Polymer Solar Cell - Architectures

Architectures

As described in Mayer's review, the simplest organic PV device features a planar heterojunction (figure 1). A film of active polymer (donor) and a film of electron acceptor is sandwiched between contacts. Excitons created in the donor region may diffuse to the junction and separate, with the hole remaining behind and the electron passing into the acceptor. Because charge carriers have diffusion lengths of just 3–10 nm in typical organic semiconductors, planar cells must be thin, but the thin cells absorb light less well. Bulk heterojunctions (BHJs) address this shortcoming. In a BHJ, a blend of electron donor and acceptor materials is cast as a mixture, which then phase-separates. Regions of each material in the device are separated by only several nanometers, a distance suited for carrier diffusion. BHJs require sensitive control over materials morphology on the nanoscale. A number of variables, are important including choice of materials, solvents, and the donor-acceptor weight ratio.

The next logical step beyond BHJs are ordered nanomaterials for solar cells, or ordered heterojunctions (OHJs). OHJs minimize the variability associated with BHJs. OHJs are generally hybrids of ordered inorganic materials and organic active regions. For example, a photovoltaic polymer can be deposited into pores in a ceramic such as TiO₂. Since holes still must diffuse the length of the pore through the polymer to a contact, OHJs do suffer thickness limitations. Mitigating the hole mobility bottleneck is key to further enhancing device performance of OHJ's.