Influence of Molecular Stacking Pattern on Excited State Dynamics of Copper Phthalocyanine Films

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 provide a low-cost, light-weight, and flexible strategy for solar energy conversion [1, 2]. Ultrafast pump-probe techniques are powerful tools for studying excited-state dynamics in OSC systems [3–6]. Phthalocyanines are archetypical electron donors which are deposited by vacuum evaporation to form films for use in such cells [7–10]. The molecular stacking configuration and crystal structure can markedly affect material characteristics, such as light absorption, charge transport, exciton diffusion, and molecular energy levels [11–14]. In our previous work, we considerably improved the 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/C$_{60}$ [15]. Herein, we report a study of the excited-state dynamics of the CuPc films with lying-down and upright molecular stacking configuration using an ultrafast transient absorption (TA) spectroscopy. 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 acetone, alcohol, and deionized water in sequence, and then blown dry with pure N$_2$ 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 pressure of $\leq$5$\times$10$^{-4}$ Pa. The film thicknesses were monitored online by a quartz-crystal microbalance. Atom force microscopy (AFM) morphologies were measured on a Bruker Dimension Icon. Steady state absorption spectra were measured on a Shimadzu UV3600 spectrophotometer. TA spectra were measured on a Femto-TA100 system (Time-Tech Spectra) using previously described method [16]. The full-width at half-maximum of the pump pulse was $\sim$80 fs, and the time resolution was $\sim$150 fs. All the TA spectra were measured with a 700-nm pump pulse. The pump beam was focused to a diameter of $\sim$300 ${\rm{ \mathsf{ μ} }}$m. The sample was moved with a velocity of 0.3 mm/s during the TA measurements. There was no evident difference in the dynamics of the samples with and without an encapsule, and all the measurements 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 is converted from upright to lying-down by introducing a 3.0-nm thick CuI buffer layer [15].

Figure  1.  Molecular structure of CuPc. Schematic diagrams of energy levels and molecular stacking configurations. The $\pi$-stacking direction is given.

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AFM surface morphologies of the films are given in FIG. 2. It can be seen that the surface morphologies are very different for the films with lying-down and upright configurations. The crystal grains can be seen clearly for the lying-down configuration. The absorption spectra of CuI and CuPc films with different stacking configurations are shown in FIG. 3. Both spectra of the CuPc films show a Soret-band at around 350 nm and a Q-band from 550 nm to 800 nm. The Soret-band is assigned to the transition of S$_0$$\rightarrow$S$_2$, and the Q-band corresponds to the transition of S$_0$$\rightarrow$S$_1$ [17]. Here, S$_0$, S$_1$, and S$_2$ are the ground, the first, and the second excited singlet states. Two peaks appear 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, two shoulders locate at the low- and high-energy absorption edge of the Q-band. The high-energy shoulder is caused by the formation of higher order aggregate, and the low-energy shoulder is related to the different torsional conformational forms of these higher order aggregates [19]. The absorbance of the film with a lying-down configuration is much higher than that with an upright configuration. For example, at the peak wavelength 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 molecular absorption cross-section $\sigma$ is satisfied by the following relation [20]:

Figure  2.  AFM surface morphologies of CuPc films with (a) an upright configuration and (b) a lying-down configuration.

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Figure  3.  Steady state absorption spectra of the CuPc films deposited on quartz with (red circle) and without (black square) a CuI buffer. The absorption spectrum of the CuI film (blue triangle) deposited on quartz is also given.

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where ${\bf E}$ is the electronic field vector of incident light, and ${\bf M}$ is the molecular transition moment. The molecular absorption cross-section $\sigma$ is proportional to the square of the dot product of ${\bf E}$ and ${\bf M}$, which is maximized when ${\bf E}$ is parallel with ${\bf M}$. For $\pi$-$\pi^*$ transition of the planar CuPc molecule, the transition moments are parallel with the molecular plane [20]. During the absorption measurements, incident light is perpendicular to the substrate plane and the electric field vector is in parallel with the substrate. Thus, the molecular absorption cross-section is larger for the lying-down stacking configuration compared with that of the upright stacking configuration.

To investigate excited state dynamics of CuPc films with different stacking configuration, we performed ultrafast 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 negligible absorption of the CuI layer at the pump wavelength of 700 nm. The band gap of CuPc ($\sim$1.6 eV) is much smaller than that of CuI ($\sim$3.1 eV), preventing the excited state energy transfer from CuPc to CuI. Moreover, the lowest unoccupied molecular (LUMO) level of CuPc ($-$3.5 eV) is much lower than the conduction band edge of CuI ($-$2.0 eV), blocking the electron transfer from CuPc to CuI. For the exciton in CuPc, the electron and the hole are bound by the Coulomb interaction. This binding energy is very large (0.3$-$0.5 eV) because of its low relatively dielectric constant ($\sim$3). So, the hole transfer from CuPc to CuI requires energy to separate 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 analyses, we can conclude that the photophysical processes following the excited pulse occurs within the CuPc films.

FIG. 4 shows the TA spectra of CuPc films with upright and lying-down configurations, recorded at different pump-probe delays under a pump fluence of 15 ${\rm{ \mathsf{ μ} }}$J/cm$^2$. Both of the TA spectra exhibit negative absorbance 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 transient signal intensity of the CuPc film with the lying-down configuration is stronger than that of the film with the upright configuration. We attribute this difference to 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 depletion of the ground state population. We rule out stimulated emission because there is no measurable fluorescence from CuPc, and its long-lived phosphorescence at around 1120 nm can be observed only at lower temperature [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 ($^2$S). The normal triplet state splits into a tripdoublet ($^2$T) and a tripquartet ($^4$T) [25]. Because the relaxation process from $^2$S$_1$ to $^2$T$_1$ or to $^4$T$_1$ is spin-allowed, it can take place rapidly with characteristic time of $\sim$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 positive TA band is mainly originated from the excited tripdoublet and tripquartet state. Here, we cannot clearly attribute the observed decay to either a tripdoublet or a tripquartet state, which is considered as a whole and is 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 dynamic traces are shown in FIG. 5. The characteristic time is 0.5 ps for both the samples. This process is assigned to intersystem crossing [21].

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

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

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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 leading to the near 100% triplet quantum yield. Hence, the excited state decay 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 pathways 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 examine the presence of TTA, we measure the TA spectra under different pump fluences. FIG. 7 shows the triplet dynamic traces under excitation intensities of 15 and 35 ${\rm{ \mathsf{ μ} }}$J/cm$^2$. The decay of these signals is highly nonexponential, and the decay rate increase as the pump fluence increases. When the TTA reaction of $^3$A$^*$+$^3$A$^*$$\rightarrow$$^3$A$^*$+A occurs, where "$^3$A$^*$" denotes the triplet excited state, and "A" denotes the ground state, the triplet population can be described by the following rate equation [29]:

Figure  6.  Schematic diagram of the main photophysical processes in CuPc films.

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

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where $n$ is the triplet exciton density, $\tau$ is the intrinsic triplet exciton lifetime in the absence of annihilation, and $\gamma$ 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 $\tau$, Eq.(1) can be reduced to

There are two TTA mechanisms. One is the exciton diffusion collision annihilation, and the other is the static annihilation via Förster long-range dipole-dipole interaction. Förster energy transfer requires that the transition of the energy donor is allowed. However, the transition of $^3$A$^*$$\rightarrow$A is spin-forbidden for CuPc. Therefore, we can conclude that the static annihilation mechanism is not important. Our conclusion consists with that the energy transport in CuPc film is dominated by short-range Dexter mechanism [21]. In contrast to the triplet exciton in CuPc, the singlet exciton annihilation in polycrystalline film of H$_2$Pc is dominated by the static annihilation via Förster energy transfer mechanism [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:

for three-dimensional diffusion [30, 31]:

where $D$ is the diffusion coefficient, $R_ {\rm{a}}$ is the critical distance at which exciton annihilation reaction takes place, and $t$ is the time. In general $R_ {\rm{a}}$ is assumed to be the separation of adjacent molecules. When $t$ is much larger than $R_ {\rm{a}}$$^2/(2\pi D)$, Eq.(4) can be reduced to

In the case of one-dimensional diffusion, Eq.(2) can be rewritten as

where $\gamma_0$ is a constant. Integration of Eq.(6) yields an expression of the time-dependent exciton density:

where $n_0$ is the initial exciton concentration at $t$=0. If we plot the kinetic traces in the form of (1/$n$$-$1/$n_0$) vs. $t^{1/2}$, kinetics obeying rate equation (Eq.(6)) will yield a straight line with a slope of 2$\gamma_0$. The initial triplet densities $n_0$ for the lying-down and upright configurations are estimated to be 6.85$\times$10$^{18}$ cm$^{-3}$ and 3.95$\times$10$^{18}$ cm$^{-3}$ based on their absorbance values of 0.13 and 0.07, respectively. FIG. 8 shows the kinetic traces in the form of (1/$n$$-$1/$n_0$) vs. $t^{1/2}$, which yield straight lines for both the lying-down and the upright configurations. The results suggest that one-dimensional diffusion collision annihilation is the dominant mechanism in CuPc films. It is similar to exciton annihilation in polycrystalline film of H$_2$Pc, which shows a clearly time-dependent annihilation rate constant with $\gamma$$\propto$$t^{-1/2}$ [38]. The kinetic traces are 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 $\gamma_0$= (1.42$\pm$0.02)$\times$10$^{-20}$ cm$^3$$\cdot$s$^{-1/2}$, which is smaller than that for upright configuration of (2.87$\pm$0.02)$\times$10$^{-20}$ cm$^3$$\cdot$s$^{-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.

Figure  8.  Kinetic traces in the form of (1/$n$$-$1/$n_0$) vs. $t^{1/2}$. The pump fluence is 15 ${\rm{ \mathsf{ μ} }}$J/cm$^2$. Solid symbols show experimental data, and solid lines correspond to the best fit.

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In our experiment the film thickness is 20 nm, which is the optimized thickness of our solar cell based on CuPc/C$_{60}$ heterojunction [15]. This thin thickness should thus limit the exciton diffuse along the direction perpendicular to the substrate. Hence, collision annihilation mainly depends on exciton hopping in parallel with the substrate. In the anisotropic CuPc film, exciton hopping along the $\pi$-$\pi$ stacking direction is more favorable owing to the stronger electron coupling, in agreement with the above conclusion of one-dimensional diffusion model. The $\pi$-$\pi$ stacking direction is given 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, $\pi$-$\pi$ stacking in direction perpendicular to the substrate is in favor of the carrier collection and the exciton diffusion to the interface of heterojunction. Therefore, the CuPc film with a lying-down configuration is more suitable for OSCs.

Ⅳ.   CONCLUSION

In summary, we investigate the photophysical processes in CuPc films with lying-down and upright molecular stacking configurations. The absorbance of the film with a lying-down configuration is much higher than that with an upright configuration. The ultrafast TA measurements indicate that the primary annihilation mechanism is one-dimensional exciton diffusion collision destruction. The decay kinetics shows a clearly time-dependent annihilation rate constant with $\gamma$$\propto$$t^{-1/2}$. For the CuPc film with a lying-down configuration, TTA rate constant is $\gamma_0$=(1.42$\pm$0.02)$\times$10$^{-20}$ cm$^3\cdot$s$^{-1/2}$, smaller than that of upright configuration which is (2.87$\pm$0.02)$\times$10$^{-20}$ cm$^3\cdot$s$^{-1/2}$. Compared with the CuPc thin film with an upright configuration, the thin film with a lying-down configuration shows a longer exciton lifetime and a stronger absorption, which are beneficial for OSCs.

Ⅴ.   ACKNOWLEDGMENTS

This work was supported by the Open Fund of the State Key Laboratory of Molecular Reaction Dynamics at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (No.SKLMRD-K202108).