Photon Factory Activity Report 2017 #35 (2018)

BL-3B, BL-13B/2016G584

Energy level alignment at hetero-interfaces in sub-phthalocyanine based solar cells
Alexandre Lira FOGGIATTO1,* Takahiro UEBA2, Satoshi KERA2 and Takeaki SAKURAI1
1 University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki, 305-8573, Japan
2Institute for Molecular Science, 38, Nishigonaka, Myodaijichi, Okazaki, Aichi, 444-8585,
Japan
1 Introduction
In the past years, many works have been dedicated tounderstand the origin of the band bending in organic thinfilms and the energy-level alignment (ELA) at organicinorganic and organic-organic heterojunction (OOH)
interfaces. [1] In our work, we are interested inunderstanding the influence of the substrate work
function (WF) at donor/acceptor interface and the role ofthe tail states in the ELA. To do so, we used X-ray andultraviolet photoelectron spectroscopy (XPS and UPS) tostudy the ELA of boron subphthalocyanine chloride(SubPc)/ α-sexithiophene (6T) deposited on MoO3, [2]SiO2, [3] and Cs2CO3. Their work function (WF) values
are 5.9, 4.0, 3.0 eV, respectively. The ionization potential(IP) of SubPc and 6T were measured as 5.7 and 4.7 eV,respectively.
2 Experiment
The UPS/XPS measurements were mainly performed
at BL-13B at Photon Factory (PF). The Si/SiO2 substrate
was grown on the surface of Si by using an UV-ozonetreatment. The other substrate layers were obtained by thedeposition of 5 nm of MoO3 and Cs2CO3 deposited oncleaned Si substrate. Subsequently, 6T and SubPc weredeposited step-by-step in a deposition chamber(<5.0x10-9 Torr), until a thickness of 5 nm. Toinvestigate
the morphology
dependence,
theheterostructures were annealed for 60 s at 150 °C in theanalysis chamber.
3 Results and Discussion
After the contact, for MoO3, the binding energy (BE) of
HOMO onsets were measured as 0.55 eV and 0.86 eV for
6T (5nm) and SubPc (5nm), respectively. For Cs2CO3,
due to its low WF of, the HOMO onsets of theorganic materials move away from the Fermi level to 1.16
eV and 1.14 eV for 6T and SubPc, respectively. And for
the SiO2, the HOMO onsets were measured as 0.73 eV and1.24 eV for 6T and SubPc, respectively. We
estimated qualitatively the charge accumulated at theOOH, analysing the evolution of S 2p and Cl 2p peaks
position, which are associate with 6T and SubPc,respectively. Figure 1 shows the peak shift of S 2p and Cl 2p(straight lines), which exhibits the same tendency as
described above by the UPS measurement. The band
bending in SubPc can be extracted by the Cl 2p – S 2p peakshift (fig. 2). We observerd that SubPc layer depositedon 6T/Cs2CO3, due to the substrate influence, thefollowing deposition showed that the band bending
occurred in upward direction. For the SubPc/6T depositedon SiO2, almost no shift was observed. The upward band
bending suggests that an electron is accumulate at the SubPc

interface due to the acceptor property. After annealing thestack layers, we observed a drastic reduction of the corelevel shift at the organic films, as displayed at Fig. 1(dashed lines), which could be explained as reduction of
the density of the SubPc tail states that controls the band
bending at the OOH interface by better molecular
stacking due to the annealing, which reduces the available
states to charge transfer to achieve the equilibrium.

Fig. 1 Energy shift in function of the SubPc thickness depositedon 6T (5 nm)/ Substrate.

Fig. 2 Energy shift difference (Cl2p-S2p) for the organic
stack layer deposited on the substrates in function of theSubPc thickness.

References[1] J.-P. Yang et al., J. Phys. D: Appl. Phys. 50 (2017) 423002.
[2] A. L. Foggiatto and T. Sakurai, Jpn. J. Appl. Phys. 57 (2018)
03EE01.
[3] J. Endres et al., Chem. Mater. 28 (2016) 794.

* alexandrelfgt@hotmail.com.br