Eran Edri

Why Lead Methylammonium Tri-Iodide Perovskite-Based Solar Cells Require a Mesoporous Electron Transporting Scaffold (but Not Necessarily a Hole Conductor)

CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC) and compared to CH3NH3PbI(3-x)Clx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI(3-Clx ones, where LD values are comparable for both charge types, do not.

High Open-Circuit Voltage Solar Cells Based on Organic-Inorganic Lead Bromide Perovskite.

Mesoscopic solar cells, based on solution-processed organic-inorganic perovskite absorbers, are a promising avenue for converting solar to electrical energy. We used solution-processed organic-inorganic lead halide perovskite absorbers, in conjunction with organic hole conductors, to form high voltage solar cells. There is a dire need for low-cost cells of this type, to drive electrochemical reactions or as the high photon energy cell in a system with spectral splitting. These perovskite materials, although spin-coated from solution, form highly crystalline materials. Their simple synthesis, along with high chemical versatility, allows tuning their electronic and optical properties. By judicious selection of the perovskite lead halide-based absorber, matching organic hole conductor, and contacts, a cell with a ∼ 1.3 V open circuit voltage was made. While further study is needed, this achievement provides a general guideline for additional improvement of cell performance.

Chloride Inclusion and Hole Transport Material Doping to Improve Methyl Ammonium Lead Bromide Perovskite-Based High Open-Circuit Voltage Solar Cells

Low-cost solar cells with high VOC, relatively small (EG - qVOC), and high qVOC/EG ratio, where EG is the absorber band gap, are long sought after, especially for use in tandem cells or other systems with spectral splitting. We report a significant improvement in CH3NH3PbBr3-based cells, using CH3NH3PbBr3-xClx, with EG = 2.3 eV, as the absorber in a mesoporous p-i-n device configuration. By p-doping an organic hole transport material with a deep HOMO level and wide band gap to reduce recombination, the cells VOC increased to 1.5 V, a 0.2 V increase from our earlier results with the pristine Br analogue with an identical band gap. At the same time, in the most efficient devices, the current density increased from ∼1 to ∼4 mA/cm(2).

Crystallization of Methyl Ammonium Lead Halide Perovskites: Implications for Photovoltaic Applications

Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH3NH3PbI3 and CH3NH3PbI3-xClx where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diffusion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH3NH3PbI3 and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable effect on the end product, because PbCl2 nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and Voc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.

Interface energetics in organo-metal halide perovskite-based photovoltaic cells

Direct and inverse photoemission spectroscopies are used to determine materials electronic structure and energy level alignment in hybrid organic–inorganic perovskite layers grown on TiO2. The results provide a quantitative basis for the analysis of perovskite-based solar cell performance and choice of an optimal hole-extraction layer.

Rain on Methylammonium Lead Iodide Based Perovskites: Possible Environmental Effects of Perovskite Solar Cells

The great promise of hybrid organic-inorganic lead halide perovskite (HOIP)-based solar cells is being challenged by its Pb content and its sensitivity to water. Here, the impact of rain on methylammonium lead iodide perovskite films was investigated by exposing such films to water of varying pH values, simulating exposure of the films to rain. The amount of Pb loss was determined using both gravimetric and inductively coupled plasma mass spectrometry measurements. Using our results, the extent of Pb loss to the environment, in the case of catastrophic module failure, was evaluated. Although very dependent on module siting, even total destruction of a large solar electrical power generating plant, based on HOIPs, while obviously highly undesirable, is estimated to be far from catastrophic for the environment.

Perovskite Solar Cells: Do We Know What We Do Not Know?

D 1−4, 2014 saw one of the largest gatherings of scientists and engineers on the topic of hybrid organic− inorganic perovskite solar cells, which dominated symposium W of the fall meeting of the Materials Research Society (MRS) in Boston on Perovskite-Based and Related Novel Material Solar Cells. The symposium included many contributions as well as invited talks, open discussion sessions (to highlight major themes and stimulate discussion of unpublished data), and a “rump session”. The latter was intended (and indeed mostly used) for last minute news in the field. Over the 4 days, well over 120 papers were presented, including some 50 posters, 3 of which were awarded the symposium’s poster prizes. Furthermore, on the first day of the symposium, Henry Snaith (University of Oxford) received the 2014 Outstanding Young Investigator award of the MRS and gave a meeting-wide lecture at Symposium X. In this Guest Commentary, we summarize the topics that, in our opinion, were the more salient ones of symposium W, with a focus on recent progress and future challenges in understanding the unique properties of hybrid perovskite solar cells. The salient issues that were discussed included hysteresis, which appears to be, at least in part, an interfacial phenomenon, primarily relevant for interfaces with oxides; ion migration, which remains a hypothesis because although it is supported by a variety of experimental results, it still lacks fully conclusive experimental evidence; materials preparation, where solvent annealing of the product of a 2-step process seems to be beneficial for cell efficiency, as is the presence of water; and electronic traps, where evidence for the presence of traps just below mid gap was presented, which is somewhat inconsistent with the high Voc of cells and may present a case of photoinduced effects. Furthermore, a variety of stability tests give promising short-term results, but the reported very low formation energy of hybrid perovskite materials (from the “binaries”) could be detrimental for long-term stability. For mixed I−Br perovskites that are relevant as “ideal bandgap” materials for spectral splitting and tandem cells, phase separation was reported, which, at least for the moment, cannot be explained well by thermodynamics or kinetics. An overall impression is that likely not everyone is, as yet, working with the same material as some effects of exposure to ambient could be explained by doping of the perovskite or holeconducting material. Other results remain puzzling, as even N2 was seen to change the material’s properties. Finally, not all ef f iciency measurements (and their results) appear to be comparable with each other. Subsequent to the first public report of hysteresis in the current−voltage curves of hybrid-perovskite-based solar cells by Hoke et al. at the MRS 2013 Fall meeting and its discussion in the 2013 “rump session” (see Figure 1), the phenomenon was reported in the literature and was the focus of a number of talks at this year’s symposium. Ideas that were brought up to explain it were analyzed in more detail in the discussion sessions of the symposium. Part of the interest stems from the need to arrive at a well-documented, easily reproducible measurement of cell efficiencies, part from the hope that revealing the origin of such “hysteretic behavior” may help to understand the mechanism behind the photovoltaic properties of hybrid perovskites. Furthermore, the hysteresis might be connected to the long-term stability of the devices. In several talks, layers of organic molecules (discussed in talk W4.06) and device geometry (see, e.g., talk W2.05) were reported to play an important, if not essential, role in the magnitude of the hysteresis effects. These findings suggest that the hysteresis is at least partly an interface phenomenon and highlight the importance of (inter-) sur-face passivation. On the other hand, several talks throughout the symposium discussed the complementary possibility that reorientation of the polar organic molecules, contained inside the inorganic cage, due to an external electric field could occur. One hypothesis is that such concerted dipole alignment results in ferroelectric domains, which could then be responsible for the observed hysteresis in the current−voltage curves (see talk W1.01). Hysteresis was shown to be independent of light intensity, implying that it is not arising entirely from stored chargesone of the other possible explanations (talk W1.06). While not stated explicitly, we got the impression that without TiO2 or other electron-conducting hole-blocking oxide layers, there is no or very little hysteresis, suggesting that interface phenomena play a dominant role. Although there is as of now no fully comprehensive explanation for the hysteretic behavior of hybrid perovskites, it was generally agreed in a number of discussion sessions that at the moment, for generating reliable physical insight in terms of I−V curves and solar cell efficiencies, it is highly important to report a number of other experimental parameters. These include a steady-state maximum power point, the scan rate, the scan direction, and the preconditioning of the device prior to measuring (e.g., was it electrically biased, illuminated, or measured immediately upon completion of cell fabrication).

Surface Photovoltage Spectroscopy Study of Organo-Lead Perovskite Solar Cells.

The field of organo-lead perovskite absorbers for solar cells is developing rapidly, with open-circuit voltage of reported devices already approaching the maximal theoretical voltage. Obtaining such high voltages on spun-cast or evaporated thin films is intriguing and calls for detailed investigation of the source of photovoltage in those devices. We present here a study of the roles of the selective contacts to methylammonium lead iodide chloride (MAPbI3-xClx) using surface photovoltage spectroscopy. By depositing and characterizing each layer at a time, we show that the electron-extracting interface is more than twice as effective as the hole-extracting interface in generating photovoltage, for several combinations of electrode materials. We further observe the existence of an electron-injection related spectral feature at 1.1 eV, which might bear significance for the cells operation. Our results illustrate the usefulness of SPV spectroscopy in highlighting gaps in cells efficiency and for deepening the understanding of charge injection processes in perovskite-based photovoltaics.

Hierarchical Inorganic Assemblies for Artificial Photosynthesis

Artificial photosynthesis is an attractive approach for renewable fuel generation because it offers the prospect of a technology suitable for deployment on highly abundant, non-arable land. Recent leaps forward in the development of efficient and durable light absorbers and catalysts for oxygen evolution and the growing attention to catalysts for carbon dioxide activation brings into focus the tasks of hierarchically integrating the components into assemblies for closing of the photosynthetic cycle. A particular challenge is the efficient coupling of the multi-electron processes of CO2 reduction and H2O oxidation. Among the most important requirements for a complete integrated system are catalytic rates that match the solar flux, efficient charge transport between the various components, and scalability of the photosynthetic assembly on the unprecedented scale of terawatts in order to have impact on fuel consumption. To address these challenges, we have developed a heterogeneous inorganic materials approach with molecularly precise control of light absorption and charge transport pathways. Oxo-bridged heterobinuclear units with metal-to-metal charge-transfer transitions absorbing deep in the visible act as single photon, single charge transfer pumps for driving multi-electron catalysts. A photodeposition method has been introduced for the spatially directed assembly of nanoparticle catalysts for selective coupling to the donor or acceptor metal of the light absorber. For CO2 reduction, a Cu oxide cluster is coupled to the Zr center of a ZrOCo light absorber, while coupling of an Ir nanoparticle catalyst for water oxidation to the Co donor affords closing of the photosynthetic cycle of CO2 conversion by H2O to CO and O2. Optical, vibrational, and X-ray spectroscopy provide detailed structural knowledge of the polynuclear assemblies. Time resolved visible and rapid-scan FT-IR studies reveal charge transfer mechanisms and transient surface intermediates under photocatalytic conditions for guiding performance improvements. Separation of the water oxidation and carbon dioxide reduction half reactions by a membrane is essential for efficient photoreduction of CO2 by H2O to liquid fuel products. A concept of a macroscale artificial photosystem consisting of arrays of Co oxide-silica core-shell nanotubes is introduced in which each tube operates as a complete, independent photosynthetic unit with built-in membrane separation. The ultrathin amorphous silica shell with embedded molecular wires functions as a proton conducting, molecule impermeable membrane. Photoelectrochemical and transient optical measurements confirm tight control of charge transport through the membrane by the orbital energetics of the wire molecules. Hierarchical arrangement of the components is accomplished by a combination of photodeposition, controlled anchoring, and atomic layer deposition methods.

Ultrafast Charge Transfer between Light Absorber and Co3O4 Water Oxidation Catalyst across Molecular Wires Embedded in Silica Membrane

The mechanism of visible light-induced hole transfer from a molecular light absorber, in the form of a free-base porphyrin, coupled to a Co3O4 nanoparticle catalyst for water oxidation by a molecular wire (p-oligo(phenylenevinylene) featuring three aryl units) is investigated by transient absorption spectroscopy. The wires are covalently anchored on the Co3O4 surface and embedded in a dense, yet ultrathin (2 nm), silica layer that separates light absorber and catalyst. The porphyrin is electrostatically adsorbed on the silica surface, and aqueous colloidal solutions of the core-shell particles are used for transient optical measurements. Pulsed optical excitation of the porphyrin results in rapid injection of the photogenerated hole onto the molecular wire and concurrent formation of reduced light absorber in less than 1 picosecond (ps). Ultrafast charge separation was monitored by transient absorption of the wire radical cation, which is given by bands in the 500 to 600 nm region and at 1130 nm, while formation of reduced porphyrin was characterized by absorption at 700 nm. Forward transfer of the hole to Co3O4 catalyst proceeds in 255 ± 23 ps. Ultrafast transfer of positive charge from the molecular assembly to a metal oxide nanoparticle catalyst for water oxidation is unprecedented. Holes on Co3O4 recombined with electrons of the reduced sensitizer with biphasic kinetics on a much longer time scale of ten to several hundred nanoseconds. The unusually efficient hole transfer coupling of a molecular light absorber with an Earth-abundant metal oxide catalyst by silica-embedded p-oligo(phenylenevinylene) offers an approach for integrated artificial photosystems featuring product separation on the nanoscale.