Unraveling the Role of Perovskite in Buried Interface Passivation

Interfaces in perovskite solar cells play a crucial role in their overall performance, and therefore, detailed fundamental studies are needed for a better understanding. In the case of the classical n–i–p architecture, TiO2 is one of the most used electron-selective layers and can induce chemical reactions that influence the performance of the overall device stack. The interfacial properties at the TiO2/perovskite interface are often neglected, owing to the difficulty in accessing this interface. Here, we use X-rays of variable energies to study the interface of (compact and mesoporous) TiO2/perovskite in such a n–i–p architecture. The X-ray photoelectron spectroscopy and X-ray absorption spectroscopy methods show that the defect states present in the TiO2 layer are passivated by a chemical interaction of the perovskite precursor solution during the formation of the perovskite layer and form an organic layer at the interface. Such passivation of intrinsic defects in TiO2 removes charge recombination centers and shifts the bands upward. Therefore, interface defect passivation by oxidation of Ti3+ states, the organic cation layer, and an upward band bending at the TiO2/perovskite interface explain the origin of an improved electron extraction and hole-blocking nature of TiO2 in the n–i–p perovskite solar cells.

Table S1.The binding energy of core level spectra for prominent peaks at two photon energies and the chemical nature of each peak.

Device Fabrication:
Fluorine-doped tin oxide (FTO) substrates with a sheet resistance of 9 Ω /sq were thoroughly cleaned with 2% Hellmanex water solution, deionized water, acetone and isopropanol in an ultrasonic bath for 15 mins respectively.Then the washed substrates were treated with the UV-Ozone cleaner (Oscilla) for 15 mins for each step.
A thickness of 30 nm of TiO 2 compact layer was deposited on the cleaned FTO substrates with the spray pyrolysis method at 450 o C from a precursor solution of titanium diisopropoxide bis(acetylacetonate) in anhydrous ethanol.Once the spraying is done, the substrates were kept at 450 o C for another 45 mins and cooled down at room temperature eventually.Then a mesoporous TiO 2 layer was deposited on the sprayed substrates by spin coating for 10s at 4000 rpm with an acceleration of 1000 rpm s -1 .A 30 nm particle paste (GreatCell Solar) diluted in ethanol was used as the precursor solution to achieve 150-200 nm of mesoporous TiO 2 layer.Once the spin coating finished, the substrates were heated for 10 mins at 100 o C followed by annealed at 450 o C for 30 mins under a dry air flow [1] .
The perovskite solution of (Cs 5 (MA 15 FA 85 ) 95 Pb(I 85 Br 15 ) 3 (CsMAFA) was prepared from a previously reported literature [2] .Initially PbBr 2 and PbI 2 solutions with 1.5 M nominal concentration were prepared in 4:1 (v:v) mixture of DMF:DMSO and were heated at 60 o C overnight.The Pb-based stock solutions were added to organic precursors such as MABr and FAI to obtain the final 1.24 M perovskite precursor solution.The FAPbI 3 and MAPbBr 3 solutions were mixed in at 5.7:1 (v:v) ratio.At last, a 1.5 M CsI solution in DMSO was added to the final perovskite solution with a 5:95 (v:v) ratio to obtain the desired CsMAFA perovskite solution.The perovskite solution was then spin coated on the TiO 2 -layered FTO substrates with 5s of acceleration to 4000 rpm followed by 35s steady rotation at 3500 rpm then annealed at 100 o C for 30 mins to obtain 400-500 nm thick perovskite layer [3] .The perovskite deposition was performed in the glovebox to avoid humidity.
Once the substrates were annealed they were cooled down at room temperature and Spiro-OMeTAD solution in chlorobenzene was spin coated dynamically with the 4000 rpm speed according to the previously reported literature [4,5] .

Figure S2 .
Figure S2.The surface morphology of perovskite films prepared in the upper panel and the cross-section with 0.2, 0.5, and 1.2 M concentrations of perovskite precursor.

Figure S4 .
Figure S4.Current density-voltage (J-V) characteristics of solar cells prepared with 0.2, 0.5, and 1.2 M perovskite solution (a) and the statistics of the number of devices for films prepared with each concentration (b).

Figure S5 :
Figure S5: The Pb4f 7/2 spectra measured with excitation energy of 2.0 keV for 0.2, 0.5 and 1.2 M perovskite films and the intensity is normalized to the highest intensity.

Figure S6 :
Figure S6: The N1s spectra perovskite film deposited with 0.2 M solution concentration measured with the excitation energy of 880 eV.

Figure S7 .
Figure S7.The normalized intensity of Ti2p spectra of TiO 2 , 0.2 M perovskite on TiO 2 measured using a photon energy of 880 eV (a) and for TiO 2 , 0.2 M and 0.5 M perovskite samples with 2.0 keV photon energy (b).The Ti 3+ intensity at the binding energy around 458 eV decreased after the deposition of 0.2 M perovskite.

Figure S8 .
Figure S8.Valence band maxima of TiO 2 , perovskite film, prepared with 0.2 M and 1.2 M precursor solution, measured using a photon energy of 460 eV.

Figure S9 .
Figure S9.Valence band maxima of TiO 2 , perovskite film prepared with 0.2 M and 1.2 M precursor solution measured using photon energies of 1100 eV (a) and 880 eV(b).