Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells
Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells
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DOI：
10.1021/jacs.9b01465
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发表时间：
2019-04-17
影响因子：
15
通讯作者：
Nelson, Jenny
中科院分区：
文献类型：
--
作者：
Eisner, Flurin D.;Azzouzi, Mohammed;Nelson, Jenny
最近的一些研究表明，有机太阳能电池中的非辐射电压损失可以在供体和受体分子状态之间具有低能量偏移的系统中得到抑制，但支撑这一点的物理原因仍不清楚。在这里，我们提出了一个系统的研究18个不同的供体/受体共混物，以确定的效果，能量偏移的辐射和非辐射复合的电荷转移（CT）状态。我们发现，对于某些共混物，低偏移导致在电荷转移和最低的供体或受体激子状态之间的杂交，这导致在非辐射电压损失的值低至0.23 V与CT状态的发光的增加相关联的强抑制。此外，我们扩展了两个状态的CT-状态复合模型，包括CT和第一激发态之间的相互作用，这使我们能够解释低的非辐射电压损耗的有效CT到基态振子强度的增加，由于强度借用机制。我们表明，低非辐射电压损耗可以实现CT和第一激发态之间的强电子耦合的材料组合，其中较低的带隙材料具有高的振荡器强度从激发态到基态的过渡。最后，从我们的模型中，我们提出实现非常低的非辐射电压损耗可能是以更高的总体复合率为代价的，这可能有助于解释高度杂交系统通常较低的FF和EQE。
A number of recent studies have shown that the nonradiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor/acceptor blends to determine the effect that energetic offset has on both radiative and nonradiative recombination of the charge-transfer (CT) state. We find that, for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the nonradiative voltage loss to values as low as 0.23 V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low nonradiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.