Stress Tests on Dye-sensitized Solar Cells with the Cs2SnI6 Defect Perovskite as Hole-transporting Material

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ScienceDirect Energy Procedia 102 (2016) 49 – 55

E-MRS Spring Meeting 2016 Symposium T - Advanced materials and characterization techniques for solar cells III, 2-6 May 2016, Lille, France

Stress tests on dye-sensitized solar cells with the Cs2SnI6 defect perovskite as hole-transporting material Andreas Kaltzogloua,*, Dorothea Pergantia,b, Maria Antoniadoua, Athanassios G. Kontosa, Polycarpos Falarasa,* a

Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, 153 10 Agia Paraskevi, Athens, Greece b School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St., 15780 Zografou, Athens, Greece

Abstract Inorganic tin perovskites appear as highly efficient materials for dye-sensitized solar cells. Despite their promising characteristics such as low cost and lack of toxicity, the stability of the corresponding devices still remains an issue to be addressed. In the current study, the chemical reactivity of the defect perovskite Cs2SnI6 at various temperatures and under illumination as well as the ageing of Cs2SnI6-based solar cells are investigated. According to X-ray powder diffraction analysis, gradual decomposition of the perovskite in ambient air only occurs at temperatures above 80 °C. Dye-sensitized solar cells were fabricated using the Z907 metal-organic complex as photosensitizer and Cs2SnI6 as hole transporter on mesoporous TiO2 substrate. The power conversion efficiency remains constant at 3.3% when the solar cell is stored at room temperature in the dark. Successive currentvoltage measurements after exposure of the device to 40 °C for up to 200 hours revealed a marked effect on the photovoltaic performance. Electrochemical impedance spectroscopy was also employed to identify the correlation between the photoelectrochemical properties and the relevant behavior of the device components upon ageing. © 2016The TheAuthors. Authors.Published Published Elsevier © 2016 by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of The European Materials Research Society (E-MRS). (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of The European Materials Research Society (E-MRS). Keywords: Thermal stability; ageing; perovskite; dye-sensitized solar cells.

* Corresponding authors. Tel.: +30-210-650-3635 (A.K.); +30-210-650-3644 (P.F.); fax: +30-210-651-1766. E-mail addresses: [email protected] (A.K.); [email protected] (P.F.)

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of The European Materials Research Society (E-MRS). doi:10.1016/j.egypro.2016.11.317

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1. Introduction Perovskite materials have become a major research topic for third generation photovoltaic devices owing to their unique structural, optical and electronic properties [1,2]. With regard to their use as hole-transporting materials (HTMs), non-toxic lead-free compounds such as CsSnI3 [ 3 ] and Cs2SnI6 [ 4 ] exhibit high power conversion efficiency (PCE) of ca. 10% and 8%, respectively, which is comparable to the PCE values of rather expensive organic HTMs, for example 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) [5]. Cs2SnI6 appears particularly promising for large-scale fabrication of dye-sensitized solar cells (DSCs) as it is also oxygen- and moisture-stable, due to the high oxidation state of tin (Sn4+). Despite the large interest in DSCs, the life time of the devices [6,7] remains an important challenge before this technology achieves large-scale commercialization, like the robust Si-based photovoltaic devices. In principal, stress in DSCs may be divided in four categories: a) electrical, b) atmospheric, c) heat and d) light [ 8 ]. The physical/chemical stability of the electrolytes in DSCs (referring here to redox couples, ionic liquids and HTMs) is the key factor that so far limits their durability. In particular, subtle changes of the microstructure on the device interfaces can alter drastically the photoelectrochemical properties. In the current work, we investigate the chemical stability of the bulk Cs2SnI6 perovskite and we also perform accelerated degradation tests on Cs2SnI6-based DSCs in order to establish its long-term efficiency as HTM.

2. Experimental 2.1. Synthesis, characterization and chemical reactivity Cs2SnI6 powder was synthesized by fusing CsI and SnI4 in a 2:1 molar ratio in evacuated silica tubes at 200 °C, according to the literature [9]. The crystal structure determination of the reaction product was performed on a Siemens D-500 X-ray powder diffractometer (XRPD), that operates with Cu KĮ1 (Ȝ = 1.5406 Å) and Cu KĮ2 (Ȝ = 1.5444 Å) radiation. Data were collected over the angular range 10° ” 2ș ” 100° with a step of 0.02° in the detector position. Small portions of the perovskite were tested for thermal stability in open Al2O3 crucibles at 50 °C for 5 days, at 80 °C for 1 and 5 days and at 150 °C for 1 day. Fresh samples were also tested separately under illumination of a Philips TLD 15W/08 lamp (350 - 390 nm radiation) at room temperature for 24 hours, and on an ATLAS sun test station (CPS+ / METEK) under 765 W m-2 at 40 °C for 72 hours. All samples were then structurally characterized with the abovementioned XRPD method, but with a shorter exposure time for each data collection compared to the as-synthesized sample (resulting to lower signal-to-background ratio). Additional stability tests against oxygen and humidity were carried out by dissolving 50 mg Cs2SnI6 in 1 mL dry DMF (Fisher, synthesis grade). Two such solutions were prepared; one was stored in an Argon-filled glove box with oxygen and humidity levels below 1 ppm, whereas the other was left in ambient air for 30 days. 2.2. Solar cells fabrication Fluorine-doped tin oxide (FTO) transparent conductive glass electrodes (7 ohms/, Pilkington) were first washed with soap (2% Hellmanex in water), deionized water and ethanol and after drying were ultrasonicated in a 1:1 v/v ethanol/acetone solution for 15 min. The electrodes were then washed with ethanol and were dried out at room temperature. For the preparation of 60 nm thick compact TiO2 layer, a precursor solution was prepared by adding 1 mL titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich) into 39 mL absolute ethanol. The solution was spin-coated on the FTO electrodes which were subsequently annealed in a furnace at 480 °C for 30 min. This procedure was repeated twice. The mesoporous nanostructured titania layer with an average thickness of 5-6 ȝm was prepared by depositing the D/SP paste (Solaronix) using the doctor-blade technique on the TiO2 compact layer/conductive glass electrodes. The modified electrodes were then gradually annealed at 125 °C for 5 min, 325 °C for 15 min and 525 °C for 30 min. Subsequently, an opaque layer of the WER4-0 (Dyesol, average nanoparticle size 400 nm) titania paste was also

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deposited. The modified electrodes were gradually annealed at 125 °C for 5 min, at 325 °C for 15 min and at 525 °C for 30 min and were subsequently post-treated at 70 °C for 60 min with an aqueous solution of TiCl4 in 40 mmol L-1 concentration. The films were cleaned with deionized water and ethanol and were left to dry in the air, prior to reannealing at 450 °C for 60 min. The anode electrodes were immersed into a 3.0 × 10-4 mol L-1 cisbis(isothiocyanato) (2,2ƍ-bipyridyl-4,4-dicarboxylato)(2ƍ-bipyridyl-4,4ƍ-dinonyl)-ruthenium(II) (known as Z907 dye from Dyesol) and 3.0 × 10-4 mol L-1 chenodeoxycholic acid solution in a 1:1 v/v mixture of acetonitrile/tert-butanol for approximately 16 hours. With respect to the use of the perovskite as hole transporter, 50 mg Cs2SnI6 were mixed with 1 mL DMF at room temperature to form an orange-brown solution. tert-Butyl pyridine (TBP) was added to this solution as dopant with a volume-to-mass ratio of 1:26 ȝL/mg TBP:Cs2SnI6. Another stock solution was prepared by dissolving 170 mg bis(trifluoromethane)sulfonimide lithium (Li-TFSI) in 1 mL acetonitrile, and then 37.5 ȝL of this solution were also introduced as additive. The perovskite solution was added drop wise to the dye-coated mesoporous TiO2, which was subsequently dried out in ambient atmosphere. A drop of the perovskite solution was also added on the Pt counter electrode (100 nm thick prepared by sputtering) and was then left to dried out. Sealed DSCs were fabricated by adjusting a hot melt 50 ȝm thick thermoplastic (Surlyn, Dyesol) between the photoanode and the counter electrode and by heating them at 110 °C. The electrolyte was injected in the sealed cell through a hole from the pre-drilled counter electrode. The active area of the DSCs was 0.64 cm2. 2.3. Solar cells photoelectrochemical measurements and stress tests The DSCs were illuminated under simulated solar light (1 sun, 1000 W m-2) from a Xenon 300 W source in combination with AM 1.5G optical filters (Oriel). Current density-voltage measurements (J-V) were recorded using linear sweep voltammetry on the Autolab PGSTAT-30 potentiostat working in a 2-electrode system at a scan rate of 20 mV sec-1. The illuminated area of the photoelectrode area was set to 0.152 cm2 by using a large black mask in front of the cell. In order to examine the stability of the fabricated DSCs, they were placed in an oven at 40 °C in the dark and they were periodically tested by J-V measurements and electrochemical impedance spectroscopy (EIS). The latter measurements were performed at open circuit potential (Voc) under 1 sun illumination using the PGSTAT30 potentiostat and its built in frequency response analyzer over the 10 mHz to 1 MHz range. The obtained spectra were fitted using the NOVA software.

3. Results and Discussion 3.1. Chemical stability of Cs2SnI6 According to the XRPD analysis, the synthesized perovskite is in high purity and crystallizes in the cubic space group Fm-3m. The compound is stable in humid air even following exposure for several months. Moreover, no alternation occurs by tempering at 50 °C for 5 days (Fig. 1). Nevertheless, Cs2SnI6 gradually decomposes when kept at 80 °C for 5 days, which is the commonly considered as maximum temperature for solar cell operation. The decomposition is evidenced by the presence of CsI (strongest peaks at 2theta = 27.6° and 48.8°) in the XRPD pattern (Fig. 1). Exposure to 150 °C for just one day caused almost complete decomposition of the perovskite into CsI and volatile SnI4, in accordance with the literature [10]. It must be pointed out that illumination of the bulk powder using either ultraviolet and visible light did not result in material degradation, as proved by XRPD analysis. Regarding the chemical stability of the perovskite in DMF, the exposure of the solution to humid air caused slow decomposition, as white CsI precipitated, whereas the solution turned gradually into a dark brown viscous gel upon stay for 30 days. On the contrary, no decomposition was observed for the perovskite solution that was stored in the inert glove box atmosphere for over 3 months.

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Fig. 1. XRPD patterns with normalized intensities for polycrystalline Cs2SnI6 before and after various thermal treatments. Theoretical diffraction patterns for CsI and Cs2SnI6 are given below as histograms.

3.2. Solar cells J-V measurements were carried out for sealed solar cells with lamellar structure FTO/compact layer TiO2/mesoporous TiO2/Dye/Perovskite/Pt. Cells exhibited consistently high current densities of ca. 13 mA cm-2 and efficiencies higher than 3% when stored in the dark. The J-V plots for a cell submitted to ageing test at 40 °C are shown in Fig. 2. Table 1 summarizes the corresponding parameters for short-circuit current density (Jsc), opencircuit voltage (Voc), fill factor (FF) and efficiency (Ș) of selected DSCs tested under 1000 W m-2 irradiation on a solar simulator (A.M. 1.5). The highest power conversion efficiency of 3.44% was recorded after 60 hours of thermal treatment. Further thermal treatment at 40 °C resulted in progressive decrease of the photocurrent density. During this ageing process and until 160 hours of exposure, the Voc value kept increasing by 130 mV, while the Jsc dropped almost inversely proportionally by about 30%. After 200 hours the photocurrent density dropped to 4.18 mA cm-2, the open circuit potential to 407 mV and the efficiency to 0.71%. No further test was performed on this particular solar cell.

a

b

Fig. 2. (a) Current-voltage curves for Cs2SnI6-based solar cells after various exposure times at 40 °C. (b) Photograph of a solar cell using Z907 as dye and Cs2SnI6 as hole-transporting material.

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Andreas Kaltzoglou et al. / Energy Procedia 102 (2016) 49 – 55 Table 1. J-V Characteristics for Cs2SnI6-based solar cells after various exposure times at 40 °C. Jsc (mA cm-2) Exposure time (h) Voc (mV) FF

Ș (%)

0

13.06

509

0.49

3.28

24

12.69

550

0.48

3.35

60

13.97

580

0.42

3.44

160

8.96

639

0.55

3.13

200

4.18

407

0.42

0.71

EIS was applied in order to obtain a better understanding of the changes in the electrical parameters of the solar cells during the thermal treatment. The devices were studied under 1 sun illumination, at dc bias equivalent to the Voc value. The Nyquist plots from the EIS results show three well distinguished semicircles which represent the charge-transfer processes across the Pt/HTM and the TiO2/dye/HTM interfaces as well as the charge transport in the HTM [11] (Fig. 3a and Table 2). The equivalent circuit is Rs(RPt/HTM QPt/HTM)(Rrec Qrec)O, where Rs is the series resistance which is determined from the intersection of the first arc at high frequency and it is attributed to the resistance of the contacts, the conductive substrate and the electrolyte, RPt/HTM is the charge-transfer resistance and QPt/HTM the capacitance described by a constant phase element at the Pt/HTM interface. Rrec and Qrec describe the recombination resistance and capacitance respectively at the TiO2-HTM interface. Finally O is an element defining the diffusion impedance (ZDif), expressed by the following equation: ZDif(Ȧ) = RDif {[coth(jȦIJ)1/2]/(jȦIJ)1/2}, with RDif = B/Yo (the diffusion resistance, RHTM) and IJ = B2 (IJ being the electron lifetime and B the reaction’s Beta coefficient) and Yo a component of the constant phase element (Fig. 3b).

a

b

Fig. 3. (a) Nyquist plots for Cs2SnI6-based solar cells after various exposure times at 40 °C. Both experimental (symbols) and fitted data are presented. (b) Equivalent electrical circuit model used for the analysis. Table 2. Charge-transfer resistance at the counter electrode/electrolyte interface (RPt/HTM), charge-transfer resistance at the photoelectrode/ electrolyte interface (Rrec) and the charge transport in the HTM (RHTM) for Cs2SnI6-based solar cells after various exposure times at 40 °C. Exposure time (h) RPt/HTM (ohm cm2) Rrec (ohm cm2) RHTM (ohm cm2) 0

2.42

4.80

1.73

24

2.31

3.98

1.47

60

2.33

4.38

1.92

160

1.70

4.76

5.88

The changes of the cell performance are depicted in the differences of the EIS spectra. For the first 60 hours of thermal treatment, the cells undergo only minor changes. The RPt/HTM, Rrec and RHTM values remain almost constant

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in line with the J-V parameters. Thus, the increase in Voc (~80 mV up to 60 hours) should not be attributed to the reduction of recombination rate, which remains constant, but to a negative shift of the semiconductor’s conduction band (Ec) energy level. On the other hand, the latter does not affect the electron injection from the excited dye to the semiconductor, therefore the Jsc values are stable. As for the impedance data at 160 h, an abrupt increase of the RHTM resistance by a factor of three is observed while the other resistances are only slightly affected. This increase is a signal of severe perovskite degradation and justifies the strong reduction of the cells photocurrent. Overall, the Cs2SnI6-based solar cells age fast at a temperature of 40 °C. Despite the fact that the bulk perovskite withstands the heat and light stresses at this temperature in ambient air, the solar cells undergo irreversible changes probably through interlayer diffusion that ultimately reduce their power conversion efficiency. A slow decomposition of the perovskite into its precursor compounds (CsI and SnI4) is also likely, and this would explain the significant increase (by a factor of three) of the RHTM value. The performance drop does not follow the linear degradation law as a function of time that was proposed for DSCs by Di Carlo et al. [12]. In a brief comparison with other types of DSCs, it is clear that solvent-based DSCs using I-/I3- redox pairs still show much higher stability, as they can pass 1000 h / 85 °C test with less than 10% performance loss [7,13].

4. Conclusions The Cs2SnI6 perovskite is an inexpensive and environmentally friendly material with high stability in air at temperatures up to 80 °C. Cs2SnI6-based solar cells are also stable at room temperature for several months without any signs of performance decline. Nevertheless, their efficiency drops fast when exposed to 40 °C. In order to render them suitable for commercial exploitation, further studies are needed to fully elucidate the mechanism of degradation, fine tune the corresponding interfaces and redesign the device for optimum long term efficiency.

Acknowledgements Financial support from FP7 European Union (Marie Curie Initial Training Network DESTINY/316494) as well as from “Advanced Materials and Devices for Energy Harvesting and Management” project within GSRT's KRIPIS action, funded by Greece and the European Regional Development Fund of the European Union under NSRF 2007– 2013 and the Regional Operational Program of Attica are acknowledged. This work has been also co-financed by “IKY fellowships of Excellence for Postgraduate Studies in Greece-Siemens Programme 2014-2015” in the framework of the Hellenic Republic-Siemens Settlement Agreement.

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