Temperature-Independent Vibrational Dynamics in an Organic Photovoltaic Material
Temperature-Independent Vibrational Dynamics in an Organic Photovoltaic Material
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DOI：
10.1021/jp105772y
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发表时间：
2010-09-30
影响因子：
3.3
通讯作者：
Asbury, John B.
中科院分区：
文献类型：
--
作者：
Pensack, Ryan D.;Banyas, Kyle M.;Asbury, John B.
与共轭聚合物 CN-MEH-PPV 混合的功能化富勒烯 PCBM 的羰基伸缩振动的超快取向运动和光谱扩散，通过二维红外和偏振分辨红外泵浦探针光谱进行了检查。在我们小组之前的贡献中，PCBM的羰基伸缩频率已被用作局部振动报告器来测量CN-MEH-PPV:PCBM聚合物共混物中电荷转移激子解离的时间尺度的温度依赖性。研究发现，电荷分离速率与温度无关，表明电荷分离是通过无活化途径发生的。这一分配得到了室温下观察的支持，即热波动不会引起皮秒和更长时间尺度上的羰基伸缩振动的光谱扩散。在这篇文章中，我们研究了羰基振动动力学的温度依赖性，以确定热波动是否可能引起其他温度下的光谱扩散。我们发现，快速摆动圆锥定向运动的时间尺度在亚皮秒时间尺度上与温度无关。类似地，在所有检查的温度下，在皮秒和更长的时间尺度上都没有观察到光谱扩散，这证实了我们之前仅根据电荷分离对频移动力学的解释。有趣的是，表征锥体摆动取向运动的半角在较高温度下确实增加，这归因于聚合物共混物的热膨胀导致自由体积增加。
Ultrafast orientational motion and spectral diffusion of the carbonyl stretch vibration of the functionalized fullerene, PCBM, blended with the conjugated polymer, CN-MEH-PPV, are examined with two-dimensional infrared and polarization-resolved IR pump probe spectroscopy. In previous contributions from our group, the carbonyl stretch frequency of PCBM has been used as a local vibrational reporter to measure the temperature dependence of the time scale for dissociation of charge transfer excitons in CN-MEH-PPV:PCBM polymer blends. It was found that the rate of charge separation is independent of temperature, indicating that charge separation occurs through an activationless pathway. This assignment was supported by the observation at room temperature that thermal fluctuations do not give rise to spectral diffusion of the carbonyl stretch vibration on the picosecond and longer time scale. In this contribution, we examine the temperature dependence of the carbonyl vibrational dynamics to determine whether thermal fluctuations might give rise to spectral diffusion at other temperatures. We find that the time scale for fast wobbling-in-cone orientational motion is independent of temperature on the subpicosecond time scale. Similarly, spectral diffusion is not observed on the picosecond and longer time scale at all temperatures examined confirming our earlier interpretation of the frequency shift dynamics exclusively in terms of charge separation. Interestingly, the half angle characterizing the wobbling-in-cone orientational motion does increase at higher temperature clue to increased free-volume resulting from thermal expansion of the polymer blend.