for Dye Sensitized Solar Cells.

Transcription

1 Supporting Information Copper Phenanthroline as Fast and High Performance Redox Mediator for Dye Sensitized Solar Cells. Marina Freitag *, Fabrizio Giordano #, Wenxing Yang, Meysam Pazoki, Yan Hao, Burkhard Zietz, Michael Grätzel #, Anders Hagfeldt and Gerrit Boschloo Department of Chemistry- Ångström Laboratory, Uppsala University, Box 523, SE Uppsala, Sweden # Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering École Polytechnique Fédérale de Lausanne 1015, Lausanne, Switzerland Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH 1015 Lausanne, Switzerland. Corresponding Author; marina.freitag@epfl.ch 1. Device fabrication for the best performing solar cell FTO Glass (Nippon sheet Glass 10 Ohm square) was cleaned in a 2% aqueous solution of Deconex 21 for 30 minutes. The substrates further were treated with UV/O 3 for 15 minutes. A dense TiO 2 blocking layer was applied to prevent the back reaction of the redox couple with the FTO by two consecutive 40mM aqueous TiCl 4 solution bath deposition (70 C, 30 min). The photoanode consists of screen printed mesoporous TiO 2 electrodes comprising a 3.5 µm mesoporous TiO 2 layer (30 NR-D Dyesol, particle size, 30 nm) and a 3.5 µm TiO 2 scattering layer (WER2-O Dyesol, particle size, nm). After sintering at 500 C, the substrates were treated again in 40mM aqueous TiCl 4 solution (70 C, 30min) and rinsed with deionized water. Consecutive sintering step at 500 C was completed the previous surface treatment. S1

2 The working electrode was prepared by immersing the TiO 2 film into the dye solution (0.1mM of LEG4 in acetonitrile/tert-butanol mixture plus 0.4 mm of CDCA) for 12 h. The counter electrode was electrodeposited PEDOT. The counter electrode and the working electrode were then sealed with a 25 mm thick hot-melt film (Surlyn, Dupont). Devices were completed by filling the electrolyte (0.2 M Cu(dmp) 2 TFSI, 0.05 M Cu(dmp) 2 TFSI/Cl, 0.5 M 4-tert-butylpyridine (TBP) and 0.1 M lithium bistrifluoromethanesulfonyl-imide (LiTFSI) in acetonitrile via pre-drilled holes in the counter electrodes and sealed with a Surlyn sheet and a thin glass cover. 2. Photovoltaic characterization A 450W xenon lamp (Oriel, USA) was used as a light source to study the current-voltage characteristics of DSC devices. The spectral output of the lamp was filtered using a Schott K113 Tempax sunlight filter (Präzisions Glas & Optik GmbH, Germany) to reduce the mismatch between the simulated and actual solar spectrum to less than 2%. The Keithley model 2400 digital source meter (Keithley, USA) was used for data acquisition. The photo-active area of 0.16 cm 2 was defined by a black mask of 4x4 mm 2 S2

3 Figure S 1 Current density J vs. applied potential V curves of best performing DSC sensitized with LEG4 Cu 1+/2+ (dmp) 2 1+/2+ at various light intensities and under dark conditions marina.freitag@epfl.ch 3. Cyclic voltammetry A three-electrode cell was used for the cyclic voltammetry experiments, with a supporting electrolyte of 0.1 M LiN(CF 3 SO 2 ) 2 in acetonitrile, stainless steel counter electrode (3 cm 2 ) and the electrode was Ag/AgCl. The reference electrode was calibrated by recording the cyclic voltammogram of ferrocene under same conditions as the electrolyte in acetonitrile (0.642V/SHE). The formal potential E 0 Cu(I/II)(dmp) 2 = 0.94 V vs SHE. S3

4 Figure S 2 Cyclic Voltammogram of Cu(I/II)(dmp) 2 TFSI/Cl 3. Impedance spectroscopy The measurements were performed in dark condition and in the forward bias region. The Figure 2 shows the Nyquist plot of devices incorporating [Cu(dmp) 2 ] 1+/2+ or [Co(bpy) 3 ] 2+/3+ based electrolyte at the same current density of 1.15 macm -2 corresponding to a forward bias voltage of 1.1 and 1.15 V respectively. The impedance spectrum of the devices employing the cobalt redox couple shows the three typical semi-circle for series resistances that contribute to the overall resistance. The TiO 2 /electrolyte charge transfer resistance, the diffusion of the electrolyte at low frequency (2Hz) (Figure S3 a)) and the charge transfer resistance at the counter electrode in the khz frequency range (Figure S3b)). The Nyquist plot of the copper based device on the contrary is dominated by the large diffusion component which is also responsible for the low fill factor in the solar cells. The charge transfer resistance at the counter electrode however is well defined and lower than 1 Ohm (Figure S3b). S4

5 Figure S 3 a) Nyquist plot of cobalt(blue solid square) and copper (red open circles) in dark at 1.15 macm -2 in forward bias. b) High frequency region of the Nyquist plot, counter electrode charge transfer resistance. It should be noted that the diffusion of redox species in a sensitized mesoporous layer is greatly affected by the porosity of the layer, the size of the dye molecule and its aggregates. Furthermore the additives used in the electrolyte such as the TBP may increase the viscosity thus ampering S5

6 the mobility of the redox. This difference in experimental conditions, the symmetrical cell, which where used to measure the diffusion coefficients, could be responsible for contrasting results in Table UV-Vis Spectroscopy Figure S 4 UV-Vis Spectroscopy of [Cu(dmp) 2 ] 1+/2+ complexes and electrolyte Table S1 Extinction coefficients for [Cu(dmp) 2 ] 1+/2+ complexes and electrolyte ε [M -1 cm -1 ] λ max cudmp2 = 285 nm ε [M -1 cm -1 ] λ max leg 4 = 541 nm Cu(I/II)(dmp) 2 electrolyte Cu I Cu II S6

7 UV-Vis spectra were recorded in a 1 cm path length quartz cell on a Cary 5 spectrophotometer. Figure 1 shows UV Vis absorption spectra of 5850 M -1 cm -1 for Cu 1+ complex, 2722 M -1 cm -1 for Cu 2+ complex and 4520 M -1 cm -1 for the complete electrolyte at the maximum wavelength. The used dye LEG4 has a molar extinction coefficients at λ max = 541 nm of M -1 cm -1, not only is this value considerably higher in comparison, but also is the extinction of the complexes at this wavelength is around 124 M -1 cm -1. The results indicate that the light absorption of the copper complexes is negligible. S7