Influence of Fluorinated Additives on the Performance of Polymer Solar Cells
Ibraheem Almansouri a, Marcus Dahlem a, Calvyn Howells  a, Azza Al Bakr  a, Pascal Andre b
a Masdar Institute of Science and Technology
b RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Oral, Azza Al Bakr , presentation 106
Publication date: 7th November 2016

Poly(3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as an interlayer between the bulk-heterojunction and the indium tin oxide (ITO) is a common organic photovoltaic (OPV) structure. A subtle change to this interlayer can lead to improved OPV performance and stability. Two anionic fluorinated molecules, a perfluorinated ionomer (PFI) and a perfluooroooctane sulfonic acid (FOS), were investigated as additives in the interlayer. A ~15 % increase in power conversion efficiency (PCE) was observed for two different OPV systems. The leading mechanism responsible for the enhancement was determined and found to be different for each system. Both enhancement mechanisms, charge extraction (Voc, τCR) and photo-generation (Jsc, n-k, Q), originate from fluorinated induced charge transfers in the P3HT:PC61BM and PTB7:PC71BM OPVs, respectively. To help understand the mechanisms responsible for the enhancement and discriminate between their relative impacts on PCE, a plethora of measurements were performed. These include, but are not limited to: atomic force microscopy (AFM), kelvin probe (KPFM, mKP), ultraviolet photoelectron spectroscopy (UPS), contact angle, grazing incidence wide-angle x-ray scattering (GIWAXS), transient absorption, ellipsometry and electro-optical modelling. The enhancement in PCE was found to vary with fluorination concentration and the type of molecule, polymer (PFI), or small molecule (FOS). While both molecules effectively tune the electronic (work-function) and optical properties (refractive index) of the interlayer, at low concentrations the polymer dissolves within the interlayer. This disrupts charge transport between the PEDOT grains, leading to S-kinks in the J-V characteristics and low fill factors. No such injurious effect is observed with the small molecule, as it is not as soluble as the polymer. A decrease in standard deviation was noticed for OPVs employing the fluorinated additives. The degradation dynamics were investigated and two mechanisms identified. The first is associated with a fast exponential decay that reduces the PCE by ~ 25 % within 30 minutes of operation. The second is a slower almost linear decay. The latter is associated with the degradation of the active layer and top electrode by photochemical and thermochemical reactions, and oxidation and delamination, respectively. These are, unsurprisingly, unaffected with the fluorinated additives. However, the initial fast decay, which is shown to result from water entering the hygroscopic interlayer, is inhibited with the use of fluorinated additives. Therefore, fluorinated additives prove a promising strategy to improve OPV efficiency and stability by tuning the optical and electronic properties and inhibiting the absorption of water in the otherwise hygroscopic interlayer, respectively.

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