Molecular Engineering Towards High Efficiency Perovskite Solar Cells

High energy demands of our society stimulate the development of sustainable tech-nologies, such as those based on solar energy conversion or photovoltaics. Perovskite solar cells (PSCs) are an emerging photovoltaic technology that consist of the perov-skite film positioned between the electron transport and hole transport layer, while the photovoltaic properties are determined by each component of the structure, as well as the corresponding interfaces. Since 2009, power conversion efficiency of solar cells has already achieved values above 22%. This rapid progress in perovskite solar cell research led to the increased performance and operational stability, which nevertheless remains a challenge towards the commercialization of PSC technologies. In the course of this PhD thesis, new strategies in the design of perovskite solar cells were established based on molecular engineering by addressing (1) perovskite, (2) electron transport, as well as (3) hole transport layers, as detailed in the subsequent chapters. The research objectives focused on two key investigation domains, namely charge recombination in the perovskite grain boundaries or interfaces of perovskite / electron transport layers, and the stability of perovskite and hole transport materials. This approach resulted in the development of highly stable and efficient PSCs. First, with the aim of achieving high performance PSCs, the investigation of crystal growth mechanism was performed to discover the methods to control the grain size distribution within perovskite films that affects their performance by employing or-ganic agents. The study revealed the crystal growth mechanisms and resulted in the achievement of high efficiency of 19.5% in a planar perovskite solar cell architecture. Subsequently, cesium-based perovskites were studied as novel perovskite systems with excellent stability. The role of the cesium in the crystal formation of perovskites was primarily explored, facilitating the rapid room temperature crystallization that led to power conversion efficiency of 18.1% under room temperature. These results are very promising for low temperatue applcation demands, such as the roll-to-roll production and the development of flexible PSCs. In addition to the perovskite absorption layer, the roles of electron and hole transport layers are very important for the PSC performance. In this regard, while the mesoscopic titanium oxide (mp-TiO2) is predominantly used for electron transporting materials, studies of mp-TiO2 are scarce despite its common use in today’s most effi-cient PSCs. Therefore, following the previous investigation of the perovskite materi-al, we further demonstrated for the first time that bimodal porous TiO2 nanoparticles and their surface doping with cesium halides further strengthens the interaction with the perovskite layer, resulting in very high fill factor of 80% and efficiencies exceed-ing 21%. Finally, a new dopant, Zn(TFSI)2, for organic hole transport materials was explored to enhance thermal stability by replacement of one of the dopant materials. This ap-proach revealed further improved stability with efficiency exceeding 22%. This work on the development of highly efficient and stable perovskite solar cells highlights the utility of molecular engineering and provides the basis for facilitating industrial applications in the near future.

Graetzel, Michael
Zakeeruddin, Shaik Mohammed
Lausanne, EPFL

 Record created 2018-07-16, last modified 2019-05-09

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