ABSTRACT
Heterojunction Cells ..................................................................380 12.4 Tandem Cells Based on CuPc:C60 Planar-Mixed Heterojunctions ... 382
12.4.1 Background of Organic Tandem Cells .................................... 382 12.4.2 Asymmetric Organic Tandem Cells Based
on CuPc-C60 PM-HJs ..................................................................383 12.5 Conclusions ...............................................................................................388 Acknowledgments .............................................................................................. 389 References ............................................................................................................. 389
In an organic donor-acceptor (D-A) heterojunction (HJ) photovoltaic (PV) cell, the entire PV process consists of four consecutive steps: photon absorption by either donor or acceptor molecules to generate excitons, diffusion of
excitons to the D-A heterointerface, charge transfer at the interface leading to the dissociation of excitons and generation of separated charge carriers, and fi nally the collection of photogenerated charge carriers at opposite electrodes (Peumans et al., 2003). In bilayer, or planar HJ, organic PV cells, exciton diffusion is generally the limiting step for the overall PV performance (Forrest, 2005). Due to the weak van der Waals-type intermolecular interactions that hold the organic molecules together, the exciton diffusion length LD is typically on the order of a few nm (Pope and Swenberg, 1999; Peumans et al., 2003). As an exciton is less likely to reach the interface if it is generated farther from the interface, the exciton diffusion effi ciency ηED, or the fraction of excitons reaching the interface (and subsequently dissociate at the interface), decreases with increasing the layer thicknesses. On the other hand, the optical absorption length LA = 1/α (where α is the absorption coeffi cient) is typically on the order of 50-100 nm. This suggests that there is a critical trade-off between using thicker layers for more effi cient light absorption and using thinner layers for a higher ηED.
