Carr Joshua M, Allen Taylor G, Larson Bryon W, Davydenko Iryna G, Dasari Raghunath R, Barlow Stephen, Marder Seth R, Reid Obadiah G, Rumbles Garry
University of Colorado Boulder, Materials Science & Engineering Program, Boulder, CO, 80303, USA.
National Renewable Energy Laboratory, Chemistry and Nanoscience Center, Golden, CO, 80401, USA.
Mater Horiz. 2022 Jan 4;9(1):312-324. doi: 10.1039/d1mh01331a.
Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies.
事实证明，理解弗伦克尔激子如何在低介电常数有机半导体中有效分裂以形成自由电荷具有挑战性，近年来人们提出了许多不同的模型来解释这一现象。在这里，我们提供证据表明，一个简单的模型，即引入适度的电荷离域、对可用微观状态求和以及电子转移的马库斯速率常数，可以解释文献中报道的许多看似矛盾的现象。我们使用一系列电子供体分子稀释敏化的电子受体富勒烯主体基质来检验这一假设。通过该供体系列，我们能够在0.7 eV的范围内调节光致电子转移的驱动力，通过时间分辨微波电导率测量，绘制出自由电荷产生效率的正常、最佳和反转区域。然而，随着驱动力增加，供体的光致发光迅速猝灭，没有反转行为的证据，也没有在没有额外竞争损失途径的情况下人们所预期的光致发光猝灭与电荷产生效率之间的线性关系。通过束缚电荷转移态和长程或离域自由电荷态的竞争形成可以自洽地解释这种行为，其中两个速率常数都由马库斯速率方程描述。此外，该模型预测高浓度共混物的反转区域会受到抑制，并且会有效产生超快自由电荷，这为在器件研究中很少观察到马库斯反转行为提供了一个机理解释。
