Advances on the Synthesis of Small Molecules. as Hole Transport Materials for Lead Halide. Perovskite Solar Cells.

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1 Supporting Information Advances on the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells. Cristina Rodríguez-Seco 1, Lydia Cabau 1, Anton Vidal-Ferran 1,2 * and Emilio Palomares 1,2 * 1 Institute of Chemical Research of Catalonia- The Barcelona Institute of Science and Technology (ICIQ-BIST). Avda. Països Catalans, 16. Tarragona. E Spain. 2 ICREA. Passeig Lluis Companys, 23. Barcelona. E Spain The Spiro-OMeTAD Characteristics as Hole Transport Material. Prior to describe the advances on the HTMs, used as selective contacts in perovskite solar cells, we would like to highlight some of the important chemical and physical parameters of spiro-ometad HTM. As illustrated in Figure 1SI, a film of 150 nm of undoped spiro-ometad (i.e, a mixture consisting of spiro-ometad M in chlorobenzene as solvent) presents a weak absorption in the visible range and strong bands in the UV region.

2 Figure 1SI. The UV-Visible absorption spectra for a 150 nm thick film deposited onto glass from a solution containing 72.3mg/mL of spiro-ometad (0.059 M), in chlorobenzene as solvent. It is important for HTMs that the molecules have outstanding solubility. For instance, spiro-ometad is often used in a concentration of 72.3 mg/ml, which allows for regular 150 nm thick films using the spin-coating technique during their preparation (2000 rpm). The electrochemical properties of spiro-ometad are often used as standard for other HTMs. The oxidation of spiro-ometad is fully reversible (Figure 2SI) and displays three waves at 0.576V/0.12V (one e - oxidation), at 0.700V/0.22V (one e - oxidation) and 0.901V/0.42V (two e - oxidation) vs the Ag/AgCl or vs Fc/Fc + reference system respectively (a cyclic voltammetry of a mixture of spiro-ometad and ferrocene has been also shown in Figure 2SI to aid comparison). It is important to notice that spiro-ometad solutions can be oxidized in air, which leads to a change in the physicochemical properties of the molecules and the thin films. When the solution is oxidized, a change from yellow to orange color is observed.

3 Figure 2SI. Cyclic voltammetry for a solution of spiro-ometad (0.1 M in CH 2 Cl 2, black curve) with 0.1 M (n-c 4 H 9 ) 4 NF 6 and the same solution with ferrocene (1 mm, in CH 2 Cl 2, red curve) with a scan rate of 100mV/s. The hole mobility is an important parameter too. What determines excellent mobility in organic thin films is, to our best understanding, not yet fully known. Often, molecules that are designed to be efficient HTMs display very low mobility when deposited as thin films, as we will show later on this Account. Our own Space Charge Limited Current (SCLC) measurements (Figure 3SI) of spiro-ometad using only-hole devices (devices with identical architecture to the solar cells, but metal contacts that only allow holes to be collected) lead to mobility values of cm 2 V -1 s -1 after fitting the experimental data to Equation 1. J =!! ε!ε! μ!!!!! e!.!"!!! Equation 1 The charge transfer from TiO 2 /dye to the spiro-ometad has been also well studied. For instance, Durrant et al. 1 measured the fast regeneration of the dye ground state by the efficient electron transfer from the spiro-ometad molecule to the oxidized dye.

4 This charge transfer reaction is alike in perovskite solar cells where the spiro- OMeTAD also is oxidized upon electron transfer to the perovskite-like material. Our group has characterized the polarons (spiro-ometad + ) that show a characteristic band in the IR region upon excitation of the mtio 2 /perovskite/spiro-ometad thin film at 535 nm with a maximum centered at λ = 1400 nm 2. Moreover, using time correlated single photon counting (TCSPC) our group has estimated that the charge transfer between the spiro-ometad and the perovskite is outstanding 3, with almost unit yield. Figure 3SI. The current vs voltage measurement for a 240 nm thick spiro-ometad film from which the hole mobility value was calculated using Space Charge Limited Current (SCLC) All these factors mentioned have resulted in the spiro-ometad being the most efficient HTM in solid state DSSC and perovskite solar cells so far. Now, we analyze different synthetic approaches that have been explored with the aim to replace spiro-ometad by a likely efficient HTM in perovskite solar cells, with the purpose of easier synthetic methodology and enhanced long term stability.

5 REFERENCES (1) Bach, U.; Tachibana, Y.; Moser, J.- E.; Haque, S. A.; Durrant, J. R.; Grättzel, M.; Klug, D. R. Charge Separation in Solid- State Dye- Sensitized Heterojunction Solar Cells. J. Am. Chem. Soc., 1999, 121, (2) Montcada, N. F.; Marin- Beloqui, J. M.; Cambarau, W.; Jimenez- Lopez, J.; Cabau, L.; Cho, K. T.; Nazeeruddin, M. K.; Palomares, E. Analysis of Photoinduced Carrier Recombination Kinetics in Flat and Mesoporous Lead Perovskite Solar Cells. ACS Energ. Lett., 2017, 2, (3) Marin- Beloqui, J. M.; Lanzetta, L.; Palomares, E. Decreasing Charge Losses in Perovskite Solar Cells Through mp- TiO2/MAPI Interface Engineering. Chem. Mat.,2016, 28,