Conspectus The optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in ...perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br0.5I0.5)3) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br0.5I0.5)3 films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br0.5I0.5)3 is considered to be an important step in inducing halide mobility in photoirradiated films. This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.
The demand for clean energy will require the design of nanostructure-based light-harvesting assemblies for the conversion of solar energy into chemical energy (solar fuels) and electrical energy ...(solar cells). Semiconductor nanocrystals serve as the building blocks for designing next generation solar cells, and metal chalcogenides (e.g., CdS, CdSe, PbS, and PbSe) are particularly useful for harnessing size-dependent optical and electronic properties in these nanostructures. This Account focuses on photoinduced electron transfer processes in quantum dot sensitized solar cells (QDSCs) and discusses strategies to overcome the limitations of various interfacial electron transfer processes. The heterojunction of two semiconductor nanocrystals with matched band energies (e.g., TiO2 and CdSe) facilitates charge separation. The rate at which these separated charge carriers are driven toward opposing electrodes is a major factor that dictates the overall photocurrent generation efficiency. The hole transfer at the semiconductor remains a major bottleneck in QDSCs. For example, the rate constant for hole transfer is 2–3 orders of magnitude lower than the electron injection from excited CdSe into oxide (e.g., TiO2) semiconductor. Disparity between the electron and hole scavenging rate leads to further accumulation of holes within the CdSe QD and increases the rate of electron–hole recombination. To overcome the losses due to charge recombination processes at the interface, researchers need to accelerate electron and hole transport. The power conversion efficiency for liquid junction and solid state quantum dot solar cells, which is in the range of 5–6%, represents a significant advance toward effective utilization of nanomaterials for solar cells. The design of new semiconductor architectures could address many of the issues related to modulation of various charge transfer steps. With the resolution of those problems, the efficiencies of QDSCs could approach those of dye sensitized solar cells (DSSC) and organic photovoltaics.
The recent surge in the utilization of semiconductor nanostructures for solar energy conversion has led to the development of high-efficiency solar cells. Some of these recent advances are in the ...areas of synthesis of new semiconductor materials and the ability to tune the electronic properties through size, shape, and composition and to assemble quantum dots as hybrid assemblies. In addition, processes such as hot electron injection, multiple exciton generation (MEG), plasmonic effects, and energy-transfer-coupled electron transfer are gaining momentum to overcome the efficiency limitations of energy capture and conversion. The recent advances as well as future prospects of quantum dot solar cells discussed in this perspective provide the basis for consideration as “The Next Big Thing” in photovoltaics.
Graphene-based assemblies are gaining attention as a viable alternate to boost the efficiency of various catalytic and storage reactions in energy conversion applications. The use of reduced graphene ...oxide has already proved useful in collecting and transporting charge in photoelectrochemical solar cells, photocatalysis, and electrocatalysis. In many of these applications, the flat carbon serves as a scaffold to anchor metal and semiconductor nanoparticles and assists in promoting selectivity and efficiency of the catalytic process. Covalent and noncovalent interaction with organic molecules is another area that is expected to provide new frontiers in graphene research. Recent advances in manipulating graphene-based two-dimensional carbon architecture for energy conversion are described.
The trail of semiconductor surface photochemistry during the past four decades has led to the emergence of new areas in chemistry (e.g., photocatalysis, solar cells, solar fuels). How can one now ...exploit the richness of surface chemistry of hybrid architectures and make a transformative leap in light energy conversion and other applications?
The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light ...energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices: (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utilization of such tailored nanostructures or ordered hybrid assemblies will play an important task in achieving the desired goal of cheap and efficient fuel production (e.g., solar hydrogen production) or electricity (photochemical solar cells).
The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent ...developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future research efforts toward utilization of tailored nanostructures will be an important challenge for the development of next generation solar cells.
The Perspective focuses on photoinduced electron transfer between semiconductor–metal and semiconductor–semiconductor nanostructures and factors that influence the rate of electron transfer at the ...interface. The storage and discharge properties of metal nanoparticles play an important role in dictating the photocatalytic performance of semiconductor–metal composite assemblies. Both electron and hole transfer across the interface with comparable rates are important in maintaining high photocatalytic efficiency and stability of the semiconductor assemblies. Coupled semiconductors of well-matched band energies are convenient to improve charge separation. Furthermore, semiconductor and metal nanoparticles assembled on reduced graphene oxide sheets offer new ways to design multifunctional catalyst mat. The fundamental understanding of charge-transfer processes is important in the future design of light-harvesting assemblies.
Graphene based two-dimensional carbon nanostructures serve as a support to disperse catalyst nanoparticles. Reduced graphene oxide is used as a support to anchor semiconductor and metal ...nanoparticles. Such a design strategy would enable the development of a multifunctional catalyst mat. This Perspective focuses on the interaction between graphene oxide−semiconductor (TiO2, ZnO) and graphene oxide−metal (Au, Pt) nanoparticles and discusses potential applications in catalysis, light energy conversion, and fuel cells.
A new chapter in the long and distinguished history of perovskites is being written with the breakthrough success of metal halide perovskites (MHPs) as solution-processed photovoltaic (PV) absorbers. ...The current surge in MHP research has largely arisen out of their rapid progress in PV devices; however, these materials are potentially suitable for a diverse array of optoelectronic applications. Like oxide perovskites, MHPs have ABX3 stoichiometry, where A and B are cations and X is a halide anion. Here, the underlying physical and photophysical properties of inorganic (A = inorganic) and hybrid organic–inorganic (A = organic) MHPs are reviewed with an eye toward their potential application in emerging optoelectronic technologies. Significant attention is given to the prototypical compound methylammonium lead iodide (CH3NH3PbI3) due to the preponderance of experimental and theoretical studies surrounding this material. We also discuss other salient MHP systems, including 2-dimensional compounds, where relevant. More specifically, this review is a critical account of the interrelation between MHP electronic structure, absorption, emission, carrier dynamics and transport, and other relevant photophysical processes that have propelled these materials to the forefront of modern optoelectronics research.
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