This PDF file contains the front matter associated with SPIE Proceedings Volume 11095, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Here we report the fabrication and lifetime analysis of PSCs applying a thin film of Pb[ZrxTi1-x]O3 (PZT) ferroelectric oxide as the ETM. The PZT was made as dense thin film and applied in planar configuration PSCs of the type: FTO/PZT/CsMAFA/ spiro-OMeTAD/Au, where CsMAFA refers to the halide perovskite material with formula Cs5(MA0.17FA0.83)(95)Pb(I0.83Br0.17)3 and PZT refers to the nominal composition Pb[Zr0.6Ti0.4]O3. For comparison purposes, reference solar cells were made applying SnO2 as dense ETM as in FTO/SnO2/CsMAFA/spiro-OMeTAD/ Au. We also explored the effect of the PZT layer on the triple cation perovskite under illumination with respect to solar cell stability. Analysis of the solar cells were made at 1-sun AM 1.5 G, including UV light, in air (no encapsulation), at 45 ºC and 55% RH. The solar cells were poled up to 2 V in order to polarize the PZT electrode. Poling resulted in a slow but steady improvement of the photovoltaic properties of the PZT-based PSC. The improvement was observed during the first 90 minutes after which the device stabilized and then maintained its photovoltaic properties under continuous illumination in air for hours. Our results demonstrate, for the first time, the possibility of applying the PZT ferroelectric oxide as ETM in air and UV-stable PSCs.

Light trapping in solar cells has been studied for decades, but study of light rejection has been much less studied. Here, we discuss the benefits of rejecting sub band gap light and the broader tradeoffs of strategies for light management in tandem solar cells. Rejection of sub band gap light can lead to reduced operating temperature, with associated benefits of higher efficiency performance. Recycling of emitted light is known to increase the photovoltage relatively more than the photocurrent. Applying these light management techniques to tandem cells is more complicated, encountering the tradeoff between “recycling” the photons within the same sub-cell versus passing them to a lower sub-cell. The talk will give an overview of implications of light management techniques (excluding light trapping) in solar cells.

Organic-inorganic metal-halide perovskites have attracted great attention in recent years due to their remarkable semiconductor properties. While great advances have been made towards understanding dynamics under steady-state conditions, the importance of non-equilibrium phenomena and their effect on device performances remains elusive. To provide experimental access to the unexplored regime of spatiotemporal dynamics occurring on ultrafast timescales, we combined the extreme temporal resolution provided by ultrafast spectroscopy with the nm-level localisation capabilities of optical microscopes. We focused our investigation on the spatial carrier dynamics of well characterised methylammonium lead iodide system (MAPI3-xClx). Intriguingly, the consecutive fs-TAM images reveal a pronounced spatial expansion of the carrier distribution within a few tens of fs. The mean-squared-displacement (MSD) profile of the non-equilibrium carriers grows non-linearly, in contrast to the linear behaviour expected for normal diffusion. Further, the MSD profile is well described by a power law fit, signifying that the non-equilibrium carriers in perovskite thin films propagate in a ballistic manner during the initial 20 fs. In addition, the linear fit reveals the ballistic transport length of 153 nm. T ballistic transport velocity of 7.5 ×106 ms-1 is in approximate agreement with the group velocity of electrons within the conduction band as modelled by density functional theory. This implies that photogenerated carriers propagate as coherent wavepackets and have a non-interacting nature at the earliest times following photon absorption. Our results suggest that at least ~25% of carriers generated in a typical perovskite PV device reach the charge collection layers ballistically

Molecular electronic materials such as conjugated polymers and small molecules have attracted intense interest for applications in solar energy conversion as well as to light emission, thin-film electronics and other fields. Their appeal lies in the potential to tune material properties (electronic, optical, mechanical and thermal) through control of chemical structure and molecular packing, whilst using facile fabrication methods. Achieving this goal has been challenging, however, due to the intrinsic disorder and structural heterogeneity of the materials and the lack of appropriate device-physics models to relate structure to physical properties. Recent developments in materials design, computational modelling and experimental characterisation have led to the demonstration of improved molecular materials systems for photovoltaic energy conversion. We will discuss the factors that control photovoltaic efficiency in molecular materials, considering the impact of chemical and physical structure on properties such as phase behaviour, electronic transport, light harvesting, and charge recombination and consider the limits to conversion efficiency in such systems. We will briefly address the application of conjugated polymers to the challenge of energy storage, as functional materials for both electrochemical devices and photocatalytic energy conversion.

In this talk, I will discuss recent progress at UCSB on the development of donor and acceptor materials for application in solution processed bulk heterojunction solar cells. Chemical structure and processing conditions can be used to tune the energy level, bandgap, solubility, molecular packing, film morphology, charge generation, charge mobility, charge recombination, and therefore, the device performance. A combination of techniques is employed to characterize material properties especially film morphology including steady-state and time-resolved spectroscopy, atomic force microscopy (AFM), photoconductive AFM, TEM, GIWAXS, and impedance spectroscopy. The results from these studies provide design guidelines for new generation of materials for applications in organic solar cells.

High energy long life rechargeable battery is considered as key enabling technology for deep de-carbonization. Energy storage in the electrochemical form is attractive because of its high efficiency and fast response time. Besides the technological importance, electrochemical devices also provide a unique platform for fundamental and applied materials research since ion movement is often accompanied by inherent complex phenomena related to phase changes, electronic structure changes. In this talk, I will discuss a few new perspectives for energy storage materials including new gas electrolytes, new Li/Na intercalation compounds and new battery architectures. I hope to demonstrate how to combine knowledge-guided synthesis/characterization and computational modeling to develop and optimize new higher energy/power density electrode and electrolytes materials for rechargeable batteries from picowatt-hour to megawatt-hour. With recent advances in characterization tools and computational methods, we are able to explore ionic mobility, charge transfer and phase transformations in electrode materials in operando, and map out the structure-properties relations in functional materials for energy storage and conversion. Scanning electron microscopy and electron energy loss spectroscopy (STEM/EELS) offers unprecedented spatial resolution, which has enabled nanoscale imaging and chemical analysis of battery materials - their surfaces, grain boundaries and phase boundaries. Combining the state-of-the-art in situ operando analytical electron microscopy with first principles (FP) computational data analysis, we reveal some insights that could not be possible to see in the past. On the other hand, coherent x-ray diffraction imaging (CXDI), a lensless form of microscopy capable of discerning electron density and strain with 10 nm resolution, can be used to map the strain evolution of a single cathode particle in a functional battery as it is cycled in-situ. By combining electron based and X-ray based novel imaging techniques, I hope to showcase the diagnostic tools developed for probing and understanding energy storage materials in operando. Last but not least, I will give an update on the Cryogenic Electron Microscopy techniques which enable to visualization of Li and Na metals and their interfaces with electrolytes at atomistic-scale.

Impedance Spectroscopy (IS) is a non-destructive characterization technique that has been extensively applied to different electronic devices, such as LEDs, photodiodes and solar cells. This technique provides access to valuable information about dynamical mechanisms (minority carrier recombination, diffusion, etc.) taking place in the different layers of the device. Besides, material and device parameters, such as dielectric constant, built-in potential, and carrier mobilities can be extracted. Impedance spectra results from applying a small AC signal over a steady DC bias and measuring the resulting small AC current over a frequency range, typically from 1 Hz to 1 MHz, 𝑍(jω) = V_AC/1_AC. The Nyquist plot of the complex impedance (imaginary part vs real part) generally presents one or more features (mainly semicircles), depending on the number of mechanisms governing the device. Fitting an electrical equivalent circuit to the complete impedance spectra provides parameters related to each feature (resistance and capacitance). In this work, IS has been used to characterize organic and perovskite solar cells (OSCs and PSCs, respectively). Measurements have been performed in dark and under illumination conditions at different bias (from 0 V to V_OC). A simple circuital model containing two resistances and two capacitances has been used to fit the measured IS spectra. The interpretation of extracted circuital parameters and its relationship with the physical model of the device will be discussed.