Organic Solar Cells: Tuning Electron Energy Level Offsets At The Anodic Interfaces

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Quentin ARNOUX, 184
Publication date: 5th February 2015

Organic photovoltaics have gained much attention during the last decade in particular due to the development of the synthesis of organic semiconductors with high absorption coefficients, light weight, flexibility and low cost¹. In such devices, the stacked layers (Figure a)) must respect an electronic level “staircase” (Figure b)) to allow the charges separations and conduction to the electrodes. Since 2007 numerous publications report the advantage of an interfacial layer to drain and/or collect charges; in particular Tortech et al. demonstrated how power generation efficiency is increased by inserting at the anodic contact (ITO)² a p-type hole-draining molecule layer of DIPO (Figure c)), a member of the dipyranylidene (DIP) family. Surface studies were previously done using atomic force microscopy (AFM) and current sensing AFM (CS-AFM). We observed that conductivity is high either at the top or on the sides of the DIPO column depending on peak height. To understand the energetics and interface properties, the growth of DIPO layers was studied using a combination of x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at SOLEIL synchrotron facility. We examined via XPS the evolution of the In 3d and O 1s core levels. We could observe two O 1s components originating from the DIPO molecule, although the two oxygen atoms are equivalent (see Figure c)). We attribute the one at higher binding energy (BE) to DIPO molecules in interaction with the ITO substrate, while the one at lower BE corresponds to bulk DIPO (e.g. in columns) (Figure d)). Besides, a rigid +100 meV BE shift was observed for all substrate core levels, indicating a change in band bending of ITO at the surface, and an electron transfer from DIPO to the substrate. The positioning of the HOMO edge with respect to the ITO valence band maximum, completed the energy level scheme of the interface. Via the secondary electron edge cutoff measurement we observed the sizeable changes in the work function. Finally, via NEXAFS measurements at the C K edge and measurement of the C 1s pi* transition intensity, we determined the orientation of DIPO molecules on the ITO surface, for various increasing thicknesses. Finally, photovoltaic studies were performed on such devices in an attempt to correlate the electron level scheme with the device performances.
a) Scheme of a typical structure of an organic solar cell (OSC); b) Staircase band diagram of a P3HT:PCBM bulk heterojunction using a molecule of tetra phenyldithiapyranylidene (DITPY-φ4). The positioning of the energetic levels is only indicative.; c) Dipyranylidene family; d) O 1s core level: ITO and ITO + DIPO comparison
[1] Hoppe, H.; Sariciftci, N. S. Organic solar cells: An overview. Journal of Materials Research 2004, 19, 1924–1945. [2] Tortech, L.; Véber, M.; Fichou, D.; Berny, S. D. Dithiopyrannylidenes as Efficient Hole Collection Interfacial Layers in Organic Solar Cells. ACS Appl. Mater. Interfaces 2010, 2, 3059–3068.

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