How Molecules with Dipole Moments Enhance the Selectivity of Electrodes In Organic Solar Cells

Moritz Unmüssig^a, Martin Sessler^a, Markus Kohlstädt^a^,^b, Uli Würfel^a^,^b

^aFraunhofer Institute for Solar Energy Systems ISE, Germany, Heidenhofstraße, 2, Freiburg im Breisgau, Germany

^bFreiburg Materials Research Center FMF, Albert-Ludwigs-University Freiburg, DE, Stefan-Meier-Straße, 25, Freiburg im Breisgau, Germany

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

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Invited Speaker Session, Uli Würfel, presentation 166
Publication date: 5th February 2015

To minimize surface recombination in organic solar cells charge carrier selective layers are used between the photoactive layer and the electrode. These layers (e.g. PEDOT:PSS, TiOx, ZnO, WoOx, MoOx) often have a rather large band gap and thus can block the “wrong” type of charge carrier quite efficiently. However, surface states within the band gap of these materials at the interface with the photoactive layer can still act as recombination centers. Recently, there are numerous examples of polar (organic) molecules providing a high degree of charge carrier selectivity, the most prominent one being the work of He et al. on PFN [1]. Some of these materials are however rather expensive. We used simple, extremely cheap and easy-to-process organic molecules with permanent dipole moments. These molecules alter the effective work function of the electrode as confirmed by scanning Kelvin probe force microscopy which revealed a corresponding shift of the surface potential. This leads to a strong increase (decrease) of the electron (hole) concentration in the adjacent photoactive layer. It will be shown in detail that this is the main reason for the enhanced selectivity causing an increase of the open-circuit voltage (and fill factor) for different active layers. A theoretical model was set up and the results of the numerical simulations are in full accordance with the experimental data. Interestingly, DFT-calculations prove that the energy levels of the dipole molecules used are not suited to conduct charge carriers from the photoactive layer to the electrode but that a tunneling mechanism is involved. Implementing this into our model, it is found that there is no necessity to assume preferential tunneling for electrons. Their accumulation and the depletion of holes due to the altered work function are sufficient to explain the observed behavior.

[1] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells, Adv. Mater. 23 (2011) 4636-4643.

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