Supporting Information Colloidal Quantum Dot Photovoltaics Enhanced by Perovskite Shelling Zhenyu Yang,, Alyf Janmohamed,, Xinzheng Lan, F. Pelayo García de Arquer, Oleksandr Voznyy, Emre Yassitepe, Gi-Hwan Kim, Zhijun Ning,, Xiwen Gong, Riccardo Comin, and Edward H. Sargent *, The Edward S. Rogers Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario M5S 3G4, Canada Indicates equal contribution School of Physical Science and Technology, ShanghaiTech University, 100 Haike Rd., Pudong New Area, Shanghai, 201210, China *Address correspondence to ted.sargent@utoronto.ca. S1
METHODS PbS CQD Synthesis and Perovskite Ligand Exchange: The CQDs used during fabrication were synthesized and purified according to previously reported method. 1. The starting concentration of CQD solution was set at ~15 mg/ml in octane. For solution ligand exchange, 5 ml of dimethylformamide (DMF) solvent containing equal amounts of MAI and PbI 2 (0.3 mol/l) were added to the vial and mixed vigorously at room temperature for about 15 minutes. After ligand exchange, CQDs weretransferred to the DMF solution phase. The octane supernatant was decanted and additional ~5 ml of octane was added and vortexed with DMF solution to remove the residual OA ligands. This solvent/anti-solvent purification process was repeated twice. Next, CQD DMF solution was transferred into 2 test tubes (~2.5 ml each). ~0.75 ml of toluene was added into each tube, resulting in a dark and non-transparent solution. The precipitate was isolated by centrifugation at 6000 rpm for 2 minutes and the supernatant was decanted. The precipitate was dried under vacuum at room temperature for 20 minutes and finally redispersed by adding dry butylamine to form a concentrated ink (350 mg/ml). Solar Cell Fabrication: The solar cells were prepared on a pre-patterned ITO substrate (2.5 cm 2.5 cm). Two layers of ZnO nanoparticles were deposited on the substrate by spin-coating at 6000 rpm. The perovskite-capped CQD film was further annealed at 70 o C for 10 min under nitrogen atmosphere. Two layers of EDT ligand exchanged CQDs were deposited on top of perovskite-capped CQD film by spin-casting following reported method. 2 For the top electrode, 100 nm Au was thermally-deposited on the PbS film to complete the device. Each ITO substrate was patterned to yield eight devices, each with an area of 7.1 mm 2. Electron Microscopy Characterization: Transmission electron microscopy/scanning transmission electron microscopy measurements (TEM/STEM) were carried out on Hitachi HF-3300 model equipped with a cold field emission gun with 300 kv operating voltage S2
PL and absorption measurement: Photoluminescence measurements were done with a Horiba Fluorolog Time Correlated Single Photon Counting system equipped with UV/VIS/NIR photomultiplier tube detectors, dual grating spectrometers, and a monochromatized xenon lamp excitation source. Optical absorption measurements were carried out in a Lambda 950500 UV-Vis-IR spectrophotometer. XRD and XPS Measurement: Powder XRD patterns were collected using a Rigaku MiniFlex 600 diffractometer equipped with a NaI scintillation counter and using monochromatized Copper Kα radiation (λ=1.5406 A ). XPS analysis was carried out using the Thermo Scientific K-Alpha XPS system with monochromated Al K α source.. J-V characterization: Current-voltage traces were acquired with a Keithley 2400 sourcemeterunit under simulated AM1.5G illumination (Sciencetech class A). The spectral mismatch was calibrated using a reference solar cell (Newport), yielding a correction multiplicative factor of M=0.848. Devices were measured under a continuous flow of nitrogen gas. The aperture was 4.9 mm 2 for solar cell measurement. EQE measurement: External-quantum-efficiency spectra were taken by measuring the photocurrent generated after subjecting the cells to monochromatic illumination (400W Xe lamp passing through a monochromator with appropriate cut-off filters, calibrated with Newport 818-UV and Newport 838-IR photodetectors). The beam was chopped at 220Hz. The response of the cell was acquired with a Lakeshore pre-amplifier connected in series to a Stanford Research 830 lock-in amplifier at short-circuit conditions (virtual-null). Capacitance-Voltage: Capacitance voltage measurements were acquired with an Agilent 4284A LCR meter at a frequency of 10 KHz and an AC signal of 50 mv, scanning from 1V to -1V. From the S3
capacitance measurement (C) the depletion width (W dep ) can be calculated as: = (Eq.1) where A is the area of the device (0.049 cm 2 ) and ε is the static permittivity of the PbS CQD solid (ε r = 20 was employed). The built-in potential and carrier density were obtained under the Mott-Schottky formulation. Diffusion-length characterization: The estimation of the diffusion length in our films is based on a previously introduced model. 3 The collection efficiency (η) is obtained at different biases as set by a Lakeshore pre-amplifier connected in series to Stanford Research 830 lock-in amplifier. By fitting the experimental data to the analytical model L diff can be estimated. S4
Figure S1. TEM images of (a) OA- and (b) MAI-capped PbS CQDs. S5
Figure S2. STEM image of MAPbI3-PbS solid. The highlighted lattice fringes spaced by 3.7 Å and 3.4 Å are correspondent to MAPbI3 {022} and PbS {111} planes, respectively. S6
Figure S3. Time-dependent short circuit current density measurement of graded structure MAPbI 3 -passivated devices under simulated AM 1.5 illumination. S7
Table S1: Device parameters of CQDs with different surface ligand treatments: annealed MAPbI 3, untreated MAPbI 3 and MAI annealed. Ligand Type MAPbI 3, annealed Untreated MAPbI 3, V oc (V) J sc (ma/cm 2 ) FF (%) PCE (%) Hysteresis (%) 0.61 21.89 63.8 8.52 6.58 0.55 17.98 52.7 5.33 10.8 MAI, annealed 4,$ 0.58 21.05 64.5 7.66 11.5 $ This process is reproduced based on the published recipe. 4 MAI-capped PbS CQD film was annealed at 70 o C under N 2 atmosphere for 10 minutes. REFERENCES 1. Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Chou, K. W.; Amassian, A.; Sargent, E. H., Hybrid passivated colloidal quantum dot solids. Nat. Nanotech. 2012, 7 577. 2. Lan, X.; Voznyy, O.; Kiani, A.; García de Arquer, F. P.; Abbas, A. S.; Kim, G.-H.; Liu, M.; Yang, Z.; Walters, G.; Xu, J.; Yuan, M.; Ning, Z.; Fan, F.; Kanjanaboos, P.; Kramer, I.; Zhitomirsky, D.; Lee, P.; Perelgut, A.; Hoogland, S.; Sargent, E. H., Passivation using molecular halides increases quantum dot solar cell performance. Adv. Mater. 2015, Submitted. 3. Kemp, K. W.; Wong, C. T. O.; Hoogland, S. H.; Sargent, E. H., Photocurrent extraction efficiency in colloidal quantum dot photovoltaics. Appl. Phys. Lett. 2013, 103 211101. 4. Ning, Z.; Dong, H.; Zhang, Q.; Voznyy, O.; Sargent, E. H., Solar Cells Based on Inks of n-type Colloidal Quantum Dots. ACS Nano 2014, 8 10321. S8