Electrodes and Buffer Layers in Organic Solar Cells

Subarna Babu Sapkota, Universität Freiburg

Organic solar cells degrade during the illumination and in the dark in ambient air when not sealed properly. Despite of having huge potential relatively low efficiency and degradation are the most challenging aspects related to the organic photovoltaics at present. During the last years, improved solar energy conversion efficiency and longterm stabiltiy of organic solar cells has been achieved. These achievements are still not enough to have roll to roll mass production of organic solar cells, some more improvement in material used, efficiency and stability is needed.

Extraction of electrons and holes from the photoactive absorber layer at the contacts without reducing their electrochemical potential is one possible route for achieving maximum efficiency and long term stability of Organic Solar Cells.

In principle, this can be achieved by using contacts whose Fermi energy matches the quasi Fermi energy of the respective charge carrier. At the metallic contacts, due to the presence of large numbers of states both for electron and holes recombination occurs (surface recombination). Thus, it is important to reduce the recombination at metallic contacts. It can be reduced by inserting a high bandgap semiconductor with low absorption as a “buffer layer” which has a valence band (conduction band) close to the valence band (conduction band) of the photovoltaically active semiconductor and which is doped to achieve a match between the contact Fermi energy and respective charge carrier quasi Fermi energy.

To date, ITO is widely used as a transparent electrode but ITO is not an optimum electrode, chemically,mechanically or economically. It is quite expensive and limits the cost of the organic solar cells. Current electrodes like Aluminum and techniques to deposit the electrodes are providing a possible route for the entrance of oxygen and water vapor inside the organic solar cells which causes degradation. Widely used buffer layer like PEDOT itself is hygroscopic in nature, so it is possible for it to act as a source for water vapor and facilitate the degradation mechanism.

Finding the electrode/deposition techniques with no/very less possible route for the entrance of oxygen and water vapor, replacement of ITO electrode by other cheap materials and replacement of hygroscopic buffer layers (like PEDOT) with non hygroscopic alternatives could be a long waited turning point in the field of organic solar cells.

The cost per kilowatt hour of electricity plays a decisive role for economical viability of organic solar cells which is directly proportional to the price of modules and inversely proportional to the efficiency and life time. Increased efficiency and increased life time could lead to reduced cost for the electricity produced with organic solar cells. Along with this, possible replacement of ITO will also lead to a huge reduction in the cost of organic solar cells.

The main objective of this thesis work is

  1. To investigate, find and develop most suitable electrode materials and buffer layers in order to increase efficiency and long term stability of organic solar cells, which could be used for roll to roll mass production.
  2. Possible replacement of expensive materials for transparent electrodes with cheaper ones.
  3. To reduce the cost per kilowatt hour of electricity through increased efficiency and increased life time.