Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 22

Scheme 2.2 Aromatic and quinoid forms of two conjugated polymers, PBDTBT and PBDTTPZ

has a smaller band gap than the aromatic form [ 12 ], the band gap of PBDTTPZ is
lower than that of PBDTBT. Besides of TPZ unit, several other conjugated
components and their derivatives with strong quinoid characteristics, as shown in
Scheme 2.3 , can also be used in LBG polymers.
To build a conjugated structure with alternative electron donor and electron
acceptor units (D/A structure) is another effective approach to get LBG polymer.
Many LBG polymers for solar cells were successfully designed based on this
method, but it is still hard to get an exact theoretical calculation for the band gap of
a newly designed conjugated structure. As known, the enhanced intra-molecular
charge transfer is helpful to get a broadened valance and conducting bands, and
hence lower band gap of a conjugated polymer with D/A structure can be reduced
by increasing electron donating effect of the electron donor units or by increasing
electron withdrawing effect of the electron acceptor units; the band gap of con-
jugated polymer with D/A structure is also tunable by changing the ratio of the
donor and acceptor moieties in the polymer backbone.

2.1.2 Molecular Energy Level

As discussed above, performance of an OPV device is closely related to molecular
energy levels of its active layer materials. Therefore, how to modulate HOMO and

Scheme 2.3 Several
aromatic building blocks that
can form stable quinoid
structures