4:00pm - 4:30pmInvited
Light-Induced Processes in Organohalide Perovskites: Interplay of Nuclear and Electronic Dynamics
1National Research Council (CNR), Institute of Molecular Science and Technologies (ISTM), Italy; 2CompuNet Italian Institute of Technology (IIT), Italy
We illustrate the results of ab initio molecular dynamics simulations coupled to first principles electronic structure calculations on the effect of light absorption on the electronic and dynamical properties of organohalide lead perovskites. The role of the organic cation dynamics and of ion/defect migration is analyzed in relation to photoinduced structural transformations and solar cell operation. It is found that Frenkel defects, relatively abundant in MAPbI3 and related perovskites, undergo a light-induced dynamical transformation which may account for the observed enhanced photoluminescence quantum yield following sample irradiation. We also show how most of perovskites unusual properties in terms of defects and trapping dynamics can be explained by the close similarity between the perovskite properties and the photochemistry of iodine, both for 3D and 2D materials. Along with the unusual defect chemistry, the role of large polarons in screening charge carriers from recombination is finally illustrated, which largely contributes to the outstanding optoelectronic properties of this class of materials.
 J.M. Azpiroz et al. Energy Env. Sci. 2015, 8, 2118.
 C. Quarti et al. Energy Env. Sci. 2016, 9, 155.
 E. Mosconi et al. ACS Energy Lett. 2016, 1, 182.
 E. Mosconi et al. Energy Environ. Sci. 2016, 9, 3180.
 D. Cortecchia et al. J. Am. Chem. Soc. 2017, 139, 39.
 K. Domansk et al. Energy Environ. Sci. 2017, DOI: 10.1039/C6EE03352K.
4:30pm - 5:00pmInvited
High-performance Mesoscopic Perovskite Solar Cells with Carbon Counter Electrode
National Chiao Tung University, Taiwan
The development of all solid-state thin-film solar cells has reached a new milestone when the devices made of organometallic lead halide perovskite materials were reported with power conversion efficiency (PCE) exceeding 20 %. The key issue to make a device with a great photovoltaic performance for this kind of solar cells is to control the film morphology of perovskite under different experimental conditions. Varied mesoporous TiO2 nanostructures were applied to show the morphological effect of the scaffold on the device performance with a mesoscopic heterojuction. Anti-solvent method was applied to improve the morphology and crystallinity of the perovskite films to obtain device performance exceeding PCE 16 %. Varied additives were applied to control the formation morphology of the perovskite nanocrystals with a planar heterojunction. Moreover, we developed a simple drop-casting method via slow crystallization to grow dense and uniform perovskite nanocrystals at room temperature for carbon-based mesoscopic solar cells free of an organic hole-transport layer. The CH3NH3PbI3/N-methyl-2-pyrrolidone (NMP) precursor solution (40 %) was first dripped onto a substrate with film configuration TiO2/Al2O3/C and infiltrated at 70 °C for 10 min. The perovskite substrate was then stored in a dry box (humidity 50 %) at 20 °C for at least 100 h to complete the crystal growth. The device attained the best efficiency of power conversion (PCE), 15.0 %, with average value (13.9 ± 0.5 %), which is much superior to those devices from either the traditional one-step thermal annealing (TA) method (5.2±1.0 %) or the traditional sequential TA method (10.1±0.7 %). For tin-rich perovskite, we designed and synthesized alloyed Sn-Pb mixed halide perovskites by dipping the precursor solutions on the mesoporous films with the TiO2/Al2O3/C configuration to form solar cells free of organic HTM. The photovoltaic performance was further improved on adding 30 mol % of tin fluoride (SnF2) with device configuration FTO/TiO2/Al2O3/NiO/C producing best power conversion efficiency 5.13 % with great reproducibility and intrinsic stability.
5:00pm - 5:15pmOral
Alcohol-Soluble Small Molecular Hole Transporting Materials for Perovskite Solar Cells
South China Normal University, China
Perovskite solar cells (PSCs) are one of the most promising technologies since the last decade due to its facile fabrication process and the high power conversion efficiency (PCE). Up to date, the best reported PCE has reached 21%, however it is still facing lots of challenges like the stability against moisture. Although PCE of 10% has been obtained based on hole transporting materials (HTMs) free PSCs, this interface can protect the active perovskite phase and more importantly increase the PCE further.
Our previous work has proved that the side chains on the nitrogen atom of diphenylamine can effectively induce the molecular self-organization and have a fundamental influence on the solid state electronic properties.
Herein, we reported the design and synthesis of a series of small molecular HTMs based on diphenylamine which is endowed with the alcohol solubility by substitution of aliphatic side chains. The characterization of the molecular structure and optoelectronic properties by single crystal X-ray diffraction, UV-Vis absorption spectroscopy, cyclic voltammetry and the hole transporting properties based on SCLC will be presented to build a clear structure-properties relationship. Last but not least, the photovoltaic properties and the stability will be discussed on a n-i-p type perovskite solar cells.
 Polman A., Knight M., Garnett E. C., Ehrler B., Sinke W. C. Science, 2016, 352, 307.
 Ponseca C. S. Jr., Savenije T. J., Abdellah M., Zheng K., Yartsev A., Pascher T., Harlang T., Chabera P., Pullerits T., Stepanov A., Wolf J-P., Sundström V. J. Am. Chem. Soc. 2014, 136, 5189.
 Jiang Y., Cabanetos C., Allain M., Jungsuttiwong S., Roncali J. Org Electron, 2016, 37, 294; Jiang Y., Cabanetos C., Allain M., Liu P., Roncali J. J. Mater. Chem. C. 2015, 0, 5145; Jiang Y., Gindre D., Allain M., Liu P., Cabanetos C., RoncaliJ. Adv. Mater. 2015, 27, 4285.
5:15pm - 5:30pmOral
Light-trapping Metallic Nanostructures for Enhanced Power Conversion Efficiency in Perovskite Photovoltaic
Department of Materials Engineering, Indian Institute of Science, India
In perovskite solar cell, light absorption is one of the critical factors for the enhancement of the power conversion efficiency. For the effective enhancement in the device efficiency more number of photons should be harvested in the given device area. Photon harvesting can be adopted by various methods in perovskite photovoltaic, one such method is improving active layer grain size to achieve high performance devices. The improved light harvesting can also be achieved by trapping the light by incorporating metallic nanostructures at the interface. Depend on the size of the metallic nanostructure either light is absorbed or scattered by it. When light is absorbed by the nanostructures, it behave like sub-wavelength antennas as result of LSPR excitation and hence plasmonic near field will coupled to active layer. While larger size particles (> 30 nm) acts as sub-wavelength scattering center of light and this helps in trapping freely propagating plane wave of incident light and couples with active layer. As a result there is an increase in the optical enhance. Under this work, three types of metallic nanostructure (Al, Cu and Ag) which is basically poly-dispersed particles were embedded at the interface and studied device performance. It is observed that device with the metallic nanostructure showed significant enhancement in optical absorption. The enhancement in optical absorption was observed for all three systems. Consequence of improved optical absorption, device with metallic nanostructure showed significant improvement in short circuit current. From this observation the metallic nano-structures at electron collecting electrode aids in light trapping and reduction in series resistance. The coupled effect of optical and electrical enhancement tends to improved power conversion efficiency in the device.
5:30pm - 5:45pmOral
Molecularly Engineered Hole Transporting Materials for High Performance Perovskite Solar Cells
Ecole Polytechnique Federale de Lausanne, Switzerland
Hybrid lead halide perovskite-based solar cells have attracted significant attention in the photovoltaics due to inexpensive precursors, simple fabrication methods, and remarkably high power conversion efficiency. Hole transporting material plays an important role facilitating the extraction and transportation of holes from perovskite to the corresponding contact and is essential to reach high light-to-current conversion efficiency. To date, a great number of new promising molecular organic HTMs have been reported, but only very few candidates reached PCE values close to or exceeding 20%. However, although PSCs have achieved high PCE values, the stability remains an issue due to dopant-induced degradation of PSC.
Traditionally, the HTM for PSCs is heavily doped with LiTFSI, TBP, and FK209. However, the use of additives is problematic, since hygroscopic nature of lithium salt makes the HTM highly hydrophilic and Co(III) dopant tends to chemical degradation, negatively influencing the stability of the entire device. Therefore, a promising solution for stabilizing PSCs is the appropriate choice of dopant-free HTMs.
In this work, we have systematically engineered three novel dopant-free star-shaped donor – π-bridge – acceptor (D–π–A) type HTMs. Such molecules feature a planar triazatruxene central core (D), inducing π-stacking for vertical hole conduction, thiophene-based multiple conjugated arms (π) and malononitrile (A). Due to strong intermolecular interaction, the molecules have a great potential to show high charge carrier mobility. We show that a highly ordered characteristic face-on organization could favor vertical charge carrier transport in the perovskite solar cell and a PCE over 19% with improved stability was achieved using KR321. This result is on par with the heavily doped spiro-OMeTAD reference, clearly showing the importance of proper molecular engineering and the great perspective of dopant-free HTMs in perovskite solar cells and outperforms most of the other dopant-free HTMs reported to date.