Fluorinated Copolymer
PCPDTBT with Enhanced Open-Circuit
Voltage and Reduced Recombination for Highly Efficient Polymer Solar
Cells
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Abstract
A novel fluorinated copolymer (F-PCPDTBT) is introduced
and shown
to exhibit significantly higher power conversion efficiency in bulk
heterojunction solar cells with PC<sub>70</sub>BM compared to the
well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer
HOMO level, resulting in high open-circuit voltages well exceeding
0.7 V. Optical spectroscopy and morphological studies with energy-resolved
transmission electron microscopy reveal that the fluorinated polymer
aggregates more strongly in pristine and blended layers, with a smaller
amount of additives needed to achieve optimum device performance.
Time-delayed collection field and charge extraction by linearly increasing
voltage are used to gain insight into the effect of fluorination on
the field dependence of free charge-carrier generation and recombination.
F-PCPDTBT is shown to exhibit a significantly weaker field dependence
of free charge-carrier generation combined with an overall larger
amount of free charges, meaning that geminate recombination is greatly
reduced. Additionally, a 3-fold reduction in non-geminate recombination
is measured compared to optimized PCPDTBT blends. As a consequence
of reduced non-geminate recombination, the performance of optimized
blends of fluorinated PCPDTBT with PC<sub>70</sub>BM is largely determined
by the field dependence of free-carrier generation, and this field
dependence is considerably weaker compared to that of blends comprising
the non-fluorinated polymer. For these optimized blends, a short-circuit
current of 14 mA/cm<sup>2</sup>, an open-circuit voltage of 0.74 V,
and a fill factor of 58% are achieved, giving a highest energy conversion
efficiency of 6.16%. The superior device performance and the low band-gap
render this new polymer highly promising for the construction of efficient
polymer-based tandem solar cells