Department of Physics, University of Oxford , Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom.
Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München , James-Franck-Strasse 1, 85748 Garching, Germany.
Nano Lett. 2017 Sep 13;17(9):5782-5789. doi: 10.1021/acs.nanolett.7b02834. Epub 2017 Aug 14.
An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CHNHPbI are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10 cms is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.
理解电荷载流子复合过程对于开发用于光伏应用的混合金属卤化物钙钛矿至关重要。我们表明，CHNHPbI 中辐射双分子复合常数的典型测量受到光子再吸收的强烈影响，这掩盖了更大的固有双分子复合速率常数。通过研究一组厚度在 50nm 至 533nm 之间变化的薄膜，我们发现双分子电荷复合速率似乎随着薄膜厚度的增加而减缓一个数量级。然而，通过使用一个考虑光子再吸收和电荷载流子扩散的动力学模型，我们确定所有样品的单个固有双分子复合系数值均为 6.8×10 cms，与薄膜厚度无关。因此，我们假设，对于此类系数，文献中报道的广泛值部分归因于不同大小的晶粒或介孔支架的样品之间的光子外耦合的差异影响光散射，而较薄的薄膜或折射率匹配的周围层可以减少光子再吸收的可能性。我们讨论了光子限制对薄光伏层中自由电荷载流子保留的关键作用，并强调了一种从瞬态光谱测量评估此类方案成功的方法。
