Polymer solar cells are very promising devices due to their distinct advantages, such as low material and fabrication costs, the flexibility and light weight of the devices and accessible technology that will help to enable mass fabrication. However, the lack of stability of these devices is one of the greatest challenges [1]. Thus, understanding the mechanisms of solar cell degradation is a key aspect for the development of this technology. The bulk heterojunction solar cell with active layer made of blends of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric-acid methyl ester (P3HT/PCBM) [2,3] is a benchmark to study the degradation over the operating lifetime of the organic photovoltaic cells. Therefore improving the stability of the organic solar cells it is mandatory to understand the transport properties of the different layers and the interface between them and how they are affected by environmental conditions, such as humidity, oxygen exposure, temperature or solar irradiation.
In the present work we use scanning force microscopy to study the typical full organic architecture formed by a transparent substrate (glass or PET), a transparent electrode (ITO), a hole injection layer (PEDOT), an active layer P3HT/PCBM and finally an aluminium top electrode. Also macroscopic characterization (I-V curves) has been carried out. In order to study the nanoscale electrical properties of these devices, Kelvin force microscopy (KPM), capacitance force microscopy (CFM), “jumping” mode SPM (local conductivity imaging) are applied. In addition, a 3-D nanoscale study of the structure and electronic properties of the fabricated devices have been carried out by means of SFM-based nanotomography and cross-sectional SFM techniques. The nanomorphology and electrical properties of the interfaces between the different layers and their relationship with the device performance have been analysed during the degradation process under different environmental conditions.
[1] Stability and Degradation of Organic and Polymer Solar Cells, F. C. Krebs, 2012, John Wiley & Sons, Ltd. [2] Campoy-Quiles M. et. al. 2008, “Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends”, Nature materials, 7: 158-164. [3]Yu G., et al., 1995. “Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor–Acceptor Heterojunctions”. Science, 270:1789–1791.