Environmental Engineering Reference
In-Depth Information
2.1.1 Band Gap and Molecular Energy Level Control
Polymers, oligomers, and small molecular compounds with conjugated backbones
can be used as electron donor material in OPV devices. Although, these three
categories of compounds are different, as photovoltaic donor materials, the
requirements for their properties are quite similar. Broad absorption bands and
appropriate molecular energy levels are first required, and also the strategies used
to modulate absorption bands, and molecular energy levels of polymers, oligo-
mers, and small molecular compounds are similar. In this section, conjugated
polymers will be used as examples. Band gap (Eg) of conjugated polymer based on
the absorption edge obtained at long wavelength direction (Eg = 1240/kedge) is
used as a ruler to evaluate the absorption band of conjugated polymers. In order to
utilize more sunlight, absorption edge of conjugated polymer should be extended
to near-infrared region. Therefore, to find efficient way to lower band gap of
conjugated polymer is crucial to molecular design. As known, the band gap and the
molecular energy levels (HOMO and LUMO) of a conjugated polymer is closely
related to the molecular weight, molecular configuration and conformation, inter-
molecular interaction, effective conjugating length, and so on, and most of LBG
polymers were designed based on these parameters.
Two strategies are well used in molecular design of LBG polymer. To make a
conjugated structure with enhanced quinoid structure is an effective way to get
LBG material. When a quinoid structure is formed, the conjugation of the back-
bone will be enhanced and the pi-electrons will be more delocalized, and hence the
polymer's band gap will be lowered greatly. For example, molecular structure of
2,1,3-benzothiadiazole (BT) is similar as that of thieno[3,4-b]pyrazine (TPZ) (see
Scheme
2.2
), but these two components exhibited much different influence on
band gaps of conjugated polymers. When TPZ is copolymerized with a conjugated
building block, band gap of the resulting polymer is much lower than that of BT-
based polymer. For instance, the onset of the absorption spectrum of the alter-
nating copolymer of benzo[1,2-b:4,5-b
0
]dithiophene (BDT) and BT (PBDTBT in
Scheme
2.2
) was at *700 nm, corresponding to a Eg of 1.7 eV; however, the
onset of the absorption spectrum of PBDTTPZ was at *1000 nm, corresponding
to a Eg of 1.2 eV. As shown in Scheme
2.2
, BT and TPZ are formed by a five-
member ring fused with a six-member ring. For PBDTBT, the polymerization was
taken place on 4 and 7 positions on the five-member ring of the BT unit; and for
PBDTTPZ, the polymerization was taken place on 5 and 7 positions on the six-
member ring of the TPZ unit. The quinoid structure of the polymer PBDTTPZ can
be stabilized by the six-member ring of the TPZ units, and thus to form a more
stable aromatic electron structure for the six-member ring, whereas the electron
structure of the five-member ring of BT has no change in its aromatic and quinoid
structures. Based on this scheme, it can be seen that the six-member ring of TPZ
will stabilize the quinoid structure of the conjugated backbone of the polymer
better than the five-member ring of BT, and thus a more stable quinoid structure
will be formed in polymer PBDTPTZ than in PBDTBT. Since, the quinoid form
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