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K. Kamada et al.
molecular species, i.e, triplet exciton migration. This is a necessary process for two
emitter triplets to encounter and to cause TTA, which has been known to occur in
the molecular crystal [15]. Triplet exciton migration may be slower than that of TTA
and be an important process governing the efficiency of overall (or wide meaning)
TTA process.
TET is another bimolecular process and a critically important process in TTA-UC.
In the solution system, the process is mostly diffusion-controlled like TTA, and the
triplet level matching between sensitizer and emitter is a remaining essential factor.
In solid system, on the other hand, direct contact between sensitizer and emitter
with keeping the energy-level matching is critically important. The details will be
discussed in the following sections.
9.2 Experimental Studies on Triplet–Triplet Annihilation
Photon Upconversion in Solid and Related Systems
9.2.1 TTA-UC in Binary Solid
TTA-UC has been developed in solution systems because molecular diffusion can
mediate TET and TTA processes during the lifetime of their triplets. However, they
have the inconvenience of handling and require hermetic sealing because they are
volatile, flammable, and easily dissolving oxygen that quenches triplets. Thus, solid
systems are suitable for device applications. There are the approaches of the diluted
solid systems, such as doped-polymer and gel; however, molecular diffusion mediated TET and TTA in these systems are slow because of the high viscosity, leading low
overall efficiency. Thus, the solid system with condensed chromophore of the emitter
is preferable media because triplet exciton migration can mediate the processes,
allowing the realization of fast and efficient bimolecular processes.
From this point of view, binary crystals can be an ideal medium for solid TTAUC because the speed of migration in a crystal can be higher than in other dense
media thanks to the crystal regularity and dense packing. As TTA-UC medium, binary
solids, however, has a problem of segregation of the sensitizer from the emitter matrix
[14, 16]. Segregation forms the dimer of sensitizers. The triplet level of the dimer is
lower than that of sensitizer, which interferes with TET. High excitation intensities
in the order of kW/cm
2 , was reported to induce TTA-UC in binary solids [17].
In order to overcome the segregation problem in binary solid, we developed a
simple method called rapid-drying casting [12]. The key idea is crystallizing host
matrix of emitter before guest molecule of sensitizer aggregating. We choose Ptoctaethylporphyrin (PtOEP) as sensitizer and 9,10-diphenyl-anthracene (DPA) as
emitter (Fig. 9.2a) because the PtOEP-DPA system has been used in many works
and can be a good benchmark. Moreover, DPA has very high fluorescence (FL) QY,
FL = 0.95, in crystals [18]. The binary crystal (PtOEP:DPA) were fabricated by
drop casting from mixed solution on a glass slide at the aerated condition. For the
K. Kamada et al.
molecular species, i.e, triplet exciton migration. This is a necessary process for two
emitter triplets to encounter and to cause TTA, which has been known to occur in
the molecular crystal [15]. Triplet exciton migration may be slower than that of TTA
and be an important process governing the efficiency of overall (or wide meaning)
TTA process.
TET is another bimolecular process and a critically important process in TTA-UC.
In the solution system, the process is mostly diffusion-controlled like TTA, and the
triplet level matching between sensitizer and emitter is a remaining essential factor.
In solid system, on the other hand, direct contact between sensitizer and emitter
with keeping the energy-level matching is critically important. The details will be
discussed in the following sections.
9.2 Experimental Studies on Triplet–Triplet Annihilation
Photon Upconversion in Solid and Related Systems
9.2.1 TTA-UC in Binary Solid
TTA-UC has been developed in solution systems because molecular diffusion can
mediate TET and TTA processes during the lifetime of their triplets. However, they
have the inconvenience of handling and require hermetic sealing because they are
volatile, flammable, and easily dissolving oxygen that quenches triplets. Thus, solid
systems are suitable for device applications. There are the approaches of the diluted
solid systems, such as doped-polymer and gel; however, molecular diffusion mediated TET and TTA in these systems are slow because of the high viscosity, leading low
overall efficiency. Thus, the solid system with condensed chromophore of the emitter
is preferable media because triplet exciton migration can mediate the processes,
allowing the realization of fast and efficient bimolecular processes.
From this point of view, binary crystals can be an ideal medium for solid TTAUC because the speed of migration in a crystal can be higher than in other dense
media thanks to the crystal regularity and dense packing. As TTA-UC medium, binary
solids, however, has a problem of segregation of the sensitizer from the emitter matrix
[14, 16]. Segregation forms the dimer of sensitizers. The triplet level of the dimer is
lower than that of sensitizer, which interferes with TET. High excitation intensities
in the order of kW/cm
2 , was reported to induce TTA-UC in binary solids [17].
In order to overcome the segregation problem in binary solid, we developed a
simple method called rapid-drying casting [12]. The key idea is crystallizing host
matrix of emitter before guest molecule of sensitizer aggregating. We choose Ptoctaethylporphyrin (PtOEP) as sensitizer and 9,10-diphenyl-anthracene (DPA) as
emitter (Fig. 9.2a) because the PtOEP-DPA system has been used in many works
and can be a good benchmark. Moreover, DPA has very high fluorescence (FL) QY,
FL = 0.95, in crystals [18]. The binary crystal (PtOEP:DPA) were fabricated by
drop casting from mixed solution on a glass slide at the aerated condition. For the