Pressure-Dependent Relaxation Dynamics of Excitons in Conjugated

May 19, 2015 - Moreover, the delocalization extent of excitons under different pressures is quantitatively calculated, indicating that the scale of ex...
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Pressure-Dependent Relaxation Dynamics of Excitons in Conjugated Polymer Film Dong-Xiao Lu, Ying-Hui Wang, Fang-Fei Li, Xiao-Li Huang, Ling-Yun Pan, Yuan-Bo Gong, Bo Han, Qiang Zhou,* and Tian Cui State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China ABSTRACT: Conjugated polymers have been widely used in polymer solar cells (PSC). The transient absorption spectra on conjugated polymer poly{2,7′-9,9-dioctylfluorene-alt-5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole1,4-dione} (PDPP-F) film are measured under pressure, and the pressure-dependent relaxation dynamics of excitons in PDPP-F film are investigated. It is found that the relaxation dynamics of excitons under high pressure could be divided into a fast relaxation process that is attributed to the exciton− exciton annihilation (EEA) and a slow relaxation process that is assigned to the diffusion of free excitons. As a result of pressure-induced delocalization of excitons and extension of lifetime of free excitons, both the EEA process and the free exciton diffusion are enhanced upon compression. Moreover, the delocalization extent of excitons under different pressures is quantitatively calculated, indicating that the scale of exciton delocalization at 30 GPa is tripled compared with that at 1 atm. These results are very important to understand the photophysical property of polymer and to optimize the efficiency of PSC.

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INTRODUCTION Conjugated polymers have attracted much attention because of their unique features like low weight, low fabrication cost, and great potential in large-area flexible device manufacture. Owing to such excellent properties, conjugated polymers have been widely used in several optoelectronic fields, such as polymer solar cells (PSC),1,2 field effect transistors,3 and light-emitting diodes.4 Particularly, in order to improve the absorption efficiency of PSC, considerable efforts were made to expand the region of light harvesting in conjugated polymer.5,6 Therefore, some narrow-band-gap conjugated polymers with excellent performance have been synthesized,7,8 Among these materials, the well-known narrow-band-gap conjugated polymer poly{2,7′-9,9-dioctylfluorene-alt-5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4-dione} (PDPP-F) is demonstrated to be a promising material in the field of PSC.9 Understanding the exciton relaxation dynamics is not only essential for studying the exciton behaviors in PSC such as exciton diffusion, delocalization, and annihilation but also beneficial for further improving the efficiency of PSC through optimization of polymer designing. Generally speaking, the exciton relaxation dynamics correlate with the configuration of polymer; thus, many typical techniques have been applied to modulate the polymer configuration, including chemical modification,7 thermal annealing,10 changing the processing conditions,11 and varying the solvent type,12 for the further understanding of the dynamics process in polymer. However, in most cases, the abovementioned techniques require complex procedures and often introduce additional unnecessary alterations. Moreover, it is difficult for the aforementioned © XXXX American Chemical Society

techniques to make consecutive adjustment on polymer conformation. As an alternative, changing the pressure on polymer aggregation is an ideal and clean method to consecutively modulate the interchain distance and polymer conformation13−17 without introducing any unnecessary interference. The combination of high-pressure technology and ultrafast spectroscopy methods is expected to open a window to observe and investigate the interaction-dependent transient physical process. Recently, several studies were reported that used highpressure femtosecond transient absorption spectroscopy (HP fs-TAS).18−21 A pressure-induced redshift in the photoluminescence spectra of the conjugated polymer F8BT (9,9di-n-octylfluorene-alt-benzothiadiazole) film caused by interchain π−electron interaction was found by Schmidtke et al. at high pressure.18 In another HP fs-TAS work on copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole), Albert-Seifried et al. found that more delocalized excited states appeared at high pressures.19 Besides, Liu et al. reported that the rate of intermolecular energy relaxation could be changed in solidified LDS698 (pyridine, C19H23N2O4Cl) molecular solution significantly.20 Despite much progress being made, the information about the relaxation dynamics of excitons in PDPP-F film aggregation under pressure, which is of great interest and fundamental for the design and optimization of optoelectronic devices based on PDPP-F material, is still unknown. Received: May 8, 2015 Revised: May 14, 2015

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DOI: 10.1021/acs.jpcc.5b04441 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C In this paper, the pressure-dependent relaxation dynamics of excitons in PDPP-F film are investigated using diamond anvil cell (DAC) high-pressure technique combined with femtosecond transient absorption spectroscopy technique. It is found that both the exciton−exciton annihilation (EEA) and the diffusion length of free excitons are enhanced upon compression because of the pressure-induced delocalization of excitons and extended lifetime of free excitons, respectively. Moreover, the quantitatively calculated scale of exciton delocalization reveals that the scale of exciton delocalization at 30 GPa is tripled compared with that at 1 atm. The enhanced delocalization extent and diffusion length of excitons will facilitate the dissociation of excitons into free carriers, which is beneficial for the solar cell performance.

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Figure 2. Schematic diagram of HP fs-TAS system: BS1 and BS2, 3:7 and 1:1 beam splitters, respectively; BBO, 0.5 mm thick β-BaB2O4 crystal; ODL, optical delay line; R, reflector; CL, convex lens; FWC, flow water cell; DAC, diamond anvil cell; M, three grating monochromator; P, photomultiplier; LIA, lock-in amplifier.

EXPERIMENTAL SECTION PDPP-F was purchased from Terthon Technology. The macromolecule of PDPP-F consists of the repeating units of one fluorine unit, one diketopyr-rolopyrrole unit, and two thiophene units,22 as shown in Figure 1a. The chloroform

i.e., high-pressure apparatus,24,25 and the coaxial pump−probe technique based on the traditional femtosecond transient absorption system described elsewhere.26−30 In brief, a regenerative amplified Ti/sapphire laser (Spectra-Physics) was used to generate an 800 nm laser pulse with a 35 fs pulse width and a 1 kHz repetition rate. The output laser beam was split by a 7:3 beam splitter. The frequency of 70% of laser beam (λ = 800 nm) was doubled through a 0.5 mm thick β-BaB2O4 crystal to provide a 400 nm pump beam. The residual 30% of the laser beam (λ = 800 nm) was focused on a 5 mm cell filled with flow water to generate the white light acting as the probe beam, which was then coaxial with the pump beam. The radius of the focal spots of pump and probe beams on the film sample in the DAC were respectively 30 and 20 μm, which were monitored by an optical microscope in real time. The time delay between the pump and probe beam was realized by a computercontrolled translation stage (M-405.DG, PI Corp.), and the signal was sent to a lock-in amplifier (SR830, Stangord Co.) to be processed. The steady-state absorption spectra of PDPP-F were obtained using an Ocean Optics QE65000 spectrometer. The steady-state emission spectra of PDPP-F were collected using a T64000 System (HORIBA Scientific). All measurements were performed at room temperature.

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RESULTS AND DISCUSSION Figure 3 shows the normalized steady-state absorption and emission spectra of PDPP-F film under ambient pressure. For the absorption spectrum, there are two main absorption bands: one is located from 350 to 480 nm (P1 band) and contains two low-intensity peaks at 380 and 430 nm and the other is located from 480 to 750 nm (P2 band) and also contains two absorption peaks, at 630 and 687 nm. The absorption range of the P2 band of PDPP-F film matches the maximum photon flux of the solar spectrum well,31 which is beneficial for the absorption efficiency of PSC. The red line in Figure 3 is the corresponding emission spectrum of PDPP-F film. The emission peak is concentrated at 707 nm, and the edge is located at 820 nm. The dynamics signal of excited-state absorption (ESA) provides the information related to the relaxation process of excitons. Thus, it is very important to recognize the signal that belongs to ESA. Herein, the change of transmissivity (ΔT) in transient absorption is defined as Tprobe − Tpump+probe, where Tprobe and Tpump+probe represent the transmissivities of probe

Figure 1. (a) Molecular structure of the PDPP-F conjugated polymer. (b) Experimental setup of diamond anvil cell and the circumstances in it. The detailed diamond anvil cell part is enlarged in the red square for clarity.

solution of PDPP-F with a concentration of 10 mg/mL was dropped onto the diamond culet to get a thin PDPP-F film after chloroform volatilization. The DAC with 400 μm diameter culet was used to generate high pressure (Figure 1b). A stainless-steel T-301 gasket with a central hole 180 μm in diameter and 40 μm in depth was used as the sample chamber, in which a small chip of ruby was loaded. The pressure was calibrated by ruby fluorescence technique.23 The pressure medium was liquid argon, which was used to guarantee a quasihydrostatic pressure on the sample. The pressure-dependent signals of relaxation dynamics of excitons were characterized by a high-pressure femtosecond transient absorption spectroscopy (HP fs-TAS) system as shown in Figure 2. Our HP fs-TAS system integrated the DAC, B

DOI: 10.1021/acs.jpcc.5b04441 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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relaxation process could be recognized in the initial relaxation stage. To clarify the origin of this fast decay component, a three-exponential function [a1exp(−t/τ1) + a2exp(−t/τ2) + a3exp(−t/τ3)] is used to fit the ESA decay dynamics curves under high pressures. All of the curves under high pressure show two-stage decay dynamics, a fast relaxation process, and are followed by a slow relaxation process, indicating that multiple species participate in the relaxation process under high pressures. It is well-known that high pressure may increase the πstacking and decrease the distance between the polymer chains as well as the torsion angle between the aromatic rings of conjugated backbone in polymer film dramatically,32 and the EEA is liable to occur when the polymer chains are closely packed.18,33 Thus, the pressure-induced fast relaxation process was considered to be an EEA process. Furthermore, the number of remaining excitons became less and less after the rapid annihilation of nearby excitons, which made it difficult to maintain the high efficiency of collision and annihilation for the remaining excitons. Therefore, the following slow relaxation process of excitons could be assigned to the diffusion of free excitons. To get better understanding of the excitons relaxation dynamics in PDPP-F film, spectral analysis at different pressures are carried out, as shown in Figure 5. It can be

Figure 3. Normalized steady-state absorption (blue, left axis) and emission spectra (red, right axis) of PDPP-F film under ambient pressure.

laser without and with pump laser, respectively. The normalized change of transmissivity difference is defined as ΔT/T = (Tprobe − Tpump+probe)/Tprobe. If the sample in excited state absorbs the photons during the excitation with pump laser, then the transmissivity of the probe laser (Tpump+probe) will decrease, leading to a positive signal of ΔT/T, whereas other physical process would produce a negative signal of ΔT/T. During our measurements on pure PDPP-F polymer film, it is found that the ultrafast dynamics signals probed at 900 nm are always positive even though the pressure increases from 1 atm to 30 GPa (Figure 4), so the ultrafast dynamics signals of PDPP-F

Figure 5. Percentage of EEA in PDPP-F film as a function of applied pressures.

seen that the percentage of EEA gradually increased with pressure from 0% (1 atm) to 24.8% (2.9 GPa) and eventually to 68% (30 GPa). The scale of exciton delocalization can be quantitatively estimated as follows according to the percentage of EEA. The number of excitons at initial time (N0) is calculated as N0 = (Eσa)/E400, where E is pump energy, σa is the absorption percentage of PDPP-F film at the excitation wavelength (400 nm), and E400 is the photon energy at 400 nm. E is adjusted according to σa under different pressures to make N0 unchanged (N0 = 9.56 × 107). The number of remaining excitons after EEA (Nr) could be calculated as Nr = N0(1 − PEEA%), where PEEA% is the percentage of EEA. On the basis of the value of PEEA%, Nr is quantitatively estimated to be 9.56 × 107 under ambient pressure and 3.11 × 107 at 30 GPa, respectively. Furthermore, the area of one exciton after EEA (Se) can be quantitatively evaluated using the formula Se = S/ Nr, where S represents the area of the pump beam on PDPP-F film and is constant (S = 1.26 × 109 nm2). In this situation, Se is quantitatively estimated to be 13.2 nm2 under ambient pressure and 41.1 nm2 at 30 GPa, indicating that the scale of exciton delocalization at 30 GPa is tripled compared with that at 1 atm. It can be therefore concluded that the delocalization extent of excitons in PDPP-F film increased as the film is compressed. A

Figure 4. Experimental (open circles) and fitted (thick lines) data for normalized ESA decay dynamics curves of PDPP-F film under pressures. The probe wavelength was 900 nm, and the excitation wavelength was 400 nm. All the ESA decay dynamics curves were normalized at time zero, being fitted with exponential function I(t) =∑aiexp(−t/τi). The delay time range is from the pump pulse to 1800 ps.

film at 900 nm derives from the ESA under different pressures.9 Thus, the wavelength of 900 nm was chosen to probe the ultrafast dynamics signals of PDPP-F film. To explore the pressure evolution of relaxation process of excitons in PDPP-F film, the ESA dynamics curves of PDPP-F film at 900 nm were detected under different pressures, as shown in Figure 4. Under ambient pressure, the ESA decay dynamics curve exhibits only a slow relaxation process that could be fitted by a monoexponential function a1exp(−t/τ1). This slow relaxation process is assigned to the free exciton relaxation, and the corresponding lifetime of exciton relaxation is 172 ps. However, the exciton relaxation process became complex after high pressure is applied. At 2.9 GPa, a fast C

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integer ranging from 2 to 3 according to the fitting data. The value of τ will increase as the pressure increases as shown in Figure 7; the value of τ was about 218 ps under 2.9 GPa and

similar conclusion has been demonstrated in previous studies by others,19,34 which proposed that the increased effective length of conjugated chain18 and the increased overlap degree of electron cloud26 caused by high pressure will lead to an increase of exciton delocalization. Moreover, the increase of exciton delocalization induced by high pressure may make excitons meet each other easily and facilitate EEA,35 which is the possible explanation for the experimental result that the percentage of EEA gradually increased with pressure. It has been reported that the increased exciton delocalization and configuration planarization may cause the red-shift and broadening of steady-state absorption spectra for some polymers under high pressure.36,37 To figure out whether this is the same in the case of PDPP-F, the pressure-dependent steady-state absorption spectra of PDPP-F film were measured, and the spectra are presented in Figure 6a. Clearly, the PDPP-F

Figure 7. Mean lifetime of the slow relaxation process as a function of applied pressures.

gradually increased to 328 ps under 30 GPa. This result agrees well with the energy−time uncertainty principle ΔtΔE ≥ ℏ/2, where Δt is lifetime, ΔE is energy gap, and ℏ is Planck’s constant. As we know, the energy corresponding to the reddest peak position in steady-state absorption spectra is the energy gap. In our experiment, the energy gap of PDPP-F film decreases with the increase of pressure (Figure 6b); thus, the lifetime should increase with pressure according to the energy− time uncertainty principle. The increased lifetime may be caused by the fact that the pressure-induced planarization in conjugated polymer may restrict the rotation of the aromatic ring around the connecting linkages, leading to a reduction of the nonradiative rotational relaxation,38 which eventually causes the long lifetime. Moreover, it is well-known that the diffusion speed of excitons from polymer chain A to polymer chain B represents the diffusion probability of excitons from one to another. As the high pressure makes the polymer chains packed more closely, the interchain interaction is enhanced accordingly. Meanwhile, the exciton delocalization extent increases when planarization of conjugated polymer chains is enhanced. These effects will facilitate the diffusion of excitons between neighboring chains; thus, the diffusion probability of excitons increases with pressure accordingly, i.e., the diffusion speed of excitons is considered to increase under high pressures. The diffusion speed and the lifetime of excitons both increase with pressure, which means that the diffusion length of excitons increases with pressure. It can be therefore concluded that the enhanced extent of polymer aggregation under pressure will cause the extension of exciton diffusion length, which plays a significance role in charge separation in polymer-based solar cells.

Figure 6. (a) Normalized steady-state absorption spectra of PDPP-F film under different pressures. (b) Absorption peak position (red, left axis) and the full width at half-maximum (fwhm; blue, right axis) of P2 band as a function of applied pressure.

film has a broad near-infrared absorption band in the longwavelength region under all pressures. The two parts of the P2 band gradually broaden and overlap with each other, eventually merging into one peak at 7.3 GPa. The maximum absorption peak position of the P2 band undergoes a significant redshift from 1.81 eV (1 atm) to approximately 1.62 eV (30 GPa); meanwhile, the corresponding full width at half-maximum (fwhm) broadens apparently from 0.33 eV (1 atm) to 0.49 eV (30 GPa), as shown in Figure 6b. Interestingly, the pressureinduced changes in steady-state absorption of PDPP-F film were reversible after releasing the pressure; thus, the PDPP-F film sample is relatively stable below 30 GPa. The observed pressure-dependent red-shift and broadening of steady-state absorption spectrum of PDPP-F film were in good agreement with the results reported by others, which further verified our inference that the delocalization extent of excitons in PDPP-F film increased as the film was compressed. Furthermore, the mean lifetime (τ) of the slow decay process can be calculated as τ = (∑aiτi)/(∑ai), where i represents the

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CONCLUSIONS The pressure-dependent relaxation dynamics of excitons in PDPP-F film was studied using high-pressure femtosecond transient absorption spectroscopy technology. The ESA decay dynamics curve under high pressure exhibited a fast relaxation process and a subsequent slow relaxation process. The pressure-dependent fast relaxation process is attributed to exciton−exciton annihilation, and the slow relaxation process is assigned to the diffusion process of free excitons. On the basis of the quantitative calculation of exciton delocalization and the lifetime of free excitons under different pressures, the delocalization extent of excitons and the diffusion length of D

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free excitons are found to be significantly enhanced with pressure. These results have important implications for understanding the photophysical property of polymers that are beneficial to improve the efficiency of PSC.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Prof. Yisong Zheng, Yongjun Bao, and Zhe Liu for many fruitful discussions. We also thank Kai Wang, Guohui Lu, and Xiaowei Li for their help during the experimental research. This work was supported by the financial support from the National Natural Science Foundation of China (nos. 11274137, 51032001, 11074090, 51025206, and 11204100), National Basic Research Program of China (no. 2011CB808200), Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1132), and National Fund for Fostering Talents of Basic Science (no. J1103202).

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