KEYWORDS: Perovskite, Solar cells, Crystals, Interfaces, External quantum efficiency, Chemical elements, Resistance, Absorption, Thin film solar cells, Thin films
Different additive materials have been extensively investigated in hybrid perovskite solar cells (PSC) to enhance the crystal growth, minimize defects and to improve the device stability. However, most of the additive engineering attempts are basically dedicated to the improvement of open circuit voltage (Voc) in PSC. There are very few investigations where PSC’s short circuit current density (Jsc) is significantly improved along with the Voc. In this work, a novel organic additive material called Phenylhydrazinium iodide (PHI) has been employed to enhance the Jsc hence the overall power conversion efficiency (PCE) of CH3NH3PbI3 PSC. Surprisingly, after PHI treatment Jsc jumps from around 20 to 23 mA/cm2 which corresponds to ~15% increase of short circuit current density resulting overall 21% improvement in PCE. PHI treatment in wide-bandgap perovskite will help to mitigate the problem of shorter Jsc in compared to low-bandgap perovskite materials in multi-junctional tandem solar cell.
Conjugated polymers doped with metal ions offer superior material properties in the development of next generation flexible PV technology [1]. Charge transport mechanism in metallated conjugated polymer with different solvent processing was described by A.F. Mitul in [1]. In this work, the modification of heavy metals e.g, Pt, Ru etc in organometllic solar devices are investigated. The variation in device performance i.e, open circuit voltage (Voc), external quantum efficiency (EQE) is explained in the light of nano scale morphology. Change of heavy metals in organicchemical structure provides differences in nanoscale morphology and hence, it describes the favorable condition for optimum device performance.
We report effects of controlled humidity in ambient condition on grain boundary potential and charge transport within the grains of Pervoskite films prepared by sequential deposited technique. Grain boundary exhibited variation of their electronic properties with change in humidity level from sample kept inside glove box to 75% RH. X-ray diffraction (XRD) indicates the formation of PbI2 phase with increasing humidity level. The degradation of Pervoskite solar cell is mainly associated with the increase of PbI2 phase with increase in humidity level and hydration of the grain boundaries with the formation of hydrated phases. Spatial mapping of surface potential in the Perovskite film exhibits higher positive potential at grain boundaries compared to the surface of the grains. Grain boundary potential barrier were found to increase from ~35 meV to 80 meV for perovskite film exposed to 75% RH level compared to perovskite film kept inside glove box. Nanoscale current sensing measurement (Cs-AFM) shows that charge transport in perovskit solar cell strongly depends in humidity level. Performances of the solar cell was maximum for 25% humidity with 14.01 %. Transient measurement shows decrease in charge carrier life time and charge transport time with increase in humidity level. Our results show strong correlation between humidity level, electronic grain boundary properties and device performance.
Smooth, compact and defect free morphology of perovskite is highly desired for enhanced device performance. Several routes such as thermal annealing, use of solvent mixtures, growth under controlled humidity has been adopted to obtain crystalline, smooth and defect free perovskite film. Herein we showed direct use of water (H2O) as co-solvent in precursor solution and have optimized the water content required to obtain smooth and dense film. Varying concentration of water was used in precursor solution of CH3NH3I and PbI2 mixed in γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO). Perovskite films were crystallized using toluene assisted solvent engineering method using GBL:DMSO:H2O as solvent mixture. The amount of water was varied from 1% to 25%, which resulted in change in film morphology and perovskite crystallinity. It was concluded that an appropriate amount of water is required to assist the crystallization process to obtain smooth pin-hole free morphology. The change in morphology led to improved fill factor in the device, with highest efficiency ~14%, which was significantly higher than devices made from perovskite film without adding water. We also showed that addition of up to 25% by volume of water does not significantly change the device performance.
KEYWORDS: Solar cells, Polymers, Annealing, Silver, Zinc oxide, Solar energy, Atomic force microscopy, 3D image processing, Photovoltaics, Electrical engineering
Solution processed tandem polymer solar cell has drawn a great deal of attention due its low cost, ease of production and capability of harvesting solar energy more efficiently. In solution processed tandem polymer solar cell, the most challenging part is the optimization of interfacial layer. In this work, we have investigated the robustness of PEDOT:PSS/AZO/PEIE interfacial layer to develop tandem polymer solar cell. While developing triple junction polymer solar cell, temperature of second interfacial layer has also a great impact on overall device performance. Here, the performance of tandem polymer solar cell was investigated on different temperature of interfacial layer.
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.
Recently, excellent solar cell device performances have been achieved with solution-processed small-molecule donor materials. Small molecules have well defined structures and thus allow better control of self-assembly in the solid state. However, the easy formation of H-type aggregates and lack of strong interactions between nanodomains could limit charge transport, device performance, and long-term stability. We have recently explored the synthesis of ring-protected small molecules (with rings surrounding the center of the molecules), studied the intermolecular interactions in solution and solid state, and conducted preliminary solar cell device fabrications. It has been found that the molecules behave very differently from conventional flat small molecules in both solution and solid states. Proton NMR study of solutions of different concentrations revealed the presence of strong intermolecular interactions as a result of absence or shortage of open-ended alkyl side chains; however, such strong interactions do not lead to precipitation of the molecules even at high concentrations. Excellent films are routinely obtained from the neat small molecules despite the much reduced number of solubilizing groups. The New findings strongly suggest that ring protection is an effective strategy to avoid Haggregation and maintain strong pi-pi interactions simultaneously. Such materials are expected to form head-tail selfassemblies that will open new possibilities for small molecule organic materials. Conceptually, thin films of such materials are potentially more isotropic in charge transport than conventional small molecule and polymer films, a property desirable for photovoltaics and some other optoelectronic applications.
The correlation between the physical properties of spin-casting solvents, film morphology, nanoscale charge transport, and device performance was studied in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends, spin cast with two halogenated aromatic solvents: chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB). 1,2-DCB-based blends exhibited fine phase separation of ∼10 to 15 nm length scale with ordered self-assembly of P3HT whereas blends spin cast from CB showed coarse phase separation with large isolated clusters of ∼25 to 100 nm of donor- and acceptor-rich regions. Higher solubility of both P3HT and PCBM in 1,2-DCB and a slower drying rate of 1,2-DCB (because of higher boiling point) facilitated self-organization and ordering of P3HT and promoted finer phase separation. Higher local hole mobility in 1,2-DCB-based blend was attributed to efficient hole transport through the ordered network of P3HT chains. Moreover, higher local illuminated current (dark + photocurrent) in 1,2-DCB-based blend suggested efficient diffusion and dissociation of excitons due to finer phase separation. As a consequence, 1,2-DCB-based devices exhibited higher short circuit current density (Jsc), external quantum efficiency and power conversion efficiency in contrast to the CB-based device. It was also observed that the device performance was not limited by light absorption and exciton generation; rather morphology dependent processes subsequent to exciton generation, primarily charge transport to the electrodes, limited device performance.
A thienylsilane molecular layer is self-assembled onto vertically aligned ZnO nanowire templates for promoting in situ electrochemical polymerization of P3HT. The silane functionalization on ZnO surface is investigated using x-ray photoelectron spectroscopy and water contact angle measurements. The silane-based surface modified layer acts as a favorable nucleation site for electrochemical polymerization. We find that the oxidation potential for electrochemical polymerization is obviously decreased compared to that without a surface modifier. The UV-visible absorption in the ZnO nanowire/P3HT film with thienylsilane molecular layer is much stronger than that without surface modification.
This work reports the study of solvent effects on the morphology for P3HT/PCBM films using chlorobenzene, 1,
2-dichlorobenzene and 1, 3-dichlorobenzene. Although extensive research has been focused on investigating devices
using chlorobenzene and 1, 2-dichlorobenzene and it was found that 1, 2-dichlorobenzene led to an improved device
performance, little work has been conducted in the morphology by comparing films fabricated via chlorobenzene, 1,
2-dichlorobenzene and 1, 3-dichlorobenzene. Atomic force microscopy (AFM) was performed to study the film
morphology using chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene as solvents. Initial studies showed
that the size of nanocrystallites in 1, 2-dichlorobenzene based films is smaller than nanoclusters in the other two films.
Kelvin probe force microscopy (KFM) images, which were used to figure out the electron transport pathway, together
with AFM images, showed the solvent effect on the morphology of these films. In addition, obvious red shifts were
observed in the UV-Vis absorption spectra for the P3HT/PCBM blend from 1, 3-dichlorobenzene and 1,
2-dichlorobenzene compared to the one from chlorobenzene.
A series of dye-sensitized solar cells (DSSCs) were fabricated using ZnO nanorod arrays as the anode electrode.
The ZnO nanorod arrays were grown on the fluorine doped tin dioxide (FTO) substrates by a hydrothermal method.
The scanning electron microscopy (SEM) images indicated that the ZnO nanorod arrays were highly oriented on
FTO substrates with an average diameter of ~40 nm and an average length of ~1 μm. After sensitized by Z-907
dye via impregnation in solution, ZnO nanorod arrays changed the color from white to pink. This indicated that the
dye had been successfully attached to ZnO nanorods. The high-aspect-ratio (~25) ZnO nanorod arrays are expected
to improve charge transport through the formation of continuous channels along the nanorods. We fabricated
photovoltaic cells based on these ZnO nanorod arrays and found the deposition time and effective area were two
important factors affecting short circuit current densities and cell efficiencies. The device performance (Voc = 0.48
V, Jsc = 5.39 mA/cm2, η = 0.73 %) showed a great potential for solar energy conversion.
Over the last decade, conjugated polymer-based semiconductors have been developed as a novel class of photovoltaic materials that have the potential to lower costs. Solvent based polymers MEH-PPV, MDMO-PPV, P3HT, and P3OT have been reported as electron donors in photovoltaic devices. In this research, we studied the use of a water soluble polythiophene - Sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate]) [PTEBS] in photovoltaic devices. Solar cells in the configuration of bilayer heterojunctions with TiO2 were prepared. The water-soluble polythiophene showed significant photovoltaic effect and potential for use in solar cells. The use of this polymer would allow safe, environmentally friendly processing. In addition, due to the covalent bonding of the counterion to the polymer backbone chain simultaneously with electron loss in the doping and oxidation, the water-soluble polymer PTEBS can be self-doped by acids. The appearance and absorption spectra of the self-doped solutions and films have also been investigated. New absorption bands in the ultraviolet and infrared have been observed after acidic doping offering the possibility of improved light harvesting. Experimental results have shown that the polymer can be used as the active layer in photovoltaic applications. These photovoltaic devices had an energy conversion efficiency of 0.23% and a fill factor of 0.41 under the illumination of an 80 mW/cm2 solar simulator. A simple mechanism has also been proposed to fit the open circuit voltage found in the devices.
At first, we made a approximate λ/4-shifted DFB Yb-doped fiber laser with double exposure method. The length of Yb-doped fiber is 10cm. Secondly, we treat λ/4-shifted Yb-doped DFB fiber laser with UV trimming. We observed the running characteristic of the laser with scanning F-P interferometer and oscilloscope at the same time. Finally, we obtained SLM Yb-doped λ/4-shifted DFB fiber laser. The laser threshold is 20mW. When pump power is 130mW, we have gained 25mW SLM laser at 1053nm. Finally, we obtained SLM Yb-doped λ/4-shifted DFB fiber laser. The threshold of which is 20mW. When pump power is 130mW, we obtained 25mW SLM laser at 1053nm.
? phase-shifted distributed feedback fiber lasers , ofwhich the power would focus on the phase-shift region ifthe coupling coefficient was high enough, was often adopted in order to acquire single-frequency output. But if the phase shift region was far away from the center of the grating, the output wavelength would be different that the fundamental mode would focus on the port which was near the phase shift region while the ±1 modes would focus on the other port due to the spatial hole burning. Both the theoretical analysis and experiments were presented.
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