Enabling High-Performance Tandem Organic Photovoltaic Cells by Balancing the Front and Rear Subcells.

In tandem organic photovoltaics, the front subcell is based on large-bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow-bandgap acceptor for infrared absorption but a large-bandgap donor to realize a high open-circuit voltage. Unfortunately, most of the ultraviolet-visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large-bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole-transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared-absorbing polymer donor as a third component is introduced, which provides a second hole-generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short-circuit current density of the two-terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7 mA cm-2 (while retaining the open-circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.