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Organic Solar Cells

In order to reduce the cost of solar cells, manufacturing techniques are required that allow the fast fabrication of thin films at low temperatures. A high speed means that the (expensive) machines are used efficiently, the low thickness of the films reduces the amount of raw material needed and the low temperatures save energy. Printing of solar cells fulfills all these requirements but it requires specific materials that can be printed from solution. One of the most thoroughly studied family of solution processable semiconductors are organic materials. Organic semiconductors are e.g. conjugated polymers but also small molecules like fullerenes (e.g. derivatives of C60 or C70). Many of those organic semiconductors are very efficient light absorbers and allow solution processing without the requirement for expensive equipment or explosive solvents like hydrazine. However, the challenge is to make highly efficient solar cells from organic semiconductors.

Organic semiconductors typically have rather small relative permittivities which implies that the Coulomb attraction between two differntly charged carriers (like electron and hole) are much stronger than in an inorganic semiconductor. Therefore, an absorbed photon does not directly create an electron-hole pair but initially a strongly bound Frenkel exciton (see Fig. 1a). This exciton has a binding energy much larger than the thermal energy kT at room temperature (~ 25 meV). Therefore a trick is required to split the exciton. This trick is a type II heterojunction as shown in Fig. 1a. A type II heterojunction has discontinuities of conduction and valence band with the same sign. Fig. 1a shows that the energy levels of the left molecule are always higher than that of the right molecule. Therefore, it is energetically favorable for the bound electron to hop from the left to the right molecule. Because of the discontinuity in the bands at the interface it is very unlikely that the electron overcomes the barrier to return to the left molecule. For the hole it is more favorable to stay on the left molecule (for holes the energy axis is inverted) which means that the type II heterojunction allows the splitting of the Frenkel exciton into an electron and a hole on two separate molecules.

Exciton diffusion lengths are typically about 10 nm while the absorber layer thickness needs to be at least around 100 nm to absorb a substantial part of the light. This means that two thin layers of molecule A and molecule B on top of each other would not be sufficient for high efficiency solar cells. If the layers are very thin, the solar cell would be essentially transparent. If the layers were thick, the solar cell would absorb the light but only a small fraction of it would be absorbed within an exciton diffusion length of the interface and therefore only few electrons and holes would be created. In both cases the photocurrent would be relatively small.

The solution for this dilemma is shown schematically in Fig. 1b. In a so called bulk heterojunction, the two molecules are mixed such that (ideally) each point in the volume is within about 10 nm to the next interface. In addition, one needs to make sure that there are percolating pathways such that electrons and holes can be transported to their respective contacts. The bulk heterojunction (BHJ) concept is a source of various scientific questions dealing e.g. with the ideal microstructure of a BHJ or the ideal energy level alignment at the interfaces.

organic bulk heterojunction

Fig. 1: Working principle of a bulk heterojunction solar cell. The energy diagram (a) shows that the used type II heterojunction allows charge generation (2) following photon absorption (1). The schematic cross section in (b) shows that the microstructure of the blend needs to allow for charge separation via separate percolating pathways for electrons and holes. The functionality of the device relies heavily on both the properties (especially the energetics) of the interface and the micro- or nanostructure of the donor:acceptor blend forming the bulk heterojunction.

The work on organic solar cells will probably start in summer 2014 with current activities focussing on building up additional characterization equipment and performing first tests. Our mid-term goal is to focus on optical, electrical and optoelectronic characterization techniques and apply them to organic solar cells to improve our current understanding of these fascinating devices. This will hopefully allow us to develop strategies to further improve the power conversion efficiencies of solution processable solar cells in general and of organic solar cells in particular.