Exciton dynamics in merocyanine/fulerene blend films made by vacuum deposition and solution processing
techniques were investigated by means of steady-state and time resolved fluorescence and
absorption spectroscopy. Intermolecular charge transfer states are formed during several ps in neat
merocianine films, which determine their fluorescence properties. Fullerene additives cause formation of
new heterogeneous charge transfer states. Even a small fullerene concentration significantly influences
the exciton dynamics by quenching inherent merocianine fluorescent states and causing appearance of new
fluorescence bands caused by the charge transfer states between merocyanine and fullerene molecules. All
fluorescence bands are quenched in films with high fulerence concentration due to the charge carrier generation,
and the quenching effect is stronger in vacuum deposited films.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3529

Devizis, A.

Gulbinas, V.

Hertel, D.

Peckus, D.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 4, 04NEA10(3pp) (2012)   2304-1862/2012/1(4)04NEA10(3) 04NEA10-1  2012 Sumy State University Excited State Relaxation in Vacuum Deposited and Solution Processed Films of  Merocyanine/Fulerene Blends  D. Peckus1,*, A. Devižis1, D. Hertel2, V. Gulbinas1,†  1 Center for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania 2 Department of Chemistry, Physical Chemistry, University of Cologne, Luxemburgerstr. 116, 50939 Cologne, Germany   (Received 22 June 2012; published online14 August 2012)  Exciton dynamics in merocyanine/fulerene blend films made by vacuum deposition and solution pro-cessing techniques were investigated by means of steady-state and time resolved fluorescence and absorption spectroscopy. Intermolecular charge transfer states are formed during several ps in neat merocianine films, which determine their fluorescence properties. Fullerene additives cause formation of new heterogeneous charge transfer states. Even a small fullerene concentration significantly influences the exciton dynamics by quenching inherent merocianine fluorescent states and causing appearance of new fluorescence bands caused by the charge transfer states between merocyanine and fullerene molecules. All fluorescence bands are quenched in films with high fulerence concentration due to the charge carrier gen-eration, and the quenching effect is stronger in vacuum deposited films.  Keywords: Merocyanine, Heterojunction, Exciton dynamics, Electron donor, Electron acceptor, Organic solar cells.    PACS numbers: 78.40.Me, 78.47.J –                                                              * domantas@ar.fi.lt  † vidgulb@ktl.mii.lt 1. INTRODUCTION   The growing need for energy stimulates scientific research directed to creation of efficient, low-cost ener-gy sources, one of them may be photovoltaic devices. Organic semiconductors have emerged as a new class of materials, which promise development low-cost and large-area solar cells. The bulk heterojunction (blend) concept has turned out to be the most promising ap-proach to highly efficient organic solar cells, since it features an extended interface between the phases of the electron donor compound and the electron acceptor compound and leads to an enhanced generation of free charge carriers [1,2]. Two main processing techniques are possible for fabrication of blend devices: vacuum deposition and solution processing. A broad variety of organic conju-gated polymers and small molecules can be used as electron donors and fullerene as an electron acceptor. Small molecules may be deposited under high vacuum conditions by thermal evaporation. On the other hand, soluble materials like polymers can be spin coated from solution. Many semiconducting polymers have been successfully applied in solution processed solar cells devices [3, 4].  Solution processing is a relatively fast and low-priced method for fabricating thin layers. However, the stacking of several layers on top of each other becomes a great problem, because new layer dissolve older one. Nevertheless, the highest solar cells made from organic polymers by solution processing technique shows effi-ciencies more then 9 % while small molecules shows efficiencies up to 6 % [3]. Polymer blends have been widely investigated, while investigations of blends of small polar molecules are less intensive. It is reliable that organic small mol-ecules have some unused potential, because of that more detail investigation of organic small molecules is necessary. Perhaps some technological improvement of vacuum deposition or solution processing methods may increase solar cells of small molecules efficiency dra-matically.  Recently, low-molecular-weight merocyanine dyes have been introduced as new absorbers and electron-donating compounds in blend solar cells [3-5]. It was found that small merocyanine molecules can be either thermally evaporated in vacuum or processed from solution. Here we address the excited state dynamics in blend films fabricated by the two processing methods.  MD376 molecules, which belong to the family of merocianine dyes, were used as samples in our meas-urements [3, 4].  Pure MD376 films and blends with fullerene deriva-tives at various concentration ratios have been investi-gated. MD376 solutions in dichloromethane and tolu-ene were also studied.  The main goal of our investigation was to compare ultrafast excitonic processes in different films and solu-tions. The charge transfer exciton formation features in films of MD376 were very interesting for us, because charge transfer exciton can be much more easily divid-ed into free charge carriers.   2.  EXPERIMENTAL  Steady-state and time resolved fluorescence spec-troscopy techniques were used for merocyanine sample investigations. The fluorescence spectra and fluores-cence decay kinetics were measured with an Edinburgh Instruments Fluorescence Spectrometer F900. A diode laser EPL-375 emitting 50 ps pulses at 375 nm with a repetition rate of 20 MHz were used.   D. PECKUS, A. DEVIŽIS, D. HERTEL, V. GULBINAS PROC. NAP 1, 04NEA10 (2012)   04NEA10-2 3. RESULTS AND DISCUTION   Steady-state fluorescence spectra of neat films reveal the presence of several molecular species, with relative concentrations strongly dependent on the film prepara-tion. Even low concentrations of fullerene or its deriva-tives cause a clear decrease of the merocianine fluores-cence intensity, while new fluorescence bands attributed to the charge transfer between merocianine and fuller-ene appear in the long wavelength region. Fluorescence intensity is quenched more significantly in vacuum de-posited blend samples (Fig. 1). This correlates with the fact that solar power conversion efficiency of merocya-nine cells prepared by vacuum deposition method is higher than of cells processes from solutions [3].   100010000100000550 600 650 700 750 8001000100001000001000000Fluorescence Sol 1:0 Sol 100:1 Sol 10:1 Sol 5:1 Sol 1:1aFluorescenceWavelength, nm Vac 1:0  Vac 9:1 Vac 7:1 Vac 1:1b  Fig. 1 – Fluorescence spectra of MD376 molecules (a) of film processed from solution (Sol) with various PCBM concentra-tions and (b) of vacuum deposited (Vac) film with various full-erene derivatives concentrations  Fluorescence spectra of films strongly depend on the film preparation conditions and aging. The spectra are composed of two main bands. Fluorescence band at about 650 nm shall be attributed to Frenkel excitons. The long wavelength fluorescence band at about 720 nm has very large Stokes shift, therefore it evidently belongs to some impurity or exciplex states. Both fluorescence bands of neat films decay almost exponentially with the identical lifetime of about 1 ns independently of the de-tection wavelength. That is for both solution processed and vacuum deposition technique made films (Fig. 2, 3).  PCBM additives cause strong quenching of the inher-ent MD376 fluorescence and cause appearance of the band with maximum at about 730 nm (Fig. 1a). Even 1% of PCBM causes quenching of the inherent fluorescence by about 30% and appearance of the new fluorescence band with comparable intensity. Surprisingly intensities of both fluorescence bands drop down in films with high PCBM concentration. This is attributed to the free charge carrier generation, which becomes efficient at high fuler-ene concentrations when its droplets are formed. Fluorescence decay in solution processed blend films strongly depends on the detection wavelength (Fig. 2). Fluorescence at 650 nm decays nonexponentially and much faster than in neat films, while fluorescence at 725 nm attributed at CT states between MD376 and fluler-ene decays almost exponentialy with about 3 ns time constant independently of the fulerene concentration.  0.010.1110100Normalized fluorescence  1:0 650 nm 100:1 650 nm 10:1 650 nm 5:1 650 nm 1:1 650 nm a0 2 4 6 8 10 12 140.010.1110Normalized fluorescencet, ns 1:0 725 nm 100:1 725 nm 10:1 725 nm 5:1 725 nm 1:1 725 nm b  Fig. 2 – Fluorescence decay kinetics at 650 nm (a) and 725 nm (b) of MD376 films processed from solution with various PCBM concentrations  Fluorescence of vacuum deposited films decays non-exponentialy at both detection wavelengths and its in-tensity decays more significantly with increase in fuler-ene concentration. (Fig. 3). It correlates with the higher solar energy conversion efficiency of the vacuum deposit-ed films. Morphology of the vacuum deposited films is evidently more favorable for the free carrier generation quenching fluorescent states.  0.11101000 2 4 6 8 10 12 140.1110100Normalized fluorescence  1:0 660 nm 9:1 660 nm 7:1 660 nm 1:1 670 nm aNormalized fluorescencet, ns 1:0 725 nm 9:1 725 nm 7:1 725 nm 1:1 725 nm b  Fig. 3 – Fluorescence decay kinetics at 650 nm (a) and 725 nm (b) of MD376 films made by vacuum deposition technique with various fullerene concentrations  4. CONCLUSIONS  Absorption and fluorescence spectra reveal different aspects of properties neat MD376 films. The spectra show different sensitivity to the material structure. Energy migration and excitation localization at low energy states makes fluorescence spectra very sensitive to minor molecular species and to formation of new low energy excited electronic states. Charge transfer states formed between merocianine molecules and those formed between merocianine and fulerene molecules  EXCITED STATE RELAXATION IN VACUUM DEPOSITED… PROC. NAP 1, 04NEA10 (2012)   04NEA10-3 determine fluorescence properties of neat films and blends. Generation of free charge carriers cause additional fluorescence quenching at high fulerence concentrations. ACKNOWLEDGEMENT   This research was funded by the European Social Fund under the Global Grant measure.   REFERENCES  1. J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376, 498 (1995). 2. G.Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 270, 1789 (1995). 3. N.M. Kronenberg, V. Steinmann, H. Bürckstümmer,  J. Hwang, D. Hertel, F. Würthner and K. Meerholz, Adv. Mater. 22, 4193 (2010).  4. V. Steinmann, N. M. Kronenberg, M.R. Lenze, S.M. Graf, D. Hertel, K. Meerholz, H. Bürckstümmer, E.V. Tulyakova,  F. Würthner, Adv. Energy Mater. 1, 888 (2011). 5. Frank Würthner, Theo E. Kaiser, and Chantu R. Saha-Möller, Angew. Chem. Int. Ed. 50, 3376 (2011).  

Excited State Relaxation in Vacuum Deposited and Solution Processed Films of Merocyanine/Fulerene Blends

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 4, 04NEA10(3pp) (2012) 
 
 
2304-1862/2012/1(4)04NEA10(3) 04NEA10-1  2012 Sumy State University 
Excited State Relaxation in Vacuum Deposited and Solution Processed Films of  
Merocyanine/Fulerene Blends 
 
D. Peckus1,*, A. Devižis1, D. Hertel2, V. Gulbinas1,† 
 
1 Center for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania 
2 Department of Chemistry, Physical Chemistry, University of Cologne, 
Luxemburgerstr. 116, 50939 Cologne, Germany  
 
(Received 22 June 2012; published online14 August 2012) 
 
Exciton dynamics in merocyanine/fulerene blend films made by vacuum deposition and solution pro-
cessing techniques were investigated by means of steady-state and time resolved fluorescence and 
absorption spectroscopy. Intermolecular charge transfer states are formed during several ps in neat 
merocianine films, which determine their fluorescence properties. Fullerene additives cause formation of 
new heterogeneous charge transfer states. Even a small fullerene concentration significantly influences 
the exciton dynamics by quenching inherent merocianine fluorescent states and causing appearance of new 
fluorescence bands caused by the charge transfer states between merocyanine and fullerene molecules. All 
fluorescence bands are quenched in films with high fulerence concentration due to the charge carrier gen-
eration, and the quenching effect is stronger in vacuum deposited films. 
 
Keywords: Merocyanine, Heterojunction, Exciton dynamics, Electron donor, Electron acceptor, Organic 
solar cells. 
 
  PACS numbers: 78.40.Me, 78.47.J –  
 
 
                                                          
* domantas@ar.fi.lt  
† vidgulb@ktl.mii.lt 
1. INTRODUCTION  
 
The growing need for energy stimulates scientific 
research directed to creation of efficient, low-cost ener-
gy sources, one of them may be photovoltaic devices. 
Organic semiconductors have emerged as a new class of 
materials, which promise development low-cost and 
large-area solar cells. The bulk heterojunction (blend) 
concept has turned out to be the most promising ap-
proach to highly efficient organic solar cells, since it 
features an extended interface between the phases of 
the electron donor compound and the electron acceptor 
compound and leads to an enhanced generation of free 
charge carriers [1,2]. 
Two main processing techniques are possible for 
fabrication of blend devices: vacuum deposition and 
solution processing. A broad variety of organic conju-
gated polymers and small molecules can be used as 
electron donors and fullerene as an electron acceptor. 
Small molecules may be deposited under high vacuum 
conditions by thermal evaporation. On the other hand, 
soluble materials like polymers can be spin coated from 
solution. Many semiconducting polymers have been 
successfully applied in solution processed solar cells 
devices [3, 4].  
Solution processing is a relatively fast and low-
priced method for fabricating thin layers. However, the 
stacking of several layers on top of each other becomes 
a great problem, because new layer dissolve older one. 
Nevertheless, the highest solar cells made from organic 
polymers by solution processing technique shows effi-
ciencies more then 9 % while small molecules shows 
efficiencies up to 6 % [3]. 
Polymer blends have been widely investigated, 
while investigations of blends of small polar molecules 
are less intensive. It is reliable that organic small mol-
ecules have some unused potential, because of that 
more detail investigation of organic small molecules is 
necessary. Perhaps some technological improvement of 
vacuum deposition or solution processing methods may 
increase solar cells of small molecules efficiency dra-
matically.  
Recently, low-molecular-weight merocyanine dyes 
have been introduced as new absorbers and electron-
donating compounds in blend solar cells [3-5]. It was 
found that small merocyanine molecules can be either 
thermally evaporated in vacuum or processed from 
solution. Here we address the excited state dynamics in 
blend films fabricated by the two processing methods.  
MD376 molecules, which belong to the family of 
merocianine dyes, were used as samples in our meas-
urements [3, 4].  
Pure MD376 films and blends with fullerene deriva-
tives at various concentration ratios have been investi-
gated. MD376 solutions in dichloromethane and tolu-
ene were also studied.  
The main goal of our investigation was to compare 
ultrafast excitonic processes in different films and solu-
tions. The charge transfer exciton formation features in 
films of MD376 were very interesting for us, because 
charge transfer exciton can be much more easily divid-
ed into free charge carriers.  
 
2.  EXPERIMENTAL 
 
Steady-state and time resolved fluorescence spec-
troscopy techniques were used for merocyanine sample 
investigations. The fluorescence spectra and fluores-
cence decay kinetics were measured with an Edinburgh 
Instruments Fluorescence Spectrometer F900. A diode 
laser EPL-375 emitting 50 ps pulses at 375 nm with a 
repetition rate of 20 MHz were used.  
 D. PECKUS, A. DEVIŽIS, D. HERTEL, V. GULBINAS PROC. NAP 1, 04NEA10 (2012) 
 
 
04NEA10-2 
3. RESULTS AND DISCUTION  
 
Steady-state fluorescence spectra of neat films reveal 
the presence of several molecular species, with relative 
concentrations strongly dependent on the film prepara-
tion. Even low concentrations of fullerene or its deriva-
tives cause a clear decrease of the merocianine fluores-
cence intensity, while new fluorescence bands attributed 
to the charge transfer between merocianine and fuller-
ene appear in the long wavelength region. Fluorescence 
intensity is quenched more significantly in vacuum de-
posited blend samples (Fig. 1). This correlates with the 
fact that solar power conversion efficiency of merocya-
nine cells prepared by vacuum deposition method is 
higher than of cells processes from solutions [3].  
 
1000
10000
100000
550 600 650 700 750 800
1000
10000
100000
1000000
F
lu
o
re
s
c
e
n
c
e
 Sol 1:0
 Sol 100:1
 Sol 10:1
 Sol 5:1
 Sol 1:1
a
F
lu
o
re
s
c
e
n
c
e
Wavelength, nm
 Vac 1:0 
 Vac 9:1
 Vac 7:1
 Vac 1:1
b
 
 
Fig. 1 – Fluorescence spectra of MD376 molecules (a) of film 
processed from solution (Sol) with various PCBM concentra-
tions and (b) of vacuum deposited (Vac) film with various full-
erene derivatives concentrations 
 
Fluorescence spectra of films strongly depend on the 
film preparation conditions and aging. The spectra are 
composed of two main bands. Fluorescence band at 
about 650 nm shall be attributed to Frenkel excitons. 
The long wavelength fluorescence band at about 720 nm 
has very large Stokes shift, therefore it evidently belongs 
to some impurity or exciplex states. Both fluorescence 
bands of neat films decay almost exponentially with the 
identical lifetime of about 1 ns independently of the de-
tection wavelength. That is for both solution processed 
and vacuum deposition technique made films (Fig. 2, 3).  
PCBM additives cause strong quenching of the inher-
ent MD376 fluorescence and cause appearance of the 
band with maximum at about 730 nm (Fig. 1a). Even 1% 
of PCBM causes quenching of the inherent fluorescence 
by about 30% and appearance of the new fluorescence 
band with comparable intensity. Surprisingly intensities 
of both fluorescence bands drop down in films with high 
PCBM concentration. This is attributed to the free charge 
carrier generation, which becomes efficient at high fuler-
ene concentrations when its droplets are formed. 
Fluorescence decay in solution processed blend films 
strongly depends on the detection wavelength (Fig. 2). 
Fluorescence at 650 nm decays nonexponentially and 
much faster than in neat films, while fluorescence at 725 
nm attributed at CT states between MD376 and fluler-
ene decays almost exponentialy with about 3 ns time 
constant independently of the fulerene concentration.  
0.01
0.1
1
10
100
N
o
rm
a
li
z
e
d
 f
lu
o
re
s
c
e
n
c
e  1:0 650 nm
 100:1 650 nm
 10:1 650 nm
 5:1 650 nm
 1:1 650 nm 
a
0 2 4 6 8 10 12 14
0.01
0.1
1
10
N
o
rm
a
li
z
e
d
 f
lu
o
re
s
c
e
n
c
e
t, ns
 1:0 725 nm
 100:1 725 nm
 10:1 725 nm
 5:1 725 nm
 1:1 725 nm 
b
 
 
Fig. 2 – Fluorescence decay kinetics at 650 nm (a) and 725 nm 
(b) of MD376 films processed from solution with various 
PCBM concentrations 
 
Fluorescence of vacuum deposited films decays non-
exponentialy at both detection wavelengths and its in-
tensity decays more significantly with increase in fuler-
ene concentration. (Fig. 3). It correlates with the higher 
solar energy conversion efficiency of the vacuum deposit-
ed films. Morphology of the vacuum deposited films is 
evidently more favorable for the free carrier generation 
quenching fluorescent states. 
 
0.1
1
10
100
0 2 4 6 8 10 12 14
0.1
1
10
100
N
o
rm
a
li
z
e
d
 f
lu
o
re
s
c
e
n
c
e  1:0 660 nm
 9:1 660 nm
 7:1 660 nm
 1:1 670 nm 
a
N
o
rm
a
li
z
e
d
 f
lu
o
re
s
c
e
n
c
e
t, ns
 1:0 725 nm
 9:1 725 nm
 7:1 725 nm
 1:1 725 nm 
b
 
 
Fig. 3 – Fluorescence decay kinetics at 650 nm (a) and 725 nm 
(b) of MD376 films made by vacuum deposition technique with 
various fullerene concentrations 
 
4. CONCLUSIONS 
 
Absorption and fluorescence spectra reveal different 
aspects of properties neat MD376 films. The spectra 
show different sensitivity to the material structure. 
Energy migration and excitation localization at low 
energy states makes fluorescence spectra very sensitive 
to minor molecular species and to formation of new low 
energy excited electronic states. Charge transfer states 
formed between merocianine molecules and those 
formed between merocianine and fulerene molecules 
 EXCITED STATE RELAXATION IN VACUUM DEPOSITED… PROC. NAP 1, 04NEA10 (2012) 
 
 
04NEA10-3 
determine fluorescence properties of neat films and 
blends. Generation of free charge carriers cause 
additional fluorescence quenching at high fulerence 
concentrations. 
ACKNOWLEDGEMENT 
 
 This research was funded by the European Social 
Fund under the Global Grant measure. 
 
 
REFERENCES 
 
1. J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, 
R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376, 498 (1995). 
2. G.Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, 
Science 270, 1789 (1995). 
3. N.M. Kronenberg, V. Steinmann, H. Bürckstümmer,  
J. Hwang, D. Hertel, F. Würthner and K. Meerholz, Adv. 
Mater. 22, 4193 (2010).  
4. V. Steinmann, N. M. Kronenberg, M.R. Lenze, S.M. Graf, D. 
Hertel, K. Meerholz, H. Bürckstümmer, E.V. Tulyakova,  
F. Würthner, Adv. Energy Mater. 1, 888 (2011). 
5. Frank Würthner, Theo E. Kaiser, and Chantu R. Saha-
Möller, Angew. Chem. Int. Ed. 50, 3376 (2011). 
 


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Excited State Relaxation in Vacuum Deposited and Solution Processed Films of Merocyanine/Fulerene Blends

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