group can be the following dyes: metal-free phthalocyanine (free-base phthalocyanine—H 2 Pc), metallic phthalocyanine (M-Pc—e.g., CuPc, ZnPc), and those
substituted with peripheral groups like zinc phthalocyanine substituted with 16
fluorine atoms (ZnPcF 16 ), zinc one substituted with 12 chlorine atoms (ZnPcCl 12 ),
zinc dye substituted with long alkyloxy chains (ZnPcAlkyl), and unsymmetrical
indium phthalocyanine (InPc) and gallium (GaPc) substituted with terminal alkynyl
group [18–21], methane groups at peripheral positions and many others moieties
[6–21].
A large energy gap between the porphyrin and phthalocyanine absorption bands
is unfavorable and can lead to reduction in the efficiency of energy transport
between the dyes. One of the good solutions to this problem is the use of other dyes
that fill the missing energy gap—e.g., merocyanine dyes which posses high
absorption coefficient in the range of 450–580 nm [23–25]. The merocyanine dye
itself is not rather good energy converter. As shown by spectroscopic studies, the
light energy absorbed by merocyanines completes the porphyrin-phthalocyanine
energy gap, and as a result it can lead to increase photovoltaic cell efficiency
[26, 27].
In many systems, photoinduced electron transport is studied by fluorescence and
electron spin resonance (ESR) [28]. The transparent and simple example could be a
mixture of porphyrin and quinone. In a mixture composed of porphyrin (H 2 TPP,
MgTPP, ZnTPP) and quinone in dimethyloxide (DMSO), energy transfer was
observed by quenching fluorescence supported by the ESR experiment. The
changes in the dye’s fluorescence quantum yield, declining dye’s lifetime in the
presence of quinone and the Stern–Volmer equation analysis confirm quinone as a
good quencher. Those results confirmed occurrence of the electron transfer process
between the dye (donor) and quinone (acceptor). Another spectacular example how
the changes in the substituent in porphyrins affect dye’s photoactivity is a series of
studies of meso-tetraphenylzincporphyrins sensitized in TiO 2 cells [29–31]. In this
point, it is worth to underline that the spectroscopic and photovoltaic experiments
done for photoactive materials in the simple models are placed in, e.g., nematic
liquid crystal (MBBA-EBBA), DMSO or polyvinyl alcohol with DMSO
(PVA-DMSO) or in polymers [5–30].
In many approaches, the experiments are carried out for the systems in the
volume phase as well as for thin-film systems (Langmuir, Langmuir-Blodgett,
Langmuir-Schaefer layers, and others). The systems based on organic chromophores absorb strongly light in the range of solar exposure. However, efficiency
of the systems based on organics is rather low. The essential confinements are:
• low mobility of electron carriers in organic materials—this effect is partially
compensated by large absorption coefficients (>10
5 cm
−1 ),
• and rather small diffusion depth of photoexcitation—it limits the thickness of
organic layers to ca. 10 nm.
102
D. Wróbel and B. Barszcz
substituted with peripheral groups like zinc phthalocyanine substituted with 16
fluorine atoms (ZnPcF 16 ), zinc one substituted with 12 chlorine atoms (ZnPcCl 12 ),
zinc dye substituted with long alkyloxy chains (ZnPcAlkyl), and unsymmetrical
indium phthalocyanine (InPc) and gallium (GaPc) substituted with terminal alkynyl
group [18–21], methane groups at peripheral positions and many others moieties
[6–21].
A large energy gap between the porphyrin and phthalocyanine absorption bands
is unfavorable and can lead to reduction in the efficiency of energy transport
between the dyes. One of the good solutions to this problem is the use of other dyes
that fill the missing energy gap—e.g., merocyanine dyes which posses high
absorption coefficient in the range of 450–580 nm [23–25]. The merocyanine dye
itself is not rather good energy converter. As shown by spectroscopic studies, the
light energy absorbed by merocyanines completes the porphyrin-phthalocyanine
energy gap, and as a result it can lead to increase photovoltaic cell efficiency
[26, 27].
In many systems, photoinduced electron transport is studied by fluorescence and
electron spin resonance (ESR) [28]. The transparent and simple example could be a
mixture of porphyrin and quinone. In a mixture composed of porphyrin (H 2 TPP,
MgTPP, ZnTPP) and quinone in dimethyloxide (DMSO), energy transfer was
observed by quenching fluorescence supported by the ESR experiment. The
changes in the dye’s fluorescence quantum yield, declining dye’s lifetime in the
presence of quinone and the Stern–Volmer equation analysis confirm quinone as a
good quencher. Those results confirmed occurrence of the electron transfer process
between the dye (donor) and quinone (acceptor). Another spectacular example how
the changes in the substituent in porphyrins affect dye’s photoactivity is a series of
studies of meso-tetraphenylzincporphyrins sensitized in TiO 2 cells [29–31]. In this
point, it is worth to underline that the spectroscopic and photovoltaic experiments
done for photoactive materials in the simple models are placed in, e.g., nematic
liquid crystal (MBBA-EBBA), DMSO or polyvinyl alcohol with DMSO
(PVA-DMSO) or in polymers [5–30].
In many approaches, the experiments are carried out for the systems in the
volume phase as well as for thin-film systems (Langmuir, Langmuir-Blodgett,
Langmuir-Schaefer layers, and others). The systems based on organic chromophores absorb strongly light in the range of solar exposure. However, efficiency
of the systems based on organics is rather low. The essential confinements are:
• low mobility of electron carriers in organic materials—this effect is partially
compensated by large absorption coefficients (>10
5 cm
−1 ),
• and rather small diffusion depth of photoexcitation—it limits the thickness of
organic layers to ca. 10 nm.
102
D. Wróbel and B. Barszcz