Unravelling halide segregation in hybrid perovskites for Si tandem photovoltaics

可再生能源为应对能源安全和气候变化带来的挑战提供了令人兴奋的机会。特别是光伏（PV）电池可以实现大规模的可持续发电：一小时内入射到地球表面的太阳能足以提供全世界目前每年的能源需求。作为光伏领域令人兴奋的新成员，有机-无机金属卤化物钙钛矿现在显示出超过22%的单结薄膜太阳能电池的认证功率转换效率。性能最好的单结电池目前都基于具有A-PbI 3式的碘化铅钙钛矿，其中阳离子A通常是甲基铵（MA）、甲脒鎓（FA）、铯（Cs）或其混合物。可再生能源领域的许多分析师认为，这些新型钙钛矿的最有效商业化是与现有的成熟硅技术相结合。在这里，钙钛矿薄膜电池与硅电池组合在2端或4端串联电池中，以较小的额外成本提高效率。对于优化的串联架构，需要平衡每个电池产生的光电流，这需要带隙接近1.75eV的钙钛矿，显著高于已建立的A-PbI 3材料显示的典型带隙约1.5eV。迄今为止，与硅串联应用相匹配的唯一高性能钙钛矿薄膜材料是基于A-Pb（Br_xI_（1-x））_3体系，当溴离子含量在x=0（仅碘离子）和x=1（仅溴离子）之间变化时，其带隙可调范围为~1.5 ~2.2eV。然而，混合卤化物钙钛矿受到一种不稳定性的影响，其起源使研究人员感到困惑。当用可见光照射时，材料自发地分离成富含碘化物和富含溴化物的域。这种效应是短暂的，在黑暗中几分钟后就会恢复。对于光伏应用，与这种效应相关的潜在电压偏移和电荷捕获对于稳定PV操作的目标是非常有害的。牛津大学和国际研究界最近的研究表明，材料有时可以通过选择A-阳离子和增强结晶度来稳定。然而，发现光稳定性敏感地依赖于加工条件，当方案或环境条件变化时，不稳定性反复出现。这些早期的研究表明，光致卤化物偏析不是固有的，因此可以补救，但如何做到这一点的全球图景仍然难以捉摸。我们的计划将确定这种影响的原因，并开拓在预计的太阳能电池寿命内对光稳定的新材料。我们将通过一个新的计划来实现这些目标，该计划汇集了一支世界领先的研究人员团队，他们在光伏材料和设备，先进的光谱学和高分辨率电子显微镜以及原位晶体结构分析方面具有互补的技能。该计划的成果将使长期光稳定，充分优化的材料，用于串联电池与既定的硅光伏技术的发展。

Renewable energy sources offer exciting opportunities to address challenges caused by energy security and climate change. Photovoltaic (PV) cells in particular can enable sustainable generation of electricity on a large scale: the solar energy incident on the surface of the earth in one hour is enough to provide the whole world's current yearly energy requirements. As an exciting newcomer to the PV landscape, organic-inorganic metal halide perovskites now show certified power conversion efficiencies for single-junctions thin film solar cells in excess of 22%. The best performing single-junction cells are currently all based on lead iodide perovskites with A-PbI3 formula, where the cation A is typically methylammonium (MA), formamidinium (FA), Caesium (Cs) or a mixture thereof. Many analysts in the renewable energy sector believe that the most effective commercialisation of these novel perovskites is in combination with existing, well-established silicon technology. Here, a perovskite thin-film cell is combined with a silicon cell in a 2- or 4-terminal tandem cell, boosting efficiency at small additional cost. For optimised tandem architectures, the photocurrents created by each cell need to be balanced, which requires a perovskite with band gap near 1.75eV, significantly above the typical bandgap of ~1.5eV displayed by the established A-PbI3 materials. To date, the only high-performance perovskite thin-film materials ideally matched for tandem applications with silicon are based on the A-Pb(Br_x I_(1-x))3 system, which allows band gap tunability from ~1.5 to ~2.2eV when the bromide content is varied between x=0 (iodide only) and x=1 (bromide only). However, the mixed halide perovskites are affected by an instability whose origin mystifies researchers. When illuminated with visible light, the material segregates spontaneously into iodide-rich and bromide-rich domains. This effect is transient, and recovers in the dark over the timescale of minutes. For photovoltaic applications, the potential voltage shifts and charge trapping associated with this effect are highly detrimental to the aim of stable PV operation. Recent research at Oxford and in the international research community has shown that materials can sometimes be stabilized through choice of A-cation and enhanced crystallinity. However, photo-stability was found to depend sensitively on processing conditions, with instability recurring when protocols or environmental conditions were varied. These incipient studies suggest that the photo-induced halide segregation is not as such intrinsic and therefore can be remedied, but a global picture of how this can be done remains elusive. Our programme will identify the causes underlying this effect and pioneer new materials that are photo-stable over projected solar cell life spans. We will achieve these aims through a novel programme that brings together a team of world-leading investigators with complementary skills in photovoltaic materials and devices, advanced spectroscopy and high-resolution electron microscopy, and in-situ crystal structure analysis. The outcomes of this programme will enable the development of long-term photo-stable, fully optimized materials for use in tandem cells with established silicon photovoltaic technology.