Optoelectronic properties of hybrid metal halide perovskites: from nanoscale to devices

人为气候变化是我们社会目前面临的最大挑战之一。为了减轻燃烧化石燃料和向大气中释放二氧化碳的有害影响，我们必须尽快转向清洁的可再生能源。太阳能是最有前途的选择之一，因为它有潜力完全满足我们整个全球的能源需求。虽然太阳能产业近年来发展迅速，但在降低或维持成本的同时提高光伏器件的效率仍然存在挑战。混合金属卤化物钙钛矿薄膜是用于实现这些目标的有前景的材料，因为与现有技术竞争的高效器件可以通过使用廉价的地球丰富的材料的溶液加工方法容易地合成。然而，目前最先进的钙钛矿材料难以在环境条件下保持长期稳定性。二维Ruddlesden-Popper相钙钛矿已经证明了上级稳定性，并且因此在钙钛矿界引起了很多关注，尽管用这些材料制成的光伏器件还不能实现与它们的3D对应物相同的高效率。由于目前尚不清楚这种低效率是由于材料的内在限制还是由于改进的加工程序可修复的外在因素，因此迫切需要对这些材料的基本光电特性进行全面研究。这项研究将通过充分表征2D钙钛矿的光电特性来填补这一知识空白，以确定其用于太阳能电池的最终可行性。然而，这项任务远非简单，因为许多竞争因素会限制这些材料中的有效电荷传输。除了表现出高激子结合能之外，2D钙钛矿还表现出与其薄膜微结构相关的掺杂密度增加和结晶度降低，所有这些先前的工作都表明限制了电荷载流子扩散长度（Milot等人，Nano Lett，2016）。理解这些材料中的光电性质的挑战是能够将本征性质（例如激子效应）的影响与可以通过改进的加工方法改变的诸如掺杂密度和结晶度的非本征性质（extrinsic properties）的影响隔离开。由于许多非本征性质可以随着并入太阳能电池而进一步改变，因此这个问题是重要的。为了解决这个问题，这项研究将开创一种新的方法，通过研究从单晶到器件的光电特性，以获得内在特性的全貌，并确定它们如何受到微观结构和太阳能电池内含物等外在因素的影响。为了最好地进行比较，它将利用THz和光致发光（PL）光谱，这是两种最通用的光电特性分析技术，包括电荷载流子迁移率和复合动力学。它将进一步利用这两种技术的多功能性，首次将太赫兹散射近场光学显微镜（THz-SNOM）和时间分辨PL显微镜分析相结合，将纳米级空间分辨率的能力添加到现有的超快时间分辨率能力中。通过与传统的光伏功率转换效率的测量比较，它将确定改善器件制造的途径。这项研究提出的对2D钙钛矿光电特性的更深入理解将直接导致高效太阳能电池的开发，以满足我们的能源需求。

Anthropogenic climate change is currently one of the biggest challenges facing our society. In order to mitigate the detrimental effects of burning fossil fuels and releasing CO2 into the atmosphere, we must switch to clean, renewable sources as quickly as possible. Solar energy is one of the most promising options available because it has the potential to completely meet our entire global energy requirements. Although the solar energy industry has seen rapid development in recent years, challenges still remain to increase the efficiency of photovoltaic devices while decreasing or maintaining costs. Hybrid metal halide perovskite thin films are promising materials for achieving these goals as high efficiency devices which rival existing technology can be easily synthesized via solution processing methods using inexpensive, earth-abundant materials. However, current state-of-the-art perovskite materials struggle to maintain long-term stability under ambient conditions. Two-dimensional, Ruddlesden-Popper phase perovskites have demonstrated superior stability, and have thus attracted much attention in the perovskite community, although photovoltaic devices made with these materials have not been able to achieve the same high efficiencies as their 3D counterparts. As it is currently unknown whether this inefficiency is due to intrinsic limitations of the material or to extrinsic factors fixable with improved processing procedures, a comprehensive study of the fundamental optoelectronic properties in these materials is desperately needed. This research will fill this knowledge gap by fully characterizing the optoelectronic properties of 2D perovskites in order to determine their ultimate viability for use in solar cells. This task is far from straightforward, however, as many competing factors can limit efficient charge transport in these materials. In addition to exhibiting high exciton binding energies, 2D perovskites also demonstrate increased doping density and decreased crystallinity associated with their thin-film microstructure, all of which previous work has shown to limit charge-carrier diffusion lengths (Milot et al, Nano Lett, 2016). The challenge for understanding the optoelectronic properties in these materials is being able to isolate the effects of the intrinsic properties (e.g excitonic effects) from extrinsic properties such as doping density and crystallinity which could be altered with improved processing methods. As many of the extrinsic properties can further change with incorporation into solar cells, this problem is nontrivial. To address this issue, this research will pioneer a new approach by studying optoelectronic properties from single crystals to devices in order to gain a full picture of intrinsic properties and determine how they are affected by extrinsic factors including microstructure and solar cell inclusion. To best enable comparisons, it will utilize THz and photoluminescence (PL) spectroscopy, two of the most versatile techniques for the analysis of optoelectronic properties including charge-carrier mobility and recombination dynamics. It will further harness the versatility of these two techniques by combining THz scattering near-field optical microscopy (THz-SNOM) and time-resolved PL microscopy analyses for the first time, adding the capability of nanoscale spatial resolution to the existing capabilities for ultrafast time resolution. Through comparison with conventional measurements of photovoltaic power conversion efficiencies, it will identify pathways to improvement in device fabrication. The greater understanding of the optoelectronic properties of 2D perovskites that this research presents will lead directly to the development of high efficiency solar cells to meet our energy needs.