Ultrafast Dynamics Group
Frédéric Laquai's Group

Introduction to Organic Solar Cells | KAUST

Introduction to Organic Solar Cells

Details

Organic solar cells offer new domains of energy production. Easy roll-to-roll production could produce large scale and therefore cheap modules. Moreover, their energy pay back time, i.e. the operating time that is needed to gain back the energy that was spent upon production, could be considerably lower than for the crystalline silicon solar cells as they are processed at low temperatures. New areas of application and consumer products can open up as organic solar cells can be produced on flexible light-weight substrates. So far certified ​power conversion efficiencies up to 11.5 % have been reached on small substrates (www.nrel.gov). Organic solar cells use semiconducting polymers as light harvesting material. Their physical properties, such as band gap and absorption spectrum, can be tailored by tuning their chemical structure through controlled synthesis and careful choice of monomers. They possess high absorption coefficients, therefore only very thin layers (~100 nm) are necessary to absorb a sufficient amount of light and they can be processed at room temperature, which are important prerequisites for the production of cheap solar modules. The standard device assembly of an organic solar cell, which can be described as a sandwich structure of different layers on a glass substrate is depicted in the figure below.


                                                          © Group Laquai

The glass substrate is coated with a transparent electrode. This is usually ITO (indium tin oxide) which is structured by etching. On top of the ITO layer a PEDOT:PSS layer is spin coated to complement the anode. PEDOT:PSS (poly(3,4-ethylendioxythiophene)poly-(styrenesulfonate)) is a transparent, highly doped polymer blend. Therefore, its conductivity is high and it forms a better hole contact to the photoactive blend layer than ITO. Moreover, it can compensate for surface roughness of the ITO layer, which could lead to short circuits in the photovoltaic cell. On top of the anode, the intimately mixed blend of a semiconducting donor polymer and an electron acceptor, typically a fullerene derivative, is prepared by spin coating. An aluminium layer is usually used as cathode due to its workfunction fitting well with the electron transport level of the acceptor. The cathode is vacuum evaporated on top of the absorber blend. Between the photoactive layer and the aluminium cathode very thin layers of calcium (Ca) or lithiumfluoride (LiF) can be vapour deposited in order to improve contact properties. In our group, we focus mainly on testing new donor and acceptor materials. The performance of the “fruitfly” system P3HT/PCBM has been surpassed by novel donor-acceptor polymers with enhanced absorption in the red and near-infrared region of the solar spectrum (e.g. PCDTBT, [1]). Also, novel acceptor materials that contribute to light absorption are investigated (e.g. perylene diimides, [2]). By using optical spectroscopy we figure out the fundamental processes of charge generation and recombination that lead to differences in device performance.

Publications

1. Etzold, F., Howard, I.A., Mauer, R., Meister, M., Kim, T., Lee, K., Baek, N., Laquai, F.
Ultrafast Exciton Dissociation Followed by Nongeminate Charge Recombination in PCDTBT:PCBM Photovoltaic Blends