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Posted: Sep 21, 2011
How phase segregation affects efficiency in organic photovoltaics
(Nanowerk News) Recent theoretical work conducted at the NIST Center for Nanoscale Science and Technology explains the surprisingly small effect of macroscale phase segregation on the overall efficiency of blended organic photovoltaic (OPV) materials by showing that electrons can effectively burrow through a skin layer to get to the device's cathode ("Organic photovoltaic bulk heterojunctions with spatially varying composition").
OPVs consist of two types of organic molecules, electron donors and electron acceptors, which are uniformly blended throughout the volume of the material. In an OPV, light photoexcites a bound electron-hole pair, which separate at the interface between donor and acceptor. The separated free charges migrate to different contacts, generating an electrical current. The choice of electrode material is crucial to the operation of the OPV. The cathode must preferentially collect electrons and the anode must preferentially collect holes.
Recent studies of OPV materials conducted at the CNST and elsewhere revealed a donor-rich hole transporting skin layer near the cathode. This phase segregation, or high hole concentration, is due to the smaller surface energy of the donor-cathode interface relative to the acceptor-cathode interface. The fact that the electron collector has mostly holes in its vicinity would seem to be an impediment to charge collection and overall efficiency. However, the ratio of collected to excited charge is still high. By extending previously developed models to account for macroscopic phase segregation, it was theoretically determined that charges can rather easily "squeeze" through regions of reduced density.
This effect explains the relatively benign influence of the skin layer on overall device performance. The work demonstrates that cathode skin layer phase segregation should not be an impediment to the development of high efficiency OPVs.