In photovoltaic cells, bimolecular recombination "competes" with other loss mechanisms, such as shunts and sub-optimum charge separation (geminate recombination). It is important to understand which of these loss mechanisms dominates a given cell parameter (Voc, Jsc, FF) in order to have a reasonable strategy for improvement. Perhaps even more important, it is critical to examine the loss routes when new materials fail to preform as desired, in order to develop design rules for future efforts. Concurrent charge density and charge lifetime measurements, along with transient absorption, can convincingly determining the major contribution to losses for both existing and new cell materials.
The "holy grail" of dye sensitized cells is a dye that has strong absorption at >700 nm and yet functions as well as the present best dye, "N3". Many hundresds of new dyes have been synthesized and tested. Despite this effort, design rules for optimized dyes remain poorly developed. By comparing charge density and recombination rates, and by examination of the literature, we have come to the conclusion that many, if not most, dyes are catalysts for the recombination of the electron with the electrolyte. This catalysis of recombination can strongly decrease the Voc and fill factor relative to N3, which does not catalyze recombination. We will present recombination measurements covering a range of dyes and dye classes, with the hope that motifs that encourage and discourage recombination will be identifiable.
Given time, new charge density, TAS, and recombination measurements in polymer/PCBM cells will also be discussed.