Organic photovoltaic devices require a defined morphology with large interfacial area. Such morphology can be achieved by vertically and laterally structured surfaces in the realm of several tens to hundreds of nanometers. In order to obtain an insight in the interface phenomena of solar cell materials we prepared cross sections by focussed ion beam methods. Scanning conductive force microscopy on these cross sections allowed the investigation of the function of different layers within the solar cell, e.g. the charge blocking layers. In addition, we could map that the titania in our system forms an electrically percolating network.

Figure 1: (a) Scheme of the conductive SFM measurement setup. Yellow: gold electrode, green: PEDOT:PSS layer, purple: TiO2 matrix, blue: PTPA (hole conducting part), red: barrier layer and light blue: FTO electrode. (b) SEM of the polished section with C-SFM current maps at a sample bias of -2 V and +1 V. For an easier comparison between the two current maps, the current for the negative sample bias has been inverted.

Arrays of nano-pillars have been proposed as an appropriate morphology for the electron donor/acceptor layer in an organic solar cell. Nano-pillars with a defined aspect ratio and pitch can be easily prepared by template assisted preparation techniques, e.g. blockcopolymer templates or anodized aluminium oxide. Here, the knowledge of the interplay between morphology and conductivity of individual nano-pillars is crucial for the functionality of the device. In particular, the analysis of free standing organic nano-pillars by standard conductive SFM is often hampered by the damage of the sample due to the forces induced by the tip. For these applications we found that Scanning Conductive Torsion Mode Microscopy is gentle enough for a non-destructive characterization of topography and conductance, simultaneously.

Furthermore, we have investigated the photo-induced changes in the surface potential and conductivity for locally degraded active layers of organic solar cell materials using electrical modes of scanning force microscopy. Samples were degraded under different partial pressures of oxygen and humidity in the presence of light. Degraded and non-degraded areas were investigated by Kelvin Probe Force Microscopy (KPFM) and conductive scanning force microscopy (cSFM). The analysis allowed us to quantify the extent of degradation and compensate the contribution of the probe tip. Two typical blends used for organic solar cell, i.e. P3HT:PCBM and PCPDTBT:PCBM were investigated. We observed that P3HT:PCBM photo-degraded significantly more than PCPDTBT:PCBM for an environment containing oxygen. For short photo-degradation times (1 hour) we verified that changes in the surface potential and conductivity of P3HT:PCBM films were fully reversible after annealing. For individual layers of P3HT and PCBM we found that only P3HT degrades. However, the blend material of P3HT and PCBM leads to an accelerated degradation supporting the interpretation that PCBM undergoes a series of oxidations in the blend.

Figure 2: cSFM image with a bias voltage of 3 V images under dark conditions of locally degraded sample (18 hours, 2 Sun, synthetic air, 20% H2O). cSFM image under illumination of the locally degraded sample. Histogram analysis of the currents measured. Upon illumination a much higher current (>200 pA) is measured in the non-degraded parts.

• M.C. Lechmann, S.A.L. Weber, J. Geserick, N. Hüsing, R. Berger and J.S. Gutmann: Investigating Morphology and Electronic Properties of Self-Assembled Hybrid systems for Solar Cells, J. Mater. Chem., 21 7765 - 7770 (2011).
• M. Zorn, S.A.L. Weber, M.N. Tahir, W. Tremel, H.-J. Butt, R. Berger, R. Zentel: Light Induced Charging of Polymer Functionalized Nanorods, Nano Letters, 10, 2812–2816 (2010).
• E. Sengupta, A.L. Domanski, S.A.L. Weber, M.B. Untch, H-J. Butt, T. Sauermann, H.J. Egelhaaf, R. Berger: Photo induced degradation studies of organic solar cell materials using Kelvin Probe Force and Conductive Scanning Force Microscopy, accepted in Journal of Physical Chemistry C 2011.