Sol-Gel Synthesis of TiO2 with Pectin and Their Efficiency in Solar Cells Sensitized by Quantum Dots.

In this study, titanium oxide TiO2 nanoparticles were produced using the sol-gel approach of green synthesis with pectin as the reducing agent. The synthetized TiO2 nanoparticles with pectin were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), visible light absorption (UV-Vis) and the BET method. The structure and morphology of the TiO2 powder were described with SEM, revealing uniform monodisperse grains with a distribution of 80% regarding sizes < 250 nm; the resulting crystal phase of synthetized TiO2 was identified as an anatase and rutile phase with a crystallinity size estimated between 27 and 40 nm. Also, the surface area was determined by nitrogen adsorption-desorption using the Brown-Emmet-Teller method, with a surface area calculated as 19.56 m2/g, typical of an IV type isotherm, indicating mesoporous NPs. UV-Vis spectra showed that sol-gel synthesis reduced the band gap from the 3.2 eV common value to 2.22 eV after estimating the optical band gap energy using the adsorption coefficient; this translates to a possible extended photo response to the visible region, improving photoactivity. In addition, the power conversion of the photoelectrode was compared based on similar assembly techniques of TiO2 electrode deposition. Quantum dot crystals were deposited ionically on the electrode surface, as two different paste formulations based on a pectin emulsifier were studied for layer deposition. The results confirm that the TiO2 paste with TiO2-synthesized powder maintained good connections between the nanocrystalline mesoporous grains and the deposited layers, with an efficiency of 1.23% with the transparent paste and 2.27% with the opaque paste. These results suggest that pectin could be used as a low-cost, functional sol-gel catalysis agent for the synthesis of controlled NPs of metal oxide. It demonstrates interesting optical properties, such as an increase in photo response, suggesting further applications to photocatalysts and biomedical features.

Copper complex molecules as dye-sensitizers: Hybrid MetaGGA and standard + van der Waals functionals.

This study focuses on the use of Density Functional Theory calculations with two main approaches: computational chemistry and computational physics. The following three cases were considered for the derivation: (I) computational chemistry using the M06 hybrid functional, (II) computational chemistry using the standard PBE functional including vdW interactions, and (III) computational physics using the standard PBE functional including vdW interactions and periodic boundary conditions. Since the approximation using hybrid functionals M06 has been extensively validated, this method was used as a reference. The second and third methods are less expensive, it is ideal for use to extend large systems. From the sensitized molecules are found in the gas phase and include solvent effects through the integral equation formalism polarizable continuum model. In a systematic analysis of 15 Cu complex molecules, a complete characterization for DSSCs has been carried out and molecular geometry, electronic and optical measurements have been reported.

Enhancing c-Si Solar Cell Efficiency in the UV Region: Photophysical Insights into the Use of Eu3+ Complexes for Down-Shifting Layer Applications.

[Eu(3DPIQC)3] (where DPIQC = 3-(diphenyl phosphoryl)-1-isoquinolinecarboxylate), a luminescent europium complex with antenna ligands, has been carefully embedded within a polyvinyl butyral (PVB) matrix and the resulting material was used to prepare films used as luminescent down-shifting layers (LDSLs) for crystalline Si-based solar cells. The films were characterized using photoluminescence spectroscopy, atomic force spectroscopy (AFM), UV-Vis spectroscopy, and fluorescence microscopy. The AFM analysis shows films with low surface roughness, while fluorescence microscopy revealed that the Eu complex embedded in PVB assumed a spheroidal configuration, a morphology especially beneficial for optical applications. The so-obtained LDSLs were utilized as energy converters in c-Si solar cells to enhance the utilization of high-energy photons, thereby improving their overall efficiency. The determination of photovoltaic parameters carried out before and after the deposition of the LDSLs on the c-Si cells confirms a positive effect on the efficiency of the cell. The Jsc increases from 121.6 mA/cm2 to 124.9 mA/cm2, and the open circuit voltage (Voc) is found to be unrelated to the complex concentration in the films. The fill factor (FF) remains constant with the Eu concentration. The EQE curves indicate an enhancement in the performance of the photovoltaic cells within the UV region of the spectrum for all coated devices. Electrochemical impedance spectroscopy (EIS) was also carried out in order to analyze the effect of the Eu complex in the charge transfer process of the devices.

Analytical Modeling and Optimization of Cu2ZnSn(S,Se)4 Solar Cells with the Use of Quantum Wells under the Radiative Limit.

In this work, we present a theoretical study on the use of Cu2ZnSn(S,Se)4 quantum wells in Cu2ZnSnS4 solar cells to enhance device efficiency. The role of different well thickness, number, and S/(S + Se) composition values is evaluated. The physical mechanisms governing the optoelectronic parameters are analyzed. The behavior of solar cells based on Cu2ZnSn(S,Se)4 without quantum wells is also considered for comparison. Cu2ZnSn(S,Se)4 quantum wells with a thickness lower than 50 nm present the formation of discretized eigenstates which play a fundamental role in absorption and recombination processes. Results show that well thickness plays a more important role than well number. We found that the use of wells with thicknesses higher than 20 nm allow for better efficiencies than those obtained for a device without nanostructures. A record efficiency of 37.5% is achieved when 36 wells with a width of 50 nm are used, considering an S/(S + Se) well compositional ratio of 0.25.

Study of the Electronic Interaction between NiO and Short Polythiophene Chains towards Solar Photon Harvesting.

The sustainable production of energy is a field of interest to which a new requirement is now imposed: the need to be respectful of the environment. New materials and techniques are being developed, but environmental concerns impose the necessity of keeping research active towards the development of green energy. For this reason, we present the study of short polythiophene (PTh) chains (three and five monomers) and their interaction with nickel oxide, looking for properties related to solar photon harvesting in order to produce electricity. The models of the molecules were developed, and the calculations were performed with an M11-L meta-GGA functional, specially developed for electronic structure calculations. The theoretical explorations demonstrated that the geometry of the PTh molecules suffer little distortion when interacting with the NiO molecule. The calculated value of Eg lies between 2.500 and 0.412 eV for a three-ring PTh chain and between 1.944 and 0.556 eV for a five-ring PTh chain. The chemical parameters indicated that, depending on the geometry of the system, the chemical potential varies from 81.27 to 102.38 kcal/mol and the highest amount of electronic charge varies from -2.94 to 21.56 a.u. for three-monomer systems. For five-monomer systems, the values lie within similar ranges as those of the three-monomer systems. The Partial Density of States (PDOS) showed that the valence and conduction electronic bands were composed of states in the NiO and PTh rings, except for a system where there was a non-bonding interaction.

Towards a CdTe Solar Cell Efficiency Promotion: The Role of ZnO:Al and CuSCN Nanolayers.

A numerical simulation is a valuable tool since it allows the optimization of both time and the cost of experimental processes for time optimization and the cost of experimental processes. In addition, it will enable the interpretation of developed measurements in complex structures, the design and optimization of solar cells, and the prediction of the optimal parameters that contribute to manufacturing a device with the best performance. In this sense, a detailed simulation study was carried out in this work by the Solar Cell Capacitance Simulator (SCAPS). In particular, we evaluate the influence of absorber and buffer thickness, absorber defect density, work function in back contact, Rs, Rsh, and carrier concentration on a CdTe/CdS cell to maximize its performance. Furthermore, the incorporation effect of ZnO:Al (TCO) and CuSCN (HTL) nanolayers was studied for the first time. As a result, the efficiency of the solar cell was maximized from 16.04% to 17.74% by increasing the Jsc and Voc. This work will play an essential role in enhancing the performance of CdTe-based devices with the best performance.

Performance simulation of solar cell based on AZO/CdTe heterostructure by SCAPS 1D software.

Simulation and analysis of solar cells based on the heterojunction of zinc oxide doped with aluminum (AZO) and cadmium telluride (CdTe) with the structure (Al/AZO/CdTe/NiO/Ni) using the Simulator of the capacitance of solar cells - 1 dimension (SCAPS-1D) has been presented in this paper. AZO is used as a window layer and Nickel oxide (NiO) has been introduced as a hole transport layer (HTL). Through the software, the effect of thickness, absorber (CdTe), and window (AZO) layers carrier concentration, operating temperature, and resistances (series and shunt) have been studied. Simulation results show that the solar cell performance can be greatly improved by adjusting the layer's thickness and carrier concentration, obtaining optimal values of 10 nm and 10 18 c m - 3 for the AZO layer, while for the CdTe layer they were 2 µm and 10 15 c m - 3 . The optimum series and shunt resistances are in the range of 1-3 Ω c m 2 and 1800-2200 Ω c m 2 respectively. A maximum power conversion efficiency (PCE) of 14.2% is achieved with an open circuit voltage (Voc) of 0.74 V, short circuit current density (Jsc) of 26.15 m A / c m 2 and a fill factor (FF) of 72.83%, this shows AZO potential to be considered as an interesting material to replace CdS window layer.

Numerical Study of a Solar Cell to Achieve the Highest InGaN Power Conversion Efficiency for the Whole In-Content Range.

A solar cell structure with a graded bandgap absorber layer based on InGaN has been proposed to overcome early predicted efficiency. Technological issues such as carrier concentration in the p- and n-type are based on the data available in the literature. The influence of carrier concentration-dependent mobility on the absorber layer has been studied, obtaining considerable improvements in efficiency and photocurrent density. Efficiency over the tandem solar cell theoretical limit has been reached. A current density of 52.95 mA/cm2, with an efficiency of over 85%, is determined for a PiN structure with an InGaN step-graded bandgap absorption layer and 65.44% of power conversion efficiency for the same structure considering piezoelectric polarization of fully-strained layers and interfaces with electron and hole surface recombination velocities of 10-3 cm/s.

Solar Cell Parameter Extraction Method from Illumination and Dark I-V Characteristics.

A novel method to extract the seven parameters of the double-diode model of solar cells using the current-voltage (I-V) characteristics under illumination and in the dark is presented. The algorithm consists of two subroutines which are alternatively run to adjust all the parameters of the cell in an iterative process. Curve fitting of the light I-V characteristics ensures accuracy in the prediction of the maximum power point, whereas simultaneously fitting the dark I-V characteristics results in a set of physically meaningful parameters that provide information about the physical performance of the photovoltaic devices. Experimental I-V curves of in-house solar cells are used to validate the proposed parameter extraction method, which can be furthermore applied to other types of p-n junction-based photovoltaic devices.

Study of a Lead-Free Perovskite Solar Cell Using CZTS as HTL to Achieve a 20% PCE by SCAPS-1D Simulation.

In this paper, a n-i-p planar heterojunction simulation of Sn-based iodide perovskite solar cell (PSC) is proposed. The solar cell structure consists of a Fluorine-doped tin oxide (FTO) substrate on which titanium oxide (TiO2) is placed; this material will act as an electron transporting layer (ETL); then, we have the tin perovskite CH3NH3SnI3 (MASnI3) which is the absorber layer and next a copper zinc and tin sulfide (CZTS) that will have the function of a hole transporting layer (HTL). This material is used due to its simple synthesis process and band tuning, in addition to presenting good electrical properties and stability; it is also a low-cost and non-toxic inorganic material. Finally, gold (Au) is placed as a back contact. The lead-free perovskite solar cell was simulated using a Solar Cell Capacitance Simulator (SCAPS-1D). The simulations were performed under AM 1.5G light illumination and focused on getting the best efficiency of the solar cell proposed. The thickness of MASnI3 and CZTS, band gap of CZTS, operating temperature in the range between 250 K and 350 K, acceptor concentration and defect density of absorber layer were the parameters optimized in the solar cell device. The simulation results indicate that absorber thicknesses of 500 nm and 300 nm for CZTS are appropriate for the solar cell. Further, when optimum values of the acceptor density (NA) and defect density (Nt), 1016 cm-3 and 1014 cm-3, respectively, were used, the best electrical values were obtained: Jsc of 31.66 mA/cm2, Voc of 0.96 V, FF of 67% and PCE of 20.28%. Due to the enhanced performance parameters, the structure of the device could be used in applications for a solar energy harvesting system.

Electrodeposited PEDOT:PSS-Al2O3 Improves the Steady-State Efficiency of Inverted Perovskite Solar Cells.

The atomic layer deposition (ALD) of Al2O3 between perovskite and the hole transporting material (HTM) PEDOT:PSS has previously been shown to improve the efficiency of perovskite solar cells. However, the costs associated with this technique make it unaffordable. In this work, the deposition of an organic-inorganic PEDOT:PSS-Cl-Al2O3 bilayer is performed by a simple electrochemical technique with a final annealing step, and the performance of this material as HTM in inverted perovskite solar cells is studied. It was found that this material (PEDOT:PSS-Al2O3) improves the solar cell performance by the same mechanisms as Al2O3 obtained by ALD: formation of an additional energy barrier, perovskite passivation, and increase in the open-circuit voltage (Voc) due to suppressed recombination. As a result, the incorporation of the electrochemical Al2O3 increased the cell efficiency from 12.1% to 14.3%. Remarkably, this material led to higher steady-state power conversion efficiency, improving a recurring problem in solar cells.

An Extended Methodology for Sizing Solar Unmanned Aerial Vehicles: Theory and Development of a Python Framework for Design Assist.

There is a growing interest in using unmanned aerial vehicles (UAVs) in the most diverse application areas from agriculture to remote sensing, that determine the need to project and define mission profiles of the UAVs. In addition, solar photovoltaic energy increases the flight autonomy of this type of aircraft, forming the term Solar UAV. This study proposes an extended methodology for sizing Solar UAVs that take off from a runway. This methodology considers mission parameters such as operating location, altitude, flight speed, flight endurance, and payload to sizing the aircraft parameters, such as wingspan, area of embedded solar cells panels, runway length required for takeoff and landing, battery weight, and the total weight of the aircraft. Using the Python language, we developed a framework to apply the proposed methodology and assist in designing a Solar UAV. With this framework, it was possible to perform a sensitivity analysis of design parameters and constraints. Finally, we performed a simulation of a mission, checking the output parameters.

Nanostructured TiO2 and PEDOT Electrodes with Photovoltaic Application.

In this work, nanostructured TiO2 and poly-3,4-ethylenedioxythiophene (PEDOT) layers were electrochemically prepared over transparent electrodes. Morphological characterization evidenced the presence of nanostructures as planed with 50-nm-wide TiO2 rod formations followed by 30-nm-wide PEDOT wires. Different characterizations were made to the deposits, establishing their composition and optic properties of the deposits. Finally, photovoltaic cells were prepared using this modified electrode, proving that the presence of PEDOT nanowires in the cell achieves almost double the efficiency of its bulk analogue.

ZnO Nanowires/N719 Dye With Different Aspect Ratio as a Possible Photoelectrode for Dye-Sensitized Solar Cells.

The vapor-liquid-solid (VLS) process was applied to fabricate zinc oxide nanowires (ZnO NWs) with a different aspect ratio (AR), morphological, and optical properties. The ZnO NWs were grown on a system that contains a quartz substrate with transparent conductive oxide (TCO) thin film followed by an Al-doped ZnO (AZO) seed layer; both films were grown by magnetron sputtering at room temperature. It was found that the ZnO NWs presented high crystalline quality and vertical orientation from different structural and morphological characterizations. Also, NWs showed a good density distribution of 69 NWs/µm2 with a different AR that offers their capability to be used as possible photoelectrode (anode) in potential future device applications. The samples optical properties were studied using various techniques such as photoluminescence (PL), absorption, and transmittance before and after sensitization with N719 dye. The results demonstrated that NW with 30 nm diameter had the best characteristics as feasible photoelectrode (anode) (high absorption, minimum recombination, high crystallinity). Also, the present samples optical properties were found to be improved due to the existence of N719 dye and Au nanoparticles on the tip of NWs. NWs grown in this work can be used in different photonic and optoelectronic applications.

Relationship between photo-physical and electrochemical properties of D-π-A compounds regarding solar cell applications. 1. Substituent type effect in photovoltaic performance.

Studying the electrochemical characteristics is an important step for determining interactions between molecules and the chemical environment. Moreover, the electrochemical evaluation of dyes is highly needed to establish the behavior of electro-active chemical species inside dye-sensitized solar cells (DSSCs). Four compounds, M8-1, M8-2, M8-O1, and M8-O2 (with a common organic structure (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(phenylamino)-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid), are studied in two solvents, tetrahydrofuran (THF) and dimethylsulfoxide (DMSO). Among the studied compounds, M8-1 has highlighted characteristics compared with the others: its ground and excited states oxidation potential are the highest (1.14 and -1.22 V, respectively). Also, it shows the lowest energy gap between the excited state oxidation potential and the TiO2 conduction band. Relating to the substituent effect, the shorter the length, the higher the energetic difference in the electronic transition (M8-1 and 2). Comparing characteristics through quantum chemistry, the values obtained in DMSO are the most predictable. The injection energies signal that M8-1 is the best injector. The performances in solar cells are measured in three TiO2 materials: Degussa (D-TiO2), active opaque (A-TiO2), and transparent (T-TiO2). The IPCE results show the A > T > D average tendency, and the family of substituted alkyl has higher values than the alcoxyl one. Furthermore, in the first family the methyl substituent has a higher value than the ethyl one. M8-1 has the highest IPCE value, on average. In terms of efficiency, the alkyl substituted family again has higher values than the alcoxyl family. On average, the methyl substituent has a higher value than the ethyl one in both families. M8-1 has the highest efficiency value.

Exploring the Properties of Niobium Oxide Films for Electron Transport Layers in Perovskite Solar Cells.

In this work, niobium oxide films were deposited by reactive magnetron sputtering under different oxygen flow rate and applied as electron transport layer in perovskite solar cells. It was found that the deposition made using 3.5 sccm of oxygen flow resulted in films with better electrical properties which helped the extraction of the photogenerated electrons to the external circuit, improving the Jsc and consequently the device efficiency. In addition, by photoluminescence measurements, we found a better charge transfer from perovskite to TiO2/niobium oxide film deposited at 3.5 sccm of oxygen flow.

Versatile ruthenium(II) dye towards blue-light emitter and dye-sensitizer for solar cells.

A versatile Ru(II) complex bearing an anthracene moiety was synthesized in our search for suitable compounds towards efficient molecular devices. The new engineered dye, cis­[Ru(dcbH2)(NCS)2(mbpy­anth)] (dcbH2=2,2'­bipyridyl­4,4'­dicarboxylic acid, mbpy­anth=4­[N­(2­anthryl)carbamoyl]­4'­methyl­2,2'­bipyridine), exhibits a blueish emission in a vibronically structured spectrum ascribed to the fluorescence of a 1LCAnth (ligand centered) excited state in the anthracene and has a potential to be exploited in the fields of smart lighting and displays. This complex was also employed in dye-sensitized solar cells with fairly efficient solar energy conversion with the use of self-assembled TiO2 compact layers beneath the TiO2 mesoporous film to prevent meso­TiO2/dye back reactions. Further photoelectrochemical investigations through incident photon-to-current efficiency and electrochemical impedance spectra showed that the all-nano-TiO2 compact layer acts as contact layers that increase the electron harvesting in the external circuit, enhancing efficiencies up to 50%.

Exploring the relevance of thiophene rings as bridge unit in acceptor-bridge-donor dyes on self-aggregation and performance in DSSCs.

The possibility of dye charge recombination in DSSCs remains a challenge for the field. This consists of: (a) back-transfer from the TiO2 to the oxidized dye and (b) intermolecular electron transfer between dyes. The latter is attributed to dye aggregation due to dimeric conformations. This leads to poor electron injection which decreases the photocurrent conversion efficiency. Most organic sensitizers are characterized by an Acceptor-Bridge-Donor (A-Bridge-D) arrangement that is commonly employed to provide charge separation and, therefore, lowering the unwanted back-transfer. Here, we address the intermolecular electron transfer by studying the dimerization and photovoltaic performance of a group of A-Bridge-D structured dyes. Specifically, eight famous sulfur containing π-bridges were analyzed (A and D remained fixed). Through quantum mechanical and molecular dynamics approaches, it was found that the formation of weakly stabilized dimers is allowed. The dyes with covalently bonded and fused thiophene rings as Bridges, 6d and 7d as well as 8d with a fluorene, would present high aggregation and, therefore, high probability of recombination processes. Conversely, using TiO2 cluster and surface models, delineated the shortest bridges to improve the adsorption energy and the stability of the system. Finally, the elongation of the bridge up to 2 and 3 units and their photovoltaic parameters were studied. These results showed that all the sensitizers are able to provide similar photocurrent outcomes, regardless of whether the bridge is elongated. © 2017 Wiley Periodicals, Inc.

Optical properties of P3HT and N2200 polymers: a performance study of an optimally tuned DFT functional.

The optical properties of systems composed of the polymers PolyeraActivInk™ N2200 and P3HT are experimentally and theoretically investigated using UV-Vis spectroscopy and time-dependent density functional theory calculations, respectively. From a theoretical point of view, we carried out an analysis considering several functionals and model oligomers of different sizes to mimic the polymers. As our studies were performed with and without solvents, a first important result regards the fact that, by considering solvent effects, a better agreement between theoretical and experimental results could be achieved. Our findings also show that an optimally tuned functional is better suited to describe the experimental absorption profile than a hybrid one for the flexible polymer (P3HT). For the almost rigid polymer considered here (N2200), on the other hand, hybrid functionals may perform better than tuned functionals.

Simultaneous Top and Bottom Perovskite Interface Engineering by Fullerene Surface Modification of Titanium Dioxide as Electron Transport Layer.

Optimization of the interface between the electron transport layer (ETL) and the hybrid perovskite is crucial to achieve high-performance perovskite solar cell (PSC) devices. Fullerene-based compounds have attracted attention as modifiers on the surface properties of TiO2, the archetypal ETL in regular n-i-p PSCs. However, the partial solubility of fullerenes in the aprotic solvents used for perovskite deposition hinders its application to low-temperature solution-processed PSCs. In this work, we introduce a new method for fullerene modification of TiO2 layers derived from nanoparticles (NPs) inks. Atomic force microscopy characterization reveals that the resulting ETL is a network of TiO2-NPs interconnected by fullerenes. Interestingly, this surface modification enhances the bottom interface of the perovskite by improving the charge transfer as well as the top perovskite interface by reducing surface trap states enhancing the contact with the p-type buffer layer. As a result, rigid PSCs reached a 17.2% power conversion efficiency (PCE), while flexible PSCs exhibited a remarkable stabilized PCE of 12.2% demonstrating the potential application of this approach for further scale-up of PSC devices.