Kelvin Probe Microscopy and in-situ macroscopic IV study of inverted organic solar cells stability

Resumen

Polymer or organic solar cells (OSCs) are very promising devices due to their distinct advantages, such as low material usage and fabrication costs, flexibility and light weight, together with scalable deposition technologies that will help to enable mass fabrication. However, the lack of stability of these devices is one of the greatest drawbacks and constitutes a major challenge nowadays [1]. To overcome this issue, many researcher groups fabricate the devices in an inverted architecture, obtaining a clear improvement of the OSCs ambient stability. Unencapsulated OSCs can be stored in these conditions during several months without an appreciable lost in their performance. In inverted OSCs the sequence of the constituting layers (and the most common materials used) is the following: indium tin oxide (ITO) as the transparent cathode, a thin film of zinc oxide as electron transport layer, the photo-active layer made of blends of poly(3- hexylthiophene)/[6,6]-phenyl-C61-butyric-acidmethyl ester (P3HT/PCBM) and finally Ag as top electrode. This architecture demonstrate better stability by removing the frequently used acid and hygroscopic hole-transporting PEDOT:PSS layer and low-work-function metal cathode like Al. An interesting observed effect in inverted architecture devices is the improvement of the device performance when stored under air in dark conditions [1-3]. To investigate this effect we use scanning force microscopy with in-situ macroscopic I-V measurements, under light and dark conditions, to study the stability during the operating lifetime of the organic photovoltaic cells. In order to correlate the macroscopic with the nanoscale electrical parameters Kelvin force microscopy (KPM) is applied.