Ordered Organic-Inorganic Bulk Heterojunction Photovoltaic Cells
- PDF / 276,143 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 3 Downloads / 317 Views
Ordered Organic–
Inorganic Bulk Heterojunction Photovoltaic Cells
Kevin M. Coakley, Yuxiang Liu, Chiatzun Goh, and Michael D. McGehee Abstract Fabrication of bulk heterojunctions with well-ordered arrays of organic and inorganic semiconductors is a promising route to increasing the efficiency of polymer photovoltaic cells. In such structures, almost all excitons formed are close enough to the organic–inorganic interface to be dissociated by electron transfer, all charge carriers have an uninterrupted pathway to the electrodes, and polymer chains are aligned to increase their charge carrier mobility. Furthermore, ordered structures are interesting because they are relatively easy to model. Studies of ordered cells are likely to lead to better design rules for making efficient photovoltaic cells. Keywords: ordered bulk heterojunctions, conjugated polymers, mesoporous titania, organic photovoltaic cells.
Introduction As discussed in other articles in this issue, an attractive approach to improving the efficiency of organic photovoltaic (PV) cells is to use a bulk heterojunction, in which semiconductors with offset energy levels interpenetrate at approximately the 10 nm length scale. Most bulk heterojunction PV cells have been made by casting solutions containing the two semiconductors (e.g., a conjugated polymer and a fullerene derivative,1 another polymer,2 CdSe nanocrystals,3 titania nanocrystals,4 or ZnO nanocrystals5) to make blends. Although these blends are easy to fabricate, and it is desirable to use this simple process to manufacture PV cells, there are problems with the disordered nanostructures that are typically created. In some cases, the two semiconductors phaseseparate on too large a length scale. Consequently, some of the excitons (electron–hole pairs) generated when light is absorbed are not able to diffuse to an interface to be dissociated by electron transfer before they decay. In other cases, there are dead ends in one of the phases that prevent charge carriers from reaching electrodes. In most
MRS BULLETIN • VOLUME 30 • JANUARY 2005
cases, charge transport is not fast enough to enable the charge carriers to reach the electrodes before the electrons and holes recombine with each other, unless the films are made so thin that they cannot absorb all of the incident light. Ordered bulk heterostructure PV cells, such as the one shown schematically in Figure 1, are more difficult to fabricate than disordered blends, but there are several good reasons to do so. First, the dimensions of both phases can be controlled to ensure that every spot in a film is within an exciton diffusion length of an interface between the two semiconductors. Second, there are no dead ends in the structure. After excitons are dissociated by electron transfer, the electrons and holes have straight pathways to the electrodes. This geometry ensures that the carriers escape the device as quickly as possible, which minimizes recombination. Third, in an ordered structure it is possible to align conjugated polymer chains, which inc
Data Loading...
