layered structure. These calculated effective masses are comparable to the benchmark of MAPbI3 [6,39], which indicate that the FA4GeIISbIIICl12 possesses good performances on carrier mobility. In order to know more about the nature of this double perovskite FA4GeIISbIIICl12, the optical absorption and photoluminescence spectra, the variable temperature conductivity and stability studies were implemented. The absorption spectrum of the FA4GeIISbIIICl12 indicates its semiconducting nature (Fig. 3a) and the Tauc plot spectrum (Fig. S2) clearly demonstrates the direct transition, which is agreed with the result by the DFT calculations. The absorption onset occurs at ~ 950 nm and RT near-infrared PL can be observed at ~ 950 nm, thus indicating

elements or other divalent cations. However, due to the oxidation (i.e., Sn2+ to Sn4+, which renders metallic conductivity) [3,5,32,33] and/or unsuitable bandgaps (effective masses) [3,5,14], only limited success was achieved. On the other hand, heterovalent substitution is possible avenue to form double perovskite with the formula An+1BnX3n+3. Meanwhile, only few of double perovskites based on alkali and rareearth metals were studied for the application as scintillators in radiation detector [12,26], For replacing Pb and maintain the overall charge neutrality in perovskite, the double metals are better to choose ca.one 1 + and one 3 + metals. However, taking into account the photovoltaic performance and feasibility of preparation method, some other heterovalent substitutions are more attractive, such as the GeII-SbIII in the perovskite FA4GeIISbIIICl12 in this study. The imbalanced valence state replacement problem can be adjusted by changing the geometry structure (i.e, from 3D to 2D layered structure). The black color microcrystalline powder and single-crystalline (~ 1 mm) FA4GeIISbIIICl12 perovskites can be obtained via the solutionprocess method and the SCXRD measurement showed the structure belonged to the monoclinic C2/m system (Fig. S1, S-Supporting information). From the crystal structure point of view, FA4GeIISbIIICl12 can be described as the alternating < 111 > -oriented triple-layered (n = 3) perovskite, with corner sharing octahedra GeCl6 and SbCl6 and FA atoms accommodated within the voids of the channel (Fig. 1). In some extent, the structure of FA4GeIISbIIICl12 can be regarded, on one hand, as defective 3D framework due to the SbIII replaced on the GeII sites, which generated vacancies and formed layered network (2D). On the other hand, the FA4GeIISbIIICl12 can also be visualized as the finetuned structure of α-Cs3Sb2Cl9 (Fig. 1) [25], in which both the GeCl6 and SbCl6 octahedra were significantly distorted with the GeCl6 octahedra was occupied in the interval of the SbCl6 layers. The penurious research before on this kind of double perovskite is related with the structure combination, which can be inferred that the oxidation state on B site should be mixture of different B cations or fractional of B for n ≥ 2 [27]. Notably, this structure is completely different with other layered perovskites, such as the < 110 > and < 100 > -oriented perovskites, where the oxidation state of B is preserved regardless of the thickness of the inorganic layers. Actually, the representative < 110 > and < 100 > -oriented perovskites have been successfully implemented as absorbers in the solar cells with PCEs up to 11.6% and 12.7%, respectively [34,35]. The electronic structure calculations of the FA4GeIISbIIICl12 were

Fig. 1. Schematic representation of the stacking of inorganic octahedral layers (n) of the < 111 > -oriented perovskites with general formula An+1BnX3n+3. Left (n = 3): the crystal structure of the FA4GeIISbIIICl12, which can be obtained by cutting along the < 111 > -direction of the 3D parent structure. FA and Cl atoms are depicted as green and red color, Ge and Sb coordinate polyhedral are draw as yellow and gray, respectively. Right: the crystal structure of α-Cs3Sb2Cl9. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). 142

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Fig. 2. The band structure (left) and DOS-pDOS (right) for FA4GeIISbIIICl12 perovskite obtained by DFT calculation.

the FA4GeIISbIIICl12 has a band gap of ~1.3 eV and suitable for using as absorber in solar cells (Fig. 3a). The conductivity (σ) performance (Fig. 3b and c) reveals that the perovskite obey the Arrhenius behavior [40,41], which shows the linear relationship between the ln(σ) and T−1

with activation energy of ~ 1.3 eV (matches well with its optical bandgap). Meanwhile, the conductivity of FA4GeIISbIIICl12 is about one order of magnitude higher that the benchmark of MAPbI3, which means that the charge mobility of this material is possible to be adopted as

Fig. 3. a) Absorbance and photoluminescence spectra, b) variable temperature conductivity and Arrhenius plot c) of the temperature dependence of conductivity of FA4GeIISbIIICl12 perovskite, d) TG curve and DSC e) measurements for the stability of compound FA4GeIISbIIICl12. 143

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photoluminescence spectra of the film were measured as shown in Fig. S5, which shows similar phenomena as the powder FA4GeIISbIIICl12 perovskite. Cross-sectional SEM image of device architecture is shown in Fig. 4b. The corresponding energy level alignment diagram is shown in Fig. 4c. Photoemission spectroscopy was employed in air condition to measure the VB energy level of the FA4GeIISbIIICl12 perovskite and with the calculated optical bandgap value, the CB value can also be obtained. All other energy levels of ingredients made up of the PSC architecture were obtained from different studies. Actually, the possible direct contact between TiO2 and Spiro-OMeTAD can be originated from nonuniform infiltration/converge of FA4GeIISbIIICl12 perovskite upon the mesoporous scaffold as shown in Fig. 4d. The current density-voltage (J-V) characteristics of the photovoltaic device are shown in Fig. 5a. The short circuit photocurrent observed from the FA4GeIISbIIICl12 based perovskite system indicates the enhanced light harvesting in the infrared region of the solar spectrum. The value of the photocurrent is higher than that reported for Pb-Sn halides perovskites [44]. The optimal PCE was calculated to be ~ 4.7% for the system with the following measured photovoltaic parameters Jsc = 23.1 mA cm−2, Voc = 0.73 V and FF = 0.53. Furthermore, the device displayed low hysteresis when measured from either voltage sweep direction (Fig. 5b and Fig. S6) in contrast to the benchmark of MAPbI3. The infrared response for the FA4GeIISbIIICl12 based sample is supported by the IPCE spectrum as shown in Fig. 5c, which obviously indicates the onset is extended to 950 nm. The IPCE value peaks at ~ 70% for the spectrum range between 450 and 550 nm. Photocurrents obtained from integrating the IPCE values correspond well with the short circuit current density obtained from the cell. Meanwhile, variety structures (such as the different thickness of FA4GeIISbIIICl12 perovskite, different HTM by utilizing m-MTDATA) of the photovoltaic devices were also detected (Figs. S7), however, their photovoltaic performances were all with relatively low quality compared to the optimal device. Although the optimal device has excellent photocurrent (Jsc), the open circuit voltage (Voc) is low (which is ~ 2 times lower than that for the

absorber in photovoltaic device though it is not in the 3D structure but another 2D layered structure. Stability is another important issue for the perovskite materials which needs to be considered before its practical application. Putting this concern in mind we studied the thermal and photo stability of FA4GeIISbIIICl12 and also involved its tolerance toward to humidity. The TG and DSC analysis indicate that FA4GeIISbIIICl12 is stable up to ~ 235 °C and no phase varieties from − 100 to 220 °C (Fig. 3d and e). Meanwhile, exposure of the material to relative humidity (RH) of 60% showed no sign of decomposition for up to three months (Fig. S3). Moreover, the photostability of the FA4GeIISbIIICl12 were evaluated under the RH condition, which shows no sign of structure change when irradiation of the material with a simulated sun (100 mW cm−2) or under UV (360 nm) irradiation for up to half month (Fig. S4). Interestingly, this is the first demonstration of the 2D layered double perovskites from a solution processed method at relatively low temperature with good characters for applying in photovoltaic devices. Till now, there is a dearth of researches associated with the double perovskites based PSCs. However, different PSC architectures and their photovoltaic properties have been exhaustively studied previously, which were typically employed of single perovskite absorber, i.e., MAPbI3 or B-γ-CsSnI3, as the light-absorbing material [3,5,14,27]. In this context, we studied the performance of the PSCs based on the double perovskite. The photovoltaic device with the FA4GeIISbIIICl12 as the absorber and using compact and mesoporous TiO2 and SpiroOMeTAD as electron and hole-selective contacts, respectively, were prepared. Notably, it is important to have an overlayer of the perovskite on top of TiO2 to prevent the recombination of electrons injected into TiO2 with the holes in the hole transporting material. FA4GeIISbIIICl12 layers were spin coated on to ~ 200 nm thick mesoporous layers. Film fabricated with the semi-transparent solutions of FA4GeIISbIIICl12 perovskites in dimethylformamide (DMF) is shown in Fig. 4a, which shows good pore filling of FA4GeIISbIIICl12 on the TiO2 scaffold and results in efficient light harvesting. Meanwhile, the absorbance and

Fig. 4. a) SEM micrograph (top view) of the FA4GeIISbIIICl12 perovskite thin film, b) schematic diagram and cross-sectional images of device architecture, c) energy level diagram of device used in this study, d) SEM micrograph of the FA4GeIISbIIICl12 perovskite layer deposited on a mesoporous-TiO2-coated FTO substrate (top view). 144

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Fig. 5. a) J-V curve of photovoltaic device, b) J-V graphs of device measured from positive to negative direction (100 mV/s scanning rate), c) IPCE spectrum and the integrated solar photocurrent density (sums up to ~ 22.5 mA cm−2), d) relationship between the Voc (Jsc) and light intensity, which indicate the first order recombination and minimal space-charge limited effect in the system.

photocurrent value of Jsc = 23.1 mA cm−2, open circuit voltage Voc = 0.73 eV and the PCE is reached as high as 4.7%. The high short circuit photocurrent is due to the absorption was extended to the 950 nm as is evident from the IPCE measurement while the low open circuit voltage Voc is because of the first order recombination of the charge carriers in the “defect” of FA4GeIISbIIICl12 perovskite. Our results indicate that the rational design/control of the solar cell architecture and the double perovskites microstructures are important for the further development of high-performance lead-free PSCs.

conventional MAPbI3 perovskite device). To understand the limitations of the photovoltaic device, light intensity dependent measurements were performed due to the Voc dependence on the incident light intensity has been found to be a useful measure to distinguish the different recombination regimes [42,43]. The Voc is associated with the splitting of the quasi-Fermi levels of electrons and holes by the free-carrier concentrations, which conversely are determined by a balance between the photo-generation and recombination rates. As shown in the Fig. 5d, the ΔVoc vs. lnI yields a slope of 1.64 (I is the light intensity), which indicating that the recombination process is closer to the first order recombination and playing the major role in the system. Meanwhile, the nearly linear dependence of Jsc with I indicates that there are no significant energy barriers in the device and minimal space-charge limited effect. The presence of this kind of recombination reveals a trap assisted recombination mechanism where the recombination rates are dependent on the concentration of only one type of the photo-generated carriers. Considering the structure of the FA4GeIISbIIICl12, the defects in the perovskite is due to the heterovalent substitution between the SbIII and PbII. Further optimization of composition, defects and microstructural characteristics in the FA4GeIISbIIICl12 perovskite may lead to further enhancements in the performance in the resultant PSCs.

Acknowledgments The authors thank funding from the Science Research Fund of Wuhan Institute of Technology (Grant No.: 16QD28), the Hubei Natural Science Foundation (Grant No.: 18S020), the Science & Technology Pillar Program of Hubei Province (Grant No.: 2015BAA105), the Project of Technology Innovation in Hubei Province (Grant No.: 2016ACA160) and the Hubei Provincial Department of Education Science and Technology Research Program (Grant No.: Q20181503). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.12.031.

4. Conclusions

References

In summary, we report a simple solution-process method for designing a new mixed-metal halide perovskite, which incorporates two nontoxic and earth abundant metal GeII and SbIII. The obtained FA4GeIISbIIICl12 perovskite shows ultra-stable to temperature, light and moisture and has a direct bandgap with 1.3 eV and high conductivity, which is suitable for application in single-absorber solar cells. The optimal solar cell device based on FA4GeIISbIIICl12 perovskite exhibited

[1] J.Y. Sun, F.T. Rabouw, X.F. Yang, X.Y. Huang, X.P. Jing, S. Ye, Q.Y. Zhang, Facile two-step synthesis of all-inorganic perovskite CsPbX3 (X = Cl, Br and I) Zeolite-Y composite phosphors for potential backlight display application, Adv. Funct. Mater. 27 (2017) 1704371. [2] E.H. Song, S. Ding, M. Wu, S. Ye, F. Xiao, S.F. Zhou, Q.Y. Zhang, Anomalous NIR luminescence in Mn2+-doped fluoride perovskite nanocrystals, Adv. Opt. Mater. 2

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W.B. Dai et al.

(2015) 498–501. [24] B. Saparov, F. Hong, J.P. Sun, H.S. Duan, W. Meng, S. Cameron, I.G. Hill, Y. Yan, D.B. Mitzi, Thin-Film preparation and characterization of Cs3Sb2I9: a lead-free layered perovskite semiconductor, Chem. Mater. 27 (2015) 5622–5632. [25] C.W.M. Timmermans, S.O. Cholakh, G.J. Blasse, The luminescence of Cs3Bi2Cl9 and Cs3Sb2Cl9, J. Solid State Chem. 46 (1983) 222–233. [26] S. Vasala, M. Karppinen, A2B/B//O6 perovskites: a review, Prog. Solid State Chem. 43 (2015) 1–36. [27] B. Saparov, D.B. Mitzi, Organic-inorganic perovskites: structural versatility for functional materials design, Chem. Rev. 116 (2016) 4558–4596. [28] H. Wang, D.H. Kim, Perovskite-based photodetectors: materials and devices, Chem. Soc. Rev. 46 (2017) 5204–5236. [29] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1993) 558. [30] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865. [31] P.E. Perdew, K. Burke, M. Ernzerhof, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953. [32] K. Wang, Z.Q. Liang, X.Q. Wang, X.D. Cui, Lead replacement in CH3NH3PbI3 perovskites, Adv. Mater. 1 (2015) 1500089. [33] T. Baikie, Y. Fang, J.M. Kadro, M. Schreyer, F. Wei, S.G. Mhaisalkar, M. Gratzel, T.J. White, Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitized solar cell applications, J. Mater. Chem. A 1 (2013) 5628–5641. [34] I.C. Smith, E.T. Hoke, D. Solis-Ibarra, M.D. McGehee, H.I. Karunadasa, A layered hybrid perovskite solar-cell absorber with enhanced moisture stability, Angew. Chem. Int. Ed. 53 (2014) 11232–11235. [35] H. Tsai, W. Nie, J.C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, A.J. Neukirch, R. Verduzco, J.J. Crochet, S. Tretiak, L. Pedesseau, J. Even, M. Alam, G. Gupta, J. Lou, P.M. Ajayan, M.J. Bedzyk, M.G. Kanatzidis, A.D. Mohite, Highefficiency two-dimensional Ruddlesden-Popper perovskite solar cells, Nature 536 (2016) 312–316. [36] S. Ruhle, Tabulated values of the Shockley-Queisser limit for single junction solar cells, Sol. Energy 130 (2016) 139–147. [37] P. Umari, E. Mosconi, F. De Angelis, Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 perovskite for solar cell applications, Sci. Rep. 4 (2014) 4467. [38] J. Feng, B. Xiao, Crystal structures, optical properties, and effective mass tensors of CH3NH3PbX3 (X = I and Br) phases predicted from HSE06, J. Phys. Chem. Lett. 5 (2014) 1278–1282. [39] W.T. Wang, S.K. Das, Y. Tai, Fully ambient-processed perovskite film for perovskite solar cells: effect of solvent polarity on lead iodide, ACS Appl. Mater. Interfaces 9 (2017) 10743–10751. [40] S.R.L. Fernando, M.Y. Ogawa, A U-shaped Arrhenius behaviour for emission decay: temperature-dependent switching between excited-state decay paths, Chem. Commun. (1996) 637–638. [41] G.V. Buxton, S.R. Mackenzie, Non-linear Arrhenius behaviour of the rate constants of some activation-controlled reactions of the hydrated electron in the temperature range 20−200 °C, J. Chem. Soc. Faraday Trans. 88 (1992) 2833–2836. [42] W.B. Dai, Y.F. Lei, M. Xu, P. Zhao, Z.H. Zhang, J. Zhou, Rare-earth free self-activated graphene quantum dots and copper-cysteamine phosphors for enhanced white light-emitting-diodes under single excitation, Sci. Rep. 7 (2017) 12872. [43] J. Bisquert, G. Garcia-Belmonte, On voltage, photovoltage, and photocurrent in bulk heterojunction organic solar cells, J. Phys. Chem. Lett. 2 (2011) 1950–1964. [44] J.D. Fan, C. Liu, H.L. Li, C.L. Zhang, W.Z. Li, Y.H. Mai, Molecular self-assembly fabrication and carrier dynamics of stable and efficient CH3NH3Pb(1-x)SnxI3 perovskite solar cells, ChemSusChem 10 (2017) 3839–3845.

(2014) 670–678. [3] S.F. Hoefler, G. Trimmel, T. Rath, Preparation and characterization of mixed formamidinium lead iodide/methyl ammonium lead bromide layers, Mon. Chem. 148 (2017) 795–826. [4] S.A. Veldhuis, P.P. Boix, N. Yantara, M.J. Li, T.C. Sum, N. Mathews, S.G. Mhaisalkar, Perovskite materials for light-emitting diodes and lasers, Adv. Mater. 28 (2016) 6804–6834. [5] Y.X. Zhao, K. Zhu, Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655–689. [6] V.S. Chirvony, S. Gonzalez-Carrero, I. Suarez, R.E. Galian, M. Sessolo, H.J. Bolink, J.P. Martinez-Pastor, J. Perez-Prieto, Delayed luminescence in lead halide perovskite nanocrystals, J. Phys. Chem. C 121 (2017) 13381–13390. [7] Q.C. Zhou, Z.L. Bai, W.G. Lu, Y.T. Wang, B.S. Zou, H.Z. Zhong, In situ fabrication of halide perovskite nanocrystal-embedded polymer composite films with enhanced photoluminescence for display backlights, Adv. Mater. 28 (2016) 9163–9168. [8] P. Liu, X.X. He, J.H. Ren, Q. Liao, J.N. Yao, H.B. Fu, Organic-inorganic hybrid perovskite nanowire laser arrays, ACS Nano 11 (2017) 5766–5773. [9] S.W. Shi, Y.F. Li, X.Y. Li, H.Q. Wang, Advancements in all-solid-state hybrid solar cells based on organometal halide perovskites, Mater. Horiz. 2 (2015) 378–405. [10] Q.F. Han, S.H. Bae, P. Sun, Y.T. Hsieh, Y. Yang, Y.S. Rim, H.X. Zhao, Q. Chen, W.Z. Shi, Y. Yang, Single crystal formamidinium lead iodide (FAPbI3): insight into the structural, optical, and electrical properties, Adv. Mater. 28 (2016) 2253–2258. [11] S.D. Yang, W.Q. Liu, L.J. Zuo, X.Q. Zhang, T. Ye, J.H. Chen, C.Z. Li, G. Wu, H.Z. Chen, Thiocyanate assisted performance enhancement of formamidinium based planar perovskite solar cells through a single one-step solution process, J. Mater. Chem. A 4 (2016) 9430–9436. [12] Y.Q. Zhao, B. Liu, Z.L. Yu, J.M. Ma, Q. Wan, P.B. He, M.Q. Cai, Strong ferroelectric polarization of CH3NH3GeI3 with high-absorption and mobility transport anisotropy: theoretical study, J. Mater. Chem. C. 5 (2017) 5356–5364. [13] K. Yamada, K. Mikawa, T. Okuda, K.S. Knight, Static and dynamic structures of CD3ND3GeCl3 studied by TOF high resolution neutron powder diffraction and solid state NMR, J. Chem. Soc. Dalton Trans. (2002) 2112–2118. [14] W. Wang, M.O. Tade, Z.P. Shao, Research progress of perovskite materials in photocatalysis- and photovoltaics-related energy conversion and environmental treatment, Chem. Soc. Rev. 44 (2015) 5371–5408. [15] M. Konstantakou, T. Stergiopoulos, A critical review on tin halide perovskite solar cells, J. Mater. Chem. A 5 (2017) 11518–11549. [16] S.Q. Luo, W.A. Daoud, Recent progress in organic-inorganic halide perovskite solar cells: mechanisms and material design, J. Mater. Chem. A 3 (2015) 8992–9010. [17] V. Zardetto, B.L. Williams, A. Perrotta, F. Di Giacomo, M.A. Verheijen, R. Andriessen, W.M.M. Kessels, M. Creatore, Atomic layer deposition for perovskite solar cells: research status, opportunities and challenges, Sustain. Energy Fuels 1 (2017) 30–55. [18] G. Volonakis, M.R. Filip, A.A. Haghighirad, N. Sakai, B. Wenger, H.J. Snaith, F. Giustino, Lead-free halide double perovskites via heterovalent substitution of noble metals, J. Phys. Chem. Lett. 7 (2016) 1254–1259. [19] R. Nechache, C. Harnagea, S. Li, L. Cardenas, W. Huang, J. Chakrabartty, F. Rosei, Bandgap tuning of multiferroic oxide solar cells, Nat. Photonics 9 (2014) 61–67. [20] R.F. Berger, J.B. Neaton, Computational design of low-band-gap double perovskites, Phys. Rev. B: Condens. Matter Mater. Phys. 86 (2012) 165211. [21] P.P. Sun, Q.S. Li, L.N. Yang, Z.S. Li, Theoretical insights into a potential lead-free hybrid perovskite: substituting Pb2+ with Ge2+, Nanoscale 8 (2016) 1503–1512. [22] L.C. Tang, Y.C. Chang, J.Y. Huang, M.H. Lee, C.S. Chang, First principles calculations of linear and second-order responses in rhombohedrally distorted perovskite ternary halides, CsGeX3 (X = Cl, Br and I), Jpn. J. Appl. Phys. 48 (2009) 112402. [23] T.B. Rhaiem, H. Boughzala, Crystal structure of a new hybrid antimony-halidebased compound for possible non-linear optical applications, Acta Crystallogr. E 71

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