  We demonstrate that blending fluorinated molecules in PEDOT:PSS hole
transport layers (HTL) induces charge transfers which impact on both charge
extraction and photogeneration within organic photovoltaic (OPV) devices. OPVs
fabricated with modified HTL and two photoactive polymer blends led
systematically to power conversion efficiencies (PCE) increases, with
PTB7:PC70BM blend exhibiting PCE of ~ 8.3 %, i.e. ~ 15 % increase compared to
pristine HTL devices. A reduced device-to-device characteristics variations was
also noticed when fluorinated additives were used to modify the PEDOT:PSS.
Shading lights onto the effect of HTL fluorination, we show that the morphology
of the polymer:PCBM blends remains surprisingly unaffected by the fluorinated
HTL surface energy but that, instead, the OPVs are impacted not only by the HTL
electronic properties (work function, dipole layer, open circuit voltage,
charge transfer dynamic) but also by alteration of the complex refractive
indices (photogeneration, short circuit current density, external quantum
efficiencies, electro-optic modelling). Both mechanisms find their origin in
fluorination induced charge transfers. This work points towards fluorination as
a promising strategy toward combining both external quantum efficiency
modulation and power conversion efficiency enhancement in OPVs. Charge
transfers could also be used more broadly to tune the optical constants and
electric field distribution, as well as to reduce interfacial charge
recombinations within OPVs.
