Double doping of conjugated polymers with monomer molecular dopants
David Kiefer, Renee Kroon, Anna I. Hofmann, Hengda Sun, Xianjie Liu, Alexander Giovannitti, ........, Thomas F. Harrelson, Michael Sommer, Adam J. Moulé, Martijn Kemerink, Seth R. Marder, Iain McCulloch, Mats Fahlman, Simone Fabiano & Christian Müller
Nature Materials, 18, pp. 149-155, (2019)
Molecular doping is a crucial tool for controlling the charge-carrier concentration in organic semiconductors. Each dopant molecule is commonly thought to give rise to only one polaron, leading to a maximum of one donor:acceptor charge-transfer complex and hence an ionization efficiency of 100%. However, this theoretical limit is rarely achieved because of incomplete charge transfer and the presence of unreacted dopant. Here, we establish that common p-dopants can in fact accept two electrons per molecule from conjugated polymers with a low ionization energy. Each dopant molecule participates in two charge-transfer events, leading to the formation of dopant dianions and an ionization efficiency of up to 200%. Furthermore, we show that the resulting integer charge-transfer complex can dissociate with an efficiency of up to 170%. The concept of double doping introduced here may allow the dopant fraction required to optimize charge conduction to be halved.
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