influence of the molecular stacking pattern on the excited

CHINESE JOURNAL OF CHEMICAL PHYSICS AUGUST 3, 2021

ARTICLE

Influence of the Molecular Stacking Pattern on the Excited State

Dynamics of Copper Phthalocyanine Films

Meng Lia, Wen-hui Lia, Yu-jie Hua, Jing Lengb, Wen-ming Tianb, Chun-yi Zhaob, Jun-xue Liub,

Rong-rong Cuib, Sheng-ye Jinb, Chuan-hui Chenga∗, Shu-lin Conga

a. School of Physics, Dalian University of Technology, Dalian 116024, Chinab. State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, ChineseAcademy of Sciences, Dalian 116023, China

(Dated: Received on March 22, 2021; Accepted on July 5, 2021)

A better understanding of the photophysical pro-

cesses occurring within organic semiconductors is

important for designing and fabricating organic so-

lar cells (OSCs). Copper phthalocyanine (CuPc)

is a typical electron acceptor. In this work, the

triplet exciton lifetime is prolonged by altering

the molecular stacking pattern of the CuPc film.

For CuPc thin films, the excited state decays are

mainly determined by the triplet-triplet annihila-

tion process. The ultrafast transient absorption

measurements indicate that the primary annihila-

tion mechanism is one-dimensional exciton diffu-

sion collision destruction. The decay kinetics show

a clearly time-dependent annihilation rate constant

with γ∝t−1/2. Annihilation rate constants are deter-

mined to be γ0=(2.87±0.02)×10−20 cm3·s−1/2 and

(1.42±0.02)×10−20 cm3·s−1/2 for upright and lying-

down configurations, respectively. Compared to the CuPc thin film with an upright config-

uration, the thin film with a lying-down configuration shows a longer exciton lifetime and a

higher absorbance, which are beneficial for OSCs. The results in this work have important

implications on the design and mechanistic understanding of organic optoelectronic devices.

Key words: CuPc, Photophysics, Excited state, Triplet-triplet annihilation, Organic solar

cell

I. INTRODUCTION

The energy problem is a serious modern concern. In

recent years, organic solar cells (OSCs) have attracted

considerable attention owing to their potential to pro-

vide a low-cost, light-weight, and flexible strategy for

solar energy conversion [1, 2]. A better understanding of

∗Author to whom correspondence should be addressed. E-mail:

[email protected]

the photophysical processes within such cells is impor-

tant for designing new materials and devices. Ultrafast

pump-probe techniques are powerful tools for studying

excited-state dynamics in OSC systems [3–6]. Phthalo-

cyanines are archetypical electron donors which are de-

posited by vacuum evaporation to form films for use

in such cells [7–10]. The molecular stacking configura-

tion and crystal structure can markedly affect material

characteristics, such as light absorption, charge trans-

port, exciton diffusion, and molecular energy levels [11–

14]. In our previous work, we considerably improved the

DOI:10.1063/1674-0068/cjcp2103052 c⃝2021 Chinese Physical Society

2 Chin. J. Chem. Phys. Meng Li et al.

device performance by altering the molecular stacking

configuration of copper phthalocyanine (CuPc) from an

upright to a lying-down configuration in OSCs based on

the heterojunction of CuPc/C60 [15]. Herein, we report

a study of the excited-state dynamics of the CuPc films

with lying-down and upright molecular stacking con-

figuration using an ultrafast transient absorption (TA)

spectroscopy. Our results suggest that compared with

the upright configuration, the excited state lifetime is

longer, and the absorbance is higher for the lying-down

configuration, which are of benefit for OSCs. We also

elucidate the excited state decay mechanism.

II. EXPERIMENTS

The quartz substrates were ultrasonicated in ace-

tone, alcohol, and deionized water in sequence, and

then blown dry with pure N2 gas. CuPc (sublimed

grade, 99.0%) and CuI (99.9%) were purchased from

Nichem and Aldrich, respectively. All the films were

prepared by the vacuum evaporation under a base pres-

sure of ≤5×10−4 Pa. The film thicknesses were mon-

itored online by a quartz-crystal microbalance. Atom

force microscopy (AFM) morphologies were measured

on a Bruker Dimension Icon. Steady state absorp-

tion spectra were measured on a Shimadzu UV3600

spectrophotometer. TA spectra (Time-Tech Spec-

tra, Femto-TA100) were measured using previously de-

scribed methods [16]. The full-width at half-maximum

of the pump pulse was ∼80 fs, and the time resolution

was ∼150 fs. All the TA spectra were measured with a

700-nm pump pulse. The pump beam was focused to a

diameter of∼300 µm. The sample was moved with a ve-

locity of 0.3 mm/s during the TA measurements. There

is no evident difference in the dynamics of the samples

with and without an encapsule, and all the measure-

ments were performed in ambient atmosphere at room

temperature without encapsulation.

III. RESULTS AND DISCUSSION

We prepared two kinds of CuPc films, one with a

lying-down and the other with an upright molecular

stacking configuration, which were determined by X-ray

diffraction in our previous study [15]. The schematic

diagrams of the molecular stacking configuration in the

film and energy levels are shown in FIG. 1. The stacking

configuration was converted from upright to lying-down

by introducing a 3.0-nm thick CuI buffer layer [15].

FIG. 1 Molecular structure of CuPc. Schematic diagramsof energy levels and molecular stacking configurations. Theπ-stacking direction is given.

FIG. 2 AFM surface morphologies of the films with (a) anupright and (b) a lying-down configuration.

AFM surface morphologies of the films are given in

FIG. 2. It can be seen that the surface morphologies

are very different from the films with lying-down and

upright configurations. The crystal grains can be seen

clearly for the lying-down configuration. The absorp-

tion spectra of CuI and CuPc films with different stack-

ing configurations are shown in FIG. 3. Both spectra

of the CuPc films showed a Soret-band at around 350

nm and a Q-band from 550 nm to 800 nm. The Soret-

band has been assigned to the transition of S0→S2, and

the Q-band corresponds to the transition of S0→S1 [17].

Here, S0, S1, and S2 are the ground, the first, and the

second excited singlet states. There are two peaks in

the Q-band. The longer wavelength feature arises from

intramolecular excitations, and the shorter wavelength

peak is from intermolecular charge transfer excitations

in molecular aggregates [18]. Furthermore, there are

two shoulders located at the low- and high-energy ab-

sorption edge of the Q-band. The high-energy shoulder


Chin. J. Chem. Phys. Influence of Molecular Stacking Pattern

FIG. 3 Steady state absorption spectra of CuPc films withdifferent stacking configurations together with that of CuI.

is caused by the formation of higher order aggregate,

and the low-energy shoulder is related to the differ-

ent torsional conformational forms of these higher or-

der aggregates [19]. The absorbance of the film with a

lying-down configuration is much higher than that with

upright configuration. For example, at the peak wave-

length of the Q-band (621 nm), the absorbance is 0.17

for the lying-down configuration, which is 1.7 times as

high as that for the upright configuration. The molecu-

lar absorption cross-section σ is satisfied by the follow-

ing relation [20]:

σ ∝ (E ·M)2

where E is the electronic field vector of incident light,

and M is the molecular transition moment. The molec-

ular absorption cross-section is proportional to the

square of the dot product of E and M, which is maxi-

mized when E is parallel with M. For π-π∗ transitions

of the planar CuPc molecule, the transition moments

are parallel with the molecular plane [20]. During the

absorption measurements, incident light is perpendicu-

lar to the substrate plane and the electric field vector

is in parallel with the substrate. Thus, the molecu-

lar absorption cross-section is larger for the lying-down

stacking configuration compared with that of the up-

right stacking configuration.

To investigate excited state dynamics of CuPc films

with different stacking configuration, we performed ul-

trafast TA measurements. The CuPc was selectively

excited by a 700-nm laser pulse, corresponding to the

intramolecular excitation. We observed no measurable

response from CuI, which is consistent with the negligi-

ble absorption of the CuI layer at the pump wavelength

of 700 nm. The band gap of CuPc (∼1.6 eV) is much

smaller than that of CuI (∼3.1 eV), preventing the ex-

cited state energy transfer from CuPc to CuI. Moreover,

the lowest unoccupied molecular (LUMO) level of CuPc

(−3.5 eV) is much lower than the conduction band edge

of CuI (−2.0 eV), blocking the electron transfer from

CuPc to CuI. For the exciton in CuPc, the electron

and the hole are bound by the Coulomb interaction.

This binding energy is very large (0.3−0.5 eV) because

of its low relatively dielectric constant (∼3). So, the

hole transfer from CuPc to CuI requires energy to sep-

arate the electron and the hole in the exciton. However,

there is no energy difference between the HOMO level

of CuPc and the valence-band edge of CuI. Therefore,

the hole transfer probability is low and can be ignored.

Based on the above analysis, we can conclude that the

photophysical processes following the excited pulse oc-

curs within the CuPc films.

FIG. 4 shows the TA spectra of CuPc films with up-

right and lying-down configurations, recorded at dif-

ferent pump-probe delays under a pump fluence of 15

µJ/cm2. Both of the TA spectra exhibit negative ab-

sorbance difference signals at around 630 and 739 nm

and positive signals from 450 nm to 500 nm, which are

similar to previous reports on phthalocyanines, such

as CuPc [21], ZnPc [22], and TiOPc [23]. The tran-

sient signal intensity of the CuPc film with the lying-

down configuration is stronger than those in the film

with the upright configuration. We attribute this dif-

ference to the more pump photons being absorbed by

the film with the lying-down configuration owing to its

greater absorbance. The negative signal is originated

from ground state bleaching (GSB) caused by deple-

tion of the ground state population. We rule out stim-

ulated emission because there is no measurable fluores-

cence from CuPc, and its long-lived phosphorescence at

around 1120 nm can be observed only at lower temper-

ature [24]. In CuPc, coupling between the unpaired d-

electron in the Cu ion and the ligand causes the singlet

state to change to a singdoublet (2S). The normal triplet

state splits into a tripdoublet (2T) and a tripquartet

(4T) [25]. Because the relaxation process from 2S1 to2T1 or

4T1 is spin-allowed, it can take place rapidly with

characteristic time of ∼0.5 ps [26], resulting in a triplet

quantum yield of near 100% [16]. Some phthalocyanines

are known to show triplet absorption at around 500 nm

[21, 27, 28]. Thus, 1.0 ps after the pump pulse, the pos-



FIG. 4 TA spectra of CuPc films with (a) upright and (b)lying-down configurations measured at the indicated timedelays.

FIG. 5 Kinetic traces probed at 738 nm for CuPc films.

itive TA band mainly originated from the excited trip-

doublet and tripquartet state. Here, we cannot clearly

attribute the observed decay to either tripdoublet or

tripquartet state, which are considered as a whole and

are referred as a triplet state in this work. As can be

seen from FIG. 4, there is an ultrafast recovery process

following the pump pulse at around 735 nm. The dy-

namic traces are shown in FIG. 5. The characteristic

time is 0.5 ps for both the samples. This process is

assigned to be intersystem crossing [21].

The main photophysical processes are shown in FIG.

6. Following the photo pumping, CuPc molecules are

excited to the first excited singlet states. Subsequently,

almost all singlet states relax to triplet states through

ultrafast intersystem crossing owing to the near 100%

triplet quantum yield. Hence, the excited state de-

FIG. 6 Schematic diagram of the main photophysical pro-cesses in CuPc films.

cay process mainly depends on the decay of excited

triplet states on the timescale of 1.0 ps after the pump

pulse. In general, there are three possible decay path-

ways from excited triplet states to ground states: (i)

phosphorescence, (ii) exciton-phonon coupling (internal

conversion), and (iii) triplet-triplet annihilation (TTA).

Because the triplet exciton lifetime is generally very

long, TTA can occur on a long timescale. To exam-

ine the presence of TTA, we measured the TA spec-

tra under different pump fluences. FIG. 7 shows the

triplet dynamic traces under excitation intensities of

15 and 35 µJ/cm2. The decay of these signals is

highly nonexponential, and the decay rate is increased

as the pump fluence increases. When the TTA reaction

of 3A∗+3A∗→3A∗+A occurs, where 3A∗ denotes the

triplet excited state, and A denotes the ground state,

the triplet population can be described by the following

rate equation [29]:

dn

dt= −n

τ− γn2 (1)

where n is the triplet exciton density, τ is the intrinsic

triplet exciton lifetime in the absence of annihilation,

and γ is the annihilation rate constant. In principle,

time-dependent exciton annihilation rates are expected.

This originates from the fact that progressively greater

inter-exciton distances result in decreasing interaction

rates. When the timescale is much shorter than the

intrinsic triplet exciton lifetime τ , Eq.(1) can be reduced

to

dn

dt= −γn2 (2)

There are two TTA mechanisms. One is the exci-

ton diffusion collision annihilation, and the other is the



static annihilation via Forster long-range dipole-dipole

interaction. Forster energy transfer requires that the

transition of the energy donor is allowed. However, the

transition of 3A∗→A is spin-forbidden for CuPc. There-

fore, we can conclude that the static annihilation mech-

anism is not important. Our conclusion consists with

that the energy transport in CuPc film is dominated

by short-range Dexter mechanism [21]. In contrast to

the triplet exciton in CuPc, the singlet exciton annihi-

lation in polycrystalline film of H2Pc is dominated by

the static annihilation via Forster energy transfer mech-

anism [38]. Since the static annihilation mechanism is

ruled out, the diffusion collision annihilation mechanism

is dominant in the TTA process in CuPc films. In this

case, annihilation rate constant can be described as,

for one-dimensional diffusion:

γ1D(t) =

√2D

πt(3)

for three-dimensional diffusion [30, 31]:

γ3D(t) = 4πRaD

(1 +

Ra√2πDt

)(4)

where D is the diffusion coefficient, Ra is the critical

distance at which exciton annihilation reaction takes

place, and t is the time. In general Ra is assumed to be

the separation of adjacent molecules. When t is much

larger than Ra2/2πD, the Eq.(4) can be reduced to

γ3D = 4πRaD (5)

In the case of one-dimensional diffusion, annihilation

rate Eq.(2) can be rewritten as

dn

dt= −γ0t

−1/2n2 (6)

where γ0 is a constant. Integration of Eq.(6) yields an

expression of the time-dependent exciton density:

n(t) =

(2γ0

√t+

1

n0

)−1

(7)

where n0 is the initial exciton concentration at t=0. If

we plot the kinetic traces in the form of (1/n−1/n0) vs.

t1/2, kinetics obeying rate equation (Eq.(6)) will yield a

straight line with a slope of 2γ0. The initial triplet den-

sities n0 for the lying-down and upright configurations

were estimated to be 6.85×1018 cm−3 and 3.95×1018

cm−3 based on their absorbance values of 0.13 and

0.07, respectively. FIG. 8 shows the kinetic traces in

the form of (1/n−1/n0) vs. t1/2, which yield straight

lines for both the lying-down and the upright configu-

rations. The results suggest that one-dimensional dif-

fusion collision annihilation is the dominant mechanism

in CuPc films. It is similar to exciton annihilation in

polycrystalline film of H2Pc, which shows a clearly time-

dependent annihilation rate constant with γ∝t−1/2 [38].

The kinetic traces were fitted to Eq.(7), and solid lines

are the fitting results as shown in FIG. 8. For the

CuPc film with a lying-down configuration, TTA rate

constant is γ0=(1.42±0.02)×10−20 cm3·s−1/2, which is

smaller than that for upright configuration of (2.87±0.02)×10−20 cm3·s−1/2. The results indicate that the

triplet exciton lifetime is longer for the CuPc film with

a lying-down configuration, which is of benefit for OSCs.

In our experiment the film thickness is 20 nm, which

is the optimized thickness of our solar cell based on

CuPc/C60 heterojunction [15]. This thin thickness

should thus limit the exciton diffuse along the direction

perpendicular to the substrate. Hence, collision anni-

hilation mainly depends on exciton hopping in parallel

with the substrate. In the anisotropic CuPc film, ex-

citon hopping along the π-π stacking direction is more

favorable owing to the stronger electron coupling, in

agreement with the above conclusion of one-dimensional

diffusion model. In order to elucidate this issue more

clearly, the π-π stacking direction is given in FIG. 1. As

shown in FIG. 1, for the CuPc film with a lying-down

configuration, exciton hopping is more difficult in the

direction parallel with the substrate, leading to a lower

collision annihilation probability and a longer exciton

lifetime, which are desirable for OSCs. Furthermore, π-

π stacking in direction perpendicular to the substrate is

in favor of the carrier collection and the exciton diffu-

sion to the interface of heterojunction. Therefore, the

CuPc film with a lying-down configuration is more suit-

able for OSCs.

IV. CONCLUSION

In summary, we investigate the photophysical pro-

cesses in CuPc films with lying-down and upright molec-

ular stacking configurations. The absorbance of the film

with a lying-down configuration is much higher than

that of the film with an upright configuration. The

ultrafast TA measurements indicate that the primary

annihilation mechanism is one-dimensional exciton dif-



FIG. 7 Kinetic traces probed at 516 nm for CuPc films with(a) a lying-down and (b) an upright configuration underdifferent excitation intensities.

FIG. 8 Kinetic traces in the form of (1/n−1/n0) vs. t1/2.

The pump fluence is 15 µJ/cm2. Solid symbols show exper-imental data, and solid lines correspond to the best fit.

fusion collision destruction. The decay kinetics shows a

clearly time-dependent annihilation rate constant with

γ∝t−1/2. For the CuPc film with a lying-down config-

uration, TTA rate constant is γ0=(1.42±0.02)×10−20

cm3·s1/2, smaller than that of upright configuration

which is (2.87±0.02)×10−20 cm3·s1/2. Compared to the

CuPc thin film with an upright configuration, the thin

film with a lying-down configuration shows a longer ex-

citon lifetime and a stronger absorption, which are ben-

eficial for OSCs.

V. ACKNOWLEDGMENTS

This work was supported by the Open Fund of the

State Key Laboratory of Molecular Reaction Dynamics

in DICP (No.SKLMRD-K202108).

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influence of the molecular stacking pattern on the excited

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