17 research outputs found
The influence of siloxane side-chains on the photovoltaic performance of a conjugated polymer
No full text\u3cp\u3eThe effect of gradually replacing the branched alkyl side chains of a diketopyrrolopyrrole (DPP) conjugated polymer by linear side chains containing branched siloxane end groups on the photovoltaic performance of blends of these polymers with a common fullerene acceptor is investigated. With an increasing proportion of siloxane side chains, the molecular weight and solubility of the polymers decreases. While the siloxane containing polymers exhibit a higher hole mobility in field-effect transistors, their performance in solar cells is less than the polymer with only alkyl sides chains. Using grazing-incidence wide-angle X-ray scattering, transmission electron microscopy, and fluorescence spectroscopy we identify two main reasons for the reduced performance of siloxane containing polymers in solar cells. The first one is a somewhat coarser phase-separated morphology with slightly wider polymer fibers. This is unexpected as often the fiber width is inversely correlated with polymer solubility. The second one is stronger non-radiative decay of the pristine polymers containing siloxane side chains.\u3c/p\u3
Aqueous nanoparticle polymer solar cells: effects of surfactant concentration and processing on device performance
No full textPolymer solar cells based on PDPP5T and PCBM as donor and acceptor materials, respectively, were processed from aqueous nanoparticle dispersions. Careful monitoring and optimization of the concentration of free and surface-bound surfactants in the dispersion, by measuring the conductivity and ζ-potential, is essential to avoid aggregation of nanoparticles at low concentration and dewetting of the film at high concentration. The surfactant concentration is crucial for creating reproducible processing conditions that aid in further developing aqueous nanoparticle processed solar cells. In addition, the effects of adding ethanol, of aging the dispersion, and of replacing [60]PCBM with [70]PCBM to enhance light absorption were studied. The highest power conversion efficiencies (PCEs) obtained are 2.0% for [60]PCBM and 2.4% for [70]PCBM-based devices. These PCEs are limited by bimolecular recombination of photogenerated charges. Cryo-TEM reveals that the two components phase separate in the nanoparticles, forming a PCBM-rich core and a PDPP5T-rich shell and causing a nonoptimal film morphology
Thermal behaviour of dicarboxylic ester bithiophene polymers exhibiting a high open-circuit voltage
No full textNine different polythiophene derivatives based on dialkyl-(2,2′-bithiophene-5,5′-diyl)-4,4′-dicarboxylate (DCB) alternating with thiophene (T), bithiophene (2T) or thienothiophene (TT) as co-monomer have been synthesized to study the effect of the polymer backbone and side chain length on the thermal properties, the tendency to aggregate, and the photovoltaic performance. Polymers incorporating DCB and 2T show increased crystallinity and a large effect of the side chain length on the morphology of the photoactive layer blends. Thermal annealing increases the crystallinity of the polymers and enhances the long-wavelength light absorption. The concomitant increase in polymer fibre width, however, deteriorates the photovoltaic performance. The best devices were made using the PDCB-2T polymer with 2-butyloctyl side chains providing a power conversion efficiency of 5.18%. The PDCB-T polymer with 2-ethylhexyl substituents shows a comparable efficiency (5.08%), but with a significantly higher open-circuit voltage due to deeper frontier orbitals levels
Thiophene Rings Improve the Device Performance of Conjugated Polymers in Polymer Solar Cells with Thick Active Layers
No full textThe role of the axial substituent in subphthalocyanine acceptors for bulk-heterojunction solar cells
No full textFour hexachlorosubphthalocyanines SubPcCl6-X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution-processed bulk-heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone-shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7-Th as the electron donor exhibit a power conversion efficiency up to 4% and an external quantum efficiency approaching 60% due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non-fullerene acceptors based on subphthalocyanines and structurally related systems
Effect of Alkyl Side Chains of Conjugated Polymer Donors on the Device Performance of Non-Fullerene Solar Cells
No full textThe influence of the chemical structure
of conjugated polymers
on the nanophase separation and device performance in fullerene-based
solar cells has been widely studied, while this is less investigated
in non-fullerene solar cells. In this work, we design three conjugated
polymers with different length of side chains, and we find that the
length of side chains has little influence on the quantum efficiencies
of non-fullerene solar cells. As a comparison, the length of side
chains has a significant effect on the quantum efficiencies of fullerene-based
solar cells. This indicates that morphology of the blended thin films
in non-fullerene solar cells is rather independent of the length of
the donor side chains, and the mechanism for morphology evolution
in the non-fullerene system is completely different from that in the
fullerene system. Our conclusion is confirmed by a variety of advanced
characterization techniques. The studies reveal that in blended thin
films based on the non-fullerene material the donor polymers with
different side chains have a similar coherence length of π–π
stacking, crystal size and domain purity, giving rise to similar internal
quantum efficiency and power conversion efficiency of the solar cells
Bilayer-Ternary Polymer Solar Cells Fabricated Using Spontaneous Spreading on Water
No full textA new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk-heterojunction layers obtained using spontaneous spreading of polymer-fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer-fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin-coated devices of similar thickness. Next, bilayer-ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer-fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer-fullerene layer by transfer from a water surface. These novel bilayer-ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near-infrared bias light illumination. Drift-diffusion calculations confirm that near-infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends
Physical and chemical degradation behavior of sputtered aluminum doped zinc oxide layers for Cu(In,Ga)Se-2 solar cells
No full textInternational audienceSputtered aluminum doped zinc oxide (ZnO:Al) layers on borosilicate glass were exposed to damp heat (85 degrees C/85% relative humidity) for 2876 h to accelerate the physical and chemical degradation behavior. The ZnO: Al samples were characterized by electrical, compositional and optical measurements before and after degradation. Hall measurements show that the carrier concentration stayed constant, while the Hall mobility decreased and the overall resistivity thus increased. This can be explained by the increase of potential barriers at the grain boundaries due to the occurrence of space charge regions caused by additional electron trapping sites. X-Ray Diffraction and optical measurements show that the crystal structure and transmission in the range 300-1100 nm do no change, hereby confirming that the bulk structure stays constant.Furthermore, on the surface, white spots appeared, containing elements that migrated from the glass, like silicon and calcium, which reacted with elements from the environment, including oxygen, carbon and chlorine. Depth profiling showed that the increase of the potential barrier is caused by the diffusion of H2O/OH- through the grain boundaries leading to the formation of Zn(OH)(2) or similar species or adsorption of species. They also indicate the presence of chloride and sulfide in the top layer and the possible presence of Zn-5(OH)(8)Cl-2 center dot H2O and Zn4SO4(OH)(6)center dot nH(2)O (C) 2013 Elsevier B.V. All rights reserved
Subnaphthalocyanines as electron acceptors in polymer solar cells: Improving device performance by modifying peripheral and axial substituents
No full textA new class of subnaphthalocyanines bearing various peripheral and axial substituents have been synthesized for use as electron acceptors in solution-processed bulk-heterojunction polymer solar cells. The resulting solar cells exhibit modest photovoltaic performance with contributions from both the polymer donor and subnaphthalocyanine acceptor to the photocurrent
Efficient thick-film polymer solar cells with enhanced fill factors via increased fullerene loading
No full textDeveloping effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices
