Dresden 2014 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 23: Organic Electronics and Photovoltaics III (DS jointly with CPP, HL, O)

O 23.4: Vortrag

Montag, 31. März 2014, 18:45–19:00, CHE 91

The effect of gradual fluorination on the opto-electronic properties of F_nZnPc/C₆₀ bilayer cells — •M. Brendel¹, A. Steindamm¹, A. Topczak¹, and J. Pflaum^1,2 — ¹Exp. Phys. VI, JMU Würzburg, 97074 Würzburg — ²ZAE Bayern, 97074 Würzburg

The respective position of energy levels at the donor/acceptor heterojunction is crucial for the resulting parameters of an organic solar cell. For instance the open circuit voltage (V_oc) is correlated to the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, the so-called effective band gap (E_g,eff). To gain insights into this correlation, tailoring of energy levels by chemical modification is a powerful approach. In this contribution, we investigate the impact of gradual fluorination of zinc phthalocyanine on the opto-electronic properties of F_nZnPc/C₆₀ (n=0,4,8,16) bilayer cells. Upon fluorination, HOMO and LUMO levels are shifted towards lower energies. The gain in V_oc for F₄ZnPc/C₆₀ and F₈ZnPc/C₆₀ by 11% and 23% respectively, compared to ZnPc/C₆₀, confirms qualitatively the expected energy level scheme. Besides, the differences between e· Δ V_oc and Δ E_g,eff hint at the occurrence of dipoles and their gain in strength with increasing degree of fluorination. As will be shown, this dipol can be correctly accounted for in a plate capacitor geometry, considering image charges at the interface induced by electronegative fluorine in the immediate vicinity of C₆₀ molecules. We thank S. Sundarray and P. Erk from BASF for providing F₄ZnPc. Financial support by the DFG (program SPP1355) and the BMBF (GREKOS program).