Solar Cells with a Porous TiO 2 Blocking Layer. under Reverse Bias

Transcription

1 Supporting Information Strong Photocurrent Amplification in Perovskite Solar Cells with a Porous TiO 2 Blocking Layer under Reverse Bias Thomas Moehl a,*, Jeong Hyeok Im a,b, Yong Hui Lee a, Konrad Domanski a, Fabrizio Giordano a, Shaik M. Zakeeruddin a, Ibrahim M. Dar, a Leo-Philipp Heiniger a, Mohammad Khaja Nazeeruddin a, Nam- Gyu Park b and Michael Grätzel a a Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École polytechnique fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland b School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University (SKKU), Suwon , Korea *Corresponding author: Thomas.moehl@epfl.ch Experimental section Spin coated TiO 2 blocking layer (devices Spin_1 and Spin_2 as well as Spin_Rutile_1 and Spin_Rutile_2): Pre-etched FTO glasses (Nippon Sheet Glass, NSG 10Ω) were cleaned in an ultrasonic bath containing ethanol for 30 min and were then treated in UV/Ozone cleaner for 30 min. TiO 2 blocking layer was spin-coated on FTO glasses at 2000 rpm for 20 s using 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt.% in isopropanol, Aldrich) in 1-butanol (99.8%, Aldrich) solution. Afterwards, the coated FTO was heated at 125 o C for 5 min. For the SEMs and the electrochemical measurements the BL coated FTO was further heated to 550 C for 30 min - similarly to the subsequent heating step which is performed for the preparation of a complete photoanode after the deposition of the mesoporous TiO 2 particle layer (see solar cell fabrication B). Spray pyrolized TiO 2 blocking layer (devices Spray_1, Spray_2): Chemically etched FTO glass (NSG 10Ω, Nippon Sheet Glass) was sequentially cleaned with detergent solution, deionized water, acetone and ethanol under ultrasonication for 30 min. A 30 nm-thick TiO 2 blocking layer was deposited with diluted titanium diisopropoxide bis(acetylacetonate) solution (Sigma-Aldrich) in ethanol by spray pyrolysis at 450.

2 Solar cell fabrication A:(spin coated and spray pyrolized BL with Dyesol 18 NRT paste, devices Spin_1, Spin_2, Spray_1 and Spray_2): 350 nm-thick mesoporous TiO 2 layers with Dyesol 18-NRT paste (particle size: 20 nm; diluted with ethanol, 1:3.5 weight ratio) was made by spin coating at 5,000 rpm for 30 sec and heating at 500 for 30 min to burn organic components. The mesoporous TiO 2 film was immersed in 0.02 M aqueous TiCl 4 (>98%, Aldrich) solution at 70 o C for 30 min. After washing with DI water and drying, the film was heated at 500 o C for 30 min. Mesoporous film thickness is about 300 to 400 nm. For the deposition of methylammonium lead iodide, 1.0 M of lead iodide solution in N,N-dimethylformamide kept at 70 was firstly spin coated at 6,500 rpm for 30 sec on mp-tio2 electrode and then dried at 70 for 15 min. After cooling to room temperature, the film was immersed in a solution of methylammonium iodide in isopropanol (8 mg ml -1 ) for 20 sec, shortly rinsed with isopropanol and dried again at 70 for 15 min. The HTM solution was prepared by dissolving 72.3 mg of (2,2`,7,7`-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene) (spiro- MeOTAD), 28.8 ul of 4-tert-butylpyridine (TBP, Aldrich), 17.5 ul of a stock solution of 520 mg/ml of lithium bis(trifluoromethylsulphonyl)imide in acetonitrile and 29 ul of a stock solution of 300 mg/ml of tris(2-(1h-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(iii) bis(trifluoromethylsulphonyl)imide in acetonitrile in 1 ml of chlorobenzene. This solution was spin-coated at 4,000 rpm for 30 sec before deposition of 60 nm-thick gold counter electrodes by evaporation method. Solar cell fabrication B:(spin coated BL with rutile particle, devices Spin_rutile_1, Spin_Rutile_2 and the device without any kind of underlayer): After cooling down to room temperature from the last heating step of the blocking layer fabrication, the TiO 2 paste (ca. 40 nm rutile TiO 2 particles made after the recipe in by Lee et al. 11 ) was spin coated on the BL layer at 2000 rpm for 10 s, where the pristine paste was diluted in ethanol (0.1 g/ml). After drying at 100 o C for 5 min, the film was annealed at 550 o C for 30 min, which led to thickness of about 100 nm. The mesoporous TiO 2 film was immersed in 0.02 M aqueous TiCl 4 (>98%, Aldrich) solution at 70 o C for 30 min. After washing with DI water and drying, the film was heated at 500 o C for 30 min. CH 3 NH 3 PbI 3 was formed using two-step spin coating procedure. 1 M PbI 2 solution was prepared by dissolving 462 mg PbI 2 (99%, Aldrich) was dissolved in 1 ml N,N-dimethylformamide (DMF) (99.8%, Sigma-Aldrich) under stirring at 70 o C. 20 µl of PbI 2 solution was spin-coated on the mesoporous TiO 2 film at 3000 rpm for 5 s and 6000 rpm for 5 s (without loading time). After spinning, the film was dried at 100 o C for 10 min. After cooling down to room temperature, 200 µl of M (7 mg/ml) CH 3 NH 3 I solution in 2-propanol was loaded on the PbI 2 -coated substrate for 20 s (loading time), which was spun at 4000 rpm for 20 s and dried at 100 o C for 5 min. 20 µl of spiro-meotad solution was spin-coated on the CH 3 NH 3 PbI 3 perovskite layer at 4000 rpm for 30 s. A spiro-meotad solution was prepared by dissolving 72.3 mg of spiro-meotad in 1 ml of chlorobenzene, to which 28.8 µl of 4-tert-butyl pyridine and 17.5 µl of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg LI-TSFI in 1 ml acetonitrile

3 (Sigma-Aldrich, 99.8%)) were added. Finally, 80 nm of gold was thermally evaporated on the spiro- MeOTAD coated film. The devices without any BL or underlayer were made similarly to the fabrication described in this section without the deposition of the BL described in the first paragraph in the experimental part. SEM: Film morphology was investigated by using a high-resolution scanning electron microscope (Merlin, Zeiss) equipped with a GEMINI II column and a Schottky Field Emission gun. Images were acquired with an In-Lens Secondary Electron Detector. Cyclic voltammetry: CV measurements were carried out in a three electrode setup with a BioLogic SP300 potentiostat under oxygen free atmosphere due to Argon bubbling. The aqueous supporting electrolyte contained 0.5M KCl (ph=2.5) and the one electron redox couple K 4 [Fe(II)(CN) 6 ]/ K 3 [Fe(III)(CN) 6 ] in a concentration of 5 mm yielding a yellowish colored solution. The reference electrode used was an Ag/AgCl (sat.), the counter electrode was a Pt wire and the scan velocity of the measurements was 50 mv/s. The measurements were performed in O 2 free atmosphere. Electrochemical Impedance Spectroscopy: Impedance measurements were performed with a BioLogic SP300 potentiostat in the similar setup as described in the cyclic voltammetry section. The frequency range applied was 50 khz to 0.1 Hz. The sinusoidal potential perturbation was 20 mv. Retention time before the actual impedance measurement started at the specific bias potential was 30s. The impedance spectra were analyzed on the basis of Randles circuit with the Zview software (Scribner). JV characterization: The solar cells were measured using a 450 W xenon light source (Oriel) with an irradiance of 100 mw/cm 2. A Schott K113 Tempax filter (Präzisions Glas & Optik GmbH) was used to reduce the spectral mismatch between AM1.5G and the simulated illumination to 4% between nm. JV characteristics of the devices were obtained by applying an external voltage bias (from forward to reverse) while measuring the current response with a source meter (Keithley 2400). The voltage step and equilibration times were 10 mv and 500 ms, respectively. The cells were covered with a thin mask (0.16 cm 2 ) to reduce scattered light. The JV measurements presented in Fig. 4 were recorded with a Bio-Logic SP300 potentiostat in 50 mv steps with a retention time of 60 s at each bias potential before the actual current measurement started. Therefore these voltammograms for the determination of the-x axis intercept of the reverse current are free of hysteresis. For the measurements under light a white light LED array was used. The mask used (0.35 cm 2 ) had an area very similar to the whole active area of the PA to maintain a similar area for the dark and photocurrent measurements. The measurements in Fig. 5 were performed with a Bio-Logic SP300 potentiostat at 1V and the white light LED array.

4 a)

5 b) Figure SI1. SEMs of a spray pyrolized BL: a) and b) show cross sections of the compact TiO 2 on FTO

6 a) b) c)

7 d) e) f)

8 g) Figure SI2. SEMs of spin coated BL a) Cross section and b) to g) top view with different magnifications Table SI1. Oxidation and reduction peak current and the peak to peak separation from the CV measurements sample Oxidation peak current ( ma/cm 2 ) Reduction peak current ( ma/cm 2 ) Peak to Peak separation (mv) FTO Sprayed BL Spin Coated BL PA with 18 NRT PA with rutile

9 Rserie R2 Wo2 CPE2 Figure SI3. Nyquist plots of the FTO electrode (black), of the FTO with spin coated BL (blue), of the FTO with spray pyrolised BL (red), the PA with 18 NRT particles (green) and the PA with rutile particles (magenta) at 288 mv vs Ag/AgCl (sat.). Inset shows the Randles circuit used for the fitting procedure.

10 a) b) c) Figure SI4. a) Modulus of the DC current during the EIS measurement (bare FTO substrate (black), FTO with a spin coated (blue) and FTO with spray coated (red) BL on top. Green represents a complete photoanode with 18 NRT particles and magenta the PA with rutile particles as mesoporous scaffold (see text)). b) Charge transfer resistance determined from the impedance measurements. c) Capacitance determined from the impedance measurement. All measurements are performed in 0.5 M KCl (ph=2.5) solution with 5 mm [Fe(CN) 6 ] 3- / [Fe(CN) 6 ] 4- as the probing redox system (Potential vs. Ag/AgCl (sat.)).

11 Figure SI5. Dark (black) and photocurrent (blue) of a device without any kind of BL with the rutile particles. Inset shows the logarithm of the dark and photocurrent. Measurement was performed with white light LEDs and a scan velocity of 10mV/s.