Silaindacenodithiophene-Based
Molecular Donor: Morphological
Features and Use in the Fabrication of Compositionally Tolerant, High-Efficiency
Bulk Heterojunction Solar Cells
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Abstract
A novel solution-processable small
molecule, namely, benzo[1,2-<i>b</i>:4,5-<i>b</i>]bis(4,4′-dihexyl-4<i>H</i>-silolo[3,2-<i>b</i>]thiophene-2,2′-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophene]-5-yl)benzo[<i>c</i>][1,2,5]thiadiazole (p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub>), was designed and synthesized by utilizing the silaindacenodithiophene
(SIDT) framework as the central D<sup>2</sup> donor unit within the
D<sup>1</sup>AD<sup>2</sup>AD<sup>1</sup> chromophore configuration.
Relative to the widely studied 7,7′-[4,4-bis(2-ethylhexyl)-4<i>H</i>-silolo[3,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl]bis[6-fluoro-4-(5′-hexyl-[2,2′-bithiophene]-5-yl)benzo[<i>c</i>][1,2,5]thiadiazole] (p-DTS(FBTTh<sub>2</sub>)<sub>2</sub>), which contains the stronger donor fragment dithienosilole (DTS)
as D<sup>2</sup>, one finds that p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub> exhibits a wider band gap and can be used to fabricate bulk heterojunction
solar cells with higher open circuit voltage (0.91 V). Most remarkably,
thin films comprising p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub> can achieve
exceptional levels of self-organization directly via solution deposition.
For example, high-resolution transmission electron microscopy analysis
shows that p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub> spin-cast from chlorobenzene
organizes into crystalline domains with lattice planes that extend
over length scales on the order of hundreds of nanometers. Such features
suggest liquid crystalline properties during the evolution of the
film. Moreover, grazing incidence wide-angle X-ray scattering analysis
shows a strong tendency for the molecules to exist with a strong “face-on”
orientation relative to the substrate plane. Similar structural features,
albeit of more restricted dimensions, can be observed within p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub>:PC<sub>71</sub>BM bulk heterojunction thin films
when the films are processed with 0.4% diiodooctane (DIO) solvent
additive. DIO use also increases the solar cell power conversion efficiencies
(PCEs) from 1.7% to 6.4%. Of significance from a practical device
fabrication perspective is that, for p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub>:PC<sub>71</sub>BM blends, there is a wide range of compositions
(from 20:80 to 70:30 p-SIDT(FBTTh<sub>2</sub>)<sub>2</sub>:PC<sub>71</sub>BM) that provide good photovoltaic response, i.e., PCE =
4–6%, indicating a robust tendency to form the necessary continuous
phases for charge carrier collection. Light intensity photocurrent
measurements, charge selective diode fabrication, and internal quantum
efficiency determinations were carried out to obtain insight into
the mechanism of device operation. Inclusion of DIO in the casting
solution results in films that exhibit much lower photocurrent dependence
on voltage and a concomitant increase in fill factor. At the optimum
blend ratio, devices show high charge carrier mobilities, while mismatched
hole and electron mobilities in blends with high or low donor content
result in reduced fill factors and device performance