Conference Programme

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U2-02: Symp U
Monday, 19/Jun/2017:
4:00pm - 6:15pm

Session Chair: Constance J. Chang-Hasnain, University of California, Berkeley
Session Chair: Tae-Woo Lee, Seoul National University
Location: Rm 332

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4:00pm - 4:30pm

Basics and Applications of Organometal Halide Perovskite Solar Cells

Hiroshi SEGAWA

The University of Tokyo, Japan

Next-generation solar cells based on new concepts and/or novel materials are currently attracting wide interests. Among them, organometal halide perovskite solar cells (PSC) based on CH3NH3PbI3 have attracted attention in the last few years due to their outstanding performance as photovoltaics. Despite the unique properties and higher efficiencies of PSCs, several important issues still remained, e.g., mysterious hysteresis in I-V curves and dispersion of the performances. It has been found the hysteresis strongly depends on the device architecture, where the planar PSCs show relatively large hysteresis than meso-structured PSCs. In this study, we found that an equivalent circuit model with a series of double diodes, capacitors, shunt resistances, and single series resistance produces simulated I-V curves with large hysteresis matching with the experimentally observed curves. On the other hand, we found that the inhomogeneous photo-physical properties of the organometal halide perovskite. In order to improve the energy conversion efficiency, we should control the properties. Additionally, we have prepared the various tandem solar cells with spectral splitting system showing a high overall power conversion efficiency.

4:30pm - 5:00pm

Lead Halide Perovskites of Different Dimensionilities: Growth, Properties and Applications in Optoelectronics

Osman M. BAKR

King Abdullah University of Science and Technology (KAUST), Saudi Arabia

Three-dimensional (3D) lead halide perovskites exhibit impressively long carrier diffusion lengths and low trap densities, while reduced-dimensionality perovskites, such as two-dimensional (2D) and zero-dimensional (0D) perovskite derivatives, possess large exciton binding energies and high photoluminescence quantum yields. These characteristics make the diverse class of perovskite materials ideal for photovoltaics, photodetection, and light emission. Here we discuss our latest advances in growing and understanding the properties of monocyrstalline 3D perovskites, as well as 0D and 2D perovskite derivatives. We also demonstrate the integration of these materials in a range of optoelectronic applications including: monocrystalline perovskite solar cells; simultaneously fast and sensitive photodetectors that can operate in both broadband and narrow-band regimes; and efficient light-emitting diodes. Thus, in these device prototypes, we showcase the importance of crystallinity, dimensionality, and composition for tailoring materials properties, and realizing novel and efficient perovskite optoelectronics.

5:00pm - 5:15pm

The Role of Nanoscale Stoichiometric Variations on Local Charge Collection in Mixed-Halide Hybrid Perovskite Solar Cells

Yanqi LUO1, Sigalit AHARON2, Michael STUCKELBERGER3, Ernesto MAGANA1, Barry LAI4, Mariana BERTONI3, Lioz ETGAR2, David FENNING1

1University of California, San Diego, United States; 2The Hebrew University of Jerusalem, Israel; 3Arizona State University, United States; 4Argonne National Laboratory, United States

The spatial distribution of absorber elemental makeup determines the local electronic properties of solar absorbers and ultimately device performance. Hybrid organic-inorganic lead halide perovskite materials have disruptive potential in solar cells and other optoelectronic applications due to their cost-efficient fabrication and excellent tunable optoelectronic properties. However, the microscopic effects of A-site cation and halide substitution on carrier-charge transport have not been fully clarified in these mixed-chemistry systems. In this study, a nanoscale spatial variation of Br incorporation is found in both iodide rich and poor FAPbI3-xBrx solar cells by means of non-destructive synchrotron-based nanoprobe X-ray fluorescence (Nano-XRF). Simultaneous collection of spatially-resolved X-ray beam induced current (XBIC) maps reveal large variations in local photocurrent collection. In the case of the mixed Br/I system, the effective charge collection drops below both FAPbI3 and FAPbBr3 control samples when x = 0.20; then it gradually recovers as Br/I ratio increased and reaches plateau when x = 0.75. The application of these characterization techniques to the mixed-halide perovskites allows us to precisely superimpose the halide heterogeneity on the corresponding photocurrent response with resolution down to 200 nm. Combining the local elemental information from Nano-XRF and the local optoelectronic response from XBIC reveals the electronic role of Br substitution and opens new directions toward understanding the tuning of mixed-cation and mixed-halide perovskite systems toward optimal device efficiency.

5:15pm - 5:30pm

Pressure-induced Sintering of CH3NH3PbBr3 Quantum Dots into Stable Nanocrystals

Tingting YIN1,2, Jiaxu YAN1, Ze Xiang SHEN1,2

1Centre for Disruptive Photonic Technologies, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore; 2Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore

Hybrid organic-inorganic perovskite nanocrystals are of great interest as promising materials for solar cells and LEDs, due to their high carrier mobility and tunable spectral absorption range. To date, synthesis methods of perovskite nanocrystals are mainly focused on chemical methods. Here, we report a clean, quick and easy way to create perovskite nanocrystals via pressure-induced sintering of CH3NH3PbBr3 quantum dots (QDs). Most bulk perovskite structures formed under pressure are not stable and the corresponding developed crystal structures and properties will disappear after release of pressure, which cause a big issue for the real applications of bulk perovskite materials under compression so far. While the sintering CH3NH3PbBr3 QDs with size of 10 nm into nanocrystals with size of 100 nm is realized here under high-pressure process, the developed structure of which is stable after release of pressure. By combining the static and time-resolved photoluminescence measurements, we investigate the corresponding optical behaviors have been changed and irreversible after release of pressure. From the in-situ XRD and TEM measurements, we indicate the nanocrystal synthesis mechanism depends on both crystal phase transitions and crystal surface kinetics. Our studies provide significant insights into pressure-induced structural mechanisms and their associated growth processes.

5:30pm - 5:45pm

Small Polarons in Layered Hybrid Perovskites

Daniele CORTECCHIA1,2, Jun YIN3,4, Shu-Zee A. LO3, Gagik G. GURZADYAN5, Annalisa BRUNO2, Jean-Luc BREDAS4, Cesare SOCI3

1Interdisciplinary Graduate School, Nanyang Technological University, Singapore; 2Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Singapore; 3School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore; 4Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Saudi Arabia; 5State Key Laboratory of Fine Chemicals, Dalian University of Technology, China

2D perovskites are multiple quantum well-like layered materials consisting of alternating organic and inorganic sheets. Their unique self-assembled structure induces confinement of charge carriers within the inorganic layers, causing a large increase of the exciton binding energy up to 400 meV. In some cases, their strong excitonic characteristics lead to unusually broad and highly Stoke shifted photoluminescence. Here we combine steady-state and time-resolved spectroscopy with density functional theory (DFT) simulations to study the emissive properties of the 2D broadband emitting perovskites (EDBE)PbX4 (where X=Cl, Br, I). Time-resolved photoluminescence and transient absorption reveal the formation of multiple photoinduced colour centres with wide energy distribution suggesting the formation of self-trapped charge carriers within few nanoseconds after photoexcitation. Consistently, DFT simulations of perturbed crystal structures and large perovskite molecular clusters have allowed identifying the emissive centres as self-trapped electrons (STEL) Pb23+ and holes (STH) such as X2- (X=Cl, Br), Pb3+ and I3- trimers (VF centres) localized at specific inorganic lattice sites in form of small polarons.[1,2,3] By comparing perovskites with different photoluminescence properties, we further demonstrate that such self-trapping phenomena are induced by structural deformations of the Pb-X framework and distortions of the PbX6 octahedra units. These studies uncover the relevance of excitonic and polaronic photoexcitations in hybrid perovskites toward the design of solution processable, broadband emitting materials for solid state lighting and displays.


[1] D. Cortecchia, J. Yin, A. Bruno, S.-Z, A. Lo, G. G. Gurzadyan, S. Mhaisalkar, J.-L. Brédas, C. Soci, arXiv:1603.01284

[2] J. Yin, H. Li, D. Cortecchia, C. Soci, J.-L. Brédas, ACS Energy Lett., 2017, 2, 417–423

[3] D. Cortecchia, S. Neutzner, A. R. S. Kandada, E. Mosconi, D. Meggiolaro, F. De Angelis, C. Soci, A. Petrozza, J. Am. Chem. Soc., 2017, 139, 39–42

5:45pm - 6:00pm

Residual Solvent Effects on the Crystal Growth and Thermal Stability of a Lead-Halide Perovskite film

Chung-Yao LIN1, Hao-Chung CHIA1, Yu-Yun HSIAO2, Je-Wei CHANG1, Hwo-Sheunn SHUE3, An-Chung SU1, Shao-Sian LI2, Chun-Wei CHEN2, U-Ser JENG1,3

1Department of Chemical Engineering, National Tsing Hua University, Taiwan; 2Department of Materials Science and Engineering, National Taiwan University, Taiwan; 3National Synchrotron Radiation Research Center, Taiwan

Crystal structures of organolead trihalide perovskite materials are found to influence significantly the material performance in solar cell application. Using time-resolved grazing incident wide- and small-angle X-ray scattering (GIWAXS/GISAXS) supplemental with XRD, we reveal residual solvent effects on the crystal structural features of a perovskite (CH3NH3Pb3-xClx) film, processed from a solution containing 40 wt% of MAI and PbCl2 (3:1 in molar ratio) in DMF. The results indicate that the as-spun film with fast removal of residual solvent under vacuum after spin-coating has a pre-matured perovskite structure together with suppressed intermediate phases. In situ GIWAXS during subsequent 110 °C annealing of the as-spun film reveals a fast decay of the pre-matured perovskite structure within first 70 s. This is followed by a slow recrystallization of perovskite crystals via a monotonic growth mechanism, as revealed by the Avrami analysis. The hence formed crystal structure after annealing has a better orientation texture and higher thermal stability. In contrast, the film without fast solvent extraction treatment exhibited faster growth with multiple growth mechanisms involving intermediate phases. The final crystal size of the film with fast solvent extraction treatment is around 234 nm, with the [110] planes preferentially oriented along the surface normal direction.

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