Operation Mechanism of Perovskite Quantum Dot. Solar Cells Probed by Impedance Spectroscopy

Transcription

1 SUPPORTING INFORMATION Operation Mechanism of Perovskite Quantum Dot Solar Cells Probed by Impedance Spectroscopy Zahra Zolfaghari, 1,2 Ehsan Hassanabadi, 1,3 Didac Pitarch-Tena, 1 Seog Joon Yoon, 1 Zahra Shariatinia, 2 Jao van de Lagemaat, 4 Joseph M. Luther 4 and Iván Mora-Seró, 1, * 1 Institute of Advanced Materials (INAM), Universitat Jaume I, Castellón de la Plana, Castellón, 12071, Spain 2 Department of Chemistry, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran 3 Textile Engineering Department, Textile Excellence & Research Centers, Amirkabir University of Technology, Hafez Ave., Tehran , Iran 4 National Renewable Energy Laboratory, Golden, Colorado 80401, United States *Corresponding author: sero@uji.es 1

2 Experimental Section Materials Cesium carbonate (Cs 2 CO 3 ; 99.9 %, Sigma-Aldrich), PbI 2 ( %, ABCR), oleic acid (OA; 90 %, Sigma-Aldrich), oleylamine (OAM; 98 %, Sigma-Aldrich), 1-octadecene (ODE; 90 %, Sigma-Aldrich), hexane ( 95 %, Sigma-Aldrich), octane ( 99 %, Sigma-Aldrich), Methyl acetate(meoac anhydrous, 99.5 %, Sigma-Aldrich), Eethyl acetate (EtOAc anhydrous, 99.8 %,Sigma-Aldrich), Pb(NO 3 ) 2 ( %, Sigma-Aldrich), N 2,N 2,N 2,N 2,N 7,N 7,N 7,N 7 - octakis(4-methoxyphenyl)-9,9 -spirobi[9h-fluorene]-2,2,7,7 -tetramine (spiro-ometad, 99 %, Merck), cholorobenzene (anhydrous, 99.8 %, Sigma-Aldrich), 4-tert-butylpridine (4-TBP; 96 %, Sigma-Aldrich), bis (trifluoromethane)-sulfonimide lithium salt (99.95 %, Sigma-Aldrich), acetonitrile (anhydrous, 99.8 %, Sigma-Aldrich), and commercial Ti-alkoxide solution (ShareChem, SC-BT060). FTO glasses (25x25 mm, Pilkington TEC15, ~15Ω ). All materials -1 were used as recived. CsPbI 3 QDs synthesis and purification CsPbI 3 QDs were synthesized following the report by Luther and coworkers 1. Cs-oleate in ODE solution was synthesized with g of Cs 2 CO 3, 2.5 ml of oleic acid, and 40 ml of ODE in 100-ml three-neck flask under vacuum at 120 C for 1 hr under stirring. After purging the nitrogen in the flask, the flask was heated to 150 C until the reaction was completed shown clear solution. The Cs-oleate in ODE solution was cooled and used for QD synthesis. In order to synthesize CsPbI 3 QDs, 0.5 g of PbI 2 and 25 ml of ODE were degassed in the 100 ml three neck flask under vacuum at 120 C for 30 min under stirring. OA and OAM (2.5 ml of each) were added to the reaction flask and put flask under vacuum for 1 minute until the PbI 2 had completely dissolved. Then, PbI 2 solution was purged with N 2 and then heated to 170 C quickly. Once the reaction temperature was reached, 2 ml of preheated Cs-Oleate in ODE (100 C) was injected into the reaction flask. The reaction was quenched by immediate immersion of the flask into an ice-water bath (5 sec after injection). To purify QDs, 70 ml of MeOAc or EtOAc was added to as-synthesized QD solution and then centrifuged at 4700 rpm for 5 min. The supernatant was discarded, and the QDs was dispersed again in 5 ml of hexane. 8 ml of MeOAc or EtOAc was added again to QDs solution and then immediately centrifuged at 4700 rpm for 5 min. the precipitate was dispersed in 10 ml of hexane. QDs solution was kept to dark at 4 C. 2

3 Before use, the solution was centrifuged 4700 rpm for 5 min, the precipitate was removed and the QDs was dried with nitrogen and dissolved in octane at concentration of ~50 and 75 mg/ml. Device fabrication A ~30 nm compact TiO 2 layer was deposited via solution onto FTO. 80 μl of Tialkoxide solution was deposited with spin coating at 4000 rpm for 30 seconds, the spin-cast films were dried at 150 C for 10 min and then annealed at 500 C for 30 min. CsPbI 3 QDs film were fabricated according to previous report. 1 In short, saturated Pb(NO 3 ) 2 in MeOAc or EtOAc was prepared by sonicating 40 mg of Pb(NO 3 ) 2 in 40 ml of MeOAc or EtOAc for 10 min and then for removing the excess salt it was centrifuged at 4000 rpm for 5 min. Each layer of CsPbI 3 QDs was spin coated from QDs solution in octane (50 mg/ml and 75 mg/ml) at spin speed of 1000 rpm for 20 s and 2000 rpm for 5 s. Then, the film was dipped 3 times into Pb(NO 3 ) 2 in MeOAc or EtOAc solution, rinsed in neat MeOAc or EtOAc, and then immediately dried with nitrogen. Devices were prepared with different layer 1-5 of 50 mg/ml and 1-7 of 75 mg/ml solution. The spiro-ometad hole transporting material was spin coating at 4000 rmp for 30 sec from a solution of 72.3 mg of spiro-ometad, 1 ml chlorobenzene, 28.8 μl of 4-TBP, and 17.5 μl of Li-TFSI solution (520 mg/ml in acetonitrile). Au was evaporated at rate ranging from A/s for a total thickness of 70 nm. Characterizations The current- voltage were measured through Abet Technologies Sun 2000 Class A solar simulator with a Keithley 2612 Source Meter with scan rate of 10 mv/s under AM 1G. The active area of the device was defined based on a mask area of 0.11 cm -2. The light intensity was matched one sun by calibrating with a Si reference cell equipped with a IR-cut off filter. IPCE measurement was measured with employing 150 W xenon lamp coupled with a monochromator. The external quantum efficiency was measured using an optical power meter from Oriel instruments, using Si photodiode to calibrate the system. FE-SEM (JSM7001F) was utilized to carry out thickness of film. The structural of the CsPbI 3 QDs was determined by using X-ray diffractometer (Bruker AXS, D4 Endeavor advanced X-ray diffractometer, with Cu Kα radiation). Absorbance was done using Varian Cary 300 Bio spectrophotometer. PLQY was measured using a Hamamatsu integrating sphere where a quartz cuvette containing the sample 3

4 was placed. PLQY is referred to QDs which were purified two times with methyl acetate were dispersed in hexane. For PLQY measurement, we the absorbance of QDs solution was kept between As reference, measurement was performed with cuvette containing only the hexane in the same excitation wavelength. PLQY of CsPbI 3 QDs solution with absorbance of at excitation wavelength of 400 nm is 40%. PL was measured by Horiba FL-100. PLQY was measured with an integrating sphere with excitation wavelength at 400 nm. The impedance spectroscopy measurements were carried out in a PGSTAT- 30 potentiostat from Autolab. A Xe lamp was used to illuminate the PSC controlling the light intensity with neutral density filters, between 0.1 and 100 mw/cm 2 in a logarithmic scale. The AC voltage perturbation was fixed at 20 mv under open-circuit conditions. The integration time was fixed at s and the number of cycles was equal to 1. Each frequency spectrum was measured ranging between 1 MHz and 0.1 Hz. The equivalent circuit we have used to fit the data is plotted in Figure S5. Further details on Impedance characterization method can be found in reference 2. (a) (b) Figure S1. (a) TEM image of CsPbI 3 QDs and (b) their diameter distributions (9.3 ± 1.5 nm). 4

5 Figure S2. XRD pattern of CsPbI 3 QDs (blue solid line). For comparison diffractions from cubic CsPbI 3, crystallographic data ICSD , are also included (black solid lines). Figure S3. J-V curves measured under forward (dashed line) and reverse scans (straight line) of PQDSCs prepared from a QD solution of 50 mg/ml as function of the number of deposited layers. 5

6 Figure S4. J-V curve measured under forward and reverse scans, of the champion cell fabricated in this study. Figure S5. Example of the IPCE measured for the fabricated PQDSCs, from same champion cell fabricated in this study. 6

7 Figure S6. Equivalent circuit used for fitting the IS experimental data. 3 R s is the series resistance originated by wires, connections and FTO sheet. C G is the geometric capacitance of the device at high frequency. C IF is the intermediate frequency capacitance. C LF is the low frequency capacitance, associated with an accumulation capacitance of majority carries mediated by the presence on mobile ions. 4, 5 Interpretation of the physical meaning of R 1, R 2 and R 3 is not straight forward. The sum of all resistances provide the DC resistance, R DC = R 1 +R 2 +R 3 +R s, where R DC is the inverse of the slope of the current-voltage, J-V, curve. R 1 and R 2 are the responsible of the high frequency feature observed in Figure 3b, that contains information about transport resistance in the selective contacts. 3, 6 J-V curve reconstruction from impedance measurements suggest that recombination resistance, R rec, cannot be directly related with low frequency resistance, R 3, but with the sum of high and low frequency resistances, 5, 7 analogously we have considered recombination resistance as R rec= R DC -R s. Note that this expression overestimates R rec as it includes a contribution from transport resistance at the selective contacts but still a valid approach for a qualitative comparison as selective contacts used in all the analyzed samples are the same. Figure S7. (a) High frequency resistance, R HF = R 1 +R 2 and (b) Low frequency resistance, R LF = R 3 used in the determination of recombination resistance, R rec = R 1 +R 2 +R 3 in Figure 3c. The value obtained for thicker samples at high voltage is indicated. R H, R LF and R rec present relatively 7

8 similar values in good agreement with the =1/m=0.42 value obtained from the dependence of the voltage with the light intensity, Figure 3a. Figure S8. Characteristic time obtained as =C LF R rec from the data in Figure 3. In a good solar cell high R rec are observed, consequently mainly depends on C LF. C LF has been related with the device hysteresis decreasing when hysteresis decrease, 8 as in inverted solar cells. In this context could be a good parameter to describe a perovskite solar cells as it is practically independent of the applied voltage and on the illumination. 9 However, a clear determination of the physical meaning of C LF is needed in order to provide with a physical insight beyond a mere phenomenological parameter. REFERENCES (1) Sanehira, E. M.; Marshall, A. R.; Christians, J. A.; Harvey, S. P.; Ciesielski, P. N.; Wheeler, L. M.; Schulz, P.; Lin, L. Y.; Beard, M. C.; Luther, J. M. Enhanced mobility CsPbI 3 quantum dot arrays for record-efficiency, 8

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