Charge collection and pore filling in solid-state dye-sensitized solar cells

The solar to electrical power conversion efficiency for dye-sensitized solar cells (DSCs) incorporating a solid-state organic hole-transporter can be over 5%. However, this is for devices significantly thinner than the optical depth of the active composites and by comparison to the liquid electrolyte based DSCs, which exhibit efficiencies in excess of 10%, more than doubling of this efficiency is clearly attainable if all the steps in the photovoltaic process can be optimized. Two issues are currently being addressed by the field. The first aims at enhancing the electron diffusion length by either reducing the charge recombination or enhancing the charge transport rates. This should enable a larger fraction of photogenerated charges to be collected. The second, though less actively investigated, aims to improve the physical composite formation, which in this instance is the infiltration of mesoporous TiO(2) with the organic hole-transporter 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (spiro-MeOTAD). Here, we perform a broad experimental study to elucidate the limiting factors to the solar cell performance. We first investigate the charge transport and recombination in the solid-state dye-sensitized solar cell under realistic working conditions via small perturbation photovoltage and photocurrent decay measurements. From these measurements we deduce that the electron diffusion length near short-circuit is as long as 20?μm. However, at applied biases approaching open-circuit potential under realistic solar conditions, the diffusion length becomes comparable with the film thickness, ～2?μm, illustrating that real losses to open-circuit voltage, fill factor and hence efficiency are occurring due to ineffective charge collection. The long diffusion length near short-circuit, on the other hand, illustrates that another process, separate from ineffective charge collection, is rendering the solar cell less than ideal. We investigate the process of TiO(2) mesopore infiltration with spiro-MeOTAD by examining the cross-sectional images of and performing photo-induced absorption spectroscopy on devices with a range of thickness, infiltrated with spiro-MeOTAD with a range of concentrations. We present our interpretation of the mechanism for material infiltration, and by improving the casting conditions demonstrate efficient charge collection through devices of over 7?μm in thickness. This investigation represents an improvement in our understanding of the limiting factors to the dye-sensitized solar cell. However, much work, focused on composite formation and improved kinetic competition, is required to realize the true potential of this concept.     

Modeling photocurrent transients in organic solar cells

We investigate the transient photocurrents of organic photovoltaic devices in response to a sharp turn-on of illumination, by numerical modeling of the drift-diffusion equations. We show that the photocurrent turn-on dynamics are determined not only by the transport dynamics of free charges, but also by the time required for the population of geminate charge pairs to reach its steady-state value. The dissociation probability of a geminate charge pair is found to be a key parameter in determining the device performance, not only by controlling the efficiency at low intensities, but also in determining the fate of charge pairs formed by bimolecular recombination at high intensities. Bimolecular recombination is shown to reduce the turn-on time at high intensities, since the typical distance traveled by a charge pair is reduced.     

Hydrothermal synthesis of cdS nanoparticle-decorated TiO2 nanobelts for solar cell

We fabricated a solar cell using a hybrid film consisting of CdS nanoparticle-decorated TiO2 nanobelts, which were synthesized via a modified alkaline hydrothermal method. The hybrid film is flexible and contains homogeneous CdS nanoparticle light absorbers. Furthermore, the type II heterostructure of CdS/TiO2 facilitates charge separation in the CdS nanoparticle-decorated TiO2 nanobelts. The solar cell demonstrated a light-electricity power conversion efficiency of 2.52%. Next, we deposited the CdS nanoparticle-decorated TiO2 nanobelts onto a ZnO nanowire array forming an antireflective hybrid structure. The power conversion efficiency of the cell with the hybrid photoanode reached 2.84%.     

Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells

Plastic solar cells bear the potential for large‐scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer:fullerene BHJ solar cells, with respect to the charge‐carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build‐up of the space–charge that will significantly reduce the power conversion efficiency. Dissociation of electron–hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open‐circuit voltage (V_OC) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in V_OC of polymer:fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short‐circuit current increase simultaneously.     

Efficiency improvement of organic solar cells by tuning hole transport layer with germanium oxide

Improving optical property is critical for optimizing the power conversion efficiency of organic solar cells. In the present research, we show that modification of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer with GeO2 leads to 15% improvement of power conversion efficiency in a polymer solar cells through enhancement of short circuit currents. Modified PEDOT:PSS layer with optimized concentration of GeO2 assists active layer absorbing much light by playing a role of optical spacer. Using AFM and grazing incidence X-ray diffraction (GIXD) data, we also present the evidence that an addition of GeO2 does not affect crystallinity of active layer.     

An organic D-π-A dye for record efficiency solid-state sensitized heterojunction solar cells

The high molar absorption coefficient organic D-π-A dye C220 exhibits more than 6% certified electric power conversion efficiency at AM 1.5G solar irradiation (100 mW cm(-2)) in a solid-state dye-sensitized solar cell using 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. This contributes to a new record (6.08% by NREL) for this type of sensitized heterojunction photovoltaic device. Efficient charge generation is proved by incident photon-to-current conversion efficiency spectra. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.     

Charge photogeneration and recombination in ternary polymer solar cells based on compatible acceptors

The strategy of ternary blend is one of the most important approaches to enhance the power conversion efficiency (PCE) for polymer solar cells (PSCs). The present work prepared a new series of ternary PSCs, PBDB-T-2F:IT-4F:ITIC, based on the perspective of compatibility of non-fullerene molecule (ITIC) and its derivative (IT-4F) and studied the effect of the third component (ITIC) on the morphology and photoelectric conversion process of PBDB-T-2F:IT-4F solar cells. The results showed that surface roughness and phase scale of the PBDB-T-2F:IT-4F changed with the ITIC content, leading to variable morphology, optical, photophysical and electrical characteristics in ternary blend active layers. When the content of ITIC reached 20%, the ternary PSCs achieved the highest PCE (13.41%) among the PSCs. Morphology and optical spectroscopy studies showed that the enhanced performance of ternary device can be attributed to the morphology optimization of ternary active layer with the content of ITIC, and consequently the improved charge photogeneration processes. This work provided another way to achieve the high-performance ternary polymer solar cells via the combination of compatible non-fullerene molecules.

     

Efficiently harvesting excitons from electronic type-controlled semiconducting carbon nanotube films

We have employed thin films of highly purified semiconducting carbon nanotubes as near-infrared optical absorbers in heterojunction photovoltaic and photodetector devices with the electron acceptor C(60). In comparison with previous implementations of more electrically heterogeneous carbon nanotube/C(60) devices, we have realized a 10× gain in zero-bias quantum efficiency (QE) and even more substantial gains in power conversion efficiency (η(p)). The semiconducting nanotube/C(60) heterojunctions are highly rectifying with a peak external QE, internal QE, and η(p) of 12.9 ± 1.3, 91 ± 22, and 0.6%, respectively, in the near-infrared. We show that the device efficiency is determined by the effective length scale for exciton migration in the nanotube films, confirm the high internal QE via photoluminescence quenching, and demonstrate that the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized for diameters <1.0 nm. These results will guide the development of next-generation high-performance carbon nanotube-based solar cells and photosensitive devices.     

11.2% All‐Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

All‐polymer solar cells (all‐PSCs) that contain both p‐type and n‐type polymeric materials blended together as light‐absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of small molecular‐based organic solar cells. In this work, the interfacial stability is studied by using highly stable all‐polymer solar cell as a platform. It is found that the thermally deposited metal electrode atoms can diffuse into the active layer during device storage, which consequently greatly decreases the power conversion efficiency. Fortunately, the diffusion of metal atoms can be slowed down and even blocked by using thicker interlayer materials, high‐glass‐transition‐temperature interlayer materials, or a tandem device structure. Learning from this, homojunction tandem all‐PSCs are successfully developed that simultaneously exhibit a record power conversion efficiency over 11% and remarkable stability with efficiency retaining 93% of the initial value after thermally aging at 80 °C for 1000 h.     

One‐Step Fabrication of Unique Mesoporous NiO Hollow Sphere Film on FTO for High‐Performance P‐Type Dye‐Sensitized Solar Cells

P‐type dye sensitized solar cells (p‐DSCs) deliver much lower overall efficiency than their inverse model, n‐DSCs. However, they have fundamental and practical significance, in particular, their tandem structured solar cells with both p‐ and n‐photoelectrodes could offer great potential to significantly improve the efficiency of existing solar cells. A facile and environmentally friendly method is developed to directly one‐step grow hollow NiO spherical structures on fluorine‐doped tin oxide (FTO) substrate, in which a Ni²⁺ and polymer complex spherical structure is self‐constructed through a controlled solvent evaporation process, followed by calcination‐converting to a unique NiO hollow sphere film. The prepared material is further used as a photocathode in p‐type dye sensitized solar cells, resulting in 41% increase of an open‐circuit voltage and 18% enhancement of power conversion efficiency than NiO nanoparticles photocathode. The improved performance can be ascribed to suppressed charge recombination at the photocathode/electrolyte interface. This template‐free approach could be universally used to fabricate other nanostructured hollow spheres for a wide range of energy conversion applications such as electrochemical capacitors, chemical sensors, and electrochromic devices.     

Singlet exciton fission-sensitized infrared quantum dot solar cells

We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.     

Intertwined aligned carbon nanotube fiber based dye-sensitized solar cells

Metal wires suffer from corrosion in fiber-shaped dye-sensitized solar cells (DSSCs). We report herein that stable, ultrastrong, and highly flexible aligned carbon nanotube fibers can be used not only as catalytic counter electrodes but also as conductive materials to support dye-loaded TiO(2) nanoparticles in DSSCs. The power conversion efficiency of this fiber solar cell can achieve 2.94%. These solar power fibers, exhibiting power conversion efficiency independent of incident light angle and cell length, can be woven into textiles via a convenient weaving technology.     

Mechanism of recombination losses in bulk heterojunction P3HT:PCBM solar cells studied using intensity modulated photocurrent spectroscopy

Intensity modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies were used to study the mechanism of photoprocesses in P3HT:PCBM bulk heterojunction organic solar cells at various light intensities. The use of the frequency domain techniques allowed us to separate the bulk and interfacial processes and gain a valuable insight into the mechanism of losses in these devices. The results provide direct evidence that interfacial nongeminate recombination is one of the dominant loss and aging mechanisms in bulk heterojunction organic solar cells. The trapping of photoexcited holes in the P3HT phase was found to contribute to the increased recombination rate. The results suggest that promising ways of improving the efficiency of bulk heterojunction solar cells may be reducing the charge trapping both at and near the P3HT:PCBM interface, as well as improving the efficiency of charge extraction at contacts.     

聚光太阳能集热场先进技术综述

聚光太阳能热发电技术因其稳定性、可控性,以及高装机容量成为太阳能热利用方式的重要形式。太阳能集热场的集热效率是影响聚光太阳能热电站发电容量和光电效率的重要因素。针对近期太阳能集热场提高光热转化性能的关键技术,从光学结构、集热器结构以及流体工质3个方面进行综述,总结了先进的集热器优化策略和性能提升技术,并指出相应优化方法的局限性,在此基础上对集热场技术未来的发展提出展望。     

Enhanced performance of P3HT/TiO2 bilayer heterojunction photovoltaic device having gold nanoparticles in the donor layer

The photovoltaic effects of blending gold nanoparticles (AuNPs) into the donor layer of a poly(3-hexylthiophene) (P3HT)/TiO2 bilayer heterojunction device have been studied. P3HT was synthesized via the modified Gragnard metathesis method and AuNPs with sizes ranging from 12 to 15 nm were formed via a reduction of HAuCl4. The blending of AuNPs into P3HT caused a lower photoluminescence (PL) intensities and a decreased energy level of the highest occupied molecular orbital (HOMO) than the pristine P3HT owing to the good electron-accepting nature of AuNPs. Upon the use of P3HT-AuNPs as the donor layer, the decreased HOMO(donor) resulted in an increased open circuit voltage (V(OC)) and thus enabled the fabricated (P3HT-AuNPs)/TiO2 bilayer heterojunction photovoltaic device to have an improved power conversion efficiency of solar energy. V(OC) as well as the overall power conversion efficiency increased with an increase in the AuNP content as a result of additional interfaces which facilitated the charge separation of excitons and percolation pathways which enhanced the electron transfer to the TiO2 acceptor. Furthermore, unannealed P3HT-AuNPs exhibited nanoholes and provided photovoltaic devices a power conversion efficiency nearly two time higher than annealed P3HT-AuNPs.     

Quantifying Polaron Formation and Charge Carrier Cooling in Lead‐Iodide Perovskites

Notwithstanding the success of lead‐halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical phenomena in this material remain debated. Here, the initial steps following photogeneration of free charge carriers in lead‐iodide perovskites are studied, and timescales of charge carrier cooling and polaron formation, as a function of temperature and charge carrier excess energy, are quantified. It is found, using terahertz time‐domain spectroscopy (THz‐TDS), that the observed femtosecond rise in the photoconductivity can be described very well using a simple model of sequential charge carrier cooling and polaron formation. For excitation above the bandgap, the carrier cooling time depends on the charge carrier excess energy and lattice temperature, with cooling rates varying between 1 and 6 meV fs^?1, depending on the cation. While carrier cooling depends on the cation, polaron formation occurs within ≈400 fs in CH₃NH₃PbI₃ (MAPbI₃), CH(NH₂)₂PbI₃ (FAPbI₃), and CsPbI₃. Its formation time is independent of temperature between 160 and 295 K. The very similar polaron formation dynamics observed for the three perovskites points to the critical role of the inorganic lattice, rather than the cations, for polaron formation.     

Solar Cells,Photodetectors, and Optical Sources from Infrared Colloidal Quantum Dots

Optoelectronic devices made via spin‐coating of soft materials onto an arbitrary substrate enable ready integration, low cost, and physical flexibility. The use of solution‐processed colloidal quantum dots offers the added advantage of quantum‐size‐effect tuning of material bandgap. Tuning across the near‐ and short‐wavelength infrared (SWIR) spectral regions enables applications in fiber‐optic communications, night vision and biomedical imaging, and efficient solar energy collection. Here we review progress in infrared solar cells, light sensors, and optical sources based on solution‐processed materials. The latest solution‐processed photovoltaics now provide 4.2% power conversion efficiencies in the infrared, placing them a factor of three away from enabling a doubling in overall solar power conversion efficiency of visible‐wavelength solution‐processed photovoltaics. The best solution‐processed photodetectors now provide sensitivities of 10¹³ Jones D* (normalized detectivity), exceeding the sensitivity of the best epitaxially grown SWIR photodetectors. Infrared optical sources, both broadband light‐emitting diodes and, more recently, lasers, have now also been reported at 1.5 µm.     

Influence of thermal annealing on the charge generation and transport in PM6-based non-fullerene solar cells

To study the influence of thermal annealing on the charge generation and transport in PM6-based non-fullerene solar cells. Morphology, optical and electrochemical properties of active layers as well as electrical properties of polymer solar cells were studied. Furthermore, the photoelectric conversion processes of annealed and unannealed devices were also examined by means of time resolved spectroscopy. The results showed that thermal annealing had a weak influence on the dynamics of exciton states. Besides, annealed device is found to suppress bimolecular recombination owing to its higher charge carrier mobility in ordered donor and acceptor aggregation phases, which led to higher photocurrent and power conversion efficiency than unannealed photovoltaic device.

     

Fundamental performance of novel power supply for HTS magnet using solar energy system

A technical fusion is an important option to establish renewal development in the mutual fields. We have proposed a novel superconducting power supply that is combined with superconducting power supply and solar energy system. An eternal electric energy can be converted by solar energy system, which contains solar panel, photovoltaic (PV) controller and energy-storing battery, can be utilized in the utility power of superconducting power applications. The novel power supply could operate without external utility power to charge the HTS load magnet due to the solar energy. We can improve the operating efficiency and install it in remote locations where utility power is not available. In this paper, as a first step of this work, we showed the possibility of technical fusion between a superconducting power supply and a solar energy system.     

Influence of chemically p-type doped active organic semiconductor on the film thickness versus performance trend in cyanine/C60 bilayer solar cells

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C₆₀ bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.