Incoherent charge separation dynamics in organic photovoltaics

Shyamal K K Prasad , Joseph K Gallaher , Alex Barker (1) , Han Young Woo , Mamatimin Abbas (2) , Lionel Hirsch (2) , Justin Hodgkiss (3)

1 MacDiarmid Institute for Advanced Materials and Nanotechnology
2 IMS - Laboratoire de l'intégration, du matériau au système
3 MacDiarmid Institute for Advanced Materials and Nanotechnology

There is mounting evidence that long-range charge separation determines the efficiency of organic photovoltaic cells, yet different mechanisms remain under debate. One class of proposed mechanism is ultrafast coherent long-range charge separation, and another is a slower process whereby charges incoherently hop apart with a transiently enhanced mobility due to morphology and disorder. Here, we use transient absorption spectroscopy to probe incoherent charge separation dynamics in two different ways. First, we use a family of polymers whose backbone structures allows us to compare 2-phase donor-acceptor morphologies with 3-phase morphologies that feature an intermixed region. In the 3-phase system, we see pronounced spectral signatures associated with charges (holes) occupying the disordered intermixed region, and we track separation via biased charge diffusion to more ordered neat regions on the timescale of hundreds of picoseconds. Secondly, by resolving bimolecular charge recombination at high excitation density, we show that charge mobilities must be substantially enhanced on early timescales, which may be sufficient for separation to occur. Together, these measurements provide support for models of incoherent and relatively slow charge separation.

Mots clés: transient absorption; Charge separation; organic photovoltaic; three phase; intermixed phase; mobility relaxation

Domaines: Science des matériaux [cond-mat.mtrl-sci]

Shyamal K K Prasad, Joseph K Gallaher, Alex Barker, Han Young Woo, Mamatimin Abbas, et al.. Incoherent charge separation dynamics in organic photovoltaics. SPIE Nanoscience + Engineering, Aug 2016, San Diego, United States. ⟨10.1117/12.2238234⟩. ⟨hal-01647913⟩