planar heterojunction perovskite solar cells to 19%

Transcription

1 Supporting Information Carbon quantum dots/tio x electron transport layer boosts efficiency of planar heterojunction perovskite solar cells to 19% Hao Li a, Weina Shi a, Wenchao Huang b, En-ping Yao b, Junbo Han c, Zhifan Chen a, Shuangshuang Liu a, Yan Shen a, Mingkui Wang a,*, Yang Yang b, * a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan , Hubei, China b Department of Materials Science and Engineering, University of California Los Angeles, 405, Hilgard Ave, Los Angeles, CA c Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan , Hubei, China d Advanced Optoelectronic Technology Center, National Cheng Kung University, No. 1, University Rd., East District, Tainan City 70101, Taiwan. mingkui.wang@mail.hust.edu.cn (M.W.), yyang@ucla.edu (Y.Y.) Device fabrication and characterization: The patterned ITO coated glass was ultrasonically cleaned with detergent water, deionized water, acetone and ethanol continuously. The substrate was treated with UV-O 3

2 for 20 minutes. Figure S1 presents the fabrication process of TiO 2 and perovskite layers on the UV-O 3 -treated ITO substrate. In brief, a 50 nm-thick compact TiO 2 or CQDs/TiO 2 layer was deposition onto the ITO substrate with 3000 rpm twice followed by 150 o C annealing 30 min. After cooling to room temperature, CH 3 NH 3 PbI 3 Cl 3-x film was prepared via a modified two-step solution process 1, where 450 mg ml -1 PbI 2 (dissolved in dimethyl formamide) was spin-coated on top of ETL at 2,500 rpm for 30 s and 50 mg ml -1 CH 3 NH 3 I (with CH 3 NH 3 Cl of 10:1 in weight ratio comparing to CH 3 NH 3 I dissolved in 2-propanol) was spin coated on top of the dried PbI 2 layer at room temperature at 3,000 rpm for 30 s. The films were annealed in air at 135 o C for 3-5 min. A HTL was coated on the perovskite film at 3000 rpm for 30 s. The HTL solution contains 72.3 mg spiro-omatd, 28.8 µl tbp, 17.5 µl Li-TFSI/acetonitrile (520 mg ml -1 ) per 1 ml chlorobenzene. Finally, a 80 nm gold counter electrode was deposited by thermal evaporation. The thickness of each layer was 50 nm for TiO 2 or CQDs/TiO 2 ETL, 350 nm for perovskite layer, 200 nm for spiro-omatd HTL, and 80 nm for gold electrode. X-ray photoelectron spectroscopy (XPS) used to study the elemental composition of the CQDs/TiO 2 and pure TiO 2. Ultraviolet photoelectron spectroscopy (UPS) measurement was carried out in a Kratos AXIS Ultra-DLD ultra-high-vacuum photoemission spectroscopy system with an Al Kα radiation source to define the change of energy levels. The crystalline morphology for perovskite films was characterized with a field-emission scanning elect` (FE-SEM). The crystal structures of the perovskite films were measured by X-ray diffraction (XRD, Shimadzu XRD-6100 diffractometer with Cu K a radiation). Tapping mode atomic force microscopy (AFM) was performed using a Veeco multimode instrument to identify surface roughness changes of TiO 2 before or

3 after the adding of CQDs. Energy dispersive spectrometer (EDS) images were obtained using FEI Nova NanoSEM 450. Time-resolved luminescence decays were recorded with time-correlated single photo counting system (PicoHarp 300, PicoQuant GmbH). The excitation light source was Ti: Sapphire laser (Mira 900, Coherent; 76 MHz, 130 fs). Steady-state photoluminescence (PL) and time-resolved PL decay of the MAPbI3-xClx film on the CQDs/TiO 2 substrates were conducted to investigate whether the photo-generated electrons have efficiently injected from perovskite film to ETL or not. A laser beam with an excitation wavelength of 450 nm (20 mw) was used to excite the perovskite layer from the air side. For the nanosecond transient absorption spectroscopy, about 50 µj of pulse energy as the fundamental output from a Ti: Sapphire femtosecond regenerative amplifier (800 nm, 35 fs FWHM, 1 khz, Newport Spectra-Physics) was used to generate pump and probe beams. By introducing the fundamental beams into an optical parametric amplifier (Light Conversion Ltd), we could select a certain wavelength from the tunable output as the pump pulses, whereas light continuum probe pulses were obtained by focusing the fundamental beams onto a sapphire plate (contained in LP920, Edinburgh Instruments). The transmitted probe light from the samples was collected and focused on the broadband VIS-NIR detector for recording the time-resolved excitation induced difference spectrum ( OD). A xenon light source solar simulator (450 W, Oriel, model 9119) with an AM 1.5G filter (Oriel, model 91192) was used to give an irradiance of 100 mw cm 2 at the surface of the solar cells. The photocurrent-voltage (J-V) characteristics of the

4 MAPbI 3-x Cl x -based solar cells were measured by recording the current through Keithley 2400 digital source meter. A xenon light source solar simulator (450W, Oriel, model 9119) with AM 1.5G filter (Oriel, model 91192) was used to give an irradiance of 100 mw cm -2 at the surface of solar cells. The devices were tested using a metal mask with an area of cm -2. A similar data-acquisition system was used to control the incident photon conversion efficiency (IPCE) measurements. A white-light bias (10 % sunlight intensity) was applied onto the sample during the IPCE measurements with the alternating current (AC) model (130 Hz). The electronic impedance measurements were performed using the PGSTAT302N frequency analyzer from Autolab (The Netherlands) together with the Frequency Response Analyzer to give voltage modulation under the giving range of frequency. The electronic impedance spectra (IS) of the MAPbI 3 -based devices were recorded at potentials varying from -1.0 V to 0 V at frequencies ranging from 0.01 Hz to 1 MHz, the oscillation potential amplitudes being adjusted to 10 mv. The Z-view software (v2.8b) was use to analyze the impedance data. Synthesis of CH 3 NH 3 I (MAI) and CH 3 NH 3 Cl (MACl): In brief (CH 3 NH 3 I) MAI was synthesized by adding 15 ml methylamine (40% in methanol, Aladdin) and ml hydroiodic acid (57% in water, Aldrich) into a beaker at 0 o C with stirring for 2 hours. To precipitate CH 3 NH 3 I, the following step was used to remove solvents by rotary evaporation and the products were washed several times with diethyl ether. White crystals were obtained after drying in vacuum for 3 days. MACl was

5 synthesized using the same method, only changed the hydroiodic acid to hydrochloric acid. Results of steady-state PL Figure S9 shows the normalized steady-state PL spectra of TiO 2 /MAPbI 3-x Cl x and (C 10 /T)/MAPbI 3-x Cl x deposited on ITO glass. The MAPbI 3-x Cl x film deposited on C 10 /T shows a stronger PL quenching (50%) evidenced by the intensity of the photoluminescence peak at 767 nm as compared to the case of TiO 2 film, proving that the addition of CQDs has efficiently enhanced the carrier extraction process. Spreading TiO 2 nano-crystalline solution 150 o C Annealing TiO 2 nano-crystalline solution Spreading PbI 2 solution 135 o C Annealing MAPbI 3 Cl 3-x film in the air Spreading MAI/MACl solution 70 o C Annealing PbI 2 film Figure S1. A diagrammatic presentation of the two-step preparation procedure of the perovskite solar cells.

6 Intensity(a.u.) TiO 2 C 10 /T C 50 /T CQD 3.42 ev ev Binding Energy(eV) Figure S2. UPS spectra describing the cut-off energy (E cut-off ) and Fermi edge (EF, edge) for TiO 2, C 10 /T, C 50 /T, CQDs, respectively.

7 a) b) c) c) Figure S3. EDS mapping images of C 10 /T substance, a) C, b) Ti and c) O;

8 Figure S4. AFM images (size: 2 2 µm) of different contents of CQDs added TiO 2 /ITO substance, (a) the TiO 2, (b) the C 0.1 /T, (c) the C 1 /T, (d) the C 10 /T, (e) the C 25 /T, and (f) the C 50 /T. R a is the root-mean-square roughness values can be estimated from Nano-Scope Analysis software.

9 Figure S5. AFM images (size: 2 2 µm) of perovskite film on different contents of CQDs added TiO 2 /ITO substance, (a) the TiO 2, (b) the C 0.1 /T, (c) the C 1 /T, (d) the C 10 /T, (e) the C 25 /T, and (f) the C 50 /T.

10 Figure S6. a) UV vis absorption spectra of the perovskite (MAPbI 3 Cl 3-x )/different content of CQDs modified TiOx deposited on ITO, b) XRD measurements of perovskite film based on TiOx and CQDs/TiOx ETL, c) and d) shows the SEM images of perovskite film based on TiOx and CQDs/TiOx ETL, respectively.

11 Intensity (a.u.) TiO 2 C 10 /T CQD only Binding energy (ev) Figure S7. XPS survey spectra of the samples.

12 1 Pure TiO CQD/TiO 2 J(A) E-3 A u A -V+ C Q D /TiO 2 G lass 1E V appl -V bi -V s (V) A u Figure S8. Current-voltage curve for an ETL-only device. The inset shows the device structure of electron-only devices.

13 IPCE(%) TiO 2 C 0.1 /T C 1.0 /T C 10 /T C 25 /T C 50 /T Wavelength (nm) Figure S9. IPCE spectra for devices made of different ratios of CQDs/TiOx ETL.

14 410nm 450nm 500nm Normalized intensity Wavelength(nm) Wavelength(nm) Wavelength(nm) 550nm 600nm 650nm Normalized intensity Wavelength(nm) Wavelength(nm) Wavelength(nm) Figure S10. Down-converted photoluminescence spectra of CQDs. The spike comes from the excitation laser, rang from nm.

15 0.020 CQDs A(%) Wavelength(nm) Figure S11. UV-visible absorption spectra of the pure CQDs (0.53 mg/ml) deposited on quartz glass.

16 Normalized PL intensity a) b) t (ns) TiO 2 /perovskite (C 0.1 (C 1 (C 10 (C 25 (C 50 PL intensity (a.u.) Wavelength(nm) TiO 2 /perovskite (C 0.1 (C 1 (C 10 (C 25 (C 50 Figure S12. Photoelectronic properties of perovskite (MAPbI 3 Cl 3-x ) films with different ratios of CQDs/TiOx ETL. PL spectra of perovskite/etl films excited by a 500 nm light source from the air side. a) Time-resolved PL decay transients measured at 770 nm after excitation at 500 nm; b) Steady-state PL.

17 e Solar light ITO Perovskite CQD TiO 2 UV light Visible light Near-infrared light Figure S13. Schematic working mechanism for the CQDs/TiO 2 composite ETL substance under UV, visible and NIR light irradiation, the down-converted character and a super-fast electron funnel brought by the adding of CQDs.

18 Reference: 1. Xu, X.; Chen, Q.; Hong, Z.; Zhou, H.; Liu, Z.; Chang, W.; Sun, P.; Chen, H.; Marco, N.; Wang, M.; Yang, Y. Nano Lett. 2015, 15,