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.

中文翻译：

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有机太阳能电池中电荷转移态发射光谱线宽的温度依赖性；静态与动态障碍

有机半导体中的高能紊乱的起源及其对光电特性的影响仍然是激烈争论的话题。尤其是对于有机光伏来说，电子供体-受体界面处的无序是至关重要的，因为它有望影响光载流子的产生，重组以及器件效率参数。在这项工作中，我们评估了小分子：富勒烯共混器件的光致发光（PL）和电致发光（EL）谱线形状的温度依赖性，以期解开动态和静态无序贡献。EL发射光谱以电荷转移（CT）状态发射为主，并被证实具有高斯特性并且几乎完全独立于电压。更重要的是，冷却到一定材料特定的低温后，持续观察到强烈的线宽变窄，在该温度以下，线宽保持恒定。因此很清楚，在室温或更高的工作条件下测得的线宽的主要部分是源于热激活或动态混乱。通过考虑高频和低频分子振动模式，可以完全描述观察到的高能发射尾部的温度依赖性，而不必依赖于静态无序。带有较大的Huang-Rhys因子的低频分子模的存在会导致高斯线形，在高温下，由于高频分子内模的热填充，该线形进一步加宽了。