Interface Energy Alignment between Lead Halide Perovskite Single Crystals and TIPS-Pentacene

At present, there is a huge development in optoelectronic applications using lead halide perovskites. Considering that device performance is largely governed by the transport of charges across interfaces and, therefore, the interfacial electronic structure, fundamental investigations of perovskite interfaces are highly necessary. In this study, we use high-resolution soft X-ray photoelectron spectroscopy based on synchrotron radiation to explore the interfacial energetics for the molecular layer of TIPS-pentacene and lead halide perovskite single crystals. We perform ultrahigh vacuum studies on multiple thicknesses of an in situ formed interface of TIPS-pentacene with four different in situ cleaved perovskite single crystals (MAPbI3, MAPbBr3, FAPbBr3, and CsxFA1–xPbBryI3–y). Our findings reveal a substantial shift of the TIPS-pentacene energy levels toward higher binding energies with increasing thickness, while the perovskite energy levels remain largely unaffected regardless of their composition. These shifts can be interpreted as band bending in the TIPS-pentacene, and such effects should be considered when assessing the energy alignment at perovskite/organic transport material interfaces. Furthermore, we were able to follow a reorganization on the MAPbI3 surface with the transformation of the surface C 1s into bulk C 1s.

1-The top layer of TIPS-Pen (t) is uniform and has a constant thickness (d) after each evaporation.2-The bottom perovskite layer (b) has a thickness much greater than the probing depth of the experiment and can therefore be modelled as a layer of infinite thickness.3-The element of which the core level is measured is uniformly distributed within one layer.4-A single inelastic mean free path () can be used for one kinetic energy.In our case,  was calculated for TIPS-Pen (details in experimental section).This treatment neglects that the mean free path might be different in the perovskite layer.However, as it is the layer thickness of the TIPS-Pen we are estimating, this effect should be small.5-A scaling factor "Aexp" can be used to describe the experimental parameters which influence the intensity such as the experiment geometry, the X-ray intensity, and the detection efficiency of the spectrometer at a given kinetic energy.6-The X-ray intensity and measurement geometry are the same for all measurements, where intensities are compared, therefore the scaling factor Aexp is the same for different measurements of the same core level.
The model is illustrated in Figure S1 with different thicknesses of the top layer t (TIPS-pen).Based on the above model, we write the following equations describing the intensity of a specific core level (j) for an element (i) at a specific photon energy for the two-layer system, where the top layer varies in thickness.The following additional parameters are used in the equations: !,# = Average atomic concentration of element "i" in the bottom layer "b". !,$ = Average atomic concentration of element "i" in the top layer "t". % = Photoelectron cross section for core level "j".λ = Inelastic mean free path as described above.Aexp = Experimental scaling factor as described above.
Intensity of core level "j" from element "i" in the bottom layer "b" when d = 0 (Figure S1 left) Intensity of core level "j" from element "i" in the bottom layer "b" when d ¹ 0 (Figure S1 centre) Intensity of core level "j" from element "i" in the top layer "t" when d ¹ 0 and d ¹ ∞ (Figure S1 centre) Intensity of core level "j" from element "i" in the top layer "t" when d = ∞ (as d >> λ, Figure S1 right) Using the above-described equations we can estimate the TIPS-Pen thickness by three different ways: 1) Using the decrease in Pb 4f and I 4d core levels intensity.
Ib,0 is determined for the measurement prior to evaporation and this calculation assumes that the X-ray intensity is the same for all measurements.
2) Considering that the last TIPS-Pen evaporation has an infinite layer thickness Results are summarized on the following table:

Figure S1 :
Figure S1: Illustrative pictures describing the parameters used in the following equations to calculate the TIPS-pentacene thickness.

Figure S2 :
Figure S2: Experimental PXRD pattern of the polycrystalline powder of grounded single crystals (Red) of CsxFA1-xPbBryI3-y, FAPbBr3, MAPbBr3 and MAPbI3 compared with the profile obtained from their single crystal structures at room temperature (black).

Figure S3 :Figure S4 :
Figure S3: Photoelectron spectra of the MAPbI3 O 1s core level measured using 758 eV at FlexPES beamline at MAX IV facility.Binding energies were energy calibrated against Au 4f7/2 at 84.0 eV

Figure S8 :
Figure S8: Linear fit of TIPS-Pen HOMO level obtained from the third evaporation of TIPS-Pen on top of MAPbI3 perovskite single crystal.Valence band region measured at 130 eV at FlexPES beamline.

Figure S9 :
Figure S9: Energy level diagram including MAPbI3 valence band and conduction band (black lines) and summarizing TIPS-Pen HOMO and LUMO band bending obtained by C 1s shifts (blue) and Si 2p shifts (red) The dashed line represents Fermi level.

Table S1 :
Summary of results obtained with the model and equations described above.

Table S2 :
Values used to calculate the average atomic concentration of carbon on MAPbI3 and TIPS-Pentacene.

Table S3 :
Values of inelastic mean free path (IMFP) used for each core level.

Table S4 :
Evaporated amount of TIPS-Pen in arbitrary units calculated with the following equation.
Photoelectron spectra of the Pb 4f, C 1s, N 1s, I 4d, Br 3d, Cs 4d and Si 2p core levels recorded using 535 eV and O 1s using 758 eV from MAPbI3, MAPbBr3, FAPbBr3 and CsxFA1-xPbBryI3y single crystals cleaved under vacuum (blue line) and after several TIPS-Pen evaporations (other lines).All core levels were measured at CoESCA endstation at the synchrotron Bessy II, Berlin.Binding energies were energy calibrated against Au 4f7/2 at 84.0 eV.Binding energy shifts of C 1s TIPS-Pen core level after several evaporations on different in-situ cleaved perovskite single crystals.All positions are internally calibrated against Pb 4f core level.Dashed lines represent data obtained from FlexPES beamline and continuous lines represent data obtained at CoESCA endstation.