Adjustment of Conduction Band Edge of. Through TiCl 4 Treatment

Transcription

1 Supporting Information Adjustment of Conduction Band Edge of Compact TiO 2 Layer in Perovskite Solar Cells Through TiCl 4 Treatment Takurou N. Murakami, *, Tetsuhiko Miyadera, Takashi Funaki, Ludmila Cojocaru, Said Kazaoui, Masayuki Chikamatsu and Hiroshi Segawa, Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba City, , Japan Research Center for Advanced Science and Technology, The University of Tokyo, Komaba, Meguro-ku, Tokyo , Japan Graduate School of Arts and Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo , Japan * Corresponding Author Takurou N. Murakami takurou-murakami@aist.go.jp S-1

2 After heating

3 Charge-extraction method The conduction band edge (CBE) potential of the compact TiO 2 layer was evaluated using the charge-extraction method. The electrochemical cells used with this method were prepared by the following procedure. The compact TiO 2 layer prepared on a fluorine-doped SnO 2 -coated glass (FTO glass) substrate and the platinized FTO glass used as the counter electrode were sandwiched with a heat-melt polymer inserted as the spacer, and sealed by heating. After the sealing process, the electrolyte was injected between the two electrodes through a hole in the counter electrode; the hole was then sealed with the heat-melt polymer and a cover glass. During the charge-extraction measurements, a certain potential was applied to the electrochemical cell using a potentiostat; this is State 1 in Schemes S1 and S2. Electrons accumulated in the TiO 2 layer and filled the density of states (DOS) in the layer until the applied potential was at a certain level (V app. ); this is State 2 in Schemes S1 and S2. At a steady potential, the TiO 2 electrode was connected to the counter electrode under short-circuit conditions immediately after removing the external potential, in order to measure the discharge current because of the accumulated electron; this is the transition from State 2 to State 3. The electron density was calculated by integrating the current and the volume of the TiO 2 layer. The relation between the electron density and V app. was obtained. Here, V app. is the difference between the redox potential (E RO ) of the electrolyte and the Fermi level (E F ) of TiO 2, as shown in Eq. (1). Further, the number of extracted electron, Q, is given by Eq. (2). N(E) is the DOS for the electrons in TiO 2 ; E RO is the redox potential of the electrolyte; and E F is the Fermi level of TiO 2. The electron density is the Q rate for a finite volume of TiO 2. S-3

4 State 1 State 2 State 3 Scheme S1. Model of electron accumulation and extraction corresponding to charge-extraction method. Open circles and red filled circles represent vacant states and electron-filled states, respectively. Red dots in State 3 represent extracted electrons. S-4

5 Scheme S2. Time chart of applied voltage (V app. ), current (I), and electron number (Q) during charge-extraction measurements. Each line representing the states corresponds to the state number in Scheme S1.. (1) (2) Evaluation of CBE The CBE of the TiO 2 layer was determined based on the relationship between the electron density and V app.. Scheme S3 shows the model for the DOS distribution. Model A has a shallower CBE than that of Model B. The open circles and red filled circles represent the vacant states and electron-filled states, respectively. The accumulated electron densities for Model A and Model B are the same, but the V app. values of the two models are different. By comparing the V app. values for the same electron density, one can compare the levels of the CBE. Figure S2 shows the model representing the relationship between the electron density and the V app. value of TiO 2 layers with different CBEs. S-5

6 Model A Model B Scheme S3. Model of charge accumulation in DOS for TiO 2 with different CBEs. The CBE of Model A is shallower than that of Model B. Open circles and red filled circles represent vacant states and electron-filled states, respectively. V app. indicates the applied potential at a certain level. S-6

7 Figure S2. Model for relationship between electron density and V app. of TiO 2 layer. S-7

8 Scheme S4. Presumed energy diagram of perovskite solar cells (PSCs) investigated in this study. Energy levels of FTO, TiO 2 without TiCl 4 treatment, CH 3 NH 3 PbI 3-x Cl x perovskite, spiro-ometad, and Au were taken from Reference S1. CBE of TiCl 4 -treated TiO 2 layer without heat treatment and same layer with heat treatment at 450 C for 30 min were 0.20 ev and 0.09 ev higher than that of TiO 2 layer without treatment. S-8

9 Figure S3. Nyquist plots obtained from EIS measurements of electrochemical cells with TiCl 4 -treated FTO glass and untreated FTO glass. The labels Bare FTO, TiCl 4, TiCl deg, TiCl deg, and TiCl deg in the upper left panel represent untreated FTO glass substrate, TiCl 4 -treated FTO glass sample, and TiCl 4 -treated FTO glass samples subjected to subsequent heating at 150 C, 300 C, and 450 C, respectively. S-9

10 Figure S4. Equivalent circuit used for fitting Nyquist plots shown in Figure S C: Quasi-capacitance T: Coefficient of constant-phase element (CPE) p: Index parameter of CPE Scheme S5. Formula for estimating quasi-capacitance from constant-phase element. S-10

11 Figure S5. Relationship between applied bias voltage (V app ) and charge-transfer resistance for TiCl 4 -treated FTO glass sample. The equivalent circuit shown was used to fit the EIS spectra. Labels Bare FTO, TiCl 4, TiCl , TiCl , and TiCl represent the untreated FTO glass substrate, TiCl 4 -treated FTO glass sample, and TiCl 4 -treated FTO glass samples subjected to subsequent heating at 150 C, 300 C, and 450 C, respectively. S-11

12 Figure S6. Relationship between V app and series resistance for TiCl 4 -treated FTO glass sample. The equivalent circuit shown was used to fit the EIS spectra. Labels are the same as in Figure S5 S-12

13 Table S1. Impedance parameters of TiCl 4 -treated FTO glass sample. Parameters were calculated from equivalent circuit in Figure S4 used to fit the EIS spectra. Electrode labels are the same as in Figure S5 Electrode V app. / mv R s / Ω R ct / Ω CPE-T /10-6 Fs (p-1) CPE-p Quiasi-C /10-6 F Bare FTO TiCl TiCl TiCl TiCl S-13

14 Table S2. Performance of cells shown in Figure 5. Jsc Voc FF PCE No treatment FS BS TiCl 4 FS BS TiCl C FS BS TiCl C FS BS TiCl C FS BS S-14

15 A B Scheme S6. Presumed electron states in photoanode of PSCs during I-V measurements performed using (A) forward and (B) backward bias sweep. Cells on right and left correspond to cases with TiO 2 layers with shallow and deep CBEs, respectively. Open circles and red filled circles represent vacant states and electron-filled states, respectively. S-15

16 Au electrode Hole-transport layer Perovskite layer Compact TiO2 layer FTO Figure S7. Cross-sectional SEM image of a PSC. The substrate was a TiCl4-treated compact TiO2 layer on FTO glass heated at 300 C for 30 min. Reference S1. Luo, S.; Daoud, W. A.; Recent Progress in Organic Inorganic Halide Perovskite Solar Cells: Mechanisms and Material Design, J. Mater. Chem. A, 2015, 3, S-16