Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; staticvs.dynamic disorder
Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; staticvs.dynamic disorder
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DOI：
10.1039/d0mh00385a
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发表时间：
2020-07
影响因子：
13.3
通讯作者：
Kristofer Tvingstedt;J. Benduhn;K. Vandewal
中科院分区：
文献类型：
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作者：
Kristofer Tvingstedt;J. Benduhn;K. Vandewal
有机半导体中能量无序的起源及其对光电性质的影响一直是一个有着激烈争议的话题。特别是在有机光致发光的电子给体-受体界面处的无序是理解的关键，因为预期它会影响光载流子产生、复合以及因此影响器件效率参数。在这项工作中，我们评估的小分子：富勒烯混合物设备的光致发光（PL）和电致发光（EL）光谱的线形的温度依赖性，与野心解开动态和静态无序的贡献。电致发光发射光谱主要由电荷转移（CT）状态的发射，并被证实是高斯字符和几乎完全电压无关。更重要的是，在冷却时持续观察到强烈的线宽变窄，直到某一材料特定的低温，低于该低温时线宽保持恒定。因此，很明显，在室温或更高的操作条件下测量的线宽的主要部分源自热激活或动态无序。所观察到的高能量发射尾的温度依赖性可以通过考虑高频和低频分子振动模式来充分描述，而不必依赖于静态无序。低频分子模式与大黄-里斯因素的存在下，结果在高斯线的形状，这是另外扩大在高温下的高频分子内模式的热布居。因此，我们对通常使用的假设，即吸收或发射尾部的单温光学测量能够提供有关静态态密度尾部形状的有意义的信息表示强烈怀疑。
The origin of energetic disorder in organic semiconductors and its impact on opto-electronic properties remains a topic of intense controversy. Particularly the disorder at electron donor–acceptor interfaces for organic photovoltaics is pivotal to understand as it is expected to affect photo-carrier generation, recombination and consequently device efficiency parameters. In this work we evaluate the temperature dependence of the line-shape of the photoluminescence (PL) and electroluminescence (EL) spectra of small molecule:fullerene blend devices, with the ambition to disentangle dynamic and static disorder contributions. The EL emission spectra are dominated by charge-transfer (CT) state emission and are confirmed to be of Gaussian character and almost completely voltage independent. More importantly, a strong line-width narrowing is persistently observed upon cooling, down to a certain material specific low temperature, below which the line-width remains constant. It is consequently clear that the main portion of the line-width measured at operating conditions of room temperature or higher, is originating from thermally activated, or dynamic, disorder. The observed temperature dependence of the high-energy emission tail can be fully described by taking into account high and low frequency molecular vibrational modes, without having to rely on static disorder. The presence of low frequency molecular modes with large Huang–Rhys factors results in a Gaussian line-shape, which is additionally broadened at high temperature by thermal population of high frequency intra-molecular modes. We therefore cast strong doubts regarding the commonly used assumption that single temperature optical measurements of absorption or emission tails are able to provide meaningful information regarding the shape of a static density of states tail.