Low-cost Solar Energy Conversion Research Articles

Inverted nanocone-based thin film photovoltaics with omnidirectionally enhanced performance.

Thin film photovoltaic (PV) technologies are highly attractive for low-cost solar energy conversion and possess a wide range of potential applications from building-integrated PV generation to portable power sources. Inverted nanocones (i-cones) have been demonstrated as a promising structure for practical thin film PV devices/modules, owning to their antireflection effect, self-cleaning function, superior mechanical robustness, and so forth. In this work, we have demonstrated a low-cost and scalable approach to achieve perfectly ordered i-cone arrays. Thereafter, thin film amorphous silicon (a-Si:H) solar cells have been fabricated based on various i-cone substrates with different aspect ratios and pitches to investigate the impact of geometry of i-cone nanostructures on the performance of the as-obtained PV devices. Intriguingly, the optical property investigations and device performance characterizations demonstrated that the 0.5-aspect-ratio i-cone-based device performed the best on both light absorption capability and energy conversion efficiency, which is 34% higher than that of the flat counterpart. Moreover, the i-cone-based device enhanced the light absorption and device performance over the flat reference device omnidirectionally. These results demonstrate a viable and convenient route toward scalable fabrication of nanostructures for high-performance thin film PV devices based on a broad range of materials.

Surface engineering of ZnO nanostructures for semiconductor-sensitized solar cells.

Semiconductor-sensitized solar cells (SSCs) are emerging as promising devices for achieving efficient and low-cost solar-energy conversion. The recent progress in the development of ZnO-nanostructure-based SSCs is reviewed here, and the key issues for their efficiency improvement, such as enhancing light harvesting and increasing carrier generation, separation, and collection, are highlighted from aspects of surface-engineering techniques. The impact of other factors such as electrolyte and counter electrodes on the photovoltaic performance is also addressed. The current challenges and perspectives for the further advance of ZnO-based SSCs are discussed.

Electron and Hole Transport Layers: Their Use in Inverted Bulk Heterojunction Polymer Solar Cells

Bulk heterojunction polymer solar cells (BHJ PSCs) are very promising organic-based devices for low-cost solar energy conversion, compatible with roll-to-roll or general printing methods for mass production. Nevertheless, to date, many issues should still be addressed, one of these being the poor stability in ambient conditions. One elegant way to overcome such an issue is the so-called “inverted” BHJ PSC, a device geometry in which the charge collection is reverted in comparison with the standard geometry device, i.e., the electrons are collected by the bottom electrode and the holes by the top electrode (in contact with air). This reverted geometry allows one to use a high work function top metal electrode, like silver or gold (thus avoiding its fast oxidation and degradation), and eliminates the need of a polymeric hole transport layer, typically of an acidic nature, on top of the transparent metal oxide bottom electrode. Moreover, this geometry is fully compatible with standard roll-to-roll manufacturing in air and is less demanding for a good post-production encapsulation process. To date, the external power conversion efficiencies of the inverted devices are generally comparable to their standard analogues, once both the electron transport layer and the hole transport layer are fully optimized for the particular device. Here, the most recent results on this particular optimization process will be reviewed, and a general outlook regarding the inverted BHJ PSC will be depicted.

Solution-processed mesoscopic Bi₂S₃:polymer photoactive layers.

The fabrication of solution-processed nontoxic mesoporous Bi2S3 structures is demonstrated and the suitability of these structures for use in hybrid solar cells investigated. Mesoporous Bi2S3 electrodes are prepared via thermal decomposition of a thin film composed of a bismuth xanthate single source precursor. The resultant Bi2S3 films are made up of regular needles with approximate dimensions of 50×500 nm, as confirmed by scanning electron microscopy (SEM). The crystallinity of the Bi2S3 is found to be dependent on the annealing temperature, as determined by X-ray diffraction. The porous Bi2S3 films are infiltrated with the hole conductor P3HT to generate novel hybrid films, and laser-based transient absorption spectroscopy is used to interrogate the charge-separation reaction at the resulting Bi2S3/P3HT heterojunction. Specifically, optical excitation of the hybrid films results in efficient and long-lived charge separation (microsecond to millisecond timescale), thereby rendering such films suitable for the development of novel low-cost solar-energy conversion devices.

Atomic-Scale Observation of Multiconformational Binding and Energy Level Alignment of Ruthenium-Based Photosensitizers on TiO2Anatase

Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dye's energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO2, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence. Here we demonstrate multiconformational adsorption and energy level alignment of single N3 dyes on TiO2 anatase (101) revealed by scanning tunnelling microscopy and spectroscopy. The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection. These findings emphasize the critical role of the interfacial coupling and suggest that further designs of dye-sensitized solar cells should target a higher selectivity in the dye-substrate binding conformations in order to ensure efficient electron injection from all photosensitizers.

A comprehensive review of the application of chalcogenide nanoparticles in polymer solar cells.

In this review the use of solution-processed chalcogenide quantum dots (CdS, CdSe, PbS, etc.) in hybrid organic-inorganic solar cells is explored. Such devices are known as potential candidates for low-cost and efficient solar energy conversion, and compose the so-called third generation solar cells. The incorporation of oxides and metal nanoparticles has also been successfully achieved in this new class of photovoltaic devices; however, we choose to explore here chalcogenide quantum dots in light of their particularly attractive optical and electronic properties. We address herein a comprehensive review of the historical background and state-of-the-art comprising the incorporation of such nanoparticles in polymer matrices. Later strategies for surface chemistry manipulation, in situ synthesis of nanoparticles, use of continuous 3D nanoparticles network (aerogels) and ternary systems are also reviewed.

Recent progress in organic photovoltaics: device architecture and optical design

Research on organic photovoltaic (OPV) materials and devices has flourished in recent years due to their potential for offering low-cost solar energy conversion. With a deepened understanding on the fundamental photovoltaic processes in organic electronic materials and the development of tailored materials and device architectures, we have seen a rapid increase in the efficiency of OPV devices to over 10%, which attracts tremendous commercial interests for further development and manufacturing. Here, we review recent progress in the field of organic photovoltaics, particularly on various innovative device architectures and optical designs to maximize the power conversion efficiency of OPV cells for a given set of photoactive donor and acceptor materials. Following an introduction of the basic device operation of organic photovoltaic cells and the advances in active materials, we firstly present different device architectures that have been used to optimize the charge generation and collection characteristics within the OPV devices. We then discuss various methods to manage and manipulate the light wave propagation in OPV devices for more complete absorption of the incident light, an important area that has been underexplored so far.

Silicon Nanowires Show Improved Performance as Photocathode for Catalyzed Carbon Dioxide Photofixation

Wired: With silicon as a photocathode and [Ni(bpy)2] as a catalyst, visible light-powered CO2 photofixation is made possible (see picture). Owing to their multifaceted surfaces, silicon nanowires show a significantly better performance compared to planar Si. The shape of a nanowire is indeed an appealing materials choice for high-efficiency, low-cost solar energy conversion applications.

Random vs regularized OPV: Limits of performance gain of organic bulk heterojunction solar cells by morphology engineering

Inexpensive solution processing of bulk heterojunction (BHJ) type organic photovoltaic (OPV) cells offers an attractive option for the low cost solar energy conversion. Solution processing creates a disordered morphology consisting of two organic semiconductors, intermixed randomly within the light-absorbing layer of the cell. In this paper, we use a detailed three-dimensional process-device co-modeling framework to show that in spite of the inherent structural randomness of the morphology, the efficiency of solution-processed BHJ cells is nearly optimal – close to those of the perfectly ordered structures. In addition, we show that the morphological randomness by itself does not increase the performance variability of large-area cells. Both the results indicate that the inexpensive solution processing of BHJ cells imposes no inherent limitation on the performance/variability and the ultimate efficiency of such solution-processed films should compare favorably to the other ordered OPV cells fabricated by more expensive techniques. Finally, we explore the theoretical optimum morphology for BHJ cells and find that fill factor is the only parameter through which efficiency can be enhanced by morphology engineering. We conclude by exploring the performance gains/limits of organic solar cells with the improvement in transport parameters.

Photoconduction action spectra of regio-regular poly(3-hexylthiopene):TiO2 diodes

The development of polymer-based photovoltaic devices brings the promise of low-cost and lightweight solar energy conversion systems. This technology requires new materials and device architectures with enhanced efficiency and lifetime, which depends on the understanding of charge-transport mechanisms. Organic films combined with electronegative nanoparticles may form systems with efficient dissociation of the photogenerated excitons, thus increasing the number of carriers to be collected by the electrodes. In this paper we investigate the steady-state photoconductive action spectra of devices formed by a bilayer of regio-regular poly(3-hexylthiophene) (RRP3HT) and TiO2 sandwiched between ITO and aluminum electrodes (ITO/TiO2:RRP3HT/Al). Photocurrents were measured for distinct bias voltages with illumination from either side of the device. Heterojunction structures were prepared by spin coating a RRP3HT film on an already deposited TiO2 layer on ITO. Symbatic and antibatic curves were obtained and a model for photocurrent action spectra was able to fit the symbatic responses. The quantum yield increased with the electric field, indicating that exciton dissociation is a field-assisted process as in an Onsager mechanism. Furthermore, the quantum yield was significantly higher when illumination was carried out through the ITO electrode onto which the TiO2 layer was deposited, as the highly electronegative TiO2 nanoparticles were efficient in exciton dissociation.

Theoretical characterization of ruthenium complexes containing functionalized bithiophene ligands for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have gained widespread interest for their potential as low-cost solar energy conversion devices. One of the key issues is the design of higher efficient light-absorbing dyes. In this paper, we present a theoretical characterization of ruthenium complexes containing functionalized bithiophene (btp) ligands ( CYC–B1 and CYC–B11) based on density functional theory (DFT) calculations. Molecular geometries, electronic structures, and optical absorption spectra are investigated both in the gas phase and in dimethylformamide (DMF) solution. Frontier orbital analysis shows the three highest HOMOs are composed of nonbonding combinations of the Ru t 2g orbitals with the p orbital and lone pairs of the SCN ligands, while the six lowest LUMOs are the π* combinations of the 4,4’-dicarboxy-2,2’-bipyridine (dcbpy) and/or btp-functionalized bipyridine (bpy) ligands calculated in the gas phase. Inclusion of solvent results in great changes in energies and compositions of the molecular orbitals of these complexes. The spectra are assigned to the intraligand π → π* transitions of the dcbpy ligand in the ultraviolet region, whereas in the visible region the spectra show multitransition character of metal-to-ligand charge transfer (MLCT), interligand π → π*, and intraligand π → π*. Our results clarify the role of the functionalized btp ligands on the absorption properties of the dyes.

Polymer-Based Solar Cells: State-of-the-Art Principles for the Design of Active Layer Components

AbstractThe vision of organic photovoltaics is that of a low cost solar energy conversion platform that provides lightweight, flexible solar cells that are easily incorporated into existing infrastructure with minimal impact on land usage. Polymer solar cells have been a subject of growing research interest over the past quarter century, and are now developed to the point where they are on the verge of introduction into the market. Towards the goal of continuing to improve the performance of polymer solar cells, a number of avenues are being explored. Here, the focus is on optimization of device performance via the development of a more fundamental understanding of device parameters. The fundamental operating principle of an organic solar cell is based on the cooperative interaction of molecular or polymeric electron donors and acceptors. Here the state-of-the-art in understanding of the physical and electronic interactions between donor and acceptor components is examined, as is important for understanding future avenues of research and the ultimate potential of this technology.

Perspectives on Organic Photovoltaics

Organic photovoltaics have been considered as a future low-cost solar energy conversion solution, and have attracted tremendous academic and industrial interests in recent years due to many technological advantages of organic semiconductors (including both small molecules and conjugated polymers). Here we describe the basic design and operation of organic photovoltaic cells as well as the important metrics for characterizing their performance. An overview of the three review articles included in this special issue on organic photovoltaics is provided, followed by discussions on the remaining significant challenges for the further development and commercialization of the organic photovoltaic technologies.

Organic Photovoltaic Cells Based on Molecular Donor-Acceptor Heterojunctions

Small molecule based organic photovoltaic cells have attracted considerable interest as a potential solution for low-cost solar energy conversion in the near future. In this review, we first discuss the operation, advantages, and limitations of three different molecular donor-acceptor heterojunction structures. We then discuss the phase separation in molecule donor-acceptor mixtures and its impact on the photovoltaic performance. Methods to achieve bulk heterojunctions with controlled nanoscale geometry are also reviewed. Finally, we provide an outlook on major remaining challenges to the further development of this photovoltaic technology.

Third-Generation Photovoltaic Technology − The Potential for Low-Cost Solar Energy Conversion

ADVERTISEMENT RETURN TO ISSUEGuest CommentaryNEXTThird-Generation Photovoltaic Technology − The Potential for Low-Cost Solar Energy ConversionRussell GaudianaView Author Information Konarka Technologies, Lowell, Massachusetts 01852Cite this: J. Phys. Chem. Lett. 2010, 1, 7, 1288–1289Publication Date (Web):April 1, 2010Publication History Received4 March 2010Accepted11 March 2010Published online1 April 2010Published inissue 1 April 2010https://doi.org/10.1021/jz100290qCopyright © 2010 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views3638Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (3 MB) Get e-AlertsSUBJECTS:Layers,Photovoltaics,Silicon,Solar cells,Solar energy Get e-Alerts

Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have gained widespread interest because of their potential for low-cost solar energy conversion. Currently, the certified record efficiency of these solar cells is 11.1%, and measurements of their durability and stability suggest lifetimes exceeding 10 years under operational conditions. The DSC is a photoelectrochemical system: a monolayer of sensitizing dye is adsorbed onto a mesoporous TiO(2) electrode, and the electrode is sandwiched together with a counter electrode. An electrolyte containing a redox couple fills the gap between the electrodes. The redox couple is a key component of the DSC. The reduced part of the couple regenerates the photo-oxidized dye. The formed oxidized species diffuses to the counter electrode, where it is reduced. The photovoltage of the device depends on the redox couple because it sets the electrochemical potential at the counter electrode. The redox couple also affects the electrochemical potential of the TiO(2) electrode through the recombination kinetics between electrons in TiO(2) and oxidized redox species. This Account focuses on the special properties of the iodide/triiodide (I(-)/I(3)(-)) redox couple in dye-sensitized solar cells. It has been the preferred redox couple since the beginning of DSC development and still yields the most stable and efficient DSCs. Overall, the iodide/triiodide couple has good solubility, does not absorb too much light, has a suitable redox potential, and provides rapid dye regeneration. But what distinguishes I(-)/I(3)(-) from most redox mediators is the very slow recombination kinetics between electrons in TiO(2) and the oxidized part of the redox couple, triiodide. Certain dyes adsorbed at TiO(2) catalyze this recombination reaction, presumably by binding iodine or triiodide. The standard potential of the iodide/triiodide redox couple is 0.35 V (versus the normal hydrogen electrode, NHE), and the oxidation potential of the standard DSC-sensitizer (Ru(dcbpy)(2)(NCS)(2)) is 1.1 V. The driving force for reduction of oxidized dye is therefore as large as 0.75 V. This process leads to the largest internal potential loss in DSC devices. We expect that overall efficiencies above 15% might be achieved if half of this internal potential loss could be gained. The regeneration of oxidized dye with iodide leads to the formation of the diiodide radical (I(2)(-*)). The redox potential of the I(2)(-*)/I(-) couple must therefore be considered when determining the actual driving force for dye regeneration. The formed I(2)(-*) disproportionates to I(3)(-) and I(-), which leads to a large loss in potential energy.

Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar cells. Their function is based on photoinduced charge separation at a dye-sensitized interface between a nanocrystalline, mesoporous metal oxide electrode and a redox electrolyte. They have been the subject of substantial academic and commercial research over the last 20 years, motivated by their potential as a low-cost solar energy conversion technology. Substantial progress has been made in enhancing the efficiency, stability, and processability of this technology and, in particular, the interplay between these technology drivers. However, despite intense research efforts, our ability to identify predictive materials and structure/device function relationships and, thus, achieve the rational optimization of materials and device design, remains relatively limited. A key challenge in developing such predictive design tools is the chemical complexity of the device. DSSCs comprise distinct materials components, including metal oxide nanoparticles, a molecular sensitizer dye, and a redox electrolyte, all of which exhibit complex interactions with each other. In particular, the electrolyte alone is chemically complex, including not only a redox couple (almost always iodide/iodine) but also a range of additional additives found empirically to enhance device performance. These molecular solutes make up typically 20% of the electrolyte by volume. As with most molecular systems, they exhibit complex interactions with both themselves and the other device components (e.g., the sensitizer dye and the metal oxide). Moreover, these interactions can be modulated by solar irradiation and device operation. As such, understanding the function of these photoelectrochemical solar cells requires careful consideration of the chemical complexity and its impact upon device operation. In this Account, we focus on the process by which electrons injected into the nanocrystalline electrode are collected by the external electrical circuit in real devices under operating conditions. We first of all summarize device function, including the energetics and kinetics of the key processes, using an "idealized" description, which does not fully account for much of the chemical complexity of the system. We then go on to consider recent advances in our understanding of the impact of these complexities upon the efficiency of electron collection. These include "catalysis" of interfacial recombination losses by surface adsorption processes and the influence of device operating conditions upon the recombination rate constant and conduction band energy, both attributed to changes in the chemical composition of the interface. We go on to discuss appropriate methodologies for quantifying the efficiency of electron collection in devices under operation. Finally, we show that, by taking into account these advances in our understanding of the DSSC function, we are able to recreate the current/voltage curves of both efficient and degraded devices without any fitting parameters and, thus, gain significant insight into the determinants of DSSC performance.

High Aspect Ratio Silicon Wire Array Photoelectrochemical Cells

In an effort to develop low-cost solar energy conversion techniques, high uniformity vertically oriented silicon wire arrays have been fabricated. These arrays, which allow for radial diffusion of minority charge carriers, have been measured in a photoelectrochemical cell. Large photovoltages (∼400 mV) have been measured, and these values are significantly greater than those obtained from the substrate alone. Additionally, the wire array samples displayed much higher current densities than the underlying substrate, demonstrating that significant energy conversion was occurring due to the absorption and charge-carrier transport in the vertically aligned Si wires. This method therefore represents a step toward the use of collection-limited semiconductor materials in a wire array format in macroscopic solar cell devices.

Cathodic Electrodeposition of Nanocrystalline ZnO Films for Dye-Sensitized Solar Cell Applications

Solar cells based on dye-sensitized nanostructured metal oxides are promising for low-cost solar energy conversion and are intensively investigated nowadays. In this paper, the possibilities to use nanocrystalline ZnO electrodes in dye-sensitized solar cells are investigated. ZnO films have been grown on fluorine-doped SnO 2 coated glass by cathodic electrodeposition from a simple aqueous zinc nitrate solution containing organic additives. Under optimal deposition conditions, nanoporous ZnO films with grain sizes between 20 and 40 nm are obtained. X-ray diffraction and scanning electron microscope are used to characterize the films. Nanoporous ZnO electrodes, dye-sensitized with a ruthenium bipyridyl complex, are used as photoanodes in solar cells. The solar cell performance is studied and the cell exhibits a short circuit photocurrent, an open-circuit photovoltage and the photoelectron conversion efficiency of 1.379 mA/cm 2 , 0.612 V and 1.47%, respectively.

Organic Solar Cells Using Transparent SnO2–F Anodes

Organic solar cells have attracted attention as a means to achieve low-cost solar-energy conversion owing to their ease of manufacture and compatibility with flexible substrates. Conventional organic molecular photovoltaic (PV) devices and light-emitting diodes (OLEDs) are typically grown on transparent indium tin oxide (ITO) anodes that are also widely used for flat-panel displays (FPDs). The scarcity of In, along with the rapid expansion of FPD production, has resulted in a soaring price of ITO-coated glass substrates, with the current price up to ten times greater than in 2003. Alternative transparent conducting oxides such as doped SnO2 or ZnO have been used as electrodes in dye-sensitized, CdTe, microcrystalline Si, and amorphous Si PV devices. Organic small-molecule or polymeric devices, with active layers typically < 1000 A thick, can readily be shorted owing to the pronounced surface-roughness characteristic of these oxide variants. Nevertheless, the cost of F-doped SnO2 (SnO2–F)coated glass is less than one third that of ITO-coated glass. While there have been reports of using SnO2–F as the transparent anode for polymeric OLEDs and solar cells, to our knowledge there has yet to be a demonstration of an organic heterojunction (HJ) PV cell based on SnO2–F anodes with an efficiency greater than 0.1 %. Here, we report on copper phthalocyanine (CuPc)/C60 HJ PV cells on SnO2–F anodes [8] with a power conversion efficiency of 2.5 % at 1 sun simulated AM 1.5 G (AM: air mass; G: Global) illumination. The organic layers were grown by organic vapor-phase deposition (OVPD) that enabled complete coverage of the rough oxide surface, effectively preventing shorts between opposing cathode and anode contacts. In addition, we show that by controlling the organic-film morphology, we can grow the donor–acceptor (D–A) interface into a three-dimensional interdigitated bulk HJ (BHJ) structure, resulting in power-conversion efficiencies nearly twice those of analogous devices with a planar heterointerface. As shown in Figure 1a, the 750 nm thick SnO2–F-coated glass substrates have 70–80 % transmittance in the visible range, or approximately 10 % less than that for glass with 150 nm thick ITO coatings. The absorption of both substrates has a high-energy cutoff at wavelengths less than 350 nm, implying a match of the transparency window to that of the solar radiation spectrum. The sheet resistance of SnO2–F-coated glass is less than 12 X/sq., lower than that of ITO-coated glass (15 X/sq.) The high transparency and small resistance C O M M U N IC A TI O N S