【作者】 高炳荣；

【导师】 孙洪波； 王海宇；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2011， 博士

【摘要】 有机发光器件通常工作在固体状态下,因此要求发光材料在固态时有较高的发光效率,于是具有聚集诱导发光(AIE)性质的新型分子不断被合成,但是关于AIE的机理,尚未能完全确定,而对AIE机理的正确认识是研制更高质量的AIE材料的基础。3-己基取代聚噻吩(P3HT)作为一种非常优秀的共轭聚合物太阳能电池材料,目前得到了广泛的研究。但由于聚合物的性质介于传统半导体和分子之间,并且可形成种类繁多的纳米结构,致使其能带结构及光电性质都非常复杂。P3HT薄膜在光激发后产生的各种激发产物的衰减机制以及它们对薄膜形貌的依赖性都是目前国际研究的热点,但仍有很多问题尚待研究。另外P3HT：PCBM体异质结薄膜中,激子是如何快速有效地到达异质结界面产生电荷分离也始终是个谜团。本论文结合使用瞬态吸收和时间分辨荧光方法对以上问题进行了详细的研究,并取得了以下成果：1.系统搭建：搭建了泵浦探测光路和飞秒荧光上转换光路,使用visua]basic编程语言实现了对系统各部件的计算机自动控制和数据采集,编写了数据处理程序,系统能够达到较高的信噪比,可以对微弱信号进行探测,运行良好。2.聚集诱导发光(AIE)机理研究：以前普遍认为AIE的机理是聚集后抑制了分子转动等非辐射驰豫过程而导致发光增强,但我们通过对含有电子给受体的CNDPASDB溶液和聚集状态的时间分辨荧光研究,首次发现其聚集发光增强的原因是聚集后抑制了粒子由局域激发态(LE态)向弱发光态——电荷转移态(ICT态)的弛豫。高极性溶剂中,LE态发光迅速衰减(1ps),并伴随ICT态的出现,由于ICT态为弱发光态,导致溶液中发光非常弱。聚集后抑制了LE激发态向ICT态的转变,从而产生聚集诱导发光增强现象。CNDPASDB的DMF溶液的泵浦探测实验显示初始荧光的衰减并不伴随基态漂白的恢复,也表明LE激发态粒子没有回到基态,而是弛豫到一个中间态：ICT态。之后我们又对不含电子给受体的CNDPDSB分子进行了泵浦探测的对比研究,发现其荧光的衰减直接对应基态漂白恢复,不存在中间态,因此它在溶液中发光很弱的原因是振／转非辐射驰豫。当分子聚集后,分子不能自由振转,因而抑制了非辐射驰豫途径,导致荧光增强。本研究阐明了两种不同的AIE机理,对AIE材料的发展具有重要意义,本工作被美国化学会专门介绍化学发展最新动态的网络杂志‘’Noteworthy Chemistry"进行了专题介绍。3. P3HT薄膜中极化子对衰减动力学研究：以往研究指出极化子对以0.7ps的时间常数衰减回到激发态,但我们通过对比荧光峰值、基态漂白和极化子对吸收的动力学,发现与荧光动力学相比基态漂白多出一个0.7ps的快过程,而此快过程与极化子对吸收快过程一致,因此认为极化子对驰豫回到基态。更近一步,我们通过比较激子(4ps)和极化子对(0.5ps与4ps的差谱)瞬态吸收光谱形状,发现极化子对是在有着特定吸收光谱形状的特定区域产生的,并且驰豫回到这个区域的基态。这一结果完整地给出了极化子对的衰减图像,对理解P3HT薄膜中的光物理过程有重要意义。4. P3HT薄膜形貌及功能的研究：聚合物长链的柔韧性使得链与链之间的不同堆积方式会形成不同程度的π共轭,因此也具有不同的电子结构和形貌。光激发产物的性质也随着形貌的不同而不同,但是到目前为止,还没有工作来区分一个薄膜中存在的不同形貌,并将其与功能联系起来。我们通过分析P3HT薄膜的瞬态吸收基态漂白光谱,发现在P3HT薄膜中至少存在四个不同形貌的区域,而每个区域都都能够产生不同的激发产物,因而具有相应的功能。(1)极化子可能是在弱耦合的J聚集区域形成的,0-0跃迁相对较强,其吸收光谱红移最多,在510nm以下几乎没有吸收；(2)极化子对是在聚合物长链扭曲打结等较高能量区产生的,在510nm以下吸收很弱,其吸收光谱比极化子区吸收谱稍蓝移,0-0跃迁也是允许的；(3)聚集区激子大多是在弱耦合的H聚集区产生,其光谱进一步蓝移,0-0跃迁是比较禁阻的。激子也会在极化子区和极化子对区产生,不同区域产生的激子淬灭速率不同；(4)非聚集区激子是在完全不聚集的区域产生的,这个区域的吸收光谱类似溶液的吸收光谱,相对聚集区是高能量的,其中产生的激子可以在100 fs以内传递到聚集区,因此在瞬态吸收光谱上看不到这部分激子的基态漂白信号。本工作首次给出了P3HT薄膜中不同形貌对应的不同光谱,并将形貌与激发产物联系起来,对理解P3HT薄膜中的光物理过程有着重要意义。5. P3HT：PCBM体系中的激子扩散和电荷转移动力学：实验发现,采用退火使P3HT：PCBM薄膜中相分离的纳米尺度为13-15 nm时,得到的是对提高光电转换效率最为有利的形貌结构。一般认为激子是通过扩散到达电子给受体界面实现电荷分离,但聚合物中激子扩散速度很慢,激子如何能在很短的时间内如此有效地扩散到电子给受体界面呢?因此激子扩散模型受到质疑。我们利用荧光上转换技术对P3HT：PCBM体异质结体系进行了不同激发功率下的荧光动力学测试,并采用三维扩散模型很好的描述了激子的扩散行为,得到了电荷转移半径Rct的范围在4.8-9nm之间,而激子在8ps内的扩散距离只有0.5nm,很显然,这样大的电荷转移半径不太可能是由激子扩散造成的,而更有可能是激子在4.8-9nm这样大的一个区域里非局域化,从而能够迅速到达P3HT/PCBM的界面。此工作对于理解聚合物太阳能电池中的电荷分离过程具有重要意义。更多还原

【Abstract】 Organic light-emitting diode usually works at solid state and requires the light-emitting molecules to have high solid state luminescence; so many new molecules with the property of aggregation induced emission (AIE) have been synthesized. However the mechanism of AIE is still far from fully understood. The understanding of AIE mechanism is the basis to develop more AIE materials with better quality.As a prominent polymer solar cell material. poly(3-hexyl thiophene) (P3HT) has been extensively studied. However, having intermediate standing between traditional semiconductor and molecules, maintaining heterogeneous nanostructures, P3HT is highly complex with its electronic properties. The dynamics and morphological dependence of various photo-excitations in P3HT film have attracted extended scientific interest, but there are still many unclear mechanisms. For P3HT:PCBM blend film, it still under debate how the excitons so efficiently reach the donor/acceptor interface to undergo charge separation.In this thesis, we applied both pump-probe and fluorescence up-conversion techniques to study these problems and the results are listed below.1. System built-up:We have built up the pump-probe and fluorescence up-conversion systems. We have write he control and data collection programs using visual basic language. Good signal to noise ratio have been achieved and can detect weak signals.2. The mechanism of aggregation induced emission (AIE):It was believed that the mechanism for AIE is the restriction of the torsional/vibrational non-radiative relaxation pathways in aggregate form, which results in the emission enhancement. However, when we applied time resolved fluorescent measurements of CNDPASDB, a molecule with electron donor and acceptor group, we firstly found that its AIE machnism is the aggregation induced restriction of the transition from the local excited (LE) state to the intramolecular charge transfer (ICT) state. In high polar solvent, LE state decays rapidly, accompanied by the emergence of an ICT state. Because the ICT state is a relatively dark state, the emission in the high polar solution is low. Aggregation of the molecules restricts the transition from the LE state to the ICT state, resulting in a enhancement of the emission. We have also performed pump-probe experiments for CNDPASDB. It shows that the initial decay of the fluorescence is not accompanied by the recovery of the ground state bleaching, but an emergence of a new intermediate excited state-ICT state. Then, for comparison, we performed pump-probe measurements for CNDPDSB, a molecule without donor acceptor group. It shows that the decay of the fluorescence is accompanied by the recovery of the ground state bleaching, without going through any intermediate state. So the reason for its low emission in solution state is the efficient torsional/vibrational non-radiative relaxation, Aggregation of the molecules restricts the free motion of the molecules and the non-radiative relaxation becomes inefficient, resulting in the enhancement of the emission. In this thesis, we have found two different AIE mechanisms for two different kinds of molecules, which is of great importance to the development of AIE materials. This work is introduced by the American Chemistry Society in its weekly web feature "Noteworthy Chemistry"3. The polaron pair dynamics in P3HT film:It was reported that the polaron pair decay to the singlet excited state with a life time of 0.7 ps. However through comparison of the dynamics of the peak emission, ground state bleaching and polaron pair absorption, we found that the ground state bleaching recovery has an additional fast decay of 0.7 ps compared to the fluorescence decay, and this additional decay is also found in the polaron pair absorption dynamics. Therefore we conclude that the polaron pairs actually decay back to the ground state. Furthermore, through comparison of the transient absorption spectra shape of the exciton (4 ps) and the polaron pair (the differential spectral of 0.5 ps and 4 ps), it is found that the polaron pairs decay back to a certain domain with specific absorption feature. This work gives a clear picture of the polaron pair dynamics, and is important to understand the photophysics in P3HT thin film.4. The morphology and function of P3HT film:Different stacking modes of the long polymer chains will result in different degree ofπ-conjugation, and so different morphologies and electronic structures. But until now, research on distinguishing different morphologies in one P3HT film and the relation of the morphology and function are still lacking. Through decomposing the transient spectra for different lifetimes, we, for the first time, found that there are at least four different morphologic domains, each with its own specific function to generate different photoexcitations. (1) Polaron is possibly generated in weakly coupled J aggregate domain with the 0-0 transition being pronounced. It has the most red-shifted absorption spectra and there is almost no absorption below the 510 nm. (2) Polaron pairs are generated from the higher energy places such as the kink, torsion or bents. The spectra is blue-shifted to the polaron spectra, and the 0-0 transition is allowed. The absorption under 510 nm is weak. (3) Excitons of the aggregate domain are mostly formed in the weakly coupled H-aggregate region, with a blue shifted absorption spectra, which has more absorption below 510 nm. The 0-0 transition is relatively restricted. Excitons can also be formed in the polaron region and polaron pair region and the exciton annihilation rate is different for excitons generated in different regions. (4) excitons from the unaggregated domain are formed in the totally unaggregated region, with a P3HT solution absorption feature. It has higher energy then the excitons in the aggregate region, and can rapidly transfer to the aggregate region through energy transfer. So there is no ground state bleaching signal for these excitons. This is the first time that different spectra signatures for different morphological places are revealed, and the relationship between the morphology and photo-excitations are built, which is of great importance to understand the photophysics in P3HT film.5. The exciton diffusion and charge transfer dynamics in P3HT:PCBM film:It was found that the phase separation of 13-15 nm in P3HT:PCBM blend film is the most favorable state to achieve high conversion efficiency. It was commonly believed that the excitons reach the donor/acceptor interface by diffusion to undergo charge separation. However the excion diffusion in polymer is slow, then how can the excitons reach the interface so efficient? So the diffusion model are in doubt. We have measured the excitation intensity dependent fluorescence dynamics and found that the exciton diffusion can be well described by a three dimensional diffusion model. We have got the charge transfer radius Rct of 4.8 to 9 nm, and the diffusion length of 0.5 nm in 8 ps, much smaller than Rct. It implies that the excitons reach the interface not by diffusion but possibly by the delocalization of the excitons in a large P3HT domain. This work is important to understand the charge separation process in polymer solar cell.更多还原