Interfaces in All-Perovskite Tandem Solar Cells

全钙钛矿型串联太阳能电池有望实现高效率和低成本。混合钙钛矿之所以脱颖而出，是因为它可以用相对简单的工艺生产出只有几百纳米和低缺陷密度的薄膜。串联结构使效率超过单结太阳能电池的辐射极限。尽管取得了快速的进展，但钙钛矿-钙钛矿串联太阳能电池仍然面临着几个挑战：(I)吸收体的稳定性。上单元高禁带所需的高溴含量有利于晶相的形成，而低能隙吸收层中的Sn2+可能被氧化成Sn4+。(Ii)在吸收层和电子传输层(ETL)之间的界面处或在各种互连层之一处的界面复合，例如由横向不均匀和泄漏引起。此外，挥发性成分的损失、离子迁移或化学反应可能导致复合活性陷阱状态或萃取势垒。(3)光学损耗，例如电荷传输层和互连层的寄生吸收损耗、反射损耗和不完全吸收，这限制了电流，特别是底部低带隙太阳能电池的电流。解决这些挑战的关键在于串联太阳能电池中的许多接口。策略包括添加剂、界面钝化、新的接触材料、电荷传输和互连层的减薄以及为增加吸收而进行的结构化。对于以目标为目标的串联单元的优化，对相关界面的透彻了解对于进一步的技术开发至关重要。因此，本项目的目的是研究导致钙钛矿-钙钛矿串联太阳能电池界面性能损失的物理和化学效应。为此，我们将制造钙钛矿-钙钛矿串联器件，并将开发先进的空间分辨表征工具，并辅之以从纳米尺度的界面效应到器件级别的理论建模。

All-perovskite tandem solar cells promise high efficiencies and low costs. Hybrid perovskites stand out because it is possible to produce films of only a few hundred nanometers and low defect density with relatively simple processes. The tandem geometry enables efficiencies beyond the radiative limit of single junction solar cells. Despite rapid progress, perovskite-perovskite tandem solar cells still face several challenges: (i) The stability of the absorbers. The high bromide content necessary for high bandgaps of top cell absorber favors phase, while Sn2+ in the low-bandgap absorber might oxidize to Sn4+. (ii) Interface recombination at the interface between absorber and the electron transport layers (ETL) or at one of the various interconnection layers, e.g. caused by lateral heterogeneity and leakage. Furthermore, the loss of volatile components, ion migration, or chemical reaction might lead to recombination-active trap states or extraction barriers. (iii) Optical losses, such as parasitic absorption losses in charge transport and interconnection layers, reflection losses and incomplete absorption, which limit the current especially of the bottom low-bandgap solar cell. The key to solving these challenges lies in the many interfaces in a tandem solar cell. Strategies include additives, interface passivation, new contact materials and thinning of charge transport and interconnection layers as well as structuring for increased absorption. For a target-oriented optimization of tandem cells, a thorough understanding of the associated interfaces is of crucial importance for the further technology development. Therefore, the aim of this project is to investigate the physical and chemical effects that lead to performance losses at the interfaces of perovskite-perovskite tandem solar cells. To this end, we will manufacture perovskite-perovskite tandem device, and will develop advanced spatially resolved characterization tools complemented by theoretical modeling all the way from nanoscale interface effects to the device level.