Chemistry Reference
In-Depth Information
FIGURE 9.29
(a) Absorption spectra of OTV-truxene dendrimers and model compounds.
(b) J-V curves of a single layer BHJSC with these dendrimers and PCBM as photoactive layers
under illumination of AM 1.5, 100mW/cm
2
[112].
to linear polymer. The reason is that dendrimers usually have shorter conjugation
length than corresponding polymers. This drawback can be overcome by introducing
small units that are able to absorb light in that region, such as diketopyrrolopyr-
role [115]. We expect to see dendrimers to emerge as the third type of materials for
OSCs, just as they did as DLEDs.
9.4.3 Two-Photon Absorption Material
Nonlinear optics (NLO) describes the behavior of light when the dielectric polar-
ization P responds nonlinearly to the electric field E of the light [116]. Here we will
mainly focus on the phenomenon of two-photon absorption (TPA), a third-order
nonlinear optical process [117]. TPA is the process that a molecule simultaneously
absorbs two photons and gets excited from one state to a higher energy electronic
state. Consequently, the strength of absorption depends on the square of the light
intensity. TPA materials have demonstrated many potential applications including
optical data storage, multiphoton microscopy, 3-D microfabrication, optical power
limiting, and frequency up-conversion lasing [118].
The most important parameter of a TPAmaterial is TPA cross-section
s
2
. Its unit is
GM, where 1 GM is 10
50
cm
4
s/photon. Great effort has been made to obtain
materials with large TPA cross-section, usually with a long conjugated system with
electron-donating group and electron-accepting group at two ends [119]. Compared
to linear polymers, a conjugated dendrimer has the ability to concentrate a large
number of chromophores, whose number grows exponentially with generation.
Meanwhile, interchromophore coupling in some conjugated dendrimers has also
been shown to be beneficial to nonlinear optical enhancement.
Goodson's group has extensively studied various photophysical processes in
triphenylamine-based dendrimers [120]. In contrast to 1,3,5-benzene or truxene
branching centers, the sp
2
-hybridized nitrogen provides much stronger inter-
branch coupling, resulting in large enhancement of TPA properties. For example,
Figure 9.30 shows a series of triphenylamine dendrimers using acetylene link-
age [121]. With the generation growing from zeroth to second, the
s
2
increases
