Highly Efficient Flexible Quantum Dot Solar. Cells with Improved Electron Extraction Using

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1 Supporting Information Highly Efficient Flexible Quantum Dot Solar Cells with Improved Electron Extraction Using MgZnO Nanocrystals Xiaoliang Zhang, a Pralay Kanti Santra, b Lei Tian, a Malin B. Johansson, a Håkan Rensmo, b and Erik M. J. Johansson a * a Department of Chemistry-Ångström, Physical Chemistry Uppsala University, Uppsala, Sweden. b Department of Physics and Astronomy, Molecular and Condensed Matter Physics Uppsala University, Uppsala, Sweden *Correspondence should be addressed to Erik M. J. Johansson (erik.johansson@kemi.uu.se) 1

2 Figure S1. Overview XPS spectra of films of ZnO-NCs and MZO-NCs collected using Al K alpha. MZO-NCs shows distinct Auger peaks for related Mg. Figure S2. Experimental valence band spectra of the film of (a) ZnO-NCs and (b) MZO- NCs. The thin red lines indicate where the linearly extrapolated experimental spectra intersect with the baselines. 2

3 Figure S3. Work function of the film of (a) ZnO-NCs and (b) MZO-NCs measured by Kelvin probe method. The films of ZnO-NCs and MZO-NCs were spin-coated on the ITO glass substrate for the measurement, and the vibrating Kelvin probe (KP Technology) was applied to measure the work function of the films. The samples were measured in a controlled chamber with N 2 KP Technology. The work function of the Kelvin probe was calibrated by two isopropanol cleaned highly ordered gold surface. 3

4 Figure S4. Absorption and normalized photoluminescence spectra of PbS CQDs. The high trap density of the CQD solid film served as a light absorber in the solar cell device significantly affects the charge carrier collection due to the charge recombination by these traps and therefore affects device performance. To decrease the trap density of CQD solid film, the PbS CQDs with improved passivation were synthesized according to the method reported by E. H. Sargent. 1 The PbS CQDs passivated with oleic acid were treated with methylammonium iodide (MAI) in a liquid system in a glovebox, to improves the passivation of traps in CQDs. Figure S4 shows the light absorption and normalized photoluminescence spectra of the PbS CQDs treated with MAI, which indicates that the maximum of the absorbance and photoluminescence related to the first exciton transition is at ~906 nm and ~963 nm, respectively. 4

5 Figure S5. (a) TEM and (b) high resolution TEM images of PbS CQDs. 5

6 Figure S6. (a) Simulated J-V curves of the CQD solar cell with ZnO-NCs and MZO-NCs as an ETL, respectively. Energy band diagram within the solar cell with (b) ZnO-NCs and (c) MZO-NCs as an ETL, respectively, at short-circuit condition. 6

7 Figure S7. Energy level diagram of the materials for the solar cell device. Figure S8. Normalized PCE of the flexible CQD solar cell under the bent state with different diameter. 7

8 Figure S9. Normalized PCE of the flexible CQD solar cell as function of mechanical bending cycles. The solar cell was consecutively mechanical bent on a curved surface with a diameter of 40 mm under ambient condition. Figure S10. SEM image of the sample of PET/ITO/MZO-NCs/PbS-TBAI/PbS-EDT. The sample was consecutively mechanical bent on a curved surface with a diameter of 40 mm under ambient condition. After consecutive bending 100 cycles, the cracks were formed on the CQD solid film, but the CQD solid maintains strongly adhering on the substrate and without any broke off. 8

9 Figure S11. Integrated photocurrent density from IPCE results of the solar cell with ZnO-NCs and MZO-NCs as an ETL, respectively. Figure S12. Statistics of photovoltaic parameters of the solar cell with ZnO-NCs and MZO-NCs as an ETL, respectively. 9

10 Figure S13. Steady-state efficiency and photocurrent density of the CQD solar cell fabricated on the glass substrate with MZO-NCs as an ETL at the MPP under AM1.5G 100 mw/cm 2 illumination provided by a solar simulator. The illumination was blocked with a mechanical shutter at ~3000 s to let the solar cell in dark condition. The shutter was removed after ~450 s letting the solar cell under the illumination again and the solar cell gives higher PCE comparing to that of before closing the shutter (at ~3000 s). That indicates the slightly decreased PCE with the illumination may not result from the material degradation. The device shows good photostability and light response. Figure S14. Stability of the CQD solar cells with MZO-NCs as an ETL. The unencapsulated solar cells were stored in a dark condition and under ambient conditions. 10

11 Figure S15. Mott-Schottky curves of the solar cell with ZnO-NCs and MZO-NCs as an ETL, respectively. The built-in potential is extracted from the curves. The solar cell with MZO-NCs as an ETL has higher built-in potential (electric field) that may aid the charge carrier collection. 11

12 SI-Note 1: Theoretical simulation of photovoltaic performance and energy bands within the solar cells To figure out the effect of an electron transporting material with different energy levels on the CQD solar cell performance, an optoelectronic model was built using the one dimensional program SCAPS to simulate the photovoltaic performance and energy bands within the solar cells. 23 Figure S16 shows the model figure for the simulation that the red, blue and red region is corresponding to the electron transporting layer (ZnO-NCs or MZO-NCs), PbS-TBAI layer and PbS-EDT layer, respectively, and the illumination comes from electron transporting layer (ETL) side. For the performance simulation, material s parameters of each layer, such as electric energy levels, density of states, light absorption, conductivity, as well as interfacial properties within the solar cell were all taken into account and the simulation was performed under AM1.5G 100 mw/cm 2 illumination. Table S3 displays details of SCAPS simulation used parameters, and parts 24, 25 of these parameters are taken from literatures. Figure S16. Model figure from SCAPS simulation. Red: PbS-EDT; blue: PbS-TBAI; Red; ETL. 12

13 Table S1. Summarized flexible heterojunction PbS CQD solar cells. Device structure V oc J sc FF PCE Ref. (V) (ma/cm 2 ) (%) PET/ITO/PbS-MPA/MoO 3 /Au PET/ITO/ZnO/PbS-MPA/P 3 HT/Au PET/SWCNTs/PEDOT:PSS/CQDs/PCBM/Ag PET/MoO 3 /Au/MoO 3 /PbS-MPA/ZnO/Al PET//ITO/MZO/PbS-TBAI/PbS-EDT/Au This work 13

14 Table S2. Photovoltaic performance of some typical efficient flexible organic solar cells (OSCs) and dye-sensitized solar cells (DSSCs). Device V oc (V) J sc (ma/cm 2 ) FF PCE (%) Ref. Flexible OSC Flexible DSSC

15 Table S3. Details of SCAPS simulation used parameters. Parts of these parameters are taken from literatures. PbS-TBAI PbS-EDT ZnO(MZO) Thickness (nm) Bandgap edge (ev) (3.35) Electron affinity (ev) (4.0) Permittivity (er) CB/VB DOS (cm -3 ) 1E19 1E19 1E19 Electron mobility (cm 2 /Vs) 2E-2 2E-4 5E-2 Ndonor (cm -3 ) 1E15 1E14 1E17 Nacceptor (cm -3 ) 1E15 1E16 0 EDT/TBAI defect (neutral) total density (integrated over all energies) (1/cm 2 ): 1.00E+16 Capture cross section (cm 2 ) 1.2E E-13 Position below Ec (ev) Density (cm -3 ) 1E16 1E16 TBAI-ZnO interface defects (neutral) total density (integrated over all energies) (1/cm 2 ): 1.00E+16 Capture cross section (cm 2 ) 1E-19 Position above E v (ev) 0.6 Density (cm -3 ) 1E16 15

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