Systematic Investigation
of Benzodithiophene- and
Diketopyrrolopyrrole-Based Low-Bandgap Polymers Designed for Single
Junction and Tandem Polymer Solar Cells
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Abstract
The tandem solar cell architecture is an effective way
to harvest
a broader part of the solar spectrum and make better use of the photonic
energy than the single junction cell. Here, we present the design,
synthesis, and characterization of a series of new low bandgap polymers
specifically for tandem polymer solar cells. These polymers have a
backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole
(DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated
onto the BDT unit to form BDTT and BDTP units, respectively; a furan
moiety was incorporated onto the DPP unit in place of thiophene to
form the FDPP unit. Low bandgap polymers (bandgap = 1.4β1.5
eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-coupling
polymerization. These structural modifications lead to polymers with
different optical, electrochemical, and electronic properties. Single
junction solar cells were fabricated, and the polymer:PC<sub>71</sub>BM active layer morphology was optimized by adding 1,8-diiodooctane
(DIO) as an additive. In the single-layer photovoltaic device, they
showed power conversion efficiencies (PCEs) of 3β6%. When the
polymers were applied in tandem solar cells, PCEs over 8% were reached,
demonstrating their great potential for high efficiency tandem polymer
solar cells