Gary Hodes

Perovskite-Based Solar Cells

Organic-inorganic hybrid semiconductors may provide the basis for the next generation of thin-film solar cells. [Also see Reports by Stranks et al. and Xing et al.] Photovoltaic (PV) cells that convert sunlight directly into electricity are becoming increasingly important in the worlds renewable energy mix. The cumulative world PV installations reached around 100 GWp (gigawatts) (1) by the end of 2012. Some 85% use crystalline Si, with the rest being polycrystalline thin film cells, mostly cadmium telluride/cadmium sulfide ones. Thin-film cells tend to be cheaper to make with a shorter energy payback time. However, they do have the disadvantage, one that may become crucial when considering the terawatt range, that most of them contain rare elements like tellurium (as rare as gold), indium, and gallium. A newcomer to the PV field (2) has rapidly reached conversion efficiencies of more than 15% (see the figure). Based on organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH3NH3PbI3), these perovskites tend to have high charge-carrier mobilities (3, 4). High mobility is important because, together with high charge carrier lifetimes, it means that the light-generated electrons and holes can move large enough distances to be extracted as current, instead of losing their energy as heat within the cell. On pages 344 and 341 of this issue, Xing et al. (5) and Stranks et al. (6) use time-resolved transient absorption and photoluminescence to show that the effective diffusion lengths are indeed relatively large in CH3NH3PbI3, about 100 nm for both electrons and holes—a high value for a semiconductor formed from solution at low temperature.

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.

Photoelectrochemical energy conversion and storage using polycrystalline chalcogenide electrodes

WE report here on a major improvement in the conversion efficiency of corrosion-free photoelectrochemical cells (PECs) and on a novel extension of such cells which allows the storage of part or all of the converted energy in situ for subsequent use.

Electrocatalytic Electrodes for the Polysulfide Redox System

Porous electrocatalytic electrodes for the polysulfide redox system, containing one of various metallic sulfides (especially of Co, Cu, Pb) are described. Emphasis is placed on their use as counterelectrodes in photoelectrochemical cells employing polysulfide electrolytes. Their activity is measured as a function of electrolyte temperature and composition. The ratio of S to S2−, and through it the local redox potential of the solution, is shown to be an important factor controlling electrode activity. The short and long term stability of the electrodes, as cathodes, is discussed, and it is shown that, when used in conjunction with photoanodes, and may poison the photoelectrode surface, thereby reducing total cell efficiency.

Nanocrystalline Photoelectrochemical Cells A New Concept In Photovoltaic Cells

Semiconductor films with small crystal size normally exhibit prohibitively large recombination losses in photovoltaic cells. In this paper, the authors show that porous nanocrystalline films of CdS and CdSe can be used as photoelectrodes in photoelectrochemical cells with relatively low recombination losses. Spectral response measurements show how the recombination losses depend on film thickness. These photoelectrochemical cells operate due to charge separation at the semiconductor-electrolyte interface rather than by a built-in space charge layer as normally occurs in photovoltaic or photoelectrochemical cells. The rapid removal of one charge by the electrolyte explains the low recombination loss.

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).

Stability of CdTe/CdS thin-film solar cells

Abstract The recent literature regarding the stability of CdTe/CdS photovoltaic cells (as distinguished from modules ) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu 2 Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu + , from either Cu 2 Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed.

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.