Large-scale Solar Energy Conversion Research Articles

Reinforcing built-in electric field to enable efficient carrier extraction for high-performance perovskite solar cells

This review presents an overview of the carrier separation and extraction mechanism influenced by the built-in electric field (BEF) in PSCs and various strategies used to enhance the BEF.

Z-scheme ZnO@PDA/CeO2 heterojunctions using polydopamine as electron transfer layer for enhanced photoelectrochemical H2 generation

In this study, polydopamine (PDA)-bridged ZnO@PDA/CeO2 Z-scheme heterojunctions are rationally designed using PDA as an electron transfer mediator for an efficient photoelectrochemical water splitting under visible LED illumination. PDA-bridged construction not only provided the completely wrapping of ZnO nanoparticles (NPs) but also provided a biomimetic electron transfer layer with well-defined core-shell morphologies. Thanks to this novel Z-scheme heterostructure, an improved photocurrent density is recorded for ZnO@PDA/CeO2 photoanodes (251 μA/cm2 at 0.25 V vs. reversible hydrogen electrode [RHE]) under LED irradiation (30 mW/cm2), whereas a quite low photocurrent density (24 μA/cm2 at 0.25 V vs. RHE) is obtained in dark due to low separation of electron–hole pairs. Our results suggest that the presence of PDA provided a solid–solid interfacial interaction (between semiconductors: ZnO and CeO2) that facilitated the separation and pumping of photogenerated charge carries for enhanced photoelectrochemical water splitting. The Z-scheme charge transfer mechanism is verified using radical scavengers, radical trapping experiments, as well as X-ray photoelectron spectroscopy methods. This three-dimensional (3D) Z-scheme ternary heterostructures delivers a new insight in next generation of photocatalysts for efficient large-scale conversion of solar energy to H2 fuels as well as renewable energy revolution.

Confining Mo-activated CoSx active sites within MCM-41 for highly efficient dye-sensitized photocatalytic H2 evolution

Although transition-metal-based sulfides have been identified as efficient catalysts to replace expensive noble metal catalysts for photocatalytic H2 evolution reaction (HER), their activities are still unsatisfied and could be further improved by controlling their microstructures and electronic structures. Herein, we present an effective strategy to confine highly active Mo-activated CoSx (Mo-CoSx) active sites within MCM-41 frameworks by sulfurization of Co-doped MCM-41 during the in situ photoreduction of [MoS4]2- in Erythrosin B-triethanolamine (ErB-TEOA) system. It is found that Co-MCM-41 offers not only abundant coordinatively unsaturated Co sites to be activated by Mo and S but also large surface area to effectively disperse the in situ generated amorphous Mo-CoSx active sites. Under 520 nm irradiation, the most efficient Mo-CoSx/MCM-41-100 (Si/Co = 100) catalyst exhibits ~7, 3, and 4 times higher H2 evolution activity than free MoSx, free Mo-CoSx, and CoSx/MCM-41-100, respectively, and an apparent quantum yield (AQY) of 12.3% for H2 evolution. Furthermore, when Mo-CoSx/MCM-41-100 was sensitized with a more stable fluorescein (FL) dye, the photocatalytic system shows a sustainable H2 evolution activity in a 20 h reaction, showing the good stability of Mo-CoSx/MCM-41-100 catalyst. This work provides a new insight into the design and development of highly active hybrid H2 evolution catalysts based on transition metals for highly efficient and large-scale solar energy conversion to clean H2 energy.

Full SnO2 double-layer dye-sensitized solar cells: Slowly increasing phenomenon of power conversion efficiency

Dye-sensitized solar cells (DSCs) have been proven as effective photovoltaic devices for low-cost and large-scale solar energy conversion applications. Fast electron transport, large specific surface area, and slow interfacial electron recombination are indispensable features for an efficient photoelectrode. In this work, we report full SnO2 double-layer DSCs. SnO2 nanoparticles as electron transport layer and large SnO2 particles as scattering layer (SL) are prepared by the hydrothermal method, respectively. Ultrathin Al2O3 barrier layer on the surface of SnO2 film is formed by immersing in Al (NO3)3 solutions and annealed at atmosphere in turn to retard back transfer of electrons to the electrolyte or to the oxidized dye molecules. Four kind cells with SnO2, SnO2/SL, SnO2/Al2O3 and SnO2/SL/Al2O3 film as photoanodes were fabricated, respectively, and their power conversion efficiency (PCE) was continuously tested for 60 days. The open-circuit voltage (Voc), fill factor (FF) and PCE of four kind cells are gradually improved, while short-circuit current (Jsc) is slightly reduced during testing. As a result, the PCE of the SnO2 based DSC increased from 1.36% (0 day) to 3.5% (20 day), and the PCE of the SnO2/SL/Al2O3 based DSC increased from 2.53% (0 day) to 4.57% (20 day). This novel slow increasing phenomenon of PCE observed in SnO2 DSCs is different from that of TiO2 and ZnO. This kind of structure of DSCs and the slowly increasing phenomenon have far-reaching significance for understanding the working mechanism and increasing efficiency.

Gallium nitride nanowire as a linker of molybdenum sulfides and silicon for photoelectrocatalytic water splitting

The combination of earth-abundant catalysts and semiconductors, for example, molybdenum sulfides and planar silicon, presents a promising avenue for the large-scale conversion of solar energy to hydrogen. The inferior interface between molybdenum sulfides and planar silicon, however, severely suppresses charge carrier extraction, thus limiting the performance. Here, we demonstrate that defect-free gallium nitride nanowire is ideally used as a linker of planar silicon and molybdenum sulfides to produce a high-quality shell-core heterostructure. Theoretical calculations revealed that the unique electronic interaction and the excellent geometric-matching structure between gallium nitride and molybdenum sulfides enabled an ideal electron-migration channel for high charge carrier extraction efficiency, leading to outstanding performance. A benchmarking current density of 40 ± 1 mA cm−2 at 0 V vs. reversible hydrogen electrode, the highest value ever reported for a planar silicon electrode without noble metals, and a large onset potential of +0.4 V were achieved under standard one-sun illumination.

Synthesis of High Purity Layered Structure Sn3O4 and Its Photocatalytic Performance for Hydrogen Evolution Reaction Under Visible-Light Irradiation

Catalytic water splitting in visible light by visible-light-sensitive photocatalysts is of confocal interest because this will realize efficient conversion of the solar energy to hydrogen fuels for sustainable energy managements. Many water-splitting photocatalysts, such as metal-doped oxides and oxynitrides, can decompose water and produce hydrogen fuel when absorbing visible light [1]. However, such transition-metal oxides and oxynitrides may be not practical catalysts for large-scale solar-energy conversion, because they contain environmentally toxic heavy metals or expensive transition metals (Rh, Nb, Ta, or La), which are much less produced than abundant metals such as iron. Herein, we report that an oxide of abundantly available and environmentally non-toxic tin, Sn3O4, can efficiently catalyze Hydrogen (H2)-evolution in a methanol solution under irradiation of visible light. Sn3O4 belongs to a series of layered tin oxides such as SnO and Sn2O3, of which crystal structure consists of stacking of alternating atomic layers of tin and oxygen, but the refined structure of Sn3O4 has not been fully elucidated. Recent studies have demonstrated the photoelectric properties and photocatalytic performance of dye degradation of Sn3O4, but none has ever shed light to its potential as a photocatalyst for water splitting. We prepared high purity nanocrystals of Sn3O4 by hydrothermal synthesis using sodium citrate as a ligand [2]. Optimized hydrothermal synthesis provided high purity and highly active Sn3O4 material. SEM and TEM have shown that the Sn3O4 material consisted of thin, highly crystalline flexes (500×500×10nm3), which were predominantly surrounded by the {110} facets (the inset of Fig.1). The Sn3O4 material was orange in color and more efficiently absorbed visible light than the control SnO2. Figure 1 shows the time courses of hydrogen (H2) evolution over the SnO-, SnO2-, and Sn3O4 materials under irradiation of visible light (λ > 420 nm). The hydrogen (H2)-evolution tests have demonstrated that Sn3O4 can promote H2 evolution at a significant efficiency with and without co-catalyst (Pt), whereas neither SnO nor SnO2 is active toward the reaction, as shown in Fig.1. The activity of Sn3O4 was increased with the purity of Sn3O4 phase, where inactive SnO2 particles inhibited the H2 evolution. Theoretical calculations have elucidated that the enhanced activity of Sn3O4 is attributed to the co-existence of Sn2+ and Sn4+ in Sn3O4 leads to a desirable band gap and band-edge position for photocatalytic H2 evolution from water solution under visible light, the conduction-band minimum of Sn3O4 is higher than the reduction potential for water, and the band gap matches the photon energy of visible light. The Sn3O4 has great potential as a practical solar-energy conversion catalyst, in terms of low impacts to the environment, abundance of mineral resource, and catalytic performance for the H2 generation from water solution. [1] M. Hara, K. Domen, et al., Stud. Surf. Sci. Catal. 2003, 145, 169−172. [2] M. Maidhily, T. Tanabe et al., ACS Appl. Mater. Interfaces 2014, 6, 3790. Figure 1

Photovoltaic and Photoelectrochemical Solar Energy Conversion with Cu2O

The amount of solar power striking the earth’s surface is vastly superior to humanity’s present day energy needs and can easily meet our increasing power demands as the world’s population grows. In order to make solar power cost competitive with fossil fuels, the conversion devices must be made as cheaply as possible, which necessitates the use of abundant raw materials and low energy intensity fabrication processes. Cuprous oxide (Cu2O) is a promising material with the capacity for low cost, large-scale solar energy conversion due to the abundant nature of copper and oxygen, suitable bandgap for absorption of visible light, as well as effective, low energy intensity fabrication processes such as electrodeposition. For photoelectrochemical (PEC) water splitting, protective overlayers have been developed that greatly extend the durability of hydrogen-evolving Cu2O-based materials. Recent developments in the advancement of protective overlayers for stabilizing photoabsorber materials for water splitting are...

Efficient planar heterojunction perovskite solar cells by vapour deposition

Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.

Silicon protected with atomic layer deposited TiO2: durability studies of photocathodic H2 evolution

The semiconducting materials used for photoelectrochemical (PEC) water splitting must withstand the corrosive nature of the aqueous electrolyte over long time scales in order to be a viable option for large scale solar energy conversion. Here we demonstrate that atomic layer deposited titanium dioxide (TiO2) overlayers on silicon-based photocathodes generate extremely stable electrodes. These electrodes can produce an onset potential of +0.510 V vs. RHE and a hydrogen evolution saturation current of 22 mA cm−2 using the red part of the AM1.5 solar spectrum (λ > 635 nm, 38.6 mW cm−2). A PEC chronoamperometry experiment was carried out for 2 weeks under constant illumination at +0.300 V vs. RHE with negligible degradation (<5%). Further testing showed slight degradation, but the re-addition of catalyst recovered the activity. These results show that properly processed TiO2 overlayers may have the potential to be stable for the long time frames that will be necessary for commercial devices.

P-Type Cu−Ti−O Nanotube Arrays and Their Use in Self-Biased Heterojunction Photoelectrochemical Diodes for Hydrogen Generation

Copper and titanium remain relatively plentiful in the earth's crust; hence, their use for large-scale solar energy conversion technologies is of significant interest. We describe fabrication of vertically oriented p-type Cu-Ti-O nanotube array films by anodization of copper rich (60% to 74%) Ti metal films cosputtered onto fluorine doped tin oxide (FTO) coated glass. Cu-Ti-O nanotube array films 1 mum thick exhibit external quantum efficiencies up to 11%, with a spectral photoresponse indicating that the complete visible spectrum, 380 to 885 nm, contributes significantly to the photocurrent generation. Water-splitting photoelectrochemical pn-junction diodes are fabricated using p-type Cu-Ti-O nanotube array films in combination with n-type TiO 2 nanotube array films. With the glass substrates oriented back-to-back, light is incident upon the UV absorbing n-TiO 2 side, with the visible light passing to the p-Cu-Ti-O side. In a manner analogous to photosynthesis, photocatalytic reactions are powered only by the incident light to generate fuel with oxygen evolved from the n-TiO 2 side of the diode and hydrogen from the p-Cu-Ti-O side. To date, we find under global AM 1.5 illumination that such photocorrosion-stable diodes generate a photocurrent of approximately 0.25 mA/cm (2), at a photoconversion efficiency of 0.30%.

Nanowire dye-sensitized solar cells

Excitonic solar cells-including organic, hybrid organic-inorganic and dye-sensitized cells (DSCs)-are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient and stable excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

Economics of solar systems

Large scale solar energy conversion for a variety of residential, agricultural and industrial uses may emerge as a significant part of future energy supply mix on a global scale. The timing for reaching such potential depends upon the management of solar technology, the of the various solar options, and upon the organization of their diffusion in the productive areas of their integration as a viable energy source. The contemporary management of solar technology is often too fragmented and/or departmentalized, allowing a large variety of concepts to enter the market prior to sensible standardization and quality control measures. Retrofit installations to existing buildings are receiving primary attention, although the fully integrated energy supply will offer decisively better performance and economy. In fact, several preliminary studies indicate that large scale integration of solar energy could be instrumental in decoupling future energy demands from the desired economic growth trends, which ought to be the ultimate objective of future energy policies. Solar technology, is however, still in the developmental stage, which makes projections of its competitiveness and utility in the future too speculative. The lack of actual experiences with long term reliability of solar systems, their operating and maintenance cost, and the absence of data for realistic comparison with other alternate energy systems, are among the constraining factors for pragmatic evaluation of economics. The hereby outlined economics of solar systems is, therefore, only an interim effort to stimulate interest in the state-of-the-art application of solar energy for residential space and water heating, and its possible market evolution in the Countries of the European Economic Community (EEC).

Thermomechanical solar power generation

After considerations on the thermodynamics and technicalities of solar energy conversion into power, indicating the advantage of using high temperatures, the 1 MW(el) helioelectric power plant EURELIOS of the European Communities is described. The conversion of solar energy into hydrogen, and transportation of solar energy by means of hydrogen and/or hydrogenated fuels are discussed. The solar power plant is, at about 80% of the overall costs, the most cost-intensive component in the system comprising solar energy conversion into mechanical power, electrolysis, chemical reactor and transportation of chemicals over long distances (several 1000 km) over land and sea to the user's site. Liquid and gaseous hydrogen, methanol and ammonia are considered. Industrial solar hydrogen production is discussed, especially by electrolysis, because of its present readiness for application. Some qualitative considerations on the influence of large-scale solar energy conversion on local climate indicate that there should be no heating up of the atmosphere above the ground but a slight cooling of the ground underneath the mirrors.

Effect of grain boundaries on the performance of polycrystalline tunnel MIS solar cells

Tunnel MIS solar cells utilizing polycrystalline semiconductors are an attractive alternative to conventional p–n junction devices for fabricating large-scale solar energy conversion arrays. The main effect of the grain boundaries in polycrystalline semiconductors is to reduce the lifetime of the minority carriers. Experimental work indicates that the potential barrier near the grain boundary has a very limited effect on the barrier height which, in turn, controls open-circuit voltage of the device. The collection efficiency depends upon the grain size of the crystallites, in addition to other device parameters. A simple description of the performance of polycrystalline MIS solar cells will be presented, based on the above mentioned electrical characteristics.

Development of low-cost silicon crystal growth techniques for terrestrial photovoltaic solar energy conversion

Single crystal silicon solar cells are potential elements of large scale solar energy conversion systems. Current costs of these cells are too high at least in part because current production methods require single crystal wafers obtained by slicing cylindrical single crystal ingots. This paper reviews a U.S. research program aimed at reducing the cost of silicon cells by developing new methods of growing silicon ribbons and sheets from which high efficiency solar cells can be fabricated. The paper also describes novel techniques for lower cost processes for ingot growth and wafer slicing which are included in this research and development program.

Solar power from satellites

Various alternative energy sources have been proposed in the last few years, many of them figuring in the pages of PHYSICS TODAY. The recognition that no one of these energy sources will, by itself, meet all future power needs, together with the large uncertainties inherent in the achievement of full potential for each of them, has led to what might appear the most daring proposal so far: large-scale solar-energy conversion in space with a satellite solar-power station located in synchronous orbit around Earth—that is, at an altitude of 22 300 miles above the surface.

Energy from agriculture—The most economic method of large scale solar energy conversion

The economics of terrestrial growth of vegetation for its energy content is far more favorable than other more technically sophisticated methods of large-scale solar energy conversion—mirrors, photovoltaics, etc. This paper summarizes our view of what might be attainable in biomass production, hoping, thereby, to stimulate interest in the concept. The types of vegetation best suited for an intensive energy plantation, as well as vegetation selection criteria, will be discussed. The type and availability of land for growing energy crops on a conceptual terrestrial plantation, as well as the logistics and economics, are discussed. An energy budget for plant-material production and harvesting for the conceptual plantation is developed. A technoeconomic comparison of firing the crops directly for electric power generation with conversion to clean fuel gas (methane or low-BTu gas) either at the farm site or at selected markets is made.