时间分辨光谱研究RR-P3HT薄膜中极化子对动态

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本文主要探讨了在RR-P3HT（全同聚3-己基噻吩）薄膜中，通过时间分辨光谱学方法研究极化对偶子动态的过程。极化对偶子是聚合物太阳能电池中的一种重要现象，特别是在P3HT这样的共轭聚合物材料中，它们的形成与电荷传输密切相关。研究者采用了飞秒瞬态吸收和上转换荧光技术来深入理解这种动态行为。在实验中，研究人员对比了纯荧光、基态漂白恢复以及极化对偶子吸收的动态特性。结果显示，极化对偶子并非简单地回到单线态激发态，而是特定形态位置上的地面状态，这些位置具有独特的吸收特征。这一发现揭示了RR-P3HT薄膜中极化对偶子行为的复杂性，可能涉及到不同的能量转移和重组机制。上转换荧光技术在此研究中发挥了关键作用，因为它能够观察到非辐射过程中的能量转移，这在传统的荧光测量中可能被忽视。同时，通过瞬态吸收技术，研究人员得以追踪极化对偶子的形成、演化以及最终消失的整个动态过程。 RR-P3HT的多臂结构和侧链柔性对其极化对偶子行为有显著影响，这表明薄膜的微观形态对极化对偶子的寿命和动力学有着决定性的作用。了解这些特性对于优化P3HT基太阳能电池的性能至关重要，如提高光电转换效率，减少非辐射损失，以及设计更有效的能量收集机制。因此，这项工作不仅提供了关于RR-P3HT中极化对偶子行为的新见解，也为改进聚合物太阳能电池的设计提供了宝贵的科学依据。未来的研究可能会聚焦于如何调控这些动态过程，以提升材料的稳定性和性能，从而推动可再生能源技术的发展。

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 3, MARCH 2012 425

Investigation of Polaron Pair Dynamics in

Poly(3-hexylthiophene) Film by

Time Resolved Spectroscopy

Bing-Rong Gao, Hai-Yu Wang, Hai Wang, Zhi-Yong Yang, Lei Wang, Ying Jiang,

Ya-Wei Hao, Qi-Dai Chen, and Hong-Bo Sun, Member, IEEE

Abstract—Polaron pair dynamics in regioregular Poly

(3-hexylthiophene) (RR-P3HT) ﬁlm was studied by applying

both femtosecond transient absorption and ﬂuorescence

up-conversion techniques. The comparison of the dynamics of

the pure ﬂuorescence, the ground state bleaching recovery and

the polaron pair absorption reveals that the polaron pairs decay

back to the ground state of certain morphological places with

distinct absorption features, instead of the singlet excited state.

Index Terms— Fluorescence up-conversion, morphology,

polaron pair, transient absorption.

I. INTRODUCTION

OLYMER solar cells are promising candidates for future

power generation because of the advantages they offer,

such as easy processing, low cost and ﬂexibility. In particular,

regioregular Poly (3-hexylthiophene) (RR-P3HT) has attracted

wide attention as one of the most efﬁcient organic photovoltaic

materials [1-12]. This is because thin ﬁlms can organize

themselves into two-dimensional π -stacked lamellar structures

[13, 14] resulting in high mobility of the order of

0.1 cm

−1

, a property closely related to the exciton

and carrier behavior. Therefore, a clear understanding of the

generation and decay dynamics of the excitations is essential to

explore the underlying physics of photoexcitations and to fur-

ther improve device operations. Time-resolved spectroscopy is

a powerful tool to investigate the key photophysical processes

in materials [13, 15-22]. Transient absorption studies, carried

out earlier to study P3HT, reveal that exciton, polaron and

polaron pairs in RR-P3HT thin ﬁlm are the main photoexcita-

tions [23-33]. The transient absorption measurement can give

spectra signatures of different excitations, but the large spectra

overlap of the ground state bleaching (GSB), its stimulated

emission and excited state absorption make it hard to ﬁnd a

pure spectral signature for each excitation. So, the generation

Manuscript received October 6, 2011; revised November 10, 2011; accepted

December 9, 2011. Date of publication December 20, 2011; date of current

version February 3, 2012. This work was supported in part by the National

Science Foundation of China under Grant 21003060, Grant 20973081, and

Grant 61076054.

The authors are with the State Key Laboratory on Integrated

Optoelectronics, College of Electronic Science and Engineering, Jilin

University, Changchun 130012, China. Z.-Y. Yang and H.-B. Sun are also

with the College of Physics, Jilin University, Changchun 130023, China

(e-mail: haiyu_wang@jlu.edu.cn; hbsun@jlu.edu.cn).

Color versions of one or more of the ﬁgures in this paper are available

online at http://ieeexplore.ieee.org.

Digital Object Identiﬁer 10.1109/JQE.2011.2180515

and relaxation mechanism of polaron and polaron pairs are

still far from fully understood. In this work, a combina-

tion of applying both transient absorption and ﬂuorescence

up-conversion techniques is proposed to be applied to investi-

gate polaron pair dynamics in P3HT ﬁlm. Direct measurement

of photoluminescence (PL) by time-resolved emission can

facilitate a clean probe of the exciton dynamics without

interference from other species. Knowing one pure species

helps one to extract relatively pure dynamics of the other

species from the transient absorption signal.

A polaron pair is a coulomb bound pair of a negative and

a positive polaron, situated on different molecules. At the

intermediate state of an exciton and a pair of free polarons

(mutually too far to be attracted by the other), it is important

to investigate their dynamics to further understand the charge

generations in polymers. Although it is well acknowledged

that polaron pairs are generated in RR-P3HT ﬁlm after

photoexcitation and decay with a lifetime of about 0.7 ps

[34, 35], little work was done to investigate their detailed

dynamics. For example, it is believed that polaron pairs are

easily generated where polymer chain has defects like kinks,

torsions or bends [24, 25], but until now there has been no

spectroscopic signature to verify whether polaron pairs can

be generated from anywhere in the ﬁlm or just from some

speciﬁc domains like defects. Furthermore, it is still unclear

which state they decay to: Do they directly recombine to the

ground state? Or, do they just come closer to form excitons?

It is previously reported that polaron pair decays to the singlet

excited state [36]. They reached the conclusion based on the

mismatching dynamics of the GSB of 510 nm and the excited

state absorption of 660 nm, thus excluded the possibility of

the polaron pairs recombining to the ground state. However,

in the present experiments, by comparing the PL dynamics

with the GSB recovery dynamics, it is found that the polaron

pairs actually decay to the ground state. Furthermore, the

wavelength dependent GSB recovery dynamics reveal that the

polaron pairs decay to a certain morphological domain with a

distinct absorption feature.

II. E

XPERIMENTAL STUDIES

A. Materials, Steady-State Properties

Regioregular P3HT (Luminescence Technology Corp) with

MW = 60 k and regioregularity greater than 95% were

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时间分辨光谱研究RR-P3HT薄膜中极化子对动态

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