Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D46: Invited Session: Organo-Metallic Perovskites for Photovaltaic Energy Conversion |
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Sponsoring Units: DMP Chair: Andre Schleife, University of Illinois-Urbana Room: 217A |
Monday, March 2, 2015 2:30PM - 3:06PM |
D46.00001: Achieving High Performance Perovskite Solar Cells Invited Speaker: Yang Yang Recently, metal halide perovskite based solar cell with the characteristics of rather low raw materials cost, great potential for simple process and scalable production, and extreme high power conversion efficiency (PCE), have been highlighted as one of the most competitive technologies for next generation thin film photovoltaic (PV). In UCLA, we have realized an efficient pathway to achieve high performance pervoskite solar cells, where the findings are beneficial to this unique materials/devices system. Our recent progress lies in perovskite film formation, defect passivation, transport materials design, interface engineering with respect to high performance solar cell, as well as the exploration of its applications beyond photovoltaics. These achievements include: 1) development of vapor assisted solution process (VASP) and moisture assisted solution process, which produces perovskite film with improved conformity, high crystallinity, reduced recombination rate, and the resulting high performance; 2) examination of the defects property of perovskite materials, and demonstration of a self-induced passivation approach to reduce carrier recombination; 3) interface engineering based on design of the carrier transport materials and the electrodes, in combination with high quality perovskite film, which delivers 15 $\sim$ 20{\%} PCEs; 4) a novel integration of bulk heterojunction to perovskite solar cell to achieve better light harvest; 5) fabrication of inverted solar cell device with high efficiency and flexibility and 6) exploration the application of perovskite materials to photodetector. Further development in film, device architecture, and interfaces will lead to continuous improved perovskite solar cells and other organic-inorganic hybrid optoelectronics. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D46.00002: Modeling organohalide perovskites for photovoltaic applications: From materials to interfaces Invited Speaker: Filippo De Angelis The field of hybrid/organic photovoltaics has been revolutionized in 2012 by the first reports of solid-state solar cells based on organohalide perovskites, now topping at 20{\%} efficiency. First-principles modeling has been widely applied to the dye-sensitized solar cells field, and more recently to perovskite-based solar cells. The computational design and screening of new materials has played a major role in advancing the DSCs field. Suitable modeling strategies may also offer a view of the crucial heterointerfaces ruling the device operational mechanism. I will illustrate how simulation tools can be employed in the emerging field of perovskite solar cells. The performance of the proposed simulation toolbox along with the fundamental modeling strategies are presented using selected examples of relevant materials and interfaces. The main issue with hybrid perovskite modeling is to be able to accurately describe their structural, electronic and optical features. These materials show a degree of short range disorder, due to the presence of mobile organic cations embedded within the inorganic matrix, requiring to average their properties over a molecular dynamics trajectory. Due to the presence of heavy atoms (e.g. Sn and Pb) their electronic structure must take into account spin-orbit coupling (SOC) in an effective way, possibly including GW corrections. The proposed SOC-GW method constitutes the basis for tuning the materials electronic and optical properties, rationalizing experimental trends. Modeling charge generation in perovskite-sensitized TiO$_{2}$ interfaces is then approached based on a SOC-DFT scheme, describing alignment of energy levels in a qualitatively correct fashion. The role of interfacial chemistry on the device performance is finally discussed.\\[4pt] [1] P. Umari et al. Sci. Rep 2014, 4, 4467.\\[0pt] [2] A. Amat et al. Nano Lett. .2014, 14, 3608.\\[0pt] [3] V. Roiati et al. Nano Lett. 2014, 14, 2168.\\[0pt] [4] C. Quarti et al. Chem. Mater. 2014, DOI: 10.1021/cm5032046. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 4:18PM |
D46.00003: Interface Energetics in Organo-Metallic Halide Perovskite-based Photovoltaic Cells Invited Speaker: Philip Schulz In my presentation I will talk about the most recent findings on the electronic structure of methylammonium lead tri-halide (MAPbX$_{\mathrm{3}}$, X$=$I, Br) perovskite films and their interfaces to adjacent transport layers. Intricate knowledge of the electronic alignment at the contact interfaces in perovskite solar cells is essential for the understanding of the working principle as well as improving design and thus performance of respective devices. In our studies we employ ultra-violet, X-ray and inverse photoemission spectroscopy (UPS, XPS, IPES) to directly determine valence and conduction band offsets. In this way we are able to report a direct measurement of the electronic band gap as well as ionization energy and electron affinity found for perovskite surfaces. Furthermore, our findings indicate that the electronic energy level alignment of adjacent organic hole transport layers, such as spiro-MeOTAD, can limit the maximum attainable open circuit voltage (V$_{\mathrm{oc}})$ in solar cells if the highest occupied molecular orbital of the hole transport material is not well aligned to the valence band maximum of the perovskite layer. Using better suited hole transporters, like CBP, values for V$_{\mathrm{oc}}$ larger than 1.5 V could be achieved in the case of MAPbBr$_{\mathrm{3}}$ based devices. More recently, inverted perovskite solar cells based on nickel oxide bottom anodes have been reported to yield viable power conversion efficiencies and stability. We find that the interface between the p-doped NiO surface and the MAPbI$_{\mathrm{3}}$ layer on top lead to p-type perovskite filsm while the same material deposited on TiO$_{\mathrm{2}}$ in the conventional cell geometry turns out to be n-type. A further investigation of a C$_{\mathrm{60}}$ layer deposited on top of p-type perovskite films reveals an ideal alignment between the lowest unoccupied molecular orbital of the organic electron transport materials and the conduction band minimum of the perovskite film underneath. These results explain why the inverted solar cell structure could achieve similar successes as the conventional structure and highlight the versatility of perovskite sub-cells in potential tandem cell architectures. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:54PM |
D46.00004: Impact of Atomic Structure on Absolute Energy Levels of Methylammonium Lead Iodide Perovskite Invited Speaker: Joshua Choi There has been a staggeringly rapid increase in the photovoltaic performance of methylammonium lead iodide (MAPbI$_{\mathrm{3}})$ perovskite - greater than 19 percent solar cell power conversion efficiency has been reported in less than five years since the first report in 2009. Despite the progress in device performance, structure-property relationships in MAPbI$_{\mathrm{3}}$ are still poorly understood. I will present our recent findings on the impact of changing the Pb-I bond length and Pb-I-Pb bond angle on the electronic structure of MAPbI$_{\mathrm{3}}$. By using the combination of temperature dependent X-ray scattering, ultraviolet photoelectron spectroscopy, absorbance and PL spectroscopy, we show that the energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) shift in the same direction as MAPbI$_{\mathrm{3}}$ goes through tetragonal-to-cubic structural phase transition wherein the rotational angle of PbI$_{\mathrm{6}}$ octahedra is the order parameter of the transition. Our experimental results are corroborated by density functional theory calculations which show that the lattice expansion and bond angle distortion cause different degree of orbital overlap between the Pb and I atoms and the anti-bonding orbital nature of both HOMO and LUMO results in the same direction of their shift. Moreover, through pair distribution function analysis of X-ray scattering, we discovered that the majority of MAPbI$_{\mathrm{3}}$ in thin film solar cell layer has highly disordered structure with a coherence range of only 1.4 nm. The nanostructuring correlates with a blueshift of the absorption onset and increases the photoluminescence. Our results underscore the importance of understanding the structure-property relationships in order to improve the device performance of metal-organic perovskites. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:30PM |
D46.00005: Electronic structure of hybrid halide perovskite photovoltaic absorbers Invited Speaker: Mark van Schilfgaarde The performance of organometallic perovskite solar cells has rapidly surpassed those of both traditional dye-sensitized and organic photovoltaics, e.g. solar cells based on CH$_{3}$NH$_{3}$PbI$_{3}$ have recently reached 18\% conversion efficiency. We analyze its electronic structure and optical properties within the quasiparticle self-consistent $GW$ approximation (QS${GW}$). Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density approximation (LDA) or ${GW}$ based on the LDA. Several characteristics combine to make the electronic structure of this material unusual. First, there is a strong driving force for ferroelectricity, as a consequence the polar organic moiety CH$_{3}$NH$_{3}$. The moiety is only weakly coupled to the PbI$_{3}$ cage; thus it can rotate give rise to ferroelectric domains. This in turn will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and may contribute to the current-voltage hysteresis found in perovskite solar cells. Second, spin orbit modifies both valence band and conduction band dispersions in a very unusual manner: both get split at the R point into two extrema nearby. This can be interpreted in terms of a large Dresselhaus term, which vanishes at R but for small excursions about R varies linearly in $k$. Conduction bands (Pb 6${p}$ character) and valence bands (I 5${p}$) are affected differently; moreover the splittings vary with the orientation of the moiety. We will show how the splittings, and their dependence on the orientation of the moiety through the ferroelectric effect, have important consequences for both electronic transport and the optical properties of this material. [Preview Abstract] |
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