New Paper by Nicola Gasparini and Collaborators

New Paper Published in Advanced Energy Materials

10/1/2017

Polymer:Nonfullerene Bulk Heterojunction Solar Cells with Exceptionally Low Recombination Rates

Nicola Gasparini,* Michael Salvador, Thomas Heumueller, Moses Richter,
Andrej Classen, Shreetu Shrestha, Gebhard J. Matt, Sarah Holliday, Sebastian Strohm, Hans-Joachim Egelhaaf, Andrew Wadsworth, Derya Baran, Iain McCulloch, and Christoph J. Brabec*



Organic semiconductors are in general known to have an inherently lower charge carrier mobility compared to their inorganic counterparts. Bimolecular recombination of holes and electrons is an important loss mechanism and can often be described by the Langevin recombination model. Here, the device physics of bulk heterojunction solar cells based on a nonfullerene acceptor (IDTBR) in combination with poly(3-hexylthiophene) (P3HT) are elucidated, showing an unprecedentedly low bimolecular recombination

rate. The high ll factor observed (above 65%) is attributed to non-Langevin behavior with a Langevin prefactor (β/βL) of 1.9 × 104. The absence of parasitic recombination and high charge carrier lifetimes in P3HT:IDTBR solar cells inform an almost ideal bimolecular recombination behavior. This exceptional recombination behavior is explored to fabricate devices with layer thicknesses up to 450 nm without signi cant performance losses. The deter- mination of the photoexcited carrier mobility by time-of- ight measurements reveals a long-lived and nonthermalized carrier transport as the origin for the exceptional transport physics. The crystalline microstructure arrangement

of both components is suggested to be decisive for this slow recombination dynamics. Further, the thickness-independent power conversion ef ciency is of utmost technological relevance for upscaling production and reiterates the importance of understanding material design in the context of low bimo- lecular recombination.​