In celebration of the 2015 International Year of Light, we highlight major breakthroughs in photonics for energy conversion and conservation. The section on energy conversion discusses the role of light in solar light harvesting for electrical and thermal power generation; chemical energy conversion and fuel generation; as well as photonic sensors for energy applications. The section on energy conservation focuses on solid-state lighting, flat-panel displays, and optical communications and interconnects.

Organic light-emitting diodes (OLEDs) have already been proven in display applications and show promise as the next-generation solid-state lighting technology for general illumination. A major barrier to the adoption of OLEDs for solid-state lighting is the efficiency roll-off at high brightness, which occurs at much lower current densities for OLEDs than their inorganic counterparts, in large part due to the quenching of excitons. We discuss strategies to mitigate this efficiency roll-off through management of excitons in both white and monochrome devices, including the use of phosphorescent as well as thermally activated delayed fluorescent emitters. Successful high-efficiency devices are used as case studies for how to effectively manage excitons.

We review prior and on-going works in using laser annealing (LA) techniques in the development of chalcogenide-based [CdTe and Cu(In,Ga)(S,Se)₂] solar cells. LA can achieve unique processing regimes as the wavelength and pulse duration can be chosen to selectively heat particular layers of a thin film solar cell or even particular regions within a single layer. Pulsed LA, in particular, can achieve non-steady-state conditions that allow for stoichiometry control by preferential evaporation, which has been utilized in CdTe solar cells to create Ohmic back contacts. Pulsed lasers have also been used with Cu(In,Ga)(S,Se)₂ to improve device performance by surface-defect annealing as well as bulk deep-defect annealing. Continuous-wave LA shows promise for use as a replacement for furnace annealing as it almost instantaneously supplies heat to the absorbing film without wasting time or energy to bring the much thicker substrate to temperature. Optimizing and utilizing such a technology would allow production lines to increase throughput and thus manufacturing capacity. Lasers have also been used to create potentially low-cost chalcogenide thin films from precursors, which is also reviewed.

When dispersed in water, nanoscale photocatalysts can aggregate into microscale secondary particles due to their high surface energy. The interaction between the incident photons and the aggregated particles is expected to be significantly changed due to their comparable length scales. Hydrodynamics of the particles after aggregation and the resultant photonic flux distributions in slurry photocatalytic reactors are therefore essential. The magnetically stirred photocatalytic reactor is compact and simple for fabrication, and therefore can be readily employed in lab-scale photocatalytic tests. However, studies toward its optimization are rare. In our study, the photocatalyst distribution was simulated using a Eulerian–Lagrangian approach, while the evolution of free liquid surface led by magnetic stirring was modeled by volume of fluid method. Subsequently, based on the photocatalyst distribution, the photonic flux distribution was obtained through a mean free path-based Monte Carlo method. Outcomes suggest that at a stirring speed of 900 rpm, the 10  μm particles can be well suspended, and a moderate free liquid surface will also be present. Moreover, under sufficient stirring, larger catalyst particles are more likely to be densely distributed in the outer region, which contributes to an increase in the overall photonic absorption even higher than the one with evenly distributed photocatalysts.

We analyzed the conversion performance of conventional organic photovoltaic (OPV) and inverted organic photovoltaic (IOPV) devices with an active layer of polymer, PTB7:  PC₇₀BM. We computed the current density–voltage (JV) curves, short-circuit current density (J_sc), open-circuit voltage (V_oc), maximum current density (J_max), maximum power density (P_max), and fill-factor (FF) under various scenarios. We employed the one-dimensional optical transfer matrix theory to calculate the light intensity that was then used as the input at the active layer for optical carrier generation. Then we obtained electrical performance parameters from the JV curves plotted by solving Poisson and charge transport equations. The effects of adjusting the exciton generation profile by tuning the active layer width and optical spacer thickness under 100  mW·cm−² air mass 1.5 global (AM 1.5G) illumination are also analyzed. In addition, the effect on the conversion performance by using different electron and hole mobility relations in the polymers composing the active layer is computed. To identify the optimal performance, we proposed an exciton generation profile that maintains a constant amplitude when shifted through the active layer. Subsequently, by adjusting the active layer width, optical spacer thickness, and electron and hole mobility, we found that the OPV structure achieved performance characteristics previously reported only for IOPV structures.

Thin form-factor planar sunlight concentrators fabricated using low-cost materials in conjunction with high-efficiency solar cells may prove economically competitive with silicon photovoltaics. A periodic structure with planar interfaces employing an array of microlenses and coupling structures is amenable to high-volume imprint manufacturing and hence a drastic cost reduction. A numerical study of periodic prismatic void structures aligned with microlenses to efficiently couple incoming light into a constant cross-section channel waveguide is reported. Three different prism-like cut-out structures: a bare prism, a planar connected prism, and a pointed prism are proposed. Numerical results show that the proposed pointed prism design offers a high-efficiency coupling with one-sided output (90.8% and 85.4% peak efficiency at concentration ratios of 320× and 400×, respectively).

Two CdTe and two copper indium gallium (di)selenide (CIGS)-type modules were tested for potential-induced degradation (PID) with positive and negative 1000 V biases applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber. Various degradation mechanisms could be seen with signatures such as shunting, transparent conductive oxide (TCO) corrosion, charge carrier lifetime reduction, and dead active layer at edges along with resulting cell mismatch. All modules tested exhibited degradation by system voltage stress in chamber, but only one module type has degraded in parallel field tests. I−V curve data indicated that one CdTe-type module sequentially exhibited shunting followed by a recovery and then series resistance losses. This module type showed TCO delamination from the glass in the environmental chamber tests and also exhibited power degradation within 5 weeks in field tests. Relative rates of Coulomb transfer from the voltage-biased active cell circuit to ground are compared for the modules in chamber tests to those placed outdoors under system voltage stress to extrapolate the anticipated time to failure in the field. This analysis correctly indicated which module type failed in the field first.

Ultrafast energy decay kinetics of a zinc (hydr)oxide–graphite oxide (GO) composite is studied via time-resolved fluorescence spectroscopy. The time-resolved emission is spectrally decomposed into emission regions originating from the zinc (hydr)oxide optical gap, surface, and defect states of the composite material. The radiative lifetime of deep red emission becomes an order of magnitude longer than that of GO alone while the radiative lifetime of the zinc optical gap is shortened in the composite. An energy transfer scheme from the zinc (hydr)oxide to GO is considered.

Preparation of n-ZnO/p-Si heterostructures using solution-synthesized ZnO nanowire films and their photovoltaic characterization is reported. The solution-grown ZnO nanowire film is characterized using scanning electron microscope, electron dispersive x-ray, and optical absorption studies. Electrical and photovoltaic properties of the fabricated heterostructures are studied using e-beam-evaporated aluminum as metal contacts. In order to use transparent contact and to simultaneously collect the photogenerated carriers, sandwich-type solar cells were fabricated using ZnO nanorod films grown on p-silicon and indium tin oxide (ITO) coated glass as ITO/n-ZnO NR/p-Si. The electrical properties of these structures are analyzed from current-voltage (I−V) characteristics. ZnO nanowire film thickness-dependent photovoltaic properties are also studied. Indium metal was also deposited over the ZnO nanowires and its effects on the photovoltaic response of the devices were studied. The results demonstrated that all the samples exhibit a strong rectifying behavior indicating the diode nature of the devices. The sandwich-type ITO/n-ZnO NR/p-Si solar cells exhibit improved photovoltaic performance over the Al-metal-coated n-ZnO/p-Si structures. The indium deposition is found to show enhancement in photovoltaic behavior with a maximum open-circuit voltage (V_oc) of 0.3 V and short-circuit current (I_sc) of 70×10−⁶ A under ultraviolet light excitation.

An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. In the present work, thin-film amorphous-silicon samples were irradiated with a continuous-wave green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. Micro-Raman spectroscopy was used to study the crystallization induced on the irradiated surface. Both laser peak power density and irradiation time are identified as key variables in the crystallization process, but within the parametric window considered, the enhancement of the crystalline factor, is more sensitive to the power density than to the irradiation time. The optimum parameters are then used for crystallizing a large sample area by means of overlapped laser scanned lines. Ellipsometric data experimentally show that the whole volume of a micron-thick sample is crystallized.

Modeling single junction solar cells composed of III–V semiconductors such as GaAs with the effects of photon recycling yields insight into design and material criteria required for high efficiencies. For a thin-film single junction GaAs cell to reach 28.5% efficiency, simulation results using a recently developed model which accounts for photon recycling indicate that Shockley–Read–Hall (SRH) lifetimes of electrons and holes must be longer than 3 and 1 μs, respectively, in a 2-μm thin active region, and that the native substrate must be removed such that the cell is coupled to a highly reflective rear-side mirror. The model is generalized to account for luminescence coupling in tandem devices, which yields direct insight into the top cell’s nonradiative lifetimes. A heavily current mismatched GaAs/GaAs tandem device is simulated and measured experimentally as a function of concentration between 3 and 100 suns. The luminescence coupling increases from 14% to 33% experimentally, whereas the model requires increasing electron and hole SRH lifetimes to explain these results. This could be an indication of the saturating defects which mediate the SRH process. However, intermediate GaAs layers between the two subcells may also contribute to the luminescence coupling as a function of concentration.

A high open-circuit voltage dye-sensitized solar cell (DSSC), based on a ZnO/TiO₂ nanocomposite (NC) photoelectrode, was presented. This NC photoelectrode was composed of density-controlled ZnO-TiO₂ core-shell nanorod arrays and dispersed TiO₂ nanoparticles. The electrochemical impedance spectroscopy results confirmed that a reduced recombination loss was achieved by applying the proposed composite nanostructure. Thus, a higher open-circuit voltage of up to 0.93 V was achieved.

The effect of dye-aggregation-preventing co-adsorbents, cholic acid and deoxycholic acid, on the performance of dye-sensitized solar cells constructed using a metal-free sensitizer, chlorin e6 adsorbed onto TiO₂ surface is investigated. Absorption and fluorescence studies of chlorin e6 provided the spectral coverage, whereas electrochemical studies allowed estimation of the free energy of charge injection. B3LYP/6-31G* studies were performed to visualize location of the Frontier orbitals and their contribution to the charge injection when they were surface-modified on TiO₂. The concentration of the co-adsorbent and soaking time was optimized for improved cell performance. Better dye regeneration efficiency for co-adsorbed cells compared to the cells with no co-adsorbent was revealed by electrochemical impedance spectroscopy. Femtosecond transient absorption studies were performed to probe the kinetics of charge injection and charge recombination on the TiO₂/chlorin e6/co-adsorbent electrodes. Such studies showed slower by an order of magnitude charge recombination rates for electrodes co-adsorbed either with cholic acid or deoxycholic acid while maintaining almost the same charge injection rates, thus rationalizing the importance of co-adsorbents on the overall cell performance.

Organically modified silica (ORMOSIL)-coated dye-sensitized solar cells (DSSCs) with improved energy conversion efficiency are demonstrated. ORMOSIL-coated DSSC surfaces exhibit omnidirectional low reflectivity over a broad range of wavelengths (400–800 nm). The short-circuit current density (J_SC) is enhanced up to 23% at normal incidence (θ=0  deg) as a result of ORMOSIL coating. In addition, J_SC enhancement is even higher at larger angles of incidence; 84% enhancement was observed at θ=30  deg. Moreover, ORMOSIL coating is superhydrophobic with a contact angle of 155 deg.

Photovoltaic (PV) applications such as in the architectural, automotive, and aerospace industries face design contradictions because they are expected to produce a lot of energy but are constrained by available area, surface shape, incident irradiance, shadows, and other aspects that have a negative influence on the energy produced by the solar panel. Solar competition vehicles are some of these challenging PV applications. The design of such solar arrays needs to consider efficiency evaluation in order to optimize space; it is difficult not to install solar modules in areas impacted by shadows. A design procedure for a solar array configuration based on shadow analysis for competition vehicles is presented. The principle is that shadows in moving objects can be simulated, since the vehicle, the earth and the sun are are moving in semipredictable patterns, thus net energy collection can be forecast. The case study presented is the solar array design of a vehicle that participated in the World Solar Challenge 2013. The obtained results illustrate how the employment of the procedure gives insights on important aspects to consider and also delivers qualitative and quantitative information for decision making. In addition, the experience in competition highlights some issues to be considered, modified, or improved in further vehicle designs.

Having shown some demonstration of two photon absorption and multiband emission processes in quantum dots (QD), multiquantum wells (MQW), and highly mismatched alloys, intermediate band solar cells are currently the subject of numerous studies. To better understand the underlying mechanisms, our objective is to experimentally probe the multiband operation of this device. We used photoluminescence recorded with a calibrated hyperspectral imager which provides spectrally resolved images with a spatial resolution of 2  μm and spectral resolution of 2 nm on proof of concept QD and MQW solar cells samples. Device emission can be described with the generalized Planck’s law from which the quasi-Fermi level splitting of the three bands can be determined. The advantage of the technique is that it can be used to investigate the intermediate band material without the need to make contacts or a full device structure. We also discuss the usefulness of a dual-beam method.

Perovskite compounds have the potential to transform photovoltaics technology, as they are easy to fabricate, have better stability, and possess superior power conversion efficiency. In this research, a versatile solution-processing method called “spray+brush” (SB) has been adopted to achieve a power-conversion efficiency of 3.52% for pure organometal halide perovskite devices. It has been observed that this method is more efficient and cost effective than the perovskite devices fabricated by spray (1.95%) and brush (1.17%) methods alone. The SB method of solution processing can be promising for various other organic coatings.

Light-trapping structures are fabricated on crystalline (100) silicon (c-Si) surfaces by picosecond (ps) laser irradiation followed by chemical etching. First, 1064-nm ps laser scanning is used to form micropore arrays on c-Si. The ps laser processing causes little reconsolidation of the silicon surface, which is beneficial to achieve precise etching. Control of the laser scanning interval, number of scans, and etching time gives a micro-nano hierarchical structure. In this hierarchical structure, the average diameter of the micropores is 25 to 30  μm, while the size of the finer nanostructures on the micropore inner walls ranges from dozens to hundreds of nanometers. Unlike traditional laser texturing techniques, the whole laser process is carried out without mask and photolithography. The reflectance of the c-Si surface with a micro-nano hierarchical structure is as low as 6% in the wavelength range from 400 to 1000 nm without coating. Moreover, the samples also show good hydrophobicity. This is a potential method to fabricate economical antireflective structures that are ideal for applications in c-Si solar cells and self-cleaning c-Si microelectronic devices.

Metal nanoparticles (NPs) can increase the absorption of light within semiconductors and hence improve the efficiency of solar cells. We experimentally investigate the effect that gold and silver NPs have on the reflectance of silicon wafers. The NPs are fabricated using the low cost, large area technique of thermal dewetting. We show that a dielectric spacer layer between the NPs and the semiconductor is required to achieve a net reduction of reflection. Furthermore, the optimum thickness of the spacer layer is found to be independent of NP size and metal type.

Among various deposition techniques to deposit Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films, solution-based processes have attracted considerable attention because of their potentially low cost. Most of the reported solution-based methods are based on nonaqueous solvents, such as hydrazine, and organic solvents. We report the deposition of CZTSSe thin films and fabrication of CZTSSe solar cells by a water-based, solution-processed method followed by Se vapor annealing. The effects of Se vapor feeding time on the properties of CZTSSe thin films and the performance of CZTSSe solar cells are investigated. The ratio of Se/(Se+S) can be tuned by changing the Se vapor feeding time. Our results indicate that extending the Se vapor feeding time increases the band gap, slightly increases the lattice constant, and significantly improves the morphologies of the CZTSSe thin films. A remarkable enhancement in the performance was observed from the CZTSSe solar cells annealed with a longer Se vapor feeding time compared with those without Se feeding.

Density functional theory (DFT) and time-dependent DFT(TD-DFT) modeling techniques are used to conduct a computational study of the geometry and electronic structure of oxadiazole-based organic sensitizers. A DFT study on the thermodynamic aspects of the charge transport processes associated with dye-sensitized solar cells (DSSCs) suggests that the system with 1,2,4-oxadiazole has a balance among the different crucial factors and may result in the highest incident photon to charge carrier efficiency. The dye/(TiO₂)₈ anatase clusters were also simulated to illustrate the electron injection efficiency at the interface. This study provides basic understanding of the impact of molecular design on the performance of oxadiazole dyes in DSSCs.

Nanoresonators are used in photovoltaic applications to reduce the absorber volume by confining the electromagnetic fields. We employed a two-dimensional Maxwell equation solver to investigate the optical performance of a fully structured InP-based nanoresonator, allowing high optical absorption and being electrically compatible. We demonstrated a 60% broadband optical efficiency with a good angle tolerance: more than 40% optical efficiency for an incidence angle between 60 deg and 60 deg. We also addressed the fabrication process in identifying the main barriers and in proposing technological solutions.

We demonstrate a high-efficiency solar-thermophotovoltaic system (STPV) using a monolithic, planar, and spectrally selective absorber/emitter. A complete STPV system using gallium antimonide (GaSb) cells was designed and fabricated to conduct power generation tests. To produce a high-efficiency STPV, it is important to match the thermal radiation spectrum with the sensitive region of the GaSb cells. Therefore, to reach high temperatures with low incident power, a planar absorber/emitter is incorporated for controlling the thermal radiation spectrum. This multilayer coating consists of thin-film tungsten sandwiched by yttria-stabilized zirconia. The system efficiency is estimated to be 16% when accounting for the optical properties of the fabricated absorber/emitter. Power generation tests using a high-concentration solar simulator show that the absorber/emitter temperature peaks at 1640 K with an incident power density of 45  W/cm², which can be easily obtained by low-cost optics such as Fresnel lenses. The conversion efficiency became 23%, exceeding the Shockley–Queisser limit for GaSb, with a bandgap of 0.67 eV. Furthermore, a total system efficiency of 8% was obtained with the view factor between the emitter and the cell assumed to be 1.

2,3,4,5-Tetraarylsiloles are efficient solid-state luminescent materials with good electron-transporting ability. Substitution at the 2,5-positions of silole rings produce various outstanding functional materials that can be used as active layers in organic light-emitting diodes (OLEDs). In this work, two 2,5-dicarbazole-substitued siloles, (2-Cz)₂MTPS and (3-Cz)₂MTPS, are facilely synthesized and fully characterized. Their thermal, photophysical, electrochemical, and electroluminescent properties are investigated systematically. The results show that these 2,5-dicarbazole-functined siloles are thermally stable and feature aggregation-enhanced emission characteristics with high solid-state photoluminescence efficiencies. Nondoped OLEDs [ITO/N,N'-di(1-naphthyl)-N,N'-diphenyl-benzidine (NPB) (60 nm)/emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] fabricated by adopting (2-Cz)₂MTPS and (3-Cz)₂MTPS as light-emitting layers exhibit good performances, with high luminance of 28240  cd m⁻² and electroluminescence efficiency of 4.5  cd A⁻¹.

Flexible organic light-emitting diodes (OLEDs) are candidates for next-generation solid-state lighting because they have merits such as low driving voltage, various color tuning, designable form, and large-area light emission. Although OLEDs’ efficiency, luminance, and lifetime have been improved enough to be commercialized, they are still inflexible despite being based on organic materials. To achieve efficient and reliable flexible OLEDs for solid-state lighting, flexible substrates for OLEDs should be developed. For this purpose, progress must be made in developing good flexible substrates, electrode materials, and encapsulation techniques compatible with these flexible substrates. Here, we review and discuss progress made in these three technologies for solid-state lighting using flexible OLEDs. Addressing the technical challenges associated with the development of high performing flexible substrates, electrode materials compatible with these substrates and good encapsulation techniques would lead to efficient and reliable flexible OLEDs and make flexible solid-state lighting commercially feasible.

There has been a significant interest in spectrum splitting techniques to increase the overall efficiency of photovoltaic solar energy systems. In spectrum splitting, an optical system is used to spectrally separate the incident sunlight. Although systems with different methods and geometries have been proposed, they can generally be classified as either dispersive or nondispersive. Nondispersive systems are based on reflective spectral filters that have minimum optical losses due to dispersion. Dispersive systems use optical elements that spatially separate light as a function of wavelength. This class of spectrum system typically operates in transmission and is shown to have an inherent optical loss. The dispersive effects of transmission type filters are evaluated using a cross-correlation analysis. The results of the analysis are then used to evaluate different spectrum splitting geometries and to determine parameters that minimize their dispersion losses and optimize optical designs.

A spectrum splitter can be used to spatially multiplex different solar cells that have high efficiency in mutually exclusive parts of the solar spectrum. We investigated the use of a grating, comprising an array of dielectric cylinders embedded in a dielectric slab, for specularly transmitting one part of the solar spectrum while the other part is transmitted nonspecularly and the total reflectance is very low. A combination of (1) the rigorous coupled-wave approach for computing the reflection and transmission coefficients of the grating and (2) the differential evolution algorithm for optimizing the grating geometry and the refractive indices of dielectric materials was devised as a design tool. We used this tool to optimize two candidate gratings and obtained definite improvements to the initial guesses for the structural and constitutive parameters. Significant spectrum splitting can be achieved if the angle of incidence does not exceed 15 deg.

The diversity of life on Earth is made possible through an immense variety of proteins that stems from less than a couple of dozen native amino acids. Is it possible to achieve similar engineering freedom and precision to design electronic materials? What if a handful of non-native residues with a wide range of characteristics could be rationally placed in sequences to form organic macromolecules with specifically targeted properties and functionalities? Referred to as molecular electrets, dipolar oligomers and polymers composed of non-native aromatic beta-amino acids, anthranilamides (Aa) provide venues for pursuing such possibilities. The electret molecular dipoles play a crucial role in rectifying charge transfer, e.g., enhancing charge separation and suppressing undesired charge recombination, which is essential for photovoltaics, photocatalysis, and other solar-energy applications. A set of a few Aa residues can serve as building blocks for molecular electrets with widely diverse electronic properties, presenting venues for bottom-up designs. We demonstrate how three substituents and structural permutations within an Aa residue widely alter its reduction potential. Paradigms of diversity in electronic properties, originating from a few changes within a basic molecular structure, illustrate the promising potentials of biological inspiration for energy science and engineering.

We identify and evaluate a variety of efficient and feasible micro-optics for confining the radiative emission of solar cells. The key criteria used for assessing viable designs are (1) high optical efficiency for both the transmission of impinging solar beam radiation and the external recycling of isotropic cell luminescent emission; (2) liberal optical tolerance; (3) compactness; and (4) being amenable to fabrication from existing materials and manufacturing processes. Both imaging and nonimaging candidate designs are presented, and their superiority to previous proposals is quantified. The strategy of angular confinement for boosting cell open-circuit voltage—thereby enhancing conversion efficiency—is limited to cells where radiative recombination is the dominant carrier recombination pathway. Optical systems that restrict the angular range for emission of cell luminescence must, by reciprocity, commensurately restrict the angular range for the collection of solar radiation. This, in turn, mandates the introduction of concentrators, but not for the objective of delivering concentrated flux onto the cell. Rather, the optical system must project an acceptably uniform spatial distribution of solar flux onto the cell surface at a nominal averaged irradiance of 1 sun.

This PDF file contains the editorial “Special Section Guest Editorial: Nanophotonics and Plasmonics for Solar Energy Harvesting and Conversion” for JPE Vol. 5 Issue 01

Here, we review recent progress on the integration of plasmonic electrodes into bulk-heterojunction organic photovoltaic devices. Plasmonic electrodes, consisting of thin films of metallic nanostructures, can exhibit a number of optical, electrical, and morphological effects that can be exploited to improve performance parameters of ultrathin photovoltaic active layers. We review the various types of plasmonic electrodes that have been incorporated into organic photovoltaics such as nanohole, nanowire, and nanoparticle arrays and grating electrodes and their impact on various device performance parameters. The use of plasmonic back electrodes can impact device performance in a number of ways because the mechanisms of performance improvements are often a complex combination of optical, electrical, and structural effects. Inverted bulk heterojunction device architectures have been shown to benefit from the multifunctionality of plasmonic back electrodes as they can minimize space-charge effects and reduce hole carrier collection lengths in addition to providing improved light localization in the active layer. The use of semi-transparent plasmonic electrodes can also be beneficial for organic photovoltaics as they can exhibit a variety of optical properties such as light scattering, light localization, extraordinary transmission of light, and absorption-induced transparency, in addition to providing an alternative to metal oxide–based transparent electrodes.

Both metallic nanoparticles exhibiting plasmonic effects and dielectric nanoparticles coupling the light into resonant modes have shown successful applications to photovoltaics. On a larger scale, microconcentrator optics promise to enhance solar cell efficiency and to reduce material consumption. Here, we want to create a link between the concentrators on the nano- and on the microscale. From metallic nanospheres, we turn to dielectric ones and then look at increasing radii to approach the microscale. The lenses are investigated with respect to their interaction with light using three-dimensional simulations with the finite-element method. Resulting maps of local electric field distributions reveal the focusing behavior of the dielectric spheres. For larger lens sizes, ray tracing calculations, which give ray distributions in agreement with electric field intensities, can be applied. Calculations of back focal lengths in geometrical optics coincide with ray tracing results and allow insight into how the focal length can be tuned as a function of particle size, substrate refractive index, and the shape of the microlens. Despite the similarities we find for the nano- and the microlenses, integration into solar cells needs to be carefully adjusted, depending on the goals of material saving, concentration level, focal distance, and lens size.

We experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. These back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. We vary the period, unit cell, and width of the nanostructures to identify design strategies for optimized light trapping. First, a general correlation between the reduction of the period of the nanostructures down to 550 nm and an increase of the absorptance, as well as external quantum efficiency is found for various unit cells formed by nanostructures. Second, increasing the width of the nanostructures from 200 to 350 nm, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, we identify a design for improved light trapping for the given solar cell parameters within the considered variations. It consists of thin-film solar cells applying a combination of a period of 600 nm and a structure width of 350 nm. The implementation of back contacts with this configuration yields enhanced power conversion efficiency as compared to reference solar cells processed on conventionally used randomly textured substrates. In detail, the enhancement of the short-circuit current density from initially 14.7 to initially 15.6  mA/cm² improves the power conversion efficiency from 9.1 to 9.3%.

The optical and electrical properties of optically thin one-dimensional (1-D) Ag nanogratings and two-dimensional (2-D) Ag nanogrids are studied, and their use as transparent electrodes in organic photovoltaics is explored. A large broadband and polarization-insensitive optical absorption enhancement in the organic light-harvesting layers is theoretically and numerically demonstrated using either single-layer 2-D Ag nanogrids or two perpendicular 1-D Ag nanogratings, and is attributed to the excitation of surface plasmon resonances and plasmonic cavity modes. Total photon absorption enhancements of 150% and 200% are achieved for the optimized single-layer 2-D Ag nanogrids and double (top and bottom) perpendicular 1-D Ag nanogratings, respectively.

Transmission-type concentrator photovoltaic (CPV) systems are a potential candidate to achieve high efficiency and low cost solar energy. The use of optical elements in these systems creates reflection losses of incoming solar energy that account for about 8% to 12% depending on the optical design. In order to reduce these losses, we have nanostructured the air/optical-elements’ interfaces by using plasma etching methods on the Fresnel lens made of poly(methyl methacrylate) (PMMA) and the homogenizer made of glass. On flat PMMA and glass substrates, transmittance enhancement measurements are in agreement with relative J_sc gains. The field test results using a CPV module with all textured optical-elements’ interfaces achieved 8.0% and 4.3% relative J_sc and efficiency gains, respectively, demonstrating the potential of this approach to tackle the reflection losses.

Rapid prototyping of photovoltaic (PV) cells requires a method for the simultaneous simulation of the optical and electrical characteristics of the device. The development of nanomaterial-enabled PV cells only increases the complexity of such simulations. Here, we use a commercial technology computer aided design (TCAD) software, Silvaco Atlas, to design and model plasmonic gold nanoparticles integrated in optoelectronic device models of thin-film amorphous silicon (a-Si:H) PV cells. Upon illumination with incident light, we simulate the optical and electrical properties of the cell simultaneously and use the simulation to produce current–voltage (J−V) and external quantum efficiency plots. Light trapping due to light scattering and localized surface plasmon resonance interactions by the nanoparticles has resulted in the enhancement of both the optical and electrical properties due to the reduction in the recombination rates in the photoactive layer. We show that the device performance of the modeled plasmonic a-Si:H PV cells depends significantly on the position and size of the gold nanoparticles, which leads to improvements either in optical properties only, or in both optical and electrical properties. The model provides a route to optimize the device architecture by simultaneously optimizing the optical and electrical characteristics, which leads to a detailed understanding of plasmonic PV cells from a design perspective and offers an advanced tool for rapid device prototyping.

A spectral fluid filter for potential use in hybrid photovoltaic/thermal concentrating solar collectors has been developed, targeting maximum absorption above and transmission below a desired wavelength. In this application, the temperature-dependent bandgap of the potential solar cell is used in the optimization of the filter. Dispersing a mix of colloidal nanoparticles in a heat transfer fluid is shown to absorb 86% of sub-bandgap insolation while absorbing only 18% above bandgap insolation. Transmission above bandgap light would be directly absorbed into the photovoltaic (PV) cell while absorbed photons transfer energy directly into the heat transfer fluid ultimately reducing the number of heat transfer steps. Placement of a filter in front of the PV cell is shown to decrease losses by converting an additional 2% of the total solar energy into thermal energy since it allows recollection of light reflected off the receiver.

This PDF file contains the editorial “Special Section Guest Editorial: Solution-Processable Organic Solar Cells” for JPE Vol. 5 Issue 01

Conjugated donor–acceptor (D-A) molecular structures play a very important role in the significant progress of organic photovotaics. However, the reports on conjugated D-A porphyrin polymers for organic solar cells are very limited. In this work, five conjugated D-A porphyrin copolymers PEZPEBTA(C₁₂), PEZPEBT, PEZPEBT_ff, PEZPETPD(O), and PEZPETDPPT(O) were synthesized by Sonagashira coupling of a porphyrin donor unit with five typical acceptor units 2-dodecyl-2H-benzotriazole, benzo[1,2,5]thiadiazole, 5,6-difluoro-benzo[1,2,5]thiadiazole, 5-octyl-thieno[3,4-c]pyrrole-4,6-dione, and 3,6-bis-(thiophen-2-yl)-2,5-dioctyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione linked by ethynylene linkages, respectively. They possess excellent thermal stability with a decomposition temperature of around 400°C. All absorption spectra of the copolymers were significantly red shifted with enhanced Q bands at the near-infrared region both in solutions and in films due to the simultaneous introduction of ethynylene linkages and acceptor units, which make the polymer main chains coplanar and π-conjugated and enhance the intramolecular charge transfer. PEZPEBT and PEZPEBT_ff are electrochemically active in both the oxidation and reduction regions, while PEZPEBTA(C₁₂, PEZPETPD(O), and PEZPETDPPT(O) show only oxidation peaks. Power conversion efficiencies of 0.12%, 0.41%, 0.26%, 0.19%, and 0.41% were achieved for the polymer solar cells based on PEZPEBTA(C12), PEZPEBT, PEZPEBTff, PEZPETPD(O), and PEZPETDPPT(O), respectively, under AM 1.5, 100  mW/cm² with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PC₆₁BM) (1:2, w/w) as the active layer in the presence of 3% pyridine.

In ternary bulk heterojunction solar cells based on a semiconducting biphenyl-dithienyldiketopyrrolopyrrole copolymer donor and two different fullerene acceptors that distinctly differ in electron affinity, the open-circuit voltage is found to depend in a slightly sublinear fashion on the relative ratio of the two fullerenes in the blend. Similar effects have previously been observed and have been attributed to the formation of an alloyed fullerene phase possessing electronic levels that are the weighted average of the two components. By analyzing the contribution of the charge transfer (CT)-state absorption to the external quantum efficiency of the ternary blend solar cells as a function of composition, we find no evidence for a CT state formed between the polymer and an alloyed fullerene phase. Rather, the results are consistent with the presence of two distinct CT states, one for each polymer–fullerene combination. The two-state CT model does not, however, explain the sublinear behavior of the open-circuit voltage as a function of the blend composition.

The knowledge of the complex refractive indices of all thin layers in organic solar cells (OSCs) is a prerequisite for comprehensive optical device simulations that are particularly important for sophisticated device architectures, such as tandem OSCs. Therefore, refractive indices are often determined via spectroscopic ellipsometry and subsequent time-consuming modeling. Here, we investigate a modeling approach that allows for the determination of complex refractive indices of bulk-heterojunctions by superimposing the optical models of the respective fullerenes and polymers. The optical constants of neat [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM), poly{[4,4’-bis(2-ethylhexyl)dithieno(3,2-b;2’,3’-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) are determined, covering the OSC relevant spectral region from 250 to 1,000 nm. Then the blends PSBTBT : PC₇₁BM and PCPDTBT : PC₇₁BM are described within an effective medium approximation. From this approximation, the mass density ratio of polymer and fullerene can be derived. This approach furthermore allows for a uniaxial anisotropic optical description of the polymers and provides insight into thin-film morphology. In contrast to x-ray diffraction experiments, this method also allows for probing amorphous materials. Spectroscopic ellipsometry can be a valuable tool for the investigation of bulk-heterojunction morphologies of the latest high-performance OSC materials that exhibit a low degree of crystallinity.

The photochemical stability of the active layer blend for organic solar cells was explored by introducing electron withdrawing cyano groups into the backbone of poly-3-hexylthiophene (P3HT). Random copolymerization of 2-bromo-3-hexyl-5-trimethylstannylthiophene and 2-bromo-3-cyano-5-trimethylstannylthiophene enabled introduction of the cyanogroups along the polythiophene backbone. The percentage of the cyano groups was 10%. The photochemical stability of poly(3-hexylthiophene-co-3-cyanothiophene) (CN-P3HT) was shown to be significantly better than pristine P3HT and the addition of CN-P3HT to P3HT also increased the photochemical stability of the blend. The photochemical stability of bulk heterojunction mixtures of the polymers and their blends with the fullerene phenyl-C61-butyric acid methyl ester ([60]PCBM) were then studied and it was found that [60]PCBM had a significantly more stabilizing effect on P3HT than CN-P3HT and that the stabilization of the bulk heterojunction mixture was dominated by the fullerene. The mixture comprising both fullerene and CN-P3HT, however, demonstrated the highest degree of photochemical stability supporting earlier observations that the stabilizing effects are additive. Finally, the blends were explored in fully printed flexible ITO-free roll coated inverted devices (with an active area of 0.8  cm²) using two different back PEDOT:PSS electrode compositions and the operational stability of the devices was studied under ISOS-L-2 conditions. The pure P3HT:PCBM devices were found to be the most stable in operation demonstrating that photochemical stability alone is not necessarily the dominant factor for overall device stability.

We report the fabrication of cost-effective indium-free polymer solar cells (PSCs) with an inverted structure that incorporates an ultrathin copper (Cu) film as a bottom cathode via thermal evaporation. The average optical transmittance of the 15-nm Cu coated glass substrate in the visible region of the spectrum was found to be around 80% with a highest value of 84.5%. The Cu electrode was modified by an interfacial layer of an alcohol-/water-soluble conjugated polymer, poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) interlayer to ensure a very smooth surface. Upon the use of the PFN interfacial layer, the work function of Cu was decreased from 4.68 to 4.31 eV, which can form an Ohmic contact with photoactive layer and facilitate electrode transport and extraction. As a result, a power conversion efficiency of 3.6% was achieved when poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] and a [6,6]-phenyl C₇₁-butyric acid methyl ester blend were utilized as the photoactive layers, demonstrating that the thermally evaporated Cu thin-film electrode can be a promising candidate to replace indium tin oxide for highly efficient PSCs.

Conjugated polymers are potential materials for photovoltaic applications due to their high absorption coefficient, mechanical flexibility, and solution-based processing for low-cost solar cells. A bulk heterojunction (BHJ) structure made of donor–acceptor composite can lead to high charge transfer and power conversion efficiency. Active layer morphology is a key factor for device performance. Film formation processes (e.g., spray-coating, spin-coating, and dip-coating), post-treatment (e.g., annealing and UV ozone treatment), and use of additives are typically used to engineer the morphology, which optimizes physical properties, such as molecular configuration, miscibility, lateral and vertical phase separation. We will review electronic donor–acceptor interactions in conjugated polymer composites, the effect of processing parameters and morphology on solar cell performance, and charge carrier transport in polymer solar cells. This review provides the basis for selection of different processing conditions for optimized nanomorphology of active layers and reduced bimolecular recombination to enhance open-circuit voltage, short-circuit current density, and fill factor of BHJ solar cells.

We explored relationships between photophysical processes and solar cell characteristics in solution-processable bulk heterojunctions (BHJs), in particular: (1) polymer donor:fullerene acceptor:small-molecule (SM) nonfullerene acceptor, (2) polymer donor:SM donor:SM nonfullerene acceptor, and (3) SM donor:SM nonfullerene or fullerene acceptor. Addition of a nonfullerene SM acceptor to “efficient” polymer:fullerene BHJs led to a reduction in power conversion efficiency (PCE), mostly due to decreased charge photogeneration efficiency and increased disorder. By contrast, addition of an SM donor to “inefficient” polymer:SM nonfullerene acceptor BHJs led to a factor of two to three improvement in the PCE, due to improved charge photogeneration efficiency and transport. In most blends, exciplex formation was observed and correlated with a reduced short-circuit current (J_sc) without negatively impacting the open-circuit voltage (V_oc). A factor of ∼5 higher PCE was observed in SM donor:fullerene acceptor BHJs as compared to SMBHJs with the same SM donor but nonfullerene acceptor, due to enhanced charge carrier photogeneration in the blend with fullerene. Our study revealed that the HOMO and LUMO energies of molecules comprising a blend are not reliable parameters for predicting V_oc of the blend, and an understanding of the photophysics is necessary for interpreting solar cell characteristics and improving the molecular design of BHJs.

In this investigation, photovoltaic (PV) parameters improvements in poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-alt-5,5′-(3,6-bis[4-(2-ethylhexyl)thienyl-2-yl]-s-tetrazine] and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PDTSTTz:PCBM) blend were made through solvent additives and secondary donor addition. Short carbon chain solvent additives such as iodomethane (IMe), iodoethane (IEt), iodobutane (IBu), and diiodomethane (DIMe) were used. The results have shown that the short circuit current density (J_sc), as well as power conversion efficiency (PCE) of PDTSTTz:PCBM blend cell increased with the increase in length of carbon chains of the additives. IBu, with relatively the longest carbon chain, has better performance-improving impact than IMe (with the shortest carbon chain). The same trend was observed for fill factor. The other PV parameter, open circuit voltages (V_oc), did not show significant change following these solvent additives. The effect of a secondary (additional) donor on the PDTSTTz/PCBM system was investigated by adding different proportions of poly(3-hexylthiophene-2,5-diyl) (P3HT). Because P3HT (secondary donor) and PDTSTTz (hosting donor) complement each other in light absorption, the PDTSTTz/PCBM system containing an optimum proportion of P3HT could provide a wider range of light absorption, and as a result it exhibited a higher short circuit current (11.08  mA/cm²) and then a PCE of 2.42%.

Thin film photovoltaics can be beneficial for specific applications like building integrated photovoltaics. To fully exploit the differentiator of form freedom, the interconnections in thin film modules can be tuned depending on the required module output. Traditionally, an alternation of coating and scribing steps is applied, determining the form from the start. Here, we present a set of techniques to define the module design from a master substrate with homogeneously coated electroactive layers. By applying subtractive and additive laser based processes, the size and form of the module are only fixed after the manufacturing of the whole solar cell stack. By laser-induced forward transfer, an isolating dielectric material and a conductive top electrode are deposited in laser ablated scribes to enable the interconnection between two adjacent cells. After optimization of the laser settings for ablation and forward transfer, the optimal annealing time and temperature for the curing of the silver top electrode were determined. The proof of principle was demonstrated by constructing a 4-cell organic solar module of 1.0% efficiency on an area of over 3 cm2 showing the anticipated short-circuit current and open-circuit voltage.

We describe the synthesis and preliminary photovoltaic performance of a solution-processable organic small-molecule electron donor DT-DPP(TVT)₂ that consists of dithienyldiketopyrrolopyrrole (DT-DPP) as the core and thienylvinylthiophene (TVT) as the endgroups. The new compound is a crystalline solid with a T_m of approximately 216°C. Cyclic voltammetry indicates that DT-DPP(TVT)₂ exhibits two quasi-reversible one-electron oxidation waves at ca. 0.68 and 0.90 V versus an Ag/AgCl reference electrode, respectively, leading to an estimated highest occupied molecular orbital (HOMO) level of about −5.08  eV. Introducing the branched 2-hexyldecyl side chain provides DT-DPP(TVT)₂ with a high solubility in chloroform up to ca. 36  mg mL⁻¹ at room temperature. Thermal annealing increases the crystallinity of the as-cast film from chloroform solution, thereby rendering slightly red-shifted charge-transfer absorption maxima. Fitting the space-charge-limited current characteristics of the thermally annealed thin film yields an improved hole mobility of ∼2.14×10⁻⁴  cm²V⁻¹ s⁻¹ at low voltages versus ∼1.46×10⁻⁴  cm² V⁻¹s⁻¹ of the as-cast film. A first characterization of the solar cell [ITO/PEDOT: PSS/DT-DPP(TVT)₂: PC₆₁BM/Al] produces a power conversion efficiency of ∼3% with V_OC≈0.78  V, J_SC≈7.91  mA cm⁻², and FF≈48.7%, under simulated AM1.5G with an illumination intensity of 100  mW cm⁻². It should be noted that the thermal effect on the thin film absorption of DT-DPP(TVT)₂ does not seem to be completely similar to the molecular donor DPP(TFNa)₂ reported earlier, which bears 6-fluoronaphthyl endgroups.

Over the last three decades, progress in the organic photovoltaic field has resulted in some device features which make organic cells applicable in electricity generation configurations where the standard silicon-based technology is not suitable, for instance, when a semi-transparent photovoltaic panel is needed. When the thin film solar cell performance is evaluated in terms of the device’s visible transparency and power conversion efficiency, organic solar cells offer the most promising solution. During the last three years, research in the field has consolidated several approaches for the fabrication of high performance semi-transparent organic solar cells. We have grouped these approaches under three categories: devices where the absorber layer includes near-infrared absorption polymers, devices incorporating one-dimensional photonic crystals, and devices with a metal cavity light trapping configuration. We herein review these approaches.

Two π-conjugated acceptor-donor-acceptor-donor-acceptor-type (A-D-A-D-A) oligothiophenes, TT-(2T-DCV-Hex)₂ and BT-(2T-DCV-Hex)₂ were designed and synthesized with thienothiadiazole (TT) or benzothiadiazole (BT) as the core and dicyanovinyl (DCV) as the terminal acceptor groups for comprehensively investigating and understanding structure–property relationships. The resulting oligomers were first characterized by thermal analysis, UV-Vis spectroscopy, and cyclic voltammetry. By simply changing the BT to TT core in these two oligothiophenes, the highest occupied molecular orbital levels were varied from −5.55  eV for BT-(2T-DCV-Hex)₂ to −5.11  eV for TT-(2T-DCV-Hex)₂, and the optical band gaps were varied from 1.72 eV for BT-(2T-DCV-Hex)₂ to 1.25 eV for TT-(2T-DCV-Hex)₂, ascribed to the stronger electron accepting character of the TT core. However, the power conversion efficiency of bulk heterojunction organic solar cells (OSCs) with TT-(2T-DCV-Hex)₂ as donor and [6,6]-phenyl C₇₀-butyric acid methyl ester (PC₇₁BM) as acceptor was measured to be 0.04% only, which is much lower than that of BT-(2T-DCV-Hex)₂:PC₇₁BM (1.54%). Compared to the TT-(2T-DCV-Hex)₂ system, the BT-(2T-DCV-Hex)₂ based device shows smoother film surface morphology, and superior charge generation and charge carrier mobilities. Therefore, the results clearly demonstrate that in addition to modifying the alkyl side chains and π-bridge lengths, the design of new small molecules for high-performance OSCs should also aim to choose suitable acceptor units.

In spite of the impressive development achieved by organic photovoltaics throughout the last decades, especially in terms of reported power conversion efficiencies, there are still important technological and fundamental obstacles to circumvent before they can be implemented into reliable and long-lasting applications. Regarding device processing, the synthesis of highly soluble polymeric semiconductors first, and then fullerene derivatives, was initially considered as an important breakthrough that would definitely change the fabrication of photovoltaics once and for all. The potential and the expectation raised by this technology is such that it is very difficult to keep track of the most significant progresses being now published in different and even monographic journals. In this paper, we review the development of polymeric solar cells from its origin to the most efficient devices published to date. We separate these achievements into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: active materials, strategies—fabrication/processing procedures—that can mainly modify the active film morphology, and all the different cell layout/architectures that have been used in order to extract as high a photocurrent as possible from the Sun. The synthesis of new donors, the use of additives and postprocessing techniques, buffer interlayers, inverted and tandem designs are some of the most important aspects that are reviewed in detail in this paper. All have equally contributed to develop this technology and bring it at the doors of commercialization.

The synthesis and characterization of three solution processable diketopyrrolopyrrole (DPP) derivatives featuring acceptor units attached to the core by alkyne linker units is reported. Cyclic voltammetry and density functional theory calculations indicate that the DPP derivatives possess similar HOMO and LUMO energies. Solar cells were fabricated by blending the synthesized DPP derivatives with [6,6]-phenyl-C71-butyrate methyl ester. The influence of donor:acceptor blend ratio, film thickness, annealing temperature, and annealing time on device performance was studied. Differences in device performance were related to atomic force microscopy measurements of the films. The highest power conversion efficiency of 1.76% was achieved for the DPP derivative functionalized with an aldehyde electron-withdrawing group with a 1∶0.7 donor:acceptor ratio when the active layer was annealed for 10 min at 110°C.

This PDF file contains the editorial “Special Section Guest Editorial: Hybrid Organic-Inorganic Solar Cells” for JPE Vol. 5 Issue 01

The recent power-packed advent of perovskite solar cells is transforming photovoltaics (PV) with their superior efficiencies, ease of fabrication, and cost. This perovskite solar cell further boasts of many unexplored features that can further enhance its PV properties and lead to it being branded as a successful commercial product. This article provides a detailed insight of the organometal halide based perovskite structure, its unique stoichiometric design, and its underlying principles for PV applications. The compatibility of various PV layers and its fabrication methods is also discussed.

Ultrafast transient absorption spectroscopy is used in conjunction with spectroelectrochemistry and chemical doping experiments to study the photogeneration of charges in hybrid bulk heterojunction (BHJ) thin films composed of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and CdSe nanocrystals. Chemical doping experiments on hybrid and neat PCPDTBT:CdSe thin films are used to deconvolute the spectral signatures of the transient states in the near infrared. We confirm the formation and assignment of oxidized species in chemical doping experiments by comparing the spectral data to that from spectroelectrochemical measurements on hybrid and neat PCPDTBT:CdSe BHJ thin films. The deconvolution procedure allows extraction of the polaron populations in the neat polymer and hybrid thin films.

In the current study, the perovskite absorber (CH₃NH₃PbI₃) is processed via one-step deposition employing the small molecule additive, BmPyPhB, which can be dissolved in dimethylformamide along with precursors. Here, 1,3-Bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB) functions as the morphology controller to introduce an intermediate phase during perovskite film growth, which allows well-defined and precrystallized domains formed before the annealing treatment. Furthermore, a chloroform solvent wash procedure is applied afterward to remove BmPyPhB from perovskite without damaging the predetermined morphology. Thus, postannealing as low as 100°C for 5 min can achieve the optimal power conversion efficiency of 8% in a planar-structured inverted solar cell.

We herein demonstrate the viability of utilizing the hole transporting medium of solid-state dye-sensitized solar cells for light harvesting. When using a hole transporting dye (HTD) in addition to an interface dye (ID) bound to the surface of the mesoporous metal oxide scaffold, both are shown to contribute to the photocurrent. Efficient energy transfer (ET) from the HTD to the ID was accomplished by spectrally matching two triphenylamine dyes. The photoluminescence of the HTD was found to be quenched in the presence of the ID. In nanosecond transient absorption measurements, rapid formation of the oxidized HTD was observed after photoexcitation of the ID, demonstrating fast regeneration of the oxidized ID by the HTD. In solar cell devices comprising both the ID and HTD, the spectral response of the external quantum efficiency shows that both dyes contribute to the photocurrent, resulting in a doubling of the photocurrent. In comparison with devices comprising only TiO₂ and the HTD, devices with the additional ID exhibited an increased photovoltage due to more efficient charge-carrier separation and energy transfer. Combining and matching HTDs with IDs for optimal ID regeneration but also providing ET is thus a viable means to optimize hybrid solar cells based on mesoporous TiO₂.

We investigate the effect of TiO₂ nanoparticles–nanospheres and nanorods–inserted in the poly(3-hexylthiophene) (P3HT) matrix of TiO₂/P3HT inverted hybrid solar cells. X-ray diffraction, high-resolution transmission electron microscopy, small-angle x-ray scattering, photoluminescence, and photoelectrochemical experiments were employed to investigate the structure, morphology, and photoactivity of TiO₂ nanoparticles modified with 2-thiopheneacetic acid, mixed or not with P3HT. Both TiO₂ nanospheres and TiO₂ nanorods presented a good dispersion in the polymer matrix. The incorporation of TiO₂ nanospheres and nanorods has improved the photocurrent generation, and devices with efficiency values up to 1.35% were obtained. Our results reveal that the nanoscale morphology enables an enhanced interfacial area for exciton dissociation. In particular, the nanospheres contribute with their high specific area, and the nanorods contribute with their high aspect ratio.

In this study, we investigated inverted hybrid bulk-heterojunction solar cells with the following configuration: fluorine-doped tin oxide (FTO) |TiO₂/RGO|P3HT:PC₆₁BM|V₂O₅ or PEDOT:PSS|Ag. The TiO₂/GO dispersions were prepared by sol-gel method, employing titanium isopropoxide and graphene oxide (GO) as starting materials. The GO concentration was varied from 0.1 to 4.0 wt%. The corresponding dispersions were spin-coated onto FTO substrates and a thermal treatment was performed to remove organic materials and to reduce GO to reduced graphene oxide (RGO). The TiO₂/RGO films were characterized by x-ray diffraction, Raman spectroscopy, and microscopy techniques. Atomic force microscopy (AFM) images showed that the addition of RGO significantly changes the morphology of the TiO₂ films, with loss of uniformity and increase in surface roughness. Independent of the use of V₂O₅ or PEDOT: PSS films as the hole transport layer, the incorporation of 2.0 wt% of RGO into TiO₂ films was the optimal concentration for the best organic photovoltaic performance. The solar cells based on TiO₂/RGO (2.0 wt%) electrode exhibited a ∼22.3% and ∼28.9% short circuit current density (J_sc) and a power conversion efficiency enhancement, respectively, if compared with the devices based on pure TiO₂ films. Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO₂ films can promote the appearance of regions with different charge dissipation capacities.

We report photovoltaic devices based on composites of a branched nanoheterostructure containing a CdTe core and CdSe arms, CdTe(c)-CdSe(a), combined with either poly(3-hexylthiophene), P3HT, or poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, with solar power conversion efficiencies of 1.2% and 1.8%, respectively. A comparison with previously reported composite devices of a related branched nanoheterostructure: CdSe(c)-CdTe(a) reveals an improved device performance that is attributed to a better electron percolation pathway provided by the dominant, higher electron affinity CdSe arms of the nanoheterostructures.

Adsorption of PbI₂ onto a titania layer was monitored by a quartz crystal microbalance system in solution. The amount of PbI₂ adsorbed on the titania layer increased with an increase in the PbI₂ concentration in dimethylformamide (DMF). However, PbI₂ remained after being rinsed with DMF, suggesting that PbI₂ is rigidly bonded to the surface of the titania. The x-ray photoelectron spectroscopy measurement of PbI₂ adsorbed on the titania substrate showed that the Pb compound has a composition of PbI_0.33, not PbI₂, suggesting that part of the Pb-I reacts with the HO-Ti moieties of titania to form Pb-O-Ti linkages. Trap density as measured by the thermally stimulated current method decreased after PbI₂ passivation. Perovskite solar cells consisting of porous titania passivated with PbI₂ had a higher efficiency than those without the passivation. It was concluded that PbI₂ passivation of porous titania surfaces is one of the crucial approaches for enhancing the efficiency of perovskite solar cells with a scaffold layer of porous titania.

This PDF file contains the editorial “Special Section Guest Editorial:Solid-State Lighting: Photonics and Technologies” for JPE Vol. 5 Issue 01

The study of thermal properties for GaN-based light-emitting diodes (LEDs) with various last barrier thicknesses was reported. It was found that the LED output power decreased as we increased the thickness of the last barrier. It was also found that the LED output powers decrease with the increase of temperature for the LEDs with 12-, 24-, and 48-nm-thick last barriers. However, it was found that the LED output power increases with the increase of temperature for the LED with a 72-nm-thick last barrier due to the fact that more holes could enter the multiquantum well active region at elevated temperatures. Furthermore, it was found that the output power decreased by 0.15%/°C, 0.15%/°C, and 0.11%/°C for the LEDs with 12-, 24-, and 48-nm-thick last barriers, respectively, but increased by 0.09%/°C for the LED with a 72-nm-thick last barrier.

This paper presents a “hybrid” structure for the coating of yellow YAG : Ce3⁺ phosphor on blue GaN-based light-emitting diodes (LEDs). The luminous efficiency of the hybrid phosphor structure improved by 5.9% and 11.7%, compared with the conventional remote and conformal phosphor structures, respectively, because of the increased intensity of the yellow component. The hybrid structure also has an advantage in the phosphor usage reduction for the LEDs. Furthermore, the electric intensity of the hybrid phosphor structure was calculated for various thicknesses by conducting TFCalc32 simulation, and the enhanced utilization of blue rays was verified. Finally, the experimental results were consistent with the simulation results performed using the Monte-Carlo method.

Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an Al_xGa₁−_xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to <7×10⁸  cm⁻², yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE.

We report the effects of microcell layout on the performances of GaN-based high-voltage light-emitting diodes (HV-LEDs). Compared with samples with an S-type layout pattern, it was found that the samples with an I-type layout pattern exhibit smaller forward voltage, larger light output power, lower thermal temperature, and better wall-plug efficiency (WPE). It was also found that we could further improve the performances of HV-LED chips by introducing extra metal fingers to enhance current spreading. Compared with the S-type sample without metal fingers, we could reduce the efficiency droop from 38.6% to 14.8% by using an I-type sample with metal fingers. Furthermore, it was found that WPE reduced by around 40% after a 1000 h aging test for the S-type sample without metal fingers. In contrast, almost no decrease in WPE could be observed from the I-type sample with metal fingers after the same aging time.

This article presents a chip-scale package (CSP) with conformal and uniform structures for white light-emitting diodes used in lighting and backlight unit (BLU) applications. The CSP structures produce higher light extraction efficiency and lower assembly-dependent packaging compared with conventional surface-mounted devices (SMDs). Simulation results show that compared with SMDs, the luminous efficiency of CSP structures is 8.81% higher in lighting applications and 9.43% higher in BLU applications. This is likely due to light loss in the light bowl of the SMDs. Moreover, CSPs with a conformal phosphor structure exhibit low assembly dependence and redundancy, and rb-CSPs with a conformal structure are a more effective light source in both lighting and BLU applications.

Organic light-emitting diodes (OLEDs) have been successfully introduced to the smartphone display market and have geared up to become contenders for applications in general illumination where they promise to combine efficient generation of white light with excellent color quality, glare-free illumination, and highly attractive designs. Device efficiency is the key requirement for such white OLEDs, not only from a sustainability perspective, but also because at the high brightness required for general illumination, losses lead to heating and may, thus, cause rapid device degradation. The efficiency of white OLEDs increased tremendously over the past two decades, and internal charge-to-photon conversion can now be achieved at ∼100% yield. However, the extraction of photons remains rather inefficient (typically <30%). Here, we provide an introduction to the underlying physics of outcoupling in white OLEDs and review recent progress toward making light extraction more efficient. We describe how structures that scatter, refract, or diffract light can be attached to the outside of white OLEDs (external outcoupling) or can be integrated close to the active layers of the device (internal outcoupling). Moreover, the prospects of using top-emitting metal–metal microcavity designs for white OLEDs and of tuning the average orientation of the emissive molecules within the OLED are discussed.

Organic light-emitting diodes (OLEDs) have attracted considerable attention in both academic and industrial circles. Certain properties of OLEDs make them especially attractive in the lighting market, including area emission characteristics not found in other existing light sources, environmentally friendly efficient use of energy, large area, ultra-light weight, and ultra-thin shape. Fluorescent and phosphorescent materials that are being applied to white OLEDs have been categorized, and the chemical structures and device performances of the important blue, orange, and red light-emitting materials have been summarized. Such a systematic classification and understanding of the materials that have already been reported can aid the development and study of new light-emitting materials through quantitative and qualitative approaches.

This paper reviews the principles, impediments, and recent progress in the development of ultrathin flexible Ag electrodes for use in flexible optoelectronic devices. Thin Ag-based electrodes are promising candidates for next-generation flexible transparent electrodes. Thin Ag-based electrodes that have a microcavity structure show the best device performance, but have relatively low optical transmittance (OT) due to reflection and absorption of photons by the thin Ag; this trait causes problems such as spectral narrowing and change of emission color with viewing angle in white organic light-emitting diodes. Thinning the Ag electrode to <−10  nm thickness (ultrathin Ag) is an approach to overcome these problems. This ultrathin Ag electrode has a high OT, while providing comparable sheet resistance similar to indium tin oxide. As the OT of the electrode increases, the cavity is weakened, so the spectral width of the emission and the angular color stability are increased.

This study demonstrates an analytical relation that models the carrier injection in metal-organic interfaces and which considers two consecutive carriers hopping as the injection mechanism. The new formula has superior attributes and can surpass conventional relations, in particular the thermionic emission-diffusion formula. For example, the model can properly trace the temperature dependency of the injection up to temperatures as low as 30 K and the full range of electric fields. Also, the prominence of joule heating for proper modeling of the injection is presented. This study examines the validity of the introduced analytical equation by exploring the injection in several practical contacts extracted from the literature, the results of which are discussed in this paper.

Organic light-emitting diodes (OLEDs) have become a promising candidate for lighting and display applications. High efficiency OLEDs require a multilayer device architecture to provide exciton confinement and balance charge transport. Conventional OLEDs are made by vacuum process, and the manufacturing cost can be reduced by solution processing. However, unlike vacuum-deposited OLEDs, solution-processed multilayer OLEDs are more challenging to make. The key for multilayer solution processing is to have the layer structure which can withstand solvents used in subsequent processing. We review the materials’ strategies to make multilayer solution-processed OLEDs. Specifically, we will discuss the use of cross-linkable organic materials, metal oxides, and orthogonal solvent systems to deposit various functional layers in an OLED.

Optical and electrical properties of gallium nitride (GaN)-based blue light-emitting diodes (LEDs) with various indium gallium nitride (InGaN) quantum well (QW) thicknesses were investigated. As the QW thickness was increased, the light output power of GaN-based LEDs also increased. The increase can be attributed to the increase in the carrier radiative recombination rate in the active region. However, the turn-on voltages of these fabricated LEDs are different. This was attributed to the increase in the polarization field with increasing QW thickness. In regard to the hot/cold factor, LEDs with a thicker QW achieved better performance at a low-injection current owing to the lower defect density. The hot/cold factor at a high-injection current would be mainly influenced by the efficiency droop mechanism.

This PDF file contains the errata for “JPE Vol. 5 Issue 01 Paper JPE-2015-1214-ERR” for JPE Vol. 5 Issue 01

This PDF file contains the errata for “JPE Vol. 5 Issue 01 Paper JPE-2015-0210-ERR” for JPE Vol. 5 Issue 01