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Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2020-01-10 , DOI: 10.1021/jacs.9b12526 Kyra N Schwarz , Paul B Geraghty , Valerie D Mitchell , Saeed-Uz-Zaman Khan , Oskar J Sandberg 1 , Nasim Zarrabi 1 , Bryan Kudisch , Jegadesan Subbiah , Trevor A Smith , Barry P Rand , Ardalan Armin 1 , Gregory D Scholes , David J Jones , Kenneth P Ghiggino
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2020-01-10 , DOI: 10.1021/jacs.9b12526 Kyra N Schwarz , Paul B Geraghty , Valerie D Mitchell , Saeed-Uz-Zaman Khan , Oskar J Sandberg 1 , Nasim Zarrabi 1 , Bryan Kudisch , Jegadesan Subbiah , Trevor A Smith , Barry P Rand , Ardalan Armin 1 , Gregory D Scholes , David J Jones , Kenneth P Ghiggino
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Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient 20 Stark effect, caused by nanoscale electric fields of ~487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ~2000 times less than predicted by Langevin theory. This in turn leads to the build-up of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. 25 Interestingly, this signal is only experimentally obvious in thick films, due to the different scaling of electro-absorption and photo-induced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of > 8 % because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies, the bulk heterojunction can go beyond its established role in charge photogeneration, and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.
中文翻译：
减少有机异质结中的复合和类似电容器的电荷积累
有机光伏 (OPV) 的效率不断提高，提高了太阳能转换的前景。然而，研究人员长期以来一直在考虑如何通过复合不良扩散和显着的库仑吸引力来抑制自由载流子的损失，这会导致电子和空穴在靠近它们光生位置的界面处相遇。使用飞秒瞬态光谱，我们报告了大型瞬态 20 Stark 效应的纳秒增长，这是由器件活性层中光生自由载流子之间~487 kV/cm 的纳米级电场引起的。我们发现活性层的特定形态导致电荷载流子的能量级联，抑制复合途径，这比朗之万理论预测的少约 2000 倍。这反过来导致电荷在供体和受体域中积累 - 远离界面，抵抗双分子重组。25 有趣的是，由于瞬态吸收光谱中电吸收和光致吸收信号的缩放比例不同，该信号仅在厚膜中在实验上是明显的。我们没有抑制器件性能，而是表明厚度达 600 nm 的器件保持了 > 8% 的效率，因为域可以提供更高的载流子密度。这些观察结果表明，通过特定的纳米级形态，体异质结可以超越其在电荷光生中的既定作用，并且可以充当电容器，其中相邻的自由电荷远离界面并可以防止双分子重组。
更新日期：2020-01-10
中文翻译：
减少有机异质结中的复合和类似电容器的电荷积累
有机光伏 (OPV) 的效率不断提高，提高了太阳能转换的前景。然而，研究人员长期以来一直在考虑如何通过复合不良扩散和显着的库仑吸引力来抑制自由载流子的损失，这会导致电子和空穴在靠近它们光生位置的界面处相遇。使用飞秒瞬态光谱，我们报告了大型瞬态 20 Stark 效应的纳秒增长，这是由器件活性层中光生自由载流子之间~487 kV/cm 的纳米级电场引起的。我们发现活性层的特定形态导致电荷载流子的能量级联，抑制复合途径，这比朗之万理论预测的少约 2000 倍。这反过来导致电荷在供体和受体域中积累 - 远离界面，抵抗双分子重组。25 有趣的是，由于瞬态吸收光谱中电吸收和光致吸收信号的缩放比例不同，该信号仅在厚膜中在实验上是明显的。我们没有抑制器件性能，而是表明厚度达 600 nm 的器件保持了 > 8% 的效率，因为域可以提供更高的载流子密度。这些观察结果表明，通过特定的纳米级形态，体异质结可以超越其在电荷光生中的既定作用，并且可以充当电容器，其中相邻的自由电荷远离界面并可以防止双分子重组。
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