Optical analysis of CH3NH3SnxPb1-I-x(3) absorbers: a roadmap for perovskite-on-perovskite tandem solar cells

Organic-inorganic perovskite structures in which lead is substituted by tin are exceptional candidates for broadband light absorption. Herein we present a thorough analysis of the optical properties of CH3NH3SnxPb1-xI3 films, providing the field with definitive insights about the possibilities of these materials for perovskite solar cells of superior efficiency. We report a user's guide based on the first set of optical constants obtained for a series of tin/lead perovskite films, which was only possible to measure due to the preparation of optical quality thin layers. According to the Shockley-Queisser theory, CH3NH3SnxPb1-xI3 compounds promise a substantial enhancement of both short circuit photocurrent and power conversion efficiency in single junction solar cells. Moreover, we propose a novel tandem architecture design in which both top and bottom cells are made of perovskite absorbers. Our calculations indicate that such perovskite-on- perovskite tandem devices could reach efficiencies over 35%. Our analysis serves to establish the first roadmap for this type of cells based on actual optical characterization data. We foresee that this study will encourage the research on novel near-infrared perovskite materials for photovoltaic applications, which may have implications in the rapidly emerging field of tandem devices.

Mechanosynthesis of the hybrid perovskite CH3NH3PbI3: characterization and the corresponding solar cell efficiency

Marius Franckevicius, Michael Graetzel, Daniel Prochowicz, Shaik Mohammed Zakeeruddin

We present a facile mechanochemical route for the preparation of hybrid CH3NH3PbI3 (MAPbI(3)) perovskite particles with the size of several hundred nanometers for high-efficiency thin-film photovoltaic devices. Powder X-ray diffraction measurements demonstrate that mechanosynthesis is a suitable strategy to produce a highly crystalline CH3NH3PbI3 material showing no detectable amounts of the starting CH3NH3I and PbI2 reagents. Thermal stability measurements based on the thermogravimetric analysis data of mechanosynthesized perovskite particles indicated that the as-ground MAPbI(3) is stable up to 300 degrees C with no detectable material loss at lower temperatures. The optical properties of newly synthesized perovskite particles were characterized by applying steady state absorption and fluorescence spectroscopy, which confirmed a direct band-gap of 1.48 eV. Time resolved single photon counting measurements revealed that 70% of charges undergo recombination with a 61 ns lifetime. The solar cell devices made from mechanosynthesized perovskite particles achieved a power conversion efficiency of 9.1% when applying a one step deposition method.

Royal Society of Chemistry2015

Molecular Engineering Towards High Efficiency Perovskite Solar Cells

Jiyoun Seo

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 todayâs 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.

EPFL2018

Molecular Engineering Towards High Efficiency Perovskite Solar Cells

Jiyoun Seo

EPFL2018